[ { "text": "Topological Nature of Anomalous Hall Effect in Ferromagnet: The anomalous Hall effect in two-dimensional ferromagnets is discussed to be\nthe physical realization of the parity anomaly in (2+1)D, and the band crossing\npoints behave as the topological singularity in the Brillouin zone. This\nappears as the sharp peaks and the sign changes of the transverse conductance\n$\\sigma_{xy}$ as a function of the Fermi energy and/or the magnetization. The\nrelevance to the experiments including the three dimensional systems is also\ndiscussed.", "category": "cond-mat_str-el" }, { "text": "CsMn$_4$As$_3$: A layered tetragonal transition-metal pnictide compound\n with antiferromagnetic ground state: We report the synthesis and properties of a new layered tetragonal ternary\ncompound CsMn$_4$As$_3$ (structure: KCu$_4$S$_3$-type, space group: $P4/mmm$,\nNo. 123 and $Z = 2$). The material is a small band-gap semiconductor and\nexhibits an antiferromagnetic ground state associated with Mn spins. The\ncompound exhibits a signature of a distinct magnetic moment canting event at\n150(5)~K with a canting angle of $\\approx 0.3^{\\circ}$. Although, some features\nof the magnetic characteristics of this new compound are qualitatively similar\nto those of the related BaMn$_2$As$_2$, the underlying Mn sublattices of the\ntwo materials are quite different. While the Mn square-lattice layers in\nBaMn$_2$As$_2$ are equally spaced along the $c$-direction with the interlayer\ndistance $d_{\\rm L\\,Ba} = 6.7341(4)$ Ang., the Mn sublattice forms bilayers in\nCsMn$_4$As$_3$ with the interlayer distance within a bilayer $d_{\\rm L\\,Cs} =\n3.1661(6)$ Ang. and the distance between the two adjacent bilayers $d_{\\rm B} =\n7.290(6)$ Ang. This difference in the Mn sublattice is bound to significantly\nalter the energy balance between the $J_{1}$, $J_{2}$ and $J_{c}$ exchange\ninteractions within the J1-J2-Jc model compared to that in BaMn$_2$As$_2$ and\nthe other related 122 compounds including the well-known iron-arsenide\nsuperconductor parent compound BaFe$_2$As$_2$. Owing to the novelty of its\ntransition metal sublattice, this new addition to the family of tetragonal\nmaterials related to the iron-based superconductors brings prospects for doping\nand pressure studies in the search of new superconducting phases as well as\nother exciting correlated-electron properties.", "category": "cond-mat_str-el" }, { "text": "The phase diagram and the structure of CDW state in high magnetic field\n in quasi-1D materials: mean-field approach: We develop the mean-field theory of a charge-density wave (CDW) state in\nmagnetic field and study the properties of this state below the transition\ntemperature. We show that the CDW state with shifted wave vector in high\nmagnetic field (CDW$_x$ phase) has at least double harmonic modulation on the\nmost part of the phase diagram. In the perfect nesting case the single harmonic\nCDW state with shifted wave vector exists only in a very narrow region near the\ntricritical point where the fluctuations are very strong. We show that the\ntransition from CDW$_0$ to CDW$_x$ state below the critical temperature is\naccompanied by a jump of the CDW order parameter and of the wave vector rather\nthan by their continuous increase. This implies a first order transition\nbetween these CDW states and explains the strong hysteresis accompanying this\ntransition in many experiments. We examine how the phase diagram changes in the\ncase of imperfect nesting.", "category": "cond-mat_str-el" }, { "text": "Chern and $Z_{2}$ topological insulating phases in perovskite-derived\n $4d$ and $5d$ oxide buckled honeycomb lattices: Based on density functional theory calculations including a Coulomb repulsion\nparameter $U$, we explore the topological properties of\n(La$X$O$_3$)$_2$/(LaAlO$_3$)$_4$(111) with $X=$ $4d$ and $5d$ cations. The\nmetastable ferromagnetic phases of LaTcO$_3$ and LaPtO$_3$ preserve P321\nsymmetry and emerge as Chern insulators (CI) with $C$=2 and 1 and band gaps of\n41 and 38 meV at the lateral lattice constant of LaAlO$_3$, respectively. Berry\ncurvatures, spin textures as well as edge states provide additional insight\ninto the nature of the CI states. While for $X$=Tc the CI phase is further\nstabilized under tensile strain, for $X$=Pd and Pt a site disproportionation\ntakes place when increasing the lateral lattice constant from $a_{\\rm LAO}$ to\n$a_{\\rm LNO}$. The CI phase of $X$=Pt shows a strong dependence on the Hubbard\n$U$ parameter with sign reversal for higher values associated with the change\nof band gap opening mechanism. Parallels to the previously studied\n($X_2$O$_3$)$_1$/(Al$_2$O$_3$)$_5$(0001) honeycomb corundum layers are\ndiscussed. Additionally, non-magnetic systems with $X$=Mo and W are identified\nas potential candidates for $Z_2$ topological insulators at $a_{\\rm LAO}$ with\nband gaps of 26 and 60 meV, respectively. The computed edge states and $Z_{2}$\ninvariants underpin the non-trivial topological properties.", "category": "cond-mat_str-el" }, { "text": "Constraint Effective Potential of the Magnetization in the Quantum XY\n Model: Using an improved estimator in the loop-cluster algorithm, we investigate the\nconstraint effective potential of the magnetization in the spin $\\tfrac{1}{2}$\nquantum XY model. The numerical results are in excellent agreement with the\npredictions of the corresponding low-energy effective field theory. After its\nlow-energy parameters have been determined with better than permille precision,\nthe effective theory makes accurate predictions for the constraint effective\npotential which are in excellent agreement with the Monte Carlo data. This\nshows that the effective theory indeed describes the physics in the low-energy\nregime quantitatively correctly.", "category": "cond-mat_str-el" }, { "text": "Phonon anomalies due to stripe collective modes in high T_c cuprates: Phonon anomalies observed in various high $T_c$ cuprates by neutron\nexperiments are analyzed theoretically in terms of the stripe concept. The\nphonon self-energy correction is evaluated by taking into account the charge\ncollective modes of stripes, giving rise to dispersion gap, or kink and shadow\nphonon modes at twice the wave number of spin stripe. These features coincide\nprecisely with observations. The gapped branches of the phonon are found to be\nin-phase and out-of-phase oscillations relative to the charge collective mode.", "category": "cond-mat_str-el" }, { "text": "Hole-pair hopping in arrangements of hole-rich/hole-poor domains in a\n quantum antiferromagnet: We study the motion of holes in a doped quantum antiferromagnet in the\npresence of arrangements of hole-rich and hole-poor domains such as the\nstripe-phase in high-$T_C$ cuprates. When these structures form, it becomes\nenergetically favorable for single holes, pairs of holes or small bound-hole\nclusters to hop from one hole-rich domain to another due to quantum\nfluctuations. However, we find that at temperature of approximately 100 K, the\nprobability for bound hole-pair exchange between neighboring hole-rich regions\nin the stripe phase, is one or two orders of magnitude larger than single-hole\nor multi-hole droplet exchange. As a result holes in a given hole-rich domain\npenetrate further into the antiferromagnetically aligned domains when they do\nit in pairs. At temperature of about 100 K and below bound pairs of holes hop\nfrom one hole-rich domain to another with high probability. Therefore our main\nfinding is that the presence of the antiferromagnetic hole-poor domains act as\na filter which selects, from the hole-rich domains (where holes form a\nself-bound liquid), hole pairs which can be exchanged throughout the system.\nThis fluid of bound hole pairs can undergo a superfluid phase ordering at the\nabove mentioned temperature scale.", "category": "cond-mat_str-el" }, { "text": "Analysis of the magnetic response of the edge-sharing chain cuprate\n Li$_2$CuO$_2$ within TMRG: It is widely accepted that the low-energy physics in edge-sharing cuprate\nmaterials has one-dimensional (1D) character. The relevant model to study such\nsystems is believed to be the 1D extended Heisenberg model with ferromagnetic\nnearest-neighbor (NN) interaction and antiferromagnetic next-nearest-neighbor\none. Thus far, however, theoretical studies of such materials have been\nconfined to the case of isotropic interactions. In the present work, we compare\nthe spin susceptibility of the 1D extended Heisenberg model with anisotropy in\nthe NN channel, obtained by means of the Transfer Matrix Renormalization Group\nmethod, with that of the edge-sharing chain cuprate Li$_2$CuO$_2$.", "category": "cond-mat_str-el" }, { "text": "Functional renormalization group for frustrated magnets with nondiagonal\n spin interactions: In the field of quantum magnetism, the advent of numerous spin-orbit assisted\nMott insulating compounds, such as the family of Kitaev materials, has led to a\ngrowing interest in studying general spin models with non-diagonal interactions\nthat do not retain the SU(2) invariance of the underlying spin degrees of\nfreedom. However, the exchange frustration arising from these non-diagonal and\noften bond-directional interactions for two- and three-dimensional lattice\ngeometries poses a serious challenge for numerical many-body simulation\ntechniques. In this paper, we present an extended formulation of the\npseudo-fermion functional renormalization group that is capable of capturing\nthe physics of frustrated quantum magnets with generic (diagonal and\noff-diagonal) two-spin interaction terms. Based on a careful symmetry analysis\nof the underlying flow equations, we reveal that the computational complexity\ngrows only moderately, as compared to models with only diagonal interaction\nterms. We apply the formalism to a kagome antiferromagnet which is augmented by\ngeneral in-plane and out-of-plane Dzyaloshinskii-Moriya (DM) interactions, as\nargued to be present in the spin liquid candidate material herbertsmithite. We\ncalculate the complete ground state phase diagram in the strength of in-plane\nand out-of-plane DM couplings, and discuss the extended stability of the spin\nliquid of the unperturbed kagome antiferromagnet in the presence of these\ncouplings.", "category": "cond-mat_str-el" }, { "text": "The stability of 3D skyrmions under mechanical stress studied via Monte\n Carlo calculations: Using Monte Carlo (MC) simulations, we study the skyrmion\nstability/instability as a response to uniaxial mechanical stresses. Skyrmions\nemerge in chiral magnetic materials as a stable spin configuration under\nexternal magnetic field $\\vec{B}$ with the competition of ferromagnetic\ninteraction and Dzyaloshinskii-Moriya interaction (DMI) at low temperature $T$.\nSkyrmion configurations are also known to be stable (unstable) under a\ncompressive stress applied parallel (perpendicular) to $\\vec{B}$. To understand\nthe origin of such experimentally confirmed stability/instability, we use the\nFinsler geometry modeling technique with a new degree of freedom for strains,\nwhich plays an essential role in DMI being anisotropic. We find from MC data\nthat the area of the skyrmion state on the $B$-$T$ phase diagram increases\n(decreases) depending on the direction of applied stresses, in agreement with\nreported experimental results. This change in the area of the skyrmion state\nindicates that skyrmions become more (less) stable if the tensile strain\ndirection is parallel (perpendicular) to $\\vec{B}$. From the numerical data in\nthis paper, we find that the so-called magneto-elastic effect is suitably\nimplemented in the effective DMI theory with the strain degree of freedom\nwithout complex magneto-elastic coupling terms for chiral magnetic materials.\nThis result confirms that experimentally-observed skyrmion stability and\ninstability are caused by DMI anisotropy.", "category": "cond-mat_str-el" }, { "text": "Luttinger liquid coupled to Bose-Einstein condensation reservoirs: We investigate the transport properties for a Luttinger liquid coupled to two\nidentical Bose-Einstein condensation reservoirs. Using the approach of equation\nof motion for the Green function of the system, we find that the distance\nbetween the two resonant transmission probability peaks of the system is\ndetermined by the bosonic interaction strengths, and the sharpness of these\nresonant peaks is mainly determined by the Rabi frequency and phase of the\nBose-Einstein condensation reservoir. These results for the proposed system\ninvolving a Luttinger liquid may build a bridge between the controling\ntransport properties of cold atom in atom physics and the interacting boson\ntransport in low-dimensional condensed matter physics.", "category": "cond-mat_str-el" }, { "text": "Planar Thermal Hall Effects in Kitaev Spin Liquid Candidate Na2Co2TeO6: We investigate both the longitudinal thermal conductivity ($\\kappa_{xx}$) and\nthe planar thermal Hall conductivity ($\\kappa_{xy}$) in the Kitaev spin liquid\ncandidate of Co-based honeycomb antiferromagnet Na$_2$Co$_2$TeO$_6$ in a\nmagnetic field ($B$) applied along the $a$ and $a^*$ axes. A finite\n$\\kappa_{xy}$ is resolved for both field directions in the antiferromagnetic\n(AFM) phase below the N\\'eel temperature of 27 K. The temperature dependence of\n$\\kappa_{xy}/T$ shows the emergence of topological bosonic excitations. In\naddition, the field dependence of $\\kappa_{xy}$ shows sign reversals at the\ncritical fields in the AFM phase, suggesting the changes in the Chern number\ndistribution of the topological magnons. Remarkably, a finite $\\kappa_{xy}$ is\nobserved in $B \\parallel a^*$ between the first-order transition field in the\nAFM phase and the saturation field, which is prohibited in a disordered state\nby the two-fold rotation symmetry around the $a^*$ axis of the honeycomb\nlattice, showing the presence of a magnetically ordered state that breaks the\ntwo-fold rotation symmetry. Our results demonstrate the presence of topological\nmagnons in this compound in the whole field range below the saturation field.", "category": "cond-mat_str-el" }, { "text": "Charge, Lattice, and Spin Dynamics in Photoinduced Phase Transitions\n from Charge-Order-Insulator to Metal in Quasi-Two-Dimensional Organic\n Conductors: To elucidate different photoinduced melting dynamics of charge orders\nobserved in quasi-two-dimensional organic conductors $ \\theta\n$-(BEDT-TTF)$_2$RbZn(SCN)$_4$ and $ \\alpha $-(BEDT-TTF)$_2$I$_3$\n[BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene], we theoretically study\nphotoinduced time evolution of charge and spin correlation functions on the\nbasis of exact many-electron wave functions coupled with classical phonons in\nextended Peierls-Hubbard models on anisotropic triangular lattices. In both\nsalts, the so-called horizontal-stripe charge order is stabilized by\nnearest-neighbor repulsive interactions and by electron-lattice interactions.\nIn $ \\theta $-(BEDT-TTF)$_2$RbZn(SCN)$_4$ (abbreviated as $ \\theta $-RbZn), the\nstabilization energy due to lattice distortion is larger, so that larger\nquantity of energy needs to be absorbed for the melting of the charge and\nlattice orders. The photoinduced charge dynamics shows a complex behavior owing\nto a substantial number of nearly degenerate eigenstates involved. This is\nrelated to the high structural symmetry when the lattice is undistorted. In $\n\\alpha $-(BEDT-TTF)$_2$I$_3$ (abbreviated as $ \\alpha $-I$_3$), the lattice\nstabilization energy is smaller, and smaller quantity of energy is sufficient\nto melt the charge and lattice orders leading to a metallic phase. The\nphotoinduced charge dynamics shows a sinusoidal oscillation. In $ \\alpha\n$-I$_3$, the low structural symmetry ensures nearly spin-singlet bonds between\nhole-rich sites, where the spin correlation survives even after\nphotoexcitation.", "category": "cond-mat_str-el" }, { "text": "Spin-glass like dynamics of ferromagnetic clusters in\n La$_{0.75}$Ba$_{0.25}$CoO$_3$: We report the magnetization study of the compound\nLa$_{0.75}$Ba$_{0.25}$CoO$_3$ where Ba$^{2+}$ doping is just above the critical\nlimit for percolation of ferromagnetic clusters. The field cooled (FC) and zero\nfield cooled (ZFC) magnetization exhibit a thermomagnetic irreversibility and\nthe ac susceptibility show a frequency dependent peak at the ferromagnetic\nordering temperature (T$_C$$\\approx$203~K) of the clusters. These features\nindicate about the presence of a non-equilibrium state below T$_C$. In the\nnon-equilibrium state, the dynamic scaling of the imaginary part of ac\nsusceptibility and the static scaling of the nonlinear susceptibility clearly\nestablish a spin-glass like cooperative freezing of ferromagnetic clusters at\n200.9(2)~K. The existence of spin-glass like freezing of ferromagnetic clusters\nis further substantiated by the ZFC aging and memory experiments. We also\nobserve certain dynamical features which are not present in a typical\nspin-glass, such as, initial magnetization after ZFC aging first increases and\nthen decreases with the wait time and an imperfect recovery of relaxation in\nnegative temperature cycling experiments. This imperfect recovery transforms to\nperfect recovery on concurrent field cycling. Our analysis suggests that these\nadditional dynamical features have their origin in inter-cluster exchange\ninteraction and cluster size distribution. The inter-cluster exchange\ninteraction above the magnetic percolation gives a superferromagnetic state in\nsome granular thin films but our results show the absence of typical\nsuperferromagnetic like state in La$_{0.75}$Ba$_{0.25}$CoO$_3$.", "category": "cond-mat_str-el" }, { "text": "Theory of the field-revealed Kitaev spin liquid: Elementary excitations in highly entangled states such as quantum spin\nliquids may exhibit exotic statistics, different from those obeyed by\nfundamental bosons and fermions. Excitations called non-Abelian anyons are\npredicted to exist in a Kitaev spin liquid - the ground state of an exactly\nsolvable model proposed by Kitaev almost a decade ago. A smoking-gun signature\nof such non-Abelian anyons, namely a half-integer quantized thermal Hall\nconductivity, was recently reported in $\\alpha$-RuCl$_3$. While fascinating, a\nmicroscopic theory for this phenomenon in $\\alpha$-RuCl$_3$ remains elusive\nbecause the pure Kitaev phase cannot capture these anyons appearing in an\nintermediate magnetic field. Here we present a microscopic theory of the Kitaev\nspin liquid emerging between the low- and high-field states. Essential to this\nresult is an antiferromagnetic off-diagonal symmetric interaction that permits\nthe Kitaev spin liquid to protrude from the pure ferromagnetic Kitaev limit\nunder a magnetic field. This generic model captures a field-revealed Kitaev\nspin liquid, and displays strong anisotropy of field effects. A wide regime of\nnon-Abelian anyon Kitaev spin liquid is predicted when the magnetic field is\nperpendicular to the honeycomb plane.", "category": "cond-mat_str-el" }, { "text": "Unconventional spin transport in strongly correlated kagome systems: Recent progress in material design enables the study of correlated,\nlow-temperature phases and associated anomalous transport in two-dimensional\nkagome systems. Here, we show that unconventional spin transport can arise in\nsuch systems even at elevated temperatures due to emergent dynamical\nconstraints. To demonstrate this effect, we consider a strong-coupling limit of\nan extended Hubbard model on the kagome lattice with density of $2/3$. We\nnumerically investigate the charge and spin transport by a cellular automaton\ncircuit, allowing us to perform simulations on large systems to long times\nwhile preserving the essential conservation laws. The charge dynamics reflects\nthe constraints and can be understood by a Gaussian field theory of a scalar\nheight field. Moreover, the system exhibits a hidden spin conservation law with\na dynamic sublattice structure, which enables additional slow relaxation\npathways for spin excitations. These features can be directly tested by\nmeasuring the dynamic spin structure factor with neutron scattering.", "category": "cond-mat_str-el" }, { "text": "Dynamical Conductivity of Dirac Materials: For graphene (a Dirac material) it has been theoretically predicted and\nexperimentally observed that DC resistivity is proportional to $ T^4$ when the\ntemperature is much less than Bloch- Gr\\\"{u}neisen ($\\Theta_{BG}$) temperature\nand T linear in opposite case ($T>>\\Theta_{BG}$). Going beyond the DC case, we\ninvestigate the dynamical conductivity in graphene using the powerful method of\nmemory function formalism. In the DC (zero frequency regime) limit, we obtained\nthe above mention behavior which was previously obtained using the\nBloch-Boltzmann kinetic equation. In the finite frequency regime, we obtained\nseveral new results: (1) the generalized Drude scattering rate, in the zero\ntemperature limit, shows $\\omega^4 $ behavior at low frequencies ($\\omega <<\nk_B \\Theta_{BG}/ \\hbar$) and saturates at higher frequencies. We also observed\nthe Holstein Mechanism, however, with different power laws from that in the\ncase of metals; (2) At higher frequencies, $\\omega>>k_B \\Theta_{BG}/ \\hbar$,\nand higher temperatures $T>>\\Theta_{BG}$, we observed that the generalized\nDrude scattering rate is linear in temperature. In addition, several other\nresults are also obtained. With the experimental advancement of this field,\nthese results should be experimentally tested.", "category": "cond-mat_str-el" }, { "text": "Metal-Mott insulator interfaces: Motivated by the direct observation of electronic phase separation in\nfirst-order Mott transitions, we model the interface between the\nthermodynamically coexisting metal and Mott insulator. We show how to model the\nrequired slab geometry and extract the electronic spectra. We construct an\neffective Landau free energy and compute the variation of its parameters across\nthe phase diagram. Finally, using a linear mixture of the density and\ndouble-occupancy, we identify a natural Ising order parameter which unifies the\ntreatment of the bandwidth and filling controlled Mott transitions.", "category": "cond-mat_str-el" }, { "text": "Magnetic Field Induced Exotic Phases in Isotropic Frustrated Spin-1/2\n chain: The frustrated isotropic $J_1-J_2$ model with ferromagnetic $J_1$ and\nanti-ferromagnetic $J_2$ interactions in presence of an axial magnetic field\nshows many exotic phases, such as vector chiral and multipolar phases. The\nexisting studies of the phase boundaries of these systems are based on the\nindirect evidences such as correlation functions {\\it etc}. In this paper, the\nphase boundaries of these exotic phases are calculated based on order\nparameters and jumps in the magnetization. In the strong magnetic field, $Z_2$\nsymmetry is broken, therefore, order parameter of the vector chiral phase is\ncalculated using the broken symmetry states. Our results obtained using the\nmodified density matrix renormalization group and exact diagonalization\nmethods, suggest that the vector chiral phase exist only in narrow range of\nparameter space $J_2/J_1$.", "category": "cond-mat_str-el" }, { "text": "Pomeranchuk instability in doped graphene: The density of states of graphene has Van Hove singularities that can be\nreached by chemical doping and have already been explored in photoemission\nexperiments. We show that in the presence of Coulomb interactions the system at\nthe Van Hove filling is likely to undergo a Pomeranchuk instability breaking\nthe lattice point group symmetry. In the presence of an on--site Hubbard\ninteraction the system is also unstable towards ferromagnetism. We explore the\ncompetition of the two instabilities and build the phase diagram. We also\nsuggest that, for doping levels where the trigonal warping is noticeable, the\nFermi liquid state in graphene can be stable up to zero temperature avoiding\nthe Kohn--Luttinger mechanism and providing an example of two dimensional Fermi\nliquid at zero temperature.", "category": "cond-mat_str-el" }, { "text": "Accessing the spectral function in a current-carrying device: The presence of an electrical transport current in a material is one of the\nsimplest and most important realisations of non-equilibrium physics. The\ncurrent density breaks the crystalline symmetry and can give rise to dramatic\nphenomena, such as sliding charge density waves [1], insulator-to-metal\ntransitions [2,3] or gap openings in topologically protected states [4]. Almost\nnothing is known about how a current influences the electron spectral function,\nwhich characterizes most of the solid's electronic, optical and chemical\nproperties. Here we show that angle-resolved photoemission spectroscopy with a\nnano-scale light spot (nanoARPES) provides not only a wealth of information on\nlocal equilibrium properties, but also opens the possibility to access the\nlocal non-equilibrium spectral function in the presence of a transport current.\nUnifying spectroscopic and transport measurements in this way allows\nnon-invasive local measurements of the composition, structure, many-body\neffects and carrier mobility in the presence of high current densities.", "category": "cond-mat_str-el" }, { "text": "Parametrization of LSDA+$U$ for noncollinear magnetic configurations:\n Multipolar magnetism in UO$_2$: To explore the formation of noncollinear magnetic configurations in materials\nwith strongly correlated electrons, we derive a noncollinear LSDA+$U$ model\ninvolving only one parameter $U$, as opposed to the difference between the\nHubbard and Stoner parameters $U-J$. Computing $U$ in the constrained random\nphase approximation, we investigate noncollinear magnetism of uranium dioxide\nUO$_2$ and find that the spin-orbit coupling (SOC) stabilizes the 3$\\textbf{k}$\nordered magnetic ground state. The estimated SOC strength in UO$_2$ is as large\nas 0.73 eV per uranium atom, making spin and orbital degrees of freedom\nvirtually inseparable. Using a multipolar pseudospin Hamiltonian, we show how\noctupolar and dipole-dipole exchange coupling help establish the 3$\\textbf{k}$\nmagnetic ground state with canted ordering of uranium $f$-orbitals. The\ncooperative Jahn-Teller effect does not appear to play a significant part in\nstabilizing the noncollinear 3$\\textbf{k}$ state, which has the lowest energy\neven in an undistorted lattice. The choice of parameter $U$ in the LSDA+$U$\nmodel has a notable quantitative effect on the predicted properties of UO$_2$,\nin particular on the magnetic exchange interaction and, perhaps trivially, on\nthe band gap: The value of $U=3.46$ eV computed fully $ab$ $initio$ delivers\nthe band gap of 2.11~eV in good agreement with experiment, and a balanced\naccount of other pertinent energy scales.", "category": "cond-mat_str-el" }, { "text": "Characterizing The Many-Body Localization via Studying State Sensitivity\n to Boundary Conditions: We introduce novel characterizations for many-body phase transitions between\ndelocalized and localized phases based on the system's sensitivity to boundary\nconditions. In particular, we change boundary conditions from periodic to\nantiperiodic and calculate shift in the system's energy and shifts in the\nsingle-particle density matrix eigenvalues in the corresponding energy window.\nWe employ the typical model for studying MBL, a one-dimensional disordered\nsystem of fermions with nearest-neighbor repulsive interaction where disorder\nis introduced as randomness on on-site energies. By calculating numerically the\nshifts in the system's energy and eigenvalues of the single-particle density\nmatrix, we observe that in the localized regime, both shifts are vanishing;\nwhile in the extended regime, both shifts are on the order of the corresponding\nlevel spacing. We also applied these characterizations of the phase transition\nto the case of having next-nearest-neighbor interactions in addition to the\nnearest-neighbor interactions, and studied its effect on the transition.", "category": "cond-mat_str-el" }, { "text": "Ab initio downfolding for electron-phonon coupled systems: constrained\n density-functional perturbation theory (cDFPT): We formulate an ab initio downfolding scheme for electron-phonon coupled\nsystems. In this scheme, we calculate partially renormalized phonon frequencies\nand electron-phonon coupling, which include the screening effects of\nhigh-energy electrons, to construct a realistic Hamiltonian consisting of\nlow-energy electron and phonon degrees of freedom. We show that our scheme,\nwhich we call constrained density-functional perturbation theory (cDFPT), can\nbe implemented by slightly modifying the conventional DFPT, which is one of the\nstandard methods to calculate phonon properties from first principles. Our\nscheme can be applied to various phonon-related problems, such as\nsuperconductivity, electron and thermal transport, thermoelectricity,\npiezoelectricity, dielectricity and multiferroicity. We believe that the cDFPT\nprovides a firm basis for the understanding of the role of phonons in strongly\ncorrelated materials. Here, we apply the scheme to the fullerene\nsuperconductors and discuss how the realistic low-energy Hamiltonian is\nconstructed.", "category": "cond-mat_str-el" }, { "text": "Gutzwiller-correlated wave functions for degenerate bands: exact results\n in infinite dimensions: We introduce Gutzwiller-correlated wave functions for the variational\ninvestigation of general multi-band Hubbard models. We set up a diagrammatic\nformalism which allows us to evaluate analytically ground-state properties in\nthe limit of infinite spatial dimensions. In this limit recent results obtained\nwithin the Gutzwiller approximation are seen to become exact for these wave\nfunctions. We further show that the Slave Boson mean-field theory for\ndegenerate bands becomes variationally controlled at zero temperature in\ninfinite dimensions. Lastly, we briefly comment on the variational approach to\nthe Anderson transition in strongly correlated electron systems.", "category": "cond-mat_str-el" }, { "text": "Field-induced intermediate phase in $\u03b1$-RuCl$_3$: Non-coplanar\n order, phase diagram, and proximate spin liquid: Frustrated magnets with strong spin-orbit coupling promise to host\ntopological states of matter, with fractionalized excitations and emergent\ngauge fields. Kitaev's proposal for a honeycomb-lattice Majorana spin liquid\nhas triggered an intense search for experimental realizations, with\nbond-dependent Ising interaction being the essential building block. A prime\ncandidate is $\\alpha$-RuCl$_3$ whose phase diagram in a magnetic field is,\nhowever, not understood to date. Here we present conclusive experimental\nevidence for a novel field-induced ordered phase in $\\alpha$-RuCl$_3$,\nsandwiched between the zigzag and quantum disordered phases at low and high\nfields, respectively. We provide a detailed theoretical study of the relevant\neffective spin model which we show to display a field-induced intermediate\nphase as well. We fully characterize the intermediate phase within this model,\nincluding its complex spin structure, and pinpoint the parameters relevant to\n$\\alpha$-RuCl$_3$ based on the experimentally observed critical fields. Most\nimportantly, our study connects the physics of $\\alpha$-RuCl$_3$ to that of the\nKitaev-$\\Gamma$ model, which displays a quantum spin liquid phase in zero\nfield, and hence reveals the spin liquid whose signatures have been detected in\na variety of dynamical probes of $\\alpha$-RuCl$_3$.", "category": "cond-mat_str-el" }, { "text": "Entanglement entropy of the $\u03bd=1/2$ composite fermion non-Fermi liquid\n state: The so-called ``non-Fermi liquid'' behavior is very common in strongly\ncorrelated systems. However, its operational definition in terms of ``what it\nis not'' is a major obstacle against theoretical understanding of this\nfascinating correlated state. Recently there has been much interest in\nentanglement entropy as a theoretical tool to study non-Fermi liquids. So far\nexplicit calculations have been limited to models without direct experimental\nrealizations. Here we focus on a two dimensional electron fluid under magnetic\nfield and filling fraction $\\nu=1/2$, which is believed to be a non-Fermi\nliquid state. Using the composite fermion (CF) wave-function which captures the\n$\\nu=1/2$ state very accurately, we compute the second R\\'enyi entropy using\nvariational Monte-Carlo technique and an efficient parallel algorithm. We find\nthe entanglement entropy scales as $L\\log L$ with the length of the boundary\n$L$ as it does for free fermions, albeit with a pre-factor twice that of the\nfree fermion. We contrast the results against theoretical conjectures and\ndiscuss the implications of the results.", "category": "cond-mat_str-el" }, { "text": "Phonon spectral function of the one-dimensional Holstein-Hubbard model: We use the continuous-time interaction expansion (CT-INT) quantum Monte Carlo\nmethod to calculate the phonon spectral function of the one-dimensional\nHolstein-Hubbard model at half-filling. Our results are consistent with a\nsoft-mode Peierls transition in the adiabatic regime, and the existence of a\ncentral peak related to long-range order in the Peierls phase. We explain a\npreviously observed feature at small momenta in terms of a hybridization of\ncharge and phonon excitations. Tuning the system from a Peierls to a metallic\nphase with a nonzero Hubbard interaction suppresses the central peak, but a\nsignificant renormalization of the phonon dispersion remains. In contrast, the\ndispersion is only weakly modified in the Mott phase. We discuss finite-size\neffects, the relation to the dynamic charge structure factor, as well as\nadditional sum rules and their implications. Finally, we reveal the existence\nof a discrete symmetry in a continuum field theory of the Holstein model, which\nis spontaneously broken in the Peierls phase.", "category": "cond-mat_str-el" }, { "text": "Evolution of magnetism in Pd-substituted Ce$_2$RhIn$_8$ single crystals: The evolution of magnetism and superconductivity in\nCe$_2$Rh$_{1-x}$Pd$_x$In$_8$ solid solutions has been studied within the entire\nconcentration range by means of thermodynamic and magnetic measurements at\nambient pressure and at temperatures between 0.35 K and room temperature. For\nthis purpose, single crystals with Pd concentrations x = 0, 0.10, 0.15, 0.30,\n0.45, 0.55, 0.85 and 1 have been grown from In self-flux and characterized by\nx-ray diffraction and microprobe analysis. Starting from the antiferromagnet\nCe$_2$RhIn$_8$, the N\\'eel temperature gradually decreases with increasing Pd\nconcentration and the antiferromagnetism has disappeared for $x \\ge 0.45$.