[
    {
        "text": "Possible superconductivity in multi-layer-graphene by application of a\n  gate voltage: The carrier density in tens of nanometers thick graphite samples\n(multi-layer-graphene, MLG) has been modified by applying a gate voltage\n($V_g$) perpendicular to the graphene planes. Surface potential microscopy\nshows inhomogeneities in the carrier density ($n$) in the sample near surface\nregion and under different values of $V_g$ at room temperature. Transport\nmeasurements on different MLG samples reveal that under a large enough applied\nelectric field these regions undergo a superconducting-like transition at $T\n\\lesssim 17$ K. A magnetic field applied parallel or normal to the graphene\nlayers suppresses the transition without changing appreciably the transition\ntemperature.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magneto-elastic coupling model of deformable anisotropic superconductors: We develop a magneto-elastic (ME) coupling model for the interaction between\nthe vortex lattice and crystal elasticity. The theory extends the Kogan-Clem's\nanisotropic Ginzburg-Landau (GL) model to include the elasticity effect. The\nanisotropies in superconductivity and elasticity are simultaneously considered\nin the GL theory frame. We compare the field and angular dependences of the\nmagnetization to the relevant experiments. The contribution of the ME\ninteraction to the magnetization is comparable to the vortex-lattice energy, in\nmaterials with relatively strong pressure dependence of the critical\ntemperature. The theory can give the appropriate slope of the field dependence\nof magnetization near the upper critical field. The magnetization ratio along\ndifferent vortex frame axes is independent with the ME interaction. The\ntheoretical description of the magnetization ratio is applicable only if the\napplied field moderately close to the upper critical field.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Temperature Dependence of Superconducting Gaps in Mg(Al)B2 System\n  Investigated by SnS-Andreev Spectroscopy: Detailed temperature dependence of both superconducting gaps was obtained\ndirectly by means of SnS-Andreev spectroscopy. The \\Delta \\sigma,\\pi(T) -curves\nwere shown to be deviated from standard BCS-like behavior, due to k-space\nproximity effect between \\sigma - and \\pi - condensates, which could give a key\nto experimental determination of interband electron-phonon coupling constants.\nFor the first time, an excellent qualitative agreement with theoretical\npredictions of Nicol and Carbotte, and Moskalenko and Suhl was shown.\ndI(V)/dV-spectra of SnS-Andreev contacts based on MgB2 samples (with defects of\ncrystal structure), and Mg(Al)B2 polycrystalline samples (with the local\ncritical temperatures Tc variation 10 K < Tc < 37 K) were studied by means of\nthe \"break-junction\" technique within the temperature range 4.2 K < T < Tc.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Insulating regime of an underdamped current-biased Josephson junction\n  supporting $\\mathbb{Z}_3$ and $\\mathbb{Z}_4$ parafermions: We study analytically a current-biased topological Josephson junction\nsupporting $\\mathbb{Z}_n$ parafermions. First, we show that in an infinite-size\nsystem a pair of parafermions on the junction can be in $n$ different states;\nthe $2\\pi{n}$ periodicity of the phase potential of the junction results in a\nsignificant suppression of the maximal current $I_m$ for an insulating regime\nof the underdamped junction. Second, we study the behaviour of a realistic\nfinite-size system with avoided level crossings characterized by splitting\n$\\delta$. We consider two limiting cases: when the phase evolution may be\nconsidered adiabatic, which results in decreased periodicity of the effective\npotential, and the opposite case, when Landau-Zener transitions restore the\n$2\\pi{n}$ periodicity of the phase potential. The resulting current $I_m$ is\nexponentially different in the opposite limits, which allows us to propose a\nnew detection method to establish the appearance of parafermions in the system\nexperimentally, based on measuring $I_m$ at different values of the splitting\n$\\delta$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Evidence for field-induced excitations in low-temperature thermal\n  conductivity of Bi_2Sr_2CaCu_2O_8: The thermal conductivity ,$\\kappa$, of Bi_2Sr_2CaCu_2O_8 was studied as a\nfunction of magnetic field. Above 5 K, after an initial decrease, $\\kappa(H)$\npresents a kink followed by a plateau, as recently reported by Krishana et al..\nBy contrast, below 1K, the thermal conductivity was found to \\emph{increase}\nwith increasing field. This behavior is indicative of a finite density of\nstates and is not compatible with the existence of a field-induced fully gapped\n$d_{x^{2}-y^{2}}+id_{xy}$ state which was recently proposed to describe the\nplateau regime. Our low-temperature results are in agreement with recent works\npredicting a field-induced enhancement of thermal conductivity by Doppler shift\nof quasi-particle spectrum.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quantum phase transitions in two-dimensional superconductors: a review\n  on recent experimental progress: Superconductor-insulator/metal transition (SIT/SMT) as a paradigm of quantum\nphase transition has been a research highlight over the last three decades.\nBenefit from recent developments in the fabrication and measurements of 2D\nsuperconducting films and nanodevices, unprecedented quantum phenomena have\nbeen revealed in the quantum phase transitions of 2D superconductors. In this\nreview, we introduce the recent progress on quantum phase transitions in 2D\nsuperconductors, focusing on the quantum Griffiths singularity (QGS) and\nanomalous metal state. Characterized by a divergent critical exponent when\napproaching zero temperature, QGS of SMT is discovered in ultrathin crystalline\nGa films and subsequently detected in various 2D superconductors. The\nuniversality of QGS indicates the profound influence of quenched disorder on\nquantum phase transitions. Besides, in a 2D superconducting system, whether a\nmetallic ground state can exist is a long-sought mystery. Recently, the\ncharge-2e quantum oscillations are observed in nanopatterned superconducting\nfilms, indicating the bosonic nature of the anomalous metal state and ending\nthe debate on whether bosons can exist as a metal. The evidences of the\nanomalous metal states have also been reported in crystalline epitaxial thin\nfilms and exfoliated nanoflakes, as well as granular composite films. High\nquality filters are used in these works to exclude the influence of external\nhigh frequency noises in ultralow temperature measurements. The observations of\nQGS and metallic ground states in 2D superconductors not only reveal the\nprominent role of quantum fluctuations and dissipations but also provide new\nperspective to explore quantum phase transitions in superconducting systems.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetic field dependence of pairing interaction in ferromagnetic\n  superconductors with triplet pairing: It is developed a microscopic description of superconductivity in\nferromagnetic materials with triplet pairing triggered by the exchange of\nmagnetic fluctuations. Instead widely used paramagnon model we work with\nphenomenological spectrum of fluctuations in the orthorhombic ferromagnet with\nstrong magnetic anisotropy. Depending of the field orientation parallel or\nperpendicular to the direction of spontaneous magnetization the effective\namplitude of pairing interaction proves to be decreasing or increasing function\nof magnetic field that allows to explain the drastic difference in magnitudes\nof upper critical field in these directions.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The Microstrain-Doping Phase Diagram of the Iron Pnictides\n  Heterostructures at Atomic Limit: The 3D phase diagram of iron pnictides where the critical temperature depends\non charge density and microstrain in the active FeAs layers is proposed. The\niron pnictides superconductors are shown to be a practical realization of a\nheterostructure at the atomic limit made of a superlattice of FeAs layers\nintercalated by spacer layers. We have focussed our interest on the A 1-x\nBxFe2As2 (122) families and we show that FeAs layers have a tensile microstrain\ndue to the misfit strain between the active layers and the spacers. We have\nidentified the critical range of doping and microstrain where the critical\ntemperature gets amplified to its maximum value.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The pseudogap and doping dependent magnetic properties of\n  La2-xSrxCu1-yZnyO4: The effects of planar hole content, p (= x), on the static magnetic\nsusceptibility, chi(T), of polycrystalline La2-xSrxCu1-yZnyO4 compounds were\ninvestigated over a wide range of Sr (x) and Zn (y) contents. The magnetic\nbehavior caused by Zn was found to depend strongly on the hole content. The\napparent magnetic moment induced by Zn was larger in underdoped\nLa2-xSrxCu1-yZnyO4, decreased quite sharply around p ~ 0.19, and did not change\nmuch for further overdoping. This is interpreted in terms of the effect of the\npseudogap on the Zn-induced magnetic behavior, as there is growing evidence\nthat the pseudogap vanishes quite abruptly at p ~ 0.19 +/- 0.01. From a\ndetailed analysis of chi(T) data the Zn-induced magnetic contribution was found\nto be rather complex and showed non-Curie-like features over a wide range of\ntemperature. The observed behavior was scrutinized in terms of two scenarios\n(a) that of independent localized-moments and (b) low energy quasiparticle\nresonances associated with each Zn atom. Our study points towards the latter\nscenario and more generally suggests that there is a re-distribution of\nquasiparticle spectral weight due to Zn substitution, the features of which are\ngreatly influenced by the presence and magnitude of the pseudogap.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Minimal timing jitter in superconducting nanowire single photon\n  detectors: Using two-temperature model coupled with modified time-dependent\nGinzburg-Landau equation we calculate the delay time $\\tau_d$ in appearance of\ngrowing normal domain in the current-biased superconducting strip after\nabsorption of the single photon. We demonstrate that $\\tau_d$ depends on the\nplace in the strip where photon is absorbed and monotonically decreases with\nincreasing of the current. We argue, that the variation of $\\tau_d$ (timing\njitter), connected either with position-dependent response or Fano fluctuations\ncould be as small as the lowest relaxation time of the superconducting order\nparameter $\\sim \\hbar/k_BT_c$ ($T_c$ is the critical temperature of the\nsuperconductor) when the current approaches the depairing current.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quasiparticle spectrum near the gap node directions in the mixed state\n  of d-wave superconductors: We report on a calculation of the quantized energy spectrum and quasiparticle\neigenfunctions for low lying excitations in the mixed state of clean d-wave\nsuperconductors. Our study is based on an approximate analytical solution of\nthe Bogolubov-de Gennes equations for both rectangular and triangular flux\nlattices with one of the primitive translations chosen parallel to the gap node\ndirection. For excitations with momenta close to a certain gap node we have\nobtained a set of eigenfunctions which appear to be extended along the chosen\ngap node direction and localized along the perpendicular one on a scale\ndetermined by the intervortex distance. The periodic superfluid velocity field\ninduces a band structure in the spectrum, which depends essentially on the\nvortex lattice geometry.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Nickelate superconductors: an ongoing dialog between theory and\n  experiments: After decades of fundamental research, unconventional superconductivity has\nrecently been demonstrated in rare-earth infinite-layer nickelates. The current\nview depicts these systems as a new category of superconducting materials, as\nthey appear to be correlated metals with distinct multiband features in their\nphase diagram. Here, we provide an overview of the state of the art in this\nrapidly evolving topic.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Theory of the Transition at 0.2 K in Ni-doped Bi2Sr2CaCu2O8: A theory is put forward that the electronic phase transition at 0.2 K in\nNi-doped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ is result of the formation of a spin\ndensity wave in the system of Ni impurities. The driving force for the\ntransition is the exchange interaction between the impurity spins and the spins\nof the conduction electrons. This creates a small gap at two of the four nodes\nof the superconducting gap. The effect is to reduce the thermal conductivity by\na factor of two, as observed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Topological Phase Transition in Superconductors with Mirror Symmetry: We provide analytical and numerical evidence that the attractive\ntwo-dimensional Kitaev model on a lattice with mirror symmetry demonstrates\nunusual 'intrinsic' phase at half filling. This phase emerges in the phase\ndiagram at the boundary separating two topological superconductors with\nopposite Chern numbers and exists due to condensation of non-zero momentum\nCooper pairs. Unlike Fulde-Ferrell-Larkin-Ovchinnikov superconductivity, the\nCooper pairs momenta are lying along two lines in the Brillouin zone meaning\nsimultaneous condensation of a continuum of Cooper pairs.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pseudogap phase of cuprate superconductors confined by Fermi surface\n  topology: The properties of cuprate high-temperature superconductors are largely shaped\nby competing phases whose nature is often a mystery. Chiefly among them is the\npseudogap phase, which sets in at a doping $p^*$ that is material-dependent.\nWhat determines $p^*$ is currently an open question. Here we show that the\npseudogap cannot open on an electron-like Fermi surface, and can only exist\nbelow the doping $p_{FS}$ at which the large Fermi surface goes from hole-like\nto electron-like, so that $p^*$ $\\leq$ $p_{FS}$. We derive this result from\nhigh-magnetic-field transport measurements in\nLa$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$ under pressure, which reveal a large and\nunexpected shift of $p^*$ with pressure, driven by a corresponding shift in\n$p_{FS}$. This necessary condition for pseudogap formation, imposed by details\nof the Fermi surface, is a strong constraint for theories of the pseudogap\nphase. Our finding that $p^*$ can be tuned with a modest pressure opens a new\nroute for experimental studies of the pseudogap.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Enhanced low-energy magnetic excitations evidencing the Cu-induced\n  localization in an Fe-based superconductor Fe$_{0.98}$Te$_{0.5}$Se$_{0.5}$: We have performed inelastic neutron scattering measurements on\noptimally-doped Fe$_{0.98}$Te$_{0.5}$Se$_{0.5}$ and 10% Cu-doped\nFe$_{0.88}$Cu$_{0.1}$Te$_{0.5}$Se$_{0.5}$ to investigate the substitution\neffects on the spin excitations in the whole energy range up to 300 meV. It is\nfound that substitution of Cu for Fe enhances the low-energy spin excitations\n($\\le$ 100 meV), especially around the (0.5, 0.5) point, and leaves the\nhigh-energy magnetic excitations intact. In contrast to the expectation that Cu\nwith spin 1/2 will dilute the magnetic moments contributed by Fe with a larger\nspin, we find that the 10% Cu doping enlarges the effective fluctuating moment\nfrom 2.85 to 3.13 $\\mu_{\\rm B}$/Fe, although there is no long- or short-range\nmagnetic order around (0.5, 0.5) and (0.5, 0). The presence of enhanced\nmagnetic excitations in the 10% Cu doped sample which is in the insulating\nstate indicates that the magnetic excitations must have some contributions from\nthe local moments, reflecting the dual nature of the magnetism in iron-based\nsuperconductors. We attribute the substitution effects to the localization of\nthe itinerant electrons induced by Cu dopants. These results also indicate that\nthe Cu doping does not act as electron donor as in a rigid-band shift model,\nbut more as scattering centers that localize the system.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Calculation of Leggett-Takagi relaxation in vortices of superfluid 3He-B: We calculate the relaxation of Brinkman-Smith mode via Leggett-Takagi\nrelaxation in the presence of an isolated vortex in superfluid 3He-B. The\ncalculation is based on an analytical solution of the order parameter far from\nthe vortex axis. We obtain an expression for the dissipated power per vortex\nlength as a function of the tipping angle of the magnetization and the\norientation of the static magnetic field with respect to the vortex.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Structure and Magnetization of Two-Dimensional Vortex Arrays in the\n  Presence of Periodic Pinning: Ground-state properties of a two-dimensional system of superconducting\nvortices in the presence of a periodic array of strong pinning centers are\nstudied analytically and numerically. The ground states of the vortex system at\ndifferent filling ratios are found using a simple geometric argument under the\nassumption that the penetration depth is much smaller than the spacing of the\npin lattice. The results of this calculation are confirmed by numerical studies\nin which simulated annealing is used to locate the ground states of the vortex\nsystem. The zero-temperature equilibrium magnetization as a function of the\napplied field is obtained by numerically calculating the energy of the ground\nstate for a large number of closely spaced filling ratios. The results show\ninteresting commensurability effects such as plateaus in the B-H diagram at\nsimple fractional filling ratios.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Transport properties of the new Fe-based superconductor KxFe2Se2 (Tc =\n  33 K): We synthesized the new Fe-based superconductor K0.8Fe2Se2 single crystals.\nThe obtained single crystal exhibited a sharp superconducting transition, and\nthe onset and zero-resistivity temperature was estimated to be 33 and 31.8 K,\nrespectively. A high upper critical field of 192 T was obtained. Anisotropy of\nsuperconductivity of K0.8Fe2Se2 was ~3.6. Both the high upper critical field\nand comparably low anisotropy are advantageous for the application under high\nmagnetic field.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The crystal structure of FeSe0.44Te0.56: The crystal structure of the superconductor FeSe0.44Te0.56 was redetermined\nby high-resolution X-ray single crystal diffraction at 173 K (anti-PbO-type,\nP4/nmm, a=3.7996(2), c=5.9895(6) A, R1=0.022, wR2=0.041, 173 F^2).\nSignificantly different z-coordinates of tellurium and selenium at the 2c site\nare clearly discernible and were refined to z_Te=0.2868(3) and z_Se=0.2468(7).\nThus the chalcogen heights differ by 0.24 A and the Fe-Se bonds are by 0.154 A\nshorter than the Fe-Te bonds, while three independent (Te,Se)-Fe-(Te,Se) bond\nangles occur. An elevated U33 displacement parameter of the iron atom is\nsuggestive of a slightly puckered Fe layer resulting from different\ncombinations of Se or Te neighbors. Such strong disorder underlines the\nrobustness of superconductivity against structural randomness and has not yet\nbeen considered in theoretical studies of this system.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Hydrogen-induced high-temperature superconductivity in two-dimensional\n  materials: Example of hydrogenated monolayer MgB$_2$: Hydrogen-based compounds under ultra-high pressure, such as the polyhydrides\nH$_3$S and LaH$_{10}$, superconduct through the conventional electron-phonon\ncoupling mechanism to attain the record critical temperatures known to date. We\ndemonstrate here that the intrinsic advantages of hydrogen for phonon-mediated\nsuperconductivity can be exploited in a completely different system, namely\ntwo-dimensional (2D) materials. We find that hydrogen adatoms can strongly\nenhance superconductivity in 2D materials due to flatband states originating\nfrom atomic-like hydrogen orbitals, with a resulting high density of states,\nand due to the emergence of high-frequency hydrogen-related phonon modes that\nboost the electron-phonon coupling. As a concrete example, we investigate the\neffect of hydrogen adatoms on the superconducting properties of monolayer\nMgB$_2$, by solving the fully anisotropic Eliashberg equations, in conjunction\nwith a first-principles description of the electronic and vibrational states,\nand the coupling between them. We show that hydrogenation leads to a high\ncritical temperature of 67 K, which can be boosted to over 100 K by biaxial\ntensile strain.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Charge-density wave transition in magnetic topological semimetal\n  EuAl$_4$: The interplay among topology, charge-density wave (CDW), and magnetism can\ngive rise to a plethora of exotic quantum phenomena. Recently, a group of\nmagnetic topological semimetals with tetragonal lattices and CDW order were\nfound to exhibit anomalous magnetic instability, helical spin ordering, and the\npresence of skyrmions. However, the underlying mechanism responsible for these\nobservations remains unclear. Here, we conducted a comprehensive investigation\ninto the impact of CDW on the topological and magnetic properties of EuAl$_4$\nusing optical spectroscopy and the first-principles calculations. Through\noptical spectroscopy, we observed a partial gap (60~meV) on the Fermi surface\nand an enhanced mid-infrared absorption around 0.4~eV after the CDW transition.\nMagneto-optical spectroscopy and the first-principles calculations proved that,\nby affecting the band structure, the CDW order frustrates the antiferromagnetic\ninteractions but strengthened the ferromagnetic ones, which can destabilize the\nmagnetism. With lower symmetry in the CDW ordered state, carriers from the Weyl\nbands will mediate the anisotropic magnetic interactions promoting the\nformation of chiral spin textures. Conversely, without the CDW order, the\ncounterpart EuGa$_4$ shows robust collinear antiferromagnetic order. Our\nfindings uncover the pivotal role played by CDW order in arousing intricate\nmagnetism in topological materials and provide valuable insights into\ncontrolling topological and magnetic properties through the manipulation of CDW\norders.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity in undoped CaFe2As2 single crystals: Single crystals of undoped CaFe2As2 were grown by a FeAs self-flux method,\nand the crystals were quenched in ice-water rapidly after high temperature\ngrowth. The quenched crystal undergoes a collapsed tetragonal structural phase\ntransition around 80 K revealed by the temperature dependent X-ray diffraction\nmeasurements. Superconductivity below 25 K was observed in the collapsed phase\nby resistivity and magnetization measurements. The isothermal magnetization\ncurve measured at 2 K indicates that this is a typical type-II superconductor.\nFor comparison, we systematically characterized the properties of the furnace\ncooled, quenched, and post-annealed single crystals, and found strong internal\ncrystallographic strain existing in the quenched samples, which is the key for\nthe occurrence of superconductivity in the undoped CaFe2As2 single crystals.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Unveiling Mechanisms of Electric Field Effects on Superconductors by\n  Magnetic Field Response: We demonstrate that superconducting aluminium nano-bridges can be driven into\na state with complete suppression of the critical supercurrent via\nelectrostatic gating. Probing both in- and out-of-plane magnetic field\nresponses in the presence of electrostatic gating can unveil the mechanisms\nthat primarily cause the superconducting electric field effects. Remarkably, we\nfind that a magnetic field, independently of its orientation, has only a weak\ninfluence on the critical electric field that identifies the transition from\nthe superconducting state to a phase with vanishing critical supercurrent. This\nobservation points to the absence of a direct coupling between the electric\nfield and the amplitude of the superconducting order parameter or $2\\pi$-phase\nslips via vortex generation. The magnetic field effect observed in the presence\nof electrostatic gating is described within a microscopic model where a\nspatially uniform inter-band $\\pi$-phase is stabilized by the electric field.\nSuch an intrinsic superconducting phase rearrangement can account for the\nsuppression of the supercurrent, as well as for the weak dependence of the\ncritical magnetic fields on the electric field.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Theory of superconductivity due to Ngai's mechanism in lightly doped\n  SrTiO3: We develop a theory of superconducting pairing in low-density Strontium\ntitanate due to quadratic coupling of electron density to soft transverse\noptical phonons. It leads to static attractive potential between electrons\nwhich decay length scales inversely with soft optical gap. For low electron\ndensities attraction between electrons is local and transition temperature Tc\nwas found. The Tc(n) dependence in agreement with experimental data for low\ndoping was calculated. Next, we show that suppression of Tc by hydrostatic\npressure and strong increase of Tc due to isotop substitution are explained\nwithin our theory.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Macroscopic character of composite high temperature superconducting\n  wires: The \"d-wave\" symmetry of the superconducting order in the cuprate high\ntemperature superconductors is a well established fact, and one which\nidentifies them as \"unconventional.\" However, in macroscopic contexts --\nincluding many potential applications ({\\it i.e.} superconducting \"wires\") --\nthe material is a composite of randomly oriented superconducting grains in a\nmetallic matrix, in which Josephson coupling between grains mediates the onset\nof long-range phase coherence. Here, we analyze the physics at length scales\nlarge compared to the size of such grains, and in particular the macroscopic\ncharacter of the long-range order that emerges. While XY-glass order and\nmacroscopic d-wave superconductivity may be possible, we show that under many\ncircumstances -- especially when the d-wave superconducting grains are embedded\nin a metallic matrix -- the most likely order has global s-wave symmetry.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Two energy scales and close relationship between the pseudogap and\n  superconductivity in underdoped cuprate superconductors: By measuring the low temperature specific heat, the low energy quasi-particle\nexcitation has been derived and analyzed in systematically doped\nLa$_{2-x}$Sr$_{x}$CuO$_{4}$ single crystals. The Volovik's relation predicted\nfor a d-wave superconductor has been well demonstrated in wide doping regime,\nshowing a robust evidence for the d-wave pairing symmetry. Furthermore the\nnodal gap slope $v_\\Delta$ of the superconducting gap is derived and is found\nto follow the same doping dependence of the pseudogap obtained from ARPES and\ntunnelling measurement. This strongly suggests a close relationship between the\npseudogap and superconductivity. Taking the entropy conservation into account,\nwe argue that the ground state of the pseudogap phase should have Fermi arcs\nwith finite density of states at zero K, and the transport data show that it\nbehaves like an insulator due to probably weak localization. A nodal metal\npicture for the pseudogap phase cannot interpret the data. Based on the Fermi\narc picture for the pseudogap phase it is found that the superconducting energy\nscale or $T_c$ in underdoped regime is governed by both the maximum gap and the\nspectral weight from the Fermi arcs. This suggests that there are two energy\nscales: superconducting energy scale and the pseudogap. The superconductivity\nmay be formed by the condensation of Fermi arc quasiparticles through pairing\nby exchanging virtue bosons.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Synthesis and superconductivity of new BiS2 based superconductor\n  PrO0.5F0.5BiS2: We report synthesis and superconductivity at 3.7K in PrO0.5F0.5BiS2. The\nnewly discovered material belongs to the layered sulfide based REO0.5F0.5BiS2\ncompounds having ZrCuSiAs type structure. The bulk polycrystalline compound is\nsynthesized by vacuum encapsulation technique at 7800C in single step. Detailed\nstructural analysis has shown that the as synthesized PrO0.5F0.5BiS2 is\ncrystallized in tetragonal P4/nmm space group with lattice parameters a =\n4.015(5) {\\AA}, c = 13.362(4) {\\AA}. Bulk superconductivity is observed in\nPrO0.5F0.5BiS2 below 4K from magnetic and transport measurements. Electrical\ntransport measurements showed superconducting transition temperature (Tc) onset\nat 3.7K and Tc ({\\rho}=0) at 3.1K. Hump at Tc related to superconducting\ntransition is not observed in heat capacity measurement and rather a\nSchottky-type anomaly is observed at below ~6K. The compound is slightly\nsemiconducting in normal state. Isothermal magnetization (MH) exhibited typical\ntype II behavior with lower critical field (Hc1) of around 8Oe.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Parity effect in a small superconducting grain: A rigorous result: The parity effect in an ultra-small superconducting grain is examined. By\napplying a generalized version of Lieb's spin-reflection positivity technique,\nwe show rigorously that the parity parameter $\\Delta_P$ is nonvanishing in such\na system. A positive lower bound for $\\Delta_P$ is derived.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetic characterization of sintered MgB2 samples: effect of the\n  substitution or doping with Li, Al and Si: Powdered and sintered MgB2 samples have been characterized through magnetic\nmeasurements performed from T = 5 K up to few degrees above the transition\ntemperature of about 39 K. We found that the sintered samples behave as\nwell-connected bodies, showing no trace of granularity. In order to obtain the\ncritical current density value Jc the Critical State Model has been therefore\nemployed in a straightforward way. With the aim either to decrease the electron\nmean free path or to increase its Jc we have attempted to introduce defects in\nthe MgB2 structure by different procedures: substitution of Lithium on the\nMagnesium site and doping of the precursor Boron powders with Aluminum and\nSilicon. The best result in terms of Jc has been achieved by Silicon doping\nthat, moreover, does not significantly affect the transition temperature.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "What does the Josephson effect tell us about the superconducting state\n  of the cuprates?: We review the recent measurements of the current-phase relation in cuprate\nJosephson junctions. Special attention is paid to 45 degree grain boundary\njunctions and to c-axis junctions between YBCO and Nb. It is shown that the\nanomalous properties of such junctions which have been found experimentally\n(large second harmonic of the current phase relation, sign change of the first\nharmonic as a function of temperature) are qualitatively consistent with the\nd-wave symmetry of pairing in the cuprates. It is argued that in order to\ndescribe the data quantitatively, quantum fluctuations of the order parameter\nneed to be taken into account. It is pointed out that the Josephson effect is\nan ideal tool for measuring the reduced phase stiffness in the cuprates.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "High Temperature Superconductivity: the explanation: Soon after the discovery of the first high temperature superconductor by\nGeorg Bednorz and Alex Mueller in 1986 the late Sir Nevill Mott answering his\nown question \"Is there an explanation?\" [Nature v 327 (1987) 185] expressed a\nview that the Bose-Einstein condensation (BEC) of small bipolarons, predicted\nby us in 1981, could be the one. Several authors then contemplated BEC of real\nspace tightly bound pairs, but with a purely electronic mechanism of pairing\nrather than with the electron-phonon interaction (EPI). However, a number of\nother researchers criticized the bipolaron (or any real-space pairing) scenario\nas incompatible with some angle-resolved photoemission spectra (ARPES), with\nexperimentally determined effective masses of carriers and unconventional\nsymmetry of the superconducting order parameter in cuprates. Since then the\ncontroversial issue of whether the electron-phonon interaction (EPI) is crucial\nfor high-temperature superconductivity or weak and inessential has been one of\nthe most challenging problems of contemporary condensed matter physics. Here I\noutline some developments in the bipolaron theory suggesting that the true\norigin of high-temperature superconductivity is found in a proper combination\nof strong electron-electron correlations with a significant finite-range\n(Froehlich) EPI, and that the theory is fully compatible with the key\nexperiments.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Phenomenological study of spin-triplet ferromagnetic superconductors: Unconventional superconductivity with spin-triplet Cooper pairing is reviewed\non the basis of the quasi-phenomenological Ginzburg-Landau theory. The\nsuperconductivity, in particular, the mixed phase of coexistence of\nferromagnetism and unconventional superconductivity is triggered by the\nspontaneous magnetization. The mixed phase is stable whereas the other\nsuperconducting phases that usually exist in unconventional superconductors are\neither unstable, or, for particular values of the parameters of the theory,\nsome of these phases are metastable at relatively low temperatures in a quite\nnarrow domain of the phase diagram. The phase transitions from the normal phase\nto the phase of coexistence is of first order while the phase transition from\nthe ferromagnetic phase to the coexistence phase can be either of first or\nsecond order depending on the concrete substance. The Cooper pair and crystal\nanisotropy are relevant to a more precise outline of the phase diagram shape\nand reduce the degeneration of the ground states of the system but they do not\ndrastically influence the phase stability domains and the thermodynamic\nproperties of the respective phases.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "On the Experimental Evidence for Possible Superconductivity in LK99: The desire to create an energy efficient world is bound to be incomplete\nwithout the discovery of a room temperature superconductor at ambient pressure.\nA recent report on the room-temperature ambient-pressure superconductor has\ninspired scientists to study the Cu doped Lead apatite named as LK-99. Here, we\nhave synthesized Cu doped LK-99 and Ni-doped LK-99 compounds and studied their\ntemperature dependent transport and magnetization behavior. In spite of the\npresence of impurity phase Cu$_2$S, the temperature dependent resistance shows\nan insulating nature of the sample. The radio frequency penetration depth\nmeasurement unveils the absence of diamagnetic flux expulsion in this sample.\nThe temperature dependent ac susceptibility measurements reveal the\nparamagnetic nature of the Ni doped LK-99.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "ARPES kink is a \"smoking gun\" for the theory of high-Tc superconductors:\n  dominance of the electron-phonon interaction with forward scattering peak: The ARPES spectra in high-Tc superconductors show four distinctive features\nin the quasiparticle self-energy. All of them can be explained consistently by\nthe theory in which the electron phonon interaction (EPI) with the forward\nscattering peak dominates over the Coulomb scattering. In particular, this\ntheory explains why there is no shift of the nodal kink at 70 meV in the\nsuperconducting state, contrary to the clear shift of the anti-nodal\nsingularity at 40 meV. The theory predicts a ``knee''-like structure of the\nimaginary part of the self-energy, which is phonon dominated for $\\omega\n\\approx \\omega^{(70)}_{ph}$, and shows linear behavior for $\\omega >\n\\omega^{(70)}_{ph}$ - due to the Coulomb scattering. Recent ARPES spectra give\nthat the EPI coupling constant is much larger than the Coulomb one. The\ndip-hump structure in the spectral function comes out naturally from the\nproposed theory.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Psuedo-isotropic upper critical field in cobalt-doped SrFe2As2 epitaxial\n  films: The temperature and angular dependence of the upper critical field (Hc2) is\nreported for cobalt-doped SrFe2As2 epitaxial films between Tc and 0.5 K in\npulsed magnetic fields up to 50 T. For H parallel c, Hc2 is close to a linear\nfunction of temperature, while in the perpendicular direction there is\nsignificant downward curvature that results in an Hc2 ratio (gamma =\nHc2(perpendicular)/Hc2(parallel) that decreases nearly linearly with\ntemperature, approaching gamma = 1 at low temperature with Hc2(0) = 47 T. We\nmeasure the complete upper-critical field phase diagram including angular\ndependence and model the data using a two band theory allowing us to determine\nthe anisotropy of both bands, their relative diffusivities, and the\nrelationship between BCS coupling constant matrix elements. We find an unusual\nrelationship between the diffusivities of the two bands, with two anisotropic\nand opposite bands. This relationship is supported by the observation of a\nlocal maximum for Hc2(parallel) at low temperature.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Proximity Eliashberg theory of electrostatic field-effect-doping in\n  superconducting films: We calculate the effect of a static electric field on the critical\ntemperature of a s-wave one band superconductor in the framework of proximity\neffect Eliashberg theory. In the weak electrostatic field limit the theory has\nno free parameters while, in general, the only free parameter is the thickness\nof the surface layer where the electric field acts. We conclude that the best\nsituation for increasing the critical temperature is to have a very thin film\nof a superconducting material with a strong increase of electron-phonon (boson)\nconstant upon charging.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Nernst effect of the new iron-based superconductor\n  LaO$_{1-x}$F$_{x}$FeAs: We report the first Nernst effect measurement on the new iron-based\nsuperconductor LaO$_{1-x}$F$_{x}$FeAs $(x=0.1)$. In the normal state, the\nNernst signal is negative and very small. Below $T_{c}$ a large positive peak\ncaused by vortex motion is observed. The flux flowing regime is quite large\ncompared to conventional type-II superconductors. However, a clear deviation of\nthe Nernst signal from normal state background and an anomalous depression of\noff-diagonal thermoelectric current in the normal state between $T_{c}$ and 50\nK are observed. We propose that this anomaly in the normal state Nernst effect\ncould correlate with the SDW fluctuations.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Transport evidence for the surface state and spin-phonon interaction in\n  FeTe$_{0.5}$Se$_{0.5}$: The iron chalcogenides have been proved to be intrinsic topological\nsuperconductors to implement quantum computation because of their unique\nelectronic structures. The topologically nontrivial surface states of\nFeTe$_{0.5}$Se$_{0.5}$ have been predicted by several calculations and then\nconfirmed by high-resolution photoemission and scanning tunneling experiments.\nHowever, so far, the shreds of the electrical transport evidence for\ntopological surface states are still in absence. By carrying out electrical\ntransport experiments, we observe a topological transition with a nonlinear\nHall conductivity and simultaneous linear magnetoresistance near the\nsuperconducting transition temperature. Furthermore, we observe a sign reversal\nof the Hall coefficient accompanied by a concurrently softening of the\n${A}_{1g}$ phonon mode at about 40 K, indicating a nematic transition. The\nsynchronized phonon softening with nematicity manifests an enhanced fluctuation\nstate through spin-phonon interaction. Our results solidly corroborate the\ntopological surface states of FeTe$_{0.5}$Se$_{0.5}$ and provide an\nunderstanding of the mechanism of the superconductivity in iron chalcogenides.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Screening magnetic fields by a superconducting disk: a simple model: We introduce a simple approach to evaluate the magnetic field distribution\naround superconducting samples, based on the London equations; the elementary\nvariable is the vector potential. This procedure has no adjustable parameters,\nonly the sample geometry and the London length, $\\lambda$, determine the\nsolution. The calculated field reproduces quantitatively the measured induction\nfield above MgB$_2$ disks of different diameters, at 20K and for applied fields\nlower than 0.4T. The model can be applied if the flux line penetration inside\nthe sample can be neglected when calculating the induction field distribution\noutside the superconductor. Finally we show on a cup-shape geometry how one can\ndesign a magnetic shield satisfying a specific constraint.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "AC Loss in Striped (Filamentary) YBCO Coated Conductors Leading to\n  Designs for High Frequencies and Field-Sweep Amplitudes: AC losses of YBCO coated conductors are investigated by calculation and\nexperiment for the higher frequency regime. Previous research using YBCO film\ndeposited onto single crystal substrates demonstrated the effectiveness of\nfilamentary subdivision as a technique for AC loss reduction. As a result of\nthese studies the idea of subdividing YBCO coated conductors (both YBCO,\noverlayer, and even underlayer) into such stripes suggested itself. The\nsuggestion was implemented by burning grooves into samples of coated conductor\nusing laser micromachining. Various machining parameters were investigated, and\nthe striping and slicing characteristics are presented. Loss measurements were\nperformed on unstriped as well as striped samples by the pick-up coil technique\nat frequencies of from 50-200 Hz at field sweep amplitudes of up to 150 mT. The\neffect of soft ferromagnetic Fe shielding was also investigated. The results of\nthe experiments form a starting point for a more general study of reduced loss\ncoated conductor design (including hysteretic, coupling, normal eddy current,\nand transport losses) projected into higher ranges of frequency and field sweep\namplitude with transformer and all cryogenic motor/generator applications in\nmind.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Non-equilibrium phenomena in superconductors probed by femtosecond\n  time-domain spectroscopy: Development of ultrafast lasers and non-linear optical techniques over the\nlast two decades provides tools to access real-time dynamics of low energy\nexcitations in superconductors. For example, time-resolved THz spectroscopy and\ntime- and angular-resolved photoemission spectroscopy provide access to the\nreal-time dynamics of the superconducting gap amplitude. Such studies enable\ndetermination of microscopic parameters like quasi-particle recombination\nrates, pair-breaking rates and electron-boson coupling constants. Recently,\nintense THz pulses have been used to probe the non-linear dynamics, including\nobservation of collective modes. Moreover, using low frequency electromagnetic\npulses, there are several reports of amplification of superconductivity in both\nconventional and unconventional superconductors. Starting with a brief\nhistorical overview of the pioneering work, where non-equilibrium phenomena in\nsuperconductors were investigated using quasi-continuous excitation, we review\nsome of the insights that are provided by using real-time approaches. We focus\non conventional BCS superconductors, whose ground state is reasonably well\nunderstood, and address similarities and open questions related to the\ncorresponding studies in high-T$_{c}$ superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Carbon substitution effect in MgB$_2$: We investigated carbon substitution effect on boron plane of superconducting\nMgB$_2$. MgB$_2$ and MgB$_{1.8}$C$_{0.2}$ samples are synthesized under high\npressure furnace. MgB$_{1.8}$C$_{0.2}$ are characterized as AlB$_2$-type single\nphase with smaller B-B distance. During the superconducting transition, two\ndistinct onset temperatures are observed in MgB$_{1.8}$C$_{0.2}$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Why an ac magnetic field shifts the irreversibility line in type-II\n  superconductors: We show that for a thin superconducting strip placed in a transverse dc\nmagnetic field - the typical geometry of experiments with high-Tc\nsuperconductors - the application of a weak ac magnetic field perpendicular to\nthe dc field generates a dc voltage in the strip. This voltage leads to the\ndecay of the critical currents circulating in the strip, and eventually the\nequilibrium state of the superconductor is established. This relaxation is not\ndue to thermally activated flux creep but to the \"walking\" motion of vortices\nin the two-dimensional critical state of the strip with in-plane ac field. Our\ntheory explains the shaking effect that was used for detecting phase\ntransitions of the vortex lattice in superconductors with pinning. Some recent\nexperiments on this subject are discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Reply to \"Comment on Yerin et al., Phys. Rev. Lett. 121, 077002 (2018),\n  and Mironov et al., Phys. Rev. Lett. 109, 237002 (2012)\" by A. F. Volkov, F.\n  S. Bergeret, and K. B. Efetov: Here we reply to the comment by A. F. Volkov, F. S. Bergeret, and K. B.\nEfetov.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Lithium Beryllium Phosphide (LiBeP): A possible MgB2-like Superconductor: The search for materials similar to magnesium diboride, MgB2, based on\nstructure and electronic similarity did not produce close enough\nsuperconductors in terms of Tc. Changing the search to iso-electronic and\niso-atomic number equivalents opened new doors to very many possible MgB2-like\nsuperconductors. Here we present LiBeP which meets these new conditions. We\nestimate its Tc to be 34.5K if two-gapped or 17.2K if single gapped.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electronic Phase Separation Transition as the Origin of the\n  Superconductivity and the Pseudogap Phase of Cuprates: We propose a new phase of matter, an electronic phase separation transition\nthat starts near the upper pseudogap and segregates the holes into high and low\ndensity domains. The Cahn-Hilliard approach is used to follow quantitatively\nthis second order transition. The resulting grain boundary potential confines\nthe charge in domains and favors the development of intragrain superconducting\namplitudes. The zero resistivity transition arises only when the intergrain\nJosephson coupling $E_J$ is of the order of the thermal energy and phase\nlocking among the superconducting grains takes place. We show that this\napproach explains the pseudogap and superconducting phases in a natural way and\nreproduces some recent scanning tunneling microscopy data",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electronic band structure and chemical bonding in the novel\n  antiperovskite ZnCNi3 as compared with 8-K superconductor MgCNi3: Energy band structure of the discovered ternary perovskite-like compound\nZnCNi3 reported by Park et al (2004) as a non-superconducting paramagnetic\nmetal was investigated using the FLMTO-GGA method. The electronic bands,\ndensity of states, Fermi surface, charge density and electron localization\nfunction distributions for ZnCNi3 are obtained and analyzed in comparison with\nthe isoelectronic and isostructural 8K superconductor MgCNi3. The effect of\nexternal pressure on the electronic states of ZnCNi3 and MgCNi3 is studied.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity in $\u03b2$-pyrochlore superconductor KOs$_{2}$O$_{6}$:\n  treatment within strong-coupling Eliashberg theory: We study the influence of the rattling phonons on superconductivity in\n$\\beta$-pyrochlore KOs$_{2}$O$_{6}$ compound based on the strong-coupling\nEliashberg approach. In particular, analyzing the specific heat data we find\nthat the rattling phonon frequency changes discontinuously at the critical\ntemperature of the first order phase transition. Solving the strong-coupling\nEliashberg equations with temperature dependent $\\alpha^{2}F(\\omega)$, we\ninvestigate the consequence of this first order phase transition for the\nanomalous temperature dependence of the superconducting gap. We discuss our\nresults in context of the recent experimental data.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "KFe_2Se_2 is the parent compound of K-doped iron selenide\n  superconductors: We elucidate the existing controversies in the newly discovered K-doped iron\nselenide (KxFe2-ySe2-z) superconductors. The stoichiometric KFe2Se2 with\n\\surd2\\times\\surd2 charge ordering was identified as the parent compound of\nKxFe2-ySe2-z superconductor using scanning tunneling microscopy and\nspectroscopy. The superconductivity is induced in KFe2Se2 by either Se\nvacancies or interacting with the anti-ferromagnetic K2Fe4Se5 compound. Totally\nfour phases were found to exist in KxFe2-ySe2-z: parent compound KFe2Se2,\nsuperconducting KFe2Se2 with \\surd2\\times\\surd5 charge ordering,\nsuperconducting KFe2Se2-z with Se vacancies and insulating K2Fe4Se5 with\n\\surd5\\times\\surd5 Fe vacancy order. The phase separation takes place at the\nmesoscopic scale under standard molecular beam epitaxy condition.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The Gradient Expansion for the Free-Energy of a Clean Superconductor: We describe a novel method for obtaining the gradient expansion for the free\nenergy of a clean BCS superconductor. We present explicit results up to fourth\norder in the gradients of the order parameter.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Zurek-Kibble domain structures: The Dynamics of Spontaneous Vortex\n  formation in Annular Josephson Tunnel Junctions: Phase transitions executed in a finite time show a domain structure with\ndefects, that has been argued by Zurek and Kibble to depend in a characteristic\nway on the quench rate. In this letter we present an experiment to measure the\nZurek-Kibble scaling exponent sigma. Using symmetric and long Josephson Tunnel\nJunctions, for which the predicted index is sigma = 0.25, we find sigma = 0.27\n+/- 0.05. Further, there is agreement with the ZK prediction for the overall\nnormalisation.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity of the New Medium-Entropy Alloy V4Ti2W with a\n  Body-Centered Cubic Structure: Medium- and high-entropy alloy (MEA and HEA) superconductors have attracted\nconsiderable interest since their discovery. This paper reports the\nsuperconducting properties of ternary tungsten-containing MEA V4Ti2W for the\nfirst time. V4Ti2W is a type II superconductor with a body-centered cubic (BCC)\nstructure. Experimental results of resistivity, magnetization, and heat\ncapacity indicate that the superconducting transition temperature of the MEA\nV4Ti2W is roughly 5.0 K. The critical magnetic fields at the upper and lower\nends are 9.93(2) T and 40.7(3) mT, respectively. Interestingly, few BCC MEA\nsuperconductors with VEC greater than 4.8 have been found. The addition of\ntungsten leads to a VEC of 4.83 e/a for V4Ti2W, which is rarely higher than the\n4.8 value. Adding tungsten element expands the variety of MEA alloys, which may\nimprove the microstructure and mechanical properties of materials and even\nsuperconducting properties. This material could potentially offer a new\nplatform for the investigation of innovative MEA and HEA superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Elliptical vortex and oblique vortex lattice in the FeSe superconductor\n  based on the nematicity and mixed superconducting orders: The electronic nematic phase is characterized as an ordered state of matter\nwith rotational symmetry breaking, and has been well studied in the quantum\nHall system and the high-$T_c$ superconductors, regardless of cuprate or\npnictide family. The nematic state in high-$T_c$ systems often relates to the\nstructural transition or electronic instability in the normal phase.\nNevertheless, the electronic states below the superconducting transition\ntemperature is still an open question. With high-resolution scanning tunneling\nmicroscope measurements, direct observation of vortex core in FeSe thin films\nrevealed the nematic superconducting state by Song \\emph{et al}. Here,\nmotivated by the experiment, we construct the extended Ginzburg-Landau free\nenergy to describe the elliptical vortex, where a mixed \\emph{s}-wave and\n\\emph{d}-wave superconducting order is coupled to the nematic order. The\nnematic order induces the mixture of two superconducting orders and enhances\nthe anisotropic interaction between the two superconducting orders, resulting\nin a symmetry breaking from $C_4$ to $C_2$. Consequently, the vortex cores are\nstretched into an elliptical shape. In the equilibrium state, the elliptical\nvortices assemble a lozenge-like vortex lattice, being well consistent with\nexperimental results.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "New Superconductivity Dome in LaFeAsO$_{1-x}$F$_{x}$ Accompanied by\n  Structural Transition: High temperature superconductivity is often found in the vicinity of\nantiferromagnetism. This is also true in LaFeAsO$_{1-x}$F$_{x}$ ($x \\leq$ 0.2)\nand many other iron-based superconductors, which leads to proposals that\nsuperconductivity is mediated by fluctuations associated with the nearby\nmagnetism. Here we report the discovery of a new superconductivity dome without\nlow-energy magnetic fluctuations in LaFeAsO$_{1-x}$F$_{x}$ with 0.25$\\leq x\n\\leq$0.75, where the maximal critical temperature $T_c$ at $x_{opt}$ =\n0.5$\\sim$0.55 is even higher than that at $x \\leq$ 0.2. By nuclear magnetic\nresonance and Transmission Electron Microscopy, we show that a C4 rotation\nsymmetry-breaking structural transition takes place for $x>$ 0.5 above $T_c$.\nOur results point to a new paradigm of high temperature superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Two-dimensional magnetism in the pnictide superconductor parent material\n  SrFeAsF probed by muon-spin relaxation: We report muon-spin relaxation measurements on SrFeAsF, which is the parent\ncompound of a newly discovered iron-arsenic-fluoride based series of\nsuperconducting materials. We find that this material has very similar magnetic\nproperties to LaFeAsO, such as separated magnetic and structural transitions\n(TN = 120 K, Ts = 175 K), contrasting with SrFe2As2 where they are coincident.\nThe muon oscillation frequencies fall away very sharply at TN, which suggests\nthat the magnetic exchange between the layers is weaker than in comparable\noxypnictide compounds. This is consistent with our specific heat measurements,\nwhich find that the entropy change S = 0.05 J/mol/K largely occurs at the\nstructural transition and there is no anomaly at TN.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Flux Pinning by Nano Particles Embedded in Polycrystalline Y-123\n  Superconductors: Bulk superconductor samples of YBa2Cu3-xZnxO7-{\\delta} with x = 0, 0.01, 0.03\nare synthesized by solid-state reaction route. The structural characterisation\nof all samples has been carried out by x-ray-diffraction (XRD) and transmission\nelectron microscopy (TEM) techniques. The x-ray diffraction patterns indicate\nthat the gross structure/phase of YBa2Cu3-xZnxO7-{\\delta} do not change with\nthe substitution of Zn up to x=0.03. In TEM investigations of Zn-doped Y-based\ncuprates a number of ZnO nano-flower and nano-rod of Y-211 phase are found\ndispersed in regular YBa2Cu3-xZnxO7 matrix. These dispersed nano-flowers of ZnO\nand nano-rods of Y-211 phase may serve as flux-pinning centers. These pinning\ncenters enhance critical current density (Jc) value of these HTSC samples.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Microwave transmissions through superconducting coplanar waveguide\n  resonators with different coupling configurations: We design and fabricate two types of superconducting niobium coplanar\nwaveguide microwave resonators with different coupling capacitors on high\npurity Si substrates. Their microwave transmissions are measured at the\ntemperatures of 20 mK. It is found that these two types of resonators possess\nsignificantly-different loaded quality factors; one is $5.6\\times{10}^{3}$, and\nthe other is $4.0\\times{10}^{4}$. The measured data are fitted well by\nclassical ABCD matrix approach. We found that the transmission peak deviates\nfrom the standard Lorentizian with a frequency broadening.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Chemically gated electronic structure of a superconducting doped\n  topological insulator system: Angle resolved photoemission spectroscopy is used to observe changes in the\nelectronic structure of bulk-doped topological insulator Cu$_x$Bi$_2$Se$_3$ as\nadditional copper atoms are deposited onto the cleaved crystal surface. Carrier\ndensity and surface-normal electrical field strength near the crystal surface\nare estimated to consider the effect of chemical surface gating on atypical\nsuperconducting properties associated with topological insulator order, such as\nthe dynamics of theoretically predicted Majorana Fermion vortices.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Vortex dynamics in two-dimensional Josephson junction arrays: The dynamic response of unfrustrated two-dimensional Josephson junction\narrays close to, but above the Kosterlitz-Thouless($KT$) transition temperature\nis described in terms of the vortex dielectric function $\\epsilon(\\omega)$. The\nlatter is calculated by considering separately the contribution of\n$\\ll$free$\\gg$ vortices interacting by a screened Coulomb potential, and the\n$\\ll$pair motion$\\gg$ of vortices that are closer to each other than the $KT$\ncorrelation length. This procedure allows to understand various anomalous\nfeatures in $\\epsilon(\\omega)$ and in the flux noise spectra that have been\nobserved experimentally and in dynamic simulations.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Impurity-induced quasiparticle interference in the parent compounds of\n  iron-pnictide superconductors: The impurity-induced quasiparticle interference(QPI) in the parent compounds\nof iron-pnictide superconductors is investigated based on a phenomenological\ntwo-orbital four-band model and T-matrix method. We find the QPI is sensitive\nto the value of the magnetic order which may vary from one compound to another.\nFor small value of the magnetic order, the pattern of oscillation in the local\ndensity of states (LDOS) induced by the QPI exhibits two-dimensional\ncharacteristics, consistent with the standing wave state observed in the 1111\ncompound. For larger value of the magnetic order, the main feature of the\nspatial modulation of the LDOS is the existence of one-dimensional stripe\nstructure which is in agreement with the nematic structure in the parent\ncompound of the 122 system. In both cases the system shows $C_2$ symmetry and\nonly in the larger magnetic order case, there exist in-gap bound states. The\ncorresponding QPI in $q$-space is also presented. The patterns of modulation in\nthe LDOS at nonzero energies are attributed to the interplay between the\nunderlying band structure and Fermi surfaces.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Neutron Scattering Studies of spin excitations in hole-doped\n  Ba0.67K0.33Fe2As2 superconductor: We report inelastic neutron scattering experiments on single crystals of\nsuperconducting Ba0.67K0.33Fe2As2 (Tc = 38 K). In addition to confirming the\nresonance previously found in powder samples, we find that spin excitations in\nthe normal state form longitudinally elongated ellipses along the QAFM\ndirection in momentum space, consistent with density functional theory\npredictions. On cooling below Tc, while the resonance preserves its momentum\nanisotropy as expected, spin excitations at energies below the resonance become\nessentially isotropic in the in-plane momentum space and dramatically increase\ntheir correlation length. These results suggest that the superconducting gap\nstructures in Ba0.67Ka0.33Fe2As2 are more complicated than those suggested from\nangle resolved photoemission experiments.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Scanning tunnelling spectroscopy of the vortex state in NbSe2 using a\n  superconducting tip: The vortex electronic structure in the multiband superconductor NbSe2 is\nstudied by means of Scanning Tunneling Spectroscopy (STS) using a\nsuperconducting tip. The use of a superconducting tip (Pb) as a probe provides\nan enhancement of the different features related to the DOS of NbSe2 in the\ntunneling conductance curves. This use allows the observation of rich patterns\nof electronic states in the conductance images around the vortex cores in a\nwide range of temperature, as well as the simultaneous acquisition of Josephson\ncurrent images in the vortex state.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity without inversion and time-reversal symmetries: The traditional symmetries that protect superconductivity are time-reversal\nand inversion. Here, we examine the minimal symmetries protecting\nsuperconductivity in two dimensions and find that time-reversal symmetry and\ninversion symmetry are not required, and having a combination of either\nsymmetry with a mirror operation on the basal plane is sufficient. We classify\nsuperconducting states stabilized by these two symmetries, when time-reversal\nand inversion symmetries are not present, and provide realistic minimal models\nas examples. Interestingly, several experimentally realized systems, such as\ntransition metal dichalcogenides and the two-dimensional Rashba system belong\nto this category, when subject to an applied magnetic field.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Effect of metal vacancies on the electronic band structure of hexagonal\n  Nb, Zr and Y diborides: Energy band structures of metal-deficient hexagonal diborides M$_{0.75}$B$_2$\n(M = Nb, Zr and Y) were calculated using the full-potential LMTO method. The\nmetal vacancies change the density of states near the Fermi level and this\neffect is quite different for III-V group transition metal diborides.\nContradictory data on superconductivity in diborides may be supposed to be\nconnected with nonstoichiometry of samples. Vacancy formation energies are\nestimated and analyzed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Trilayer multi-orbital models of $\\mathrm{La_{4}Ni_{3}O_{10}}$: Recently, the discovery of superconductivity in Ruddlesden-Popper (RP)\n$\\mathrm{La_4Ni_3O_{10}}$ under pressure has further expanded the realm of\nnickelate-based superconductor family. In this paper, we performed a\nfirst-principle study of $\\mathrm{La_4Ni_3O_{10}}$ for both $P2_1/a$ phase at\nambient pressure and $I4/mmm$ phase at high pressure, with $U$=0, 3.5 eV. Our\nresults confirmed the characteristic upward shift of Ni-$d_{z^2}$ bonding band\nunder pressure. Moreover, our analysis of electronic spectrum and orbital\noccupancy unveil the dynamical mechanism of electronic reconstructions under\npressure, embedded in a critical dual effect. Based on our DFT results, we\nfurther proposed a trilayer two-orbital model by performing Wannier downfolding\non Ni-$e_g$ orbitals. Our model reveals four Fermi surface sheets with\n$\\alpha,\\beta,\\beta^\\prime,\\gamma$ pockets, bearing resemblance to that of\nbilayer $\\mathrm{La_3Ni_2O_7}$. According to the model, our calculated spin\nsusceptibility under random phase approximation predicts an analogous magnetic\nsignal at ${\\rm q}=(\\frac{\\pi}{2},\\frac{\\pi}{2})$, which is more associated\nwith nesting within $\\beta,\\beta^\\prime$ pockets. Finally, a high energy\nsixteen-orbital model with direct $dp,pp$ hoppings is proposed, which implies\nthat $\\mathrm{La_4Ni_3O_{10}}$ also lies in charge-transfer picture within\nZaanen-Sawatzky-Allen scheme. Our exposition of electronic reconstructions and\nmulti-orbital models shed light on theoretical electronic correlation study and\nexperimental exploration for lower pressure in RP series.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Route to Observable Fulde-Ferrell-Larkin-Ovchinnikov Phases in 3D\n  Spin-Orbit Coupled Degenerate Fermi Gases: The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase was first predicted in 2D\nsuperconductors about 50 years ago, but so far unambiguous experimental\nevidences are still lacked. The recently experimentally realized\nspin-imbalanced Fermi gases may potentially unveil this elusive state, but\nrequire very stringent experimental conditions. In this Letter, we show that\nFFLO phases may be observed even in a 3D degenerate Fermi gas with spin-orbit\ncoupling and in-plane Zeeman field. The FFLO phase is driven by the interplay\nbetween asymmetry of Fermi surface and superfluid order, instead of the\ninterplay between magnetic and superconducting order in solid materials. The\npredicted FFLO phase exists in a giant parameter region, possesses a stable\nlong-range superfluid order due to the 3D geometry, and can be observed with\nexperimentally already achieved temperature ($T\\sim 0.05E_{F}$), thus opens a\nnew fascinating avenue for exploring FFLO physics.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pseudogap-like phase in Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ revealed by\n  $^{75}$As NQR: We report $^{75}$As NQR measurements on single crystalline\nCa(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($0\\leq x \\leq 0.09$). The nuclear spin-lattice\nrelaxation rate $T_1^{-1}$ as a function of temperature $T$ and Co dopant\nconcentration $x$ reveals a normal-state pseudogap-like phase below a crossover\ntemperature $T^*$ in the under- and optimally-doped region. The resulting\n$x$-$T$ phase diagram shows that, after suppression of the spin-density-wave\norder, $T^*$ intersects $T_c$ falling to zero rapidly near the optimal doping\nregime. Possible origins of the pseudogap behavior are discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Enhancement of functional properties of V$_{0.6}$Ti$_{0.4}$ alloy\n  superconductor by the addition of yttrium: We show here that the yttrium is immiscible and precipitates with various\nsizes in the body centred cubic V$_{0.6}$Ti$_{0.4}$ alloy superconductor. The\nnumber and size of the precipitates are found to depend on the amount of\nyttrium added. Precipitates with various sizes up to 30~$\\mu$m are found in the\nV$_{0.6}$Ti$_{0.4}$ alloy containing 5 at.\\% yttrium. The large amount of line\ndisorders generated by the addition of yttrium in this alloy are found to be\neffective in pinning the magnetic flux lines. While the superconducting\ntransition temperature increases with the increasing amount of yttrium in the\nV$_{0.6}$Ti$_{0.4}$ alloy, the critical current density is maximum for the\nalloy containing 2 at. \\% yttrium, where it is more than 7.5 times the parent\nalloy in fields higher than 1~T. We found that the effectiveness of each type\nof defect in pinning the flux lines is dependent on the temperature and the\napplied magnetic filed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "High-$T$$_\\textrm{C}$ Superconductivity in Hydrogen Clathrates Mediated\n  by Coulomb Interactions between Hydrogen and Central-Atom Electrons: The uniquely characteristic macrostructures of binary hydrogen-clathrate\ncompounds $M$H$_\\textrm{n}$ formed at high pressure, a cage of hydrogens\nsurrounding a central-atom host, is theoretically predicted in various studies\nto include structurally stable phonon-mediated superconductors. High\nsuperconductive transition temperatures $T$$_\\textrm{C}$ have thus far been\nmeasured for syntheses with $M$ = La, Y, and Th. In compressed\nLaH$_\\textrm{10}$, independent studies report $T$$_\\textrm{C}$ of 250 K and\nover 260 K, a maximum in $T$$_\\textrm{C}$ with pressure $P$, and normal-state\nresistance scaling with temperature (suggesting unconventional pairing).\nAccording to reported band structure calculations of $Fm$$\\bar{3}$$m$-phase\nLaH$_\\textrm{10}$, the La is anionic, with the chemical valence electrons\nappearing evenly split between La and H$_\\textrm{10}$. Thus, compressed\nLaH$_\\textrm{10}$ contains the combination of structure, charge separation, and\noptimal balanced allocation of valence electrons for supporting unconventional\nhigh-$T$$_\\textrm{C}$ superconductivity mediated by Coulomb interactions\nbetween electronic charges associated with La and H$_\\textrm{10}$. A general\nexpression for the optimal superconducting transition temperature for\n$M$H$_\\textrm{n}$ clathrates is derived as $T$$_\\textrm{C0}$ =\n$k$$_\\textrm{B}$$^{-1}$$\\Lambda$[(n + $v$)/2$A$]$^{1/2}$$e$$^{2}$/$\\zeta$,\nwhere $\\Lambda$ is a universal constant, (n + $v$) is the chemical valence sum\nper formula unit, taking unity for H and $v$ for atom $M$, $A$ is the surface\narea of the H-polyhedron cage, and $\\zeta$ is the mean distance between the $M$\nsite and the centroids of the polyhedron faces. Applied to $Fm$$\\bar{3}$$m$\nLaH$_\\textrm{10}$, $T$$_\\textrm{C0}$ values of 249.8(1.3) K and 260.7(2.0) K\nare found for the two experiments. Associated attributes of charge allocation,\nstructure, effective Coulomb potential, . . .",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Optimal interlayer hopping and high temperature Bose-Einstein\n  condensation of local pairs in quasi 2D superconductors: Both FeSe and cuprate superconductors are quasi 2D materials with high\ntransition temperatures and local fermion pairs. Motivated by such systems, we\ninvestigate real space pairing of fermions in an anisotropic lattice model with\nintersite attraction, V, and strong local Coulomb repulsion, U, leading to a\ndetermination of the optimal conditions for superconductivity from\nBose-Einstein condensation. Our aim is to gain insight as to why high\ntemperature superconductors tend to be quasi 2D. We make both analytically and\nnumerically exact solutions for two body local pairing applicable to\nintermediate and strong V. We find that the Bose Einstein condensation\ntemperature of such local pairs is maximal when hopping between layers is\nintermediate relative to in-plane hopping, indicating that the quasi 2D nature\nof unconventional superconductors has an important contribution to their high\ntransition temperatures.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Reversible transitions in high-$T_c$ cuprates based point contacts: The influence of electric felds and currents has been investigated in the\nhigh-$T_c$ superconductors $YBaCuO$ and $BiSrCaCuO$ using a point-contact\ngeometry with $Ag$ as the counterelectrode, which reveal switching transitions\nbetween states of a different resistance. The origin of this effect in point\ncontacts is associated with electromigration of the oxygen, driven by the\nelectric feld as well as by the currentinduced \"electron wind\". The switching\neffect preserves its basic features at elevated temperatures up to room\ntemperature and in high magnetic felds up to 10 T.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Finite energy spin fluctuation as a pairing glue in systems with\n  coexisting electron and hole bands: We study, within the fluctuation exchange approximation, the\nspin-fluctuation-mediated superconductivity in Hubbard-type models possessing\nelectron and hole bands, and compare them with a model on a square lattice with\na large Fermi surface. In the square lattice model, superconductivity is more\nenhanced for better nesting for a fixed band filling. By contrast, in the\nmodels with electron and hole bands, superconductivity is optimized when the\nFermi surface nesting is degraded to some extent, where finite energy spin\nfluctuation around the nesting vector develops. The difference lies in the\nrobustness of the nesting vector, namely, in models with electron and hole\nbands, the wave vector at which the spin susceptibility is maximized is fixed\neven when the nesting is degraded, whereas when the Fermi surface is large, the\nnesting vector varies with the deformation of the Fermi surface. We also\ndiscuss the possibility of realizing in actual materials the bilayer Hubbard\nmodel, which is a simple model with electron and hole bands, and is expected to\nhave a very high T_c.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Cold atoms near superconductors: Atomic spin coherence beyond the\n  Johnson noise limit: We report on the measurement of atomic spin coherence near the surface of a\nsuperconducting niobium wire. As compared to normal conducting metal surfaces,\nthe atomic spin coherence is maintained for time periods beyond the Johnson\nnoise limit. The result provides experimental evidence that magnetic near field\nnoise near the superconductor is strongly suppressed. Such long atomic spin\ncoherence times near superconductors open the way towards the development of\ncoherently coupled cold atom / solid state hybrid quantum systems with\npotential applications in quantum information processing and precision force\nsensing.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Majorana-assisted nonlocal electron transport through a floating\n  topological superconductor: The nonlocal nature of the fermionic mode spanned by a pair of Majorana bound\nstates in a one-dimensional topological superconductor has inspired many\nproposals aiming at demonstrating this property in transport. In particular,\ntransport through the mode from a lead attached to the left bound state to a\nlead attached to the right will result in current cross-correlations. For ideal\nzero modes on a grounded superconductor, the cross-correlations are however\ncompletely suppressed in favor of purely local Andreev reflection. In order to\nobtain a non-vanishing cross-correlation, previous studies have required the\npresence of an additional global charging energy. Adding nonlocal terms in the\nform of a global charging energy to the Hamiltonian when testing the intrinsic\nnonlocality of the Majorana modes seems to be conceptually troublesome. Here,\nwe show that a floating superconductor allows to observe nonlocal current\ncorrelations in the absence of charging energy. We show that the\nnon-interacting and the Coulomb-blockade regime have the same peak conductance\n$e^2/h$ but different shot-noise power; while the shot noise is sub-Poissonian\nin the Coulomb-blockade regime in the large bias limit, Poissonian shot noise\nis generically obtained in the non-interacting case.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Single crystal growth and optical conductivity of SrPt$_2$As$_2$\n  superconductors: SrPt$_2$As$_2$ single crystals with CaBe$_2$Ge$_2$-type structure were\nsynthesized by self-melting technique. X-ray diffraction, transmission electron\nmicroscopy, electrical resistivity, specific heat and optical spectroscopy\nmeasurements were conducted to elucidate the properties of SrPt$_2$As$_2$.\nSrPt$_2$As$_2$ single crystals exhibit a superconducting transition at 5.2K,\nexperiencing a structural phase transition well above room temperature (about\n455K). The superconducting and structural phase transition temperatures are\nboth reduced by 6% Iridium doping. Both pure SrPt$_2$As$_2$ and the doped\nsingle crystals are revealed to be highly metallic with rather high plasma\nfrequencies. In particular, the optical spectroscopy measurement revealed two\ngap-like suppression features. We elaborated that the one at higher energy\nscale could be ascribed to the correlation effect, while the other one at lower\nenergy scale is related to the structural phase transition, leading to the\nremoval of a small portion of the Fermi surfaces.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "S-shaped suppression of the superconducting transition temperature in\n  Cu-intercalated NbSe2: 2H-NbSe2 is the prototype and most frequently studied of the well-known\ntransition metal dichalcogenide (TMDC) superconductors. Widely acknowledged to\nbe a conventional superconductor, its transition temperature to the\nsuperconducting state (Tc) is 7.3 K - a Tc that is substantially higher than\nthose seen for the majority of TMDCs, where Tcs between 2 and 4 K are the norm.\nHere we report the intercalation of Cu into 2H-NbSe2 to make CuxNbSe2. As is\ntypically found when chemically altering an optimal superconductor, Tc\ndecreases with increasing x, but the way that Tc is suppressed in this case is\nunusual - an S-shaped character is observed, with an inflection point near x =\n0.03 and, at higher x, a leveling off of the Tc near 3 K - down to the usual\nvalue for a layered TMDC. Electronic characterization reveals corresponding\nS-like behavior for many of the materials parameters that influence Tc. To\nillustrate its character, the superconducting phase diagram for CuxNbSe2 is\ncontrasted to those of FexNbSe2 and NbSe2-xSx.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity and topological behavior in gallenene: Among the large variety of two-dimensional (2D) materials discovered to date,\nelemental monolayers that host superconductivity are very rare. Using ab initio\ncalculations we show that recently synthesized gallium monolayers, coined\ngallenene, are intrinsically superconducting through electron-phonon coupling.\nWe reveal that Ga-100 gallenene, a planar monolayer isostructural with\ngraphene, is the structurally simplest 2D superconductor to date, furthermore\nhosting topological edge states due to its honeycomb structure. Our anisotropic\nEliashberg calculations show distinctly three-gap superconductivity in Ga-100,\nin contrast to the alternative buckled Ga-010 gallenene which presents a single\nanisotropic superconducting gap. Strikingly, the critical temperature ($T_c$)\nof gallenene is in the range of $7-10$ K, exceeding the $T_c$ of bulk gallium\nfrom which it is exfoliated. Finally we explore chemical functionalization of\ngallenene with hydrogen, and report induced multigap superconductivity with an\nenhanced $T_c$ in the resulting gallenane compound.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity at 4K in Pd deficient layered Ta_2Pd_xS_6: Here in this short note, we report on the low dimensional 4d and 5d\ntransition metals-chalcogenide based compounds i.e., Ta2PdxS6, showing\nsemiconducting to superconducting transition around 4K with upper critical\nfield outside Pauli paramagnetic limit. It seems couple of different\nsuperconducting phases do exist in these new set of compounds. Our short note\nin this regards is thought provoking, asking to explore various unearthed\npossible superconducting phases in (Nb/Ta)2Pdx(S/Se/Te)y systems.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "A-15 type superconducting hydride $La_4H_{23}$: Nanograined structure\n  with low strain, strong electron-phonon interaction, and moderate level of\n  nonadiabaticity: For seven decades by A-15 superconductors we meant metallic $A_3B$ alloys\n(where A is a transition metal, and B is groups IIIB and IVB element)\ndiscovered by Hardy and Hulm (Phys. Rev. 89, 884 (1953)). Nb3Ge exhibited the\nhighest superconducting transition temperature, $T_c = 23 K$, among these\nalloys. One of these alloys, $Nb_3Sn$, is primary material in modern applied\nsuperconductivity. Recently Guo et al (arXiv:2307.13067) extended the family of\nsuperconductors where the metallic ions arranged in the beta tungsten (A-15)\nsublattice by observation of $T_{c,zero} = 81 K$ in $La_4H_{23}$ phase\ncompressed at $P = 118 GPa$. Despite the $La_4H_{23}$ has much lower $T_c$ in\ncomparison with near-room-temperature superconducting $LaH_{10}$ phase\n($T_{c,zero} = 250 K$ at $P = 200 GPa$) discovered by Drozdov et al (Nature\n569, 531 (2019)), the $La_4H_{23}$ holds the record high $T_c$ within A-15\nfamily. Cross et al (Phys. Rev. B 109, L020503 (2024)) confirmed the\nhigh-temperature superconductivity in the compressed $La_4H_{23}$. In this\npaper, we analyzed available experimental data measured in $La_4H_{23}$ and\nfound that this superconductor exhibits nanograined structure, 6 nm < D < 27\nnm, low crystalline strain < 0.005, high electron-phonon coupling constant, 1.5\n< $\\lambda_{e-ph}$ < 2.7, and moderate level of the nonadiabaticity\n$\\Theta_{D}/T_{F}$. We found that derived $\\Theta_{D}/T_{F}$ and $T_c/T_F$\nvalues for the $La_4H_{23}$ phase are similar to the ones in cuprates,\npnictides, and near-room-temperature superconductors $H_3S$ and $LaH_{10}$,\nwhich implies that the $La_4H_{23}$ phase falls to unconventional\nsuperconductors band in the Uemura plot.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Doping dependence of the coupling of electrons to bosonic modes in the\n  single-layer high-temperature Bi2Sr2CuO6 superconductor: A recent highlight in the study of high-Tc superconductors is the observation\nof band renormalization / self-energy effects on the quasiparticles. This is\nseen in the form of kinks in the quasiparticle dispersions as measured by\nphotoemission and interpreted as signatures of collective bosonic modes\ncoupling to the electrons. Here we compare for the first time the self-energies\nin an optimally doped and strongly overdoped, non-superconducting single-layer\nBi-cuprate (Bi2Sr2CuO6). Besides the appearance of a strong overall weakening,\nwe also find that weight of the self-energy in the overdoped system shifts to\nhigher energies. We present evidence that this is related to a change in the\ncoupling to c-axis phonons due to the rapid change of the c-axis screening in\nthis doping range.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Tuning the static spin-stripe phase and superconductivity in\n  La_(2-x)Ba_xCuO_4 (x = 1/8) by hydrostatic pressure: Magnetization and muon spin rotation experiments were performed in\nLa_(2-x)Ba_xCuO_4 (x = 1/8) as a function of hydrostatic pressure up to p = 2.2\nGPa. It was found that the magnetic volume fraction of the static stripe phase\nstrongly decreases linearly with pressure, while the superconducting volume\nfraction increases by the same amount. This demonstrates competition between\nbulk superconductivity and static magnetic order in the stripe phase of\nLa_1.875Ba_0.125CuO_4 and that these phenomena occur in mutually exclusive\nspatial regions. The present results also reveal that the static spin-stripe\nphase still exists at pressures, where the long-range low-temperature\ntetragonal (LTT) structure is completely suppressed. This indicates that the\nlong-range LTT structure is not necessary for stabilizing the static spin order\nin La_1.875Ba_0.125CuO_4.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Strong charge fluctuations manifested in the high-temperature Hall\n  coefficient of high-T_c cuprates: By measuring the Hall coefficient R_H up to 1000 K in La_2CuO_4,\nPr_{1.3}La_{0.7}CuO_4, and La_{2-x}Sr_xCuO_4 (LSCO), we found that the\ntemperature (T) dependence of R_H in LSCO for x = 0 - 0.05 at high temperature\nundoubtedly signifies a gap over which the charge carriers are thermally\nactivated, which in turn indicates that in lightly-doped cuprates strong charge\nfluctuations are present at high temperature and the carrier number is not a\nconstant. At higher doping (x = 0.08 - 0.21), the high-temperature R_H(T)\nbehavior is found to be qualitatively the same, albeit with a weakened\ntemperature dependence, and we attempt to understand its behavior in terms of a\nphenomenological two-carrier model where the thermal activation is considered\nfor one of the two species. Despite the crude nature of the model, our analysis\ngives a reasonable account of R_H both at high temperature and at 0 K for a\nwide range of doping, suggesting that charge fluctuations over a gap remain\nimportant at high temperature in LSCO deep into the superconducting doping\nregime. Moreover, our model gives a perspective to understand the seemingly\ncontradicting high-temperature behavior of R_H and the in-plane resistivity\nnear optimum doping in a consistent manner. Finally, we discuss possible\nimplications of our results on such issues as the scattering-time separation\nand the large pseudogap.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Number theory, periodic orbits and superconductivity in nano-cubes: We study superconductivity in isolated superconducting nano-cubes and\nnano-squares of size $L$ in the limit of negligible disorder, $\\delta/\\Delta_0\n\\ll 1$ and $k_F L \\gg 1$ for which mean-field theory and semiclassical\ntechniques are applicable, with $k_F$ the Fermi wave vector, $\\delta$ the mean\nlevel spacing and $\\Delta_0$ the bulk gap. By using periodic orbit theory and\nnumber theory we find explicit analytical expressions for the size dependence\nof the superconducting order parameter. Our formalism takes into account\ncontributions from both the spectral density and the interaction matrix\nelements in a basis of one-body eigenstates. The leading size dependence of the\nenergy gap in three dimensions seems to be universal as it agrees with the\nresult for chaotic grains. In the region of parameters corresponding to\nconventional metallic superconductors, and for sizes $L \\gtrsim 10$nm, the\ncontribution to the superconducting gap from the matrix elements is substantial\n($\\sim 20\\%$). Deviations from the bulk limit are still clearly observed even\nfor comparatively large grains $L \\sim 50$nm. These analytical results are in\nexcellent agreement with the numerical solution of the mean-field gap equation.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Probing itinerant antiferromagnetism with $d$-wave Andreev reflection\n  spectroscopy: To study how Andreev reflection (AR) is affected by itinerant\nantiferromagnetism, we perform $d$-wave AR spectroscopy with superconducting\nYBa$_2$Cu$_3$O$_{7-\\delta}$ on TiAu and on variously-oxidized Nb (NbO$_x$)\nsamples. X-ray photoelectron spectroscopy is also used on the latter to measure\ntheir surface oxide composition. Below the N\\'eel temperatures ($T_N$) of both\nTiAu and NbO$_x$, the conductance spectra show a dip-like structure instead of\na zero-bias peak within the superconducting energy gap; for NbO$_x$,\nhigher-oxidized samples show a stronger spectral dip at zero bias. These\nobservations indicate that itinerant antiferromagnetic order suppresses the AR\nprocess. Interestingly, the spectral dip persists above $T_N$ for both TiAu and\nNbO$_x$, implying that spin fluctuations can also suppress AR. Our results\nsuggest that $d$-wave AR spectroscopy may be used to probe the degree of spin\nordering in itinerant antiferromagnets.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Reentrant AC magnetic susceptibility in Josephson-junction arrays: An\n  alternative explanation for the paramagnetic Meissner effect: The paramagnetic Meissner effect (PME) measured in high $T_{C}$ granular\nsuperconductors has been attributed to the presence of $\\pi$-junctions between\nthe grains. Here we present measurements of complex AC magnetic susceptibility\nfrom two-dimensional arrays of conventional (non $\\pi$) Nb/Al/AlOx/Nb Josephson\njunctions. We measured the susceptibility as a function of the temperature $T$,\nthe AC amplitude of the excitation field, $h_{AC}$, and the external magnetic\nfield, $H_{DC}$. The experiments show a strong paramagnetic contribution from\nthe multi-junction loops, which manifests itself as a reentrant screening at\nlow temperature, for values of $h_{AC}$ higher than 50 mOe. A highly simplified\nmodel, based on a single loop containing four junction, accounts for this\nparamagnetic contribution and the range of parameters in which it appears. This\nmodel offers an alternative explanation of PME which does not involve\n$\\pi$-junctions.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Suppression of superconductivity by Neel-type magnetic fluctuations in\n  the iron pnictides: Motivated by recent experimental detection of Neel-type ($(\\pi,\\pi)$)\nmagnetic fluctuations in some iron pnictides, we study the impact of competing\n$(\\pi,\\pi)$ and $(\\pi,0)$ spin fluctuations on the superconductivity of these\nmaterials. We show that, counter-intuitively, even short-range, weak Neel\nfluctuations strongly suppress the $s^{+-}$ state, with the main effect arising\nfrom a repulsive contribution to the $s^{+-}$ pairing interaction, complemented\nby low frequency inelastic scattering. Further increasing the strength of the\nNeel fluctuations leads to a low-$T_{c}$ d-wave state, with a possible\nintermediate $s+id$ phase. The results suggest that the absence of\nsuperconductivity in a series of hole-doped pnictides is due to the combination\nof short-range Neel fluctuations and pair-breaking impurity scattering, and\nalso that $T_{c}$ of optimally doped pnictides could be further increased if\nresidual $(\\pi,\\pi)$ fluctuations were reduced.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Strong electron-lattice coupling and orbital fluctuations in iron\n  pnictide superconductor Ba(Fe1-xCox)2As2: This paper has been withdrawn by the author due to some experimental\nmistakes. In this paper, we reported that C66, C44 and (C11-C12)/2 show\nremarkable softening toward the structural transition temperature TS. The data\nreported in this paper were acquired using the ultrasonic frequency lower than\n25 MHz. Recently, we performed high-frequency measurements for the same system.\nWe found that the anomaly of C44 and (C11-C12)/2 tend to disappear rapidly with\nincreasing the frequency. On the other hand, C66 anomaly is still there at high\nfrequencies. Therefore, we concluded that the observed anomalies in C44 and\n(C11-C12)/2 are not true. They would be ascribed to certain influence by the\nlarge softening of C66. So, we have checked our data through careful\nmeasurements by using ultrasonic frequency higher than 60 MHz, so far. Then, it\nhas been found that C66 shows still nice softening toward TS, but that its\ntemperature dependence is slightly different from the results of this paper. We\nhave accumulated reliable data now. They will be reported in near future.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Comparison of the fluctuation influence on the resistive properties of\n  the mixed state of BiSrCaCuO and of thin films of conventional superconductor: The resistive properties of layered HTSC BiSrCaCuO in the mixed state are\ncompared with those of thin films of conventional superconductors with weak\ndisorders (amorphous Nb_{1-x}0_{x} films) and with strong disorders\n(Nb_{1-x}O_{x} films with small grain structure). The excess conductivity is\nconsidered as a function of superconducting electron density and phase\ncoherence length. It is shown that the transition to the Abrikosov state\ndiffers from the ideal case both in BiSrCaCuO and Nb_{1-x}O_{x} films, i.e. the\nappearance of long-range phase coherence is continuous transition in both\ncases. The quantitative difference between thin films with weak and strong\ndisorders is greater than the one between layered HTSC and conventional\nsuperconductors, showing that the dimensionality of the system, rather than the\ncritical temperature, is the key factor ruling fluctuation effects",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetic field induced polarization effects in intrinsically granular\n  superconductors: Based on the previously suggested model of nanoscale dislocations induced\nJosephson junctions and their arrays, we study the magnetic field induced\nelectric polarization effects in intrinsically granular superconductors. In\naddition to a new phenomenon of chemomagnetoelectricity, the model predicts\nalso a few other interesting effects, including charge analogues of Meissner\nparamagnetism (at low fields) and \"fishtail\" anomaly (at high fields). The\nconditions under which these effects can be experimentally measured in\nnon-stoichiometric high-T_c superconductors are discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Justification of the canonical quantization of the Josephson effect: Quantum devices based on Josephson effect in superconductors are usually\ndescribed by a Hamiltonian obtained by commonly used canonical quantization.\nHowever, this recipe has not been yet rigorously justified. We show that this\napproach is indeed correct in certain range of parameters. We find the\ncondition of the validity of such quantization and the lowest corrections to\nthe Josephson energy.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Geometrical Probability Distribution Functions for Cable-in-Conduit\n  Conductors with Simply and Multiply Connected Cross-Sections: A geometrical method is presented for the calculation of the strand\ndistribution functions for cable-in-conduit superconductors with simple and\nmultiply connected cross-sections. The method is illustrated on different cable\ndesigns with simply and multiply connected structures.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Clocking the Onset of Bilayer Coherence in a High-$\\mathrm{T_C}$ Cuprate: In cuprates, a precursor state of superconductivity is speculated to exist\nabove the critical temperature $\\mathrm{T_C}$. Here we show via a combination\nof far-infrared ellipsometry and ultrafast broadband optical spectroscopy that\nsignatures of such a state can be obtained via three independent observables in\nan underdoped sample of NdBa$_2$Cu$_3$O$_{6+\\delta}$. The pseudogap\ncorrelations were disentangled from the response of laser-broken pairs by\nclocking their characteristic time-scales. The onset of a superconducting\nprecursor state was found at a temperature $\\mathrm{T_{ONS}}$ $>$\n$\\mathrm{T_C}$, consistent with the temperature scale identified via static\noptical spectroscopy. Furthermore, the temperature evolution of the coherent\nvibration of the Ba ion, strongly renormalized by the onset of\nsuperconductivity, revealed a pronounced anomaly at the same temperature\n$\\mathrm{T_{ONS}}$. The microscopic nature of such a precursor state is\ndiscussed in terms of pre-formed pairs and enhanced bilayer coherence.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "A first principles study on FeAs single layers: FeAs- single layer is tested as a simple model for LaFeAsO and BaFe2As2 based\non first-principles calculations using generalized gradient approximation (GGA)\nand GGA+U. The calculated single- layer geometric and electronic structures are\ninconsistent with that of bulk materials. The bulk collinear antiferromagnetic\nground state is failed to be obtained in the FeAs- single layer. The monotonous\nbehavior of the Fe-As distance in z direction upon electron or hole doping is\nalso in contrast with bulk materials. Our results indicate that, in LaFeAsO and\nBaFe2As2, interactions between FeAs layer and other layers beyond simple charge\ndoping are important, and a single FeAs layer may not represent a good model\nfor Fe based superconducting materials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Roles of Antiferromagnetic Fluctuation in High Field Phase Diagram of\n  Superconductors with Strong Paramagnetic Depairing: The high field phase diagram and magnetic properties of CeCoIn_5 below\nH_c2(0) are examined from the picture regarding the high field and low\ntemperature (HFLT) phase as a possible Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)\nvortex lattice. Crucial roles of antiferromagnetic (AFM) fluctuations enhanced\nclose to H_c2(0) are stressed. The FFLO vortex lattice with a longitudinal\nmodulation parallel to the field is stabilized compared with those with lateral\nmodulations as a consequence of the presence of AFM fluctuations. Further, an\nunusual field-induced enhancement of the flux distribution is argued to be a\nconsequence of interplay between the paramagnetic depairing and AFM\nfluctuation, both of which are enhanced with increasing field.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Where are holes in Y_{1-x}Pr_xBa2Cu3O7 ?: Recent experiments by Mertz et al (Phys. Rev. B55, 9160, 1997) demonstrated\nthat the loss of superconductivity upon Pr doping is associated with the change\nof the character of the oxygen holes from $p_{\\sigma}$ to $p_{\\pi}.$ This\nexperiment sheds new light onto the long-standing problem of $T_{c}$\nsuppression by Pr, and helps to rule out a number of theoretical models,\nleaving only those which predict such a transfer of the O holes. To distinguish\nbetween the two models that do predict such an effect, one has to access the\nratio of the planar and axial character of the holes in both models. We do so\nin this paper.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Orientation-dependent Josephson effect in spin-singlet\n  superconductor/altermagnet/spin-triplet superconductor junctions: We study the Josephson effect in the spin-singlet\nsuperconductor/altermagnet/spin-triplet superconductor junctions using the\nGreen's function method. The current-phase difference relationships in the\njunctions strongly depend on the orientation of altermagnet and the types of\nthe Cooper pairs. For the orientation angle equal to odd multiples of $\\pi/4$,\nthe current-phase difference relationships are of the $\\sin{2\\phi}$ type, which\nare irrespective of the pairing wave functions in superconductors. For the\nother orientation angles, the emergence of the lowest order current becomes\npossible and its form, $\\sin\\phi$ or $\\cos\\phi$, depends on the pairing wave\nfunctions in superconductors. The $\\phi_{0}$ phase and the $0$-$\\pi$ transition\ncan be realized in our junctions due to the appearance of the lowest order\ncurrent. The selection rules for the lowest order current are presented. The\nsymmetric relations satisfied by the current-phase difference relationships are\nanalyzed through considering the transformations of the junctions under the\nmirror reflection, the time-reversal and the spin rotation operations. Our\nresults not only provide a method to detect the intrinsic spin-triplet\nsuperconductivity but also possess application values in the design of the\nfield-free quantum devices.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "On the coupling of magnetic moments to superconducting quantum\n  interference devices: We investigate the coupling factor $\\phi_\\mu$ that quantifies the magnetic\nflux $\\Phi$ per magnetic moment $\\mu$ of a point-like magnetic dipole that\ncouples to a superconducting quantum interference device (SQUID). Representing\nthe dipole by a current-carrying loop, the reciprocity of mutual inductances of\nSQUID and loop provides a way of calculating $\\phi_\\mu(\\vec{r}, \\vec{e}_\\mu)$\nvs.~position $\\vec{r}$ and orientation $\\vec{e}_\\mu$ of the dipole anywhere in\nspace from the magnetic field $B(\\vec{r})$ produced by a supercurrent\ncirculating in the SQUID loop. We use numerical simulations based on London and\nGinzburg-Landau theory to calculate $\\phi_\\mu$ from the supercurrent density\ndistributions in various SQUID geometries. We treat the far-field regime\n($r\\gtrsim a=$ inner size of the SQUID loop) with the dipole placed on the\nsymmetry axis of circular or square shaped loops. We compare expressions for\n$\\phi_\\mu$ from filamentary loop models with simulation results for loops with\nfinite width $w$ (outer size $A>a$), thickness $d$ and London penetration depth\n$\\lambda_L$ and show that for thin ($d\\ll a$) and narrow ($w < a$) loops the\nintroduction of an effective loop size $a_{\\rm eff}$ in the filamentary\nloop-model expressions results in agreement with simulations. For a dipole\nplaced in the center of the loop, simulations provide an expression\n$\\phi_\\mu(a,A,d,\\lambda_L)$ that covers a wide parameter range. In the\nnear-field regime (dipole centered at small distance $z$ above one SQUID arm)\nonly coupling to a single strip representing the SQUID arm has to be\nconsidered. Here, we compare simulations with an analytical expression derived\nfor a homogeneous current density distribution, which yields excellent\nagreement for $\\lambda_L>w,d$. Moreover, we analyze $\\phi_\\mu$ provided by the\nintroduction of a constriction in the SQUID arm below the magnetic dipole.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Conventional mechanisms for \"exotic\" superconductivity: We consider the pairing state due to the usual BCS mechanism in substances of\ncubic and hexagonal symmetry where the Fermi surface forms pockets around\nseveral points of high symmetry. We find that the symmetry imposed on the\nmultiple pocket positions could give rise to a multidimensional nontrivial\nsuperconducting order parameter. The time reversal symmetry in the pairing\nstate is broken. We suggest several candidate substances where such ordering\nmay appear, and discuss means by which such a phase may be identified.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Anisotropic Superconductivity in the Induced Pairing Model: The model of local electron pairs and itinerant fermions coupled via charge\nexchange mechanism, which mutually induces superconductivity in both subsystems\nis studied for anisotropic pairing symmetry. The phase diagram is presented and\nthe phase fluctuations effects are analyzed within the Kosterlitz-Thouless\nscenario.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Structural Modulation in LaO0.9F0.1BiSe2 Single Crystals Revealed by\n  Scanning Tunneling Microscopy/Spectroscopy: We present scanning tunneling microscopy and spectroscopy measurements on a\ncleaved surface of the LaO0.9F0.1BiSe2 single crystals. Tunneling spectra show\na finite local density of states at EF, which is consistent with metallic\nconductivity in bulk. In addition, the existence of the supermodulation running\nalong the diagonal directions of Bi square lattice was revealed. The period of\nthe supermodulation was about 3 to 5 times the length of the lattice constant.\nThis period is close to that observed in LaO0.5F0.5BiSe2.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Charge creation and nucleation of longitudinal plasma wave in coupled\n  Josephson junctions: We study the phase dynamics in coupled Josephson junctions describing by\nsystem of nonlinear differential equations. Results of detailed numerical\nsimulations of charge creation in the superconducting layers and the\nlongitudinal plasma wave (LPW) nucleation are presented. We demonstrate the\ndifferent time stages in the development of the LPW and present results of FFT\nanalysis at different values of bias current. The correspondence between the\nbreakpoint position on the outermost branch of current voltage characteristics\n(CVC) and the growing region in time dependence of the electric charge in the\nsuperconducting layer is established. The effects of noise in the bias current\nand the external microwave radiation on the charge dynamics of the coupled\nJosephson junctions are found. These effects introduce a way to regulate the\nprocess of LPW nucleation in the stack of IJJ.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Zero-crossing Shapiro steps in focused-ion-beam-tailored high-$T_c$\n  superconducting microstructures: Microwave response of S-shaped Bi$_2$Sr$_2$CaCu2O$_{8+x}$ (Bi-2212)\nmicron-scale samples, in which the supercurrent was forced to flow\nperpendicular to the crystal layers, was investigated. A treatment with a\nfocused ion beam allowed us to reduce the plasma frequency down to $f_p$$\\sim$5\nGHz at $T$=0.3 K in naturally stacked Josephson junctions in a crystal. We\nobserved Shapiro steps at frequencies as low as $\\sim$5 GHz. Well-developed\nzero-crossing Shapiro steps were observed at frequencies as low as $\\sim$10\nGHz. They appeared as constant-voltage plateaus with a non-zero voltage\noccurring at zero bias current. We confirmed that zero-crossing Shapiro steps\nin the Bi-2212 stacked junctions can be observed when the irradiated frequency\nis sufficiently larger than $f_p$. The observed high-order fractional steps in\nthe microwave responses indicate that the interlayer-coupled Bi-2212 Josephson\njunctions have nonsinusoidal current-phase relation. Based on the temperature\ndependence of the steps we also showed that the finite slope of the steps is\ndue to the enhancement of the phase diffusion effect.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The circulation radius and critical current density in type-II\n  superconductors: A method is proposed for estimating the length scale of currents circulating\nin superconductors. The estimated circulation radius is used to determine the\ncritical current density on the basis of magnetic measurements. The obtained\nformulas are applicable to samples with negligibly small demagnetizing factors\nand to polycrystalline superconductors. The proposed method has been verified\nusing experimental magnetization loops measured for polycrystalline\nYBa$_2$Cu$_3$O$_{7-d}$ and Bi$_{1.8}$Pb$_{0.3}$Sr$_{1.9}$Ca$_2$Cu$_3$O$_x$\nsuperconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Giant shot noise from Majorana zero modes in topological trijunctions: The clear-cut experimental identification of Majorana bound states in\ntransport measurements still poses experimental challenges. We here show that\nthe zero-energy Majorana state formed at a junction of three topological\nsuperconductor wires is directly responsible for giant shot noise amplitudes,\nin particular at low voltages and for small contact transparency. The only\nintrinsic noise limitation comes from the current-induced dephasing rate due to\nmultiple Andreev reflection processes.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Cooper pairs localization in tree-like networks of superconducting\n  islands: We study inhomogeneous Cooper pairs distribution and localization effects in\ntree-like networks of superconducting islands coupled via Josephson weak links.\nUsing a generalized Feynman's approach, reminiscent of the Bose-Hubbard model,\nwe demonstrate that the Cooper pairs fraction which localizes on a specific\nnetwork's island is limited by the network topology and, if present, by the\nrepulsive interaction. These findings contribute to clarify the interplay\nbetween confinement effects induced by the network's topology and interaction\nand shed some light on recent experiments dealing with networks of Josephson\njunctions.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pinning properties of FeSeTe thin film through multifrequency\n  measurements of the surface impedance: We present high frequency measurements of the vortex dynamics of a\nFeSe$_x$Te$_{1-x}$ ($x=0.5$) thin film grown on a CaF$_2$ substrate and with a\ncritical temperature $T_c\\simeq18\\;$K, performed by means of a dual frequency\ndielectric resonator at 16.4 GHz and 26.6 GHz. We extract and discuss various\nimportant vortex parameters related to the pinning properties of the sample,\nsuch as the characteristic frequency $\\nu_c$, the pinning constant $k_p$ and\nthe pinning barrier height $U$ relevant for creep phenomena. We find that the\nvortex system is in the single-vortex regime, and that pinning attains\nrelatively high values in terms of $k_p$, indicating significant pinning at the\nhigh frequencies here studied. The pinning barrier energy $U$ is quite small\nand exhibits a non-monotonous temperature dependence with a maximum near 12 K.\nThis result is discussed in terms of core pinning of small portion of vortices\nof size $\\propto\\xi^3$ jumping out of the pinning wells over very small\ndistances, a process which is favoured in the high frequency, short ranged\nvortex oscillations here explored.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Nonequilibrium transport via spin-induced sub-gap states in\n  superconductor/quantum dot/normal metal cotunnel junctions: We study low-temperature transport through a Coulomb blockaded quantum dot\n(QD) contacted by a normal (N), and a superconducting (S) electrode. Within an\neffective cotunneling model the conduction electron self energy is calculated\nto leading order in the cotunneling amplitudes and subsequently resummed to\nobtain the nonequilibrium T-matrix, from which we obtain the nonlinear\ncotunneling conductance. For even occupied dots the system can be conceived as\nan effective S/N-cotunnel junction with subgap transport mediated by Andreev\nreflections. The net spin of an odd occupied dot, however, leads to the\nformation of sub-gap resonances inside the superconducting gap which gives rise\nto a characteristic peak-dip structure in the differential conductance, as\nobserved in recent experiments.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Some Global Properties of the Attractive Hubbard Model in the\n  Superconducting Phase: T-Matrix Approximation in 2D: We have applied the Fast Fourier transform (FFT), which allows to compute\nefficiently convolution sums, to solve the set of self-consistent T-matrix\nequations to get the Green function of the two dimensional attractive-U Hubbard\nmodelbelow $T_c$, extending previous calculations of the same authors. Using a\nconstant order parameter $\\Delta(T)$, we calculated $T_c$ as a function of\nelectron density and interaction strength $U$. These global results deviate\nfrom the BCS behavior remarkably.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Unusual Relationship between Magnetism and Superconductivity in\n  FeTe$_{0.5}$Se$_{0.5}$: We use neutron scattering, to study magnetic excitations in crystals near the\nideal superconducting composition of FeTe$_{0.5}$Se$_{0.5}$. Two types of\nexcitations are found, a resonance at (0.5, 0.5, 0) and incommensurate\nfluctuations on either side of this position. We show that the two sets of\nmagnetic excitations behave differently with doping, with the resonance being\nfixed in position while the incommensurate excitations move as the doping is\nchanged. These unusual results show that a common behavior of the low energy\nmagnetic excitations is not necessary for pairing in these materials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Control of Correlations in Sr4V2O6Fe2As2 by Chemical Stoichiometry: We show using a combination of powder X-ray and neutron diffraction, first\nprinciples calculations, temperature- and field-dependent magnetization, heat\ncapacity and resistivity data that the superconducting behavior of\n`Sr$_4$V$_2$O$_6$Fe$_2$As$_2$' is dependent on synthesis conditions,\nparticularly, heating profiles result in unintentional chemical doping. This\ncompound can be tuned from a state in which the vanadium electrons are\nitinerant with a high electronic density of states, to a state where the\nvanadium-oxide layers are insulating and presumably antiferromagnetic.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Small Fermi energy, zero point fluctuations and nonadiabaticity in\n  MgB$_2$: Small Fermi energy effects are induced in MgB$_2$ by the low hole doping in\nthe $\\sigma$ bands which are characterized by a Fermi energy $E_{\\rm F}^\\sigma\n\\sim 0.5$ eV. We show that, due to the particularly strong deformation\npotential relative to the $E_{2g}$ phonon mode, lattice fluctuations are\nreflected in strong fluctuations in the electronic band structure. Quantum\nfluctuations associated to the zero-point lattice motion are responsible for an\nuncertainty of the Fermi energy of the order of the Fermi energy itself,\nleading to the breakdown of the adiabatic principle underlying the\nBorn-Oppenheimer approximation in MgB$_2$ even if $\\omega_{\\rm ph}/E_{\\rm F}\n\\sim 0.1-0.2$, where $\\omega_{\\rm ph}$ are the characteristic phonon\nfrequencies. This amounts to a new nonadiabatic regime, which could be relevant\nto other unconventional superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity in WO2.6F0.4 synthesized by reaction of WO3 with\n  Teflon: WO3-xFx (x < 0.45) perovskite-like oxyfluorides were prepared by a chemically\nreducing fluorination route using the polymer polytetrafluoroethylene (Teflon).\nThe symmetry of the crystal structures of WO3-xFx changes from monoclinic to\ntetragonal to cubic as the fluorine content increases. Fluorine doping changes\ninsulating WO3 to a metallic conductor, and superconductivity (Tc = 0.4 K) was\ndiscovered in the samples with fluorine contents of 0.41 < x < 0.45. This easy\nfluorination method may be applicable to other systems and presents an\nopportunity for finding new oxyfluoride superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Observation of pseudogap-like feature above Tc in LiFeAs and\n  (Ba0.6K0.4)Fe2As2 by ultrafast optical measurement: We utilize ultrafast optical measurement to study the quasiparticle\nrelaxation in stoichiometric LiFeAs and nearly optimally doped (BaK)Fe2As2\ncrystals. According to our temperature-dependent studies of LiFeAs, we have\nobserved pseudogap-like feature at onset temperature of ~ 55 K, which is above\nTc = 15 K. In addition, the onset temperature of pseudogap ~90K was also\nobserved in Ba0.6K0.4Fe2As2 (Tc = 36 K). Our findings seem implying that the\npseudogap feature, which is due to antiferromagnetic fluctuations, is universal\nfor the largely studied 11, 111, 122, and 1111 iron-based superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Ring Oscillators for Clocking Reversible Superconducting Circuits and\n  Fabrication Process Benchmarking: Existing concepts of reversible superconducting circuits as well as\ndemonstrated adiabatic circuits require three-phase bias/clock signals\ngenerated by room temperature sources. A while ago, we suggested that a\nmulti-phase bias/clock could be provided by a local Josephson junction-based\ngenerator. The generator looks like a long annular Josephson junction, only\ncomposed of discreet elements - junctions and inductors, and closed into a ring\nvia a flux pump to inject any required number of vortices into the ring. A\nsteady motion of the vortices forced by a uniformly distributed dc bias current\napplied to the ring is accompanied by a nearly harmonic ac currents flowing via\nthe Josephson junctions (JJs) connected in series with small inductors. These\nac currents serve as multi-phase bias/clock for nSQUID-based circuitry. To\nverify this concept and compare the dissipated energy with kBTln2 threshold, we\ndeveloped a ring composed of 256 unshunted JJs with 20 {\\mu}A target critical\ncurrent, Ic. We investigated the behavior of the ring oscillator at each vortex\ncount from 0 to 256. The measured critical current of the ring with vortices\nwas about 0.1 {\\mu}A per one JJ, which can be explained by unavoidable\nnonuniformity of the ring components and the influence of fluxes frozen near\nthe ring. The corresponding energy dissipation, about 10kBT per passage of one\nvortex through one JJ, should be reduced further for prospective experiments\nwith reversible circuits. However, obtained I-V characteristics could be of\ninterest for scientists working with long Josephson junctions. Superiority of\nthe fabrication process used in this work is demonstrated by the obtained about\n200 times reduction of Ic of the ring with vortices with respect to a single\ncomprising JJ, much larger than in any previously described case.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Topological superconductivity on the surface of Fe-based superconductors: As one of the simplest systems for realizing Majorana fermions, topological\nsuperconductor plays an important role in both condensed matter physics and\nquantum computations. Based on \\emph{ab~initio} calculations and the analysis\nof an effective 8-band model with the superconducting pairing, we demonstrate\nthat the three dimensional extended $s$-wave Fe-based superconductors such as\nFe$_{1+\\text{y}}$Se$_{0.5}$Te$_{0.5}$ have a metallic topologically nontrivial\nband structure, and exhibit a normal-topological-normal superconductivity phase\ntransition on the ($001$) surface by tuning the bulk carrier doping level. In\nthe topological superconductivity (TSC) phase, a Majorana zero mode is trapped\nat the end of a magnetic vortex line. We further show that, the surface TSC\nphase only exists up to a certain bulk pairing gap, and there is a\nnormal-topological phase transition driven by the temperature, which has not\nbeen discussed before. These results pave an effective way to realize the TSC\nand Majorana fermions in a large class of superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Mechanism of Cooper-pairing in layered high temperature superconductors: In this study, the pairing mechanism for layered HTS materials based on\nattraction between electrons from adjacent layers is proposed. Initially, each\nlayer has expanded Fermi sphere owing to ridged geometry. When the two layers\nare close enough for tunneling, it becomes energetically advantageous to form\ncorrelated quantum states (CQS), reducing the Fermi sphere volume. Cooper\npairs, comprising inter-tunneling electrons, occupy the CQS. The image force is\nresponsible for the electron-electron attraction. Pair-binding energy and the\ncorresponding effective mass vary in a wide range. At T>0, some heavy pairs do\nnot condense. Such pairs are responsible for pseudogap. Light pairs get Bose\ncondensed and are responsible for superconductivity. The proposed mechanism\nprovides clarification of superconductivity in cuprates, iron based\nsuperconductors and LSCO/LCO interfaces. It provides explanation of two energy\ngaps and two characteristic temperatures in layered superconducting materials.\nIt also provides clarification on the Fermi surface pockets, anisotropy of\ncharge transport in pseudogap state, and other properties of HTS materials. The\npseudogap, estimated within the model, fits the experimental values for the\ntwo-layer cuprates, such as YBCO, Bi2212, Tl2212, and Hg1212.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Scenario of Superconducting Transition for quasi-2D HTS: We discuss the scenario of superconducing transition for quasi two dimension\nHTS with spin fluctuation pairing mechanism. At mean field temperature of 2D\nsuperconducting transition the interaction of fluctuation spin waves with holes\nin copper-oxigen planes leads to the pairing of holes and to the fluctuation\ngeneration of superconducting regions, and also to the essential temperature\ndependence of the strength of the interlayer coupling. At decreasing\ntemperature and sufficiently small interlayer coupling the transition of the\nsample to coherent superconducting state occurs.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Connection between the semiconductor--superconductor transition and the\n  spin-polarized superconducting phase in the honeycomb lattice: The band structure of noninteracting fermions in the honeycomb lattice\nexhibits the Dirac cones at the corners of the Brillouin zone. As a\nconsequence, fermions in this lattice manifest a semiconducting behavior below\nsome critical value of the onsite attraction, $U_{c}$. However, above $U_{c}$,\nthe superconducting phase can occur. We discuss an interplay between the\nsemiconductor--superconductor transition and the possibility of realization of\nthe spin-polarized superconductivity (the so-called Sarma phase). We show that\nthe critical interaction can be tuned by the next-nearest-neighbor (NNN)\nhopping in the absence of the magnetic field. Moreover, a critical value of the\nNNN hopping exists, defining a range of parameters for which the semiconducting\nphase can emerge. In the weak coupling limit case, this quantum phase\ntransition occurs for the absolute value of the NNN hopping equal to one third\nof the hopping between the nearest neighbors. Similarly, in the presence of the\nmagnetic field, the Sarma phase can appear, but only in a range of parameters\nfor which initially the semiconducting state is observed. Both of these aspects\nare attributed to the Lifshitz transition, which is induced by the NNN hopping\nas well as the external magnetic field.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Stability conditions for a large anharmonic bipolaron: A large polaron is a quasiparticle that consists of a nearly free electron\ninteracting with the phonons of a material, whose lattice parameters are much\nsmaller than the polaron scale. The electron-phonon interaction also leads to\nan attractive interaction between electrons, which can allow two polarons to\npair up and form a bipolaron. It has been shown that large bipolarons can form\nin theory due to strong 1-electron-1-phonon coupling, but they have not been\nseen in real materials because the critical value of the required\nelectron-phonon interaction is too large. Here, we investigate the effect of\n1-electron-2-phonon coupling on the large bipolaron problem.\n  Starting from a generalization of the Fr\\\"ohlich Hamiltonian that includes\nboth the standard 1-electron-1-phonon interaction as well as an anharmonic\n1-electron-2-phonon interaction, we use the path integral method to find a\nsemi-analytical upper bound for the bipolaron energy that is valid at all\nvalues of the Fr\\\"ohlich coupling strength $\\alpha$. We find the bipolaron\nphase diagram and conditions for the bipolaron stability by comparing the\nbipolaron energy to the energy of two free polarons. The critical value of the\nFr\\\"ohlich coupling strength $\\alpha_{\\text{crit}}$ is calculated as a function\nof the strength of the 1-electron-2-phonon interaction. The results suggest\nthat large bipolaron formation is more likely in materials with significant\n1-electron-2-phonon interaction as well as strong 1-electron-1-phonon\ninteraction, such as strontium titanate.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Universal suppression of superfluid weight by disorder independent of\n  quantum geometry and band dispersion: Motivated by the experimental progress in controlling the properties of the\nenergy bands in superconductors, significant theoretical efforts have been\ndevoted to study the effect of the quantum geometry and the flatness of the\ndispersion on the superfluid weight. In conventional superconductors, where the\nenergy bands are wide and the Fermi energy is large, the contribution due to\nthe quantum geometry is negligible, but in the opposite limit of flat-band\nsuperconductors the superfluid weight originates purely from the quantum\ngeometry of Bloch wave functions. Here, we study how the energy band dispersion\nand the quantum geometry affect the disorder-induced suppression of the\nsuperfluid weight. Surprisingly, we find that the disorder-dependence of the\nsuperfluid weight is universal across a variety of models, and independent of\nthe quantum geometry and the flatness of the dispersion. Our results suggest\nthat a flat-band superconductor is as resilient to disorder as a conventional\nsuperconductor.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Tunneling conductance in Superconductor/Ferromagnet junctions: a self\n  consistent approach: We evaluate the tunneling conductance of clean Ferromomagnet/Superconductor\njunctions via a fully self-consistent numerical solution of the microscopic\nBogoliubov-DeGennes equations. We present results for a relevant range of\nvalues of the Fermi wavevector mismatch (FWM), the spin polarization, and the\ninterfacial scattering strength. For nonzero spin polarization, the conductance\ncurves vary nonmonotonically with FWM. The FWM dependence of the\nself-consistent results is stronger than that previously found in\nnon-self-consistent calculations, since, in the self-consistent case, the\neffective scattering potential near the interface depends on the FWM. The\ndependence on interfacial scattering is monotonic. These results confirm that\nit is impossible to characterize both the the FWM and the interfacial\nscattering by a single effective parameter and that analysis of experimental\ndata via the use of such one-parameter models is unreliable.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Bias current dependence of superconducting transition temperature in\n  superconducting spin valve nanowires: Competition between superconducting and ferromagnetic ordering at interfaces\nbetween ferromagnets (F) and superconductors (S) gives rise to several\nproximity effects such as odd-triplet superconductivity and spin-polarized\nsupercurrents. A prominent example of an S/F proximity effect is the spin\nswitch effect (SSE) observed in S/F/N/F superconducting spin-valve multilayers,\nin which the superconducting transition temperature T$_c$ is controlled by the\nangle $\\phi$ between the magnetic moments of the F layers separated by a\nnonmagnetic metallic spacer N. Here we present an experimental study of SSE in\nNb/Co/Cu/Co/CoO$_x$ nanowires measured as a function of bias current flowing in\nthe plane of the layers. These measurements reveal an unexpected dependence of\nT$_c(\\phi)$ on the bias current: T$_c(\\pi)$--T$_c(0)$ changes sign with\nincreasing current bias. We attribute the origin of this bias dependence of the\nSSE to a spin Hall current flowing perpendicular to the plane of the\nmultilayer, which suppresses T$_c$ of the multilayer. The bias dependence of\nSSE can be important for hybrid F/S devices such as those used in cryogenic\nmemory for superconducting computers as device dimensions are scaled down to\nthe nanometer length scale.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Doping evolution of itinerant magnetic excitations in Fe-based\n  oxypnictides: Employing the four-band tight-binding model we study theoretically the doping\ndependence of the spin response in the normal state of novel Fe-based pnictide\nsuperconductors. We show that the commensurate spin density wave (SDW)\ntransition that arises due to interband scattering between the hole\n$\\alpha$-pockets and the electron $\\beta$-pockets disappears already at the\ndoping concentration $x \\approx 0.04$ reflecting the evolution of the Fermi\nsurfaces. Correspondingly, with further increase of the doping the\nantiferromagnetic fluctuations are suppressed for $x > 0.1$ and the\nIm$\\chi({\\bf Q_{AFM}},\\omega)$ becomes nearly temperature independent. At the\nsame time, we observe that the uniform susceptibility deviates from the\nPauli-like behavior and is increasing with increasing temperature reflecting\nthe activation processes for the $\\alpha$-Fermi surfaces up to temperatures of\nabout T=800K. With increase of the doping the absolute value of the uniform\nsusceptibility lowers and its temperature dependence changes. In particular, it\nis a constant at low temperatures and then decreases with increasing\ntemperature. We discuss our results in a context of recent experimental data.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity with hard-core repulsion: BCS-Bose crossover and\n  s-/d-wave competition: We consider fermions on a 2D lattice interacting repulsively on the same site\nand attractively on the nearest neighbor sites. The model is relevant, for\ninstance, to study the competition between antiferromagnetism and\nsuperconductivity in a Kondo lattice. We first solve the two-body problem to\nshow that in the dilute and strong coupling limit the s-wave Bose condensed\nstate is always the ground state. We then consider the many-body problem and\ntreat it at mean-field level by solving exactly the usual gap equation. This\nguarantees that the superconducting wave-function correctly vanishes when the\ntwo fermions (with antiparallel spin) sit on the same site. This fact has\nimportant consequences on the superconducting state that are somewhat unusual.\nIn particular this implies a radial node-line for the gap function. When a next\nneighbor hopping t' is present we find that the s-wave state may develop nodes\non the Fermi surface.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Large gap, a pseudogap and proximity effect in the Bi2Te3/Fe1+yTe\n  interfacial superconductor: We report directional point-contact spectroscopy data on the novel\nBi2Te3/Fe1+yTe interfacial superconductor for a Bi2Te3 thickness of 9 quintuple\nlayers, bonded by van der Waals epitaxy to a Fe1+yTe film at an atomically\nsharp interface. Our data show a very large superconducting twin-gap structure\nwith an energy scale exceeding that of bulk FeSe or FeSe1-xTex by a factor of\n4. While the larger gap is isotropic and attributed to a thin FeTe layer in\nproximity of the interface, the smaller gap has a pronounced anisotropy and is\nassociated with proximity-induced superconductivity in the topological\ninsulator Bi2Te3. Zero resistance is lost above 8 K, but superconducting\nfluctuations are visible up to at least 12 K and the large gap is replaced by a\npseudogap that persists up to 40 K. The spectra show a pronounced zero-bias\nconductance peak in the superconducting state, which may be a signature of an\nunconventional pairing mechanism.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Anisotropy of the upper critical field in MgB2: the two-gap\n  Ginzburg-Landau theory: The upper critical field in MgB2 is investigated in the framework of the\ntwo-gap Ginzburg-Landau theory. A variational solution of linearized\nGinzburg-Landau equations agrees well with the Landau level expansion and\ndemonstrates that spatial distributions of the gap functions are different in\nthe two bands and change with temperature. The temperature variation of the\nratio of two gaps is responsible for the upward temperature dependence of\nin-plane Hc2 as well as for the deviation of its out-of-plane behavior from the\nstandard angular dependence. The hexagonal in-plane modulations of Hc2 can\nchange sign with decreasing temperature.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Anisotropic angle-dependent Andreev reflection at the\n  ferromagnet/superconductor junction on the surface of topological insulators: We theoretically demonstrate that a ferromagnetic/superconductor junction on\nthe surface of three-dimensional topological insulators (3D TIs) has an\nanisotropic angle-dependent Andreev reflection when the in-plane magnetization\nhas a component perpendicular to the junction. In the presence of in-plane\nmagnetization, the Dirac cone's location adjusts in the $k$-space, whereas its\nout-of-plane component induces a gap. This movement leads to the anisotropic\nangle-dependent Andreev reflection and creates transverse conductance flows\nparallel to the interface. Also, an indirect gap induces in the junction, which\nremoves the transport signatures of Majorana bound states. Because of the full\nspin-momentum locking of Dirac fermions on the surface of 3DTIs, a torque that\ncalled \\textit{Andreev Transfer Torque} (ATT) imposes on the junction.\nMoreover, we propose a setup to detect them experimentally.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Symmetry protected line nodes in non-symmorphic magnetic space groups:\n  Applications to UCoGe and UPd$_2$Al$_3$: We present the group-thoretical classification of gap functions in\nsuperconductors coexisting with some magnetic order in non-symmorphic magnetic\nspace groups. Based on the weak-coupling BCS theory, we show that UCoGe-type\nferromagnetic superconductors must have horizontal line nodes on either $k_z=0$\nor $\\pm\\pi/c$ plane. Moreover, it is likely that additional Weyl point nodes\nexist at the axial point. On the other hand, in UPd$_2$Al$_3$-type\nantiferromagnetic superconductors, gap functions with $A_g$ symmetry possess\nhorizontal line nodes in antiferromagnetic Brillouin zone boundary\nperpendicular to $c$-axis. In other words, the conventional fully-gapped\n$s$-wave superconductivity is forbidden in this type of antiferromagnetic\nsuperconductors, irrelevant to the pairing mechanism, as long as the Fermi\nsurface crosses a zone boundary. UCoGe and UPd$_2$Al$_3$ are candidates for\nunconventional superconductors possessing hidden symmetry-protected line nodes,\npeculiar to non-symmorphic magnetic space groups.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Emergence of an incipient ordering mode in FeSe: The structurally simplest Fe-based superconductor FeSe with a critical\ntemperature $T_{c}\\approx$ 8.5 K displays a breaking of the four-fold\nrotational symmetry at a temperature $T_{s}\\approx 87$ K. We investigated the\nelectronic properties of FeSe using scanning tunneling microscopy/spectroscopy\n(STM/S), magnetization, and electrical transport measurements. The results\nindicated two new energy scales (i) $T^{*} \\approx$ 75 K denoted by an onset of\nelectron-hole asymmetry in STS, enhanced spin fluctuations, and increased\npositive magnetoresistance; (ii) $T^{**} \\approx$ 22 - 30 K, marked by opening\nup of a partial gap of about 8 meV in STS and a recovery of Kohler's rule. Our\nresults reveal onset of an incipient ordering mode at $T^{*}$ and its\nnucleation below $T^{**}$. The ordering mode observed here, both in spin as\nwell as charge channels, suggests a coupling between the spins with charge,\norbital or pocket degrees of freedom.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Engineering Quantum Interference: A model for describing interference and diffraction of wave functions of\none-dimensional Josephson array interferometers is presented. The derived\nexpression for critical current modulations accounts for an arbitrary number of\nsquare junctions, variable distance between these, and variable size of their\narea. Predictions are tested on real arrays containing up to 20 equally spaced\nand identical junctions and on arrays shaped with peculiar geometries. Very\ngood agreement with the modulations predicted by the model and the experimental\nresults is obtained for all the tested configurations. It is shown that\nspecific designs of the arrays generate significant differences in their static\nand dynamical (non-zero voltage) properties. The results demonstrate that the\nmagnetic field dependence of Josephson supercurrents shows how interference and\ndiffraction of macroscopic quantum wavefunctions can be manipulated and\ncontrolled.