\nSuperconductivity has been observed only for Ce$_2$PdIn$_8$.", "category": "cond-mat_str-el" }, { "text": "Interpretation of high-pressure experiments on FeAs superconductors: In two recent articles (cond-mat/0606177 and arXiv:0804.1615), we have\nsuggested a unified theory of superconductivity based on the real-space\nspin-parallel electron pairing and superconducting mechanism and have shown\nthat the stable hexagonal and tetragonal vortex lattices (the optimal doping\nphases) can be expected in the newly discovered LaO{1-x}F{x}FeAs\n(x0=1/7=0.1428) and SmO{1-x}F{x}FeAs (x0=1/6=0.1667), respectively. In this\npaper, we present a theoretical study of the effects of hydrostatic and\nanisotropic pressure on the superconducting transition temperature Tc of the\nFe-based layered superconductors based on the above mentioned theory. Our\nresults indicate a strong doping-dependent pressure effects on the Tc of this\ncompound system. Under high hydrostatic pressure, we find that dTc/dP is\nnegative when x>x0 (the so-called overdoped region) and is positive when x1/6)\n(arXiv:0804.1582). Furthermore, Tc of both overdoped and underdoped samples\nshows an increase with uniaxial pressure in the charge stripe direction and a\ndecrease with pressure in the direction perpendicular to the stripes. We\nsuggest that the mechanism responsible for the pressure effect is not specific\nto the iron-based family and it may also be applicable to other superconducting\nmaterials.", "category": "cond-mat_str-el" }, { "text": "Integrable Extended Hubbard Hamiltonians from Symmetric Group Solutions: We consider the most general form of extended Hubbard Hamiltonian conserving\nthe total spin and number of electrons, and find all the 1-dimensional\ncompletely integrable models which can be derived from first degree polynomial\nsolution of the Yang-Baxter equation. It is shown that such models are 96. They\nare identified with the 16-dimensional representation of a new class of\nsolutions of symmetric group relations, acting as generalized permutators. As\nparticular examples, the EKS and some other known models are obtained.", "category": "cond-mat_str-el" }, { "text": "Topological Magnons: A Review: At sufficiently low temperatures magnetic materials often enter a correlated\nphase hosting collective, coherent magnetic excitations such as magnons or\ntriplons. Drawing on the enormous progress on topological materials of the last\nfew years, recent research has led to new insights into the geometry and\ntopology of these magnetic excitations. Berry phases associated to magnetic\ndynamics can lead to observable consequences in heat and spin transport while\nanalogues of topological insulators and semimetals can arise within magnon band\nstructures from natural magnetic couplings. Magnetic excitations offer a\nplatform to explore the interplay of magnetic symmetries and topology, to drive\ntopological transitions using magnetic fields. examine the effects of\ninteractions on topological bands and to generate topologically protected spin\ncurrents at interfaces. In this review, we survey progress on all these topics,\nhighlighting aspects of topological matter that are unique to magnon systems\nand the avenues yet to be fully investigated.", "category": "cond-mat_str-el" }, { "text": "Pair Superfluid and Supersolid of Correlated Hard-Core Bosons on a\n Triangular Lattice: We have systematically studied the hard-core Bose-Hubbard model with\ncorrelated hopping on a triangular lattice using density-matrix renormalization\ngroup method. A rich ground state phase diagram is determined. In this phase\ndiagram there is a supersolid phase and a pair superfluid phase due to the\ninterplay between the ordinary frustrated boson hopping and an unusual\ncorrelated hopping. In particular, we find that the quantum phase transition\nbetween the supersolid phase and the pair superfluid phase is continuous.", "category": "cond-mat_str-el" }, { "text": "Coulomb matrix elements in multi-orbital Hubbard models: Coulomb matrix elements are needed in all studies in solid-state theory that\nare based on Hubbard-type multi-orbital models. Due to symmetries, the matrix\nelements are not independent. We determine a set of independent Coulomb\nparameters for a $d$-shell and a $f$-shell and all point groups with up to $16$\nelements ($O_h$, $O$, $T_d$, $T_h$, $D_{6h}$, and $D_{4h}$). Furthermore, we\nexpress all other matrix elements as a function of the independent Coulomb\nparameters. Apart from the solution of the general point-group problem we\ninvestigate in detail the spherical approximation and first-order corrections\nto the spherical approximation.", "category": "cond-mat_str-el" }, { "text": "The electronic structure of La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$\n probed by high- and low-energy angle-resolved photoelectron spectroscopy:\n evolution with probing depth: We present angle-resolved photoelectron spectroscopy data probing the\nelectronic structure of the Nd-substituted high-$T_c$ cuprate\nLa$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$ (Nd-LSCO). Data have been acquired at\nlow and high photon energies, $h\\nu$ = 55 and 500 eV, respectively. Earlier\ncomparable low-energy studies of La$_{1.4-x}$Nd$_{0.6}$Sr$_{x}$CuO$_4$ ($x =\n0.10, 0.12, 0.15$) have shown strongly suppressed photoemission intensity, or\nabsence thereof, in large parts of the Brillouin zone. Contrary to these\nfindings we observe spectral weight at all points along the entire Fermi\nsurface contour at low and high photon energies. No signs of strong charge\nmodulations are found. At high photon energy, the Fermi surface shows obvious\ndifferences in shape as compared to the low-energy results presented here and\nin similar studies. The observed difference in shape and the high\nbulk-sensitivity at this photon energy suggest intrinsic electronic structure\ndifferences between the surface and bulk regions.", "category": "cond-mat_str-el" }, { "text": "Terahertz Electrodynamics of Mixed-Valent YbAl$_3$ and LuAl$_3$ Thin\n Films: We present THz measurements of thin films of mixed-valent YbAl$_3$ and its\nstructural analogue LuAl$_3$. Combined with traditional Fourier transform\ninfrared (FTIR) spectroscopy, the extended Drude formalism is utilized to study\nthe low-frequency transport of these materials. We find that LuAl$_3$\ndemonstrates conventional Drude transport whereas at low temperatures YbAl$_3$\ndemonstrates a sharply renormalized Drude peak and a mid-infrared (MIR) peak in\nthe conductivity, indicative of the formation of a heavy Fermi liquid. In\nYbAl$_3$ the extended Drude framework shows a consistency of the scattering\nrate with Fermi-liquid behavior below $T < 40$ K and a moderate mass\nenhancement. While a $\\omega^2$ Fermi liquid-like frequency dependence is not\nclearly exhibited, the temperature dependence of the Drude scattering rate and\neffective mass is consistent with the formation of a low-temperature moderately\nheavy Fermi liquid, albeit one with a smaller mass than observed in single\ncrystals. The extended Drude analysis also supports a slow crossover between\nthe Fermi liquid state and the normal state in YbAl$_3$.", "category": "cond-mat_str-el" }, { "text": "Magnetic bubble crystal in tetragonal magnets: A magnetic bubble crystal is a two-dimensional soliton lattice consisting of\nmultiple spin density waves similar to a magnetic skyrmion crystal.\nNevertheless, the emergence of the bubble crystal with a collinear spin texture\nis rare compared to that of the skyrmion crystal with a noncoplanar spin\ntexture. Here we theoretically report the stabilization mechanisms of the\nbubble crystal in tetragonal magnets. By performing numerical calculations\nbased on an efficient steepest descent method for an effective spin model with\nmagnetic anisotropy and multiple spin interactions in momentum space on a\ntwo-dimensional square lattice, we construct magnetic field-temperature phase\ndiagrams for various sets of model parameters. We find that the bubble crystal\nis stabilized at finite temperatures near the skyrmion crystal by an easy-axis\nanisotropic two-spin interaction. Through a detailed analysis, we also show\nthat the high-harmonic wave-vector interaction and the biquadratic interaction\nplay important roles in the stability of the bubble crystal. Our results\nindicate a close relationship between the bubble crystal and the skyrmion\ncrystal in terms of the stabilization mechanisms, which suggests the\npossibility of the bubble crystal in the skyrmion-hosting materials by\ncontrolling the easy-axis magnetic anisotropy through external and/or chemical\npressure.", "category": "cond-mat_str-el" }, { "text": "The Square-Lattice Heisenberg Antiferromagnet at Very Large Correlation\n Lengths: The correlation length of the square-lattice spin-1/2 Heisenberg\nantiferromagnet is studied in the low-temperature (asymptotic-scaling) regime.\nOur novel approach combines a very efficient loop cluster algorithm --\noperating directly in the Euclidean time continuum -- with finite-size scaling.\nThis enables us to probe correlation lengths up to $\\xi \\approx 350,000$\nlattice spacings -- more than three orders of magnitude larger than any\nprevious study. We resolve a conundrum concerning the applicability of\nasymptotic-scaling formulae to experimentally- and numerically-determined\ncorrelation lengths, and arrive at a very precise determination of the\nlow-energy observables. Our results have direct implications for the\nzero-temperature behavior of spin-1/2 ladders.", "category": "cond-mat_str-el" }, { "text": "Singlet Ground State and Magnetization Plateaus in Ba$_3$Mn$_2$O$_8$: Magnetic susceptibility and the magnetization process have been measured in\n\\green polycrystal. In this compound, the magnetic manganese ion exists as\nMn$^{5+}$ in a tetrahedral environment, and thus the magnetic interaction can\nbe described by an S=1 Heisenberg model. The ground state was found to be a\nspin singlet with an excitation gap $\\Delta/k_{\\rm B}=11.2$ K. Magnetization\nplateaus were observed at zero and at half of the saturation magnetization.\nThese results indicate that the present system can be represented by a coupled\nantiferromagnetic dimer model.", "category": "cond-mat_str-el" }, { "text": "Two-stage spin-flop transitions in S = 1/2 antiferromagnetic spin chain\n BaCu_2Si_2O_7: Two-stage spin-flop transitions are observed the in quasi-one-dimensional\nantiferromagnet, BaCu${}_2$Si${}_2$O${}_7$. A magnetic field applied along the\neasy axis induces a spin-flop transition at 2.0 T followed by a second\ntransition at 4.9 T. The magnetic susceptibility indicates the presence of\nDzyaloshinskii-Moriya (DM) antisymmetric interactions between the intrachain\nneighboring spins. We discuss a possible mechanism whereby the geometrical\ncompetition between DM and interchain interactions, as discussed for the\ntwo-dimensional antiferromagnet La${}_2$CuO${}_4$, causes the two-stage\nspin-flop transitions.", "category": "cond-mat_str-el" }, { "text": "Curvature induced drift and deformation of magnetic skyrmions:\n comparison of ferro- and antiferromagnetic cases: The influence of the geometrical curvature of chiral magnetic films on the\nstatic and dynamic properties of hosted skyrmions are studied theoretically. We\npredict the effects of the curvature-induced drift of skyrmions under the\naction of the curvature gradients without any external stimuli. The strength of\nthe curvature-induced driving force essentially depends on the skyrmion type,\nNeel or Bloch, while the trajectory of motion is determined by the type of\nmagnetic ordering: ferro- or antiferromagnetic. When moving on the surface,\nskyrmions undergo deformations that depend on the type of skyrmion. In the\nsmall-curvature limit, using the collective-variable approach we show, that the\ndriving force acting on a Neel skyrmion is linear in the gradient of the mean\ncurvature. The driving acting on a Bloch skyrmion is much smaller: it is\nproportional to the product of the mean curvature and its gradient. In contrast\nto the fast Neel skyrmions, the dynamics of the slow Bloch skyrmions is\nessentially affected by the skyrmion profile deformation. For the sake of\nsimplicity, we restrict ourselves to the case of zero Gaussian curvature and\nconsider cylindrical surfaces of general type. Equations of motion for\nferromagnetic and antiferromagnetic skyrmions in curved magnetic films are\nobtained in terms of collective variables. All analytical predictions are\nconfirmed by numerical simulations.", "category": "cond-mat_str-el" }, { "text": "An Ising model on a 3D honeycomb zigzag-ladder lattice: a solution to\n the ground-state problem and application to the SrRE$_2$O$_4$ and\n BaRE$_2$O$_4$ magnets: An exact solution (incomplete) of the ground-state problem for an Ising model\nin an external field on a 3D honeycomb zigzag-ladder lattice with two types of\nsites is found. It is shown that the geometrical frustration due to the\npresence of triangle elements leads to the emergence of a variety of magnetic\nphases. The majority of these are partially disordered (highly degenerate). The\ntheoretical results are used to explain the sequence of experimentally observed\nphase transitions in the honeycomb zigzag-ladder magnets and to predict the\nappearance of new phases.", "category": "cond-mat_str-el" }, { "text": "Photoemission spectra of LaMnO3 controlled by orbital excitations: We investigate the spectral function of a hole moving in the orbital-ordered\nferromagnetic planes of LaMnO$_3$, and show that it depends critically on the\ntype of orbital ordering. While the hole does not couple to the spin\nexcitations, it interacts strongly with the excitations of $e_g$ orbitals\n(orbitons), leading to new type of quasiparticles with a dispersion on the\norbiton energy scale and with strongly enhanced mass and reduced weight.\nTherefore we predict a large redistribution of spectral weight with respect to\nthe bands found in local density approximation (LDA) or in LDA+U.", "category": "cond-mat_str-el" }, { "text": "Finite-temperature dynamic structure factor of the spin-1 XXZ chain with\n single-ion anisotropy: Improving matrix-product state techniques based on the purification of the\ndensity matrix, we are able to accurately calculate the finite-temperature\ndynamic response of the infinite spin-1 XXZ chain with single-ion anisotropy in\nthe Haldane, large-$D$ and antiferromagnetic phases. Distinct thermally\nactivated scattering processes make a significant contribution to the spectral\nweight in all cases. In the Haldane phase intraband magnon scattering is\nprominent, and the onsite anisotropy causes the magnon to split into singlet\nand doublet branches. In the large-$D$ phase response, the intraband signal is\nseparated from an exciton-antiexciton continuum. In the antiferromagnetic\nphase, holons are the lowest-lying excitations, with a gap that closes at the\ntransition to the Haldane state. At finite temperatures, scattering between\ndomain-wall excitations becomes especially important and strongly enhances the\nspectral weight for momentum transfer $\\pi$.", "category": "cond-mat_str-el" }, { "text": "Local atomic and magnetic structure of multiferroic (Sr,Ba)(Mn,Ti)O$_3$: We present a detailed study of the local atomic and magnetic structure of the\ntype-I multiferroic perovskite system (Sr,Ba)(Mn,Ti)O$_3$ using x-ray and\nneutron pair distribution function (PDF) analysis, polarized neutron\nscattering, and muon spin relaxation ($\\mu$SR) techniques. The atomic PDF\nanalysis reveals widespread nanoscale tetragonal distortions of the crystal\nstructure even in the paraelectric phase with average cubic symmetry,\ncorresponding to incipient ferroelectricity in the local structure. Magnetic\nPDF analysis, polarized neutron scattering, and $\\mu$SR likewise confirm the\npresence of short-range antiferromagnetic correlations in the paramagnetic\nstate, which grow in magnitude as the temperature approaches the magnetic\ntransition. We show that these short-range magnetic correlations coincide with\na reduction of the tetragonal (i.e. ferroelectric) distortion in the average\nstructure, suggesting that short-range magnetism can play an important role in\nmagnetoelectric and/or magnetostructural phenomena even without genuine\nlong-range magnetic order. The reduction of the tetragonal distortion scales\nlinearly with the local magnetic order parameter, pointing to spontaneous\nlinear magnetoelectric coupling in this system. These findings provide greater\ninsight into the multiferroic properties of (Sr,Ba)(Mn,Ti)O$_3$ and demonstrate\nthe importance of investigating the local atomic and magnetic structure to gain\na deeper understanding of the intertwined degrees of freedom in multiferroics.", "category": "cond-mat_str-el" }, { "text": "A robust but disordered collapsed-volume phase in a cerium alloy under\n the application of pulsed magnetic fields: We report synchrotron x-ray powder diffraction measurements of\nCe0.8La0.1Th0.1 subject to pulsed magnetic fields as high as 28 Tesla. This\nalloy is known to exhibit a continuous volume collapse on cooling at ambient\npressure, which is a modification of the gamma -> alpha transition in elemental\ncerium. Recently, it has been suggested on the basis of field-cooled\nresistivity and pulsed field magnetization measurements that the volume\ncollapse in this alloy can be suppressed by the application of magnetic fields.\nConversely, our direct diffraction measurements show a robust collapsed phase,\nwhich persists in magnetic fields as high as 28 Tesla. We also observe\nnanoscale disorder in the collapsed phase, which increasingly contaminates the\nhigh temperature phase on thermal cycling.", "category": "cond-mat_str-el" }, { "text": "The Effect of Randomness on the Mott State: We reinvestigate the competition between the Mott and the Anderson insulator\nstate in a one-dimensional disordered fermionic system by a combination of\ninstanton and renormalization group methods. Tracing back both the\ncompressibility and the ac-conductivity to a vanishing kink energy of the\nelectronic displacement field we do not find any indication for the existence\nof an intermediate (Mott glass) phase.", "category": "cond-mat_str-el" }, { "text": "Magnetic effects at the interface between nonmagnetic oxides: The electronic reconstruction at the interface between two insulating oxides\ncan give rise to a highly-conductive interface. In analogy to this remarkable\ninterface-induced conductivity we show how, additionally, magnetism can be\ninduced at the interface between the otherwise nonmagnetic insulating\nperovskites SrTiO3 and LaAlO3. A large negative magnetoresistance of the\ninterface is found, together with a logarithmic temperature dependence of the\nsheet resistance. At low temperatures, the sheet resistance reveals magnetic\nhysteresis. Magnetic ordering is a key issue in solid-state science and its\nunderlying mechanisms are still the subject of intense research. In particular,\nthe interplay between localized magnetic moments and the spin of itinerant\nconduction electrons in a solid gives rise to intriguing many-body effects such\nas Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions, the Kondo effect, and\ncarrier-induced ferromagnetism in diluted magnetic semiconductors. The\nconducting oxide interface now provides a versatile system to induce and\nmanipulate magnetic moments in otherwise nonmagnetic materials.", "category": "cond-mat_str-el" }, { "text": "Global phase diagram of the spin-1 antiferromagnet with uniaxial\n anisotropy on the kagome lattice: The phase diagram of the XXZ spin-1 quantum magnet on the kagome lattice is\nstudied for all cases where the $J_z$ coupling is antiferromagnetic. In the\nzero magnetic field case, the six previously introduced phases, found using\nvarious methods, are: the nondegenerate gapped photon phase which breaks no\nspace symmetry or spin symmetry; the six-fold degenerate phase with plaquette\norder, which breaks both time reversal symmetry and translational symmetry; the\n\"superfluid\" (ferromagnetic) phase with an in-plane global U(1) symmetry\nbroken, when $J_{xy} < 0$; the $\\sqrt{3}\\times\\sqrt{3}$ order when $J_{xy} >\n0$; the nematic phase when $D < 0$ and large; and a phase with resonating\ndimers on each hexagon. We obtain all of these phases and partial information\nabout their quantum phase transitions in a single framework by studying\ncondensation of defects in the six-fold plaquette phases. The transition\nbetween nematic phase and the six-fold degenerate plaquette phase is\npotentially an unconventional second-order critical point. In the case of a\nnonzero magnetic field along $\\hat{z}$, another ordered phase with translation\nsymmetry broken is opened up in the nematic phase. Due to the breaking of\ntime-reversal symmetry by the field, a supersolid phase emerges between the\nsix-fold plaquette order and the superfluid phase. This phase diagram might be\naccessible in nickel compounds, BF$_4$ salts, or optical lattices of atoms with\nthree degenerate states on every site.", "category": "cond-mat_str-el" }, { "text": "Strong electronic correlations in superconducting organic charge\n transfer salts: We review the role of strong electronic correlations in\nquasi--two-dimensional organic charge transfer salts such as (BEDT-TTF)$_2X$,\n(BETS)$_2Y$ and $\\beta'$-[Pd(dmit)$_2$]$_2Z$. We begin by defining minimal\nmodels for these materials. It is necessary to identify two classes of\nmaterial: the first class is strongly dimerised and is described by a\nhalf-filled Hubbard model; the second class is not strongly dimerised and is\ndescribed by a quarter filled extended Hubbard model. We argue that these\nmodels capture the essential physics of these materials. We explore the phase\ndiagram of the half-filled quasi--two-dimensional organic charge transfer\nsalts, focusing on the metallic and superconducting phases. We review work\nshowing that the metallic phase, which has both Fermi liquid and `bad metal'\nregimes, is described both quantitatively and qualitatively by dynamical mean\nfield theory (DMFT). The phenomenology of the superconducting state is still a\nmatter of contention. We critically review the experimental situation, focusing\non the key experimental results that may distinguish between rival theories of\nsuperconductivity, particularly probes of the pairing symmetry and measurements\nof the superfluid stiffness. We then discuss some strongly correlated theories\nof superconductivity, in particular, the resonating valence bond (RVB) theory\nof superconductivity. We conclude by discussing some of the major challenges\ncurrently facing the field.", "category": "cond-mat_str-el" }, { "text": "Critical Metal Phase at the Anderson Metal-Insulator Transition with\n Kondo Impurities: It is well-known that magnetic impurities can change the symmetry class of\ndisordered metallic systems by breaking spin and time-reversal symmetry. At low\ntemperature these symmetries can be restored by Kondo screening. It is also\nknown that at the Anderson metal-insulator transition, wave functions develop\nmultifractal fluctuations with power law correlations. Here, we consider the\ninterplay of these two effects. We show that multifractal correlations open\nlocal pseudogaps at the Fermi energy at some random positions in space. When\ndilute magnetic impurities are at these locations, Kondo screening is strongly\nsuppressed. We find that when the exchange coupling J is smaller than a certain\nvalue J*, the metal-insulator transition point extends to a critical region in\nthe disorder strength parameter and to a band of critical states. The width of\nthis critical region increases with a power of the concentration of magnetic\nimpurities.", "category": "cond-mat_str-el" }, { "text": "Hidden Charge Order in an Iron Oxide Square-Lattice Compound: Since the discovery of charge disproportionation in the FeO$_2$\nsquare-lattice compound Sr$_3$Fe$_2$O$_7$ by M\\\"ossbauer spectroscopy more than\nfifty years ago, the spatial ordering pattern of the disproportionated charges\nhas remained \"hidden\" to conventional diffraction probes, despite numerous\nx-ray and neutron scattering studies. We have used neutron Larmor diffraction\nand Fe K-edge resonant x-ray scattering to demonstrate checkerboard charge\norder in the FeO$_2$ planes that vanishes at a sharp second-order phase\ntransition upon heating above 332 K. Stacking disorder of the checkerboard\npattern due to frustrated interlayer interactions broadens the corresponding\nsuperstructure reflections and greatly reduces their amplitude, thus explaining\nthe difficulty to detect them by conventional probes. We discuss implications\nof these findings for research on \"hidden order\" in other materials.", "category": "cond-mat_str-el" }, { "text": "Ground state of S=1 zigzag spin-orbital chain: We investigate ground-state properties of a $t_{\\rm 2g}$-orbital Hubbard\nmodel on a zigzag chain relevant for CaV$_{2}$O$_{4}$, by exploiting numerical\ntechniques such as Lanczos diagonalization and density-matrix renormalization\ngroup. Assuming a V$^{3+}$ ion, a local spin $S$=$1$ state is formed by two\nelectrons in the $t_{\\rm 2g}$ orbitals. That is, the system is a Haldane system\nwith active $t_{\\rm 2g}$-orbital degrees of freedom. We observe orbital-state\ntransitions, yielding a distinct spin system under the orbital-ordered\nbackground. We also discuss the orbital structure induced by open edges,\noriginating in the spatial anisotropy of the $t_{\\rm 2g}$ orbitals.", "category": "cond-mat_str-el" }, { "text": "Evidence for a fractional quantum Hall state with anisotropic\n longitudinal transport: At high magnetic fields, where the Fermi level lies in the N=0 lowest Landau\nlevel (LL), a clean two-dimensional electron system (2DES) exhibits numerous\nincompressible liquid phases which display the fractional quantized Hall effect\n(FQHE) (Das Sarma and Pinczuk, 1997). These liquid phases do not break\nrotational symmetry, exhibiting resistivities which are isotropic in the plane.\nIn contrast, at lower fields, when the Fermi level lies in the $N\\ge2$ third\nand several higher LLs, the 2DES displays a distinctly different class of\ncollective states. In particular, near half filling of these high LLs the 2DES\nexhibits a strongly anisotropic longitudinal resistance at low temperatures\n(Lilly et al., 1999; Du et al., 1999). These \"stripe\" phases, which do not\nexhibit the quantized Hall effect, resemble nematic liquid crystals, possessing\nbroken rotational symmetry and orientational order (Koulakov et al., 1996;\nFogler et al., 1996; Moessner and Chalker, 1996; Fradkin and Kivelson, 1999;\nFradkin et al, 2010). Here we report a surprising new observation: An\nelectronic configuration in the N=1 second LL whose resistivity tensor\nsimultaneously displays a robust fractionally quantized Hall plateau and a\nstrongly anisotropic longitudinal resistance resembling that of the stripe\nphases.", "category": "cond-mat_str-el" }, { "text": "How do we interrogate the electrons without roughing them up?: Electrons are indistinguishable, but the energy of each electron is different\nin different materials and if we can exploit this energy, then we can\nsystematically study the changes of electronic properties in non free-electron\nmetals.", "category": "cond-mat_str-el" }, { "text": "Entanglement and Topology in Su-Schrieffer-Heeger Cavity Quantum\n Electrodynamics: Cavity materials are a frontier to investigate the role of light-matter\ninteractions on the properties of electronic phases of matter. In this work, we\nraise a fundamental question: can non-local interactions mediated by cavity\nphotons destabilize a topological electronic phase? We investigate this\nquestion by characterizing entanglement, energy spectrum and correlation\nfunctions of the topological Su-Schrieffer-Heeger (SSH) chain interacting with\nan optical cavity mode. Employing density-matrix renormalization group (DMRG)\nand exact diagonalization (ED), we demonstrate the stability of the edge state\nand establish an area law scaling for the ground state entanglement entropy,\ndespite long-range correlations induced by light-matter interactions. These\nfeatures are linked to gauge invariance and the scaling of virtual photon\nexcitations entangled with matter, effectively computed in a low-dimensional\nKrylov subspace of the full Hilbert space. This work provides a framework for\ncharacterizing novel equilibrium phenomena in topological cavity materials.", "category": "cond-mat_str-el" }, { "text": "From order to randomness: Onset and evolution of the random-singlet\n state in bond-disordered BaCu$_2$(Si$_{1-x}$Ge$_x$)$_2$O$_7$ spin-chain\n compounds: Heisenberg-type spin-chain materials have been extensively studied over the\nyears, yet not much is known about their behavior in the presence of disorder.\nStarting from BaCu$_2$Si$_2$O$_7$, a typical spin-1/2 chain system, we\ninvestigate a series of compounds with different degrees of bond disorder,\nwhere the systematic replacement of Si with Ge results in a re-modulation of\nthe Cu$^{2+}$ exchange interactions. By combining magnetometry measurements\nwith nuclear magnetic resonance studies we follow the evolution of the\ndisorder-related properties from the well-ordered BaCu$_2$Si$_2$O$_7$ to the\nmaximally disordered BaCu$_2$SiGeO$_7$. Our data indicate that already a weak\ndegree of disorder of only 5% Ge, apart from reducing the 3D magnetic ordering\ntemperature $T_\\mathrm{N}$ quite effectively, induces a qualitatively different\nstate in the paramagnetic regime. At maximum disorder our data indicate that\nthis state may be identified with the theoretically predicted random singlet\n(RS) state. With decreasing disorder the extension of the RS regime at\ntemperatures above $T_\\mathrm{N}$ is reduced, yet its influence is clearly\nmanifest, particularly in the features of NMR relaxation data.", "category": "cond-mat_str-el" }, { "text": "Quantum-critical transport of marginal Fermi-liquids: We present exact results for the electrical and thermal conductivity and\nSeebeck coefficient at low temperatures and frequencies in the quantum-critical\nregion for fermions on a lattice scattering with the collective fluctuations of\nthe quantum xy model. This is done by the asymptotically exact solution of the\nvertex equation in the Kubo formula for these transport properties. The model\nis applicable to the fluctuations of the loop-current order in cuprates as well\nas to a class of quasi-two dimensional heavy-fermion and other metallic\nantiferromagnets, and proposed recently also for the possible loop-current\norder in Moir\\'{e} twisted bi-layer graphene and bi-layer WSe$_2$. All these\nmetals have a linear in temperature electrical resistivity in the\nquantum-critical region of their phase diagrams, often termed \"Planckian\"\nresistivity. The solution of the integral equation for the vertex in the Kubo\nequation for transport shows that all vertex renormalizations except due to\nAslamazov-Larkin processes are absent. The latter appear as an Umklapp\nscattering matrix, which is shown to give only a temperature independent\nmultiplicative factor for electrical resistivity which is non-zero in the pure\nlimit only if the Fermi-surface is large enough, but do not affect thermal\nconductivity. We also show that the mass renormalization which gives a\nlogarithmic enhancement of the marginal Fermi-liquid specific heat does not\nappear in the electrical resistivity as well as in the thermal conductivity. On\nthe other hand the mass renormalization appears in the Seebeck coefficient. The\nresults for transport properties are derived for any Fermi-surface on any\nlattice. As an example, the linear in $T$ electrical resistivity is explicitly\ncalculated for large enough circular Fermi-surfaces on a square lattice. We\nalso discuss in detail the conservation laws that play a crucial role in all\ntransport properties.", "category": "cond-mat_str-el" }, { "text": "The chemical bond as an emergent phenomenon: We first argue that the covalent bond and the various closed-shell\ninteractions can be thought of as symmetry broken versions of one and the same\ninteraction, viz., the multi-center bond. We use specially chosen molecular\nunits to show that the symmetry breaking is controlled by density and\nelectronegativity variation. We show that the bond order changes with bond\ndeformation but in a step-like fashion, regions of near constancy separated by\nelectronic localization transitions. These will often cause displacive\ntransitions as well so that the bond strength, order, and length are\nestablished self-consistently. We further argue on the inherent relation of the\ncovalent, closed-shell, and multi-center interactions with ionic and metallic\nbonding. All of these interactions can be viewed as distinct sectors on a phase\ndiagram with density and electronegativity variation as control variables; the\nionic and covalent/secondary sectors are associated with on-site and bond-order\ncharge density wave respectively, the metallic sectorwith an electronic fluid.\nWhile displaying a contiguity at low densities, the metallic and ionic\ninteractions represent distinct phases separated by discontinuous transitions\nat sufficiently high densities. Multi-center interactions emerge as a hybrid of\nthe metallic and ionic bond that results from spatial coexistence of\ndelocalized and localized electrons. In the present description, the issue of\nthe stability of a compound is that of mutual miscibility of electronic fluids\nwith distinct degrees of electron localization, supra-atomic ordering in\ncomplex inorganic compounds comes about naturally. The notions of electronic\nlocalization advanced hereby suggest a high throughput, automated procedure for\nscreening candidate compounds and structures with regard to stability, without\nthe need for computationally costly geometric optimization.", "category": "cond-mat_str-el" }, { "text": "First-Order Reversal Curves of the Magnetostructural Phase Transition in\n FeTe: We apply the first-order reversal curve (FORC) method, borrowed from studies\nof ferromagnetic materials, to the magneto-structural phase transition of FeTe.\nFORC measurements reveal two features in the hysteretic phase transition, even\nin samples where traditional temperature measurements display only a single\ntransition. For Fe1.13Te, the influence of magnetic field suggests that the\nmain feature is primarily structural while a smaller, slightly\nhigher-temperature transition is magnetic in origin. By contrast Fe1.03Te has a\nsingle transition which shows a uniform response to magnetic field, indicating\na stronger coupling of the magnetic and structural phase transitions. We also\nintroduce uniaxial stress, which spreads the distribution width without\nchanging the underlying energy barrier of the transformation. The work shows\nhow FORC can help disentangle the roles of the magnetic and structural phase\ntransitions in FeTe.", "category": "cond-mat_str-el" }, { "text": "Two ferromagnetic phases in La1-xSrxMnO3(x ~1/8): It was discovered in La1-xSrxMnO3(x~1/8) that a field induced phase\ntransition occurs from a ferromagnetic metal(FM) phase to a ferromagnetic\ninsulator (FI) phase. The magnetization shows a sharp jump at the transition\nfield accompanying with a remarkable increase of magnetoresistance. Striction\nmeasurements clarified that this transition is associated with the structural\nchange from a Jahn-Teller(JT) distorted orthorhombic phase to a pseudo cubic\nphase. These results evidently show that the FI phase with a pseudo cubic\nsymmetry is more stable in high fields than the FM phase due to the double\nexchange interaction. The driving force of this transition is explained by the\nenhancement of the ferromagnetic superexchange interaction induced by an\nantiferromagnetic type orbital ordering in the pseudo cubic phase, which was\nrecently found in the anomalous X-ray scattering experiments.", "category": "cond-mat_str-el" }, { "text": "Doping evolution of the electron-hole asymmetric s-wave pseudogap in\n underdoped high-Tc cuprate superconductors: We study the doping evolution of the electronic structure in the pseudogap\nstate of high-Tc cuprate superconductors, by means of a cluster extension of\nthe dynamical mean-field theory applied to the two-dimensional Hubbard model.\nThe calculated single-particle excitation spectra in the strongly underdoped\nregime show a marked electron-hole asymmetry and reveal a \"s-wave\" pseudogap,\nwhich display a finite amplitude in all the directions in the momentum space\nbut not always at the Fermi level: The energy location of the gap strongly\ndepends on momentum, and in particular in the nodal region, it is above the\nFermi level. With increasing hole doping, the pseudogap disappears everywhere\nin the momentum space. We show that the origin and the \"s-wave\" structure of\nthe pseudogap can be ascribed to the emergence of a strong-scattering surface,\nwhich appears in the energy-momentum space close to the Mott insulator.", "category": "cond-mat_str-el" }, { "text": "The spatial range of the Kondo effect: a numerical analysis: The spatial length of the Kondo screening is still a controversial issue\nrelated to Kondo physics. While renormalization group and Bethe Anzats\nsolutions have provided detailed information about the thermodynamics of\nmagnetic impurities, they are insufficient to study the effect on the\nsurrounding electrons, i.e., the spatial range of the correlations created by\nthe Kondo effect between the localized magnetic moment and the conduction\nelectrons. The objective of this work is to present a quantitative way of\nmeasuring the extension of these correlations by studying their effect directly\non the local density of states (LDOS) at arbitrary distances from the impurity.\nThe numerical techniques used, the Embedded Cluster Approximation, the Finite U\nSlave Bosons, and Numerical Renormalization Group, calculate the Green\nfunctions in real space. With this information, one can calculate how the local\ndensity of states away from the impurity is modified by its presence, below and\nabove the Kondo temperature, and then estimate the range of the disturbances in\nthe non-interacting Fermi sea due to the Kondo effect, and how it changes with\nthe Kondo temperature $T_{\\rm K}$. The results obtained agree with results\nobtained through spin-spin correlations, showing that the LDOS captures the\nphenomenology of the Kondo cloud as well. To the best of our knowledge, it is\nthe first time that the LDOS is used to estimate the extension of the Kondo\ncloud.", "category": "cond-mat_str-el" }, { "text": "On the existence of the excitonic insulator phase in the extended\n Falicov-Kimball model: an SO(2)-invariant slave-boson approach: We re-examine the three-dimensional spinless Falicov-Kimball model with\ndispersive $f$ electrons at half-filling, addressing the dispute about the\nformation of an excitonic condensate, which is closely related to the problem\nof electronic ferroelectricity. To this end, we work out a slave-boson\nfunctional integral representation of the suchlike extended Falicov-Kimball\nmodel that preserves the $SO(2)\\otimes U(1)^{\\otimes 2}$ invariance of the\naction. We find a spontaneous pairing of $c$ electrons with $f$ holes, building\nan excitonic insulator state at low temperatures, also for the case of\ninitially non-degenerate orbitals. This is in contrast to recent predictions of\nscalar slave-boson mean-field theory but corroborates previous Hartree-Fock and\nRPA results. Our more precise treatment of correlation effects, however, leads\nto a substantial reduction of the critical temperature. The different behavior\nof the partial densities of states in the weak and strong inter-orbital Coulomb\ninteraction regimes supports a BCS-BEC transition scenario.", "category": "cond-mat_str-el" }, { "text": "Thermal Conductivity due to Spinons in the One-Dimensional Quantum Spin\n System Sr2V3O9: We have measured the thermal conductivity along different directions of the S\n= 1/2 one-dimensional (1D) spin system Sr2V3O9 in magnetic fields up to 14 T.\nIt has been found that the thermal conductivity along the [10-1] direction,\n\\k{appa}[10-1], is large and markedly suppressed by the application of magnetic\nfield, indicating that there is a large contribution of spinons to\n\\k{appa}[10-1] and that the spin chains run along the [10-1] direction. The\nmaximum value of the thermal conductivity due to spinons is ~14 W/Km along the\n[10-1] direction, supporting the empirical law that the magnitude of the\nthermal conductivity due to spinons is roughly proportional to the\nantiferromagnetic interaction between the nearest neighboring spins.", "category": "cond-mat_str-el" }, { "text": "Effects of electron coupling to intra- and inter-molecular vibrational\n modes on the transport properties of single crystal organic semiconductors: Electron coupling to intra- and inter-molecular vibrational modes is\ninvestigated in models appropriate to single crystal organic semiconductors,\nsuch as oligoacenes. Focus is on spectral and transport properties of these\nsystems beyond perturbative approaches. The interplay between different\ncouplings strongly affects the temperature band renormalization that is the\nresult of a subtle equilibrium between opposite tendencies: band narrowing due\nto interaction with local modes, band widening due to electron coupling to non\nlocal modes. The model provides an accurate description of the mobility as\nfunction of temperature: indeed, it has the correct order of magnitude, at low\ntemperatures, it scales as a power-law $T^{-\\delta}$ with the exponent $\\delta$\nlarger than unity, and, at high temperatures, shows an hopping behavior with a\nsmall activation energy.", "category": "cond-mat_str-el" }, { "text": "Effects of Dissipation on Solitons in the Hydrodynamic Regime of\n Graphene: We use hydrodynamic techniques to analyze the one-dimensional propagation of\nsolitons in gated graphene on an arbitrary uniform background current. Results\nare derived for both the Fermi liquid and Dirac fluid regimes. We find that\nthese solutions satisfy the Korteweg-de Vries-Burgers equation. Viscous\ndissipation and ohmic heating are included, causing the solitons to decay.\nExperiments are proposed to measure this decay and thereby quantify the shear\nviscosity in graphene.", "category": "cond-mat_str-el" }, { "text": "Conserving quasiparticle calculations for small metal clusters: A novel approach for GW-based calculations of quasiparticle properties for\nfinite systems is presented, in which the screened interaction is obtained\ndirectly from a linear response calculation of the density-density correlation\nfunction. The conserving nature of our results is shown by explicit evaluation\nof the $f$-sum rule. As an application, energy renormalizations and level\nbroadenings are calculated for the closed-shell Na$_9^+$ and Na$_{21}^+$\nclusters, as well as for Na$_4$. Pronounced improvements of conserving\napproximations to RPA-level results are obtained.", "category": "cond-mat_str-el" }, { "text": "Laser-excited ultrahigh-resolution photoemission spectroscopy of\n NaxCoO2.yH2O:Evidence for pseudogap formation: We have studied the temperature-dependent electronic structure near the Fermi\nlevel (EF) of the layered cobaltate superconductor, Na0.35CoO2.1.3H2O, and\nrelated materials, using laser-excited ultrahigh-resolution photoemission\nspectroscopy. We observe the formation of a pseudogap with an energy scale of ~\n20 meV in Na0.35CoO2.1.3H2O and Na0.35CoO2.0.7H2O, which is clearly absent in\nNa0.7CoO2. The energy scale of the pseudogap is larger than the expected value\nfor the superconducting gap, suggesting an additional competing order parameter\nat low temperatures. We discuss implications of the pseudogap in relation to\navailable transport and magnetic susceptibility results.", "category": "cond-mat_str-el" }, { "text": "Current response of nonequilibrium steady states in Landau-Zener\n problem: Nonequilibrium Green's function approach: The carrier generation in insulators subjected to strong electric fields is\ncharacterized by the Landau-Zener formula for the tunneling probability with a\nnonperturbative exponent. Despite its long history with diverse applications\nand extensions, study of nonequilibrium steady states and associated current\nresponse in the presence of the generated carriers has been mainly limited to\nnumerical simulations so far. Here, we develop a framework to calculate the\nnonequilibrium Green's function of generic insulating systems under a DC\nelectric field, in the presence of a fermionic reservoir. Using asymptotic\nexpansion techniques, we derive a semi-quantitative formula for the Green's\nfunction with nonperturbative contribution. This formalism enables us to\ncalculate dissipative current response of the nonequilibrium steady state,\nwhich turns out to be not simply characterized by the intraband current\nproportional to the tunneling probability. We also apply the present formalism\nto noncentrosymmetric insulators, and propose nonreciprocal charge and spin\ntransport peculiar to tunneling electrons.", "category": "cond-mat_str-el" }, { "text": "Anisotropic optical properties of detwinned BaFe$_{2}$As$_{2}$: The optical properties of a large, detwinned single crystal of BaFe$_2$As$_2$\nhave been examined over a wide frequency range above and below the structural\nand magnetic transition at $T_{\\rm N}\\simeq 138$ K. Above $T_{\\rm N}$ the real\npart of the optical conductivity and the two infrared-active lattice modes are\nalmost completely isotropic; the lattice modes show a weak polarization\ndependence just above $T_{\\rm N}$. For $T0. The construction is\nextended to spin-dependent phenomena by parametrizing the\nmagnetization-dependence of the ground-state energy using further exact results\nand numerical benchmarking. Lastly, the parametrizations developed for the\nspatially uniform model are extended by means of a simple local-density-type\napproximation to spatially inhomogeneous models, e.g., in the presence of\nimpurities, external fields or trapping potentials. Results are shown to be in\nexcellent agreement with independent many-body calculations, at a fraction of\nthe computational cost.", "category": "cond-mat_str-el" }, { "text": "Metal-insulator transition and local-moment collapse in negative\n charge-transfer CaFeO$_3$ under pressure: We compute the electronic structure, spin and charge state of Fe ions, and\nstructural phase stability of paramagnetic CaFeO$_3$ under pressure using a\nfully self-consistent in charge density DFT+dynamical mean-field theory method.\nWe show that at ambient pressure CaFeO$_3$ is a negative charge-transfer\ninsulator characterized by strong localization of the Fe $3d$ electrons. It\ncrystallizes in the monoclinic $P2_1/n$ crystal structure with a cooperative\nbreathing mode distortion of the lattice. While the Fe $3d$ Wannier occupations\nand local moments are consistent with robust charge disproportionation of Fe\nions in the insulating $P2_1/n$ phase, the physical charge density difference\naround the structurally distinct Fe A and Fe B ions with the ``contracted'' and\n``expanded'' oxygen octahedra, respectively, is rather weak, $\\sim$0.04. This\nimplies the importance of the Fe $3d$ and O $2p$ negative charge transfer and\nsupports the formation of a bond-disproportionated state characterized by the\nFe A $3d^{5-\\delta}\\underline{L}^{2-\\delta}$ and Fe B $3d^5$ valence\nconfigurations with $\\delta \\ll 1$, in agreement with strong hybridization\nbetween the Fe $3d$ and O $2p$ states. Upon compression above $\\sim$41 GPa\nCaFeO$_3$ undergoes the insulator-to-metal phase transition (IMT) which is\naccompanied by a structural transformation into the orthorhombic $Pbnm$ phase.\nThe phase transition is accompanied by suppression of the cooperative breathing\nmode distortion of the lattice and, hence, results in the melting of bond\ndisproportionation of the Fe ions. Our analysis suggests that the IMT\ntransition is associated with orbital-dependent delocalization of the Fe $3d$\nelectrons and leads to a remarkable collapse of the local magnetic moments. Our\nresults imply the crucial importance of the interplay of electronic\ncorrelations and structural effects to explain the properties of CaFeO$_3$.", "category": "cond-mat_str-el" }, { "text": "Dynamical Signature of Fractionalization at the Deconfined Quantum\n Critical Point: Deconfined quantum critical points govern continuous quantum phase\ntransitions at which fractionalized (deconfined) degrees of freedom emerge.\nHere we study dynamical signatures of the fractionalized excitations in a\nquantum magnet (the easy-plane J-Q model) that realize a deconfined quantum\ncritical point with emergent O(4) symmetry. By means of large-scale quantum\nMonte Carlo simulations and stochastic analytic continuation of imaginary-time\ncorrelation functions, we obtain the dynamic spin structure factors in the\n$S^{x}$ and $S^{z}$ channels. In both channels, we observe broad continua that\noriginate from the deconfined excitations. We further identify several distinct\nspectral features of the deconfined quantum critical point, including the lower\nedge of the continuum and its form factor on moving through the Brillouin Zone.\nWe provide field-theoretical and lattice model calculations that explain the\noverall shapes of the computed spectra, which highlight the importance of\ninteractions and gauge fluctuations to explaining the spectral-weight\ndistribution. We make further comparisons with the conventional Landau O(2)\ntransition in a different quantum magnet, at which no signatures of\nfractionalization are observed. The distinctive spectral signatures of the\ndeconfined quantum critical point suggest the feasibility of its experimental\ndetection in neutron scattering and nuclear magnetic resonance experiments.", "category": "cond-mat_str-el" }, { "text": "From Kosterlitz-Thouless to Pokrovsky-Talapov transitions in spinless\n fermions and spin chains with next-nearest neighbour interactions: We investigate the nature of the quantum phase transition out of\ncharge-density-wave phase in the spinless fermion model with nearest- and\nnext-nearest-neighbor interaction at one-third filling. Using an extensive\nDensity Matrix Renormalization Group (DMRG) simulations we show that the\ntransition changes it nature. We show that for weak next-nearest-neighbor\ncoupling the transition is of Kosterlitz-Thouless type in agreement with\nbosonisation predictions. We also provide strong numerical evidences that for\nlarge next-nearest-neighbor repulsion the transition belongs to the\nPokrovsky-Talapov univerality class describing a non-conformal\ncommensurate-incommensurate transition. Finally, we argue that the change of\nthe nature of the transition is a result of incommensurability induced by\nfrustration and realized even at zero doping. The implications in the context\nof XXZ chain with next-nearest-neighbor Ising interaction is briefly discussed.", "category": "cond-mat_str-el" }, { "text": "A finite-frequency functional RG approach to the single impurity\n Anderson model: We use the Matsubara functional renormalization group (FRG) to describe\nelectronic correlations within the single impurity Anderson model. In contrast\nto standard FRG calculations, we account for the frequency-dependence of the\ntwo-particle vertex in order to address finite-energy properties (e.g, spectral\nfunctions). By comparing with data obtained from the numerical renormalization\ngroup (NRG) framework, the FRG approximation is shown to work well for\narbitrary parameters (particularly finite temperatures) provided that the\nelectron-electron interaction U is not too large. We demonstrate that aspects\nof (large U) Kondo physics which are described well by a simpler\nfrequency-independent truncation scheme are no longer captured by the\n'higher-order' frequency-dependent approximation. In contrast, at small to\nintermediate U the results obtained by the more elaborate scheme agree better\nwith NRG data. We suggest to parametrize the two-particle vertex not by three\nindependent energy variables but by introducing three functions each of a\nsingle frequency. This considerably reduces the numerical effort to integrate\nthe FRG flow equations.", "category": "cond-mat_str-el" }, { "text": "Graded Projected Entangled-Pair State Representations and An Algorithm\n for Translationally Invariant Strongly Correlated Electronic Systems on\n Infinite-Size Lattices in Two Spatial Dimensions: An algorithm to find a graded Projected Entangled-Pair State representation\nof the ground state wave functions is developed for translationally invariant\nstrongly correlated electronic systems on infinite-size lattices in two spatial\ndimensions. It is tested for the two-dimensional t-J model at and away from\nhalf filling, with truncation dimensions up to 6. We are able to locate a line\nof phase separation, which qualitatively agrees with the results based on the\nhigh-temperature expansions. We find that the model exhibits an extended s-wave\nsuperconductivity for J=0.4t at quarter filling. However, we emphasize that the\ncurrently accessible truncation dimensions are not large enough, so it is\nnecessary to incorporate the symmetry of the system into the algorithm, in\norder to achieve results with higher precision.", "category": "cond-mat_str-el" }, { "text": "Spin and orbital ordering in double-layered manganites: We study theoretically the phase diagram of the double-layered perovskite\nmanganites taking into account the orbital degeneracy, the strong Coulombic\nrepulsion, and the coupling with the lattice deformation. Observed spin\nstructural changes as the increased doping are explained in terms of the\norbital ordering and the bond-length dependence of the hopping integral along\n$c$-axis. Temperature dependence of the neutron diffraction peak corresponding\nto the canting structure is also explained. Comparison with the 3D cubic system\nis made.", "category": "cond-mat_str-el" }, { "text": "Electric field-induced Skyrmion distortion and giant lattice rotation in\n the magnetoelectric insulator Cu2OSeO3: Uniquely in Cu2OSeO3, the Skyrmions, which are topologically protected\nmagnetic spin vortex-like objects, display a magnetoelectric coupling and can\nbe manipulated by externally applied electric (E) fields. Here, we explore the\nE-field coupling to the magnetoelectric Skyrmion lattice phase, and study the\nresponse using neutron scattering. Giant E-field induced rotations of the\nSkyrmion lattice are achieved that span a range of $\\sim$25$^{\\circ}$.\nSupporting calculations show that an E-field-induced Skyrmion distortion lies\nbehind the lattice rotation. Overall, we present a new approach to Skyrmion\ncontrol that makes no use of spin-transfer torques due to currents of either\nelectrons or magnons.", "category": "cond-mat_str-el" }, { "text": "Effects of Backflow Correlation in the Three-Dimensional Electron Gas:\n Quantum Monte Carlo Study: The correlation energy of the homogeneous three-dimensional interacting\nelectron gas is calculated using the variational and fixed-node diffusion Monte\nCarlo methods, with trial functions that include backflow and three-body\ncorrelations. In the high density regime the effects of backflow dominate over\nthose due to three-body correlations, but the relative importance of the latter\nincreases as the density decreases. Since the backflow correlations vary the\nnodes of the trial function, this leads to improved energies in the fixed-node\ndiffusion Monte Carlo calculations. The effects are comparable to those found\nfor the two-dimensional electron gas, leading to much improved variational\nenergies and fixed-node diffusion energies equal to the release-node energies\nof Ceperley and Alder within statistical and systematic errors.", "category": "cond-mat_str-el" }, { "text": "Tuning the Magnetic Ground State by Charge Transfer Energy in SrCoO2.5\n via Strain Engineering: SrCoO2.5 (SCO) is a charge transfer insulator with 3d6 ground state\nconfiguration leading to antiferromagnetic nature. It is observed that\nsubstrate induced strain engineering modifies the ground state of SCO thin film\nwith 3d7L (L:O-2p hole) configuration causing negative charge transfer\nenergy.The consequent strong hybridization between O-2p and Co-3d bands causes\na hole in O-2p band leading to hole mediated unconventional ferromagnetic\nordering in SrCoO2.5 thin film. This opens up a new avenue to tune the\nelectronic structure vis a vis magnetic property via strain engineering.", "category": "cond-mat_str-el" }, { "text": "Role of surface states in STM spectroscopy of (111) metal surfaces with\n Kondo adsorbates: A nearly-free-electron (NFE) model to describe STM spectroscopy of (111)\nmetal surfaces with Kondo impurities is presented. Surface states are found to\nplay an important role giving a larger contribution to the conductance in the\ncase of Cu(111) and Au(111) than Ag(111) surfaces. This difference arises from\nthe farther extension of the Ag(111) surface state into the substrate. The\ndifferent line shapes observed when Co is adsorbed on different substrates can\nbe explained from the position of the surface band onset relative to the Fermi\nenergy. The lateral dependence of the line shape amplitude is found to be\nbulk-like for R|| < 4 Amstrongs and surface-like at larger distances, in\nagreement with experimental data.", "category": "cond-mat_str-el" }, { "text": "Interacting surface states of three-dimensional topological insulators: We numerically investigate the surface states of a strong topological\ninsulator in the presence of strong electron-electron interactions. We choose a\nspherical topological insulator geometry to make the surface amenable to a\nfinite size analysis. The single-particle problem maps to that of Landau\norbitals on the sphere with a magnetic monopole at the center that has unit\nstrength and opposite sign for electrons with opposite spin. Assuming\ndensity-density contact interactions, we find superconducting and anomalous\n(quantum) Hall phases for attractive and repulsive interactions, respectively,\nas well as chiral fermion and chiral Majorana fermion boundary modes between\ndifferent phases. Our setup is preeminently adapted to the search for\ntopologically ordered surface terminations that could be microscopically\nstabilized by tailored surface interaction profiles.", "category": "cond-mat_str-el" }, { "text": "Spectroscopic signatures and origin of a hidden order in Ba$_2$MgReO$_6$: Clarifying the underlying mechanisms that govern ordering transitions in\ncondensed matter systems is crucial for comprehending emergent properties and\nphenomena. While transitions are often classified as electronically driven or\nlattice-driven, we present a departure from this conventional paradigm in the\ncase of the double perovskite Ba$_2$MgReO$_6$. Leveraging resonant and\nnon-resonant elastic x-ray scattering techniques, we unveil the simultaneous\nordering of structural distortions and charge quadrupoles at a critical\ntemperature of $T_\\mathrm{q}$$\\sim$33 K. Using a variety of complementary\nfirst-principles-based computational techniques, we demonstrate that while\nelectronic interactions drive the ordering at $T_\\mathrm{q}$, it is ultimately\nthe lattice that dictates the specific ground state that emerges. Our findings\nhighlight the crucial interplay between electronic and lattice degrees of\nfreedom, providing a unified framework to understand and predict unconventional\nemergent phenomena in quantum materials.", "category": "cond-mat_str-el" }, { "text": "Dynamical vertex approximation for the two-dimensional Hubbard model: Recently, diagrammatic extensions of dynamical mean field theory (DMFT) have\nbeen proposed for including short- and long-range correlations beyond DMFT on\nan equal footing. We employ one of these, the dynamical vertex approximation\n(D$\\Gamma$A), and study the two-dimensional Hubbard model on a square lattice.\nWe define two transition lines in the phase diagram which correspond,\nrespectively, to the opening of the gap in the nodal direction and throughout\nthe Fermi surface. Our self-energy data show that the evolution between the two\nregimes occurs in a gradual way (crossover) and also that at low enough\ntemperatures the whole Fermi surface is always gapped. Furthermore, we present\na comparison of our D$\\Gamma$A calculations at a parameter set where data\nobtained by other techniques are available.", "category": "cond-mat_str-el" }, { "text": "Effect of Li doping on magnetic and transport properties of CoV2O4 and\n FeV2O4: The structural, magnetic and transport properties have been studied of Li\ndoped CoV2O4 and FeV2O4. Li doping increases the ferri-magnetic ordering\ntemperature of both the samples but decreases the spin-glass transition\ntemperature of CoV2O4. The Li-doping decreases the V-V distance which in effect\nincreases the A-V coupling. Thus the increased A-V coupling dominate over the\ndecrease in A-V coupling due to doping of non-magnetic Li.", "category": "cond-mat_str-el" }, { "text": "Quasiparticle evolution and pseudogap formation in V2O3: An infrared\n spectroscopy study: The infrared conductivity of V2O3 is measured in the whole phase diagram.\nQuasiparticles appear above the Neel temperature TN and eventually disappear\nfurther enhancing the temperature, leading to a pseudogap in the optical\nspectrum above 425 K. Our calculations demonstrate that this loss of coherence\ncan be explained only if the temperature dependence of lattice parameters is\nconsidered. V2O3 is therefore effectively driven from the metallic to the\ninsulating side of the Mott transition as the temperature is increased.", "category": "cond-mat_str-el" }, { "text": "Hubbard nanoclusters far from equilibrium: The Hubbard model is a prototype for strongly correlated many-particle\nsystems, including electrons in condensed matter and molecules, as well as for\nfermions or bosons in optical lattices. While the equilibrium properties of\nthese systems have been studied in detail, the nonequilibrium dynamics\nfollowing a strong non-perturbative excitation only recently came into the\nfocus of experiments and theory. It is of particular interest how the dynamics\ndepend on the coupling strength and on the particle number and whether there\nexist universal features in the time evolution. Here, we present results for\nthe dynamics of finite Hubbard clusters based on a selfconsistent\nnonequilibrium Green functions (NEGF) approach invoking the generalized\nKadanoff--Baym ansatz (GKBA). We discuss the conserving properties of the GKBA\nwith Hartree--Fock propagators in detail and present a generalized form of the\nenergy conservation criterion of Baym and Kadanoff for NEGF. Furthermore, we\ndemonstrate that the HF-GKBA cures some artifacts of prior two-time NEGF\nsimulations. Besides, this approach substantially speeds up the numerical\ncalculations and thus presents the capability to study comparatively large\nsystems and to extend the analysis to long times allowing for an accurate\ncomputation of the excitation spectrum via time propagation. Our data obtained\nwithin the second Born approximation compares favorably with exact\ndiagonalization results (available for up to 13 particles) and are expected to\nhave predictive capability for substantially larger systems in the weak\ncoupling limit.", "category": "cond-mat_str-el" }, { "text": "Partition Functions of Strongly Correlated Electron Systems as\n \"Fermionants\": We introduce a new mathematical object, the \"fermionant\"\n${\\mathrm{Ferm}}_N(G)$, of type $N$ of an $n \\times n$ matrix $G$. It\nrepresents certain $n$-point functions involving $N$ species of free fermions.\nWhen N=1, the fermionant reduces to the determinant. The partition function of\nthe repulsive Hubbard model, of geometrically frustrated quantum\nantiferromagnets, and of Kondo lattice models can be expressed as fermionants\nof type N=2, which naturally incorporates infinite on-site repulsion. A\ncomputation of the fermionant in polynomial time would solve many interesting\nfermion sign problems.", "category": "cond-mat_str-el" }, { "text": "Superfluid--Insulator Transition in Strongly Disordered One-dimensional\n Systems: We present an asymptotically exact renormalization-group theory of the\nsuperfluid--insulator transition in one-dimensional disordered systems, with\nemphasis on an accurate description of the interplay between the\nGiamarchi--Schulz (instanton--anti-instanton) and weak-link (scratched-XY)\ncriticalities. Combining the theory with extensive quantum Monte Carlo\nsimulations allows us to shed new light on the ground-state phase diagram of\nthe one-dimensional disordered Bose-Hubbard model at unit filling.", "category": "cond-mat_str-el" }, { "text": "Repulsive Fermi gases in a two-dimensional lattice with non-Abelian\n gauge fields: Motivated by the recent experiment realizing bidirectional spin-orbit-coupled\nBose-Einstein condensates (BEC), we theoretically explore the properties of\nrepulsive fermions in the two-dimensional (2D) optical lattice with such\nnon-Abelian gauge fields. Within the mean-field level, we find a novel phase of\ntopological antiferromagnetic (TAFM) order which incorporates both the\nnon-trivial topology due to spin-flip hopping and spontaneous symmetry breaking\n(SSB) for the in-plane spin order. We argue that the appearance of such a phase\nis generic for repulsive fermions in Chern bands achieved through spin-orbit\ncoupling. Our work paves the way for further studies of fermionic\ngeneralization of 2D non-Abelian spin-orbit-coupled quantum gases.", "category": "cond-mat_str-el" }, { "text": "Topological Equivalence between the Fibonacci Quasicrystal and the\n Harper Model: One-dimensional quasiperiodic systems, such as the Harper model and the\nFibonacci quasicrystal, have long been the focus of extensive theoretical and\nexperimental research. Recently, the Harper model was found to be topologically\nnontrivial. Here, we derive a general model that embodies a continuous\ndeformation between these seemingly unrelated models. We show that this\ndeformation does not close any bulk gaps, and thus prove that these models are\nin fact topologically equivalent. Remarkably, they are equivalent regardless of\nwhether the quasiperiodicity appears as an on-site or hopping modulation. This\nproves that these different models share the same boundary phenomena and\nexplains past measurements. We generalize this equivalence to any\nFibonacci-like quasicrystal, i.e., a cut and project in any irrational angle.", "category": "cond-mat_str-el" }, { "text": "Two-particle self-consistent approach for broken symmetry phases: Spontaneous symmetry breaking of interacting fermion systems constitutes a\nmajor challenge for many-body theory due to the proliferation of new\nindependent scattering channels once absent or degenerate in the symmetric\nphase. One example is given by the ferro/antiferromagnetic broken symmetry\nphase (BSP) of the Hubbard model, where vertices in the spin-transverse and\nspin-longitudinal channels become independent with a consequent increase in the\ncomputational power for their calculation. Here we generalize the formalism of\nthe non-perturbative Two-Particle-Self-Consistent method (TPSC) to treat broken\nSU(2) magnetic phases of the Hubbard model, providing with a efficient yet\nreliable method. We show that in the BSP, the sum-rule enforcement of\nsusceptibilities must be accompanied by a modified gap equation resulting in a\nrenormalisation of the order parameter, vertex corrections and the preservation\nof the gap-less feature of the Goldstone modes. We then apply the theory to the\nantiferromagnetic phase of the Hubbard model in the cubic lattice at\nhalf-filling. We compare our results of double occupancies and staggered\nmagnetisation to the ones obtained using Diagrammatic Monte Carlo showing\nexcellent quantitative agreement. We demonstrate how vertex corrections play a\ncentral role in lowering the Higgs resonance with respect to the quasi-particle\nexcitation gap in the spin-longitudinal susceptibility, yielding a well visible\nHiggs-mode.", "category": "cond-mat_str-el" }, { "text": "Quantum phase transitions in antiferromagnets and superfluids: We present a general introduction to the non-zero temperature dynamic and\ntransport properties of low-dimensional systems near a quantum phase\ntransition. Basic results are reviewed in the context of experiments on the\nspin-ladder compounds, insulating two-dimensional antiferromagnets, and\ndouble-layer quantum Hall systems. Recent large N computations on an extended\nt-J model (cond-mat/9906104) motivate a global scenario of the quantum phases\nand transitions in the high temperature superconductors, and connections are\nmade to numerous experiments.", "category": "cond-mat_str-el" }, { "text": "Raman response in the nematic phase of FeSe: Raman experiments on bulk FeSe revealed that the low-frequency part of\n$B_{1g}$ Raman response $R_{B_{1g}}$, which probes nematic fluctuations,\nrapidly decreases below the nematic transition at $T_n \\sim 85$K. Such behavior\nis usually associated with the gap opening and at a first glance is\ninconsistent with the fact that FeSe remains a metal below $T_n$, with sizable\nhole and electron pockets. We argue that the drop of $R_{B_{1g}}$ in a nematic\nmetal comes about because the nematic order drastically changes the orbital\ncontent of the pockets and makes them nearly mono-orbital. In this situation\n$B_{1g}$ Raman response gets reduced by the same vertex corrections that\nenforce charge conservation. The reduction holds at low frequencies and gives\nrise to gap-like behavior of $R_{B_{1g}}$, in full agreement with the\nexperimental data.", "category": "cond-mat_str-el" }, { "text": "Defect-Induced Low-Energy Majorana Excitations in the Kitaev Magnet\n $\u03b1$-RuCl$_3$: The excitations in the Kitaev spin liquid (KSL) can be described by Majorana\nfermions, which have characteristic field dependence of bulk gap and\ntopological edge modes. In the high-field state of layered honeycomb magnet\n$\\alpha$-RuCl$_3$, experimental results supporting these Majorana features have\nbeen reported recently. However, there are challenges due to sample dependence\nand the impact of inevitable disorder on the KSL is poorly understood. Here we\nstudy how low-energy excitations are modified by introducing point defects in\n$\\alpha$-RuCl$_3$ using electron irradiation, which induces site vacancies and\nexchange randomness. High-resolution measurements of the temperature dependence\nof specific heat $C(T)$ under in-plane fields $H$ reveal that while the\nfield-dependent Majorana gap is almost intact, additional low-energy states\nwith $C/T=A(H)T$ are induced by introduced defects. At low temperatures, we\nobtain the data collapse of $C/T\\sim H^{-\\gamma}(T/H)$ expected for a\ndisordered quantum spin system, but with an anomalously large exponent\n$\\gamma$. This leads us to find a power-law relationship between the\ncoefficient $A(H)$ and the field-sensitive Majorana gap. These results are\nconsistent with the picture that the disorder induces low-energy linear\nMajorana excitations, which may be considered as a weak localization effect of\nMajorana fermions in the KSL.", "category": "cond-mat_str-el" }, { "text": "Anisotropy of Kondo-lattice coherence in momentum space for CeCoIn5: We study the electronic and phononic excitations of heavy-fermion metal\nCeCoIn$_5$ by polarization-resolved Raman spectroscopy to explore the\nKondo-lattice coherence. Below the coherence temperature T*\\,=\\,45\\,K, the\ncontinuum of electronic excitations in the XY scattering geometry is suppressed\nat frequencies below 50\\,cm$^{-1}$, whereas the low-frequency continuum in the\nX'Y' geometry exhibits no change across T*. We relate the suppression to the\nreduced electron-electron scattering rate resulting from the coherence effect.