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Phase transitions in a three dimensional $U(1) \\times U(1)$ lattice\n  London superconductor: We consider a three-dimensional lattice $U(1) \\times U(1)$ superconductor in\nthe London limit, with two individually conserved condensates. The problem,\ngenerically, has two types of intercomponent interactions of different\ncharacters. First, the condensates are interacting via a minimal coupling to\nthe same fluctuating gauge field. A second type of coupling is the direct\ndissipationless drag represented by a local intercomponent current-current\ncoupling term in the free energy functional. The interplay between these two\ntypes of interactions produces a number of physical effects not present in\npreviously investigated $U(1)\\times U(1)$ models with only one kind of\nintercomponent interaction. In this work, we present a study of the phase\ndiagram of a $U(1) \\times U(1)$ superconductor which includes both of these\ninteractions. We study phase transitions and two types of competing paired\nphases which occur in this general model: (i) a metallic superfluid phase\n(where there is order only in the gauge invariant phase difference of the order\nparameters), (ii) a composite superconducting phase where there is order in the\nphase sum of the order parameters which has many properties of a\nsingle-component superconductor but with a doubled value of electric charge. We\ninvestigate the phase diagram with particular focus on what we call \"preemptive\nphase transitions\". These are phase transitions {\\it unique to multicomponent\ncondensates with competing topological objects}. A sudden proliferation of one\nkind of topological defects may come about due to a fluctuating background of\ntopological defects in other sectors of the theory.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quantum Coherence of Electrons Field-Emitted from a Superconductor:\n  Correlations and Entanglement: The correlations of the electrons field-emitted from a superconductor are\nfully analyzed, both in space and time. It is proposed that a coincidence\nexperiment would reveal a positive correlation between the electrons emitted in\nopposite directions. The electrons can be entangled and can even violate Bell's\ninequality. The crucial role played by Andreev's process is scrutinized,\nanalytical formulas are derived for the correlations, and the physics behind\nthe phenomenon is clarified.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Using Josephson junctions to determine the pairing state of\n  superconductors without crystal inversion symmetry: Theoretical studies of a planar tunnel junction between two superconductors\nwith antisymmetric spin-orbit coupling are presented. The half-space Green's\nfunction for such a superconductor is determined. This is then used to derive\nexpressions for the dissipative current and the Josephson current of the\njunction. Numerical results are presented in the case of the Rashba spin-orbit\ncoupling, relevant to the much studied compound CePt$_3$Si. Current-voltage\ndiagrams, differential conductance and the critical Josephson current are\npresented for different crystallographic orientations and different weights of\nsinglet and triplet components of the pairing state. The main conclusion is\nthat Josephson junctions with different crystallographic orientations may\nprovide a direct connection between unconventional pairing in superconductors\nof this kind and the absence of inversion symmetry in the crystal.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Interplay between spin density wave and superconductivity in '122' iron\n  pnictides: 57Fe M\u00f6ssbauer study: Iron-based superconductors Ba0.7Rb0.3Fe2As2 and CaFe1.92Co0.08As2 of the\n'122' family have been investigated by means of the 14.41-keV Moessbauer\ntransition in 57Fe versus temperature ranging from the room temperature till\n4.2 K. A comparison is made with the previously investigated parent compounds\nBaFe2As2 and CaFe2As2. It has been found that Moessbauer spectra of these\nsuperconductors are composed of the magnetically split component due to\ndevelopment of spin density wave (SDW) and non-magnetic component surviving\neven at lowest temperatures. The latter component is responsible for\nsuperconductivity. Hence, the superconductivity occurs in the part of the\nsample despite the sample is single phase. This phenomenon is caused by the\nslight variation of the dopant concentration across the sample (crystal).",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Itinerant approach to magnetic neutron scattering of FeSe: effect of\n  orbital selectivity: Recent STM experiments and theoretical considerations have highlighted the\nrole of interaction-driven orbital selectivity in FeSe, and its role in\ngenerating the extremely anisotropic superconducting gap structure in this\nmaterial. We study the magnetic excitation spectrum resulting from the coherent\nquasiparticles within the same renormalized random phase approximation approach\nused to explain the STM experiments, and show that it agrees well with the\nlow-energy momentum and energy dependent response measured by inelastic neutron\nscattering experiments. We find a correlation-induced suppression of\n$(\\pi,\\pi)$ scattering due to a small quasiparticle weight of states of\n$d_{xy}$ character. We compare predictions for twinned and untwinned crystals,\nand predict in particular a strongly $(\\pi,0)$-dominated response at low\nenergies in untwinned systems, in contrast to previous itinerant theories.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The Nernst effect in high-$T_c$ superconductors: The observation of a large Nernst signal $e_N$ in an extended region above\nthe critical temperature $T_c$ in hole-doped cuprates provides evidence that\nvortex excitations survive above $T_c$. The results support the scenario that\nsuperfluidity vanishes because long-range phase coherence is destroyed by\nthermally-created vortices (in zero field), and that the pair condensate\nextends high into the pseudogap state in the underdoped (UD) regime. We present\na series of measurements to high fields $H$ which provide strong evidence for\nthis phase-disordering scenario.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Absence of structural transition in TM0.5IrTe2 (TM=Mn, Fe, Co, Ni): TM-doped IrTe2(TM=Mn, Fe, Co, Ni) compounds were synthesized by solid state\nreaction. Single crystal x-ray diffraction experiments indicate that part of\nthe doped TM ions (TM=Fe, Co, and Ni) substitute for Ir, and the rest\nintercalate into the octahedral interstitial sites located in between IrTe2\nlayers. Due to the lattice mismatch between MnTe2 and IrTe2, Mn has limited\nsolubility in IrTe2 lattice. The trigonal structure is stable in the whole\ntemperature range 1.80<T<300K for all doped compositions. No long range\nmagnetic order or superconductivity was observed in any doped compositions\nabove 1.80K. A spin glass behavior below 10K was observed in Fe-doped IrTe2\nfrom the temperature dependence of magnetization, electrical resistivity, and\nspecific heat. The low temperature specific heat data suggest the electron\ndensity of states is enhanced in Fe- and Co-doped compositions but reduced in\nNi-doped IrTe2. With the 3d transition metal doping the trigonal a-lattice\nparameters increases but the c-lattice parameter decreases. Detailed analysis\nof the single crystal x-ray diffraction data shows that interlayer Te-Te\ndistance increases despite a reduced c-lattice. The importance of the Te-Te,\nTe-Ir, and Ir-Ir bonding is discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetic properties of undoped and 15%F doped SmFeAsO compounds: In this paper the magnetic behaviour of SmFeAsO and SmFeAs(O0.85F0.15)\nsamples is presented and discussed. Molar susceptibility of SmFeAsO exhibits a\nlocal peak at T around 140K due to the establishment of a long range\nantiferromagnetic ordering of the Fe moments in Fe-As layers. This feature has\nalready been observed with different techniques, and frequently ascribed to the\nonset of a Spin Density Wave (SDW). At TN around 6K another peak, which we\nattribute to the establishment of antiferromagnetic ordering of Sm ion\nsublattice, is observed. Furthermore, a temperature independent signal (Pauli\nparamagnetism, Landau and core diamagnetism...) is also present in the magnetic\nbehaviour of this sample. In SmFeAs(O0.85F0.15) the antiferromagnetic ordering\nin Fe-As plane is suppressed and superconductivity occurs at T = 52 K, whereas\nthe antiferromagnetic ordering of Sm ions at low temperature persists, leading\nto the coexistence and competition between superconducting and magnetic\norderings. Above the transition temperature, after the subtraction of the Sm\nion sublattice paramagnetic contribution and of the temperature independent\ncontribution to the experimental susceptibility data, a Curie-Weiss behaviour\nfor Fe is observed, with a magnetic moment of 1.4 Bohr magneton.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Bulk properties and electronic structure of PuFeAsO: Here we present bulk property measurements and electronic structure\ncalculations for PuFeAsO, an actinide analogue of the iron-based rare-earth\nsuperconductors RFeAsO. Magnetic susceptibility and heat capacity data suggest\nthe occurrence of an antiferromagnetic transition at TN=50 K. No further\nanomalies have been observed down to 2 K, the minimum temperature that we have\nbeen able to achieve. Structural measurements indicate that PuFeAsO, with its\nmore localized 5f electrons, bears a stronger resemblance to the RFeAsO\ncompounds with larger R ions, than NpFeAsO does.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pseudogap from ARPES experiment: three gaps in cuprates and topological\n  superconductivity: A term first coined by Mott back in 1968 a `pseudogap' is the depletion of\nthe electronic density of states at the Fermi level, and pseudogaps have been\nobserved in many systems. However, since the discovery of the high temperature\nsuperconductors (HTSC) in 1986, the central role attributed to the pseudogap in\nthese systems has meant that by many researchers now associate the term\npseudogap exclusively with the HTSC phenomenon. Recently, the problem has got a\nlot of new attention with the rediscovery of two distinct energy scales\n(`two-gap scenario') and charge density waves patterns in the cuprates. Despite\nmany excellent reviews on the pseudogap phenomenon in HTSC, published from its\nvery discovery up to now, the mechanism of the pseudogap and its relation to\nsuperconductivity are still open questions. The present review represents a\ncontribution dealing with the pseudogap, focusing on results from angle\nresolved photoemission spectroscopy (ARPES) and ends up with the conclusion\nthat the pseudogap in cuprates is a complex phenomenon which includes at least\nthree different `intertwined' orders: spin and charge density waves and\npreformed pairs, which appears in different parts of the phase diagram. The\ndensity waves in cuprates are competing to superconductivity for the electronic\nstates but, on the other hand, should drive the electronic structure to\nvicinity of Lifshitz transition, that could be a key similarity between the\nsuperconducting cuprates and iron based superconductors. One may also note that\nsince the pseudogap in cuprates has multiple origins there is no need to recoin\nthe term suggested by Mott.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity driven by the screening of long-distance Coulomb\n  interaction: The pair-fluctuation contribution reduces the electrostatic screening length\nin superconductivity as compared to the normal state. When a conductor\npossesses a static background charge distribution, superconductivity arises\neven in the absence of an explicit pairing interaction, such that the Coulomb\nrepulsion is reduced and the total energy is lowered. We demonstrate that the\nsuperconducting gap increases with increased background charge at first, after\nwhich the mixing of the Higgs and plasma modes suppresses superconductivity in\nthe pseudogap phase. This indicates that the mechanism may be relevant to the\ncuprates and iron pnictides. When the background charge is identified with the\nincoherent component of optical conductivity in the cuprates, our results\nreproduce the shape, size and position of the superconducting dome with zero\nfree parameters. A superconducting critical temperature of about 1000 K is\npossible in ion-doped conductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Microwave emission from superconducting vortices in Mo/Si superlattices: Most of superconductors in a magnetic field are penetrated by a lattice of\nquantized flux vortices. In the presence of a transport current causing the\nvortices to cross sample edges, emission of electromagnetic waves is expected\ndue to the continuity of tangential components of the fields at the surface.\nYet, such a radiation has not been observed so far due to low radiated power\nlevels and lacking coherence in the vortex motion. Here, we report emission of\nelectromagnetic waves from vortices crossing the layers of a\nsuperconductor/insulator Mo/Si superlattice. The emission spectra consist of\nnarrow harmonically related peaks which can be finely tuned in the GHz range by\nthe dc bias current and, coarsely, by the in-plane magnetic field value. Our\nfindings show that superconductor/insulator superlattices can act as dc-tunable\nmicrowave generators bridging the frequency gap between conventional\nradiofrequency oscillators and (sub-)terahertz generators relying upon the\nJosephson effect.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Combined effects of pressure and Ru substitution on BaFe2As2: The ab-plane resistivity of Ba(Fe1-xRux)2As2 (x = 0.00, 0.09, 0.16, 0.21, and\n0.28) was studied under nearly hydrostatic pressures, up to 7.4 GPa, in order\nto explore the T-P phase diagram and to compare the combined effects of\niso-electronic Ru substitution and pressure. The parent compound BaFe2As2\nexhibits a structural/magnetic phase transition near 134 K. At ambient\npressure, progressively increasing Ru concentration suppresses this phase\ntransition to lower temperatures at the approximate rate of ~5 K/% Ru and is\ncorrelated with the emergence of superconductivity. By applying pressure to\nthis system, a similar behavior is seen for each concentration: the\nstructural/magnetic phase transition is further suppressed and\nsuperconductivity induced and ultimately, for larger x Ru and P, suppressed. A\ndetailed comparison of the T-P phase diagrams for all Ru concentrations shows\nthat 3 GPa of pressure is roughly equivalent to 10% Ru substitution.\nFurthermore, due to the sensitivity of Ba(Fe1-xRux)2As2 to pressure conditions,\nthe melting of the liquid media, 4 : 6 light mineral oil : n-pentane and 1 : 1\niso-pentane : n-pentane, used in this study could be readily seen in the\nresistivity measurements. This feature was used to determine the freezing\ncurves for these media and infer their room temperature, hydrostatic limits:\n3.5 and 6.5 GPa, respectively.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Visualizing supercurrents in ferromagnetic Josephson junctions with\n  various arrangements of 0 and \u03c0segments: Josephson junctions with ferromagnetic barrier can have positive or negative\ncritical current depending on the thickness $d_F$ of the ferromagnetic layer.\nAccordingly, the Josephson phase in the ground state is equal to 0 (a\nconventional or 0 junction) or to $\\pi$ ($\\pi$ junction). When 0 and $\\pi$\nsegments are joined to form a \"0-$\\pi$ junction\", spontaneous supercurrents\naround the 0-$\\pi$ boundary can appear. Here we report on the visualization of\nsupercurrents in superconductor-insulator-ferromagnet-superconductor (SIFS)\njunctions by low-temperature scanning electron microscopy (LTSEM). We discuss\ndata for rectangular 0, $\\pi$, 0-$\\pi$, 0-$\\pi$-0 and 20 \\times 0-$\\pi$\njunctions, disk-shaped junctions where the 0-$\\pi$ boundary forms a ring, and\nan annular junction with two 0-$\\pi$ boundaries. Within each 0 or $\\pi$ segment\nthe critical current density is fairly homogeneous, as indicated both by\nmeasurements of the magnetic field dependence of the critical current and by\nLTSEM. The $\\pi$ parts have critical current densities $j_c^\\pi$ up to\n$35\\units{A/cm^2}$ at $T = 4.2\\units{K}$, which is a record value for SIFS\njunctions with a NiCu F-layer so far. We also demonstrate that SIFS technology\nis capable to produce Josephson devices with a unique topology of the 0-$\\pi$\nboundary.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Multiple Andreev Reflections in Weak Links of Superfluid 3He-B: We calculate the current-pressure characteristics of a ballistic pinhole\naperture between two volumes of B-phase superfluid 3He. The most important\nmechanism contributing to dissipative currents in weak links of this type is\nthe process of multiple Andreev reflections. At low biases this process is\nsignificantly affected by relaxation due to inelastic\nquasiparticle-quasiparticle collisions. In the numerical calculations,\nsuppression of the superfluid order parameter at surfaces is taken into account\nself-consistently. When this effect is neglected, the theory may be developed\nanalytically like in the case of s-wave superconductors. A comparison with\nexperimental results is presented.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetic properties of magnetically textured Bi-2212 ceramics: This paper aims at reporting magnetic properties of bulk polycrystalline\nBi2Sr2Ca0.8Dy0.2Cu2O8-y samples textured under a magnetic field. The\nmicrostructure of these materials is highly anisotropic and exhibits particular\nfeatures needed to be taken into account in order to interpret their magnetic\nand electrical properties. First the AC magnetic susceptibility c = c ' - j c\"\nhas been measured for several magnetic fields (H // ab and H // c) and compared\nto the electrical resistivity data. The structure of the c\" peak is shown to be\nrelated to the chemical content distribution of the superconducting grains.\nNext, the magnetic flux profiles have been extracted from the magnetic\nmeasurements using the Campbell - Rollins procedure. The anisotropy of the flux\nprofiles and their peculiar curvature behaviour for H // c point out the role\nof both grain platelet structure and the presence of secondary phases. From\nthese results, we conclude that the magnetic properties of such magnetically\ntextured materials do not allow for a reliable extraction of the critical\ncurrent density Jc but essentially probe geometric effects. Such effects have\nto be taken into account for improving the manufacture of attractive high-Tc\nmaterials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Tunable artificial vortex ice in nanostructured superconductors with\n  frustrated kagome lattice of paired antidots: Theoretical proposals for spin ice analogs based on nanostructured\nsuperconductors have suggested larger flexibility for probing the effects of\nfluctuations and disorder than in the magnetic systems. In this work, we unveil\nthe particularities of a vortex ice system by direct observation of the vortex\ndistribution in a kagome lattice of paired antidots using scanning Hall probe\nmicroscopy. The theoretically suggested vortex ice distribution, lacking long\nrange order, is observed at half matching field (H_{1}/2). Moreover, the vortex\nice state formed by the pinned vortices is still preserved at 2H_{1}/3. This\nunexpected result is attributed to the introduction of interstitial vortices at\nthese magnetic field values. Although the interstitial vortices increase the\nnumber of possible vortex configurations, it is clearly shown that the vortex\nice state observed at 2H_{1}/3 is less prone to defects than at $H_{1}/2$. In\naddition, the non-monotonic variations of the vortex ice quality on the lattice\nspacing indicates that a highly ordered vortex ice state cannot be attained by\nsimply reducing the lattice spacing. The optimal design to observe defect free\nvortex ice is discussed based on the experimental statistics. The direct\nobservations of a tunable vortex ice state provides new opportunities to\nexplore the order-disorder transition in artificial ice systems.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Evidence for interfacial superconductivity in a bi-collinear\n  antiferromagnetically ordered FeTe monolayer on a topological insulator: The discovery of high-temperature superconductivity in Fe-based compounds\n[1,2] has triggered numerous investigations on the interplay between\nsuperconductivity and magnetism [3] and, more recently, on the enhancement of\ntransition temperatures through interface effects [4]. It is widely believed\nthat the emergence of optimal superconductivity is intimately linked to the\nsuppression of long-range antiferromagnetic (AFM) order, although the exact\nmicroscopic picture of this relationship remains elusive [1] due to the lack of\ndata with atomic spatial resolution [5-7]. Here, we present a spin-polarized\nscanning tunneling spectroscopy (SP-STS) study of ultrathin FeTe$_{1-x}$Se$_x$\n(x = 0, 0.5) films grown on prototypical Bi-based bulk topological insulators.\nSurprisingly, we find an energy gap at the Fermi level indicating\nsuperconducting correlations up to Tc ~ 6 K for one unit cell thin FeTe layers\ngrown on Bi2Te3 substrates, in contrast to the non-superconducting FeTe bulk\ncompound [8]. Moreover, SP-STS reveals that the energy gap spatially coexists\nwith bicollinear AFM order. This finding opens novel perspectives for\ntheoretical studies of competing orders in Fe-based superconductors as well as\nfor experimental investigations of exotic phases in heterostructures of\ntopological insulators and superconducting layers.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Vortex Dipole in a Trapped Atomic Bose-Einstein Condensate: We calculate the angular momentum and energy of a vortex dipole in a trapped\natomic Bose-Einstein condensate. Fully analytic expressions are obtained. We\napply the results to understand a novel phenomenon in the MIT group experiment,\nan excellent agreement is achieved, and further experimental investigation is\nproposed to confirm this vortex dipole mechanism. We then suggest an effective\ngeneration and detection of vortex dipole for experimental realization.\nApplication of the sum rule to calculate collective mode frequency splitting is\nalso discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Tuning orbital-selective correlation effects in superconducting\n  Rb$_{0.75}$Fe$_{1.6}$Se$_{2-z}$S$_z$: We report on terahertz time-domain spectroscopy on superconducting and\nmetallic iron chalcogenides Rb$_{0.75}$Fe$_{1.6}$Se$_{2-z}$S$_z$. The\nsuperconducting transition is reduced from $T_c=$ 32 K ($z=0$) to 22 K\n($z=1.0$), and finally suppressed ($z=1.4$) by isoelectronic substitution of Se\nwith S. Dielectric constant and optical conductivity exhibit a\nmetal-to-insulator transition associated with an orbital-selective Mott phase.\nThis orbital-selective Mott transition appears at higher temperature $T_{met}$\nwith increasing sulfur content, identifying sulfur substitution as an efficient\nparameter to tune orbital-dependent correlation effects in iron-chalcogenide\nsuperconductors. The reduced correlations of the $d_{xy}$ charge carriers can\naccount for the suppression of the superconductivity and the pseudogap-like\nfeature between $T_c$ and $T_{met}$ that was observed for $z=0$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Unexpected impact of magnetic disorder on multiband superconductivity: We analyze how the magnetic disorder affects the properties of the two-band\n$s_\\pm$ and $s_{++}$ models, which are subject of hot discussions regarding\niron-based superconductors and other multiband systems like MgB$_2$. We show\nthat there are several cases when the transition temperature $T_c$ is not fully\nsuppressed by magnetic impurities in contrast to the Abrikosov-Gor'kov theory,\nbut a saturation of $T_c$ takes place in the regime of strong disorder. These\ncases are: (1) the purely interband impurity scattering, (2) the unitary\nscattering limit. We show that in the former case the $s_\\pm$ gap is preserved,\nwhile the $s_{++}$ state transforms into the $s_\\pm$ state with increasing\nmagnetic disorder. For the case (2), the gap structure remains intact.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electrodynamic response of MgB2 sintered pellets and thin films: We present a study of the electrodynamic response of MgB2 pellets and thin\nfilm samples exhibiting critical temperatures ranging between 26 and 38 K. We\nhave performed accurate measurements of the surface impedance ZS =RS+iXS as a\nfunction of the temperature and of the magnitude of the electromagnetic field.\nThe temperature variation and the field dependence of ZS was measured by a\ndielectric resonator cavity technique in the microwave region. In particular,\nthe temperature variation of the magnetic penetration depth was also determined\nin the RF region by a single coil mutual inductance method. In the case of the\nfilms, for T<TC/2 a clear exponential behavior of the penetration depth is\nobserved, which can be explained by a simple BCS s-wave model with a reduced\nvalue of the energy gap. On the contrary, pellets show no evidence of\nsaturation, and the experimental results strictly follow a quadratic dependence\ndown to the lowest temperatures. This behavior can be induced by the presence\nof metallic Mg inclusions that may locally depress the gap. The analysis of the\nfield dependence of the surface impedance in the microwave region confirms that\nthe electrodynamic response of MgB2 is dominated by different sources of\ndissipation, depending on the sample history, likely to be ascribed to the\npredominance of grain boundaries or normal regions on its surface.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "On the extraction of paramagnon excitations from resonant inelastic\n  X-ray scattering experiments: Resonant X-ray scattering experiments on high-temperature superconductors and\nrelated cuprates have revealed the presence of intense paramagnon scattering at\nhigh excitation energies, of the order of several hundred meV. The excitation\nenergies appear to show very similar behavior across all compounds, ranging\nfrom magnetically ordered, via superconductors, to heavy fermion systems.\nHowever, we argue that this apparent behavior has been inferred from the data\nthrough model fitting which implicitly imposes such similarities. Using model\nfitting that is free from such restrictions, we show that the paramagnons are\nnot nearly as well-defined as has been asserted previously, and that some\nparamagnons might not represent propagating excitations at all. Our work\nindicates that the data published previously in the literature will need to be\nre-analyzed with proper models.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "On the interpretation of the equilibrium magnetization in the mixed\n  state of high-Tc superconductors: We apply a recently developed scaling procedure to the analysis of\nequilibrium magnetization M(H) data that were obtained for T-2212 and\nBi-2212single crystals and were reported in the literature. The temperature\ndependencies of the upper critical field and the magnetic field penetration\ndepth resulting from our analysis are distinctly different from those obtained\nin the original publications. We argue that theoretical models, which are\nusually employed for analyses of the equilibrium magnetization in the mixed\nstate of type-II superconductors are not adequate for a quantitative\ndescription of high-Tc superconductors. In addition, we demonstrate that the\nscaled equilibrium magnetization M(H) curve for a Tl-2212 sample reveals a\npronounced kink, suggesting a phase transition in the mixed state.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Vortex dynamic, pinning and irreversibility field investigation in\n  EuRbFe4As4 superconductor: We performed systematic AC susceptibility and magnetic moment measurements to\ninvestigate the vortex dynamics and pinning in the $EuRbFe_4As_4$ single\ncrystal as a function of temperature, frequency, and DC magnetic field. The\nvortex solid-liquid line was determined and it fits well with\n$H(T_p)=H_0(1-t_p)^\\beta$ using $\\beta$=1.74-1.91, for $H\\parallel ab$. It\nindicates a rather high pinning strength of the vortex system. The activation\nenergy $U_0$ was determined from thermally activated flux creep theory and\nreached 6700 K at low fields, suggesting strong vortex pinning. A field\ndependence of $U_0(H\\parallel ab)\\sim H^a$ with $a=0.47$ suggests thermally\nactivated plastic pinning or caused by planar defects. Magnetic moment\nmeasurements also confirmed strong pinning in a $EuRbFe_4As_4$ superconductor\nand the superconducting response gives the main contribution to the $M(H)$\nhysteresis. Additionally, we found evidence of long-range magnetic interactions\nin $Eu^{2+}$ sublattice and the FM-like nature of $Eu^{2+}$ atoms ordering.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Imaging the Meissner effect and flux trapping in a hydride\n  superconductor at megabar pressures using a nanoscale quantum sensor: By directly altering microscopic interactions, pressure provides a powerful\ntuning knob for the exploration of condensed phases and geophysical phenomena.\nThe megabar regime represents an exciting frontier, where recent discoveries\ninclude novel high-temperature superconductors, as well as structural and\nvalence phase transitions. However, at such high pressures, many conventional\nmeasurement techniques fail. Here, we demonstrate the ability to perform local\nmagnetometry inside of a diamond anvil cell with sub-micron spatial resolution\nat megabar pressures. Our approach utilizes a shallow layer of Nitrogen-Vacancy\n(NV) color centers implanted directly within the anvil; crucially, we choose a\ncrystal cut compatible with the intrinsic symmetries of the NV center to enable\nfunctionality at megabar pressures. We apply our technique to characterize a\nrecently discovered hydride superconductor, CeH$_9$. By performing simultaneous\nmagnetometry and electrical transport measurements, we observe the dual\nsignatures of superconductivity: local diamagnetism characteristic of the\nMeissner effect and a sharp drop of the resistance to near zero. By locally\nmapping the Meissner effect and flux trapping, we directly image the geometry\nof superconducting regions, revealing significant inhomogeneities at the micron\nscale. Our work brings quantum sensing to the megabar frontier and enables the\nclosed loop optimization of superhydride materials synthesis.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Appearance of fluctuating stripes at the onset of the pseudogap in the\n  high-Tc Superconductor Bi2Sr2CaCu2O8+x: Doped Mott insulators have been shown to have a strong propensity to form\npatterns of holes and spins often referred to as stripes. In copper-oxides,\ndoping also gives rise to the pseudogap state, which transforms into a high\ntemperature superconductor with sufficient doping or by reducing the\ntemperature. A long standing question has been the interplay between pseudogap,\nwhich is generic to all hole-doped cuprates, and stripes, whose static form\noccurs in only one family of cuprates over a narrow range of the phase diagram.\nHere we examine the spatial reorganization of electronic states with the onset\nof the pseudogap state at T* in the high-temperature superconductor\nBi2Sr2CaCu2O8+x using spectroscopic mapping with the scanning tunneling\nmicroscope (STM). We find that the onset of the pseudogap phase coincides with\nthe appearance of electronic patterns that have the predicted characteristics\nof fluctuating stripes. As expected, the stripe patterns are strongest when the\nhole concentration in the CuO2 planes is close to 1/8 (per Cu). While\ndemonstrating that the fluctuating stripes emerge with the onset of the\npseudogap state and occur over a large part of the cuprate phase diagram, our\nexperiments indicate that they are a consequence of pseudogap behavior rather\nthan its cause.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Doping-Dependent and Orbital-Dependent Band Renormalization in\n  Ba(Fe_1-xCo_x)_2As_2 Superconductors: Angle resolved photoemission spectroscopy of Ba(Fe1-xCox)2As2 (x = 0.06,\n0.14, and 0.24) shows that the width of the Fe 3d yz/zx hole band depends on\nthe doping level. In contrast, the Fe 3d x^2-y^2 and 3z^2-r^2 bands are rigid\nand shifted by the Co doping. The Fe 3d yz/zx hole band is flattened at the\noptimal doping level x = 0.06, indicating that the band renormalization of the\nFe 3d yz/zx band correlates with the enhancement of the superconducting\ntransition temperature. The orbital-dependent and doping-dependent band\nrenormalization indicates that the fluctuations responsible for the\nsuperconductivity is deeply related to the Fe 3d orbital degeneracy.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Groove-rolling as an alternative process to fabricate Bi-2212 wires for\n  practical applications: Bi2Sr2CaCu2O8+x (Bi-2212) superconducting long-length wires are mainly\nlimited in obtaining high critical currents densities (JC) by the internal gas\npressure generated during the heat treatment, which expands the wire diameter\nand dedensifies the superconducting filaments. Several ways have been developed\nto increase the density of the superconducting filaments and therefore\ndecreasing the bubble density: much higher critical currents have been reached\nalways acting on the final as-drawn wires. We here try to pursue the same goal\nof having a denser wire by acting on the deformation technique, through a\npartial use of the groove-rolling at different wire processing stages. Such\ntechnique has a larger powders compaction power, is straightforwardly adaptable\nto long length samples, and allows the fabrication of samples with round,\nsquare or rectangular shape depending on the application requirements. In this\npaper we demonstrate the capability of this technique to increase the density\nin Bi-2212 wires which leads to a three-fold increase in Jc with respect to\ndrawn wires, making this approach very promising for fabricating Bi-2212 wires\nfor high magnetic field magnets, i.e. above 25 T.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Determining the absolute value of magnetic penetration depth in\n  small-sized superconducting films: In the previous four decades, a two-coil mutual inductance (MI) technique has\nbeen widely employed in characterizing magnetic penetration depth, $\\lambda$,\nof superconducting films. However, the conventional methods used to obtain\n$\\lambda$ are not applicable to small-sized films with common shapes, which\nlimits the application of the MI technique in superconductivity research. Here,\nwe first employed the fast wavelet collocation (FWC) method to a two-coil\nsystem and then proposed the possibility of directly obtaining the absolute\n$\\lambda$ of polygonal superconducting films with arbitrary sizes. To verify\nits accuracy and advantages, we extracted the $\\lambda$ values of square NbN\nfilms with different sizes using the FWC and conventional flux leakage\nsubtraction (FLS) methods. Notably, the FLS method fails for a $5\\times 5 \\,\n\\rm mm^2$ film, which is attributed to the significant current peak at the film\nedge. In contrast, the absolute $\\lambda$ extracted using the FWC method was\nindependent of the film size. Finally, we established the applicability of the\nFWC method to large coil spacings, which may pave the way for integrating\nhigh-accuracy $\\lambda$ measurements with the ionic liquid gating technique.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Type-II Ising superconductivity in two-dimensional materials with strong\n  spin-orbit coupling: Recent discovery of Ising superconductivity protected against in-plane\nmagnetic field by spin-orbit coupling (SOC) has stimulated intensive research\ninterests. The effect, however, was only expected to appear in two-dimensional\n(2D) noncentrosymmetric materials with spin-valley locking. In this work, we\nproposed a new type of Ising superconductivity in 2D materials with $C_{nz}$\nrotational symmetry ($n=3,4,6$). This mechanism, dubbed as type-II Ising\nsuperconductivity, is applicable for centrosymmetric materials. Type-II Ising\nsuperconductivity relies on the SOC-induced spin-orbital locking characterized\nby Ising-type Zeeman-like fields displaying opposite signs for opposing\norbitals. We found that type-II Ising superconductivity are most prominent\naround time-reversal invariant momenta and is not sensitive to inversion\nsymmetry breaking. By performing high-throughput first-principles calculations,\nabout one hundred candidate materials were identified. Our work significantly\nenriches the physics and materials of Ising superconductor, opening new\nopportunities for fundamental research and practical applications of 2D\nmaterials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Plain s-wave superconductivity in single-layer FeSe on SrTiO3 probed by\n  scanning tunneling microscopy: Single-layer FeSe film on SrTiO3(001) was recently found to be the champion\nof interfacial superconducting systems, with a much enhanced superconductivity\nthan the bulk iron-based superconductors. Its superconducting mechanism is of\ngreat interest. Although the film has a simple Fermi surface topology, its\npairing symmetry is unsettled. Here by using low-temperature scanning tunneling\nmicroscopy (STM), we systematically investigated the superconductivity of\nsingle-layer FeSe/SrTiO3(001) films. We observed fully gapped tunneling\nspectrum and magnetic vortex lattice in the film. Quasi-particle interference\n(QPI) patterns reveal scatterings between and within the electron pockets, and\nput constraints on possible pairing symmetries. By introducing impurity atoms\nonto the sample, we show that the magnetic impurities (Cr, Mn) can locally\nsuppress the superconductivity but the non-magnetic impurities (Zn, Ag and K)\ncannot. Our results indicate that single-layer FeSe/SrTiO3 has a plain s-wave\nparing symmetry whose order parameter has the same phase on all Fermi surface\nsections.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Surface Specific Heat of $^{3}$He and Andreev Bound States: High resolution measurements of the specific heat of liquid $^{3}$He in the\npresence of a silver surface have been performed at temperatures near the\nsuperfluid transition in the pressure range of 1 to 29 bar. The surface\ncontribution to the heat capacity is identified with Andreev bound states of\n$^{3}$He quasiparticles that have a range of half a coherence length.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Universal scaling in BCS superconductivity in two dimensions in non-s\n  waves: The solutions of a renormalized BCS model are studied in two space dimensions\nin $s$, $p$ and $d$ waves for finite-range separable potentials. The gap\nparameter, the critical temperature $T_c$, the coherence length $\\xi$ and the\njump in specific heat at $T_c$ as a function of zero-temperature condensation\nenergy exhibit universal scalings. In the weak-coupling limit, the present\nmodel yields a small $\\xi$ and large $T_c$ appropriate to those for high-$T_c$\ncuprates. The specific heat, penetration depth and thermal conductivity as a\nfunction of temperature show universal scaling in $p$ and $d$ waves.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Temperature dependence of clusters with attracting vortices in\n  superconducting Niobium studied by neutron scattering: We have investigated the intermediate mixed state of a superconducting\nniobium sample by Very Small Angle Neutron Scattering.We show that this state\nis stabilized through a sequence where a regular vortex lattice appears, which\nthen coexists with vortex clusters before vanishing at low temperature.\nVortices in clusters have a constant periodicity regardless of the applied\nfield, exhibit a temperature dependence close to the one of the penetration\ndepth. The clusters disappear in the high temperature limit. All the results\nagree with an explanation in terms of vortex attraction due non local effects,\nand indicate a negligible role of pinning. Phase coexistence between Abrikosov\nvortex lattice and vortex clusters is reported showing the first order nature\nof the boundary line.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superfluid density of Ba(Fe$_{1-x}M_x$)$_2$As$_2$ from optical\n  experiments: The temperature dependence of the $ab$-plane optical reflectivity of\nBa(Fe$_{0.92}$Co$_{0.08})_2$As$_{2}$ and Ba(Fe$_{0.95}$Ni$_{0.05})_2$As$_{2}$\nsingle crystals is measured in a wide spectral range. Upon entering the\nsuperconducting regime, the reflectivity in both compounds increases\nconsiderably at low frequency, leading to a clear gap in the optical\nconductivity below 100 cm$^{-1}$. From the analysis of the complex conductivity\nspectra we obtain the penetration depth $\\lambda(T)=(3500\\pm 350)$ \\AA for\nBa(Fe$_{0.92}$Co$_{0.08})_2$As$_{2}$ and $(3000\\pm 300)$ \\AA for\nBa(Fe$_{0.95}$Ni$_{0.05})_2$As$_{2}$. The calculated superfluid density\n$\\rho_s$ of both compounds nicely fits the scaling relation $\\rho_s=(125\\pm\n25)\\sigma_{dc}T_c$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Enhanced superconductivity and superconductor to insulator transition in\n  nano-crystalline molybdenum thin films: Disorder driven superconductor to insulator transition via intermediate\nmetallic regime is reported in nano-crystalline thin films of molybdenum. The\nnano-structured thin films have been deposited at room temperature using DC\nmagnetron sputtering at different argon pressures. The grain size has been\ntuned using deposition pressure as the sole control parameter. A variation of\nparticle sizes, room temperature resistivity and superconducting transition has\nbeen studied as a function of deposition pressure. The nano-crystalline\nmolybdenum thin films are found to have large carrier concentration but very\nlow mobility and electronic mean free path. Hall and conductivity measurements\nhave been used to understand the effect of disorder on the carrier density and\nmobilities. Ioffe-Regel parameter is shown to correlate with the continuous\nmetal-insulator transition in our samples.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Imbalanced Superfluid Phase of a Trapped Fermi Gas in the BCS-BEC\n  Crossover Regime: We theoretically investigate the ground state of trapped neutral fermions\nwith population imbalance in the BCS-BEC crossover regime. On the basis of the\nsingle-channel Hamiltonian, we perform full numerical calculations of the\nBogoliubov-de Gennes equation coupled with the regularized gap and number\nequations. The zero-temperature phase diagram in the crossover regime is\npresented, where the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing state\ngoverns the weak-coupling BCS region of a resonance. It is found that the FFLO\noscillation vanishes in the BEC side, in which the system under population\nimbalance turns into a phase separation (PS) between locally binding superfluid\nand fully polarized spin domains. We also demonstrate numerical calculations\nwith a large particle number O(10^5), comparable to that observed in recent\nexperiments. The resulting density profile on a resonance yields the PS, which\nis in good agreement with the recent experiments, while the FFLO modulation\nexists in the pairing field. It is also proposed that the most favorable\nlocation for the detection of the FFLO oscillation is in the vicinity of the\ncritical population imbalance in the weak coupling BCS regime, where the\noscillation periodicity becomes much larger than the interparticle spacing.\nFinally, we analyze the radio-frequency (RF) spectroscopy in the imbalanced\nsystem. The clear difference in the RF spectroscopy between BCS and BEC sides\nreveals the structure of the pairing field and local ``magnetization''.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Turning insulators into superconductors: The recent observation by Batlogg and colleagues of superconductivity in an\norganic field-effect transistor is reviewed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Momentum-Resolved Electronic Relaxation Dynamics in D-wave\n  Superconductors: Motivated by recent development in time-resolved angle-resolved photoemission\nspectroscopy (trARPES) for d-wave superconductors, we analyze the\nnon-equilibrium relaxation dynamics of the laser pulse excited sample within\nthe scenario of two-temperature model. It is shown that the main features\nreported in the trARPES technique may be understood within this\nphenomenological picture. The momentum dependence of the excited quasiparticle\ndensity and the relaxation rate is associated with the dynamics of the nodal\nd-wave superconducting gap, and the fluence dependence of the relaxation rate\nis related to the recombination process of quasiparticles into Cooper pairs.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Increasing d-wave superconductivity by on site repulsion: We study by Variational Monte Carlo an extended Hubbard model away from half\nfilled band density which contains two competing nearest-neighbor interactions:\na superexchange $J$ favoring d-wave superconductivity and a repulsion $V$\nopposing against it. We find that the on-site repulsion $U$ effectively\nenhances the strength of $J$ meanwhile suppressing that of $V$, thus favoring\nsuperconductivity. This result shows that attractions which do not involve\ncharge fluctuations are very well equipped against strong electron-electron\nrepulsion so much to get advantage from it.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Metal-to-Insulator Crossover in YBa_{2}Cu_{3}O_{y} Probed by\n  Low-Temperature Quasiparticle Heat Transport: It was recently demonstrated that in La_{2-x}Sr_{x}CuO_{4} the magnetic-field\n(H) dependence of the low-temperature thermal conductivity \\kappa up to 16 T\nreflects whether the normal state is a metal or an insulator. We measure the H\ndependence of \\kappa in YBa_{2}Cu_{3}O_{y} (YBCO) at subkelvin temperatures for\na wide doping range, and find that at low doping the \\kappa(H) behavior\nsignifies the change in the ground state in this system as well. Surprisingly,\nthe critical doping is found to be located deeply inside the underdoped region,\nabout the hole doping of 0.07 hole/Cu; this critical doping is apparently\nrelated to the stripe correlations as revealed by the in-plane resistivity\nanisotropy.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Impurity states in multiband s-wave superconductors: analysis of iron\n  pnictides: We examine the effect of a single, non-magnetic impurity in a multiband,\nextended s-wave superconductor allowing for anisotropy of the gaps on the Fermi\nsurfaces. We derive analytic expressions for the Green's functions in the\ncontinuum and analyse the conditions for the existence of sharp\nimpurity-induced resonant states. Underlying band structure is more relevant\nfor the multiband than for single band case, and mismatch between the bands\ngenerically makes the formation of the impurity states less likely in the\nphysical regime of parameters. We confirm these conclusions by numerically\nsolving the impurity problem in a tight-binding parameterization of the bands\nrelevant to pnictide superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electronic Structure, Localization and Spin-State Transition in\n  Cu-substituted FeSe: Fe$_{1-x}$Cu$_x$Se: We report density functional studies of the Fe$_{1-x}$Cu$_x$Se alloy done\nusing supercell and coherent potential approximation methods. Magnetic behavior\nwas investigated using the disordered local moment approach. We find that Cu\noccurs in a nominal $d^{10}$ configuration and is highly disruptive to the\nelectronic structure of the Fe sheets. This would be consistent with a metal\ninsulator transition due to Anderson localization. We further find a strong\ncross over from a weak moment itinerant system to a local moment magnet at $x\n\\approx 0.12$. We associate this with the experimentally observed jump near\nthis concentration. Our results are consistent with the characterization of\nthis concentration dependent jump as a transition to a spin-glass.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Implementation of Grover search algorithm with Josephson charge qubits: A scheme of implementing the Grover search algorithm based on Josephson\ncharge qubits has been proposed, which would be a key step to scale more\ncomplex quantum algorithms and very important for constructing a real quantum\ncomputer via Josephson charge qubits. The present scheme is simple but fairly\nefficient, and easily manipulated because any two-charge-qubit can be\nselectively and effectively coupled by a common inductance. More manipulations\ncan be carried out before decoherence sets in. Our scheme can be realized\nwithin the current technology.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Implications evinced by the phase diagram, anisotropy, magnetic\n  penetration depths, isotope effects and conductivities of cuprate\n  superconductors: Anisotropy, thermal and quantum fluctuations and their dependence on dopant\nconcentration appear to be present in all cuprate superconductors, interwoven\nwith the microscopic mechanisms responsible for superconductivity. Here we\nreview anisotropy, in-plane and c-axis penetration depths, isotope effect and\nconductivity measurements to reassess the universal behavior of cuprates as\nrevealed by the doping dependence of these phenomena and of the transition\ntemperature.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Distinct Pairing Symmetries in $Nd_{1.85}Ce_{0.15}CuO_{4-y}$ and\n  $La_{1.89}Sr_{0.11}CuO_{4}$ Single Crystals: Evidence from Comparative\n  Tunnelling Measurements: We used point-contact tunnelling spectroscopy to study the superconducting\npairing symmetry of electron-doped $Nd_{1.85}Ce_{0.15}CuO_{4-y}$ (NCCO) and\nhole-doped $La_{1.89}Sr_{0.11}CuO_{4}$ (LSCO). Nearly identical spectra without\nzero bias conductance peak (ZBCP) were obtained on the (110) and (100) oriented\nsurfaces (the so-called nodal and anti-nodal directions) of NCCO. In contrast,\nLSCO showed a remarkable ZBCP in the nodal direction as expected from a d-wave\nsuperconductor. Detailed analysis reveals an s-wave component in the pairing\nsymmetry of the NCCO sample with $\\Delta/k_BT_c=1.66$, a value remarkable close\nto that of a weakly coupled BCS superconductor. We argue that this s-wave\ncomponent is formed at the Fermi surface pockets centered at ($\\pm\\pi$,0) and\n(0,$\\pm\\pi$) although a d-wave component may also exist.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Possible observation of energy level quantization in an intrinsic\n  Josephson junction: Energy level quantization (ELQ) is studied to clarify the macroscopic quantum\ndynamics of the d-wave Josephson junction (JJ). The influences of the nodal\nquasiparticles of d-wave superconductivity on the damping effect are\nnumerically evaluated on the basis of a phenomenological model. The\ncalculation, based on realistic parameters for a Bi2Sr2CaCu2O8+d (Bi2212)\nintrinsic JJ, shows that the observation of ELQ is possible when the sweep rate\nof the bias current exceeds 10 A/sec. High-sweep- rate measurements (121A/sec)\nperformed on a Bi2212 intrinsic JJ result in the appearance of multiple peaks\nin the switching current distribution suggesting the realization of ELQ in the\nd-wave JJ.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "s+is State with Broken Time Reversal Symmetry in Fe-Based\n  Superconductors: We analyze the evolution of the superconducting gap structure in strongly\nhole doped Ba_{1-x}K_xFe_2As_2 between x=1 and x ~ 0.4 (optimal doping). In the\nlatter case, the pairing state is most likely s+-, with different gap signs on\nhole and electron pockets, but with the same signs of the gap on the two\nGamma-centered hole pockets (a ++ state on hole pockets). In a pure KFe_2As_2\n(x=1), which has only hole pockets, laser ARPES data suggested another s+-\nstate, in which the gap changes sign between hole pockets (a +- state). We\nanalyze how ++ gap transforms into a +- gap as x -> 1. We found that this\ntransformation occurs via an intermediate s+is, state in which the gaps on the\ntwo hole pockets differ in phase by `phi', which gradually involves from `phi'\n= pi (the +- state) to phi =0 (the ++ state). This state breaks time-reversal\nsymmetry and has huge potential for applications. We compute the dispersion of\ncollective excitations and show that two different Leggett-type phase modes\nsoften at the two end points of TRSB state.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Supergap and subgap enhanced currents in asymmetric {S_1FS_2} Josephson\n  junctions: We have theoretically studied the supercurrent profiles in three-dimensional\nnormal metal and ferromagnetic Josephson configurations, where the magnitude of\nthe superconducting gaps in the superconducting leads are unequal, i.e.,\n$\\Delta_1\\neq \\Delta_2$, creating asymmetric $S_1NS_2$ and $S_1FS_2$ systems.\nOur results reveal that by increasing the ratio of the superconducting gaps\n$\\Delta_2/\\Delta_1$, the critical supercurrent in a ballistic $S_1NS_2$ system\ncan be enhanced by more than $100\\%$, and reaches a saturation point, or decays\naway, depending on the junction thickness, magnetization strength, and chemical\npotential. The total critical current in a diffusive $S_1NS_2$ system was found\nto be enhanced by more than $50\\%$ parabolically, and reaches saturation by\nincreasing one of the superconducting gaps. In a uniform ferromagnetic\njunction, the supercurrent undergoes reversal by increasing\n$\\Delta_2/\\Delta_1>1$. Through decomposing the total supercurrent into its\nsupergap and subgap components, our results illustrate their crucial relative\ncontributions to the Josephson current flow. It was found that the competition\nof subgap and supergap currents in a $S_1FS_2$ junction results in the\nemergence of second harmonics in the current-phase relation. In contrast to a\ndiffusive asymmetric Josephson configuration, the behavior of the supercurrent\nin a ballistic system with $\\Delta_2/\\Delta_1=1$ can be properly described by\nthe subgap current component only, in a wide range of parameter sets, including\nFermi level mismatch, magnetization strength, and junction thickness.\nInterestingly, when $\\Delta_2/\\Delta_1>1$, our results have found multiple\nparameter sets where the total supercurrent is driven by the supergap\ncomponent. Therefore, our comprehensive study highlights the importance of\nsubgap and supergap supercurrent components in both the ballistic and diffusive\nregimes.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Fluctuation, insulation and superconductivity: the pressure-dependent\n  phase-diagram of Rb$_2$Mo$_6$Se$_6$: The quasi-one-dimensional (q1D) material Rb$_2$Mo$_6$Se$_6$ has been proposed\nto display a nontrivial combination of low-dimensional fluctuations and a\ndynamical charge density wave (CDW) at ambient pressure. This may lead to a\nprogressive metal to insulator cross over at low temperature. To explore the\nlink between the crystal dimensionality and this insulating instability, we\nhave performed hydrostatic pressure-dependent electrical transport measurements\non single crystals of Rb$_2$Mo$_6$Se$_6$. At low pressure, we observe\nthermally-activated behaviour consistent with a temperature-dependent gap\n$E_g(T)$ opening below a characteristic temperature $T_{Rmin}$. Upon increasing\nthe pressure $T_{Rmin}$ initially rises, indicating a reinforcement of the low\ntemperature insulating state despite a continuous reduction in $E_g(P)$. We\ninterpret this as a signature of suppressed fluctuations as the dimensionality\nof the electronic structure rises. However, $T_{Rmin}$ drops above 8.8 GPa and\nsuperconductivity emerges at 12 GPa. Between 12-24.2 GPa the superconducting\nand insulating instabilities coexist, with superconductivity surviving up to\nthe maximum attained pressure (52.8 GPa). Analysis of the magneto-transport\nreveals two distinct regions: at high pressures the anisotropy gradually falls\nand the superconducting state appears unremarkable. In contrast, coexistence\nwith the gapped insulating phase creates a superconducting dome. The emergence\nof a peak in the critical temperature Tc despite the depleted density of states\nis indicative of enhanced coupling. Our journey from the extreme 1D to 3D\nlimits in this prototypical q1D metal reveals an intriguing relationship\nbetween superconducting and insulating ground states which is simultaneously\ncompetitive and symbiotic.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Physical Properties of Metallic Antiferromagnetic CaCo{1.86}As2 Single\n  Crystals: We report studies of CaCo{1.86}As2 single crystals. The electronic structure\nis probed by angle-resolved photoemission spectroscopy (ARPES) measurements of\nCaCo{1.86}As2 and by full-potential linearized augmented-plane-wave\ncalculations for the supercell Ca8Co15As16 (CaCo{1.88}As2). Our XRD crystal\nstructure refinement is consistent with the previous combined refinement of\nx-ray and neutron powder diffraction data showing a collapsed-tetragonal\nThCr2Si2-type structure with 7(1)% vacancies on the Co sites corresponding to\nthe composition CaCo{1.86}As2 [D. G. Quirinale et al., Phys. Rev. B 88, 174420\n(2013)]. The anisotropic magnetic susceptibility chi(T) data are consistent\nwith the magnetic neutron diffraction data of Quirianale et al. that\ndemonstrate the presence of A-type collinear antiferromagnetic order below the\nNeel temperature TN = 52(1) K with the easy axis being the tetragonal c axis.\nHowever, no clear evidence from the resistivity rho(T) and heat capacity Cp(T)\ndata for a magnetic transition at TN is observed. A metallic ground state is\ndemonstrated from band calculations and the rho(T), Cp(T) and ARPES data, and\nspin-polarized calculations indicate a competition between the A-type AFM and\nFM ground states. The Cp(T) data exhibit a large Sommerfield electronic\ncoefficient reflecting a large density of states at the Fermi energy D(EF),\nconsistent with the band structure calculations which also indicate a large\nD(EF) arising from Co 3d bands. At 1.8 K the M(H) data for H|| c exhibit a\nwell-defined first-order spin-flop transition at an applied field of 3.5 T. The\nsmall ordered moment of 0.3 muB/Co obtained from the M(H) data at low T, the\nlarge exchange enhancement of chi and the lack of a self-consistent\ninterpretation of the chi(T) and M(H,T) data in terms of a local moment\nHeisenberg model together indicate that the magnetism of CaCo{1.86}As2 is\nitinerant.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Finite energy spin fluctuation as a pairing glue in systems with\n  coexisting electron and hole bands: We study, within the fluctuation exchange approximation, the\nspin-fluctuation-mediated superconductivity in Hubbard-type models possessing\nelectron and hole bands, and compare them with a model on a square lattice with\na large Fermi surface. In the square lattice model, superconductivity is more\nenhanced for better nesting for a fixed band filling. By contrast, in the\nmodels with electron and hole bands, superconductivity is optimized when the\nFermi surface nesting is degraded to some extent, where finite energy spin\nfluctuation around the nesting vector develops. The difference lies in the\nrobustness of the nesting vector, namely, in models with electron and hole\nbands, the wave vector at which the spin susceptibility is maximized is fixed\neven when the nesting is degraded, whereas when the Fermi surface is large, the\nnesting vector varies with the deformation of the Fermi surface. We also\ndiscuss the possibility of realizing in actual materials the bilayer Hubbard\nmodel, which is a simple model with electron and hole bands, and is expected to\nhave a very high T_c.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "High-Density Superconductive Logic Circuits Utilizing 0 and $\u03c0$\n  Josephson Junctions: Superconductor Electronics (SCE) is a fast and power-efficient technology\nwith great potential for overcoming conventional CMOS electronics' scaling\nlimits. Nevertheless, the primary challenge confronting SCE today pertains to\nits integration level, which lags several orders of magnitude behind CMOS\ncircuits. In this study, we have innovated and simulated a novel logic family\ngrounded in the principles of phase shifts occurring in 0 and $\\pi$ Josephson\njunctions. The fast phase logic (FPL) eliminates the need for large inductor\nloops and shunt resistances by combining the half-flux and phase logic.\nTherefore, the Josephson junction (JJ) area only limits the integration\ndensity. The cells designed with this paradigm are fast, and the clock-to-Q\ndelay is about 4ps while maintaining over 50% parameter margins. This logic is\npower efficient and can increase the integration by at least 100$\\times$ in the\nSCE chips.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The direct Cu NQR Study of the Stripe Phase in the Lanthanum Cuprates: Using Cu NQR in Eu-doped La_(2-x)Sr_xCuO_4 we find the evidence of the pinned\nstripe phase at 1.3K for 0.08<x<0.18. The pinned fraction increases by one\norder of magnitude near hole doping x=1/8. The NQR lineshape reveals three\ninequivalent Cu positions. A dramatic change of the NQR signal for x > 0.18\ncorrelating with the onset of bulk superconductivity corresponds to the\ndepinning of the stripe phase.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Non-monotonic behaviour of the superconducting order parameter in\n  Nb/PdNi bilayers observed through point contact spectroscopy: Point contact spectroscopy measurements have been performed on Nb/PdNi\nbilayers in which the thickness of the Nb layer, dNb, was kept constant to 40\nnm while the thickness of PdNi, dPdNi, was changed from 2 nm to 9 nm. Features\nrelated to the superconducting gap induced in the ferromagnet have been\nobserved in the dV/dI versus V curves. These structures show a non-monotonic\nbehaviour as a function of dPdNi as a consequence of the damped oscillatory\nbehaviour of the superconducting order parameter in the ferromagnetic layer.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Approaching ideal rectification in superconducting diodes through\n  multiple Andreev reflections: We analyze the rectification properties of voltage-biased Josephson junctions\nexhibiting the superconducting diode effect. Taking into account multiple\nAndreev reflection (MAR) processes in our scattering theory, we consider a\nshort weak link of arbitrary transparency between two superconductors with\nfinite Cooper pair momentum $2q$. In equilibrium, the diode efficiency is\nbounded from above in this model, with maximal efficiency $\\eta_0\\approx 0.4$.\nOut of equilibrium, we find a rich subharmonic structure in the current-voltage\ncurve. For high transparency and low bias voltage $V$, the rectification\nefficiency $\\eta(V)$ approaches the ideal value $\\eta=1$ for $q\\xi\\to 1$ (with\ncoherence length $\\xi$).",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Size driven phase transitions in pinned vortex systems: We model a tridimensional vortex system in a sample with square superficial\npinning in the top surface and obtain the ground state structures as a function\nof the sample thickness. Using a simple Frenkel-Kontorova like model and no\nadjustable parameters, we reproduce the experimental vortex configurations seen\nin the bottom surface and their range of stability. We find three phases with\ntwo transitions between them, including a continuous one from square to\ndistorted hexagonal structure and a discontinuous one from distorted hexagonal\nto hexagonal structure.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Epitaxial Growth of NdFeAsO Thin Films by Molecular Beam Epitaxy: Epitaxial films of NdFeAsO were grown on GaAs substrates by molecular beam\nepitaxy (MBE). All elements including oxygen were supplied from solid sources\nusing Knudsen cells. The x-ray diffraction pattern of the film prepared with\nthe optimum growth condition showed no indication of impurity phases. Only\n(00l) peaks were observed, indicating that NdFeAsO was grown with the c-axis\nperpendicular to the substrate. The window of optimum growth condition was very\nnarrow, but the NdFeAsO phase was grown with a very good reproducibility.\nDespite the absence of any appreciable secondary phase, the resistivity showed\nan increase with decreasing temperature.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Absence of the Pauli-Paramagnetic Limit in a Superconducting U$_6$Co: We performed $^{59}$Co nuclear magnetic resonance (NMR) measurements of\nsingle-crystalline U$_6$Co. There is a small decrease in the Knight shift in\nthe superconducting (SC) state, but this change mainly arises from the SC\ndiamagnetic effect. The negligible change of the spin part of the Knight shift,\ntogether with the absence of the Pauli-paramagnetic effect in the SC U$_6$Co,\nis understood as a consequence of the small spin susceptibility. The nuclear\nspin-lattice relaxation rate $1/T_1$ is also measured in the SC state under the\nmagnetic field, and exhibits a tiny Hebel-Slichter peak just below the SC\ntransition temperature and exponential behavior at lower temperatures. These\nbehaviors are in agreement with the full-gap s-wave pairing in U$_6$Co.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Theory of the Resistive Transition in Overdoped $Tl_2Ba_2CuO_{6+x}$:\n  Implications for the angular dependence of the quasiparticle scattering rate\n  in High-$T_c$ superconductors: We show that recent measurements of the magnetic field dependence of the\nmagnetization, specific heat and resistivity of overdoped $T_c \\sim 17K$\n$Tl_{2}Ba_{2}CuO_{6+\\delta}$ in the vicinity of the superconducting $H_{c2}$\nimply that the vortex viscosity is anomalously small and that the material\nstudied is inhomogeneous with small, a few hundred $\\AA$, regions in which the\nlocal $T_{c}$ is much higher than the bulk $T_{c}$. The anomalously small\nvortex viscosity can be derived from a microscopic model in which the\nquasiparticle lifetime varies dramatically around the Fermi surface, being\nsmall everywhere except along the zone diagonal (``cold spot''). We propose\nexperimental tests of our results.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Ultimate on-chip quantum amplifier: We report amplification of electromagnetic waves by a single artificial atom\nin open 1D space. Our three-level artificial atom -- a superconducting quantum\ncircuit -- coupled to a transmission line presents an analog of a natural atom\nin open space. The system is the most fundamental quantum amplifier whose gain\nis limited by a spontaneous emission mechanism. The noise performance is\ndetermined by the quantum noise revealed in the spectrum of spontaneous\nemission, also characterized in our experiments.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Highly responsive Y-Ba-Cu-O thin film THz detectors with picosecond time\n  resolution: High-temperature superconducting YBa2Cu3O7-d (YBCO) thin-film detectors with\nimproved responsivities were developed for fast time-domain measurements in the\nTHz frequency range. YBCO thin films of 30 nm thickness were patterned to\nmicro- and nanobridges and embedded into planar log-spiral THz antennas. The\nYBCO thin-film detectors were characterized with continuous wave radiation at\n0.65 THz. Responsivity values as high as 710 V/W were found for the YBCO\nnanobridges. Pulsed measurements in the THz frequency range were performed at\nthe electron storage ring ANKA from the Karlsruhe Institute of Technology\n(KIT). Due to the high responsivities of the nanobridges no biasing was\nrequired for the detection of the coherent synchrotron radiation pulses\nachieving very good agreement between the measured pulse shapes and\nsimulations.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Stripe-like Inhomogeneities, Coherence, and the Physics of the High Tc\n  Cuprates: The carriers in the high-Tc cuprates are found to be polaron-like \"stripons\"\ncarrying charge and located in stripe-like inhomogeneities, \"quasi-electrons\"\ncarrying charge and spin, and \"svivons\" carrying spin and some lattice\ndistortion. The anomalous spectroscopic and transport properties of the\ncuprates are understood. The stripe-like inhomogeneities result from the Bose\ncondensation of the svivon field, and the speed of their dynamics is determined\nby the width of the double-svivon neutron-resonance peak. The connection of\nthis peak to the peak-dip-hump gap structure observed below Tc emerges\nnaturally. Pairing results from transitions between pair states of stripons and\nquasi-electrons through the exchange of svivons. The pairing symmetry is of the\nd_{x^2-y^2} type; however, sign reversal through the charged stripes results in\nfeatures not characteristic of this symmetry. The phase diagram is determined\nby pairing and coherence lines within the regime of a Mott transition.\nCoherence without pairing results in a Fermi-liquid state, and incoherent\npairing results in the pseudogap state where localized electron and electron\npair states exist within the Hubbard gap. A metal-insulator-transition quantum\ncritical point occurs between these two states at T=0 when the superconducting\nstate is suppressed. An intrinsic heterogeneity is expected of superconducting\nand pseudogap nanoscale regions.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "A second phase transition and superconductivity in the beta-pyrochlore\n  oxide KOs2O6: Another phase transition that is probably of first order is found in the\nbeta-pyrochlore oxide superconductor KOs2O6 with a superconducting transition\ntemperature Tc of 9.6 K. It takes place at Tp=7.5 K in the superconducting\nstate in a zero magnetic field. By applying magnetic fields of up to 140 kOe,\nthe Tc gradually decreased to 5.2 K, while Tp changed little, eventually\nbreaking through the Hc2 line at approximately 65 kOe in the H-T diagram. Both\nthe normal-state resistivity and Hc2 change slightly but significantly across\nthe second phase transition. It is suggested that the transition is associated\nwith the rattling of potassium ions located in an oversized cage of osmium and\noxide ions.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "One-electron scattering rate and normal-state linear-$T$ resistivity of\n  the cuprates: Here we use a description of the electronic correlations contained in the\nHubbard model on the square-lattice perturbed by very weak three-dimensional\nuniaxial anisotropy in terms of the residual interactions of charge $c$\nfermions and spin-neutral composite two-spinon $s1$ fermions. Excellent\nquantitative agreement with the anisotropic linear-$\\omega$ one-electron\nscattering rate and normal-state linear-$T$ resistivity observed in experiments\non hole-doped cuprates with critical concentrations $x_c\\approx 0.05$ and\n$x_*\\approx 0.27$ is achieved. Our results provide strong evidence that the\nnormal-state linear-$T$ resistivity is a manifestation of low-temperature\nscale-invariant physics.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Weyl superconductor phases in a Weyl-semimetal/superconductor multilayer: Topologically nontrivial superconducting phases have been engineered in\ntopological materials by the proximity effect in contact with conventional\nsuperconductors. In this paper, by using the method of the Kronig-Penney model,\nwe study the superconducting proximity effect in the bulk electronic states of\nWeyl semimetals by considering a multilayer structure consisting of\nWeyl-semimetal and superconductor layers. Due to the proximity effect, two Weyl\nnodes are decoupled into four nodes of Majorana fermions resulting in\nWeyl-superconductor phases or three-dimensional extension of\ntopological-superconductor phases. We find that mismatch of the Fermi velocity\nand potential barriers at the interface gap out Majorana nodes, thus turn\nWeyl-superconductor phases with four Majorana nodes into Weyl-superconductor\nphases with half of Majorana nodes and topological-superconductor phases with\nodd integer Chern numbers.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Nodal band-off-diagonal superconductivity in twisted graphene\n  superlattices: The superconducting state and mechanism are among the least understood\nphenomena in twisted graphene systems. For instance, recent tunneling\nexperiments indicate a transition between nodal and gapped pairing with\nelectron filling, which is not naturally understood within current theory. We\ndemonstrate that the coexistence of superconductivity and flavor polarization\nleads to pairing channels that are guaranteed by symmetry to be entirely\nband-off-diagonal, with a variety of unusual consequences: most notably, the\npairing invariant under all symmetries can have protected nodal lines or be\nfully gapped, depending on parameters, and the band-off-diagonal chiral d-wave\nstate exhibits transitions between gapped and nodal regions upon varying the\nchemical potential. We demonstrate that nodal band-off-diagonal pairing can be\nthe leading state when only phonons are considered, and is also uniquely\nfavored by fluctuations of a time-reversal-symmetric intervalley-coherent order\nmotivated by recent experiments. Consequently, band-off-diagonal\nsuperconductivity allows for the reconciliation of several key experimental\nobservations in graphene moir\\'e systems.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Critical Fields and Anisotropy of NdO0.82F0.18FeAs Single Crystals: The newly discovered iron-based superconductors have stimulated enormous\ninterests in the field of superconductivity. Since the new superconductor is a\nlayered system, the anisotropy is a parameter with the first priority to know.\nMeanwhile any relevant message about the critical fields (upper critical field\nand irreversibility line) are essentially important. By using flux method, we\nhave successfully grown the single crystals NdO0.82F0.18FeAs at ambient\npressure. Resistive measurements reveal a surprising discovery that the\nanisotropy \\Gamma = (mc/mab)^{1/2} is below 5, which is much smaller than the\ntheoretically calculated results. The data measured up to 400 K show a\ncontinuing curved feature which prevents a conjectured linear behavior for an\nunconventional metal. The upper critical fields determined based on the\nWerthamer-Helfand-Hohenberg formula are H_{c2}^{H||ab}(T=0 K) = 304 T and\nH_{c2}^{H||c}(T=0 K)=62-70 T, indicating a very encouraging application of the\nnew superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Triplet superconductivity and spin density wave in biased AB bilayer\n  graphene: We examine spin density wave and triplet superconductivity as possible ground\nstates of the Bernal bilayer graphene. The spin density wave is stable for the\nunbiased and undoped bilayer. Both the doping and the applied bias voltage\ndestroy this phase. We show that, when biased and slightly doped, bilayer can\nhost a triplet superconducting phase. The mechanisms for both ordered phases\nrely on the renormalized Coulomb interaction. Consistency of our theoretical\nconclusions with recent experimental results are discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Interpretation of Inelastic Neutron Scattering Data Using the Phase\n  Diagram of Hole-Doped Cuprates: Inelastic Neutron Scattering (INS) data in LSCO, YBCO and Bi2212 are\ndiscussed. In the literature, the INS spectra remain far from being\ncomprehensively understood. We show that local (Q-integrated) susceptibility\ndata and the energy dependence of the spin susceptibility at antiferromagnetic\nvector, Q = (p,p), can be interpreted by using the phase diagram for hole-doped\ncuprates. We analyze also the origin of the resonant peak which relates to the\norder parameter for long-range phase coherence in hole-doped cuprates. Thus, we\npresent an interpretation of neutron data in LSCO, YBCO and Bi2212 published so\nfar.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity in iron-based F-doped layered quaternary compound\n  Nd[O1-xFx]FeAs: The recently discovered quaternary arsenide oxide superconductor\nLa[O1-xFx]FeAs with the superconducting critical transition temperature (Tc) of\n26 K [1], has been quickly expanded to another high-Tc superconducting system\nbeyond copper oxides by the replacement of La with other rare earth elements,\nsuch as Sm, Ce, and Pr etc. [2-4], and the Pr[O1-xFx]FeAs has become to be the\nfirst non-cuprate superconductor that holding a Tc above 50 K. All these\narsenide (including phosphide) superconductors formed in a same tetragonal\nlayered structure with the space group P4/nmm which has an alternant stacked\nFe-As layer and RO (R = rare earth metals) layer. Here we report the discovery\nof another superconductor in this system, the neodymium-arsenide Nd[O1-xFx]FeAs\nwith an resistivity onset Tc of 51.9 K, which is the second non-cuprate\ncompound that superconducts above 50 K.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Spectroscopy of electron-phonon interaction of superconducting point\n  contacts: experimental aspects: The recovering procedure of the electron-phonon interaction (EPI) functions\nfrom the additional nonlinearities of the current-voltage curve ($I-V$ curve)\nof point contacts associated with an excess current is considered. The approach\nproposed takes into account both inelastic scattering, which causes suppression\nof the excess current in the reabsorption of nonequilibrium phonons by\nelectrons undergoing Andreev reflection (Andreev electrons), and elastic\nprocesses associated with the electron-phonon renormalization of the energy\nspectrum in a superconductor. The results obtained are systematically expounded\nfor both the ballistic contacts, wherein the second derivatives of the $I-V$\ncurve in the normal state are proportional to the EPI functions, and\ninhomogeneous contacts (with dirty constrictions and clean banks), whose second\nderivatives in the normal state are either free of phonon singularities or\nweakly pronounced.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Muon-Spin Rotation Spectra in the Mixed Phase of High-T_c\n  Superconductors : Thermal Fluctuations and Disorder Effects: We study muon-spin rotation (muSR) spectra in the mixed phase of highly\nanisotropic layered superconductors, specifically Bi_2+xSr_2-xCaCu_2O_8+delta\n(BSCCO), by modeling the fluid and solid phases of pancake vortices using\nliquid-state and density functional methods. The role of thermal fluctuations\nin causing motional narrowing of muSR lineshapes is quantified in terms of a\nfirst-principles theory of the flux-lattice melting transition. The effects of\nrandom point pinning are investigated using a replica treatment of liquid state\ncorrelations and a replicated density functional theory. Our results indicate\nthat motional narrowing in the pure system, although substantial, cannot\naccount for the remarkably small linewidths obtained experimentally at\nrelatively high fields and low temperatures. We find that satisfactory\nagreement with the muSR data for BSCCO in this regime can be obtained through\nthe ansatz that this ``phase'' is characterized by frozen short-range\npositional correlations reflecting the structure of the liquid just above the\nmelting transition. This proposal is consistent with recent suggestions of a\n``pinned liquid'' or ``glassy'' state of pancake vortices in the presence of\npinning disorder. Our results for the high-temperature liquid phase indicate\nthat measurable linewidths may be obtained in this phase as a consequence of\ndensity inhomogeneities induced by the pinning disorder. The results presented\nhere comprise a unified, first-principles theoretical treatment of muSR spectra\nin highly anisotropic layered superconductors in terms of a controlled set of\napproximations.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Tunable mechanically-induced hysteresis in suspended Josephson junctions: The coupling of superconducting systems to mechanical resonators is an\nemerging field, with wide reaching implications including high precision\nsensing and metrology. Experimental signatures of this coupling have so far\nbeen small, seldom and often reliant on high frequency AC electronics. To\novercome this limitation, in this work we consider a mechanical resonator\nsuspended between two superconducting contacts to form a suspended Josephson\njunction in which the electronic normal- and super-currents can be coupled to\nmechanical motion via the Lorentz force due to an external magnetic field. We\nshow both analytically and numerically that this electro-mechanical coupling\nproduces unprecedented mechanically-induced hysteresis loops in the junction's\nDC I-V characteristic. Firstly, we unveil how this new hysteresis may be\nexploited to access a huge mechanically-induced Shapiro-like voltage plateau,\nextending over a current range comparable with the junction's critical current.\nWe then investigate a sudden mechanically-induced retrapping that occurs at\nstrong coupling. Our analytical treatment provides a clear explanation for the\neffects above and allows us to derive simple relationships between the features\nin the DC I-V characteristic and the resonance frequency and quality factor of\nthe mechanical resonator. We stress that our setup requires only DC current\nbias and voltage measurements, allowing the activation and detection of\nhigh-frequency mechanical oscillations in state of the art devices and without\nthe need of any AC equipment.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Strong local moment antiferromagnetic spin fluctuations in V-doped\n  LiFeAs: We use neutron scattering to study vanadium (hole)-doped LiFe$_{1-x}$V$_x$As.\nIn the undoped state, LiFeAs exhibits superconductivity at $T_c=18$ K and\ntransverse incommensurate spin excitations similar to electron overdoped iron\npnictides. Upon vanadium-doping to form LiFe$_{0.955}$V$_{0.045}$, the\ntransverse incommensurate spin excitations in LiFeAs transform into\nlongitudinally elongated in a similar fashion as that of potassium (hole) doped\nBa$_{0.7}$K$_{0.3}$Fe$_2$As$_2$, but with dramatically enhanced magnetic\nscattering and elimination of superconductivity. This is different from the\nsuppression of the overall magnetic excitations in hole doped BaFe$_2$As$_2$\nand the enhancement of superconductivity near optimal hole doping. These\nresults are consistent with density function theory plus dynamic mean field\ntheory calculations, suggesting that vanadium-doping in LiFeAs may induce an\nenlarged effective magnetic moment $S_{eff}$ with a spin crossover ground state\narising from the inter-orbital scattering of itinerant electrons.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Crossover of the high-energy spin fluctuations from collective triplons\n  to localized magnetic excitations in doped Sr14-xCaxCu24O41 cuprate ladders: We studied the magnetic excitations in the quasi-one-dimensional (q-1D)\nladder subsystem of Sr_(14-x) Ca_x Cu_24 O_41(SCCO) using Cu L_3-edge resonant\ninelastic X-ray scattering (RIXS). By comparing momentum-resolved RIXS spectra\nwith (x=12.2) and without (x=0) high Ca content, we track the evolution of the\nmagnetic excitations from collective two-triplon (2T) excitations (x=0) to\nweakly-dispersive gapped modes at an energy of 280 meV (x=12.2). Density matrix\nrenormalization group (DMRG) calculations of the RIXS response in the doped\nladders suggest that the flat magnetic dispersion and damped excitation profile\nobserved at x=12.2 originates from enhanced hole localization. This\ninterpretation is supported by polarization-dependent RIXS measurements, where\nwe disentangle the spin-conserving {\\Delta}S=0 scattering from the predominant\n{\\Delta}S=1 spin-flip signal in the RIXS spectra. The results show that the\nlow-energy weight in the {\\Delta}S=0 channel is depleted when Sr is replaced by\nCa, consistent with a reduced carrier mobility. Our results demonstrate that\noff-ladder impurities can affect both the low-energy magnetic excitations and\nsuperconducting correlations in the CuO_4 plaquettes. Finally, our study\ncharacterizes the magnetic and charge fluctuations in the phase from which\nsuperconductivity emerges in SCCO at elevated pressures.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "MgB2 single crystals: high pressure growth and anisotropic properties: Single crystals of MgB2 with a size up to 1.5x0.9x0.2 mm3 have been grown\nwith a high pressure cubic anvil technique. The crystal growth process is very\npeculiar and involves an intermediate nitride, namely MgNB9. Single crystals of\nBN and MgB2 grow simultaneously by a peritectic decomposition of MgNB9.\nMagnetic measurements in fields of 1-5 Oe show sharp transitions to the\nsuperconducting state at 37-38.6 K with width of ~0.5 K. The high quality of\nthe crystals allowed the accurate determination of magnetic, transport and\noptical properties as well as scanning tunnelling spectroscopy (STS) and\ndecoration studies. Investigations of crystals with torque magnetometry show\nthat Hc2//c is very low (24 kOe at 15 K), while Hc2//ab increases up to 140 kOe\nat 15 K. The upper critical field anisotropy gamma = Hc2//ab/ Hc2//c was found\nto be temperature dependent (decreasing from 6 at 15 K to 2.8 at 35 K). The\neffective anisotropy gamma_eff, as calculated from reversible torque data near\nTc, is field dependent (increasing roughly linearly from 2 in zero field to 3.7\nin 10 kOe). The temperature and field dependence of the anisotropy can be\nrelated to the double gap structure of MgB2 with a large two-dimensional gap\nand small three-dimensional gap, the latter being rapidly suppressed in a\nmagnetic field. Torque magnetometry investigations show a pronounced peak\neffect, indicating an order-disorder transition of vortex matter. Decoration\nexperiments and STS visualise a hexagonal vortex lattice. STS spectra evidence\ntwo gaps (3 meV/6 meV) with direction dependent weight. Magneto-optic\ninvestigations with H//c show a clear signature of the smaller of the two gaps,\ndisappearing in fields higher than Hc2//c.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Multi-Dimensional Coherent Spectroscopy of Light-Driven States and their\n  Collective Modes in Multi-Band Superconductors: We present a comprehensive theory of light-controlled multi-band\nsuperconductivity and apply it to predict distinctive signatures of\nlight-driven superconducting (SC) states in terahertz multi-dimensional\ncoherent spectroscopy (THz-MDCS) experiments. We first derive gauge-invariant\nMaxwell-Bloch equations for multi-band BCS superconductors. For this, we go\nbeyond previously considered Anderson pseudo-spin precession models to include\nquantum transport effects. By calculating the THz-MDCS spectra measured\nexperimentally, we then identify unique signatures of finite-momentum\nCooper-pairing states that live longer than the laser pulse. These\nnon-equilibrium SC states are characterized by long-lived canting of Anderson\npseudo-spins. The pseudo-spin oscillators that describe the properties of these\nSC states are parametrically driven by both finite-momentum Cooper pairing and\nby time oscillations of the order parameter relative phase. We show that such\nstrong parametric driving leads to drastic changes in the THz-MDCS spectral\nshape from the predictions of third-order nonlinear susceptibility\ncalculations. These spectral changes strongly depend on the\ninterband-to-intraband interaction ratio and on the collective modes of the\nlight-driven state.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electron-phonon superconductivity in $A$Pt$_3$P compounds: from weak to\n  strong coupling: We study the newly discovered Pt phosphides $A$Pt$_3$P ($A$=Sr, Ca, La) [ T.\nTakayama et al. Phys. Rev. Lett. 108, 237001 (2012)] using first-principles\ncalculations and Migdal-Eliashberg theory. Given the remarkable agreement with\nthe experiment, we exclude the charge-density wave scenario proposed by\nprevious first-principles calculations, and give conclusive answers concerning\nthe superconducting state in these materials. The pairing increases from La to\nCa and Sr due to changes in the electron-phonon matrix elements and\nlow-frequency phonons. Although we find that all three compounds are well\ndescribed by conventional s-wave superconductivity and spin-orbit coupling of\nPt plays a marginal role, we show that it could be possible to tune the\nstructure from centrosymmetric to noncentrosymmetric opening new perspectives\ntowards the understanding of unconventional superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Giant anisotropic magnetoresistance in Ising superconductor-magnetic\n  insulator tunnel junctions: Superconductivity and magnetism are generally incompatible because of the\nopposing requirement on electron spin alignment. When combined, they produce a\nmultitude of fascinating phenomena, including unconventional superconductivity\nand topological superconductivity. The emergence of two-dimensional (2D)layered\nsuperconducting and magnetic materials that can form nanoscale junctions with\natomically sharp interfaces presents an ideal laboratory to explore new\nphenomena from coexisting superconductivity and magnetic ordering. Here we\nreport tunneling spectroscopy under an in-plane magnetic field of\nsuperconductor-ferromagnet-superconductor (S/F/S) tunnel junctions that are\nmade of 2D Ising superconductor NbSe2 and ferromagnetic insulator CrBr3. We\nobserve nearly 100% tunneling anisotropic magnetoresistance (AMR), that is,\ndifference in tunnel resistance upon changing magnetization direction from\nout-of-plane to inplane. The giant tunneling AMR is induced by\nsuperconductivity, particularly, a result of interfacial magnetic exchange\ncoupling and spin-dependent quasiparticle scattering. We also observe an\nintriguing magnetic hysteresis effect in superconducting gap energy and\nquasiparticle scattering rate with a critical temperature that is 2 K below the\nsuperconducting transition temperature. Our study paves the path for exploring\nsuperconducting spintronic and unconventional superconductivity in van der\nWaals heterostructures.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Properties of a diagonal 2-orbital ladder model of the Fe-pnictide\n  superconductors: We study a diagonal 2-orbital ladder model of the Fe based superconductors\nusing the density matrix renormalization group method. At half filling, we find\na close competition between a \"spin-striped\" state and a non-collinear\n\"spin-checkerboard\" state, as well as significant nematic correlations. Upon\nfinite hole or electron doping, the dominant pairing correlations are found to\nhave A$_{1,g}$ ($S-$wave) symmetry.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Shaping graphene superconductivity with nanometer precision: Graphene holds great potential for superconductivity due to its pure\ntwo-dimensional nature, the ability to tune its carrier density through\nelectrostatic gating, and its unique, relativistic-like electronic properties.\nAt present, we are still far from controlling and understanding graphene\nsuperconductivity, mainly because the selective introduction of superconducting\nproperties to graphene is experimentally very challenging. Here, we have\ndeveloped a method that enables shaping at will graphene superconductivity\nthrough a precise control of graphene-superconductor junctions. The method\ncombines the proximity effect with scanning tunnelling microscope (STM)\nmanipulation capabilities. We first grow Pb nano-islands that locally induce\nsuperconductivity in graphene. Using a STM, Pb nano-islands can be selectively\ndisplaced, over different types of graphene surfaces, with nanometre scale\nprecision, in any direction, over distances of hundreds of nanometres. This\nopens an exciting playground where a large number of predefined\ngraphene-superconductor hybrid structures can be investigated with atomic scale\nprecision. To illustrate the potential, we perform a series of experiments,\nrationalized by the quasi-classical theory of superconductivity, going from the\nfundamental understanding of superconductor-graphene-superconductor\nheterostructures to the construction of superconductor nanocorrals, further\nused as \"portable\" experimental probes of local magnetic moments in graphene.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Percolative nature of the transition from 60 K to 90 K phase in\n  YBa2Cu3O6+d: We have measured the heat capacity of YBa2Cu3O6+d for 0.7<d<0.8 between 1.8\nand 300K. It was found that doping dependences of specific heat jump and\ntemperature of heat capacity jump contradict to the assumption of spatially\nhomogeneous electronic density. The results suggest that the transition from\n60K to 90K phase has a percolative nature and the structure of underdoped 60K\nphase can be considered as array of superconducting nanoclusters embedded in\nthe insulating matrix.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Spin Fluctuation-Induced Superconductivity in \u03ba-BEDT-TTF Compounds: Spin fluctuation-induced superconductivity in quasi-two dimensional organic\ncompounds, \\kappa-BEDT-TTF salts, is investigated within a fluctuation exchange\n(FLEX) approximation using a half-filled Hubbard model with a right-angled\nisosceles triangular lattice (transfer matrices -\\tau, -\\tau^\\prime), extending\na previous work above T_c. An energy gap of A_2 or (x^2-y^2)-type develops with\ndecreasing temperature below T_c more rapidly than in the BCS model. The\ncalculated dynamical susceptibilities enough below T_c show sharp resonance\npeaks like those in certain cuprates superconductors. The calculated nuclear\nspin-lattice relaxation rate 1/T_1 shows a T^3 behavior below T_c in accordance\nwith experiment. Estimated values of 1/T_1 are roughly consistent with\nexperimental results. A prediction is made for the doping concentration\ndependence of T_c and the antiferromagnetic and superconductive instability\npoints are calculated in the U/\\tau vs. \\tau^\\prime/\\tau plane.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Controlling the superconducting transition by spin-orbit coupling: Whereas there exists considerable evidence for the conversion of singlet\nCooper pairs into triplet Cooper pairs in the presence of inhomogeneous\nmagnetic fields, recent theoretical proposals have suggested an alternative way\nto exert control over triplet generation: intrinsic spin-orbit coupling in a\nhomogeneous ferromagnet coupled to a superconductor. Here, we proximity-couple\nNb to an asymmetric Pt/Co/Pt trilayer, which acts as an effective spin-orbit\ncoupled ferromagnet owing to structural inversion asymmetry. Unconventional\nmodulation of the superconducting critical temperature as a function of\nin-plane and out-of- plane applied magnetic fields suggests the presence of\ntriplets that can be controlled by the magnetic orientation of a single\nhomogeneous ferromagnet. Our studies demonstrate for the first time an active\nrole of spin-orbit coupling in controlling the triplets -- an important step\ntowards the realization of novel superconducting spintronic devices.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconducting density of states and bandstructure at the surface of\n  the candidate topological superconductor Au2Pb: The electronic bandstructure of Au$_2$Pb has a Dirac cone which gaps when\nundergoing a structural transition into a low temperature superconducting\nphase. This suggests that the superconducting phase ($T_c=1.1$ K) might hold\ntopological properties at the surface. Here we make Scanning Tunneling\nMicroscopy experiments on the surface of superconducting Au$_2$Pb. We measure\nthe superconducting gap and find a sizeable superconducting density of states\nat the Fermi level. We discuss possible origins for this finding in terms of\nsuperconductivity induced into surface states.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Dynamic Phase Transitions in Superconductivity: In this Letter, the dynamic phase transitions of the time-dependent\nGinzburg-Landau equations are analyzed using a newly developed dynamic\ntransition theory and a new classification scheme of dynamics phase\ntransitions. First, we demonstrate that there are two type of dynamic\ntransitions, jump and continuous, dictated by the sign of a nondimensional\nparameter R. This parameter is computable, and depends on the material\nproperty, the applied field, and the geometry of domain that the sample\noccupies. Second, using the parameter R, precise analytical formulas for\ncritical domain size, and for critical magnetic fields are derived.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Observation of enhanced coherence in Josephson SQUID cavities using a\n  hybrid fabrication approach: We study the coherence of flux-tunable Josephson junction resonators made\nwith two different fabrication processes. In the first process, devices are\nmade using a single step of evaporation in which the resonator and the\njunctions of the SQUID are made at the same time. In the second process,\ndevices are made with an identical geometry, but in which the resonators are\nmade from a MoRe superconding layer to which an the junctions are added later\nin a second step. To characterize the coherence of the two types of SQUID\ncavities, we observe and analyze the quality factor of their resonances as a\nfunction of flux and photon number. Despite a detailed cleaning process applied\nduring fabrication, the single-step Al devices show significantly worse quality\nfactor than the hybrid devices, and conclude that a the hybrid technique\nprovides a much more reliable approach for fabricating high-Q flux-tunable\nresonators.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pre-formed Cooper pairs and Bose-Einstein condensation in cuprate\n  superconductors: A two-dimensional (2D) assembly of noninteracting, temperature-dependent,\npre-formed Cooper pairs in chemical/thermal equilibrium with unpaired fermions\nis examined in a binary boson-fermion statistical model as the Bose-Einstein\ncondensation (BEC) singularity temperature $T_{c}$ is approached from above.\nCompared with BCS theory (which is {\\it not} a BEC theory) substantially higher\n$T_{c}$'s are obtained without any adjustable parameters, that fall roughly\nwithin the range of empirical $T_{c}$'s for quasi-2D cuprate superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetic Field Effect on the Supercurrent of an SNS junction: In this paper we study the effect of a Zeeman field on the supercurrent of a\nmesoscopic SNS junction. It is shown that the supercurrent suppression is due\nto a redistribution of current-carrying states in energy space. A dramatic\nconsequence is that (part of the) the suppressed supercurrent can be recovered\nwith a suitable non-equilibrium distribution of quasiparticles.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Spin-excitation anisotropy in the nematic state of detwinned FeSe: The origin of the electronic nematicity in FeSe is one of the most important\nunresolved puzzles in the study of iron-based superconductors. In both spin-\nand orbital-nematic models, the intrinsic magnetic excitations at\n$\\mathbf{Q}_1=(1, 0)$ and $\\mathbf{Q}_2=(0, 1)$ of twin-free FeSe are expected\nto provide decisive criteria for clarifying this issue. Although a\nspin-fluctuation anisotropy below 10 meV between $\\mathbf{Q}_1$ and\n$\\mathbf{Q}_2$ has been observed by inelastic neutron scattering around\n$T_c\\sim 9$ K ($<<T_s\\sim 90$ K), it remains unclear whether such an anisotropy\nalso persists at higher energies and associates with the nematic transition\n$T_{\\rm s}$. Here we use resonant inelastic x-ray scattering (RIXS) to probe\nthe high-energy magnetic excitations of uniaxial-strain detwinned FeSe and\n{\\BFA}. A prominent anisotropy between the magnetic excitations along the $H$\nand $K$ directions is found to persist to $\\sim200$ meV in FeSe, which is even\nmore pronounced than the anisotropy of spin waves in {\\BFA}. This anisotropy\ndecreases gradually with increasing temperature and finally vanishes at a\ntemperature around the nematic transition temperature $T_{\\rm s}$. Our results\nreveal an unprecedented strong spin-excitation anisotropy with a large energy\nscale well above the $d_{xz}/d_{yz}$ orbital splitting, suggesting that the\nnematic phase transition is primarily spin-driven. Moreover, the measured\nhigh-energy spin excitations are dispersive and underdamped, which can be\nunderstood from a local-moment perspective. Our findings provide the\nmuch-needed understanding of the mechanism for the nematicity of FeSe and\npoints to a unified description of the correlation physics across seemingly\ndistinct classes of Fe-based superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Self-Consistent Vertex Correction Analysis for Iron-Based\n  Superconductors: Mechanism of Coulomb-Interaction-Driven Orbital Fluctuations: We study the mechanism of orbital/spin fluctuations due to multiorbital\nCoulomb interaction in iron-based superconductors, going beyond the\nrandom-phase-approximation. For this purpose, we develop a self-consistent\nvertex correction (SC-VC) method, and find that multiple orbital fluctuations\nin addition to spin fluctuations are mutually emphasized by the \"multimode\ninterference effect\" described by the VC. Then, both the antiferro-orbital and\nferro-orbital (=nematic) fluctuations simultaneously develop for $J/U \\sim\n0.1$, both of which contribute to the s-wave superconductivity. Especially, the\nferro-orbital fluctuations give the orthorhombic structure transition as well\nas the softening of shear modulus $C_{66}$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetic flux pinning in superconductors with hyperbolic-tesselation\n  arrays of pinning sites: We study magnetic flux interacting with arrays of pinning sites (APS) placed\non vertices of hyperbolic tesselations (HT). We show that, due to the gradient\nin the density of pinning sites, HT APS are capable of trapping vortices for a\nbroad range of applied magnetic fluxes. Thus, the penetration of magnetic field\nin HT APS is essentially different from the usual scenario predicted by the\nBean model. We demonstrate that, due to the enhanced asymmetry of the surface\nbarrier for vortex entry and exit, this HT APS could be used as a \"capacitor\"\nto store magnetic flux.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Topological invariants beyond symmetry indicators: Boundary diagnostics\n  for twofold rotationally symmetric superconductors: Topological crystalline superconductors are known to have possible\nhigher-order topology, which results in Majorana modes on $d-2$ or\nlower-dimensional boundaries. Given the rich possibilities of boundary\nsignatures, it is desirable to have topological invariants that can predict the\ntype of Majorana modes from band structures. Although symmetry indicators, a\ntype of invariant that depends only on the band data at high-symmetry points,\nhave been proposed for certain crystalline superconductors, there exist\nsymmetry classes in which symmetry indicators fail to distinguish\nsuperconductors with different Majorana boundaries. Here, we systematically\nobtain topological invariants for an example of this kind, two-dimensional\ntime-reversal symmetric superconductors with twofold rotational symmetry $C_2$.\nFirst, we show that the nontrivial topology is independent of band data on the\nhigh-symmetry points by conducting a momentum-space classification study. Then,\nfrom the resulting K groups, we derive calculable expressions for four\n$\\mathbb{Z}_2$ invariants defined on high-symmetry lines or general points in\nthe Brillouin zone. Finally, together with a real-space classification study,\nwe establish the bulk-boundary correspondence and show that the four\n$\\mathbb{Z}_2$ invariants can predict Majorana boundary types from band\nstructures. Our proposed invariants can fuel practical material searches for\n$C_2$-symmetric topological superconductors featuring Majorana edge and corner\nmodes.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Evolution of correlation strength in KxFe(2-y)Se2 superconductor doped\n  with S: We report the evolution of thermal transport properties of iron-based\nsuperconductor K$_x$Fe$_{2-y}$Se$_2$ with sulfur substitution at Se sites.\nSulfur doping suppresses the superconducting $T_c$ as well as the Seebeck\ncoefficient. The Seebeck coefficient of all crystals in the low temperature\nrange can be described very well by diffusive thermoelectric response model.\nThe zero-temperature extrapolated value of Seebeck coefficient divided by\ntemperature $S/T$ gradually decreases from $-0.48 \\mu V/K^2$ to a very small\nvalue $\\sim$ 0.03 $\\mu$V/K$^2$ where $T_c$ is completely suppressed. The normal\nstate electron Sommerfeld term ($\\gamma_n$) of specific heat also decreases\nwith the increase of sulfur content. The dcrease of $S/T$ and $\\gamma_n$\nreflects a suppression of the density of states at the Fermi energy, or a\nchange in the Fermi surface that would induce the suppression of correlation\nstrength.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconducting transition temperatures of the elements related to\n  elastic constants: For a given crystal structure, say body-centred-cubic, the many-body\nHamiltonian in which nuclear and electron motions are to be treated from the\noutset on the same footing, has parameters, for the elements, which can be\nclassified as (i) atomic mass M, (ii) atomic number Z, characterizing the\nexternal potential in which electrons move, and (iii) bcc lattice spacing, or\nequivalently one can utilize atomic volume, Omega. Since the thermodynamic\nquantities can be determined from H, we conclude that Tc, the superconducting\ntransition temperature, when it is non-zero, may be formally expressed as Tc =\nTc^(M) (Z, Omega). One piece of evidence in support is that, in an atomic\nnumber vs atomic volume graph, the superconducting elements lie in a well\ndefined region. Two other relevant points are that (a) Tc is related by BCS\ntheory, though not simply, to the Debye temperature, which in turn is\ncalculable from the elastic constants C_{11}, C_{12}, and C_{44}, the atomic\nweight and the atomic volume, and (b) Tc for five bcc transition metals is\nlinear in the Cauchy deviation C* = (C_{12} - C_{44})/(C_{12} + C_{44}).\nFinally, via elastic constants, mass density and atomic volume, a correlation\nbetween C* and the Debye temperature is established for the five bcc transition\nelements.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Critical Current Oscillations in Strong Ferromagnetic Pi-Junctions: We report magnetic and electrical measurements of Nb Josephson junctions with\nstrongly ferromagnetic barriers of Co, Ni and Ni80Fe20 (Py). All these\nmaterials show multiple oscillations of critical current with barrier thickness\nimplying repeated 0-pi phase-transitions in the superconducting order\nparameter. We show in particular that the Co barrier devices can be accurately\nmodelled using existing clean limit theories and so that, despite the high\nexchange energy (309 meV), the large IcRN value in the pi-state means Co\nbarriers are ideally suited to the practical development of superconducting\npi-shift devices.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Topological states in normal and superconducting $p$-wave chains: We study a two-band model of fermions in a 1d chain with an antisymmetric\nhybridization that breaks inversion symmetry. We find that for certain values\nof its parameters, the $sp$-chain maps formally into a $p$-wave superconducting\nchain, the archetypical 1d system exhibiting Majorana fermions. The\neigenspectra, including the existence of zero energy modes in the topological\nphase, agree for both models. The end states too share several similarities in\nboth models, such as the behavior of the localization length, the non-trivial\ntopological index and robustness to disorder. However, we show by mapping the\n$s$- and $p$- fermions to two copies of Majoranas, that the excitations in the\nends of a finite $sp$ chain are indeed conventional fermions though endowed\nwith protected topological properties. Our results are obtained by a scattering\napproach in a semi-infinite chain with an edge defect treated within the\n$T$-matrix approximation. We augment the analytical results with exact\nnumerical diagonalization that allow us to extend our results to arbitrary\nparameters and also to disordered systems.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Studies on Fabrication of Ag/HgBaCaCuO/CdSe Heterostructures by\n  Pulse-Electrodeposition Route: Metal/superconductor/semiconductor (Ag/HgBaCaCuO/CdSe) heterostructures have\nbeen successfully fabricated using pulse-electrodeposition technique. The\nelectrochemical parameters are optimized and diffusion free growth of CdSe onto\nAg/HgBaCaCuO was obtained by employing under-potential deposition and by\nstudying nucleation and growth mechanism during deposition. The\nheterostructures are characterized by X-ray diffraction (XRD), full-width at\nhalf-maximum (FWHM), scanning electron microscopy (SEM) studies and low\ntemperature four probe electrical resistivity measurements. After the\ndeposition of CdSe the critical transition temperature of HgBaCaCuO films was\nfound be increased from 115 K with Jc = 1.7 x 103 A/cm2 to 117.2 K with Jc =\n1.91 x 103 A/cm2. When the heterostructure was irradiated with red He-Ne laser\n(2 mW), the Tc was further enhanced to 120.3 K with Jc = 3.7 x 103 A/cm2. This\nincrease in superconducting parameters of HgBaCaCuO in Ag/ HgBaCaCuO/CdSe\nheterostructure has been explained at length in this paper.\n  Keywords. Electrodeposition; Hg-based cuprate; semiconductor;\nheterostructures; electrical properties.\n  PACS Nos 81.15.Pq; 74.72.Gr; 78.40.Fy; 84.37; 73.40 *E-mail:\npawar_s_h@yahoo.com, shpawar_phy@unishivaji.ac.in",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Effects of Disorder on Superconductivity of Systems with Coexisting\n  Itinerant Electrons and Local Pairs: We study the influence of diagonal disorder (random site energy) of local\npair (LP) site energies on the superconducting properties of a system of\ncoexisting local pairs and itinerant electrons described by the (hard-core)\nboson-fermion model. Our analysis shows that the properties of such a model\nwith s-wave pairing can be very strongly affected by the diagonal disorder in\nLP subsystem (the randomness of the LP site energies). This is in contrast with\nthe conventional s-wave BCS superconductors, which according to the Anderson's\ntheorem are rather insensitive to the diagonal disorder (i.e. to nonmagnetic\nimpurities). It has been found that the disorder effects depend in a crucial\nway on the total particle concentration n and the LP level position DELTA_o and\ndepending on the parameters the system can exhibit various types of\nsuperconducting behaviour, including the LP-like, intermediate (MIXED)and the\n'BCS'-like. In the extended range of {n,DELTA_o} the superconducting ordering\nis suppressed by the randomness of the LP site energies and the increasing\ndisorder induces a changeover from the MIXEDlike behaviour to the BCS-like one,\nconnected with abrupt reduction of T_c and energy gap to zero. However, there\nalso exist a definite range of {n,DELTA_o} in which the increasing disorder has\na quite different effect: namely it can substantially enhance T_c or even lead\nto the phenomenon which can be called disorder induced superconductivity.\nAnother interesting effect is a possibility of a disorder induced bound pair\nformation of itinerant electrons, connected with the change-over to the LP-like\nregime.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Thermodynamic phase diagram and phase competition in BaFe2(As1-xPx)2\n  studied by thermal expansion: High-resolution thermal-expansion and specific-heat measurements were\nperformed on single crystalline BaFe2(As1-xPx)2 (0 < x < 0.33, x = 1). The\nobservation of clear anomalies allows to establish the thermodynamic phase\ndiagram which features a small coexistence region of SDW and superconductivity\nwith a steep rise of Tc on the underdoped side. Samples that undergo the\ntetragonal-orthorhombic structural transition are detwinned in situ, and the\nresponse of the sample length to the magneto-structural and superconducting\ntransitions is studied for all three crystallographic directions. It is shown\nthat a reduction of the magnetic order by superconductivity is reflected in all\nlattice parameters. On the overdoped side, superconductivity affects the\nlattice parameters in much the same way as the SDW on the underdoped side,\nsuggesting an intimate relation between the two types of order. Moreover, the\nuniaxial pressure derivatives of Tc are calculated using the Ehrenfest relation\nand are found to be large and anisotropic. A correspondence between\nsubstitution and uniaxial pressure is established, i.e., uniaxial pressure\nalong the b-axis (c-axis) corresponds to a decrease (increase) of the P\ncontent. By studying the electronic contribution to the thermal expansion we\nfind evidence for a maximum of the electronic density of states at optimal\ndoping.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pairing competition in a quasi-one-dimensional model of organic\n  superconductors (TMTSF)$_{2}X$ in magnetic field: We microscopically study the effect of the magnetic field (Zeeman splitting)\non the superconducting state in a model for quasi-one-dimensional organic\nsuperconductors (TMTSF)$_{2}X$. We investigate the competition between spin\nsinglet and spin triplet pairings and the\nFulde-Ferrell-Larkin-Ovchinnikov(FFLO) state by random phase approximation.\nWhile we studied the competition by comparison with the eigenvalue of the gap\nequation at a fixed temperature in our previous study (Phys. Rev. Lett.\n\\textbf{102} (2009) 016403), here we obtain both the $T_c$ for each pairing\nstate and a phase diagram in the $T$(temperature)-$h_z$(field)-$V_y$(strength\nof the charge fluctuation) space. The phase diagram shows that consecutive\ntransitions from singlet pairing to the FFLO state and further to $S_z=1$\ntriplet pairing can occur upon increasing the magnetic field when $2k_{F}$\ncharge fluctuations coexist with $2k_{F}$ spin fluctuations. In the FFLO state,\nthe singlet d-wave and $S_{z}=0$ triplet $f$-wave components are strongly mixed\nespecially when the charge fluctuations are strong.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Two-dimensional topological superconductivity candidate in van der Waals\n  layered material: Two-dimensional (2D) topological superconductors are highly desired because\nthey not only offer opportunities for exploring novel exotic quantum physics,\nbut also possesses potential applications in quantum computation. However,\nthere are few reports on 2D superconductors, let alone topological\nsuperconductors. Here, we find a 2D monolayer W$_2$N$_3$, which can be\nexfoliated from its real van der Waals bulk material with much lower\nexfoliation energy than MoS$_2$, to be a topological metal with exotic\ntopological states at different energy level. Due to the Van Hove\nsingularities, the density of states near Fermi level are high, making the\nmonolayer a compensate metal. Moreover, the monolayer W$_2$N$_3$ is unveiled to\nbe a superconductor with the superconducting transition temperature Tc $\\sim$\n22 K and a superconducting gap of about 5 meV based on the anisotropic\nMigdal-Eliashberg formalism, arising from the strong electron-phonon coupling\naround the $\\Gamma$ point. Because of the strong electron and lattice coupling,\nthe monolayer displays a non-Fermi liquid behavior in its normal states at\ntemperatures lower than 80 K, where the specific heat exhibit T$^3$ behavior\nand the Wiedemann-Franz law dramatically violates. Our findings not only\nprovide the platform to study the emergent phenomena in 2D topological\nsuperconductors, but also open a door to discover more 2D high-temperature\ntopological superconductors in van der Waals materials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Asymmetric Little-Parks Oscillations in Full Shell Double Nanowires: Little-Parks oscillations of a hollow superconducting cylinder are of\ninterest for flux-driven topological superconductivity in single Rashba\nnanowires. The oscillations are typically symmetric in the orientation of the\napplied magnetic flux. Using double InAs nanowires coated by an epitaxial\nsuperconducting Al shell which, despite the non-centro-symmetric geometry,\nbehaves effectively as one hollow cylinder, we demonstrate that a small\nmisalignment of the applied parallel field with respect to the axis of the\nnanowires can produce field-asymmetric Little-Parks oscillations. These are\nrevealed by the simultaneous application of a magnetic field perpendicular to\nthe misaligned parallel field direction. The asymmetry occurs in both the\ndestructive regime, in which superconductivity is destroyed for half-integer\nquanta of flux through the shell, and in the non-destructive regime, where\nsuperconductivity is depressed but not fully destroyed at these flux values.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Detection of the phase shift from a single quantized superconducting\n  vortex: An Abrikosov vortex in a superconductor carries a flux quantum, Phi_0 =\nhc/2e, localized at its center, but induces a global 2pi phase rotation in the\nsuperconducting condensate. This long-range gauge field outside the area\npierced by a magnetic field is due to the Aharonov-Bohm effect, which is a\nnon-classical phenomenon that illustrates the significance of potentials rather\nthan forces in quantum mechanics. In the London gauge, the phase of the\ncondensate is given by the polar angle around the vortex. Here we raise the\nquestion whether this phase shift could be detected by means of Cooper pair\ninterferometry using Josephson junctions as phase-sensitive detectors. We\nintroduce a single Abrikosov vortex into a superconducting lead with a detector\njunction made at the edge of the lead. We observe that the vortex induces a\nJosephson phase shift equal to the polar angle of the vortex within the\njunction length. When the vortex is close to the junction it induces a pi-step\nin the Josephson phase difference, leading to a controllable and reversible\nswitching of the junction into the 0 - pi state. This in turn results in an\nunusual Phi_0/2 quantization of the flux in the junction. The vortex may hence\nact as a tunable \"phase battery\" for quantum electronics.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconducting fluctuations in the reversible magnetization of the\n  iron-pnictide $Ba_{1-x}K_xFe_2As_2$: We report on isofield magnetization curves obtained as a function of\ntemperature in two single crystals of $Ba_{1-x}K_xFe_2As_2$ with\nsuperconducting transition temperature $T_c$=28K and 32.7 K. Results obtained\nfor fields above 20 kOe show a well defined rounding effect on the reversible\nregion extending 1-3 K above $T_c(H)$ masking the transition. This rounding\nappears to be due to three-dimensional critical fluctuations, as the higher\nfield curves obey a well know scaling law for this type of critical\nfluctuations. We also analysed the asymptotic behavior of $\\sqrt M$vs.T curves\nin the reversible region which probes the shape of the gap near $T_c(H)$.\nResults of the analysis suggests that phase fluctuations are important in\n$Ba_{1-x}K_xFe_2As_2$ which is consistent with nodes in the gap.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Synthesis and physical characterization of superconductivity-magnetism\n  crossover compound RuSr2EuCeCu2O10: We carefully studied the nonsuperconducting sample of the\nmagneto-superconducting RuSr2(Eu1-xCex)Cu2O10 series with composition\nRuSr2EuCeCu2O10. This compound seems to exhibit a complex magnetic state as\nrevealed by host of techniques like resistivity, thermopower, magnetic\nsusceptibility and MR measurements.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electron and Superconducting Properties of the AFeAs (A= Li, Na) Family\n  Alkali-Metal Pnictides: Current Stage of the Research (mini-review): The review is focused on one of the most exotic families of iron-based\nsuperconductors belonging to the AFeAs structural type, where A is alkali\nmetal. We briefly concern physical and electron properties of the typical\nmembers of this family, LiFeAs and NaFeAs, discuss the theoretical models\ndescribing the multiple-gap superconducting state, and the experimental data\navailable in literature. As well, we specify the main unsolved problems, that\nseem crucial for both the AFeAs family and for iron-based superconductors in\ngeneral.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The role of domain walls on the vortex creep dynamics in unconventional\n  superconducors: We investigate the influence of domain walls on the vortex dynamics in\nsuperconductors with multi-component order parameters. We show that, due to\ntheir complex structure domain walls can carry vortices with fractional flux\nquanta. The decay of conventional vortices into fractional ones on domain walls\nis examined. This decay presents an extraordinarily strong pinning mechanism\nfor vortices and turns domain walls occupied with pinned fractional vortices\ninto efficient barriers for the vortex motion. Therefore, domain walls can act\nas fences for the flux flow, preventing the decay of the remnant magnetic flux\nenclosed by them. Furthermore, the consequences of this property of domain\nwalls on the vortex dynamics are discussed in connection with observed noise in\nthe hysteresis cycle, using the Bean model of the critical vortex state. Based\non this picture experimental data in the unconventional superconductors\nUPt$_3$, U$_{1-x}$Th$_x$Be$_{13}$ and Sr$_2$RuO$_4$ are interpreted.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Computational Materials Design for High Critical Temperature\n  Superconductivity in Hole-Doped Delafossite CuAlO$_2$: Transparent\n  Superconductors: We have calculated the superconducting critical temperature $T_c$ of\nhole-doped delafossite CuAlO$_2$ based on the first-principles calculations.\nAccording our calculation, $0.2\\sim0.3$ hole-doped CuAlO$_2$ can become a\nphonon-mediated high-$T_c$ superconductor with $T_c\\sim50$ K. In the hole-doped\nCuAlO$_2$, the A$_1$L$_1$ phonon mode that stretches O-Cu-O dumbbell has a\nstrong electron-phonon interaction with electrons of the flat band in Cu\n3d$_{3z^2-r^2}$ and the O 2p$_z$ anti-bonding $\\pi$-band.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The spin exchange interaction effect on Tc equation of anisotropic\n  impure superconductors: We study the influence of spin exchange interaction of impurity scattering on\ncritical temperature of anisotropic impure superconductors. The model of random\nnonmagnetic and magnetic impurity are revised to cover the effect of spin\nexchange interaction . The sign of magnitude of the second-order Born\nscattering have been changed after consideration the spin exchange interaction\neffect that also effect to form of Tc equation. We can get the general Tc\nequation that can be described anisotropic impure superconductors well and\ncover all model done before.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Vortex dynamics and losses due to pinning: Dissipation from trapped\n  magnetic flux in resonant superconducting radio-frequency cavities: We use a model of vortex dynamics and collective weak pinning theory to study\nthe residual dissipation due to trapped magnetic flux in a dirty\nsuperconductor. Using simple estimates, approximate analytical calculations,\nand numerical simulations, we make predictions and comparisons with experiments\nperformed in CERN and Cornell on resonant superconducting radio-frequency NbCu,\ndoped-Nb and Nb$_3$Sn cavities. We invoke hysteretic losses originating in a\nrugged pinning potential landscape to explain the linear behavior of the\nsensitivity of the residual resistance to trapped magnetic flux as a function\nof the amplitude of the radio-frequency field. Our calculations also predict\nand describe the crossover from hysteretic-dominated to viscous-dominated\nregimes of dissipation. We propose simple formulas describing power losses and\ncrossover behavior, which can be used to guide the tuning of material\nparameters to optimize cavity performance.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Energy dependence of the spin excitation anisotropy in uniaxial-strained\n  BaFe1.9Ni0.1As2: We use inelastic neutron scattering to study the temperature and energy\ndependence of the spin excitation anisotropy in uniaxial-strained\nelectron-doped iron pnictide BaFe$_{1.9}$Ni$_{0.1}$As$_2$ near optimal\nsuperconductivity ($T_c=20$ K). Our work has been motivated by the observation\nof in-plane resistivity anisotropy in the paramagnetic tetragonal phase of\nelectron-underdoped iron pnictides under uniaxial pressure, which has been\nattributed to a spin-driven Ising-nematic state or orbital ordering. Here we\nshow that the spin excitation anisotropy, a signature of the spin-driven\nIsing-nematic phase, exists for energies below $\\sim$60 meV in\nuniaxial-strained BaFe$_{1.9}$Ni$_{0.1}$As$_2$. Since this energy scale is\nconsiderably larger than the energy splitting of the $d_{xz}$ and $d_{yz}$\nbands of uniaxial-strained Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ near optimal\nsuperconductivity, spin Ising-nematic correlations is likely the driving force\nfor the resistivity anisotropy and associated electronic nematic correlations.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Dynamic cluster quantum Monte Carlo simulations of a two-dimensional\n  Hubbard model with stripe-like charge density wave modulations: Interplay\n  between inhomogeneity and superconductivity: Using dynamic cluster quantum Monte Carlo simulations, we study the\nsuperconducting behavior of a 1/8 doped two-dimensional Hubbard model with\nimposed uni-directional stripe-like charge density wave modulation. We find a\nsignificant increase of the pairing correlations and critical temperature\nrelative to the homogeneous system when the modulation length-scale is\nsufficiently large. With a separable form of the irreducible particle-particle\nvertex, we show that optimized superconductivity is obtained for moderate\nmodulation strength due to a delicate balance between the modulation enhanced\npairing interaction, and a concomitant suppression of the bare\nparticle-particle excitations by a modulation reduction of the quasi-particle\nweight.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Scanning SQUID study of ferromagnetism and superconductivity in\n  infinite-layer nickelates: Infinite-layer nickelates $R_{1-x}$Sr$_{x}$NiO$_{2}$ ($R$ = La, Pr, Nd) are a\nclass of superconductors with structural similarities to cuprates. Although\nlong-range antiferromagnetic order has not been observed for these materials,\nmagnetic effects such as antiferromagnetic spin fluctuations and spin-glass\nbehavior have been reported. Different experiments have drawn different\nconclusions about whether the pairing symmetry is $s$- or $d$ wave. In this\npaper, we applied a scanning superconducting quantum interference device\n(SQUID) to probe the magnetic behavior of film samples of three infinite-layer\nnickelates (La$_{0.85}$Sr$_{0.15}$NiO$_2$, Pr$_{0.8}$Sr$_{0.2}$NiO$_2$, and\nNd$_{0.775}$Sr$_{0.225}$NiO$_2$) grown on SrTiO$_3$ (STO), each with a nominal\nthickness of 20 unit cells. In all three films, we observed a ferromagnetic\nbackground. We also measured the magnetic susceptibility above the\nsuperconducting critical temperature in Pr$_{0.8}$Sr$_{0.2}$NiO$_2$ and\nLa$_{0.85}$Sr$_{0.15}$NiO$_2$ and identified a non-Curie-Weiss dynamic\nsusceptibility. Both magnetic features are likely due to NiO$_x$ nanoparticles.\nAdditionally, we investigated superconductivity in Pr$_{0.8}$Sr$_{0.2}$NiO$_2$\nand Nd$_{0.775}$Sr$_{0.225}$NiO$_2$, which exhibited inhomogeneous diamagnetic\nscreening. The superfluid density inferred from the diamagnetic susceptibility\nin relatively homogeneous regions shows $T$-linear behavior in both samples.\nFinally, we observed superconducting vortices in\nNd$_{0.775}$Sr$_{0.225}$NiO$_2$. We determined a Pearl length of 330 $\\upmu$m\nfor Nd$_{0.775}$Sr$_{0.225}$NiO$_2$ at 300 mK both from the strength of the\ndiamagnetism and from the size and shape of the vortices. These results\nhighlight the importance of considering NiO$_x$ particles when interpreting\nexperimental results for these films.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Comparative Study of Dense Bulk MgB$_2$ Materials Prepared by Different\n  Methods: We report on the results of a comparative investigation of highly dense bulk\nMgB$_2$ samples prepared by three methods: (i) hot deformation; (ii) high\npressure sintering; and (iii) mechanical alloying of Mg and B powders with\nsubsequent hot compaction. All types of samples were studied by\nac-susceptibility, dc-magnetization and resistivity measurements in magnetic\nfields up to $\\mu_0H=160$ kOe. A small but distinct anisotropy of the upper\ncritical field $H_{c2}^{a,b}/H_{c2}^{c}\\sim1.2$ connected with some texture of\nMgB$_2$ grains was found for the hot deformed samples. The samples prepared by\nhigh pressure sintering as well as by mechanical alloying show improved\nsuperconducting properties, including high upper critical fields $H_{c2}$\n($\\mu_0H_{c2}(0)\\sim23$ T), irreversibility fields $H_{irr}$ which are strongly\nshifted towards higher values $H_{irr}(T)\\sim0.8H_{c2}(T)$ and high critical\ncurrent $J_c$ ($J_c=10^5$ A/cm$^2$ at 20 K and 1 T).",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Generating Giant Spin Currents Using Nodal Topological Superconductors: In this work, we show that a giant spin current can be injected into a nodal\ntopological superconductor, using a normal paramagnetic lead, through a large\nnumber of zero energy Majorana fermions at the superconductor edge. The giant\nspin current is caused by the selective equal spin Andreev reflections (SESAR)\ninduced by Majorana fermions. In each SESAR event, a pair of electrons with\ncertain spin polarization are injected into the nodal topological\nsuperconductor, even though the pairing in the bulk of the nodal superconductor\nis spin-singlet s-wave. We further explain the origin of the spin current by\nshowing that the pairing correlation at the edge of a nodal topological\nsuperconductor is predominantly equal spin-triplet at zero energy. The\nexperimental consequences of SESAR in nodal topological superconductors are\ndiscussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Characterization of superconducting nanowire single-photon detector with\n  artificial constrictions: Statistical studies on the performance of different superconducting nanowire\nsingle-photon detectors (SNSPDs) on one chip suggested that random\nconstrictions existed in the nanowire that were barely registered by scanning\nelectron microscopy. With the aid of advanced e-beam lithography, artificial\ngeometric constrictions were fabricated on SNSPDs as well as single nanowires.\nIn this way, we studied the influence of artificial constrictions on SNSPDs in\na straight forward manner. By introducing artificial constrictions with\ndifferent wire widths in single nanowires, we concluded that the dark counts of\nSNSPDs originate from a single constriction. Further introducing artificial\nconstrictions in SNSPDs, we studied the relationship between detection\nefficiency and kinetic inductance and the bias current, confirming the\nhypothesis that constrictions exist in SNSPDs.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Angular Position of Nodes in the Superconducting Gap of Quasi-2D\n  Heavy-Fermion Superconductor CeCoIn_5: The thermal conductivity of the heavy-fermion superconductor CeCoIn_5 has\nbeen studied in a magnetic field rotating within the 2D planes. A clear\nfourfold symmetry of the thermal conductivity which is characteristic of a\nsuperconducting gap with nodes along the (+-pi,+-pi)-directions is resolved.\nThe thermal conductivity measurement also reveals a first order transition at\nH_c2, indicating a Pauli limited superconducting state. These results indicate\nthat the symmetry most likely belongs to d_{x^2-y^2}, implying that the\nanisotropic antiferromagnetic fluctuation is relevant to the superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "A model for critical current effects in point-contact Andreev-reflection\n  spectroscopy: It is well known that point-contact Andreev reflection spectroscopy provides\nreliable measurements of the energy gap(s) in a superconductor when the contact\nis in the ballistic or nearly-ballistic regime. However, especially when the\nmean free path of the material under study is small, obtaining ballistic\ncontacts can be a major challenge. One of the signatures of a Maxwell\ncontribution to the contact resistance is the presence of \"dips\" in the\ndifferential conductance, associated to the sudden appearance of a Maxwell\nterm, in turn due to the attainment of the critical current of the material in\nthe contact region. Here we show that, using a proper model for the $R(I)$ of\nthe material under study, it is possible to fit the experimental curves\n(without the need of normalization) obtaining the correct values of the gap\namplitudes even in the presence of such dips, as well as the temperature\ndependence of the critical current in the contact. We present a test of the\nprocedure in the case of Andreev-reflection spectra in\nMg$_{0.85}$Al$_{0.15}$B$_2$ single crystals.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Third harmonic generation from collective modes in disordered\n  superconductors: Recent experiments with strong THz fields in both conventional and\nunconventional superconductors have clearly evidenced a marked third-harmonic\ngeneration below the superconducting temperature $T_c$. Its interpretation\nchallenged substantial theoretical work aimed at establishing the relative\nefficiency of quasiparticle excitations and collective modes in triggering such\na resonant response. Here we compute the non-linear current by implementing a\ntime-dependent Bogoljubov de-Gennes approach, with the twofold aim to account\nnon-perturbatively for the effect of local disorder, and to include the\ncontribution of all collective modes, i.e. superconducting amplitude (Higgs)\nand phase fluctuations, and charge fluctuations. We show that, in agreement\nwith previous work, already at small disorder the quasiparticle response is\ndominated by paramagnetic effects. We further demonstrate that paramagnetic\nprocesses mediate also the response of all collective modes, with a substantial\ncontribution of charge/phase fluctuations. These processes, which have been\noverlooked so far, turn out to dominate the third-order current at strong\ndisorder. In addition, we show that disorder strongly influences the\npolarization dependence of the non-linear response, with a marked difference\nbetween the clean and the disordered case. Our results are particularly\nrelevant for recent experiments in cuprates, whose band structure is in a first\napproximation reproduced by our lattice model.