\nThe presence of suppression in the XY geometry and absence of it in the X'Y'\ngeometry implies that the $\\alpha$ and $\\beta$ bands become coherent below T*,\nwhereas the $\\gamma$ band remains largely incoherent down to 10\\,K. Moreover,\ntwo optical phonon modes exhibit anomalies in their temperature dependence of\nthe frequency and linewidth below T*, which results from developing coherent\nspectral weight near the Fermi level and reduced electron-phonon scattering\nrate. Our results further support the key role of anisotropic hybridization in\nCeCoIn$_5$.", "category": "cond-mat_str-el" }, { "text": "Excessive Noise as a Test for Many-Body Localization: Recent experimental reports suggested the existence of a finite-temperature\ninsulator in the vicinity of the superconductor-insulator transition. The rapid\ndecay of conductivity over a narrow temperature range was theoretically linked\nto both a finite-temperature transition to a many-body-localized state, and to\na charge-Berezinskii Kosterlitz Thouless transition. Here we report of\nlow-frequency noise measurements of such insulators to test for many body\nlocalization. We observed a huge enhancement of the low-temperatures noise when\nexceeding a threshold voltage for nonlinear conductivity and discuss our\nresults in light of the theoretical models.", "category": "cond-mat_str-el" }, { "text": "Quantum Disordered Ground States in Frustrated Antiferromagnets with\n Multiple Ring Exchange Interactions: We present a certain class of two-dimensional frustrated quantum Heisenberg\nspin systems with multiple ring exchange interactions which are rigorously\ndemonstrated to have quantum disordered ground states without magnetic\nlong-range order. The systems considered in this paper are s=1/2\nantiferromagnets on a honeycomb and square lattices, and an s=1 antiferromagnet\non a triangular lattice. We find that for a particular set of parameter values,\nthe ground state is a short-range resonating valence bond state or a valence\nbond crystal state. It is shown that these systems are closely related to the\nquantum dimer model introduced by Rokhsar and Kivelson as an effective\nlow-energy theory for valence bond states.", "category": "cond-mat_str-el" }, { "text": "Spin-phonon coupling effects in transition-metal perovskites:a DFT+$U$\n and hybrid-functional study: Spin-phonon coupling effects, as reflected in phonon frequency shifts between\nferromagnetic (FM) and G-type antiferromagnetic (AFM) configurations in cubic\nCaMnO$_3$, SrMnO$_3$, BaMnO$_3$, LaCrO$_3$, LaFeO$_3$ and La$_2$(CrFe)O$_6$,\nare investigated using density-functional methods. The calculations are carried\nout both with a hybrid-functional (HSE) approach and with a DFT+$U$ approach\nusing a $U$ that has been fitted to HSE calculations. The phonon frequency\nshifts obtained in going from the FM to the AFM spin configuration agree well\nwith those computed directly from the more accurate HSE approach, but are\nobtained with much less computational effort. We find that in the $A$MnO$_3$\nmaterials class with $A$=Ca, Sr, and Ba, this frequency shift decreases as the\nA cation radius increases for the $\\Gamma$ phonons, while it increases for\nR-point phonons. In La$M$O$_3$ with $M$=Cr, Fe, and Cr/Fe, the phonon\nfrequencies at $\\Gamma$ decrease as the spin order changes from AFM to FM for\nLaCrO$_3$ and LaFeO$_3$, but they increase for the double perovskite\nLa$_2$(CrFe)O$_6$. We discuss these results and the prospects for bulk and\nsuperlattice forms of these materials to be useful as multiferroics.", "category": "cond-mat_str-el" }, { "text": "Spinon Confinement and a Sharp Longitudinal Mode in Yb$_2$Pt$_2$Pb in\n Magnetic Fields: The fundamental excitations in an antiferromagnetic chain of spins-1/2 are\nspinons, de-confined fractional quasiparticles that when combined in pairs,\nform a triplet excitation continuum. In an Ising-like spin chain the continuum\nis gapped and the ground state is N{\\'e}el ordered. Here, we report high\nresolution neutron scattering experiments, which reveal how a magnetic field\ncloses this gap and drives the spin chains in \\YPP\\ to a critical, disordered\nLuttinger-liquid state. In \\YPP\\ the effective spins-1/2 describe the dynamics\nof large, Ising-like Yb magnetic moments, ensuring that the measured\nexcitations are exclusively longitudinal, which we find to be well described by\ntime-dependent density matrix renormalization group calculations. The\ninter-chain coupling leads to the confinement of spinons, a condensed matter\nanalog of quark confinement in quantum chromodynamics. Insensitive to\ntransverse fluctuations, our measurements show how a gapless, dispersive\nlongitudinal mode arises from confinement and evolves with magnetic order.", "category": "cond-mat_str-el" }, { "text": "Relationship between single-particle excitation and spin excitation at\n the Mott Transition: An intuitive interpretation of the relationship between the dispersion\nrelation of the single-particle excitation in a metal and that of the spin\nexcitation in a Mott insulator is presented, based on the results for the one-\nand two-dimensional Hubbard models obtained by using the Bethe ansatz,\ndynamical density-matrix renormalization group method, and cluster perturbation\ntheory. The dispersion relation of the spin excitation in the Mott insulator is\nnaturally constructed from that of the single-particle excitation in the\nzero-doping limit in both one- and two-dimensional Hubbard models, which allows\nus to interpret the doping-induced states as the states that lose charge\ncharacter toward the Mott transition. The characteristic feature of the Mott\ntransition is contrasted with the feature of a Fermi liquid and that of the\ntransition between a band insulator and a metal.", "category": "cond-mat_str-el" }, { "text": "Anomalous State Sandwiched between Fermi Liquid and Charge Ordered\n Mott-Insulating Phases of Ti4O7: The Magneli phase Ti4O7 exhibits two sharp jumps in resistivity with coupled\nstructural transitions as a function of temperature at Tc1=142 K and Tc2=154 K.\nWe have studied electronic structure changes across the two transitions using 7\neV laser, soft x-ray and hard x-ray (HX) photoemission spectroscopy (PES). Ti\n2p-3d resonant PES and HX-PES show a clear metallic Fermi-edge and mixed\nvalency above Tc2. The low temperature phase below Tc1 shows a clear insulating\ngap of 100 meV. The intermediate phase between Tc1 and Tc2 indicates a\npseudogap coexisting with remnant coherent states. HX-PES and complementary\ncalculations have confirmed the coherent screening in the strongly correlated\nintermediate phase. The results suggest existence of a highly anomalous state\nsandwiched between the mixed-valent Fermi liquid and charge ordered\nMott-insulating phase in Ti4O7.", "category": "cond-mat_str-el" }, { "text": "Emergence of Jack ground states from two-body pseudopotentials in\n fractional quantum Hall systems: The family of \"Jack states\" related to antisymmetric Jack polynomials are the\nexact zero-energy ground states of particular model short-range {\\em many-body}\nrepulsive interactions, defined by a few non-vanishing leading\npseudopotentials. Some Jack states are known or anticipated to accurately\ndescribe many-electron incompressible ground states emergent from the {\\em\ntwo-body} Coulomb repulsion in fractional quantum Hall effect. By extensive\nnumerical diagonalization we demonstrate emergence of Jack states from suitable\npair interactions. We find empirically a simple formula for the optimal\ntwo-body pseudopotentials for the series of most prominent Jack states\ngenerated by {\\em contact} many-body repulsion. Furthermore, we seek\nrealization of arbitrary Jack states in realistic quantum Hall systems with\nCoulomb interaction, i.e., in partially filled lowest and excited Landau levels\nin quasi-two-dimensional layers of conventional semiconductors like GaAs or in\ngraphene.", "category": "cond-mat_str-el" }, { "text": "Emergent Dipole Gauge Fields and Fractons: We present a realization of fracton-elasticity duality purely formulated in\nterms of ordinary gauge fields, encompassing standard elasticity and\nincommensurate crystals as those describing twisted bilayer graphene,\nquasicrystals or more general moir\\'e lattices. Our construction comprises a\ndescription of all types of two-dimensional defects: disclinations,\ndislocations, discompressions and point-like defects, and takes into account\nbody forces and impurities. The original form of the duality in terms of tensor\ngauge fields is recovered after partial gauge fixing. We identify the coupling\nof each type of defect to the dual gauge fields, and from gauge invariance we\nderive generalized continuity equations for the defect currents and the\nexpected mobility restrictions of elasticity defects.", "category": "cond-mat_str-el" }, { "text": "Interactions in Quasicrystals: Although the effects of interactions in solid state systems still remains a\nwidely open subject, some limiting cases such as the three dimensional Fermi\nliquid or the one-dimensional Luttinger liquid are by now well understood when\none is dealing with interacting electrons in {\\it periodic} crystalline\nstructures. This problem is much more fascinating when periodicity is lacking\nas it is the case in {\\it quasicrystalline} structures. Here, we discuss the\ninfluence of the interactions in quasicrystals and show, on a controlled\none-dimensional model, that they lead to anomalous transport properties,\nintermediate between those of an interacting electron gas in a periodic and in\na disordered potential.", "category": "cond-mat_str-el" }, { "text": "Holographic Dynamics from Multiscale Entanglement Renormalization Ansatz: The Multiscale Entanglement Renormalization Ansatz (MERA) is a tensor network\nbased variational ansatz that is capable of capturing many of the key physical\nproperties of strongly correlated ground states such as criticality and\ntopological order. MERA also shares many deep relationships with the AdS/CFT\n(gauge-gravity) correspondence by realizing a UV complete holographic duality\nwithin the tensor networks framework. Motivated by this, we have re-purposed\nthe MERA tensor network as an analysis tool to study the real-time evolution of\nthe 1D transverse Ising model in its low energy excited state sector. We\nperformed this analysis by allowing the ancilla qubits of the MERA tensor\nnetwork to acquire quantum fluctuations, which yields a unitary transform\nbetween the physical (boundary) and ancilla qubit (bulk) Hilbert spaces. This\nthen defines a reversible quantum circuit which is used as a `holographic\ntransform' to study excited states and their real-time dynamics from the point\nof the bulk ancillae. In the gapped paramagnetic phase of the transverse field\nIsing model, we demonstrate the holographic duality between excited states\ninduced by single spin-flips (Ising `magnons') acting on the ground state and\nsingle ancilla qubit spin-flips. The single ancillae qubit excitation is shown\nto be stable in the bulk under real-time evolution and hence defines a stable\nholographic quasiparticle which we have named the `hologron'. The `dictionary'\nbetween the bulk and boundary is determined and realizes many features of the\nholographic correspondence in a non-CFT limit of the boundary theory. As an\nadded spin-off, this dictionary together with the extension to multi-hologron\nsectors gives us a systematic way to construct quantitatively accurate low\nenergy effective Hamiltonians.", "category": "cond-mat_str-el" }, { "text": "Enhancement of superconductivity by pressure-induced critical\n ferromagnetic fluctuations in UCoGe: A $^{59}$Co nuclear quadrupole resonance (NQR) was performed on a\nsingle-crystalline ferromagnetic (FM) superconductor UCoGe under pressure. The\nFM phase vanished at a critical pressure $P_c$, and the NQR spectrum just below\n$P_c$ showed phase separation of the FM and paramagnetic (PM) phases below\nCurie temperature $T_{\\textrm{Curie}}$, suggesting first-order FM quantum phase\ntransition (QPT). We found that the internal field was absent above $P_c$, but\nthe superconductivity is almost unchanged. This result suggests the existence\nof the nonunitary to unitary transition of the superconductivity around $P_c$.\nNuclear spin-lattice relaxation rate $1/T_1$ showed the FM critical\nfluctuations around $P_c$, which persist above $P_c$ and are clearly related to\nsuperconductivity in the PM phase. This FM QPT is understood to be a weak first\norder with critical fluctuations. $1/T_1$ sharply decreased in the\nsuperconducting (SC) state above $P_c$ with a single component, in contrast to\nthe two-component $1/T_1$ in the FM SC state, indicating that the inhomogeneous\nSC state is a characteristic feature of the FM SC state in UCoGe.", "category": "cond-mat_str-el" }, { "text": "Spin Coulomb Drag: We introduce a distinctive feature of spin-polarized transport, the Spin\nCoulomb Drag: there is an intrinsic source of friction for spin currents due to\nthe Coulomb interaction between spin ``up'' and spin ``down'' electrons. We\ncalculate the associated ``spin transrestistivity'' in a generalized random\nphase approximation and discuss its dependence on temperature, frequency, and\nelectron density. We show that, in an appropriate range of parameters, such\nresistivity is measurable and propose an experiment to measure it.", "category": "cond-mat_str-el" }, { "text": "Dynamical Mean Field Theory, Density-Matrix Embedding Theory and\n Rotationally Invariant Slave Bosons: a Unified Perspective: We present a unified perspective on Dynamical Mean Field Theory (DMFT),\nDensity-Matrix Embedding Theory (DMET) and Rotationally Invariant Slave Bosons\n(RISB). We show that DMET can be regarded as a simplification of the RISB\nmethod where the quasiparticle weight is set to unity. This relation allows to\neasily transpose extensions of a given method to another: for instance, a\ntemperature-dependent version of RISB can be used to derive a\ntemperature-dependent free-energy formula for DMET.", "category": "cond-mat_str-el" }, { "text": "Interaction-driven Spontaneous Ferromagnetic Insulating States with Odd\n Chern Numbers: Motivated by recent experimental work on moir\\'e systems in a strong magnetic\nfield, we compute the compressibility as well as the spin correlations and\nHofstadter spectrum of spinful electrons on a honeycomb lattice with Hubbard\ninteractions using the determinantal quantum Monte Carlo method. While the\ninteractions in general preserve quantum and anomalous Hall states, emergent\nfeatures arise corresponding to an antiferromagnetic insulator at half-filling\nand other incompressible states following the Chern sequence $\\pm (2N+1)$.\nThese odd integer Chern states exhibit strong ferromagnetic correlations and\narise spontaneously without any external mechanism for breaking the\nspin-rotation symmetry. Analogs of these magnetic states should be observable\nin general interacting quantum Hall systems. In addition, the interacting\nHofstadter spectrum is qualitatively similar to the experimental data at\nintermediate values of the on-site interaction.", "category": "cond-mat_str-el" }, { "text": "A Quantum Monte Carlo algorithm for out-of-equilibrium Green's functions\n at long times: We present a quantum Monte-Carlo algorithm for computing the perturbative\nexpansion in power of the coupling constant $U$ of the out-of-equilibrium\nGreen's functions of interacting Hamiltonians of fermions. The algorithm\nextends the one presented in Phys. Rev. B 91 245154 (2015), and inherits its\nmain property: it can reach the infinite time (steady state) limit since the\ncomputational cost to compute order $U^n$ is uniform versus time; the computing\ntime increases as $2^n$. The algorithm is based on the Schwinger-Keldysh\nformalism and can be used for both equilibrium and out-of-equilibrium\ncalculations. It is stable at both small and long real times including in the\nstationary regime, because of its automatic cancellation of the disconnected\nFeynman diagrams. We apply this technique to the Anderson quantum impurity\nmodel in the quantum dot geometry to obtain the Green's function and\nself-energy expansion up to order $U^{10}$ at very low temperature. We\nbenchmark our results at weak and intermediate coupling with high precision\nNumerical Renormalization Group (NRG) computations as well as analytical\nresults.", "category": "cond-mat_str-el" }, { "text": "Kondo destruction in a quantum paramagnet with magnetic frustration: We report results of isothermal magnetotransport and susceptibility\nmeasurements at elevated magnetic fields B down to very low temperatures T on\nhigh-quality single crystals of the frustrated Kondo-lattice system CePdAl.\nThey reveal a B*(T) line within the paramagnetic part of the phase diagram.\nThis line denotes a thermally broadened 'small'-to-'large' Fermi surface\ncrossover which substantially narrows upon cooling. At B_0* = B*(T=0) = (4.6\n+/- 0.1) T, this B*(T) line merges with two other crossover lines, viz. Tp(B)\nbelow and T_FL(B) above B_0*. Tp characterizes a frustration-dominated\nspin-liquid state, while T_FL is the Fermi-liquid temperature associated with\nthe lattice Kondo effect. Non-Fermi-liquid phenomena which are commonly\nobserved near a 'Kondo destruction' quantum critical point cannot be resolved\nin CePdAl. Our observations reveal a rare case where Kondo coupling,\nfrustration and quantum criticality are closely intertwined.", "category": "cond-mat_str-el" }, { "text": "Spin-rotationally symmetric domain flux phases in underdoped cuprates: We propose a new form of inhomogeneous phases consisting of out-of-phase\nstaggered flux domains separated by diagonal charged domain walls centered on\nbonds or on sites. Remarkably, such domain flux phases are spin-rotationally\nsymmetric and exhibit cone-like quasiparticle dispersion as well as\nincommensurate order of orbital currents. Such features are consistent with the\npseudogap behavior and the diagonal stripes observed experimentally in lightly\ndoped cuprates. A renormalized mean field theory shows that such solutions are\ncompetitive candidates within the $t$--$J$ model.", "category": "cond-mat_str-el" }, { "text": "Spectral form factors of clean and random quantum Ising chains: We compute the spectral form factor of two integrable quantum-critical many\nbody systems in one spatial dimension. The spectral form factor of the quantum\nIsing chain is periodic in time in the scaling limit described by a conformal\nfield theory; we also compute corrections from lattice effects and deviation\nfrom criticality. Criticality in the random Ising chain is described by rare\nregions associated with a strong randomness fixed point, and these control the\nlong time limit of the spectral form factor.", "category": "cond-mat_str-el" }, { "text": "Transport Properties of the One Dimensional Ferromagnetic Kondo Lattice\n Model : A Qualitative Approach to Oxide Manganites: The transport properties of the ferromagnetic Kondo lattice model in one\ndimension are studied via bosonization methods. The antiferromagnetic\nfluctuations, which normally appear because of the RKKY interactions, are\nexplicitly taken into account as a direct exchange between the ``core'' spins.\nIt is shown that in the paramagnetic regime with the local antiferromagnetic\nfluctuations, the resistivity decays exponentially as the temperature increases\nwhile in the ferromagnetic regime the system is an almost perfect conductor. %A\nnon-perturbative description of localized spin polarons %in the paramagnetic\nregion is obtained.\n The effect of a weak applied field is discussed to be reduced to the case of\nthe ferromagnetic state leading to band splitting. The qualitative relevance of\nthe results for the problem of the Oxide Manganites is emphasized.", "category": "cond-mat_str-el" }, { "text": "Magnetic Phase Diagram of Spin-1/2 Two-Leg Ladder with Four-Spin Ring\n Exchange: We study the spin-1/2 two-leg Heisenberg ladder with four-spin ring exchanges\nunder a magnetic field. We introduce an exact duality transformation which is\nan extension of the spin-chirality duality developed previously and yields a\nnew self-dual surface in the parameter space. We then determine the magnetic\nphase diagram using the numerical approaches of the density-matrix\nrenormalization-group and exact diagonalization methods. We demonstrate the\nappearance of a magnetization plateau and the Tomonaga-Luttinger liquid with\ndominant vector-chirality quasi-long-range order for a wide parameter regime of\nstrong ring exchange. A \"nematic\" phase, in which magnons form bound pairs and\nthe magnon-pairing correlation functions dominate, is also identified.", "category": "cond-mat_str-el" }, { "text": "Spin-Dependent Correlations of Fermi Liquids at Nonzero Temperatures\n within Correlated Density-Matrix Approach: Correlated density matrix theory is generalized to investigate equilibrium\nproperties of normal Fermi Liquids such as 3He and nuclear matter at nonzero\ntemperatures. The results also generalize the Fermi-hypernetted-chain technique\nthat is familiar from studies of the ground state of correlated fermions. By\nemploying the concept of renormalized bosons and fermions the formal results\nare cast in a form that permits the direct evaluation of the statistical\nproperties of the correlated liquid such as the entropy and the specific heat\nat constant volume among other quantities.", "category": "cond-mat_str-el" }, { "text": "Symmetric Fracton Matter: Twisted and Enriched: In this paper, we explore the interplay between symmetry and fracton order,\nmotivated by the analogous close relationship for topologically ordered\nsystems. Specifically, we consider models with 3D planar subsystem symmetry,\nand show that these can realize subsystem symmetry protected topological phases\nwith gapless boundary modes. Gauging the planar subsystem symmetry leads to a\nfracton order in which particles restricted to move along lines exhibit a new\ntype of statistical interaction that is specific to the lattice geometry. We\nshow that both the gapless boundary modes of the ungauged theory, and the\nstatistical interactions after gauging, are naturally captured by a higher-rank\nversion of Chern-Simons theory. We also show that gauging only part of the\nsubsystem symmetry can lead to symmetry-enriched fracton orders, with\nquasiparticles carrying fractional symmetry charge.", "category": "cond-mat_str-el" }, { "text": "Observation of Incompressibility at $\u03bd=4/11$ and $\u03bd=5/13$: The region of filling factors $1/3<\\nu<2/5$ is predicted to support new types\nof fractional quantum Hall states with topological order different from that of\nthe Laughlin-Jain or the Moore-Read states. Incompressibility is a necessary\ncondition for the formation of such novel topological states. We find that at\n6.9~mK incompressibility develops only at $\\nu=4/11$ and $5/13$, while the\nstates at $\\nu=6/17$ and $3/8$ remain compressible. Our observations at\n$\\nu=4/11$ and $5/13$ are first steps towards understanding emergent\ntopological order in these fractional quantum Hall states.", "category": "cond-mat_str-el" }, { "text": "Electronic structure of the $Sr_{0.4}Ca_{13.6}Cu_{24}O_{41}$\n incommensurate compound: We extracted, from strongly-correlated ab-initio calculations, a complete\nmodel for the chain subsystem of the $Sr_{0.4}Ca_{13.6}Cu_{24}O_{41}$\nincommensurate compound. A second neighbor $t-J+V$ model has been determined as\na function of the fourth crystallographic parameter $\\tau$, for both low and\nroom temperature crystallographic structures. The analysis of the obtained\nmodel shows the crucial importance of the structural modulations on the\nelectronic structure through the on-site energies and the magnetic\ninteractions. The structural distortions are characterized by their long range\neffect on the cited parameters that hinder the reliability of analyses such as\nBVS. One of the most striking results is the existence of antiferromagnetic\nnearest-neighbor interactions for metal-ligand-metal angles of $90^\\circ$. A\ndetailed analysis of the electron localization and spin arrangement is\npresented as a function of the chain to ladder hole transfer and of the\ntemperature. The obtained spin arrangement is in agreement with\nantiferromagnetic correlations in the chain direction at low temperature.", "category": "cond-mat_str-el" }, { "text": "TU$^2$FRG -- a scalable approach for truncated unity functional\n renormalization group in generic fermionic models: Describing the emergence of phases of condensed matter is one of the central\nchallenges in physics. For this purpose many numerical and analytical methods\nhave been developed, each with their own strengths and limitations. The\nfunctional renormalization group is one of these methods bridging between\nefficiency and accuracy. In this paper we derive a new truncated unity (TU)\napproach unifying real- and momentum space TU, called TU$^2$FRG. This formalism\nsignificantly improves the scaling compared to conventional momentum (TU)FRG\nwhen applied to large unit-cell models and models where the translational\nsymmetry is broken.", "category": "cond-mat_str-el" }, { "text": "Fermionology in the Kondo-Heisenberg model: the case of CeCoIn$_{5}$: Fermi surface of heavy electron systems plays a fundamental role in\nunderstanding their variety of puzzling phenomena, for example, quantum\ncriticality, strange metal behavior, unconventional superconductivity and even\nenigmatic phases with yet unknown order parameters. The spectroscopy\nmeasurement of typical heavy fermion superconductor CeCoIn$_{5}$ has\ndemonstrated multi-Fermi surface structure, which has not been in detail\nstudied theoretically in a model system like the Kondo-Heisenberg model. In\nthis work, we make a step toward such an issue with revisiting the\nKondo-Heisenberg model. It is surprising to find that the usual self-consistent\ncalculation cannot reproduced the fermionology of the experimental observation\nof the system due to the unfounded sign binding between the hopping of the\nconduction electrons and the mean-field valence-bond order. To overcome such\ninconsistency, we assume that the sign binding should be relaxed and the\nmean-field valence-bond order can be considered as a free/fit parameter so as\nto meet with real-life experiments. Given the fermionology, the calculated\neffective mass enhancement, entropy, superfluid density and Knight shift are\nall in qualitative agreement with the experimental results of CeCoIn$_{5}$,\nwhich confirms our assumption. Our result supports a $d_{x^{2}-y^{2}}$-wave\npairing structure in heavy fermion material CeCoIn$_{5}$. In addition, we have\nalso provided the scanning tunneling microscopy (STM) spectra of the system,\nwhich is able to be tested by the present STM experiments.", "category": "cond-mat_str-el" }, { "text": "Interplay of Fractional Chern Insulator and Charge-Density-Wave Phases\n in Twisted Bilayer Graphene: We perform an extensive exact diagonalization study of interaction driven\ninsulators in spin- and valley-polarized moir\\'{e} flat bands of twisted\nbilayer graphene aligned with its hexagonal boron nitride substrate. In\naddition to previously reported fractional Chern insulator phases, we provide\ncompelling evidence for competing charge-density-wave phases at multiple\nfractional fillings of a realistic single-band model. A thorough analysis at\ndifferent interlayer hopping parameters, motivated by experimental variability,\nand the role of kinetic energy at various Coulomb interaction strengths\nhighlight the competition between these phases. The interplay of the\nsingle-particle and the interaction induced hole dispersion with the inherent\nBerry curvature of the Chern bands is intuitively understood to be the driving\nmechanism for the ground-state selection. The resulting phase diagram features\nremarkable agreement with experimental findings in a related moir\\'{e}\nheterostructure and affirms the relevance of our results beyond the scope of\ngraphene based materials.", "category": "cond-mat_str-el" }, { "text": "Anderson localization of spinons in a spin-1/2 antiferromagnetic\n Heisenberg chain: Anderson localization is a general phenomenon of wave physics, which stems\nfrom the interference between multiple scattering paths1,2. It was originally\nproposed for electrons in a crystal, but later was also observed for light3-5,\nmicrowaves6, ultrasound7,8, and ultracold atoms9-12. Actually, in a crystal,\nbesides electrons there may exist other quasiparticles such as magnons and\nspinons. However the search for Anderson localization of these magnetic\nexcitations is rare so far. Here we report the first observation of spinon\nlocalization in copper benzoate, an ideal compound of spin-1/2\nantiferromagnetic Heisenberg chain, by ultra-low-temperature specific heat and\nthermal conductivity measurements. We find that while the spinon specific heat\nCs displays linear temperature dependence down to 50 mK, the spinons thermal\nconductivity ks only manifests the linear temperature dependence down to 300\nmK. Below 300 mK, ks/T decreases rapidly and vanishes at about 100 mK, which is\na clear evidence for Anderson localization. Our finding opens a new window for\nstudying such a fundamental phenomenon in condensed matter physics.", "category": "cond-mat_str-el" }, { "text": "Structural Transitions in a Classical Two-Dimensional Molecule System: The ground state of a classical two-dimensional (2D) system with finite\nnumber of charged particles, trapped by two positive impurities charges\nlocalized at a distance (zo) from the 2D plane and separated from each other by\na distance xp are obtained. The impurities are allowed to carry more than one\npositive charge. This classical system can form a 2D-like classical molecule\nthat exhibits structural transitions and spontaneous symmetry breaking as a\nfunction of the separation between the positive charges before it transforms\ninto two independent 2D-like classical atoms. We also observe structural\ntransitions as a function of the dielectric constant of the substrate which\nsupports the charged particles, in addition to broken symmetry states and\nunbinding of particles.", "category": "cond-mat_str-el" }, { "text": "Crystallization in the Fractional Quantum Hall Regime Induced by\n Landau-level Mixing: The interplay between strongly correlated liquid and crystal phases for\ntwo-dimensional electrons exposed to a high transverse magnetic field is of\nfundamental interest. Through the non-perturbative fixed phase diffusion Monte\nCarlo method, we determine the phase diagram of the Wigner crystal in the\n$\\nu-\\kappa$ plane, where $\\nu$ is the filling factor and $\\kappa$ is the\nstrength of Landau level mixing. The phase boundary is seen to exhibit a\nstriking $\\nu$ dependence, with the states away from the magic filling factors\n$\\nu=n/(2pn+1)$ being much more susceptible to crystallization due to Landau\nlevel mixing than those at $\\nu=n/(2pn+1)$. Our results explain the qualitative\ndifference between the experimental behaviors observed in n-doped and p-doped\nGaAs quantum wells, and, in particular, the existence of an insulating state\nfor $\\nu<1/3$ and also for $1/3 <\\nu< 2/5$ in low density p-doped systems. We\npredict that in the vicinity of $\\nu=1/5$ and $\\nu=2/9$, increasing LL mixing\ncauses a transition not into an ordinary electron Wigner crystal but rather\ninto a strongly correlated crystal of composite fermions carrying two vortices.", "category": "cond-mat_str-el" }, { "text": "Ab-initio determination of the localized/delocalized f-manifold in\n UPd_2Al_3: The electronic structure of UPd_2Al_3 is described using the self-interaction\ncorrected local-spin-density approximation to density functional theory. The\ngroundstate is found to be characterized by the coexistence of localized (f^2)\nand delocalized U f electrons, in agreement with experimental evidence. We\nobserve significant difference in electronic structure between UPd_2Al_3 and\nthe previously studied UPt_3 compound. Even though a trend towards localization\nexists in UPt_3, the total energies and the density of states at the Fermi\nlevel favor a groundstate with localized f^1, rather than f^2 U ions.", "category": "cond-mat_str-el" }, { "text": "Temperature dependent bilayer ferromagnetism in Sr3Ru2O7: The Ruthenium based perovskites exhibit a wide variety of interesting\ncollective phenomena related to magnetism originating from the Ru 4d electrons.\nMuch remains unknown concerning the nature of magnetic fluctuations and\nexcitations in these systems. We present results of detailed inelastic neutron\nscattering measurements of Sr3Ru2O7 as a function of temperature, probing the\nferromagnetic fluctuations of the bilayer structure. A magnetic response is\nclearly visible for a range of temperatures, T = 3.8 K up to T = 100 K, and for\nenergy transfers between 2 and 14 meV. These measurements indicate that the\nferromagnetic fluctuations manifest in the bilayer structure factor persist to\nsurprisingly large temperatures. This behavior may be related to the proximity\nof the system in zero magnetic field to the metamagnetic/ferromagnetic\ntransition.", "category": "cond-mat_str-el" }, { "text": "Search for a quantum phase transition in U(Pt_(1-x)Pd_x)_3: Pd in U(Pt_{1-x}Pd_x)_3 suppresses the superconducting T_c to 0 K at critical\nconcentration x_c of 0.007 and induces a conventional AFM state for x > x_c.\nThe resistivity below 1 K shows a deviation from Fermi liquid behavior\ndescribed by a power law where the exponent ranges from 2 at x=0 to 1.6 for x =\nx_c. This suggests that a quantum phase transition (QPT) may exist near x_c\nassociated with either the magnetic or superconducting transition temperature =\n0 K. Transport for a sample with x = 0.004 < x_c has constant exponent of 1.77\nas increasing pressure suppresses T_c to 0 K, suggesting that if a QPT exists\nit may be associated with the magnetic transition.", "category": "cond-mat_str-el" }, { "text": "Ground States of the Ising Model on the Shastry-Sutherland Lattice and\n the Origin of the Fractional Magnetization Plateaus in Rare-Earth\n Tetraborides: A complete and exact solution of the ground-state problem for the Ising model\non the Shastry-Sutherland lattice in the applied magnetic field is found. The\nmagnetization plateau at the one third of the saturation value is shown to be\nthe only possible fractional plateau in this model. However, stripe magnetic\nstructures with magnetization 1/2 and $1/n$ ($n > 3$), observed in the\nrare-earth tetraborides RB$_4$, occur at the boundaries of the\nthree-dimensional regions of the ground-state phase diagram. These structures\ngive rise to new magnetization plateaus if interactions of longer ranges are\ntaken into account. For instance, an additional third-neighbor interaction is\nshown to produce a 1/2 plateau. The results obtained significantly refine the\nunderstanding of the magnetization process in RB$_4$ compounds, especially in\nTmB$_4$ and ErB$_4$ which are strong Ising magnets.", "category": "cond-mat_str-el" }, { "text": "Nature of the glassy magnetic state in Cu$_{2.84}$Mn$_{0.44}$Al$_{0.72}$\n shape memory alloy: The magnetic ground state of the ferromagnetic shape memory alloy of nominal\ncomposition Cu$_{2.84}$Mn$_{0.44}$Al$_{0.72}$ was investigated. The sample\nshows reentry of a glassy magnetic phase below the martensitic transition\ntemperature, which is found to have complex character with two distinct\nanomalies in the temperature dependent ac susceptibility data. The sample\nretains its glassy phase even below the second transition as evident from the\nmagnetic memory measurements in different protocols. Existence of two\ntransitions along with their observed nature suggest that the system can be\ndescribed by the mean field Heisenberg model of reentrant spin glass as\nproposed by Gabay and Toulous. \\cite{rsg-GT1} The sample provides a fascinating\nexample where a Gabay-Toulous type spin glass state is triggered by a first\norder magneto-structural transition.", "category": "cond-mat_str-el" }, { "text": "High-Temperature Criticality in Strongly Constrained Quantum Systems: The exotic nature of many strongly correlated materials at reasonably high\ntemperatures, for instance cuprate superconductors in their normal state, has\nlead to the suggestion that such behavior occurs within a quantum critical\nregion where the physics is controlled by the influence of a phase transition\ndown at zero temperature. Such a scenario can be thought of as a bottom-up\napproach, with the zero temperature mechanisms finding a way to manifest\ncritical behavior at high temperatures. Here we propose an alternative,\ntop-down, mechanism by which strong kinematic constraints that can only be\nbroken at extremely high temperatures are responsible for critical behavior at\nintermediate but still high temperatures. This critical behavior may extend all\nthe way down to zero temperature, but this outcome is not one of necessity, and\nthe system may order at low temperatures. We provide explicit examples of such\nhigh-temperature criticality when additional strong interactions are introduced\nin quantum Heisenberg, transverse field Ising, and some bosonic lattice models.", "category": "cond-mat_str-el" }, { "text": "Analytic continuation of the self-energy via Machine Learning techniques: We develop a novel analytic continuation method for self-energies on the\nMatsubara domain as computed by quantum Monte Carlo simulations within\ndynamical mean field theory (QMC+DMFT). Unlike a maximum entropy (maxEn)\nprocedure employed for the last thirty years, our approach is based on a\nmachine learning (ML) technique in combination with the iterative perturbative\ntheory impurity solver of the dynamical mean field theory self-consistent\nprocess (IPT+DMFT). The input and output training datasets for ML are\nsimultaneously obtained from IPT+DMFT calculations on Matsubara and real\nfrequency domains, respectively. The QMC+DMFT self-energy on real frequencies\nis determined from the -- usually noisy -- input QMC+DMFT self-energy on the\nMatsubara domain and the trained ML kernel. Our approach is free from both,\nbias of ML training datasets and from fitting parameters present in the maxEn\nmethod. We demonstrate the efficiency of the method on the testbed frustrated\nHubbard model on the square lattice.", "category": "cond-mat_str-el" }, { "text": "Probing light-driven quantum materials with ultrafast resonant inelastic\n X-ray scattering: Ultrafast optical pulses are an increasingly important tool for controlling\nquantum materials and triggering novel photo-induced phase transitions.\nUnderstanding these dynamic phenomena requires a probe sensitive to spin,\ncharge, and orbital degrees of freedom. Time-resolved resonant inelastic X-ray\nscattering (trRIXS) is an emerging spectroscopic method, which responds to this\nneed by providing unprecedented access to the finite-momentum fluctuation\nspectrum of photoexcited solids. In this Perspective, we briefly review\nstate-of-the-art trRIXS experiments on condensed matter systems, as well as\nrecent theoretical advances. We then describe future research opportunities in\nthe context of light control of quantum matter.", "category": "cond-mat_str-el" }, { "text": "Commensurate and Incommensurate Structure of the Neutron Cross Section\n in LaSrCuO and YBaCuO: We study the evolution of the d-wave neutron cross-section with variable\nfrequency \\omega and fixed T (below and above Tc) in two different cuprate\nfamilies. The evolution from incommensurate to commensurate to incommensurate\npeaks is rather generic within an RPA-like scheme. This behavior seems to be in\nreasonable accord with experiments, and may help distinguish between this and\nthe \"stripe\" scenario.", "category": "cond-mat_str-el" }, { "text": "Phase control of magnons in the van der Waals antiferromagnet NiPS$_3$: We demonstrate phase control of magnons in the van der Waals antiferromagnet\nNiPS$_3$ using optical excitation by polarized light. The sign of the coherent\nprecession of spin amplitude changes upon (1) reversing the helicity of a\ncircularly polarized pump beam, or (2) rotating the polarization of a linearly\npolarized pump by $\\pi/2$. Because these two excitation pathways have\ncomparable generation efficiency, the phase of spin precession can be\ncontinuously tuned from 0 to $2\\pi$ by controlling the polarization state of\nthe pump pulse. The ability to excite magnons with a desired phase has\npotential applications in the design of a spin-wave phased array and ultrafast\nspin information processing.", "category": "cond-mat_str-el" }, { "text": "Optimizing configurations for determining the magnetic model based on\n ab-initio calculations: In this paper, it is presented a novel strategy to optimize the determination\nof magnetic couplings by using ab-initio calculations of the energy. This\napproach allows determining efficiently, in terms of a proposed effective\nmagnetic spin model, an optimal set of magnetic configurations to be simulated\nby DFT methods. Moreover, a procedure to estimate the values of the coupling\nconstants and their error bounds from the estimated energies is proposed. This\nmethod, based on Monte Carlo sampling, takes into account the accuracy of the\nab - initio simulations. A strategy to refine models reusing previously\ncomputed configuration energies is also presented. We apply the method to\ndetermine a magnetic model for the recently synthesized material\nBi$_3$Mn$_4$O$_{12}$(NO$_3$). Finally, an open source software that implements\nand automatizes the whole process is presented.", "category": "cond-mat_str-el" }, { "text": "Order by disorder in classical kagome antiferromagnets with chiral\n interactions: The Heisenberg antiferromagnet on the kagome lattice is an archetypal\ninstance of how large ground state degeneracies arise, and how they may get\nresolved by thermal and quantum fluctuations. Augmenting the Heisenberg model\nby chiral spin interactions has proved to be of particular interest in the\ndiscovery of certain chiral quantum spin liquids. Here we consider the\nclassical variant of this chiral kagome model and find that it exhibits,\nsimilar to the classical Heisenberg antiferromagnet, a remarkably large and\nstructured ground-state manifold, which combines continuous and discrete\ndegrees of freedom. This allows for a rich set of order-by-disorder phenomena.\nDegeneracy lifting occurs in a highly selective way, choosing already at the\nharmonic level specific triaxial states which however retain an emergent $Z_2$\ndegree of freedom (absent in the conventional Heisenberg model). We also study\nthe competition of entropic and energetic ground state selection as the model\ninterpolates between the purely chiral and Heisenberg cases. For this mixed\nmodel, we find a \"proximate ordered-by-disorder\" finite-temperature regime\nwhere fluctuations overcome the energetic ground state preference of the\nperturbation. Finally, a semiclassical route to a spin liquid is provided by\nquantum order by disorder in the purely chiral models, where the aforementioned\n$Z_2$ degrees of freedom are elevated to the role of an emergent gauge field.", "category": "cond-mat_str-el" }, { "text": "The structure of spinful quantum Hall states: a squeezing perspective: We provide a set of rules to define several spinful quantum Hall model\nstates. The method extends the one known for spin polarized states. It is\nachieved by specifying an undressed root partition, a squeezing procedure and\nrules to dress the configurations with spin. It applies to both the\nexcitation-less state and the quasihole states. In particular, we show that the\nnaive generalization where one preserves the spin information during the\nsqueezing sequence, may fail. We give numerous examples such as the Halperin\nstates, the non-abelian spin-singlet states or the spin-charge separated\nstates. The squeezing procedure for the series (k=2,r) of spinless quantum Hall\nstates, which vanish as r powers when k+1 particles coincide, is generalized to\nthe spinful case. As an application of our method, we show that the counting\nobserved in the particle entanglement spectrum of several spinful states\nmatches the one obtained through the root partitions and our rules. This\ncounting also matches the counting of quasihole states of the corresponding\nmodel Hamiltonians, when the latter is available.", "category": "cond-mat_str-el" }, { "text": "Electronic correlations in organometallic complexes: We investigate an effective model for organometallic complexes (with\npotential uses in optoelectronic devices) via both exact diagonalisation and\nthe configuration interaction singles (CIS) approximation. This model captures\na number of important features of organometallic complexes, notably the\nsensitivity of the radiative decay rate to small chemical changes. We find that\nfor large parameter ranges the CIS approximation accurately reproduces the low\nenergy excitations and hence the photophysical properties of the exact\nsolution. This suggests that electronic correlations do \\emph{not} play an\nimportant role in these complexes. This explains why time-dependent density\nfunctional theory works surprisingly well in these complexes.", "category": "cond-mat_str-el" }, { "text": "Non-Hermitian Mott Skin Effect: We propose a novel type of skin effects in non-Hermitian quantum many-body\nsystems which we dub a non-Hermitian Mott skin effect. This phenomenon is\ninduced by the interplay between strong correlations and the non-Hermitian\npoint-gap topology. The Mott skin effect induces extreme sensitivity to the\nboundary conditions only in the spin degree of freedom (i.e., the charge\ndistribution is not sensitive to boundary conditions), which is in sharp\ncontrast to the ordinary non-Hermitian skin effect in non-interacting systems.\nConcretely, we elucidate that a bosonic non-Hermitian chain exhibits the Mott\nskin effect in the strongly correlated regime by closely examining an effective\nHamiltonian. The emergence of the Mott skin effect is also supported by\nnumerical diagonalization of the bosonic chain. The difference between the\nordinary non-Hermitian skin effect and the Mott skin effect is also reflected\nin the time-evolution of physical quantities; under the time-evolution spin\naccumulation is observed while the charge distribution remains spatially\nuniform.", "category": "cond-mat_str-el" }, { "text": "63,65Cu Nuclear Resonance Study of the Coupled Spin Dimers and Chains\n Compound Cu2Fe2Ge4O13: Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) of Cu\nhave been measured in a coupled spin dimers and chains compound Cu2Fe2Ge4O13.\nCu NQR has also been measured in an isostructural material Cu2Sc2Ge4O13\nincluding only spin dimers. Comparison of the temperature dependence of the\n63Cu nuclear spin-lattice relaxation rate between the two compounds reveals\nthat the Fe chains in Cu2Fe2Ge4O13 do not change a spin gap energy of the Cu\ndimers from that in Cu2Sc2Ge4O13, contributing additionally to the relaxation\nrate at the Cu site. A modestly large internal field of 3.39 T was observed at\nthe Cu site in the antiferromagnetic state of Cu2Fe2Ge4O13 at 4.2 K, which is\npartly because of quantum reduction of the ordered moment of a Cu atom. The\ninternal field and the ordered moment of Cu are noncollinear due to large\nanisotropy of the hyperfine interaction at the Cu site. A model analysis of the\ninternal field based on the fourfold planar coordination of Cu suggests that a\n3d hole of the Cu2+ ion is mainly in the d(x2-y2) orbital state.", "category": "cond-mat_str-el" }, { "text": "Arrested Kondo effect and hidden order in URu_2Si_2: Complex electronic matter exhibit subtle forms of self organization which are\nalmost invisible to the available experimental tools, but which have dramatic\nphysical consequences. One prominent example is provided by the actinide based\nheavy fermion material URu_2Si_2. At high temperature, the U-5f electrons in\nURu_2Si_2 carry a very large entropy. This entropy is released at 17.5K via a\nsecond order phase transition to a state which remains shrouded in mystery, and\nwhich was termed a \"hidden order\" state. Here we develop a first principles\ntheoretical method to analyze the electronic spectrum of correlated materials\nas a function of the position inside the unit cell of the crystal, and use it\nto identify the low energy excitations of the URu_2Si_2. We identify the order\nparameter of the hidden order state, and show that it is intimately connected\nwith magnetism. We present first principles results for the temperature\nevolution of the electronic states of the material. At temperature below 70K\nU-5f electrons undergo a multichannel Kondo effect, which is arrested at low\ntemperature by the crystal field splitting. At lower temperatures, two broken\nsymmetry states emerge, characterized by a complex order parameter \\psi. A real\n$\\psi$ describes the hidden order phase, and an imaginary \\psi corresponds to\nthe large moment antiferromagnetic phase, thus providing a unified picture of\nthe two broken symmetry phases, which are realized in this material.", "category": "cond-mat_str-el" }, { "text": "Valence instability across magnetostructural transition in USb$_2$: We have performed pressure dependent X-ray diffraction and resonant X-ray\nemission spectroscopy experiments on USb$_2$ to further characterize the AFM-FM\ntransition occurring near 8 GPa. We have found the magnetic transition\ncoincides with a tetragonal to orthorhombic transition resulting in a 17%\nvolume collapse as well as a transient $\\textit{f}$-occupation enhancement.\nCompared to UAs$_2$ and UAsS, USb$_2$ shows a reduced bulk modulus and\ntransition pressure and an increased volume collapse at the structural\ntransition. Except for an enhancement across the transition region, the\n$\\textit{f}$-occupancy decreases steadily from 1.96 to 1.75.", "category": "cond-mat_str-el" }, { "text": "Revisiting the electronic properties of dislocated graphene sheets: The interplay between topological defects, such as dislocations or\ndisclinations, and the electronic degrees of freedom in graphene has been\nextensively studied. In the literature, for the study of this kind of problems,\nit is in general used either a gauge theory or a curved spatial Riemannian\ngeometry approach, where, in the geometric case, the information about the\ndefects is contained in the metric and the spin-connection. However, these\ntopological defects can also be associated to a Riemann-Cartan geometry where\ncurvature and torsion plays an important role. In this article we study the\ninterplay between a wedge dislocations in a planar graphene sheet and the\nproperties of its electronic degrees of freedom. Our approach relies in its\nrelation with elasticity theory through the so called elastic-gauge, where\ntheir typical coefficients, as for example the Poisson's ratio, appear directly\nin the metric, and consequently also in the electronic spectrum.", "category": "cond-mat_str-el" }, { "text": "Exact SO(8) Symmetry in the Weakly-Interacting Two-Leg Ladder: A perturbative renormalization group analysis of interacting electrons on a\ntwo-leg ladder reveals that at half-filling any weakly repulsive system scales\nonto an exactly soluble Gross-Neveu model with a hidden SO(8) symmetry. The\nhalf-filled ground state is a Mott insulator with short-range d-wave pair\ncorrelations. We extract the exact energies, degeneracies, and quantum numbers\nof *all* the low energy excited multiplets. One energy (mass) m octets contains\nCooper pair, magnon, and density-wave excitations, two more octets contain\nsingle-particle excitations, and a mass \\sqrt{3}m antisymmetric tensor contains\n28 \"bound states\". Exact single-particle and spin gaps are found for the\nlightly-doped (d-wave paired one-dimension Bose fluid) system. We also\ndetermine the four other robust phases occuring at half-filling for partially\nattractive interactions. All 5 phases have distinct SO(8) symmetries, but share\nS.C. Zhang's SO(5) as a common subgroup.", "category": "cond-mat_str-el" }, { "text": "Multiple supersonic phase fronts launched at a complex-oxide\n hetero-interface: Selective optical excitation of a substrate lattice can drive phase changes\nacross hetero-interfaces. This phenomenon is a non-equilibrium analogue of\nstatic strain control in heterostructures and may lead to new applications in\noptically controlled phase change devices. Here, we make use of time-resolved\nnon-resonant and resonant x-ray diffraction to clarify the underlying physics,\nand to separate different microscopic degrees of freedom in space and time. We\nmeasure the dynamics of the lattice and that of the charge disproportionation\nin NdNiO3, when an insulator-metal transition is driven by coherent lattice\ndistortions in the LaAlO3 substrate. We find that charge redistribution\npropagates at supersonic speeds from the interface into the NdNiO3 film,\nfollowed by a sonic lattice wave. When combined with measurements of magnetic\ndisordering and of the metal-insulator transition, these results establish a\nhierarchy of events for ultrafast control at complex oxide hetero-interfaces.", "category": "cond-mat_str-el" }, { "text": "Critical charge fluctuations in a pseudogap Anderson model: The Anderson impurity model with a density of states $\\rho(\\varepsilon)\n\\propto |\\varepsilon|^r$ containing a power-law pseudogap centered on the Fermi\nenergy ($\\varepsilon = 0$) features for $01.8 T) the system is\nnot fully polarized, but has a small canting of the moments induced by the\ndipolar interaction. We show that the degree of canting is accurately predicted\nby the standard Hamiltonian which includes the dipolar interaction. The\ninelastic scattering is dominated at large momentum transfers by a band of\nalmost dispersionless excitations. We show that these correspond to the spin\nwaves localized on ten site rings, expected for a system described by a nearest\nneighbor interaction, and that the spectrum at high fields B>1.8 T is\nwell-described by a spin wave theory. The phase for fields <1.8 T is\ncharacterized by an antiferromagnetic Bragg peak at (210) and an incommensurate\npeak.", "category": "cond-mat_str-el" }, { "text": "Optimum ground states of generalized Hubbard models with next-nearest\n neighbour interaction: We investigate the stability domains of ground states of generalized Hubbard\nmodels with next-nearest neighbour interaction using the optimum groundstate\napproach. We focus on the $\\eta$-pairing state with momentum P=0 and the fully\npolarized ferromagnetic state at half-filling. For these states exact lower\nbounds for the regions of stability are obtained in the form of inequalities\nbetween the interaction parameters. For the model with only nearest neighbour\ninteraction we show that the bounds for the stability regions can be improved\nby considering larger clusters. Additional next-nearest neighbour interactions\ncan lead to larger or smaller stability regions depending on the parameter\nvalues.", "category": "cond-mat_str-el" }, { "text": "High-resolution neutron depolarization microscopy of the ferromagnetic\n transitions in Ni$_3$Al and HgCr$_2$Se$_4$ under pressure: We performed neutron imaging of ferromagnetic transitions in Ni$_3$Al and\nHgCr$_2$Se$_4$ crystals. These neutron depolarization measurements revealed\nbulk magnetic inhomogeneities in the ferromagnetic transition temperature with\nspatial resolution of about 100~$\\mu$m. To obtain such spatial resolution, we\nemployed a novel neutron microscope equipped with Wolter mirrors as a neutron\nimage-forming lens and a focusing neutron guide as a neutron condenser lens.\nThe images of Ni$_3$Al show that the sample does not homogeneously go through\nthe ferromagnetic transition; the improved resolution allowed us to identify a\ndistribution of small grains with slightly off-stoichiometric composition.\nAdditionally, neutron depolarization imaging experiments on the chrome spinel,\nHgCr$_2$Se$_4$, under pressures up to 15~kbar highlight the advantages of the\nnew technique especially for small samples or sample environments with\nrestricted sample space. The improved spatial resolution enables one to observe\ndomain formation in the sample while decreasing the acquisition time despite\nhaving a bulky pressure cell in the beam.", "category": "cond-mat_str-el" }, { "text": "Microscopic phase separation in triangular-lattice quantum spin magnet\n kappa-(BEDT-TTF)2Cu2(CN)3 probed by muon spin relaxation: The ground state of the quantum spin system kappa-(BEDT-TTF)2Cu2(CN)3 in\nwhich antiferromagnetically-interacting S=1/2 spins are located on a nearly\nequilateral triangular lattice attracts considerable interest both from\nexperimental and theoretical aspects, because a simple antiferromagnetic order\nmay be inhibited because of the geometrical frustration and hence an exotic\nground state is expected. Furthermore, recent two reports on the ground state\nof this system have made it further intriguing by showing completely\ncontroversial results; one indicates the gapless state and the other gapped. By\nutilizing microscopic probe of muSR, we have investigated its spin dynamics\nbelow 0.1 K, unveiling its microscopically phase separated ground state at zero\nfield.", "category": "cond-mat_str-el" }, { "text": "Destabilization of the Zhang-Rice singlet at optimal doping: The construction of the Zhang-Rice singlet is revisited in the light of\nrecent understanding of high-temperature superconductors at optimal doping. A\nminimal local model is derived which contains the physical regime found\nrelevant for ARPES experiments, characterized by significant direct\noxygen-oxygen hopping. For the values of orbital parameters indicated by\nexperiment, the Zhang-Rice singlet is strongly mixed with a pure oxygen singlet\nof the same symmetry. The destabilization of the Zhang-Rice ground state is due\nto the oxygen singlet having twice as large a coherence factor with respect to\noxygen-oxygen hopping. An analogous quantum phase transition is identified in\nthe t-t'-J model. The orbital-antisymmetric copper-oxygen singlet is confirmed\nto be irrelevant, as found originally. The usual symmetry analysis is extended\nto include dynamical symmetries.", "category": "cond-mat_str-el" }, { "text": "Quadriexcitons and excitonic condensate in a symmetric electron-hole\n bilayer with valley degeneracy: Using quantum Monte Carlo simulations we have mapped out the zero temperature\nphase diagram of a symmetric electron-hole bilayer with twofold valley\ndegeneracy, as function of the interlayer distance $d$ and in-layer density\n$n$. We find that the effect of the valley degeneracy is to shrink the region\nof stability of the excitonic condensate, in favor of quadriexcitons at small\n$d$ and of the four-component plasma at large $d$, with minor effects on the\nvalue of the excitonic condensate fraction. The enclosure of the condensate in\na density window possibly explains why anomalous tunnelling conductivity,\ninterpreted as signature of condensation, is observed only between two finite\nvalues of carrier density in graphene bilayers. Our phase diagram may provide\ndirections to select device parameters for future experiments.", "category": "cond-mat_str-el" }, { "text": "Thermoelectric power in one-dimensional Hubbard model: The thermoelectric power S is studied within the one-dimensional Hubbard\nmodel using the linear response theory and the numerical exact-diagonalization\nmethod for small systems. While both the diagonal and off-diagonal dynamical\ncorrelation functions of particle and energy current are singular within the\nmodel even at temperature T>0, S behaves regularly as a function of frequency\n$\\omega$ and T. Dependence on the electron density n below the half-filling\nreveals a change of sign of S at n_0=0.73+/-0.07 due to strong correlations, in\nthe whole T range considered. Approaching half-filling S is hole-like and can\nbecome large for U>>t although decreasing with T.", "category": "cond-mat_str-el" }, { "text": "Remarkably robust and correlated coherence and antiferromagnetism in\n (Ce$_{1-x}$La$_x$)Cu$_2$Ge$_2$: We present magnetic susceptibility, resistivity, specific heat, and\nthermoelectric power measurements on (Ce$_{1-x}$La$_x$)Cu$_2$Ge$_2$ single\ncrystals (0 $\\leq x\\leq$ 1). With La substitution, the antiferromagnetic\ntemperature $T_N$ is suppressed in an almost linear fashion and moves below\n0.36 K, the base temperature of our measurements for $x>$ 0.8. Surprisingly, in\naddition to robust antiferromagnetism, the system also shows low temperature\ncoherent scattering below $T_{coh}$ up to $\\sim$ 0.9 of La, indicating a small\npercolation limit $\\sim$ 9$\\%$ of Ce that separates a coherent regime from a\nsingle-ion Kondo impurity regime. $T_{coh}$ as a function of magnetic field was\nfound to have different behavior for $x$< 0.9 and $x$> 0.9. Remarkably,\n$(T_{coh})^2$ at $H$ = 0 was found to be linearly proportional to $T_N$. The\njump in the magnetic specific heat $\\delta C_{m}$ at $T_N$ as a function of\n$T_K/T_N$ for (Ce$_{1-x}$La$_x$)Cu$_2$Ge$_2$ follows the theoretical prediction\nbased on the molecular field calculation for the $S$ = 1/2 resonant level\nmodel.", "category": "cond-mat_str-el" }, { "text": "Induced-Moment Weak Antiferromagnetism and Orbital Order on the\n Itinerant-Localized Duality Model with Nested Fermi Surface: A Possible\n Origin of Exotic Magnetism in URu${}_{2}$Si$_{2}$: The weak antiferromagnetism of URu${}_{2}$Si${}_{2}$ is discussed on the\nbasis of a duality model which takes into account salient features of both\nitinerant fermions and \"localized\" component of spin degrees of freedom. The\nproblem is analyzed in the framework of induced-moment mechanism by taking a\nsinglet-singlet crystal field scheme together with the nesting property of\npartial Fermi surface of itinerant fermions . It is shown that the extremely\nsmall ordered moment $m$ of ${\\cal O}$($10^{-2}$$\\times$$\\mu_{B}$) can be\ncompatible with the large specific-heat jump at the transition temperature\n$T_{N}$. Analysis performed in the presence of external magnetic field shows\nthat the field dependence of $m$ in the limit T\\to 0 and T_{N}$ do not scale\nexcept very near the critical field B which is consistent with a recent\nobservation by Mentink. It is also shown that the antiferromagnetic magnetic\norder gives rise to a tiny amount of antiferromagnetic orbital order of\nf-electrons.", "category": "cond-mat_str-el" }, { "text": "Spin--orbital interaction for face-sharing octahedra: Realization of a\n highly symmetric SU(4) model: Specific features of orbital and spin structure of transition metal compounds\nin the case of the face-sharing MO$_6$ octahedra are analyzed. In this\ngeometry, we consider the form of the spin--orbital Hamiltonian for transition\nmetal ions with double ($e_g^{\\sigma}$) or triple ($t_{2g}$) orbital\ndegeneracy. Trigonal distortions typical of the structures with face-sharing\noctahedra lead to splitting of $t_{2g}$ orbitals into an $a_{1g}$ singlet and\n$e_g^{\\pi}$ doublet. For both doublets ($e_g^{\\sigma}$ and $e_g^{\\pi}$), in the\ncase of one electron or hole per site, we arrive at a symmetric model with the\norbital and spin interaction of the Heisenberg type and the Hamiltonian of\nunexpectedly high symmetry: SU(4). Thus, many real materials with this geometry\ncan serve as a testing ground for checking the prediction of this interesting\ntheoretical model. We also compare general trends in spin--orbital\n(\"Kugel--Khomskii\") exchange interaction for three typical situations: those of\nMO$_6$ octahedra with common corner, common edge, and the present case of\ncommon face, which has not been considered yet.", "category": "cond-mat_str-el" }, { "text": "A Lattice Model of Intercalation: The thermodynamics of the lattice model of intercalation of ions in crystals\nis considered in the mean field approximation. Pseudospin formalism is used for\nthe description of interaction of electrons with ions and the possibility of\nhopping of intercalated ions between different positions is taken into account.\nPhase diagrams are built. It is shown that the effective interaction between\nintercalated ions can lead to phase separation or to appearance of modulated\nphase (it depends on filling of the electron energy band). At high values of\nthe parameter of ion transfer the ionic subsystem can pass to the\nsuperfluid-like state.", "category": "cond-mat_str-el" }, { "text": "Complex Quantum Phenomena in a Bilayered Calcium Ruthenate: Ca$_3$Ru$_2$O$_7$ undergoes an antiferromagnetic transition at\n$T_{\\text{N}}=56 $K, followed by a Mott-like (MI) transition at\n$T_{\\text{MI}}=48$ K. This nonmetallic ground state, with a charge gap of 0.1\neV, is suppressed by a highly anisotropic metamagnetic transition that leads to\na fully spin-polarized metallic state. We report the observation of\nShubnikov-de Haas oscillations in the \\textit{gapped} state, colossal\nmagnetoresistance in the inter-plane resistivity with a large anisotropy\ndifferent from that observed in the magnetization, and non-Fermi liquid\nbehavior in the metallic state at high magnetic fields.", "category": "cond-mat_str-el" }, { "text": "Collective charge excitations and the metal-insulator transition in the\n square lattice Hubbard-Coulomb model: In this article, we discuss the non-trivial collective charge excitations\n(plasmons) of the extended square-lattice Hubbard model. Using a fully\nnon-perturbative approach, we employ the hybrid Monte Carlo algorithm to\nsimulate the system at half-filling. A modified Backus-Gilbert method is\nintroduced to obtain the spectral functions via numerical analytic\ncontinuation. We directly compute the single-particle density of states which\ndemonstrates the formation of Hubbard bands in the strongly-correlated phase.