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Effect of La Doping on the Crystal Structure, Electric, Magnetic and\n  Morphologic Properties of the BSCCO System: Studies of the doping process can provide a better understanding of the\nsuperconducting mechanisms in cuprous oxide materials. In this work, we studied\nthe doping effects on the crystal structure, electric, morphologic and magnetic\nproperties of the BSCCO system with the nominal composition\n$Bi_{1.6}Pb_{0.4}Sr_{(2-x)}RE_xCa_2Cu_3O_{(10-{\\delta})}$. Here, the rare earth\nelement (RE) was replaced by La in the sites of Sr. The x was ranged from 0.0\nto 2.0, in steps of 0.5. The samples were prepared based on Pechini's method.\nThe resulting powder was pressed at room temperature and the pellets were\nsubmitted to several heat treatments. The characterizations confirm the La in\nthe sites of Sr however, the superconducting properties of the sample were not\nimproved.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Coupling between dynamic magnetic and charge-order correlations in the\n  cuprate superconductor Nd$_{2-x}$Ce$_{x}$CuO$_4$: Charge order has now been observed in several cuprate high-temperature\nsuperconductors. We report a resonant inelastic x-ray scattering experiment on\nthe electron-doped cuprate Nd$_{2-x}$Ce$_{x}$CuO$_4$ that demonstrates the\nexistence of dynamic correlations at the charge order wave vector. Upon cooling\nwe observe a softening in the electronic response, which has been predicted to\noccur for a d-wave charge order in electron-doped cuprates. At low\ntemperatures, the energy range of these excitations coincides with that of the\ndispersive magnetic modes known as paramagnons. Furthermore, measurements where\nthe polarization of the scattered photon is resolved indicate that the dynamic\nresponse at the charge order wave vector primarily involves spin-flip\nexcitations. Overall, our findings indicate a coupling between dynamic magnetic\nand charge-order correlations in the cuprates.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pair correlations of the hybridized orbitals in a ladder model for the\n  bilayer nickelate La$_3$Ni$_2$O$_7$: To clarify the nature of high-temperature superconductivity in the bilayer\nnickelate La$_3$Ni$_2$O$_7$ under pressure, we investigate, using the\ndensity-matrix renormalization group method, the pair correlations in the\ntwo-orbital $t$-$J$ ladder model. While the interchain-intraorbital pair\ncorrelations exhibit a slow power-law decay in both orbitals, the interorbital\npair correlation also develops due to the hybridization via interorbital\nhopping. These intra and interorbital pair correlations are enhanced by Hund's\ncoupling, but more importantly, the interorbital pairing correlation develops\neven without Hund's coupling, where apparent interorbital pairing glue is\nabsent. Our finding suggests that the hybridized two-orbital picture precisely\ndescribes superconductivity in the bilayer nickelate.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pairing state in the rutheno-cuprate superconductor RuSr2GdCu2O8: A\n  point contact Andreev Reflection Spectroscopy study: The results of Point Contact Andreev Reflection\n  Spectroscopy on polycrystalline RuSr$_2$GdCu$_2$O$_8$ pellets are presented.\nThe wide variety of the measured spectra are all explained in terms of a\nmodified BTK model considering a \\emph{d-wave} symmetry of the superconducting\norder parameter. Remarkably low values of the energy gap $\\Delta=(2.8\\pm\n0.2)meV$ and of the $2\\Delta/k_BT_c\\simeq 2$ ratio are inferred. From the\ntemperature evolution of the $dI/dV$ vs $V$ characteristics we extract a\nsublinear temperature dependence of the superconducting energy gap. The\nmagnetic field dependence of the conductance spectra at low temperatures is\nalso reported. From the $\\Delta$ vs $H$ evolution, a critical magnetic field\n$H_{c_2}\\simeq 30 T$ is inferred. To properly explain the curves showing\ngap-like features at higher voltages, we consider the formation of a Josephson\njunction in series with the Point Contact junction, as a consequence of the\ngranularity of the sample.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Epitaxial growth and electronic structure of Ruddlesden-Popper\n  nickelates ($ \\mathrm{La}_{n+1}\\mathrm{Ni}_{n}\\mathrm{O}_{3n+1}, n=1-5 $): We report the epitaxial growth of Ruddlesden-Popper nickelates, $\n\\mathrm{La}_{n+1}\\mathrm{Ni}_{n}\\mathrm{O}_{3n+1} $, with $ n $ up to 5 by\nreactive molecular beam epitaxy (MBE). X-ray diffractions indicate high\ncrystalline quality of these films and transport measurements show strong\ndependence on the $ n $ values. Angle-resolved photoemission spectroscopy\n(ARPES) reveals the electronic structure of $\n\\mathrm{La}_{5}\\mathrm{Ni}_{4}\\mathrm{O}_{13} $, showing a large hole-like\npocket centered around the Brillouin zone corner with a $ (\\pi, \\pi) $\nback-folded copy.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Phase segregation of superconductivity and ferromagnetism at\n  LaAlO$_3$/SrTiO$_3$ interface: The highly conductive two-dimensional electron gas formed at the interface\nbetween insulating SrTiO$_3$ and LaAlO$_3$ shows low-temperature\nsuperconductivity coexisting with inhomogeneous ferromagnetism. The Rashba\nspin-orbit interaction with in-plane Zeeman field of the system favors $p_x \\pm\ni p_y$-wave superconductivity at finite momentum. Owing to the intrinsic\ndisorder at the interface, the role of spatial inhomogeneity on the\nsuperconducting and ferromagnetic states becomes important. We find that for\nstrong disorder, the system breaks up into mutually excluded regions of\nsuperconductivity and ferromagnetism. This inhomogeneity-driven electronic\nphase separation accounts for the unusual coexistence of superconductivity and\nferromagnetism observed at the interface.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "A splitting integrator for the BCS equations of superconductivity: The BCS equations are the centerpiece of the microscopic description of\nsuperconductivity. Their space discretization yields a system of coupled\nordinary differential equations. In this work, we come up with fast time\nevolution schemes based on a splitting approach. One of the schemes only\nrequires basic operations. For the physically important case of the BCS\nequations for a contact interaction potential, the computational cost of the\nschemes increases only linearly with the dimension of the space discretization.\nTheir accuracy is demonstrated in extensive numerical experiments. These\nexperiments also show that the physical energy of the system is preserve up to\nvery small errors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Anomalous optical absorption in overdoped cuprates near the\n  charge-ordering instability: We propose an interpretation for the hump observed in the optical\nconductivity at or below a few hundreds of cm$^{-1}$, in overdoped cuprates\nlike the electron-doped Nd_{2-x}Ce_xCuO_{4-y} at x\\gtrsim 0.15 and the\nhole-doped Bi_2Sr_2CuO_6 and La_{2-x}Sr_xCuO_4. This interpretation is based on\nthe direct excitation of charge collective modes, which become nearly critical\nin the proximity to a charge-ordering instability. The nearly critical\ncharacter of these excitations entails a peculiar temperature dependence and a\npseudo-scaling form of the lineshapes, which are in agreement with the\nexperimental data.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The excitation spectrum of mesoscopic proximity structures: We investigate one aspect of the proximity effect, viz., the local density of\nstates of a superconductor-normal metal sandwich. In contrast to earlier work,\nwe allow for the presence of an arbitrary concentration of impurities in the\nstructure. The superconductor induces a gap in the normal metal spectrum that\nis proportional to the inverse of the elastic mean free path l_N for rather\nclean systems. For a mean free path much shorter than the thickness of the\nnormal metal, we find a gap size proportional to l_N that approaches the\nbehavior predicted by the Usadel equation (diffusive limit). We also discuss\nthe influence of interface and surface roughness, the consequences of a\nnon-ideal transmittivity of the interface, and the dependence of our results on\nthe choice of the model of impurity scattering.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Flux growth and physical properties of Mo3Sb7 single crystals: Millimeter sized single crystals of Mo3Sb7 are grown using the self-flux\ntechnique and a thorough characterization of their structural, magnetic,\nthermal and transport properties is reported. The structure parameters for the\nhigh-temperature cubic phase and the low-temperature tetragonal phase were, for\nthe first time, determined with neutron single crystal diffraction. Both X-ray\npowder diffraction and neutron single crystal diffraction at room temperature\nconfirmed that Mo3Sb7 crystallizes in Ir3Ge7-type cubic structure with space\ngroup Im-3m. The cubic-tetragonal structure transition at 53K is verified by\nthe peak splitting of (4 0 0) reflection observed by X-ray single crystal\ndiffraction and the dramatic intensity change of (12 0 0) peak observed by\nneutron single crystal diffraction. The structural transition is accompanied by\na sharp drop in magnetic susceptibility, electrical resistivity, and\nthermopower while cooling. A weak lambda anomaly was also observed around 53K\nin the temperature dependence of specific heat and the entropy change across\nthe transition is estimated to be 1.80J/molMoK. The temperature dependence of\nmagnetic susceptibility was measured up to 750K and it follows a Curie-Weiss\nbehavior above room temperature. Analysis of the low-temperature magnetic\nsusceptibility suggests a spin gap of 110K around 53K. A typical phonon thermal\nconductivity was observed in the low temperature tetragonal phase. A glassy\nphonon thermal conductivity above 53K suggests a structural instability in a\nwide temperature range. Superconductivity was observed at 2.35K in the as-grown\ncrystals and the dimensionless specific heat jump was determined to be 1.49,\nwhich is slightly larger than the BCS value of 1.43 for the weak-coupling\nlimit.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Possible \"Magn\u00e9li\" phases and self-alloying in the superconducting\n  sulfur hydride: We theoretically give an infinite number of metastable crystal structures for\nthe superconducting sulfur hydride H$_{x}$S under pressure. It has been thought\nthat theoretically predicted structures of H$_{2}$S and H$_{3}$S exhibit low\nand high $T_{\\rm c}$ in the experiment, respectively. The newly found\nstructures are long-period modulated crystals where slab-like H$_{2}$S and\nH$_{3}$S regions intergrow in a microscopic scale. The extremely small\nformation enthalpy for the H$_{2}$S--H$_{3}$S boundary indicated with the\nfirst-principles calculations suggests possible alloying of these phases\nthrough formation of local H$_{3}$S regions. The modulated structures and\ngradual alloying transformations between them not only explain the peculiar\npressure dependence of $T_{\\rm c}$ in sulfur hydride observed experimentally,\nbut also could prevail in the experimental samples under various compression\nschemes.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Measurements of surface impedance of superconductors as a function of\n  frequency in microwave range: We report measurements of the complex resistivity in YBCO and MgB$_2$ thin\nfilms over a continuous frequency spectrum in the microwave range, making use\nof a Corbino disk geometry. The paper mainly focuses on the extraction of the\nresistivity from raw data, displaying data analysis procedure and its limits of\nvalidity. We obtain and show resistivity curves as a function of frequency and\ntemperature denoting a frequency dependent widening of the superconducting\ntransition.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Consequences of the peculiar intrinsic properties of MgB2 on its\n  macroscopic current flow: The influence of two important features of magnesium diboride on the\nmacroscopic transport properties of polycrystalline MgB2 is discussed in the\nframework of a percolation model. While two band superconductivity does not\nhave significant consequences in the field and temperature range of possible\npower applications, the opposite is true for the anisotropy of the upper\ncritical field. The field dependence of the critical current densities strongly\nincreases and the macroscopic supercurrents disappear well below the apparent\nupper critical field. The common scaling laws for the field dependence of the\nvolume pinning force are altered and Kramer's plot is no longer linear,\nalthough grain boundary pinning dominates in nearly all polycrystalline MgB2\nconductors. In contrast to the conventional superconductors NbTi and Nb3Sn, a\nsignificant critical current anisotropy can be induced by the preparation\ntechnique of MgB2 tapes.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Anomalous Surface Impedance due to Odd-frequency Cooper Pairs: We discuss dynamical response of odd-frequency Cooper pairs to\nelectromagnetic field. By using the quasiclassical Green function method, we\ncalculate the surface impedance (Z=R-iX) of a normal metal thin film which\ncovers a superconductor. In contrast to the standard relation (i.e., R << X),\nthe surface impedance in spin-triplet proximity structures shows anomalous\nbehavior (i.e., R>X) at low temperatures. This unusual relation is a result of\nthe penetration of odd-frequency pairs into the normal metal and reflects the\nnegative Cooper pair density.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Phase-sensitive determination of nodal $\\mathbf{d}$-wave order parameter\n  in single-band and multiband superconductors: Determining the exact pairing symmetry of the superconducting order parameter\nin candidate unconventional superconductors remains an important challenge.\nRecently a new method, based on phase sensitive quasiparticle interference\nmeasurements, was developed to identify gap sign changes in isotropic multiband\nsystems. Here we extend this approach to the single-band and multiband nodal\n$d$-wave superconducting cases relevant, respectively, for the cuprates and\nlikely for the infinite-layer nickelate superconductors. Combining analytical\nand numerical calculations, we show that the antisymmetrized correction to the\ntunneling density of states due to nonmagnetic impurities in the Born and\nintermediate-scattering limits shows characteristic features for sign-changing\nand sign-preserving scattering wavevectors, as well as for the\nmomentum-integrated quantity. Furthermore, using a realistic approach\naccounting for the Wannier orbitals, we model scanning tunneling microscopy\ndata of $\\text{Bi}_2\\text{Sr}_2\\text{CaCu}_2\\text{O}_{8+\\delta}$, which should\nallow the comparison of our theory with experimental data.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Inversion symmetry of Josephson current as test of chiral domain wall\n  motion in Sr$_{2}$RuO$_{4}$: Clarifying the chiral domains structure of superconducting Sr$_{2}$RuO$_{4}$\nhas been a long-standing issue in identifying its peculiar topological\nsuperconducting state. We evaluated the critical current $I_{c}$ versus the\nmagnetic field $H$ of Nb/Sr$_{2}$RuO$_{4}$ Josephson junctions, changing the\njunction dimension in expectation of that the number of domains in the junction\nis controlled. $I_{c}(H)$ exhibits a recovery from inversion symmetry breaking\nto invariance when the dimension is reduced to several microns. This inversion\ninvariant behavior indicates the disappearance of domain walls; thus, the size\nof a single domain is estimated at approximately several microns.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Field-Induced Boson Insulating States in a 2D Superconducting Electron\n  Gas with Strong Spin-Orbit Scatterings: The phenomenon of field-induced superconductor-to-insulator transitions\nobserved experimentally in electron-doped SrTiO$_{3}$/LaAlO$_{3}$ interfaces,\nanalyzed recently by means of 2D superconducting fluctuations theory (Phys.\nRev. B \\textbf{104}, 054503 (2021)), is revisited with new insights associating\nit with the appearance at low temperatures of field-induced boson insulating\nstates. Within the framework of the time-dependent Ginzburg-Landau functional\napproach, we pinpoint the origin of these states in field-induced extreme\nsoftening of fluctuation modes over a large region in momentum space, upon\ndiminishing temperature, which drives Cooper-pair fluctuations to condense into\nmesoscopic puddles in real space. Dynamical quantum tunneling of Cooper-pair\nfluctuations out of these puddles, introduced within a phenomenological\napproach, which break into mobile single-electron states, contains the\nhigh-field resistance onset predicted by the exclusive boson theory.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Divergent Vortex Mass in a Superconducting Film in Proximity to a Metal: We consider a moving vortex in a two dimensional superconductor located at a\ndistance $d$ from a metallic overlayer. Starting from the microscopic imaginary\ntime action we integrate out the electronic degrees of freedom to obtain a low\nenergy, long wavelength effective action for the vortex. We focus our attention\non the vortex kinetic energy and derive a general expression for the vortex\nmass. We find that in the limit $d\\to \\infty$ the Coulomb screening of the\ndensity fluctuations, associated with vortex motion, results in a very small\nvortex mass as has been obtained in earlier studies. In the opposite limit of\n$d\\ll \\xi $ where $\\xi$ is the coherence length of the superconductor we find\nthat the vortex mass diverges logarithmically with the size of the system as\nthe proximity to the metal makes the screening processes, that usually make the\nmass small, ineffective. We comment on the relevance of our results to recent\nexperiments which show a dramatic fall in resistance when a metallic gate is\nplaced near a supeconducting film in a magnetic field at low temperature.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Emergence of Superconductivity from Fully Incoherent Normal State in an\n  Iron-Based Superconductor (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$: In unconventional superconductors, it is generally believed that\nunderstanding the physical properties of the normal state is a pre-requisite\nfor understanding the superconductivity mechanism. In conventional\nsuperconductors like niobium or lead, the normal state is a Fermi liquid with a\nwell-defined Fermi surface and well-defined quasipartcles along the Fermi\nsurface. Superconductivity is realized in this case by the Fermi surface\ninstability in the superconducting state and the formation and condensation of\nthe electron pairs (Cooper pairing). The high temperature cuprate\nsuperconductors, on the other hand, represent another extreme case that\nsuperconductivity can be realized in the underdoped region where there is\nneither well-defined Fermi surface due to the pseudogap formation nor\nquasiparticles near the antinodal regions in the normal state. Here we report a\nnovel scenario that superconductivity is realized in a system with well-defined\nFermi surface but without quasiparticles along the Fermi surface in the normal\nstate. High resolution laser-based angle-resolved photoemission measurements\nhave been performed on an optimally-doped iron-based superconductor\n(Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$. We find that, while sharp superconducting\ncoherence peaks emerge in the superconducting state on the hole-like Fermi\nsurface sheets, no quasiparticle peak is present in the normal state. Its\nelectronic behaviours deviate strongly from a Fermi liquid system. The\nsuperconducting gap of such a system exhibits an unusual temperature dependence\nthat it is nearly a constant in the superconducting state and abruptly closes\nat T$_c$. These observations have provided a new platform to study\nunconventional superconductivity in a non-Fermi liquid system.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pre-pairing and the \"Filling\" Gap in the Cuprates From the Tomographic\n  Density of States: We use the tomographic density of states (TDoS), which is a measure of the\ndensity of states for a single slice through the band structure of a solid, to\nstudy the temperature evolution of the superconducting gap in the cuprates. The\nTDoS provides unprecedented accuracy in determining both the superconducting\npair-forming strength, $\\Delta$, and the pair-breaking rate, $\\Gamma$. In both\noptimally- and under-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\\delta}$, we find the\nnear-nodal $\\Delta$ smoothly evolves through the superconducting transition\ntemperature - clear evidence for the existence of pre-formed pairs.\nAdditionally, we find the long observed `filling' of the superconducting gap in\nthe cuprates is due to the strongly temperature dependent $\\Gamma$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Emergent superconductivity in van der Waals Kagome material Pd3P2S8\n  under high pressure: Kagome lattice systems have been proposed to host rich physics, which provide\nan excellent platform to explore unusual quantum states. Here, we report on the\ndiscovery of superconductivity in van der Waals material Pd3P2S8 under\npressure. The superconductivity is observed in Pd3P2S8 for those pressures\nwhere the temperature dependence of the resistivity changes from a\nsemiconducting-like behavior to that of a normal metal. The superconducting\ntransition temperature Tc increases with applied pressure and reaches ~ 6.83 K\nat 79.5 GPa. Combining high-pressure XRD, Raman spectroscopy and theoretical\ncalculations, our results demonstrate that the observed superconductivity\ninduced by high pressure in Pd3P2S8 is closely related to the formation of\namorphous phase, which results from the structural instability due to the\nenhanced coupling between interlayer Pd and S atoms upon compression.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Character of ground state of an aperiodic frustrated Josephson junction\n  array: We study the energy spectrum for an aperiodic Josephson junction ladder, as a\nfunction of frustration. Frustration is brought about by application of a\ntransverse magnetic field, and aperiodicity is imposed by the arrangement of\nplaquettes with two incommensurate areas. We study the effect of the\nincommensurate plaquette areas in conjunction with that of the aperiodicity.\nThe structure of the energy spectrum at deep minima is shown to be described by\na model that treats the plaquettes independently. The energy spectrum is a\nquasiperiodic function of frustration; short range correlations in the\narrangement of plaquettes have a small effect on the energy power spectrum.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Antiferromagnetic Correlation and the Pairing Mechanism of the Cuprates\n  and Iron Pnictides : a View From the Functional Renormalization Group Studies: We compare the one-loop functional renormalization group results for the\ncuprates and the iron pnictides. Interestingly a coherent picture suggesting\nthat antiferromagnetic correlation causes pairing for both materials emerges.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Approximate scaling relation for the anharmonic electron-phonon problem: An approximate scaling relation is found for the transition temperature to a\ncharge-density-wave instability in the anharmonic electron-phonon problem,\nwhich maps a wide range of interaction strengths, anharmonicities, and phonon\nfrequencies onto a common functional form. The relation employs the\nwave-function renormalization parameter and is valid even for systems that are\nnot Fermi liquids.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Temperature dependence of the spectral weight in p- and n-type cuprates:\n  a study of normal state partial gaps and electronic kinetic energy: The optical conductivity of CuO2 (copper-oxygen) planes in p- and n-type\ncuprates thin films at various doping levels is deduced from highly accurate\nreflectivity data. The temperature dependence of the real part sigma1(omega) of\nthis optical conductivity and the corresponding spectral weight allow to track\nthe opening of a partial gap in the normal state of n-type Pr{2-x}Ce(x)CuO4\n(PCCO), but not of p-type Bi2Sr2CaCu2O(8+delta} (BSCCO) cuprates. This is a\nclear difference between these two families of cuprates, which we briefly\ndiscuss. In BSCCO, the change of the electronic kinetic energy Ekin - deduced\nfrom the spectral weight- at the superconducting transition is found to cross\nover from a conventional BCS behavior (increase of Ekin below Tc to an\nunconventional behavior (decrease of Ekin below Tc) as the free carrier density\ndecreases. This behavior appears to be linked to the energy scale over which\nspectral weight is lost and goes into the superfluid condensate, hence may be\nrelated to Mott physics.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Stabilization of an ambient pressure, collapsed tetragonal phase in\n  CaFe2As2 and tuning of the orthorhombic / antiferromagnetic transition\n  temperature by over 70 K by control of nano-precipitates: We have found a remarkably large response of the transition temperature of\nCaFe2As2 single crystals grown out of excess FeAs to annealing / quenching\ntemperature. Whereas crystals that are annealed at 400 C exhibit a first order\nphase transition from a high temperature tetragonal to a low temperature\northorhombic and antiferromagnetic state near 170 K, crystals that have been\nquenched from 960 C exhibit a transition from a high temperature tetragonal\nphase to a low temperature, non-magnetic, collapsed tetragonal phase below 100\nK. By use of temperature dependent electrical resistivity, magnetic\nsusceptibility, X-ray diffraction, Mossbauer spectroscopy and nuclear magnetic\nresonance measurements we have been able to demonstrate that the transition\ntemperature can be reduced in a monotonic fashion by varying the annealing /\nquenching temperature from 400 to 850 C with the low temperature state\nremaining antiferromagnetic for transition temperatures larger than 100 K and\nbecoming collapsed tetragonal / non-magnetic for transition temperatures below\n90 K. This suppression of the orthorhombic / antiferromagnetic phase transition\nand its ultimate replacement with the collapsed tetragonal / non-magnetic phase\nis similar to what has been observed for CaFe2As2 under hydrostatic pressure.\nTransmission electron microscopy studies indicate that there is a temperature\ndependent, width of formation of CaFe2As2 with a decreasing amount of excess Fe\nand As being soluble in the single crystal at lower annealing temperatures. For\nsamples quenched from 960 C there is a fine (of order 10 nm), semi-uniform\ndistribution of precipitate that can be associated with an average strain field\nwhereas for samples annealed at 400 C the excess Fe and As form mesoscopic\ngrains that induce little strain throughout the CaFe2As2 lattice.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Phase fluctuations and the pseudogap in YBa2Cu3Ox: The thermodynamics of the superconducting transition is studied as a function\nof doping using high-resolution expansivity data of YBa2Cu3Ox single crystals\nand Monte-Carlo simulations of the anisotropic 3D-XY model. We directly show\nthat Tc of underdoped YBa2Cu3Ox is strongly suppressed from its mean-field\nvalue (Tc-MF) by phase fluctuations of the superconducting order parameter. For\noverdoped YBa2Cu3Ox fluctuation effects are greatly reduced and Tc ~ Tc-MF . We\nfind that Tc-MF exhibits a similar doping dependence as the pseudogap energy,\nnaturally suggesting that the pseudogap arises from phase-incoherent Cooper\npairing.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Surface density of states of s+-wave Cooper pairs in a two-band model: We calculate surface density of state (SDOS) of s+-wave Cooper pair in\ntwo-band superconductor model, where gap functions have different signs between\ntwo bands. We find that Andreev bound state appears at surface due to the sign\nchange in the gap function in the interband quasiparticle scattering. However,\nwe do not obtain the zero-energy peak of SDOS in contrast to the d-wave case.\nThe tunneling spectroscopy of s+-wave is much more complex as compared to the\nd-wave case realized in high-Tc cuprates.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The effect of {11}^B substitution on the superconductivity in MgCNi3: The crystal structure of boron doped superconducting\nMgC_{1-x}{11}^B_{x}Ni_{3}, studied by powder neutron diffraction, is reported.\nThe solubility limit of boron is determined to be approximately x=0.16. The\nunit cell expands from a = 3.81089(2) to 3.81966(2) Angstroms as x increases\nfrom x=0 to x=0.155. Boron ({11}^B) doping decreases Tc with increasing x: from\n7.09K (x=0) to 6.44K (x=0.155).",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "First-Order Reorientation of the Flux-Line Lattice in CaAlSi: The flux line lattice in CaAlSi has been studied by small angle neutron\nscattering. A well defined hexagonal flux line lattice is seen just above Hc1\nin an applied field of only 54 Oe. A 30 degree reorientation of this vortex\nlattice has been observed in a very low field of 200 Oe. This reorientation\ntransition appears to be of first-order and could be explained by non-local\neffects. The magnetic field dependence of the form factor is well described by\na single penetration depth of 1496(1) angstroms and a single coherence length\nof 307(1) angstroms at 2 K. At 1.5 K the penetration depth anisotropy is 2.7(1)\nwith the field applied perpendicular to the c axis and agrees with the\ncoherence length anisotropy determined from critical field measurements.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity in cuprates governed by topological constraints: The remarkable universality of the cuprate $T_c$ dome suggests a very\nfundamental unifying principle. Moreover, the superconducting gap is known to\npersist above $T_c$ in the pseudogap phase of all cuprates. So, contrary to\nBCS, the gap cannot be the order parameter of the transition.\n  In this work, we show that both the $T_c$-dome and the pseudogap line\n$T^*(p)$ arise from a unique and identifiable principle: the interaction of\nlocalized `pairons' on an antiferromagnetic square lattice. The topological\nconstraints on such preformed pairons give rise to both the $T_c$ dome and the\npairing energy {\\it simultaneously}. It also provides a natural explanation for\nthe critical doping points of the phase diagram.\n  The model matches perfectly both the $T^*$ and $T_c$ experimental lines, with\nonly one adjustable parameter.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Optical phonons along the c axis of YBa_2Cu_3O_{6+x}, for x=0.5 -> 0.95: The c-axis polarized phonon spectra of single crystals of YBCO_{6+x} have\nbeen measured for the doping range x=0.5 -> 0.95, between 10 K and 300 K. The\nlow background electronic conductivity, determined by Kramers-Kronig analysis\nof the reflectance, leads to a rich phonon structure. With decreased doping the\nfive normally-active B_{1u} modes broaden and the high-frequency apical oxygen\nmode splits into two components. We associate the higher of these with the\ntwo-fold coordinated copper-oxygen sticks. The 155 cm^{-1} low-frequency mode,\nwhich involves the apical and chain-oxygens, splits into at least three\ncomponents with decreasing doping. Some phonon anomalies which occur near T_c\nin the highly-doped material occur well above T_c in the oxygen-reduced\nsystems. An unusual broad phonon band develops in the normal state at approx\n400 cm^{-1}, which becomes more intense at low doping and low temperatures,\nborrowing oscillator strength from apical and plane oxygen modes resulting in a\nmajor transformation of the phonon spectrum below approx 150 K.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Phase transition in a chain of quantum vortices: We consider interacting vortices in a quasi-one-dimensional array of\nJosephson junctions with small capacitance. If the charging energy of a\njunction is of the order of the Josephson energy, the fluctuations of the\nsuperconducting order parameter in the system are considerable, and the\nvortices behave as quantum particles. Their density may be tuned by an external\nmagnetic field, and therefore one can control the commensurability of the\none-dimensional vortex lattice with the lattice of Josephson junctions. We show\nthat the interplay between the quantum nature of a vortex, and the long-range\ninteraction between the vortices leads to the existence of a specific\ncommensurate-incommensurate transition in a one-dimensional vortex lattice. In\nthe commensurate phase an elementary excitation is a soliton, with energy\nseparated from the ground state by a finite gap. This gap vanishes in the\nincommensurate phase. Each soliton carries a fraction of a flux quantum; the\npropagation of solitons leads to a finite resistance of the array. We find the\ndependence of the resistance activation energy on the magnetic field and\nparameters of the Josephson array. This energy consists of the above-mentioned\ngap, and also of a boundary pinning term, which is different in the\ncommensurate and incommensurate phases. The developed theory allows us to\nexplain quantitatively the available experimental data.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Multiple optical gaps and laser with magnonic pumping in 2D Ising\n  superconductors: Ising superconductivity has been recently discovered in 2D transition metal\ndichalcogenides. We report that such superconductors have unusual optical\nproperties controlled by the in-plane Zeeman field. First, we find several\noptical gaps visible as peaks of the conductivity and the Raman susceptibility.\nMoreover, we find that the Ising spin splitting in the spectrum of Bogolubov\nquasiparticles enables strong population inversion generated by the\ntime-dependent Zeeman field. Ultimately this leads to the possibility of the\nsuperconducting laser with magnonic pumping which can be realized in the van\nder Waals structures consisting of the Ising superconductor and the\nferromagnetic insulator layers.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Eliashberg theory for spin-fluctuations mediated superconductivity --\n  Application to bulk and monolayer FeSe: We present a novel method for embedding spin and charge fluctuations in an\nanisotropic, multi-band and full-bandwidth Eliashberg treatment of\nsuperconductivity. Our analytical framework, based on the random phase\napproximation, allows for a selfconsistent calculation of material specific\ncharacteristics in the interacting, and more specifically, the superconducting\nstate. We apply this approach to bulk FeSe as representative for the iron-based\nsuperconductors and successfully solve for the superconducting transition\ntemperature $T_c$, the gap symmetry and the gap magnitude. We obtain $T_c\n\\approx 6$ K, consistent with experiment ($T_c \\approx 8$ K), as well as other\nquantities in good agreement with experimental observations, thus supporting\nspin fluctuations mediated pairing in bulk FeSe. On the contrary, applying our\napproach to monolayer FeSe on SrTiO$_3$ we find that spin fluctuations within\nthe full Eliashberg framework give a $d$-wave gap with $T_c\\le 11$ K and\ntherefore cannot provide an explanation for a critical temperature as high as\nobserved experimentally ($T_c \\approx 70$ K). Our results hence point towards\ninterfacial electron-phonon coupling as the dominant Cooper pairing mediator in\nthis system.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Microwave properties of YBa$_2$Cu$_3$O$_{7-\u03b4}$ films with BaZrO$_3$\n  nanoinclusions: We present measurements of the microwave complex surface impedance at 47.7\nGHz in YBa$_2$Cu$_3$O$_{7-\\delta}$ (YBCO) films deposited by pulsed laser\ndeposition with the explicit goal to introduce BaZrO$_3$ (BZO) nanoinclusions.\nComposite targets obtained by addition of BZO powder in molar percents ranging\nfrom 2.5 to 7 mol.% have been prepared and characterized. Measurements of the\nmicrowave surface impedance indicate a broadened transition in zero field,\nhowever compensated by a very large pinning frequency, indicating that while\nintergrain properties are still to be optimized the effect of nanometric\ninclusions substantially enhances the intragrain vortex pinning.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pseudogap and Amplitude Fluctuations in High Temperature Superconductors: Amplitude fluctuations of the pairing field are responsible together with\nphase fluctuations for the pseudogap phenomena in high temperature\nsuperconductors. Here we present the more detailed theory of the amplitude and\nphase fluctuations approach in the framework of a fermionic pairing model. New\nexperimental comparisons are presented for the specific heat of the curprate\nLSCO confirming the generality of this phenomenological approach. The strong\ndecrease of amplitude fluctuations near optimal doping induces the illusion of\na \"quantum critical point\", which in fact does not exist since the pseudogap\nenergy scale is always different from zero even in the overdoped regime.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Soliton induced critical current oscillations in two-band\n  superconducting bridges: Using time-dependent Ginzburg-Landau theory we find oscillations of critical\ncurrent density $j_c$ as a function of the length $L$ of the bridge formed from\ntwo-band superconductor. We explain this effect by appearance of the phase\nsolitons in the bridge at $j<j_c$, those number changes with change of $L$. In\ncase of sufficiently strong interband coupling oscillations of $j_c$ disappear.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Evidence for superconducting gap nodes in the zone-centered hole bands\n  of KFe2As2 from magnetic penetration-depth measurements: Among the iron-based pnictide superconductors the material KFe$_2$As$_2$ is\nunusual in that its Fermi surface does not consist of quasi-nested electron and\nhole pockets. Here we report measurements of the temperature dependent London\npenetration depth of very clean crystals of this compound with residual\nresistivity ratio $>1200$. We show that the superfluid density at low\ntemperatures exhibits a strong linear-in-temperature dependence which implies\nthat there are line nodes in the energy gap on the large zone-centered hole\nsheets. The results indicate that KFe$_2$As$_2$ is an unconventional\nsuperconductor with strong electron correlations.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity with angular dependent coupling: stripes, Coulomb\n  repulsion and enhanced Tc: We have analysed the effect of intrinsic doping inhomogeneity and the\npresence of stripes in high-T_c superconductors on coupling \\lambda by using a\nsimple analytically solvable model with an angular dependent \\lambda (\\phi)\nrepresented by a square-well form. We have found that the introduction of the\nCoulomb repulsion \\lambda_C, increasing the ''contrast'' | \\lambda | + |\n\\lambda_C |, or the depth of the angular modulation of \\lambda (\\phi), leads to\na remarkable enhancement of T_c. This effect can be optimized by combining\nattractive (\\lambda <0) and repulsive (\\lambda >0) interactions along stripes\nand perpendicular to them.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The Fulde-Ferrell-Larkin-Ovchinnikov state in pnictides: Fe-based superconductors (FeSC) exhibit all the properties of systems that\nallow the formation of a superconducting phase with oscillating order\nparameter, called the Fulde--Ferrell--Larkin--Ovchinnikov (FFLO) phase. By the\nanalysis of the Cooper pair susceptibility in two-band FeSC, such systems are\nshown to support the existence of a FFLO phase, regardless of the exhibited\norder parameter symmetry. We also show the state with nonzero Cooper pair\nmomentum, in superconducting FeSC with $\\sim \\cos(k_{x}) \\cdot \\cos (k_{y})$\nsymmetry, to be the ground state of the system in a certain parameter range.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Optical determination of the superconducting energy gap in\n  electron-doped Pr_{1.85}Ce_{0.15}CuO_4: The optical properties of single crystal Pr_{1.85}Ce_{0.15}CuO_4 have been\nmeasured over a wide frequency range above and below the critical temperature\n(T_c \\simeq 20 K). In the normal state the coherent part of the conductivity is\ndescribed by the Drude model, from which the scattering rate just above T_c is\ndetermined to be 1/\\tau \\simeq 80 cm^{-1}. The condition that \\hbar/\\tau\n\\approx 2k_B T near T_c appears to be a general result in many of the cuprate\nsuperconductors. Below T_c the formation of a superconducting energy gap is\nclearly visible in the reflectance, from which the gap maximum is estimated to\nbe \\Delta_0 \\simeq 35 cm^{-1} (4.3 meV). The ability to observe the\nsuperconducting energy gap in the optical properties favors the nonmonotonic\nover the monotonic description of the d-wave gap. The penetration depth for\nT\\ll T_c is \\lambda \\simeq 2000 \\AA, which when taken with the estimated value\nfor the dc conductivity just above T_c of \\sigma_{dc} \\simeq 35 \\times 10^3\n\\Omega^{-1}cm^{-1} places this material on the general scaling line for the\ncuprates defined by 1/\\lambda^2 \\propto \\sigma_{dc}(T\\simeq T_c) \\times T_c.\nThis result is consistent with the observation that 1/\\tau \\approx 2\\Delta_0,\nwhich implies that the material is not in the clean limit.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Evidence of Strong Correlations and Coherence-Incoherence Crossover in\n  the Iron Pnictide Superconductor KFe2As2: Using resistivity, heat-capacity, thermal-expansion, and susceptibility\nmeasurements we study the normal-state behavior of KFe2As2. We find that both\nthe Sommerfeld coefficient gamma = 103 mJ mol-1 K-2 and the Pauli\nsusceptibility chi = 4x10-4 are strongly enhanced, which confirm the existence\nof heavy quasiparticles inferred from previous de Haas-van Alphen and ARPES\nexperiments. We discuss this large enhancement using a Gutzwiller slave-boson\nmean-field calculation, which reveals the proximity of KFe2As2 to an\norbital-selective Mott transition. The temperature dependence of the magnetic\nsusceptibility and the thermal expansion provide strong experimental evidence\nfor the existence of a coherence-incoherence crossover, similar to what is\nfound in heavy fermion and ruthenate compounds, due to Hund's coupling between\norbitals.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetoresistance and Hall effect in e-doped superconducting SrLaCuO\n  thin films: We have epitaxially grown c-axis oriented SrxLa1-xCuO2 thin films by rf\nsputtering on KTaO3 substrates with x = 0.12. The as-grown deposits are\ninsulating and a series of superconducting films with various Tc(R=0) up to 26\nK have been obtained by in-situ oxygen reduction. Transport measurements in the\nab plane of these samples have been undertaken. We report original results on\nthe temperature dependence of the Hall effect and on the anisotropic\nmagnetoresistance (T > Tc). We discuss the magnitude of upper critical fields\nand anisotropy, the Hall effect, which presents changes of sign indicative of\nthe existence of two types of carriers, the normal state magnetoresistance,\nnegative in parallel magnetic field, a possible signature of spin scattering.\nThese properties are compared to those of hole-doped cuprates, such as\nBiSr(La)CuO with comparable Tc.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "d-Wave superconductivity on the checkerboard Hubbard model at weak and\n  strong coupling: It has been argued that inhomogeneity generally can enhance superconductivity\nin the cuprate high-Tc materials. To check the validity of this claim, we study\nd-wave superconductivity on the checkerboard Hubbard model on a square lattice\nusing the Cellular Dynamical Mean Field theory method with an exact\ndiagonalization solver at zero temperature. The d-wave order parameter is\ncomputed for various inhomogeneity levels over the entire doping range of\ninterest in both strong and weak coupling regimes. At a given doping, the size\nof the d-wave order parameter manifests itself directly in the height of the\ncoherence peaks and hence is an appropriate measure of the strength of\nsuperconductivity. The weak coupling results reveal a suppression of the order\nparameter in the presence of inhomogeneity for small to intermediate hole\ndopings, while it is enhanced for large dopings. In contrast, for strong\ncoupling there is a monotonic decrease in the maximum amplitude of the\nsuperconducting order parameter with inhomogeneity over the entire doping range\nof interest. Furthermore, at moderately high inhomogeneity, the system\nundergoes a first-order transition from the superconducting to the normal state\nin the underdoped regime. In the overdoped regime, the change in the value of\nthe superconducting order parameter correlates with the height of the lowest\nenergy peak in the spectral weight of antiferromagnetic spin fluctuations,\nconfirming the connection between antiferromagnetic fluctuations and d-wave\nsuperconductivity found in earlier studies on the homogeneous case. Our results\nare benchmarked by comparisons with numerically exact results on the\ncheckerboard Hubbard ladder.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Imaging Flux Vortices in MgB2 using Transmission Electron Microscopy: We report the successful imaging of flux vortices in single crystal MgB2\nusing transmission electron microscopy. The specimen was thinned to electron\ntransparency (350 nm thickness) by focussed ion beam milling. An artefact of\nthe thinning process was the production of longitudinal thickness undulations\nof height 1-2 nm in the sample which acted as pinning sites due to the energy\nrequired for the vortices to cross them. These had a profound effect on the\npatterns of vortex order observed which we examine here.\n  Supplementary information can be downloaded from\nhttp://www-hrem.msm.cam.ac.uk/people/loudon/#publications",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Iron-based superconductors: teenage, complex, challenging: The advent of iron-based superconductors in 2008 came as a complete surprise\nto the condensed matter community. Now 15 years later, they are beginning to\nimpart some of their new-found wisdom on a slew of emerging superconductors\nthat boast similar traits.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Point-contact spectroscopy of superconductors in the nonequilibrium\n  state: A phase transition of the region of the superconductor near the point contact\ninto a new nonequi- librium state at the critical density of\nnonequilibriumquasiparticles is observed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Oxygen adsorption induced superconductivity in ultrathin FeTe film on\n  SrTiO3(001): The phenomenon of oxygen incorporation induced superconductivity in iron\ntelluride (Fe1+yTe, with antiferromagnetic (AFM) orders) is intriguing and\nquite different from the case of FeSe. Until now, the microscopic origin of the\ninduced superconductivity and the role of oxygen are far from clear. Here, by\ncombining in-situ scanning tunneling microscopy/spectroscopy (STM/STS) and\nx-ray photoemission spectroscopy (XPS) on oxygenated FeTe, we found physically\nadsorbed O2 molecules crystallized into c(2/3x2) structure as an oxygen\noverlayer at low temperature, which was vital for superconductivity. The O2\noverlayer were not epitaxial on the FeTe lattice, which implied weak O2-FeTe\ninteraction but strong molecular interactions. Energy shift observed in the STS\nand XPS measurements indicated hole doping effect from the O2 overlayer to the\nFeTe layer, leading to a superconducting gap of 4.5 meV opened across the Fermi\nlevel. Our direct microscopic probe clarified the role of oxygen on FeTe and\nemphasized the importance of charge transfer effect to induce superconductivity\nin iron-chalcogenide thin films.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Prediction of superconducting properties of materials based on machine\n  learning models: The application of superconducting materials is becoming more and more\nwidespread. Traditionally, the discovery of new superconducting materials\nrelies on the experience of experts and a large number of \"trial and error\"\nexperiments, which not only increases the cost of experiments but also prolongs\nthe period of discovering new superconducting materials. In recent years,\nmachine learning has been increasingly applied to materials science. Based on\nthis, this manuscript proposes the use of XGBoost model to identify\nsuperconductors; the first application of deep forest model to predict the\ncritical temperature of superconductors; the first application of deep forest\nto predict the band gap of materials; and application of a new sub-network\nmodel to predict the Fermi energy level of materials. Compared with our known\nsimilar literature, all the above algorithms reach state-of-the-art. Finally,\nthis manuscript uses the above models to search the COD public dataset and\nidentify 50 candidate superconducting materials with possible critical\ntemperature greater than 90 K.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Field-angle dependence reveals odd-parity superconductivity in\n  CeRh$_2$As$_2$: CeRh$_2$As$_2$ is an unconventional superconductor with multiple\nsuperconducting phases and $T_\\mathrm{c} = 0.26$ K. When $H\\parallel c$, it\nshows a field-induced transition at $\\mu_0H^* = 4$ T from a low-field\nsuperconducting state SC1 to a high-field state SC2 with a large critical field\nof $\\mu_0H_\\mathrm{c2} = 14$ T. In contrast, for $H\\perp c$, only the SC1 with\n$\\mu_0H_\\mathrm{c2} = 2$ T is observed. A simple model based on the crystal\nsymmetry was able to reproduce the phase-diagrams and their anisotropy,\nidentifying SC1 and SC2 with even and odd parity superconducting states,\nrespectively. However, additional orders were observed in the normal state\nwhich might have an influence on the change of the superconducting state at\n$H^*$. Here, we present a comprehensive study of the angle dependence of the\nupper critical fields using magnetic ac-susceptibility, specific heat and\ntorque on single crystals of CeRh$_2$As$_2$. The experiments show that the\nstate SC2 is strongly suppressed when rotating the magnetic field away from the\n$c$ axis and it disappears for an angle of 35$^{\\circ}$. This behavior agrees\nperfectly with our extended model of a pseudospin triplet state with $\\vec{d}$\nvector in the plane and hence allows to nail down that SC2 is indeed the\nsuggested odd-parity state.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Robust topological superconductivity in spin-orbit coupled systems at\n  higher-order van Hove filling: Van Hove singularities (VHSs) in proximity to the Fermi level promote\nelectronic interactions and generate diverse competing instabilities. It is\nalso known that a nontrivial Berry phase derived from spin-orbit coupling (SOC)\ncan introduce an intriguing decoration into the interactions and thus alter\ncorrelated phenomena. However, it is unclear how and what type of new physics\ncan emerge in a system featured by the interplay between VHSs and the Berry\nphase. Here, based on a general Rashba model on the square lattice, we\ncomprehensively explore such an interplay and its significant influence on the\ncompeting electronic instabilities by performing a parquet renormalization\ngroup analysis. Despite the existence of a variety of comparable fluctuations\nin the particle-particle and particle-hole channels associated with\nhigher-order VHSs, we find that the chiral $p \\pm ip$ pairings emerge as two\nstable fixed trajectories within the generic interaction parameter space,\nnamely the system becomes a robust topological superconductor. The chiral\npairings stem from the hopping interaction induced by the nontrivial Berry\nphase. The possible experimental realization and implications are discussed.\nOur work sheds new light on the correlated states in quantum materials with\nstrong SOC and offers fresh insights into the exploration of topological\nsuperconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Microwave spectroscopy of Majorana vortex modes: The observation of zero-bias conductance peaks in vortex cores of certain\nFe-based superconductors has sparked renewed interest in vortex-bound Majorana\nstates. These materials are believed to be intrinsically topological in their\nbulk phase, thus avoiding potentially problematic interface physics encountered\nin superconductor-semiconductor heterostructures. However, progress toward a\nvortex-based topological qubit is hindered by our inability to measure the\ntopological quantum state of a non-local vortex Majorana state, i.e., the\ncharge of a vortex pair. In this paper, we theoretically propose a\nmicrowave-based charge parity readout of the Majorana vortex pair charge. A\nmicrowave resonator above the vortices can couple to the charge allowing for a\ndispersive readout of the Majorana parity. Our technique may also be used in\nvortices in conventional superconductors and allows one to probe the lifetime\nof vortex-bound quasiparticles, which is currently beyond existing scanning\ntunneling microscopy capabilities.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Microscopic derivation of Ginzburg-Landau equations for coexistent\n  states of superconductivity and magnetism: Ginzburg-Landau (GL) equations for the coexistent states of superconductivity\nand magnetism are derived microscopically from the extended Hubbard model with\non-site repulsive and nearest-neighbor attractive interactions. In the derived\nGL free energy a cubic term that couples the spin-singlet and spin-triplet\ncomponents of superconducting order parameters (SCOP) with magnetization\nexists. This term gives rise to a spin-triplet SCOP near the interface between\na spin-singlet superconductor and a ferromagnet, consistent with previous\ntheoretical studies based on the Bogoliubov de Gennes method and the\nquasiclassical Green's function theory. In coexistent states of singlet\nsuperconductivity and antiferromagnetism it leads to the occurrence of\npi-triplet SCOPs.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Induced unconventional superconductivity on the surface states of\n  Bi$_2$Te$_3$ topological insulator: Topological superconductivity is central to a variety of novel phenomena\ninvolving the interplay between topologically ordered phases and\nbroken-symmetry states. The key ingredient is an unconventional order\nparameter, with an orbital component containing a chiral $p_x$ + i$p_y$ wave\nterm. Here we present phase-sensitive measurements, based on the quantum\ninterference in nanoscale Josephson junctions, realized by using Bi$_2$Te$_3$\ntopological insulator. We demonstrate that the induced superconductivity is\nunconventional and consistent with a sign-changing order parameter, such as a\nchiral $p_x$ + i$p_y$ component. The magnetic field pattern of the junctions\nshows a dip at zero externally applied magnetic field, which is an\nincontrovertible signature of the simultaneous existence of 0 and $\\pi$\ncoupling within the junction, inherent to a non trivial order parameter phase.\nThe nano-textured morphology of the Bi$_2$Te$_3$ flakes, and the dramatic role\nplayed by thermal strain are the surprising key factors for the display of an\nunconventional induced order parameter.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Noise Performance of Lumped Element Direct Current Superconducting\n  Quantum Interference Device Amplifiers in the 4 GHz-8 GHz Range: We report on the noise of a lumped element Direct Current Superconducting\nQuantum Interference Device amplifier. We show that the noise temperature in\nthe 4 GHz-8 GHz range over ranges of 10's of MHz is below 1 kelvin (three\nphotons of added noise), characterize the overall behavior of the noise as a\nfunction of bias parameters, and discuss potential mechanisms which determine\nthe noise performance in this amplifier. We show that this device can provide\nmore than a factor of 10 improvement in practical system noise over existing\nphase-preserving microwave measurement systems in this frequency band.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Ambient Temperature Growth and Superconducting Properties of Ti-V Alloy\n  Thin Films: A study on the optimization of ambient temperature growth and superconducting\nproperties of Ti-V alloy thin films grown on SiO2-coated Si substrate is\nreported here. These films have been synthesized by co-sputtering of Ti and V\ntargets, and films having different Ti concentrations were deposited to get the\noptimized critical temperature (TC) of thin films close to the bulk value. The\nmaximum TC of 5.2 K has been obtained in the Ti40V60 composition, which is\nfurther increased to 6.2 K when a 10 nm thick Ti underlayer is added below the\nTi-V film. GIXRD measurements confirm the formation of Ti-V alloys in the\ndesired crystal structure. The upper critical field (HC2) of the thin films has\nbeen estimated with the help of magnetotransport measurements. The utility of\nTi-V alloy thin films in superconducting radiation detection applications is\nascertained.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Impact of nonlocal electrodynamics on flux noise and inductance of\n  superconducting wires: We present exact numerical calculations of supercurrent density, inductance,\nand impurity-induced flux noise of cylindrical superconducting wires in the\nnonlocal Pippard regime, which occurs when the Pippard coherence length is\nlarger than the London penetration depth. In this regime the supercurrent\ndensity displays a peak away from the surface, and changes sign inside the\nsuperconductor, signalling a breakdown of the usual approximation of local\nLondon electrodynamics with a renormalized penetration depth. Our calculations\nshow that the internal inductance and the bulk flux noise power is enhanced in\nnonlocal superconductors. In contrast, the kinetic inductance is reduced and\nthe surface flux noise remains the same. As a result, impurity spins in the\nbulk may dominate the flux noise in superconducting qubits in the Pippard\nregime, such as the ones using aluminum superconductors with large electron\nmean free path.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Imaging Local Sources of Intermodulation in Superconducting Microwave\n  Devices: This work presents new experimental results on low-temperature (LT)\ncharacterization of local rf properties of passive superconducting (SC)\nmicrowave devices using a novel Laser Scanning Microscope (LSM). In this\ntechnique, a modulated laser beam is focused onto and scanned over the surface\nof a resonant SC device to probe the spatial distribution of rf current. The\nhighly localized photo-induced change of the kinetic inductance of the SC\ndevice produces both a shift of the resonant frequency f_0 and change of the\nquality factor Q. An image of these changes is recorded as the laser spot is\nscanned over the device. We present the first measurements of spatially\nresolved intermodulation response in a High Temperature Superconducting (HTS)\nco-planar waveguide resonator, opening up a new window into the local origins\nof nonlinearity in the HTS materials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Room temperature Peierls distortion in small radius nanotubes: By means of {\\it ab initio} simulations, we investigate the phonon band\nstructure and electron-phonon coupling in small 4-\\AA diameter nanotubes. We\nshow that both the C(5,0) and C(3,3) tubes undergo above room temperature a\nPeierls transition mediated by an acoustical long-wavelength and an optical\n$q=2k_F$ phonons respectively. In the armchair geometry, we verify that the\nelectron-phonon coupling parameter $\\lambda$ originates mainly from phonons at\n$q=2k_F$ and is strongly enhanced when the diameter decreases. These results\nquestion the origin of superconductivity in small diameter nanotubes.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "A new kind of vortex pinning in superconductor / ferromagnet\n  nanocomposites: This paper reports the observation of hysteresis in the vortex pinning in a\nsuperconductor / ferromagnetic epitaxial nanocomposite consisting of fcc Gd\nparticles incorporated in a Nb matrix. We show that this hysteretic pinning is\nassociated with magnetic reversal losses in the Gd particles and is\nfundamentally different in origin to pinning interactions previously observed\nfor ferromagnetic particles or other microstructural features.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The effects of Zn Impurity on the Properties of Doped Cuprates in the\n  Normal State: We study the interplay of quantum impurity, and collective spinon and holon\ndynamics in Zn doped high-T$_c$ cuprates in the normal state. The\ntwo-dimensional t-t$^{\\prime}$-J models with one and a small amount of Zn\nimpurity are investigated within a numerical method based on the double-time\nGreen function theory. We study the inhomogeneities of holon density and\nantiferromagnetic correlation background in cases with different Zn\nconcentrations, and obtain that doped holes tend to assemble around the Zn\nimpurity with their mobility being reduced. Therefore a bound state of holon is\nformed around the nonmagnetic Zn impurity with the effect helping Zn to\nintroduce local antiferromagnetism around itself. The incommensurate peaks we\nobtained in the spin structure factor indicate that Zn impurities have effects\non mixing the q=($\\pi$, $\\pi$) and q=0 components in spin excitations.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Odd triplet superconductivity in clean and moderately disordered SFFS\n  junctions: We study the Josephson effect and pairing correlations in SFFS junctions that\nconsist of conventional superconductors (S) connected through two metallic\nmonodomain ferromagnets (F) with transparent and spin inactive interfaces. We\nsolve the Eilenberger equations for arbitrary relative orientation of\nmagnetizations of the two F layers in the clean limit and for moderate disorder\nin ferromagnets. Spatial variation of pair amplitudes, singlet $f_s$, and odd\nin frequency triplet $f_{t0}$ and $f_{t1}$, with 0 and $\\pm1$ spin projections,\nas well as the Josephson current-phase relations are calculated for different\nvalues of the ferromagnetic layers thickness and angle $\\alpha$ between\nin-plane magnetizations. In contrast to the dirty limit case, we find that for\n$0<\\alpha<\\pi$ both spin singlet and triplet pair amplitudes in F layers\npower-law decay in the same oscillatory manner with distance from the FS\ninterfaces. This decay gets faster as the impurity-scattering rate in\nferromagnets is increased. The computed triplet amplitude $f_{t1}$ has opposite\nsigns in the two magnet regions, penetrates into the superconductors and\nmonotonically decays over the same distance, which is the superconducting\ncoherence length, as the singlet amplitude $f_s$ saturates to the bulk value.\nWe point out that influence of misorientation of magnetizations on the\nJosephson current can not be attributed directly to the appearance of odd\ntriplet correlations.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Double-dome superconductivity under pressure in the V-based Kagome\n  metals AV3Sb5 (A = Rb and K): We present high-pressure electrical transport measurements on the newly\ndiscovered V-based superconductors $A$V$_3$Sb$_5$ ($A$ = Rb and K), which have\nan ideal Kagome lattice of vanadium. Two superconducting domes under pressure\nare observed in both compounds, as previously observed in their sister compound\nCsV$_3$Sb$_5$. For RbV$_3$Sb$_5$, the $T_c$ increases from 0.93 K at ambient\npressure to the maximum of 4.15 K at 0.38 GPa in the first dome. The second\nsuperconducting dome has the highest $T_c$ of 1.57 K at 28.8 GPa. KV$_3$Sb$_5$\ndisplays a similar double-dome phase diagram, however, its two maximum $T_c$s\nare lower, and the $T_c$ drops faster in the second dome than RbV$_3$Sb$_5$. An\nintegrated temperature-pressure phase diagram of $A$V$_3$Sb$_5$ ($A$ = Cs, Rb\nand K) is constructed, showing that the ionic radius of the intercalated\nalkali-metal atoms has a significant effect. Our work demonstrates that\ndouble-dome superconductivity under pressure is a common feature of these\nV-based Kagome metals.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Numerical extension of the power law Jc(B) to zero field in thin\n  superconducting films: Numerical simulations of the current and field distribution in thin\nsuperconducting films are carried out for a given material law Jc(B) and as a\nfunction of the applied field H, taking the sample's self-field into account.\nThe dependence of the critical current density on the applied field Jc(H) is\ncomputed for comparison with experiment, considering the geometry of transport\nmeasurements.\n  We show that extrapolating the high field power law Jc \\propto B^{-\\alpha} to\nthe lowest fields results in a finite critical current at zero applied field\nJc(H=0), despite the singularity of Jc(B). Moreover, particular features of the\nexperiment, such as a low field plateau in Jc(H), are reproduced and found to\nbe determined by the self-field.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Impact of the transport supercurrent on the Josephson effect: We study the weak link between current-carrying superconductors, both\nconventional and d-wave. The state of the system is controlled by two\nparameters: the order parameter phase difference $\\phi$ and the superfluid\nvelocity $v_{s}$, which parameterizes the parallel to the boundary transport\nsupercurrent which is injected externally. The low-temperature current-phase\nrelations are derived. We consider two models of weak links: a constriction\nbetween two conventional superconductors and a plane boundary between two\ndifferently orientated d-wave superconductors. We show that for some relation\nbetween $\\phi $ and $v_{s}$ quasiparticles create the current along the\nboundary which flows in the direction opposite to the transport supercurrent.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "An attempt at a resonating mean-field theoretical description of thermal\n  behavior of two-gap superconductivity: The resonating mean-field theory (Res-MFT) has been applied and shown to\neffectively describe two-gap superconductivity (SC). Particularly at T=0 using\na suitable chemical potential, the two-gap SC in MgB2 has been well described\nby the Res-Hartree-Bogoliubov theory (Res-HBT). The Res-HB ground state\ngenerated with HB wave functions almost exhausts the ground-state correlation\nenergy in all the correlation regimes. In this paper we make an attempt at a\nRes-MF theoretical description of thermal behavior of the two-gap SC. In an\nenergy-gap case we find a new formula leading to a higher Tc than the Tc of the\nusual HB formula.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Diboride compounds doped with transition metals$\\unicode{x2013}$a route\n  to superconductivity through structure stabilization as well as defects: Recent investigations into MoB$_{2}$ have unveiled a direct connection\nbetween a pressure-induced structural transition to a P6/mmm space group\nstructure and the emergence of superconductivity, producing critical\ntemperatures up to 32 K at 100 GPa. This pressure-induced superconducting state\nunderscores the potential of doped MoB$_{2}$ as a possible candidate for\nmetastable superconductivity at ambient pressure. In this work, we demonstrate\nthat doping by Zr, Hf, or Ta stabilizes the P6/mmm structure at ambient\npressure and results in the realization of a superconducting state with\ncritical temperatures ranging from 2.4 up to 8.5 K depending on the specific\ndoping. We estimate the electron-phonon coupling $\\lambda$ and the density of\nstates based on resistivity and specific heat data, finding that $\\lambda$\nranges from 0.4 - 0.6 for these compounds. Finally, to investigate the role of\npossible metastable defect structures on the critical temperature, we analyze\nMoB$_{2}$, MoB$_{2.5}$, and Nb/Zr-doped MoB$_{2}$ using rapid cooling\ntechniques. Notably, splat-quenching produces samples with higher critical\ntemperatures and even retains superconductivity in MoB$_{2}$ at ambient\npressure, achieving a critical temperature of 4.5 K.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Phase Separation, Competition, and Volume Fraction Control in\n  NaFe$_{1-x}$Co$_x$As: We report a detailed nuclear magnetic resonance (NMR) study by combined\n$^{23}$Na and $^{75}$As measurements over a broad range of doping to map the\nphase diagram of NaFe$_{1-x}$Co$_x$As. In the underdoped regime ($x \\le$\n0.017), we find a magnetic phase with robust antiferromagnetic (AFM) order,\nwhich we denote the {\\it s}-AFM phase, cohabiting with a phase of weak and\npossibly proximity-induced AFM order ({\\it w}-AFM) whose volume fraction $V\n\\simeq 8$\\% is approximately constant. Near optimal doping, at $x = 0.0175$, we\nobserve a phase separation between static antiferromagnetism related to the\n{\\it s}-AFM phase and a paramagnetic (PM) phase related to {\\it w}-AFM. The\nvolume fraction of AFM phase increases upon cooling, but both the N{\\'e}el\ntemperature and the volume fraction can be suppressed systematically by\napplying a $c$-axis magnetic field. On cooling below $T_c$, superconductivity\noccupies the PM region and its volume fraction grows at the expense of the AFM\nphase, demonstrating a phase separation of the two types of order based on\nvolume exclusion. At higher dopings, static antiferromagnetism and even\ncritical AFM fluctuations are completely suppressed by superconductivity. Thus\nthe phase diagram we establish contains two distinct types of phase separation\nand reflects a strong competition between AFM and superconducting phases both\nin real space and in momentum space. We suggest that both this strict mutual\nexclusion and the robustness of superconductivity against magnetism are\nconsequences of the extreme two-dimensionality of NaFeAs.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Topological superconducting phases and Josephson effect in curved\n  time-reversal-invariant superconductors: We consider a Rashba spin-orbit coupled nanowire with anisotropic\nspin-singlet superconducting pairing and time-reversal-invariant symmetry. We\nexplore the evolution of the topological superconducting phases of this system\ndue to geometric deformations for the representative case of a wire bent in a\nsemielliptical shape. We find that when the system is in its topological\nsuperconducting phase, strong inhomogeneities in the profile curvature can\nproduce a pair of localized eigenmodes, which can be attributed to a nonuniform\ntopological phase. The curved geometric profile also allows to tune the spin\ncorrelations of the superconducting state via the induced inhomogeneity of the\nspin-orbit coupling (SOC). The geometric control of the superconducting pair\ncorrelations allows to manipulate the critical current in Josephson junctions\nmade up of two time reversal invariant topological superconductors separated by\na spin-orbit coupled normal metal. In particular, we find that the curvature\ninhomogeneity can be exploited for amplifying the current intensity, but also\nto generate a $0-\\pi$ transition, and a second harmonic contribution, which\ngenerates, for some specific geometric configurations, a $\\varphi$-junction\nbehavior.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pressure effects on the electronic structure and magnetic properties of\n  infinite-layer nickelates: Motivated by the discovery of superconductivity in infinite-layer nickelates\nRNiO$_2$ (R= rare-earth), and the subsequent enhancement of their T$_c$ with\npressure, we investigate the evolution of the electronic structure and magnetic\nproperties of this family of materials via first-principles calculations\nemploying hydrostatic and chemical pressure as tuning knobs. Overall, our\nanalysis shows that pressure tends to increase the R-$5d$ self-doping effect,\nas well as the Ni-$d _{x^{2}-y^{2}}$ bandwidth, the $e_g$ energy splitting, the\ncharge transfer energy, and the superexchange ($J$). Using the energy scale of\n$J$ as a predictor of superconducting tendencies, we anticipate that pressure\ncan indeed be a feasible means to further increase the T$_c$ in this family of\nmaterials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Interplay between atomic fluctuations and charge density waves in\n  La$_{2-x}$Sr$_{x}$CuO$_{4}$: In the cuprate superconductors, the spatial coherence of the charge density\nwave (CDW) state grows rapidly below a characteristic temperature\n$T_\\mathrm{CDW}$, the nature of which is debated. We have combined a set of\nx-ray scattering techniques to study La$_{1.88}$Sr$_{0.12}$CuO$_{4}$\n($T_\\mathrm{CDW}$~$\\approx$~80\\,K) to shed light on this discussion. We observe\nthe emergence of a crystal structure, which is consistent with the CDW\nmodulation in symmetry, well above $T_\\mathrm{CDW}$. This global structural\nchange also induces strong fluctuations of local atomic disorder in the\nintermediate temperature region. At $T_\\mathrm{CDW}$, the temperature\ndependence of this structure develops a kink, while the atomic disorder is\nminimized. We find that the atomic relaxation dynamics cross over from a\ncooperative to an incoherent response at $T_\\mathrm{CDW}$. These results reveal\na rich interplay between the CDWs and atomic fluctuations of distinct\nspatio-temporal scales. For example, the CDW coherence is enhanced on\nquasi-elastic timescales by incoherent atomic relaxation.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Compartmentalizing the cuprate strange metal: It has long been recognized that the key to unlocking the mystery of cuprate\nhigh-Tc superconductivity lies in understanding the anomalous normal state from\nwhich pairs form and condense. While many of its defining properties have been\nidentified, they are often considered either at a singular doping level or as\nan isolated phenomenon as a function of doping. As a result, their relation to\neach other and to the pseudogap (PG), strange metal (SM) and\nnon-superconducting (non-SC) regimes that define the cuprate phase diagram has\nyet to be elucidated. Here, we report a high-field in-plane MR study on several\ncuprate families spanning all 3 regimes that reveal a complex yet nonetheless\nsystematic evolution of the form of the MR, with each regime possessing its own\ndistinct scaling behavior. In the PG regime, the MR exhibits pure H/T^2 scaling\nat low fields and H-linearity at the highest field strengths. While the\nH-linearity persists inside the SM regime, the scaling changes abruptly to H/T.\nThe size of the H-linear slope, meanwhile, is found to be correlated with both\nthe T-linear resistivity coefficient and Tc, strengthening the characterization\nof the SM regime as a quantum critical phase. We interpret the omnipresence of\nH-linear MR across both regimes as a signature of highly anisotropic, possibly\ndiscontinuous features on the Fermi surface. Finally, within the non-SC,\nFermi-liquid regime, we observe a recovery of conventional Kohler scaling. This\ncomprehensive study establishes the distinct nature of the magnetotransport\nwithin each regime and identifies power-law scaling of the normal state MR as a\ndefining feature of SC hole-doped cuprates. The incompatibility of such\npower-law scaling with any known variant of Boltzmann transport theory\nmotivates the quest for an altogether new theoretical framework, one in which\nthe MR is entirely decoupled from elastic impurity scattering.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Violation of Kohler rule in Ta2PdTe6 and absence of same in Nb2PdS5- A\n  high field magneto transport study: Here, we present the comparative study of magnetotransport properties of\nrecently discovered Ta2PdTe6 and Nb2PdS5 superconductors. The XRD and\nmagnetotransport measurements are performed on these samples to investigate\nstructure and superconducting properties as well as normal state transport\nproperties of these compounds. Both the compounds are crystallized in\nmonoclinic structure within space group C2m. Here, we observe superconductivity\nin both the compounds Ta2PdTe6 (Tc =4.4 K) and Nb2PdS5 (Tc =6.6 K). We see a\nlinear magnetoresistance in Ta2PdTe6 as well as violation of Kohler rule in\nsame compound. On the other hand, we find the absence of same in Nb2PdS5\ncompound.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Why the lowest Landau level approximation works in strongly type II\n  superconductors: Higher than the lowest Landau level contributions to magnetization and\nspecific heat of superconductors are calculated using Ginzburg - Landau\nequations approach. Corrections to the excitation spectrum around solution of\nthese equations (treated perturbatively) are found. Due to symmetries of the\nproblem leading to numerous cancellations the range of validity of the LLL\napproximation in mean field is much wider then a naive range and extends all\nthe way down to $H = {H_{c2}(T)}/13$. Moreover the contribution of higher\nLandau levels is significantly smaller compared to LLL than expected naively.\nWe show that like the LLL part the lattice excitation spectrum at small\nquasimomenta is softer than that of usual acoustic phonons. This enhanses the\neffect of fluctuations. The mean field calculation extends to third order,\nwhile the fluctuation contribution due to HLL is to one loop. This complements\nthe earlier calculation of the LLL part to two loop order.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "A Model to Describe Transport Properties in\n  $Bi_2Sr_2(Ca_zPr_{1-z})Cu_2O_{8+y}$: A pseudo-spin model is proposed, as a means to describe some transport\nproperties (resistivity and Hall mobility) in\n$Bi_2Sr_2(Ca_zPr_{1-z})Cu_2O_{8+y}$. Our model is based in a double-well\npotential where tunneling in a given site and interaction between different\nlattice sites are allowed only through the excited states. Doping of the pure\nsystem by the addition of $Pr$ increases the ratio between the activation\nenergy and the tunneling constant. The model Hamiltonian displays some features\nwhich are present in the hydrogen-bonded ferroelectrics. Its dynamics is\ntreated in the random phase approximation and the characteristic frequency\n(time) is used in a Drude formula in order to obtain some transport properties\nof the system, namely the electric resistivity and the Hall mobility. The\nquantities calculated in this work are compared with the experimental data of\nB. Beschoten, S. Sadewasser, G. G\\\"{u}ntherodt and C. Quitmann [Phys. Rev.\nLett.77, 1837(1996)].",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Laser beam focusing by layered superconductor tuned by DC magnetic field: We develop a theory of the propagation and focusing of the THz Gaussian laser\nbeam through the layered superconductor slab of finite thickness in the\npresence of an external DC magnetic field in a nonlinear regime. We show that,\nin this case, focusing of radiation, which results from the specific\nnonlinearity of the medium, can be flexibly tuned by the external magnetic\nfield providing a new way to control THz waves. We analytically study the main\ncharacteristics of the Gaussian beam, its waist, and the focusing distance as\nthe functions of wave frequency, amplitude, and the external magnetic field.\nThe results of analytic calculations are supported by the numerical simulation\nof the electromagnetic field distribution in the slab.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Strongly correlated s-wave pairing in the n-type infinite-layer cuprate: Quasiparticle tunneling spectra of the electron-doped (n-type) infinite-layer\ncuprate Sr_{0.9}La_{0.1}CuO_2 reveal characteristics that counter a number of\ncommon phenomena in the hole-doped (p-type) cuprates. The optimally doped\nSr_{0.9}La_{0.1}CuO_2 with T_c = 43 K exhibits a momentum-independent\nsuperconducting gap \\Delta = 13.0 +- 1.0 meV that substantially exceeds the BCS\nvalue, and the spectral characteristics indicate insignificant quasiparticle\ndamping by spin fluctuations and the absence of pseudogap. The response to\nquantum impurities in the Cu-sites also differs fundamentally from that of the\np-type cuprates with d_{x^2-y^2}-wave pairing symmetry.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Corrections to the Higgs Mode Masses in Superfluid 3He: Superfluid 3He has a rich spectrum of collective modes with both massive and\nmassless excitations. The masses of these modes can be precisely measured using\nacoustic spectroscopy and fit to theoretical models. Prior comparisons of the\nexperimental results with theory did not include strong-coupling effects beyond\nthe weak-coupling-plus BCS model, so-called non-trivial strong-coupling\ncorrections. In this work we utilize recent strong-coupling calculations to\ndetermine the Higgs masses and find consistency between experiments that relate\nthem to a sub-dominant $f$-wave pairing strength.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductor strip with transport current: Magneto-optical study of\n  current distribution and its relaxation: The dynamics of magnetic flux distributions across a YBaCuO strip carrying\ntransport current is measured using magneto-optical imaging at 20 K. The\ncurrent is applied in pulses of 40-5000 ms duration and magnitude close to the\ncritical one, 5.5 A. During the pulse some extra flux usually penetrates the\nstrip, so the local field increases in magnitude. When the strip is initially\npenetrated by flux, the local field either increases or decreases depending\nboth on the spatial coordinate and the current magnitude. Meanwhile, the\ncurrent density always tends to redistribute more uniformly. Despite the\nrelaxation, all distributions remain qualitatively similar to the Bean model\npredictions.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quantum liquid crystals in imbalanced Fermi gas: fluctuations and\n  fractional vortices in Larkin-Ovchinnikov states: We develop a low-energy model of a unidirectional Larkin-Ovchinnikov (LO)\nstate. Because the underlying rotational and translational symmetries are\nbroken spontaneously, this gapless superfluid is a smectic liquid crystal, that\nexhibits fluctuations that are qualitatively stronger than in a conventional\nsuperfluid, thus requiring a fully nonlinear description of its Goldstone\nmodes. Consequently, at nonzero temperature the LO superfluid is an algebraic\nphase even in 3d. It exhibits half-integer vortex-dislocation defects, whose\nunbinding leads to transitions to a superfluid nematic and other phases. In 2d\nat nonzero temperature, the LO state is always unstable to a charge-4 nematic\nsuperfluid. We expect this superfluid liquid-crystal phenomenology to be\nrealizable in imbalanced resonant Fermi gases trapped isotropically.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Effects of Magnetic and non-Magnetic Impurities on the Superconducting\n  State of YBa$_{2}$Cu$_{3}$O$_{7-\u03b4}$: We report Electronic Raman Scattering measurements on optimally doped Zn and\nNi substituted YBCO in A$_{1g}$ and B$_{1g}$ channels. We show that the\nsuperconducting gap energy is independant of magnetic Ni and non-magnetic Zn\nsubstitutions. On the contrary the collective A$_{1g}$ mode follows the\ncritical temperature $T_c$ with two distinct slopes for Ni and Zn, and tracks\nthe magnetic resonance seen in inelastic neutron scattering. We explain the\nunconventionnal energy dependence of the superconducting gap and discuss the\nbehaviour of the A$_{1g}$ mode within magnetic and non-magnetic impurities.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Low-Temperature Rapid Synthesis and Superconductivity of Fe-Based\n  Oxypnictide Superconductors: we were able to develop a novel method to synthesize Fe-based oxypnictide\nsuperconductors. By using LnAs and FeO as the starting materials and a\nball-milling process prior to solid-state sintering, Tc as high as 50.7 K was\nobtained with the sample of Sm 0.85Nd0.15FeAsO0.85F0.15 prepared by sintering\nat temperatures as low as 1173 K for times as short as 20 min.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Robust creation of entangled states of two coupled flux qubits via\n  dynamic control of the transition frequencies: Coherent control and the creation of entangled states are discussed in a\nsystem of two superconducting flux qubits interacting with each other through\ntheir mutual inductance and identically coupling to a reservoir of harmonic\noscillators. We present different schemes using continuous-wave control fields\nor Stark-chirped rapid adiabatic passages, both of which rely on a dynamic\ncontrol of the qubit transition frequencies via the external bias flux in order\nto maximize the fidelity of the target states. For comparison, also special\narea pulse schemes are discussed. The qubits are operated around the optimum\npoint, and decoherence is modelled via a bath of harmonic oscillators. As our\nmain result, we achieve controlled robust creation of different Bell states\nconsisting of the collective ground and excited state of the two-qubit system.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Experimental study of intrinsic multiple Andreev reflections effect in\n  GdO(F)FeAs superconductor array junctions: We report the first observation of the intrinsic multiple Andreev reflections\neffect (IMARE) in S-n-S-...-S-arrays (S = superconductor, n = normal metal)\nformed by \"break-junction\" technique in GdO(F)FeAs superconductor (Tc = 48 - 53\nK). We show that superconducting gap peculiarities at dI/dV-spectra sharpen\ndramatically in the arrays as compared with that in the single-contact spectra;\nthis enables to improve significantly accuracy of the bulk superconducting\nparameters determination. Using IMARE, we determined the large and the small\ngap values \\Delta_L = 11 +- 1.1 meV and \\Delta_S = 2.6 +- 0.4 meV. The\nBCS-ratio 2\\Delta_L/kTc^{local} = 5.0 - 5.9 > 3.52 (Tc^{local} is the contact\narea critical temperature) evidences for a strong electron-boson coupling. The\nresults obtained agree well with our previous data by Andreev spectroscopy for\nsingle SnS-contacts.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Non-Fermi liquid behavior due to U(1) gauge field in two dimensions: We study the damping rate of massless Dirac fermions due to the U(1) gauge\nfield in (2+1)-dimensional quantum electrodynamics. In the absence of a Maxwell\nterm for the gauge field, the fermion damping rate\n$\\mathrm{Im}\\Sigma(\\omega,T)$ is found to diverge in both perturbative and\nself-consistent results. In the presence of a Maxwell term, there is still\ndivergence in the perturbative results for $\\mathrm{Im}\\Sigma(\\omega,T)$. Once\nthe Maxwell term is included into the self-consistent equations for fermion\nself-energy and vacuum polarization functions, the fermion damping rate is free\nof divergence and exhibits non-Fermi liquid behavior:\n$\\mathrm{Im}\\Sigma(\\omega,T) \\propto \\mathrm{max}(\\sqrt{\\omega},\\sqrt{T})$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Observation of diamagnetic strange-metal phase in sulfur-copper codoped\n  lead apatite: By codoping sulfur and copper into lead apatite, the crystal grains are\ndirectionally stacked and the room-temperature resistivity is reduced from\ninsulating to $2\\times10^{-5}~\\Omega\\cdot$m. The resistance-temperature curve\nexhibits a nearly linear relationship suggesting the presence of strange-metal\nphase. A possible Meissner effect is observed in dc magnetic measurements. An\neven-in-field transverse voltage is found indicating the possible contribution\nof individual vortex dynamics. A clear pathway towards superconductivity in\nthis material is subsequently benchmarked.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Ac$_3$Ni$_2$O$_7$ and La$_2$$Ae$Ni$_2$O$_6$F ($Ae$ = Sr, Ba): Benchmark\n  Materials for Bilayer Nickelate Superconductivity: We theoretically propose Ac$_3$Ni$_2$O$_7$, La$_2$BaNi$_2$O$_6$F, and\nLa$_2$SrNi$_2$O$_6$F compounds to be benchmark materials for bilayer nickelate\nsuperconductivity. The stable phase of Ac$_3$Ni$_2$O$_7$ and\nLa$_2$BaNi$_2$O$_6$F are found to be $I4/mmm$ without the lattice distortion\ncaused by octahedra rotation at ambient pressure, where as the lattice\ndistortion in La$_2$SrNi$_2$O$_6$F can be suppressed with relatively small\nexternal pressure of 4 GPa. The magnetism, electronic structure and spin\nsusceptibilities of Ac$_3$Ni$_2$O$_7$ are extremely close to those of\nLa$_3$Ni$_2$O$_7$ at 30 GPa. The ground state of La$_2$BaNi$_2$O$_6$F and\nLa$_2$SrNi$_2$O$_6$F are antiferromagnetically coupled checkerboard bilayer\nwith sizable magnetic moment on Ni. In addition, the inter-layer coupling\n$J_{\\perp}$ between Ni-bilayers in La$_2$BaNi$_2$O$_6$F or La$_2$SrNi$_2$O$_6$F\nis only $\\sim$ 1/10 of that in Ac$_3$Ni$_2$O$_7$ or La$_3$Ni$_2$O$_7$ at 30\nGPa. We argue that these compounds may serve as superconducting candidates at\nambient pressure and can be employed to testify theoretical proposals for\nbilayer nickelate superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Majorana bound states in vortex lattices on iron-based superconductors: Majorana quasi-particles may arise as zero-energy bound states in vortices on\nthe surface of a topological insulator that is proximitized by a conventional\nsuperconductor. Such a system finds its natural realization in the iron-based\nsuperconductor FeTe$_{0.55}$Se$_{0.45}$ that combines bulk $s$-wave pairing\nwith spin helical Dirac surface states, and which thus comprises the\ningredients for Majorana modes in absence of an additional proximitizing\nsuperconductor. In this work, we investigate the emergence of Majorana vortex\nmodes and lattices in such materials depending on parameters like the magnetic\nfield strength and vortex lattice disorder. A simple 2D square lattice model\nhere allows us to capture the basic physics of the underlying materials system.\nTo address the problem of disordered vortex lattice, which occurs in real\nsystems, we adopt the technique of the singular gauge transformation which we\nmodify such that it can be used in a system with periodic boundary conditions.\nThis approach allows us to go to larger vortex lattices than otherwise\naccessible, and is successful in replicating several experimental observations\nof Majorana vortex bound states in the FeTe$_{0.55}$Se$_{0.45}$ platform.\nFinally it can be related to a simple disordered Majorana lattice model that\nshould be useful for further investigations on the role of interactions, and\ntowards topological quantum computation.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The Distribution of the Energy Gap and Josephson IcRn Product in Bi2212\n  by Tunneling Spectroscopy: We present direct measurements of the density of states by tunneling\nspectroscopy on slightly overdoped Bi2212 single crystals at low temperature\nusing break-junction and point-contact techniques. We find that (i) the\nvariation of the gap magnitude, Delta, between 20 and 36 meV is likely to be\nintrinsic to the Bi2212, and (ii) there is a correlation between the maximum\nvalue of the Josephson IcRn product and the gap magnitude: IcRn decreases with\nthe increase of Delta. The maximum IcRn value of 26 mV is observed at Delta =\n20.5 meV. For Delta = 36.5 meV, the maximum measured value of IcRn is 7.3 mV.\nWe conclude that (i) the distribution of the Josephson IcRn product as a\nfunction of gap magnitude can not be explained by the presence of a single\nenergy gap in Bi2212, and (ii) the coherence energy scale in Bi2212 has the\nmaximum Josephson strength.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Chiral-Flux-Phase-Based Topological Superconductivity in Kagome Systems\n  with Mixed Edge Chiralities: Recent studies have attracted intense attention on the quasi-2D kagome\nsuperconductors $ A\\text{V}_3\\text{Sb}_5 $ ($ A = $ K, Rb, and Cs) where the\nunexpected chiral flux phase (CFP) associates with the spontaneous\ntime-reversal symmetry breaking in charge density wave (CDW) states. Here,\ncommencing from the 2-by-2 CDW phases, we bridge the gap between topological\nsuperconductivity (TSC) and time-reversal asymmetric CFP in kagome systems.\nSeveral chiral TSC states featuring distinct Chern numbers emerge for an s-wave\nor a d-wave superconducting pairing symmetry. Importantly, these CFP-based TSC\nphases possess unique gapless edge modes with mixed chiralities (i.e., both\npositive and negative chiralities), but with the net chiralities consistent\nwith the Bogoliubov-de Gennes Chern numbers. We further study the transport\nproperties of a two-terminal junction, using Chern insulator or normal metal\nleads via atomic Green's function method with Landauer-B\\\"uttiker formalism. In\nboth cases, the normal electron tunneling and the crossed Andreev reflection\noscillate as the chemical potential changes, but together contribute to plateau\ntransmissions (1 and 3/2, respectively). These behaviors can be regarded as the\nsignature of a topological superconductor hosting edge states with mixed\nchiralities.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity and multiple pressure-induced phases in BaPt$_2$As$_2$: The newly discovered BaPt$_2$As$_2$ shows a structural distortion at around\n275~K, followed by the emergence of superconductivity at lower temperatures.\nHere we identify the presence of charge density wave (CDW) order at room\ntemperature and ambient pressure using single crystal x-ray diffraction, with\nboth a superlattice and an incommensurate modulation, where there is a change\nof the superlattice structure below $\\simeq$ 275~K. Upon applying pressure,\nBaPt$_2$As$_2$ shows a rich temperature-pressure phase diagram with multiple\npressure-induced transitions at high temperatures, the emergence or\ndisappearance of which are correlated with sudden changes in the\nsuperconducting transition temperature $T_c$. These findings demonstrate that\nBaPt$_2$As$_2$ is a promising new system for studying competing interactions\nand the relationship between high-temperature electronic instabilities and\nsuperconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Remote Sensing and Control of Phase Qubits: We demonstrate a remote sensing design of phase qubits by separating the\ncontrol and readout circuits from the qubit loop. This design improves\nmeasurement reliability because the control readout chip can be fabricated\nusing more robust materials and can be reused to test different qubit chips.\nTypical qubit measurements such as Rabi oscillations, spectroscopy, and\nexcited-state energy relaxation are presented.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Particle-hole symmetry breaking in the pseudogap state of\n  Pb0.55Bi1.5Sr1.6La0.4CuO6+d: A quantum-chemical perspective: Two Bi2201 model systems are employed to demonstrate how, beside the Cu-O\n\\sigma-band, a second band of purely O2p\\pi character can be made to cross the\nFermi level owing to its sensitivity to the local crystal field. This result is\nemployed to explain the particle-hole symmetry breaking across the pseudo-gap\nrecently reported by Shen and co-workers, see M. Hashimoto et al., Nature\nPhysics 6, (2010) 414. Support for a two-bands-on-a-checkerboard candidate\nmechanism for High-Tc superconductivity is claimed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Dynamic spin-triplet order induced by alternating electric fields in\n  superconductor-ferromagnet-superconductor Josephson junctions: Dynamic states offer extended possibilities to control the properties of\nquantum matter. Recent efforts are focused on studying the ordered states which\nappear exclusively under the time-dependent drives. Here we demonstrate a class\nof systems which feature dynamic spin-triplet superconducting order stimulated\nby the alternating electric field. The effect is based on the interplay of\nferromagnetism, interfacial spin-orbital coupling (SOC) and the condensate\nmotion driven by the field, which converts hidden static p-wave order, produced\nby the joint action of the ferromagnetism and the SOC, into dynamical s-wave\nequal-spin triplet correlations. We demonstrate that the critical current of\nJosephson junctions hosting these states is proportional to the electromagnetic\npower, supplied either by the external irradiation or by the ac current source.\nBased on these unusual properties we propose the scheme of a Josephson\ntransistor which can be switched by the ac voltage and demonstrates an\neven-numbered sequence of Shapiro steps. Combining the photo-active Josephson\njunctions with recently discovered Josephson phase batteries we find\nphoto-magnetic SQUID devices which can generate spontaneous magnetic fields\nwhile being exposed to irradiation.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Bound States of Defects in Superconducting LiFeAs Studied by Scanning\n  Tunneling Spectroscopy: Defects in LiFeAs are studied by scanning tunneling microscopy (STM) and\nspectroscopy (STS). Topographic images of the five predominant defects allow\nthe identification of their position within the lattice. The most commonly\nobserved defect is associated with an Fe site and does not break the local\nlattice symmetry, exhibiting a bound state near the edge of the smaller gap in\nthis multi-gap superconductor. Three other common defects, including one also\non an Fe site, are observed to break local lattice symmetry and are\npair-breaking indicated by clear in-gap bound states, in addition to states\nnear the smaller gap edge. STS maps reveal complex, extended real-space bound\nstate patterns, including one with a chiral distribution of the local density\nof states (LDOS). The multiple bound state resonances observed within the gaps\nand at the inner gap edge are consistent with theoretical predictions for\ns$^{\\pm}$ gap symmetry proposed for LiFeAs and other iron pnictides.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Anomalous Electronic Susceptibility in Bi2sr2cuo6+d: We report magnetic susceptibility performed on overdoped Bi2Sr2CuO6+d powders\nas a function of oxygen doping d and temperature T. The decrease of the spin\nsusceptibility chis with increasing T is confirmed. At sufficient high\ntemperature, chis presents an unusual linear temperature dependence chis=chis0\n-chi1T . Moreover, a linear correlation between chi1 and chis0 for increasing\nhole concentration has been displayed. These non conventional metal features\nwill be discussed in terms of a singular narrow-band structures.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "NMR Investigation of the Quasi One-dimensional Superconductor\n  K$_{2}$Cr$_{3}$As$_{3}$: We report $^{75}$As NMR measurements on the new quasi one-dimensional\nsuperconductor K$_{2}$Cr$_{3}$As$_{3}$ ($T_{c} \\sim 6.1$~K) [J.\\ K.\\ Bao et\nal., Phys. Rev. X {\\bf 5}, 011013 (2015)]. We found evidence for strong\nenhancement of Cr spin fluctuations above $T_c$ in the\n[Cr$_{3}$As$_{3}$]$_{\\infty}$ double-walled subnano-tubes based on the nuclear\nspin-lattice relaxation rate $1/T_{1}$. The power law temperature dependence,\n$1/T_{1}T \\sim T^{-\\gamma}$ ($\\gamma \\sim 0.25$), is consistent with the\nTomonaga-Luttinger liquid. Moreover, absence of the Hebel-Slichter coherence\npeak of $1/T_{1}$ just below $T_{c}$ suggests unconventional nature of\nsuperconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Effects of strain on the electronic structure, superconductivity, and\n  nematicity in FeSe studied by angle-resolved photoemission spectroscopy: One of central issues in iron-based superconductors is the role of structural\nchange to the superconducting transition temperature (T_c). It was found in\nFeSe that the lattice strain leads to a drastic increase in T_c, accompanied by\nsuppression of nematic order. By angle-resolved photoemission spectroscopy on\ntensile- or compressive-strained and strain-free FeSe, we experimentally show\nthat the in-plane strain causes a marked change in the energy overlap\n(DeltaE_{h-e}) between the hole and electron pockets in the normal state. The\nchange in DeltaE_{h-e} modifies the Fermi-surface volume, leading to a change\nin T_c. Furthermore, the strength of nematicity is also found to be\ncharacterized by DeltaE_{h-e}. These results suggest that the key to\nunderstanding the phase diagram is the fermiology and interactions linked to\nthe semimetallic band overlap.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Vortex-antivortex nucleation in magnetically nanotextured\n  superconductors: Magnetic-field-driven and thermal scenarios: Within the Ginzburg-Landau formalism, we predict two novel mechanisms of\nvortex-antivortex nucleation in a magnetically nanostructured superconductor.\nAlthough counterintuitive, nucleation of vortex-antivortex pairs can be\nactivated in a superconducting (SC) film covered by arrays of submicron\nferromagnets (FMs) when exposed to an external homogeneous magnetic field. In\nanother scenario, we predict the thermal induction of vortex-antivortex\nconfigurations in SC/FM samples. This phenomenon leads to a new type of\nLittle-Parks oscillations of the FM magnetization-temperature phase boundary of\nthe superconducting film.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Enhanced Superconductivity on the Tetragonal Lattice in FeSe under\n  Hydrostatic Pressure: Superconductivity under pressure in FeSe ($T_{\\rm c}$$\\sim$7.5 K) has been\ninvestigated using single-crystal specimens through the measurements of DC\nmagnetization and electrical resistivity. A characteristic three-step increase\nin $T_{\\rm c}$ has been found under hydrostatic pressure up to $\\sim$34 K above\n7 GPa. The structural transition from a tetragonal phase to an orthorhombic\nphase ($T_{\\rm s}$$\\sim$87 K) is found to disappear at $P$$\\sim$2.3 GPa, above\nwhich $T_{\\rm c}$ increases rapidly, suggesting that the superconductivity is\nenhanced by the tetragonal environment. Under non-hydrostatic pressure, the\nincrease in $T_{\\rm c}$ is suppressed and the superconductive volume fraction\nis considerably reduced above 2 GPa, probably owing to the breaking of the\ntetragonal lattice symmetry by the uniaxial stress. The intimate correlation\nbetween the enhanced (suppressed) superconductivity and the tetragonality\n(orthorhombicity) in the phase diagram is a common feature of FeSe and other\niron-pnictide superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Triplanar Model for the Gap and Penetration Depth in YBCO: YBaCuO_7 is a trilayer material with a unit cell consisting of a CuO_2\nbilayer with a CuO plane of chains in between. Starting with a model of\nisolated planes coupled through a transverse matrix element, we consider the\npossibility of intra as well as interplane pairing within a nearly\nantiferromagnetic Fermi liquid model. Solutions of a set of three coupled BCS\nequations for the gap exhibit orthorhombic symmetry with s- as well as d-wave\ncontributions. The temperature dependence and a-b in plane anisotropy of the\nresulting penetration depth is discussed and compared with experiment.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Reduction of the electrodynamics of superconductors to those for\n  conductors with the incorporation of spatial dispersion: We derive general frequency dependencies of the surface impedance modulus for\nconductors without the dc dissipation, i. e. for superconductors or perfect\nconductors. The frequency-dependent surface impedance was applied for the\nsolutions corresponding to the spatially dispersive eigenvalues of the\npermittivity operator for conductors. We demonstrate that appropriately taken\ninto account effects of the spatial dispersion can give the general frequency\ndependence of the surface impedance for the obtained solutions including that\nfor superconductor. It is shown that an incorporation of the spatial dispersion\nleads to an appearance of the Meissner effect in perfect conductors in the same\nmanner as in superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The maximal superconductivity in proximity to charge density wave\n  quantum critical point in Cu$_x$TiSe$_2$: Superconductivity emerges in $1T$-TiSe$_2$ when its charge density wave (CDW)\norder is suppressed by Cu intercalation or pressure. Since the CDW state is\nthought to be an excitonic insulator, an interesting question is whether the\nsuperconductivity is also mediated by the excitonic fluctuations. We\ninvestigated this question as to the nature of doping induced superconductivity\nin Cu$_x$TiSe$_2$ by asking if it is consistent with the phonon-mediated\npairing. We employed the {\\it ab initio} density functional theory and density\nfunctional perturbation theory to compute the electron-phonon coupling\nEliashberg function from which to calculate the superconducting (SC) critical\ntemperature $T_c$. The calculated $T_c $ as a function of the doping\nconcentration $x$ exhibits a dome shape with the maximum $T_c$ of $2-6$ K at $x\n\\approx 0.05$ for the Coulomb pseudopotential $0 \\leq \\mu^* \\leq 0.1$. The\nmaximal $T_c$ was found to be pinned to the quantum critical point at which the\nCDW is completely suppressed and the corresponding phonon mode becomes soft.\nUnderlying physics is that the reduced phonon frequency enhances the\nelectron-phonon coupling constant $\\lambda$ which overcompensates the frequency\ndecrease to produce a net increase of $T_c$. The doping induced\nsuperconductivity in Cu$_x$TiSe$_2$ seems to be consistent with the\nphonon-mediated pairing. Comparative discussion was made with the pressure\ninduced superconductivity in TiSe$_2$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pressure-induced magnetic collapse and metallization of\n  $\\mathrm{TlF}{\\mathrm{e}}_{1.6}\\mathrm{S}{\\mathrm{e}}_{2}$: The crystal structure, magnetic ordering, and electrical resistivity of\nTlFe1.6Se2 were studied at high pressures. Below ~7 GPa, TlFe1.6Se2 is an\nantiferromagnetically ordered semiconductor with a ThCr2Si2-type structure. The\ninsulator-to-metal transformation observed at a pressure of ~ 7 GPa is\naccompanied by a loss of magnetic ordering and an isostructural phase\ntransition. In the pressure range ~ 7.5 - 11 GPa a remarkable downturn in\nresistivity, which resembles a superconducting transition, is observed below 15\nK. We discuss this feature as the possible onset of superconductivity\noriginating from a phase separation in a small fraction of the sample in the\nvicinity of the magnetic transition.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Hidden symmetry and knot solitons in a charged two-condensate Bose\n  system: We show that a charged two-condensate Ginzburg-Landau model or equivalently a\nGross-Pitaevskii functional for two charged Bose condensates, can be mapped\nonto a version of the nonlinear O(3) $\\sigma$-model. This implies in particular\nthat such a system possesses a hidden O(3) symmetry and allows for the\nformation of stable knotted solitons. The results, in particular, should be\nrelevant to the superconducting MgB_2.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Effects of the Spin-Orbit Coupling and the Superconductivity in\n  simple-cubic alpha-Polonium: We have investigated the mechanism of stabilizing the simple-cubic (SC)\nstructure in polonium (alpha- Po), based on the phonon dispersion calculations\nusing the first-principles all-electron band method. We have demonstrated that\nthe stable SC structure results from the suppression of the Peierls instability\ndue to the strong spin-orbit coupling (SOC) in alpha-Po. Further, we have\nexplored the possible superconductivity in alpha-Po, and predicted that it\nbecomes a superconductor with Tc ~ 4 K. The transverse soft phonon mode at q ~\n2/3 R, which is greatly influenced by the SOC, plays an important role both in\nthe structural stability and the superconductivity in alpha-Po. We have\ndiscussed effects of the SOC and the volume variation on the phonon dispersions\nand superconducting properties of alpha-Po.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Evidence for quasi-two-dimensional superconductivity in infinite-layer\n  nickelates: After being expected as a promising analogue to cuprates for decades,\nsuperconductivity was recently discovered in infinite-layer nickelates,\nproviding new opportunities to explore mechanisms of high-temperature\nsuperconductivity. However, in sharp contrast to the single-band\nquasi-two-dimensional superconductivity in cuprates, nickelates exhibit a\nmulti-band electronic structure and an unexpected isotropic superconductivity\nas reported recently, which challenges the cuprate-like picture in nickelates.\nHere, we show the superconductivity in nickelates is actually anisotropic and\nquasi-two-dimensional in nature, as that in cuprates. By synthesizing\nhigh-quality lanthanide nickelate films with enhanced crystallinity and\nsuperconductivity ($T_{c}^{onset}$ = 18.8 K, $T_{c}^{zero}$ = 16.5 K), strong\nanisotropic magnetotransport behaviors have been observed. The\nquasi-two-dimensional nature is further confirmed by the existence of a\ncusp-like peak of the angle-dependent $T_{c}$, and a\nBerezinskii-Kosterlitz-Thouless transition near $T_{c}$. Our work thus suggests\na quasi-two-dimensional superconductivity in infinite-layer nickelates,\nimplying a single-3$d_{x^2-y^2}$-band cuprate-like picture may remain valid in\nthese compounds.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electronic structure studies of BaFe2As2 by angle-resolved photoemission\n  spectroscopy: We report high resolution angle-resolved photoemission spectroscopy (ARPES)\nstudies of the electronic structure of BaFe$_2$As$_2$, which is one of the\nparent compounds of the Fe-pnictide superconductors. ARPES measurements have\nbeen performed at 20 K and 300 K, corresponding to the orthorhombic\nantiferromagnetic phase and the tetragonal paramagnetic phase, respectively.\nPhoton energies between 30 and 175 eV and polarizations parallel and\nperpendicular to the scattering plane have been used. Measurements of the Fermi\nsurface yield two hole pockets at the $\\Gamma$-point and an electron pocket at\neach of the X-points. The topology of the pockets has been concluded from the\ndispersion of the spectral weight as a function of binding energy. Changes in\nthe spectral weight at the Fermi level upon variation of the polarization of\nthe incident photons yield important information on the orbital character of\nthe states near the Fermi level. No differences in the electronic structure\nbetween 20 and 300 K could be resolved. The results are compared with density\nfunctional theory band structure calculations for the tetragonal paramagnetic\nphase.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Correlation of Fe-based Superconductivity and Electron-Phonon Coupling\n  in an FeAs/Oxide Heterostructure: Interfacial phonons between iron-based superconductors (FeSCs) and perovskite\nsubstrates have received considerable attention due to the possibility of\nenhancing preexisting superconductivity. Using scanning tunneling spectroscopy,\nwe studied the correlation between superconductivity and e-ph interaction with\ninterfacial-phonons in an iron-based superconductor Sr$_2$VO$_3$FeAs ($T_c\n\\approx$ 33 K) made of alternating FeSC and oxide layers. The quasiparticle\ninterference measurement over regions with systematically different average\nsuperconducting gaps due to the e-ph coupling locally modulated by O vacancies\nin VO$_2$ layer, and supporting self-consistent momentum-dependent Eliashberg\ncalculations provide a unique real-space evidence of the forward-scattering\ninterfacial phonon contribution to the total superconducting pairing.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Fragmentation of Fast Josephson Vortices and Breakdown of Ordered States\n  by Moving Topological Defects: Topological defects such as vortices, dislocations or domain walls define\nmany important effects in superconductivity, superfluidity, magnetism, liquid\ncrystals, and plasticity of solids. Here we address the breakdown of the\ntopologically-protected stability of such defects driven by strong external\nforces. We focus on Josephson vortices that appear at planar weak links of\nsuppressed superconductivity which have attracted much attention for electronic\napplications, new sources of THz radiation, and low-dissipative computing. Our\nnumerical simulations show that a rapidly moving vortex driven by a constant\ncurrent becomes unstable with respect to generation of vortex-antivortex pairs\ncaused by Cherenkov radiation. As a result, vortices and antivortices become\nspatially separated and accumulate continuously on the opposite sides of an\nexpanding dissipative domain. This effect is most pronounced in thin film edge\nJosephson junctions at low temperatures where a single vortex can switch the\nwhole junction into a resistive state at currents well below the Josephson\ncritical current. Our work gives a new insight into instability of a moving\ntopological defect which destroys global long-range order in a way that is\nremarkably similar to the crack propagation in solids.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Persistent Charge Density Wave Memory in a Cuprate Superconductor: Although charge density wave (CDW) correlations appear to be a ubiquitous\nfeature of the superconducting cuprates, their disparate properties suggest a\ncrucial role for coupling or pinning of the CDW to lattice deformations and\ndisorder. While diffraction intensities can demonstrate the occurrence of CDW\ndomain formation, the lack of scattering phase information has limited our\nunderstanding of this process. Here, we report coherent resonant x-ray speckle\ncorrelation analysis, which directly determines the reproducibility of CDW\ndomain patterns in La1.875Ba0.125CuO4 (LBCO 1/8) with thermal cycling. While\nCDW order is only observed below 54 K, where a structural phase transition\nresults in equivalent Cu-O bonds, we discover remarkably reproducible CDW\ndomain memory upon repeated cycling to temperatures well above that transition.\nThat memory is only lost on cycling across the transition at 240(3) K that\nrestores the four-fold symmetry of the copper-oxide planes. We infer that the\nstructural-domain twinning pattern that develops below 240 K determines the CDW\npinning landscape below 54 K. These results open a new view into the complex\ncoupling between charge and lattice degrees of freedom in superconducting\ncuprates.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Intrinsic Properties of Stoichiometric LaOFeP: DC and ac magnetization, resistivity, specific heat, and neutron diffraction\ndata reveal that stoichiometric LaOFeP is metallic and non-superconducting\nabove T = 0.35 K, with gamma = 12.5 mJ/mol*K. Neutron diffraction data at room\ntemperature and T = 10 K are well described by the stoichiometric, tetragonal\nZrCuSiAs structure and show no signs of structural distortions or long range\nmagnetic ordering, to an estimated detectability limit of 0.07 uB/Fe. We\npropose a model, based on the shape of the iron-pnictide tetrahedron, that\nexplains the differences between LaOFeP and LaOFeAs, the parent compound of the\nrecently discovered high-Tc oxyarsenides, which, in contrast, shows both\nstructural and spin density wave (SDW) transitions.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Striking Zn impurity effect on the Fe-based superconductor\n  BaFe1.87Co0.13As2: Nonmagnetic impurity effect was studied on the n-type Fe-based superconductor\nBaFe1.87Co0.13As2 (Tc = 25 K) by a successful Zn substitution for Fe up to 7\nat.%. Magnetic susceptibility, electrical resistivity, specific heat, and Hall\ncoefficient measurements indicated that Tc linearly decreases with the Zn\nconcentration and disappears at 7 at.%. The result is quantitatively comparable\nwith what was observed for YBCO, while it disagrees with a recent report for\nthe p-type Ba0.5K0.5Fe2As2. Fragile SC against a nonmagnetic impurity was first\nconfirmed for the n-type 122 Fe-based superconductor.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Huge magnetostriction in superconducting single-crystalline\n  BaFe$_{1.908}$Ni$_{0.092}$As$_{2}$: The performance of iron-based superconductors in high magnetic fields plays\nan important role for their practical application. In this work, we measured\nthe magnetostriction and magnetization of BaFe$_{1.908}$Ni$_{0.092}$As$_{2}$\nsingle crystals using pulsed magnetic fields up to 60 T and static magnetic\nfields up to 33 T, respectively. A huge longitudinal magnetostriction (of the\norder of 10$ ^{-4} $) was observed in the direction of the twin boundaries. The\nmagnetization measurements evidence a high critical-current density due to\nstrong bulk pinning. By using magnetization data with an exponential\nflux-pinning model, we can reproduce the magnetostriction curves qualitatively.\nThis result shows that the magnetostriction of\nBaFe$_{1.908}$Ni$_{0.092}$As$_{2}$ can be well explained by a\nflux-pinning-induced mechanism.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Manifesto for a higher Tc -- lessons from pnictides and cuprates: We explore energy scales, features in the normal state transport, relevant\ninteractions and constraints for the pairing mechanisms in the high-Tc cuprates\nand Fe-pnictides. Based on this analysis we attempt to identify a number of\nattributes of superconductors with a higher T_c. Expanded version of the\narticle published in Nature Physics, 7, 271 (2011).",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetic and structural properties of the iron silicide superconductor\n  LaFeSiH: The magnetic and structural properties of the recently discovered\npnictogen/chalcogen-free superconductor LaFeSiH ($T_c\\simeq10$~K) have been\ninvestigated by $^{57}$Fe synchrotron M{\\\"o}ssbauer source (SMS) spectroscopy,\nx-ray and neutron powder diffraction and $^{29}$Si nuclear magnetic resonance\nspectroscopy (NMR). No sign of long range magnetic order or local moments has\nbeen detected in any of the measurements and LaFeSiH remains tetragonal down to\n2 K. The activated temperature dependence of both the NMR Knight shift and the\nrelaxation rate $1/T_1$ is analogous to that observed in strongly overdoped\nFe-based superconductors. These results, together with the\ntemperature-independent NMR linewidth, show that LaFeSiH is an homogeneous\nmetal, far from any magnetic or nematic instability, and with similar Fermi\nsurface properties as strongly overdoped iron pnictides. This raises the\nprospect of enhancing the $T_c$ of LaFeSiH by reducing its carrier\nconcentration through appropriate chemical substitutions. Additional SMS\nspectroscopy measurements under hydrostatic pressure up to 18.8~GPa found no\nmeasurable hyperfine field.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Interplay between collective modes in hybrid electron gas-superconductor\n  structures: We study hybridization of collective plasmon and\nCarlson-Goldman-Artemenko-Volkov modes in a hybrid system, consisting of a\ntwo-dimensional layers of electron gas in the normal state and superconductor,\ncoupled by long-range Coulomb forces. The interaction between these collective\nmodes is not possible in a regular single-layer two-dimensional system since\nthey exist in non-overlapping domains of dimensionless parameter $\\omega\\tau$,\nwhere $\\omega$ is the external electromagnetic field frequency and $\\tau$ is\nelectron scattering time. Thus, in a single-layer structure, these modes are\nmutually exclusive. However, the coupling may become possible in a hybrid\nsystem consisting of two separated in space materials with different\nproperties, in particular, the electron scattering time. We investigate the\nelectromagnetic power absorption by the hybrid system and reveal the conditions\nnecessary for the hybridization of collective modes.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Non-thermal origin of nonlinear transport across magnetically induced\n  superconductor-metal-insulator transition: We have studied the effect of perpendicular magnetic fields and temperatures\non the nonlinear electronic transport in amorphous Ta superconducting thin\nfilms. The films exhibit a magnetic field induced metallic behavior intervening\nthe superconductor-insulator transition in the zero temperature limit. We show\nthat the nonlinear transport in the superconducting and metallic phase is of\nnon-thermal origin and accompanies an extraordinarily long voltage response\ntime.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Modification of the effective action approach for the Leggett mode: The modified procedure to get the effective action is discussed. Careful\nobservation of phase of electrons leads to the conclusion that\nHubbard-Stratonovich field should be pair wavefunction, instead of gap. The\nmodified theory is valid for arbitrary strong interband coupling while the\nearlier approach only deals with weak interband coupling. This paper also\ndiscusses experimental observation of the Leggett mode: Raman spectroscopy. The\nspectral peak which corresponds to the Leggett mode is calculated in earlier\nthe earlier and the modified theory, which shows that they are much different.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Collision Dynamics of Two Bose-Einstein Condensates in the Presence of\n  Raman Coupling: A collision of two-component Bose-Einstein condensates in the presence of\nRaman coupling is proposed and studied by numerical simulations. Raman\ntransitions are found to be able to reduce collision-produced irregular\nexcitations by forming a time-averaged attractive optical potential. Raman\ntransitions also support a kind of dark soliton pairs in two-component\nBose-Einstein condensates. Soliton pairs and their remnant single solitons are\nshown to be controllable by adjusting the initial relative phase between the\ntwo colliding condensates or the two-photon detuning of Raman transitions.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Hall-Lorenz ratio of YBa2Cu3O7 using Ionization energy based Fermi-Dirac\n  statistics and charge-spin separation: The temperature dependent properties of heat capacity, heat conductivity and\nHall-Lorenz ratio have been solved numerically after taking the previously\nproposed ionization energy based Fermi-Dirac statistics and the coexistence of\nFermi and charge-spin separated liquid into account. The\nthermo-magneto-electronic properties are entirely for spin and charge carriers,\nhence the phonon contribution has been neglected. A linear dependence between\nthe Hall-Lorenz ratio and temperature ($T$) is also obtained in accordance with\nthe experimental results for overdoped YBa$_2$Cu$_3$O$_{7-\\delta}$, if these\nconditions, $E_I$ $<$ $T_c$ with respect to resistivity and there are no spinon\npairings ($T^*$ = 0) are satisfied. Heat conductivity based on both pure and\nelectron-contaminated charge-spin separated liquid in $ab$-planes above $T_c$\nare found to increase with decreasing $T$ as a consequence of inverse\nproportionality with $T$. The $T$-dependence of heat capacity are also\nhighlighted, which qualitatively complies with the experimental findings.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Comment on \"Enhanced two-dimensional properties of the four-layered\n  cuprate high-Tc superconductor TlBa-2Ca-3Cu-4O-y\": We reanalyze published magnetization data and demonstrate that the conclusion\nof the original authors, claiming enhanced two-dimensional properties of the\ncuprate superconductor Tl-1234, is not supported by the experimental results.\nOur analysis shows that the magnetic field dependence of the mixed-state\nmagnetization for this particular sample is amazingly close to the results of\nnumerical calculations by E. H. Brandt for an ideal vortex lattice without\nfluctuations. This good agreement between experiment and theory allows for the\nevaluation of the absolute values of the upper critical field.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Effect of biaxial strain on the phase transitions of Ca(Fe1-xCox)2As2: We study the effect of applied strain as a physical control parameter for the\nphase transitions of Ca(Fe1-xCox)2As2 using resistivity, magnetization, x-ray\ndiffraction and 57Fe M\\\"ossbauer spectroscopy. Biaxial strain, namely\ncompression of the basal plane of the tetragonal unit cell, is created through\nfirm bonding of samples to a rigid substrate, via differential thermal\nexpansion. This strain is shown to induce a magneto-structural phase transition\nin originally paramagnetic samples; and superconductivity in previously\nnon-superconducting ones. The magneto-structural transition is gradual as a\nconsequence of using strain instead of pressure or stress as a tuning\nparameter.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Optical sum increase due to electron undressing: For a system with a fixed number of electrons, the total optical sum is a\nconstant, independent of many-body interactions, of impurity scattering and of\ntemperature. For a single band in a metal, such a sum rule is no longer\nindependent of the interactions or temperature, when the dispersion and/or\nfinite bandwidth is accounted for. We adopt such a model, with electrons\ncoupled to a single Einstein oscillator of frequency $\\omega_{E}$, and study\nthe optical spectral weight. The optical sum depends on both the strength of\nthe coupling and on the characteristic phonon frequency, $\\omega_{E}$. A\nhardening of $\\omega_{E}$, due, for example, to a phase transition, leads to\nelectron undressing and translates into a decrease in the electron kinetic\nenergy and an increase in the total optical sum, as observed in recent\nexperiments in the cuprate superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity in layered CeO0.5F0.5BiS2: We report appearance of superconductivity in CeO0.5F0.5BiS2. The bulk\npolycrystalline samples CeOBiS2 and CeO0.5F0.5BiS2 are synthesized by\nconventional solid state reaction route via vacuum encapsulation technique.\nDetailed structural analysis showed that the studied CeO0.5F0.5BiS2 compound is\ncrystallized in tetragonal P4/nmm space group with lattice parameters a =\n4.016(3) A, c = 13.604(2) A. DC magnetization measurement (MT-curve) shows the\nferromagnetic signal at the low temperature region. The superconductivity is\nestablished in CeO0.5F0.5BiS2 at Tconset = 2.5K by electrical transport\nmeasurement. Under applied magnetic field both Tc onset and Tc ({\\rho} =0)\ndecrease to lower temperatures and an upper critical field [Hc2(0)] above\n1.2Tesla is estimated. The results suggest coexistence of ferromagnetism and\nsuperconductivity for the CeO0.5F0.5BiS2 sample.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Negative in-plane and out-of-plane magnetoresistivities in optimally\n  doped Bi2Sr2Ca0.8Y0.2Cu2O8+d single crystal: Both the in-plane and out-of-plane magnetoresistivities have been measured in\nthe normal state of an optimally doped Bi2Sr2Ca0.8Y0.2Cu2O8+d single crystal\nwith a magnetic field applied parallel and perpendicular to the CuO2 planes.\n  Whatever the magnetic field and the current directions are, a negative\nmagnetoresistivity is obtained over a wide range of temperature above the\ncritical temperature Tc. For the in-plane and out-of-plane measurements, the\nnon-dominant orbital contribution to magnetoresistivity suggests the\nsubstantial role played by the spin degrees of freedom.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity of the New Medium-Entropy Alloy V4Ti2W with a\n  Body-Centered Cubic Structure: Medium- and high-entropy alloy (MEA and HEA) superconductors have attracted\nconsiderable interest since their discovery. This paper reports the\nsuperconducting properties of ternary tungsten-containing MEA V4Ti2W for the\nfirst time. V4Ti2W is a type II superconductor with a body-centered cubic (BCC)\nstructure. Experimental results of resistivity, magnetization, and heat\ncapacity indicate that the superconducting transition temperature of the MEA\nV4Ti2W is roughly 5.0 K. The critical magnetic fields at the upper and lower\nends are 9.93(2) T and 40.7(3) mT, respectively. Interestingly, few BCC MEA\nsuperconductors with VEC greater than 4.8 have been found. The addition of\ntungsten leads to a VEC of 4.83 e/a for V4Ti2W, which is rarely higher than the\n4.8 value. Adding tungsten element expands the variety of MEA alloys, which may\nimprove the microstructure and mechanical properties of materials and even\nsuperconducting properties. This material could potentially offer a new\nplatform for the investigation of innovative MEA and HEA superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Spectrum of low energy excitations in the vortex state: comparison of\n  Doppler shift method to quasiclassical approach: We present a detailed comparison of numerical solutions of the quasiclassical\nEilenberger equations with several approximation schemes for the density of\nstates of s- and d-wave superconductors in the vortex state, which have been\nused recently. In particular, we critically examine the use of the Doppler\nshift method, which has been claimed to give good results for d-wave\nsuperconductors. Studying the single vortex case we show that there are\nimportant contributions coming from core states, which extend far from the\nvortex cores into the nodal directions and are not present in the Doppler shift\nmethod, but significantly affect the density of states at low energies. This\nleads to sizeable corrections to Volovik's law, which we expect to be sensitive\nto impurity scattering. For a vortex lattice we also show comparisons with the\nmethod due to Brandt, Pesch, and Tewordt and an approximate analytical method,\ngeneralizing a method due to Pesch. These are high field approximations\nstrictly valid close to the upper critical field Bc2. At low energies the\napproximate analytical method turns out to give impressively good results over\na broad field range and we recommend the use of this method for studies of the\nvortex state at not too low magnetic fields.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Phase diagram of nickelate superconductors calculated by dynamical\n  vertex approximation: We review the electronic structure of nickelate superconductors with and\nwithout effects of electronic correlations. As a minimal model we identify the\none-band Hubbard model for the Ni 3$d_{x^2-y^2}$ orbital plus a pocket around\nthe $A$-momentum. The latter however merely acts as a decoupled electron\nreservoir. This reservoir makes a careful translation from {nominal} Sr-doping\nto the doping of the one-band Hubbard model mandatory. Our dynamical mean-field\ntheory calculations, in part already supported by experiment, indicate that the\n$\\Gamma$ pocket, Nd 4$f$ orbitals, oxygen 2$p$ and {the} other Ni 3$d$ orbitals\nare not relevant in the superconducting doping regime. The physics is\ncompletely different if topotactic hydrogen is present or the oxygen reduction\nis incomplete. Then, a two-band physics hosted by the Ni 3$d_{x^2-y^2}$ and\n3$d_{3z^2-r^2}$ orbitals emerges. Based on our minimal modeling we calculated\nthe superconducting $T_c$ vs. Sr-doping $x$ phase diagram prior to experiment\nusing the dynamical vertex approximation. For such a notoriously difficult to\ndetermine quantity as $T_c$, the agreement with experiment is astonishingly\ngood. The prediction that $T_c$ is enhanced with pressure or compressive\nstrain, has been confirmed experimentally as well. This supports that the\none-band Hubbard model plus an electron reservoir is the appropriate minimal\nmodel.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Thermodynamic evidence for nematic superconductivity in\n  Cu$_x$Bi$_2$Se$_3$: Unconventional superconductivity is characterized by the spontaneous symmetry\nbreaking of the macroscopic superconducting wavefunction in addition to the\ngauge symmetry breaking, such as rotational-symmetry breaking with respect to\nthe underlying crystal-lattice symmetry. Particularly, superconductivity with\nspontaneous rotational-symmetry breaking in the wavefunction amplitude and thus\nin bulk properties, not yet reported previously, is intriguing and can be\ntermed \"nematic\" superconductivity in analogy to nematic liquid-crystal phases.\nHere, based on specific-heat measurements of the single-crystalline\nCu$_x$Bi$_2$Se$_3$ under accurate magnetic-field-direction control, we report\nthermodynamic evidence for nematic superconductivity, namely, clear\ntwo-fold-symmetric behavior in a trigonal lattice. The results indicate\nrealization of an \"odd-parity nematic\" state, feasible only by macroscopic\nquantum condensates and distinct from nematic states in liquid crystals. The\nresults also confirm topologically non-trivial superconductivity in\nCu$_x$Bi$_2$Se$_3$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetic hysteresis of a superconducting microstrip resonator with a\n  high edge barrier: We investigate the magnetic hysteresis of a superconducting microstrip\nresonator with a high edge barrier. We measure the magnetic hysteresis while\neither sweeping a magnetic field or tuning the edge barrier by high microwave\ncurrent. We show that the magnetic hysteresis of such a device is qualitatively\ndifferent from that of one without an edge barrier and can be understood based\non the generalized critical-state model. In particular, we propose and\ndemonstrate a simple and intuitive method that relies on a plot of the quality\nfactor versus the resonance frequency for revealing the physical processes\nbehind those hysteretic behaviors. Based on this, we find that the interplay\nbetween the Meisser current and vortex pinning is essential for understanding\nthe magnetic hysteresis of such a device.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Thermally assisted quantum vortex tunneling in the Hall and dissipative\n  regime: Quantum vortex tunneling is studied for the case where the Hall and the\ndissipative dynamics are simultaneously present. For a given temperature, the\nmagnetization relaxation rate is calculated as a function of the external\ncurrent and the quasiparticle scattering time. The relaxation rate is solved\nanalytically at zero temperature and obtained numerically at finite\ntemperatures by the variational method. In the moderately clean samples, we\nhave found that a minimum in the relaxation rate exists at zero temperature,\nwhich tends to disappear with increase in the temperature.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Transport signatures of fragile-glass dynamics in the melting of the\n  two-dimensional vortex lattice: In two-dimensional (2D) systems, the melting from a solid to an isotropic\nliquid can occur via an intermediate phase that retains orientational order.\nHowever, in 2D superconducting vortex lattices, the effect of orientational\ncorrelations on transport, and their interplay with disorder remain open\nquestions. Here we study a 2D weakly pinned vortex system in amorphous MoGe\nfilms over an extensive range of temperatures ($\\bm{T}$) and perpendicular\nmagnetic fields ($\\bm{H}$) using linear and nonlinear transport measurements.\nWe find that, at low fields, the resistivity obeys the Vogel-Fulcher-Tamman\n(VFT) form, $\\bm{\\rho(T)\\propto\\exp[-{W}(H)/(T-T_0(H))]}$, characteristic of\nfragile glasses. As $\\bm{H}$ increases, $\\bm{T_0(H)}$ is suppressed to zero,\nand a standard vortex liquid behavior consistent with a $\\bm{T=0}$\nsuperconducting transition is observed. Our findings, supported also by\nsimulations, suggest that the presence of orientational correlations gives rise\nto a heterogeneous dynamics responsible for the VFT behavior. The effects of\nquenched disorder become dominant at high $\\bm{H}$, where a crossover to a\nstrong-glass behavior is observed. This is a new insight into the dynamics of\nmelting in 2D systems with competing orders.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconducting mechanism for the cuprate Ba$_2$CuO$_{3+\u03b4}$ based\n  on a multiorbital Lieb lattice model: For the recently discovered cuprate superconductor\n$\\mathrm{Ba_{2}CuO_{3+\\delta}}$, we propose a lattice structure which resembles\nthe model considered by Lieb to represent the vastly oxygen-deficient material.\nWe first investigate the stability of the Lieb-lattice structure, and then\nconstruct a multiorbital Hubbard model based on first-principles calculation.\nBy applying the fluctuation-exchange approximation to the model and solving the\nlinearized Eliashberg equation, we show that $s$-wave and $d$-wave pairings\nclosely compete with each other, and, more interestingly, that the\nintra-orbital and inter-orbital pairings coexist. We further show that, if the\nenergy of the $d_{3z^2-r^2}$ band is raised to make it \"incipient\" with the\nlower edge of the band close to the Fermi level within a realistic band filling\nregime, $s\\pm$-wave superconductivity is strongly enhanced. We reveal an\nintriguing relation between the Lieb model and the two-orbital model for the\nusual K$_2$NiF$_4$ structure where a close competition between $s-$ and\n$d-$wave pairings is known to occur. The enhanced superconductivity in the\npresent model is further shown to be related to an enhancement found previously\nin the bilayer Hubbard model with an incipient band.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Helium-Three in Aerogel: Liquid 3He confined in silica aerogel provides us with a unique system to\nstudy the effects of quenched disorder on the properties of a strongly\ncorrelated quantum liquid. The superfluid phases display interplay between\ndisorder and complex symmetry-breaking.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pressure effects on the unconventional superconductivity of the\n  noncentrosymmetric LaNiC2: The unconventional superconductivity in the noncentrosymmetric LaNiC$_2$, and\nits evolution with pressure, is analyzed basing on the {\\it ab initio}\ncomputations and the full Eliashberg formalism. First principles calculations\nof the electronic structure, phonons and the electron-phonon coupling are\nreported in the pressure range 0-15 GPa. The thermodynamic properties of the\nsuperconducting state were determined numerically solving the Eliashberg\nequations. We found that already at $p=0$ GPa, the superconducting parameters\ndeviate from the BCS-type, and a large value of the Coulomb pseudopotential\n$\\mu^{\\star}=0.22$ is required to get the critical temperature $T_c = 2.8$~K\nconsistent with experiment. If such $\\mu^{\\star}$ is used, the Eliashberg\nformalism reproduces also the experimentally observed values of the\nsuperconducting order parameter, the electronic specific heat jump at the\ncritical temperature, and the change of the London penetration depth with\ntemperature. This shows, that deviation of the above-mentioned parameters from\nthe BCS predictions do not prejudge on the triplet or multiple gap nature of\nthe superconductivity in this compound. Under the external pressure,\ncalculations predict continuous increase of the electron-phonon coupling\nconstant in the whole pressure range 0-15~GPa, consistent with the\nexperimentally observed increase in $T_c$ for the pressure range 0-4~GPa, but\ninconsistent with the drop of $T_c$ above 4~GPa and the disappearance of the\nsuperconductivity above 7~GPa, reported experimentally. The disappearance of\nsuperconductivity may be accounted for by increasing the $\\mu^{\\star}$ to 0.36\nat 7~GPa, which supports the hypothesis of the formation of a new high-pressure\nelectronic phase, which competes with the superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "NMR evidence for an intimate relationship between antiferromagnetic spin\n  fluctuations and extended s-wave superconductivity in mono-crystalline\n  SrFe2(As_{1-x}P_{x})2: We report systematic 31P-NMR study on iron (Fe)-based superconductors\nSrFe2(As_{1-x}P_{x})2 (Sr122AsP), in which a superconducting (SC) transition\ntemperature Tc at x=0.35 increases from Tc=26 K up to 33 K by annealing an\nas-grown mono-crystalline sample. The present NMR study has unraveled that Tc\nreaches a highest value of 33 K at x=0.35 around a quantum critical point at\nwhich antiferromagnetic (AFM) order disappears. When noting that the SC\ntransition disappears at x=0.6 where the AFM spin fluctuations (SFs) are no\nlonger present, we remark that the onset and increase of Tc are apparently\nassociated with the emergence and enhancement of AFM-SFs, respectively. In the\nSC state, the residual density of state (RDOS) at the Fermi energy EF in the SC\nstate becomes much smaller for the annealed sample than for the as-grown one,\nsuggesting that some inhomogeneity and/or imperfection for the latter increases\nRDOS as expected for unconventional SC state with nodal gap. These findings in\nSr122AsP are consistent with the unconventional s(+-)-wave Cooper pairing state\nthat is mediated by AFM-SFs. We also discuss other key-ingredients besides the\nAFM-SFs to increase Tc further.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Origin of Topological Surface Superconductivity in\n  FeSe$_{0.45}$Te$_{0.55}$: The engineering of Majorana zero modes in topological superconductors, a new\nparadigm for the realization of topological quantum computing and\ntopology-based devices, has been hampered by the absence of materials with\nsufficiently large superconducting gaps. Recent experiments, however, have\nprovided enthralling evidence for the existence of topological surface\nsuperconductivity in the iron-based superconductor FeSe$_{0.45}$Te$_{0.55}$\npossessing a full $s_\\pm$-wave gap of a few meV. Here, we propose a mechanism\nfor the emergence of topological superconductivity on the surface of\nFeSe$_{0.45}$Te$_{0.55}$ by demonstrating that the interplay between the\n$s_\\pm$-wave symmetry of the superconducting gap, recently observed surface\nmagnetism, and a Rashba spin-orbit interaction gives rise to several\ntopological superconducting phases. Moreover, the proposed mechanism explains a\nseries of experimentally observed hallmarks of topological superconductivity,\nsuch as the emergence of Majorana zero modes in the center of vortex cores and\nat the end of line defects, as well as of chiral Majorana edge modes along\ncertain types of domain walls. We also propose that the spatial distribution of\nsupercurrents near a domain wall is a characteristic signature measurable via a\nscanning superconducting quantum interference device that can distinguish\nbetween chiral Majorana edge modes and trivial in-gap states.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Probing the topological band structure of diffusive multiterminal\n  Josephson junction devices with conductance measurements: The energy of an Andreev bound state in a clean normal metal in contact with\ntwo superconductors disperses with the difference $\\Delta \\phi$ in the\nsuperconducting phase between the superconductors in much the same way as the\nenergies of electrons in a one-dimensional crystal disperse with the crystal\nmomentum $k$ of the electrons. A normal metal with $n$ superconductors maps on\nto a $n-1$ dimensional crystal, each dimension corresponding to the phase\ndifference $\\phi_i$ between a specific pair of superconductors. The resulting\nband structure as a function of the phase differences $\\{\\Delta \\phi_i\\}$ has\nbeen proposed to have a topological nature, with gapped regions characterized\nby different Chern numbers separated by regions where the gap in the\nquasiparticle spectrum closes. A similar complex evolution of the quasiparticle\nspectrum with $\\{\\Delta \\phi_i\\}$ has also been predicted for diffusive normal\nmetals in contact with multiple superconductors. Here we show that the\nvariation of the density of states at the Fermi energy of such a system can be\ndirectly probed by relatively simple conductance measurements, allowing rapid\ncharacterization of the energy spectrum.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Controlling superconductivity of CeIrIn$_5$ microstructures by substrate\n  selection: Superconductor/metal interfaces are usually fabricated in heterostructures\nthat join these dissimilar materials. A conceptually different approach has\nrecently exploited the strain sensitivity of heavy-fermion superconductors,\nselectively transforming regions of the crystal into the metallic state by\nstrain gradients. The strain is generated by differential thermal contraction\nbetween the sample and the substrate. Here, we present an improved\nfinite-element model that reliably predicts the superconducting transition\ntemperature in CeIrIn$_5$ even in complex structures. Different substrates are\nemployed to tailor the strain field into the desired shapes. Using this\napproach, both highly complex and strained as well as strain-free\nmicrostructures are fabricated to validate the model. This enables full control\nover the microscopic strain fields, and forms the basis for more advanced\nstructuring of superconductors as in Josephson junctions.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Generic Finite Size Enhancement of Pairing in Mesoscopic Fermi Systems: The finite size dependent enhancement of pairing in mesoscopic Fermi systems\nis studied under the assumption that the BCS approach is valid and that the two\nbody force is size independent. Different systems are investigated such as\nsuperconducting metallic grains and films as well atomic nuclei. It is shown\nthat the finite size enhancement of pairing in these systems is in part due to\nthe presence of a surface which accounts quite well for the data of nuclei and\nexplains a good fraction of the enhancement in Al grains.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pair Density Waves from Local Band Geometry: A band-projection formalism is developed for calculating the superfluid\nweight in two-dimensional multi-orbital superconductors with an\norbital-dependent pairing. It is discovered that, in this case, the band\ngeometric superfluid stiffness tensor can be locally non-positive-definite in\nsome regions of the Brillouin zone. When these regions are large enough or\ninclude nodal singularities, the total superfluid weight becomes\nnon-positive-definite due to pairing fluctuations, resulting in the transition\nof a BCS state to a pair-density wave (PDW). This geometric BCS-PDW transition\nis studied in the context of two-orbital superconductors, and proof of the\nexistence of a geometric BCS-PDW transition in a generic topological flat band\nis established.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electron-Phonon Superconductivity in LaO$_{0.5}$F$_{0.5}$BiSe$_{2}$: We report density functional calculations of the electronic structure, Fermi\nsurface, phonon spectrum and electron--phonon coupling for newly discovered\nsuperconductor LaO$_{0.5}$F$_{0.5}$BiSe$_{2}$. Significant similarity between\nLaO$_{0.5}$F$_{0.5}$BiS$_{2}$ and LaO$_{0.5}$F$_{0.5}$BiSe$_{2}$ is found, i.e.\nthere is a strong Fermi surface nesting at ($\\pi $,$\\pi $,0), which results in\nunstable phonon branches. Combining the frozen phonon total energy calculations\nand an anharmonic oscillator model, we find that the quantum fluctuation\nprevents the appearance of static long--range order. The calculation shows that\nLaO$_{0.5}$F$_{0.5}$BiSe$_{2}$ is highly anisotropic, and same as\nLaO$_{0.5}$F$_{0.5}$BiS$_{2}$, this compound is also a conventional\nelectron-phonon coupling induced superconductor.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Critical Velocity in the Presence of Surface Bound States in Superfluid\n  $^3$He-B: A microelectromechanical oscillator with a gap of 1.25 $\\mu$m was immersed in\nsuperfluid $^3$He-B and cooled below 250 $\\mu$K at various pressures.\nMechanical resonances of its shear motion were measured at various levels of\ndriving force. The oscillator enters into a nonlinear regime above a certain\nthreshold velocity. The damping increases rapidly in the nonlinear region and\neventually prevents the velocity of the oscillator from increasing beyond the\ncritical velocity which is much lower than the Landau critical velocity. We\npropose that this peculiar nonlinear behavior stems from the escape of\nquasiparticles from the surface bound states into the bulk fluid.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Reinterpretation of the equilibrium magnetization of a Tl-based single\n  crystal. Another phase transition in the mixed state of high-Tc\n  superconductors?: We apply a recently developed scaling procedure for the analysis of the\nequilibrium magnetization M that was measured on a Tl-based single crystal and\nwas recently reported in the literature. The results of our analysis are\ndistinctly different from those obtained in the original publication where the\nHao-Clem model served to analyze the magnetization data. We argue that the\nHao-Clem model is not adequate for a quantitative description of the mixed\nstate in high-Tc superconductors especially in high magnetic fields. The scaled\nequilibrium magnetization data reveal a pronounced kink in the M(H) dependence\nthat might be indicative of a phase transition in the mixed state.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Finite-size effects in hyperuniform vortex matter: Novel hyperuniform materials are emerging as an active field of applied and\nbasic research since they can be designed to have exceptional physical\nproperties. This ubiquitous state of matter presents a hidden order that is\ncharacterized by the density of constituents of the system being uniform at\nlarge scales, as in a perfect crystal, although they can be isotropic and\ndisordered like a liquid. In the quest for synthesizing hyperuniform materials\nin experimental conditions, the impact of finite-size effects remains as an\nopen question to be addressed. We use vortex matter in type-II superconductors\nas a toy model system to study this issue. We previously reported that vortex\nmatter nucleated in samples with point disorder is effectively hyperuniform and\nthus presents the interesting physical properties inherent to hyperuniform\nsystems. In this work we present experimental evidence that on decreasing the\nthickness of the vortex system its hyperuniform order is depleted. By means of\nhydrodynamic arguments we show that the experimentally observed depletion can\nbe associated to two crossovers that we describe within a hydrodynamic\napproximation. The first crossover length is thickness-dependent and separates\na class-II hyperuniform regime at intermediate lengthscales from a regime that\ncan become asymptotically non-hyperuniform for large wavelengths in very thin\nsamples. The second crossover takes place at smaller lengthscales and marks the\nonset of a faster increase of density fluctuations due to the dispersivity of\nthe elastic constants. Our work points to a novel mechanism of emerging\nhyperuniformity controlled by the thickness of the host sample, an issue that\nhas to be taken into account when growing hyperuniform structures for\ntechnological applications.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Diamagnetism of real-space pairs above Tc in hole doped cuprates: The nonlinear normal state diamagnetism reported by Lu Li et al. [Phys. Rev.\nB 81, 054510 (2010)] is shown to be incompatible with an acclaimed Cooper\npairing and vortex liquid above the resistive critical temperature. Instead it\nis perfectly compatible with the normal state Landau diamagnetism of real-space\ncomposed bosons, which describes the nonlinear magnetization curves in less\nanisotropic cuprates La-Sr-Cu-O (LSCO) and Y-Ba-Cu-O (YBCO) as well as in\nstrongly anisotropic bismuth-based cuprates in the whole range of available\nmagnetic fields.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Shaping graphene superconductivity with nanometer precision: Graphene holds great potential for superconductivity due to its pure\ntwo-dimensional nature, the ability to tune its carrier density through\nelectrostatic gating, and its unique, relativistic-like electronic properties.\nAt present, we are still far from controlling and understanding graphene\nsuperconductivity, mainly because the selective introduction of superconducting\nproperties to graphene is experimentally very challenging. Here, we have\ndeveloped a method that enables shaping at will graphene superconductivity\nthrough a precise control of graphene-superconductor junctions. The method\ncombines the proximity effect with scanning tunnelling microscope (STM)\nmanipulation capabilities. We first grow Pb nano-islands that locally induce\nsuperconductivity in graphene. Using a STM, Pb nano-islands can be selectively\ndisplaced, over different types of graphene surfaces, with nanometre scale\nprecision, in any direction, over distances of hundreds of nanometres. This\nopens an exciting playground where a large number of predefined\ngraphene-superconductor hybrid structures can be investigated with atomic scale\nprecision. To illustrate the potential, we perform a series of experiments,\nrationalized by the quasi-classical theory of superconductivity, going from the\nfundamental understanding of superconductor-graphene-superconductor\nheterostructures to the construction of superconductor nanocorrals, further\nused as \"portable\" experimental probes of local magnetic moments in graphene.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Vortex Interaction with Mesoscopic Irregularities in High Temperature\n  Superconductors: The conformal mapping method is used to study the problem of flux line\ninteraction with surface cavities having cylindrical profile and characteristic\nsize much less than the penetration length, i.e, within mesoscopic scale. It is\nshown that the metastable states are achieved when the dimensions of the\nsurface irregularities do not exceed the coherence length. Our study shows that\nthe surface barrier may vanished at some weak point at which the surface\nirregularities have mesoscopic scales. On the other hand, the surface barrier\nis completely disappeared when the surface defects size is more greater than\npenetration length. Our results are compared with the available experimental\ndata and theoretical results.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetic-Field-Induced Localization of Quasiparticles in Underdoped\n  La$_{2-x}$Sr$_x$CuO$_4$ Single Crystals: Magnetic-field-induced ordering of electrons around vortices is a striking\nphenomenon recently found in high-$T_c$ cuprates. To identify its consequence\nin the quasiparticle dynamics, the magnetic-field ($H$) dependence of the\nlow-temperature thermal conductivity $\\kappa$ of La$_{2-x}$Sr$_x$CuO$_4$\ncrystals is studied for a wide doping range. It is found that the behavior of\n$\\kappa(H)$ in the sub-Kelvin region changes drastically across optimum doping,\nand the data for underdoped samples are indicative of unusual\nmagnetic-field-induced localization of quasiparticles; this localization\nphenomenon is probably responsible for the unusual \"insulating normal state\"\nunder high magnetic fields.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The dual nature of magnetism in a uranium heavy fermion system: The duality between localized and itinerant nature of magnetism in\n$5\\textit{f}$ electron systems has been a longstanding puzzle. Here, we report\ninelastic neutron scattering measurements, which reveal both local and\nitinerant aspects of magnetism in a single crystalline system of\nUPt$_{2}$Si$_{2}$. In the antiferromagnetic state, we observe broad continuum\nof diffuse magnetic scattering with a resonance-like gap of $\\approx$ 7 meV,\nand surprising absence of coherent spin-waves, suggestive of itinerant\nmagnetism. While the gap closes above the Neel temperature, strong dynamic spin\ncorrelations persist to high temperature. Nevertheless, the size and\ntemperature dependence of the total magnetic spectral weight can be well\ndescribed by local moment with $J=4$. Furthermore, polarized neutron\nmeasurements reveal that the magnetic fluctuations are mostly transverse, with\nlittle or none of the longitudinal component expected for itinerant moments.\nThese results suggest that a dual description of local and itinerant magnetism\nis required to understand UPt$_{2}$Si$_{2}$, and by extension, other 5$f$\nsystems in general.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Vortex pattern in a nanoscopic cylinder: A superconducting nanoscopic cylinder, with radius $R = 4.0\\xi$ and height $D\n= 4.0\\xi$ is submitted to an applied field along the cylinder axis. The\nGinzburg-Landau theory is solved in three-dimensions using the simulated\nannealing technique to minimize the free energy functional. We obtain different\nvortex patterns, some of which are giant vortices and up to twelve vortices are\nable to fit inside the cylinder",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Thermal Conductivity as a Probe of Quasi-Particles in the Cuprates: In underdoped YBa_2Cu_3O_x (x=6.63), the low-T thermal conductivity Kappa_xx\nvaries steeply with field B at small B, and saturates to a nearly\nfield-independent value at high fields. The simple expression [1+p(T)|B|]^(-1)\nprovides an excellent fit to Kappa_xx(B) over a wide range of fields. From the\nfit, we extract the zero-field mean-free-path, and the low temperature behavior\nof the QP current. The procedure also allows the QP Hall angle Theta_QP to be\nobtained. We find that Theta_QP falls on the 1/T^2 curve extrapolated from the\nelectrical Hall angle above Tc. Moreover, it shares the same T dependence as\nthe field scale p(T) extracted from Kappa_xx. We discuss implications of these\nresults.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Comment on ``Critical current density from magnetization hysteresis data\n  using the critical-state model'': A recent paper [Physical Review {\\bf B 64} 014508 (2001)] claims to present\nan exact method to extract the critical current density J$_C$ from the M-H\nhysteresis curve. We show that this claim appears unjustified.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Vortex Phase Diagram of weakly pinned YBa$_2$Cu$_3$O$_{7-\u03b4}$ for H\n  $\\parallel$ c: Vortex phase diagram in a weakly pinned crystal of YBCO for H $\\parallel$ c\nis reviewed in the light of a recent elucidation of the process of `inverse\nmelting' in a Bismuth cuprate system and the imaging of an interface between\nthe ordered and the disordered regions across the peak effect in 2H-NbSe$_2$.\nIn the given YBCO crystal, a clear distinction can be made between the second\nmagnetization peak (SMP) and the peak effect (PE) between 65 K and 75 K. The\nfield region between the peak fields of the SMP (H$^m_{smp}$) and the onset\nfields of the PE (H$^{on}_{pe}$)is not only continuously connected to the Bragg\nglass phase at lower fields but it is also sandwiched between the higher\ntemperature vortex liquid phase and the lower temperature vortex glass phase.\nThus, an ordered vortex state between H$^m_{smp}$ and H$^{on}_{pe}$ can get\ntransformed to the (disordered) vortex liquid state on heating as well as to\nthe (disordered) vortex glass state on cooling, a situation analogous to the\nthermal melting and the inverse melting phenomenon seen in a Bismuth cuprate.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Microwave absorption in s- and d-wave disordered superconductors: We model s- and d-wave ceramic superconductors with a three-dimensional\nlattice of randomly distributed Josephson junctions with finite\nself-inductance. The field and temperature dependences of the microwave\nabsoption are obtained by solving the corresponding Langevin dynamical\nequations. We find that at magnetic field H=0 the microwave absoption of the\ns-wave samples, when plotted against the field, has a minimum at any\ntemperature. In the case of d-wave superconductors one has a peak at H=0 in the\ntemperature region where the paramagnetic Meissner effect is observable. These\nresults agree with experiments. The dependence of the microwave absorption on\nthe screening strength was found to be nontrivial due to the crossover from the\nweak to the strong screening regime.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "A Trapped Field of 17.6 T in Melt-Processed, Bulk Gd-Ba-Cu-O Reinforced\n  with Shrink-Fit Steel: The ability of large grain, REBa$_{2}$Cu$_{3}$O$_{7-\\delta}$ [(RE)BCO; RE =\nrare earth] bulk superconductors to trap magnetic field is determined by their\ncritical current. With high trapped fields, however, bulk samples are subject\nto a relatively large Lorentz force, and their performance is limited primarily\nby their tensile strength. Consequently, sample reinforcement is the key to\nperformance improvement in these technologically important materials. In this\nwork, we report a trapped field of 17.6 T, the largest reported to date, in a\nstack of two, silver-doped GdBCO superconducting bulk samples, each of diameter\n25 mm, fabricated by top-seeded melt growth (TSMG) and reinforced with\nshrink-fit stainless steel. This sample preparation technique has the advantage\nof being relatively straightforward and inexpensive to implement and offers the\nprospect of easy access to portable, high magnetic fields without any\nrequirement for a sustaining current source.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Low-temperature high-frequency dynamic magnetic susceptibility of\n  classical spin-ice Dy$_2$Ti$_2$O$_7$: Radio-frequency (14.6 MHz) AC magnetic susceptibility, $\\chi^{\\prime}_{AC}$,\nof \\dytio\\ was measured using a self-oscillating tunnel-diode resonator.\nMeasurements were made with the excitation AC field parallel to the\nsuperimposed DC magnetic field up 5 T in a wide temperature range from 50 mK to\n100 K. At 14.6 MHz a known broad peak of $\\chi^{\\prime}_{AC}(T)$ from kHz -\nrange audio-frequency measurements around 15~K for both [111] and [110]\ndirections shifts to 45~K, continuing the Arrhenius activated behavior with the\nsame activation energy barrier of $E_a \\approx 230$~K. Magnetic field\ndependence of $\\chi^{\\prime}_{AC}$ along [111] reproduces previously reported\nlow-temperature two-in-two-out to three-in-one-out spin configuration\ntransition at about 1~T, and an intermediate phase between 1 and 1.5~T. The\nboundaries of the intermediate phase show reasonable overlap with the\nliterature data and connect at a critical endpoint of the first-order\ntransition line, suggesting that these low-temperature features are frequency\nindependent. An unusual upturn of magnetic susceptibility at $T \\to 0$ was\nobserved in magnetic fields between 1.5~T and 2~T for both magnetic field\ndirections, before fully polarized configuration sets in above 2~T.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Single particle tunneling spectroscopy and superconducting gaps in\n  layered iron based superconductor KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$: We perform scanning tunneling microscopy/spectroscopy study on the layered\niron based superconductor KCa$_2$Fe$_4$As$_4$F$_2$ with a critical temperature\nof about 33.5 K. Two types of terminated surfaces are generally observed after\ncleaving the samples in vacuum. On one commonly obtained surface, we observe a\nfull gap feature with energy gap values close to 4.6 meV. This type of spectrum\nshows a clean and uniform full gap in space, which indicates the absence of gap\nnodes in this superconductor. Quasiparticle interference patterns have also\nbeen measured, which show no scattering patterns between the hole and tiny\nelectron pockets, but rather an intra-band scattering pattern is observed\npossibly due to the hole-like $\\alpha$ pocket. The Fermi energy of this band is\nonly about $24\\pm6$ meV as derived from the energy dispersion result.\nMeanwhile, impurity induced bound-state peaks can be observed at about $\\pm2.2$\nmeV on some spectra, and the peak value seems to be independent to magnetic\nfield. On the second type of surface which is rarely obtained, the fully gapped\nfeature can still be observed on the tunneling spectra, although multiple gaps\nare obtained either from a single spectrum or separate ones, and the gap values\ndetermined from coherence peaks locate mainly in the range from 4 to 8 meV. Our\nresults clearly indicate multiple and nodeless superconducting gap nature in\nthis layered superconductor KCa$_2$Fe$_4$As$_4$F$_2$, and the superfluid is\nmainly contributed by the hole-like Fermi surfaces near $\\Gamma$ point. This\nwould inspire further consideration on the effect of the shallow and incipient\nbands near M point, and help to understand the pairing mechanism in this highly\nlayered iron-based superconductor.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pair correlations in the two-orbital Hubbard ladder: Implications on\n  superconductivity in the bilayer nickelate La$_3$Ni$_2$O$_7$: Motivated by high-temperature superconductivity in pressurized\nLa$_3$Ni$_2$O$_7$, we investigate the pair correlations in the two-orbital\nHubbard ladder, which consists of the nearly half-filled and nearly\nquarter-filled orbitals. By employing the density matrix renormalization group\nmethod, we demonstrate that the pair correlation exhibits a power-law decay\nagainst the distance while the spin correlation decays exponentially. The decay\nexponent of the pair correlation of the nearly half-filled orbital is\ncomparable to the exponent of the quasi-long-range superconducting correlation\nin the doped single-orbital Hubbard ladder, which suggests the importance of\nthe $d_{3z^2-r^2}$ orbital in La$_3$Ni$_2$O$_7$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Topological Josephson Heat Engine: The promise of fault-tolerant quantum computing has made topological\nsuperconductors the focus of intense research during the past decade. In this\ncontext, topological Josephson junctions based on nanowires or on topological\ninsulators provide an alternative route for probing topological\nsuperconductivity. As a hallmark of their topological nature, such junctions\nexhibit a ground-state fermion parity that is $4\\pi$-periodic in the\nsuperconducting phase difference $\\phi$. Finding unambiguous experimental\nevidence for this $4\\pi$-periodicity still proves a difficult task, however.\nHere we propose a topological Josephson heat engine implemented by a\nJosephson-Stirling cycle as an alternative thermodynamic approach to test the\nground-state parity. Using a Josephson junction based on a quantum spin Hall\n(QSH) insulator, we show how the thermodynamic cycle can be used to test the\n$4\\pi$-periodicity of the topological ground state and to distinguish between\nparity-conserving and non-parity-conserving engines. Interestingly, we find\nthat parity conservation generally boosts both the efficiency and power of the\ntopological heat engine with respect to its non-topological counterpart. Our\nresults, applicable not only to QSH-based junctions but also to any topological\nJosephson junction, demonstrate the potential of the intriguing and fruitful\nmarriage between topology and coherent thermodynamics.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Polar Kerr effect from a time-reversal symmetry breaking unidirectional\n  charge density wave: We analyze the Hall conductivity $\\sigma_{xy}(\\omega)$ of a charge ordered\nstate with momentum $\\mathbf{Q}=(0,2Q)$ and calculate the intrinsic\ncontribution to the Kerr angle $\\Theta_K$ using the fully reconstructed\ntight-binding band structure for layered cuprates beyond the low energy hot\nspots model and particle hole symmetry. We show that such a unidirectional\ncharge density wave (CDW), which breaks time reversal symmetry as recently put\nforward by Wang and Chubukov [Phys. Rev. B {\\bf 90}, 035149 (2014)], leads to a\nnonzero polar Kerr effect as observed experimentally. In addition, we model a\nfluctuating CDW via a large quasiparticle damping of the order of the CDW gap\nand discuss possible implications for the pseudogap phase. We can qualitatively\nreproduce previous measurements of underdoped cuprates but making quantitative\nconnections to experiments is hampered by the sensitivity of the polar Kerr\neffect with respect to the complex refractive index $n(\\omega)$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "A possible cooling effect in high temperature superconductors: We show that an adiabatic increase of the supercurrent along a superconductor\nwith lines of nodes of the order parameter on the Fermi surface can result in a\ncooling effect. The maximum cooling occurs if the supercurrent increases up to\nits critical value. The effect can also be observed in a mixed state of a bulk\nsample. An estimate of the energy dissipation shows that substantial cooling\ncan be performed during a reasonable time even in the microkelvin regime.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Spin-pumping from a ferromagnetic insulator to an unconventional\n  superconductor with interfacial Andreev bound-states: Spin-pumping from a ferromagnetic insulator into a high-$T_c$ superconductor\nwith a $d$-wave superconducting order parameter has recently been\nexperimentally observed. Such unconventional superconducting order is known to\nproduce interfacial bound-states for certain crystallographic orientations.\nHere, we present a methodology which can be used to study spin-pumping into\nunconventional superconductors, including the role of interfacial bound-states.\nAs an example, we determine how the crystallographic orientation of the\n$d$-wave order parameter relative the interface changes the spin-pumping\neffect. We find that the spin-pumping effect is slightly enhanced at low\ntemperatures for orientations hosting interfacial bound-states compared to\nother superconducting states. However, the spin-pumping effect does not show a\ncoherence peak close to $T_c$ for such orientations, and instead remains\nsmaller than the normal state value for all $T$. For orientations not hosting\ninterfacial bound-states, we find that the pumped spin current can be increased\nto several times the normal-state spin current at frequencies that are small\ncompared to the superconducting gap. Our results show that the spin-pumping\ndependency on frequency and temperature changes qualitatively depending on the\ncrystallographic orientation of unconventional superconducting order parameters\nrelative the interface.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Impurity bands in magnetic superconductors with spin density wave: Magnetic superconductors define a broad class of strongly correlated\nmaterials in which superconductivity may coexist with either localized or\nitinerant long-range magnetic order. In this work we consider a multiband model\nof a disordered magnetic superconductor which realizes coexistence of\nunconventional superconductivity and a spin-density-wave. We derive an exact\n$T$-matrix and compute a single particle density of states in this system. In a\npurely superconducting state the interband scattering potential leads to an\nappearance of the localized Yu-Shiba-Rusinov bound states. Our main finding is\nthat in the fairly broad swath of the coexistence region superconductivity\nremains fully gapped despite the presence of the impurity bands. We also\ndiscuss the effects of spatial inhomogeneities on the density of states in\nstrongly contaminated superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quasi-1-Dimensional Superconductivity in Highly Disordered NbN Nanowires: The topic of superconductivity in strongly disordered materials has attracted\na significant attention. In particular vivid debates are related to the subject\nof intrinsic spatial inhomogeneity responsible for non-BCS relation between the\nsuperconducting gap and the pairing potential. Here we report experimental\nstudy of electron transport properties of narrow NbN nanowires with effective\ncross sections of the order of the debated inhomogeneity scales. We find that\nconventional models based on phase slip concept provide reasonable fits for the\nshape of the R(T) transition curve. Temperature dependence of the critical\ncurrent follows the text-book Ginzburg-Landau prediction for\nquasi-one-dimensional superconducting channel Ic~(1-T/Tc)^3/2. Hence, one may\nconclude that the intrinsic electronic inhomogeneity either does not exist in\nour structures, or, if exist, does not affect their resistive state properties.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Overscreening in Hubbard electron systems: We show that doping-induced charge fluctuations in strongly correlated\nHubbard electron systems near the 1/2-filled, insulating limit cause\noverscreening of the electron-electron Coulomb repulsion. The resulting\nattractive screened interaction potential supports d_{x^2-y^2}-pairing with a\nstrongly peaked, doping dependent pairing strength at lower doping, followed by\ns-wave pairing at larger doping levels.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Unconventional dynamical covalency driven superconductivity in Nb doped\n  SrTiO3: Nb doped SrTiO3, the first discovered two-gap superconductor, is shown to be\nthe most unconventional one of the known multiband superconductors, since the\nsmaller of the two superconducting gaps follows a non BCS temperature\ndependence. Such a behavior stems from two cooperating effects: an extreme\nanisotropy in the frequency dependent interactions, involving one very soft\nmode and an almost vanishing interband interaction. In contrast to all other\nmultiband superconductors, the temperature dependence of the superfluid density\nof Nb doped SrTiO3 is predicted to exhibit an inflection point close to Tc and\nnot close to T=0.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Optimization of parameters of nanostructure for study inverse proximity\n  effects on \"superconductor-ferromagnetic\" interface using Polarized Neutron\n  Reflectometry in enhanced standing wave regime: This work is devoted to experimental study of influence of superconductivity\n(S) on ferromagnetism (FM) (inverse proximity effects) with the help of\nPolarized Neutron Reflectivity. Combining meausurements of specular and diffuse\nintensities it is possible to obtain full picture of magnetization change in\nS/FM layered systems like magnetization rotation, domain state formation,\ninducing of magnetization in S layer, etc. To increase weak magnetic signal we\npropose to use enhanced neutron standing wave regime (e.g. waveguides). Choose\nof materials, optimization of thicknesses of layers, estimation of roughnesses\ninfluence is presented in this work.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Evidence for an angular dependent contribution from columnar defects to\n  the equilibrium magnetization of YBa_2Cu_3O_{7-d}: We have measured an angle-dependent contribution to the equilibrium\nmagnetization of a YBa_2Cu_3O_{7-d} single crystal with columnar defects\ncreated by 5.8GeV Pb ions. This contribution manifests itself as a jump in the\nequilibrium torque signal, when the magnetic field crosses that of the defects.\nThe magnitude of the signal, which is observed in a narrow temperature interval\nless than 2K and for fields up to about twice the irradiation field, is used to\nestimate the the energy gained by vortex pinning on the defects. The vanishing\nof the effective pinning energy at a temperature below Tc is attributed to its\nrenormalization by thermal fluctuations.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Ground state angular momentum, spectral asymmetry, and topology in\n  chiral superfluids and superconductors: Recently it was discovered that the ground state orbital angular momentum in\ntwo-dimensional chiral superfluids with pairing symmetry $(p_x+ip_y)^\\nu$\ndepends on the winding number $\\nu$ in a striking manner. The ground state\nvalue for the $\\nu=1$ case is $L_z=\\hbar N/2$ as expected by counting the\nCooper pairs, while a dramatic cancellation takes place for $\\nu>1$. The origin\nof the cancellation is associated with the topological edge states that appear\nin a finite geometry and give rise to a spectral asymmetry. Here we study the\nreduction of orbital angular momentum for different potential profiles and\npairing strengths, showing that the result $L_z=\\hbar N/2$ is robust for\n$\\nu=1$ under all studied circumstances. We study how angular momentum depends\non the gap size $\\Delta/E_F$ and obtain the result $L_z=\\frac{\\hbar\\nu}{2}\nN(1-\\frac{\\mu}{E_F})$ for $\\nu=2,3$. Thus, the gap-dependence of $L_z$ for\n$\\nu<4$ enters at most through the chemical potential while $\\nu\\geq4$ is\nqualitatively different. In addition, we generalize the spectral asymmetry\narguments to \\emph{total} angular momentum in the ground state of triplet\nsuperfluids where due to a spin-orbit coupling $L_z$ is not a good quantum\nnumber. We find that the ground state total angular momentum also behaves very\ndifferently depending on total angular momentum of the Cooper pairs.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Bifurcation structure and chaos in nanomagnet coupled to Josephson\n  junction: Recently an irregular easy axis reorientation demonstrating the Kapitza\npendulum features were observed in numerical simulations of nanomagnet coupled\nto the Josephson junction. To explain its origin we study the magnetization\nbifurcations and chaos which appear in this system due to interplay of\nsuperconductivity and magnetism. The bifurcation structure of the magnetization\nunder the variation of Josephson to magnetic energy ratio as a control\nparameter demonstrates several precessional motions. They are related to\nchaotic behavior, bistability, and multiperiodic orbits in the ferromagnetic\nresonance region. Effect of external periodic drive on the bifurcation\nstructure is investigated. The results demonstrate high-frequency modes of\nperiodic motion and chaotic response near resonance. Far from the ferromagnetic\nresonance we observe a quasiperiodic behavior.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Change of the vortex core structure in two-band superconductors at\n  impurity-scattering-driven $s_\\pm/s_{++}$ crossover: We report a nontrivial transition in the core structure of vortices in\ntwo-band superconductors as a function of interband impurity scattering. We\ndemonstrate that, in addition to singular zeros of the order parameter, the\nvortices there can acquire a circular nodal line around the singular point in\none of the superconducting components. It results in the formation of the\npeculiar \"moat\"-like profile in one of the superconducting gaps. The moat-core\nvortices occur generically in the vicinity of the impurity-induced crossover\nbetween $s_{\\pm}$ and $s_{++}$ states.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Temperature dependence of iron local magnetic moment in phase-separated\n  superconducting chalcogenide: We have studied local magnetic moment and electronic phase separation in\nsuperconducting K$_{x}$Fe$_{2-y}$Se$_2$ by x-ray emission and absorption\nspectroscopy. Detailed temperature dependent measurements at the Fe K-edge have\nrevealed coexisting electronic phases and their correlation with the transport\nproperties. By cooling down, the local magnetic moment of Fe shows a sharp drop\nacross the superconducting transition temperature (T$_c$) and the coexisting\nphases exchange spectral weights with the low spin state gaining intensity at\nthe expense of the higher spin state. After annealing the sample across the\niron-vacancy order temperature, the system does not recover the initial state\nand the spectral weight anomaly at T$_c$ as well as superconductivity\ndisappear. The results clearly underline that the coexistence of the low spin\nand high spin phases and the transitions between them provide unusual magnetic\nfluctuations and have a fundamental role in the superconducting mechanism of\nelectronically inhomogeneous K$_{x}$Fe$_{2-y}$Se$_2$ system.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Time relaxation of microwave second order response of superconductors in\n  the critical state: Relaxation of the microwave second order response of YBa2Cu3O7 and\nBa0.6K0.4BiO3 crystals in the critical state is studied. The samples are\nexposed to static and pulsed microwave magnetic fields. The second harmonic\nsignals decay during the time in which the microwave pulse endures. The decay\ntimes depend on the supercon-ductor investigated and on the way the value of\nthe static field has been reached.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Paramagnetic Effects in the Vortex Lattice Field Distribution of\n  Strongly Type-II Superconductors: We present an analysis of the magnetic field distribution in the Abrikosov\nlattice of high-$\\kappa$ superconductors with singlet pairing in the case where\nthe critical field is mainly determined by the Pauli limit and the superfluid\ncurrents partly come from the paramagnetic interaction of electron spins with\nthe local magnetic field. The derivation is performed in the frame of the\ngeneralized Clem variational method which is valid not too close to the\ncritical field and furthermore with the Abrikosov type theory in the vicinity\nof it. The found vortex lattice form factor increases with increasing field and\nthen falls down at approach of the upper critical field where the\nsuperconducting state is suppressed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quasiparticle dispersion and lineshape in a strongly correlated liquid\n  with the fermion condensate: A model of a strongly correlated electron liquid based on the fermion\ncondensation (FC) is applied to the consideration of high temperature\nsuperconductors in its superconducting and normal states. Within our model the\nappearance of FC presents a boundary, separating the region of strongly\ninteracting electron liquid from the region of strongly correlated electron\nliquid. We show that at temperatures $T<T_c$ the quasiparticle dispersion in\nsystems with FC can be presented by two straight lines, characterized by\neffective masses $M^*_{FC}$ and $M^*_L$, respectively, and intersecting near\nthe binding energy which is of the order of the superconducting gap. This same\npicture persists in the normal state being modified only by the presence of the\nquasiparticle width. We argue that this strong change of the quasiparticle\ndispersion can be enhanced in underdoped samples due to strengthening the FC\ninfluence. The single particle excitation width and lineshape are also studied.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Stacked Josephson junction SQUID: Operation of a Superconducting Quantum Interference Device (SQUID) made of\nstacked Josephson junctions is analyzed numerically for a variety of junction\nparameters. Due to a magnetic coupling of junctions in the stack, such a SQUID\nhas certain advantages as compared to an uncoupled multi-junction SQUID.\nNamely, metastability of current-flux modulation can be reduced and a\nvoltage-flux modulation can be improved if junctions in the stack are\nphase-locked. Optimum operation of the SQUID is expected for moderately long,\nstrongly coupled stacked Josephson junctions. A possibility of making a stacked\nJosephson junction SQUID based on intrinsic Josephson junctions in high-Tc\nsuperconductor is discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Depinning of a vortex chain in a disordered flow channel: We study depinning of vortex chains in channels formed by static, disordered\nvortex arrays. Depinning is governed either by the barrier for defect\nnucleation or for defect motion, depending on whether the chain periodicity is\ncommensurate or incommensurate with the surrounding arrays. We analyze the\nreduction of the gap between these barriers as function of disorder. At large\ndisorder, commensurability becomes irrelevant and the pinning force is reduced\nto a small fraction of the ideal shear strength of ordered channels.\nImplications for experiments on channel devices are discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Antiferromagnetically Driven Electronic Correlation in Iron Pnictides\n  and Cuprates: The iron pnictides and the cuprates represent two families of materials,\nwhere strong antiferromagnetic correlation drives three other distinct ordering\ntendencies: (1) superconducting pairing, (2) Fermi surface distortion, and (3)\norbital current order. We propose that (1)-(3) and the antiferromagnetic\ncorrelation are the hallmarks of a class of strongly correlated materials to\nwhich the cuprates and pnictides belong. In this paper we present the results\nof the functional renormalization group studies to support the above claim. In\naddition, we show that as a function of the interlayer hopping parameter, the\ndouble layer Hubbard model nicely interpolates between the cuprate and the iron\npnictide physics. Finally, as a check, we will present the renormalization\ngroup study of a ladder version of the iron pnictide, and compare the results\nto those of the two-dimensional model.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Emergence of Superconductivity from Fully Incoherent Normal State in an\n  Iron-Based Superconductor (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$: In unconventional superconductors, it is generally believed that\nunderstanding the physical properties of the normal state is a pre-requisite\nfor understanding the superconductivity mechanism. In conventional\nsuperconductors like niobium or lead, the normal state is a Fermi liquid with a\nwell-defined Fermi surface and well-defined quasipartcles along the Fermi\nsurface. Superconductivity is realized in this case by the Fermi surface\ninstability in the superconducting state and the formation and condensation of\nthe electron pairs (Cooper pairing). The high temperature cuprate\nsuperconductors, on the other hand, represent another extreme case that\nsuperconductivity can be realized in the underdoped region where there is\nneither well-defined Fermi surface due to the pseudogap formation nor\nquasiparticles near the antinodal regions in the normal state. Here we report a\nnovel scenario that superconductivity is realized in a system with well-defined\nFermi surface but without quasiparticles along the Fermi surface in the normal\nstate. High resolution laser-based angle-resolved photoemission measurements\nhave been performed on an optimally-doped iron-based superconductor\n(Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$. We find that, while sharp superconducting\ncoherence peaks emerge in the superconducting state on the hole-like Fermi\nsurface sheets, no quasiparticle peak is present in the normal state. Its\nelectronic behaviours deviate strongly from a Fermi liquid system. The\nsuperconducting gap of such a system exhibits an unusual temperature dependence\nthat it is nearly a constant in the superconducting state and abruptly closes\nat T$_c$. These observations have provided a new platform to study\nunconventional superconductivity in a non-Fermi liquid system.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Enhancement of the retrapping current of superconducting microbridges of\n  finite length: We theoretically find that the resistance of a superconducting\nmicrobridge/nanowire {\\it decreases} while the retrapping current $I_r$ for the\ntransition to the superconducting state {\\it increases} when one suppresses the\nmagnitude of the order parameter $|\\Delta|$ in the attached superconducting\nleads. This effect is a consequence of the increased energy interval for\ndiffusion of the 'hot' nonequilibrium quasiparticles (induced by the\noscillations of $|\\Delta|$ in the center of the microbridge) to the leads. The\neffect is absent in short microbridges (with length less than the coherence\nlength) and it is relatively weak in long microbridges (with length larger than\nthe inelastic relaxation length of the nonequilibrium distribution function). A\nnonmonotonous dependence of $I_r$ on the length of the microbridge is\npredicted. Our results are important for the explanation of the enhancement of\nthe critical current and the appearance of negative magnetoresistance observed\nin many recent experiments on superconducting microbridges/nanowires.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Hysteresis in mesoscopic superconducting disks: the Bean-Livingston\n  barrier: The magnetization behavior of mesoscopic superconducting disks can show\nhysteretic behavior which we explain by using the Ginzburg-Landau (GL) theory\nand properly taking into account the de-magnetization effects due to\ngeometrical form factors. In large disks the Bean-Livingston surface barrier is\nresponsible for the hysteresis. While in small disks a volume barrier is\nresponsible for this hysteresis. It is shown that although the sample\nmagnetization is diamagnetic (negative), the measured magnetization can be\npositive at certain fields as observed experimentally, which is a consequence\nof the de-magnetization effects and the experimental set up.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity and Charge Density Wave of CuIr2Te4 by Iodine Doping: Here we report a systematic investigation on the evolution of the structural\nand physical properties, including the charge density wave and\nsuperconductivity of the polycrystalline CuIr2Te4-xIx. X-ray diffraction\nresults indicate that both of a and c lattice parameters increase linearly. The\nresistivity measurements indicate that the charge density wave is destabilized\nwith slight x but reappears when x is large than 0.9. Meanwhile, the\nsuperconducting transition temperature enhances as x raises and reaches a\nmaximum value of around 2.95 K for the optimal composition CuIr2Te3.9I0.1\nfollowed by a slight decrease with higher iodine doping content. The specific\nheat jump for the optimal composition CuIr2Te3.9I0.1 is approximately 1.46,\nwhich is close to the Bardeen Cooper Schrieffer value which is 1.43, indicating\nit is a bulk superconductor. The results of thermodynamic heat capacity\nmeasurements under different magnetic fields, magnetization and\nmagneto-transport measurements further suggest that CuIr2Te4-xIx bulks are type\nII superconductors. Finally, an electronic phase diagram for this CuIr2Te4-xIx\nsystem has been constructed. The present study provides a suitable material\nplatform for further investigation of the interplay of the CDW and\nsuperconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quantum phase slip phenomenon in superconducting nanowires with\n  low-Ohmic environment: In a number of recent experiments it has been demonstrated that in\nultra-narrow superconducting channels quantum fluctuations of the order\nparameter, alternatively called quantum phase slips, are responsible for the\nfinite resistance well below the critical temperature. The acceptable agreement\nbetween those experiments and the models describing quantum fluctuations in\nquasi-one-dimensional superconductors has been established. However the very\nconcept of the phase slip is justified when these fluctuations are the\nrelatively rare events, meaning that the effective resistance of the system\nshould be much smaller than the normal state equivalent. In this paper we study\nthe limit of the strong quantum fluctuations where the existing models are not\napplicable. In particular case of ultra-thin titanium nanowires it is\ndemonstrated that below the expected critical temperature the resistance does\nnot demonstrate any trend towards the conventional for a superconductor\nzero-resistivity state even at negligibly small measuring currents. Application\nof a small magnetic field leads to an unusual negative magnetoresistance, which\nbecomes more pronounced at lower temperatures. The origin of the negative\nmagnetoresistance effect is not clear.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Doping dependence of electromagnetic response in cuprate superconductors: The study of the electromagnetic response in cuprate superconductors plays a\ncrucial role in the understanding of the essential physics of these materials.\nHere the doping dependence of the electromagnetic response in cuprate\nsuperconductors is studied within the kinetic-energy driven superconducting\nmechanism. The kernel of the response function is evaluated based on the linear\nresponse approximation for a purely transverse vector potential, and can be\nbroken up into its diamagnetic and paramagnetic parts. In particular, this\nparamagnetic part exactly cancels the corresponding diamagnetic part in the\nnormal-state, and then the Meissner effect is obtained within the entire\nsuperconducting phase. Following this kernel of the response function, the\nelectromagnetic response calculation in terms of the specular reflection model\nqualitatively reproduces many of the striking features observed in the\nexperiments. In particular, the local magnetic-field profile follows an\nexponential law, while the superfluid density exhibits the nonlinear\ntemperature behavior at the lowest temperatures, followed by the linear\ntemperature dependence extending over the most of the superconducting\ntemperature range. Moreover, the maximal value of the superfluid density occurs\nat around the critical doping $\\delta_{\\rm critical}\\sim 0.16$, and then\ndecreases in both lower doped and higher doped regimes. The theory also shows\nthat the nonlinear temperature dependence of the superfluid density at the\nlowest temperatures can be attributed to the nonlocal effects induced by the\nd-wave gap nodes on the electron Fermi surface.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Critical behavior at superconductor-insulator phase transitions near one\n  dimension: I argue that the system of interacting bosons at zero temperature and in\nrandom external potential possesses a simple critical point which describes the\nproliferation of disorder-induced topological defects in the superfluid ground\nstate, and which is located at weak disorder close to and above one dimension.\nThis makes it possible to address the critical behavior at the superfluid-Bose\nglass transition in dirty boson systems by expanding around the lower critical\ndimension d=1. Within the formulated renormalization procedure near d=1 the\ndynamical critical exponent is obtained exactly and the correlation length\nexponent is calculated as a Laurent series in the parameter \\sqrt{\\epsilon},\nwith \\epsilon=d-1: z=d, \\nu=1/\\sqrt{3\\epsilon} for the short range, and z=1,\n\\nu=\\sqrt{2/3\\epsilon}, for the long-range Coulomb interaction between bosons.\nThe identified critical point should be stable against the residual\nperturbations in the effective action for the superfluid, at least in\ndimensions 1\\leq d \\leq 2, for both short-range and Coulomb interactions. For\nthe superfluid-Mott insulator transition in the system in a periodic potential\nand at a commensurate density of bosons I find \\nu=(1/2\\sqrt{\\epsilon})+\n1/4+O(\\sqrt{\\epsilon}), which yields a result reasonably close to the known XY\ncritical exponent in d=2+1. The critical behavior of the superfluid density,\nphonon velocity and the compressibility in the system with the short-range\ninteractions is discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Fractional Josephson Vortices and Braiding of Majorana Zero Modes in\n  Planar Superconductor-Semiconductor Heterostructures: We consider the one-dimensional (1D) topological superconductor that may form\nin a planar superconductor-metal-superconductor Josephson junction in which the\nmetal is is subjected to spin orbit coupling and to an in-plane magnetic field.\nThis 1D topological superconductor has been the subject of recent theoretical\nand experimental attention. We examine the effect of perpendicular magnetic\nfield and a supercurrent driven across the junction on the position and\nstructure of the Majorana zero modes that are associated with the topological\nsuperconductor. In particular, we show that under certain conditions the\nJosephson vortices fractionalize to half-vortices, each carrying half of the\nsuperconducting flux quantum and a single Majorana zero mode. Furthemore, we\nshow that the system allows for a current-controlled braiding of Majorana zero\nmodes.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Is the anisotropy of the upper critical field of Sr$_2$RuO$_4$\n  consistent with a helical $p$-wave state?: We calculate the angular and temperature $T$ dependencies of the upper\ncritical field $H_{c2}(\\theta,\\phi,T)$ for the $C_{4v}$ point group helical\n$p$-wave states, assuming a single uniaxial ellipsoidal Fermi surface, Pauli\nlimiting, and strong spin-orbit coupling that locks the spin-triplet $\\vec{\\bf\nd}$-vectors onto the layers. Good fits to the Sr$_2$RuO$_4$\n$H_{c2,a}(\\theta,T)$ data of Kittaka {\\it et al.} [Phys. Rev. B {\\bf 80},\n174514 (2009)] are obtained. Helical states with $\\vec{\\bf d}(\\vec{\\bf\nk})=k_x\\vec{\\bf x}-k_y\\vec{\\bf y}$ and $k_y\\vec{\\bf x}+k_x\\vec{\\bf y}$ (or\n$k_x\\vec{\\bf x}+k_y\\vec{\\bf y}$ and $k_y\\vec{\\bf x}-k_x\\vec{\\bf y}$) produce\n$H_{c2}(90^{\\circ},\\phi,T)$ that greatly exceed (or do not exhibit) the\nfour-fold azimuthal anisotropy magnitudes observed in Sr$_2$RuO$_4$ by Kittaka\n{\\it et al.} and by Mao {\\it et al.} [Phys. Rev. Lett. {\\bf 84}, 991 (2000)],\nrespectively.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Renormalized q-dependent Spin Susceptibility by inverting the Random\n  Phase Approximation: Implications for quantitative assessment of the role of\n  spin fluctuations in 2D Ising superconductor NbSe$_{2}$: Accurate determination of the full momentum-dependent spin susceptibility\n$\\chi(\\mathbf{q}) $ is very important for the description of magnetism and\nsuperconductivity. While in principle the formalism for calculating\n$\\chi(\\mathbf{q})$ in the linear response density functional theory (DFT) is\nwell established, hardly any publicly available code includes this capability.\nHere, we describe an alternative way to calculate the static\n$\\chi(\\mathbf{q})$, which can be applied to most common DFT codes without\nadditional programming. The method combined standard fixed-spin-moment\ncalculations of $\\chi(\\mathbf{0}) $ with direct calculations of the energy of\nspin spirals stabilized by an artificial Hubbard interaction. From these\ncalculations, $\\chi_{DFT}(\\mathbf{q} )$ can be extracted by inverting the RPA\nformula. We apply this recipe to the recently discovered Ising\nsuperconductivity in NbSe$_2$ monolayer, one of the most exciting findings in\nsuperconductivity in recent years. It was proposed that spin fluctuations may\nstrongly affect the parity of the order parameter. Previous estimates suggested\nproximity to ferromagnetism, $i.e.$, $\\chi(\\mathbf{q})$ peaked at\n$\\mathbf{q}=0$. We find that the structure of spin fluctuations is more\ncomplicated, with the fluctuation spectrum sharply peaked at $\\mathbf{q}\\approx\n(0.2,0)$. Such a spectrum would change the interband pairing interaction and\nconsiderably affect the superconducting state.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Nonlinear Coupling in Nb/NbN Superconducting Microwave Resonators: In this experimental study we show that the coupling between Nb/NbN\nsuperconducting microwave resonators and their feedline can be made amplitude\ndependent. We employ this mechanism to tune the resonators into critical\ncoupling condition, a preferable mode of operation for a wide range of\napplications. Moreover we examine the dependence of critical coupling state on\nother parameters such as temperature and magnetic field. Possible novel\napplications based on such nonlinear coupling are briefly discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Local dynamical lattice instabilities: Prerequisites for resonant\n  pairing superconductivity: Fluctuating local diamagnetic pairs of electrons, embedded in a Fermi sea,\nare candidates for non-phonon-mediated superconductors without the stringent\nconditions on Tc which arise in phonon-mediated BCS classical low-Tc\nsuperconductors. The local accumulations of charge, from which such diamagnetic\nfluctuations originate, are irrevocably coupled to local dynamical lattice\ninstabilities and form composite charge-lattice excitations of the system. For\na superconducting phase to be realized, such excitations must be itinerant\nspatially phase-coherent modes. This can be achieved by resonant pair tunneling\nin and out of polaronic cation-ligand sites. Materials in which\nsuperconductivity driven by such local lattice instability can be expected,\nhave a Tc which is controlled by the phase stiffness rather than the amplitude\nof the diamagnetic pair fluctuations. Above Tc, a pseudogap phase will be\nmaintained up to a T*, where this pairing amplitude disappears. We discuss the\ncharacteristic local charge and lattice properties which characterize this\npseudogap phase and which form the prerequisites for establishing a\nphase-coherent macroscopic superconducting state.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "A close look at antiferromagnetism in multidimensional phase diagram of\n  electron-doped copper oxide: Emergency of superconductivity at the instabilities of antiferromagnetism\n(AFM), spin/charge density waves has been widely recognized in unconventional\nsuperconductors. In copper-oxide superconductors, spin fluctuations play a\npredominant role in electron pairing with electron dopants yet composite orders\nveil the nature of superconductivity for hole-doped family. However, in\nelectron-doped ones the ending point of AFM is still in controversy for\ndifferent probes or its sensitivity to oxygen content. Here, by carefully\ntuning the oxygen content, a systematic study of Hall signal and\nmagnetoresistivity up to 58 Tesla on optimally doped La2-xCexCuO4+-{\\delta} (x\n= 0.10) thin films identifies two characteristic temperatures at 62.5+-7.5 K\nand 25+-5 K. The former is quite robust whereas the latter becomes flexible\nwith increasing magnetic field, thereby linked to two- and three-dimensional\nAFM, evident from the multidimensional phase diagram as a function of oxygen as\nwell as Ce dopants. Consequently, the observation of extended AFM phase in\ncontrast to {\\mu}SR probe corroborates an elevated critical doping in field,\nproviding an unambiguous picture to understand the interactions between AFM and\nsuperconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Sensitivity of the superconducting state and magnetic susceptibility to\n  key aspects of electronic structure in ferropnictides: Experiments on the iron-pnictide superconductors appear to show some\nmaterials where the ground state is fully gapped, and others where low-energy\nexcitations dominate, possibly indicative of gap nodes. Within the framework of\na 5-orbital spin fluctuation theory for these systems, we discuss how changes\nin the doping, the electronic structure or interaction parameters can tune the\nsystem from a fully gapped to nodal sign-changing gap with s-wave ($A_{1g}$)\nsymmetry ($s^\\pm$). In particular we focus on the role of the hole pocket at\nthe $(\\pi,\\pi)$ point of the unfolded Brillouin zone identified as crucial to\nthe pairing by Kuroki {\\it et al.}, and show that its presence leads to\nadditional nesting of hole and electron pockets which stabilizes the isotropic\n$s^\\pm$ state. The pocket's contribution to the pairing can be tuned by doping,\nsurface effects, and by changes in interaction parameters, which we examine.\nAnalytic expressions for orbital pairing vertices calculated within the RPA\nfluctuation exchange approximation allow us to draw connections between aspects\nof electronic structure, interaction parameters, and the form of the\nsuperconducting gap.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "A new seeding technique for the reliable fabrication of large, SmBCO\n  single grains containing silver using top seeded melt growth: Silver (Ag) is an established additive for improving the mechanical\nproperties of single grain, (RE)BCO bulk superconductors (where RE = Sm, Gd and\nY). The presence of Ag in the (RE)BCO bulk composition, however, typically\nreduces the melting temperature of the single crystal seed in the top seeded\nmelt growth (TSMG) process, which complicates significantly the controlled\nnucleation and subsequent epitaxial growth of a single grain, which is\nessential for high field engineering applications. The reduced reliability of\nthe seeding process in the presence of Ag is particularly acute for the SmBCO\nsystem, since the melting temperature of SmBCO is very close to that of the\ngeneric NdBCO(MgO) seed. SmBCO has the highest superconducting transition\ntemperature, Tc, and exhibits the most pronounced \"peak\" effect at higher\nmagnetic field of all materials in the family of (RE)BCO bulk superconductors\nand, therefore, has the greatest potential for use in practical applications\n(compared to GdBCO and YBCO, in particular). Development of an effective\nseeding process, therefore, is one of the major challenges of the TSMG process\nfor the growth of large, high quantity single grain superconductors. In this\npaper, we report a novel technique that involves introducing a buffer layer\nbetween the seed crystal and the precursor pellet, primarily to inhibit the\ndiffusion of Ag from the green body to the seed during melt processing in order\nto prevent the melting of the seed. The success rate of the seeding process\nusing this technique is 100% for relatively small batch samples. The\nsuperconducting properties, Tc, Jc and trapped fields, of the single grains\nfabricated using the buffers are reported and the micro-structures in the\nvicinity of the buffer of single grains fabricated by the modified technique\nare analysed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electronic Structure of\n  $\\mathrm{Sr}_{1-y}\\mathrm{Ca}_{y}\\mathrm{Fe_{2}}(\\mathrm{As}_{1-x}\\mathrm{P}_{x}\\mathrm{)_{2}}$\n  ($x$ = 0.25, $y$ = 0.08) Revealed by Angle-Resolved Photoemission\n  Spectroscopy: We have investigated the electronic structure of\n$\\mathrm{Sr}_{1-y}\\mathrm{Ca}_{y}\\mathrm{Fe_{2}}(\\mathrm{As}_{1-x}\\mathrm{P}_{x}\\mathrm{)_{2}}$\n($x$ = 0.25, $y$ = 0.08) by means of angle-resolved photoemission spectroscopy.\nFrom the comparison with the results of\n$\\mathrm{BaFe_{2}}(\\mathrm{As}_{1-x}\\mathrm{P}_{x}\\mathrm{)_{2}}$, the effects\nof smaller structural anisotropy ($c/a$) on the Fermi surfaces (FSs) and the\ngap structures are discussed. The observed FSs have three dimensional shapes.\nOne of the hole FSs is strongly warped between the $\\Gamma$ and $Z$ points, and\nthe innermost FS observed at the $Z$ point disappears at the $\\Gamma$ point,\nwhich is similar to the FS features of\n$\\mathrm{SrFe_{2}}(\\mathrm{As}_{1-x}\\mathrm{P}_{x}\\mathrm{)_{2}}$ ($x$ = 0.35).\nIn the superconducting state, the node like gap-minimum is present for the\n$d_{xy}$ electron FS near the $X$ point, while the gaps around the other high\nsymmetry points are isotropic. Several theoretical models based on the spin\nand/or the orbital fluctuation are examined to explain all these experimental\nresults.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Nonlinear Seebeck Effect in a Model Granular Superconductor: The change of the Josephson supercurrent density of a weakly-connected\ngranular superconductor in response to externally applied arbitrary thermal\ngradient dT/dx (nonlinear Seebeck effect) is considered within a model of 3D\nJosephson junction arrays. For dT/dx>(dT/dx)_c, where (dT/dx)_c is estimated to\nbe of the order of 10^4 K/m for YBCO ceramics with an average grain's size of\n10 microns, the weak-links-dominated thermopower S (Seebeck coefficient) is\npredicted to become strongly dT/dx-dependent.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Stability of Mixed-Symmetry Superconducting States with Broken\n  Time-Reversal Symmetry against Lattice Distortions: We examine the stability of mixed-symmetry superconducting states with broken\ntime-reversal symmetry in spatial-symmetry-broken systems, including chiral\nstates, on the basis of the free-energy functional derived in the weak-coupling\ntheory. We consider a generic a_1 + i a_2 wave state, with a_1 and a_2 being\ndifferent symmetry indices such as (a_1,a_2) = (d,s), (p_x,p_y), and (d,d').The\ntime-reversal symmetry of the mixed-symmetry state with the a_1- and a_2-wave\ncomponents is broken when the phases of these components differ, and such a\nstate is called the time-reversal-symmetry breaking (TRSB) state. However,\ntheir phases are equated by Cooper-pair scattering between these components if\nit occurs; i.e., when the off-diagonal elements S_{a_1 a_2} = S_{a_2 a_1} of\nthe scattering matrix are nonzero, they destabilize the TRSB state. Hence, it\nhas often been believed that the TRSB state is stable only in systems with a\nspatial symmetry that guarantees S_{a_1 a_2}=0. We note that, contrary to this\nbelief, the TRSB state can remain stable in systems without the spatial\nsymmetry when the relative phase shifts so that S_{a_1 a_2} = 0 is restored,\nwhich results in a distorted TRSB (a_1 + a_2) + i a_2 wave state. Here, note\nthat the restoration of S_{a_1 a_2} = 0 does not imply that the symmetry of the\nquasi-particle energy E_k is recovered. This study shows that such\nstabilization of the TRSB state occurs when the distortion is sufficiently\nsmall and \\Delta_{a_1} \\Delta_{a_2} is sufficiently large, where \\Delta_a is\nthe amplitude of the a-wave component in the TRSB state in the absence of the\ndistortion. We clarify the manner in which the shift in the relative phase\neliminates S_{a_1 a_2} and prove that such a state yields a free-energy\nminimum. We also propose a formula for the upper bound of the degree of lattice\ndistortion, below which the TRSB state can be stable.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "History effects and pinning regimes in solid vortex matter: We propose a phenomenological model that accounts for the history effects\nobserved in ac susceptibility measurements in YBa2Cu3O7 single crystals [Phys.\nRev. Lett. 84, 4200 (2000) and Phys. Rev. Lett. 86, 504 (2001)]. Central to the\nmodel is the assumption that the penetrating ac magnetic field modifies the\nvortex lattice mobility, trapping different robust dynamical states in\ndifferent regions of the sample. We discuss in detail on the response of the\nsuperconductor to an ac magnetic field when the vortex lattice mobility is not\nuniform inside the sample. We begin with an analytical description for a simple\ngeometry (slab) and then we perform numerical calculations for a strip in a\ntransverse magnetic field which include relaxation effects. In calculations,\nthe vortex system is assumed to coexist in different pinning regimes. The\nvortex behavior in the regions where the induced current density j has been\nalways below a given threshold (j_c^>) is described by an elastic Campbell-like\nregime (or a critical state regime with local high critical current density,\nj_c^>). When the VS is shaken by symmetrical (e.g. sinusoidal) ac fields, the\ncritical current density is modified to j_c^< (which is smaller than j_c^>) at\nregions where vortices have been forced to oscillate by a current density\nlarger than j_c^>. Experimentally, an initial state with high critical current\ndensity (j_c^>) can be obtained by zero field cooling, field cooling (with no\napplied ac field) or by shaking the vortex lattice with an asymmetrical (e.g.\nsawtooth) field. We compare our calculations with experimental ac\nsusceptibility results in YBa2Cu3O7 single crystals.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Spin-dependent quasiparticle reflection and bound states at interfaces\n  with itinerant antiferromagnets: We present a formulation of the quasiclassical theory of junctions between\nitinerant antiferromagnets (AF) and s-wave (sSC) and d-wave superconductors\n(dSC). For the simplest two-sublattice antiferromagnet on a bipartite lattice,\nwe derive Andreev-type equations and show that their solutions lead to a novel\nchannel of quasiparticle reflection. In particular, quasiparticles in a normal\nmetal with energies less than or comparable to the antiferromagnetic gap\nexperience spin-dependent retroreflection at antiferromagnet-normal metal\n(AF/N) transparent (100) and (110) interfaces. A relative phase difference of\npi between up spin and down spin quasiparticle reflection amplitudes is shown\nto lead to zero-energy interface bound states on AF/sSC interfaces. For an\nsSC/AF/sSC junction, these bound states are found to be split, due to a finite\nwidth of the AF interlayer, and carry the supercurrent. At AF/dSC interfaces we\nfind no zero-energy bound states for both interface orientations we considered,\nin contrast with the case of (110) impenetrable surface of a dSC.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetic dipole induced guided vortex motion: We present evidence of magnetically controlled guided vortex motion in a\nhybrid superconductor/ferromagnet nanosystem consisting of an Al film on top of\na square array of permalloy square rings. When the rings are magnetized with an\nin-plane external field H, an array of point-like dipoles with moments\nantiparallel to H, is formed. The resulting magnetic template generates a\nstrongly anisotropic pinning potential landscape for vortices in the\nsuperconducting layer. Transport measurements show that this anisotropy is able\nto confine the flux motion along the high symmetry axes of the square lattice\nof dipoles. This guided vortex motion can be either re-routed by 90 degrees by\nsimply changing the dipole orientation or even strongly suppressed by inducing\na flux-closure magnetic state with very low stray fields in the rings.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Coherent Modulation of the YBa2Cu3O6+x Atomic Structure by Displacive\n  Stimulated Ionic Raman Scattering: We discuss the mechanism of coherent phonon generation by Stimulated Ionic\nRaman Scattering, a process different from conventional excitation with near\nvisible optical pulses. Ionic Raman scattering is driven by anharmonic coupling\nbetween a directly excited infrared-active phonon mode and other Raman modes.\nWe experimentally study the response of YBa2Cu3O6+x to the resonant excitation\nof apical oxygen motions at 20 THz by mid-infrared pulses, which has been shown\nin the past to enhance the interlayer superconducting coupling. We find\ncoherent oscillations of four totally symmetric (Ag) Raman modes and make a\ncritical assessment of the role of these oscillatory motions in the enhancement\nof superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Theory of the in-plane anisotropy of magnetic excitations in\n  YBa_{2}Cu_{3}O_{6+y}: A pronounced xy-anisotropy was observed in recent neutron scattering\nexperiments for magnetic excitations in untwinned YBa_{2}Cu_{3}O_{6+y} (YBCO).\nThe small anisotropy of the bare band structure due to the orthorhombic crystal\nsymmetry seems to be enhanced by correlation effects. A natural possibility is\nthat the system is close to a Pomeranchuk instability associated with a d-wave\nFermi surface deformation (dFSD). We investigate this possibility in the\nbilayer t-J model within a self-consistent slave-boson mean-field theory. We\nshow that the dFSD correlations drive a pronounced xy-anisotropy of magnetic\nexcitations at low doping and at relatively high temperatures, providing a\nscenario for the observed xy-anisotropy in optimally doped as well as\nunderdoped YBCO, including the pseudogap phase.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Universal Fermi velocity in highly compressed hydride superconductors: Fermi velocity, $v_F$, is one of the primary characteristics of any\nconductor, including superconductors. For conductors at ambient pressure\nseveral experimental techniques have been developed to measure $v_F$ and, for\ninstance, Zhou et al (Nature 423 398 (2003)) reported that high-Tc cuprates\nexhibit universal nodal Fermi velocity of $v_F$ = $2.7*10^5$ m/s. However,\nthere were no experimental techniques applied to measure $v_F$ in\nhighly-compressed near-room-temperature superconductors (NRTS), due to\nexperimental challenges. Here to answer a question about the existence of the\nuniversal Fermi velocity in NRTS materials, we analyzed full inventory of the\nground-state upper critical field data, Bc2(0), for these materials and found\nthat this class of superconductors exhibits universal Fermi velocity of $v_F$ =\n(1/1.3)*(2${\\Delta}$(0)/(kBTc))*10^5 m/s (where ${\\Delta}(0)$ is ground state\namplitude of the energy gap). Due to the ratio of 2${\\Delta}$(0)/$({k_B}{T_c})$\nis varying within a narrow arrange of 3.2 < 2${\\Delta}$(0)/$({k_B}{T_c})$ < 5,\nthen $v_F$ in NRTS materials is within the same ballpark with its high-Tc\ncuprates counterpart.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The Ground-state Inter-plane Superconducting Coherence Length of\n  La$_{1.875}$Sr$_{0.125}$CuO$_4$ Measured by a \"Xiometer\": A long excitation coil piercing a superconducting (SC) ring is used to\ngenerate ever increasing persistent current in the ring, until the current\ndestroys the order parameter. Given that the penetration depth $\\lambda$ is\nknown, this experiment measures, hypothetically, the coherence length $\\xi$,\nhence the name \"Xiometer\". We examine various aspects of this theoretically\ndriven hypothesis by testing niobium rings with different dimensions, and by\ncomparing the results to the known values of $\\xi$. We then apply the method to\ntwo La$_{1.875}$Sr$_{0.125}$CuO$_4$ rings at $T \\rightarrow 0$. In one, the\ncurrent flows in the CuO$_2$ planes hence it is set by $\\xi_{ab}$. In the\nother, the current must cross planes and is determined by $\\xi_{c}$. We find\nthat $\\xi_{c}=1.3 \\pm 0.1$~nm, and $\\xi_{ab}<2.3~$nm indicating that at low\ntemperatures the Cooper pairs are three dimensional.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The influence of the spin-dependent phases of tunneling electrons on the\n  conductance of a point ferromagnet/isolator/d-wave superconductor contact: The influence of the spin-dependent phase shifts (SDPS) associated to the\nelectronic reflection and transmission amplitudes acquired by electrons upon\nscattering on the potential barrier on the Andreev reflection probability of\nelectron and hole excitations for a ferromagnet/isolator/d-wave superconductor\n(FIS) contact and the charge conductance of the FIS contact is studied. Various\nsuperconductor orientations are considered. It is found that SDPS can suppress\nthe zero-potential peak and restore finite-potential peaks in the charge\nconductance of the F/I/d-wave superconductor contact for the (110) orientation\nof the d-wave superconductor and, on the contrary, can restore the\nzero-potential peak and suppress finite-potential peaks for the $\\{100\\}$\norientation of the d-wave superconductor.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetoelectric effects in superconductors due to spin-orbit scattering:\n  a non-linear $\u03c3$-model description: We suggest a generalization of nonlinear $\\sigma$-model for diffusive\nsuperconducting systems to account for magnetoelectric effects due to\nspin-orbit scattering. In the leading orders of spin-orbit strength and\ngradient expansion it includes two additional terms responsible for the\nspin-Hall effect and the spin-current swapping. First, assuming a\ndelta-correlated disorder we derive the new terms from the Keldysh path\nintegral representation of the generating functional. Then we argue\nphenomenologically that they exhaust all invariants allowed in the effective\naction to the leading order in the spin-orbit coupling (SOC). Finally, the\nresults are confirmed by a direct derivation of the saddle-point (Usadel)\nequation from the quantum kinetic equations in the presence of randomly\ndistributed impurities with SOC. At this point we correct a recent derivation\nof the Usadel equation that includes magneto-electric effects and does not\nresort to the Born approximation.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Collective mode in the SU(2) theory of cuprates: Recent advances in momentum-resolved electron energy-loss spectroscopy\n(MEELS) and resonant inelastic X-ray scattering (RIXS) now allow one to access\nthe charge response function with unprecedented versatility and accuracy. This\nallows for the study of excitations which were inaccessible recently, such as\nlow-energy and finite momentum collective modes. The SU(2) theory of the\ncuprates is based on a composite order parameter with SU(2) symmetry\nfluctuating between superconductivity and charge order. The phase where it\nfluctuates is a candidate for the pseudogap phase of the cuprates. This theory\nhas a signature, enabling its strict experimental test, which is the\nfluctuation between these two orders, corresponding to a charge 2 spin 0 mode\nat the charge ordering wave-vector. Here we derive the influence of this SU(2)\ncollective mode on the charge susceptibility in both strong and weak coupling\nlimits, and discuss its relation to MEELS, RIXS and Raman experiments. We find\ntwo peaks in the charge susceptibility at finite energy, whose middle is the\ncharge ordering wave-vector, and discuss their evolution in the phase diagram.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Exotic Superconducting Phases of Ultracold Atom Mixtures on Triangular\n  Lattices: We study the phase diagram of two-dimensional Bose-Fermi mixtures of\nultracold atoms on a triangular optical lattice, in the limit when the velocity\nof bosonic condensate fluctuations is much larger than the Fermi velocity.\n  We contrast this work with our previous results for a square lattice system\nin Phys. Rev. Lett. {\\bf 97}, 030601 (2006).\n  Using functional renormalization group techniques we show that the phase\ndiagrams for a triangular lattice contain exotic superconducting phases. For\nspin-1/2 fermions on an isotropic lattice we find a competition of $s$-, $p$-,\nextended $d$-, and $f$-wave symmetry, as well as antiferromagnetic order. For\nan anisotropic lattice, we further find an extended p-wave phase. A Bose-Fermi\nmixture with spinless fermions on an isotropic lattice shows a competition\nbetween $p$- and $f$-wave symmetry.\n  These phases can be traced back to the geometric shapes of the Fermi surfaces\nin various regimes, as well as the intrinsic frustration of a triangular\nlattice.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity in the non-oxide Perovskite MgCNi3: The oxide perovskites are a large family of materials with many important\nphysical properties. Of particular interest has been the fact that this\nstructure type provides an excellent structural framework for the existence of\nsuperconductivity. The high Tc copper oxides are the most famous examples of\nsuperconducting perovskites, but there are many others [1]. Intermetallic\ncompounds have been the source of many superconducting materials in the past,\nbut they have been eclipsed in recent years by the perovskite oxides. The\nrecent discovery of superconductivity in MgB2 [2] suggests that intermetallic\ncompounds with simple structure types are worth serious reconsideration as\nsources of new superconducting materials. Here we report the observation of\nsuperconductivity at 8 K in the perovskite structure intermetallic compound\nMgCNi3, linking what appear at first sight to be mutually exclusive classes of\nsuperconducting materials. The observation of superconductivity in MgCNi3\nindicates that MgB2 will not be the only one of its kind within the chemical\nparadigm that it suggests for new superconducting materials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Theory of Local Density of States of d-Wave Superconducting State Near\n  the Surfaces of the t-J Model: Spatial dependencies of the pair potential and the local density of states\nnear the surfaces of $d_{x^{2}-y^{2}}$-wave superconductors are studied\ntheoretically. The calculation is based on the t-J model within a mean-field\ntheory with Gutzwiller approximation. Various types of surface geometries are\nconsidered. Similar to our result in the extended Hubbard model, it is found\nthat the formation of zero-energy states strongly depends on the surface\ngeometry. In addition to this feature, the zero-energy states give peak\nsplitting for the (110) surfaces when the super-exchange interaction $J$ is\nlarge. This is due to the induced s-wave component near the surface. The\npresent result explains the microscopic origin of the spontaneous time-\nreversal symmetry breaking at the surfaces of high-$T_{c}$ superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconducting and thermal properties of ex-situ Glidcop sheathed\n  multifilamentary MgB2 wires: In DC and AC practical applications of MgB2 superconducting wires an\nimportant role is represented by the material sheath which has to provide,\namong other things, a suitable electrical and thermal stabilization. A way to\nobtain a large enough amount of low resistivity material in to the conductor\narchitecture is to use it as external sheath. In this paper we study ex-situ\nmultifilamentary MgB2 wires using oxide-dispersion-strengthened copper\n(GlidCop) as external sheath in order to reach a good compromise between\ncritical current density and thermal properties. We prepared three GlidCop\nsamples differing by the content of dispersed sub-microscopic Al2O3 particles.\nWe characterized the superconducting and thermal properties and we showed that\nthe good thermal conductivity together the good mechanical properties and a\nreasonable critical current density make of GlidCop composite wire a useful\nconductor for applications where high thermal conductivity is request at\ntemperature above 30K, such as Superconducting-FCL.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Temperature, RF Field, and Frequency Dependence Performance Evaluation\n  of Superconducting Niobium Half-Wave Cavity: Recent advancement in superconducting radio frequency cavity processing\ntechniques, with diffusion of impurities within the RF penetration depth,\nresulted in high quality factor with increase in quality factor with increasing\naccelerating gradient. The increase in quality factor is the result of a\ndecrease in the surface resistance as a result of nonmagnetic impurities doping\nand change in electronic density of states. The fundamental understanding of\nthe dependence of surface resistance on frequency and surface preparation is\nstill an active area of research. Here, we present the result of RF\nmeasurements of the TEM modes in a coaxial half wave niobium cavity resonating\nat frequencies between 0.3-1.3 GHz. The temperature dependence of the surface\nresistance was measured between 4.2 K and 1.6 K. The field dependence of the\nsurface resistance was measured at 2.0 K. The baseline measurements were made\nafter standard surface preparation by buffered chemical polishing.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electronic structure of CaFe2As2: Contribution of itinerant Fe 3d-states\n  to the Fermi Level: We present density functional theory (DFT) calculations and a full set of\nX-ray spectra (resonant inelastic X-ray scattering and X-ray photoelectron\nspectra) measurements of single crystal CaFe2As2. The experimental valence band\nspectra are consistent with our DFT calculations. Both theory and experiment\nshow that the Fe 3d-states dominate the Fermi level and hybridize with Ca\n3d-states. The simple shape of Xray photoelectron (XPS) Fe 2p-core level\nspectrum (without any satellite structure typical for correlated systems)\nsuggests itinerant character of the Fe 3d-electrons. Based on the similarity of\nthe calculated and experimental Fe 3d-states distribution in LaOFeAs and\nCaFe2As2 we conclude that superconductivity in the FeAs-systems can be\ndescribed within a minimal model, taking into account only Fe 3d-bands close to\nthe Fermi level.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "ARPES experiment in fermiology of quasi-2D metals (Review Article): Angle resolved photoemission spectroscopy (ARPES) enables direct observation\nof the Fermi surface and underlying electronic structure of crystals---the\nbasic concepts to describe all the electronic properties of solids and to\nunderstand the key electronic interactions involved. The method is the most\neffective to study quasi-2D metals, to which the subjects of almost all hot\nproblems in modern condensed matter physics have happened to belong. This has\nforced incredibly the development of the ARPES method which we face now. The\naim of this paper is to introduce to the reader the state-of-the-art ARPES,\nreviewing the results of its application to such topical problems as high\ntemperature superconductivity in cuprates and iron based superconductors, and\nelectronic ordering in the transition metal dichalcogenides and manganites.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electronic theory for the normal state spin dynamics in Sr$_2$RuO$_4$:\n  anisotropy due to spin-orbit coupling: Using a three-band Hubbard Hamiltonian we calculate within the\nrandom-phase-approximation the spin susceptibility, $\\chi({\\bf q},\\omega)$, and\nNMR spin-lattice relaxation rate, 1/T$_1$, in the normal state of the triplet\nsuperconductor Sr$_2$RuO$_4$ and obtain quantitative agreement with\nexperimental data. Most importantly, we find that due to spin-orbit coupling\nthe out-of-plane component of the spin susceptibility $\\chi^{zz}$ becomes at\nlow temperatures two times larger than the in-plane one. As a consequence\nstrong incommensurate antiferromagnetic fluctuations of the\nquasi-one-dimensional $xz$- and $yz$-bands point into the $z$-direction. Our\nresults provide further evidence for the importance of spin fluctuations for\ntriplet superconductivity in Sr$_2$RuO$_4$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "DFT+U study of magnetic order in doped La$_2$CuO$_4$ crystals: This article presents the results of several magnetic phases of doped\nLa$_{2-x}$Sr$_x$CuO$_4$ using density-functional theory with an added Hubbard\nterm (DFT+U). Doping factors from $x=0$ to 0.25 were examined. We found that a\nbond centered stripe is the magnetic ground state for $x=1/8$ and $x=1/4$. No\nstable stripe order was found for $x=1/6$. Analysis of the electron density\nrevealed that apical oxygen atoms, those located above and below the copper\natoms in the CuO$_2$ planes, hold a non negligible part of the holes at large\ndoping and present a small spin polarization. Finally, the charge\nreorganization caused by the magnetic stripe modulation was studied for bond\ncentered and atom centered stripes.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Onset of Superfluidity in 4He Films Adsorbed on Disordered Substrates: We have studied 4He films adsorbed in two porous glasses, aerogel and Vycor,\nusing high precision torsional oscillator and DC calorimetry techniques. Our\ninvestigation focused on the onset of superfluidity at low temperatures as the\n4He coverage is increased. Torsional oscillator measurements of the 4He-aerogel\nsystem were used to determine the superfluid density of films with transition\ntemperatures as low as 20 mK. Heat capacity measurements of the 4He-Vycor\nsystem probed the excitation spectrum of both non-superfluid and superfluid\nfilms for temperatures down to 10 mK. Both sets of measurements suggest that\nthe critical coverage for the onset of superfluidity corresponds to a mobility\nedge in the chemical potential, so that the onset transition is the bosonic\nanalog of a superconductor-insulator transition. The superfluid density\nmeasurements, however, are not in agreement with the scaling theory of an onset\ntransition from a gapless, Bose glass phase to a superfluid. The heat capacity\nmeasurements show that the non-superfluid phase is better characterized as an\ninsulator with a gap.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Controlling the Intrinsic Josephson Junction Number in a\n  $\\mathbf{Bi_2Sr_2CaCu_2O_{8+\u03b4}}$ Mesa: In fabricating $\\mathrm{Bi_2Sr_2CaCu_2O_{8+\\delta}}$ intrinsic Josephson\njunctions in 4-terminal mesa structures, we modify the conventional fabrication\nprocess by markedly reducing the etching rates of argon ion milling. As a\nresult, the junction number in a stack can be controlled quite satisfactorily\nas long as we carefully adjust those factors such as the etching time and the\nthickness of the evaporated layers. The error in the junction number is within\n$\\pm 1$. By additional ion etching if necessary, we can controllably decrease\nthe junction number to a rather small value, and even a single intrinsic\nJosephson junction can be produced.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconducting electronic state in optimally doped YBa2Cu3O7-d observed\n  with laser-excited angle-resolved photoemission spectroscopy: Low energy electronic structure of optimally doped YBa2Cu3O7-d is\ninvestigated using laser-excited angle-resolved photoemission spectroscopy. The\nsurface state and the CuO chain band that usually overlap the CuO2 plane\nderived bands are not detected, thus enabling a clear observation of the bulk\nsuperconducting state. The observed bilayer splitting of the Fermi surface is\n~0.08 angstrom^{-1} along the (0,0)-(pi,pi) direction, significantly larger\nthan Bi2Sr2CaCu2O8+d. The kink structure of the band dispersion reflecting the\nrenormalization effect at ~60 meV shows up similarly as in other hole-doped\ncuprates. The momentum-dependence of the superconducting gap shows\nd_{x^2-y^2}-wave like amplitude, but exhibits a nonzero minimum of ~12 meV\nalong the (0,0)-(pi,pi) direction. Possible origins of such an unexpected\n\"nodeless\" gap behavior are discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Optical Conductivity Anisotropy in the Undoped Three-Orbital Hubbard\n  Model for the Pnictides: The resistivity anisotropy unveiled in the study of detwinned single crystals\nof the undoped 122 pnictides is here studied using the two-dimensional\nthree-orbital Hubbard model in the mean-field approximation. Calculating the\nDrude weight in the x and y directions at zero temperature for a Q=(\\pi,0)\nmagnetically ordered state, the conductance along the antiferromagnetic\ndirection is shown to be larger than along the ferromagnetic direction. This\neffect is caused by the suppression of the d_{yz} orbital at the Fermi surface,\nbut additional insight based on the momentum dependence of the transitions\ninduced by the current operator is provided. It is shown that the effective\nsuppression of the inter-orbital hopping d_{xy} and d_{yz} along the y\ndirection is the main cause of the anisotropy.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Probing d-wave pairing correlations in the pseudogap regime of the\n  cuprate superconductors via low-energy states near impurities: The issue of probing the pseudogap regime of the cuprate superconductors,\nspecifically with regard to the existence and nature of superconducting pairing\ncorrelations of d-wave symmetry, is explored theoretically. It is shown that if\nthe d-wave correlations believed to describe the superconducting state persist\ninto the pseudogap regime, but with pair-potential phase-fluctuations that\ndestroy their long-range nature, then the low-energy quasiparticle states\nobserved near extended impurities in the truly superconducting state should\nalso persist as resonances in the pseudogap regime. The scattering of\nquasiparticles by these phase-fluctuations broadens what was (in the\nsuperconducting state) a sharp peak in the single-particle spectral function at\nlow energy, as we demonstrate within the context of a simple model. This peak\nand its broadening is, in principle, accessible via scanning tunneling\nspectroscopy near extended impurities in the pseudogap regime. If so, such\nexperiments would provide a probe of the extent to which d-wave superconducting\ncorrelations persist upon entering the pseudogap regime, thus providing a\nstringent diagnostic of the phase-fluctuation scenario.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Hund's coupling stabilized superconductivity in the presence of\n  spin-orbit interactions: The intraorbital repulsive Hubbard interaction cannot lead to attractive\nsuperconducting pairing states, except through the Kohn-Luttinger mechanism.\nThis situation may change when we include additional local interactions such as\nthe interorbital repulsion $U^\\prime$ and Hund's interactions $J$. Adding these\nlocal interactions, we study the nature of the superconducting pairs in systems\nwith tetragonal crystal symmetry including the $d_{xz}$ and $d_{yz}$ orbitals,\nand in octahedral systems including all three of $d_{xz}$, $d_{yz}$, and\n$d_{xy}$ orbitals. In the tetragonal case, spin-orbit interactions can\nstabilize attractive pairing channels containing spin triplet, orbital singlet\ncharacter. Depending on the form of spin-orbit coupling, pairing channels\nbelonging to degenerate, non-trivial irreducible representations may be\nstabilized. In the octahedral case, the pairing interactions of superconducting\nchannels are found to depend critically on the number of bands crossing the\nFermi energy.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pairing, Pseudogap and Fermi Arcs in Cuprates: We use Angle Resolved Photoemission Spectroscopy (ARPES) to study the\nrelationship between the pseudogap, pairing and Fermi arcs in cuprates. High\nquality data measured over a wide range of dopings reveals a consistent picture\nof Fermiology and pairing in these materials. The pseudogap is due to an\nordered state that competes with superconductivity rather then preformed pairs.\nPairing does occur below Tpair~150K and significantly above Tc, but well below\nT* and the doping dependence of this temperature scale is distinct from that of\nthe pseudogap. The d-wave gap is present below Tpair, and its interplay with\nstrong scattering creates \"artificial\" Fermi arcs for Tc<T<Tpair. However,\nabove Tpair, the pseudogap exists only at the antipodal region. This leads to\npresence of real, gapless Fermi arcs close to the node. The length of these\narcs remains constant up to T*, where the full Fermi surface is recovered. We\ndemonstrate that these findings resolve a number of seemingly contradictory\nscenarios.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Theoretical study on the correlation between the spin fluctuation and\n  $T_c$ in the isovalent-doped 1111 iron-based superconductors: Motivated by recent experiments on isovalent-doped 1111 iron-based\nsuperconductors LaFeAs$_{1-{x}}$P$_{x}$O$_{1-{y}}$F$_{y}$ and the theoretical\nstudy that followed, we investigate, within the five orbital model, the\ncorrelation between the spin fluctuation and the superconducting transition\ntemperature, which exhibits a double dome feature upon varying the Fe-As-Fe\nbond angle. Around the first dome with higher $T_c$, the low energy spin\nfluctuation and $T_c$ are not tightly correlated because the finite energy spin\nfluctuation also contributes to superconductivity. On the other hand, the\nstrength of the low-energy spin fluctuation originating from the $d_{xz/yz}$\norbital is correlated with $T_c$ in the second dome with lower $T_c$. These\ncalculation results are consistent with recent NMR study, and hence strongly\nsuggest that the pairing in the iron-based superconductors is predominantly\ncaused by the multi-orbital spin fluctuation.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "First order $0$ - $\u03c0$ phase transitions in\n  superconductor/ferromagnet/superconductor trilayers: We study the thermodynamics of the diffusive SFS trilayer composed of thin\nsuperconductor (S) and ferromagnet (F) layers. On the basis the self-consistent\nsolutions of nonlinear Usadel equations in the F and S layers we obtain the\nGinzburg--Landau expansion and compute the condensation free energy and entropy\nof the $0$ (even) and $\\pi$ (odd) order parameter configurations. The first\norder $0-\\pi$ transition as a function of temperature $T$ occurs, which is\nresponsible for a jump of the averaged magnetic field penetration depth\n$\\lambda(T)$ recently observed on experiments [N.Pompeo, et. al., Phys. Rev. B\n90, 064510 (2014)]. The generalized Ginzburg-Landau functional was proposed to\ndescribe SFS trilayer for arbitrary phase difference between the\nsuperconducting order parameters in the S layers. The temperature dependence of\nthe SFS Josephson junction critical current demonstrates the strong\nanharmonicity of the corresponding current--phase relation in the vicinity of\nthe $0-\\pi$ transition. In rf SQUID, coexistence of stable and metastable $0$\nand $\\pi$ states provides integer and half--integer fluxoid configurations.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Constrain relations for superfluid weight and pairings in a chiral flat\n  band superconductor: Within ten years, flat band (FB) superconductivity has gained a huge interest\nfor its remarkable features and connection to quantum geometry. We investigate\nthe superconductivity in a FB system whose orbitals are inequivalent and in\nwhich the gap and the quantum metric are tunable. The key feature of the\npresent theoretical study is to show a unique and simple constrain relation\nthat pairings obey. Furthermore, pairings and superfluid weight in partially\nfilled FB are shown to be controlled by those of the half-filled lattice. We\nargue that the geometry of the lattice or the complexity of the hopping terms\nhave no impact on the features revealed in this work as far as the system is\nbipartite.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Non-London electrodynamics in a multiband London model:\n  anisotropy-induced non-localities and multiple magnetic field penetration\n  lengths: The London model describes strongly type-2 superconductors as massive vector\nfield theories, where the magnetic field decays exponentially at the length\nscale of the London penetration length. This also holds for isotropic\nmulti-band extensions, where the presence of multiple bands merely renormalises\nthe London penetration length. We show that, by contrast, the magnetic\nproperties of anisotropic multi-band London models are not this simple, and the\nanisotropy leads to the inter-band phase differences becoming coupled to the\nmagnetic field. This results in the magnetic field in such systems having N+1\npenetration lengths, where N is the number of field components or bands. That\nis, in a given direction, the magnetic field decay is described by N+1 modes\nwith different amplitudes and different decay length scales. For certain\nanisotropies we obtain magnetic modes with complex masses. That means that\nmagnetic field decay is not described by a monotonic exponential increment set\nby a real penetration length but instead is oscillating. Some of the\npenetration lengths are shown to diverge away from the superconducting phase\ntransition when the mass of the phase-difference mode vanishes. Finally the\nanisotropy-driven hybridization of the London mode with the Leggett modes can\nprovide an effectively non-local magnetic response in the nominally local\nLondon model. Focusing on the two-component model, we discuss the magnetic\nfield inversion that results from the effective non-locality, both near the\nsurface of the superconductor and around vortices. In the regime where the\nmagnetic field decay becomes non-monotonic, the multiband London superconductor\nis shown to form weakly-bound states of vortices.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Ferromagnetic-superconducting hybrid films and their possible\n  applications: A direct study in a model combinatorial film: Model combinatorial films (CFs) which host a pure superconductor adjacent to\na ferromagneticsuperconducting hybrid film (HF) are manufactured for the study\nof the influence of ferromagnetic nanoparticles (FNs) on the nucleation of\nsuperconductivity. Careful resistance measurements were performed\nsimultaneously on two different sites of the CFs. Enhancement of\nsuperconductivity and magnetic memory effects were observed only on the hybrid\nsite of the CFs but were absent on their purely superconducting part. Our\nresults give direct proof that the FNs modulate the superconducting order\nparameter in an efficient and controlled way giving us the possibility of\nmiscellaneous practical applications.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The resonance peak in the electron-doped cuprate superconductors: We study the emergence of a magnetic resonance in the superconducting state\nof the electron-doped cuprate superconductors. We show that the recently\nobserved resonance peak in the electron-doped superconductor\nPr$_{0.88}$LaCe$_{0.12}$CuO$_{4-\\delta}$ is consistent with an overdamped spin\nexciton located near the particle-hole continuum. We present predictions for\nthe magnetic-field dependence of the resonance mode as well as its temperature\nevolution in those parts of the phase diagram where $d_{x^2-y^2}$-wave\nsuperconductivity may coexist with an antiferromagnetic spin-density wave.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Comment on \"Large energy gaps in CaC6 from tunneling spectroscopy:\n  possible evidence of strong-coupling superconductivity\": Comment on \"Large energy gaps in CaC6 from tunneling spectroscopy: possible\nevidence of strong-coupling superconductivity\"",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "In-gap states of magnetic impurity in quantum spin Hall insulator\n  proximitized to a superconductor: We study in-gap states of a single magnetic impurity embedded in a honeycomb\nmonolayer which is deposited on superconducting substrate. The intrinsic\nspin-orbit coupling induces the quantum spin Hall insulating (QSHI) phase\ngapped around the Fermi energy. Under such circumstances we consider the\nemergence of Shiba-like bound states driven by the superconducting proximity\neffect. We investigate their topography, spin-polarization and signatures of\nthe quantum phase transition manifested by reversal of the local currents\ncirculating around the magnetic impurity. These phenomena might be important\nfor more exotic in-gap quasiparticles in such complex nanostructures as\nmagnetic nanowires or islands, where the spin-orbit interaction along with the\nproximity induced electron pairing give rise to topological phases hosting the\nprotected boundary modes.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Nonlinear features of the superconductor--ferromagnet--superconductor\n  $\\varphi_0$ Josephson junction in ferromagnetic resonance region: We demonstrate the manifestations of the nonlinear features in magnetic\ndynamics and IV-characteristics of the $\\varphi_0$ Josephson junction in the\nferromagnetic resonance region. We show that at small values of system\nparameters, namely, damping, spin-orbit interaction, and Josephson to magnetic\nenergy ratio, the magnetic dynamics is reduced to the dynamics of the scalar\nDuffing oscillator, driven by the Josephson oscillations. The role of\nincreasing superconducting current in the resonance region is clarified.\nShifting of the ferromagnetic resonant frequency and the reversal of its\ndamping dependence due to nonlinearity are demonstrated by the full\nLandau-Lifshitz-Gilbert-Josephson system of equations, and in its different\napproximations. Finally, we demonstrate the negative differential resistance in\nthe IV--characteristics, and its correlation with the foldover effect.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Diamagnetic susceptibility obtained from the six-vertex model and its\n  implications for the high-temperature diamagnetic state of cuprate\n  superconductors: We study the diamagnetism of the 6-vertex model with the arrows as directed\nbond currents. To our knowledge, this is the first study of the diamagnetism of\nthis model. A special version of this model, called F model, describes the\nthermal disordering transition of an orbital antiferromagnet, known as\nd-density wave (DDW), a proposed state for the pseudogap phase of the high-Tc\ncuprates. We find that the F model is strongly diamagnetic and the\nsusceptibility may diverge in the high temperature critical phase with power\nlaw arrow correlations. These results may explain the surprising recent\nobservation of a diverging low-field diamagnetic susceptibility seen in some\noptimally doped cuprates within the DDW model of the pseudogap phase.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Evidence of zero point fluctuation of vortices in a very weakly pinned\n  a-MoGe thin film: In a Type II superconductor, the vortex core behaves like a normal metal.\nConsequently, the single-particle density of states in the vortex core of a\nconventional Type II superconductor remains either flat or (for very clean\nsingle crystals) exhibits a peak at zero bias due to the formation of Caroli-de\nGennes-Matricon bound state inside the core. Here we report an unusual\nobservation from scanning tunneling spectroscopy measurements in a weakly\npinned thin film of the conventional s-wave superconductor a-MoGe, namely, that\na soft gap in the local density of states continues to exist even at the center\nof the vortex core. We ascribe this observation to rapid fluctuation of\nvortices about their mean position that blurs the boundary between the gapless\nnormal core and the gapped superconducting region outside. Analyzing the data\nas a function of magnetic field we show that the variation of fluctuation\namplitude as a function of magnetic field is consistent with quantum zero-point\nmotion of vortices.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Persistent electrical doping of Bi2Sr2CaCu2O8+x mesa structures: Application of a significantly large bias voltage to small Bi2Sr2CaCu2O8+x\nmesa structures leads to persistent doping of the mesas. Here we employ this\neffect for analysis of the doping dependence of the electronic spectra of\nBi-2212 single crystals by means of intrinsic tunneling spectroscopy. We are\nable to controllably and reversibly change the doping state of the same single\ncrystal from underdoped to overdoped state, without changing its chemical\ncomposition. It is observed that such physical doping is affecting\nsuperconductivity in Bi-2212 similar to chemical doping by oxygen impurities:\nwith overdoping the critical temperature and the superconducting gap decrease,\nwith underdoping the c-axis critical current rapidly decreases due to\nprogressively more incoherent interlayer tunneling and the pseudogap rapidly\nincreases, indicative for the presence of the critical doping point. We\ndistinguish two main mechanisms of persistent electric doping: (i) even in\nvoltage contribution, attributed to a charge transfer effect, and (ii) odd in\nvoltage contribution, attributed to reordering of oxygen impurities.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The Topological Relation Between Bulk Gap Nodes and Surface Bound States\n  : Application to Iron-based Superconductors: In the past few years materials with protected gapless surface (edge) states\nhave risen to the central stage of condensed matter physics. Almost all\ndiscussions centered around topological insulators and superconductors, which\npossess full quasiparticle gaps in the bulk. In this paper we argue systems\nwith topological stable bulk nodes offer another class of materials with robust\ngapless surface states. Moreover the location of the bulk nodes determines the\nMiller index of the surfaces that show (or not show) such states. Measuring the\nspectroscopic signature of these zero modes allows a phase-sensitive\ndetermination of the nodal structures of unconventional superconductors when\nother phase-sensitive techniques are not applicable. We apply this idea to\ngapless iron based superconductors and show how to distinguish accidental from\nsymmetry dictated nodes. We shall argue the same idea leads to a method for\ndetecting a class of the elusive spin liquids.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Unconventional superconductivity induced in Nb films by adsorbed chiral\n  molecules: Motivated by recent observations of chiral-induced magnetization and\nspin-selective transport we studied the effect of chiral molecules on\nconventional BCS superconductors. By applying scanning tunneling spectroscopy,\nwe demonstrate that the singlet-pairing s-wave order parameter of Nb is\nsignificantly altered upon adsorption of chiral polyalanine alpha-helix\nmolecules on its surface. The tunneling spectra exhibit zero-bias conductance\npeaks embedded inside gaps or gap-like features, suggesting the emergence of\nunconventional triplet-pairing components with either d-wave or p-wave\nsymmetry, as corroborated by simulations. These results may open a way for\nrealizing simple superconducting spintroinics devices.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Comment on \"Microwave vortex dissipation of superconducting Nd-Ce-Cu-O\n  epitaxial films in high magnetic fields\": While the electrodynamic treatment of a superconducting thin film by Yeh et\nal. [Phys. Rev. B 48, 9861 (1993)] ostensibly has application for the\n\"thin-film limit,\" their work is shown to employ bulk-like instead of thin-film\nelectrodynamics, and as such obtains an ostensible violation of the first law\nof thermodynamics and a surface resistance independent of the resistivity.\nOther theoretical and experimental difficulties are briefly noted.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Cooper-pair size and binding energy for unconventional superconducting\n  systems: The main proposal of this paper is to analyze the size of the Cooper pairs\ncomposed by unbalanced mass fermions from different electronic bands along the\nBCS-BEC crossover and study the binding energy of the pairs. We are considering\nan interaction between fermions with different masses leading to an inter-band\npairing. In addiction to the attractive interaction we have an hybridization\nterm to couple both bands, which in general acts unfavorable for the pairing\nbetween the electrons. We get first order phase transitions as the\nhybridization break the Cooper pairs for the the $s$-wave symmetry of the gap\namplitude. The results show the dependence of the Cooper-pair size as a\nfunction of the hybridization for $T=0$. We also propose the structure of the\nbinding energy of the inter-band system as a function of the two-bands\nquasi-particle energies.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Upper limit on spontaneous supercurrents in Sr$_2$RuO$_4$: It is widely believed that the perovskite Sr$_2$RuO$_4$ is an unconventional\nsuperconductor with broken time reversal symmetry. It has been predicted that\nsuperconductors with broken time reversal symmetry should have spontaneously\ngenerated supercurrents at edges and domain walls. We have done careful imaging\nof the magnetic fields above Sr$_2$RuO$_4$ single crystals using scanning Hall\nbar and SQUID microscopies, and see no evidence for such spontaneously\ngenerated supercurrents. We use the results from our magnetic imaging to place\nupper limits on the spontaneously generated supercurrents at edges and domain\nwalls as a function of domain size. For a single domain, this upper limit is\nbelow the predicted signal by two orders of magnitude. We speculate on the\ncauses and implications of the lack of large spontaneous supercurrents in this\nvery interesting superconducting system.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetism and Superconductivity in Ferromagnetic Heavy Fermion System\n  UCoGe under In-plane Magnetic Fields: We study the ferromagnetic superconductor UCoGe at ambient pressure under\n$ab$-plane magnetic fields $\\vec{H}$ which are perpendicular to the\nferromagnetic easy axis. It is shown that, by taking into account the\nDyaloshinskii-Moriya interaction arising from the zigzag chain crystal\nstructure of UCoGe, we can qualitatively explain the experimentally observed\nin-plane anisotropy for critical magnetic fields of the paramagnetic\ntransition. Because of this strong dependence on the magnetic field direction,\nupper critical fields of superconductivity, which is mediated by ferromagnetic\nspin fluctuations, also become strongly anisotropic. The experimental\nobservation of \"S-shaped\" $H_{c2}\\parallel b$-axis is qualitatively explained\nas a result of enhancement of the spin fluctuations due to decreased Curie\ntemperature by the $b$-axis magnetic field. We also show that the S-shaped\n$H_{c2}$ is accompanied by a rotation of the $d$-vector, which would be a key\nto understand the experiments not only at ambient pressure but also under\npressure.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "M\u00f6ssbauer spectroscopy study of magnetic fluctuations in\n  superconducting RbGd$_2$Fe$_4$As$_4$O$_2$: $^{57}$Fe M\\\"ossbauer spectra were measured at different temperatures between\n5.9 K and 300 K on the recently discovered self-doped superconducting\nRbGd$_2$Fe$_4$As$_4$O$_2$ with T$_c$ as high as 35 K. Singlet pattern was\nobserved down to the lowest temperature measured in this work, indicating the\nabsence of static magnetic order on the Fe site. The intermediate isomer shift\nin comparison with that of the samples RbFe$_2$As$_2$ and GdFeAsO confirms the\nself doping induced local electronic structure change. Surprisingly, we observe\ntwo magnetic fluctuation induced spectral broadenings below $\\sim$15 K and\n$\\sim$100 K which are believed to be originated from the transferred magnetic\nfluctuations of the Gd$^{3+}$ moments and that of the magnetic fluctuations of\nthe Fe atoms, respectively.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Three-band superconductivity and the order parameter that breaks\n  time-reversal symmetry: We consider a model of multiband superconductivity, inspired by iron\npnictides, in which three bands are connected via repulsive pair-scattering\nterms. Generically, three distinct superconducting states arise within such a\nmodel. Two of them are straightforward generalizations of the two-gap order\nparameter while the third one corresponds to a time-reversal symmetry breaking\norder parameter, altogether absent within the two-band model. Potential\nobservation of such a genuinely frustrated state would be a particularly vivid\nmanifestation of the repulsive interactions being at the root of iron-based\nhigh temperature superconductivity. We construct the phase diagram of this\nmodel and discuss its relevance to the iron pnictides family of high\ntemperature superconductors. We also study the case of the Josephson coupling\nbetween a two-band s' (or extended s-wave) superconductor and a single-gap\ns-wave superconductor, and the associated phase diagram.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pressure Driven Fractionalization of Ionic Spins Results in Cupratelike\n  High-$T_c$ Superconductivity in La$_3$Ni$_2$O$_7$: Beyond 14GPa of pressure, bi-layered La$_3$Ni$_2$O$_7$ was recently found to\ndevelop strong superconductivity above the liquid nitrogen boiling temperature.\nAn immediate essential question is the pressure-induced qualitative change of\nelectronic structure that enables the exciting high-temperature\nsuperconductivity. We investigate this timely question via a numerical\nmulti-scale derivation of effective many-body physics. At the atomic scale, we\nfirst clarify that the system has a strong charge transfer nature with\nitinerant carriers residing mainly in the in-plane oxygen between spin-1\nNi$^{2+}$ ions. We then elucidate in eV- and sub-eV-scale the key physical\neffect of the applied pressure: It induces a cupratelike electronic structure\nthrough partially screening the Ni spin from 1 to 1/2. This suggests a\nhigh-temperature superconductivity in La$_3$Ni$_2$O$_7$ with microscopic\nmechanism and ($d$-wave) symmetry similar to that in the cuprates.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Observation of Higher-Order Sideband Transitions and First-Order\n  Sideband Rabi Oscillations in a Superconducting Flux Qubit Coupled to a SQUID\n  Plasma Mode: We report results of spectroscopic measurements and time-domain measurements\nof a superconducting flux qubit. The dc superconducting quantum interference\ndevice (SQUID), used for readout of the qubit, and a shunt capacitor formed an\nLC resonator generating a SQUID plasma mode. Higher-order red and blue\nsidebands were observed in a simple measurement scheme because the resonant\nenergy of the resonator, 600 MHz, was comparable to the thermal energy. We also\nobserved Rabi oscillations on the carrier transition and the first-order\nsideband transitions. Because the qubit was coupled to a single arm of the dc\nSQUID, the qubit-SQUID coupling was significant at zero bias current, where\nthese phenomena were observed. The ratios between the Rabi periods for the\ncarrier transition and the sideband transitions are compared with those\nestimated from the coupling constant, which was separately determined. The\nresult may be explained by assuming initial excitation of the resonator.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Correlated trends of coexisting magnetism and superconductivity in\n  optimally electron-doped oxy-pnictides: We report on the recovery of the short-range static magnetic order and on the\nconcomitant degradation of the superconducting state in optimally F-doped\nSmFe_(1-x)Ru_(x)AsO_0.85F_0.15 for 0.1< x<0.6. The two reduced order parameters\ncoexist within nanometer-size domains in the FeAs layers and finally disappear\naround a common critical threshold x_c=0.6. Superconductivity and magnetism are\nshown to be closely related to two distinct well-defined local electronic\nenvironments of the FeAs layers. The two transition temperatures, controlled by\nthe isoelectronic and diamagnetic Ru substitution, scale with the volume\nfraction of the corresponding environments. This fact indicates that\nsuperconductivity is assisted by magnetic fluctuations, which are frozen\nwhenever a short-range static order appears, and totally vanish above the\nmagnetic dilution threshold x_c.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Fractional Flux Plateau in Magnetization Curve of Multicomponent\n  Superconductor Loop: Time-reversal symmetry (TRS) may be broken in superconductors with three or\nmore condensates interacting repulsively, yielding two degenerate states\nspecified by chirality of gap functions. We consider a loop of such\nsuperconductor with two halves occupied by the two states with opposite\nchiralities. Fractional flux plateaus are found in magnetization curve\nassociated with free-energy minima, where the two domain walls between the two\nhalves accommodate different inter-component phase kinks leading to finite\nwinding numbers in a part of the whole condensates around the loop. Fractional\nflux plateaus form pairs with their heights related to the flux quantum {\\Phi}0\n= hc/2e. This phenomenon is a clear evidence of time-reversal symmetry broken\n(TRSB) superconductivity, which in a general point of view provides a novel\nchance to explore relative phase difference, phase kink and soliton in\nubiquitous multi-component superconductivity such as that in iron pnicitides.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quantum detector of noise based on a system of asymmetric Al\n  superconducting rings: The quantum rectification effect observed on asymmetric superconducting loops\nis proposed to use as a basic of a noise detector with maximum sensitivity. The\nmeasurements show that a critical amplitude of noise or ac current decreases\ndown to zero near superconducting transition, Tc, just as superconducting\ncritical current. Therefore any how weak noise, right down to the equilibrium\none, can induce the quantum oscillations of the dc voltage near Tc. The\ntransformation of the power of random noise into the dc power observed on\nasymmetric superconducting loops gives very important advantage: even very weak\nnoise can give enough high output power in a system with large number of loops.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Upper bounds on the superfluid stiffness and superconducting $T_c$:\n  Applications to twisted-bilayer graphene and ultra-cold Fermi gases: Understanding the material parameters that control the superconducting\ntransition temperature $T_c$ is a problem of fundamental importance. In many\nnovel superconductors, phase fluctuations determine $T_c$, rather than the\ncollapse of the pairing amplitude. We derive rigorous upper bounds on the\nsuperfluid phase stiffness for multi-band systems, valid in any dimension. This\nin turn leads to an upper bound on $T_c$ in two dimensions (2D), which holds\nirrespective of pairing mechanism, interaction strength, or order-parameter\nsymmetry. Our bound is particularly useful for the strongly correlated regime\nof low-density and narrow-band systems, where mean field theory fails. For a\nsimple parabolic band in 2D with Fermi energy $E_F$, we find that $k_BT_c \\leq\nE_F/8$, an exact result that has direct implications for the 2D BCS-BEC\ncrossover in ultra-cold Fermi gases. Applying our multi-band bound to\nmagic-angle twisted bilayer graphene (MA-TBG), we find that band structure\nresults constrain the maximum $T_c$ to be close to the experimentally observed\nvalue. Finally, we discuss the question of deriving rigorous upper bounds on\n$T_c$ in 3D.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Spin flip scattering engendered quantum spin torque in a Josephson\n  junction: We examine a Josephson junction with two Ferromagnets and a spin flipper\nsandwiched between two superconductors. In such Ferromagnetic Josephson\njunctions, equilibrium spin torque exists only when Ferromagnets are\nmisaligned. This is explained via the \"conventional\" mechanism of spin transfer\ntorque, which owes its origin to the misalignment of two Ferromagnets. However,\nwe see surprisingly when the magnetic moments of the Ferromagnets are aligned\nparallel or antiparallel, there is a finite equilibrium spin torque due to the\nquantum mechanism of spin-flip scattering. We explore the properties of this\nunique spin-flip scattering induced equilibrium quantum spin torque, especially\nits tunability via exchange coupling and phase difference across the\nsuperconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Continuum dual theory of the transition in 3D lattice superconductor: A recently proposed form of dual theory for the three dimensional\nsuperconductor is rederived starting from the lattice electrodynamics and\nstudied by renormalization group. The superfluid density below and close to the\ntransition vanishes as inverse of the correlation length of the disorder field.\nThe corresponding universal amplitude is given by the fixed point value of the\ndual charge, and it is calculated to the leading order. The continuum dual\ntheory predicts the divergence of the magnetic field penetration depth with the\nXY exponent, in contradiction to the results obtained from the Ginzburg-Landau\ntheory for the superconducting order-parameter. Possible reasons for this\ndifference are discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Fermi surface reconstruction due to hidden rotating antiferromagnetism\n  in n and p-type high-$T_C$ cuprates: The Fermi surface calculated within the rotating antiferromagentism theory\nundergoes a topological change when doping changes from p-type to n-type, in\nqualitative agreement with experimental data for n-type cuprate\nNd$_{2-x}$Ce$_x$CuO$_4$ and p-type La$_{2-x}$Sr$_x$CuO$_4$. Also, the\nreconstruction of the Fermi surface observed experimentally close to optimal\ndoing in p-type cuprates, and slightly higher than optimal doping in the\noverdoped regime for this n-type high-$T_C$ cuprate is well accounted for in\nthis theory, and is a consequence of quantum criticality caused by the\ndisappearance of rotating antiferromagnetism. The present results are in\nqualitative agreement with the recently observed quantum oscillations in some\nhigh-$T_C$ cuprates regarding the change in the size of the Fermi surface as\ndoping evolves and the location of its reconstruction. This paper presents new\nresults about the application of the rotating antiferromagnetism theory to the\nstudy of electronic structure for n-type materials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Giant Triplet Proximity Effect in $\u03c0$-biased Josephson Junctions with\n  Spin-Orbit Coupling: In diffusive Josephson junctions the phase-difference $\\phi$ between the\nsuperconductors strongly influences the spectroscopic features of the layer\nseparating them. The observation of a uniform minigap and its phase modulation\nwere only recently experimentally reported, demonstrating agreement with\ntheoretical predictions up to now - a vanishing minigap at $\\phi=\\pi$.