\nThe momentum-resolved charge susceptibility is also computed on the basis of\nthe Euclidean charge density-density correlator. In agreement with previous\nEDMFT studies, we find that at large strength of the electron-electron\ninteraction, the plasmon dispersion develops two branches.", "category": "cond-mat_str-el" }, { "text": "Fate of entanglement in magnetism under Lindbladian or non-Markovian\n dynamics and conditions for their transition to Landau-Lifshitz-Gilbert\n classical dynamics: It is commonly assumed in spintronics and magnonics that localized spins\nwithin antiferromagnets are in the N\\'{e}el ground state (GS), as well as that\nsuch state evolves, when pushed out of equilibrium by current or external\nfields, according to the Landau-Lifshitz-Gilbert (LLG) equation viewing\nlocalized spins as classical vectors of fixed length. On the other hand, the\ntrue GS of antiferromagnets is highly entangled, as confirmed by very recent\nneutron scattering experiments witnessing their entanglement. Although GS of\nferromagnets is always unentangled, their magnonic low-energy excitation are\nsuperpositions of many-body spin states and, therefore, entangled. In this\nstudy, we initialize quantum Heisenberg ferro- or antiferromagnetic chains\nhosing localized spins $S=1/2$, $S=1$ or $S=5/2$ into unentangled pure state\nand then evolve them by quantum master equations (QMEs) of Lindblad or\nnon-Markovian type, derived by coupling localized spins to a bosonic bath (such\nas due to phonons) or by using additional ``reaction coordinate'' in the latter\ncase. The time evolution is initiated by applying an external magnetic field,\nand entanglement of time-evolving {\\em mixed} quantum states is monitored by\ncomputing its logarithmic negativity. We find that non-Markovian dynamics\nmaintains some degree of entanglement, which shrinks the length of the vector\nof spin expectation values, thereby making the LLG equation inapplicable.\nConversely, Lindbladian (i.e., Markovian) dynamics ensures that entanglement\ngoes to zero, thereby enabling quantum-to-classical (i.e., to LLG) transition\nin all cases -- $S=1/2$, $S=1$ and $S=5/2$ ferromagnet or $S=5/2$\nantiferromagnet -- {\\em except} for $S=1/2$ and $S=1$ antiferromagnet. We also\ninvestigate the stability of entangled antiferromagnetic GS upon suddenly\ncoupling it to the bosonic bath.", "category": "cond-mat_str-el" }, { "text": "Spin-Fluctuation Drag Thermopower of Nearly Ferromagnetic Metals: We investigate theoretically the Seebeck effect in materials close to a\nferromagnetic quantum critical point to explain anomalous behaviour at low\ntemperatures. It is found that the main effect of spin fluctuations is to\nenhance the coefficient of the leading $T$-linear term, and a quantum critical\nbehaviour characterized by a spin-fluctuation temperature appears in the\ntemperature dependence of correction terms as in the specific heat.", "category": "cond-mat_str-el" }, { "text": "Simulation of braiding anyons using Matrix Product States: Anyons exist as point like particles in two dimensions and carry braid\nstatistics which enable interactions that are independent of the distance\nbetween the particles. Except for a relatively few number of models which are\nanalytically tractable, much of the physics of anyons remain still unexplored.\nIn this paper, we show how U(1)-symmetry can be combined with the previously\nproposed anyonic Matrix Product States to simulate ground states and dynamics\nof anyonic systems on a lattice at any rational particle number density. We\nprovide proof of principle by studying itinerant anyons on a one dimensional\nchain where no natural notion of braiding arises and also on a two-leg ladder\nwhere the anyons hop between sites and possibly braid. We compare the result of\nthe ground state energies of Fibonacci anyons against hardcore bosons and\nspinless fermions. In addition, we report the entanglement entropies of the\nground states of interacting Fibonacci anyons on a fully filled two-leg ladder\nat different interaction strength, identifying gapped or gapless points in the\nparameter space. As an outlook, our approach can also prove useful in studying\nthe time dynamics of a finite number of nonabelian anyons on a finite\ntwo-dimensional lattice.", "category": "cond-mat_str-el" }, { "text": "Double Exchange in a Magnetically Frustrated System: This work examines the magnetic order and spin dynamics of a double-exchange\nmodel with competing ferromagnetic and antiferromagnetic Heisenberg\ninteractions between the local moments. The Heisenberg interactions are\nperiodically arranged in a Villain configuration in two dimensions with\nnearest-neighbor, ferromagnetic coupling $J$ and antiferromagnetic coupling\n$-\\eta J$. This model is solved at zero temperature by performing a\n$1/\\sqrt{S}$ expansion in the rotated reference frame of each local moment.\nWhen $\\eta $ exceeds a critical value, the ground state is a magnetically\nfrustrated, canted antiferromagnet. With increasing hopping energy $t$ or\nmagnetic field $B$, the local moments become aligned and the ferromagnetic\nphase is stabilized above critical values of $t$ or $B$. In the canted phase, a\ncharge-density wave forms because the electrons prefer to sit on lines of sites\nthat are coupled ferromagnetically. Due to a change in the topology of the\nFermi surface from closed to open, phase separation occurs in a narrow range of\nparameters in the canted phase. In zero field, the long-wavelength spin waves\nare isotropic in the region of phase separation. Whereas the average spin-wave\nstiffness in the canted phase increases with $t$ or $\\eta $, it exhibits a more\ncomplicated dependence on field. This work strongly suggests that the jump in\nthe spin-wave stiffness observed in Pr$_{1-x}$Ca$_x$MnO$_3$ with $0.3 \\le x \\le\n0.4$ at a field of 3 T is caused by the delocalization of the electrons rather\nthan by the alignment of the antiferromagnetic regions.", "category": "cond-mat_str-el" }, { "text": "A neutron scattering study of two-magnon states in the quantum magnet\n copper nitrate: We report measurements of the two-magnon states in a dimerized\nantiferromagnetic chain material, copper nitrate (Cu(NO3)2*2.5D2O). Using\ninelastic neutron scattering we have measured the one and two magnon excitation\nspectra in a large single crystal. The data are in excellent agreement with a\nperturbative expansion of the alternating Heisenberg Hamiltonian from the\nstrongly dimerized limit. The expansion predicts a two-magnon bound state for q\n~ (2n+1)pi*d which is consistent with the neutron scattering data.", "category": "cond-mat_str-el" }, { "text": "Collective spin excitations in a quantum spin ladder probed by\n high-resolution Resonant Inelastic X-ray Scattering: We investigate magnetic excitations in the spin-ladder compound\nSr$_{14}$Cu$_{24}$O$_{41}$ using high-resolution Cu $L_3$-edge Resonant\nInelastic X-ray Scattering (RIXS). Our findings demonstrate that RIXS couples\nto collective spin excitations from a quantum spin-liquid ground state. In\ncontrast to Inelastic Neutron Scattering (INS), the RIXS cross section changes\nonly moderately over the entire Brillouin Zone (BZ), revealing a high\nsensitivity also at small momentum transfers. The two-triplon energy gap is\nfound to be $100\\pm 30$ meV. Our results are supported by calculations within\nan effective Hubbard model for a finite-size cluster.", "category": "cond-mat_str-el" }, { "text": "Competition between three-sublattice order and superfluidity in the\n quantum 3-state Potts model of ultracold bosons and fermions on a square\n optical lattice: We study a quantum version of the three-state Potts model that includes as\nspecial cases the effective models of bosons and fermions on the square lattice\nin the Mott insulating limit. It can be viewed as a model of quantum\npermutations with amplitudes J_parallel and J_perp for identical and different\ncolors, respectively. For J_parallel=J_perp>0, it is equivalent to the SU(3)\nHeisenberg model, which describes the Mott insulating phase of 3-color\nfermions, while the parameter range J_perp0 properties for J_perp<0. For dominant antiferromagnetic\nJ_parallel interactions, a three-sublattice long-range ordered stripe state is\nselected out of the ground state manifold of the antiferromagnetic Potts model\nby quantum fluctuations. Upon increasing |J_perp|, this state is replaced by a\nuniform superfluid for J_perp<0, and by an exotic three-sublattice superfluid\nfollowed by a two-sublattice superfluid for J_perp>0. The transition out of the\nuniform superfluid (that can be realized with bosons) is shown to be a peculiar\ntype of Kosterlitz-Thouless transition with three types of elementary vortices.", "category": "cond-mat_str-el" }, { "text": "Pairing Correlations on t-U-J Ladders: Pairing correlations on generalized t-U-J two-leg ladders are reported. We\nfind that the pairing correlations on the usual t-U Hubbard ladder are\nsignificantly enhanced by the addition of a nearest-neighbor exchange\ninteraction J. Likewise, these correlations are also enhanced for the t-J model\nwhen the onsite Coulomb interaction is reduced from infinity. Moreover, the\npairing correlations are larger on a t-U-J ladder than on a t-Jeff ladder in\nwhich Jeff has been adjusted so that the two models have the same spin gap at\nhalf-filling. This enhancement of the pairing correlations is associated with\nan increase in the pair-binding energy and the pair mobility in the t-U-J model\nand point to the importance of the charge transfer nature of the cuprate\nsystems.", "category": "cond-mat_str-el" }, { "text": "Slow scrambling and hidden integrability in a random rotor model: We analyze the out-of-time-order correlation functions of a solvable model of\na large number, $N$, of $M$-component quantum rotors coupled by\nGaussian-distributed random, infinite-range exchange interactions. We focus on\nthe growth of commutators of operators at a temperature $T$ above the zero\ntemperature quantum critical point separating the spin-glass and paramagnetic\nphases. In the large $N,~M$ limit, the squared commutators of the rotor fields\ndo not display any exponential growth of commutators, in spite of the absence\nof any sharp quasiparticle-like excitations in the disorder-averaged theory. We\nshow that in this limit, the problem is integrable and point out interesting\nconnections to random-matrix theory. At leading order in $1/M$, there are no\nmodifications to the critical behavior but an irrelevant term in the\nfixed-point action leads to a small exponential growth of the squared\ncommutator. We also introduce and comment on a generalized model involving\n$p$-pair rotor interactions.", "category": "cond-mat_str-el" }, { "text": "Symmetry Enriched U(1) Topological Orders for Dipole-Octupole Doublets\n on a Pyrochlore Lattice: Symmetry plays a fundamental role in our understanding of both conventional\nsymmetry breaking phases and the more exotic quantum and topological phases of\nmatter. We explore the experimental signatures of symmetry enriched U(1)\nquantum spin liquids (QSLs) on the pyrochlore lattice. We point out that the Ce\nlocal moment of the newly discovered pyrochlore QSL candidate\nCe$_2$Sn$_2$O$_7$, is a dipole-octupole doublet. The generic model for these\nunusual doublets supports two distinct symmetry enriched U(1) QSL ground states\nin the corresponding quantum spin ice regimes. These two U(1) QSLs are dubbed\ndipolar U(1) QSL and octupolar U(1) QSL. While the dipolar U(1) QSL has been\ndiscussed in many contexts, the octupolar U(1) QSL is rather unique. Based on\nthe symmetry properties of the dipole-octupole doublets, we predict the\npeculiar physical properties of the octupolar U(1) QSL, elucidating the unique\nspectroscopic properties in the externalmagnetic fields. We further predict the\nAnderson-Higgs transition from the octupolar U(1) QSL driven by the external\nmagnetic fields. We identify the experimental relevance with the candidate\nmaterial Ce$_2$Sn$_2$O$_7$ and other dipole-octupole doublet systems.", "category": "cond-mat_str-el" }, { "text": "Anomalous heavy-fermion and ordered states in the filled skutterudite\n PrFe4P12: Specific heat and magnetization measurements have been performed on\nhigh-quality single crystals of filled-skutterudite PrFe_4P_{12} in order to\nstudy the high-field heavy-fermion state (HFS) and low-field ordered state\n(ODS). From a broad hump observed in C/T vs T in HFS for magnetic fields\napplied along the <100> direction, the Kondo temperature of ~ 9 K and the\nexistence of ferromagnetic Pr-Pr interactions are deduced. The {141}-Pr nuclear\nSchottky contribution, which works as a highly-sensitive on-site probe for the\nPr magnetic moment, sets an upper bound for the ordered moment as ~ 0.03\n\\mu_B/Pr-ion. This fact strongly indicates that the primary order parameter in\nthe ODS is nonmagnetic and most probably of quadrupolar origin, combined with\nother experimental facts. Significantly suppressed heavy-fermion behavior in\nthe ODS suggests a possibility that the quadrupolar degrees of freedom is\nessential for the heavy quasiparticle band formation in the HFS. Possible\ncrystalline-electric-field level schemes estimated from the anisotropy in the\nmagnetization are consistent with this conjecture.", "category": "cond-mat_str-el" }, { "text": "Controlling structural distortion in the geometrically frustrated\n layered cobaltate YBaCo4O7+\u03b4 by Fe substitution and its role on\n magnetic correlations: Effects of Fe-substitution on the crystal structure and magnetic correlations\nof the geometrically frustrated antiferromagnets YBaCo4-xFexO7+{\\delta} (x = 0,\n0.2, 0.4, 0.5, 0.6, and 0.8) have been studied by neutron diffraction,\nM\\\"ossbauer spectroscopy, and ac susceptibility. The compounds\nYBaCo4-xFexO7+{\\delta} have a special layered-type crystal structure with an\nalternating Kagom\\'e (6c site) and triangular (2a site) layers along the c\naxis. Fe3+ ions are found to be substituted at both the crystallographic 2a and\n6c sites of Co ions. M\\\"ossbauer results show a high spin state of Fe3+ ions in\na tetrahedral coordination. A reduction in the distortion of the Kagom\\'e\nlattice has been observed with the Fe-substitution. The correlation length of\nthe short-range antiferromagnetic ordering decreases with the Fe-substitution.\nThe sharpness of the magnetic transition also decreases with the\nFe-substitution.", "category": "cond-mat_str-el" }, { "text": "Colossal Positive Magnetoresistance in a Doped Nearly Magnetic\n Semiconductor: We report on a positive colossal magnetoresistance (MR) induced by\nmetallization of FeSb$_{2}$, a nearly magnetic or \"Kondo\" semiconductor with 3d\nions. We discuss contribution of orbital MR and quantum interference to\nenhanced magnetic field response of electrical resistivity.", "category": "cond-mat_str-el" }, { "text": "Photoinduced Structural Phase Transitions in Polyacene: There exist two types of structural instability in polyacene: double bonds in\na cis pattern and those in a trans pattern. They are isoenergetic but\nspectroscopically distinct. We demonstrate optical characterization and\nmanipulation of Peierls-distorted polyacene employing both correlated and\nuncorrelated Hamiltonians. We clarify the phase boundaries of the cis- and\ntrans-distorted isomers, elucidate their optical-conductivity spectra, and then\nexplore their photoresponses. There occurs a photoinduced transformation in the\npolyacene structure, but it is one-way switching: The trans configuration is\nwell convertible into the cis one, whereas the reverse conversion is much less\nfeasible. Even the weakest light irradiation can cause a transition of\nuncorrelated electrons, while correlated electrons have a transition threshold\nagainst light irradiation.", "category": "cond-mat_str-el" }, { "text": "Magnetic state dynamics in itinerant paramagnet UM3B2 (M= Co, Ir) probed\n by 11B NMR: We have carried out the $^{11}$B NMR measurement on the itinerant\nparamagnetic systems U$M_{3}$B$_{2}$ ($M =$ Co, Ir) to investigate the\nlow-dimensional characteristics of the $5f$-electrons due to the structural\nanisotropy. The recent X-ray analysis suggests that UIr$_3$B$_2$ has a\ndifferent structure modulated from the ever-known superlattice. The azimuth\nangle variation of NMR spectrum within the $ab$-plane clarified that B atoms\noccupy the single site, and a certain ligands arrangement surrounding B atom\nturns to the same orientation as the another one through the three- or six-fold\nrotation around the c-axis. These results have been consistent with the X-ray\nproposition. To evaluate the temperature ($T$) development of general\nsusceptibility ($\\chi_{q,\\omega}$), Knight shift and nuclear spin-lattice\nrelaxation rates measurements were performed and the similar variations of\n$\\chi_{q,\\omega}$ were identified in both UCo$_{3}$B$_{2}$ and\nUIr$_{3}$B$_{2}$. Above a crossover point defined as $T^{*}\\simeq50$ K, the\nevolution of $\\chi_{q,\\omega}$ is dominant at $q=0$, suggesting that\nferromagnetic correlations develop in high-$T$ regimes; meanwhile, below\n$T^{*}$, the $q=0$ part in $\\chi_{q,\\omega}$ shows the saturation tendency, and\na different class of dispersion at finite-$q$ suddenly emerges. This particular\nmagnetic correlations are interpreted as the antiferromagnetic correlations,\nand notable feature of the magnetic state dynamics in low-$T$ regimes is that\nthe antiferromagnetic correlations arise together with the ferromagnetic\ncomponent at the same time. The unique magnetic correlations obtained from NMR\nexperiment will be discussed by the possible low-dimensionality of\nU$M_{3}$B$_{2}$ lattice.", "category": "cond-mat_str-el" }, { "text": "$Z_{2}$ fractionalized Chern/topological insulators in an exactly\n soluble correlated model: In this paper we propose an exactly soluble model in two-dimensional\nhoneycomb lattice, from which two phases are found. One is the usual\nChern/topological insulating state and the other is an interesting $Z_2$\nfractionalized Chern/topological insulator. While their bulk properties are\nsimilar, the edge-states of physical electrons are quite different. The single\nelectron excitation of the former shows a free particle-like behavior while the\nlatter one is gapped, which provides a definite signature to identify the\nfractionalized states. The transition between these two phases is found to fall\ninto the 3D Ising universal class. Significantly, near the quantum transition\npoint the physical electron in the edge-states shows strong Luttinger liquid\nbehavior. An extension to the interesting case of the square lattice is also\nmade. In addition, we also discuss some relationship between our exactly\nsoluble model and various Hubbard-like models existing in the literature. The\nessential difference between the proposed $Z_{2}$ fractionalized Chern\ninsulator and the hotly pursued fractional Chern insulator is also pointed out.\nThe present work may be helpful for further study on the fractionalized\ninsulating phase and related novel correlated quantum phases.", "category": "cond-mat_str-el" }, { "text": "Magnetoelectric behavior from cluster multipoles in square cupolas:\n Study of Sr(TiO)Cu$_4$(PO$_4$)$_4$ in comparison with Ba and Pb\n isostructurals: We report our combined experimental and theoretical study of magnetoelectric\nproperties of an antiferromagnet Sr(TiO)Cu$_4$(PO$_4$)$_4$, in comparison with\nthe isostructurals Ba(TiO)Cu$_4$(PO$_4$)$_4$ and Pb(TiO)Cu$_4$(PO$_4$)$_4$. The\nfamily of compounds commonly possesses a low-symmetric magnetic unit called the\nsquare cupola, which is a source of magnetoelectric responses associated with\nthe magnetic multipoles activated under simultaneous breaking of spatial\ninversion and time reversal symmetries. Measuring the full magnetization curves\nand the magnetic-field profiles of dielectric constant for\nSr(TiO)Cu$_4$(PO$_4$)$_4$ and comparing them with the theoretical analyses by\nthe cluster mean-field theory, we find that the effective $S=1/2$ spin model,\nwhich was used for the previous studies for Ba(TiO)Cu$_4$(PO$_4$)$_4$ and\nPb(TiO)Cu$_4$(PO$_4$)$_4$, well explains the experimental results by tuning the\nmodel parameters. Furthermore, elaborating the phase diagram of the model, we\nfind that the square cupolas could host a variety of magnetic multipoles, i.e.,\nmonopole, toroidal moment, and quadrupole tensor, depending on the parameters\nthat could be modulated by deformations of the magnetic square cupolas. Our\nresults not only provide a microscopic understanding of the series of the\nsquare cupola compounds, but also stimulate further exploration of the\nmagnetoelectric behavior arising from cluster multipoles harboring in\nlow-symmetric magnetic units.", "category": "cond-mat_str-el" }, { "text": "Observation of orbital order in the Van der Waals material 1T-TiSe2: Besides magnetic and charge order, regular arrangements of orbital occupation\nconstitute a fundamental order parameter of condensed matter physics. Even\nthough orbital order is difficult to identify directly in experiments, its\npresence was firmly established in a number of strongly correlated,\nthree-dimensional Mott insulators. Here, reporting resonant X-ray scattering\nexperiments on the layered Van der Waals compound $1T$-TiSe$_2$, we establish\nthe emergence of orbital order in a weakly correlated, quasi-two-dimensional\nmaterial. Our experimental scattering results are consistent with\nfirst-principles calculations that bring to the fore a generic mechanism of\nclose interplay between charge redistribution, lattice displacements, and\norbital order. It demonstrates the essential role that orbital degrees of\nfreedom play in TiSe$_2$, and their importance throughout the family of\ncorrelated Van der Waals materials.", "category": "cond-mat_str-el" }, { "text": "Signatures of broken parity and time-reversal symmetry in generalized\n string-net models: We study indicators of broken time-reversal and parity symmetries in gapped\ntopological phases of matter. We focus on phases realized by Levin-Wen\nstring-net models, and generalize the string-net model to describe phases which\nbreak parity and time-reversal symmetries. We do this by introducing an extra\ndegree of freedom into the string-net graphical calculus, which takes the form\nof a branch cut located at each vertex of the underlying string-net lattice. We\nalso work with string-net graphs defined on arbitrary (non-trivalent) graphs,\nwhich reveals otherwise hidden information about certain configurations of\nanyons in the string-net graph. Most significantly, we show that objects known\nas higher Frobenius-Schur indicators can provide several efficient ways to\ndetect whether or not a given topological phase breaks parity or time-reversal\nsymmetry.", "category": "cond-mat_str-el" }, { "text": "The ubiquitous 1100 charge ordering in organic charge-transfer solids: Charge and spin-orderings in the 1/4-filled organic CT solids are of strong\ninterest, especially in view of their possible relations to organic\nsuperconductivity. We show that the charge order (CO) in both 1D and 2D CT\nsolids is of the ...1100... type, in contradiction to mean field prediction of\n>...1010... CO. We present detailed computations for metal-insulator and\nmagnetic insulator-insulator transitions in the theta-ET materials. Complete\nagreement with experiments in several theta systems is found. Similar\ncomparisons between theory and experiments in TCNQ, TMTTF, TMTSF, and ET\nmaterials prove the ubiquity of this phenomenon.", "category": "cond-mat_str-el" }, { "text": "Magnetism and metal-insulator transitions in the Rashba-Hubbard model: The nature of metal-insulator and magnetic transitions is still a subject\nunder intense debate in condensed matter physics. Amongst the many possible\nmechanisms, the interplay between electronic correlations and spin-orbit\ncouplings is an issue of a great deal of interest, in particular when dealing\nwith quasi-2D compounds. In view of this, here we use a Hartree-Fock approach\nto investigate how the Rashba spin-orbit coupling, $V_\\text{SO}$, affects the\nmagnetic ordering provided by a Hubbard interaction, $U$, on a square lattice.\nAt half-filling, we have found a sequence of transitions for increasing\n$V_\\text{SO}$: from a Mott insulator to a metallic antiferromagnet, and then to\na paramagnetic Rashba metal. Also, our results indicate that the Rashba\ncoupling favors magnetic striped phases in the doped regime. By analyzing\nspectral properties, we associate the rearrangement of the magnetic ordering\nwith the emerging chirality created by the spin-orbit coupling. Our findings\nprovide insights towards clarifying the competition between these tendencies.", "category": "cond-mat_str-el" }, { "text": "Physical realization of the four color problem in quantum systems: A multi-component electron model on a lattice is constructed whose ground\nstate exhibits a spontaneous ordering which follows the rule of map-coloring\nused in the solution of the four color problem. The number of components is\ndetermined by the Euler characteristics of a certain surface into which the\nlattice is embedded. Combining the concept of chromatic polynomials with the\nHeawood-Ringel-Youngs theorem, we derive an index theorem relating the\ndegeneracy of the ground state with a hidden topology of the lattice. The\nsystem exhibits coloring transition and hidden-topological structure\ntransition. The coloring phase exhibits a topological order.", "category": "cond-mat_str-el" }, { "text": "Fermi arc criterion for surface Majorana modes in superconducting\n time-reversal symmetric Weyl semimetals: Many clever routes to Majorana fermions have been discovered by exploiting\nthe interplay between superconductivity and band topology in metals and\ninsulators. However, realizations in semimetals remain less explored. We ask,\n``under what conditions do superconductor vortices in time-reversal symmetric\nWeyl semimetals -- three-dimensional semimetals with only time-reversal\nsymmetry -- trap Majorana fermions on the surface?'' If each constant-$k_{z}$\nplane, where $z$ is the vortex axis, contains equal numbers of Weyl nodes of\neach chirality, we predict a generically gapped vortex and derive a topological\ninvariant $\\nu=\\pm1$ in terms of the Fermi arc structure that signals the\npresence or absence of surface Majorana fermions. In contrast, if certain\nconstant-$k_{z}$ planes contain a net chirality of Weyl nodes, the vortex is\ngapless. We analytically calculate $\\nu$ within a perturbative scheme and\nprovide numerical support with a lattice model. The criteria survive the\npresence of other bulk and surface bands and yield phase transitions between\ntrivial, gapless and topological vortices upon tilting the vortex. We propose\nLi(Fe$_{0.91}$Co$_{0.09}$)As and Fe$_{1+y}$Se$_{0.45}$Te$_{0.55}$ with broken\ninversion symmetry as candidates for realizing our proposals.", "category": "cond-mat_str-el" }, { "text": "An Operational Definition of Topological Order: The unrivaled robustness of topologically ordered states of matter against\nperturbations has immediate applications in quantum computing and quantum\nmetrology, yet their very existence poses a challenge to our understanding of\nphase transitions. However, a comprehensive understanding of what actually\nconstitutes topological order is still lacking. Here we show that one can\ninterpret topological order as the ability of a system to perform topological\nerror correction. We find that this operational approach corresponding to a\nmeasurable both lays the conceptual foundations for previous classifications of\ntopological order and also leads to a successful classification in the hitherto\ninaccessible case of topological order in open quantum systems. We demonstrate\nthe existence of topological order in open systems and their phase transitions\nto topologically trivial states. Our results demonstrate the viability of\ntopological order in nonequilibrium quantum systems and thus substantially\nbroaden the scope of possible technological applications.", "category": "cond-mat_str-el" }, { "text": "Disturbing the dimers: electron- and hole-doping in the intermetallic\n insulator FeGa$_3$: Insulating FeGa$_3$ poses peculiar puzzles beyond the occurrence of an\nelectronic gap in an intermetallic compound. This Fe-based material has a very\ndistinctive structural characteristic with the Fe atoms occurring in dimers.\nThe insulating gap can be described comparably well in either the weakly\ncorrelated limit or the strongly correlated limit within density functional\ntheory viewpoints, where the latter corresponds to singlet formation on the\nFe$_2$ dimers. Though most of the calculated occupied Wannier functions are an\nadmixture of Fe $3d$ and Ga $4s$ or $4p$ states, there is a single bonding-type\nWannier function per spin centered on each Fe$_2$ dimer. Density functional\ntheory methods have been applied to follow the evolution of the magnetic\nproperties and electronic spectrum with doping, where unusual behavior is\nobserved experimentally. Both electron and hole doping are considered, by Ge\nand Zn on the Ga site, and by Co and Mn on the Fe site, the latter introducing\ndirect disturbance of the Fe$_2$ dimer. Results from weakly and strongly\ncorrelated pictures are compared. Regardless of the method, magnetism including\nitinerant phases appears readily with doping. The correlated picture suggests\nthat in the low doping limit Mn (for Fe) produces an in-gap hole state, while\nCo (for Fe) introduces a localized electronic gap state.", "category": "cond-mat_str-el" }, { "text": "Magnetic excitations of the field induced states in BaCo2(AsO4)2 probed\n by time-domain terahertz spectroscopy: Searching for Kitaev quantum spin liquid (QSL) is a fascinating and\nchallenging problem. Much effort has been devoted to honeycomb lattice\ncandidates with strong spin-orbit coupling in 5d-electron iridates and\n4delectron RuCl3. Recently, theoretical studies suggested that the 3d7 Co-based\nhoneycomb materials with high spin state S=3/2 and effective orbital angular\nmomentum L=1 could also be promising candidates of Kitaev QSL. One of the\ncandidates, BaCo2(AsO4)2, was revisited recently. The long range magnetic order\nin BaCo2(AsO4)2 can be suppressed by very weak in-plane magnetic field,\nsuggesting its proximity to Kitaev QSL. Here we perform time domain terahertz\nspectroscopy measurement to study the magnetic excitations on BaCo2(AsO4)2. We\nobserve different magnon excitations upon increasing external magnetic field.\nIn particular, the system is easily driven to a field-polarized paramagnetic\nphase, after the long range magnetic order is suppressed by a weak field Hc 2.\nThe spectra beyond Hc2 are dominated by single magnon and two magnon\nexcitations without showing signature of QSL. We discuss the similarity and\ndifference of the excitation spectra between BaCo2(AsO4)2 and the widely\nstudied Kitaev QSL candidate RuCl3.", "category": "cond-mat_str-el" }, { "text": "Phonon-induced topological insulation: We develop an approximate theory of phonon-induced topological insulation in\nDirac materials. In the weak coupling regime, long wavelength phonons may favor\ntopological phases in Dirac insulators with direct and narrow bandgaps. This\nphenomenon originates from electron-phonon matrix elements, which change\nqualitatively under a band inversion. A similar mechanism applies to weak\nCoulomb interactions and spin-independent disorder; however, the influence of\nthese on band topology is largely independent of temperature. As applications\nof the theory, we evaluate the temperature-dependence of the critical thickness\nand the critical stoichiometric ratio for the topological transition in\nCdTe/HgTe quantum wells and in BiTl(S$_{1-\\delta}$Se$_{\\delta})_2$,\nrespectively.", "category": "cond-mat_str-el" }, { "text": "The Effect of $f$-$d$ Magnetic Coupling in Multiferroic $R$MnO$_3$\n Crystals: We have established detailed magnetoelectric phase diagrams of\n(Eu$_{0.595}$Y$_{0.405}$)$_{1-x}$Tb$_x$MnO$_3$ ($0 \\le x \\le 1$) and\n(Eu,Y)$_{1-x}$Gd$_x$MnO$_3$ ($0 \\le x \\le 0.69$), whose average ionic radii of\n$R$-site ($R$: rare earth) cations are equal to that of Tb$^{3+}$, in order to\nreveal the effect of rare earth 4$f$ magnetic moments on the magnetoelectric\nproperties. In spite of the same $R$-site ionic radii, the magnetoelectric\nproperties of the two systems are remarkably different from each other. A small\namount of Tb substitution on $R$ sites ($x \\sim 0.2$) totally destroys\nferroelectric polarization along the a axis ($P_a$), and an increase in Tb\nconcentration stabilizes the $P_c$ phase. On the other hand, Gd substitution\n($x \\sim 0.2$) extinguishes the $P_c$ phase, and slightly suppresses the $P_a$\nphase. These results demonstrate that the magnetoelectric properties of\n$R$MnO$_3$ strongly depend on the characteristics of the rare earth 4$f$\nmoments.", "category": "cond-mat_str-el" }, { "text": "Competition between Interactions and Randomness in Photoinduced\n Synchronization of Charge Oscillations on a Dimer Lattice: The synchronization of charge oscillations after photoexcitation that has\nbeen realized through the emergence of an electronic breathing mode on dimer\nlattices is studied here from the viewpoint of the competition between\ninteractions and randomness. We employ an extended Hubbard model at\nthree-quarter filling on a simple dimer lattice and add random numbers to all\ntransfer integrals between nearest-neighbor sites. Photoinduced dynamics are\ncalculated using the time-dependent Schr\\\"odinger equation by the exact\ndiagonalization method. Although the randomness tends to unsynchronize charge\noscillations on different bonds during and after photoexcitation, sufficiently\nstrong on-site repulsion $U$ overcomes this effect and synchronizes these\ncharge oscillations some time after strong photoexcitation. The degree of\nsynchronization is evaluated using an order parameter that is derived from the\ntime profiles of the current densities on all bonds. As to the nearest-neighbor\ninteraction $V$, if $V$ is weakly attractive, it increases the order parameter\nby facilitating the charge oscillations. The relevance of these findings to\npreviously reported experimental and theoretical results for the organic\nconductor $\\kappa$-(bis[ethylenedithio]tetrathiafulvalene)$_2$Cu[N(CN)$_2$]Br\nis discussed.", "category": "cond-mat_str-el" }, { "text": "New gap equation for a marginal Fermi liquid: Assuming a phenomenological self-energy $Im \\Sigma(\\omega) \\sim\n|\\omega|^{\\beta\\}, (\\beta=1 $), which becomes gapped below $T_c$, we derived a\nnew gap equation. The new gap equation contains the effect of the kinetic\nenergy gain upon developing a superconducting order parameter. However, this\nnew kinetic energy gain mechanism works only for a repulsive pairing potential\nleading to a s-wave state. In this case, compared to the usual potential energy\ngain in the superconducting state as in the BCS gap equation, the kinetic\nenergy gain is more effective to easily achieve a high critical temperature\n$T_c$, since it is naturally Fermi energy scale. In view of the experimental\nevidences of the d-wave pairing state in the hole-doped copper-oxide high-$T_c$\nsuperconductors, we discuss the implications of our results.", "category": "cond-mat_str-el" }, { "text": "New Paired-Wavefunction for the Frustrated Antiferromagnetic Spin-Half\n Chain: I propose a new paired-wavefunction with a parameter that continuously\ninterpolates from the 1D Jastrow-product to the Majumdar-Ghosh\ndimer-wavefunction appropriate for the frustrated Heisenberg $S = 1/2$\nantiferromagnet. This spin paired-state constructed in $S_z$ basis is an\nalternative to the well-known resonating-valence-bond basis state for\ndescribing the $S = 0$ ground-state with no apparent long-range spin order.