\nRemarkably, we find that in the presence of intrinsic spin-orbit coupling a\ngiant proximity effect due to spin-triplet Cooper pairs can develop at\n$\\phi=\\pi$, in complete contrast to the suppressed proximity effect without\nspin-orbit coupling. We here report a combined numerical and analytical study\nof this effect, proving its presence solely based on symmetry arguments, which\nmakes it independent of the specific parameters used in experiments. We show\nthat the spectroscopic signature of the triplets is present throughout the\nentire ferromagnetic layer. Our finding offers a new way to artificially\ncreate, control and isolate spin-triplet superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Fixed Number and Quantum Size Effects in Nanoscale Superconductors: In recent experiments on nanoscale Al particles, whose electron number was\nfixed by charging effects, a ``negative gap'' was observed in particles with an\nodd number of electrons. This observation has called into question the use of a\ngrand canonical ensemble in describing superconductivity in such ultrasmall\nparticles.\n  We have studied the effects of fixed electron number and finite size in\nnanoscale superconductors, by applying the canonical BCS theory for the\nattractive Hubbard model. The ground state energy and the energy gap are\ncompared with the conventional and parity-projected grand canonical BCS\nresults, and in one dimension also with the exact solutions by the Bethe\nansatz. The crossover from the bulk to quantum limit is studied for various\nregimes of electron density and coupling strength. The effects of boundary\nconditions and different lattice structures are also examined.\n  A ``negative gap'' for odd electron number emerges most naturally in the\ncanonical scheme. For even electron number, the gap is particularly large for\n``magic numbers'' of electrons for a given system size or of atoms for a fixed\nelectron density. These features are in accordance with the exact solutions,\nbut are essentially missed in the grand canonical results.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The dc-Josephson effect with more than four superconducting leads: By definition, the $p$-terminal dc-Josephson current is sensitive to the\nsuperconducting phase variables of $p$ terminals. In the paper, we establish\nprotocol for direct detection of the $p$-terminal dc-Josephson effect with\n$p\\ge 3$ in a device containing $N$ superconducting leads\n$S_1,\\,S_2,\\,...,\\,S_N$ having the phase variables\n$\\varphi_1,\\,\\varphi_2,\\,...,\\,\\varphi_N$. The calculated signal ${\\chi}^{(N)}$\ncan be probed in microwave experiments, and it corresponds to the higher-order\nnonlocal inverse inductance obtained from differentiating the current $I_1$\nthrough $S_1$ with respect to the remaining $N-2$ independent phase differences\n$\\varphi_2-\\varphi_N,\\,\\varphi_3-\\varphi_N,\\, ...,\\varphi_{N-1}-\\varphi_N$. We\nfind that the values $p\\le N-2$ do not contribute to ${\\chi}^{(N)}$, and that\n${\\chi}^{(N)}\\ne 0$ implies evidence for the $p=N-1$ or the $p=N$-terminal\ndc-Josephson currents. For $N=4$ superconducting leads, we demonstrate that\n${\\chi}^{(4)}\\ne 0$ implies evidence for the $p=3$ or $p=4$ dc-Josephson\neffect, irrespective of the $p=2$-terminal dc-Josephson current. Thus, we\nprovide a way to demonstrate the dc-Josephson effect with more than three\nterminals (i.e. with $p\\ge 3$) in a device containing more than four\nsuperconducting leads (i.e. with $N\\ge 4$). The predicted ${\\chi}^{(4)}$ is\n\"yes or no\" answer to the $p\\ge 3$ dc-Josephson effect, i.e. for $N=4$,\nnonvanishingly small $\\chi^{(4)}\\ne 0$ implies the $p=3$ or $p=4$-terminal\ndc-Josephson effect and vanishingly small $\\chi^{(4)}=0$ implies absence of the\n$p=3$ and $p=4$-terminal dc-Josephson effect. The paper can be viewed as\ngeneralizing the recently considered $\\varphi$-junctions in Andreev molecules\nto arbitrary number $N$ of the superconducting leads, and it relies on basic\nproperties of the dc-Josephson effect that are not directly related to\nnontrivial topology and Weyl point singularities.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Dynamical Structure Factor of Fulde-Ferrell-Larkin-Ovchinnikov\n  Superconductors: Superconductor with a spatially-modulated order parameter is known as\nFulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductor. Using the\ntime-dependent Ginzburg-Landau (TDGL) formalism we have theoretically studied\nthe temporal behaviour of the equal-time correlation function, or the structure\nfactor, of a FFLO superconductor following a sudden quench from the unpaired,\nor normal, state to the FFLO state. We find that quenching into the ordered\nFFLO phase can reveal the existence of a line in the mean-field phase diagram\nwhich cannot be accessed by static properties.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Identifying the pairing mechanism in Fe-As based superconductors: gaps\n  and isotope effects: The temperature dependencies of the coupled superconducting gaps, observed in\nFe-As based superconducting compounds is calculated and a universal temperature\nscaling observed which is only present if the coupled order parameters both\nhave s-wave symmetry. Predictions for possible isotope effects on the\ntransition temperature are made if phonons are involved in the pairing or\npolaronic effects are of importance. Comparison to experimental data is given\nwhere these are available.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Controllable tunnelling of single flux-quanta mediated by quantum\n  phase-slip in disordered superconducting loops: Quantum phase-slip (QPS) is the exact dual to the well-known Josephson\neffect. Although there are numerous proposals for applications of QPS devices,\nexperimental work to develop these remains in the relatively early stages.\nSignificant barriers to exploiting QPS nanowires for useful technologies still\nexist, such as establishing robust nanowire fabrication methods that allow\ncoupling to low-loss circuits, and demonstrating control over the QPS process\nwith an experimenter-controlled external bias. Here we report experiments which\nshow that both of these barriers have been overcome. We present measurements at\n300 mK of NbN coplanar waveguide (CPW) resonators embedded with nanowires\nfabricated using a neon focused ion-beam. The internal quality factor exceeds\n$2\\times10^{4}$ -- significantly higher than previously reported in comparable\nexperiments. The resonator frequency tunes periodically with an applied\nmagnetic field, revealing tunnelling of the order parameter that always occurs\nat half-integer values of the applied flux. In contrast to previous studies of\nsingle QPS, the order-parameter tunnelling is shown to be adiabatic,\ndemonstrating improved control over energy dissipation in nanowire QPS\ncircuits. Our results highlight a promising pathway towards realising low-loss\nnanowire-based QPS devices.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Chiral CP^2 skyrmions in three-band superconductors: It is shown that under certain conditions, three-component superconductors\n(and in particular three-band systems) allow stable topological defects\ndifferent from vortices. We demonstrate the existence of these excitations,\ncharacterized by a $CP^2$ topological invariant, in models for three-component\nsuperconductors with broken time reversal symmetry. We term these topological\ndefects \"chiral $GL^{(3)}$ skyrmions\", where \"chiral\" refers to the fact that\ndue to broken time reversal symmetry, these defects come in inequivalent left-\nand right-handed versions. In certain cases these objects are energetically\ncheaper than vortices and should be induced by an applied magnetic field. In\nother situations these skyrmions are metastable states, which can be produced\nby a quench. Observation of these defects can signal broken time reversal\nsymmetry in three-band superconductors or in Josephson-coupled bilayers of\n$s_\\pm$ and s-wave superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Improved Single-Crystal Growth of Sr$_2$RuO$_4$: High-quality single crystals are essentially needed for the investigation of\nthe novel bulk properties of unconventional superconductors. The availability\nof such crystals grown by the floating-zone method has helped to unveil the\nunconventional superconductivity of the layered perovskite Sr$_2$RuO$_4$, which\nis considered as a strong candidate of a topological spin-triplet\nsuperconductor. Yet, recent progress of investigations urges further efforts to\nobtain ultimately high-quality crystalline samples. In this paper, we focus on\nthe method of preparation of feed rods for the floating-zone melting and report\non the improvements of the crystal growth. We present details of the improved\nmethods used to obtain crystals with superconducting transition temperatures\n$T_\\mathrm{c}$ that are consistently as high as 1.4 K, as well as the\nproperties of these crystals.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pressure tuning of Fermi surface topology of optimally doped\n  BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$: The superconducting, transport, and structural properties of optimally\nelectron-doped BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ are investigated by combining the\nelectrical resistance and synchrotron X-ray diffraction measurements at high\npressures. The superconducting transition temperature of this system is found\nto decrease in a similar way of the axial ratio of $c/a$ with increasing\npressure but vanishing at a critical pressure of 7.5 GPa where $c/a$ has a dip\nand an isostructural transformation from a tetragonal to a collapsed tetragonal\nphase takes place. The resistance is found to obey a linear temperature\ndependence, evidencing the antiferromagnetic spin-fluctuations transport\nmechanism. The pressure effects are interpreted within the framework of\npressure-induced Fermi surface topology modification in which pressure\nsuppresses both the quasiparticle effective mass and the strength of the\nantiferromagnetic spin fluctuations leading to the reduction of\nsuperconductivity, accordingly. The absent superconductivity in the collapsed\ntetragonal phase is suggested to result from the complete suppression of the\nantiferromagnetic spin fluctuations.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Nature of carrier doping in T'-La1.8-xEu0.2SrxCuO4 studied by X-Ray\n  Photoemission and Absorption Spectroscopy: Recently, hole-doped superconducting cuprates with the T'-structure\nLa1.8-xEu0.2SrxCuO4 (LESCO) have attracted a lot of attention. We have\nperformed x-ray photoemission and absorption spectroscopy measurements on\nas-grown and reduced T0-LESCO. Results show that electrons and holes were doped\nby reduction annealing and Sr substitution, respectively. However, it is shown\nthat the system remains on the electron-doped side of the Mott insulator or\nthat the charge-transfer gap is collapsed in the parent compound.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Substrate mediated nitridation of niobium into superconducting Nb2N thin\n  films for phase slip study: Here we report a novel nitridation technique for transforming niobium into\nhexagonal Nb2N which appears to be superconducting below 1K. The nitridation is\nachieved by high temperature annealing of Nb films grown on Si3N4/Si (100)\nsubstrate under high vacuum. The structural characterization directs the\nformation of a majority Nb2N phase while the morphology shows granular nature\nof the films. The temperature dependent resistance measurements reveal a wide\nmetal-to-superconductor transition featuring two distinct transition regions.\nThe region close to the normal state varies strongly with the film thickness,\nwhereas, the second region in the vicinity of the superconducting state remains\nalmost unaltered but exhibiting resistive tailing. The current-voltage\ncharacteristics also display wide transition embedded with intermediate\nresistive states originated by phase slip lines. The transition width in\ncurrent and the number of resistive steps depend on film thickness and they\nboth increase with decrease in thickness. The broadening in transition width is\nexplained by progressive establishment of superconductivity through proximity\ncoupled superconducting nano-grains while finite size effects and quantum\nfluctuation may lead to the resistive tailing. Finally, by comparing with Nb\ncontrol samples, we emphasize that Nb2N offers unconventional superconductivity\nwith promises in the field of phase slip based device applications.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Generic nodeless Larkin Ovchinnikov states due to singlet-triplet mixing: Larkin-Ovchinnikov (LO) states typically have a singlet-gap that vanishes\nalong real-space lines. These real-space nodes lead to Andreev midgap states\nwhich can serve as a signature of LO pairing. We show that at these nodes, an\nodd-parity, spin-triplet component is always induced, leading to a nodeless LO\nphase. We find the two-dimensional weak coupling, clean limit s-wave phase\ndiagram when this spin-triplet part is included. The triplet component is large\nand increases the stability of the FFLO phase. We also show that the\nspin-triplet contribution pushes the midgap states away from zero energy.\nFinally, we show how our results can be explained phenomenologically though\nLifshitz invariants. These invariants provide a simple approach to understand\nthe role of unconventional pairing states, spin-orbit coupling, and\ninhomogeneous mixed singlet-triplet states that are not due to a FFLO\ninstability. We discuss our results in the context of organic superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Upper critical magnetic field of $Ln$O$_{0.5}$F$_{0.5}$BiS$_2$ ($Ln$ =\n  La, Nd) superconductors at ambient and high pressure: The upper critical field $H_{c2}$ of polycrystalline samples of\n$Ln$O$_{0.5}$F$_{0.5}$BiS$_{2}$ ($Ln$ = La, Nd) at ambient pressure (tetragonal\nstructure) and high pressure (HP) (monoclinic structure) have been investigated\nvia electrical resistivity measurements at various magnetic fields up to 8.5 T.\nThe $H_{c2}$($T$) curves for all the samples show an uncharacteristic concave\nupward curvature at temperatures below $T_c$, which cannot be described by the\nconventional one-band Werthamer-Helfand-Hohenberg theory. For the\nLaO$_{0.5}$F$_{0.5}$BiS$_{2}$ sample under HP, as temperature is decreased, the\nupper critical field $H_{onset}$, estimated from the onset of the\nsuperconducting transitions, increases slowly between 4.9 and 5.8 T compared\nwith the slope of $H_{onset}$($T$) below 4.9 T and above 5.8 T. This anomalous\nbehavior reveals a remarkable similarity in superconductivity between\nLaO$_{0.5}$F$_{0.5}$BiS$_{2}$ samples measured under HP and synthesized under\nHP, although the crystal structures of the two samples were reported to be\ndifferent. The experimental results support the idea that local atomic\nenvironment, which can be tuned by applying external pressure and can be\nquenched to ambient pressure via high temperature-pressure annealing, is\npossibly more essential to the enhancement of $T_c$ for BiS$_2$-based\nsuperconductors than the structural phase transition. On the other hand, such\nanomalous behavior is very subtle in the case of NdO$_{0.5}$F$_{0.5}$BiS$_{2}$\nunder HP, suggesting that the anisotropy of the upper critical field in the\n$ab$-plane and the possible lattice deformation induced by external pressure is\nweak. This explains why the pressure-induced enhancement of $T_c$ for\nNdO$_{0.5}$F$_{0.5}$BiS$_{2}$ is not as large as that for\nLaO$_{0.5}$F$_{0.5}$BiS$_{2}$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Fermi surface reconstruction in electron-doped cuprates without\n  antiferromagnetic long-range order: Fermi surface (FS) topology is a fundamental property of metals and\nsuperconductors. In electron-doped cuprate Nd2-xCexCuO4 (NCCO), an unexpected\nFS reconstruction has been observed in optimal- and over-doped regime\n(x=0.15-0.17) by quantum oscillation measurements (QOM). This is all the more\npuzzling because neutron scattering suggests that the antiferromagnetic (AFM)\nlong-range order, which is believed to reconstruct the FS, vanishes before\nx=0.14. To reconcile the conflict, a widely discussed external magnetic\nfield-induced AFM long-range order in QOM explains the FS reconstruction as an\nextrinsic property. Here, we report angle-resolved photoemission (ARPES)\nevidence of FS reconstruction in optimal- and over-doped NCCO. The observed FSs\nare in quantitative agreement with QOM, suggesting an intrinsic FS\nreconstruction without field. This reconstructed FS, despite its importance as\na basis to understand electron-doped cuprates, cannot be explained under the\ntraditional scheme. Furthermore, the energy gap of the reconstruction decreases\nrapidly near x=0.17 like an order parameter, echoing the quantum critical\ndoping in transport. The totality of the data points to a mysterious order\nbetween x=0.14 and 0.17, whose appearance favors the FS reconstruction and\ndisappearance defines the quantum critical doping. A recent topological\nproposal provides an ansatz for its origin.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Tuning competing orders in La2-xSrxCuO4 cuprate superconductors by the\n  application of an external magnetic field: We report the results of a combined muon spin rotation and neutron scattering\nstudy on La2-xSrxCuO4 in the vicinity of the so-called 1/8-anomaly. Application\nof a magnetic field drives the system towards a magnetically ordered\nspin-density-wave state, which is fully developed at 1/8 doping. The results\nare discussed in terms of competition between antiferromagnetic and\nsuperconducting order parameters.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Bosonic Spectral Function in HTSC Cuprates: Part I - Experimental\n  Evidence for Strong Electron-Phonon Interaction: In Part I we discuss accumulating experimental evidence related to the\nstructure and origin of the bosonic spectral function \\alpha ^{2}F in\nhigh-temperature superconducting (HTSC) cuprates near optimal doping. Some\nglobal properties of \\alpha ^{2}F such as number and positions of peaks, are\nextracted by combining optics, neutron scattering, ARPES and tunnelling\nmeasurements. These methods give convincing evidence for strong electron-phonon\ninteraction (EPI) with 1<\\lambda \\lesssim 3 in cuprates near optimal doping.\nHere we clarify how these results are in favor of the Eliashberg-like theory\nfor HTSC cuprates near optimal doping.We argue that the neglect of EPI in some\nprevious studies of HTSC was based on a number of deceptive prejudices related\nto the strength of EPI, on some physical misconceptions and misleading\ninterpretation of experimental results.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Effect of doping on the magnetostructural ordered phase of iron\n  arsenides: A comparative study of the resistivity anisotropy in the doped\n  BaFe$_2$As$_2$ with doping into three different sites: In order to unravel a role of doping in the iron-based superconductors, we\ninvestigated the in-plane resistivity for BaFe$_2$As$_2$ doped at either of the\nthree different lattice sites, Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$,\nBaFe$_2$(As$_{1-x}$P$_x$)$_2$, and Ba$_{1-x}$K$_x$Fe$_2$As$_2$, focusing on the\ndoping effect in the low-temperature antiferromagnetic/orthorhombic (AFO)\nphase. A major role of doping in the high-temperature paramagnetic/tetragonal\n(PT) phase is known to change the Fermi surface by supplying charge carriers or\nby exerting chemical pressure. In the AFO phase, we found a clear correlation\nbetween the magnitude of residual resistivity and resistivity anisotropy. This\nindicates that the resistivity anisotropy originates from the anisotropic\nimpurity scattering from dopant atoms. The magnitude of residual resistivity is\nalso found to be a parameter controlling the suppression rate of AFO ordering\ntemperature $T_s$. Therefore, the dominant role of doping in the AFO phase is\nto introduce disorder to the system, distinct from that in the PT phase.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Transport and magnetic properties of La-doped CaFe$_2$As$_2$: We measured the transport properties and susceptibility of single crystals\nCa$_{1-x}$La$_x$Fe$_2$As$_2$(x=0, 0.05, 0.1, 0.15, 0.19 and 0.25). Large\nin-plane resistivity anisotropy similar to that in Co-doped 122 iron-pnictides\nis observed although no transition metals were introduced in the FeAs-plane.\nThe in-plane resistivity anisotropy gradually increases with La doping below\nT$_{SDW}$, being different from the hole-doped 122 superconductors. The\nsusceptibilities of the samples show that La doping leads to suppression of SDW\nand induces a Curie-Weiss-like behavior at low temperature, which is much\nstronger than the other 122 iron-based superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Domes of $T_c$ in single-band and multiband superconductors with\n  finite-range attractive interactions: The rise and fall of the superconducting transition temperature $T_c$ upon\ntuning carrier density or external parameters, such as pressure or magnetic\nfield, is ubiquitously observed in a wide range of quantum materials. In order\nto investigate such domes of $T_c$, we go beyond the prototypical attractive\nHubbard model, and consider a lattice model of electrons coupled via\ninstantaneous, spatially extended, attractive interactions. By numerically\nsolving the mean-field equations, as well as going beyond mean field theory\nusing a functional renormalization group approach, we find that for a\ncharacteristic interaction range $\\ell$, there exists a dome in $T_c$ around\n$k_F \\ell \\! \\sim \\! {\\mathcal{O}}(1)$. For multiband systems, our mean field\ntheory shows the presence of additional domes in the vicinity of Lifshitz\ntransitions. Our results hold in both two and three dimensions and can be\nintuitively understood from the geometric relation between the Fermi surface\nand the interaction range. Our model may be relevant for domes of $T_c$ in\ndilute weakly coupled superconductors or in engineered cold atom systems.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Numerical Study of Energy Loss by a Nanomechanical Oscillator Coupled to\n  a Cooper Pair Box: We calculate the dynamics of a nanomechanical oscillator (NMO) coupled\ncapacitively to a Cooper pair box (CPB), by solving a stochastic Schrodinger\nequation with two Lindblad operators. Both the NMO and the CPB are assumed\ndissipative, and the coupling is treated within the rotating wave\napproximation. We show numerically that, if the CPB decay time is smaller than\nthe NMO decay time, the coupled NMO will lose energy faster, and the coupled\nCPB more slowly, than do the uncoupled NMO and CPB. The results show that the\nefficiency of energy loss by an NMO can be substantially increased if the NMO\nis coupled to a CPB.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "High-energy ball milling and synthesi temperature study to improve\n  superconducting properties of MgB2 ex-situ tapes and wires: MgB2 monofilamentary nickel-sheated tapes and wires were fabricated by means\nof the ex-situ powder-in-tube method using either high-energy ball milled and\nlow temperature synthesized powders. All sample were sintered at 920 C in Ar\nflow. The milling time and the revolution speed were tuned in order to maximize\nthe critical current density in field (Jc): the maximum Jc value of 6 x 10e4\nA/cm2 at 5 K and 4 T was obtained corresponding to the tape prepared with\npowders milled for 144h at 180rpm. Vorious synthesis temperature were also\ninvestigated (730-900 C) finding a best Jc value for the wire prepared with\npowders synthesized at 745 C. We speculate that this optimal temperature is due\nto the fluidifying effect of unreacted magnesium content before the sintering\nprocess which could better connect the grains.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Spectral signatures of modulated d-wave superconducting phases: We calculate within a mean-field theory the spectral signatures of various\nstriped d-wave superconducting phases. We consider both in-phase and anti-phase\nmodulations of the superconducting order across a stripe boundary, and the\neffects of coexisting inhomogeneous orders, including spin stripes, charge\nstripes, and modulated d-density-wave. We find that the anti-phase modulated\nd-wave superconductor exhibits zero-energy spectral weight, primarily along\nextended arcs in momentum space. Concomitantly, a Fermi surface appears and\ntypically includes both open segments and closed pockets. When weak homogeneous\nsuperconductivity is also present the Fermi surface collapses onto nodal\npoints. Among them are the nodal points of the homogeneous d-wave\nsuperconductor, but others typically exist at positions which depend on the\ndetails of the modulation and the band structure. Upon increasing the amplitude\nof the constant component these additional points move towards the edges of the\nreduced Brillouin zone where they eventually disappear. The above signatures\nare also manifested in the density of states of the clean, and the disordered\nsystem. While the presence of coexisting orders changes some details of the\nspectral function, we find that the evolution of the Fermi-surface and the\ndistribution of the low-energy spectral weight are largely unaffected by them.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Sublattice-sensitive Majorana Modes: For two- and three-dimensional topological insulators whose unit cells\nconsist of multiple sublattices, the boundary terminating at which type of\nsublattice can affect the time-reversal invariant momentum at which the Dirac\npoints of helical boundary states are located. Through a general theory and a\nrepresentative model, we reveal that this interesting property allows the\nrealization of Majorana modes at sublattice domain walls forming on the\nboundary when the boundary Dirac points of the topological insulator are gapped\nby an unconventional superconductor in proximity. Intriguingly, we find that\nthe sensitive sublattice-dependence of the Majorana modes allows their\npositions to be precisely manipulated by locally controlling the terminating\nsublattices or boundary potential. Our work reveals that the common sublattice\ndegrees of freedom in materials open a new route to realize and manipulate\nMajorana modes.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "On a possibility of effective electron attraction without \"a glue\": With the means of Schrodinger equation a two electron problem is considered.\nIt is shown that electrons in a periodic potential might experience effective\nattraction without any mediating agents; which means electrons in HTSC might\nhave effective attraction potential.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "High-energy magnetic excitations in overdoped\n  La$_{2-x}$Sr$_{x}$CuO$_{4}$ studied by neutron and resonant inelastic X-ray\n  scattering: We have performed neutron inelastic scattering and resonant inelastic X-ray\nscattering (RIXS) at the Cu-$L_3$ edge to study high-energy magnetic\nexcitations at energy transfers of more than 100 meV for overdoped\nLa$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x=0.25$ ($T_c=15$ K) and $x=0.30$\n(non-superconducting) using identical single crystal samples for the two\ntechniques. From constant-energy slices of neutron scattering cross-sections,\nwe have identified magnetic excitations up to ~250 meV for $x=0.25$. Although\nthe width in the momentum direction is large, the peak positions along the (pi,\npi) direction agree with the dispersion relation of the spin-wave in the\nnon-doped La$_{2}$CuO$_{4}$ (LCO), which is consistent with the previous RIXS\nresults of cuprate superconductors. Using RIXS at the Cu-$L_3$ edge, we have\nmeasured the dispersion relations of the so-called paramagnon mode along both\n(pi, pi) and (pi, 0) directions. Although in both directions the neutron and\nRIXS data connect with each other and the paramagnon along (pi, 0) agrees well\nwith the LCO spin-wave dispersion, the paramagnon in the (pi, pi) direction\nprobed by RIXS appears to be less dispersive and the excitation energy is lower\nthan the spin-wave of LCO near (pi/2, pi/2). Thus, our results indicate\nconsistency between neutron inelastic scattering and RIXS, and elucidate the\nentire magnetic excitation in the (pi, pi) direction by the complementary use\nof two probes. The polarization dependence of the RIXS profiles indicates that\nappreciable charge excitations exist in the same energy range of magnetic\nexcitations, reflecting the itinerant character of the overdoped sample. A\npossible anisotropy in the charge excitation intensity might explain the\napparent differences in the paramagnon dispersion in the (pi, pi) direction as\ndetected by the X-ray scattering.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Analysis of the paraconductivity in underdoped $La_{2-x}Sr_xCuO_4$ thin\n  films using high magnetic fields: The contribution of superconducting fluctuations to the conductivity, or\nparaconductivity is studied in the underdoped regime of $La_{2-x}Sr_xCuO_4$\ncuprates. A perpendicular magnetic field up to 50 T is applied to suppress the\nsuperconductivity and obtain the normal state resistivity which is then used to\ncalculate the paraconductivity. Surprisingly enough, it is consistent with a\ntwo-dimensional Aslamazov-Larkin (AL) regime of Gaussian fluctuations close to\nthe critical temperature. At higher temperature, the paraconductivity shows a\npower-law decrease in temperature (as $T^{-\\alpha}$) as was previously shown\nfor underdoped $YBa_2Cu_3O_{7-\\delta}$ and $Bi_2Sr_2CaCu_2O_{8+\\delta}$\nsamples. Our observations are not consistent with the existence of\nKosterlitz-Thouless fluctuations. This tends to indicate that the\nsuperconducting pair amplitude is not already defined above $T_C$ in the\npseudogap state.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Evidence for unusual superconducting correlations coexisting with stripe\n  order in La(1.875)Ba(0.125)CuO(4): We present new x-ray and neutron scattering measurements of stripe order in\nLa(1.875)Ba(0.125)CuO(4), along with low-field susceptibility, thermal\nconductivity, and specific heat data. We compare these with previously reported\nresults for resistivity and thermopower. Temperature-dependent features\nindicating transitions (or crossovers) are correlated among the various\nexperimental quantities. Taking into account recent spectroscopic studies, we\nargue that the most likely interpretation of the complete collection of results\nis that an unusual form of two-dimensional superconducting correlations appears\ntogether with the onset of spin-stripe order. Recent theoretical proposals for\na sinusoidally-modulated superconducting state compatible with stripe order\nprovide an intriguing explanation of our results and motivate further\nexperimental tests. We also discuss evidence for one-dimensional pairing\ncorrelations that appear together with the charge order. With regard to the\noverall phenomenology, we consider the degree to which similar behavior may\nhave been observed in other cuprates, and describe possible connections to\nvarious puzzling phenomena in cuprate superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Supercurrent induced domain wall motion: We study the dynamics of a magnetic domain wall, inserted in, or juxtaposed\nto, a conventional superconductor, via the passage of a spin polarized current\nthrough a FSF junction. Solving the Landau-Lifshitz-Gilbert equation of motion\nfor the magnetic moments we calculate the velocity of the domain wall and\ncompare it with the case of a FNF junction. We find that in several regimes the\ndomain wall velocity is larger when it is driven by a supercurrent.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Simulation of some quantum gates, with decoherence: Methods and results for numerical simulations of one and two interacting\nrf-Squid systems suitable for adiabatic quantum gates are presented. These are\nbased on high accuracy numerical solutions to the static and time dependent\nSchroedinger equation for the full Squid Hamiltonian in one and two variables.\nAmong the points examined in the static analysis is the range of validity of\nthe effective two-state or ``spin 1/2'' picture. A range of parameters is\ndetermined where the picture holds to good accuracy as the energy levels\nundergo gate manipulations. Some general points are presented concerning the\nrelations between device parameters and ``good'' quantum mechanical state\nspaces.\n  The time dependent simulations allow the examination of suitable conditions\nfor adiabatic behavior, and permits the introduction of a random noise to\nsimulate the effects of decoherence. A formula is derived and tested relating\nthe random noise to the decoherence rate. Sensitivity to device and operating\nparameters for the logical gates NOT and CNOT are examined, with particular\nattention to values of the tunnel parameter beta slightly above one. It appears\nthat with values of beta close to one, a quantum CNOT gate is possible even\nwith rather short decoherence times.\n  Many of the methods and results will apply to coupled double-potential well\nsystems in general.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Switching of intra-orbital spin excitations in electron-doped iron\n  pnictide superconductors: We investigate the doping dependence of the magnetic excitations in\ntwo-superconducting-dome-system LaFeAsO1-xDx. Using inelastic neutron\nscattering, spin fluctuations at different wavenumbers were observed under both\nsuperconducting domes around x = 0.1 and 0.4, but vanished at x = 0.2\ncorresponding to the Tc valley. Theoretical calculations indicate that the\ncharacteristic doping dependence of spin fluctuations is rationally explained\nas a consequence of the switching of the two intra-orbital nestings within\nFe-3dYZ, ZX and 3dX2-Y2 by electron doping. The present results imply that the\nmulti-orbital nature plays an important role in the doping and / or material\ndependence of the Tc of the iron pnictide superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Topological superconducting transition driven by time-reversal-symmetry\n  breaking: Three-dimensional line-nodal superconductors exhibit nontrivial topology,\nwhich is protected by the time-reversal symmetry. Here we investigate four\ntypes of short-range interaction between the gapless line-nodal fermionic\nquasiparticles by carrying renormalization group analysis. We find that such\ninteractions can induce the dynamical breaking of time-reversal symmetry, which\nalters the topology and might lead to six possible distinct superconducting\nstates, distinguished by the group representations. After computing the\nsusceptibilities for all the possible phase-transition instabilities, we\nestablish that the superconducting pairing characterized by $id_{xz}$-wave gap\nsymmetry is the leading instability in noncentrosymmetric superconductors.\nAppropriate extension of this approach is promising to pick out the most\nfavorable superconducting pairing during similar topology-changing transition\nin the polar phase of $^3$He.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superfluid transition in disordered dipolar Fermi gases: We consider a weakly interacting two-component Fermi gas of dipolar particles\n(magnetic atoms or polar molecules) in the two-dimensional geometry. The\ndipole-dipole interaction (together with the short-range interaction at\nFeshbach resonances) for dipoles perpendicular to the plane of translational\nmotion may provide a superfluid transition. The dipole-dipole scattering\namplitude is momentum dependent, which violates the Anderson theorem claiming\nthe independence of the transition temperature on the presence of weak\ndisorder. We have shown that the disorder can strongly increase the critical\ntemperature (up to 10 nK at realistic densities). This opens wide possibilities\nfor the studies of the superfluid regime in weakly interacting Fermi gases,\nwhich was not observed so far.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Tunneling spectra for ($d_{x^2-y^2}+is$)-wave superconductors versus\n  tunneling spectra for ($d_{x^2-y^2}+id_{xy}$)-wave superconductors: The tunneling conductance spectra of a normal metal / insulator / singlet\nsuperconductor is calculated from the reflection amplitudes using the\nBlonder-Tinkham-Klapwijk (BTK) formulation. The pairing symmetry of the\nsuperconductor is assumed to be $d_{x^2-y^2}+is$, or $d_{x^2-y^2}+id_{xy}$. It\nis found that in the ($d_{x^2-y^2}+is$)-wave case there is a well defined\nconductance peak in the conductance spectra, in the amplitude of the secondary\ns-wave component. In the ($d_{x^2-y^2}+id_{xy}$)-wave case the tunneling\nconductance has residual values within the gap, due to the formation of bound\nstates. The bound state energies depend on the angle of the incident\nquasiparticles, and also on the boundary orientation. On the basis of this\nobservation an electron focusing experiment is proposed to probe the\n($d_{x^2-y^2}+id_{xy}$)-wave state.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Ab initio calculations of the electronic structure of cuprates using\n  large scale cluster techniques: The local electronic structures of La2CuO4, three members of the\nYttrium-family (YBa2Cu3O6, YBa2Cu3O7, and YBa2Cu4O8), and to some extent of\nNd2CuO4 have been determined using all-electron ab-initio cluster calculations\nfor clusters comprising up to thirteen planar copper atoms associated with\ntheir nearest planar and apical oxygen atoms. Spin-polarized calculations in\nthe framework of density functional theory have enabled an estimation of the\nsuperexchange couplings J. Electric field gradients at the planar copper sites\nare determined and their dependence on the occupation of the various atomic\norbitals are investigated in detail. The changes of the electronic field\ngradient and of the occupation of orbitals upon doping are studied and\ndiscussed. Furthermore, magnetic hyperfine fields are evaluated and\ndisentangled into on-site and transferred contributions, and the chemical\nshifts at the copper nucleus are calculated. In general the results are in good\nagreement with values deduced from experiments except for the value of the\nchemical shift with applied field perpendicular to the CuO2-plane.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Disorder-induced time effect in the antiferromagnetic domain state of\n  Fe1+yTe: We report on temperature-dependent soft X-ray absorption spectroscopy (XAS)\nmeasurements utilizing linearly polarized synchrotron radiation to probe\nmagnetic phase transitions in iron-rich Fe1+yTe. X-ray magnetic linear\ndichroism (XMLD) signals, which sense magnetic ordering processes at surfaces,\nstart to increase monotonically below the N\\'eel temperature TN = 57 K. This\nincrease is due to a progressive bicollinear antiferromagnetic (AFM) alignment\nof Fe spins of the monoclinic Fe1+yTe parent phase. This AFM alignment was\nachieved by a [100]-oriented biasing field favoring a single-domain state\nduring cooling across TN. Our specific heat and magnetization measurements\nconfirm the bulk character of this AFM phase transition. On longer time scales,\nhowever, we observe that the field-biased AFM state is highly unstable even at\nthe lowest temperature of T = 3 K. After switching off the biasing field, the\nXMLD signal decays exponentially with a time constant {\\tau} = 1506 s. The\ninitial XMLD signal is restored only upon repeating a cycle consisting of\nheating and field-cooling through TN. We explain the time effect by a gradual\nformation of a multi-domain state with 90 deg rotated AFM domains, promoted by\nstructural disorder, facilitating the motion of twin-domains. Significant\ndisorder in our Fe1+yTe sample is evident from our X-ray diffraction and\nspecific heat data. The stability of magnetic phases in Fe-chalcogenides is an\nimportant material property, since the Fe(Te1-xSex) phase diagram shows\nmagnetism intimately connected with superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Thermal Conductivity in 3D NJL Model Under External Magnetic Field: The thermal conductivity of the (2+1)-dimensional NJL model in the presence\nof a constant magnetic field is calculated in the mean-field approximation and\nits different asymptotic regimes are analyzed. Taking into account the\ndynamical generation of a fermion mass due to the magnetic catalysis\nphenomenon, it is shown that for certain relations among the theory's\nparameters (particle width, temperature and magnetic field), the profile of the\nthermal conductivity versus the applied field exhibits kink- and plateau-like\nbehaviors. We point out possible applications to planar condensed matter.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quasiparticle Thermal Conductivities in a Type-II Superconductor at High\n  Magnetic Field: We present a calculation of the quasiparticle contribution to the\nlongitudinal thermal conductivities as well as transverse (Hall) thermal\nconductivity of an extreme type-II superconductor in a high magnetic field and\nat low temperatures. In the limit of frequency and temperature approaching\nzero, both longitudinal and transverse conductivities upon entering the\nsuperconducting state undergo a reduction from their respective normal state\nvalues by the factor $(\\Gamma /\\Delta)^2$, which measures the size of the\nregion at the Fermi surface containing gapless quasiparticle excitations. We\nuse our theory to numerically compute the longitudinal transport coefficient in\nborocarbide and A-15 superconductors. The agreement with recent experimental\ndata on LuNi_2B_2C is very good.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Anomalous Dimension of Dirac's Gauge-Invariant Nonlocal Order Parameter\n  in Ginzburg-Landau Field Theory: In a Ginzburg-Landau theory with $n$ fields, the anomalous dimension of the\ngauge-invariant nonlocal order parameter defined by the long-distance limit of\nDirac's gauge-invariant two-point function is calculated. The result is exact\nfor all $n$ to first order in $\\epsilon \\equiv 4-d$, and for all $d\\in (2,4)$\nto first order in $1/n$, and coincides with the previously calculated\ngauge-dependent exponent in the Landau gauge.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Carbonaceous sulfur hydride system: the strong-coupled room-temperature\n  superconductor with a low value of Ginzburg-Landau parameter: The superconducting state in Carbonaceous Sulfur Hydride (C-S-H) system is\ncharacterized by the record-high critical temperature of $288$~K experimentally\nobserved at $\\sim$267 GPa. Herein, we determined the properties of the\n\\mbox{C-S-H} superconducting phase within the scope of both classical\nEliashberg equations (CEE) and the Eliashberg equations with vertex corrections\n(VCEE). We took into account the scenarios pertinent to either the intermediate\nor the high value of electron-phonon coupling constant ($\\lambda\\sim 0.75$ or\n$\\lambda\\sim 3.3$, respectively). The scenario for the intermediate value,\nhowever, cannot be actually realized due to the anomally high value of\nlogarithmic phonon frequency ($\\omega_{\\rm ln}/k_{B}=7150$~K) it would require.\nOn the other hand, we found it possible to reproduce correctly the value of\n$T_{C}$ and other thermodynamic quantities in the case of strong coupling.\nHowever, the vertex corrections lower the order parameter values within the\nrange from $\\sim 50$~K to $\\sim275$~K. For the upper critical field $H_{C2}\\sim\n27$~T, the Ginzburg-Landau parameter $\\kappa$ is of the order of $1.7$. This\ncorrelates well with the sharp drop of resistance observed by Hirsch and\nMarsiglio at the critical temperature. The strong-coupling scenario for C-S-H\nsystem is also suggested by the high values of $\\lambda$ estimated for ${\\rm\nH_{3}S}$ ($\\lambda\\sim 2.1$, $\\kappa\\sim 1.5$), ${\\rm LaH_{10}}$ ($\\lambda\\sim\n2.8$-$3.9$, $\\kappa\\sim 1.6$), and ${\\rm YH_{6}}$ ($\\lambda\\sim 1.7$,\n$\\kappa\\sim 1.3$) compounds.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Flux growth at ambient pressure of millimeter-sized single crystals of\n  LaFeAsO, LaFeAsO1-xFx, and LaFe1-xCoxAsO: Millimeter-sized single crystals of LaFeAsO, LaFeAsO1-xFx, and LaFe1-xCoxAsO\nwere grown in NaAs flux at ambient pressure. The detailed growth procedure and\ncrystal characterizations are reported. The as-grown crystals have typical\ndimensions of 3 * 4 * 0.05-0.3 mm3 with the crystallographic c-axis\nperpendicular to the plane of the plate-like single crystals. Some crystals\nmanifest linear dimensions as large as 4-5 mm. X-ray and neutron single crystal\nscattering confirmed that LaFeAsO crystals exhibit a structural phase\ntransition at Ts ~ 154 K and a magnetic phase transition at TSDW ~ 140 K. The\ntransition temperatures agree with those determined by anisotropic\nmagnetization, in-plane electrical resistivity and specific heat measurements\nand are consistent with previous reports on polycrystalline samples. Co and F\nwere successfully introduced into the lattice leading to superconducting\nLaFe1-xCoxAsO and LaFeAsO1-xFx single crystals, respectively. This growth\nprotocol has been successfully employed to grow single crystals of NdFeAsO.\nThus it is expected to be broadly applicable to grow other RMAsO (R = rare\nearth, M = transition metal) compounds. These large crystals will facilitate\nthe efforts of unraveling the underlying physics of iron pniticide\nsuperconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Temperature-doping phase diagrams for Ba(Fe1-xTMx)2As2(TM=Ni, Cu, Cu /\n  Co) single crystals: Microscopic, structural, transport and thermodynamic measurements of single\ncrystalline Ba(Fe1-xTMx)2As2 (TM = Ni and Cu) series, as well as two mixed TM =\nCu / Co series, are reported. All the transport and thermodynamic measurements\nindicate that the structural and magnetic phase transitions at 134 K in pure\nBaFe2As2 are monotonically suppressed and increasingly separated in a similar\nmanner by these dopants. In the Ba(Fe1-xNix)2As2 (x =< 0.072),\nsuperconductivity, with Tc up to 19 K, is stabilized for 0.024 =< x =< 0.072.\nIn the Ba(Fe1-xCux)2As2 (x =< 0.356) series, although the structural and\nmagnetic transitions are suppressed, there is only a very limited region of\nsuperconductivity: a sharp drop of the resistivity to zero near 2.1 K is found\nonly for the x = 0.044 samples. In the Ba(Fe1-x-yCoxCuy)2As2 series,\nsuperconductivity, with Tc values up to 12 K (x ~ 0.022 series) and 20 K (x ~\n0.047 series), is stabilized. Quantitative analysis of the detailed\ntemperature-dopant concentration (T-x) and temperature-extra electrons (T-e)\nphase diagrams of these series shows that there exists a limited range of the\nnumber of extra electrons added, inside which the superconductivity can be\nstabilized if the structural and magnetic phase transitions are suppressed\nenough. Moreover, comparison with pressure-temperature phase diagram data, for\nsamples spanning the whole doping range, further reenforces the conclusion that\nsuppression of the structural / magnetic phase transition temperature enhances\nTc on the underdoped side, but for the overdoped side Tcmax is determined by e.\nTherefore, by choosing the combination of dopants that are used, we can adjust\nthe relative positions of the upper phase lines (structural and magnetic phase\ntransitions) and the superconducting dome to control the occurrence and\ndisappearance of the superconductivity in transition metal, electron-doped\nBaFe2As2.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pairing Mechanism of the Heavily Electron Doped FeSe Systems: Dynamical\n  Tuning of the Pairing Cutoff Energy: We studied pairing mechanism of the heavily electron doped FeSe (HEDIS)\nsystems, which commonly have one incipient hole band -- a band top below the\nFermi level by a finite energy distance $\\epsilon_b$ -- at $\\Gamma$ point and\nordinary electron bands at $M$ points in Brillouin zone (BZ). We found that the\nsystem allows two degenerate superconducting solutions with the exactly same\n$T_c$ in clean limit: the incipient $s^{\\pm}_{he}$-gap ($\\Delta_h^{-} \\neq 0$,\n$\\Delta_e^{+} \\neq 0$) and $s_{ee}^{++}$-gap ($\\Delta_h =0$, $\\Delta_e^{+} \\neq\n0$) solutions with different pairing cutoffs, $\\Lambda_{sf}$ (spin fluctuation\nenergy) and $\\epsilon_b$, respectively. The $s_{ee}^{++}$-gap solution, in\nwhich the system dynamically renormalizes the original pairing cutoff\n$\\Lambda_{sf}$ to $\\Lambda_{phys}=\\epsilon_b$ ($< \\Lambda_{sf}$), therefore\nactively eliminates the incipient hole band from forming Cooper pairs, but\nwithout loss of $T_c$, becomes immune to the impurity pair-breaking. As a\nresult, the HEDIS systems, by dynamically tuning the pairing cutoff and\nselecting the $s_{ee}^{++}$-pairing state, can always achieve the maximum $T_c$\n-- the $T_c$ of the degenerate $s^{\\pm}_{he}$ solution in the ideal clean limit\n-- latent in the original pairing interactions, even in dirty limit.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Structure Re-determination and Superconductivity Observation of Bulk 1T\n  MoS2: 2H MoS2 has been intensively studied because of layer-dependent electronic\nstructures and novel physical properties. Though the metastable 1T MoS2 with\nthe [MoS6] octahedron was observed from the microscopic area, the true crystal\nstructure of 1T phase has not been determined strictly. Moreover, the true\nphysical properties have not been demonstrated from experiments due to the\nchallenge for the preparation of pure 1T MoS2 crystals. Here, we successfully\nsynthesized the 1T MoS2 single crystals and re-determined the crystal structure\nof 1T MoS2 from single-crystal X-ray diffraction. 1T MoS2 crystalizes in space\ngroup P-3m1 with a cell of a = b = 3.190(3) {\\AA} and c = 5.945(6) {\\AA}. The\nindividual MoS2 layer consists of MoS6 octahedron sharing edge with each other.\nMore surprisingly, the bulk 1T MoS2 crystals undergo a superconducting\ntransition of Tc = 4 K, which is the first observation of superconductivity in\npure 1T MoS2 phase.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "$^{23}$Na NMR Study of Layered Superconductor $Na_xCoO_2- yH_2O$: We measured $^{23}$Na-NMR spectra and nuclear spin-lattice relaxation rates\n$^{23}(1/T_1)$ of superconducting and non-superconducting bilayer hydrate\n$Na_xCoO_2-yH_2O$ (x ~ 0.3, y ~ 1.3). The central resonance frequency shows a\nsmall but various shift due to the difference in the shielding effect by\nintercalated $H_2O$ molecules. The different shielding effect also gives a\nlarge difference in the magnitude of $^{23}(1/T_1)$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Multi-component strongly attractive Fermi gas: a color superconductor in\n  a one-dimensional harmonic trap: Recent advances in ultra-cold atomic Fermi gases make it possible to achieve\na fermionic superfluid with multiple spin components. In this context, any\nmean-field description is expected to fail, owing to the presence of tightly\nbound clusters or molecules that consist of more than two particles. Here we\npresent a detailed study of a strongly interacting multi-component Fermi gas in\na highly elongated or quasi-one-dimensional harmonic trap, which could be\nreadily obtained in experiment. By using the exact Bethe ansatz solution and a\nlocal density approximation treatment of the harmonic trap, we investigate the\nequation of state of the multi-component Fermi gas in both a homogeneous and\ntrapped environment, as well as the density profiles and low-energy collective\nmodes. The binding energy of multi-component bound clusters is also given. We\nshow that there is a peak in the collective mode frequency at the critical\ndensity for a deconfining transition to a many-body state that is analogous to\nthe quark color superconductor state expected in neutron stars.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Resistivity tensor of vortex-lattice states in Josephson junction arrays: Two-dimensional Josephson junction arrays frustrated by a perpendicular\nmagnetic field are predicted to form a cascade of distinct vortex lattice\nstates. Here, we show that the resistivity tensor provides both structural and\ndynamical information on the vortex-lattice states and intervening phase\ntransitions, which allows for experimental identification of these\nsymmetry-breaking ground states. We illustrate our general approach by a\nmicroscopic theory of the resistivity tensor for a range of magnetic fields\nexhibiting a rich set of vortex lattices as well as transitions to\nliquid-crystalline vortex states.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quantum Superconductor-Metal Transition in a 2D Proximity-Coupled Array: We construct a theory of quantum fluctuatons in a regular array of small\nsuperconductive islands connected via low-resistance tunnel contacts to a dirty\nthin metal film.\n  Electron-electron interaction in the film is assumed to be repulsive. The\nsystem is macroscopically superconductive when the distance between\nneighbouring islands is short enough. The zero-temperature phase transition\nfrom superconductive to normal-conductive state is shown to occur with the\nincrease of distance between superconductive islands; the logarithm of the\ncritical distance is proportional to the low-frequency zero-voltage Andreev\nconductance between the SC island and the film. This critical distance is\nalways much less the than the two-dimensional localization length, so the\nconsidered effect develops when weak-localization corrections are still small.\nThe dependence of the critical temperature on the film conductance and\ninter-island distance is found.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Spectroscopy of Magnetic Excitations in Magnetic Superconductors Using\n  Vortex Motion: In magnetic superconductors a moving vortex lattice is accompanied by an ac\nmagnetic field which leads to the generation of spin waves. At resonance\nconditions the dynamics of vortices in magnetic superconductors changes\ndrastically, resulting in strong peaks in the dc I-V characteristics at\nvoltages at which the washboard frequency of vortex lattice matches the spin\nwave frequency $\\omega_s({\\bf g})$, where ${\\bf g}$ are the reciprocal vortex\nlattice vectors. We show that if washboard frequency lies above the magnetic\ngap, peaks in the I-V characteristics in borocarbides and cuprate layered\nmagnetic superconductors are strong enough to be observed over the background\ndetermined by the quasiparticles.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electronic Structure of Superconducting Infinite-Layer Lanthanum\n  Nickelates: Revealing the momentum-resolved electronic structure of infinite-layer\nnickelates is essential for understanding this new class of unconventional\nsuperconductors, but has been hindered by the formidable challenges in\nimproving the sample quality. In this work, we report for the first time the\nangle-resolved photoemission spectroscopy of superconducting\nLa$_{0.8}$Sr$_{0.2}$NiO$_{2}$ films prepared by molecular beam epitaxy and\n${\\mathrm{\\textit{in situ}}}$ atomic-hydrogen reduction. The measured Fermi\ntopology closely matches theoretical calculations, showing a large\nNi-$d_{x^2-y^2}$ derived Fermi sheet that evolves from hole-like to\nelectron-like along $k_{z}$, and a three-dimensional (3D) electron pocket\ncentered at Brillouin zone corner. The Ni-$d_{x^2-y^2}$ derived bands show a\nmass enhancement ($m^*/m_{\\rm{DFT}}$) of 2-3,while the 3D electron band shows\nnegligible band renormalization. Moreover, the Ni-$d_{x^2-y^2}$ derived states\nalso display a band dispersion anomaly at higher binding energy, reminiscent of\nthe waterfall feature and kinks observed in cuprates.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Coexistence of Two Sharp-Mode Couplings and Their Unusual Momentum\n  Dependence in the Superconducting State of Bi2Sr2CaCu2O8+d Superconductor\n  Revealed by Laser-Based Angle-Resolved Photoemission: Super-high resolution laser-based angle-resolved photoemission measurements\nhave been carried out on Bi2Sr2CaCu2O8+d (Bi2212) superconductors to\ninvestigate momentum dependence of electron coupling with collective\nexcitations (modes). Two coexisting energy scales are clearly revealed over a\nlarge momentum space for the first time in the superconducting state of an\noverdoped Bi2212 superconductor. These two energy scales exhibit distinct\nmomentum dependence: one keeps its energy near 78 meV over a large momentum\nspace while the other changes its energy from $\\sim$40 meV near the antinodal\nregion to $\\sim$70 meV near the nodal region. These observations provide a new\npicture on momentum evolution of electron-boson coupling in Bi2212 that\nelectrons are coupled with two sharp modes simultaneously over a large momentum\nspace in the superconducting states. Their unusual momentum dependence poses a\nchallenge to our current understanding of electron-mode-coupling and its role\nfor high temperature superconductivity in cuprate superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Intrinsic superconducting diode effects in tilted Weyl and Dirac\n  semimetals: We explore Weyl and Dirac semimetals with tilted nodes as platforms for\nrealizing an intrinsic superconducting diode effect. Although tilting breaks\nsufficient spatial and time-reversal symmetries, we prove that -- at least for\nconventional $s$-wave singlet pairing -- the effect is forbidden by an emergent\nparticle-hole symmetry at low energies if the Fermi level is tuned to the\nnodes. Then, as a stepping stone to the three-dimensional semimetals, we\nanalyze a minimal one-dimensional model with a tilted helical node using\nGinzburg-Landau theory. While one might naively expect a drastic enhancement of\nthe effect when the node turns from type-I to type-II, we find that the\npresence of multiple Fermi pockets is more important as it enables multiple\npairing amplitudes with indepedent contributions to supercurrents in opposite\ndirections. Equipped with this insight, we construct minimal lattice models of\nWeyl and Dirac semimetals and study the superconducting diode effect in them.\nOnce again, we see a substantial enhancement when the normal state has multiple\nFermi pockets per node that can accommodate more than one pairing channel. In\nsummary, this study sheds light on the key factors governing the intrinsic\nsuperconducting diode effect in systems with asymmetric band structures and\npaves the way for realizing it in topological semimetals.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity in the doped topological insulator Cu$_x$Bi$_2$Se$_3$\n  under high pressure: We report a high-pressure single crystal study of the topological\nsuperconductor Cu$_x$Bi$_2$Se$_3$. Resistivity measurements under pressure show\nsuperconductivity is depressed smoothly. At the same time the metallic behavior\nis gradually lost. The upper critical field data $B_{c2}(T)$ under pressure\ncollapse onto a universal curve. The absence of Pauli limiting and the\ncomparison of $B_{c2}(T)$ to a polar state function point to spin-triplet\nsuperconductivity, but an anisotropic spin-singlet state cannot be discarded\ncompletely.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Single-crystal growth and extremely high H_c2 of 12442-type Fe-based\n  superconductor KCa_2Fe_4As_4F_2: Millimeter sized single crystals of KCa_2Fe_4As_4F_2 were grown using a\nself-flux method. The chemical compositions and crystal structure were\ncharacterized carefully. Superconductivity with the critical transition T_c =\n33.5 K was confirmed by both the resistivity and magnetic susceptibility\nmeasurements. Moreover, the upper critical field H_c2 was studied by the\nresistivity measurements under different magnetic fields. A rather steep\nincrease for the in-plane H_c2^ab with cooling, d\\mu_0H_c2^a/dT|T_c = -50.9\nT/K, was observed, indicating an extremely high upper critical field. Possible\norigins for this behavior were discussed. The findings in our work is a great\npromotion both for understanding the physical properties and applications of\n12442-type Fe-based superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Complex superconductivity in the noncentrosymmetric compound Re$_{6}$Zr: We report the electronic structure, synthesis, and measurements of the\nmagnetic, transport, and thermal properties of the polycrystalline\nnoncentrosymmetric compound Re$_{6}$Zr. We observed a bulk superconducting\ntransition at temperature $T_{c}$ $\\sim$ 6.7 K, and measured the resistivity,\nheat capacity, thermal conductivity, and the London penetration depth below the\ntransition, as well as performed doping and pressure studies. From these\nmeasurements we extracted the critical field, and the superconducting\nparameters of Re$_{6}$Zr. Our measurements indicate a relatively weak to\nmoderate contribution from a triplet component to the order parameter, and\nfavor a full superconducting gap, although we cannot exclude the existence of\npoint nodes based on our data.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconducting Gap Structure of LaFePO Studied by Thermal Conductivity: The superconducting gap structure of LaFePO ($T_c=7.4 $K) is studied by\nthermal conductivity ($\\kappa$) at low temperatures in fields $H$ parallel and\nperpendicular to the c axis. A clear two-step field dependence of $\\kappa(H)$\nwith a characteristic field $H_s(\\sim 350$ Oe) much lower than the upper\ncritical field $H_{c2}$ is observed. In spite of large anisotropy of $H_{c2}$,\n$\\kappa(H)$ in both $H$-directions is nearly identical below $H_s$. Above\n$H_s$, $\\kappa(H)$ grows gradually with $H$ with a convex curvature, followed\nby a steep increase with strong upward curvature near $H_{c2}$. These results\nindicate the multigap superconductivity with active two-dimensional (2D) and\npassive 3D bands having contrasting gap values. Together with the recent\npenetration depth results, we suggest that the 2D bands consist of nodal and\nnodeless ones, consistent with the extended s-wave symmetry.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Raman scattering from a superconductivity-induced bound state in $MgB_2$: It is shown that the sharp peak in the $E_{2g}$ Raman spectrum of\nsuperconducting $MgB_2$ is due to a bound state caused by the electron-phonon\ncoupling. Our theory explains why this peak appears only in the spectra with\n$E_{2g}$ symmetry and only in the $\\sigma$ but not $\\pi$ bands. The properties\nof the bound state and the Raman spectrum are investigated, also in the\npresence of impurity scattering.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Stripe Disordering Transition: We have recently begun Monte Carlo simulations of the dynamics of stripe\nphases in the cuprates. A simple model of spinodal decomposition of the holes\nallows us to incorporate Coulomb repulsion and coherency strains. We find\nevidence for a possible stripe disordering transition, at a temperature below\nthe pseudogap onset. Experimental searches for such a transition can provide\nconstraints for models of stripe formation.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Correcting 100 years of misunderstanding: electric fields in\n  superconductors, hole superconductivity, and the Meissner effect: From the outset of superconductivity research it was assumed that no\nelectrostatic fields could exist inside superconductors, and this assumption\nwas incorporated into conventional London electrodynamics. Yet the London\nbrothers themselves initially (in 1935) had proposed an electrodynamic theory\nof superconductors that allowed for static electric fields in their interior,\nwhich they unfortunately discarded a year later. I argue that the Meissner\neffect in superconductors necessitates the existence of an electrostatic field\nin their interior, originating in the expulsion of negative charge from the\ninterior to the surface when a metal becomes superconducting. The theory of\nhole superconductivity predicts this physics, and associated with it a\nmacroscopic spin current in the ground state of superconductors (\"Spin Meissner\neffect\"), qualitatively different from what is predicted by conventional\nBCS-London theory. A new London-like electrodynamic description of\nsuperconductors is proposed to describe this physics. Within this theory\nsuperconductivity is driven by lowering of quantum kinetic energy, the fact\nthat the Coulomb repulsion strongly depends on the character of the charge\ncarriers, namely whether electron- or hole-like, and the spin-orbit\ninteraction. The electron-phonon interaction does not play a significant role,\nyet the existence of an isotope effect in many superconductors is easily\nunderstood. In the strong coupling regime the theory appears to favor local\ncharge inhomogeneity. The theory is proposed to apply to all superconducting\nmaterials, from the elements to the high $T_c$ cuprates and pnictides, is\nhighly falsifiable, and explains a wide variety of experimental observations.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Long-range spin and charge accumulation in mesoscopic superconductors\n  with Zeeman splitting: We describe the far from equilibrium non-local transport in a diffusive\nsuperconducting wire with a Zeeman splitting, taking into account the different\nspin relaxation mechanisms. We demonstrate that due to the Zeeman splitting an\ninjection of a current in a superconducting wire creates a spin accumulation\nthat can only relax via thermalization. In addition the Zeeman splitting also\ncauses a suppression of the spin-orbit and spin-flip scattering rates. These\ntwo effects lead to long-range spin and charge accumulations detectable in the\nnon-local signal. Our model explains the main qualitative features of recent\nexperimental results in terms of realistic parameters and predicts a strong\ndependence of the non-local signal on the orbital depairing effect from an\ninduced magnetic field.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Suppression of superconductivity at nematic critical point in underdoped\n  cuprates: A nematic quantum critical point is anticipated to exist in the\nsuperconducting dome of some high-temperature superconductors. The nematic\norder competes with the superconducting order and hence reduces the\nsuperconducting condensate at $T = 0$. Moreover, the critical fluctuations of\nnematic order can excite more nodal quasiparticles out of the condensate. We\naddress these two effects within an effective field theory and show that\nsuperfluid density $\\rho^s(T)$ and superconducting temperature $T_c$ are both\nsuppressed strongly by the critical fluctuations. The strong suppression of\nsuperconductivity provides a possible way to determine the nematic quantum\ncritical point.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Orbital selective spin excitations and their impact on superconductivity\n  of LiFe1-xCoxAs: We use neutron scattering to study spin excitations in single crystals of\nLiFe$_{0.88}$Co$_{0.12}$As, which is located near the boundary of the\nsuperconducting phase of LiFe$_{1-x}$Co$_{x}$As and exhibits non-Fermi-liquid\nbehavior indicative of a quantum critical point. By comparing spin excitations\nof LiFe$_{0.88}$Co$_{0.12}$As with a combined density functional theory (DFT)\nand dynamical mean field theory (DMFT) calculation, we conclude that\nwave-vector correlated low energy spin excitations are mostly from the $d_{xy}$\norbitals, while high-energy spin excitations arise from the $d_{yz}$ and\n$d_{xz}$ orbitals. Unlike most iron pnictides, the strong orbital selective\nspin excitations in LiFeAs family cannot be described by anisotropic Heisenberg\nHamiltonian. While the evolution of low-energy spin excitations of\nLiFe$_{1-x}$Co$_x$As are consistent with electron-hole Fermi surface nesting\ncondition for the $d_{xy}$ orbital, the reduced superconductivity in\nLiFe$_{0.88}$Co$_{0.12}$As suggests that Fermi surface nesting conditions for\nthe $d_{yz}$ and $d_{xz}$ orbitals are also important for superconductivity in\niron pnictides.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Critical Temperature Enhancement of Topological Superconductors: A\n  Dynamical Mean Field Study: We show that a critical temperature Tc for spin-singlet two-dimensional\nsuperconductivity is enhanced by a cooperation between the Zeeman magnetic\nfield and the Rashba spin-orbit coupling, where a superconductivity becomes\ntopologically non-trivial below Tc. The dynamical mean field theory (DMFT) with\nthe segment-based hybridization-expansion continuous-time quantum Monte Carlo\nimpurity solver (ct-HYB) is used for accurately evaluating a critical\ntemperature, without any Fermion sign problem. A strong-coupling approach shows\nthat spin-flip driven local pair hopping leads to part of this enhancement,\nespecially effects of the magnetic field. We propose physical settings suitable\nfor verifying the present calculations, one-atom-layer system on Si(111) and\nionic-liquid based electric double-layer transistors (EDLTs).",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Tunnel and thermal c-axis transport in BSCCO in the normal and pseudogap\n  state: We consider the problem of c-axis transport in double-layered cuprates, in\nparticular with reference to Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\\delta}$\ncompounds. We exploit the effect of the two barriers on the thermal and tunnel\ntransport. The resulting model is able to describe accurately the normal state\nc-axis resistivity in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\\delta}$, from the\nunderdoped side up to the strongly overdoped. We extend the model, without\nintroducing additional parameters, in order to allow for the decrease of the\nbarrier when an external voltage bias is applied. The extended model is found\nto describe properly the c-axis resistivity for small voltage bias above the\npseudogap temperature $T^{*}$, the c-axis resistivity for large voltage bias\neven below $T_c$, and the differential $dI/dV$ curves taken in mesa structures.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Nuclear ferromagnetism induced Fulde-Ferell-Larkin-Ovchinnikov state: We present a theoretical study of the influence of the nuclear ferromagnetism\non superconductivity in the presence of the electron-nuclear spin interaction.\nIt is demonstrated that in some metals, e.g. Rh, W, the BCS condensate imbedded\nin a matrix of ferromagneticaly ordered nuclear spins should manifest the FFLO\n(Fulde-Ferel-Larkin-Ovchinniov) state. We outline that the optimal experimental\nconditions for observation of FFLO could be achieved by creation, via adiabatic\nnuclear demagnetization, of the negative nuclear spin temperatures. In this\ncase the nuclear polarization points in the opposite to the external magnetic\nfield direction and the electromagnetic part of the nuclear spin magnetization\ncompensates the external magnetic field, while the exchange part creates the\nnonhomogeneous superconducting order parameter.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Fermiological Interpretation of Superconductivity/Non-superconductivity\n  of FeTe_{1-x}Se_{x} Thin Crystal Determined by Quantum Oscillation\n  Measurement: We have successfully observed quantum oscillation (QO) for FeTe_{1-x}Se_{x}.\nQO measurements were performed using non-superconducting and superconducting\nthin crystals of FeTe_{0.65}Se_{0.35} fabricated by the scotch-tape method. We\nshow that the Fermi surfaces (FS) of the non-superconducting crystal are in\ngood agreement with the rigid band shift model based on electron doping by\nexcess Fe while that of the superconducting crystal is in good agreement with\nthe calculated FS with no shift. From the FS comparison of both crystals, we\ndemonstrate the change of the cross-sectional area of the FS, suggesting that\nthe suppression of the FS nesting with the vector Q_{s} = (\\pi, \\pi) due to\nexcess Fe results in the disappearance of the superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Accurate Measurements of the Intrinsic Surface Impedance of Thin YBCO\n  Films Using a Modified Two-tone Resonator Method: We propose a modified two-tone method that could be used for sensitive\nmeasurements of the intrinsic microwave surface impedance (Zs) of thin\nsuperconductor films and the tand of a low-loss dielectric. An open-gap\nresonator scheme is used to measure the penetration depth of thin\nsuperconductor films and extract the intrinsic Zs of the superconductor films\nfrom its measured effective surface resistance and penetration depth. We use a\nvery small gap of 10 um between the top plate and the rest parts of the\nresonator. The loss tangent of rutile in the low 10^-7 range and the dielectric\nconstant as high as ~110 are observed at temperatures below 10 K at ~15.2 GHz,\nwhich enable to measure the Rs of the 10 mm-in-diameter YBCO films as low as\n\\~100 micro-ohm at the same frequency (f). The discrepancy between the\neffective surface resistance at ~15.2 GHz and that at ~8.5 GHz scaled to ~15.2\nGHz appears less than 6 % when the relations of Rs ~ f^2 and loss tangent ~ f\nare used. We describe usefulness of our measurement method for measuring the\nintrinsic microwave properties of various superconductor samples.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Carrier relaxation dynamics in intra-gap states: the case of the\n  superconductor Y Ba_2 Cu_3O_7-delta and the charge-density-wave semiconductor\n  K_0.3MoO_3: The unusual slow carrier relaxation dynamics - observed in femtosecond\npump-probe experiments on high-temperature superconductors and recently also in\na charge-density-wave system - is analyzed in terms of a model for relaxation\nof carriers in intra-gap states. The data on YBa_2Cu_3O_7-delta near optimum\ndoping and K_0.3$MoO_3 are found to be described very well with the model using\na BCS-like gap which closes at T_c. From the analysis of the data we conclude\nthat a significant intra-gap density of localized states exists in these\nmaterials, which can be clearly distinguished from quasiparticle states by the\ntime-resolved optical experiments because of the different time- and\ntemperature-dependences of the photoinduced transmission or reflection.\nLocalized charges are suggested to be the most likely origin of the intra-gap\nstates, while the similarity of the response in the two materials appear to\nexclude spin and vortex excitations.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Possible origin of the pseudogap end point in the high-T$_{c}$ cuprates: Recent experiments find that the pseudogap phase of the high-T$_{c}$ cuprates\nends suddenly at an electron doping $x^{*}$ when the Fermi surface change its\nshape from hole-like to electron-like. In this short note, we argue that the\nantiferromagnetic(AF) spin correlation of the system should drop abruptly at\nthe same doping. At the same time, we argue that the critical behavior observed\nat $x^{*}$ in the specific heat measurement should be attributed to the strong\nrenormalization of the quasiparticle excitation in the anti-nodal region by the\ncritical AF spin fluctuation. This picture also predicts that any\npseudogap-like spectral feature related to AF scattering in the electron-doped\ncuprates should terminate at the doping level when the Fermi surface becomes\ntangent to the boundary of AF Brillouin zone. Recent ARPES measurement on the\nelectron-doped cuprate Nd$_{2-x}$Ce$_{x}$CuO$_{4}$ seems to be consistent with\nsuch a prediction.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Type-II Ising Pairing in Few-Layer Stanene: Spin-orbit coupling has proven indispensable in realizing topological\nmaterials and more recently Ising pairing in two-dimensional superconductors.\nThis pairing mechanism relies on inversion symmetry breaking and sustains\nanomalously large in-plane polarizing magnetic fields whose upper limit is\nexpected to diverge at low temperatures, although experimental demonstration of\nthis has remained elusive due to the required fields. In this work, the\nrecently discovered superconductor few-layer stanene, i.e. epitaxially strained\n$\\alpha$-Sn, is shown to exhibit a new type of Ising pairing between carriers\nresiding in bands with different orbital indices near the $\\Gamma$-point. The\nbands are split as a result of spin-orbit locking without the participation of\ninversion symmetry breaking. The in-plane upper critical field is strongly\nenhanced at ultra-low temperature and reveals the sought for upturn.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Reentrant topological transitions with Majorana end states in 1D\n  superconductors by lattice modulation: The possibility to observe and manipulate Majorana fermions as end states of\none-dimensional topological superconductors has been actively discussed\nrecently. In a quantum wire with strong spin-orbit coupling placed in proximity\nto a bulk superconductor, a topological superconductor has been expected to be\nrealized when the band energy is split by the application of a magnetic field.\nWhen a periodic lattice modulation is applied multiple topological\nsuperconductor phases appear in the phase diagram. Some of them occur for\nhigher filling factors compared to the case without the modulation. We study\nthe effects of phase jumps and argue that the topologically nontrivial state of\nthe whole system is retained even if they are present. We also study the effect\nof the spatial modulation in the hopping parameter.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Charge-4e supercurrent in a two-dimensional InAs-Al\n  superconductor-semiconductor heterostructure: Superconducting qubits with intrinsic noise protection offer a promising\napproach to improve the coherence of quantum information. Crucial to such\nprotected qubits is the encoding of the logical quantum states into\nwavefunctions with disjoint support. Such encoding can be achieved by a\nJosephson element with an unusual charge-4e supercurrent emerging from the\ncoherent transfer of pairs of Cooper-pairs. In this work, we demonstrate the\ncontrolled conversion of a conventional charge-2e dominated to a charge-4e\ndominated supercurrent in a superconducting quantum interference device (SQUID)\nconsisting of gate-tunable planar Josephson junctions (JJs). We investigate the\nac Josephson effect of the SQUID and measure a dominant photon emission at\ntwice the fundamental Josephson frequency together with a doubling of the\nnumber of Shapiro steps, both consistent with the appearance of charge-4e\nsupercurrent. Our results present a step towards novel protected\nsuperconducting qubits based on superconductor-semiconductor hybrid materials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quantum anomalous vortex and Majorana zero mode in iron-based\n  superconductor Fe(Te,Se): In topological insulators doped with magnetic ions, spin-orbit coupling and\nferromagnetism give rise to the quantum anomalous Hall effect. Here we show\nthat in s-wave superconductors with strong spin-orbit coupling, magnetic\nimpurity ions can generate topological vortices in the absence of external\nmagnetic fields. Such vortices, dubbed quantum anomalous vortices, support\nrobust Majorana zero-energy modes when superconductivity is induced in the\ntopological surface states. We demonstrate that the zero-energy bound states\nobserved in Fe(Te,Se) superconductors are possible realizations of the Majorana\nzero modes in quantum anomalous vortices produced by the interstitial magnetic\nFe. The quantum anomalous vortex matter not only advances fundamental\nunderstandings of topological defect excitations of Cooper pairing, but also\nprovides new and advantageous platforms for manipulating Majorana zero modes in\nquantum computing.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity in the orthorhombic phase of thermoelectric\n  CsPbxBi4-xTe6 with 0.3=<x=<1.0: Experimental measurements clearly reveal the presence of bulk\nsuperconductivity in the CsPbxBi4-xTe6 (0.3=<x=<1.0) materials, i.e. the first\nmember of the thermoelectric series of Cs[PbmBi3Te5+m], these materials have\nthe layered orthorhombic structure containing infinite anionic [PbBi3Te6]-\nslabs separated with Cs+ cations. Temperature dependences of electrical\nresistivity, magnetic susceptibility, and specific heat have consistently\ndemonstrated that the superconducting transition in CsPb0.3Bi3.7Te6 occurs at\nTc=3.1K, with a superconducting volume fraction close to 100% at 1.8 K.\nStructural study using aberration-corrected STEM/TEM reveals a rich variety of\nmicrostructural phenomena in correlation with the Pb-ordering and chemical\ninhomogeneity. The superconducting material CsPb0.3Bi3.7Te6 with the highest Tc\nshows a clear ordered structure with a modulation wave vector of q=a*/2+\nc*/1.35 on the a-c plane. Our study evidently demonstrates that\nsuperconductivity deriving upon doping of narrow-gap semiconductor is a viable\napproach for exploration of novel superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Anomalous Josephson effect in p-wave dirty junctions: The Josephson effect in p-wave superconductor / diffusive normal metal /\np-wave superconductor junctions is studied theoretically. Amplitudes of\nJosephson currents are several orders of magnitude larger than those in s-wave\njunctions. Current-phase (J-\\phi) relations in low temperatures are close to\nthose in ballistic junctions such as J\\propto\\sin(\\phi/2) and J\\propto\\phi even\nin the presence of random impurity potentials. A cooperative effect between the\nmidgap Andreev resonant states and the proximity effect causes such anomalous\nproperties and is a character of the spin-triplet superconductor junctions.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Vortex configurations and metastability in mesoscopic superconductors: The vortex dynamics in mesoscopic superconducting cylinders with rectangular\ncross section under an axially applied magnetic field is investigated in the\nmultivortex London regime. The rectangles considered range from a square up to\nan infinite slab. The flux distribution and total flux carried by a vortex\nplaced in an arbitrary position of the sample is calculated analytically by\nassuming Clem's solution for the vortex core. The Bean-Livingston energy\nbarrier is also analytically calculated in this framework. A Langevin algorithm\nsimulates the flux penetration and dynamical evolution of the vortices as the\nexternal field is slowly cycled. The simulated magnetization process is\ngoverned by metastable states. The magnetization curves are hysteretic, with\nparamagnetic response in part of the downward branch, and present a series of\npeaks corresponding to the entry or expulsion of a single vortex. For elongated\nrectangles, the vortices arrange themselves into parallel vortex chains and an\nadditional modulation of the magnetization, corresponding to creation or\ndestruction of a vortex chain, comes out.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Proximity effect induced intriguing superconductivity in van der Waals\n  heterostructure of magnetic topological insulator and conventional\n  superconductor: Nontrivial topological superconductivity has received enormous research\nattentions due to its potential for diverse applications in topological quantum\ncomputing. The intrinsic issue concerning the correlation between a topological\ninsulator and a superconductor is, however, still widely open. Here, we\nsystemically report an emergent superconductivity in a cross-junction composed\nof a magnetic topological insulator MnBi2Te4 and a conventional superconductor\nNbSe2. Remarkably, the interface indicates existence of a reduced\nsuperconductivity at surface of NbSe2 and a proximity-effectinduced\nsuperconductivity at surface of MnBi2Te4. Furthermore, the in-plane\nangular-dependent magnetoresistance measurements reveal the fingerprints of the\nparing symmetry behaviors for these superconducting gaps as a unconventional\nnature. Our findings extend our views and ideas of topological\nsuperconductivity in the superconducting heterostructures with time-reversal\nsymmetry breaking, offering an exciting opportunity to elucidate the\ncooperative effects on the surface state of a topological insulator aligning a\nsuperconductor.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Hole Localization in Underdoped Superconducting Cuprates Near 1/8th\n  Doping: Measurements of thermal conductivity versus temperature over a broad range of\ndoping in YBa$_2$Cu$_3$O$_{6+x}$ and HgBa$_2$Ca$_{n-1}$Cu$_n$O$_{2n+2+\\delta}$\n($n$=1,2,3) suggest that small domains of localized holes develop for hole\nconcentrations near $p$=1/8. The data imply a mechanism for localization that\nis intrinsic to the CuO$_2$-planes and is enhanced via pinning associated with\noxygen-vacancy clusters.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Shapiro Step on the rc-Branch in the IV-characteristic of a Josephson\n  Junction with an LC-shunting: The effect of external radiation on the phase dynamics of Josephson junctions\nshunted by an LC circuit is examined. It is shown that additional resonant\ncircuit (rc) branches appear in the current voltage characteristics of the\njunctions when the Josephson frequency is equal to the frequency of the formed\nresonance circuit. We show that the amplitude dependence of the Shapiro step\nwidth crucially changes when the Shapiro step is on the rc-branch in comparison\nto the case of Josephson junction without shunt. The experimental\nimplementation of these effects might give very important advantages for\nexisting methods and technologies.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Nonequilibrium Josephson effect in mesoscopic ballistic multiterminal\n  SNS junctions: We present a detailed study of nonequilibrium Josephson currents and\nconductance in ballistic multiterminal SNS-devices. Nonequilibrium is created\nby means of quasiparticle injection from a normal reservoir connected to the\nnormal part of the junction. By applying a voltage at the normal reservoir the\nJosephson current can be suppressed or the direction of the current can be\nreversed. For a junction longer than the thermal length, $L\\gg\\xi_T$, the\nnonequilibrium current increases linearly with applied voltage, saturating at a\nvalue equal to the equilibrium current of a short junction. The conductance\nexhibits a finite bias anomaly around $eV \\sim \\hbar v_F/L$. For symmetric\ninjection, the conductance oscillates $2\\pi$-periodically with the phase\ndifference $\\phi$ between the superconductors, with position of the minimum\n($\\phi=0$ or $\\pi$) dependent on applied voltage and temperature. For\nasymmetric injection, both the nonequilibrium Josephson current and the\nconductance becomes $\\pi$-periodic in phase difference. Inclusion of barriers\nat the NS-interfaces gives rise to a resonant behavior of the total Josephson\ncurrent with respect to junction length with a period $\\sim \\lambda_F$. Both\nthree and four terminal junctions are studied.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetic Exciton Mediated Superconductivity in the Hidden-Order Phase of\n  URu2Si2: We propose the magnetic exciton mediated superconductivity occurring in the\nenigmatic hidden-order phase of URu2Si2. The characteristic of the massive\ncollective excitation observed only in the hidden-order phase is well\nreproduced by the antiferro hexadecapole ordering model as the trace of the\ndispersive crystalline-electric-field excitation. The disappearance of the\nsuperconductivity in the high-pressure antiferro magnetic phase can naturally\nbe understood by the sudden suppression of the magnetic-exciton intensity. The\nanalysis of the momentum dependence of the magnetic-exciton mode leads to the\nexotic chiral d-wave singlet pairing in the Eg symmetry. The Ising-like\nmagnetic-field response of the mode yields the strong anisotropy observed in\nthe upper critical field even for the rather isotropic 3-dimensional Fermi\nsurfaces of this compound.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Direct penetration of spin-triplet superconductivity into a ferromagnet\n  in Au/SrRuO3/Sr2RuO4 junctions: Efforts have been ongoing to establish superconducting spintronics utilizing\nferromagnet/superconductor heterostructures1. Previously reported devices are\nbased on spin-singlet superconductors (SSCs), where the spin degree of freedom\nis lost. Spin-polarized supercurrent induction in ferromagnetic metals (FMs) is\nachieved even with SSCs, but only with the aid of interfacial complex magnetic\nstructures, which severely affect information imprinted to the electron spin.\nUse of spin-triplet superconductors (TSCs) with active spins potentially\novercomes this difficulty and further leads to novel functionalities. Here, we\nreport spin-triplet superconductivity induction into a FM SrRuO3 from a leading\nTSC candidate Sr2RuO4, by fabricating microscopic devices using an epitaxial\nSrRuO3/Sr2RuO4 hybrid. The differential conductance, exhibiting\nAndreev-reflection features with multiple energy scales up to around half\ntesla, indicates the penetration of superconductivity over a considerable\ndistance of 15 nm across the SrRuO3 layer without help of interfacial complex\nmagnetism. This demonstrates the first FM/TSC device exhibiting the\nspin-triplet proximity effect.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Improved superconducting properties in nanocrystalline bulk MgB_2: Highly dense nanocrystalline MgB_2 bulk superconductors with distinctly\nimproved pinning were prepared by mechanical alloying of Mg and B powders and\nhot compaction at ambient temperatures. The nanocrystalline samples reveal high\ncritical current densities of 105 A/cm2 at 20 K and 1 T together with a\nstrongly shifted irreversibility line towards higher fields resulting in\nH_irr(T) ~ 0.8 H_c2(T), whereas typically H_irr(T) ~ 0.5 H_c2(T) is observed\nfor bulk untextured samples. These values exceed that of all other so far\nreported bulk samples and are in the range of the values of thin films. The\nimproved pinning of this material which mainly consists of spherical grains of\nabout 40-100 nm in size is attributed to the large number of grain boundaries.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Alignment of Carbon Nanotube Additives for Improved Performance of\n  Magnesium Diboride Superconductors: The rapid progress on MgB2 superconductor since its discovery[1] has made\nthis material a strong competitor to low and high temperature superconductors\n(HTS) for applications with a great potential to catch the niche market such as\nin magnetic resonant imaging (MRI). Thanks to the lack of weak links and the\ntwo-gap superconductivity of MgB2 [2,3] a number of additives have been\nsuccessfully used to enhance the critical current density, Jc and the upper\ncritical field, Hc2.[4-12] Carbon nanotubes (CNTs) have unusually electrical,\nmechanical and thermal properties[13-16] and hence is an ideal component to\nfabricate composites for improving their performance. To take advantages of the\nextraordinary properties of CNTs it is important to align CNTs in the\ncomposites. Here we report a method of alignment of CNTs in the CNT/MgB2\nsuperconductor composite wires through a readily scalable drawing technique.\nThe aligned CNT doped MgB2 wires show an enhancement in magnetic Jc(H) by more\nthan an order of magnitude in high magnetic fields, compared to the undoped\nones. The CNTs have also significantly enhanced the heat transfer and\ndissipation. CNTs have been used mainly in structural materials, but here the\nadvantage of their use in functional composites is shown and this has wider\nramifications for other functional materials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "d-wave superconductivity and its coexistence with antiferromagnetism in\n  t-J-U model: Statistically consistent Gutzwiller approach: We discuss the coexistence of antiferromagnetism and d-wave superconductivity\nwithin the so-called statistically-consistent Gutzwiller approximation (SGA)\napplied to the t-J-U model. In this approach, the averages calculated in a\nself-consistent manner coincide with those determined variationally. Such\nconsistency is not guaranteed within the standard renormalized mean field\ntheory. With the help of SGA, we show that for the typical value J/|t| = 1/3,\ncoexistence of antiferromagnetism (AF) and superconductivity (SC) appears only\nfor U/|t| > 10.6 and in a very narrow range of doping (\\delta < 0.006) in the\nvicinity of the Mott insulating state, in contrast to some previous reports. In\nthe coexistent AF+SC phase, a staggered spin-triplet component of the\nsuperconducting gap appears also naturally; its value is very small.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetization-dependent $T_c$ shift in F/S/F trilayers with a strong\n  ferromagnet: We have measured the superconducting transition temperature $T_c$ of Ni/Nb/Ni\ntrilayers when the magnetizations of the two outer Ni layers are parallel (P)\nand antiparallel (AP). The largest difference in $T_c$ occurs when the Nb\nthickness is just above the critical thickness at which superconductivity\ndisappears completely. We have observed a difference in $T_c$ between the P and\nAP states as large as 41 mK - a significant increase over earlier results in\nsamples with higher $T_c$ and with a CuNi alloy in place of the Ni. Our result\nalso demonstrates that strong elemental ferromagnets are promising candidates\nfor future investigations of ferromagnet/superconductor heterostructures.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Structural Evolution of One-dimensional Spin Ladder Compounds\n  Sr14-xCaxCu24O41 with Ca doping and Related Hole Redistribution Evidence: Incommensurate crystal structures of spin ladder series Sr14-xCaxCu24O41\n(x=3, 7, 11, 12.2) were characterized by powder neutron scattering method and\nrefined using the superspace group Xmmm(00{\\gamma})ss0 (equivalent to\nsuperspace group Fmmm(0,0,1+{\\gamma})ss0); X stands for non-standard centering\n(0,0,0,0), (0,1/2,1/2,1/2), (1/2,1/2,0,0), (1/2,0,1/2,1/2)) with a modulated\nstructure model. The Ca doping effects on the lattice parameters, atomic\ndisplacement, Cu-O distances, Cu-O bond angles and Cu bond valence sum were\ncharacterized. The refined results show that the CuO4 planar units in both\nchain and ladder sublattices become closer to square shape with an increase of\nCa doping. The Cu bond valence sum calculation provided new evidence for the\ncharge transfer from the chains to ladders (approximately 0.16 holes per Cu\nfrom x=0 to 12.2). The charge transfer was attributed to two different\nmechanisms: (a) the Cu-O bond distance shrinkage on the ladder; (b) increase of\nthe interaction between two sublattices, resulting in Cu-O bonding between the\nchains and ladders. The low temperature structural refinement resulted in the\nsimilar conclusion, with a slight charge backflow to the chains.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "A chemist's view of chemical bonding in the mechanism of high\n  temperature superconductivity: Through a 2-dimensionnal tight-binding crystal orbital approach and a (CuO2)2\nsquare unit cell of parameter a, we show that a Cu2+/O2- -> Cu+/O- charge\ntransfer is likely to occur at the M($\\pi/a$, $\\pi/a$) point of the Brillouin\nzone, for O4 groups with antibonding b1g symmetry. This approach emphasises the\nrole of oxygen-oxygen interactions in avoiding the nesting of the Fermi surface\nand agrees with its observed topology. At the M point of the Brillouin zone,\noxygen atoms are strongly dissymmetric (\"Janus\" atoms) and link copper atoms\nwith different environments (a1g vs. b1g symmetries). Further hole doping\ngenerates two situations: two holes (S=0) and/or a single hole (S=+/- 1/2) in\nthe O4 b1g groups ($\\sigma$ or $\\pi$), with a possible equilibrium between\nthem; the former can be considered as a \"hole lone pair\" by analogy with\nelectron lone pairs. Mulliken-Jaffe electronegativity considerations justify\nthe nearly zero U Hubbard parameter value. The mixing of the pair-occupied with\npair-iunoccupied wave functions is realised via an electronic hamiltonian in\nplace of the electron-phonon coupling of the pristine BCS theory.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Theoretical explanation of electric field-induced superconductive\n  critical temperature shifts in Indium thin films: We calculate the effect of a static electric field on the superconductive\ncritical temperature of Indium thin films in the framework of proximity effect\nEliashberg theory, in order to explain 60 years old experimental data. Since in\nthe theoretical model we employ all quantities of interest can be computed\nab-initio (i.e. electronic densities of states, Fermi energy shifts and\nEliashberg spectral functions), the only free parameter is in general the\nthickness of the surface layer where the electric field acts. However, in the\nweak electrostatic field limit Thomas-Fermi approximation is still valid and\ntherefore no free parameters are left, as this perturbed layer is known to have\na thickness of the order of the Thomas-Fermi screening length. We show that the\ntheoretical model can reproduce experimental data, even when the magnitude of\nthe induced charge densities are so small to be usually neglected.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetic field effect on Fe-induced short-range magnetic correlation and\n  electrical conductivity in\n  Bi$_{1.75}$Pb$_{0.35}$Sr$_{1.90}$Cu$_{0.91}$Fe$_{0.09}$O$_{6+y}$: We report electrical resistivity measurements and neutron diffraction studies\nunder magnetic fields of\nBi$_{1.75}$Pb$_{0.35}$Sr$_{1.90}$Cu$_{0.91}$Fe$_{0.09}$O$_{6+y}$, in which hole\ncarriers are overdoped. This compound shows short-range incommensurate magnetic\ncorrelation with incommensurability $\\delta=0.21$, whereas a Fe-free compound\nshows no magnetic correlation. Resistivity shows an up turn at low temperature\nin the form of $ln(1/T)$ and shows no superconductivity. We observe reduction\nof resistivity by applying magnetic fields (i.e., a negative magnetoresistive\neffect) at temperatures below the onset of short-range magnetic correlation.\nApplication of magnetic fields also suppresses the Fe induced incommensurate\nmagnetic correlation. We compare and contrast these observations with two\ndifferent models: 1) stripe order, and 2) dilute magnetic moments in a metallic\nalloy, with associated Kondo behavior. The latter picture appears to be more\nrelevant to the present results.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electron Phonon Interaction and Strong Correlations in High-Temperature\n  Superconductors: One can not avoid unavoidable: The important role of the electron-phonon interaction (EPI) in explaining the\nproperties of the normal state and pairing mechanism in high-T$_{c}$\nsuperconductors (HTSC) is discussed. A number of experimental results are\nanalyzed such as: dynamical conductivity, Raman scattering, neutron scattering,\nARPES, tunnelling measurements, isotope effect and etc. They give convincing\nevidence that the EPI is strong and dominantly contributes to pairing in HTSC\noxides. It is argued that strong electronic correlations in conjunction with\nthe pronounced (in relatively weakly screened materials) EPI are unavoidable\ningredients for the microscopic theory of pairing in HTSC oxides. I present the\nwell defined and controllable theory of strong correlations and the EPI. It is\nshown that strong correlations give rise to the pronounced \\textit{forward\nscattering peak} in the EPI - the FSP theory. The FSP theory explains in a\nconsistent way several (crucial) puzzles such as much smaller transport\ncoupling constant than the pairing one ($\\lambda_{tr}\\ll \\lambda$), which are\npresent if one interprets the results in HTSC oxides by the old\nMigdal-Eliashberg theory for the EPI. The ARPES shift puzzle where the nodal\nkink at 70 meV is unshifted in the superconducting state, while the anti-nodal\none at 40 meV is shifted can be explained at present only by the FSP theory. A\nnumber of other interesting predictions of the FSP theory are also discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magic Doping Fractions in High-Temperature Superconductors: We report hole-doping dependence of the in-plane resistivity \\rho_{ab} in a\ncuprate superconductor La_{2-x}Sr_{x}CuO_{4}, carefully examined using a series\nof high-quality single crystals. Our detailed measurements find a tendency\ntowards charge ordering at particular rational hole doping fractions of 1/16,\n3/32, 1/8, and 3/16. This observation appears to suggest a specific form of\ncharge order and is most consistent with the recent theoretical prediction of\nthe checkerboard-type ordering of the Cooper pairs at rational doping fractions\nx = (2m+1)/2^n, with integers m and n.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Three-dimensional energy gap and origin of charge-density wave in kagome\n  superconductor KV3Sb5: Kagome lattices offer a fertile ground to explore exotic quantum phenomena\nassociated with electron correlation and band topology. The recent discovery of\nsuperconductivity coexisting with charge-density wave (CDW) in the kagome\nmetals KV3Sb5, RbV3Sb5, and CsV3Sb5 suggests an intriguing entanglement of\nelectronic order and superconductivity. However, the microscopic origin of CDW,\na key to understanding the superconducting mechanism and its possible\ntopological nature, remains elusive. Here, we report angle-resolved\nphotoemission spectroscopy of KV3Sb5 and demonstrate a substantial\nreconstruction of Fermi surface in the CDW state that accompanies the formation\nof small three-dimensional pockets. The CDW gap exhibits a periodicity of\nundistorted Brillouin zone along the out-of-plane wave vector, signifying a\ndominant role of the in-plane inter-saddle-point scattering to the mechanism of\nCDW. The characteristics of experimental band dispersion can be captured by\nfirst-principles calculations with the inverse star-of-David structural\ndistortion. The present result indicates a direct link between the low-energy\nexcitations and CDW, and puts constraints on the microscopic theory of\nsuperconductivity in alkali-metal kagome lattices.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Depth profile of London length induced by nonuniform scattering rate\n  maximizing breakdown magnetic field in type II superconductors: The breakdown magnetic field is the key parameter which determines the\nperformance of superconducting radio-frequency cavities. This is the maximum\nfield up to which the Meissner state remains stable and in uniform material, it\nis approximately given by the thermodynamic field. There are several recent\nsuggestions to use nonuniform structures to enhance the breakdown field. One of\npossible realizations of such structure is depth profile of the scattering rate\nwhich, in the first approximation, modifies the London penetration depth but\ndoes not change the thermodynamic field. In this paper, we evaluate the optimal\nprofile of the London penetration depth for which the screening current density\nreaches the local depairing value \\emph{simultaneously at every point within\nfinite-size region}. Such profile is realized for close-to-linear decrease of\nthe London penetration depth within the length scale proportional to its value\nat the surface. Achieving noticeable enhancement of the breakdown field,\nhowever, requires strong enhancement of the London length within large region\nwithout affecting the thermodynamic field.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Stability of Elastic Glass Phases in Random Field XY Magnets and Vortex\n  Lattices in Type II Superconductors: A description of a dislocation-free elastic glass phase in terms of domain\nwalls is developed and used as the basis of a renormalization group analysis of\nthe energetics of dislocation loops added to the system. It is found that even\nafter optimizing over possible paths of large dislocation loops, their energy\nis still very likely to be positive when the dislocation core energy is large.\nThis implies the existence of an equilibrium elastic glass phase in three\ndimensional random field X-Y magnets, and a dislocation free,\nbond-orientationally ordered ``Bragg glass'' phase of vortices in dirty Type II\nsuperconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Signatures of Stripe Phases in Hole Doped $La_2NiO_4$: We study nickelate-centered and oxygen-centered stripe phases in doped\nLa$_{2}$NiO$_{4}$ materials. We use an inhomogeneous Hartree-Fock and\nrandom-phase approximation approach including both electron-electron and\nelectron-lattice(e-l) coupling for a layer of La$_{2}$NiO$_{4}$. We find that\nwhether the ground state after commensurate hole doping comprises Ni-centered\nor O-centered charge-localized stripes depends sensitively on the e-l\ninteraction. With increasing e-l interaction strength, a continuous transition\nfrom an O-centered stripe phase to a Ni-centered one is found. Various low- and\nhigh-energy signatures of these two kinds of stripe phases are predicted, which\ncan clearly distinguish them. These signatures reflect the strongly correlated\nspin-charge-lattice features in the vicinity of Ni-centered or O-centered\nstripe domains. The importance of e-l interaction for recent experiments on\nstripe phases is discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Effects of Disorder in Mg_{1-x}Ta_{x}B_2 Alloys using Coherent-Potential\n  Approximation: Using Korringa-Kohn-Rostoker coherent-potential approximation in the\natomic-sphere approximation (KKR-ASA CPA) method for taking into account the\neffects of disorder, Gaspari-Gyorffy formalism for calculating the\nelectron-phonon coupling constant $\\lambda$, and Allen-Dynes equation for\ncalculating T_c, we have studied the variation of T_c in Mg_{1-x}Ta_{x}B_2\nalloys as a function of Ta) concentration. Our results show that the T_c\ndecreases with the addition of Ta for upto 40 at% and remains essentially zero\nfrom 60 at% to 80 at% of Ta. We also find TaB_2 to be superconducting, albeit\nat a lower temperature. Our analysis shows that the variation in T_c in\nMg_{1-x}Ta_{x}B_2 is mostly dictated by the changes in the B p density of\nstates with the addition of Ta.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Nanoscale ferromagnet-superconductor-ferromagnet switches controlled by\n  magnetization orientation: We study clean ferromagnet-superconductor-ferromagnet (FSF) nanostructures in\nwhich the magnetization of the F layers can be parallel (P) or antiparallel\n(AP). We consider the case where the thickness of the S layer is of order of\nthe coherence length, with thinner F layers. We find that reversing the\ndirection of the magnetization in one of the F layers leads in general to\ndrastic changes in the superconductor's state. Under a wide variety of\nconditions, the AP geometry favors superconductivity. Magnetization reversal in\none of the F layers can lead to the superconductivity turning on and off, or to\nswitching between different states. Our results are obtained via self\nconsistent solution of the Bogoliubov-de Gennes equations and evaluation of the\ncondensation energies of the system.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pseudogap Phenomena and Phase Diagram in the 2-Band Hubbard Model: High-Tc superconducting materials (HTSC) have anomalous properties such as\nthe pseudo-gap or spin-gap etc., in Hall coefficient, 1/T1T and the density of\nstates etc. First including effects of strong on-site repulsion between\nd-electrons at Cu-sites, we obtain quasi-particles with super-exchange\ninteraction Js, whose band width tends to zero, i.e., the system goes to\ninsulator, as the hole-doping rate tends to zero. The quasi-particles\ncorrespond to Zhang-Rice singlet states. Js larger than the band width combined\nwith the 2-dimensional character of the system induces strong\nantiferro-magnetic (AF) and superconducting (SC) fluctuations. We treat effects\nof the AF ones in the FLEX approximation and those of the SC ones in the\nself-consistent t-matrix approximation to show that both fluctuations in the\nunder-doped region start to increase at T0 as T decreases from T>>Tc, the AF\nones dominate the SC ones at T>Tsg, while SC ones dominate at T<Tsg. This\ncross-over of the fluctuations causes the anomalous phenomena in the\nunder-doped region. We also obtain the phase diagram of HTSC consistent to one\nobserved in experiments.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Phonon linewidth due to electron-phonon interactions with strong forward\n  scattering in FeSe thin films on oxide substrates: The discovery of an enhanced superconducting transition temperature $T_c$ in\nmonolayers of FeSe grown on several oxide substrates has opened a new route to\nhigh-$T_c$ superconductivity through interface engineering. One proposal for\nthe origin of the observed enhancement is an electron-phonon (e-ph) interaction\nacross the interface that peaked at small momentum transfers. In this paper, we\nexamine the implications of such a coupling on the phononic properties of the\nsystem. We show that a strong forward scattering leads to a sizable broadening\nof phonon lineshape, which may result in charge instabilities at\nlong-wavelengths. However, we further find that the inclusion of Coulombic\nscreening significantly reduces the phonon broadening. Our results show that\none might not expect anomalously broad phonon linewidths in the FeSe interface\nsystems, despite the fact that the e-ph interaction has a strong peak in the\nforward scattering (small $q$) direction.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Lattice dynamics of the cluster chain compounds M2Mo6Se6: The lattice dynamics of members of the M2Mo6Se6 family of materials with\nguest ions M = K, Rb, Cs, In, and Tl has been studied using inelastic x-ray\nscattering and Raman spectroscopy at room temperature, as well as by ab-initio\ncalculations. We find a good match between calculations and experiment, both\nfor structure factors (Eigenvectors) and for the calculated phonon frequencies.\nThe observed lattice dynamics for Tl2Mo6Se6 show no signs of anharmonicity or\nabsence of avoided dispersion crossings, thus ruling out previously\nhypothesised rattling phonon modes. The reduced mode energies for In2Mo6Se6 are\nidentified as only partially responsible for the lower superconducting\ntransition temperature Tc in this material when compared to Tl2Mo6Se6.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Multipole superconductivity in nonsymmorphic Sr$_2$IrO$_4$: Discoveries of marked similarities to high-$T_{\\text{c}}$ cuprate\nsuperconductors point to the realization of superconductivity in the doped\n$J_{\\text{eff}} = 1 / 2$ Mott insulator Sr$_2$IrO$_4$. Contrary to the mother\ncompound of cuprate superconductors, several stacking patterns of in-plane\ncanted antiferromagnetic moments have been reported, which are distinguished by\nthe ferromagnetic components as $-++-$, $++++$, and $-+-+$. In this paper, we\nclarify unconventional features of the superconductivity coexisting with $-++-$\nand $-+-+$ structures. Combining the group theoretical analysis and numerical\ncalculations for an effective $J_{\\text{eff}} = 1 / 2$ model, we show unusual\nsuperconducting gap structures in the $-++-$ state protected by nonsymmorphic\nmagnetic space group symmetry. Furthermore, our calculation shows that the\nFulde-Ferrell-Larkin-Ovchinnikov superconductivity is inevitably stabilized in\nthe $-+-+$ state since the odd-parity magnetic $-+-+$ order makes the band\nstructure asymmetric by cooperating with spin-orbit coupling. These unusual\nsuperconducting properties are signatures of magnetic multipole order in\nnonsymmorphic crystal.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Intertype superconductivity evoked by the interplay of disorder and\n  multiple bands: Nonmagnetic impurity scattering is known to shift up the Ginzburg-Landau\nparameter $\\kappa$ of a superconductor. In this case, when the system is\ninitially in type I, it can change its magnetic response, crossing the\nintertype domain with $\\kappa \\sim 1$ between the two standard\nsuperconductivity types and arriving at type II. In the present work we\ndemonstrate that the impact of disorder can be much more profound in the\npresence of the multiband structure of the charge carrier states. In\nparticular, when the band diffusivities differ from each other, the intertype\ndomain tends to expand significantly, including points with $\\kappa \\gg 1$ that\nbelong to deep type-II in conventional single-band superconductors. Our finding\nsheds light on the nontrivial disorder effect and significantly complements\nearlier results on the enlargement of the intertype domain in clean multiband\nsuperconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Stochastic resonance in an RF SQUID with shunted ScS junction: Using a point (superconductor-constriction-superconductor, ScS) contact in a\nsingle-Josephson-junction superconducting quantum interference device (RF\nSQUID) provides stochastic resonance conditions at any arbitrary small value of\nloop inductance and contact critical current, unlike SQUIDs with more\ntraditional tunnel (superconductor-insulator-superconductor, SIS) junctions.\nThis is due to the unusual potential energy of the ScS RF SQUID which always\nhas a barrier between two wells thus making the device bistable. This paper\npresents the results of a numerical simulation of the stochastic dynamics of\nthe magnetic flux in an ScS RF SQUID loop affected by band-limited white\nGaussian noise and low-frequency sine signals of small and moderate amplitudes.\nThe difference in stochastic amplification of RF SQUID loops incorporating ScS\nand SIS junctions is discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Preparation of electron-doped La(2-x)CexCuO4 thin films with various Ce\n  doping by dc magnetron sputtering: A series of c-axis oriented electron-doped high-Tc superconducting\nLa(2-x)CexCuO4 thin films, from heavily underdoped x=0.06 to heavily overdoped\nx=0.19, have been synthesized by dc magnetron sputtering technique on (100)\nSrTiO3 substrates. The influence of various fabrication conditions, such as the\ndeposition temperature and the deposition rate, etc., on the quality of the\nthin films has been scrutinized. We find that the quality of the films is less\nsensitive to the deposition temperature in the overdoped region than that in\nthe underdoped region. In the phase diagram of Tc(x), the superconducting dome\nindicates that the optimally doping level is at the point x=0.105 with the\ntransition temperature Tc0 = 26.5 K. Further more, both the disappearance of\nthe upturn in the $\\rho_{xx}$(T) curve at low temperature under H=10 T and the\npositive differential Hall coefficient, $R_H'=d \\rho_{xy}/dH$, are observed\naround x = 0.15, implying a possible rearrangement of Fermi surface at this\ndoping level.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Spontaneous Long-range Vortex-antivortex Pair in the Two-band 2D\n  Superconductor: The relaxation iterative method is used to minimize Ginzburg-Landau model for\nthe two-band superconductor with Josephson-coupling. A stable spontaneous\nvortex-antivortex pair with long range order has been revealed. Our result\nappears due to the Josephson-coupling effect, which leads to a\nlinearly-dependence of total free energy on the distance between\nvortex-antivortex pair, hence accounts for this phenomenon.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity with High Upper Critical Field in the Cubic\n  Centrosymmetric $\u03b7$-Carbide Nb$_4$Rh$_2$C$_{1-\u03b4}$: The upper critical field is a fundamental measure of the strength of\nsuperconductivity in a material. It is also a cornerstone for the realization\nof superconducting magnet applications. The critical field arises because of\nthe Copper pair breaking at a limiting field, which is due to the Pauli\nparamagnetism of the electrons. The maximal possible magnetic field strength\nfor this effect is commonly known as the Pauli paramagnetic limit given as\n$\\mu_0 H_{\\rm Pauli} \\approx 1.86{\\rm [T/K]} \\cdot T_{\\rm c}$ for a\nweak-coupling BCS superconductor. The violation of this limit is only rarely\nobserved. Exceptions include some low-temperature heavy fermion and some\nstrongly anisotropic superconductors. Here, we report on the superconductivity\nat 9.75 K in the centrosymmetric, cubic $\\eta$-carbide-type compound\nNb$_4$Rh$_2$C$_{1-\\delta}$, with a normalized specific heat jump of $\\Delta\nC/\\gamma T_{\\rm c} =$ 1.64. We find that this material has a remarkably high\nupper critical field of $\\mu_0 H_{\\rm c2}{\\rm (0)}$ =~28.5~T, which is\nexceeding by far its weak-coupling BCS Pauli paramagnetic limit of $\\mu_0\nH_{\\rm Pauli}$~=~18.1 T. Determination of the origin and consequences of this\neffect will represent a significant new direction in the study of critical\nfields in superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Hidden Pseudogap and Excitation Spectra in a Strongly Coupled Two-Band\n  Superfluid/Superconductor: We investigate single-particle excitation properties in the normal state of a\ntwo-band superconductor or superfluid throughout the Bardeen-Cooper-Schrieffer\n(BCS) to Bose-Einstein-condensation (BEC) crossover, within the many-body\nT-matrix approximation for multi-channel pairing fluctuations. We address the\nsingle-particle density of states and the spectral functions consisting of two\ncontributions associated with a waekly interacting deep band and a strongly\ninteracting shallow band, relevant for iron-based multiband superconductors and\nmulticomponent fermionic superfluids. We show how the pseudogap state in the\nshallow band is hidden by the deep band contribution throughout the two-band\nBCS-BEC crossover. Our results could explain the missing pseudogap in recent\nscanning tunneling microscopy experiments in FeSe superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Visualization of vortex bound states in polarized Fermi gases at\n  unitarity: We analyse theoretically a single vortex in 3D trapped atomic Fermi gases\nwith population polarization near a broad Feshbach resonance. Above a critical\npolarization the Andreev-like bound states inside the core become occupied for\nthe majority component. As a result, the local density difference at the core\ncenter acquires a sudden rise at low temperautres. This provides a\nvisualization of the lowest bound state within the absorption imaging\ntechnique. As the polarization increases, the core expands gradually, and\ncorrespondingly, the energy of the lowest bound state decreases.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Measurement of the dynamic charge response of materials using\n  low-energy, momentum-resolved electron energy-loss spectroscopy (M-EELS): One of the most fundamental properties of an interacting electron system is\nits frequency- and wave-vector-dependent density response function, $\\chi({\\bf\nq},\\omega)$. The imaginary part, $\\chi''({\\bf q},\\omega)$, defines the\nfundamental bosonic charge excitations of the system, exhibiting peaks wherever\ncollective modes are present. $\\chi$ quantifies the electronic compressibility\nof a material, its response to external fields, its ability to screen charge,\nand its tendency to form charge density waves. Unfortunately, there has never\nbeen a fully momentum-resolved means to measure $\\chi({\\bf q},\\omega)$ at the\nmeV energy scale relevant to modern elecronic materials. Here, we demonstrate a\nway to measure $\\chi$ with quantitative momentum resolution by applying\nalignment techniques from x-ray and neutron scattering to surface\nhigh-resolution electron energy-loss spectroscopy (HR-EELS). This approach,\nwhich we refer to here as \"M-EELS,\" allows direct measurement of $\\chi''({\\bf\nq},\\omega)$ with meV resolution while controlling the momentum with an accuracy\nbetter than a percent of a typical Brillouin zone. We apply this technique to\nfinite-q excitations in the optimally-doped high temperature superconductor,\nBi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (Bi2212), which exhibits several phonons\npotentially relevant to dispersion anomalies observed in ARPES and STM\nexperiments. Our study defines a path to studying the long-sought collective\ncharge modes in quantum materials at the meV scale and with full momentum\ncontrol.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Elastic properties and inter-atomic bonding in new superconductor\n  KFe2Se2 from first principles calculations: Very recently (November, 2010, PRB, 82, 180520R) the first 122-like ternary\nsuperconductor KxFe2Se2 with enhanced TC ~ 31K has been discovered. This\nfinding has stimulated much activity in search of related materials and\ntriggered the intense studies of their properties. Indeed already in 2010-2011\nthe superconductivity (TC ~ 27-33K) was also found in the series of new\nsynthesized 122 phases such as CsxFe2Se2, RbxFe2Se2, (TlK)xFeySe2 etc. which\nhave formed today the new family of superconducting iron-based materials\nwithout toxic As. Here, using the ab initio FLAPW-GGA method we have predicted\nfor the first time the elastic properties for KFe2Se2 and discussed their\ninterplay with inter-atomic bonding for this system. Our data reveal that the\nexamined phase is relatively soft material. In addition, this system is\nmechanically stable, adopts considerable elastic anisotropy, and demonstrates\nbrittleness. These conclusions agree with the bonding picture for KFe2Se2,\nwhere the inter-atomic bonding is highly anisotropic and includes ionic,\ncovalent and metallic contributions.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Competition of electron-phonon mediated superconductivity and Stoner\n  magnetism on a flat band: The effective attractive interaction between electrons, mediated by\nelectron-phonon coupling, is a well-established mechanism of conventional\nsuperconductivity. In metals exhibiting a Fermi surface, the critical\ntemperature of superconductivity is exponentially smaller than the\ncharacteristic phonon energy. Therefore such superconductors are found only at\ntemperatures below a few Kelvin. Systems with flat energy bands have been\nsuggested to cure the problem and provide a route to room-temperature\nsuperconductivity, but previous studies are limited to only BCS models with an\neffective attractive interaction. Here we generalize Eliashberg's theory of\nstrong-coupling superconductivity to systems with flat bands and relate the\nmean-field critical temperature to the microscopic parameters describing\nelectron-phonon and electron-electron interaction. We also analyze the\nstrong-coupling corrections to the BCS results, and construct the phase diagram\nexhibiting superconductivity and magnetic phases on an equal footing. Our\nresults are especially relevant for novel quantum materials where electronic\ndispersion and interaction strength are controllable.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Hollow carbon spheres as an efficient dopant for enhancing critical\n  current density of MgB2 based tapes: A significant enhancement of Jc and Hirr in MgB2 tapes has been achieved by\nthe in situ powder-in-tube method utilizing hollow carbon spheres (HCS) as\ndopants. At 4.2 K, the transport Jc for the 850C sintered samples reached\n3.1x10^4, and 1.4x10^4 A/cm^2 at 10 and 12 T, respectively, and were better\nthan those of optimal nano-SiC doped tapes. Furthermore, the Hirr for doped\nsample was raised up to 16.8 T at 10 K due to the carbon substitution effect.\nThe results demonstrate that HCS is one of the most promising dopants besides\nnano-carbon and SiC for the enhancement of current capacity for MgB2 in high\nfields.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "A tunable rf SQUID manipulated as flux and phase qubit: We report on two different manipulation procedures of a tunable rf SQUID.\nFirst, we operate this system as a flux qubit, where the coherent evolution\nbetween the two flux states is induced by a rapid change of the energy\npotential, turning it from a double well into a single well. The measured\ncoherent Larmor-like oscillation of the retrapping probability in one of the\nwells has a frequency ranging from 6 to 20 GHz, with a theoretically expected\nupper limit of 40 GHz. Furthermore, here we also report a manipulation of the\nsame device as a phase qubit. In the phase regime, the manipulation of the\nenergy states is realized by applying a resonant microwave drive. In spite of\nthe conceptual difference between these two manipulation procedures, the\nmeasured decay times of Larmor oscillation and microwave-driven Rabi\noscillation are rather similar. Due to the higher frequency of the Larmor\noscillations, the microwave-free qubit manipulation allows for much faster\ncoherent operations.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "NMR Investigation of the Quasi One-dimensional Superconductor\n  K$_{2}$Cr$_{3}$As$_{3}$: We report $^{75}$As NMR measurements on the new quasi one-dimensional\nsuperconductor K$_{2}$Cr$_{3}$As$_{3}$ ($T_{c} \\sim 6.1$~K) [J.\\ K.\\ Bao et\nal., Phys. Rev. X {\\bf 5}, 011013 (2015)]. We found evidence for strong\nenhancement of Cr spin fluctuations above $T_c$ in the\n[Cr$_{3}$As$_{3}$]$_{\\infty}$ double-walled subnano-tubes based on the nuclear\nspin-lattice relaxation rate $1/T_{1}$. The power law temperature dependence,\n$1/T_{1}T \\sim T^{-\\gamma}$ ($\\gamma \\sim 0.25$), is consistent with the\nTomonaga-Luttinger liquid. Moreover, absence of the Hebel-Slichter coherence\npeak of $1/T_{1}$ just below $T_{c}$ suggests unconventional nature of\nsuperconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Effects of Iodine Annealing on Fe1+yTe0.6Se0.4: Effects of iodine annealing to induce bulk superconductivity in\nFe1+yTe0.6Se0.4 have been systematically studied by changing the molar ratio of\niodine to the sample and annealing temperature. The optimal condition to induce\nbulk superconductivity with Tc ~14.5 K and self-field Jc(2 K) ~ 5x10^5 A/cm2 is\nfound to be a molar ratio of iodine of 5-7 % at the annealing temperature of\n400 C. Furthermore, the fact that no compounds containing iodine are detected\nin the crystal and a significant amount of FeTe2 is produced after the iodine\nannealing strongly indicate that the excess iron is consumed to form FeTe2 and\niodine works as a catalyst in this process.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "ARPES in the normal state of the cuprates: comparing the marginal Fermi\n  liquid and spin fluctuation scenarios: We address the issue whether ARPES measurements of the spectral function $A_k\n(\\omega)$ near the Fermi surface in the normal state of near optimally doped\ncuprates can distinguish between the marginal Fermi liquid scenario and the\nspin-fluctuation scenario. We argue that the data for momenta near the Fermi\nsurface are equally well described by both theories, but this agreement is\nnearly meaningless as in both cases one has to add to $\\Sigma^{\\prime \\prime}\n(\\omega)$ a large constant of yet unknown origin. We show that the data can be\nwell fitted by keeping only this constant term in the self-energy. To\ndistinguish between the two scenarios, one has to analyze the data away from\nthe Fermi surface, when the intrinsic piece in $\\Sigma (\\omega)$ becomes\ndominant.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Effect of damage by 2-MeV He ions on the normal and superconducting\n  properties of magnesium diboride: We have studied the effect of damage induced by 2-MeV alpha particles on the\ncritical temperature, Tc, and resistivity of MgB2 thin films. This technique\nallows defects to be controllably introduced into MgB2 in small successive\nsteps. Tc decreases linearly as the intragrain resistivity at 40 K increases,\nwhile the intergrain connectivity is not changed. Tc is ultimately reduced to\nless than 7 K and we see no evidence for a saturation of Tc at about 20 K,\ncontrary to the predictions of the Tc of MgB2 in the dirty limit of interband\nscattering.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Antiferromagnetic Phases in the Fulde-Ferrell-Larkin-Ovchinnikov State\n  of CeCoIn_5: The antiferromagnetic (AFM) order in the Fulde-Ferrell-Larkin-Ovchinnikov\n(FFLO) superconducting state is analyzed on the basis of a Ginzburg-Landau\ntheory. To examine the possible AFM-FFLO state in CeCoIn_5, we focus on the\nincommensurate AFM order characterized by the wave vector Q = Q_{0} \\pm q_inc\nwith Q_0 =(\\pi,\\pi,\\pi) and q_inc \\parallel [110] or [1-10] in the tetragonal\ncrystal structure. We formulate the two component Ginzburg-Landau theory and\ninvestigate the two degenerate incommensurate AFM order. We show that the\npinning of AFM moment due to the FFLO nodal planes leads to multiple phases in\nmagnetic fields along [100] or [010]. The phase diagrams for various coupling\nconstants between the two order parameters are shown for the comparison with\nCeCoIn_5. Experimental results of the NMR and neutron scattering measurements\nare discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Localization-delocalization dichotomy: Inherent spectral properties of\n  the cuprates: We consider hole pairing in the pseudopgap phase of High T_c cuprates, as\narising from resonant scattering on dynamically deformable molecular units. As\na result, localized and delocalized features coexist in the one-particle\nspectra: the pseudogap and propagating diffusive Bogoliubov modes. Due to the\nanisotropy of the electron dispersion and pairing interaction, these two\nmanifestations have different impact in the different regions of the Brillouin\nzone. We illustrate that for k-vectors crossing the arc, determined by the\nchemical potential, joining the anti-nodal and the nodal point.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "On the generation of multipartite entangled states in Josephson\n  architectures: We propose and analyze a scheme for the generation of multipartite entangled\nstates in a system of inductively coupled Josephson flux qubits. The qubits\nhave fixed eigenfrequencies during the whole process in order to minimize\ndecoherence effects and their inductive coupling can be turned on and off at\nwill by tuning an external control flux. Within this framework, we will show\nthat a W state in a system of three or more qubits can be generated by\nexploiting the sequential one by one coupling of the qubits with one of them\nplaying the role of an entanglement mediator.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Picosecond Trajectory of Two-dimensional Vortex Motion in\n  FeSe$_{0.5}$Te$_{0.5}$ Visualized by Terahertz Second Harmonic Generation: We have investigated the vortex dynamics in a thin film of an iron-based\nsuperconductor FeSe$_{0.5}$Te$_{0.5}$ by observing second-harmonic generation\n(SHG) in the THz frequency range. We visualized the picosecond trajectory of\ntwo-dimensional vortex motion in a pinning potential tilted by Meissner\nshielding current. The SHG perpendicular to the driving field is observed,\ncorresponding to the nonreciprocal nonlinear Hall effect under the\ncurrent-induced inversion symmetry breaking, whereas the linear Hall effect is\nnegligible. The estimated vortex mass, as light as a bare electron, suggests\nthat the vortex core moves independently from quasiparticles at such a high\nfrequency and large velocity $\\approx$300 km/s.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconducting properties of SmO1-xFxFeAs wires with Tc = 52 K prepared\n  by the powder-in-tube method: We demonstrate that Ta sheathed SmO1-xFxFeAs wires were successfully\nfabricated by the powder-in-tube (PIT) method for the first time. Structural\nanalysis by mean of x-ray diffraction shows that the main phase of SmO1-xFxFeAs\nwas obtained by this synthesis method. The transition temperature of the\nSmO0.65F0.35FeAs wires was confirmed to be as high as 52 K. Based on\nmagnetization measurements, it is found that a globe current can flow on\nmacroscopic sample dimensions with Jc of ~3.9x10^3 A/cm^2 at 5 K and self\nfield, while a high Jc about 2x10^5 A/cm^2 is observed within the grains,\nsuggesting that a significant improvement in the globle Jc is possible. It\nshould be noted that the Jc exhibits a very weak field dependence behavior.\nFurthermore, the upper critical fields (Hc2) determined according to the\nWerthamer-Helfand-Hohenberg formula are (T= 0 K) = 120 T, indicating a very\nencouraging application of the new superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "$^{11}B$ NMR and Relaxation in $MgB_2$ Superconductor: $^{11}B$ NMR and nuclear spin-lattice relaxation rate (NSLR) are reported at\n7.2 Tesla and 1.4 Tesla in powder samples of the intermetallic compound $MgB_2$\nwith superconducting transition temperature in zero field $T_c$ = 39.2 K. From\nthe first order quadrupole perturbed NMR specrum a quadrupole coupling\nfrequency of 835 $\\pm$ 5 kHz is obtained. The Knight shift is very small and it\ndecreases to zero in the superconducting phase. The NSLR follows a linear law\nwith $T_1T$ = 165 $\\pm$ 10 (sec K) . The results in the normal phase indicate a\nnegligible $s$-character of the wave function of the conduction electrons at\nthe Fermi level. Below $T_c$ the NSLR is strongly field dependent indicating\nthe presence of an important contribution related to the density and the\nthermal motion of flux lines. No coherence peak is observed at the lower field\ninvestigated (1.4 T).",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Microscopic evidence for anisotropic multigap superconductivity in the\n  CsV$_3$Sb$_5$ kagome superconductor: The recently discovered kagome superconductor CsV$_3$Sb$_5$ ($T_c \\simeq 2.5$\nK) has been found to host charge order as well as a non-trivial band topology,\nencompassing multiple Dirac points and probable surface states. Such a complex\nand phenomenologically rich system is, therefore, an ideal playground for\nobserving unusual electronic phases. Here, we report on microscopic studies of\nits anisotropic superconducting properties by means of transverse-field muon\nspin rotation ($\\mu$SR) experiments. The temperature dependences of the\nin-plane and out-of-plane components of the magnetic penetration depth\n$\\lambda_{ab}^{-2}(T)$ and $\\lambda_{c}^{-2}(T)$ indicate that the\nsuperconducting order parameter exhibits a two-gap ($s+s$)-wave symmetry,\nreflecting the multiple Fermi surfaces of CsV3Sb5. The multiband nature of its\nsuperconductivity is further validated by the different temperature dependences\nof the anisotropic magnetic penetration depth $\\gamma_\\lambda(T)$ and upper\ncritical field $\\gamma_{\\rm B_{c2}}(T)$, both in close analogy with the well\nknown two-gap superconductor MgB$_2$. Remarkably, the high value of the\n$T_c/\\lambda^{-2}(0)$ ratio in both field orientations strongly suggests the\nunconventional nature of superconductivity. The relaxation rates obtained from\nzero field $\\mu$SR experiments do not show noticeable change across the\nsuperconducting transition, indicating that superconductivity does not break\ntime reversal symmetry.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Type-1.5 superconductivity in multicomponent systems: In general a superconducting state breaks multiple symmetries and, therefore,\nis characterized by several different coherence lengths $\\xi_i$, $i=1,...,N$.\nMoreover in multiband material even superconducting states that break only a\nsingle symmetry are nonetheless described, under certain conditions by\nmulti-component theories with multiple coherence lengths. As a result of that\nthere can appear a state where some coherence lengths are larger and some are\nsmaller than the magnetic field penetration length $\\lambda$: $\\xi_1\\leq\n\\xi_2... < \\sqrt{2}\\lambda<\\xi_M\\leq...\\xi_N$. That state was recently termed\n\"type-1.5\" superconductivity. This breakdown of type-1/type-2 dichotomy is\nrather generic near a phase transition between superconducting states with\ndifferent symmetries. The examples include the transitions between $U(1)$ and\n$U(1)\\times U(1)$ states or between $U(1)$ and $U(1)\\times Z_2$ states. The\nlater example is realized in systems that feature transition between s-wave and\n$s+is$ states. The extra fundamental length scales have many physical\nconsequences. In particular in these regimes vortices can attract one another\nat long range but repel at shorter ranges. Such a system can form vortex\nclusters in low magnetic fields. The vortex clustering in the type-1.5 regime\ngives rise to many physical effects, ranging from macroscopic phase separation\nin domains of different broken symmetries, to unusual transport properties.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Structural and electronic response upon hole-doping of rare-earth iron\n  oxyarsenides Nd1-xSrxFeAsO (0 < x < 0.2): Hole-doping of NdFeAsO via partial replacement of Nd3+ by Sr2+ is a\nsuccessful route to obtain superconducting phases (Tc = 13.5 K for a Sr2+\ncontent of 20%); however, the structural and electronic response with doping is\ndifferent from and non-symmetric to that in the electron-doped side of the\nphase diagram.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Growth, Characterization, Vortex Pinning and Vortex Flow Properties of\n  Single Crystals of Iron Chalcogenide Superconductor FeCr$_{0.02}$Se: We report the growth and characterization of single crystals of iron\nchalcogenide superconductor FeCr$_{0.02}$Se. There is an enhancement of the\nsuperconducting transition temperature (T$_{\\rm c}$) as compared to the T$_{\\rm\nc}$ of the single crystals of the parent compound Fe$_{1+x}$Se by about 25%.\nThe superconducting parameters such as the critical fields, coherence length,\npenetration depth and the Ginzburg-Landau parameter have been estimated for\nthese single crystals. Analysis of the critical current data suggests a\nfluctuation in electronic mean free path induced ($\\delta l$) pinning mechanism\nin this material. Thermally activated transport across the superconducting\ntransition in the presence of external magnetic fields suggests a crossover\nfrom a single vortex pinning regime at low fields to a collective flux creep\nregime at higher magnetic fields. The nature of charge carriers in the normal\nstate estimated from the Hall effect and thermal transport measurements could\nprovide crucial information on the mechanism of superconductivity in Fe-based\nmaterials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Dynamics of metastable vortex states in weakly pinned superconductors: A\n  phenomenological model: We present a phenomenological model for the vortex dynamics in the peak\neffect region of weakly pinned superconductors. We explain the history\ndependent dynamic response of the metastable vortex states subjected to a\ntransport current and the hysteretic voltage-current characteristics observed\nin the vicinity of peak effect in weakly pinned superconductors. A strong\nvariation in voltage current characteristics with the current sweep rate and\nthe anomalous dependence of critical current density $J_c$ on the magnetic\nfield sweep rate have also been accounted for by this model.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Interplay between unconventional superconductivity and heavy-fermion\n  quantum criticality: CeCu$_2$Si$_2$ versus YbRh$_2$Si$_2$: In this paper the low-temperature properties of two isostructural canonical\nheavy-fermion compounds are contrasted with regards to the interplay between\nantiferromagnetic (AF) quantum criticality and superconductivity. For\nCeCu$_2$Si$_2$, fully-gapped d-wave superconductivity forms in the vicinity of\nan itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum\ncritical point (QCP). Inelastic neutron scattering results highlight that both\nquantum critical SDW fluctuations as well as Mott-type fluctuations of local\nmagnetic moments contribute to the formation of Cooper pairs in CeCu$_2$Si$_2$.\nIn YbRh$_2$Si$_2$, superconductivity appears to be suppressed at $T\\gtrsim~10$\nmK by AF order ($T_N$ = 70 mK). Ultra-low temperature measurements reveal a\nhybrid order between nuclear and 4f-electronic spins, which is dominated by the\nYb-derived nuclear spins, to develop at $T_A$ slightly above 2 mK. The hybrid\norder turns out to strongly compete with the primary 4f-electronic order and to\npush the material towards its QCP. Apparently, this paves the way for\nheavy-fermion superconductivity to form at $T_c$ = 2 mK. Like the pressure -\ninduced QCP in CeRhIn$_5$, the magnetic field - induced one in YbRh$_2$Si$_2$\nis of the local Kondo-destroying variety which corresponds to a Mott-type\ntransition at zero temperature. Therefore, these materials form the link\nbetween the large family of about fifty low-$T$ unconventional heavy - fermion\nsuperconductors and other families of unconventional superconductors with\nhigher $T_c$s, notably the doped Mott insulators of the cuprates, organic\ncharge-transfer salts and some of the Fe-based superconductors. Our study\nsuggests that heavy-fermion superconductivity near an AF QCP is a robust\nphenomenon.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Josephson Junctions with a synthetic antiferromagnetic interlayer: We report measurements of the critical current vs. Co thickness in\nNb/Cu/Co/Ru/Co/Cu/Nb Josephson junctions, where the inner Co/Ru/Co trilayer is\na \"synthetic antiferromagnet\" with the magnetizations of the two Co layers\ncoupled antiparallel to each other via the 0.6 nm-thick Ru layer. Due to the\nantiparallel magnetization alignment, the net intrinsic magnetic flux in the\njunction is nearly zero, and such junctions exhibit excellent Fraunhofer\npatterns in the critical current vs. applied magnetic field, even with total Co\nthicknesses as large as 23 nm. There are no apparent oscillations in the\ncritical current vs. Co thickness, consistent with theoretical expectations for\nthis situation. The critical current of the junctions decays over 4 orders of\nmagnitude as the total Co thickness increases from 3 to 23 nm. These junctions\nmay serve as useful templates for future explorations of spin-triplet\nsuperconducting correlations, which are predicted to occur in supercon-\nducting/ferromagnetic hybrid systems in the presence of certain types of\nmagnetic inhomogeneity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Majorana bands, Berry curvature, and thermal Hall conductivity in the\n  vortex state of a chiral p-wave superconductor: Majorana quasiparticles localized in vortex cores of a chiral p-wave\nsuperconductor hybridize with one another to form bands in a vortex lattice. We\nbegin by solving a fully microscopic theory describing all quasiparticle bands\nin a chiral p-wave superconductor in magnetic field, then use this solution to\nbuild localized Wannier wavefunctions corresponding to Majorana quasiparticles.\nA low-energy tight-binding theory describing the intervortex hopping of these\nis then derived, and its topological properties---which depend crucially on the\nsigns of the imaginary intervortex hopping parameters---are studied. We show\nthat the energy gap between the Majorana bands may be either topologically\ntrivial or nontrivial, depending on whether the Chern number contributions from\nthe Majorana bands and those from the background superconducting condensate add\nconstructively or destructively. This topology directly affects the\ntemperature-dependent thermal Hall conductivity, which we also calculate.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Asymmetrical solutions and role of thermal fluctuations in dc current\n  driven extended Josephson junction: Extended Josephson junction driven by dc bias current is studied numerically.\nTwo types of solutions, symmetrical and asymmetrical, are found. The\ncurrent-voltage characteristic (IVC) is calculated. The symmetrical solutions\nform main histeretic IVC and asymmetrical one create an additional branch.\nDepending on the bias current value periodic, quasiperiodic and chaotic modes\nof the junction motion was observed. Dynamics of the junction affected by\nthermal fluctuations was analyzed. Stability of different states of the\njunction is discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "A Unified Description of Cuprate and Iron Arsenide Superconductors: We propose a unified description of cuprate and iron-based superconductivity.\nConsistency with magnetic structure inferred from neutron scattering implies\nsignificant constraints on the symmetry of the pairing gap for the iron-based\nsuperconductors. We find that this unification requires the orbital pairing\nformfactors for the iron arsenides to differ fundamentally from those for\ncuprates at the microscopic level.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Similarity of slow stripe fluctations between Sr-doped cuprates and\n  oxygen-doped nickelates: Stripe fluctuations in La2NiO4.17 have been studied by 139La NMR using the\nfield and temperature dependence of the linewidth and relaxation rates. In the\nformation process of the stripes the NMR line intensity is maximal below 230K,\nstarts to diminish around 140K, disappears around 50K and recovers at 4K. These\nresults are shown to be consistent with, but completely complementary to\nneutron measurements, and to be generic for oxygen doped nickelates and\nunderdoped cuprates.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Doping dependence of heat transport in the iron-arsenide superconductor\n  Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$: from isotropic to strongly $k$-dependent gap\n  structure: The temperature and magnetic field dependence of the in-plane thermal\nconductivity $\\kappa$ of the iron-arsenide superconductor\nBa(Fe$_{1-x}$Co$_x$)$_2$As$_2$ was measured down to $T \\simeq 50$ mK and up to\n$H = 15$ T as a function of Co concentration $x$ in the range 0.048 $ \\leq x\n\\leq $ 0.114. In zero magnetic field, a negligible residual linear term in\n$\\kappa/T$ as $T \\to 0$ at all $x$ shows that there are no zero-energy\nquasiparticles and hence the superconducting gap has no nodes in the $ab$-plane\nanywhere in the phase diagram. However, the field dependence of $\\kappa$\nreveals a systematic evolution of the superconducting gap with doping $x$, from\nlarge everywhere on the Fermi surface in the underdoped regime, as evidenced by\na flat $\\kappa (H)$ at $T \\to 0$, to strongly $k$-dependent in the overdoped\nregime, where a small magnetic field can induce a large residual linear term,\nindicative of a deep minimum in the gap magnitude somewhere on the Fermi\nsurface. This shows that the superconducting gap structure has a strongly\n$k$-dependent amplitude around the Fermi surface only outside the\nantiferromagnetic/orthorhombic phase.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Coulomb blockade and Bloch oscillations in superconducting Ti nanowires: Quantum fluctuations in quasi-one-dimensional superconducting channels\nleading to spontaneous changes of the phase of the order parameter by $2\\pi$,\nalternatively called quantum phase slips (QPS), manifest themselves as the\nfinite resistance well below the critical temperature of thin superconducting\nnanowires and the suppression of persistent currents in tiny superconducting\nnanorings. Here we report the experimental evidence that in a current-biased\nsuperconducting nanowire the same QPS process is responsible for the insulating\nstate -- the Coulomb blockade. When exposed to RF radiation, the internal Bloch\noscillations can be synchronized with the external RF drive leading to\nformation of quantized current steps on the I-V characteristic. The effects\noriginate from the fundamental quantum duality of a Josephson junction and a\nsuperconducting nanowire governed by QPS -- the QPS junction (QPSJ).",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Multiband behavior and non-metallic low-temperature state of\n  K$_{0.50}$Na$_{0.24}$Fe$_{1.52}$Se$_{2}$: We report evidence for multiband transport and an insulating low-temperature\nnormal state in superconducting K$_{0.50}$Na$_{0.24}$Fe$_{1.52}$Se$_{2}$ with\n$T_{c}\\approx 20$ K. The temperature-dependent upper critical field, $H_{c2}$,\nis well described by a two-band BCS model. The normal-state resistance,\naccessible at low temperatures only in pulsed magnetic fields, shows an\ninsulating logarithmic temperature dependence as $T \\rightarrow 0$ after\nsuperconductivity is suppressed. This is similar as for high-$T_{c}$ copper\noxides and granular type-I superconductors, suggesting that the\nsuperconductor-insulator transition observed in high magnetic fields is related\nto intrinsic nanoscale phase separation.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Switchable resonant coupling of flux qubits: We propose a coupling scheme, where two or more flux qubits with different\neigenfrequencies share Josephson junctions with a coupler loop devoid of its\nown quantum dynamics. Switchable two-qubit coupling is realized by tuning the\nfrequency of the AC magnetic flux through the coupler to a combination\nfrequency of two of the qubits. The coupling allows any or all of the qubits to\nbe simultaneously at the degeneracy point and can change sign.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Odd-parity superconductors with two-component order parameters: nematic\n  and chiral, full gap and Majorana node: Motivated by the recent experiment indicating that superconductivity in the\ndoped topological insulator Cu$_x$Bi$_2$Se$_3$ has an odd-parity pairing\nsymmetry with rotational symmetry breaking, we study the general class of\nodd-parity superconductors with two-component order parameters in trigonal and\nhexagonal crystal systems. In the presence of strong spin-orbit interaction, we\nfind two possible superconducting phases below $T_c$, a time-reversal-breaking\n(i.e., chiral) phase and an anisotropic (i.e., nematic) phase, and determine\ntheir relative energetics from the gap function in momentum space. The nematic\nsuperconductor generally has a full quasi-particle gap, whereas the chiral\nsuperconductor with a three-dimensional (3D) Fermi surface has point nodes with\nlifted spin degeneracy, resulting in itinerant Majorana fermions in the bulk\nand topological Majorana arcs on the surface.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Fluctuation conductivity in superconductors in strong electric fields: We study the effect of a strong electric field on the fluctuation\nconductivity within the time-dependent Ginzburg-Landau theory for the case of\narbitrary dimension. Our results are based on the analytical derivation of the\nvelocity distribution law for the fluctuation Cooper pairs, from the Boltzmann\nequation. Special attention is drawn to the case of small nonlinearity of\nconductivity, which can be investigated experimentally. We obtain a general\nrelation between the nonlinear conductivity and the temperature derivative of\nthe linear Aslamazov-Larkin conductivity, applicable to any superconductor. For\nthe important case of layered superconductors we derive an analogous relation\nbetween the small nonlinear correction for the conductivity and the\nfluctuational magnetoconductivity. On the basis of these relations we provide\nnew experimental methods for determining both the lifetime constant of\nmetastable Cooper pairs above T_c and the coherence length. A systematic\ninvestigation of the 3rd harmonic of the electric field generated by a harmonic\ncurrent can serve as an alternative method for the examination of the\nmetastable Cooper-pair relaxation time.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Structure and superconductivity of LiFeAs: The lithium ions in Lithium iron arsenide phases with compositions close to\nLiFeAs have been located using powder neutron diffraction. These phases exhibit\nsuperconductivity at temperatures at least as high as 16 K demonstrating that\nsuperconductivity in compounds with [FeAs]- anti-PbO-type anionic layers occurs\nin compounds with at least three different structure types and occurs for a\nwide range of As-Fe-As bond angles.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Effect of Cu doping on superconductivity in TaSe$_3$: Relationship\n  between superconductivity and induced charge density wave: By measuring the temperature dependence of the resistance, we investigated\nthe effect of Cu doping on superconductivity (SC) in Cu-doped TaSe$_3$ in which\nthe charge density wave (CDW) transition is induced by Cu doping. We observed\nan emergence of a region where the SC transition temperature ($T_\\mathrm{C}$)\ndecreased in samples with higher Cu concentrations and found that the region\ntended to expand with increasing Cu concentration. In addition, the temperature\ndependence of the upper critical field ($H_\\mathrm{C2}$) of Cu-doped TaSe$_3$\nwas found to differ from that of pure TaSe$_3$. Based on these experimental\nresults and the fact that the SC of TaSe$_3$ is filamentary, we conclude that\nSC is suppressed locally by Cu doping and competes with the CDW in Cu-doped\nTaSe$_3$. The resistance anomaly due to the CDW transition was extremely small\nand the size of the anomaly was enhanced with increasing Cu concentration but\nthe temperature at which the anomaly appeared hardly changed. This result of\nthe anomaly and the local suppression of SC imply that the induced CDWs are\nshort-range order in the vicinity of Cu atoms. We also discuss the effect of\nthe pinning of CDWs on the relationship between SC and short-range order CDWs.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "What are the elementary excitations of the BCS model in the canonical\n  ensemble?: We have found the elementary excitations of the exactly solvable BCS model\nfor a fixed number of particles. These turn out to have a peculiar dispersion\nrelation, some of them with no counterpart in the Bogoliubov picture, and\nunusual counting properties related to an old conjecture made by Gaudin. We\ngive an algorithm to count the number of excitations for each excited state and\na graphical interpretation in terms of paths and Young diagrams. For large\nsystems the excitations are described by an effective Gaudin model, which\naccounts for the finite size corrections to BCS.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Positive Seebeck coefficient in highly doped La$_{2-x}$Sr$_x$CuO$_4$\n  ($x$=0.33); its origin and implication: We present a study of the thermoelectric (Seebeck and Nernst) response in\nheavily overdoped, non-superconducting La$_{1.67}$Sr$_{0.33}$CuO$_4$. In spite\nof the electron-like curvature of the Fermi surface, the Seebeck coefficient is\npositive at low temperatures. Such a feature, previously observed in copper,\nsilver, gold and lithium, is caused by a non-trivial energy dependence of the\nscattering time. We argue that this feature implies a strong asymmetry between\nthe lifetime of occupied and unoccupied states along the zone diagonals and\nsuch an electron-hole asymmetry impedes formation of Cooper pairs along the\nnodal direction in the superconducting ground state emerging at lower doping\nlevels.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Unifying Magnons and Triplons in Stripe-Ordered Cuprate Superconductors: Based on a two-dimensional model of coupled two-leg spin ladders, we derive a\nunified picture of recent neutron scattering data of stripe-ordered\nLa_(15/8)Ba_(1/8)CuO_4, namely of the low-energy magnons around the\nsuperstructure satellites and of the triplon excitations at higher energies.\nThe resonance peak at the antiferromagnetic wave vector Q_AF in the\nstripe-ordered phase corresponds to a saddle point in the dispersion of the\nmagnetic excitations. Quantitative agreement with the neutron data is obtained\nfor J= 130-160 meV and J_cyc/J = 0.2-0.25.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Vortex Glass is a Metal: Unified Theory of the Magnetic Field and\n  Disorder-Tuned Bose Metals: We consider the disordered quantum rotor model in the presence of a magnetic\nfield. We analyze the transport properties in the vicinity of the multicritical\npoint between the superconductor, phase glass and paramagnetic phases. We find\nthat the magnetic field leaves metallic transport of bosons in the glassy phase\nin tact. In the vicinity of the vicinity of the superconductivity-to-Bose metal\ntransition, the resistitivy turns on as $(H-H_c)^{2}$ with $H_c$. This\nfunctional form is in excellent agreement with the experimentally observed\nturn-on of the resistivity in the metallic state in MoGe, namely $R\\approx\nR_c(H-H_c)^\\mu$, $1<\\mu<3$. The metallic state is also shown to presist in\nthree spatial dimensions. In addition, we also show that the metallic state\nremains intact in the presence of Ohmic dissipation in spite of recent claims\nto the contrary. As the phase glass in $d=3$ is identical to the vortex glass,\nwe conclude that the vortex glass is, in actuality, a metal rather than a\nsuperconductor at T=0. Our analysis unifies the recent experiments on vortex\nglass systems in which the linear resistivity remained non-zero below the\nputative vortex glass transition and the experiments on thin films in which a\nmetallic phase has been observed to disrupt the direct transition from a\nsuperconductor to an insulator.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Probability of the resistive state formation caused by absorption of a\n  single-photon in current-carrying superconducting nano-strips: We have studied supercurrent-assisted formation of the resistive state in\nnano-structured Nb and NbN superconducting films after absorption of a single\nphoton. In amorphous narrow NbN strips the probability of the resistive state\nformation has a pronounced spectral cut-off. The corresponding threshold photon\nenergy decreases with the bias current. Analysis of the experimental data in\nthe framework of the generalized hot-spot model suggests that the quantum yield\nfor near-infrared photons increases faster than the photon nergy. Relaxation of\nthe resistive state depends on the photon energy making the phenomenon feasible\nfor the development of energy resolving single-photon detectors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconducting topological Dirac semimetals: $P6/m$-Si$_6$ and\n  $P6/m$-NaSi$_6$: We theoretically propose that hexagonal silicon-based crystals, $P6/m$-Si$_6$\nand $P6/m$-NaSi$_6$, are topological Dirac semimetals with superconducting\ncritical temperatures of 12 K and 13 K, respectively, at ambient pressure. Band\ninversion occurs with the Fu-Kane topological invariant $\\mathbb{Z}_2=1$, even\nin the absence of spin-orbit coupling. The Dirac nodes protected by $C_6$\ncrystal rotational symmetry remain gapless with spin-orbit coupling. Using\nfirst-principles calculations, we find pressure-induced topological phase\ntransitions for $P6/m$-Si$_6$ and $P6/m$-NaSi$_6$ with critical external\npressures of 11.5 GPa and 14.9 GPa, respectively. Above the critical pressures,\nthe Dirac bands are gapped with $\\mathbb{Z}_2=0$, while the superconducting\nstates and the crystal symmetries are retained.Our results may shed light into\na search for silicon-based topological materials with superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quantum decay of the persistent current in a Josephson junction ring: We study the persistent current in a ring consisting of N >> 1 Josephson\njunctions threaded by the magnetic flux. When the dynamics of the ring is\ndominated by the capacitances of the superconducting islands the system is\nequivalent to the xy spin system in 1+1 dimensions at the effective temperature\nT*=(2JU)^(1/2), with J being the Josephson energy of the junction and U being\nthe charging energy of the superconducting island. The numerical problem is\nchallenging due to the absence of thermodynamic limit and slow dynamics of the\nKosterlitz-Thouless transition. It is investigated on lattices containing up to\none million sites. At T << J the quantum phase slips are frozen. The low-T*\ndependence of the persistent current computed numerically agrees quantitatively\nwith the analytical formula provided by the spin-wave approximation. The high-\nT* behavior depends strongly on the magnetic flux and on the number of\nsuperconducting islands N. Depending on the flux, the persistent current gets\ndestroyed by the phase slips and/or by the superconductor-insulator transition\non increasing T*.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pressure-induced Superconductivity and Topological Quantum Phase\n  Transitions in the Topological Semimetal ZrTe2: Topological transition metal dichalcogenides (TMDCs) have attracted much\nattention due to its potential applications in spintronics and quantum\ncomputations. In this work, we systematically investigate the structural and\nelectronic properties of topological TMDCs candidate ZrTe2 under high pressure.\nA pressure-induced Lifshitz transition is evidenced by the change of charge\ncarrier type as well as the Fermi surface. Superconductivity was observed at\naround 8.3 GPa without structural phase transition. A typical dome-shape phase\ndiagram is obtained with the maximum Tc of 5.6 K for ZrTe2. Furthermore, our\ntheoretical calculations suggest the presence of multiple pressure-induced\ntopological quantum phase transitions, which coexists with emergence of\nsuperconductivity. The results demonstrate that ZrTe2 with nontrivial topology\nof electronic states display new ground states upon compression.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quantum Pairing Time Orders: We propose the concept of the time-independent correlators for the even- and\nodd-frequency pairing states that can be defined for both bosonic and fermionic\nquasiparticles. These correlators explicitly capture the existence of two\ndistinct classes of pairing states and provide a direct probe of the hidden\nBerezinskii order. This concept is illustrated in the cases of pairings for\nMajorana fermions and quasiparticles in Dirac semimetals. It is shown that the\ntime-independent correlator is able to effectively capture the energy scale\nrelevant for pairing.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Type-1.5 Superconductors: We demonstrate the existence of a novel superconducting state in high quality\ntwo-component MgB2 single crystalline superconductors where a unique\ncombination of both type-1 (kappa_1 < 0.707) and type-2 (kappa_2 > 0.707)\nsuperconductor conditions is realized for the two components of the order\nparameter. This condition leads to a vortex-vortex interaction attractive at\nlong distances and repulsive at short distances, which stabilizes\nunconventional stripe- and gossamer-like vortex patterns that we have\nvisualized in this type-1.5 superconductor using Bitter decoration and also\nreproduced in numerical simulations.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Fermi-liquid state in $T$*-type La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$CuO$_4$\n  revealed via element substitution effects on magnetism: Despite its unique structural features, the magnetism of single-layered\ncuprate with five oxygen coordination ($T$*-type structure) has not been\ninvestigated thus far. Here, we report the results of muon spin relaxation and\nmagnetic susceptibility measurements to elucidate the magnetism of $T$*-type\nLa$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$CuO$_4$ (LESCO) via magnetic Fe- and non-magnetic\nZn-substitution. We clarified the inducement of the spin-glass (SG)-like\nmagnetically ordered state in\nLa$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$Cu$_y$Fe$_{1-y}$O$_4$ with $x = 0.24 + y$, and\nthe non-magnetic state in La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$Cu$_y$Zn$_{1-y}$O$_4$\nwith $x$ = 0.24 after the suppression of superconductivity for $y$ $\\geq$\n0.025. The SG state lies below $\\sim$7 K in a wide Sr concentration range\nbetween 0.19 and 0.34 in 5$\\%$ Fe-substituted LESCO. The short-range SG state\nis consistent with that originating from the Ruderman-Kittel-Kasuya-Yosida\ninteraction in a metallic state. Thus, the results provide the first evidence\nfor Fermi liquid (FL) state in the pristine $T$*-type LESCO. Taking into\naccount the results of an oxygen $K$-edge X-ray absorption spectroscopy\nmeasurement $[$J. Phys. Soc. Jpn. 89, 075002 (2020)$]$ reporting the actual\nhole concentrations in LESCO, our results demonstrate the existence of the FL\nstate in a lower hole-concentration region, compared to that in $T$-type\nLa$_{2-x}$Sr$_x$CuO$_4$. The emergence of the FL state in a lower\nhole-concentration region is possibly associated with a smaller charge transfer\ngap energy in the parent material with five oxygen coordination.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Capturing Complex Behaviour in Josephson Travelling Wave Parametric\n  Amplifiers: We present an analysis of wave-mixing in recently developed Josephson\nTravelling Wave Parametric Amplifiers (JTWPAs). Circuit simulations performed\nusing WRspice show the full behaviour of the JTWPA allowing propagation of all\ntones. The Coupled Mode Equations (CMEs) containing only pump, signal, and\nidler propagation are shown to be insufficient to completely capture complex\nmixing behaviour in the JTWPA. Extension of the CMEs through additional state\nvectors in the analytic solutions allows closer agreement with WRspice. We\nconsider an ordered framework for the systematic inclusion of extended\neigenmodes and make a qualitative comparison with WRspice at each step. The\nagreement between the two methods validates both approaches and provides\ninsight into the operation of the JTWPA. We show that care should be taken when\nusing the CMEs and propose that WRspice should be used as a design tool for\nnon-linear superconducting circuits such as the JTWPA.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Possible coexistence of double-Q magnetic order and chequerboard charge\n  order in the re-entrant tetragonal phase of Ba0.76K0.24Fe2As2: We investigate the re-entrant tetragonal phase in the iron-based\nsuperconductor Ba0 .76K0.24Fe2As2 by DC magnetization and thermoelectrical\nmeasurements. The reversible magnetization confirms by a thermodynamic method\nthat the spin alignment in the re-entrant C4 phase is out-of-plane, in\nagreement with an itinerant double-Q magnetic order [Allred et al., Nat. Phys.\n12, 493 (2016)]. The Nernst coefficient shows the typical unusually large\nnegative value in the stripe-type spin density wave (SDW) state owing to the\nFermi surface reconstruction associated with SDW and nematic order. At the\ntransition into the re-entrant C4 tetragonal phase it hardly changes, which\ncould indicate that instead of a complete vanishing of the associated charge\norder, the spin reorientation could trigger a redistribution of the charges to\nform a secondary charge order, e.g. in form of a chequerboard-like pattern that\nno longer breaks the rotational C4 symmetry.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Stable Thermomagnetic Waves in Hard Superconductors: The problem of the stability of a nonlinear thermomagnetic wave with respect\nto small thermal and electromagnetic perturbations in hard superconductors was\nstudied. It is shown that spatially bounded solutions may correspond only to\nthe perturbations decaying with time, which implies stability of the nonlinear\nthermomagnetic wave.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Role of interactions in the energy of the spin resonance peak in\n  Fe-based superconductors: We consider the spin response within the five-orbital model for iron-based\nsuperconductors and study two cases: equal and unequal gaps in different bands.\nIn the first case, the spin resonance peak in the superconducting state appears\nbelow the characteristic energy scale determined by the gap magnitude,\n$2\\Delta_L$. In the second case, the energy scale corresponds to the sum of\nsmaller and larger gap magnitudes, $\\Delta_L + \\Delta_S$. Increasing the values\nof the Hubbard interaction and the Hund's exchange, we observe a shift of the\nspin resonance energy to lower frequencies.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Four-legged starfish-shaped Cooper pairs with ultrashort antinodal\n  length scales in cuprate superconductors: Cooper pairs of mutually attracting electrons form the building blocks of\nsuperconductivity. Thirty years after the discovery of high-temperature\nsuperconductivity in cuprates, many details of the pairs remain unknown,\nincluding their size and shape. Here we apply brand new ARPES-based methods\nthat allow us to reconstruct the shape and size of the pairs in\nBi$_2$Sr$_2$CaCu$_2$O$_{8+{\\delta}}$. The pairs are seen to form a\ncharacteristic starfish shape that is very long (>50{\\AA}) in the near-nodal\ndirection but extremely short (~4.5{\\AA}) in the antinodal (Cu-O) direction. We\nfind that this ultrashort antinodal length scale, which is of order a lattice\nconstant, is approximately constant over a wide range of doping levels even as\nmany other parameters including the pairing strength change. This suggests that\nthis new length scale, along with the pair shape, is one of the most\nfundamental characteristics of the pairs. Further, the shape and ultrashort\nlength scale should make the pairs create or intertwine with variations in\ncharge and pair density, center on various types of lattice positions, and\npotentially explain aspects of the nematic order in these materials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pressure-induced Superconductivity in Noncentrosymmetric Weyl Semimetals\n  LaAlX (X = Si and Ge): In topological materials, Dirac fermions can split into two Weyl fermions\nwith opposite chiralities due to the breaking of space inversion symmetry,\nwhile in non-centrosymmetric superconductors, novel superconducting electron\npairing mechanisms arise because of the antisymmetric spin-orbit coupling. In\nthis work, we report the pressure-introduced superconductivity in a typical\nnoncentrosymmetric Weyl semimetal LaAlX (X=Si and Ge). Superconductivity was\nobserved at around 65 GPa without structural phase transition. A typical\ndome-shape phase diagram is obtained with the maximum Tc of 2.5 K (2.1 K) for\nLaAlSi (LaAlGe). Furthermore, the application of pressure does not destroy the\nnontrivial band topology of LaAlSi up to 80.4 GPa, making such materials as\npotential candidates for realizing topological superconductivity. Our discovery\nof superconductivity in LaAlX (X=Si and Ge) will provide critical insight in\nnoncentrosymmetric superconductors and stimulate further study on\nsuperconductivity in Weyl semimetals.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Three-Dimensional Phase-Kink State in Thick Stack of Josephson Junctions\n  and Terahertz Radiation: The dynamics of superconductivity phase in thick stack of Josephson junctions\nwith strong inductive coupling, such as the one realized in layered high-$T_c$\ncuprates and possibly the recently discovered FeAs-based superconductors, is\ninvestigated under a c-axis bias voltage and in the absence of an external\nmagnetic field. The kink state found previously by the present authors is\nextended to three dimensions for both rectangular and cylindrical geometries.\nThe IV characteristics are calculated and the distributions of electromagnetic\nfield inside the samples are clarified. The solution for a cylindrical mesa\nexhibits a higher resonating frequency than that of a square mesa with the same\nlinear size by a factor of $\\sim 2.4$. More importantly, from the radius\ndependence of the resonance frequency for the cylinder geometry it is possible\nto confirm directly the kink state, and thus to reveal the mechanism of the\nstrong radiation discovered in recent experiments.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Interplay between superconductivity and itinerant magnetism in\n  underdoped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ ($x=$ 0.2) probed by the response to\n  controlled point-like disorder: The response of superconductors to controlled introduction of point-like\ndisorder is an important tool to probe their microscopic electronic collective\nbehavior. In the case of iron-based superconductors (IBS), magnetic\nfluctuations presumably play an important role in inducing high temperature\nsuperconductivity. In some cases, these two seemingly incompatible orders\ncoexist microscopically. Therefore, understanding how this unique coexistence\nstate is affected by disorder can provide important information about the\nmicroscopic mechanisms involved. In one of the most studied pnictide family,\nhole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ (BaK122), this coexistence occurs over a\nwide range of doping levels, 0.16~$\\lesssim x \\lesssim $~0.25. We used\nrelativistic 2.5 MeV electrons to induce vacancy-interstitial (Frenkel) pairs\nthat act as efficient point-like scattering centers. Upon increasing dose of\nirradiation, the superconducting transition temperature $T_c$ decreases\ndramatically. In the absence of nodes in the order parameter this provides a\nstrong support for a sign-changing $s_{\\pm}$ pairing. Simultaneously, in the\nnormal state, there is a strong violation of the Matthiessen's rule and a\ndecrease (surprisingly, at the same rate as $T_c$) of the magnetic transition\ntemperature $T_{sm}$, which indicates the itinerant nature of the long-range\nmagnetic order. Comparison of the hole-doped BaK122 with electron-doped\nBa(Fe$_x$Co$_{1-x}$)$_2$As$_2$ (FeCo122) with similar $T_{sm}\\sim$110~K,\n$x=$0.02, reveals significant differences in the normal states, with no\napparent Matthiessen's rule violation above $T_{sm}$ on the electron-doped\nside. We interpret these results in terms of the distinct impact of impurity\nscattering on the competing itinerant antiferromagnetic and $s_{\\pm}$\nsuperconducting orders.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Optical symmetries and anisotropic transport in high-Tc superconductors: A simple symmetry analysis of in-plane and out-of-plane transport in a family\nof high temperature superconductors is presented. It is shown that generalized\nscaling relations exist between the low frequency electronic Raman response and\nthe low frequency in-plane and out-of-plane conductivities in both the normal\nand superconducting states of the cuprates. Specifically, for both the normal\nand superconducting state, the temperature dependence of the low frequency\n$B_{1g}$ Raman slope scales with the $c-$axis conductivity, while the $B_{2g}$\nRaman slope scales with the in-plane conductivity. Comparison with experiments\nin the normal state of Bi-2212 and Y-123 imply that the nodal transport is\nlargely doping independent and metallic, while transport near the BZ axes is\ngoverned by a quantum critical point near doping $p\\sim 0.22$ holes per\nCuO$_{2}$ plaquette. Important differences for La-214 are discussed. It is also\nshown that the $c-$ axis conductivity rise for $T\\ll T_{c}$ is a consequence of\npartial conservation of in-plane momentum for out-of-plane transport.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Numerical simulation of thermal noise in Josephson circuits: We present a method to numerically add thermal noise to the equations of\nmotion for a circuit of Josephson junctions. A new noise term, which we call\n\"linearly interpolated Gaussian noise,\" replaces the usual white noise process.\nIt consists of random noise values spaced at a chosen time interval and\nlinearly interpolated in-between. This method can be used with variable time\nstep solvers, allowing more precise control over the error while ensuring that\nfast dynamics are not missed by the solver. We derive the spectral density of\nsuch a noise term and compare it to a white noise process. Then we demonstrate\nthe technique by computing the switching dynamics of a circuit of two Josephson\njunctions and comparing the results to the traditional method.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Bi-directional ultrafast electric-field gating of interlayer transport\n  in a cuprate superconductor: In cuprate superconductors, tunneling between planes makes possible\nthree-dimensional coherent transport. However, the interlayer tunnelling\namplitude is reduced when an order-parameter phase gradient between planes is\nestablished. As such, c-axis superconductivity can be weakened if a strong\nelectric field is applied along the c axis. We use high-field single-cycle\nterahertz pulses to gate interlayer coupling in La1.84Sr0.16CuO4. We induce\nultrafast oscillations between superconducting and resistive states and switch\nthe plasmon response on and off, without reducing the density of Cooper pairs.\nIndeed, in-plane superconductivity remains unperturbed throughout, revealing a\nnon-equilibrium state in which the dimensionality of the superconductor is time\ndependent. The gating frequency is determined by the electric field strength,\nin the spirit of the ac Josephson effect. Non-dissipative, bi-directional\ngating of superconductive coupling is of interest for device applications in\nultrafast nanoelectronics. It is also a new example of nonlinear terahertz\nphysics, applicable to nanoplasmonics and active metamaterials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Properties of High-Tc Single Crystals as Natural Interferometers in the\n  THz Frequency Range: We consider oblique incidence of (p)TM-polarized wave on the anisotropic\nsuperconducting slab, immersed on a dielectric media, such that its uniaxial\n(c) axis is perpendicular to the surfaces. The below and above plasma frequency\ntransmissivity patterns are studied and several of its properties determined,\nwithin the context of the Maxwell-London theory. Below, the regime is\nattenuated for any incident angle, and there is a transmissivity maximum, quite\npronounced in case of a very high external dielectric constant. Above, a\npropagative regime exists where the superconductor is a natural optical\nresonator, and we show here that the minimum of the transmission peaks are\nmodulated by an envelope function associated to the Brewster condition. We\npropose this set-up to obtain light with an extremely small transverse\nwavelength inside the superconductor.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "A Comment on ``Superconducting-Normal Phase Transition in (Ba1-xKx)BiO3,\n  x = 0.40, 0.47'' by B. F. Woodfield, D. A. Wright, R. A. Fisher, N. E.\n  Phillips and H. Y. Tang, Phys. Rev. Lett. 83, 4622 (1999): This comment addresses criticisms of our PRL 82 (1999) p. 4532-4535\n(cond-mat/9904288) raised by Woodfield et al. in their recent PRL. We find that\nthe critical fields in single crystals of Ba1-xKxBiO3 (x = 0.4) have anomalous\ntemperature dependecies which suggests that the superconducting phase\ntransition in this material is not of order II.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Design and testing of high-speed interconnects for Superconducting\n  multi-chip modules: Superconducting single flux quantum (SFQ) circuits can process information at\nextremely high speeds, in the range of hundreds of GHz. SFQ circuits are based\non Josephson junction cells for switching logic and ballistic transmission for\ntransferring SFQ pulses. Multi-chip modules (MCM) are often used to implement\nlarger complex designs, which cannot be fit onto a single chip. We have\noptimized the design of wideband interconnects for transferring signals and SFQ\npulses between chips in flip-chip MCMs and evaluated the importance of several\ndesign parameters such as the geometry of bump pads on chips, length of passive\nmicro-strip lines (MSL)s, number of corners in MSLs as well as flux trapping\nand fabrication effects on the operating margins of the MCMs. Several test\ncircuits have been designed to evaluate the above mentioned features and\nfabricated in the framework of 4.5-kA/cm2 HYPRES process. The MCMs bumps for\nelectrical connections have been deposited using the waferlevel electroplating\nprocess. We have found that, at the optimized configuration, the maximum\noperating frequency of the MCM test circuit, a ring oscillator with\nchip-to-chip connections, approaches 100 GHz and is not noticeably affected by\nthe presence of MCM interconnects, decreasing only about 3% with respect to the\nsame circuit with no inter-chip connections.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quasiparticle scattering in a superconductor near a nematic critical\n  point: resonance mode and multiple attractive channels: We analyze the scattering rate for 2D fermions interacting via soft nematic\nfluctuations. The ground state is an s-wave superconductor, but other pairing\nchannels are almost equally attractive. This strongly alters the scattering\nrate: At energies beyond the pairing gap $\\Delta$, it is renormalized by\ncontributions from all pairing channels. At energies of order $\\Delta$, it is\ndetermined by the competition between scattering into a gapped continuum and\ndispersing nematic resonance. The outcome is a \"peak-peak-dip-hump\" spectrum,\nsimilar, but not identical, to the \"peak-dip-hump\" structure in the cuprates.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Boron Spectral Density and Disorder Broadening in B-doped Diamond: Comparison of periodic B dopants with a random alloy of substitional boron in\ndiamond is carried out using several supercells and the coherent potential\napproximation (CPA) for the random alloy case. The main peak in the B local\ndensity of states is shifted to lower binding energy compared to the\ncorresponding C peak in intrinsic diamond. In supercells, this shows up as\nstrongly B-character bands split from bulk C bands away from the zone center,in\nan energy region around -1 eV. Even for a 4*4*4 supercell (BC$_{127}$), effects\nof the dopant order are evident in the form of primarily B-character bands just\nbelow the Fermi level at the supercell zone boundary. The bands resulting from\nthe CPA are of continuous mixed C-B character. They resemble virtual crystal\nbands, but broadened somewhat reflecting the disorder-induced lifetime, and are\nconsistent with angle-resolved photoemission band maps. The B character is 1.7\ntimes larger than for C (per atom) near the top of the valence bands for CPA,\nand roughly the same for supercells. CPA results are particularly useful since\nthey characterize the wavevector and energy dependence of disorder broadening.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Upper critical field Hc2 in Bechgaard salts (TMTSF)2PF6: The symmetry of the superconductivity in Bechgaard salts is still unknown,\nthough the triplet pairing has been established by Hc2 and NMR for (TMTSF)2PF6.\nThe large upper critical field at T = 0K (Hc2 ~ 5 Tesla) both for H || a and H\n|| b also indicates strongly the triplet pairing.\n  Here we start with a low energy effective Hamiltonian and study the\ntemperature dependence of the corresponding Hc2(T)'s.\n  The present analysis suggests that one chiral f-wave superconductor should be\nthe most likely candidate near the upper critical field.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Possible Superconductivity in Fe-Sb Based Materials: Density Functional\n  Study of LiFeSb: We investigate the electronic and other properties of the hypothetical\ncompound LiFeSb in relation to superconducting LiFeAs and FeSe using density\nfunctional calculations. The results show that LiFeSb in the LiFeAs structure\nwould be dynamically stable in the sense of having no unstable phonon modes,\nand would have very similar electronic and magnetic properties to the layered\nFe based superconductors. Importantly, a very similar structure for the Fermi\nsurface and a spin density wave related to but stronger than that in the\ncorresponding As compound is found. These results are indicative of possible\nsuperconductivity analogous to the Fe-As based compounds if the spin density\nwave can be suppressed by doping or other means. Prospects for synthesizing\nthis material in pure form or in solid solution with FeTe are discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Ultrasensitive interplay between ferromagnetism and superconductivity in\n  NbGd composite thin films: A model binary hybrid system composed of a randomly distributed rare-earth\nferromagnetic (Gd) part embedded in an s-wave superconducting (Nb) matrix is\nbeing manufactured to study the interplay between competing superconducting and\nferromagnetic order parameters. The normal metallic to superconducting phase\ntransition appears to be very sensitive to the magnetic counterpart and the\nmodulation of the superconducing properties follow closely to the\nAbrikosov-Gorkov (AG) theory of magnetic impurity induced pair breaking\nmechanism. A critical concentration of Gd is obtained for the studied NbGd\nbased composite films (CFs) above which superconductivity disappears. Besides,\na magnetic ordering resembling the paramagnetic Meissner effect (PME) appears\nin DC magnetization measurements at temperatures close to the superconducting\ntransition temperature. The positive magnetization related to the PME emerges\nupon doping Nb with Gd. The temperature dependent resistance measurements\nevolve in a similar fashion with the concentration of Gd as that with an\nexternal magnetic field and in both the cases, the transition curves accompany\nseveral intermediate features indicating the traces of magnetism originated\neither from Gd or from the external field. Finally, the signatures of magnetism\nappear evidently in the magnetization and transport measurements for the CFs\nwith very low (less than 1 at. %) doping of Gd.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconducting electronic state in optimally doped YBa2Cu3O7-d observed\n  with laser-excited angle-resolved photoemission spectroscopy: Low energy electronic structure of optimally doped YBa2Cu3O7-d is\ninvestigated using laser-excited angle-resolved photoemission spectroscopy. The\nsurface state and the CuO chain band that usually overlap the CuO2 plane\nderived bands are not detected, thus enabling a clear observation of the bulk\nsuperconducting state. The observed bilayer splitting of the Fermi surface is\n~0.08 angstrom^{-1} along the (0,0)-(pi,pi) direction, significantly larger\nthan Bi2Sr2CaCu2O8+d. The kink structure of the band dispersion reflecting the\nrenormalization effect at ~60 meV shows up similarly as in other hole-doped\ncuprates. The momentum-dependence of the superconducting gap shows\nd_{x^2-y^2}-wave like amplitude, but exhibits a nonzero minimum of ~12 meV\nalong the (0,0)-(pi,pi) direction. Possible origins of such an unexpected\n\"nodeless\" gap behavior are discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Long-lived, radiation-suppressed superconducting quantum bit in a planar\n  geometry: We present a superconducting qubit design that is fabricated in a 2D geometry\nover a superconducting ground plane to enhance the lifetime. The qubit is\ncoupled to a microstrip resonator for readout. The circuit is fabricated on a\nsilicon substrate using low loss, stoichiometric titanium nitride for capacitor\npads and small, shadow-evaporated aluminum/aluminum-oxide junctions. We observe\nqubit relaxation and coherence times ($T_1$ and $T_2$) of 11.7 $\\pm$ 0.2 $\\mu$s\nand 8.7 $\\pm$ 0.3 $\\mu$s, respectively. Calculations show that the proximity of\nthe superconducting plane suppresses the otherwise high radiation loss of the\nqubit. A significant increase in $T_1$ is projected for a reduced\nqubit-to-superconducting plane separation.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Impurity-induced broadening of the transition to a\n  Fulde-Ferrell-Larkin-Ovchinnikov phase: Recent study on doping effects in the heavy fermion superconductor CeCoIn$_5$\nhas shown that a small amount of doping induces unexpectedly large broadening\nof the second order transition into the high field and low temperature (HFLT)\nphase of this material. Motivated by this observation, effects of quenched\ndisorder on the second order transition into a longitudinal\nFulde-Ferrell-Larkin-Ovchinnikov (FFLO) state are examined. The observed large\nbroadening of the transition is naturally explained as a consequence of\nsoftness of each FFLO nodal plane. The present results strongly support the\nscenario identifying the HFLT phase of CeCoIn$_5$ with a longitudinal FFLO\nvortex state.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Impurity bound states in fully gapped $d$-wave superconductors with\n  subdominant order parameters: Impurities in superconductors and their induced bound states are important\nboth for engineering novel states such as Majorana zero-energy modes and for\nprobing bulk properties of the superconducting state. The high-temperature\ncuprates offer a clear advantage in a much larger superconducting order\nparameter, but the nodal energy spectrum of a pure $d$-wave superconductor only\nallows virtual bound states. Fully gapped $d$-wave superconducting states have\nhowever been proposed in several cuprate systems thanks to subdominant order\nparameters producing $d+is$- or $d+id'$-wave superconducting states. Here we\nstudy both magnetic and potential impurities in these fully gapped $d$-wave\nsuperconductors. Using analytical T-matrix and complementary numerical\ntight-binding lattice calculations, we show that magnetic and potential\nimpurities behave fundamentally different in $d+is$- and $d+id'$-wave\nsuperconductors. In a $d+is$-wave superconductor, there are no bound states for\npotential impurities, while a magnetic impurity produces one pair of bound\nstates, with a zero-energy level crossing at a finite scattering strength. On\nthe other hand, a $d+id'$-wave symmetry always give rise to two pairs of bound\nstates and only produce a reachable zero-energy level crossing if the normal\nstate has a strong particle-hole asymmetry.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Observation of an isotropic superconducting gap at the Brillouin zone\n  center of Tl$_{0.63}$K$_{0.37}$Fe$_{1.78}$Se$_2$: We performed a high-resolution angle-resolved photoemission spectroscopy\nstudy on superconducting (SC) Tl$_{0.63}$K$_{0.37}$Fe$_{1.78}$Se$_2$ ($T_c=29$\nK) in the whole Brillouin zone (BZ). In addition to a nearly isotropic $\\sim$\n8.2 meV 2-dimensional (2D) SC gap ($2\\Delta/k_BT_c\\sim7$) on quasi-2D electron\nFermi surfaces (FSs) located around M$(\\pi,0,0)$-A$(\\pi,0,\\pi)$, we observe a\n$\\sim 6.2$ meV isotropic SC gap ($2\\Delta/k_BT_c\\sim5$) on the Z-centered\nelectron FS that rules out any d-wave pairing symmetry and rather favors an\ns-wave symmetry. All isotropic SC gap amplitudes can be fit by a single gap\nfunction derived from a local strong coupling approach suggesting an\nenhancement of the next-next neighbor exchange interaction in the\nferrochalcogenide superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Size of stripe domains in a superconducting ferromagnet: In a superconducting ferromagnet, the superconducting state appears in the\nferromagnetic phase where usually a domain structure has already developed. We\nstudy the influence of the superconducting screening currents on a stripe\nstructure with out-of-plane magnetization, in a film of arbitrary thickness. We\nfind that superconductivity always induces a shrinkage of the domains, and\nthere is a critical value of penetration depth below which a mono-domain\nstructure is more stable than the periodic one. Furthermore we investigate the\npossible different effects of singlet and triplet superconductivity on the\ndomain width, as well as the conditions for the existence of vortices in the\ndomains. The obtained results are then discussed in light of the experimental\ndata of superconducting ferromagnets URhGe, UGe2, and UCoGe.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Andreev reflection and spin polarization measurement of Co/YBCO junction: We report temperature dependent Andreev reflection measurements of Co/\nY$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-\\delta}$ (YBCO) heterostructure samples with\njunction areas of 1 $\\mu$m diameter. Modelling of the 5-70 K conductivity data\naccording to a modified Blonder-Tinkham-Klapwijk theory yields a spin\npolarization in Co film amounting to 34% which is almost constant up to 70 K.\nThe YBCO films have been grown by pulsed laser deposition on sapphire\nsubstrates. The Co films are deposited by thermal evaporation on YBCO. The film\nis characterized by powder X-ray diffraction measurements which shows YBCO is\ngrown in (001) direction.The critical current density, 5 x 10$^{6}$ A/cm$^{2}$,\nin YBCO remains nearly constant after deposition of Co at zero field and 77 K.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetism and superconductivity in Ru(1-x)Sr2RECu(2+x)O(8-d) (RE=Gd, Eu)\n  and RuSr2Gd(1-y)Ce(y)Cu2O8 compounds: We discuss the properties of new superconducting compositions of\nRu(1-x)Sr2RECu(2+x)O(8-d) (RE=Gd, Eu) ruthenocuprates that were synthesized at\n600 atm. of oxygen at 1080 C. By changing ratio between the Ru and Cu, the\ntemperature of superconducting transition (Tc) raises up to Tc max=72 K for\nx=0.3, 0.4. The hole doping achieved along the series increases with Cu->Ru\nsubstitution. For x different than x=0, Tc can be subsequently tuned between Tc\nmax and 0 K by changing oxygen content in the compounds. The magnetic\ncharacteristics of the RE=Gd and Eu based compounds are interpreted as\nindicative of constrained dimensionality of the superconducting phase. Muon\nspin rotation experiments reveal the presence of the magnetic transitions at\nlow temperatures (Tm=14-2 K for x=0.1-0.4) that can originate in the response\nof Ru/Cu sublattices. RuSr2Gd(1-y)Ce(1-y)Cu2O8 (0<y<0.1) compounds show the\nsimultaneous increase of TN and decrease of Tc with y. The effect should be\nexplained by the electron doping that occurs with Ce->Gd substitution.\nProperties of these two series allow us to propose phase diagram for 1212-type\nruthenocuprates that links their properties to the hole doping achieved in the\nsystems. Non-superconducting single-phase RuSr2GdCu2O8 and RuSr2EuCu2O8 are\nreported and discussed in the context of the properties of substituted\ncompounds.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity in dilute SrTiO$_3$: a review: Doped SrTiO$_3$, one of the most dilute bulk systems to display\nsuperconductivity, is perhaps the first example of an unconventional\nsuperconductor, as it does not fit into the standard BCS paradigm. More than\nfive decades of research has revealed a rich temperature-carrier concentration\nphase diagram that showcases a superconducting dome, proximity to a putative\nquantum critical point, Lifshitz transitions, a multi-gap pairing state and\nunusual normal-state transport properties. Research has also extended beyond\nbulk SrTiO$_3$, ushering the new field of SrTiO$_3$-based heterostructures.\nBecause many of these themes are also featured in other quantum materials of\ncontemporary interest, recent years have seen renewed interest in SrTiO$_3$.\nHere, we review the challenges and recent progress in elucidating the\nsuperconducting state of this model system. At the same time that its extreme\ndilution requires to revisit several of the approximations that constitute the\nsuccessful Migdal-Eliashberg description of electron-phonon superconductivity,\nincluding the suppression of the Coulomb repulsion via the\nTolmachev-Anderson-Morel mechanism, it opens interesting routes for alternative\npairing mechanisms whose applicability remains under debate. For instance,\npairing mechanisms involving longitudinal optical phonons have to overcome the\nhurdles created by the anti-adiabatic nature of the pairing interaction,\nwhereas mechanisms that rely on the soft transverse optical phonons associated\nwith incipient ferroelectricity face challenges related to the nature of the\nelectron-phonon coupling. Proposals in which pairing is mediated by plasmons or\npromoted locally by defects are also discussed. We finish by surveying the\nexisting evidence for multi-band superconductivity and outlining promising\ndirections that can potentially shed new light on the rich problem of\nsuperconductivity in SrTiO$_3$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electronic phase separation in iron selenide (Li, Fe)OHFeSe\n  superconductor system: The phenomenon of phase separation into antiferromagnetic (AFM) and\nsuperconducting (SC) or normal-state regions has great implication for the\norigin of high-temperature (high-Tc) superconductivity. However, the occurrence\nof an intrinsic antiferromagnetism above the Tc of (Li, Fe)OHFeSe\nsuperconductor is questioned. Here we report a systematic study on a series of\n(Li, Fe)OHFeSe single crystal samples with Tc up to ~41 K. We observe an\nevident drop in the static magnetization at Tafm ~125 K, in some of the SC (Tc\n< ~38 K, cell parameter c < ~9.27 {\\AA}) and non-SC samples. We verify that\nthis AFM signal is intrinsic to (Li, Fe)OHFeSe. Thus, our observations indicate\nmesoscopic-to-macroscopic coexistence of an AFM state with the normal (below\nTafm) or SC (below Tc) state in (Li, Fe)OHFeSe. We explain such coexistence by\nelectronic phase separation, similar to that in high-Tc cuprates and iron\narsenides. However, such an AFM signal can be absent in some other samples of\n(Li, Fe)OHFeSe, particularly it is never observed in the SC samples of Tc > ~38\nK, owing to a spatial scale of the phase separation too small for the\nmacroscopic magnetic probe. For this case, we propose a microscopic electronic\nphase separation. It is suggested that the microscopic static phase separation\nreaches vanishing point in high-Tc (Li, Fe)OHFeSe, by the occurrence of\ntwo-dimensional AFM spin fluctuations below nearly the same temperature as Tafm\nreported previously for a (Li, Fe)OHFeSe (Tc ~42 K) single crystal. A complete\nphase diagram is thus established. Our study provides key information of the\nunderlying physics for high-Tc superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Theoretical investigation of magnetic order in ReOFeAs, Re = Ce, Pr: Density functional theory (DFT) calculations are carried out on ReOFeAs, Re =\nCe, Pr, the parent compounds of the high-T$_c$ superconductors\nReO$_{1-x}$F$_{x}$FeAs, in order to determine the magnetic order of the ground\nstate. It is found that the magnetic moments on the Fe sites adopt a collinear\nantiferromagnetic order, similar to the case of LaOFeAs. Within the generalized\ngradient approximation along with Coulomb onsite repulsion (GGA+U), we show\nthat the Re magnetic moments also adopt an antiferromagnetic order for which,\nwithin the ReO layer, same spin Re sites lie along a zigzag line perpendicular\nto the Fe spin stripes. While within GGA the Re 4f band crosses the Fermi\nlevel, upon inclusion of onsite Coulomb interaction the 4f band splits and\nmoves away from the Fermi level, making ReOFeAs a Mott insulator.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Hole doping dependences of the magnetic penetration depth and vortex\n  core size in YBa2Cu3Oy: Evidence for stripe correlations near 1/8 hole doping: We report on muon spin rotation measurements of the internal magnetic field\ndistribution n(B) in the vortex solid phase of YBa2Cu3Oy (YBCO) single\ncrystals, from which we have simultaneously determined the hole doping\ndependences of the in-plane Ginzburg-Landau (GL) length scales in the\nunderdoped regime. We find that Tc has a sublinear dependence on\n1/lambda_{ab}^2, where lambda_{ab} is the in-plane magnetic penetration depth\nin the extrapolated limits T -> 0 and H -> 0. The power coefficient of the\nsublinear dependence is close to that determined in severely underdoped YBCO\nthin films, indicating that the same relationship between Tc and the superfluid\ndensity is maintained throughout the underdoped regime. The in-plane GL\ncoherence length (vortex core size) is found to increase with decreasing hole\ndoping concentration, and exhibit a field dependence that is explained by\nproximity-induced superconductivity on the CuO chains. Both the magnetic\npenetration depth and the vortex core size are enhanced near 1/8 hole doping,\nsupporting the belief by some that stripe correlations are a universal property\nof high-Tc cuprates.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Effect of Coulomb Interaction and Disorder on Density of States in\n  Conventional Superconductors: The density of states of the disordered s-wave superconductor is calculated\nperturbatively. The effect of Coulomb interaction on diffusively moving\nelectrons in the normal state has been known before, but in the superconducting\nstate both diffuson and the screened Coulomb interaction are modified.\nTherefore, the correction to the density of states in the superconducting state\nexhibits an energy dependence different from that of the normal state. There is\na dip structure in the correction part because the interaction has a peak at\ntwice the energy of the superconducting gap. The Coulomb interaction and the\nsuperconducting fluctuation cannot be treated separately because the density\nfluctuation is coupled to the phase fluctuation in the superconducting state.\nThis coupling results in the absence of divergence around the gap edge in the\ncorrection part, which suggests the validity of this perturbation calculation.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Hall Anomaly and Vortex-Lattice Melting in Superconducting Single\n  Crystal YBa2Cu3O7-d: Sub-nanovolt resolution longitudinal and Hall voltages are measured in an\nultra pure YBa2Cu3O7-d single crystal. The Hall anomaly and the first-order\nvortex-lattice melting transition are observed simultaneously. Changes in the\ndynamic behavior of the vortex solid and liquid are correlated with features of\nthe Hall conductivity sxy. With the magnetic field oriented at an angle from\nthe twin-boundaries, the Hall conductivity sharply decreases toward large\nnegative values at the vortex-lattice melting transition.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Nernst effect anisotropy as a sensitive probe of Fermi surface\n  distortions from electron-nematic order: We analyze the thermoelectric response in layered metals with spontaneously\nbroken rotation symmetry. We identify the anisotropy of the quasiparticle\nNernst signal as an extremely sensitive probe of Fermi surface distortions\ncharacteristic of the ordered state. This is due to a subtle interplay of\ndifferent transport anisotropies which become additionally enhanced near\nvan-Hove singularities. Applied to recent experiments, our results reinforce\nthe proposal that the underdoped cuprate superconductor YBCO displays such\n``electron-nematic'' order in the pseudogap regime.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Experimental Evidences for Static Charge Density Waves in Iron\n  Oxy-pnictides: In this Letter we report high-resolution synchrotron X-ray powder diffraction\nand transmission electron microscope analysis of Mn-substituted LaFeAsO\nsamples, demonstrating that a static incommensurate modulated structure\ndevelops across the low-temperature orthorhombic phase, whose modulation\nwave-vector depends on the Mn content. The incommensurate structural distortion\nis likely originating from a charge-density-wave instability, a periodic\nmodulation of the density of conduction electrons associated with a modulation\nof the atomic positions. Our results add a new component in the physics of\nFe-based superconductors, indicating that the density wave ordering is\ncharge-driven.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The pseudogap in Bi2212 single crystals from tunneling measurements: a\n  possible evidence for the Cooper pairs above Tc: We present electron-tunneling spectroscopy of slightly overdoped Bi2212\nsingle crystals with Tc = 87 - 90 K in a temperature range between 14 K and 290\nK using a break-junction technique. The pseudogap which has been detected above\nTc appears at T* = 280 K. The analysis of the spectra shows that there is a\ncontribution to the pseudogap above Tc, which disappears approximately at 110 -\n115 K. We associate this contribution with the presence of incoherent Cooper\npairs.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Macroscale three-dimensional proximity effect in disordered\n  normal/superconductor nanocomposites: Recently, interest in Superconductor (S)-Normal (N) interfaces was renewed by\nthe observation of exotic proximity effects in various systems, including\nS/semiconductor, S/ferromagnet, and S/topological insulator. In general, the\nproximity effect is enhanced in transparent weak links where coherent Andreev\nreflection is possible. Also, it is a common knowledge that the proximity\neffect is, by definition, is a localized phenomenon that can only be active in\neach S/N interface region. However, here we show that a three-dimensional (3D)\nmacroscale proximity effect is realized in few-micrometer-thick MgO/Mg2Si/MgB2\nnanocomposite layers with atomically smooth and clean heterointerfaces. We\nfound from scanning superconducting quantum interference device (SQUID)\nmicroscopy measurements that a normal region of more than 100x100 square\nmicrometers totally undergoes transition into a bulk-like superconducting state\nalthough the normal host originally contains less than ~10 vol % of\nsuperconducting MgB2 nanograins in a dispersed manner. In the proximity-induced\nsuperconducting region, vortex formation and annihilation processes as well as\nvortex-free Meissner regions were observed with respect to applied fields in a\nsimilar manner as Abrikosov vortices in type-II superconductors. Furthermore,\nwe found that the induced superconducting layers exhibit an anisotropic\nmagnetization behavior, in consistent with the formation of the large-scale\nsuperconducting coherence. This unusually extended proximity effect suggests\nthat disorder-induced interaction and coupling of Andreev bound states, which\nare coherent superposition of time reversed electron hole pairs, is realized in\nthe nanocomposite. Thus, the present results not only expand the limit of the\nproximity effect to bulk scales, but also provides a new route to obtain a\nproximity-induced superconducting state from disordered systems.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Transport properties of Lix(NH3)yFe2(TezSe1-z)2 single crystals in the\n  mixed state: We study the electric transport properties of Lix(NH3)yFe2(TezSe1-z)2 single\ncrystals with z= 0 and 0.6 in the mixed state. Thermally-activated flux-flow,\nvortex glass and flux-flow Hall effect (FFHE) behaviors are observed.\nExperimental results show that there are rich vortex phases existing in these\nsystems and the vortex liquid states occupy broad regions of phase diagrams.\nFurther analysis suggests that thermal fluctuation plays an important role in\nthe vortex phase diagrams of Lix(NH3)yFe2(TezSe1-z)2. Moreover, for\nLix(NH3)yFe2Se2, there is no sigh reversal of FFHE in the mixed state and a\nscaling behavior |rhoxy(mu0H)|=Arhoxx(mu0H)^beta with beta ~ 2.0 is observed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "On the Mechanism of Superconductivity in HTSC from Tunneling\n  Spectroscopy Measurements on Bi-2212 Single Crystals: The paper has been withdrawn by the author because the observed effect has a\ndifferent origin.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Anomalous thermodynamic power laws near topological transitions in nodal\n  superconductors: Unconventional superconductors are most frequently identified by the\nobservation of power-law behaviour on low-temperature thermodynamic or\ntransport properties, such as specific heat. Here we show that, in addition to\nthe usual point and line nodes, a much wider class of different nodal types can\noccur. These new types of nodes typically occur when there are transitions\nbetween different types of gap node topology, for example when point or line\nnodes first appear as a function of some physical parameter. We identify\nanomalous, non-integer thermodynamic power laws associated with these new nodal\ntypes, and give physical examples of superconductors in which they might be\nobserved experimentally, including the noncentrosymmetric superconductor\nLi$_2$Pd$_{3-x}$Pt$_x$B.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Thermal and electrical conductivity of Fermi pocket models of underdoped\n  cuprates: Several models of the electronic spectrum in the pseudogap state of\nunderdoped cuprates have been proposed to explain ARPES and STM measurements,\nwhich reveal only truncated Fermi pockets instead of a full metallic Fermi\nsurface. We consider the transport properties expected of four physically\ndistinct models, and calculate the thermal and electrical conductivity of the\nelectronic quasiparticles. By proposing transport currents that reflect the\nclose correspondence between quasiparticles on the Fermi pockets in the\npseudogap and those near nodes in the superconducting state, we show that\nmeasurable transport coefficients provide stringent tests of pseudogap models.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Non-Abelian Anyons and Non-Abelian Vortices in Topological\n  Superconductors: Anyons are particles obeying statistics of neither bosons nor fermions.\nNon-Abelian anyons, whose exchanges are described by a non-Abelian group acting\non a set of wave functions, are attracting a great attention because of\npossible applications to topological quantum computations. Braiding of\nnon-Abelian anyons corresponds to quantum computations. The simplest\nnon-Abelian anyons are Ising anyons which can be realized by Majorana fermions\nhosted by vortices or edges of topological superconductors, $\\nu =5/2$ quantum\nHall states, spin liquids, and dense quark matter. While Ising anyons are\ninsufficient for universal quantum computations, Fibonacci anyons present in\n$\\nu =12/5$ quantum Hall states can be used for universal quantum computations.\nYang-Lee anyons are non-unitary counterparts of Fibonacci anyons. Another\npossibility of non-Abelian anyons (of bosonic origin) is given by vortex\nanyons, which are constructed from non-Abelian vortices supported by a\nnon-Abelian first homotopy group, relevant for certain nematic liquid crystals,\nsuperfluid $^3$He, spinor Bose-Einstein condensates, and high density quark\nmatter. Finally, there is a unique system admitting two types of non-Abelian\nanyons, Majorana fermions (Ising anyons) and non-Abelian vortex anyons. That is\n$^3P_2$ superfluids (spin-triplet, $p$-wave paring of neutrons), expected to\nexist in neutron star interiors as the largest topological quantum matter in\nour universe.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Double path interference and magnetic oscillations in Cooper pair\n  transport through a single nanowire: We show that the critical current of the Josephson junction consisting of\nsuperconducting electrodes coupled through a nanowire with two conductive\nchannels can reveal the multi-periodic magnetic oscillations. The\nmulti-periodicity originates from the quantum mechanical interference between\nthe channels affected by both the strong spin-orbit coupling and Zeeman\ninteraction. This minimal two-channel model is shown to explain the complicated\ninterference phenomena observed recently in Josephson transport through Bi\nnanowires.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Ballistic magnon transport and phonon scattering in the antiferromagnet\n  Nd$_2$CuO$_4$: The thermal conductivity of the antiferromagnet Nd$_2$CuO$_4$ was measured\ndown to 50 mK. Using the spin-flop transition to switch on and off the acoustic\nNd magnons, we can reliably separate the magnon and phonon contributions to\nheat transport. We find that magnons travel ballistically below 0.5 K, with a\nthermal conductivity growing as $T^3$, from which we extract their velocity. We\nshow that the rate of scattering of acoustic magnons by phonons grows as $T^3$,\nand the scattering of phonons by magnons peaks at twice the average Nd magnon\nfrequency.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "First Order Premelting Transition of Vortex Lattices: Vortex lattices in the high temperature superconductors undergo a first order\nphase transition which has thus far been regarded as melting from a solid to a\nliquid. We point out an alternative possibility of a two step process in which\nthere is a first order transition from an ordinary vortex lattice to a soft\nvortex solid followed by another first order melting transition from the soft\nvortex solid to a vortex liquid. We focus on the first step. This premelting\ntransition is induced by vacancy and interstitial vortex lines. We obtain good\nagreement with the experimental transition temperature versus field, latent\nheat, and magnetization jumps for YBCO and BSCCO.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Predicted formation of superconducting platinum-hydride crystals under\n  pressure in the presence of molecular hydrogen: Noble metals adopt close-packed structures at ambient pressure and rarely\nundergo structural transformation at high pressures. Platinum (Pt), in\nparticular, is normally considered to be unreactive and is therefore not\nexpected to form hydrides under pressure. We predict that platinum hydride\n(PtH) has a lower enthalpy than its constituents solid Pt and molecular\nhydrogen at pressures above 21.5 GPa. We have calculated structural phase\ntransitions from tetragonal to hexagonal close-packed or face-centered cubic\n(fcc) PtH between 70 and 80 GPa. Linear response calculations indicate that PtH\nis a superconductor at these pressures with a critical temperature of about\n10--25 K. These findings help to shed light on recent observations of\npressure-induced metallization and superconductivity in hydrogen-rich\nmaterials. We show that formation of fcc metal hydrides under pressure is\ncommon among noble metal hydrides and examine the possibility of\nsuperconductivity in these materials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Periodicity in Al/Ti superconducting single electron transistors: We present experiments on single Cooper-pair transistors made of two\ndifferent superconducting materials. We chose Ti and Al to create an energy gap\nprofile such that the island has a higher gap than the leads, thereby acting as\na barrier to quasiparticle tunneling. Our transport measurements demonstrate\nthat quasiparticle poisoning is suppressed in all our TiAlTi structures (higher\ngap for the island) with clear 2e periodicity observed, whereas full\nquasiparticle poisoning is observed in all AlTiAl devices (higher gap for the\nleads) with e periodicity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "A Fully Quantum Mechanical Model of a SQUID Ring Coupled to an\n  Electromagnetic Field: A quantum system comprising of a monochromatic electromagnetic field coupled\nto a SQUID ring with sinusoidal non-linearity, is studied. A magnetostatic flux\n$\\Phi_{x}$ is also threading the SQUID ring, and is used to control the\ncoupling between the two systems. It is shown that for special values of\n$\\Phi_{x}$ the system is strongly coupled. The time evolution of the system is\nstudied. It is shown that exchange of energy takes place between the two modes\nand that the system becomes entangled. A second quasi-classical model that\ntreats the electromagnetic field classically is also studied. A comparison\nbetween the fully quantum mechanical model with the electromagnetic field\ninitially in a coherent state and the quasi-classical model, is made.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Vortices and quasiparticles near the \"superconductor-insulator\"\n  transition in thin films: We study the low temperature behavior of an amorphous superconducting film\ndriven normal by a perpendicular magnetic field (B). For this purpose we\nintroduce a new two-fluid formulation consisting of fermionized field induced\nvortices and electrically neutralized Bogoliubov quasiparticles (spinons)\ninteracting via a long-ranged statistical interaction. This approach allows us\nto access a novel non-Fermi liquid phase which naturally interpolates between\nthe low B superconductor and the high B normal metal. We discuss the transport,\nthermodynamic, and tunneling properties of the resulting \"vortex metal\" phase.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Study of grain boundary transparency in (Yb1-xCax)Ba2Cu3O bi-crystal\n  thin films over a wide temperature, field and field orientation range: The residual low angle grain boundary (GB) network is still the most\nimportant current-limiting mechanism operating in bi-axially textured rare\nearth barium copper oxide (REBCO) coated conductors. While Ca-doping is well\nestablished to improve super-current flow across low angle GBs in weak fields\nat high temperatures, Ca-doping also depresses Tc, making it so far impractical\nfor high temperature applications of REBCO coated conductors. On the other\nhand, high field magnet applications of REBCO require low temperatures. Here we\nsystematically evaluate the effectiveness of Ca-doping in improving the GB\ntransparency, r$^{GB}$= Jc$^{GB}$/Jc$^{grain}$ , of low angle Yb1-xCaxBaCuO\n[001] tilt bi-crystal films down to 10K and with magnetic fields perpendicular\nand parallel to the film surfaces, while varying the Ca and oxygen doping\nlevel. Using Low Temperature Scanning Laser Microscopy (LTSLM) and\nMagneto-Optical Imaging (MOI), we found rGB to strongly depend on the angle\nbetween magnetic field and the GB plane and clearly identified regimes in which\nJc$^{GB}$ can exceed Jc$^{grain}$ (r$^{GB}$>1) where the GB pinning is\noptimized by the field being parallel to the GB dislocations. However, even in\nthis favorable situation, we found that r$^{GB}$ became much smaller at lower\ntemperatures. Calculations of the GB Ca segregation profile predict that the\nhigh Jc channels between the GB dislocation cores are almost Ca-free. It may be\ntherefore that the positive effects of Ca doping seen by many authors near Tc\nare partly a consequence of the higher Tc of these Ca-free channels.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electron-phonon coupling in cuprate and iron-based superconductors\n  revealed by Raman scattering: Electron-phonon coupling (EPC) is one of the most common and fundamental\ninteractions in solids. It not only dominates many basic dynamic processes like\nresistivity, thermal conductivity etc, but also provides the pairing glue in\nconventional superconductors. But in high-temperature superconductors (HTSC),\nit is still controversial whether or not EPC is in favor of paring. Despite the\ncontroversies, many experiments have provided clear evidence for EPC in HTSC.\nIn this paper, we briefly review EPC in cuprate and iron-based superconducting\nsystems revealed by Raman scattering. We introduce how to extract the coupling\ninformation through phonon lineshape. Then we discuss the strength of EPC in\ndifferent HTSC systems and possible factors affecting the strength. The\ncomparative study between Raman phonon theories and experiments allows us to\ngain insight into some crucial electronic properties, especially\nsuperconductivity. Finally we summarize and compare EPC in the two existing\nHTSC systems, and discuss what role it may play in HTSC.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Novel phase diagram for antiferromagnetism and superconductivity in\n  pressure-induced heavy-fermion superconductor Ce$_2$RhIn$_8$ probed by In-NQR: We present a novel phase diagram for the antiferromagnetism and\nsuperconductivity in Ce$_2$RhIn$_8$ probed by In-NQR studies under pressure\n($P$). The quasi-2D character of antiferromagnetic spin fluctuations in the\nparamagnetic state at $P$ = 0 evolves into a 3D character because of the\nsuppression of antiferromagnetic order for $P > P_{\\rm QCP}\\sim$ 1.36 GPa (QCP:\nantiferromagnetic quantum critical point). Nuclear-spin-lattice-relaxation rate\n$1/T_1$ measurements revealed that the superconducting order occurs in the $P$\nrange 1.36 -- 1.84 GPa, with maximum $T_c\\sim$ 0.9 K around $P_{\\rm QCP}\\sim$\n1.36 GPa.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetic Excitations in Strained Infinite-layer Nickelate PrNiO2: Strongly correlated materials often respond sensitively to the external\nperturbations. In the recently discovered superconducting infinite-layer\nnickelates, the superconducting transition temperature can be dramatically\nenhanced via only ~1% compressive strain-tuning enabled by substrate design.\nHowever, the root of such enhancement remains elusive. While the\nsuperconducting pairing mechanism is still not settled, magnetic Cooper pairing\n- similar to the cuprates has been proposed. Using resonant inelastic x-ray\nscattering, we investigate the magnetic excitations in infinite-layer PrNiO2\nthin films for different strain conditions. The magnon bandwidth of PrNiO2\nshows only marginal response to strain-tuning, in sharp contrast to the\nstriking enhancement of the superconducting transition temperature Tc in the\ndoped superconducting samples. These results suggest the enhancement of Tc is\nnot mediated by spin excitations and thus provide important empirics for the\nunderstanding of superconductivity in infinite-layer nickelates.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Response to comment on \"Broken translational and rotational symmetry via\n  charge stripe order in underdoped YBa2Cu3O6+y\": Fine questions our interpretation of unidirectional-stripes over\nbidirectional-checkerboard, and illustrates his criticism by simulating a\nmomentum space structure consistent with our data and corresponding to a\ncheckerboard-looking real space density. Here we use a local\nrotational-symmetry analysis to demonstrate that the simulated image is in\nactuality composed of locally unidirectional modulations of the charge density,\nconsistent with our original conclusions.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Charge-Stripe Order and Superconductivity in $\\mathrm{Ir_{1-x}Pt_xTe_2}$: A combined resistivity and hard x-ray diffraction study of superconductivity\nand charge ordering in $\\mathrm{Ir_{1-x}Pt_xTe_2}$, as a function of Pt\nsubstitution and externally applied hydrostatic pressure, is presented.\nExperiments are focused on samples near the critical composition $x_c\\sim\n0.045$ where competition and switching between charge order and\nsuperconductivity is established. We show that charge order as a function of\npressure in $\\mathrm{Ir_{0.95}Pt_{0.05}Te_{2}}$ is preempted - and hence\ntriggered - by a structural transition. Charge ordering appears uniaxially\nalong the short crystallographic (1,0,1) domain axis with a\n$\\mathrm{(\\frac{1}{5},0,\\frac{1}{5})}$ modulation. Based on these results we\ndraw a charge-order phase diagram and discuss the relation between stripe\nordering and superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Optical conductivity of CuO_2 infinite-layer films: The infrared conductivity of CaCuO_2, SrCuO_{2-y}, and\nSr_{0.85}Nd_{0.15}CuO_2 infinite-layer films is obtained from reflectivity\nmeasurements by taking into account the substrate contribution. SrCuO_{2-y} and\nSr_{0.85}Nd_{0.15}CuO_2 exhibit extra-phonon modes and structured bands in the\nmidinfrared, not found in stoichiometric CaCuO_2. These features mirror those\nobserved in the perovskitic cuprates, thus showing that the polaronic\nproperties of high-T_c superconductors are intrinsic to the CuO_2 planes.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Annealing, acid, and alcoholic beverage effects on Fe1+yTe0.6Se0.4: We have systematically investigated and compared different methods to induce\nsuperconductivity in iron chalcogenide Fe1+yTe0.6Se0.4 including annealing in\nvacuum, N2, O2, I2 atmosphere, and immersing samples into acid and alcoholic\nbeverages. Vacuum and N2 annealing are proved to be ineffective to induce\nsuperconductivity in Fe1+yTe0.6Se0.4 single crystal. O2 and I2 annealing, acid\nand alcoholic beverages can induce superconductivity by oxidizing the excess Fe\nin the sample. Superconductivity in O2 annealed sample is in bulk nature, while\nI2, acid and alcoholic beverages can only induce superconductivity near the\nsurface. By comparing different effects of O2, I2, acid and alcoholic\nbeverages, we propose a scenario to explain how the superconductivity is\ninduced in the non-superconducting as-grown Fe1+yTe0.6Se0.4.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Multiple nodeless superconducting gaps in noncentrosymmetric\n  superconductor PbTaSe2 with topological bulk nodal lines: Low-temperature thermal conductivity measurements were performed on single\ncrystal of PbTaSe$_2$, a noncentrosymmetric superconductor with topological\nbulk nodal lines in the electronic band structure. It is found that the\nresidual linear term $\\kappa_0/T$ is negligible in zero magnetic field.\nFurthermore, the field dependence of $\\kappa_0/T$ exhibits a clear \"$S$\"-shape\ncurve. These results suggest that PbTaSe$_2$ has multiple nodeless\nsuperconducting gaps. Therefore, the spin-triplet state with gap nodes does not\nplay an important role in this noncentrosymmetric superconductor with strong\nspin-orbital coupling. The fully gapped superconducting state also meets the\nrequirement of a topological superconductor, if PbTaSe$_2$ is indeed the case.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Tunneling studies in a homogeneously disordered s-wave superconductor:\n  NbN: We report the evolution of superconducting properties as a function of\ndisorder in homogeneously disordered epitaxial NbN thin films grown on (100)\nMgO substrates, studied through a combination of electrical transport, Hall\nEffect and tunneling measurements. The thickness of all our films are >50nm\nmuch larger than the coherence length ~5nm. The effective disorder in different\nfilms encompasses a large range, with the Ioffe-Regel parameter varying in the\nrange kFl~1.38-8.77. Tunneling measurements on films with different disorder\nreveals that for films with large disorder the bulk superconducting transition\ntemperature (Tc) is not associated with a vanishing of the superconducting\nenergy gap, but rather a large broadening of the superconducting density of\nstates. Our results provide strong evidence of the loss of superconductivity\nvia phase-fluctuations in a disordered s-wave superconductor.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electronic Structure, Electron-Phonon Coupling, and Multiband Effects in\n  MgB2: We review the current situation in the theory of superconducting and\ntransport properties of MgB2. First principle calculations of of the electronic\nstructure and electron-phonon coupling are discussed and compared with the\nexperiment. We also present a brief description of the multiband effects in\nsuperconductivity and transport, and how these manifest themselves in MgB2. We\nalso mention some yet open questions.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Odd-frequency pairing and Ising spin susceptibility in time-reversal\n  invariant superfluids and superconductors: We here examine the relation between odd-frequency spin-triple even-parity\n(OTE) Cooper pairs and anomalous surface magnetic response in time-reversal\ninvariant (TRI) spin-triplet superfluids and superconductors. The spin\nsusceptibility generally consists of two contributions: Even-frequency\nodd-parity pair amplitudes and odd-frequency even-parity pair amplitudes. The\nOTE pair amplitudes are absent in the bulk region, but ubiquitously exist in\nthe surface and interface region as Andreev bound states. We here clarify that\nadditional discrete symmetries, originating from the internal symmetry and\npoint group symmetry, impose strong constraint on the OTE pair amplitudes\nemergent in the surface of TRI superfluids and superconductors. As a result of\nthe symmetry constraint, the magnetic response of the OTE pairs yields\nIsing-like anisotropy. For the topological phase of the $^3$He-B in a\nrestricted geometry, the coupling of the OTE pair amplitudes to an applied\nfield is prohibited by an additional discrete symmetry. Once the discrete\nsymmetry is broken, however, the OTE pairs start to couple to the applied\nfield, which anomalously enhances surface spin susceptibility. Furthermore, we\nextend this theory to TRI superconductors, where the corresponding discrete\nsymmetry is the mirror reflection symmetry.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "On the Excitations of a Balian-Werthamer Superconductor: My contribution to this collection of articles in honor of David Lee and John\nReppy on their 90th birthdays is a reflection on the remarkable phenomenology\nof the excitation spectra of superfluid $^3$He, in particular the B-phase which\nwas identified by NMR and acoustic spectroscopy as Balian-Werthamer state shown\nin 1963 to be the ground state of a spin-triplet, p-wave superconductor within\nweak-coupling BCS theory. The superfluid phases of $^3$He provide paradigms for\nelectronic superconductors with broken space-time symmetries and non-trivial\nground-state topology. Indeed broken spin- and orbital rotation symmetries lead\nto a rich spectrum of collective modes of the order parameter that can be\ndetected using NMR, acoustic and microwave spectroscopies. The topology of the\nBW state implies its low-temperature, low-energy transport properties are\ndominated by gapless Majorana modes confined on boundaries or interfaces. Given\nthe central role the BW state played I discuss the acoustic and electromagnetic\nsignatures of the BW state, the latter being relevant if an electronic analog\nof superfluid $^3$He-B is realized.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Fully dense MgB_2 superconductor textured by hot deformation: Bulk textured MgB_2 material of nearly full density showing a weak c-axis\nalignment of the hexagonal MgB_2 grains parallel to the pressure direction was\nobtained by hot deformation of a stoichiometric MgB_2 pellet prepared by a\ngas-solid reaction. The texture of the material was verified by comparing the\nx-ray diffraction patterns of the hot deformed material with isotropic MgB_2\npowder. A small, but distinct anisotropy of the upper critical field up to\nHc2^{a,b}/Hc2^{c}~1.2 depending on degree of texture was found by resistance\nand susceptibility measurements. No anisotropy of the critical current density\ndetermined from magnetization measurements was found for the textured material.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Critical Current Peaks at $3B_\u03a6$ in Superconductors with Columnar\n  Defects: Recrystalizing the Interstitial Glass: The role of commensurability and the interplay of correlated disorder and\ninteractions on vortex dynamics in the presence of columnar pins is studied via\nmolecular dynamics simulations. Simulations of dynamics reveal substantial\ncaging effects and a non-monotonic dependence of the critical current with\nenhancements near integer values of the matching field $B_{\\phi}$ and\n$3B_{\\phi}$ in agreement with experiments on the cuprates. We find qualitative\ndifferences in the phase diagram for small and large values of the matching\nfield.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The Garber Current Pattern: An Additional Contribution to AC Losses in\n  Helical HTS Cables?: Conductors made of high-temperature (HTS) wires helically wound in one or\nmore layers on round tubes (CORT) are compact, flexible, and can carry a large\namount of current. Although these conductors were initially developed for DC\napplications, e.g. in magnets, it is worth considering their use for AC, e.g.\nin underground cables for medium voltage grids and with currents in the\nkA-range. In these cases, the major challenge is reducing AC losses. In\ncontrast to a straight superconducting wire, in a helical arrangement, due to\nsuperconducting shielding, the current does not follow the direction of the\nwires, but takes a non-trivial zig-zag path within the individual HTS wires\n(Garber pattern). This includes current components across the thickness of the\nsuperconducting layers, so that the often used thin-shell approximation does\nnot hold. In this contribution, we studied a one-layer three-wire CORT by means\nof fully three-dimensional simulations, based on the H-formulation of Maxwell's\nequations implemented in the commercial software package COMSOL Multiphysics.\nAs a result of our simulations, the peculiar current profiles were confirmed.\nIn addition, the influence of current, pitch angle, and frequency on the AC\nlosses was studied. We found an optimum for the pitch angle and that the\ncurrent profiles strongly depend on frequency.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The Thermodynamics and the Inverse Isotope Effect of superconducting PdH\n  and PdD under pressure: We present in this paper the thermodynamics of superconducting PdH and PdD\nunder pressure. We make use of a general method to calculate the thermodynamics\nunder pressure within the Migdal-Eliashberg theory. We have considered the\ncrystal lattice to be zincblende taking into account the experimental evidence\nfor both PdH and PdD at temperatures below 55 K. We have studied, in\nparticular, the changes induced by pressure in the critical temperature, $T_c$,\nin the specific heat jump at $T_c$, in the energy gap at $T=0K$, in the\ndeviation function $D(t)$ and in the isotope effect coefficient, $\\alpha$. We\nget a very good agreement with experiment where this data exist. This method\nrepresents a basis on which the thermodynamics of other hydrides under pressure\ncan be calculated.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Encapsulating high-temperature superconducting twisted van der Waals\n  heterostructures blocks detrimental effects of disorder: High-temperature cuprate superconductors-based van der Waals (vdW)\nheterostructures hold high technological promise. One of the obstacles\nhindering progress is the detrimental effect of disorder on the properties of\nJosephson junctions (JJ) realized by the vdW devices. Here we report the new\nmethod of fabricating twisted vdW heterostructures made of Bi2Sr2CuCa2O8+d,\ncrucially improving the JJ characteristics, pushing them up to those of the\nintrinsic JJs in bulk samples. The method combines a cryogenic stacking using a\nsolvent-free stencil mask technique and covering interface by the insulating\nhexagonal boron nitride crystals. Despite the high-vacuum condition down to\n10-6 mbar in the evaporation chamber, the interface appears to be protected\nfrom water molecules during the in-situ metal deposition only when fully\nencapsulated. Comparing the current-voltage curves of encapsulated and\nunencapsulated interfaces, we reveal that the encapsulated interfaces'\ncharacteristics are crucially improved so that the corresponding JJs\ndemonstrate high critical currents and sharpness of the superconducting\ntransition comparable to those of the intrinsic JJs. Finally, we show that the\nencapsulated heterostructures are more stable in time.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Ab-initio Studies of (Li$_{0.8}$Fe$_{0.2}$)OHFeSe Superconductors:\n  Revealing the Dual Roles of Fe$_{0.2}$ in Structural Stability and Charge\n  Transfer: The recently discovered (Li$_{0.8}$Fe$_{0.2}$)OHFeSe superconductor provides\na new platform for exploiting the microscopic mechanisms of high-$T_c$\nsuperconductivity in FeSe-derived systems. Using density functional theory\ncalculations, we first show that substitution of Li by Fe not only\nsignificantly strengthens the attraction between the (Li$_{0.8}$Fe$_{0.2}$)OH\nspacing layers and the FeSe superconducting layers along the \\emph{c} axis, but\nalso minimizes the lattice mismatch between the two in the \\emph{ab} plane,\nboth favorable for stabilizing the overall structure. Next we explore the\nelectron injection into FeSe from the spacing layers, and unambiguously\nidentify the Fe$_{0.2}$ components to be the dominant atomic origin of the\ndramatically enhanced interlayer charge transfer. We further reveal that the\nsystem strongly favors collinear antiferromagnetic ordering in the FeSe layers,\nbut the spacing layers can be either antiferromagnetic or ferromagnetic\ndepending on the Fe$_{0.2}$ spatial distribution. Based on these\nunderstandings, we also predict (Li$_{0.8}$Co$_{0.2}$)OHFeSe to be structurally\nstable with even larger electron injection and potentially higher $T_c$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "$\u03b7$ collective mode as A$_{1g}$ Raman resonance in cuprate\n  superconductors: We discuss the possible existence a spin singlet excitation with charge\n$\\pm2$ ($\\eta$-mode) originating the $A_{1g}$ Raman resonance in cuprate\nsuperconductors. This $\\eta$-mode relates the $d$-wave superconducting singlet\npairing channel to a $d$-wave charge channel. We show that the $\\eta$ boson\nforms a particle-particle bound state below the $2\\Delta$ threshold of the\nparticle-hole continuum where $\\Delta$ is the maximum $d$-wave gap. Within a\ngeneralized random phase approximation and Bethe-Salpether approximation study,\nwe find that this mode has energies similar to the resonance observed by\nInelastic Neutron Scattering (INS) below the superconducting (SC) coherent peak\nat $2\\Delta$ in various SC cuprates compounds. We show that it is a very good\ncandidate for the resonance observed in Raman scattering below the $2\\Delta$\npeak in the $A_{1g}$ symmetry. Since the $\\eta$-mode sits in the $S=0$ channel,\nit may be observable via Raman, X -ray or Electron Energy Loss Spectroscopy\nprobes.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Anomalous metallic state above the upper critical field of the\n  conventional three-dimensional superconductor AgSnSe2 with strong intrinsic\n  disorder: We report superconducting properties of AgSnSe2 which is a conventional\ntype-II superconductor in the very dirty limit due to intrinsically strong\nelectron scatterings. While this material is an isotropic three-dimensional\n(3D) superconductor with a not-so-short coherence length where strong vortex\nfluctuations are NOT expected, we found that the magnetic-field-induced\nresistive transition at fixed temperatures becomes increasingly broader toward\nzero temperature and, surprisingly, that this broadened transition is taking\nplace largely ABOVE the upper critical field determined thermodynamically from\nthe specific heat. This result points to the existence of an anomalous metallic\nstate possibly caused by quantum phase fluctuations in a strongly-disordered 3D\nsuperconductor.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Induced ferromagnetic moment at the interface between epitaxial\n  SrRuO$_3$ film and Sr$_2$RuO$_4$ single crystal: SrRuO$_3$ (SRO113) is an important material for device physics particularly\nas one of the best metallic oxide electrodes for ferroelectric devices. This\noxide has moderate electron correlations with novel properties including\nferromagnetic ordering, which can be utilized in future to spintronics and\nsuperconducting spintronics devices. Recently, we observed strongly enhanced\nmagnetization of SRO113 thin films grown on single crystals of the spin-triplet\nsuperconductor Sr$_2$RuO$_4$ (SRO214). To clarify the origin of such an\nenhancement, we conducted systematic investigations of magnetic properties of\nSRO113 films deposited on a variety of oxide substrates. We carefully\nsubtracted the substrate contributions and found that the enhanced 2\nmagnetization occurs only for SRO113/SRO214 films. We further found that\nneither strain nor metallicity of the substrate plays any significant roles in\nthe enhancement. The X-ray magnetic circular dichroism reveals that the\nsubstrate-induced strain does not switch the Ru4+ state from the low-spin to\nhigh-spin states. The film-thickness dependence of the magnetization of\nSRO113/SRO214 films strongly suggest that the additional magnetization arises\ndue to the induction of magnetic moment into the SRO214 substrate over 20-nm\ndepth. Our results imply new magnetic functionality that can trigger studies\nsearching for yet unknown physical phenomena in magnetic ruthenates.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconducting Quantum Interference at the Atomic Scale: A single spin in a Josephson junction can reverse the flow of the\nsupercurrent. At mesoscopic length scales, such $\\pi$-junctions are employed in\nvarious instances from finding the pairing symmetry to quantum computing. In\nYu-Shiba-Rusinov (YSR) states, the atomic scale counterpart of a single spin in\na superconducting tunnel junction, the supercurrent reversal so far has\nremained elusive. Using scanning tunneling microscopy (STM), we demonstrate\nsuch a 0 to $\\pi$ transition of a Josephson junction through a YSR state as we\ncontinuously change the impurity-superconductor coupling. We detect the sign\nchange in the critical current by exploiting a second transport channel as\nreference in analogy to a superconducting quantum interference device (SQUID),\nwhich provides the STM with the required phase sensitivity. The measured change\nin the Josephson current is a signature of the quantum phase transition and\nallows its characterization with unprecedented resolution.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Comment on \"Quantum Melting of the Quasi-Two-Dimensional Vortex Lattice\n  in $\u03ba-(ET)_2Cu(NCS)_2$\": In a recent Letter Mola et al. \\cite{mola} reported magnetization\nmeasurements $M(H,\\theta)$ performed on the organic superconductor\n$\\kappa-$(ET)$_2$Cu(NCS)$_2 (T_c = 9.1$ K) as a function of the magnetic field\n$H$ applied at different angles $\\theta$ with respect to the $a$-axis\ndirection. The results \\cite{mola} demonstrate:\n  (a) the occurrence of pronounced irreversible magnetization $M_{\\rm irr}(H)$\njumps and (b) their sudden cessation for $H \\ge H_m(T,\\theta)$. The boundary\nline $H_m(T)$ has been interpreted by Mola et al. as the Q2D vortex lattice\n(VL) quantum melting phase transition line \\cite{mola}. The purpose of this\ncomment is to show that the results can be understood in a simple way without\ninvoking \"quantum melting phase transition\".",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Exactly solvable pairing models in two dimensions: The BCS theory models electron correlations with pure zero-momentum pairs.\nHere we consider a family of pairing Hamiltonians, where the electron\ncorrelations are modelled with pure arbitrary-momentum pairs. We find all\nmodels in the family are exactly solvable, and present these solutions. It is\ninteresting to note that the $\\eta$ pair or the $d$ -wave pair condensate in\n$T_{c}$ superconductivity can be the ground state of a Hamiltonian in the\nfamily. These models are two-dimensional because only the z-component of the\ntotal electron spin $S_z$ is conserved. Significantly, for the $\\eta$ pair or\n$d$ -wave pairing model in the family we find an analytical expression of\nenergy and an abrupt ground state change from independent particle state to the\n$d$ -wave pair condensate, suggesting a quantum phase transition.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Single Crystal Growth of Cuprate Superconductor\n  (Lu$_{0.8}$Nd$_{0.2}$)Ba$_2$Cu$_4$O$_8$ by KOH Flux Method: Single crystals of Nd-substituted LuBa$_2$Cu$_4$O$_8$ were successfully grown\nby the KOH flux method. The single phase of Lu$_{1-x}$Nd$_x$Ba$_2$Cu$_4$O$_8$\n[(Lu,Nd)124] formed at $x = 0.2$. The compound crystallized as orthorhombic\nAmmm structure with lattice constants of a = 3.835(3) {\\AA}, b = 3.879(2) {\\AA}\nand c = 27.195(6) {\\AA}. Single crystal structural analysis demonstrated that\nthe Nd ion partially occupied the Lu site. The (Lu,Nd)124 exhibited the\nsuperconducting transition at ~75 K in the magnetic susceptibility and\nresistivity. The $T_c$ and the c-axis of the (Lu,Nd)124 corresponded to Tm124.\nThese results indicate that RE124 equivalent to the one composed of single RE\nelement is obtained by combination of two RE elements.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Observation of novel gapped phases in potassium doped single layer\n  p-terphenyl on Au (111): Recently, superconductivity in potassium (K) doped p-terphenyl (C18H14) has\nbeen suggested by the possible observation of the Meissner effect and\nsubsequent photoemission spectroscopy measurements, but the detailed lattice\nstructure and more-direct evidence are still lacking. Here we report a low\ntemperature scanning tunneling microscopy/spectroscopy (STM/STS) study on\nK-doped single layer p-terphenyl films grown on Au (111). We observe several\nordered phases with different morphologies and electronic behaviors, in two of\nwhich a sharp and symmetric low-energy gap of about 11 meV opens below 50 K. In\nparticular, the gap shows no obvious response to a magnetic field up to 11\nTesla, which would caution against superconductivity as an interpretation in\nprevious reports of K-doped p-terphenyl materials. Such gapped phases are\nrarely (if ever) observed in single layer hydrocarbon molecular crystals. Our\nwork also paves the way for fabricating doped two-dimensional (2D) hydrocarbon\nmaterials, which will provide a platform to search for novel emergent\nphenomena.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The Effects of d_{x^2-y^2}-d_{xy} Mixing on Vortex Structures and\n  Magnetization: The structure of an isolated single vortex and the vortex lattice, and the\nmagnetization in a $d$-wave superconductor are investigated within a\nphenomenological Ginzburg-Landau (GL) model including the mixture of the\n$d_{x^2-y^2}$-wave and $d_{xy}$-wave symmetry. The isolated single vortex\nstructure in a week magnetic field is studied both numerically and\nasymptotically. Near the upper critical field $H_{c2}$, the vortex lattice\nstructure and the magnetization are calculated analytically.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Interplay between kinetic inductance, non-linearity and quasiparticle\n  dynamics in granular aluminum MKIDs: Microwave kinetic inductance detectors (MKIDs) are thin film, cryogenic,\nsuperconducting resonators. Incident Cooper pair-breaking radiation increases\ntheir kinetic inductance, thereby measurably lowering their resonant frequency.\nFor a given resonant frequency, the highest MKID responsivity is obtained by\nmaximizing the kinetic inductance fraction $\\alpha$. However, in circuits with\n$\\alpha$ close to unity, the low supercurrent density reduces the maximum\nnumber of readout photons before bifurcation due to self-Kerr non-linearity,\ntherefore setting a bound for the maximum $\\alpha$ before the noise equivalent\npower (NEP) starts to increase. By fabricating granular aluminum MKIDs with\ndifferent resistivities, we effectively sweep their kinetic inductance from\ntens to several hundreds of pH per square. We find a NEP minimum in the range\nof $25\\; \\text{aW}/\\sqrt{\\text{Hz}}$ at $\\alpha \\approx 0.9$, which results\nfrom a tradeoff between the onset of non-linearity and a non-monotonic\ndependence of the noise spectral density vs. resistivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Multimodal synchrotron X-ray diffraction across the superconducting\n  transition of Sr$_{0.1}$Bi$_2$Se$_3$: In the doped topological insulator Sr$_x$Bi$_2$Se$_3$, a pronounced in-plane\ntwo-fold symmetry is observed in electronic properties below the\nsuperconducting transition temperature $T_c \\sim$ 3 K, despite the three-fold\nsymmetry of the observed $R\\bar{3}m$ space group. The axis of two-fold symmetry\nis nominally pinned to one of three rotational equivalent directions and\ncrystallographic strain has been proposed to be the origin of this pinning. We\ncarried out multimodal synchrotron diffraction and resistivity measurements\ndown to $\\sim$0.68 K and in magnetic fields up to 45 kG on a single crystal of\nSr$_{0.1}$Bi$_2$Se$_3$ to probe the effect of superconductivity on the\ncrystallographic distortion. Our results indicate that there is no in-plane\ncrystallographic distortion at the level of $1x10^{-5}$ associated with the\nsuperconducting transition. These results further support the model that the\nlarge two-fold in-plane anisotropy of superconducting properties of\nSr$_x$Bi$_2$Sr$_3$ is not structural in origin but electronic, namely it is\ncaused by a nematic superconducting order parameter of Eu symmetry.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity-insensitive order at $q$~1/4 in electron doped\n  cuprates: One of the central questions in the cuprate research is the nature of the\n\"normal state\" which develops into high temperature superconductivity (HTSC).\nIn the normal state of hole-doped cuprates, the existence of charge density\nwave (CDW) is expected to shed light on the mechanism of HTSC. With evidence\nemerging for CDW order in the electron-doped cuprates, the CDW would be thought\nto be a universal phenomenon in high-$T_c$ cuprates. However, the CDW phenomena\nin electron-doped cuprate are quite different than those in hole-doped\ncuprates. Here we study the nature of the putative CDW in an electron-doped\ncuprate through direct comparisons between as-grown and post-annealed\nNd$_{1.86}$Ce$_{0.14}$CuO$_4$ (NCCO) single crystals using Cu $L_3$-edge\nresonant soft x-ray scattering (RSXS) and angle resolved photoemission\nspectroscopy (ARPES). The RSXS result reveals that the non-superconducting NCCO\nshows the same reflections at the wavevector (~1/4, 0, $l$) as like the\nreported superconducting NCCO. This superconductivity-insensitive signal is\nquite different with the characteristics of the CDW reflection in hole-doped\ncuprates. Moreover, the ARPES result suggests that the fermiology cannot\naccount for such wavevector. These results call into question the universality\nof CDW phenomenon in the cuprates.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The c axis optical conductivity of layered systems in the\n  superconducting state: In this paper, we discuss the c axis optical conductivity Re [sigma_c(omega)]\nin the high T_c superconductors, in the superconducting state. The basic\npremise of this work is that electrons travelling along the c axis between\nadjacent CuO_2 layers must pass through several intervening layers. In earlier\nwork we found that, for weak inter-layer coupling, it is preferable for\nelectrons to travel along the c axis by making a series of interband\ntransitions rather than to stay within a single (and very narrow) band.\nMoreover, we found that many of the properties of the normal state optical\nconductivity, including the pseudogap could be explained by interband\ntransitions. In this work we examine the effect of superconductivity on the\ninterband conductivity. We find that, while the onset of superconductivity is\nclearly evident in the spectrum, there is no clear signature of the symmetry of\nthe superconducting order parameter.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Group-9 Transition Metal Suboxides Adopting the Filled-Ti$_2$Ni\n  Structure: A Class of Superconductors Exhibiting Exceptionally High Upper\n  Critical Fields: The Ti$_2$Ni and the related $\\eta$-carbide structure are known to exhibit\nvarious intriguing physical properties. The Ti$_2$Ni structure with the cubic\nspace group $Fd\\bar{3}m$ is surprisingly complex, consisting of a unit cell\nwith 96 metal atoms. The related $\\eta$-carbide compounds correspond to a\nfilled version of the Ti$_2$Ni structure. Here, we report on the structure and\nsuperconductivity in the $\\eta$-carbide type suboxides Ti$_4$M$_2$O with M =\nCo, Rh, Ir. We have successfully synthesized all three compounds in single\nphase form. We find all three compounds to be type-II bulk superconductors with\ntransition temperatures of $T_{\\rm c}$ = 2.7, 2.8, and 5.4 K, and with\nnormalized specific heat jumps of $\\Delta C/\\gamma T_{\\rm c}$ = 1.65, 1.28, and\n1.80 for Ti$_4$Co$_2$O, Ti$_4$Rh$_2$O, and Ti$_4$Ir$_2$O, respectively. We find\nthat all three superconductors, exhibit high upper-critical fields.\nParticularly noteworthy is Ti$_4$Ir$_2$O with an upper critical field of $\\mu_0\nH_{\\rm c2}{\\rm (0)}$ =~16.06~T, which exceeds by far the weak-coupling Pauli\nlimit of 9.86~T. The role of the void filling light atom X has so far been\nuncertain for the overall physical properties of these materials. Herein, we\nhave successfully grown single crystals of Ti$_2$Co. In contrast to the\nmetallic $\\eta$-carbide type suboxides Ti$_4$M$_2$O, we find that Ti$_2$Co\ndisplays a semimetallic behavior. Hence, the octahedral void-filling oxygen\nplays a crucial role for the overall physical properties, even though its\neffect on the crystal structure is small. Our results indicate that the design\nof new superconductors by incorporation of electron-acceptor atoms may in the\nTi$_2$Ni-type structures and other materials with crystallographic void\nposition be a promising future approach. The remarkably high upper critical\nfields, in this family of compounds, may furthermore spark significant future\ninterest.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Materials Informatics Reveals Unexplored Structure Space in Cuprate\n  Superconductors: High-temperature superconducting cuprates have the potential to be\ntransformative in a wide range of energy applications. In this work we analyse\nthe corpus of historical data about cuprates using materials informatics and\nre-examine how their structures are related to their critical temperatures\n(Tc). The available data is highly clustered and no single database contains\nall the features of interest to properly examine trends. To work around these\nissues we employ a linear calibration approach that allows us to utilise\nmultiple data sources -- combining fine resolution data for which the Tc is\nunknown with coarse resolution data where it is known. The hybrid data set\nconstructed enables us to explore the trends in Tc with the apical and in-plane\ncopper-oxygen distances. We show that large regions of the materials space have\nyet to be explored and highlight how novel experiments relying on\nnano-engineering of the crystal structure may enable us to explore such new\nregions. Based on the trends identified we propose that single layer Bi-based\ncuprates are good candidate systems for such experiments.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "High-Temperature Superconductivity Mechanism for Cuprates: Egorov and March plotted the product of resistivity and the copper\nspin-lattice relaxation time vs. temperature for yttrium barium copper oxide\nfinding a minimum at temperature T greater than the superconducting\ntemperature, heralding an electronic phase change which can be interpreted as\nthe formation of a \"preformed\" pair. In this context we propose a\nsuperconducting mechanism based on the notion that the preformed pair is a\n\"soft\" boson (a localized, different type of Cooper pair) which dissociates\nabove the classical BKT transition temperature, resulting in two circular\ncharge density waves. The model suggests explanations for considerable\nexperimental work and offers a physical explanation for the basis of the\nUemura-Homes law.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "STM/STS study on large pseudogap and nodal superconducting gap in\n  Bi2201(La) and Bi2212: In the present work, scanning tunneling microscopy/spectroscopy (STM/STS)\nmeasurements were carried out on underdoped $\\rm Bi_2Sr_{2-{\\it x}}La_{\\it\nx}CuO_{6+\\delta}$ and $\\rm Bi_2Sr_2CaCu_2O_{8+\\delta}$ to clarify the origin of\nthe pseudogap, in particular, the inhomogeneous large pseudogap. The nodal part\nof a d-wave pairing gap, which is under no influence of the inhomogeneous large\npseudogap, was also examined by relating the homogeneous bottom part of the STS\ngap to a nodal d-wave gap in momentum space. We report that the inhomogeneous\nlarge pseudogap in the antinodal region links to a two-dimensional electronic\ncharge order, and that the gap size of the nodal d-wave part $\\rm\n{\\Delta}_{sc}$ scales with the superconducting critical temperature $\\rm {\\it\nT}_c$ in the pseudogap regime.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Ginzburg-Landau expansion and the upper critical field in disordered\n  attractive Hubbard model: We present a short review of our studies of disorder influence upon Ginzburg\n- Landau expansion coefficients in Anderson - Hubbard model with attraction in\nthe framework of the generalized DMFT+$\\Sigma$ approximation. A wide range of\nattractive potentials $U$ is considered - from weak coupling limit, where\nsuperconductivity is described by BCS model, to the limit of very strong\ncoupling, where superconducting transition is related to Bose - Einstein\ncondensation (BEC) of compact Cooper pairs, which are formed at temperatures\nsignificantly higher than the temperature of superconducting transition, as\nwell as the wide range of disorders - from weak to strong, when the system is\nin the vicinity of Anderson transition. For the same range of parameters we\nstudy in detail the temperature behavior of orbital and paramagnetic upper\ncritical field $H_{c2}(T)$, which demonstrates the anomalies both due to the\ngrowth of attractive potential and the effects of strong disordering.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pairing mechanism in Fe pnictide superconductors: By applying an exact unitary transformation to a two-band hamiltonian which\nalso includes the effects due to large pnictogen polarizabilities, we show that\nan attractive spin-mediated Hubbard term appears in the $d_{xz}$, $d_{yz}$\nnearest-neighbour channel. This pairing mechanism implies a singlet\nsuperconducting order parameter in iron pnictides.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Comment on the paper by D. Efremov and Yu.N. Ovchinnikov \"Singular\n  ground state of multiband inhomogeneous superconductors\", Phys. Rev. B 99,\n  224508 (2019): We show that the conclusion reported in Ref. 1, that there are no spontaneous\nmagnetic fields in multiband superconductors that break time reversal symmetry,\nis incorrect. We demonstrate that the state proposed in Ref. 1 is not a\nsolution of the Ginzburg-Landau equations for the considered model. The reason\nis that in Ref. 1 one of the Ginzburg-Landau equations is neglected and\nsubstituted by the spurious zero current restriction. This restriction together\nwith all of the Ginzburg-Landau equations leads to an overdetermined system\nwhich does not have a solution. This inconsistency invalidates all the results\nof the paper.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Signatures of a pair density wave at high magnetic fields in cuprates\n  with charge and spin orders: In underdoped cuprates, the interplay of the pseudogap, superconductivity,\nand charge and spin ordering can give rise to exotic quantum states, including\nthe pair density wave (PDW), in which the superconducting (SC) order parameter\nis oscillatory in space. However, the evidence for a PDW state remains\ninconclusive and its broader relevance to cuprate physics is an open question.\nTo test the interlayer frustration, the crucial component of the PDW picture,\nwe performed transport measurements on La$_{1.7}$Eu$_{0.2}$Sr$_{0.1}$CuO$_{4}$\nand La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_{4}$, cuprates with \"striped\" spin and\ncharge orders, in perpendicular magnetic fields ($H_\\perp$), and also with an\nadditional field applied parallel to CuO$_2$ layers ($H_\\parallel$). We\ndetected several phenomena predicted to arise from the existence of a PDW,\nincluding an enhancement of interlayer SC phase coherence with increasing\n$H_\\parallel$. Our findings are consistent with the presence of local, PDW\npairing correlations that compete with the uniform SC order at $T_{c}^{0}<\nT<(2-6) T_{c}^{0}$, where $T_{c}^{0}$ is the $H=0$ SC transition temperature,\nand become dominant at intermediate $H_\\perp$ as $T\\rightarrow 0$. These data\nalso provide much-needed transport signatures of the PDW in the regime where\nsuperconductivity is destroyed by quantum phase fluctuations.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Measuring the Localization Length through the superconductor-insulator\n  transition in ultrathin amorphous beryllium films: Electron transport and tunneling across the superconductor-insulator (SI)\ntransition have been measured simultaneously for quench-condensed ultrathin\namorphous beryllium films. The anomalous negative magnetoresistance previously\nobserved in insulating films disappears when Mn impurities are introduced to\nthe films, restoring a rather clean Efros-Shklovskii type hopping behavior. The\ncombination of transport and tunneling data allows us to determine,\nindependently and up to a constant on the order of unity, the localization\nlength, \\xi_{L}, and the dielectric constant, \\kappa, for the films. As the\nnormal-state sheet resistance of the films at 20 K is reduced with increasing\nfilm thickness, \\xi_{L} increases exponentially. The SI transition occurs when\n\\xi_{L} crosses the Ginzburg-Landau coherence length, \\xi_{S}.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetic resonance peak and nonmagnetic impurities: Nonmagnetic Zn impurities are known to strongly suppress superconductivity.\nWe review their effects on the spin excitation spectrum in $\\rm\nYBa_2Cu_3O_{7}$, as investigated by inelastic neutron scattering measurements.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pressure effects on the electronic structure and superconductivity of\n  (TaNb)$_{0.67}$(HfZrTi)$_{0.33}$ high entropy alloy: Effects of pressure on the electronic structure, electron-phonon interaction,\nand superconductivity of the high entropy alloy\n(TaNb)$_{0.67}$(HfZrTi)$_{0.33}$ are studied in the pressure range 0 - 100 GPa.\nThe electronic structure is calculated using the Korringa-Kohn-Rostoker method\nwith the coherent potential approximation. Effects of pressure on the lattice\ndynamics are simulated using the Debye-Gr\\\"{u}neisen model and the\nGr\\\"{u}neisen parameter at ambient conditions. In addition, the Debye\ntemperature and Sommerfeld electronic heat capacity coefficient were\nexperimentally determined. The electron-phonon coupling parameter $\\lambda$ is\ncalculated using the McMillan-Hopfield parameters and computed within the rigid\nmuffin tin approximation. We find, that the system undergoes the Lifshitz\ntransition, as one of the bands crosses the Fermi level at elevated pressures.\nThe electron-phonon coupling parameter $\\lambda$ decreases above 10 GPa. The\ncalculated superconducting $T_c$ increases up to 40 - 50 GPa and, later, is\nstabilized at the larger value than for the ambient conditions, in agreement\nwith the experimental findings. Our results show that the experimentally\nobserved evolution of $T_c$ with pressure in (TaNb)$_{0.67}$(HfZrTi)$_{0.33}$\ncan be well explained by the classical electron-phonon mechanism.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Positive current cross-correlations in a highly transparent\n  normal-superconducting beam splitter due to synchronized Andreev and inverse\n  Andreev reflections: Predictions are established for linear differential current-current\ncross-correlations dSab/dV in a symmetrically biased three-terminal normal\nmetal-superconductor-normal metal (NSN) device. Highly transparent contacts\nturn out to be especially interesting because they feature positive dSab/dV. At\nhigh transparency, processes based on Crossed Andreev Reflection (CAR)\ncontribute only negligibly to the current and to dSab/dV. Under these\ncircumstances, current-current cross-correlations can be plausibly interpreted\nas a coherent coupling between the two NS interfaces in the form of\nsynchronized Andreev and inverse Andreev reflections, corresponding to the\nprocess where a pair of electron-like quasi-particles and a pair of hole-like\nquasi-particles arrive from the normal electrodes and annihilate in the\nsuperconductor. Hence, positive dSab/dV does not automatically imply CAR. For\ntunnel contacts, dSab/dV is positive because of CAR. In between these two\nextremities, at intermediate transparencies, dSab/dV is negative because both\nprocesses which cause positive correlations, occur only with small amplitude.\nWe use scattering theory to obtain analytic expressions for current and noise,\nand microscopic calculation using a tight binding model in order to obtain a\nclear interpretation of the physical processes.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Bosonic spectral density of epitaxial thin-film La1.83Sr0.17CuO4\n  superconductors from infrared conductivity measurements: We use optical spectroscopy to investigate the excitations responsible for\nthe structure in the optical self-energy of thin epitaxial films of\nLa1.83Sr0.17CuO4. Using Eliashberg formalism to invert the optical spectra we\nextract the electron-boson spectral function and find that at low temperature\nit has a two component structure closely matching the spin excitation spectrum\nrecently measured by magnetic neutron scattering. We contrast the temperature\nevolution of the spectral density and the two-peak behavior in La2-xSrxCuO4\nwith another high temperature superconductor Bi2Sr2CaCu2O8+d. The bosonic\nspectral functions of the two materials account for the low Tc of LSCO as\ncompared to Bi-2212.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Theory of heterotic SIS Josephson junctions between single- and\n  multi-gap superconductors: Using the functional integral method, we construct a theory of heterotic SIS\nJosephson junctions between single- and two-gap superconductors. The theory\npredicts the presence of in-phase and out-of-phase collective oscillation modes\nof superconducting phases. The former corresponds to the Josephson plasma mode\nwhose frequency is drastically reduced for $\\pm$ s-wave symmetry, and the\nlatter is a counterpart of Leggett's mode in Josephson junctions. We also\nreveal that the critical current and the Fraunhofer pattern strongly depend on\nthe symmetry type of the two-gap superconductor.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Bimagnon studies in cuprates with Resonant Inelastic X-ray Scattering at\n  the O K edge. I - An assessment on La2CuO4 and a comparison with the\n  excitation at Cu L3 and Cu K edges: We assess the capabilities of magnetic Resonant Inelastic X-ray Scattering\n(RIXS) at the O $K$ edge in undoped cuprates by taking La_{2}CuO_{4} as a\nbenchmark case, based on a series of RIXS measurements that we present here. By\ncombining the experimental results with basic theory we point out the\nfingerprints of bimagnon in the O $K$ edge RIXS spectra. These are a dominant\npeak around 450 meV, the almost complete absence of dispersion both with $\\pi$\nand $\\sigma$ polarization and the almost constant intensity vs. the transferred\nmomentum with $\\sigma$ polarization. This behavior is quite different from Cu\n$L_3$ edge RIXS giving a strongly dispersing bimagnon tending to zero at the\ncenter of the Brillouin zone. This is clearly shown by RIXS measurements at the\nCu $L_3$ edge that we present. The Cu $L_3$ bimagnon spectra and those at Cu\n$K$ edge - both from the literature and from our data - however, have the same\nshape. These similarities and differences are understood in terms of different\nsampling of the bimagnon continuum. This panorama points out the unique\npossibilities offered by O $K$ RIXS in the study of magnetic excitations in\ncuprates near the center of the BZ.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Single vortex fluctuations in a superconducting chip as generating\n  dephasing and spin flips in cold atom traps: We study trapping of a cold atom by a single vortex line in an extreme type\nII superconducting chip, allowing for pinning and friction. We evaluate the\natom's spin flip rate and its dephasing due to the vortex fluctuations in\nequilibrium and find that they decay rapidly when the distance to the vortex\nexceeds the magnetic penetration length. We find that there are special spin\norientations, depending on the spin location relative to the vortex, at which\nspin dephasing is considerably reduced while perpendicular directions have a\nreduced spin flip rate. We also show that the vortex must be perpendicular to\nthe surface for a general shape vortex.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Hidden Reentrant and Larkin-Ovchinnikov-Fulde-Ferrell Superconducting\n  Phases in a Magnetic Field in (TMTSF)$_2$ClO$_4$: We solve a long-standing problem about a theoretical description of the upper\ncritical magnetic field, parallel to conducting layers and perpendicular to\nconducting chains, in (TMTSF)$_2$ClO$_4$ superconductor. In particular, we\nexplain why the experimental upper critical field, $H^{b'}_{c2} \\simeq 6 \\ T$,\nis higher than both the quasi-classical upper critical field and Clogston\nparamagnetic limit. We show that this property is due to the coexistence of the\nhidden Reentrant and Larkin-Ovchinnikov-Fulde-Ferrell phases in a magnetic\nfield in a form of three plane waves with non-zero momenta of the Cooper pairs.