\nSome numerical evidences are presented.", "category": "cond-mat_str-el" }, { "text": "Fragile Mott Insulators: We prove that there exists a class of crystalline insulators, which we call\n\"fragile Mott insulators\" which are not adiabatically connected to any sort of\nband insulator provided time-reversal and certain point-group symmetries are\nrespected, but which are otherwise unspectacular in that they exhibit no\ntopological order nor any form of fractionalized quasiparticles. Different\nfragile Mott insulators are characterized by different nontrivial\none-dimensional representations of the crystal point group. We illustrate this\nnew type of insulators with two examples: the d-Mott insulator discovered in\nthe checkerboard Hubbard model at half-filling and the\nAffleck-Kennedy-Lieb-Tasaki insulator on the square lattice.", "category": "cond-mat_str-el" }, { "text": "Unusual behaviors in the transport properties of REFe$_{4}$P$_{12}$ (RE:\n La, Ce, Pr, and Nd): We have investigated the resistivity ($\\rho$), thermoelectric power (TEP) and\nHall coefficient ($R_{H}$) on high quality single crystals of\nREFe$_{4}$P$_{12}$. TEP in CeFe$_{4}$P$_{12}$ is extremely large ($\\sim$\n0.5mV/K at 290K) with a peak of $\\sim$ 0.75mV/K at around 65K. The Hall\nmobility also shows a peak at $\\sim$ 65K, suggesting carriers with heavy masses\ndeveloped at lower temperatures related with the f-hybridized band. Both Pr-\nand Nd- systems exhibit an apparent increase of $\\rho$ with decreasing\ntemperature far above their magnetic transition temperatures. In the same\ntemperature ranges, TEP exhibits unusually large absolute values of -50$\\mu$V/K\nfor PrFe$_{4}$P$_{12}$ and -15$\\mu$V/K for NdFe$_{4}$P$_{12}$, respectively.\nFor PrFe$_{4}$P$_{12}$, such anomalous transport properties suggest an unusual\nground state, possibly related with the Quadrupolar Kondo effect.", "category": "cond-mat_str-el" }, { "text": "Antiferromagnetic chiral spin density wave and strain-induced Chern\n insulator in the square lattice Hubbard model with frustration: We employ the Hartree-Fock approximation to identify the magnetic ground\nstate of the Hubbard model on a frustrated square lattice. We investigate the\nphase diagram as a function of the Coulomb repulsion's strength $U$, and the\nratio $t'/t$ between the nearest and next nearest neighbor hoppings $t$ and\n$t'$. At half-filling and for a sufficiently large $U$, an antiferromagnetic\nchiral spin density wave order with nonzero spin chirality emerges as the\nground state in a wide regime of the phase diagram near $t'/t=1/\\sqrt{2}$,\nwhere the Fermi surface is well-nested for both $(\\pi,\\pi)$ and\n$(\\pi,0)/(0,\\pi)$ wave vectors. This triple-${\\bf Q}$ chiral phase is\nsandwiched by a single-${\\bf Q}$ N\\'{e}el phase and a double-${\\bf Q}$ coplanar\nspin-vortex crystal phase, at smaller and larger $t'/t$, respectively. The\nenergy spectrum in the chiral spin density wave phase consists of four pairs of\ndegenerate bands. These give rise to two pairs of Dirac cones with the same\nchirality at the point $({\\pi \\over 2},{\\pi\\over 2})$ of the Brillouin zone. We\ndemonstrate that the application of a diagonal strain induces a $d_{xy}$-wave\nnext nearest neighbor hopping which, in turn, opens gaps in the two Dirac cones\nwith opposite masses. As a result, four pairs of well-separated\ntopologically-nontrivial bands emerge, and each pair of those contributes with\na Chern number $\\pm1$. At half-filling, this leads to a zero total Chern number\nand renders the topologically-notrivial properties observable only in the ac\nresponse regime. Instead, we show that at $3/4$ filling, the triple-${\\bf Q}$\nchiral phase yields a Chern insulator exhibiting the quantum anomalous Hall\neffect.", "category": "cond-mat_str-el" }, { "text": "Localized moments and the stability of antiferromagnetic order in Yb3Pt4: We present here the results of electrical resistivity {\\rho}, magnetization\nM, ac susceptibility \\c{hi}ac', and specific heat CM measurements that have\nbeen carried out on single crystals of Yb3Pt4 over a wide range of fields and\ntemperatures. The 2.4-K N\\'eel temperature that is found in zero field\ncollapses under field to a first-order transition TN=0 at BCEP=1.85 T. In the\nabsence of antiferromagnetic order, the specific heat CM(T,B), the\nmagnetization M(T,B), and even the resistivity {\\rho}(T,B) all display B/T\nscaling, indicating that they are dominated by strong paramagnetic\nfluctuations, where the only characteristic energy scale results from the\nZeeman splitting of an energetically isolated, Yb doublet ground state. This\nparamagnetic scattering disappears with the onset of antiferromagnetic order,\nrevealing Fermi liquid behavior {\\Delta}{\\rho}=AT2 that persists up to the\nantiferromagnetic phase line TN(B), but not beyond. The first-order character\nof TN=0 and the ubiquity of the paramagnetic fluctuations imply that\nnon-Fermi-liquid behaviors are absent in Yb3Pt4. In contrast to heavy fermions\nsuch as YbRh2Si2, Yb3Pt4 represents an extremely simple regime of f-electron\nbehavior where the Yb moments and conduction electrons are almost decoupled,\nand where Kondo physics plays little role.", "category": "cond-mat_str-el" }, { "text": "An effect of the uniaxial strain on the temperature of Bose-Einstein\n condensation of the intersite bipolarons: We have studied an effect of uniaxial strain to the temperature of\nBose-Einstein condensation of intersite bipolarons within the framework of\nExtended Holstein-Hubbard model. Uniaxial lattice strains are taken into an\naccount by introducing a generalized density-displacement type force for\nelectron-lattice interaction. Associating the superconducting critical\ntemperature $T_c$ with the temperature of Bose-Einstein condensation $T_{BEC}$\nof intersite bipolarons we have calculated strain derivatives of $T_{BEC}$ and\nsatisfactorily explained the results of the experiments on La-based high-$T_c$\nfilms.", "category": "cond-mat_str-el" }, { "text": "Staggered Flux State in Two-Dimensional Hubbard Models: The stability and other properties of a staggered flux (SF) state or a\ncorrelated d-density wave state are studied for the Hubbard (t-t'-U) model on\nextended square lattices, as a low-lying state that competes with the\nd(x2-y2)-wave superconductivity (d-SC) and possibly causes the pseudogap\nphenomena in underdoped high-Tc cuprates and organic kappa-BEDT-TTF salts. In\ncalculations, a variational Monte Carlo method is used. In the trial wave\nfunction, a configuration-dependent phase factor, which is vital to treat a\ncurrent-carrying state for a large U/t, is introduced in addition to ordinary\ncorrelation factors. Varying U/t, t'/t, and the doping rate (delta)\nsystematically, we show that the SF state becomes more stable than the normal\nstate (projected Fermi sea) for a strongly correlated (U/t\\gtrsim 5) and\nunderdoped (delta\\lesssim 0.16) area. The decrease in energy is sizable,\nparticularly in the area where Mott physics prevails and the circular current\n(order parameter) is strongly suppressed. These features are consistent with\nthose for the t-J model. The effect of the frustration t'/t plays a crucial\nrole in preserving charge homogeneity and appropriately describing the behavior\nof hole- and electron-doped cuprates and kappa-BEDT-TTF salts. We argue that\nthe SF state does not coexist with d-SC and is not a `normal state' from which\nd-SC arises. We also show that a spin current (flux or nematic) state is never\nstabilized in the same regime.", "category": "cond-mat_str-el" }, { "text": "Nuclear magnetic resonance signature of the spin-nematic phase in\n LiCuVO$_{4}$ at high magnetic fields: We report a 51V nuclear magnetic resonance investigation of the frustrated\nspin-1/2 chain compound LiCuVO4, performed in pulsed magnetic fields and\nfocused on high-field phases up to 55 T. For the crystal orientations H // c\nand H // b we find a narrow field region just below the magnetic saturation\nwhere the local magnetization remains uniform and homogeneous, while its value\nis field dependent. This behavior is the first microscopic signature of the\nspin-nematic state, breaking spin-rotation symmetry without generating any\ntransverse dipolar order, and is consistent with theoretical predictions for\nthe LiCuVO4 compound.", "category": "cond-mat_str-el" }, { "text": "Stability of the doped antiferromagnetic state of the t-t'-Hubbard model: The next-nearest-neighbour hopping term t' is shown to stabilize the AF state\nof the doped Hubbard model with respect to transverse perturbations in the\norder- parameter by strongly suppressing the intraband particle-hole processes.\nFor a fixed sign of t', this stabilization is found to be significantly\ndifferent for electron and hole doping, which qualitatively explains the\nobserved difference in the degree of robustness of the AF state in the\nelectron-doped (Nd_{2-x}Ce_{x}CuO_{4}) and hole-doped (La_{2-x}Sr_{x}CuO_{4})\ncuprates. The t'-U phase diagram is obtained for both signs of the t' term,\nshowing the different regions of stability and instability of the doped\nantiferromagnet. Doping is shown to suppress the t'-induced frustration due to\nthe competing interaction J'. A study of transverse spin fluctuations in the\nmetallic AF state reveals that the decay of magnons into particle-hole\nexcitations yields an interesting low-energy result \\Gamma \\sim \\omega for\nmagnon damping.", "category": "cond-mat_str-el" }, { "text": "First order transition from ferromagnetism to antiferromagnetism in\n Ce(Fe$_{0.96}$Al$_{0.04}$)$_2$: a magnetotransport study: The magnetotransport behaviour is investigated in detail across the first\norder magnetic phase transition from ferromagnetic to antiferromagnetic state\nin polycrystalline Ce(Fe$_{0.96}$Al$_{0.04}$)$_2$ sample. The study clearly\nbrings out various generic features associated with a first order transition,\nviz., hysteresis, phase coexistence, supercooling and superheating, presence\nand limits of the metastable regimes. These results of magnetotransport study\nexhibit and support all the interesting thermomagnetic history effects that\nwere observed in our earlier dc-magnetisation study on the same sample. Most\nnotable here is the initial (or virgin) resistivity vs. field curve lying\noutside the hysteretic \"butterfly shaped\" magnetoresistivity loops obtained on\ncyclying the magnetic field between high enough positive and negative\nstrengths. These findings, bearing one-to-one similarity with the data obtained\nin their magnetic counterpart (i.e., dc-magnetisation), have been ascribed an\norigin due to the arresting of this first order transition kinetics at low\ntemperature and high magnetic field.", "category": "cond-mat_str-el" }, { "text": "$\\require{mhchem}$Quantum paramagnetism in the decorated square-kagome\n antiferromagnet $\\ce{Na6Cu7BiO4(PO4)4Cl3}$: $\\require{mhchem}$The square-kagome lattice Heisenberg antiferromagnet is a\nhighly frustrated Hamiltonian whose material realizations have been scarce. We\ntheoretically investigate the recently synthesized $\\ce{Na6Cu7BiO4(PO4)4Cl3}$\nwhere a Cu$^{2+}$ spin-$1/2$ square-kagome lattice (with six site unit cell) is\ndecorated by a seventh magnetic site alternatingly above and below the layers.\nThe material does not show any sign of long-range magnetic order down to 50 mK\ndespite a Curie-Weiss temperature of $-212$ K indicating a quantum paramagnetic\nphase. Our DFT energy mapping elicits a purely antiferromagnetic Hamiltonian\nthat features longer range exchange interactions beyond the pure square-kagome\nmodel and, importantly, we find the seventh site to be strongly coupled to the\nplane. We combine two variational Monte Carlo approaches,\npseudo-fermion/Majorana functional renormalization group and Schwinger-Boson\nmean field calculations to show that the complex Hamiltonian of\n$\\ce{Na6Cu7BiO4(PO4)4Cl3}$ still features a nonmagnetic ground state. We\nexplain how the seventh Cu$^{2+}$ site actually aids the stabilization of the\ndisordered state. We predict static and dynamic spin structure factors to guide\nfuture neutron scattering experiments.", "category": "cond-mat_str-el" }, { "text": "Symmetry Protected Topological Order by Folding a One-Dimensional\n Spin-$1/2$ Chain: We present a toy model with a Hamiltonian $H^{(2)}_T$ on a folded\none-dimensional spin chain. The non-trivial ground states of $H^{(2)}_T$ are\nseparated by a gap from the excited states. By analyzing the symmetries in the\nmodel, we find that the topological order is protected by a $\\mathbb{Z}_2$\nglobal symmetry. However, by using perturbation series and excluding thermal\neffects, we show that the $\\mathbb{Z}_2$ symmetry is stable in comparison to a\nstandard nearest-neighbor Ising model with a Hamiltonian $H_I$. We find that\n$H^{(2)}_T$ is a member of a family of Hamiltonians that are adiabatically\nconnected to $H_I$. Furthermore, the generalizations of this class of\nHamiltonians, their adiabatic connection to $H_I$, and the relation to quantum\nerror-correcting codes are discussed. Finally, we show the correspondence\nbetween the two ground states of $H^{(2)}_T$ and the unpaired Majorana modes,\nand provide numerical examples.", "category": "cond-mat_str-el" }, { "text": "Polaron with Quadratic Electron-phonon Interaction: We present the first numerically exact study of a polaron with quadratic\ncoupling to the oscillator displacement, using two alternative methodological\ndevelopments. Our results cover both anti-adiabatic and adiabatic regimes and\nthe entire range of electron-phonon coupling $g_2$, from the system's stability\nthreshold at attractive $g_2=-1$ to arbitrary strong repulsion at $g_2 \\gg 1$.\nKey properties of quadratic polarons prove dramatically different from their\nlinear counterparts. They (i) are insensitive even to large quadratic coupling\nexcept in the anti-adiabatic limit near the threshold of instability at\nattraction; (ii) depend only on the adiabatic ratio but are insensitive to the\nelectron dispersion and dimension of space; (iii) feature weak lattice\ndeformations even at the instability point. Our results are of direct relevance\nto properties of electrons at low densities in polar materials, including\nrecent proposals for their superconducting states.", "category": "cond-mat_str-el" }, { "text": "Optical manipulation of electronic dimensionality in a quantum material: Exotic phenomenon can be achieved in quantum materials by confining\nelectronic states into two dimensions. For example, relativistic fermions are\nrealised in a single layer of carbon atoms, the quantized Hall effect can\nresult from two-dimensional (2D) systems, and the superconducting transition\ntemperature can be enhanced significantly in a one-atomic-layer material.\nOrdinarily, 2D electronic system can be obtained by exfoliating the layered\nmaterials, growing monolayer materials on substrates, or establishing\ninterfaces between different materials. Herein, we use femtosecond infrared\nlaser pulses to invert the periodic lattice distortion sectionally in a\nthree-dimensional (3D) charge density wave material, creating macroscopic\ndomain walls of transient 2D ordered electronic states with exotic properties.\nThe corresponding ultrafast electronic and lattice dynamics are captured by\ntime- and angle-resolved photoemission spectroscopy and MeV ultrafast electron\ndiffraction. Surprisingly, a novel energy gap state, which might be a signature\nof light-induced superconductivity, is identified in the photoinduced 2D domain\nwall near the surface. Such optical modulation of atomic motion is a new path\nto realise 2D electronic states and will be a new platform for creating novel\nphases in quantum materials.", "category": "cond-mat_str-el" }, { "text": "Mott Transition of MnO under Pressure: Comparison of Correlated Band\n Theories: The electronic structure, magnetic moment, and volume collapse of MnO under\npressure are obtained from four different correlated band theory methods; local\ndensity approximation + Hubbard U (LDA+U), pseudopotential self-interaction\ncorrection (pseudo-SIC), the hybrid functional (combined local exchange plus\nHartree-Fock exchange), and the local spin density SIC (SIC-LSD) method. Each\nmethod treats correlation among the five Mn 3d orbitals (per spin), including\ntheir hybridization with three O $2p$ orbitals in the valence bands and their\nchanges with pressure. The focus is on comparison of the methods for rocksalt\nMnO (neglecting the observed transition to the NiAs structure in the 90-100 GPa\nrange). Each method predicts a first-order volume collapse, but with variation\nin the predicted volume and critical pressure. Accompanying the volume collapse\nis a moment collapse, which for all methods is from high-spin to low-spin (5/2\nto 1/2), not to nonmagnetic as the simplest scenario would have. The specific\nmanner in which the transition occurs varies considerably among the methods:\npseudo-SIC and SIC-LSD give insulator-to-metal, while LDA+U gives\ninsulator-to-insulator and the hybrid method gives an insulator-to-semimetal\ntransition. Projected densities of states above and below the transition are\npresented for each of the methods and used to analyze the character of each\ntransition. In some cases the rhombohedral symmetry of the\nantiferromagnetically ordered phase clearly influences the character of the\ntransition.", "category": "cond-mat_str-el" }, { "text": "Conductance and Kondo effect of a controlled single atom contact: The tip of a low-temperature scanning tunneling microscope is brought into\ncontact with individual Kondo impurities (cobalt atoms) adsorbed on a Cu(100)\nsurface. A smooth transition from the tunneling regime to a point contact with\na conductance of $G\\approx\\text{G}_0$ occurs. Spectroscopy in the contact\nregime, {\\it i. e.}, at currents in a $\\mu\\text{A}$ range was achieved. A\nmodified line shape is observed indicating a significant change of the Kondo\ntemperature $T_{\\text{K}}$ at contact. Model calculations indicate that the\nproximity of the tip shifts the cobalt $d$-band and thus affects\n$T_{\\text{K}}$.", "category": "cond-mat_str-el" }, { "text": "Probing Critical Surfaces in Momentum Space Using Real-Space\n Entanglement Entropy: Bose versus Fermi: A co-dimension one critical surface in the momentum space can be either a\nfamiliar Fermi surface, which separates occupied states from empty ones in the\nnon-interacting fermion case, or a novel Bose surface, where gapless bosonic\nexcitations are anchored. Their presence gives rise to logarithmic violation of\nentanglement entropy area law. When they are convex, we show that the shape of\nthese critical surfaces can be determined by inspecting the leading logarithmic\nterm of real space entanglement entropy. The fundamental difference between a\nFermi surface and a Bose surface is revealed by the fact that the logarithmic\nterms in entanglement entropies differ by a factor of two: $S^{Bose}_{log} = 2\nS^{Fermi}_{log}$, even when they have identical geometry. Our method has\nremarkable similarity with determining Fermi surface shape using quantum\noscillation. We also discuss possible probes of concave critical surfaces in\nmomentum space.", "category": "cond-mat_str-el" }, { "text": "Spin-orbit coupling and ESR theory for carbon nanotubes: A theoretical description of ESR in 1D interacting metals is given, with\nprimary emphasis on carbon nanotubes. The spin-orbit coupling is derived, and\nthe resulting ESR spectrum is analyzed by field theory and exact\ndiagonalization. Drastic differences in the ESR spectra of single-wall and\nmulti-wall nanotubes are found. For single-wall tubes, the predicted double\npeak spectrum could reveal spin-charge separation.", "category": "cond-mat_str-el" }, { "text": "Quantum phase transition in the one-dimensional compass model: We introduce a one-dimensional model which interpolates between the Ising\nmodel and the quantum compass model with frustrated pseudospin interactions\n$\\sigma_i^z\\sigma_{i+1}^z$ and $\\sigma_i^x\\sigma_{i+1}^x$, alternating between\neven/odd bonds, and present its exact solution by mapping to quantum Ising\nmodels. We show that the nearest neighbor pseudospin correlations change\ndiscontinuosly and indicate divergent correlation length at the first order\nquantum phase transition. At this transition one finds the disordered ground\nstate of the compass model with high degeneracy $2\\times 2^{N/2}$ in the limit\nof $N\\to\\infty$.", "category": "cond-mat_str-el" }, { "text": "Hidden Integrability of a Kondo Impurity in an Unconventional Host: We study a spin-1/2 Kondo impurity coupled to an unconventional host in which\nthe density of band states vanishes either precisely at (``gapless'' systems)\nor on some interval around the Fermi level (``gapped''systems). Despite an\nessentially nonlinear band dispersion, the system is proven to exhibit hidden\nintegrability and is diagonalized exactly by the Bethe ansatz.", "category": "cond-mat_str-el" }, { "text": "A High Pressure Neutron Study of Colossal Magnetoresistant\n NdMnAsO0.95F0.05: A high pressure neutron diffraction study of the oxypnictide NdMnAsO0.95F0.05\nhas been performed at temperatures of 290 K - 383 K and pressures up to 8.59\nGPa. The results demonstrate that the antiferromagnetic order of the Mn spins\nis robust to pressures of up to 8.59 GPa. TN is enhanced from 360 K to 383 K\nupon applying an external pressure of 4.97 GPa, a rate of 4.63 K/GPa.\nNdMnAsO0.95F0.05 is shown to violate Bloch's rule which would suggest that\nNdMnAsO0.95F0.05 is on the verge of a localised to itinerant transition. There\nis no evidence of a structural transition but applied pressure tends to result\nin more regular As-Mn-As and Nd-O-Nd tetrahedra. The unit cell is significantly\nmore compressible along the c-axis than the a-axis, as the inter-layer coupling\nis weaker than the intrinsic bonds contained within NdO and MnAs slabs.", "category": "cond-mat_str-el" }, { "text": "Spin Configuration in the 1/3 Magnetization Plateau of Azurite\n Determined by NMR: High magnetic field $^{63,65}$Cu NMR spectra were used to determine the local\nspin polarization in the 1/3 magnetization plateau of azurite,\nCu$_3$(CO$_3$)$_2$(OH)$_2$, which is a model system for the distorted diamond\nantiferromagnetic spin-1/2 chain. The spin part of the hyperfine field of the\nCu2 (dimer) sites is found to be field independent, negative and strongly\nanisotropic, corresponding to $\\approx$10 % of fully polarized spin in a\n$d$-orbital. This is close to the expected configuration of the \"quantum\"\nplateau, where a singlet state is stabilized on the dimer. However, the\nobserved non-zero spin polarization points to some triplet admixture, induced\nby strong asymmetry of the diamond bonds $J_1$ and $J_3$.", "category": "cond-mat_str-el" }, { "text": "Exploring the spin-orbital ground state of Ba3CuSb2O9: Motivated by the absence of both spin freezing and a cooperative Jahn-Teller\neffect at the lowest measured temperatures, we study the ground state of\nBa3CuSb2O9. We solve a general spin-orbital model on both the honeycomb and the\ndecorated honeycomb lattice, revealing rich phase diagrams. The spin-orbital\nmodel on the honeycomb lattice contains an SU(4) point, where previous studies\nhave shown the existence of a spin-orbital liquid with algebraically decaying\ncorrelations. For realistic parameters on the decorated honeycomb lattice, we\nfind a phase that consists of clusters of nearest-neighbour spin singlets,\nwhich can be understood in terms of dimer coverings of an emergent square\nlattice. While the experimental situation is complicated by structural\ndisorder, we show qualitative agreement between our theory and a range of\nexperiments.", "category": "cond-mat_str-el" }, { "text": "The one dimensional Kondo lattice model at partial band filling: The Kondo lattice model introduced in 1977 describes a lattice of localized\nmagnetic moments interacting with a sea of conduction electrons. It is one of\nthe most important canonical models in the study of a class of rare earth\ncompounds, called heavy fermion systems, and as such has been studied\nintensively by a wide variety of techniques for more than a quarter of a\ncentury. This review focuses on the one dimensional case at partial band\nfilling, in which the number of conduction electrons is less than the number of\nlocalized moments. The theoretical understanding, based on the bosonized\nsolution, of the conventional Kondo lattice model is presented in great detail.\nThis review divides naturally into two parts, the first relating to the\ndescription of the formalism, and the second to its application. After an\nall-inclusive description of the bosonization technique, the bosonized form of\nthe Kondo lattice hamiltonian is constructed in detail. Next the\ndouble-exchange ordering, Kondo singlet formation, the RKKY interaction and\nspin polaron formation are described comprehensively. An in-depth analysis of\nthe phase diagram follows, with special emphasis on the destruction of the\nferromagnetic phase by spin-flip disorder scattering, and of recent numerical\nresults. The results are shown to hold for both antiferromagnetic and\nferromagnetic Kondo lattice. The general exposition is pedagogic in tone.", "category": "cond-mat_str-el" }, { "text": "LaMnO$_3$ is a Mott Insulator: an precise definition and an evaluation\n of the local interaction strength: We compare the interaction parameters measured on LaMnO$_3$ to single site\ndynamical mean field estimates of the critical correlation strength needed to\ndrive a Mott transition, finding that the total correlation strength\n(electron-electron plus electron-lattice) is very close to but slightly larger\nthan the critical value, while if the electron lattice interaction is neglected\nthe model is metallic. Our results emphasize the importance of additional\nphysics including the buckling of the Mn-O-Mn bonds.", "category": "cond-mat_str-el" }, { "text": "\"Self-Dual\" Quantum Critical Point on the surface of $3d$ Topological\n Insulator: In the last few years a lot of exotic and anomalous topological phases were\nconstructed by proliferating the vortex like topological defects on the surface\nof the $3d$ topological insulator (TI). In this work, rather than considering\ntopological phases at the boundary, we will study quantum critical points\ndriven by vortex like topological defects. In general we will discuss a\n$(2+1)d$ quantum phase transition described by the following field theory:\n$\\mathcal{L} = \\bar{\\psi}\\gamma_\\mu (\\partial_\\mu - i a_\\mu) \\psi +\n|(\\partial_\\mu - i k a_\\mu)\\phi|^2 + r |\\phi|^2 + g |\\phi|^4$, with tuning\nparameter $r$, arbitrary integer $k$, Dirac fermion $\\psi$ and complex scalar\nbosonic field $\\phi$ which both couple to the same $(2+1)d$ dynamical\nnoncompact U(1) gauge field $a_\\mu$. The physical meaning of these\nquantities/fields will be explained in the text. We demonstrate that this\nquantum critical point has a quasi self-dual nature. And at this quantum\ncritical point, various universal quantities such as the electrical\nconductivity, and scaling dimension of gauge invariant operators can be\ncalculated systematically through a $1/k^2$ expansion, based on the observation\nthat the limit $k \\rightarrow + \\infty$ corresponds to an ordinary $3d$ XY\ntransition.", "category": "cond-mat_str-el" }, { "text": "Hydrostatic pressure effect on Co-based honeycomb magnet BaCo2(AsO4)2: The honeycomb antiferromagnet BaCo2(AsO4)2, in which small in-plane magnetic\nfields (H1 = 0.26 T and H2 = 0.52 T at T = 1.8 K < TN = 5.4 K) induce two\nmagnetic phase transitions, has attracted attention as a possible candidate\nmaterial for the realization of Kitaev physics based on the 3d element Co2+.\nHere, we report on the change of the transition temperature TN and the critical\nfields H1 and H2 of BaCo2(AsO4)2 with hydrostatic pressure up to ~ 20 kbar, as\ndetermined from magnetization and specific heat measurements. Within this\npressure range, a marginal increase of the magnetic ordering temperature is\nobserved. At the same time, the critical fields are changed significantly (up\nto ~ 25-35 %). Specifically, we find that H1 is increased with hydrostatic\npressure, i.e., the antiferromagnetic state is stabilized with hydrostatic\npressure, whereas H2, which was previously associated with a transition into a\nproposed Kitaev spin liquid state, decreases with increasing pressure. These\nresults put constraints on the magnetic models that are used to describe the\nlow-temperature magnetic properties of BaCo2(AsO4)2.", "category": "cond-mat_str-el" }, { "text": "An exactly size consistent geminal power via Jastrow factor networks in\n a local one particle basis: The accurate but expensive product of geminals ansatz may be approximated by\na geminal power, but this approach sacrifices size consistency. Here we show\nboth analytically and numerically that a size consistent form very similar to\nthe product of geminals can be recovered using a network of location specific\nJastrow factors. Upon variational energy minimization, the network creates\nparticle number projections that remove the charge fluctuations responsible for\nsize inconsistency. This polynomial cost approach captures strong many-electron\ncorrelations, giving a maximum error of just 1.8 kcal/mol during the\ndouble-bond dissociation of H2O in an STO-3G atomic orbital basis.", "category": "cond-mat_str-el" }, { "text": "Deconfined quantum criticality driven by Dirac fermions in SU(2)\n antiferromagnets: Quantum electrodynamics in 2+1 dimensions is an effective gauge theory for\nthe so called algebraic quantum liquids. A new type of such a liquid, the\nalgebraic charge liquid, has been proposed recently in the context of\ndeconfined quantum critical points [R. K. Kaul {\\it et al.}, Nature Physics\n{\\bf 4}, 28 (2008)]. In this context, we show by using the renormalization\ngroup in $d=4-\\epsilon$ spacetime dimensions, that a deconfined quantum\ncritical point occurs in a SU(2) system provided the number of Dirac fermion\nspecies $N_f\\geq 4$. The calculations are done in a representation where the\nDirac fermions are given by four-component spinors. The critical exponents are\ncalculated for several values of $N_f$. In particular, for $N_f=4$ and\n$\\epsilon=1$ ($d=2+1$) the anomalous dimension of the N\\'eel field is given by\n$\\eta_N=1/3$, with a correlation length exponent $\\nu=1/2$. These values change\nconsiderably for $N_f>4$. For instance, for $N_f=6$ we find $\\eta_N\\approx\n0.75191$ and $\\nu\\approx 0.66009$. We also investigate the effect of chiral\nsymmetry breaking and analyze the scaling behavior of the chiral holon\nsusceptibility, $G_\\chi(x)\\equiv<\\bar \\psi(x)\\psi(x)\\bar \\psi(0)\\psi(0)>$.", "category": "cond-mat_str-el" }, { "text": "Flat bands and $Z_2$ topological phases in a non-Abelian kagome lattice: We introduce a non-Abelian kagome lattice model that has both time-reversal\nand inversion symmetries and study the flat band physics and topological phases\nof this model. Due to the coexistence of both time-reversal and inversion\nsymmetries, the energy bands consist of three doubly degenerate bands whose\nenergy and conditions for the presence of flat bands could be obtained\nanalytically, allowing us to tune the flat band with respect to the other two\ndispersive bands from the top to the middle and then to the bottom of the three\nbands. We further study the gapped phases of the model and show that they\nbelong to the same phase as the band gaps only close at discrete points of the\nparameter space, making any two gapped phases adiabatically connected to each\nother without closing the band gap. Using the Pfaffian approach based on the\ntime-reversal symmetry and parity characterization from the inversion symmetry,\nwe calculate the bulk topological invariants and demonstrate that the unique\ngapped phases belong to the $Z_2$ quantum spin Hall phase, which is further\nconfirmed by the edge state calculations.", "category": "cond-mat_str-el" }, { "text": "Defect-induced edge ferromagnetism and fractional spin excitations of\n the SU(4) $\u03c0$-flux Hubbard model on honeycomb lattice: Recently, a SU(4) $\\pi$-flux Hubbard model on the honeycomb lattice has been\nproposed to study the spin-orbit excitations of $\\alpha$-ZrCl$_3$\n[Phys.~Rev.