\nOur results are in good qualitative and quantitative agreement with the recent\nexperimental measurements of $H^{b'}_{c2}$ and support a singlet d-wave like\nscenario of superconductivity in (TMTSF)$_2$ClO$_4$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Phase stiffness in an antiferromagnetic superconductor: We analyze the suppression of the phase stiffness in a superconductor by\nantiferromagnetic order. The analysis is based on a general expression for the\nphase stiffness in a mean-field state with coexisting spin-singlet\nsuperconductivity and spiral magnetism. Neel order is included as a special\ncase. Close to half-filling, where the pairing gap is much smaller than the\nmagnetic gap, a simple formula for the phase stiffness in terms of magnetic\nquasi-particle bands is derived. The phase stiffness is determined by charge\ncarriers in small electron or hole pockets in this regime. The general analysis\nis complemented by a numerical calculation for the two-dimensional Hubbard\nmodel with nearest and next-to-nearest neighbor hopping amplitudes at a\nmoderate interaction strength. The resulting phase stiffness exhibits a\nstriking electron-hole asymmetry. In the ground state, it is larger than the\npairing gap on the hole-doped side, and smaller for electron doping. Hence, in\nthe hole-doped regime near half-filling the ground state pairing gap sets the\nscale for the Kosterlitz-Thouless temperature T_c^{KT}, while in the slightly\nelectron-doped regime T_c^{KT} is determined essentially by the ground state\nphase stiffness.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Inhomogeneity of charge density wave order and quenched disorder in a\n  high Tc superconductor: It has recently been established that the high temperature (high-Tc)\nsuperconducting state coexists with short-range charge-density-wave order and\nquenched disorder arising from dopants and strain. This complex, multiscale\nphase separation invites the development of theories of high temperature\nsuperconductivity that include complexity. The nature of the spatial interplay\nbetween charge and dopant order that provides a basis for nanoscale phase\nseparation remains a key open question, because experiments have yet to probe\nthe unknown spatial distribution at both the nanoscale and mescoscale (between\natomic and macroscopic scale). Here we report micro X-ray diffraction imaging\nof the spatial distribution of both the charge-density-wave puddles (domains\nwith only a few wavelengths) and quenched disorder in HgBa2CuO4+y, the single\nlayer cuprate with the highest Tc, 95 kelvin. We found that the\ncharge-density-wave puddles, like the steam bubbles in boiling water, have a\nfat-tailed size distribution that is typical of self-organization near a\ncritical point. However, the quenched disorder, which arises from oxygen\ninterstitials, has a distribution that is contrary to the usual assumed random,\nuncorrelated distribution. The interstitials-oxygen-rich domains are spatially\nanti-correlated with the charge-density-wave domains, leading to a complex\nemergent geometry of the spatial landscape for superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Evidence for a universal length scale of dynamic charge inhomogeneity in\n  cuprate superconductors: Time-resolved optical experiments can give unique information on the\ncharacteristic length scales of dynamic charge inhomogeneity on femtosecond\ntimescales. From data on the effective quasiparticle relaxation time &#61556;r\nin La2-xSrxCuO4 and Nd2-xCexCuO4 we derive the temperature- and doping-\ndependence of the intrinsic phonon escape length le, which, under certain\ncircumstances, can be shown to be a direct measure of charge inhomogeneity.\nRemarkably, a common feature of both p and n-type cuprates - which has\nimportant consequences for superconductivity - is that as T &#61614; Tc from\nabove, the escape length approaches the zero-temperature superconducting\ncoherence length, le &#61614; &#61560;s(0). In close vicinity of Tc, le appears\nto follow the critical behaviour of the Ginsburg-Landau coherence length,\n&#61560;GL(T). In the normal state le is found to be in excellent agreement\nwith the mean free path lm obtained from the resistivity data. The data on le\nalso agree well with the data on structural coherence lengths ls obtained from\nneutron scattering experiments, implying the existence of complex intrinsic\ntextures on different length scales which may have a profound effect on the\nfunctional properties of these materials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Josephson squelch filter for quantum nanocircuits: We fabricated and tested a squelch circuit consisting of a copper powder\nfilter with an embedded Josephson junction connected to ground. For small\nsignals (squelch-ON), the small junction inductance attenuates strongly from DC\nto at least 1 GHz, while for higher frequencies dissipation in the copper\npowder increases the attenuation exponentially with frequency. For large\nsignals (squelch-OFF) the circuit behaves as a regular metal powder filter. The\nmeasured ON/OFF ratio is larger than 50dB up to 50 MHz. This squelch can be\napplied in low temperature measurement and control circuitry for quantum\nnanostructures such as superconducting qubits and quantum dots.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quantum phase slip junctions: In this paper we demonstrate that, if it exists, coherent quantum phase slip\nis the exact dual to Josephson tunneling. We use the duality to predict kinetic\ncapacitance and a sharp resonance in narrow wires. Biased resistively and\ndriven at high frequency, quantum phase slip junctions should exhibit current\nplateaus of interest for a fundamental standard.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Type I superconductivity in the Dirac semimetal PdTe2: The superconductor PdTe$_2$ was recently classified as a Type II Dirac\nsemimetal, and advocated to be an improved platform for topological\nsuperconductivity. Here we report magnetic and transport measurements conducted\nto determine the nature of the superconducting phase. Surprisingly, we find\nthat PdTe$_2$ is a Type I superconductor with $T_c = 1.64$ K and a critical\nfield $\\mu_0 H_c (0) = 13.6$ mT. Our crystals also exhibit the intermediate\nstate as demonstrated by the differential paramagnetic effect. For $H > H_c$ we\nobserve superconductivity of the surface sheath. This calls for a close\nexamination of superconductivity in PdTe$_2$ in view of the presence of\ntopological surface states.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Collective excitations in two-band superconductors: We investigate the eigen oscillations of internal degrees of freedom (Higgs\nmode and Goldstone mode) of two-band superconductors using the extended\ntime-dependent Ginzburg-Landau theory, formulated in a work Grigorishin (2021)\n\\cite{grig2}, for the case of two coupled order parameters by both the internal\nproximity effect and the drag effect. It is demonstrated, that the Goldstone\nmode splits into two branches: common mode oscillations with the acoustic\nspectrum, which is absorbed by the gauge field, and anti-phase oscillations\nwith the energy gap (mass) in the spectrum determined by the interband\ncoupling, which can be associated with the Leggett mode. The Higgs mode splits\ninto two branches also: a massive one, whose energy gap vanishes at the\ncritical temperature $T_{c}$, another massive one, whose energy gap does not\nvanish at $T_{c}$. It is demonstrated, that the second branch of the Higgs mode\nis nonphysical, and it, together with the Leggett mode, can be removed by the\nspecial choice of coefficient at the \"drag\" term in the Lagrangian. In the same\ntime, such a choice leaves only one coherence length, thereby prohibiting\nso-called type-1.5 superconductors. We analyze experimental data about the\nJosephson effect between two-band superconductors. In particular, it is\ndemonstrated, that the resonant enhancement of the DC current through a\nJosephson junction at a resonant bias voltage $V_{\\mathrm{res}}$, when the\nJosephson frequency matches the frequency of some internal oscillation mode in\ntwo-band superconductors (banks), can be explained with the coupling between AC\nJosephson current and Higgs oscillations. Thus, explanation of this effect does\nnot need the Leggett mode.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Dynamics of spin transport in voltage-biased Josephson junctions: We investigate spin transport in voltage-biased spin-active Josephson\njunctions. The interplay of spin filtering, spin mixing, and multiple Andreev\nreflection leads to nonlinear voltage dependence of the dc and ac spin current.\nWe compute the voltage characteristics of the spin current (I_S) for\nsuperconductor-ferromagnet-superconductor (SFS) Josephson junctions. The\nsub-harmonic gap structure of I_S(V) is shown to be sensitive to the degree of\nspin mixing generated by the ferromagnetic interface, and exhibits a pronounced\neven-odd effect associated with spin transport during multiple Andreev\nreflection processes. For strong spin mixing both the magnitude and the\ndirection of the dc spin current can be sensitively controlled by the bias\nvoltage.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "On superconducting mechanism in the iron-based layered superconductors: It is demonstrated that SC mechanism of doped Fe-based compounds is\ncharacteristic for itinerant electron systems with coexistence of both (e-e)-\nand (e-h)-pairing arising due to electron-phonon and Coulomb interactions,\nrespectively. The higher Tc of the SC transition here (as compared with\nconventional (LTSC) BCS-systems without (e-h)-pairing) is a natural consequence\nof (e-e)-pairing at the background of high density of states which arises in\nthe narrow energy range near dielectric-(SDW)-gap (pseudogap) edges due to\nremoving of electronic states from the energy region of dielectric (SDW) gap\n(pseudogap) (already formed at the part of the Fermi surface in the normal\nstate with onset temperature T* (near the same for corresponding structural\ntransition) due to (e-h)-pairing). Below Tc the system enters the coexistence\n(SC+SDW) state. The SDW formed is incommensurate with lattice and dynamic in\ncharacter. The phase diagram for such system is determined: the doping\ndependence of the SC gap (and Tc) has a maximum (optimal doping) while the\ndielectric (SDW) gap (pseudogap) (and T*) is a decreasing function of doping.\nThese conclusions follow from detailed analysis of available resistivity and\nanother data for Fe-based superconductors on the basis of model with partial\ndielectrization of electron energy spectra. The picture obtained and\nmanifestation of two order parameters (SC and SDW) in experiments, first of\nall, in threshold phenomena are discussed. The comparison with the case of\ncuprates is performed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "High-energy magnetic excitations in overdoped\n  La$_{2-x}$Sr$_{x}$CuO$_{4}$ studied by neutron and resonant inelastic X-ray\n  scattering: We have performed neutron inelastic scattering and resonant inelastic X-ray\nscattering (RIXS) at the Cu-$L_3$ edge to study high-energy magnetic\nexcitations at energy transfers of more than 100 meV for overdoped\nLa$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x=0.25$ ($T_c=15$ K) and $x=0.30$\n(non-superconducting) using identical single crystal samples for the two\ntechniques. From constant-energy slices of neutron scattering cross-sections,\nwe have identified magnetic excitations up to ~250 meV for $x=0.25$. Although\nthe width in the momentum direction is large, the peak positions along the (pi,\npi) direction agree with the dispersion relation of the spin-wave in the\nnon-doped La$_{2}$CuO$_{4}$ (LCO), which is consistent with the previous RIXS\nresults of cuprate superconductors. Using RIXS at the Cu-$L_3$ edge, we have\nmeasured the dispersion relations of the so-called paramagnon mode along both\n(pi, pi) and (pi, 0) directions. Although in both directions the neutron and\nRIXS data connect with each other and the paramagnon along (pi, 0) agrees well\nwith the LCO spin-wave dispersion, the paramagnon in the (pi, pi) direction\nprobed by RIXS appears to be less dispersive and the excitation energy is lower\nthan the spin-wave of LCO near (pi/2, pi/2). Thus, our results indicate\nconsistency between neutron inelastic scattering and RIXS, and elucidate the\nentire magnetic excitation in the (pi, pi) direction by the complementary use\nof two probes. The polarization dependence of the RIXS profiles indicates that\nappreciable charge excitations exist in the same energy range of magnetic\nexcitations, reflecting the itinerant character of the overdoped sample. A\npossible anisotropy in the charge excitation intensity might explain the\napparent differences in the paramagnon dispersion in the (pi, pi) direction as\ndetected by the X-ray scattering.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pairing symmetry in BiS$_{2}-$based superconductors: The possible pairing symmetries for BiS$_{2}-$based superconductors is\ninvestigated by using a minimal two-orbital model with onsite and\nnearest-neighbor intraorbital attractions $V_{0}$ and $V_{1}$, respectively. By\nusing the mean-field approximation and solving the self-consistent equations,\nthe phase diagram of the pairing symmetry is obtained. It is shown that the\nmodel allows three possible pairing symmetries, depending on the values of\n$V_{0}$ and $V_{1}$: the isotopic $s-$wave pairing\n[$\\Delta_{\\mathbf{k}}=\\Delta_{s}$], the anisotropic $s-$wave pairing\n[$\\Delta_{\\mathbf{k}}=\\Delta_{s}+\\frac{\\Delta_{xs}}{2}(\\cos k_{x}+\\cos k_{y})$]\nand the $d-$wave pairing [$\\Delta_{\\mathbf{k}}=\\frac{\\Delta_{d}}{2}(\\cos\nk_{x}-\\cos k_{y})$]. Furthermore the density of states for these pairing\nsymmetries exhibit different behaviors which can be used to distinguish them.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Gap and pseudogap evolution within the charge-ordering scenario for\n  superconducting cuprates: We describe the spectral properties of underdoped cuprates as resulting from\na momentum-dependent pseudogap in the normal state spectrum. Such a model\naccounts, within a BCS approach, for the doping dependence of the critical\ntemperature and for the two-parameter leading-edge shift observed in the\ncuprates. By introducing a phenomenological temperature dependence of the\npseudogap, which finds a natural interpretation within the stripe\nquantum-critical-point scenario for high-T_c superconductors, we reproduce also\nthe T_c-T^* bifurcation near optimum doping. Finally, we briefly discuss the\ndifferent role of the gap and the pseudogap in determining the spectral and\nthermodynamical properties of the model at low temperatures.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Topological Anomalous Skin Effect in Weyl Superconductors: We show that a Weyl superconductor can absorb light via a novel\nsurface-to-bulk mechanism, which we dub the topological anomalous skin effect.\nThis occurs even in the absence of disorder for a single-band superconductor,\nand is facilitated by the topological splitting of the Hilbert space into bulk\nand chiral surface Majorana states. In the clean limit, the effect manifests as\na characteristic absorption peak due to surface-bulk transitions. We also\nconsider the effects of bulk disorder, using the Keldysh response theory. For\nweak disorder, the bulk response is reminiscent of the Mattis-Bardeen result\nfor $s$-wave superconductors, with strongly suppressed spectral weight below\ntwice the pairing energy, despite the presence of gapless Weyl points. For\nstronger disorder, the bulk response becomes more Drude-like and the $p$-wave\nfeatures disappear. We show that the surface-bulk signal survives when combined\nwith the bulk in the presence of weak disorder. The topological anomalous skin\neffect can therefore serve as a fingerprint for Weyl superconductivity. We also\ncompute the Meissner response in the slab geometry, incorporating the effect of\nthe surface states.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Is spontaneous vortex generation in superconducting 4Hb-TaS$_2$ from\n  vison-vortex nucleation with $\\mathbb{Z}_2$ topological order?: We propose the superconducting van der Waals material 4Hb-TaS$_2$ to realize\nthe $\\mathbb{Z}_2$ topological order and interpret the recent discovery of the\nspontaneous vortex generation in 4Hb-TaS$_2$ as the vison-vortex nucleation.\nFor the alternating stacking of metallic/superconducting and Mott insulating\nlayers in 4Hb-TaS$_2$, we expect the local moments in the Mott insulating\n1T-TaS$_2$ layer to form the $\\mathbb{Z}_2$ topological order. The spontaneous\nvortex generation in 4Hb-TaS$_2$ is interpreted from the transition or\nnucleation between the superconducting vortex and the $\\mathbb{Z}_2$ vison in\ndifferent phase regimes. Differing from the single vison-vortex nucleation in\nthe original Senthil-Fisher's cuprate proposal, we consider such nucleation\nprocess between the superconducting vortex lattice and the vison crystal. We\nfurther propose experiments to distinguish this proposal with the\n$\\mathbb{Z}_2$ topological order from the chiral spin liquid scenarios.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Frustrated Magnetic Interactions, Giant Magneto-Elastic Coupling, and\n  Magnetic Phonons in Iron-Pnictides: We present a detailed first principles study of Fe-pnictides with particular\nemphasis on competing magnetic interactions, structural phase transition, giant\nmagneto-elastic coupling and its effect on phonons. The exchange interactions\n$J_{i,j}(R)$ are calculated up to $\\approx 12 $\\AA $. We find that $J_{i,j}(R)$\nhas an oscillatory character with an envelop decaying as $1/R^3$ along the\nstripe-direction while it is very short range along the diagonal direction and\nantiferromagnetic. A brief discussion of the neutron scattering determination\nof these exchange constants from a single crystal sample with orthorhombic\ntwinning is given. The lattice parameter dependence of the exchange constants,\n$dJ_{i,j}/da$ are calculated for a simple spin-Peierls like model to explain\nthe fine details of the tetragonal-orthorhombic phase transition. We then\ndiscuss giant magneto-elastic effects in these systems. We show that when the\nFe-spin is turned off the optimized c-values are shorter than experimetnal\nvalues by 1.4 \\AA $ $ for CaFe$_2$As$_2$, by 0.4 \\AA $ $ for BaFe$_2$As$_2$,\nand by 0.13 \\AA $ $ for LaOFeAs. Finally, we show that Fe-spin is also required\nto obtain the right phonon energies, in particular As c-polarized and Fe-Fe\nin-plane modes. Since treating iron as magnetic ion always gives much better\nresults than non-magnetic ones and since there is no large c-axis reduction\nduring the normal to superconducting phase transition, the iron magnetic moment\nshould be present in Fe-pnictides at all times. We discuss the implications of\nour results on the mechanism of superconductivity in these fascinating\nFe-pnictide systems.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Design and implementation of a micro-coil induction magnetometer: We present a micron-sized induction magnetometer designed to measure the\nmagnetic response of superconducting mesoscopic samples. The device is\nmanufactured using the Memscap PolyMUMPs process and consists of two octagonal\nplanar parallel micro-coils covering an area of 240 microns X 240 microns,\nwhich are separated by only 2.75 microns. We show that this design is\nsufficiently sensitive to detect the Meissner transition at zero dc field, of a\nhigh-Tc superconductor Bi2Sr2CaCu2O8 disk of 40 microns in diameter and 1\nmicron in thicknesses.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "SU(4) Dynamical Symmetry and the Origin of Pseudogaps: This paper has been withdrawn by the author due to incomplete interpretation\nfor the results.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "A Terraced Scanning Superconducting Quantum Interference Device\n  Susceptometer with Sub-Micron Pickup Loops: Superconducting Quantum Interference Devices (SQUIDs) can have excellent spin\nsensitivity depending on their magnetic flux noise, pick-up loop diameter, and\ndistance from the sample. We report a family of scanning SQUID susceptometers\nwith terraced tips that position the pick-up loops 300 nm from the sample. The\n600 nm - 2 um pickup loops, defined by focused ion beam, are integrated into a\n12-layer optical lithography process allowing flux-locked feedback, in situ\nbackground subtraction and optimized flux noise. These features enable a\nsensitivity of ~70 electron spins per root Hertz at 4K.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Spin-excitation anisotropy in the nematic state of detwinned FeSe: The origin of the electronic nematicity in FeSe is one of the most important\nunresolved puzzles in the study of iron-based superconductors. In both spin-\nand orbital-nematic models, the intrinsic magnetic excitations at\n$\\mathbf{Q}_1=(1, 0)$ and $\\mathbf{Q}_2=(0, 1)$ of twin-free FeSe are expected\nto provide decisive criteria for clarifying this issue. Although a\nspin-fluctuation anisotropy below 10 meV between $\\mathbf{Q}_1$ and\n$\\mathbf{Q}_2$ has been observed by inelastic neutron scattering around\n$T_c\\sim 9$ K ($<<T_s\\sim 90$ K), it remains unclear whether such an anisotropy\nalso persists at higher energies and associates with the nematic transition\n$T_{\\rm s}$. Here we use resonant inelastic x-ray scattering (RIXS) to probe\nthe high-energy magnetic excitations of uniaxial-strain detwinned FeSe and\n{\\BFA}. A prominent anisotropy between the magnetic excitations along the $H$\nand $K$ directions is found to persist to $\\sim200$ meV in FeSe, which is even\nmore pronounced than the anisotropy of spin waves in {\\BFA}. This anisotropy\ndecreases gradually with increasing temperature and finally vanishes at a\ntemperature around the nematic transition temperature $T_{\\rm s}$. Our results\nreveal an unprecedented strong spin-excitation anisotropy with a large energy\nscale well above the $d_{xz}/d_{yz}$ orbital splitting, suggesting that the\nnematic phase transition is primarily spin-driven. Moreover, the measured\nhigh-energy spin excitations are dispersive and underdamped, which can be\nunderstood from a local-moment perspective. Our findings provide the\nmuch-needed understanding of the mechanism for the nematicity of FeSe and\npoints to a unified description of the correlation physics across seemingly\ndistinct classes of Fe-based superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Exploring the vortex phase diagram of Bogoliubov-de Gennes disordered\n  superconductors: We study the interplay of vortices and disorder in a two-dimensional\ndisordered superconductor at zero temperature described by the Bogoliubov-de\nGennes (BdG) self-consistent formalism for lattices of sizes up to\n$100\\times100$ where the magnetic flux is introduced by the Peierls's\nsubstitution. The substantial larger size than in previous approaches ($\\leq\n36\\times 36$) has allowed us to identify a rich phase diagram as a function of\nthe magnetic flux and the disorder strength. For sufficiently weak disorder,\nand not too strong magnetic flux, we observe a slightly distorted Abrikosov\ntriangular vortex lattice. An increase in the magnetic flux leads to an\nunexpected rectangular vortex lattice. A further increase in disorder, or flux\ngradually destroy the lattice symmetry though strong vortex repulsion persists.\nAn even stronger disorder leads to deformed single vortices with an\ninhomogeneous core. As number of vortices increases, vortices overlap becomes\nmore frequent. Finally, we show that global phase coherence is a feature of all\nthese phases and that disorder enhances substantially the critical magnetic\nflux with respect to the clean limit with a maximum on the metallic side of the\ninsulating transition.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Physical mechanisms of timing jitter in photon detection by current\n  carrying superconducting nanowires: We studied timing jitter in the appearance of photon counts in meandering\nnanowires with different fractional amount of bends. Timing jitter, which is\nthe probability density of the random time delay between photon absorption in\ncurrent-carrying superconducting nanowire and appearance of the normal domain,\nreveals two different underlying physical scenarios. In the deterministic\nregime, which is realized at large currents and photon energies, jitter is\ncontrolled by position dependent detection threshold in straight parts of\nmeanders and decreases with the current. At small photon energies, jitter\nincreases and its current dependence disappears. In this probabilistic regime\njitter is controlled by Poisson process in that magnetic vortices jump randomly\nacross the wire in areas adjacent to the bends.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "1D Bose Gases in an Optical Lattice: We report on the study of the momentum distribution of a one-dimensional Bose\ngas in an optical lattice. From the momentum distribution we extract the\ncondensed fraction of the gas and thereby measure the depletion of the\ncondensate and compare it with a theorical estimate. We have measured the\ncoherence length of the gas for systems with average occupation $\\bar{n}>1$ and\n$\\bar{n}<1$ per lattice site.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Band-mixing-mediated Andreev reflection of semiconductor holes: We have investigated Andreev-reflection processes occurring at a clean\ninterface between a $p$-type semiconductor and a conventional superconductor.\nOur calculations are performed within a generalized Bogoliubov-de Gennes\nformalism where the details of the semiconductor band structure are described\nby a $6\\times 6$ Kane model. It is found that Andreev reflection of light-hole\nand heavy-hole valence-band carriers is generally possible and that the two\nvalence-band hole types can be converted into each other in the process. The\nnormal-reflection and Andreev-reflection amplitudes depend strongly on the\nsemiconductor's carrier concentration and on the angle of injection. In the\nspecial case of perpendicular incidence, Andreev reflection of heavy holes does\nnot occur. Moreover, we find conversion-less Andreev reflection to be\nimpossible above some critical angle, and another critical angle exists above\nwhich the conversion of a heavy hole into a light hole cannot occur.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Extraordinary magnetoresistance in graphite: experimental evidence for\n  the time-reversal symmetry breaking: The ordinary magnetoresistance (MR) of doped semiconductors is positive and\nquadratic in a low magnetic field, B, as it should be in the framework of the\nBoltzmann kinetic theory or in the conventional hopping regime. We observe an\nunusual highly-anisotropic in-plane MR in graphite, which is neither quadratic\nnor always positive. In a certain current direction MR is negative and linear\nin B in fields below a few tens of mT with a crossover to a positive MR at\nhigher fields, while in a perpendicular current direction we observe a giant\nsuper-linear and positive MR. These extraordinary MRs are respectively\nexplained by a hopping magneto-conductance via non-zero angular momentum\norbitals, and by the magneto-conductance of inhomogeneous media. The linear\norbital NMR is a unique signature of the broken time-reversal symmetry (TRS) in\ngraphite. While some local paramagnetic centers could be responsible for the\nbroken TRS, the observed large diamagnetism suggests a more intriguing\nmechanism of this breaking, involving superconducting clusters with\nunconventional (chiral) order parameters and spontaneously generated\nnormal-state current loops in graphite.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Thermal evolution of antiferromagnetic correlations and tetrahedral bond\n  angles in superconducting FeTe$_{1-x}$Se$_x$: It has recently been demonstrated that dynamical magnetic correlations\nmeasured by neutron scattering in iron chalcogenides can be described with\nmodels of short-range correlations characterized by particular {choices of\nfour-spin plaquettes, where the appropriate choice changes as the} parent\nmaterial is doped towards superconductivity. Here we apply such models to\ndescribe measured maps of magnetic scattering as a function of two-dimensional\nwave vectors obtained for optimally superconducting crystals of\nFeTe$_{1-x}$Se$_x$. We show that the characteristic antiferromagnetic wave\nvector evolves from that of the bicollinear structure found in underdoped\nchalcogenides (at high temperature) to that associated with the stripe\nstructure of antiferromagnetic iron arsenides (at low temperature); {these can\nboth be described with the same local plaquette, but with different\ninter-plaquette correlations}. While the magnitude of the low-energy magnetic\nspectral weight is substantial at all temperatures, it actually weakens\nsomewhat at low temperature, where the charge carriers become more itinerant.\nThe observed change in spin correlations is correlated with the dramatic drop\nin the electronic scattering rate and the growth of the bulk nematic response\non cooling. Finally, we also present powder neutron diffraction results for\nlattice parameters in FeTe$_{1-x}$Se$_x$ indicating that the tetrahedral bond\nangle tends to increase towards the ideal value on cooling, in agreement with\nthe increased screening of the crystal field by more itinerant electrons and\nthe correspondingly smaller splitting of the Fe $3d$ orbitals.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electronic structure and Fermi surface of iron-based superconductors\n  R2Fe3Si5 (R = Lu;Y;Sc) from first principles: Electronic structures of three superconducting rare-earth iron silicides\n(Lu;Y;Sc)2Fe3Si5 and non-superconducting Lu2Ru3Si5, adopting a tetragonal\ncrystal structure (P4/mnc), have been calculated employing the full-potential\nlocal-orbital method within the density functional theory. The investigations\nwere focused particularly on the band structures and Fermi surfaces, existing\nin four bands and containing rather three-dimensional electronlike and holelike\nsheets. They support an idea of unconventional multi-band superconductivity in\nthese ternaries, proposed earlier by other authors for Lu2Fe3Si5, based on\nheat-capacity, resistivity, electromagnetic and muon spin rotation\nmeasurements. Finally, a discussion on differences in the electronic structures\nbetween the investigated here and other common families of iron-based\nsuperconductors is carried out.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Field dependence of the vortex core size in a multi-band superconductor: The magnetic field dependence of the vortex core size in the multi-band\nsuperconductor NbSe2 has been determined from muon spin rotation measurements.\nThe spatially extended nature of the quasiparticle core states associated with\nthe smaller gap leads to a rapid field-induced shrinkage of the core size at\nlow fields, while the more tightly bound nature of the states associated with\nthe larger gap leads to a field-independent core size for fields greater than 4\nkOe. A simple model is proposed for the density of delocalized core states that\nestablishes a direct relationship between the field-induced reduction of the\nvortex core size and the corresponding enhancement of the electronic thermal\nconductivity. We show that this model accurately describes both NbSe2 and the\nsingle-band superconductor V3Si.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electron Correlations in the High Tc-Compounds: Ab-initio correlation results for an idealized high Tc-compound are compared\nto density functional (DF) calculations for the same system. It is shown that\nand why the DF-charge distribution is wrong. The largest deficiency arises for\nthe Cu-d(x2-y2)-occupation, originating from strong atomic correlations but\nmostly from anomalous neighbor Cu-spin correlations. Both features are beyound\nthe range of the homogeneous electron gas approximation underlying the\nDF-schemes. The ab-initio results also exclude a description of the real system\nin a Mott-Hubbard scenario, that is mostly chosen in theory.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Structural and Magnetic Instabilities of La$_{2-x}$Sr$_x$CaCu$_2$O$_6$: A neutron scattering study of nonsuperconducting\nLa$_{2-x}$Sr$_x$CaCu$_2$O$_6$ (x=0 and 0.2), a bilayer copper oxide without CuO\nchains, has revealed an unexpected tetragonal-to-orthorhombic transition with a\ndoping dependent transition temperature. The predominant structural\nmodification below the transition is an in-plane shift of the apical oxygen. In\nthe doped sample, the orthorhombic superstructure is strongly disordered, and a\nglassy state involving both magnetic and structural degrees of freedom develops\nat low temperature. The spin correlations are commensurate.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Staggered Pairing Phenomenology for UPd_2Al_3 and UNi_2Al_3: We apply the staggered-pairing Ginzburg-Landau phenomenology to describe\nsuperconductivity in UPd_2Al_3 and UNi_2Al_3. The phenomenology was applied\nsuccessfully to UPt_3 so it explains why these materials have qualitatively\ndifferent superconducting phase diagrams although they have the same\npoint-group symmetry. UPd_2Al_3 and UNi_2Al_3 have a two-component\nsuperconducting order parameter transforming as an H-point irreducible\nrepresentation of the space group. Staggered superconductivity can induce\ncharge-density waves characterized by new Bragg peaks suggesting experimental\ntests of the phenomenology.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity in silicon: a review: Silicon, one of the most abundant elements found on Earth, has been an\nexcellent choice of the semiconductor industry for ages. Despite it's\nremarkable applications in modern semiconductor-based electronic devices, the\npotential of cubic silicon in superconducting electronics remained a challenge\nbecause even heavily doped silicon crystals do not superconduct under normal\nconditions. It is apparent that if superconductivity can be introduced in cubic\nsilicon, that will bring a breakthrough in low-dissipation electronic\ncircuitry. Motivated by this, attempts have been made by several research\ngroups to induce superconductivity in silicon through a number of different\nroutes. Some of the other structural phases of silicon like $\\beta$-Sn and\nsimple hexagonal are, however, known to display superconductivity. In the\npresent review article, various theoretical and experimental aspects of\nsuperconductivity in silicon are discussed. Superconductivity in different\nphases and different structural forms of silicon are also reviewed. We also\nhighlight the potential of superconducting phases of silicon for technological\napplications in super-conducting nano-electronics.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Field Dependence of the Superconducting Basal Plane Anisotropy of\n  TmNi2B2C: The superconductor TmNi2B2C possesses a significant four-fold basal plane\nanisotropy, leading to a square Vortex Lattice (VL) at intermediate fields.\nHowever, unlike other members of the borocarbide superconductors, the\nanisotropy in TmNi2B2C appears to decrease with increasing field, evident by a\nreentrance of the square VL phase. We have used Small Angle Neutron Scattering\nmeasurements of the VL to study the field dependence of the anisotropy. Our\nresults provide a direct, quantitative measurement of the decreasing\nanisotropy. We attribute this reduction of the basal plane anisotropy to the\nstrong Pauli paramagnetic effects observed in TmNi2B2C and the resulting\nexpansion of vortex cores near Hc2.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "High-pressure synthesis and superconductivity of the novel Laves phase\n  BaIr2: Superconductors comprising 5d transition metals of Ir and Pt have been widely\nexplored because they have the potential of unique superconductivity caused by\nthe strong spin-orbit interaction (SOI). We successfully synthesized BaIr2, the\nlast Laves phase remaining unsynthesized in the MgCu2-type AM2 (A = Ca, Sr, Ba;\nM = Rh, Pd, Ir, Pt). BaIr2 was crystallized at 925 C under a pressure of 3.3\nGPa via a solid-state reaction between Ba and Ir powders; it was found to have\nthe longest a-lattice constant of 8.038(1) A among AM2. BaIr2 exhibited bulk\nsuperconductivity at a transition temperature (Tc) of 2.7 K. BaIr2 was found to\nhave a type-II superconductor with an upper critical field of 67.7 kOe, which\nwas above the Pauli paramagnetic limit (50 kOe). The electron-phonon coupling\nconstant and normalized specific heat jump were measured to be 0.63 and 1.2,\nrespectively, indicating that BaIr2 is a weak-coupling superconductor. The\nelectronic-structure calculations for BaIr2 revealed that the Ir-5d states are\ndominant at the Fermi energy (EF) and the density of states at the EF is\nstrongly affected by SOI as in the case of CaIr2 and SrIr2.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Minimal timing jitter in superconducting nanowire single photon\n  detectors: Using two-temperature model coupled with modified time-dependent\nGinzburg-Landau equation we calculate the delay time $\\tau_d$ in appearance of\ngrowing normal domain in the current-biased superconducting strip after\nabsorption of the single photon. We demonstrate that $\\tau_d$ depends on the\nplace in the strip where photon is absorbed and monotonically decreases with\nincreasing of the current. We argue, that the variation of $\\tau_d$ (timing\njitter), connected either with position-dependent response or Fano fluctuations\ncould be as small as the lowest relaxation time of the superconducting order\nparameter $\\sim \\hbar/k_BT_c$ ($T_c$ is the critical temperature of the\nsuperconductor) when the current approaches the depairing current.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Switching current distributions and subgap structures of underdoped\n  (Hg,Re)Ba2Ca2Cu3O8+d intrinsic Josephson junctions: We have investigated the intrinsic Josephson properties in slightly\nunderdoped (Hg,Re)Ba2Ca2Cu3Oy [Hg(Re)1223] intrinsic Josephson junctions (IJJs)\nwith a dimension of 1.0x1.5x0.11 um^3. The current-voltage characteristics of\nthe IJJs exhibit clear multiple branches with subgap structures similar to\nthose of other cuprate superconductors. The switching current distributions\nP(I) from the zero-voltage to the nonzero-voltage state in the current-biased\nIJJs agree well with the theoretical curves of the thermally assisted escape\nmodel at temperatures above ~5 K. The plasma frequency fp of the IJJs is\nestimated to be 1.3 THz from the fluctuation-free critical current density of\n2.0x10^5 A/cm2, which is one of the highest among cuprate superconductors,\nreflecting the high Tc and a relatively low anisotropy of the Re doped Hg\nsystem. The P(I) gradually becomes independent of temperature below ~5 K, which\nsuggests a crossover of the escape process from thermal activation to quantum\ntunneling at such a high temperature.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superuid density in overdoped cuprates: thin films versus bulk samples: Recent study of overdoped La$_{2-x}$Sr$_x$CuO$_4$ cuprate superconductor thin\nfilms by Bo\\v{z}ovi\\'{c} {\\it et al.} has revealed several unexpected findings,\nmost notably the violation of the BCS description which was believed to\nadequately describe overdoped cuprates. In particular, it was found that the\nsuperfluid density in La$_{2-x}$Sr$_x$CuO$_4$ films decreases on the overdoped\nside as a linear function of critical temperature T$_c$, which was taken as\nevidence for the violation of the Homes' law. We show explicitly that the law\nis indeed violated, and as the main reason for violation we find that the\nsuperfluid density in Bo\\v{z}ovi\\'{c}'s films is suppressed more strongly than\nin bulk samples. Based on the existing literature data, we show that the\nsuperfluid density in bulk cuprate samples does not decrease with doping, but\ninstead tends to saturate on the overdoped side. The result is also supported\nby our recent measurement of a heavily overdoped bulk La$_{2-x}$Sr$_x$CuO$_4$\nsample. Moreover, this saturation of superfluid density might not be limited to\ncuprates, as we find evidence for similar behavior in two pnictide\nsuperconductor families. We argue that quantum phase fluctuations play an\nimportant role in suppressing the superfluid density in thin films.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electronic structure in one-Fe Brillouin zone of iron-pnictide\n  superconductor CsFe$_2$As$_2$: The multiband nature of iron-pnictide superconductors is one of the keys to\nthe understanding of their intriguing behavior. The electronic and magnetic\nproperties heavily rely on the multiband interactions between different\nelectron and hole pockets near the Fermi level. At the fundamental level,\nthough many theoretical models were constructed on the basis of the so-called\n1-Fe Brillouin zone (BZ) with an emphasis of the basic square lattice of iron\natoms, most electronic structure measurements were interpreted in the 2-Fe BZ.\nWhether the 1-Fe BZ is valid in a real system is still an open question. Using\nangle-resolved photoemission spectroscopy (ARPES), here we show in an extremely\nhole-doped iron-pnictide superconductor CsFe$_2$As$_2$ that the distribution of\nelectronic spectral weight follows the 1-Fe BZ, and that the emerging band\nstructure bears some features qualitatively different from theoretical band\nstructures of the 1-Fe BZ. Our analysis suggests that the interlayer separation\nis an important tuning factor for the physics of FeAs layers, the increase of\nwhich can reduce the coupling between Fe and As and lead to the emergence of\nthe electronic structure in accord with the 1-Fe symmetry of the Fe square\nlattice. Our finding puts strong constraints on the theoretical models\nconstructed on the basis of the 1-Fe BZ.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "High-temperature superconductivity in one-unit-cell FeSe films: Since the dramatic interface enhancement of superconducting transition\ntemperature (Tc) was reported in one unit-cell FeSe film grown on SrTiO3\nsubstrate (1-UC FeSe/STO) by molecular beam epitaxy (MBE), related research on\nthis system has become a new frontier in condensed matter physics. In this\npaper, we present a brief review on this rapidly developing field, mainly\nfocusing on the superconducting properties of 1-UC FeSe/STO. Experimental\nevidences for the high-temperature superconductivity in 1-UC FeSe/STO,\nincluding the direct evidences revealed by transport and diamagnetic\nmeasurements, and other evidences from scanning tunneling microscope (STM) and\nangle-resolved photoemission spectroscopy (ARPES), are overviewed. Potential\nmechanisms of the enhanced superconductivity are discussed. There are\naccumulating arguments suggesting that the strengthened Cooper pairing in 1-UC\nFeSe/STO originates from the interface effects, specifically charge transfer\nand coupling to phonon modes in TiO2 plane. The study of superconductivity in\n1-UC FeSe/STO not only sheds a new light on the mechanism of high-temperature\nsuperconductors with layered structures, but also provides the insight to\nexplore new superconductors by interface engineering.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Scanning Tunneling Spectroscopy and Vortex Imaging in the Iron-Pnictide\n  Superconductor BaFe$_{1.8}$Co$_{0.2}$As$_2$: We present an atomic resolution scanning tunneling spectroscopy study of\nsuperconducting BaFe$_{1.8}$Co$_{0.2}$As$_2$ single crystals in magnetic fields\nup to $9 \\text{Tesla}$. At zero field, a single gap with coherence peaks at\n$\\overline{\\Delta}=6.25 \\text{meV}$ is observed in the density of states. At $9\n\\text{T}$ and $6 \\text{T}$, we image a disordered vortex lattice, consistent\nwith isotropic, single flux quantum vortices. Vortex locations are uncorrelated\nwith strong scattering surface impurities, demonstrating bulk pinning. The\nvortex-induced sub-gap density of states fits an exponential decay from the\nvortex center, from which we extract a coherence length $\\xi=27.6\\pm 2.9\n\\text{\\AA}$, corresponding to an upper critical field $H_{c2}=43 \\text{T}$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electromagnetic response of superconductors and optical sum rule: The interrelation between the condensation energy and the optical sum rules\nhas been investigated. It has been shown that the so called 'partial' sum rule\nviolation is related mainly to a temperature dependence of the relaxation rate\nrather than to the appearance of superconductivity itself. Moreover, we\ndemonstrate that the experimental data on the temperature dependence of the\noptical sum rule can be explained rather well by an account of strong\nelectron-phonon interaction.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Gapless helical superconductivity on the surface of a three-dimensional\n  topological insulator: Recent angle-resolved photoemission experiments have observed a\nproximity-induced superconducting gap in the helical surface states of a thin\nfilm of the 3D topological insulator Bi$_2$Se$_3$ grown on a superconducting\nNbSe$_2$ substrate. The superconducting coherence peaks in the electronic\ndensity of states are strongly suppressed when the topological insulator is\ndoped with magnetic Mn impurities, which was interpreted as the complete\ndestruction of helical superconductivity in the topological surface states.\nMotivated by these experiments, we explore a different possibility: gapless\nhelical superconductivity, where a gapless electronic density of states\ncoexists with a nonzero helical superconducting order parameter. We study a\nmodel of superconducting Dirac fermions coupled to random magnetic impurities\nwithin the Abrikosov-Gor'kov framework, and find finite regions of gapless\nhelical superconductivity in the phase diagram of the system for both\nproximity-induced and intrinsic superconductivity. For the latter, we derive\nuniversal rates of supression of the superconducting transition temperature due\nto magnetic scattering and, for a Fermi level at the Dirac point, a universal\nrate of increase of the quantum critical attraction strength.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Microscopic mechanism for fluctuating pair density wave: In weakly coupled BCS superconductors, only electrons within a tiny energy\nwindow around the Fermi energy, $E_F$, form Cooper pairs. This may not be the\ncase in strong coupling superconductors such as cuprates, FeSe, SrTiO$_3$ or\ncold atom condensates where the pairing scale, $E_B$, becomes comparable or\neven larger than $E_F$. In cuprates, for example, a plausible candidate for the\npseudogap state at low doping is a fluctuating pair density wave, but no\nmicroscopic model has yet been found which supports such a state. In this work,\nwe write an analytically solvable model to examine pairing phases in the\nstrongly coupled regime and in the presence of anisotropic interactions.\nAlready for moderate coupling we find an unusual finite temperature phase,\nbelow an instability temperature $T_i$, where local pair correlations have\nnon-zero center-of-mass momentum but lack long-range order. At low temperature,\nthis fluctuating pair density wave can condense either to a uniform $d$-wave\nsuperconductor or the widely postulated pair-density wave phase depending on\nthe interaction strength. Our minimal model offers a unified microscopic\nframework to understand the emergence of both fluctuating and long range pair\ndensity waves in realistic systems.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Two-component uniform spin susceptibility in superconducting\n  HgBa$_{2}$CuO$_{4+\u03b4}$ single crystals determined with $^{63}$Cu and\n  $^{199}$Hg NMR: $^{63}$Cu and $^{199}$Hg NMR shifts for an optimally and underdoped\nHgBa$_{2}$CuO$_{4+\\delta}$ single crystal are reported, and the temperature\ndependence dictates a two-component description of the uniform spin\nsusceptibility. The first component, associated with the pseudogap phenomenon\nin the NMR shifts, decreases already at room temperature and continues to drop\nas the temperature is lowered, without a drastic change at the transition\ntemperature into the superconducting state. The second component is temperature\nindependent above the superconducting transition temperature and vanishes\nrapidly below it. It increases with doping and is a substantial part of the\ntotal spin susceptibility measured at both nuclei.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Fifty years of Hubbard and Anderson lattice models: from magnetism to\n  unconventional superconductivity - A brief overview: We briefly overview the importance of Hubbard and Anderson-lattice models as\napplied to explanation of high-temperature and heavy-fermion superconductivity.\nApplication of the models during the last two decades provided an explanation\nof the paired states in correlated fermion systems and thus extended\nessentially their earlier usage to the description of itinerant magnetism,\nfluctuating valence, and the metal-insulator transition. In second part, we\nalso present some of the new results concerning the unconventional\nsuperconductivity and obtained very recently in our group. A comparison with\nexperiment is also discussed, but the main emphasis is put on rationalization\nof the superconducting properties of those materials within the real-space\npairing mechanism based on either kinetic exchange and/or Kondo-type\ninteraction combined with the electron correlation effects.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Doping Dependence of Anisotropic Resistivities in Trilayered\n  Superconductor Bi2Sr2Ca2Cu3O10+delta (Bi-2223): The doping dependence of the themopower, in-plane resistivity rho_ab(T),\nout-of-plane resistivity rho_c(T), and susceptibility has been systematically\nmeasured for high-quality single crystal Bi2Sr2Ca2Cu3O10+delta. We found that\nthe transition temperature Tc and pseudogap formation temperature T_rho_c*,\nbelow which rho_c shows a typical upturn, do not change from their optimum\nvalues in the \"overdoped\" region, even though doping actually proceeds. This\nsuggests that, in overdoped region, the bulk $T_c$ is determined by the always\nunderdoped inner plane, which have a large superconducting gap, while the\ncarriers are mostly doped in the outer planes, which have a large phase\nstiffness.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Comparative study on the thermoelectric effect of parent oxypnictides\n  La$T$AsO ($T$ = Fe, Ni): The thermopower and Nernst effect were investigated for undoped parent\ncompounds LaFeAsO and LaNiAsO. Both thermopower and Nernst signal in iron-based\nLaFeAsO are significantly larger than those in nickel-based LaNiAsO.\nFurthermore, abrupt changes in both thermopower and Nernst effect are observed\nbelow the structural phase transition temperature and spin-density wave (SDW)\ntype antiferromagnetic (AFM) order temperature in Fe-based LaFeAsO. On the\nother hand, Nernst effect is very small in the Ni-based LaNiAsO and it is\nweakly temperature-dependent, reminiscent of the case in normal metals. We\nsuggest that the effect of SDW order on the spin scattering rate should play an\nimportant role in the anomalous temperature dependence of Hall effect and\nNernst effect in LaFeAsO. The contrast behavior between the LaFeAsO and LaNiAsO\nsystems implies that the LaFeAsO system is fundamentally different from the\nLaNiAsO system and this may provide clues to the mechanism of high $T_c$\nsuperconductivity in the Fe-based systems.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Structure and Composition of the 200 K-Superconducting Phase of H2S\n  under Ultrahigh Pressure: The Perovskite (SH-)(H3S+): H2S is converted under ultrahigh pressure (> 110 GPa) to a metallic phase\nthat becomes superconducting with a record Tc of 200 K. It has been proposed\nthat the superconducting phase is body-centered cubic H3S ( Im3m , a = 3.089\n{\\AA}) resulting from a decomposition reaction 3H2S --> 2H3S + S. The analogy\nof H2S and H2O leads us to a very different conclusion. The well-known\ndissociation of water into H3O+ and OH- increases by orders of magnitude under\npressure. An equivalent behavior of H2S is anticipated under pressure with the\ndissociation, 2H2S --> H3S+ + SH- forming a perovskite structure (SH-)(H3S+),\nwhich consists of corner-sharing SH6 octahedra with SH- at each A-site (i.e.,\nthe center of each S8 cube). Our DFT calculations show that the perovskite\n(SH-)(H3S+) is thermodynamically more stable than the Im3m structure of H3S,\nand suggest that the A-site H atoms are most likely fluxional even at Tc.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity in boron-doped carbon nanotube networks: By using the five Angstrom diameter pores of calcined zeolite as the\ntemplate, we have fabricated boron doped carbon nanotube networks via the\nchemical vapor deposition method. Raman data indicate the network to comprise\nsegments of interconnected carbon nano tubes. Transport measurements showed a\nsuperconducting transition initiating at 40K, with a sharp downturn around 20K\nto a low resistance state at 2K, accompanied by a low resistance plateau in the\ncurrent voltage characteristic, fluctuating around zero resistance. Magnetic\nmeasurements exhibited the Meissner effect characteristic of thin\nsuperconducting wire networks in which the superconducting wire radius is much\nsmaller than the London penetration length. At low magnetic field, the negative\ndiamagnetic susceptibility was observed to persist beyond 200K. The transport\nand magnetic data are reconciled on the basis of a physical model based on weak\nlinks comprising short, one-dimensional superconducting nano tubes, that govern\nthe global transport behavior.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quantum phase-slips in Josephson junction rings: We study quantum phase-slip (QPS) processes in a superconducting ring\ncontaining N Josephson junctions and threaded by an external static magnetic\nflux. In a such system, a QPS consists of a quantum tunneling event connecting\ntwo distinct classical states of the phases with different persistent currents\n[K. A. Matveev et al., Phys. Rev. Lett. 89, 096802 (2002)]. When the Josephson\ncoupling energy EJ of the junctions is larger than the charging energy EC =\ne2/2C where C is the junction capacitance, the quantum amplitude for the QPS\nprocess is exponentially small in the ratio EJ/EC. At given magnetic flux each\nQPS can be described as the tunneling of the phase difference of a single\njunction of almost 2pi, accompanied by a small harmonic displacement of the\nphase difference of the other N-1 junctions. As a consequence the total QPS\namplitude nu is a global property of the ring. Here we study the dependence of\nnu on the ring size N taking into account the effect of a finite capacitance C0\nto ground which leads to the appearance of low-frequency dispersive modes.\nJosephson and charging effects compete and lead to a nonmonotonic dependence of\nthe ring critical current on N. For N=infty, the system converges either\ntowards a superconducting or an insulating state, depending on the ratio\nbetween the charging energy E0 = e2/2C0 and the Josephson coupling energy EJ.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Characterization of the superconducting phase in tellurium hydride at\n  high pressure: At present, hydrogen-based compounds constitute one of the most promising\nclasses of materials for applications as a phonon-mediated high-temperature\nsuperconductors. Herein, the behavior of the superconducting phase in tellurium\nhydride (HTe) at high pressure ($p=300$ GPa) is analyzed in details, by using\nthe isotropic Migdal-Eliashberg equations. The chosen pressure conditions are\nconsidered here as a case study which corresponds to the highest critical\ntemperature value ($T_{c}$) in the analyzed material, as determined within\nrecent density functional theory simulations. It is found that the\nMigdal-Eliashberg formalism, which constitutes a strong-coupling generalization\nof the Bardeen-Cooper-Schrieffer (BCS) theory, predicts that the critical\ntemperature value ($T_{c}=52.73$ K) is higher than previous estimates of the\nMcMillan formula. Further investigations show that the characteristic\ndimensionless ratios for the the thermodynamic critical field, the specific\nheat for the superconducting state, and the superconducting band gap exceeds\nthe limits of the BCS theory. In this context, also the effective electron mass\nis not equal to the bare electron mass as provided by the BCS theory. On the\nbasis of these findings it is predicted that the strong-coupling and\nretardation effects play pivotal role in the superconducting phase of HTe at\n300 GPa, in agreement with similar theoretical estimates for the sibling\nhydrogen and hydrogen-based compounds. Hence, it is suggested that the\nsuperconducting state in HTe cannot be properly described within the mean-field\npicture of the BCS theory.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Probing pair-breaking mechanisms in proximity-induced hybrid\n  superconducting interfaces: Understanding depairing effects in a hybrid-superconducting interface\nutilizing high spin-orbit materials such as topological insulators or 1D\nsemiconducting nanowires is becoming an important research topic in the study\nof proximity-induced superconductivity. Experimentally, proximity-induced\nsuperconductivity is found to suppress at much lower magnetic fields compared\nto the superconducting layer without a good understanding of its cause. Here,\nwe provide a phenomenological tool to characterize different pair-breaking\nmechanisms, the ones that break or preserve time reversal symmetry, and show\nhow they affect the differential tunneling conductance response. Importantly,\nwe probe the properties of the SC layer at the hybrid interface and observe\nconductance peak pinning at zero bias in a larger field range with eventual\nsigns of weak peak splitting. Further, the effect of varying the spin-orbit\nscattering and the Lande g-factor in tuning the conductance peaks show\ninteresting trends.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Study of Local Nonlinear Properties Using a Near-Field Microwave\n  Microscope: We have developed a near-field microwave microscope to locally apply\nmicrowave frequency currents and fields to superconductors, and dielectric\nsubstrates, and measure the locally generated 2nd and 3rd harmonic responses.\nWe measure the local nonlinear response of a Tl_2Ba_2CaCu_2O_y film grown on an\nMgO substrate, and observe a large response due to the enhanced current density\nnear the edge. We also study the local nonlinear response of a YBa_2Cu_3O_7-d\nthin film grown on a bi-crystal SrTiO_3 (STO) substrate, and spatially identify\nthe grain boundary through higher harmonic measurements. The spatial resolution\nis determined by the magnetic loop probe size. A scaling current density JNL is\nextracted to quantify the magnitude of the nonlinearity of the superconductor.\nPreliminary results on the nonlinear properties of some commonly used\nsubstrates, e.g. MgO and STO, have also been obtained",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity from site-selective Ru doping studies in Zr$_5$Ge$_3$\n  compound: Systematical doping studies have been carried out to search for the possible\nsuperconductivity in the transition metal doped Zr$_5$Ge$_3$ system.\nSuperconductivity up to 5.7K is discovered in the Ru-doped\nZr$_5$Ge$_{2.5}$Ru$_{0.5}$ sample. Interestingly, with the same Ru-doping,\nsuperconductivity is only induced with doping at the Ge site, but remains\nabsent down to 1.8K with doping at the Zr site or interstitial site. Both\nmagnetic and transport studies have revealed the bulk superconductivity nature\nfor Ru-doped Zr$_5$Ge$_{2.5}$Ru$_{0.5}$ sample. The high upper critical field,\nenhanced electron correlation, and extremely small electron-phonon coupling,\nhave indicated possible unconventional superconductivity in this system, which\nwarrants further detailed theoretical and experimental studies.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Extracting phase information about the superconducting order parameter\n  from defect bound states: Impurity bound states and quasi-particle scattering from these can serve as\nsensitive probes for identifying the pairing state of a superconducting\ncondensate. We introduce and discuss defect bound state quasi-particle\ninterference (DBS-QPI) imaging as a tool to extract information about the\nsymmetry of the order parameter from spatial maps of the density of states\naround magnetic and non-magnetic impurities. We show that the phase information\ncontained in the scattering patterns around impurities can provide valuable\ninformation beyond what is obtained through conventional QPI imaging. Keeping\ntrack of phase, rather than just magnitudes, in the Fourier transforms is\nachieved through phase-referenced Fourier transforms that preserve both real\nand imaginary parts of the QPI images. We further compare DBS-QPI to other\napproaches which have been proposed to use either QPI or defect scattering to\ndistinguish different symmetries of the order parameter.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Effective medium approximation and the complex optical properties of the\n  inhomogeneous superconductor K_{0.8}Fe_{2-y}Se_2: The in-plane optical properties of the inhomogeneous iron-chalcogenide\nsuperconductor K_{0.8}Fe_{2-y}Se_2 with a critical temperature Tc = 31 K have\nbeen modeled in the normal state using the Bruggeman effective medium\napproximation for metallic inclusions in an insulating matrix. The volume\nfraction for the inclusions is estimated to be ~ 10%; however, they appear to\nbe highly distorted, suggesting a filamentary network of conducting regions\njoined through weak links. The value for the Drude plasma frequency in the\ninclusions is much larger than the volume average, which when considered with\nthe reasonably low values for the scattering rate, suggests that the transport\nin the grains is always metallic. Estimates for the dc conductivity and the\nsuperfluid density in the grains places the inclusions on the universal scaling\nline close to the other homogeneous iron-based superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Flatband-Induced Itinerant Ferromagnetism in RbCo$_2$Se$_2$: $A$Co$_2$Se$_2$ ($A$=K,Rb,Cs) is a homologue of the iron-based\nsuperconductor, $A$Fe$_2$Se$_2$. From a comprehensive study of RbCo$_2$Se$_2$\nvia measurements of magnetization, transport, neutron diffraction,\nangle-resolved photoemission spectroscopy, and first-principle calculations, we\nidentify a ferromagnetic order accompanied by an orbital-dependent\nspin-splitting of the electronic dispersions. Furthermore, we identify the\nordered moment to be dominated by a $d_{x^2-y^2}$ flatband near the Fermi\nlevel, which exhibits the largest spin splitting across the ferromagnetic\ntransition, suggesting an itinerant origin of the ferromagnetism. In the\nbroader context of the iron-based superconductors, we find this $d_{x^2-y^2}$\nflatband to be a common feature in the band structures of both\niron-chalcogenides and iron-pnictides, accessible via heavy electron doping.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Hidden charge-conjugation, parity, and time-reversal symmetries and\n  massive Goldstone (Higgs) modes in superconductors: A massive Goldstone (MG) mode, often referred to as a Higgs amplitude mode,\nis a collective excitation that arises in a system involving spontaneous\nbreaking of a continuous symmetry, along with a gapless Nambu-Goldstone mode.\nIt has been known in the previous studies that a pure amplitude MG mode emerges\nin superconductors if the dispersion of fermions exhibits the particle-hole\n(p-h) symmetry. However, clear understanding of the relation between the\nsymmetry of the Hamiltonian and the MG modes has not been reached. Here we\nreveal the fundamental connection between the discrete symmetry of the\nHamiltonian and the emergence of pure amplitude MG modes. To this end, we\nintroduce nontrivial charge-conjugation ($\\mathcal C$), parity ($\\mathcal P$),\nand time-reversal ($\\mathcal T$) operations that involve the swapping of pairs\nof wave vectors symmetrical with respect to the Fermi surface. The product of\n$\\mathcal{CPT}$ (or its permutations) represents an exact symmetry analogous to\nthe CPT theorem in the relativistic field theory. It is shown that a fermionic\nHamiltonian with a p-h symmetric dispersion exhibits the discrete symmetries\nunder $\\mathcal C$, $\\mathcal P$, $\\mathcal T$, and $\\mathcal{CPT}$. We find\nthat in the superconducting ground state, $\\mathcal T$ and $\\mathcal P$ are\nspontaneously broken simultaneously with the U(1) symmetry. Moreover, we\nrigorously show that amplitude and phase fluctuations of the gap function are\nuncoupled due to the unbroken $\\mathcal C$. In the normal phase, the MG and NG\nmodes become degenerate, and they have opposite parity under $\\mathcal T$.\nTherefore, we conclude that the lifting of the degeneracy in the\nsuperconducting phase and the resulting emergence of the pure amplitude MG mode\ncan be identified as a consequence of the the spontaneous breaking of $\\mathcal\nT$ symmetry but not of $\\mathcal P$ or U(1).",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quasiparticle conductance in Spin Valve Josephson Structures: We study the quasiparticle current in clean ferromagnetic Josephson\nstructures of the form $S_1/F_1/N/F_2/S_2$, where $S$, $F$, and $N$ denote\nsuperconducting, ferromagnetic or normal layers respectively. Our focus is on\nthe structure of the conductance $G$ as a function of bias $V$, emphasizing the\nsubgap region. We use a fully self consistent numerical method, coupled to a\ntransfer matrix procedure to extract $G(V)$. We choose material parameters\nappropriate to experimentally realized Co Cu Nb structures. We find a resonance\npeak structure as a function of the intermediate layer thickness and of the\nmisalignement angle $\\phi$ between $F$ layers. To understand this resonance\nstructure, we develop an approximate analytic method. For experimentally\nrelevant thicknesses, the conductance has multiple subgap peaks which oscillate\nin position between low and critical bias positions. These oscillations occur\nin both $\\phi$ and the layer thicknesses. We compare our results with those\nobtained for the spin valve structures $(F_1/N/F_2/S_2)$ and discuss the\nimplications of our results for the fabrication of spin Josephson devices.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Reverse Monte Carlo study of apical Cu-O bond distortions in\n  YBa2Cu3O6.93: A combination of neutron total scattering measurement and reverse Monte Carlo\n(RMC) refinement is applied to the study of apical Cu-O bond distortions in the\nhigh-Tc superconductor YBa2Cu3O6.93. We show that the average structure is not\nconsistent with a split-site model for the corresponding Cu and O positions,\nbut that the local structure nevertheless reveals the existence of two separate\napical Cu-O bond lengths. Using G(r) data obtained from a variety of Qmax\nvalues we show that this result is independent of the data treatment\nmethodology. We also find that the resulting 'short' and 'long' Cu-O bond\nlengths agree well with the results of previous EXAFS studies. The existence of\nbimodal apical Cu-O bond distributions in the context of a single-site average\nstructure model is interpreted in terms of correlated displacements of the Cu\nand O atoms. We find evidence also for the clustering of short apical Cu-O\nbonds within our RMC configurations.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Evolution of vortex pinning in the FeSe$_{1-x}$S$_x$ system: We present a comprehensive study of vortex matter and pinning evolution in\nthe FeSe$_{1-x}$S$_x$ system with various doping degree. The influence of\nsulphur substitution on vortex pinning and peak effect occurrence is studied.\nWe show that there is a complex interplay among various pinning contributions\nin the FeSe$_{1-x}$S$_x$ system. Additionally, we study a possible vortex\nliquid-vortex glass/lattice transition and find an evidence that the vortex\nliquid-vortex glass phase transition in FeSe has a quasi two-dimensional\nnature. We investigate the upper critical field behaviour in FeSe$_{1-x}$S$_x$\nsystem, and found that the upper critical field is higher than that predicted\nby the Werthamer-Helfand-Hohenberg (WHH) model, whereas its temperature\ndependence could be fitted within a two-band framework. Finally, a detailed H-T\nphase diagram is presented.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Elevated Tc levels in YBa2Cu3O6.5 modeled on a 3-dimensional doped bond\n  structure of chain and plane pairs: The complex phenomenology of shot quenched YBa2Cu3O6.5 with Tc=100K and 200K\nlevels is compared with laser pulsed analogs with an eye on explaining the\npresumed Tc=552K of the latter. Shot quenching can produce metastable states\nwith pronounced increases in plane metric and cell volume, accompanied by a\nrough doubling of Tc to a 100K level of an orthorhombic with 3-fold O\ncoordinated chain Cu (O3 type). These states decay over a non-superconducting\ntransition range to the conventional Tc=50K level of O24. We consider the plane\nexpanded laser pulsed materials to contain aspects of O42 plane n-doped\ncounterparts of the O3 n-doped version of shot quench preparations. In\naddition, we assume that highly charged p-doped chains of 4-fold O coordination\nform hole pairs at trijugate position, allowing close approach of the apical O\nto the electron-doped planes. They are now capable of participating in the\nbonding with the plane pairs at corresponding 3a0/2 location. The overall pair\nnumber is therefore multiplied, and the coupling strengthened, by limited 3-D\neffects within the Plane-Chain-Plane sandwich. The latter can be seen as an\nextended chemical bonding system that has the potential to equilibrate\ncontractive and expansive pairs and so obviate the need for distinction of\ndoping type as it may exchange it dynamically. It is argued that indications\nfor a Tc=200K level on shot quenching has a related origin and represents one\nin series of predicted Tc levels based on bond order principles. Predictions\nare made where similar effects can be expected in other compound classes.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Phases of Mott-Hubbard bilayers: A phase diagram of two Mott-Hubbard planes interacting with a short-range\nCoulomb repulsion is presented. Considering the case of equal amount of doping\nby holes in one layer as electrons in the other, a holon-doublon inter-layer\nexciton formation is shown to be a natural consequence of Coulomb attraction.\nQuasiparticle spectrum is gapped and incoherent below a critical doping\n$\\delta_c$ due to the formation of excitons. A spin liquid insulator (SLI)\nphase is thus realized without the lattice frustration. The critical value\n$\\delta_{c}$ sensitively depends on the inter-layer interaction strength. In\nthe $tJ$ model description of each layer with the d-wave pairing, $\\delta_{c}$\nmarks the crossover between SLI and d-wave superconductor. The SLI phase,\ndespite being non-superconducting and charge-gapped, still shows\nelectromagnetic response similar to that of a superfluid due to the exciton\ntransport. Including antiferromagnetic order in the $tJ$ model introduces\nmagnetically ordered phases at low doping and pushes the spin liquid phase to a\nlarger inter-layer interaction strength and higher doping concentrations.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Properties of the phonon-induced pairing interaction in\n  YBa$_2$Cu$_3$O$_7$ within the local density approximation: The properties of the phonon-induced interaction between electrons are\nstudied using the local density approximation (LDA). Restricting the electron\nmomenta to the Fermi surface we find generally that this interaction has a\npronounced peak for large momentum transfers and that the interband\ncontributions between bonding and antibonding band are of the same magnitude as\nthe intraband ones. Results are given for various symmetry averages of this\ninteraction over the Fermi surface. In particular, we find that the\ndimensionless coupling constant in the d-wave channel $\\lambda^d$, relevant for\nsuperconductivity, is only 0.022, i.e., even about ten times smaller than the\nsmall value of the s-wave channel. Similarly, the LDA contribution to the\nresistivity is about a factor 10 times smaller than the observed resistivity\nsuggesting that phonons are not the important low-energy excitations in\nhigh-T$_c$ oxides.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pressure induced metallization and possible unconventional\n  superconductivity in spin liquid $NaYbSe_{2}$: Beyond the conventional electron pairing mediated by phonons,\nhigh-temperature superconductivity in cuprates is believed to stem from quantum\nspin liquid (QSL). The unconventional superconductivity by doping a spin\nliquid/Mott insulator, is a long-sought goal but a principal challenge in\ncondensed matter physics because of the lack of an ideal QSL platform. Here we\nreport the pressure induced metallization and possible unconventional\nsuperconductivity in $NaYbSe_{2}$, which belongs to a large and ideal family of\ntriangular lattice spin liquid we revealed recently and is evidenced to possess\na QSL ground state. The charge gap of NaYbSe2 is gradually reduced by applying\npressures, and at ~20 GPa the crystal jumps into a superconducting (SC) phase\nwith Tc ~ 5.8 K even before the insulating gap is completely closed. The\nmetallization is confirmed by further high-pressure experiments but the sign of\nsuperconductivity is not well repeated. No symmetry breaking accompanies the SC\ntransition, as indicated by X-ray diffraction and low-temperature Raman\nexperiments under high pressures. This intrinsically connects QSL and SC\nphases, and suggests an unconventional superconductivity developed from QSL. We\nfurther observed the magnetic-field-tuned superconductor-insulator transition\nwhich is analogous to that found in the underdoped cuprate superconductor\n$La_{2-x}Sr_{x}CuO_{4}$. The study is expected to inspire interest in exploring\nnew types of superconductors and sheds light into the intriguing physics from a\nspin liquid/Mott insulator to a superconductor.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Weakly correlated electrons on a square lattice: a renormalization group\n  theory: We study the weakly interacting Hubbard model on the square lattice using a\none-loop renormalization group approach. The transition temperature T_c between\nthe metallic and (nearly) ordered states is found. In the parquet regime, (T_c\n>> |mu|), the dominant correlations at temperatures below T_c are\nantiferromagnetic while in the BCS regime (T_c << |mu|) at T_c the d-wave\nsinglet pairing susceptibility is most divergent.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconducting d-wave stripes in cuprates: Valence bond order\n  coexisting with nodal quasiparticles: We point out that unidirectional bond-centered charge-density-wave states in\ncuprates involve electronic order in both s- and d-wave channels, with\nnon-local Coulomb repulsion suppressing the s-wave component. The resulting\nbond-charge-density wave, coexisting with superconductivity, is compatible with\nrecent photoemission and tunneling data and as well as neutron-scattering\nmeasurements, once long-range order is destroyed by slow fluctuations or glassy\ndisorder. In particular, the real-space structure of d-wave stripes is\nconsistent with the scanning-tunneling-microscopy measurements on both\nunderdoped Bi2Sr2CaCu2O8+x and Ca2-xNaxCuO2Cl2 of Kohsaka et al. [Science 315,\n1380 (2007), arXiv:cond-mat/0703309].",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Andreev reflection observation in Nb/FeSi/Nb and Nb/FeSi/Si/Nb Josephson\n  junctions: Electrical properties of Josephson junctions Nb/FeSi/Nb with\nsuperconductor/ferromagnet (S/F)interfaces are presented. Due to Andreev\nreflection the nearly exact quadruple enhancement of the tunnel junction\ndifferential conductance compared with that of the normal state was achieved.\nThe transparency of the S/F interfaces in our junctions was estimated to be\nclose to unity. This almost ideal value is obtained due to the use of a very\nsmooth amorphous magnetic FeSi alloy for the barrier preparation. The real\nstructure of the Nb/FeSi/Nb tunnel junction is described as a S/F/I/F/S\njunction. Also Nb/FeSi/Si/Nb Josephson junctions were investigated and the\nresults found on these junctions confirm the effects observed in Nb/FeSi/Nb.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Time-Reversal-Symmetry-Broken Superconductivity Induced by Frustrated\n  Inter-Component Couplings: Superconductivity is associated with spontaneously broken gauge symmetry. In\nsome exotic superconductors the time-reversal symmetry is broken as well,\naccompanied with internal magnetic field. A time-reversal symmetry broken\n(TRSB) superconductivity without internal magnetic field involved can be\ninduced by frustrated inter-component couplings, which becomes a realistic\nissue recently due to the discovery of iron-pnictide superconductors. Here we\nderive stability condition for this novel TRSB state using the Ginzburg-Landau\n(GL) theory. We find that there are multiple divergent coherence lengths, and\nthat this novel superconductivity cannot be categorized by the GL number into\ntype I or type II. We reveal that the critical Josephson current of a\nconstriction junction between two bulk superconductors of different chiralities\nis suppressed significantly from that for same chirality. This effect provides\na unique way to verify experimentally this brand new superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Topological phase separation in 2D hard-core Bose-Hubbard system away\n  from half-filling: We suppose that the doping of the 2D hard-core boson system away from\nhalf-filling may result in the formation of multi-center topological defect\nsuch as charge order (CO) bubble domain(s) with Bose superfluid (BS) and extra\nbosons both localized in domain wall(s), or a {\\it topological} CO+BS {\\it\nphase separation}, rather than an uniform mixed CO+BS supersolid phase.\nStarting from the classical model we predict the properties of the respective\nquantum system. The long-wavelength behavior of the system is believed to\nremind that of granular superconductors, CDW materials, Wigner crystals, and\nmulti-skyrmion system akin in a quantum Hall ferromagnetic state of a 2D\nelectron gas.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Dislocation Majorana zero modes in perovskite oxide 2DEG: Much of the current experimental efforts for detecting Majorana zero modes\nhave been centered on probing the boundary of quantum wires with strong\nspin-orbit coupling. The same type of Majorana zero mode can also be realized\nat crystalline dislocations in 2D superconductors with the nontrivial weak\ntopological indices. Unlike at an Abrikosov vortex, at such a dislocation,\nthere are no other low-lying midgap states than the Majorana zero mode so that\nit avoids usual complications encountered in experimental detections such as\nscanning tunneling microscope (STM) measurements. We will show that, using the\nanisotropic dispersion of the $t_{2g}$ orbitals of Ti or Ta atoms, such a weak\ntopological superconductivity can be realized when the surface 2DEG of\nSrTiO$_3$ or KTaO$_3$ becomes superconducting, which can occur through either\nintrinsic pairing or proximity to existing s-wave superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Dispersion of the odd magnetic resonant mode in near-optimally doped\n  Bi2Sr2CaCu2O8+d: We report a neutron scattering study of the spin excitation spectrum in the\nsuperconducting state of slightly overdoped Bi2Sr2CaCu2O8+d system (Tc=87 K).\nWe focus on the dispersion of the resonance peak in the superconducting state\nthat is due to a S=1 collective mode. The measured spin excitation spectrum\nbears a strong similarity to the spectrum of the YBa2Cu3O6+x system for a\nsimilar doping level i.e. x= 0.95-1), which consists of intersecting upward-\nand downward-dispersing branches. A close comparison of the threshold of the\nelectron-hole spin flip continuum, deduced from angle resolved photo-emission\nmeasurements in the same system, indicates that the magnetic response in the\nsuperconducting state is confined, in both energy and momentum, below the\ngapped Stoner continuum. In contrast to YBa2Cu3O6+x, the spin excitation\nspectrum is broader than the experimental resolution. In the framework of an\nitinerant-electron model, we quantitatively relate this intrinsic energy width\nto the superconducting gap distribution observed in scanning tunnelling\nmicroscopy experiments. Our study further suggests a significant in-plane\nanisotropy of the magnetic response.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Mo$_{3}$ReRuC: A noncentrosymmetric superconductor formed in the\n  MoReRu-Mo$_{2}$C system: A quaternary compound with the composition Mo$_{3}$ReRuC is obtained in a\npreviously unexplored MoReRu-Mo$_{2}$C system. According to x-ray structural\nanalysis, Mo$_{3}$ReRuC crystallizes in the noncentrosymmetric space group\n$P$4$_{1}$32 (cubic $\\beta$-Mn type structure, $a$ = 6.8107(1) {\\AA}). Below\n7.7 K, Mo$_{3}$ReRuC becomes a bulk type-II superconductor with an upper\ncritical field close to the Pauli paramagnetic limit. The specific heat data\ngives a large normalized jump $\\Delta$$C_{\\rm p}$/$\\gamma$$T_{\\rm c}$ = 2.3 at\n$T_{\\rm c}$, which points to a strongly coupled superconducting state. First\nprinciples calculations show that its electronic states at the Fermi level are\nmainly contributed by Mo, Re and Ru atoms and strongly increased by the\nspin-orbit coupling. Our finding suggests that the intermediate phase between\nalloys and carbides may be a good place to look for $\\beta$-Mn type\nnoncentrosymmetric superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconducting energy gap in $\\rm Ba_{1-x}K_xBiO_3$: Temperature\n  dependence: The superconducting energy gap of $\\rm Ba_{1-x}K_xBiO_3$ has been measured by\ntunneling. Despite the fact that the sample was macroscopically single phase\nwith very sharp superconducting transition $T_c$ at 32~$K$, some of the\nmeasured tunnel junctions made by point contacts between silver tip and single\ncrystal of $\\rm Ba_{1-x}K_xBiO_3$ had lower transition at 20~$K$. Local\nvariation of the potassium concentration as well as oxygen deficiency in $\\rm\nBa_{1-x}K_xBiO_3$ at the place where the point contact is made can account for\nthe change of $T_c$. The conductance curves of the tunnel junctions reveal the\nBCS behavior with a small broadening of the superconducting-gap structure. A\nvalue of the energy gap scales with $T_c$. The reduced gap amounts to\n$2\\Delta/kT_c = 4\\div 4.3$ indicating a medium coupling strength. Temperature\ndependence of the energy gap follows the BCS prediction.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Stability Conditions in Gapless Superconductors: Gapless superconductivity can arise when pairing occurs between fermion\nspecies with different Fermi surface sizes, provided there is a sufficiently\nlarge mismatch between Fermi surfaces and/or at sufficiently large coupling\nconstant. In gapless states, secondary Fermi surfaces appear where\nquasiparticle excitation energy vanishes. This work focuses on homogeneous and\nisotropic superfluids in the s-wave channel, with either zero (conventional\nsuperconductor), one, or two spherical Fermi surfaces. The stability conditions\nfor these candidate phases are analyzed. It is found that gapless states with\none Fermi surface are stable in the BEC region, while gapless states with two\nFermi surfaces are unstable in all parameter space. The results can be applied\nto ultracold fermionic atom systems.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Aharonov-Bohm Effect for Quasiparticles around a Vortex Line in a D-wave\n  Superconductor: On the basis of the Bogolubov-de Gennes theory we develop an analytical\ndescription of low-energy extended quasiparticle states around an isolated flux\nline in a superconductor with gap nodes. The wavefunctions of these excitations\nand the corresponding density of states are shown to be strongly influenced by\nthe interaction with a pure gauge potential due to the Aharonov-Bohm scenario.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetic resonance in a singlet-triplet Josephson junction: We study a singlet-triplet Josephson junction between a conventional s-wave\nsuperconductor and an unconventional p$_{\\rm x}$-wave superconductor. The\nAndreev spectrum of the junction yields a spontaneous magnetization in\nequilibrium. This allows manipulating the occupation of the Andreev levels\nusing an ac Zeeman field. The induced Rabi oscillations manifest themselves as\na resonance in the current-phase relation. For a circularly polarized magnetic\nfield, we find a spin selection rule, yielding Rabi oscillations only in a\ncertain interval of the superconducting phase difference.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Anomalous lattice expansion of RuSr2Eu1.5Ce0.5Cu2O10(Ru-1222) magneto\n  superconductor: A low temperature X-ray diffraction study: This is the first report of the observation of the onset of excess volume and\nalso of the strain along the a-axis near the magnetic ordering temperature in\nRu-1222 superconductor, and indicates a coupling between the lattice and the\nmagnetism in this system. Magnetization, magneto transport and thermoelectric\npower measurements being carried out on the same sample are also reported.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Thin Spectrum States in Superconductors: We show that finite size superconductors have a spectrum of states at\nextremely low energy, i.e. inside the superconducting gap. The presence of this\n{\\it thin spectrum} is a generic feature and related to the fact that in a\nsuperconductor the global phase rotation symmetry is spontaneously broken. For\na strong coupling superconductor we find the spectrum by exactly solving the\nLieb-Mattis type Hamiltonian onto which the problem maps. One of the physical\nconsequences of the presence of thin states is that they cause quantum\ndecoherence in superconducting qubits of finite extent.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Multiorbital effects on the transport and the superconducting\n  fluctuations in LiFeAs: The resistivity, Hall effect and transverse magnetoresistance (MR) have been\nmeasured in low residual resistivity single crystals of LiFeAs. A comparison\nwith angle resolved photoemission spectroscopy and quantum oscillation data\nimplies that four carrier bands unevenly contribute to the transport. However\nthe scattering rates of the carriers all display the T^2 behavior expected for\na Fermi liquid. Near Tc low field deviations of the MR with respect to a H^2\nvariation permit us to extract the superconducting fluctuation contribution to\nthe conductivity. Though below Tc the anisotropy of superconductivity is rather\nsmall, the superconducting fluctuations display a quasi ideal two-dimensional\nbehavior which persists up to 1.4 Tc. These results call for a refined\ntheoretical understanding of the multiband behavior of superconductivity in\nthis pnictide.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Temperature and field dependence of thermally activated flux flow\n  resistance in Bi2Sr2CaCu2O8+\u03b4 superconductor: We study the temperature dependence of the resistivity as a function of\nmagnetic field in superconducting transition (Tconset - TcR=0) region for\ndifferent Bi2Sr2CaCu2O8+{\\delta} superconducting samples being synthesized\nusing sol-gel method. The superconducting transition temperature (TcR=0) of the\nstudied samples is increased from 32 K to 82K by simply increasing the final\nsintering temperature with an improved grains morphology. On the other hand,\nbroadening of transition is increased substantially with decrease in sintering\ntemperature; this is because Tconset is not affected much with grains\nmorphology. Further broadening of the superconducting transition is seen under\nmagnetic field, which is being explained on the basis of thermally activated\nflux flow (TAFF) below superconducting transition temperature (Tc). TAFF\nactivation energy (U0) is calculated using the resistive broadening of samples\nin the presence of magnetic field. Temperature dependence of TAFF activation\nenergy revealed linear temperature dependence for all the samples. Further,\nmagnetic field dependence is found to obey power law for all the samples and\nthe negative exponent is increased with increase in sintering temperature or\nthe improved grains morphology for different Bi-2212 samples. We believe that\nthe sintering temperature and the ensuing role of grain morphology is yet a key\nissue to be addressed in case of cuprate superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Continuous and reversible tuning of the disorder-driven\n  superconductor-insulator transition in bilayer graphene: The influence of static disorder on a quantum phase transition (QPT) is a\nfundamental issue in condensed matter physics. As a prototypical example of a\ndisorder-tuned QPT, the superconductor-insulator transition (SIT) has been\ninvestigated intensively over the past three decades, but as yet without a\ngeneral consensus on its nature. A key element is good control of disorder.\nHere, we present an experimental study of the SIT based on precise in-situ\ntuning of disorder in dual-gated bilayer graphene proximity-coupled to two\nsuperconducting electrodes through electrical and reversible control of the\nband gap and the charge carrier density. In the presence of a static disorder\npotential, Andreev-paired carriers formed close to the Fermi level in bilayer\ngraphene constitute a randomly distributed network of proximity-induced\nsuperconducting puddles. The landscape of the network was easily tuned by\nelectrical gating to induce percolative clusters at the onset of\nsuperconductivity. This is evidenced by scaling behavior consistent with the\nclassical percolation in transport measurements. At lower temperatures, the\nsolely electrical tuning of the disorder-induced landscape enables us to\nobserve, for the first time, a crossover from classical to quantum percolation\nin a single device, which elucidates how thermal dephasing engages in\nseparating the two regimes.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The London moment: what a rotating superconductor reveals about\n  superconductivity: The London moment is the magnetic moment acquired by a rotating\nsuperconductor. We propose that the London moment reveals the following\nfundamental properties of the superconducting state: (i) superconductors\n(unlike normal metals) know the $sign$ of the charge carriers, (ii) the\nsuperconducting charge carriers are $free$ electrons, (iii) electrons are\nexpelled from the interior to the surface in the transition to the\nsuperconducting state, (iv) superfluid electrons occupy orbits of radius\n$2\\lambda_L$ ($\\lambda_L=$London penetration depth), and (v) a spin current\nexists in the ground state of superconductors. These properties are consistent\nwith the Meissner effect, however the Meissner effect does not $directly$\nreveal the sign of the charge carriers nor the fact that the carrier's mass is\nthe free electron mass nor the fact that a spin current exists in\nsuperconductors. Note also that within the BCS theory of superconductivity none\nof the key properties of superconductors listed above are predicted. Instead,\nthese properties are predicted by the theory of hole superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity of Cobalt in Thin Films: Due to competing long range ferromagnetic order, the transition metals Fe, Co\nand Ni are not superconductors at ambient pressure. While superconductivity was\nobserved in a non-magnetic phase of Fe, stabilized under pressure, it is yet to\nbe discovered in Co and Ni under any experimental conditions. Here, we report\nemergence of superconductivity in the recently discovered high-density\nnonmagnetic face centered cubic phase in Co thin films below a transition\ntemperature (Tc) of ~5.4 K, as revealed in experiments based on point-contact\nspectroscopy and resistance, and four-probe measurements of resistance at\nambient pressure. We confirm the non-magnetic nature of the dense fcc phase of\nCo within first-principles density functional theory, and show that its\nsuperconductivity below 5 K originates from anomalous softening of\nzone-boundary phonons and their enhanced coupling with electrons upon biaxial\nstrain.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Exact solution of the Lawrence-Doniach model in parallel magnetic fields: For the first time, we obtain the complete and exact analytical solution of\nthe Lawrence-Doniach model for layered superconductors in external parallel\nmagnetic fields. By solving a nontrivial mathematical problem of exact\nminimization of the free-energy functional, we derive a closed, self-consistent\nsystem of mean-field equations involving only two variables. Exact solutions to\nthese equations prove simultaneous penetration of Josephson vortices into all\nthe barriers, yield a completely new expression for the lower critical field,\nrefute the concept of a triangular Josephson vortex lattice and clarify the\nphysics of Fraunhofer oscillations of the total critical Josephson current.\n  PACS numbers: 74.80.Dm, 74.20.De, 74.50.+r",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Emergent phenomena in multicomponent superconductivity: an introduction\n  to the focus issue: Multicomponent superconductivity is a novel quantum phenomenon in many\ndifferent superconducting materials, such as multiband ones in which different\nsuperconducting gaps open in different Fermi surfaces, films engineered at the\natomic scale to enter the quantum confined regime, multilayers, two-dimensional\nelectron gases at the oxide interfaces, and complex materials in which\ndifferent electronic orbitals or different carriers participate in the\nformation of the superconducting condensate. In all these systems the increased\nnumber of degrees of freedom of the multicomponent superconducting\nwave-function allows for emergent quantum effects that are otherwise\nunattainable in single-component superconductors. In this editorial paper we\nintroduce the present focus issue, exploring the complex but fascinating\nphysics of multicomponent superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Reduction of electron repulsion and enhancement of $T_{c}$ in small\n  diffusive superconducting grains: The superconducting properties of small metallic grains has been a topic of\nactive research for half a century now. Early experiments demonstrated a\nremarkable rise in the critical temperature, $T_{c}$, with reducing grain size\nin a variety of materials. In two dimensional diffusive superconductors,\n$T_{c}$ is decreased due to enhanced Coulomb repulsion. We propose that in\nfinite size grains, the diffusive enhancement of the Coulomb repulsion is\nweakened and leads ultimately to an increase in $T_{c}$ in isolated, disordered\ntwo dimensional grains. Our mechanism is superimposed on the possible\nenhancement in $T_{c}$ due to the change in the density of states of finite\nsize systems.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Doping evoluton of antiferromagnetic order and structural distortion in\n  LaFeAsO$_{1-x}$F$_x$: We use neutron scattering to study the structural distortion and\nantiferromagnetic (AFM) order in LaFeAsO$_{1-x}$F$_{x}$ as the system is doped\nwith fluorine (F) to induce superconductivity. In the undoped state, LaFeAsO\nexhibits a structural distortion, changing the symmetry from tetragonal (space\ngroup $P4/nmm$) to orthorhombic (space group $Cmma$) at 155 K, and then\nfollowed by an AFM order at 137 K. Doping the system with F gradually decreases\nthe structural distortion temperature, but suppresses the long range AFM order\nbefore the emergence of superconductivity. Therefore, while superconductivity\nin these Fe oxypnictides can survive in either the tetragonal or the\northorhombic crystal structure, it competes directly with static AFM order.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The Berry curvature of the Bogoliubov quasiparticle Bloch states in the\n  unconventional superconductor Sr$_2$RuO$_4$: We will extend the concept of electron band Berry curvatures to\nsuperconducting materials. We show that this can be defined for the\nBogoliubov-de Gennes equation describing the superconducting state in a\nperiodic crystal. In addition, the concept is exploited to understand the\ndriving mechanism for the optical Kerr effect in time reversal symmetry\nbreaking superconductors. Finally, we establish a sum rule analogue to the\nnormal state Hall sum rule making quantitative contact between the imaginary\npart of the optical conductivity and the Berry curvature. The general theory\nwill be applied and tested against the drosophila of the p-wave paired\nmaterials Sr$_2$RuO$_4$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Field-driven transition in the Ba$_{1-x}$K$_x$Fe$_2$As$_2$\n  superconductor with splayed columnar defects: Through 2.6 GeV U irradiations, we have induced bimodal splayed columnar\ndefects in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ single crystals with splay angles, $\\pm\n5 ^\\circ$, $\\pm 10 ^\\circ$, $\\pm 15 ^\\circ$, and $\\pm 20 ^\\circ$. Critical\ncurrent densities through magnetization measurements were carefully evaluated,\nwhere a splay angle of $\\pm 5 ^\\circ$ brought about the highest $J_\\mathrm{c}$.\nMageto-optical images close to $T_\\mathrm{c}$ indicates highly anisotropic\ndiscontinuity lines in the remnant state, and with anisotropy increasing with\ngreater splay angles. Moreover, amongst those with splayed columnar defects,\nanomalous non-monotonic field dependences of $J_\\mathrm{c}$ and $S$ with an\nextrema at some fraction of the matching field are observed. We discuss that\nsuch $J_\\mathrm{c}$ enhancement arises from a field-driven coupling transition\nin which intervortex interactions reorganize the vortex structure to be\naccommodated into columnar defects, thereby increasing pinning at higher\nfields.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Ginzburg-Landau theory for the time-dependent phase field in a\n  two-dimensional d-wave superconductor: We derive a finite temperature time-dependent effective theory for the phase\n$\\theta$ of the pairing field, which is appropriate for a 2D conducting\nelectron system with non-retarded d-wave attraction. As for s-wave pairing the\neffective action contains terms with Landau damping, but their structure\nappears to be different from the s-wave case due to the fact that the Landau\ndamping is determined by the quasiparticle group velocity $v_g$, which for the\nd-wave pairing does not have the same direction as the non-interacting Fermi\nvelocity $v_F$. We show that for the d-wave pairing the Landau terms have a\nlinear low temperature dependence and in contrast to the s-wave case are\nimportant for all finite temperatures.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Even Parity, Orbital Singlet and Spin Triplet Pairing for\n  Superconducting $La(O_{1-x}F_x)FeAs$: In the present paper, we propose the parity even,orbital singlet and spin\ntriplet pairing state as the ground state of the newly discovered\nsuper-conductor $LaO_{1-x}F_xFeAs$.The pairing mechanism involves both the\nspecial shape of the electron fermi surface and the strong ferromagnetic\nfluctuation induced by Hund's rule coupling.The special behavior of the\nBogoliubov quasi-particle spectrum may leads to \"Fermi arc\" like anisotropy\nsuper-conducting gap, which can be detected by angle resolved photo\nemission(ARPES).The impurity effects are also discussed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Critical currents and vortex-unbinding transitions in quench-condensed\n  ultrathin films of Bismuth and Tin: We have investigated the I-V characteristics of strongly disordered\nultra-thin films of {\\it Bi} and {\\it Sn} produced by quench-condensation. Our\nresults show that both these sytems can be visualized as strongly disordered\narrays of Josephson junctions. The experimentally observed I-V characteristics\nof these films is hysteretic, when the injected current is ramped from zero to\ncritical current and back. These are remarkably similar to the hysteretic I-V\nof an underdamped single junction. We show by computer simulations that\nhysteresis can persist in a very strongly disordered array. It is also possible\nto estimate the individual junction parameters ($R$, $C$ and $I_c$) from the\nexperimental I-Vs of the film using this model. The films studied are in a\nregime where the Josephson-coupling energy is larger than the charging energy.\nWe find that a simple relation $I_c(T)=I_c(0)(1-(T/T_c)^4)$ describes the\ntemperature dependence of the critical current quite accurately for films with\nsheet resistance $\\sim$ 500$\\Omega$ or lower. We also find evidence of a\nvortex-unbindi",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Comment on `Strong Vortex Liquid Correlation' from Multiterminal\n  Measurements on Untwinned YBa$_2$Cu$_3$O$_{7-\u03b4}$ Single Crystals': A.Rydh and \\\"O.Rapp [Phys. Rev. Lett. {\\bf 86}, 1873 (2001).] claim that the\nvortex liquid in untwinned YBa$_2$Cu$_3$O$_{7-\\delta}$ crystals is correlated\nabove the melting transition, in striking contrast to previous work [D.L\\'opez\n{\\it et al.}, Phys. Rev. Lett. {\\bf 76}, 4034 (1996).]. In this Comment we\npresent new measurements using the same experimental technique on twinned and\nuntwinned YBa$_2$Cu$_3$O$_{7-\\delta}$ crystals with similar overall\ncharacteristics as those reported by Rydh and Rapp . The comparison of the\nvortex correlation response in both cases indicates that the central conclusion\nof their work is not correct. Our results reconfirm the work by L\\'opez {\\it et\nal.} and points on the origin of the misinterpretation in the work of Rydh and\nRapp.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Spin Excitation in d-wave Superconductors : A Fermi Liquid Picture: A detailed study of the Inelastic Neutron Scattering (INS) spectra of the\nhigh-$T_c$ cuprates based on the Fermi liquid (FL) picture is given. We focus\non the issue of the transformation between the commensurate and incommensurate\n(IC) excitation driven by frequency or $temperature$. For\nLa$_{2-x}$Sr$_x$CuO$_4$ (LSCO), the condition of small $\\Delta(0)/v_F a$ (where\n$a$ is the lattice constant, and henceforth will be set to 1) can\nsimultaneously reproduces the always existing IC peaks in the superconducting\n(SC) and normal state, and the always fixed location at temperature or\nfrequency change. For YBa$_2$Cu$_3$O$_{6+x}$ (YBCO), a moderate $\\Delta(0)/v_F\na$ and proximity of the van Hove singularity (vHS) at ${\\bar M}=(0,\\pi)$ to the\nFermi level can reproduce the frequency- and temperature-driven shifting IC\npeaks in the SC state, and the vanishing of the IC peak in the normal state.\nThe commensurate peak is found to be more appropriately described as a random\nphase approximation (RPA) effect. We address the conditional peak shifting\nbehavior to a refined consideration on the nesting effect which is previously\noverlook. As a result, both the data on LSCO and the recent data on YBCO (on\nYBa$_2$Cu$_3$O$_{6.7}$ by Arai $et$ $al.$ and YBa$_2$Cu$_3$O$_{6.85}$ by\nBourges $et$ $al.$) can be reasonably reconciled within a FL picture. We also\npoint out that the one-dimensional-like data by Mook $et$ $al.$ on a detwinned\nand more underdoped sample YBa$_2$Cu$_3$O$_{6.6}$ could be due to a gap\nanisotropy effect discussed by Rendell and Carbotte, and we proceed to suggest\na way of clarifying it.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Cooper Pairing in A Doped 2D Antiferromagnet with Spin-Orbit Coupling: We study the two-dimensional Hubbard model with the Rashba type spin-orbit\ncoupling within and beyond the mean-field theory. The antiferromagnetic ground\nstate for the model at half-filling and the Cooper pairing induced by\nantiferromagnetic spin fluctuations near half-filling are examined based on the\nrandom-phase approximation. We show that the antiferromagnetic order is\nsuppressed and the magnetic susceptibility turns out to be anisotropic in the\npresence of the spin-orbit coupling. Energy spectrums of transverse spin\nfluctuations are obtained and the effective interactions between holes mediated\nby antiferromagnetic spin fluctuations are deduced in the case of low hole\ndoping. It seems that the spin-orbit coupling tends to form s+p-wave Cooper\npairs, while the s+d-wave pairing is dominant when the spin-orbit coupling is\nabsent.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Visualization of Electron Nematicity and Unidirectional Antiferroic\n  Fluctuations at High Temperatures in NaFeAs: The driving forces behind electronic nematicity in the iron pnictides remain\nhotly debated. We use atomic-resolution variable-temperature scanning tunneling\nspectroscopy to provide the first direct visual evidence that local electronic\nnematicity and unidirectional antiferroic (stripe) fluctuations persist to\ntemperatures almost twice the nominal structural ordering temperature in the\nparent pnictide NaFeAs. Low-temperature spectroscopic imaging of\nnematically-ordered NaFeAs shows anisotropic electronic features that are not\nobserved for isostructural, non-nematic LiFeAs. The local electronic features\nare shown to arise from scattering interference around crystalline defects in\nNaFeAs, and their spatial anisotropy is a direct consequence of the structural\nand stripe-magnetic order present at low temperature. We show that the\nanisotropic features persist up to high temperatures in the nominally\ntetragonal phase of the crystal. The spatial distribution and energy dependence\nof the anisotropy at high temperatures is explained by the persistence of large\namplitude, short-range, unidirectional, antiferroic (stripe) fluctuations,\nindicating that strong density wave fluctuations exist and couple to near-Fermi\nsurface electrons even far from the structural and density wave phase\nboundaries.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Revisiting Anderson-Higgs mechanism: application of Lieb-Schultz-Mattis\n  theorem: We consider an electron model of superconductivity on a three-dimensional\nlattice where there are on-site attractive Hubbard interaction and long-range\nrepulsive Coulomb interaction. It is claimed that fully gapped $s$-wave\nsuperconductivity within this model, if present, exhibits spontaneous\ntranslation symmetry breaking possibly related to a charge order. Our\ndiscussions are based on an application of the Lieb-Schultz-Mattis theorem\nunder some physical assumptions. The inconsistency between the proposed\nsupersolid and experiments can impose some constraints on a reasonable choice\nof a theoretical model.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Unveiling the hidden nematicity and spin subsystem in FeSe: The nematic order (nematicity) is considered one of the essential ingredients\nto understand the mechanism of Fe-based superconductivity. In most Fe-based\nsuperconductors (pnictides), nematic order is reasonably close to the\nantiferromagnetic order. In FeSe, in contrast, a nematic order emerges below\nthe structure phase transition at T_s = 90 K with no magnetic order. The case\nof FeSe is of paramount importance to a universal picture of Fe-based\nsuperconductors. The polarized ultrafast spectroscopy provides a tool to probe\nsimultaneously the electronic structure and the magnetic interactions through\nquasiparticle dynamics. Here we show that this approach reveals both the\nelectronic and magnetic nematicity below and, surprisingly, its fluctuations\nfar above Ts to at least 200 K. The quantitative pump-probe data clearly\nidentify a correlation between the topology of the Fermi surface (FS) and the\nmagnetism in all temperature regimes, thus providing profound insight into the\ndriving factors of nematicity in FeSe and the origin of its uniqueness.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Impurity effects on the spin excitation spectra in a d-wave\n  superconductor: The effects of nonmagnetic impurity on the spin excitation spectra in a\n$d_{x^2-y^2}$-wave superconductor are examined, using the self-consistent\n$t$-matrix approximation. It is shown that the impurity self-energy acts to\nshift the position of the resonance peak to low frequencies and broaden the\npeak. While the impurity vertex correction causes a broad spectral weight in\nthe spin gap at the impurity concentrations where no clear resonance peak is\nobserved. The gaplike feature still remains in low frequency region upon the\nintroduction of impurities. Incorporating these two effects, we find that the\nresult is in qualitative agreement with experiments on\nYBa_{2}(Cu_{1-x}Zn_{x})_{3}O_{6+y}.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Deviations from the extended London model at high magnetic fields in\n  YBa$_2$Cu$_3$O$_7$: We report on the evolution with magnetic field and temperature of the vortex\nlattice (VL) in fully-oxygenated YBa2Cu3O7 as studied by time-of-flight small\nangle neutron scattering. Using the HFM/EXED beamline, we have obtained data up\nto 25.9 T - much higher than that available previously. Our VL structure\nresults indicate the progressive suppression by field of the superconductivity\nalong the crystallographic b (CuO chain) direction. The intensity of the\ndiffracted signal reveals the spatial variation of magnetization caused by the\nVL (the \"form factor\"). Instead of a rapid fall-off with field, as seen in\nsuperconductors with smaller upper critical fields, we find that the form\nfactor is almost constant with field above about 12 T. We speculate that this\nis due to Pauli paramagnetic moments, which increase at high fields due to\nalignment of the spins of quasiparticles in the vortex cores.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Organic Superconductors: when correlations and magnetism walk in: This survey provides a brief account for the start of organic\nsuperconductivity motivated by the quest for high Tc superconductors and its\ndevelopment since the eighties'. Besides superconductivity found in 1D organics\nin 1980, progresses in this field of research have contributed to better\nunderstand the physics of low dimensional conductors highlighted by the wealth\nof new remarkable properties. Correlations conspire to govern the low\ntemperature properties of the metallic phase. The contribution of\nantiferromagnetic fluctuations to the interchain Cooper pairing proposed by the\ntheory is borne out by experimental investigations and supports\nsupercondutivity emerging from a non Fermi liquid background. Quasi one\ndimensional organic superconductors can therefore be considered as simple\nprototype systems for the more complex high Tc materials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Normal State Nernst Effect in Electron-doped Pr2-xCexCuO4:\n  Superconducting Fluctuations and Two-band Transport: We report a systematic study of normal state Nernst effect in the\nelectron-doped cuprates Pr$_{2-x}$Ce$_x$CuO$_{4-\\delta}$ over a wide range of\ndoping (0.05$\\leq x \\leq$0.21) and temperature. At low temperatures, we\nobserved a notable vortex Nernst signal above T$_c$ in the underdoped films,\nbut no such normal state vortex Nernst signal is found in the overdoped region.\nThe superconducting fluctuations in the underdoped region are most likely\nincoherent phase fluctuations as found in hole-doped cuprates. At high\ntemperatures, a large normal state Nernst signal is found at dopings from\nslightly underdoped to highly overdoped. Combined with normal state\nthermoelectric power, Hall effect and magnetoresistance measurements, the large\nNernst effect is compatible with two-band model. For the highly overdoped\nfilms, the large Nernst effect is anomalous and not explainable with a simple\nhole-like Fermi surface seen in photoemission experiments.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "An elastic lattice in a random potential: Using Monte Carlo simulations, we study the properties of an elastic\ntriangular lattice subject to a random background potential. As the cooling\nrate is reduced, we observe a rather sudden crossover between two different\nglass phases, one with exponential decay of correlations, the other with\npower-law decay. Contrary to predictions derived from continuum models, no\nevidence of a crossover in the mean-square displacement, B(r), from quadratic\ngrowth at small r, to logarithmic growth at large r is found.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "High-pressure flux growth, structural, and superconducting properties of\n  LnFeAsO (Ln = Pr, Nd, Sm) single crystals: Single crystals of the LnFeAsO (Ln1111, Ln = Pr, Nd, and Sm) family with\nlateral dimensions up to 1 mm were grown from NaAs and KAs flux at high\npressure. The crystals are of good structural quality and become\nsuperconducting when O is partially substituted by F (PrFeAsO1-xFx and\nNdFeAsO1-xFx) or when Fe is substituted by Co (SmFe1-xCoxAsO). From\nmagnetization measurements, we estimate the temperature dependence and\nanisotropy of the upper critical field and the critical current density of\nunderdoped PrFeAsO0.7F0.3 crystal with Tc = 25 K. Single crystals of\nSmFe1-xCoxAsO with maximal Tc up to 16.3 K for x = 0.08 were grown for the\nfirst time. From transport and magnetic measurements we estimate the critical\nfields and their anisotropy, and find these superconducting properties to be\nquite comparable to the ones in SmFeAsO1-xFx with a much higher Tc of = 50 K.\nThe magnetically measured critical current densities are as high as 109 A/m2 at\n2 K up to 7 T, with indication of the usual fishtail effect. The upper critical\nfield estimated from resistivity measurements is anisotropic with slopes of\n-8.7 T/K (H // ab-plane) and -1.7 T/K (H // c-axis). This anisotropy (= 5) is\nsimilar to that in other Ln1111 crystals with various higher Tc s.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Absence of Cooper-type bound states in three- and few-electron systems: It is shown that the appearance of a fixed-point singularity in the kernel of\nthe two-electron Cooper problem is responsible for the formation of the Cooper\npair for an arbitrarily weak attractive interaction between two electrons. This\nsingularity is absent in the problem of three and few superconducting electrons\nat zero temperature on the full Fermi sea. Consequently, such three- and\nfew-electron systems on the full Fermi sea do not form Cooper-type bound states\nfor an arbitrarily weak attractive pair interaction.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Observation of the Nernst signal generated by fluctuating Cooper pairs: Long-range order is destroyed in a superconductor warmed above its critical\ntemperature (Tc). However, amplitude fluctuations of the superconducting order\nparameter survive and lead to a number of well established phenomena such as\nparaconductivity : an excess of charge conductivity due to the presence of\nshort-lived Cooper pairs in the normal state. According to an untested theory,\nthese pairs generate a transverse thermoelectric (Nernst) signal. In amorphous\nsuperconducting films, the lifetime of Cooper pairs exceeds the elastic\nlifetime of quasi-particles in a wide temperature range above Tc; consequently,\nthe Cooper pairs Nernst signal dominate the response of the normal electrons\nwell above Tc. In two dimensions, the magnitude of the expected signal depends\nonly on universal constants and the superconducting coherence length, so the\ntheory can be unambiguously tested. Here, we report on the observation of a\nNernst signal in such a superconductor traced deep into the normal state. Since\nthe amplitude of this signal is in excellent agreement with the theoretical\nprediction, the result provides the first unambiguous case for a Nernst effect\nproduced by short-lived Cooper pairs.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "AC losses in macroscopic thin-walled superconducting niobium cylinders: Measurements of the ac response represent a widely-used method for probing\nthe properties of superconductors. In the surface superconducting state (SSS),\nincrease of the current beyond the surface critical current $I_c$ leads to\nbreakdown of SSS and penetration of external magnetic field into the sample\nbulk. An interesting free-of-bulk system in SSS is offered by thin-walled\nsuperconducting cylinders. The critical state model (CSM) asserts the ac\nsusceptibility $\\chi$ to exhibit jumps as a function of the external ac field\namplitude $H_{ac}$, because of the periodic destruction and restoration of SSS\nin the cylinder wall. Here, we investigate experimentally the low-frequency\n(128-8192\\,Hz) ac response of thin-walled superconducting cylinders in\nsuperimposed dc and ac magnetic fields applied parallel to the cylinder axis.\nDistinct from the CSM predictions, experiments reveal that $\\chi$ is a smooth\nfunction of $H_{ac}$. For the explanation of our observations we propose a\nphenomenological model of partial penetration of magnetic flux (PPMF). The PPMF\nmodel implies that after a restoration of the superconducting state, the\nmagnetic fields inside and outside the cylinder are not equal, and the value of\nthe penetrating flux is random for each penetration. This model fits very well\nto the experimental data on the temperature dependence of the first-harmonic\n$\\chi_1$ at any $H_{ac}$ and dc field magnitude. However, in a certain\ntemperature range the values of physical parameters deduced within the\nframework of the PPMF model are questionable.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Columnar defects acting as passive internal field detectors: We have studied the angular dependence of the irreversible magnetization of\nseveral YBa$_2$Cu$_3$O$_7$ and 2H-NbSe$_2$ single crystals with columnar\ndefects tilted off the c-axis. At high magnetic fields, the irreversible\nmagnetization $M_i(\\Theta_H)$ exhibits a well known maximum when the applied\nfield is parallel to the tracks. As the field is decreased below $H \\sim 0.02\nH_{c2}$, the peak shifts away from the tracks' direction toward either the\nc-axis or the ab-planes. We demonstrate that this shift results from the\nmisalignment between the external and internal field directions due to the\ncompetition between anisotropy and geometry effects.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Roles of anisotropic and unequal gaps in the quasiparticle interference\n  of superconducting iron pnictides: We investigate the role of gap characteristics such as anisotropy and\ninequality of the gaps in the quasiparticle interferences of iron pnictides\nusing a five-orbital tight-binding model. We examine how the difference in the\nsensitivities exhibited by the sign-changing and -preserving $s$-wave\nsuperconductivity in an annular region around ($\\pi, 0$), which can be used to\ndetermine the sign change of the superconducting gap, gets affected when the\ngaps are unequal on the electron and hole pocket. In addition, we also discuss\nhow robust these differentiating features are on changing the quasiparticle\nenergy or when the gap is anisotropic.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Possible realization of an antiferromagnetic Griffiths phase in\n  Ba[Fe(1-x)Mn(x)](2)As(2): We investigate magnetic ordering in metallic Ba[Fe(1-x)Mn(x)](2)As(2) and\ndiscuss the unusual magnetic phase, which was recently discovered for Mn\nconcentrations x > 10%. We argue that it can be understood as a Griffiths-type\nphase that forms above the quantum critical point associated with the\nsuppression of the stripe-antiferromagnetic spin-density-wave (SDW) order in\nBaFe2As2 by the randomly introduced localized Mn moments acting as strong\nmagnetic impurities. While the SDW transition at x = 0, 2.5% and 5% remains\nequally sharp, in the x = 12% sample we observe an abrupt smearing of the\nantiferromagnetic transition in temperature and a considerable suppression of\nthe spin gap in the magnetic excitation spectrum. According to our\nmuon-spin-relaxation, nuclear magnetic resonance and neutron-scattering data,\nantiferromagnetically ordered rare regions start forming in the x = 12% sample\nsignificantly above the N\\'eel temperature of the parent compound. Upon\ncooling, their volume grows continuously, leading to an increase in the\nmagnetic Bragg intensity and to the gradual opening of a partial spin gap in\nthe magnetic excitation spectrum. Using neutron Larmor diffraction, we also\ndemonstrate that the magnetically ordered volume is characterized by a finite\northorhombic distortion, which could not be resolved in previous diffraction\nstudies most probably due to its coexistence with the tetragonal phase and a\nmicrostrain-induced broadening of the Bragg reflections. We argue that\nBa[Fe(1-x)Mn(x)](2)As(2) could represent an interesting model spin-glass\nsystem, in which localized magnetic moments are randomly embedded into a SDW\nmetal with Fermi surface nesting.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity in High-Entropy-Alloy Telluride AgInSnPbBiTe5: A polycrystalline sample of the high-entropy-alloy-type telluride\nAgInSnPbBiTe5 was synthesized using high-pressure synthesis. Superconductivity\nwith a transition temperature (Tc) of 2.6 K was observed in AgInSnPbBiTe5.\nElemental and structural analyses revealed that five metals are mixed in a\nmetal site of an NaCl-type structure. Since AgInSnPbBiTe5 has a cation site\n(Ag, In, Sn, Pb, and Bi) and an anion site (Te), this is the first example of a\nhigh-entropy-alloy (HEA) pseudo-binary superconductor.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Anomalous doping dependence of the fluctuation-induced diamagnetism in\n  superconductors of YBCO family: SQUID magnetization measurements in oriented powders of Y$_{1-x}$Ca$_{x}$Ba$%\n_{2}$Cu$_{3}$O$_{y}$, with $x$ ranging from 0 to 0.2, for $y\\approx 6.1$ and\n$y\\approx 6.97$, have been performed in order to study the doping dependence of\nthe fluctuating diamagnetism above the superconducting transition temperature\n$T_{c}$. While for optimally doped compounds the diamagnetic susceptibility and\nthe magnetization curves $-M_{fl}(T=const$) vs. $H$ are rather well justified\non the basis of an anisotropic Ginzburg-Landau (GL) functional, in underdoped\nand overdoped regimes an anomalous diamagnetism is observed, with a large\nenhancement with respect to the GL scenario. Furthermore the shape of\nmagnetization curves differs strongly from the one derived in that scheme. The\nanomalies are discussed in terms of phase fluctuations of the order parameter\nin a layered system of vortices and in the assumption of charge inhomogeneities\ninducing local, non percolating, superconducting regions with $T_{c}^{(loc)}$\nhigher than the resistive transition temperature $T_{c}$. The susceptibility\ndisplays activated temperature behavior, a mark characteristic of the\nvortex-antivortex description, while history dependent magnetization, with\nrelaxation after zero-field cooling, is consistent with the hypothesis of\nsuperconducting droplets in the normal state. Thus the theoretical picture\nconsistently accounts for most experimental findings.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The Field Perturbation Theory of Pseudogaps in HTSC: Here I establish the field perturbation theory of pseudogaps in HTSC. The\nproposed ground state suggests an internal particle-hole field, which is normal\nto nesting surfaces, and having twice the Fermi wave-number. It is proved that\nthe system violates momentum conservation by the wave-vector of this internal\nfield. This violation applies to the quasi-particle propagators, as well as to\nthe interactions. Interaction vertices via the Pauli matrix- are established.\nThis, in turn, establishes the validity of the pseudogap Hartree self-energy.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Effect of superlattice modulation of electronic parameters on\n  superconducting density of states in cuprate superconductors: Recent scanning tunneling microscopy on BSCCO 2212 has revealed a substantial\nspatial supermodulation of the energy gap in the superconducting state. We\npropose that this gap modulation is due to the superlattice modulations of the\natoms in the structure, and hence the parameters in a microscopic model of the\nCuO2 plane. The gap modulation is estimated using renormalized mean field\ntheory for a t-t'-J model on a superlattice. The results compare well with\nexperiment.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Evidence for Pauli-limiting behaviour at high fields and enhanced upper\n  critical fields near T_c in several disordered FeAs based Superconductors: We report resistivity and upper critical field B_c2(T) data for disordered\n(As deficient) LaO_0.9F_0.1FeAs_1-delta in a wide temperature and high field\nrange up to 60 T. These samples exhibit a slightly enhanced superconducting\ntransition at T_c = 28.5 K and a significantly enlarged slope dB_c2/dT = -5.4\nT/K near T_c which contrasts with a flattening of B_c2(T) starting near 23 K\nabove 30 T. The latter evidences Pauli limiting behaviour (PLB) with B_c2(0)\napproximately 63 T. We compare our results with B_c2(T)-data from the\nliterature for clean and disordered samples. Whereas clean samples show almost\nno PLB for fields below 60 to 70 T, the hitherto unexplained pronounced\nflattening of B_c2(T) for applied fields H II ab observed for several\ndisordered closely related systems is interpreted also as a manifestation of\nPLB. Consequences are discussed in terms of disorder effects within the frames\nof (un)conventional superconductivity, respectively.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Microstructural and transport properties of superconducting\n  FeTe0.65Se0.35 crystals: The issue concerning the nature and the role of microstructural\ninhomogeneities in iron chalcogenide superconducting crystals of FeTe0.65Se0.35\nand their correlation with transport properties of this system was addressed.\nPresented data demonstrate that chemical disorder originating from the kinetics\nof the crystal growth process significantly influences the superconducting\nproperties of an Fe-Te-Se system. Transport measurements of the transition\ntemperature and critical current density performed for microscopic bridges\nallow us to deduce the local properties of a superconductor with\nmicrostructural inhomogeneities, and significant differences were noted. The\nvariances observed in the local properties were explained as a consequence of\nweak superconducting links existing in the studied crystals. The results\nconfirm that inhomogeneous spatial distribution of ions and small hexagonal\nsymmetry nanoscale regions with nanoscale phase separation also seem to enhance\nthe superconductivity in this system with respect to the values of the critical\ncurrent density. Magnetic measurements confirm the conclusions drawn from the\ntransport measurements.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Periodicity of magnetization reversals in $\\varphi_0$ Josephson junction: The magnetization reversal in ${\\varphi_0}$-Josephson junction with direct\ncoupling between magnetic moment and Josephson current has been studied. By\nadding pulse signal, the dynamics of magnetic moment components have been\nsimulated and the full magnetization reversal at different parameters of the\njunction has been demonstrated. We obtain a detailed pictures representing the\nintervals of the damping parameter $\\alpha$, Josephson to magnetic energy\nrelation $G$ and spin-orbit coupling parameter $r$ with full magnetization\nreversal. A periodicity in the appearance of magnetization reversal intervals\nwith increase in Josephson to magnetic energy relation is found. The obtained\nresults might be used in different fields of superconducting spintronics.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quantum vortex fluctuations in cuprate superconductors: We study the effects of quantum vortex fluctuations in two-dimensional\nsuperconductors using a dual theory of vortices, and investigate the relevance\nto underdoped cuprates where the superconductor-insulator transition (SIT) is\npossibly driven by quantum vortex proliferation. We find that a broad enough\nphase fluctuation regime may exist for experimental observation of the quantum\nvortex fluctuations near SIT in underdoped cuprates. We propose that this\nscenario can be tested via pair-tunneling experiments which measure the\ncharacteristic resonances in the zero-temperature pair-field susceptibility in\nthe vortex-proliferated insulating phase.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Energy gaps in high-$T_c$ superconductors: BCS after all?: A major impediment to solving the problem of high-$T_c$ superconductivity is\nthe ongoing confusion about the magnitude, structure and doping dependence of\nthe superconducting gap, $\\Delta_0$, and of the mysterious pseudogap found in\nunderdoped samples\\cite{TallonLoram}. The pseudogap opens around the ($\\pi$,0)\nantinodes below a temperature $T^*$ leaving Fermi arcs across the remnant Fermi\nsurface\\cite{Kanigel} on which the superconducting gap forms at $T_c$. One\nthing that seems agreed is that the ratio $2\\Delta_0/k_BT_c$ well exceeds the\nBCS value and grows with underdoping\\cite{Miyakawa1,Miyakawa2}, suggesting\nunconventional, non-BCS superconductivity. Here we re-examine data from many\nspectroscopies, especially Raman $B_{1g}$ and $B_{2g}$\nscattering\\cite{Sacuto,Guyard}, and reconcile them all within a two-gap\nscenario showing that the points of disagreement are an artefact of\nspectral-weight loss arising from the pseudogap. Crucially, we find that\n$\\Delta_0(p)$, or more generally the order parameter, now scales with the\nmean-field $T_c$ value, adopting the weak-coupling BCS ratio across the entire\nphase diagram.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Stretched exponential spin relaxation in organic superconductors: Proton NMR measurements on the organic superconductor\n$\\kappa-(ET)_2Cu[N(CN)_2]Br$ ($T_C = 11.6$ K) exhibit stretched exponential\nspin-lattice relaxation below $T\\approx 25$ K, suggestive of an inhomogeneous\nmagnetic phase that develops in the normal state and coexists with\nsuperconductivity. The onset of this phase coincides approximately with a large\nnormal state Nernst signal reported previously. By contrast, the closely\nrelated superconductor $\\kappa-(ET)_2Cu[(NCS)_2]Br$ ($T_C = 10.5$ K) shows\nsingle exponential spin-lattice relaxation and a conventional Nernst effect.\nThe temperature range $T_C < T < 30$ K encompasses several phenomena in the\n$\\kappa-(ET)_2X$ conductors, including changes in conduction electron spin\nresonance, electronic phase separation and the onset of antiferromagnetic\norder. Analogous behavior in $La_{2-x}Sr_xCuO_4$ suggests that a density wave\nmay develop in $\\kappa-(ET)_2Cu[N(CN)_2]Br$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Tunneling Studies of Pseudogaps: a Comment: Recent tunneling observations of a pseudogap in Bi_2Sr_2CaCu_2O_{8+delta} are\nnot necessarily evidence that the pseudogap is associated with superconducting\nfluctuations. The data are here analyzed as a competition between\nsuperconductivity and a density wave (taken as a simplified model for a striped\nphase).",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Hole-Doped Room-Temperature Superconductivity in H$_{3}$S$_{1-x}$Z$_x$\n  (Z=C, Si): We examine the effects of the low-level substitution of S atoms by C and Si\natoms on the superconductivity of H$_3$S with the $Im\\bar{3}m$ structure at\nmegabar pressure. The hole doping can fine-tune the Fermi energy to reach the\nelectronic density-of-states peak maximizing the electron-phonon coupling. This\ncan boost the critical temperature from the original 203 K to 289 K and 283 K,\nrespectively, for H$_3$S$_{0.962}$C$_{0.038}$ at 260 GPa and\nH$_3$S$_{0.960}$Si$_{0.040}$ at 230 GPa. The former may provide an explanation\nfor the recent experimental observation of room-temperature superconductivity\nin a highly compressed C-S-H system [Nature 586, 373-377 (2020)]. Our work\nopens a new avenue for substantially raising the critical temperatures of\nhydrogen-rich materials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The Superfluid Glass Phase of 3He-A: It is established theoretically that an ordered state with continuous\nsymmetry is inherently unstable to arbitrarily small amounts of disorder [1,\n2]. This principle is of central importance in a wide variety of condensed\nsystems including superconducting vortices [3, 4], Ising spin models [5] and\ntheir dynamics [6], and liquid crystals in porous media [7, 8], where some\ndegree of disorder is ubiquitous, although its experimental observation has\nbeen elusive. Based on these ideas it was predicted [9] that 3He in high\nporosity aerogel would become a superfluid glass. We report here our nuclear\nmagnetic resonance measurements on 3He in aerogel demonstrating destruction of\nlong range orientational order of the intrinsic superfluid orbital angular\nmomentum, confirming the existence of a superfluid glass. In contrast, 3He-A\ngenerated by warming from superfluid 3He-B has perfect long-range orientational\norder providing a mechanism for switching off this effect.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Effective Vortex Pinning in MgB2 thin films: We discuss pinning properties of MgB2 thin films grown by pulsed-laser\ndeposition (PLD) and by electron-beam (EB) evaporation. Two mechanisms are\nidentified that contribute most effectively to the pinning of vortices in\nrandomly oriented films. The EB process produces low defected crystallites with\nsmall grain size providing enhanced pinning at grain boundaries without\ndegradation of Tc. The PLD process produces films with structural disorder on a\nscale less that the coherence length that further improves pinning, but also\ndepresses Tc.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electronic structure, lattice dynamics and magnetism of new ThXAsN\n  (X=Fe,Co,Ni) superconductors: A First Principles Study: In this work, we present a comparative first principles study of mechanical\nproperties, electronic structure, phonon dispersion relation, electron-phonon\ncoupling and magnetism in three isostructural superconductors, namely, ThFeAsN,\nThCoAsN and ThNiAsN. Experimentally, ThFeAsN and ThNiAsN show superconducting\nproperties, while ThCoAsN has not been synthesized. Our calculated elastic\nconstants show that all these systems are mechanically stable. Significant\ndifferences in the electronic structures of these three compounds in terms of\ndensity of states, band structures and Fermi surfaces, are found. Our phonon\ncalculations reveal that all the systems including ThCoAsN, are dynamically\nstable. Phonon dispersion relations indicate that the optical modes of all the\nthree systems are almost the same while there are significant variations in the\nlow frequency manifold consisting of mixed modes. The electron-phonon coupling\nconstants and superconducting transition temperatures calculated based on the\nEliashberg formalism, predict a rather high $T_c$ of 6.4 K for ThCoAsN and also\na $T_c$ of 3.4 K for ThNiAsN which agrees well with the experimental value of\n4.3 K. Nevertheless, we find a $T_c$ of 0.05 K for ThFeAsN, which is much\nsmaller than the experimental $T_c$ of $\\sim$30 K. However, a simple analysis\nconsidering the amplifying effects of spin density wave order and out-of-plane\nsoft phonon modes suggests that the $T_c$ could be increased considerably to\n$\\sim$10 K. Finally, we also discuss the effect of anion As height on the\nelectronic structures and study possible magnetic states in these three\ncompounds.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "2D orbital-like magnetic order in ${\\rm La_{2-x}Sr_xCuO_4}$: In high temperature copper oxides superconductors, a novel magnetic order\nassociated with the pseudogap phase has been identified in two different\ncuprate families over a wide region of temperature and doping. We here report\nthe observation below 120 K of a similar magnetic ordering in the archetypal\ncuprate ${\\rm La_{2-x}Sr_xCuO_4}$ (LSCO) system for x=0.085. In contrast to the\nprevious reports, the magnetic ordering in LSCO is {\\it\\bf only} short range\nwith an in-plane correlation length of $\\sim$ 10 \\AA\\ and is bidimensional\n(2D). Such a less pronounced order suggests an interaction with other\nelectronic instabilities. In particular, LSCO also exhibits a strong tendency\ntowards stripes ordering at the expense of the superconducting state.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Tunable nodal kagome superconductivity in charge ordered RbV3Sb5: Unconventional superconductors often feature competing orders, small\nsuperfluid density, and nodal electronic pairing. While unusual\nsuperconductivity has been proposed in the kagome metals AV3Sb5, key\nspectroscopic evidence has remained elusive. Here we utilize pressure-tuned (up\nto 1.85 GPa) and ultra-low temperature (down to 18 mK) muon spin spectroscopy\nto uncover the unconventional nature of superconductivity in RbV3Sb5. At\nambient pressure, we detect an enhancement of the width of the internal\nmagnetic field distribution sensed by the muon ensemble, indicative of\ntime-reversal symmetry breaking charge order. Remarkably, the superconducting\nstate displays nodal energy gap and a reduced superfluid density, which can be\nattributed to the competition with the novel charge order. Upon applying\npressure, the charge-order transitions are suppressed, the superfluid density\nincreases, and the superconducting state progressively evolves from nodal to\nnodeless. Once charge order is eliminated, we find a superconducting pairing\nstate that is not only fully gapped, but also spontaneously breaks\ntime-reversal symmetry. Our results point to unprecedented tunable nodal kagome\nsuperconductivity competing with time-reversal symmetry-breaking charge order\nand offer unique insights into the nature of the pairing state.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The decrease of the critical current of coated conductors when a\n  perpendicular magnetic field is applied : a Josephson effect point of view: A large decrease is observed in the critical current density of YBCO coated\nconductors (CC) and related compounds when a strong perpendicular magnetic\nfield is applied. While measurements are generally carried out at 77K only,\nhere we present a magnetic technique permitting to determine the critical\ncurrent per unit width of conductor (Icr/w) in a large temperature range. We\nreport measurements carried out on various CCs that show that, in addition to\nthe reduction in the critical temperature that can be attributed to the low\nvalue of the irreversibility field near Tc, the field application results in a\nlarge decrease of Icr/w at all temperatures. We ascribe this reduction to the\nJosephson behaviour of the twin boundaries included in the YBCO layer.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "In-Plane and Out-of-Plane Optical Properties of NdBa2Cu3Ox Single\n  Crystals close to x = 7.0: We present results of reflectivity measurements with E parallel to c and E\nperpendicular to c on NdBa2Cu3Ox (Nd123) single crystals close to full oxygen\ndoping. Along the c-axis the optical conductivity shows a well developed\nabsorption band around 450 cm^-1 at all temperatures. The in-plane optical\nproperties are dominated by crystal-field excitations at low energies, a\nprominent step at 400 cm^-1 and a weaker feature between 500 - 550 cm^-1. A\ncomparison of the c-axis optical conductivity and the in-plane scattering rate\nwith neutron scattering derived spin susceptibility spectra suggests that the\nin-plane and out-of-plane anomalies are caused by the same mechanism, probably\nelectron--spin scattering.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Nesting, spin-fluctuations, and odd-gap superconductivity in NaxCoO2\n  yH2O: We have calculated the one-electron susceptibility of hydrated NaxCoO2 and\nfind strong nesting nearly commensurate with a 2X2 superstructure. The nesting\ninvolves about 70% of all electrons at the Fermi level and is robust with\nrespect to doping. This nesting creates a tendency to a charge density wave\ncompatible with the charge order often seen at x approx 0.5, which is usually\nascribed to electrostatic repulsion of Na ions. In the spin channel, it gives\nrise to strong spin-fluctuations, which should be important for\nsuperconductivity. The superconducting state most compatible with this nesting\nstructure is an odd-gap triplet s-wave state.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Probing the electron-phonon interaction in correlated systems with\n  coherent lattice fluctuation spectroscopy: Tailoring the properties of correlated oxides is accomplished by chemical\ndoping, pressure, temperature or magnetic field. Photoexcitation is a valid\nalternative to reach out-of-equilibrium states otherwise inaccessible. Here, we\nquantitatively estimate the coupling between a lattice distortion and the\ncharge-transfer excitation in (La$_2$CuO$_{4+\\delta}$). We photoinduce a\ncoherent La ion vibration and monitor the response of the optical constants in\na broad energy range, providing quantitative information on the electron-phonon\nmatrix element that can be compared to theoretical models. We propose the same\nmethodology to probe electron-electron interactions in other materials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity and unusual magnetic behavior in amorphous carbon: Traces of superconductivity (SC) at elevated temperatures (up to 65 K) were\nobserved by magnetic measurements in three different inhomogeneous sulfur doped\namorphous carbon (a-C) systems: (a) in commercial and (b) synthesized powders\nand (c) in a-C thin films. (a) Studies performed on commercial (a-C) powder\nwhich contains 0.21% of sulfur, revealed traces of non-percolated\nsuperconducting phases below Tc = 65 K. The SC volume fraction is enhanced by\nthe sulfur doping. (b) a-C powder obtained by pyrolytic decomposition of\nsucrose did not show any sign for SC above 5 K. This powder was mixed with\nsulfur and synthesized at 400 C (a-CS). The inhomogeneous products obtained,\nshow traces of SC phases at TC= 17 and 42 K. (c) Non-superconducting composite\na-C-W thin films were grown by electron-beam induced deposition. SC emerged at\nTc = 34.4 K only after heat treatment with sulfur. Other parts of the pyrolytic\na-CS powder, show unusual magnetic features. (i) Pronounced irreversible peaks\naround 55-75 K appear in the first zero-field-cooled (ZFC) sweep only. Their\norigin is not known. (ii) Unexpectedly these peaks are totally suppressed in\nthe second ZFC runs measured a few minutes later. (iii) Around the peak\nposition the field-cooled (FC) curves cross the ZFC plots (ZFC>FC). These\npeculiar magnetic observations also ascribed to a-CS powder prepared from the\ncommercial a-C powder and are connected to each other. All SC and magnetic\nphenomena observed are intrinsic properties of the sulfur doped a-C materials.\nIt is proposed that the a-CS systems behave similarly to well known high TC\ncurates and/or pnictides in which SC emerges from magnetic states.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Nature of 45 degree vortex lattice reorientation in tetragonal\n  superconductors: The transformation of the vortex lattice in a tetragonal superconductor which\nconsists of its 45 degree reorientation relative to the crystal axes is studied\nusing the nonlocal London model. It is shown that the reorientation occurs as\ntwo successive second order (continuous) phase transitions. The transition\nmagnetic fields are calculated for a range of parameters relevant for\nborocarbide superconductors in which the reorientation has been observed.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Possible Triplet Superconducting Order in Magnetic Superconducting Phase\n  induced by Paramagnetic Pair-Breaking: Motivated by recent thermal conductivity measurements in the superconductor\nCeCoIn5, we theoretically examine a possible staggered spin-triplet\nsuperconducting order to be induced by the coupled spin-density-wave (SDW) and\nd-wave superconducting (SC) orders in the high field and low temperature (HFLT)\nSC phase peculiar to this material with strong paramagnetic pair-breaking\n(PPB). It is shown that one type of the $\\pi$-triplet order is consistent with\nthe thermal conductivity data and can naturally be incorporated in the picture\nthat the Q-phase is a consequence of the strong PPB effect inducing the SDW\norder and the FFLO spatial modulation parallel to the applied magnetic field.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Half-quantum vortex and d-soliton in Sr$_2$RuO$_4$: Assuming that the superconductivity in Sr$_2$RuO$_4$ is described by a planar\np-wave order parameter, we consider possible topological defects in\nSr$_2$RuO$_4$. In particular, it is shown that both of the ${\\hat d}$-soliton\nand half-quantum vortex can be created in the presence of the magnetic field\nparallel to the $a$-$b$ plane. We discuss how one can detect the ${\\hat\nd}$-soliton and half-quantum vortex experimentally.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Uniaxial \"nematic-like\" electronic structure and Fermi surface of\n  untwinned CaFe2As2: Obtaining the electronic structure of the newly discovered iron-based\nsuperconductors is the key to understanding the mechanism of their\nhigh-temperature superconductivity. We used angle-resolved photoemission\nspectroscopy (ARPES) to make direct measurements of the electronic structure\nand Fermi surface (FS) of the untwinned uniaxial state of CaFe2As2, the parent\ncompound of iron-based superconductors. We observed unequal dispersions and FS\ngeometries along the orthogonal Fe-Fe bond directions. More importantly,\nunidirectional straight and flat FS segments are observed near the zone center,\nwhich indicates the existence of a unidirectional nematic charge density wave\norder, strengthening the case for a quantum electronic liquid crystalline\n\"nematic\" phase. Further, the doping dependence extrapolates to a possible\nquantum critical point of the disappearance of this order in the heavily\noverdoped regime of these materials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Theory of the superconductivity of $UGe_2$ revisited: We present a unified theory of magnetism and superconductivity of $UGe_2$. To\nthis end, we consider part of $5f$ uranium electrons as mostly itinerant and\nother ones as mostly localized. The main feature that distinguishes the\nlocalized from the itinerant electrons is the effect of the pressure on them.\nThe pressure strongly screens the itinerant electrons while the localized ones\nare almost unaffected. The screening of itinerant electrons leads to decreasing\nof their Coulomb repulsion, therefore to formation of doubly occupied and empty\nstates. These states are spin-singlet and the effective spin of itinerant\nelectrons, the zero-temperature magnetization in units of Bohr magneton,\ndecreases. We obtain an effective two-spin Heisenberg model, which explains the\nmagnetization-temperature diagram of $UGe_2$. It is shown that the\nexperimentally observed characteristic temperature $T_x$, is a partial order\ntransition temperature. Below the Curie temperature $(T_x<T_C)$ the system\nundergoes a transition from high temperature phase, were only localized\nelectrons contribute the magnetization, to the low temperature one, where both\nitinerant and localized electrons contribute the magnetization. The\ncharacteristic temperature decreases when pressure increases. At the quantum\npartial order point $T_x=0$, the Zeeman splitting of the itinerant electrons is\nzero. This permits formation of Cooper pairs and an onset of superconductivity\ninduced by the transversal fluctuations of the localized electrons. Small\ndeviation from the quantum partial ordered state leads to suppression of\nsuperconductivity. This explains the dome form of the superconducting\ntransition temperature. The very low superconducting critical temperature is a\nconsequence of the Ising ferromagnetism.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Fractal superconductivity near localization threshold: We develop a semi-quantitative theory of electron pairing and resulting\nsuperconductivity in bulk \"poor conductors\" in which Fermi energy $E_F$ is\nlocated in the region of localized states not so far from the Anderson mobility\nedge $E_c$. We review the existing theories and experimental data and argue\nthat a large class of disordered films is described by this model.\n  Our theoretical analysis is based on the analytical treatment of pairing\ncorrelations, described in the basis of the exact single-particle eigenstates\nof the 3D Anderson model, which we combine with numerical data on eigenfunction\ncorrelations. Fractal nature of critical wavefunction's correlations is shown\nto be crucial for the physics of these systems.\n  We identify three distinct phases: 'critical' superconductive state formed at\n$E_F=E_c$, superconducting state with a strong pseudogap, realized due to\npairing of weakly localized electrons and insulating state realized at $E_F$\nstill deeper inside localized band. The 'critical' superconducting phase is\ncharacterized by the enhancement of the transition temperature with respect to\nBCS result, by the inhomogeneous spatial distribution of superconductive order\nparameter and local density of states. The major new feature of the\npseudo-gaped state is the presence of two independent energy scales:\nsuperconducting gap $\\Delta$, that is due to many-body correlations and a new\n\"pseudogap\" energy scale $\\Delta_P$ which characterizes typical binding energy\nof localized electron pairs and leads to the insulating behavior of the\nresistivity as a function of temperature above superconductive $T_c$. Two gap\nnature of the \"pseudo-gaped superconductor\" is shown to lead to a number of\nunusual physical properties.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Structural phase transition and superlattice misfit strain of RFeAsO (R\n  = La, Pr, Nd and Sm): The tetragonal-to-orthorhombic structural phase transition (SPT) in LaFeAsO\n(La-1111) and SmFeAsO (Sm-1111) single crystals measured by high resolution\nx-ray diffraction is found to be sharp while the RFeAsO (R=La, Nd, Pr, Sm)\npolycrystalline samples show a broad continuous SPT. Comparing the\npolycrystalline and the single crystal 1111 samples, the critical exponents of\nthe SPT are found to be the same while the correlation length critical\nexponents are found to be very different. These results imply that the lattice\nfluctuations in 1111 systems change in samples with different surface to volume\nratio that is assigned to the relieve of the temperature dependent superlattice\nmisfit strain between active iron layers and the spacer layers in 1111 systems.\nThis phenomenon that is missing in the AFe2As2 (A=Ca, Sr, Ba) \"122\" systems,\nwith the same electronic structure but different for the thickness and the\nelastic constant of the spacer layers, is related with the different maximum\nsuperconducting transition temperature in the 1111 (55 K) versus 122 (35 K)\nsystems and implies the surface reconstruction in 1111 single crystals.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quantum mechanics of superconducting nanowires: In a short superconducting nanowire connected to bulk superconducting leads,\nquantum phase slips behave as a system of linearly (as opposed to\nlogarithmically) interacting charges. This system maps onto quantum mechanics\nof a particle in a periodic potential. We show that, while the state with a\nhigh density of phase slips is not a true insulator (a consequence of Josephson\ntunneling between the leads), for a range of parameters it behaves as such down\nto unobservably small temperatures. We also show that quantum phase slips give\nrise to multiple branches (bands) in the energy-current relation and to an\ninterband (\"exciton\") mode.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Nematic state stabilized by off-site Coulomb interaction in iron-based\n  superconductors: Using a variational Monte Carlo method, we investigate the nematic state in\niron-base superconductors based on a three-band Hubbard model. Our results\ndemonstrate that the nematic state, formed by introducing an anisotropic\nhopping order into the projected wave function, can arise in the underdoped\nregime when a realistic off-site Coulomb interaction $V$ is considered. {\\color\n{red} We demonstrate that the off-site Coulomb interaction $V$, which is\nneglected so far in the analysis of iron-base superconductors, make a dominant\ncontribution to the stabilization of nematic state. We calculate the doping\ndependencies of the anisotropic properties such as the unequal occupation of\n$d_{xz}$ and $d_{yz}$ orbitals, anisotropies of kinetic energy and spin\ncorrelations, and show that they are all suppressed upon electron doping, which\nare consistent with the intrinsic anisotropies observed by optical spectrum\nmeasurement and ARPES experiments.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Transition Temperature and Upper Critical Field in SmFeAsO1-xFx\n  Synthesized at Low Heating Temperatures: Low-temperature synthesis is a promising and potentially effective method for\nimproving superconducting properties. We report on the fabrication of\npolycrystalline samples of SmFeAsO1-xFx with nominal x content varying in a\nwide range of x = 0-0.35 synthesized at 900 deg C. This synthesis temperature\nis around 300 deg C lower than the conventional synthesis temperature. The\nvariation in the lattice parameters and transition temperature (Tc) of various\nF-doped samples indicates that reduction of the unit cell volume (V) seems to\nbe the main reason for the rise of Tc up to 57.8 K. Magnetoresistance\nmeasurements showed that the upper critical field slope (dHc2/dT) increased\nwith increasing F concentration up to x = 0.2, where it reached a maximum value\nof -8 T/K corresponding to a coherence length of 10 angstrom. At still higher F\ndoping levels, dHc2/dT and the low field Jc decreased; above 0.5 T, however, Jc\nhad almost the same value. Compared with previous reports, the present\nsynthesis route with low synthesis temperatures and commonly available FeF2 as\nthe source of F is more effective at introducing F into the SmFeAsO system and\nthereby resulting in improved superconducting properties for the system. In\naddition, this new sample preparation method also reduces unnecessary problems\nsuch as the evaporation of F and reaction between the crucible and\nsuperconductor during the solid-state reaction.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Generalized boundary conditions for the circuit theory of mesoscopic\n  transport: The circuit theory of mesoscopic transport provides a unified framework to\ndescribe spin-dependent or superconductivity-related phenomena. We extend this\ntheory to hybrid systems of normal metals, ferromagnets and superconductors.\nOur main result is an expression for the current through an arbitrary contact\nbetween two general isotropic \"nodes\", which is suitable to describe the\npresence of superconducting and ferromagnetic elements in the system, as well\nas magnetically active interfaces/contacts. In certain cases (weak ferromagnet\nand magnetic tunnel junction) we derive transparent and simple results for the\nmatrix current.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "On the roles of graphene oxide doping for enhanced supercurrent in MgB2\n  based superconductors: Due to their graphene-like properties after oxygen reduction, incorporation\nof graphene oxide (GO) sheets into correlated-electron materials offers a new\npathway for tailoring their properties. Fabricating GO nanocomposites with\npolycrystalline MgB2 superconductors leads to an order of magnitude enhancement\nof the supercurrent at 5 K/8 T and 20 K/4 T. Herein, we introduce a novel\nexperimental approach to overcome the formidable challenge of performing\nquantitative microscopy and microanalysis of such composites, so as to unveil\nhow GO doping influences the structure and hence the material properties. Atom\nprobe microscopy and electron microscopy were used to directly image the GO\nwithin the MgB2, and we combined these data with computational simulations to\nderive the property-enhancing mechanisms. Our results reveal synergetic effects\nof GO, namely, via localized atomic (carbon and oxygen) doping as well as\ntexturing of the crystals, which provide both inter and intra granular flux\npinning. This study opens up new insights into how low-dimensional\nnanostructures can be integrated into composites to modify the overall\nproperties, using a methodology amenable to a wide range of applications.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The underdoped-overdoped transition in YBa_2Cu_3O_x: Oxygen doping in metallic YBa_2Cu_3O_x induces quadrupolar \"alpha-ortho'',\nand breathing \"beta-ortho\" deformations of the CuO_2 planes. Breathing\nbeta-ortho deformations favour hybridizations of the pd sigma Cu3d_x^2-y^2 -\nO2p_x,y with the pd pi Cu3d_x,z, 3d_y,z-O2p_z bands relaxing the confinement of\nthe carriers in the overdoped regime, x>6.95.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetization Measurement of a Possible High-Temperature Superconducting\n  State in Amorphous Carbon Doped with Sulfur: Magnetization M(T,H) measurements performed on thoroughly characterized\ncommercial amorphous carbon powder doped with sulfur (AC-S), revealed the\noccurrence of an inhomogeneous superconductivity (SC) below T_c = 38 K. The\nconstructed magnetic field-temperature (H-T) phase diagram resembles that of\ntype-II superconductors. However, AC-S demonstrates a number of anomalies. In\nparticular, we observed (1) a non-monotonic behavior of the lower critical\nfield H_c1(T); (2) a pronounced positive curvature of the \"upper critical field\nboundary\" that we associated with the flux lattice melting line Hm(T); (3) a\nspontaneous ferromagnetic-like magnetization M0 coexisting with SC. Based on\nthe analysis of experimental results we propose a nonstandard SC state in AC-S.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Coexistence of orbital degeneracy lifting and superconductivity in\n  iron-based superconductors: In contrast to conventional superconducting (SC) materials, superconductivity\nin high-temperature superconductors (HTCs) usually emerges in the presence of\nother fluctuating orders with similar or higher energy scales, thus instigating\ndebates over their relevance for the SC pairing mechanism. In iron-based\nsuperconductors (IBSCs), local orbital fluctuations have been proposed to be\ndirectly responsible for the structural phase transition and closely related to\nthe observed giant magnetic anisotropy and electronic nematicity. However,\nwhether superconductivity can emerge from, or even coexist with orbital\nfluctuations, remains unclear. Here we report the angle-resolved photoemission\nspectroscopy (ARPES) observation of the lifting of symmetry-protected band\ndegeneracy, and consequently the breakdown of local tetragonal symmetry in the\nSC state of Li(Fe1-xCox)As. Supported by theoretical simulations, we analyse\nthe doping and temperature dependences of this band-splitting and demonstrate\nan intimate connection between ferro-orbital correlations and\nsuperconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Nonlatching Superconducting Nanowire Single-Photon Detection with\n  Quasi-Constant-Voltage Bias: Latching is a serious issue in superconducting nanowire single-photon\ndetector (SNSPD) technology. By extensively studying the electrical\ntransportation characteristics of SNSPD with different bias schemes, we\nconclude that latching is a result of the improper bias to SNSPD. With the\nquasi-constant-voltage bias scheme, the intrinsic nonlatching nature of SNSPD\nis observed and discussed. The SNSPD working in the nonlatching bias shows a\nsmaller jitter and a higher pulse amplitude than that in the previous\nanti-latching method. The quantum efficiency of SNSPD with the pulsed photon\nfrequency up to 3 GHz is measured successfully, which further proves the\nnonlatching operation of SNSPD.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Anomalous Fano factor as a signature of Bogoliubov Fermi surfaces: Noise spectroscopy is a key technique to investigate the nature and dynamics\nof charge carriers in superconductors. The recently discovered superconducting\nhybrids with Bogoliubov Fermi surfaces exhibit a particularly intriguing and\nrich charge dynamics, as their charge carriers consist of both Cooper pairs and\nan extensive number of Bogoliubov quasiparticles. Motivated by this, we compute\nthe noise spectra of Bogoliubov Fermi surfaces and identify their key\nsignatures in the differential conductance and the Fano factor. Specifically,\nwe consider a semiconductor/superconductor hybrid device with an in-plane\nmagnetic field, which exhibits several Bogoliubov Fermi surfaces. The number\nand orientation of the Bogoliubov Fermi surfaces in this device can be readily\ncontrolled by the applied magnetic field, which in turn alters the noise\nsignal. In particular, we find that the Fano factor exhibits a reduced value,\nsubstantially lower than two, whenever the charge dynamics is governed by a\nlarge number of Bogoliubov quasiparticles. Using experimentally relevant\nparameters, we make a number of specific predictions for the noise spectra,\nthat can be used as direct evidence of Bogoliubov Fermi surfaces. In\nparticular, we find that the Fano factor as a function of magnetic field and\nspin-orbit coupling exhibits characteristic discontinuities at the transition\nlines that separate phases with different number of Bogoliubov Fermi surfaces.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Josephson junctions of topological nodal superconductors: Transition metal dichalcogenides (TMDs) offer a unique platform to study\nunconventional superconductivity, owing to the presence of strong spin-orbit\ncoupling and a remarkable stability to an in-plane magnetic field. A recent\nstudy found that when an in-plane field applied to a superconducting monolayer\nTMD is increased beyond the Pauli critical limit, a quantum phase transition\noccurs into a topological nodal superconducting phase which hosts Majorana flat\nbands. We study the current-phase relation of this nodal superconductor in a\nJosephson junction geometry. We find that the nodal superconductivity is\nassociated with an energy-phase relation that depends on the momentum\ntransverse to the current direction, with a $4\\pi$ periodicity in between pairs\nof nodal points. We interpret this response as a result of a series of quantum\nphase transitions, driven by the transverse momentum, which separate a\ntopological trivial phase and two distinct topologically non-trivial phases\ncharacterized by different winding invariants. This analysis sheds light on the\nstability of the Majorana flat bands to symmetry-breaking perturbations.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "High-temperature surface superconductivity in rhombohedral graphite: Surface superconductivity in rhombohedral graphite is a robust phenomenon\nwhich can exist even when higher order hoppings between the layers lift the\ntopological protection of the surface flat band and introduce a quadratic\ndispersion of electrons with a heavy effective mass. We show that for weak\npairing interaction, the flat band character of the surface superconductivity\ntransforms into a BCS-like relation with high critical temperature\ncharacterized by a higher coupling constant due to a much larger density of\nstates than in the bulk. Our results offer an explanation for the recent\nfindings of graphite superconductivity with an unusually high transition\ntemperature.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Competing Superconducting States in Strong Ferromagnets: We report results from a systematic study of the competition of exotic\ntriplet pair density wave (PDW) superconductivity (SC) with homogeneous (zero\npair momentum) SC in strongly polarized media such as half metallic systems.\nFrom the two different PDW states allowed by symmetry in this background only\none may dominate or even coexist with homogeneous SC. We propose a direct\nexperimental identification of PDW SC in this context. Our results suggest that\nthese exotic states may plausibly emerge in heterostructures involving\nproximity of SC with half-metallic CrO$_2$ where induced SC is established in\nthe half metallic region and in strongly ferromagnetic superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Thermodynamic signatures of quantum criticality in cuprates: The three central phenomena of cuprate superconductors are linked by a common\ndoping $p^{\\star}$, where the enigmatic pseudogap phase ends, around which the\nsuperconducting phase forms a dome, and at which the resistivity exhibits an\nanomalous linear dependence on temperature as $T \\to 0$. However, the\nfundamental nature of $p^{\\star}$ remains unclear, in particular whether it\nmarks a true quantum phase transition. We have measured the specific heat $C$\nof the cuprates Eu-LSCO and Nd-LSCO at low temperature in magnetic fields large\nenough to suppress superconductivity, over a wide doping range across\n$p^{\\star}$. As a function of doping, we find that the electronic term $C_{\\rm\nel}$ is strongly peaked at $p^{\\star}$, where it exhibits a $-T$log$T$\ndependence as $T \\to 0$. These are the classic signatures of a quantum critical\npoint, as observed in heavy-fermion and iron-based superconductors where their\nantiferromagnetic phase ends. We conclude that the pseudogap phase of cuprates\nends at a quantum critical point, whose associated fluctuations are most likely\ninvolved in the $d$-wave pairing and the anomalous scattering.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Theory of thermally activated vortex bundles flow over the\n  directional-dependent potential barriers in type-II superocnductors: The thermally activated vortex bundle flow over the directional-dependent\nenergy barrier in type-II superconductors is investigated. The coherent\noscillation frequency and the mean direction of the random collective pinning\nforce of the vortex bundles are evaluated by applying the random walk theorem.\nThe thermally activated vortex bundle flow velocity is obtained.The\ntemperature- and field-dependent Hall and longitudinal resistivities induced by\nthe bundle flow for type-II superconducting bulk materials and thin films are\ncalculated. All the results are in agreement with the experiments.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Bound states and impurity averaging in unconventional superconductors: The question of anomalous transport due to a band of impurity states in\nunconventional superconductors is discussed. In general, the bound state\nenergies are not in midgap, even in the unitarity limit. This implies that,\ngenerically, the states associated with impurities are broad resonances, not\ntrue bound states. There is no impurity band in the usual sense of the phrase.\nThe wavefunctions of these resonances possess interesting anisotropies in real\nspace, but this does not result in anomalous hopping between impurities. I\nconclude that the system of resonances produces no qualitative modifications to\nthe T-matrix theory with impurity averaging which is normally used to treat the\nlow-temperature transport of unconventional superconductors. However, users of\nthis method often assume a density of states which is symmetric around the\nchemical potential. This is not normally the case. It is found that the\nnon-crossing approximation is not valid in a strictly two-dimensional system.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Existence of the Abrikosov vortex state in two-dimensional type-II\n  superconductors without pinning: Theory alternative to the vortex lattice melting theories is advertised. The\nvortex lattice melting theories are science fiction cond-mat/9811051 because\nthe Abrikosov state is not the vortex lattice with crystalline long-range\norder. Since the fluctuation correction to the Abrikosov solution is infinite\nin the thermodynamic limit (K.Maki and H.Takayama, 1972) any fluctuation theory\nof the mixed state should consider a superconductor with finite sizes. Such\nnonperturbative theory for the easiest case of two-dimensional superconductor\nin the lowest Landau level approximation is presented in this work. The\nthermodynamic averages of the spatial average order parameter and of the\nAbrikosov parameter $\\beta_{a}$ are calculated. It is shown that the position\nH_{c4} of the transition into the Abrikosov state (i.e. in the mixed state with\nlong-range phase coherence) depends strongly on sizes of two-dimensional\nsuperconductor. Fluctuations eliminate the Abrikosov vortex state in a wide\nregion of the mixed state of thin films with real sizes and without pinning\ndisorders, i.e. H_{c4} << H_{c2}. The latter has experimental corroboration in\nPhys.Rev.Lett. 75, 2586 (1995).",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Suppression of Tunneling of Superconducting Vortices Caused by a Remote\n  Gate: Example of an Extended Object Tunneling: We discuss a recent experiment in which the resistance of a superconducting\nfilm has been measured in magnetic field. A strong decrease of the\nsuperconducting film resistance has been observed when a metallic gate is\nplaced above the film. We study how the magnetic coupling between vortices in a\nthin superconducting film and electrons in a remote unbiased gate suppresses\nthe tunneling rate of the vortices. We examine two general approaches to\nanalyze tunneling in the presence of slow low-energy degrees of freedom: the\nfunctional-integral and scattering formalisms. In the first one, the response\nof the electrons inside the metallic gate to a change in the vortex position is\ndescribed by the \"tunneling with dissipation\". We consider the Eddy current\ninduced in the gate by the magnetic flux of the vortex as a result of\ntunneling. In the second approach, the response is given in terms of scattering\nof the electrons by the magnetic flux of the vortex in a way similar to the\nAharonov-Bohm scattering. A sudden change in the vortex position leads to the\nOrthogonality Catastrophe that opposes the vortex tunneling. We show that the\nmagnetic coupling between the vortices and the electrons inside the gate can\nlead to a dramatic suppression of the vortex tunneling, restoring the\nsuperconducting property in accord with the experiment.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Role of Sb in the superconducting kagome metal CsV$_3$Sb$_5$ revealed by\n  its anisotropic compression: Pressure evolution of the superconducting kagome metal CsV$_3$Sb$_5$ is\nstudied with single-crystal x-ray diffraction and density-functional\nband-structure calculations. A highly anisotropic compression observed up to 5\nGPa is ascribed to the fast shrinkage of the Cs-Sb distances and suppression of\nCs rattling motion. This prevents Sb displacements required to stabilize the\nthree-dimensional charge-density-wave (CDW) order and elucidates the\ndisappearance of the CDW already at 2 GPa despite only minor changes in the\nelectronic structure of the normal state. At higher pressures, vanadium bands\nstill change only marginally, whereas antimony bands undergo a major\nreconstruction caused by the gradual formation of the interlayer Sb-Sb bonds.\nOur results exclude pressure tuning of vanadium kagome bands as the main\nmechanism for the non-trivial evolution of superconductivity in real-world\nkagome metals. Concurrently, we establish the central role of Sb atoms in the\nstabilization of a three-dimensional CDW and Fermi surface reconstruction.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Energy dependence of a vortex line length near a zigzag of pinning\n  centers: A vortex line, shaped by a zigzag of pinning centers, is described here\nthrough a three-dimensional unit cell containing two pinning centers positioned\nsymmetrically with respect to its center. The unit cell is a cube of side\n$L=12\\xi$, the pinning centers are insulating spheres of radius $R$, taken\nwithin the range $0.2\\xi$ to $3.0\\xi$, $\\xi$ being the coherence length. We\ncalculate the free energy density of these systems in the framework of the\nGinzburg-Landau theory.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Searching for Superconductivity in High Entropy Oxide Ruddlesden-Popper\n  Cuprate Films: In this work, the high entropy oxide A2CuO4 Ruddlesden-Popper\n(La0.2Pr0.2Nd0.2Sm0.2Eu0.2)2CuO4 is explored by charge doping with Ce+4 and\nSr+2 at concentrations known to induce superconductivity in the simple parent\ncompounds, Nd2CuO4 and La2CuO4. Electron doped\n(La0.185Pr0.185Nd0.185Sm0.185Eu0.185Ce0.075)2CuO4 and hole doped\n(La0.18Pr0.18Nd0.18Sm0.18Eu0.18Sr0.1)2CuO4 are synthesized and shown to be\nsingle crystal, epitaxially strained, and highly uniform. Transport\nmeasurements demonstrate that all as-grown films are insulating regardless of\ndoping. Annealing studies show that resistivity can be tuned by modifying\noxygen stoichiometry and inducing metallicity but without superconductivity.\nThese results in turn are connected to extended x-ray absorption fine structure\n(EXAFS) results indicating that the lack of superconductivity in the high\nentropy cuprates likely originates from a large distortion within the Cu-O\nplane ({\\sigma}2>0.015 {\\AA}2) due to A-site cation size variance, which drives\nlocalization of charge carriers. These findings describe new opportunities for\ncontrolling charge- and orbital-mediated functional responses in\nRuddlesden-Popper crystal structures, driven by balancing of cation size and\ncharge variances that may be exploited for functionally important behaviors\nsuch as superconductivity, antiferromagnetism, and metal-insulator transitions,\nwhile opening less understood phase spaces hosting doped Mott insulators,\nstrange metals, quantum criticality, pseudogaps, and ordered charge density\nwaves.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Tunneling spectroscopy for ferromagnet/superconductor junctions: In tunneling spectroscopy studies of ferromagnet/superconductor (F/S)\njunctions, the effects of spin polarization, Fermi wavevector mismatch (FWM)\nbetween the F and S regions, and interfacial resistance play a crucial role. We\nstudy the low bias conductance spectrum of these junctions, governed by Andreev\nreflection at the F/S interface. We consider both d- and s-wave superconductors\nas well as mixed states of the $d+is$ form. We present results for a range of\nvalues of the relevant parameters and find that a rich variety of features\nappears, depending on pairing state and other conditions. We show that in the\npresence of FWM, spin polarization can enhance Andreev reflection and give rise\nto a zero bias conductance peak for an s-wave superconductor.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Suppression of surface barrier in superconductors by columnar defects: We investigate the influence of columnar defects in layered superconductors\non the thermally activated penetration of pancake vortices through the surface\nbarrier. Columnar defects, located near the surface, facilitate penetration of\nvortices through the surface barrier, by creating ``weak spots'', through which\npancakes can penetrate into the superconductor. Penetration of a pancake\nmediated by an isolated column, located near the surface, is a two-stage\nprocess involving hopping from the surface to the column and the detachment\nfrom the column into the bulk; each stage is controlled by its own activation\nbarrier. The resulting effective energy is equal to the maximum of those two\nbarriers. For a given external field there exists an optimum location of the\ncolumn for which the barriers for the both processes are equal and the\nreduction of the effective penetration barrier is maximal. At high fields the\neffective penetration field is approximately two times smaller than in\nunirradiated samples. We also estimate the suppression of the effective\npenetration field by column clusters. This mechanism provides further reduction\nof the penetration field at low temperatures.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Detection of Geometric Phases in Flux Qubits with Coherent Pulses: We propose a experimentally feasible scheme to demonstrate the geometric\nphase in flux qubits by means of detuning coherent microwave pulse techniques.\nThrough measuring the probability of the persistent current state in flux\nqubits, one can detect the Berry phase that is acquired with system's\nHamiltonian adiabatical circular evolution in the parameter space. Furthermore,\nwe show that one should choose an appropriate amplitude of pulses in an\nexperiment to obtain high readout resolution when detuning frequency of pulses\nis fixed and controlled phase shift gates can be implemented based on the\ngeometric phases by inductance coupling two flux qubits.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Probing the Order Parameter of Superconducting LiFeAs using Pb/LiFeAs\n  and Au/LiFeAs Point-Contact Spectroscopy: We have fabricated c-axis point contact junctions between high-quality LiFeAs\nsingle crystals and Pb or Au tips in order to study the nature of the\nsuperconducting order parameter of LiFeAs, one of the few stoichiometric\niron-based superconductors. The observation of the Josephson current in c-axis\njunctions with a conventional s-wave superconductor as the counterelectrode\nindicates that the pairing symmetry in LiFeAs is not pure d-wave or pure\nspin-triplet p-wave. A superconducting gap is clearly observed in point contact\nAndreev reflection measurements performed on both Pb/LiFeAs and Au/LiFeAs\njunctions. The conductance spectra can be well described by the\nBlonder-Tinkham-Klapwijk model with a lifetime broadening term, resulting in a\ngap value of \\approx 1.6 meV (2{\\Delta}/kBTC \\approx 2.2).",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Josephson current through a long quantum wire: The dc Josephson current through a long SNS junction receives contributions\nfrom both Andreev bound states localized in the normal region as well as from\nscattering states incoming from the superconducting leads. We show that in the\nlimit of a long junction, this current, at low temperatures, can be expressed\nentirely in terms of properties of the Andreev bound states at the Fermi\nenergy: the normal and Andreev reflection amplitudes at the left-hand and at\nthe right-hand S-N interface. This has important implications for treating\ninteractions in such systems.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Electronic structure of new quaternary superconductors LaONiBi and\n  LaOCuBi from first principles: Based on first-principles FLAPW-GGA calculations, we have investigated the\nelectronic structure of newly synthesized novel superconductors LaONiBi and\nLaOCuBi, the first bismuth-containing compounds from the family of quaternary\noxypnictides which attract now a great deal of interest in search for novel\n26-55K superconductors. The band structure, density of states and Fermi\nsurfaces are discussed. Our results indicate that the bonding inside of the\n(La-O) and (Ni(Cu)-Bi) layers is covalent whereas the bonding between the\n(La-O) and (Ni(Cu)- Bi) blocks is mostly ionic. For both oxybismuthides, the\nDOSs at the Fermi level are formed mainly by the states of the (Ni(Cu)-Bi)\nlayers, the corresponding Fermi surfaces have a twodimensional character and\nthe conduction should be strongly anisotropic andhappen only on the (Ni(Cu)-Bi)\nlayers. As a whole, the new oxybismuthides may be described as low-TC\nsuperconducting non-magnetic ionic metals.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Josephson critical currents in annular superconductors with Pearl\n  vortices: We investigate the influence of Pearl vortices in the vicinity of an\nedge-type Josephson junction for a superconducting thin-film loop in the form\nof an annulus, under uniform magnetic field. Specifically, we obtain the exact\nanalytic formulation that allows to describe the circulating current density\nand the gauge invariant phase increment $\\Delta\\phi$ across the junction. The\nmain properties of $\\Delta\\phi$ and their influence on the critical current\npattern $I_c(B)$ are described quantitatively in terms of the loop's width to\nradius ratio $W/R$ and of the vortex position within the loop ${\\bf r}_v$. It\nis shown that narrow loops ($W/R < 0.3$) may be well described by the straight\ngeometry limit. However, such approximation fails to predict a number of\ndistinctive features captured by our formulation, as the node lifting effect of\nthe $I_c(B)$ pattern in wide loops or the actual influence of a vortex pinned\nat different positions.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Band-selective clean- and dirty-limit superconductivity with nodeless\n  gaps in the bilayer iron-based superconductor CsCa$_2$Fe$_4$As$_4$F$_2$: The optical properties of the new iron-based superconductor\nCsCa$_2$Fe$_4$As$_4$F$_2$ with $T_c \\sim 29$~K have been determined. In the\nnormal state a good description of the low-frequency response is obtained with\na superposition of two Drude components of which one has a very low scattering\nrate (narrow Drude-peak) and the other a rather large one (broad Drude-peak).\nWell below $T_c \\sim 29$~K, a pronounced gap feature is observed which involves\na complete suppression of the optical conductivity below $\\sim$ 110~cm$^{-1}$\nand thus is characteristic of a nodeless superconducting state. The optical\nresponse of the broad Drude-component can be described with a dirty-limit\nMattis-Bardeen-type response with a single isotropic gap of $2\\Delta \\simeq\n14$~meV. To the contrary, the response of the narrow Drude-component is in the\nultra-clean-limit and its entire spectral weight is transferred to the\nzero-frequency $\\delta(\\omega)$ function that accounts for the loss-free\nresponse of the condensate. These observations provide clear evidence for a\nband-selective coexistence of clean- and dirty-limit superconductivity with\nnodeless gaps in CsCa$_2$Fe$_4$As$_4$F$_2$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Evidence from $^{77}$Se Knight shifts for triplet superconductivity in\n  (TMTSF)$_2$PF$_6$: The layered quasi-one-dimensional molecular superconductor (TMTSF)$_2$PF$_6$\nis a very exotic material with a superconducting order parameter whose ground\nstate symmetry has remained ill-defined. Here we present a pulsed NMR Knight\nshift (K) study of $^{77}$Se measured simultaneously with transport in\npressurized (TMTSF)$_2$PF$_6$. The Knight shift is linearly dependent on the\nelectron spin susceptibility $\\chi_s$, and is therefore a direct measure of the\nspin polarization in the superconducting state. For a singlet superconductor,\nthe spin contribution to the Knight shift, K$_s$, falls rapidly on cooling\nthrough the transition. The present experiments indicate no observable change\nin K between the metallic and superconducting states, and thus strongly support\nthe hypothesis of triplet p-wave superconductivity in (TMTSF)$_2$PF$_6$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Complete zero-energy flat bands of surface states in fully gapped chiral\n  noncentrosymmetric superconductors: Noncentrosymmetric superconductors can support flat bands of zero-energy\nsurface states in part of their surface Brillouin zone. This requires that they\nobey time-reversal symmetry and have a sufficiently strong\ntriplet-to-singlet-pairing ratio to exhibit nodal lines in the bulk. These\nbands are protected by a winding number that relies on chiral symmetry, which\nis realized as the product of time-reversal and particle-hole symmetry. We\nreveal a way to stabilize a flat band in the entire surface Brillouin zone,\nwhile the bulk dispersion is fully gapped. This idea could lead to a robust\nplatform for quantum computation and represents an alternative route to\nstrongly correlated flat bands in two dimensions, besides twisted bilayer\ngraphene. The necessary ingredient is an additional spin-rotation symmetry that\nforces the direction of the spin-orbit-coupling vector not to depend on the\nmomentum component normal to the surface. We define a winding number which\nleads to flat zero-energy surface bands due to bulk-boundary correspondence. We\ndiscuss under which conditions this winding number is nonzero in the entire\nsurface Brillouin zone and verify the occurrence of zero-energy surface states\nby exact numerical diagonalization of the Bogoliubov-de Gennes Hamiltonian for\na slab. In addition, we consider how a weak breaking of the additional symmetry\naffects the surface band, employing first-order perturbation theory and a\nquasiclassical approximation. We find that the surface states and the bulk gap\npersist for weak breaking of the additional symmetry but that the band does not\nremain perfectly flat. The broadening of the band strongly depends on the\ndeviation of the spin-orbit-coupling vector from its unperturbed direction as\nwell as on the spin-orbit-coupling strength and the triplet-pairing amplitude.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity in novel BiS2-based layered superconductor\n  LaO1-xFxBiS2: Layered superconductors have provided some interesting fields in condensed\nmatter physics owing to the low dimensionality of their electronic states. For\nexample, the high-Tc (high transition temperature) cuprates and the Fe-based\nsuperconductors possess a layered crystal structure composed of a stacking of\nspacer (blocking) layers and conduction (superconducting) layers, CuO2 planes\nor Fe-Anion layers. The spacer layers provide carriers to the conduction layers\nand induce exotic superconductivity. Recently, we have reported\nsuperconductivity in the novel BiS2-based layered compound Bi4O4S3. It was\nfound that superconductivity of Bi4O4S3 originates from the BiS2 layers. The\ncrystal structure is composed of a stacking of BiS2 superconducting layers and\nthe spacer layers, which resembles those of high-Tc cuprate and the Fe-based\nsuperconductors. Here we report a discovery of a new type of BiS2-based layered\nsuperconductor LaO1-xFxBiS2, with a Tc as high as 10.6 K.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Advanced surface characterization of\n  Ba(Fe$_{0.92}$Co$_{0.08}$)$_2$As$_2$ epitaxial thin films: We report on the systematic characterization of\nBa(Fe$_{0.92}$Co$_{0.08}$)$_2$As$_2$ epitaxial thin films on CaF$_2$ substrate\nin view of their possible use for superconducting electronic applications. By\nusing different and complementary techniques we studied the morphological\ncharacteristics of the surface, the structural properties, the magnetic\nresponse, and the superconducting properties in terms of critical temperature,\ncritical current, and energy gaps. Particular attention was paid to the\nhomogeneity of the films and to the comparison of their superconducting\nproperties with those of single crystals of the same compound.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity and abnormal pressure effect in Sr0.5La0.5FBiSe2\n  superconductor: Through the solid state reaction method, we synthesized a new BiSe2-based\nsuperconductor Sr0:5La0:5FBiSe2 with superconducting transition temperature\nTc?3.8 K. A strong diamagnetic signal below Tc in susceptibility ?(T) is\nobserved indicating the bulk nature of superconductivity. Different to most\nBiS2-based compounds where superconductivity develops from a\nsemiconducting-like normal state, the present compound exhibits a metallic\nbehavior down to Tc. Under weak magnetic field or pressure, however, a\nremarkable crossover from metallic to insulating behaviors takes place around\nTmin where the resistivity picks up a local minimum. With increasing pressure,\nTc decreases monotonously and Tmin shifts to high temperatures, while the\nabsolute value of the normal state resistivity at low temperatures first\ndecreases and then increases with pressure up to 2.5 GPa. These results imply\nthat the electronic structure of Sr0:5La0:5FBiSe2 may be different to those in\nthe other BiS2-based systems.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Majorana spintronics: We propose a systematic magnetic-flux-free approach to detect, manipulate and\nbraid Majorana fermions in a semiconductor nanowire-based topological Josephson\njunction by utilizing the Majorana spin degree of freedom. We find an intrinsic\n$\\pi$-phase difference between spin-triplet pairings enforced by the Majorana\nzeros modes (MZMs) at the two ends of a one-dimensional spinful topological\nsuperconductor. This $\\pi$-phase is identified to be a spin-dependent\nsuperconducting phase, referred to as the spin-phase, which we show to be\ntunable by controlling spin-orbit coupling strength via electric gates. This\nelectric controllable spin-phase not only affects the coupling energy between\nMZMs but also leads to a fractional Josephson effect in the absence of any\napplied magnetic flux, which enables the efficient topological qubit readout.\nWe thus propose an all-electrically controlled superconductor-semiconductor\nhybrid circuit to manipulate MZMs and to detect their non-Abelian braiding\nstatistics properties. Our work on spin properties of topological Josephson\neffects potentially opens up a new thrust for spintronic applications with\nMajorana-based semiconductor quantum circuits.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetism in SQUIDs at Millikelvin Temperatures: We have characterized the temperature dependence of the flux threading dc\nSQUIDs cooled to millikelvin temperatures. The flux increases as 1/T as\ntemperature is lowered; moreover, the flux change is proportional to the\ndensity of trapped vortices. The data is compatible with the thermal\npolarization of surface spins in the trapped fields of the vortices. In the\nabsence of trapped flux, we observe evidence of spin-glass freezing at low\ntemperature. These results suggest an explanation for the \"universal\" 1/f flux\nnoise in SQUIDs and superconducting qubits.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "In-situ fabrication of cobalt-doped SrFe2As2 thin films by using pulsed\n  laser deposition with excimer laser: The remarkably high superconducting transition temperature and upper critical\nfield of iron(Fe)-based layered superconductors, despite ferromagnetic material\nbase, open the prospect for superconducting electronics. However, success in\nsuperconducting electronics has been limited because of difficulties in\nfabricating high-quality thin films. We report the growth of high-quality\nc-axis-oriented cobalt(Co)-doped SrFe2As2 thin films with bulk\nsuperconductivity by using an in-situ pulsed laser deposition technique with a\n248-nm-wavelength KrF excimer laser and an arsenic(As)-rich phase target. The\ntemperature and field dependences of the magnetization showing strong\ndiamagnetism and transport critical current density with superior Jc-H\nperformance are reported. These results provide necessary information for\npractical applications of Fe-based superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quantum Phase Transitions in Superconducting Arrays with General\n  Capacitance Matrices: We investigate quantum phase transitions in two-dimensional superconducting\narrays with general capacitance matrices and discrete charge states. We use the\nperturbation theory together with the simulated annealing method to obtain the\nzero-temperature phase diagrams, which display various lobe-like structures of\ninsulating solid phases, and examine the possibility of supersolid phase. At\nnonzero temperatures, an effective classical Hamiltonian is obtained through\nthe use of the variational method in the path-integral formalism, and the\ncorresponding phase diagram is found approximately. The insulating lobes of the\nsolid phases are shown to exist at sufficiently low temperatures, and results\nof Monte Carlo simulations are also presented.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Quasiparticle entropy in superconductor/normal metal/superconductor\n  proximity junctions in the diffusive limit: We discuss the quasiparticle entropy and heat capacity of a dirty\nsuperconductor-normal metal-superconductor junction. In the case of short\njunctions, the inverse proximity effect extending in the superconducting banks\nplays a crucial role in determining the thermodynamic quantities. In this case,\ncommonly used approximations can violate thermodynamic relations between\nsupercurrent and quasiparticle entropy. We provide analytical and numerical\nresults as a function of different geometrical parameters. Quantitative\nestimates for the heat capacity can be relevant for the design of caloritronic\ndevices or radiation sensor applications.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Amplitude `Higgs' mode in 2H-NbSe2 Superconductor: We report experimental evidences for the observation of the superconducting\namplitude mode, so-called `Higgs' mode in the charge density wave\nsuperconductor 2H-NbSe2 using Raman scattering. By comparing 2H-NbSe2 and its\niso-structural partner 2H-NbS2 which shows superconductivity but lacks the\ncharge density wave order, we demonstrate that the superconducting mode in\n2H-NbSe2 owes its spectral weight to the presence of the coexisting charge\ndensity wave order. In addition, temperature dependent measurements in 2H-NbSe2\nshow a full spectral weight transfer from the charge density wave mode to the\nsuperconducting mode upon entering the superconducting phase. Both observations\nare fully consistent with a superconducting amplitude mode or Higgs mode.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Colossal thermomagnetic response in the exotic superconductor URu2Si2: When a superconductor is heated above its critical temperature $T_c$,\nmacroscopic coherence vanishes, leaving behind droplets of thermally\nfluctuating Cooper pair. This superconducting fluctuation effect above $T_c$\nhas been investigated for many decades and its influence on the transport,\nthermoelectric and thermodynamic quantities in most superconductors is well\nunderstood by the standard Gaussian fluctuation theories. The transverse\nthermoelectric (Nernst) effect is particularly sensitive to the fluctuations,\nand the large Nernst signal found in the pseudogap regime of the underdoped\nhigh-$T_c$ cuprates has raised much debate on its connection to the origin of\nsuperconductivity. Here we report on the observation of a colossal Nernst\nsignal due to the superconducting fluctuations in the heavy-fermion\nsuperconductor URu$_2$Si$_2$. The Nernst coefficient is enhanced by as large as\none million times over the theoretically expected value within the standard\nframework of superconducting fluctuations. This, for the first time in any\nknown material, results in a sizeable thermomagnetic figure of merit\napproaching unity. Moreover, contrary to the conventional wisdom, the\nenhancement in the Nernst signal is more significant with the reduction of the\nimpurity scattering rate. This anomalous Nernst effect intimately reflects the\nhighly unusual superconducting state embedded in the so-called hidden-order\nphase of URu$_2$Si$_2$. The results invoke possible chiral or Berry-phase\nfluctuations originated from the topological aspect of this superconductor,\nwhich are associated with the effective magnetic field intrinsically induced by\nbroken time-reversal symmetry of the superconducting order parameter.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Characterizing two-dimensional superconductivity via nanoscale noise\n  magnetometry with single-spin qubits: We propose nanoscale magnetometry via isolated single-spin qubits as a probe\nof superconductivity in two-dimensional materials. We characterize the magnetic\nfield noise at the qubit location, arising from current and spin fluctuations\nin the sample and leading to measurable polarization decay of the qubit. We\nshow that the noise due to transverse current fluctuations studied as a\nfunction of temperature and sample-probe distance can be used to extract useful\ninformation about the transition to a superconducting phase and the pairing\nsymmetry of the superconductor. Surprisingly, at low temperatures, the dominant\ncontribution to the magnetic noise arises from longitudinal current\nfluctuations and can be used to probe collective modes such as monolayer\nplasmons and bilayer Josephson plasmons. We also characterize the noise due to\nspin fluctuations, which allows probing the spin structure of the pairing wave\nfunction. Our results provide a non-invasive route to probe the rich physics of\ntwo-dimensional superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Common Electronic Origin of Superconductivity in (Li,Fe)OHFeSe Bulk\n  Superconductor and Single-Layer FeSe/SrTiO3 Films: The mechanism of high temperature superconductivity in the iron-based\nsuperconductors remains an outstanding issue in condensed matter physics. The\nelectronic structure, in particular the Fermi surface topology, is considered\nto play an essential role in dictating the superconductivity. Recent revelation\nof distinct electronic structure and possible high temperature\nsuperconductivity with a transition temperature Tc above 65 K in the\nsingle-layer FeSe films grown on the SrTiO3 substrate provides key information\non the roles of Fermi surface topology and interface in inducing or enhancing\nsuperconductivity. Here we report high resolution angle-resolved photoemission\nmeasurement on the electronic structure and superconducting gap of a novel\nFeSe-based superconductor, (Li0.84Fe0.16)OHFe0.98Se, with a Tc at 41 K. We find\nthat this single-phase bulk superconductor shows remarkably similar electronic\nbehaviors to that of the superconducting single-layer FeSe/SrTiO3 film in terms\nof Fermi surface topology, band structure and nearly isotropic superconducting\ngap without nodes. These observations provide significant insights in\nunderstanding high temperature superconductivity in the single-layer\nFeSe/SrTiO3 film in particular, and the mechanism of superconductivity in the\niron-based superconductors in general.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Multiband superconductivity and a deep gap minimum from the specific\n  heat in KCa$_2$(Fe$_{1-x}$Ni$_x$)$_4$As$_4$F$_2$ ($x$ = 0, 0.05, 0.13): Specific heat can explore low-energy quasiparticle excitations of\nsuperconductors, so it is a powerful tool for bulk measurement on the\nsuperconducting gap structure and pairing symmetry. Here, we report an in-depth\ninvestigation on the specific heat of the multiband superconductors\nKCa$_2$(Fe$_{1-x}$Ni$_x$)$_4$As$_4$F$_2$ ($x$ = 0, 0.05, 0.13) single crystals\nand the overdoped non-superconducting one with $x$ = 0.17. Clear specific heat\nanomalies can be observed at the superconducting transition temperature of 33.6\nK and 28.8 K for the samples with $x$ = 0 and $x$ = 0.05, respectively. For the\ntwo samples, the magnetic field induced specific heat coefficient\n$\\Delta\\gamma(H)$ in the low-temperature limit increases rapidly below 2 T,\nthen it rises slowly above 2 T. Using the non-superconducting sample with $x$ =\n0.17 as a reference, the specific heat of phonon background for various\nsuperconducting samples can be obtained and subtracted, which allows us to\nextract the electronic specific heat of the superconducting samples. Through\ncomparative analyses, it is found that the energy gap structure including two\n$s$-wave gaps and an extended $s$-wave gap with large anisotropy can reasonably\ndescribe the electronic specific heat data. According to these results, we\nsuggest that at least one anisotropic superconducting gap with a deep gap\nminimum should exist in this multiband system. With the doping of Ni, the\nsuperconducting transition temperature of the sample decreases along with the\ndecrease of the large $s$-wave gap, but the extended $s$-wave gap increases due\nto the enlarged electron pockets via adding more electrons. Despite these\nchanges, the general properties of the gap structure remain unchanged versus\ndoping Ni.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Strain-driven nematicity of the odd-parity superconductivity in\n  Sr$_x$Bi$_2$Se$_3$: We present a novel experimental evidence for the odd-parity nematic\nsuperconductivity in high-quality single crystals of doped topological\ninsulator Sr$_x$Bi$_2$Se$_3$. The X-ray diffraction shows that the grown single\ncrystals are either weakly stretched or compressed uniaxially in the basal\nplane along one of the crystal axis. We show that in the superconducting state,\nthe upper critical magnetic field $H_{c2}$ has a two-fold rotational symmetry\nand depends on the sign of the strain: in the stretched samples, the maximum of\n$H_{c2}$ is achieved when the in-plane magnetic field is transverse to the\nstrain axis, while in the compressed samples this maximum is observed when the\nfield is along the strain direction. This result is naturally explained within\na framework of the odd-parity nematic superconductivity coupled to the strain.\nMagnetoresistance in the normal state is independent of the current direction\nand also has a two-fold rotational symmetry that demonstrates the nematicity of\nthe electronic system in the normal state.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Chiral and helical $p$-wave superconductivity in doped bilayer BiH: We investigate the superconductivity (SC) driven by correlation effects in\nelectron-doped bilayer BiH near a type-II van Hove singularity (vHS). By\nfunctional renormalization group, we find triplet $p$-wave pairing prevails in\nthe interaction parameter space, except for spin density wave (SDW) closer to\nthe vHS or when the interaction is too strong. Because of the large atomic\nspin-orbital coupling (SOC), the $p$-wave pairing occurs between equal-spin\nelectrons, and is chiral and two-fold degenerate. The chiral state supports\nin-gap edge states, even though the low energy bands in the SC state are\ntopologically trivial. The absence of mirror symmetry allows Rashba SOC that\ncouples unequal spins, but we find its effect is of very high order, and can\nonly drive the chiral $p$-wave into helical $p$-wave deep in the SC state.\nInterestingly, there is a six-fold degeneracy in the helical states, reflected\nby the relative phase angle $\\theta=n\\pi/3$ (for integer $n$) between the spin\ncomponents of the helical pairing function. The phase angle is shown to be\nstable in the vortex state.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Instability of Majorana states in Shiba chains due to leakage into a\n  topological substrate: We revisit the problem of Majorana states in chains of scalar impurities\ndeposited on a superconductor with a mixed s-wave and p-wave pairing. We also\nstudy the formation of Majorana states for magnetic impurity chains. We find\nthat the magnetic impurity chains exhibit well-localized Majorana states when\nthe substrate is trivial, but these states hybridize and get dissolved in the\nbulk when the substrate is topological. Most surprisingly, and contrary to\nprevious predictions, the scalar impurity chain does not support fully\nlocalized Majorana states except for very small and finely tuned parameter\nregimes, mostly for a non-topological substrate close to the topological\ntransition. Our results indicate that a purely p-wave or a dominant p-wave\nsubstrate are not good candidates to support either magnetic or scalar impurity\ntopological Shiba chains.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Mixed higher-order topology and nodal and nodeless flat band topological\n  phases in a superconducting multiorbital model: We investigate the topological phases that appear in an orbital version of\nthe Benalcazar-Bernevig-Hughes (BBH) model in the presence of conventional\nspin-singlet $s$-wave superconductivity and with the possibility of tuning an\nin-plane magnetic field. We chart out the phase diagram by considering\ndifferent boundary conditions, with the topology of the individual phases\nfurther examined by considering both the Wannier and entanglement spectra, as\nwell as the Majorana polarization.\n  For weak to moderate values of magnetic field and superconducting pairing\namplitude, we find a second-order topological superconductor phase with eight\nzero-energy corner modes. Further increasing field or pairing, half of the\ncorner states can be turned into zero-energy edge-localized modes, thus forming\na type of hybrid-order phase. Then, we find two different putative first-order\ntopological phases, a nodal and a nodeless phase, both with zero-energy flat\nbands localized along mirror-symmetric open edges. For the nodal phase, the\nflat bands are localized between the nodes in reciprocal space, while in the\nnodeless phase, with its a full bulk gap, the zero-energy boundary flat band\nspans the whole Brillouin zone.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Spin Transport in Half-Metallic Ferromagnet-Superconductor Junctions: We investigate the charge and spin transport in half-metallic ferromagnet\n($F$) and superconductor ($S$) nanojunctions. We utilize a self-consistent\nmicroscopic method that can accommodate the broad range of energy scales\npresent, and ensures proximity effects that account for the interactions at the\ninterfaces are accurately determined. Two experimentally relevant half-metallic\njunction types are considered: The first is a $F_1 F_2 S$ structure, where a\nhalf-metallic ferromagnet $F_1$ adjoins a weaker conventional ferromagnet\n$F_2$. The current is injected through the $F_1$ layer by means of an applied\nbias voltage. The second configuration involves a $S F_1 F_2 F_3 S$ Josephson\njunction whereby a phase difference $\\Delta\\varphi$ between the two\nsuperconducting electrodes generates the supercurrent flow. In this case, the\ncentral half-metallic $F_2$ layer is surrounded by two weak ferromagnets $F_1$\nand $F_3$. By placing a ferromagnet with a weak exchange field adjacent to an\n$S$ layer, we are able to optimize the conversion process in which\nopposite-spin triplet pairs are converted into equal-spin triplet pairs that\npropagate deep into the half-metallic regions in both junction types. For the\ntunnel junctions, we study the bias-induced local magnetization, spin currents,\nand spin transfer torques for various orientations of the relative\nmagnetization angle $\\theta$ in the $F$ layers. We find that the bias-induced\nequal-spin triplet pairs are maximized in the half-metal for\n$\\theta\\approx90^\\circ$ and as part of the conversion process, are\nanticorrelated with the opposite-spin pairs. We show that the charge current\ndensity is maximized, corresponding to the occurrence of a large amplitude of\nequal-spin triplet pairs, when the exchange interaction of the weak ferromagnet\nis about $0.1E_F.$",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Hall coefficient and angle-resolved photoemission in systems with strong\n  pair fluctuations: We examine the normal-state temperature and doping dependence of the Hall\ncoefficient in the context of a pair-fluctuation scenario, based on a model\nwhere itinerant electrons are hybridized with localized electron pairs via a\ncharge exchange term. We show that an anomalous behavior of the Hall effect,\nqualitatively similar to that observed in high-Tc superconductors, can be\nattributed to the non-Fermi liquid properties of the single-particle spectral\nfunction which exhibits pseudogap features. Our calculations are based on a\ndynamical mean-field procedure which relates the transport coefficients to the\nsingle-particle spectral function in an exact way.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "From Cooper-pair glass to unconventional superconductivity: a unified\n  approach to cuprates and pnictides: We report a microscopic model wherein the unconventional superconductivity\nemerges from an incoherent `Cooper-pair glass' state. Driven by the pair-pair\ninteraction, a new type of quasi-Bose phase transition is at work. The\ninteraction leads to the unconventional coupling of the quasiparticles to\nexcited pair states, or `super-quasiparticles', with a non-retarded\nenergy-dependent gap. The model describes quantitatively the quasiparticle\nexcitation spectra of both cuprates and pnictides, including the universal\n`peak-dip-hump' signatures, and for the pseudogap phase above $T_c$. The\nresults show that instantaneous pair-pair interactions account for the SC\ncondensation without a collective mode.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Resilience of d-wave superconductivity to nearest-neighbor repulsion: Many theoretical approaches find d-wave superconductivity in the prototypical\none-band Hubbard model for high-temperature superconductors. At strong-coupling\n(U > W, where U is the on-site repulsion and W=8t the bandwidth) pairing is\ncontrolled by the exchange energy J=4t^2/U. One may then surmise, ignoring\nretardation effects, that near-neighbor Coulomb repulsion V will destroy\nsuperconductivity when it becomes larger than J, a condition that is easily\nsatisfied in cuprates for example. Using Cellular Dynamical Mean-Field theory\nwith an exact diagonalization solver for the extended Hubbard model, we show\nthat pairing, at strong coupling, is preserved even when V>>J, as long as\nV<U/2. While at weak coupling V always reduces the spin fluctuations and hence\nd-wave pairing, at strong coupling, in the underdoped regime, the increase of\nJ=4t^2/(U-V) caused by V increases binding at low frequency while the\npair-breaking effect of V is pushed to high frequency. These two effects\ncompensate in the underdoped regime, in the presence of a pseudogap. While the\npseudogap competes with superconductivity, the proximity to the Mott transition\nthat leads to the pseudogap, and retardation effects, protect d-wave\nsuperconductivity from V.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Key Ingredients for Superconductivity in Cuprates: Using high resolution angle-resolved photoemission data in conjunction with\nthat from neutron and other probes, we show that electron-phonon (el-ph)\ncoupling is strong in cuprates superconductors and it plays an important role\nin pairing. In addition to the strong electron correlation, the inclusion of\nphonons provides a theoretical framework explaining many important phenomena\nthat cannot be understood by a strongly correlated electronic model alone.\nEspecially it is indispensable to explain the difference among materials. The\nphonons with the wave number around the (0,qx) and (qx,0) axes create the\nd-wave pairing while that near (pi,pi) are pair breaking. Therefore the\nhalf-breathing mode of the oxygen motions helps d-wave superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Interplay between magnetism and superconductivity in EuFe(2-x)Co(x)As2\n  studied by 57Fe and 151Eu M\u00f6ssbauer spectroscopy: The compound EuFe(2-x)Co(x)As2 was investigated by means of the 57Fe and\n151Eu Moessbauer spectroscopy versus temperature (4.2 - 300 K) for x=0\n(parent), x=0.34 - 0.39 (superconductor) and x=0.58 (overdoped). It was found\nthat spin density wave (SDW) is suppressed by Co-substitution, however it\nsurvives in the region of superconductivity, but iron spectra exhibit some\nnon-magnetic component in the superconducting region. Europium orders\nanti-ferromagnetically regardless of the Co concentration with the spin\nre-orientation from the a-axis in the parent compound toward c-axis with the\nincreasing replacement of iron by cobalt. The re-orientation takes place close\nto the a-c plane. Some trivalent europium appears in EuFe(2-x)Co(x)As2 versus\nsubstitution due to the chemical pressure induced by Co-atoms and it\nexperiences some transferred hyperfine field from Eu2+. Iron experiences some\ntransferred field due to the europium ordering for substituted samples in the\nSDW and non-magnetic state both, while the transferred field is undetectable in\nthe parent compound. Superconductivity coexists with the 4f-europium magnetic\norder within the same volume. It seems that superconductivity has some\nfilamentary character in EuFe(2-x)Co(x)As2 and it is confined to the\nnon-magnetic component seen by the iron Moessbauer spectroscopy.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Resistive transition in disordered superconductors with varying\n  intergrain coupling: The effect of disorder is investigated in granular superconductive materials\nwith strong and weak links. The transition is controlled by the interplay of\nthe \\emph{tunneling} $g$ and \\emph{intragrain} $g_{intr}$ conductances, which\ndepend on the strength of the intergrain coupling. For $g \\ll g_{intr}$, the\ntransition involves first the grain boundary, while for $g \\sim g_{intr}$ the\ntransition occurs into the whole grain. The different intergrain coupling is\nconsidered by modelling the superconducting material as a disordered network of\nJosephson junctions. Numerical simulations show that on increasing the\ndisorder, the resistive transition occurs for lower temperatures and the curve\nbroadens. These features are enhanced in disordered superconductors with strong\nlinks. The different behaviour is further checked by estimating the average\nnetwork resistance for weak and strong links in the framework of the effective\nmedium approximation theory. These results may be relevant to shed light on\nlong standing puzzles as: (i) enhancement of the superconducting transition\ntemperature of many metals in the granular states; (ii) suppression of\nsuperconductivity in homogeneously disordered films compared to standard\ngranular systems close to the metal-insulator transition; (iii) enhanced\ndegradation of superconductivity by doping and impurities in strongly linked\nmaterials, such as magnesium diboride, compared to weakly-linked\nsuperconductors, such as cuprates.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity of cerium under pressures up to 54GPa: Cerium is a fascinating element due to its diverse physical properties, which\ninclude forming various crystal structures ($\\gamma$, $\\alpha$, $\\alpha^{'}$,\n$\\alpha^{''}$ and $\\epsilon$), mixed valence behavior and superconductivity,\nmaking it an ideal platform for investigating the interplay between different\nelectronic states. Here, we present a comprehensive transport study of cerium\nunder hydrostatic pressures up to 54 GPa. Upon applying pressure, cerium\nundergoes the $\\alpha$ $\\rightarrow$ $\\alpha^{''}$ transition at around 4.9\nGPa, which is accompanied by the appearance of superconductivity with $T_{\\rm\nc}$ of 0.4 K, and $T_{\\rm c}$ slightly increases to 0.5 K at 11.4 GPa. At 14.3\nGPa, $T_{\\rm c}$ suddenly increases when the $\\alpha^{''}$ phase transforms\ninto the $\\epsilon$ phase, reaching a maximum value of 1.25 K at around 17.2\nGPa. Upon further increasing the pressure, $T_{\\rm c}$ monotonically decreases.\nTogether with the results of previous studies, our findings suggest that the\nevolution of superconductivity in cerium is closely correlated with the\nmultiple pressure-induced structural transitions and corresponding unusual\nelectronic structures.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Surface states and local spin susceptibility in doped three-dimensional\n  topological insulators with odd-parity superconducting pairing symmetry: We investigate characteristic features in the spin response of doped\nthree-dimensional topological insulators with odd-parity unequal-spin\nsuperconducting pairing, which are predicted to have gapless Majorana surface\nmodes. These Majorana modes contribute to the local spin susceptibility, giving\nrise to a characteristic temperature behavior of the Knight shift and the\nspin-lattice relaxation time in magnetic resonance experiments. Because of\ntheir different decay lengths, the Majorana modes can be observed and clearly\ndistinguished from the Dirac modes of the topological insulator by local probes\nwhich allow for a depth-controlled study of the electron spins on the nanometer\nlength scale.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Charge distribution in C$_{60}$ crystal doped by electric field: The calculations of the charge distribution in C$_{60}$-based FET structure\nare presented. A simple model is proposed to describe the distribution of the\ninjected electrons or holes between two-dimensional layers. The calculations\nshow that the relative charge distribution between the layers turns to be\nindependent on the total amount of injected charges. The charge density is\nmaximal on the surface layer and drops exponentially with the depth increase.\nThe relative amounts of injected charge involved in the top layer are 73 and 64\nper cent in the case of electron and hole injection, respectively. Thus, the\ncharge localization on the crystal surface turns to be markedly different from\nnear complete one that was obtained earlier within tight- binding model for the\ncharge concentration providing superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Half-integer Shapiro-steps in superconducting qubit with a\n  $\u03c0$-Josephson junction: A superconducting quantum interference device (SQUID) comprising 0- and\n$\\pi$-Josephson junctions (JJs), called $\\pi$-SQUID, is studied by the\nresistively shunted junction model. The $\\pi$-SQUID shows half-integer\nShapiro-steps (SS) under microwave irradiation at the voltage $V$ =\n$(\\hbar/2e)\\Omega (n/2)$, with angular frequency $\\Omega$ and half-integer\n$n$/2 in addition to integer $n$. We show that the $\\pi$-SQUID can be a\n$\\pi$-qubit with spontaneous loop currents by which the half-integer SS are\ninduced. Making the 0- and $\\pi$-JJs equivalent is a key for the half-integer\nSS and realizing the $\\pi$-qubit.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Controlling Factors of Tc-Dome Structure in 1111-Type Iron Arsenide\n  Superconductors: We investigated the effects of phosphorus substitution on the shape of the\nTc(x) dome in 1111-type SmFeAs1-yPyO1-xHx (0 < x < 0.5). Hydride ion\nsubstitution of oxide sites (O2- -> H-) exerts a chemical pressure effect,\ni.e., a structural reduction of the Pn-Fe-Pn angle {\\alpha} (Pn = P, As) and\nalso dopes electrons into the FePn layer to induce superconductivity. Isovalent\nphosphorus substitution (P3- -> As3-) can induce only a chemical pressure\neffect, i.e., an increase of {\\alpha} for La-substitution of Sm-sites. As y\nincreases from 0.0 to 0.5, the single Tc dome gradually splits into two domes,\nsimilar to those of LaFeAsO1-xHx with a Tc valley at x ~ 0.16. We found that\nthe Tc valley is located around (x, {\\alpha}) ~ (0.16, 113{\\deg}) for both\nSmFeAs1-yPyO1-xHx and LaFeAsO1-xHx series, irrespective of changes in the Pn\nanion and Ln cation species. This result suggests that suppression of Tc leads\nto the emergence of a Tc valley when both the shape of FePn4 tetrahedra\nrepresented by {\\alpha} and electron doping level of x meet the above criterion\nin 1111 type iron oxypnictide superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Properties of a two orbital model for oxypnictide superconductors:\n  Magnetic order, B_2g spin-singlet pairing channel, and its nodal structure: A two orbital model for the new Fe-based superconductors is studied using the\nLanczos method as well as pairing mean-field approximations. Our main goals are\n(i) to provide a comprehensive analysis of this model using numerical\ntechniques with focus on half-filling and on the state with two more electrons\nthan half-filling and (ii) to investigate the nodal structure of the mean-field\nsuperconducting state and compare the results with angle-resolved photoemission\ndata. In particular, we provide evidence that at half-filling spin 'stripes',\nas observed experimentally, dominate over competing states.\n  Depending on parameters, the state with two more electrons added to half\nfilling is either triplet or singlet. Since experiments suggest spin singlet\npairs, our focus is on this state. Under rotation, it transforms as the B_2g\nrepresentation of the D_4h group. We also show that the s+/- pairing operator\ntransforms as A_1g and becomes dominant only in an unphysical regime of the\nmodel where the undoped state is an insulator. For robust values of the\neffective electronic attraction producing the Cooper pairs, assumption\ncompatible with recent angle-resolved photoemission (ARPES) results that\nsuggesting small Cooper-pair size, the nodes of the two-orbital model are found\nto be located only at the electron pockets. Since recent ARPES efforts have\nsearched for nodes at the hole pockets or only in a few directions at the\nelectron pockets, our results for the nodal distribution may help to guide\nfuture experiments. More in general, the investigations reported here aim to\nestablish several of the properties of the two orbital model. Only a detailed\ncomparison with experiments will clarify how far this simple model present a\nvalid description of the Fe pnictides.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Doping dependent optical properties of Bi2201: An experimental study of the in-plane optical conductivity of\n(Pb$_{x}$,Bi$_{2-x}$)(La$_{y}$Sr$_{2-y}$)CuO$_{6+\\delta}$ (Bi2201) is presented\nfor a broad doping and temperature range. The in-plane conductivity is analyzed\nwithin a strong coupling formalism. We address the interrelationship between\nthe optical conductivity ($\\sigma(\\omega)$), the single particle self energy,\nand the electron-boson spectral function. We find that the frequency and\ntemperature dependence can be well described within this formalism. We present\na universal description of optical, ARPES and tunneling spectra. The full\nfrequency and temperature dependence of the optical spectra and single particle\nself-energy is shown to result from an electron-boson spectral function, which\nshows a strong doping dependence and weak temperature dependence.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Proximity of LaOFeAs to a magnetic instability: We investigate the effect of external pressure on the Fe magnetic moment in\nundoped LaOFeAs within the framework of density functional theory and show that\nthis system is close to a magnetic instability: The Fe moment is found to drop\nby nearly a factor of 3 within a pressure range of $\\pm$ 5 GPa around the\ncalculated equilibrium volume. While the Fe moments show an unusually strong\nsensitivity to the spin arrangement (type of anti-ferromagnetic structure), the\nlow temperature structural distortion is found to have only a minor influence\non them. Analysis of the Fermi surface topology and nesting features shows that\nthese properties change very little up to pressures of at least 10 GPa. We\ndiscuss the magnetic instability in terms of the itinerancy of this system.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Local characterization of superconductivity in BaFe2(As1-xPx)2: We use magnetic force microscopy (MFM) to characterize superconductivity\nacross the superconducting dome in ${\\rm {BaFe}_2 \\left(As_{1-x}P_x\\right)_2}$,\na pnictide with a peak in the penetration depth ($\\lambda_{ab}$) at optimal\ndoping ($\\rm x_{opt}$), as shown in sample-wide measurements. Our local\nmeasurements show a peak at $\\rm x_{opt}$ and a $T_C$ vs. $\\lambda_{ab}^{-2}$\ndependence similar on both sides of $\\rm x_{opt}$. Near the underdoped edge of\nthe dome $\\lambda_{ab}$ increases sharply suggesting that superconductivity\ncompetes with another phase. Indeed MFM vortex imaging shows correlated defects\nparallel to twin boundaries only in underdoped samples and not for $\\rm x\\ge\nx_{opt}$.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Controlling spin supercurrents via nonequilibrium spin injection: We propose a mechanism whereby spin supercurrents can be manipulated in\nsuperconductor/ferromagnet proximity systems via nonequilibrium spin injection.\nWe find that if a spin supercurrent exists in equilibrium, a nonequilibrium\nspin accumulation will exert a torque on the spins transported by this current.\nThis interaction causes a new spin supercurrent contribution to manifest out of\nequilibrium, which is proportional to and polarized perpendicularly to both the\ninjected spins and equilibrium spin current. This is interesting for several\nreasons: as a fundamental physical effect; due to possible applications as a\nway to control spin supercurrents; and timeliness in light of recent\nexperiments on spin injection in proximitized superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Condensate entanglement and multigap superconductivity in nanoscale\n  superconductors: A Green functions approach is used to study superconductivity in nanofilms\nand nanowires. We show that the superconducting condensate results from the\nmultimodal entanglement, or internal Josephson coupling, of the subcondensates\nassociated with the manifold of Fermi surface subparts resulting from\nsize-quantisation. This entanglement is of critical importance in these\nsystems, since without it superconductivity would be extremely weak, if not\ncompletely negligible. Further, the multimodal character of the condensate\ngenerally results in multigap superconductivity, with great quantitative\nconsequence for the value of the critical parameters. Our approach suggests\nthat these are universal characteristics of confined superconductors.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Determining Pair Interactions from Structural Correlations: We examine metastable configurations of a two-dimensional system of\ninteracting particles on a quenched random potential landscape and ask how the\nconfigurational pair correlation function is related to the particle\ninteractions and the statistical properties of the potential landscape.\n  Understanding this relation facilitates quantitative studies of magnetic flux\nline interactions in type II superconductors, using structural information\navailable from Lorentz microscope images or Bitter decorations.\n  Previous work by some of us supported the conjecture that the relationship\nbetween pair correlations and interactions in pinned flux line ensembles is\nanalogous to the corresponding relationship in the theory of simple liquids.\n  The present paper aims at a more thorough understanding of this relation. We\nreport the results of numerical simulations and present a theory for the low\ndensity behavior of the pair correlation function which agrees well with our\nsimulations and captures features observed in experiments. In particular, we\nfind that the resulting description goes beyond the conjectured classical\nliquid type relation and we remark on the differences.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Andreev bound states in rounded corners of d-wave superconductors: Andreev bound states at boundaries of d-wave superconductors are strongly\ninfluenced by the boundary geometry itself. In this work, the zero-energy\nspectral weight of the local quasiparticle density of states is presented for\nthe case of wedge-shaped boundaries with rounded corners. Generally, both\norientation of the d-wave and the specific local reflection properties of the\nrounded wedges determine, whether Andreev bound states exist or not. For the\nbisecting line of the wedge being parallel to the nodal direction of the d-wave\ngap function, strong zero-energy Andreev bound states are expected at the round\npart of the boundary.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Magnetic properties of magnetically textured Bi-2212 ceramics: This paper aims at reporting magnetic properties of bulk polycrystalline\nBi2Sr2Ca0.8Dy0.2Cu2O8-y samples textured under a magnetic field. The\nmicrostructure of these materials is highly anisotropic and exhibits particular\nfeatures needed to be taken into account in order to interpret their magnetic\nand electrical properties. First the AC magnetic susceptibility c = c ' - j c\"\nhas been measured for several magnetic fields (H // ab and H // c) and compared\nto the electrical resistivity data. The structure of the c\" peak is shown to be\nrelated to the chemical content distribution of the superconducting grains.\nNext, the magnetic flux profiles have been extracted from the magnetic\nmeasurements using the Campbell - Rollins procedure. The anisotropy of the flux\nprofiles and their peculiar curvature behaviour for H // c point out the role\nof both grain platelet structure and the presence of secondary phases. From\nthese results, we conclude that the magnetic properties of such magnetically\ntextured materials do not allow for a reliable extraction of the critical\ncurrent density Jc but essentially probe geometric effects. Such effects have\nto be taken into account for improving the manufacture of attractive high-Tc\nmaterials.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Anisotropy of the upper critical fields and the paramagnetic Meissner\n  effect in La1.85Sr0.15CuO4 single Crystals: Optimally-doped La1.85Sr0.15CuO4 single crystals have been investigated by dc\nand ac magnetic measurements. These crystals have rectangular needle-like\nshapes with the long needle axis parallel to the crystallographic c axis\n(c-crystal) or parallel to the basal planes (a-crystal). In both crystals, the\ntemperature dependence of the upper critical fields (HC2) and the surface\ncritical field (HC3) were measured. The H-T phase diagram is presented. Close\nto TC =35 K, for the c-crystal, {\\gamma}c = / = 1.80(2), whereas for the\na-crystal the {\\gamma}a = / =4.0(2) obtained, is much higher than the\ntheoretical value 1.69. At low applied dc fields, positive field-cooled\nbranches known as the \"paramagnetic Meissner effect\" (PME) are observed, their\nmagnitude is inversely proportional to H. The anisotropic PME is observed in\nboth a- and c-crystals, only when the applied field is along the basal planes.\nIt is speculated that the high {\\gamma}a and the PME are connected to each\nother.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Metastability in Josephson transmission lines: Thermal activation and macroscopic quantum tunneling in current-biased\ndiscrete Josephson transmission lines are studied theoretically. The degrees of\nfreedom under consideration are the phases across the junctions which are\ncoupled to each other via the inductances of the system. The resistively\nshunted junctions that we investigate constitute a system of N interacting\ndegrees of freedom with an overdamped dynamics. We calculate the decay rate\nwithin exponential accuracy as a function of temperature and current. Slightly\nbelow the critical current, the decay from the metastable state occurs via a\nunique (\"rigid\") saddlepoint solution of the Euclidean action describing the\nsimultaneous decay of the phases in all the junctions. When the current is\nreduced, a crossover to a regime takes place, where the decay occurs via an\n\"elastic\" saddlepoint solution and the phases across the junctions leave the\nmetastable state one after another. This leads to an increased decay rate\ncompared with the rigid case both in the thermal and the quantum regime. The\nrigid-to-elastic crossover can be sharp or smooth analogous to first- or\nsecond- order phase transitions, respectively. The various regimes are\nsummarized in a current-temperature decay diagram.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Deterministic Josephson Vortex Ratchet with a load: We investigate experimentally a deterministic underdamped Josephson vortex\nratchet -- a fluxon-particle moving along a Josephson junction in an asymmetric\nperiodic potential. By applying a sinusoidal driving current one can compel the\nvortex to move in a certain direction, producing average dc voltage across the\njunction. Being in such a rectification regime we also load the ratchet, i.e.,\napply an additional dc bias current I_dc (counterforce) which tilts the\npotential so that the fluxon climbs uphill due to the ratchet effect. The value\nof the bias current at which the fluxon stops climbing up defines the strength\nof the ratchet effect and is determined experimentally. This allows us to\nestimate the loading capability of the ratchet, the output power and\nefficiency. For the quasi-static regime we present a simple model which\ndelivers simple analytic expressions for the above mentioned figures of merit.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Tunable two-dimensional superconductivity and spin-orbit coupling at the\n  EuO/KTaO3(110) interface: Unconventional quantum states, most notably the two-dimensional (2D)\nsuperconductivity, have been realized at the interfaces of oxide\nheterostructures where they can be effectively tuned by the gate voltage\n($V_G$). Here we report that the interface between high-quality EuO (111) thin\nfilm and KTaO3 (KTO) (110) substrate shows superconductivity with onset\ntransition temperature $T_c^{onset}$ = 1.35 K. The 2D nature of\nsuperconductivity is verified by the large anisotropy of the upper critical\nfield and the characteristics of a Berezinskii-Kosterlitz-Thouless transition.\nBy applying $V_G$, $T_c^{onset}$ can be tuned from ~ 1 to 1.7 K; such an\nenhancement can be possibly associated with a boosted spin-orbit energy\n${\\epsilon}_{so}$ = $\\hbar$ / ${\\tau}_{so}$, where ${\\tau}_{so}$ is the\nspin-orbit relaxation time. Further analysis of ${\\tau}_{so}$ based on the\nupper critical field ($H_{c2}$) and magnetoconductance reveals complex nature\nof spin-orbit coupling (SOC) at the EuO/KTO(110) interface with different\nmechanisms dominate the influence of SOC effects for the superconductivity and\nthe magnetotransport in the normal state. Our results demonstrate that the SOC\nshould be considered as an important factor determining the 2D\nsuperconductivity at oxide interfaces.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconducting energy gap and c-axis plasma frequency of\n  (Nd,Sm)O0.82F0.18FeAs superconductors from infrared ellipsometry: We present ellipsometric measurements of the far-infrared dielectric response\nof polycrystalline samples of the new pnictide superconductor RO0.82F0.18FeAs\n(R=Nd and Sm). We find evidence that the electronic properties are strongly\nanisotropic such that the optical spectra are dominated by the weakly\nconducting c-axis response similar as in the cuprate high-temperature\nsuperconductors (HTSC). Accordingly, we obtain an upper limit of the c-axis\nsuperconducting plasma frequency of $\\omega_{{\\rm pl},c}^{\\rm SC}\\leq 260\\cm$\nwhich corresponds to a lower limit of the c-axis magnetic penetration depth of\n$\\lambda_c\\geq6\\mum$ and an anisotropy of $\\lambda_c/\\lambda_{ab}\\geq 30$ as\ncompared to $\\lambda_{ab}=185$ nm from muon spin rotation [A. Drew {\\it et\nal.}, arXiv:0805.1042]. Also in analogy to the cuprate HTSC, our spectra\nexhibit the signatures of a gap-like suppression of the conductivity in the\nsuperconducting state with a large gap magnitude of $2\\Delta\\approx300\\cm$ (37\nmeV) and a ratio of $2\\Delta/k_{\\rm B}\\tc\\approx8$ that is suggestive of strong\ncoupling.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Scaling of the Fano effect of the in-plane Fe-As phonon and the\n  superconducting critical temperature in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$: By means of infrared spectroscopy we determine the temperature-doping phase\ndiagram of the Fano effect for the in-plane Fe-As stretching mode in\nBa$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$. The Fano parameter $1/q^2$, which is a\nmeasure of the phonon coupling to the electronic particle-hole continuum, shows\na remarkable sensitivity to the magnetic/structural orderings at low\ntemperatures. More strikingly, at elevated temperatures in the\nparamagnetic/tetragonal state we find a linear correlation between $1/q^2$ and\nthe superconducting critical temperature $T_c$. Based on theoretical\ncalculations and symmetry considerations, we identify the relevant interband\ntransitions that are coupled to the Fe-As mode. In particular, we show that a\nsizable $xy$ orbital component at the Fermi level is fundamental for the Fano\neffect and possibly also for the superconducting pairing.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Ab initio Study of Cross-Interface Electron-Phonon Couplings in FeSe\n  Thin Films on SrTiO$_3$ and BaTiO$_3$: We study the electron-phonon coupling strength near the interface of\nmonolayer and bilayer FeSe thin films on SrTiO$_3$, BaTiO$_3$, and\noxygen-vacant SrTiO$_3$ substrates, using ab initio methods. The calculated\ntotal electron-phonon coupling strength $\\lambda=0.2\\text{--}0.3$ cannot\naccount for the high $T_c\\sim 70$ K observed in these systems through the\nconventional phonon-mediated pairing mechanism. In all of these systems,\nhowever, we find that the coupling constant of a polar oxygen branch peaks at\n$\\mathbf{q}=0$ with negligible coupling elsewhere, while the energy of this\nmode coincides with the offset energy of the replica bands measured recently by\nangle-resolved photoemission spectroscopy experiments. But the integrated\ncoupling strength for this mode from our current calculations is still too\nsmall to produce the observed high $T_c$, even through the more efficient\npairing mechanism provided by the forward scattering. We arrive at the same\nqualitative conclusion when considering a checkerboard antiferromagnetic\nconfiguration in the Fe layer. In light of the experimental observations of the\nreplica band feature and the relatively high $T_c$ of FeSe monolayers on polar\nsubstrates, our results point towards a cooperative role for the\nelectron-phonon interaction, where the cross-interface interaction acts in\nconjunction with a purely electronic interaction. We also discuss a few\nscenarios where the coupling strength obtained here may be enhanced.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "High-Density Superconductive Logic Circuits Utilizing 0 and $\u03c0$\n  Josephson Junctions: Superconductor Electronics (SCE) is a fast and power-efficient technology\nwith great potential for overcoming conventional CMOS electronics' scaling\nlimits. Nevertheless, the primary challenge confronting SCE today pertains to\nits integration level, which lags several orders of magnitude behind CMOS\ncircuits. In this study, we have innovated and simulated a novel logic family\ngrounded in the principles of phase shifts occurring in 0 and $\\pi$ Josephson\njunctions. The fast phase logic (FPL) eliminates the need for large inductor\nloops and shunt resistances by combining the half-flux and phase logic.\nTherefore, the Josephson junction (JJ) area only limits the integration\ndensity. The cells designed with this paradigm are fast, and the clock-to-Q\ndelay is about 4ps while maintaining over 50% parameter margins. This logic is\npower efficient and can increase the integration by at least 100$\\times$ in the\nSCE chips.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Z$_2$ topology and superconductivity from symmetry lowering of a 3D\n  Dirac Metal Au$_2$Pb: 3D Dirac semi-metals (DSMs) are materials that have massless Dirac electrons\nand exhibit exotic physical properties It has been suggested that structurally\ndistorting a DSM can create a Topological Insulator (TI), but this has not yet\nbeen experimentally verified. Furthermore, quasiparticle excitations known as\nMajorana Fermions have been theoretically proposed to exist in materials that\nexhibit superconductivity and topological surface states. Here we show that the\ncubic Laves phase Au$_2$Pb has a bulk Dirac cone above 100 K that gaps out upon\ncooling at a structural phase transition to create a topologically non trivial\nphase that superconducts below 1.2 K. The nontrivial Z$_2$ = -1 invariant in\nthe low temperature phase indicates that Au$_2$Pb in its superconducting state\nmust have topological surface states. These characteristics make Au$_2$Pb a\nunique platform for studying the transition between bulk Dirac electrons and\ntopological surface states as well as studying the interaction of\nsuperconductivity with topological surface states.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Generation of Teraherz Oscillations by Thin Superconducting Film in\n  Fluctuation Regime: Explicit analytical expressions for conductivity of a superconducting film\nabove and below critical temperature in an arbitrary electric field are derived\nin the frameworks of the time dependent Ginzburg-Landau theory. It is confirmed\nthat slightly below critical temperature the differential conductivity of\nsuperconducting film can become negative for small enough values of electric\nfield. This fact may cause generation of electromagnetic oscillations if the\nsuperconducting film is appropriately coupled of with a resonator. Their\nmaximal frequency is proportional to the value of critical temperature of\nsuperconducting transition. The obtained results can stimulate the development\nof Terahertz generators on the basis of high temperature superconducting films.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "High Tc superconductivity in heavy Rare Earth Hydrides: correlation\n  between the presence of the f states on the Fermi surface, nesting and the\n  value of Tc: Lanthanum hydrides, containing hydrogen framework structures under\ncompression, display a superconducting state with a high observed critical\ntemperature. However, this phenomenon has so far only been observed at very\nhigh pressures. Here, we computationally search for superconductors with very\nhigh critical temperatures, but at much lower pressures. We uncover two such\nsodalite-type clathrate hydrides, YbH6 and LuH6 (Tc =145 K at P=70 GPa for YbH6\nand, especially, Tc =273 K at P=100 GPa for LuH6). These striking properties\nare a consequence of the strong interrelationship between the f states present\nat the Fermi surface, structural stability and Tc value. For example, TmH6,\nwith unfilled 4f orbitals, is stable at 50 GPa, while it has a relatively low\nvalue of Tc of 25 K. As for the YbH6 and LuH6 compounds, they have filled\nf-shells and the decrease of the f-energy below the Fermi level leads to\nformation of the nesting regions on the Fermi surface and, as a result, to\nphonon \"softening\" and an increase in Tc.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity at 15.6 K in Calcium-doped Tb_{1-x}Ca_xFeAsO: the\n  structure requirement for achieving superconductivity in the hole-doped 1111\n  phase: Superconductivity at about 15.6 K was achieved in Tb_{1-x}Ca_xFeAsO by\npartially substituting Tb^{3+} with Ca^{2+} in the nominal doping region x =\n0.40 \\sim 0.50. A detailed investigation was carried out in a typical sample\nwith doping level of x = 0.44. The upper critical field of this sample was\nestimated to be 77 Tesla from the magnetic field dependent resistivity data.\nThe domination of hole-like charge carriers in the low-temperature region was\nconfirmed by Hall effect measurements. The comparison between the calcium-doped\nsample Pr_{1-x}Ca_xFeAsO (non-superconductive) and the Strontium-doped sample\nPr_{1-x}Sr_xFeAsO (superconductive) suggests that a lager ion radius of the\ndoped alkaline-earth element compared with that of the rare-earth element may\nbe a necessary requirement for achieving superconductivity in the hole-doped\n1111 phase.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Robust nodal superconductivity induced by isovalent doping in\n  Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ and BaFe$_2$(As$_{1-x}$P$_x$)$_2$: We present the ultra-low-temperature heat transport study of iron-based\nsuperconductors Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ and\nBaFe$_2$(As$_{1-x}$P$_x$)$_2$. For optimally doped\nBa(Fe$_{0.64}$Ru$_{0.36}$)$_2$As$_2$, a large residual linear term $\\kappa_0/T$\nat zero field and a $\\sqrt{H}$ dependence of $\\kappa_0(H)/T$ are observed,\nwhich provide strong evidences for nodes in the superconducting gap. This\nresult demonstrates that the isovalent Ru doping can also induce nodal\nsuperconductivity, as P does in BaFe$_2$(As$_{0.67}$P$_{0.33}$)$_2$.\nFurthermore, in underdoped Ba(Fe$_{0.77}$Ru$_{0.23}$)$_2$As$_2$ and heavily\nunderdoped BaFe$_2$(As$_{0.82}$P$_{0.18}$)$_2$, $\\kappa_0/T$ manifests similar\nnodal behavior, which shows the robustness of nodal superconductivity in the\nunderdoped regime and puts constraint on theoretical models.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "How to measure the spatial correlations in disordered\n  Berezinski-Kosterlitz-Thouless transition ?: The Berezinski-Kosterlitz-Thouless transition is a unique two dimensional\nphase transition, separating two phases with exponentially and power-law\ndecaying correlations, respectively. In disordered systems, these correlations\npropagate along favorable paths, with the transition marking the point where\nglobal coherence is lost. Here we propose an experimental method to probe\nlocally these particular paths in superconducting thin films, which exhibit\nthis transition, and demonstrate theoretically that close to the transition the\ncoherence propagate along a ramified network, reminiscent of a percolation\ntransition. We suggest and calculate experimentally accessible quantities that\ncan shed light on the spatial correlations in the system as it approaches the\ncritical point.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Percolative nature of the dc paraconductivity in the cuprate\n  superconductors: We present an investigation of the planar direct-current (dc)\nparaconductivity of the model cuprate material HgBa$_2$CuO$_{4+\\delta}$ in the\nunderdoped part of the phase diagram. The simple quadratic\ntemperature-dependence of the Fermi-liquid normal-state resistivity enables us\nto extract the paraconductivity above the macroscopic $T_c$ with great\naccuracy. The paraconductivity exhibits unusual exponential temperature\ndependence, with a characteristic temperature scale that is distinct from\n$T_c$. In the entire temperature range where it is discernable, the\nparaconductivity is quantitatively explained by a simple superconducting\npercolation model, which implies that underlying gap disorder dominates the\nemergence of superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Double-helix magnetic order in CrAs with Pnma space group: The transition metal pnictide CrAs exhibits superconductivity in the vicinity\nof a helimagnetic phase, where it has been found that the propagation vector is\nparallel to the c axis and the magnetic moments lie in the ab plane. Here we\nuse ab initio calculations to study the magnetic interactions in the material.\nMapping onto a Heisenberg Hamiltonian, we calculate the magnetic exchanges by\nusing LDA+U calculations and we unveil the origin of the magnetic frustration.\nFinally, we reproduce the double helix magnetic order with a propagation vector\nQ = (0, 0, 0.456) and we obtain the magnetic transition temperature TN through\nMonte-Carlo simulations of the specific heat. Due to the limitations of the use\nof the Heisenberg Hamiltonian for itinerant magnetic systems, the theoretical\nTN underestimated the experimental value of the pure CrAs. However, our results\nare in good agreement with those found for the alloy CrAs0.5Sb0.5 belonging to\nthe same space group, showing that our result can describe this material class.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Unusual Hole-doping-dependent Electronic Instability and Electron-Phonon\n  Coupling in Infinite-layer Nickelates: The interplay between superconductivity and charge density waves (CDWs) under\nhole doping in cuprates is one of the central phenomena in condensed matter\nphysics. Recently, CDWs are also observed in CaCuO$_2$-analogous nickelates\nRNiO$_2$ (R = La, Nd) but exhibit fundamentally different hole-doping-dependent\nbehaviors compared to that in cuprates, raising a challenging question on its\norigin. In this article, we propose that electronic instability (EI) and\nmoment-dependent electron-phonon coupling (MEPC), mainly contributed by Ni\n3dx2-y2 and R 5dz2, respectively, may be the possible reasons for CDW formation\nin RNiO$_2$. Without hole doping, a strong Fermi surface nesting (FSN) induced\nby the unique feature of van Hove singularity (VHS) across Fermi level exists\nin RNiO$_2$ but not in CaCuO$_2$, and the unusual temperature-insensitive\nfeature of EI and MEPC could result in rather high temperature CDWs in\nRNiO$_2$. Under hole doping, the reduced FSN of Ni 3dx2-y2 by the shift of VHS\nand decreased occupation of R 5dz2 largely weaken EI and MEPC in RNiO$_2$,\nrespectively, suppressing the CDW formation. Our theory not only offers\npossible explanations to some puzzling experimental observations, but also\nestablishes a unified understanding on the hole-doping-dependent EI and MEPC in\nnickelates and cuprates.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Optical conductivity of iron-based superconductors: The new family of unconventional iron-based superconductors discovered in\n2006 immediately relieved their copper-based high-temperature predecessors as\nthe most actively studied superconducting compounds in the world. The\nexperimental and theoretical effort made in order to unravel the mechanism of\nsuperconductivity in these materials has been overwhelming. Although our\nunderstanding of their microscopic properties has been improving steadily, the\npairing mechanism giving rise to superconducting transition temperatures up to\n55 K remains elusive. And yet the hope is strong that these materials, which\npossess a drastically different electronic structure but similarly high\ntransition temperatures compared to the copper-based compounds, will shed\nessential new light onto the several-decade-old problem of unconventional\nsuperconductivity. In this work we review the current understanding of the\nitinerant-charge-carrier dynamics in the iron-based superconductors and parent\ncompounds largely based on the optical-conductivity data the community has\ngleaned over the past seven years using such experimental techniques as\nreflectivity, ellipsometry, and terahertz transmission measurements and analyze\nthe implications of these studies for the microscopic properties of the\niron-based materials as well as the mechanism of superconductivity therein.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Paramagnon dispersion and damping in doped\n  Na$_{x}$Ca$_{2-x}$CuO$_2$Cl$_2$: Using Resonant Inelastic X-ray Scattering, we measure the paramagnon\ndispersion and damping of undoped, antiferromagnetic Ca$_2$CuO$_2$Cl$_2$ as\nwell as doped, superconducting Na$_{x}$Ca$_{2-x}$CuO$_2$Cl$_2$. Our estimation\nof the spin-exchange parameter and width of the paramagnon peak at the zone\nboundary $X=(0.5,0)$ confirms that no simple relation can be drawn between\nthese parameters and the critical temperature $T_\\mathrm{c}$. Consistently with\nother cuprate compounds, we show that upon doping there is a slight softening\nat $(0.25,0)$, but not at the zone boundary $X$. In combination with these\nmeasurements we perform calculations of the dynamical spin structure factor of\nthe one-band Hubbard model using cluster dynamical mean-field theory. The\ncalculations are in excellent agreement with the experiment in the undoped\ncase, both in terms of energy position and width. While the increase in width\nis also captured upon doping, the dynamical spin structure factor shows a\nsizable softening at $X$, which provides insightful information on the\nlength-scale of the spin fluctuations in doped cuprates.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Shell Structure and Strengthening of Superconducting Pair Correlation in\n  Nanoclusters: The existence of shell structure and the accompanying high degeneracy of\nelectronic levels leads to the possibility of strong superconducting pairing in\nmetallic nanoclusters with N~100-1000 delocalized electrons. The most favorable\ncases correspond to (a) \"magic\" clusters with strongly degenerate highest\noccupied and lowest unoccupied shells and a relatively small energy spacing\nbetween them as well as to (b) clusters with slightly incomplete shells and\nsmall Jahn-Teller splitting. It is shown that realistic sets of parameters lead\nto very high values of Tc as well as to a strong alteration of the energy\nspectrum. The impact of fluctuations is analyzed. Spectroscopic experiments\naimed at detecting the presence of pair correlations are proposed. The pairing\nshould also manifest itself via odd-even effects in cluster spectra, similar to\nthe case of nuclei.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Hidden magnetic excitation in the pseudogap phase of a model cuprate\n  superconductor: The elucidation of the pseudogap phenomenon of the cuprates, a set of\nanomalous physical properties below the characteristic temperature T* and above\nthe superconducting transition temperature Tc, has been a major challenge in\ncondensed matter physics for the past two decades. Following initial\nindications of broken time-reversal symmetry in photoemission experiments,\nrecent polarized neutron diffraction work demonstrated the universal existence\nof an unusual magnetic order below T*. These findings have the profound\nimplication that the pseudogap regime constitutes a genuine new phase of matter\nrather than a mere crossover phenomenon. They are furthermore consistent with a\nparticular type of order involving circulating orbital currents, and with the\nnotion that the phase diagram is controlled by a quantum critical point. Here\nwe report inelastic neutron scattering results for HgBa2CuO4+x (Hg1201) that\nreveal a fundamental collective magnetic mode associated with the unusual\norder, and that further support this picture. The mode's intensity rises below\nthe same temperature T* and its dispersion is weak, as expected for an\nIsing-like order parameter. Its energy of 52-56 meV and its enormous integrated\nspectral weight render it a new candidate for the hitherto unexplained\nubiquitous electron-boson coupling features observed in spectroscopic studies.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Microwave Surface Impedance Measurements of the Electronic State and\n  Dissipation of Magnetic Vortices in Superconducting Iron-Based LiFeAs Single\n  Crystals: LiFeAs is one of the iron-based superconductors having multiple gaps with the\npossible sign reversal. To clarify how those novel natures affect the energy\ndissipation of magnetic vortices, we investigated the microwave surface\nimpedance of LiFeAs single crystals under finite magnetic fields. The flux-flow\nresistivity enhanced rapidly at low magnetic fields, which is similar to the\ncase of MgB$_{2}$. This is probably the consequence of the multiple-gap nature\nand the gap anisotropy. This suggest that the sign-reversal is not important\nfor the flux-flow even for multiple-gap superconductors. As for the electronic\nstate, the vortex core of LiFeAs turned out to be \"moderately clean\".\nFurthermore, the mean free path inside the vortex core was much shorter than\nthat outside, and was close to the core radius. These results strongly suggest\na process specific to the core boundary is important for a scattering mechanism\ninside the vortex core.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Superconductivity at 17 K in (Fe2P2)(Sr4Sc2O6): a new superconducting\n  layered pnictide oxide with a thick perovskite oxide layer: A new layered oxypnictide (Fe2P2)(Sr4Sc2O6) have been synthesized by\nsolid-state reaction. This material has an alternating layer stacking structure\nof anti-fluorite Fe2P2 and perovskite-based Sr4Sc2O6 oxide layers. Space group\nof the material is P4/nmm and lattice constants a and c are 4.016 A and 15.543\nA, respectively. The interlayer Fe-Fe distance corresponding to the c-axis\nlength is the longest ever reported in the iron-based oxypnictide systems. In\nboth magnetization and resistivity measurements, the present compound exhibited\nsuperconductivity below 17 K, which is much higher than that of LaFePO and the\nhighest in arsenic-free iron-based oxypnictide systems under ambient pressure.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Direct observation of melting in a 2-D superconducting vortex lattice: Topological defects such as dislocations and disclinations are predicted to\ndetermine the twodimensional (2-D) melting transition. In 2-D superconducting\nvortex lattices, macroscopic measurements evidence melting close to the\ntransition to the normal state. However, the direct observation at the scale of\nindividual vortices of the melting sequence has never been performed. Here we\nprovide step by step imaging through scanning tunneling spectroscopy of a 2-D\nsystem of vortices up to the melting transition in a focused-ion-beam\nnanodeposited W-based superconducting thin film. We show directly the\ntransition into an isotropic liquid below the superconducting critical\ntemperature. Before that, we find a hexatic phase, characterized by the\nappearance of free dislocations, and a smectic-like phase, possibly originated\nthrough partial disclination unbinding. These results represent a significant\nstep in the understanding of melting of 2-D systems, with impact across several\nresearch fields, such as liquid crystal molecules, or lipids in membranes.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "New Series of Nickel-Based Pnictide Oxide Superconductors\n  (Ni2Pn2)(Sr4Sc2O6) (Pn = P, As): We have discovered new nickel-based pnictide oxide superconductors,\n(Ni2Pn2)(Sr4Sc2O6) (Pn = P, As). These compounds have a tetragonal unit cell\nwith a space group of P4/nmm and they consist of alternate stacking of\nanti-fluorite Ni2Pn2 layers and K2NiF4-type Sr4Sc2O6 blocking layers. Lattice\nparameters were a = 4.044 A and c = 15.23 A for (Ni2P2)(Sr4Sc2O6) and a = 4.078\nA and c = 15.41 A for (Ni2As2)(Sr4Sc2O6), indicating their thicker blocking\nlayers than that of LaNiPO (c ~ 8.1 A). Both (Ni2P2)(Sr4Sc2O6) and\n(Ni2As2)(Sr4Sc2O6) exhibited superconductivity with zero resistivity at 3.3 K\nand 2.7 K, respectively. The perfect diamagnetism observed in both compounds\nguaranteed their bulk superconductivity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Pressure-Induced Superconductivity and Its Scaling with Doping-Induced\n  Superconductivity in the Iron Pnictide with Skutterudite Intermediary Layers: The Ca10(PtnAs8)(Fe2As2)5 (n=3,4) compounds are a new type of iron pnictide\nsuperconductors whose structures consist of stacking Ca-PtnAs8-Ca-Fe2As2 layers\nin a unit cell. When n=3 (the 10-3-8 phase), the undoped compound is an\nantiferromagnetic (AFM) semiconductor, while, when n=4 (the 10-4-8 phase), the\nundoped compound is a superconductor with the transition temperature of 26K.\nHere we report the results of high-pressure studies on the 10-3-8 compound\nobtained through a combination of in-situ resistance, magnetic susceptibility,\nand Hall coefficient measurements. We find that its AFM order can be suppressed\ncompletely at 3.5 GPa and then superconducting state appears in the pressure\nrange of 3.5-7 GPa. The pressure dependence of superconducting transition\ntemperature displays a dome-like shape.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Possible Dynamic States in Inductively Coupled Intrinsic Josephson\n  Junctions of Layered High-$T_c$ Superconductors: Based on computer simulations and theoretical analysis, a new dynamic state\nis found in inductively coupled intrinsic Josephson junctions in the absence of\nan external magnetic field. In this state, the plasma oscillation is uniform\nalong the c axis and there are $(2m+1)\\pi$ phase kinks, with $m$ being an\ninteger, periodic and thus non-uniform in the $c$ direction. In the IV\ncharacteristics, the state manifests itself as current steps occurring at all\ncavity modes. Inside the current steps, the plasma oscillation becomes strong,\nwhich generates several harmonics in frequency spectra at a given voltage. The\nrecent experiments on terahertz radiations from the mesa of a BSCCO single\ncrystal can be explained in terms of this state.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Critical currents in superconductors with quasiperiodic pinning arrays:\n  One-dimensional chains and two-dimensional Penrose lattices: We study the critical depinning current J_c, as a function of the applied\nmagnetic flux Phi, for quasiperiodic (QP) pinning arrays, including\none-dimensional (1D) chains and two-dimensional (2D) arrays of pinning centers\nplaced on the nodes of a five-fold Penrose lattice. In 1D QP chains of pinning\nsites, the peaks in J_c(Phi) are shown to be determined by a sequence of\nharmonics of long and short periods of the chain. This sequence includes as a\nsubset the sequence of successive Fibonacci numbers. We also analyze the\nevolution of J_c(Phi) while a continuous transition occurs from a periodic\nlattice of pinning centers to a QP one; the continuous transition is achieved\nby varying the ratio gamma = a_S/a_L of lengths of the short a_S and the long\na_L segments, starting from gamma = 1 for a periodic sequence. We find that the\npeaks related to the Fibonacci sequence are most pronounced when gamma is equal\nto the \"golden mean\". The critical current J_c(Phi) in QP lattice has a\nremarkable self-similarity. This effect is demonstrated both in real space and\nin reciprocal k-space. In 2D QP pinning arrays (e.g., Penrose lattices), the\npinning of vortices is related to matching conditions between the vortex\nlattice and the QP lattice of pinning centers. Although more subtle to analyze\nthan in 1D pinning chains, the structure in J_c(Phi) is determined by the\npresence of two different kinds of elements forming the 2D QP lattice. Indeed,\nwe predict analytically and numerically the main features of J_c(Phi) for\nPenrose lattices. Comparing the J_c's for QP (Penrose), periodic (triangular)\nand random arrays of pinning sites, we have found that the QP lattice provides\nan unusually broad critical current J_c(Phi), that could be useful for\npractical applications demanding high J_c's over a wide range of fields.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Nonequilibrium transport via spin-induced sub-gap states in\n  superconductor/quantum dot/normal metal cotunnel junctions: We study low-temperature transport through a Coulomb blockaded quantum dot\n(QD) contacted by a normal (N), and a superconducting (S) electrode. Within an\neffective cotunneling model the conduction electron self energy is calculated\nto leading order in the cotunneling amplitudes and subsequently resummed to\nobtain the nonequilibrium T-matrix, from which we obtain the nonlinear\ncotunneling conductance. For even occupied dots the system can be conceived as\nan effective S/N-cotunnel junction with subgap transport mediated by Andreev\nreflections. The net spin of an odd occupied dot, however, leads to the\nformation of sub-gap resonances inside the superconducting gap which gives rise\nto a characteristic peak-dip structure in the differential conductance, as\nobserved in recent experiments.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Temperature and polarization dependence of low-energy magnetic\n  fluctuations in nearly-optimal-doped NaFe$_{0.9785}$Co$_{0.0215}$As: We use unpolarized and polarized neutron scattering to study the temperature\nand polarization dependence of low-energy magnetic fluctuations in\nnearly-optimal-doped NaFe$_{0.9785}$Co$_{0.0215}$As, with coexisting\nsuperconductivity ($T_{\\rm c}\\approx19$ K) and weak antiferromagnetic order\n($T_{\\rm N}\\approx30$ K, ordered moment $\\approx0.02$ $\\mu_{\\rm B}$/Fe). A\nsingle spin resonance mode with intensity tracking the superconducting order\nparameter is observed, although energy of the mode only softens slightly on\napproaching $T_{\\rm c}$. Polarized neutron scattering reveals that the single\nresonance is mostly isotropic in spin space, similar to overdoped\nNaFe$_{0.935}$Co$_{0.045}$As but different from optimal electron-, hole-, and\nisovalent-doped BaFe$_2$As$_2$ compounds, all featuring an additional prominent\nanisotropic component. Spin anisotropy in NaFe$_{0.9785}$Co$_{0.0215}$As is\ninstead present at energies below the resonance, which becomes partially gapped\nbelow $T_{\\rm c}$, similar to the situation in optimal-doped\nYBa$_2$Cu$_3$O$_{6.9}$. Our results indicate that anisotropic spin fluctuations\nin NaFe$_{1-x}$Co$_x$As appear in the form of a resonance in the underdoped\nregime, become partially gapped below $T_{\\rm c}$ near optimal doping and\ndisappear in overdoped compounds.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Elementary vortex pinning potential in a chiral p-wave superconductor: The elementary vortex pinning potential is studied in a chiral p-wave\nsuperconductor with a pairing d=z(k_x + i k_y) on the basis of the\nquasiclassical theory of superconductivity. An analytical investigation and\nnumerical results are presented to show that the vortex pinning potential is\ndependent on whether the vorticity and chirality are parallel or antiparallel.\nMutual cancellation of the vorticity and chirality around a vortex is\nphysically crucial to the effect of the pinning center inside the vortex core.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "The geometric theory of low dimensional superconductivity: The study of the electromagnetic properties of 2D materials is an area of\nintensive research. Most of the efforts in the subject have been oriented\ntowards the understanding and modelling of the microscopic behaviour of the\ncharge carriers within the medium. Albeit there is a well established manner to\nexpress London's equations in the language of differential geometry, an\neffective geometric theory of the macroscopic phenomenon is still lacking. In\nthis work we concentrate in obtaining the underlying geometric structure of\nsuperconductivity in two dimensional materials. To this effect, we produce an\nintrinsic framework which allows us to clearly identify the geometric\nassumptions leading to London's equations and the Meissner state. Specifically,\nwe show that any two-dimensional medium whose response to an externally applied\nelectromagnetic field results in a divergence free geodesically flowing induced\ncurrent must be a superconductor. In this manner, we conclude that the\nunderlying geometry of this type of media is that of a three dimensional\nLorentzian contact manifold. Moreover, we show that the geometric condition\nmacroscopically encoding the superconducting phenomena emerges from a\nvariational principle, exhibiting that the macroscopic hallmark of\nsuperconductivity is expressed as the non-vanishing of the induced current's\nhelicity.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Further detailing of the Bose-Einstein negative-U state in the high\n  temperature superconducting cuprates: The cause of the sharp and universal optimization of the HTSC condensation\nenergy at the hole doping concentration of p = 0.19 is identified within the\ncontext of the boson-fermion negative-U modelling and stripe phase electronic\norganization. Some recent structural, optical and positron annihilation\nexperiments adding further support to this type of modelling of HTSC are\nbriefly examined.",
        "category": "cond-mat_supr-con"
    },
    {
        "text": "Melting of the vortex lattice in high $T_{c}$ superconductors: The precise measurements of vortex melting point towards a need to develop a\nquantitative theoretical description of thermal fluctuations in vortex matter.\nTo tackle the difficult problem of melting, the description of both the solid\nand the liquid phase should reach the precision level well below 0.1%. Such a\ntheory in the framework of the Ginzburg - Landau approach is presented. The\nmelting line location is determined and magnetization, specific heat jumps\nalong it are calculated. We find that the magnetization in the liquid is larger\nthan that in the solid by 1.8% regardless of the melting temperature, while the\nspecific heat jump is about 6% and slowly decreases with temperature. The\nmagnetization curves agree with experimental results on $YBCO$ and Monte Carlo\nsimulations.",
        "category": "cond-mat_supr-con"
    }
]