~Lett. 121.097201~(2017)]. Based on this model with a zigzag edge,\nwe show the edge defects can induce edge flat bands that result in a SU(4) edge\nferromagnetism. We develop an effective one-dimensional interaction Hamiltonian\nto study the corresponding SU(4) spin excitations. Remarkably, SU(4) spin\nexcitations of the edge ferromagnet appear as a continuum covering the entire\nenergy region rather than usual magnons. Through further entanglement entropy\nanalysis, we suggest that the continuum consists of fractionalized spin\nexcitations from the disappeared magnons, except for that from the\nparticle-hole Stoner excitations. Moreover, in ribbon systems with finite\nwidths, the disappeared magnons can be restored in the gap formed by the\nfinite-size effect and the optical branch of the restored magnons are found to\nbe topological nontrivial.", "category": "cond-mat_str-el" }, { "text": "Phase-Space Berry Phases in Chiral Magnets: Dzyaloshinskii-Moriya\n Interaction and the Charge of Skyrmions: The semiclassical motion of electrons in phase space, x=(R, k), is influenced\nby Berry phases described by a 6-component vector potential, A=(A^R, A^k). In\nchiral magnets Dzyaloshinskii-Moriya (DM) interactions induce slowly varying\nmagnetic textures (helices and skyrmion lattices) for which all components of A\nare important inducing effectively a curvature in mixed position and momentum\nspace. We show that for smooth textures and weak spin-orbit coupling phase\nspace Berry curvatures determine the DM interactions and give important\ncontributions to the charge. Using ab initio methods we calculate the strength\nof DM interactions in MnSi in good agreement with experiment and estimate the\ncharge of skyrmions.", "category": "cond-mat_str-el" }, { "text": "Visualizing Strange Metallic Correlations in the 2D Fermi-Hubbard Model\n with AI: Strongly correlated phases of matter are often described in terms of\nstraightforward electronic patterns. This has so far been the basis for\nstudying the Fermi-Hubbard model realized with ultracold atoms. Here, we show\nthat artificial intelligence (AI) can provide an unbiased alternative to this\nparadigm for phases with subtle, or even unknown, patterns. Long- and\nshort-range spin correlations spontaneously emerge in filters of a\nconvolutional neural network trained on snapshots of single atomic species. In\nthe less well-understood strange metallic phase of the model, we find that a\nmore complex network trained on snapshots of local moments produces an\neffective order parameter for the non-Fermi-liquid behavior. Our technique can\nbe employed to characterize correlations unique to other phases with no obvious\norder parameters or signatures in projective measurements, and has implications\nfor science discovery through AI beyond strongly correlated systems.", "category": "cond-mat_str-el" }, { "text": "Exciton doublet in the Mott-Hubbard LiCuVO$_4$ insulator identified by\n spectral ellipsometry: Spectroscopic ellipsometry was used to study the dielectric function of\nLiCuVO$_{4}$, a compound comprised of chains of edge-sharing CuO$_4$\nplaquettes, in the spectral range (0.75 - 6.5) eV at temperatures (7-300) K.\nFor photon polarization along the chains, the data reveal a weak but\nwell-resolved two-peak structure centered at 2.15 and 2.95 eV whose spectral\nweight is strongly enhanced upon cooling near the magnetic ordering\ntemperature. We identify these features as an exciton doublet in the\nMott-Hubbard gap that emerges as a consequence of the Coulomb interaction\nbetween electrons on nearest and next-nearest neighbor sites along the chains.\nOur results and methodology can be used to address the role of the long-range\nCoulomb repulsion for compounds with doped copper-oxide chains and planes.", "category": "cond-mat_str-el" }, { "text": "Persistence of Ising-like easy-axis spin correlations in the\n paramagnetic state of the spin-1 chain compound NiTe$_2$O$_5$: A $^{125}$Te nuclear magnetic resonance (NMR) study was carried out in the\nparamagnetic state of the recently discovered quasi-one-dimensional spin-1\nchain compound NiTe$_2$O$_5$. We observed that the $^{125}$Te NMR spectrum\nsplits into two in a magnetic field applied along the $c$ axis. Based on the\nstrong temperature variation of the relative intensity ratio of the split\nlines, we infer that the line splitting arises from the two sublattice\nsusceptibilities induced in opposite directions along the chains. In great\nsupport of this interpretation, a quantitative analysis of the spin-lattice\nrelaxation rate $T_1^{-1}$ and the Knight shift data unravels dominant\ntransverse spin fluctuations. We conclude that Ising-like uniaxial spin\ncorrelations persist up to surprisingly high temperatures compared to the\nexchange energy scales. Spin-charge coupling mechanism via a self-doping effect\nmay be important.", "category": "cond-mat_str-el" }, { "text": "Thermodynamic behavior of the XXZ Heisenberg s=1/2 chain around the\n factorizing magnetic field: We have investigated the zero and finite temperature behaviors of the\nanisotropic antiferromagnetic Heisenberg XXZ spin-1/2 chain in the presence of\na transverse magnetic field (h). The attention is concentrated on an interval\nof magnetic field between the factorizing field (h_f) and the critical one\n(h_c). The model presents a spin-flop phase for 01$ only for small doping and strong\ncorrelations.", "category": "cond-mat_str-el" }, { "text": "Emergence of nontrivial magnetic excitations in a spin liquid state of\n kagome volborthite: When quantum fluctuations destroy underlying long-range ordered states, novel\nquantum states emerge. Spin-liquid (SL) states of frustrated quantum\nantiferromagnets, in which highly-correlated spins keep to fluctuate down to\nvery low temperatures, are prominent examples of such quantum states. SL states\noften exhibit exotic physical properties, but the precise nature of the\nelementary excitations behind such phenomena remains entirely elusive. Here we\nutilize thermal Hall measurements that can capture the unexplored property of\nthe elementary excitations in SL states, and report on the observation of\nanomalous excitations that may unveil the unique features of the SL state. Our\nprincipal finding is a negative thermal Hall conductivity (k_xy) which the\ncharge-neutral spin excitations in a gapless SL state of the two-dimensional\nkagome insulator volborthite Cu_3V_2O_7(OH)_2 \\cdot 2H_2O exhibit, in much the\nsame way in which charged electrons give rise to the conventional electric Hall\neffect. We find that k_xy is absent in the high-temperature paramagnetic state\nand develops upon entering the SL state in accordance with the growth of the\nshort-range spin correlations, demonstrating that k_xy is a key signature of\nthe elementary excitation formed in the SL state. These results suggest the\nemergence of nontrivial elementary excitations in the gapless SL state which\nfeel the presence of fictitious magnetic flux, whose effective Lorentz force is\nfound to be less than 1/100 of that experienced by free electrons.", "category": "cond-mat_str-el" }, { "text": "Low-energy excitations of the Hubbard model on the Kagom\u00e9 lattice: The Hubbard model on the Kagom\\'e lattice is investigated in a metallic phase\nat half-filling. By introducing anisotropic electron hopping on the lattice, we\ncontrol geometrical frustration and clarify how the lattice geometry affects\nphysical properties. By means of the fluctuation exchange (FLEX) approximation,\nwe calculate the spin and charge susceptibilities, the one-particle spectral\nfunction, the quasi-particle renormalization factor, and the Fermi velocity. It\nis found that geometrical frustration of the Kagom\\'e lattice suppresses the\ninstability to various ordered states through the strong reduction of the\nwavevector dependence of susceptibilities, thereby stabilizing the formation of\nquasi-particles due to the almost isotropic spin fluctuations in the Brillouin\nzone. These characteristic properties are discussed in connection with the\neffects of geometrical frustration in the strong coupling regime.", "category": "cond-mat_str-el" }, { "text": "Local and nonlocal order parameters in the Kitaev chain: We have calculated order parameters for the phases of the Kitaev chain with\ninteraction and dimerization at a special symmetric point applying the\nJordan-Wigner and other duality transformations. We use string order parameters\n(SOPs) defined via the correlation functions of the Majorana string operators.\nThe SOPs are mapped onto the local order parameters of some dual Hamiltonians\nand easily calculated. We have shown that the phase diagram of the interacting\ndimerized chain comprises the phases with the conventional local order as well\nas the phases with nonlocal SOPs. From the results for the critical indices we\ninfer the 2D Ising universality class of criticality at the particular symmetry\npoint where the model is exactly solvable.", "category": "cond-mat_str-el" }, { "text": "Quantum criticality with a twist - interplay of correlations and Kohn\n anomalies in three dimensions: A general understanding of quantum phase transitions in strongly correlated\nmaterials is still lacking. By exploiting a cutting-edge quantum many-body\napproach, the dynamical vertex approximation, we make an important progress,\ndetermining the quantum critical properties of the antiferromagnetic transition\nin the fundamental model for correlated electrons, the Hubbard model in three\ndimensions. In particular, we demonstrate that -in contradiction to the\nconventional Hertz-Millis-Moriya theory- its quantum critical behavior is\ndriven by the Kohn anomalies of the Fermi surface, even when electronic\ncorrelations become strong.", "category": "cond-mat_str-el" }, { "text": "Doped Mott phase and charge correlations in monolayer 1T-NbSe$_2$: The doped Hubbard model is one of the paradigmatic platforms to engineer\nexotic quantum many-body states, including charge-ordered states, strange\nmetals and unconventional superconductors. While undoped and doped correlated\nphases have been experimentally realized in a variety twisted van der Waals\nmaterials, experiments in monolayer materials, and in particular 1T transition\nmetal dichalcogenides, have solely reached the conventional insulating undoped\nregime. Correlated phases in monolayer two-dimensional materials have much\nhigher associated energy scales than their twisted counterparts, making doped\ncorrelated monolayers an attractive platform for high temperature correlated\nquantum matter. Here, we demonstrate the realization of a doped Mott phase in a\nvan der Waals dichalcogenide 1T-NbSe$_2$ monolayer. The system is electron\ndoped due to electron transfer to a monolayer van der Waals substrate via\nproximity, leading to a correlated triangular lattice with both half-filled and\nfully-filled sites. We analyze the distribution of the half-filled and filled\nsites and show the arrangement is unlikely to be controlled by disorder alone,\nand we show that the presence of competing non-local many-body correlations\nwould account for the charge correlations found experimentally. Our results\nestablish 1T-NbSe$_2$ as a potential monolayer platform to explore correlated\ndoped Mott physics in a frustrated lattice.", "category": "cond-mat_str-el" }, { "text": "Hydrodynamic spin fluctuations in the antiferromagnetic Heisenberg chain: We study the finite temperature, low energy, long wave-length spectrum of the\ndynamic structure factor of the spin-$1/2$ antiferromagnetic Heisenberg chain\nin the presence of exchange anisotropy and external magnetic fields. Using\nimaginary-time quantum Monte-Carlo we extract parameters, relevant to\ncharacterize a {\\it renormalized} Luttinger liquid. For small momentum our\nresults are consistent with a change from propagating spinon density waves to\nspin diffusion, described by a finite-frequency spin-current relaxation rate.\nResults for this relaxation rate as well as other Luttinger liquid parameters\nare presented versus temperature, momentum, magnetic field, and anisotropy,\nincluding finite-size analysis, and checks for anomalous diffusion. Our results\nare consistent with exact diagonalization and Bethe Ansatz, where available,\nand with corroborate findings of other previous studies using bosonization,\ntransfer matrix renormalization group, and quantum Monte-Carlo.", "category": "cond-mat_str-el" }, { "text": "Lock-in of a Chiral Soliton Lattice by Itinerant Electrons: Chiral magnets often show intriguing magnetic and transport properties\nassociated with their peculiar spin textures. A typical example is a chiral\nsoliton lattice, which is found in monoaxial chiral magnets, such as\nCrNb$_3$S$_6$ and Yb(Ni$_{1-x}$Cu$_x$)$_3$Al$_9$ in an external magnetic field\nperpendicular to the chiral axis. Here, we theoretically investigate the\nelectronic and magnetic properties in the chiral soliton lattice by a minimal\nitinerant electron model. Using variational calculations, we find that the\nperiod of the chiral soliton lattice can be locked at particular values\ndictated by the Fermi wave number, in stark contrast to spin-only models. We\ndiscuss this behavior caused by the spin-charge coupling as a possible\nmechanism for the lock-in discovered in Yb(Ni$_{1-x}$Cu$_x$)$_3$Al$_9$. We also\nshow that the same mechanism leads to the spontaneous formation of the chiral\nsoliton lattice even in the absence of the magnetic field.", "category": "cond-mat_str-el" }, { "text": "Recent Progress of Point Contact Spectroscopy as a Probe of Correlated\n Electron States: We review recent progress in point contact spectroscopy (PCS) to extract\nspectroscopic information out of correlated electron materials, with the\nemphasis on non-superconducting states. PCS has been used to detect bosonic\nexcitations in normal metals, where signatures (e.g. phonons) are usually less\nthan 1$\\%$ of the measured conductance. In the superconducting state, point\ncontact Andreev reflection (PCAR) has been widely used to study properties of\nthe superconducting gap in various superconductors. In the last decade, there\nhave been more and more experimental results suggesting that the point contact\nconductance could reveal new features associated with the unusual single\nelectron dynamics in non-superconducting states, shedding a new light on\nexploring the nature of the competing phases in correlated materials. We will\nsummarize the theories for point contact spectroscopy developed from different\napproaches and highlight these conceptual differences distinguishing point\ncontact spectroscopy from tunneling-based probes. Moreover, we will show how\nthe Schwinger-Kadanoff-Baym-Keldysh (SKBK) formalism together with the\nappropriate modeling of the nano-scale point contacts randomly distributed\nacross the junction leads to the conclusion that the point contact conductance\nis proportional to the {\\it effective density of states}, a physical quantity\nthat can be computed if the electron self energy is known. The experimental\ndata on iron based superconductors and heavy fermion compounds will be analyzed\nin this framework. These recent developments have extended the applicability of\npoint contact spectroscopy to correlated materials, which will help us achieve\na deeper understanding of the single electron dynamics in strongly correlated\nsystems.", "category": "cond-mat_str-el" }, { "text": "Magnetic Structure and Spin Fluctuations in Colossal Magnetoresistance\n Ferrimagnet Mn3Si2Te6: The ferrimagnetic insulator Mn3Si2Te6, which features a Curie temperature Tc\nat 78 K and a delicate yet consequential magnetic frustration, exhibits\ncolossal magnetoresistance (CMR) when the magnetic field is applied along the\nmagnetic hard axis, surprisingly inconsistent with existing precedents [Y. Ni,\nH. Zhao, Y. Zhang et al. Phys. Rev. B 103, L161105 (2021)]. This discovery\nmotivates a thorough single-crystal neutron diffraction study in order to gain\ninsights into the magnetic structure and its hidden correlation with the new\ntype of CMR. Here we report a noncollinear magnetic structure below the Tc\nwhere the moments lie predominantly within the basal plane but tilt toward the\nc axis by ~10o at ambient conditions. A substantial magnetic diffuse scattering\ndecays slowly and persists well above the Tc. The evolution of the spin\ncorrelation lengths agrees well with the electrical resistivity, underscoring\nthe role of spin fluctuation contributing to the magnetoresistivity near the\ntransition. Application of magnetic field along the c axis, renders a swift\noccurrence of CMR but only a slow tilting of the magnetic moments toward the c\naxis. The unparalleled changes indicate a non-consequential role of magnetic\npolarization.", "category": "cond-mat_str-el" }, { "text": "Kondo and anti-Kondo coupling to local moments in EuB$_6$: With a treatment of the 4$f$ states of EuB$_6$ based on LDA+U method, the\nmixing of Eu $f$ states with B $p$ states around the X point of the Brillouin\nzone is shown to have unexpected consequences for the effective exchange\ninteractions. We analyze in detail the orbital character of electronic states\nclose to the Fermi level and discuss the effective exchange between the\nitinerant electrons and the local $4f$ moments. The analysis suggests that the\nordered phase may provide the first example of a {\\it half metallic semimetal},\nand that the physics of EuB$_6$ should be described in terms of a two band\nKondo lattice model with parallel (ferromagnetic) coupling of the conduction\nelectrons and antiparallel (antiferromagnetic) coupling of the valence\nelectrons to the local $4f$ moments.", "category": "cond-mat_str-el" }, { "text": "Optical studies of Cr$^{3+}$-Cr$^{2+}$ pair center in KZnF$_{3}$ crystal: Optical absorption spectra of Cr$^{3+}$-Cr$^{2+}$ pair center in\nKZnF$_{3}$:Cr$^{3+}$,Cr$^{2+}$ crystal were investigated in wide temperature\nrange. Broad band at 30800 cm$^{-1}$ is attributed to cation-cation\n(e_g)-electron transfer transition. Narrow lines with maxima at 16720 cm$^{-1}$\nand 19880 cm$^{-1}$ have been assigned to purely electronic exchange-induced\nelectric-dipole transitions from the ground\n(Cr$^{3+}$,(^4A_{2g});Cr$^{2+}$,(^5E_g)) state to excited\n(Cr$^{3+}$,(^4A_{2g});Cr$^{2+}$,(^3E_g^a)) and\n(Cr$^{3+}$,(^4A_{2g});Cr$^{2+}$,(^3E_g^b)) states, respectively. It's vibronic\nsatellites corresponding to (a_{1g}) local mode of Cr$^{3+}$ fluorine\noctahedron of the pair are also observed. Energy of the local mode for the\nground and mentioned excited states are 580, 540 and 530 cm$^{-1}$. Instead of\nexpected double exchange for mixed valence pair ferromagnetic superexchange for\nCr$^{3+}$-Cr$^{2+}$ pair in KZnF$_{3}$ crystal is realized. Exchange integral\n(J=-14.9\\pm0.4) cm$^{-1}$ and Jahn-Teller splitting (\\Delta_{JT}=340\\pm40)\ncm$^{-1}$ for the ground state of the pair were obtained by analysis of the\ntemperature dependence of absorption lines. Important features of the crossover\ndouble exchange - ferromagnetic superexchange are discussed.", "category": "cond-mat_str-el" }, { "text": "Quantum Mott Transition and Multi-Furcating Criticality: Phenomenological theory of the Mott transition is presented. When the\ncritical temperature of the Mott transition is much higher than the quantum\ndegeneracy temperature, the transition is essentially described by the Ising\nuniversality class. Below the critical temperature, phase separation or\nfirst-order transition occurs. However, if the critical point is involved in\nthe Fermi degeneracy region, a marginal quantum critical point appears at zero\ntemperature. The originally single Mott critical point generates subsequent\nmany unstable fixed points through various Fermi surface instabilities induced\nby the Mott criticality characterized by the diverging charge susceptibility or\ndoublon susceptibility. This occurs in marginal quantum-critical region.\nCharge, magnetic and superconducting instabilitites compete severely under\nthese critical charge fluctuations. The quantum Mott transition triggers\nmulti-furcating criticality, which goes beyond the conventional concept of\nmulticriticality in quantum phase transitions. Near the quantum Mott\ntransition, the criticality generically drives growth of inhomogeneous\nstructure in the momentum space with singular points of flat dispersion on the\nFermi surface. The singular points determine the quantum dynamics of the Mott\ntransition by the dynamical exponent $z=4$. We argue that many of\nfilling-control Mott transitions are classified to this category. Recent\nnumerical results as well as experimental results on strongly correlated\nsystems including transition metal oxides, organic materials and $^3$He layer\nadsorbed on a substrate are consistently analyzed especially in two-dimensional\nsystems.", "category": "cond-mat_str-el" }, { "text": "Spatio-temporal dynamics of quantum-well excitons: We investigate the lateral transport of excitons in ZnSe quantum wells by\nusing time-resolved micro-photoluminescence enhanced by the introduction of a\nsolid immersion lens. The spatial and temporal resolutions are 200 nm and 5 ps,\nrespectively. Strong deviation from classical diffusion is observed up to 400\nps. This feature is attributed to the hot-exciton effects, consistent with\nprevious experiments under cw excitation. The coupled transport-relaxation\nprocess of hot excitons is modelled by Monte Carlo simulation. We prove that\ntwo basic assumptions typically accepted in photoluminescence investigations on\nexcitonic transport, namely (i) the classical diffusion model as well as (ii)\nthe equivalence between the temporal and spatial evolution of the exciton\npopulation and of the measured photoluminescence, are not valid for\nlow-temperature experiments.", "category": "cond-mat_str-el" }, { "text": "Low-temperature spin Coulomb drag in a two-dimensional electron gas: The phenomenon of low-temperature spin Coulomb drag in a two-dimensional\nelectron gas is investigated. The spin transresistivity coefficient is\nessentially enhanced in the diffusive regime, as compared to conventional\npredictions. The origin of this enhancement is the quantum coherence of spin-up\nand spin-down electrons propagating in the same random impurity potential and\ncoupled via the Coulomb interaction. A comprehensive analysis of spin and\ninterlayer Coulomb drag effects is presented.", "category": "cond-mat_str-el" }, { "text": "Energy Scale Deformation on Regular Polyhedra: A variant of energy scale deformation is considered for the S = 1/2\nantiferromagnetic Heisenberg model on polyhedra. The deformation is induced by\nthe perturbations to the uniform Hamiltonian, whose coefficients are determined\nby the bond coordinates. On the tetrahedral, octahedral, and cubic clusters,\nthe perturbative terms do not affect the ground state of the uniform\nHamiltonian when they are sufficiently small. On the other hand, for the\nicosahedral and dodecahedral clusters, it is numerically confirmed that the\nground state of the uniform Hamiltonian is almost insensitive to the\nperturbations unless they lead to a discontinuous change in the ground state.\nThe obtained results suggest the existence of a generalization of sine-square\ndeformation in higher dimensions.", "category": "cond-mat_str-el" }, { "text": "A generalization of the injectivity condition for Projected Entangled\n Pair States: We introduce a family of tensor network states that we term semi-injective\nProjected Entangled-Pair States (PEPS). They extend the class of injective PEPS\nand include other states, like the ground states of the AKLT and the CZX models\nin square lattices. We construct parent Hamiltonians for which semi-injective\nPEPS are unique ground states. We also determine the necessary and sufficient\nconditions for two tensors to generate the same family of such states in two\nspatial dimensions. Using this result, we show that the third cohomology\nlabeling of Symmetry Protected Topological phases extends to semi-injective\nPEPS.", "category": "cond-mat_str-el" }, { "text": "Quantitative functional renormalization for three-dimensional quantum\n Heisenberg models: We employ a recently developed variant of the functional renormalization\ngroup method for spin systems, the so-called pseudo Majorana functional\nrenormalization group, to investigate three-dimensional spin-1/2 Heisenberg\nmodels at finite temperatures. We study unfrustrated and frustrated Heisenberg\nsystems on the simple cubic and pyrochlore lattices. Comparing our results with\nother quantum many-body techniques, we demonstrate a high quantitative accuracy\nof our method. Particularly, for the unfrustrated simple cubic lattice\nantiferromagnet ordering temperatures obtained from finite-size scaling of\none-loop data deviate from error controlled quantum Monte Carlo results by\n$\\sim5\\%$ and we further confirm the established values for the critical\nexponent $\\nu$ and the anomalous dimension $\\eta$. As the PMFRG yields results\nin good agreement with QMC, but remains applicable when the system is\nfrustrated, we next treat the pyrochlore Heisenberg antiferromagnet as a\nparadigmatic magnetically disordered system and find nearly perfect agreement\nof our two-loop static homogeneous susceptibility with other methods. We\nfurther investigate the broadening of pinch points in the spin structure factor\nas a result of quantum and thermal fluctuations and confirm a finite width in\nthe extrapolated limit $T\\rightarrow0$. While extensions towards higher loop\norders $\\ell$ seem to systematically improve our approach for magnetically\ndisordered systems we also discuss subtleties when increasing $\\ell$ in the\npresence of magnetic order. Overall, the pseudo Majorana functional\nrenormalization group is established as a powerful many-body technique in\nquantum magnetism with a wealth of possible future applications.", "category": "cond-mat_str-el" }, { "text": "Quantum critical point in the spin glass-antiferromagnetism competition\n in Kondo-lattice systems: A theory is proposed to describe the competition among antiferromagnetism\n(AF), spin glass (SG) and Kondo effect. The model describes two Kondo\nsublattices with an intrasite Kondo interaction strength $J_{K}$ and an\ninterlattice quantum Ising interaction in the presence of a transverse field\n$\\Gamma$. The interlattice coupling is a random Gaussian distributed variable\n(with average $-2J_0/N$ and variance $32 J^{2}/N$) while the $\\Gamma$ field is\nintroduced as a quantum mechanism to produce spin flipping. The path integral\nformalism is used to study this fermionic problem where the spin operators are\nrepresented by bilinear combinations of Grassmann fields. The disorder is\ntreated within the framework of the replica trick. The free energy and the\norder parameters of the problem are obtained by using the static ansatz and by\nchoosing both $J_0/J$ and $\\Gamma/J \\approx (J_k/J)^2$ to allow, as previously,\na better comparison with the experimental findings.\n The results indicate the presence of a SG solution at low $J_K/J$ and for\ntemperature $T4.76.", "category": "cond-mat_str-el" }, { "text": "Emergence of the XY-like phase in the deformed spin-3/2 AKLT systems: Affleck, Kennedy, Lieb and Taski (AKLT) constructed an exemplary spin-3/2\nvalence-bond solid (VBS) state on the hexagonal lattice, which is the ground\nstate of an isotropic quantum antiferromagnet and possesses no spontaneous\nmagnetization but finite correlation length. This is distinct from the N\\'eel\nordered state of the spin-3/2 Heisenberg model on the same lattice. Niggemann,\nKl\\\"umper and Zittartz then generalized the AKLT Hamiltonian to one family\ninvariant under spin rotation about the z-axis. The ground states of this\nfamily can be parameterized by a single parameter that deforms the AKLT state,\nand this system exhibits a quantum phase transition between the VBS and N\\'eel\nphases, as the parameter increases from the AKLT point to large anisotropy. We\ninvestigate the opposite regime when the parameter decreases from the AKLT\npoint and find that there appears to be a Berezinskii-Kosterlitz-Thouless-like\ntransition from the VBS phase to an XY phase. Such a transition also occurs in\nthe deformation of other types of AKLT states with triplet-bond constructions\non the same lattice. However, we do not find such an XY-like phase in the\ndeformed AKLT models on other trivalent lattices, such as square-octagon, cross\nand star lattices. On the star lattice, the deformed family of AKLT states\nremain in the same phase as the isotropic AKLT state throughout the whole\nregion of the parameter. However, for two triplet-bond generalizations, the\ntriplet VBS phase is sandwiched between two ferromagnetic phases (for large and\nsmall deformation parameters, respectively), which are characterized by\nspontaneous magnetizations along different axes. Along the way, we also discuss\nhow various deformed AKLT states can be used for the purpose of universal\nquantum computation.", "category": "cond-mat_str-el" }, { "text": "Crucial role of Internal Collective Modes in Underdoped Cuprates: The enigmatic cuprate superconductors have attracted resurgent interest with\nseveral recent reports and discussions of competing orders in the underdoped\nside. Motivated by this, here we address the natural question of fragility of\nthe d-wave superconducting state in underdoped cuprates. Using a combination of\ntheoretical approaches we study t-J like models, and discover an - as yet\nunexplored - instability that is brought about by an \"internal\" (anti-symmetric\nmode) fluctuation of the d-wave state. This new theoretical result is in good\nagreement with recent STM and ARPES studies of cuprates. We also suggest\nexperimental directions to uncover this physics.", "category": "cond-mat_str-el" }, { "text": "Kondo resonances and Fano antiresonances in transport through quantum\n dots: The transmission of electrons through a non-interacting tight-binding chain\nwith an interacting side quantum dot (QD) is analized. When the Kondo effect\ndevelops at the dot the conductance presents a wide minimum, reaching zero at\nthe unitary limit. This result is compared to the opposite behaviour found in\nan embedded QD. Application of a magnetic field destroys the Kondo effect and\nthe conductance shows pairs of dips separated by the charging energy U. The\nresults are discussed in terms of Fano antiresonances and explain qualitatively\nrecent experimental results.", "category": "cond-mat_str-el" }, { "text": "Theory of Large Intrinsic Spin Hall Effect in Iridate Semimetals: We theoretically investigate the mechanism to generate large intrinsic spin\nHall effect in iridates or more broadly in 5d transition metal oxides with\nstrong spin-orbit coupling. We demonstrate such a possibility by taking the\nexample of orthorhombic perovskite iridate with nonsymmorphic lattice symmetry,\nSrIrO$_3$, which is a three-dimensional semimetal with nodal line spectrum. It\nis shown that large intrinsic spin Hall effect arises in this system via the\nspin-Berry curvature originating from the nearly degenerate electronic spectra\nsurrounding the nodal line. This effect exists even when the nodal line is\ngently gapped out, due to the persistent nearly degenerate electronic\nstructure, suggesting a distinct robustness. The magnitude of the spin Hall\nconductivity is shown to be comparable to the best known example such as doped\ntopological insulators and the biggest in any transition metal oxides. To gain\nfurther insight, we compute the intrinsic spin Hall conductivity in both of the\nbulk and thin film systems. We find that the geometric confinement in thin\nfilms leads to significant modifications of the electronic states, leading to\neven bigger spin Hall conductivity in certain cases. We compare our findings\nwith the recent experimental report on the discovery of large spin Hall effect\nin SrIrO$_3$ thin films.", "category": "cond-mat_str-el" }, { "text": "A non-perturbative study of bulk photovoltaic effect enhanced by an\n optically induced phase transition: Solid systems with strong correlations and interactions under light\nillumination have the potential for exhibiting interesting bulk photovoltaic\nbehavior in the non-perturbative regime, which has remained largely unexplored\nin the past theoretical studies. We investigate the bulk photovoltaic response\nof a perovskite manganite with strongly coupled electron-spin-lattice dynamics,\nusing real-time simulations performed with a tight-binding model. The transient\nchanges in the band structure and the photoinduced phase transitions, emerging\nfrom spin and phonon dynamics, result in a nonlinear current versus intensity\nbehavior beyond the perturbative limit. The current rises sharply across a\nphotoinduced magnetic phase transition, which later saturates at higher light\nintensities due to excited phonon and spin modes. The predicted peak\nphotoresponsivity is orders of magnitude higher than other known ferroelectric\noxides such as BiFeO$_3$. We disentangle phonon-and spin-assisted components to\nthe ballistic photocurrent, showing that they are comparable in magnitude. Our\nresults illustrate a promising alternative way for controlling and optimizing\nthe bulk photovoltaic response through the photoinduced phase transitions in\nstrongly-correlated systems.", "category": "cond-mat_str-el" }, { "text": "Competeing orders in spin-1 and spin-3/2 XXZ Kagome antiferromagnets: A\n series expansion study: We study the competition between $\\sqrt{3} \\times \\sqrt{3}$ (RT3) and $q=0$\n(Q0) magnetic orders in spin-one and spin-$3/2$ Kagome-lattice XXZ\nantiferromagnets with varying XY anisotropy parameter $\\Delta$, using series\nexpansion methods. The Hamiltonian is split into two parts: an $H_0$ which\nfavors the classical order in the desired pattern and an $H_1$, which is\ntreated in perturbation theory by a series expansion. We find that the ground\nstate energy series for the RT3 and Q0 phases are identical up to sixth order\nin the expansion, but ultimately a selection occurs, which depends on spin and\nthe anisotropy $\\Delta$. Results for ground state energy and the magnetization\nare presented. These results are compared with recent spin-wave theory and\ncoupled-cluster calculations. The series results for the phase diagram are\nclose to the predictions of spin-wave theory. For the spin-one model at the\nHeisenberg point ($\\Delta=1$), our results are consistent with a vanishing\norder parameter, that is an absence of a magnetically ordered phase. We also\ndevelop series expansions for the ground state energy of the spin-one\nHeisenberg model in the trimerized phase. We find that the ground state energy\nin this phase is lower than those of magnetically ordered ones, supporting the\nexistence of a spontaneously trimerized phase in this model.", "category": "cond-mat_str-el" }, { "text": "Diagnostics for plasmon satellites and Hubbard bands in transition metal\n oxides: Coulomb correlations between the electrons imprint characteristic signatures\nto the spectral properties of materials. Among others, they are at the origin\nof a rich phenomenology of satellite features, either stemming from atomic-like\nmultiplets or from interactions with particle-hole excitations or plasmons.\nWhile in many cases the latter lie at considerably higher energies than the\nformer, suggesting clear distinction criteria, this picture has recently become\nblurred by indications that satellites of different types can coexist in the\nsame energy range. It is now generally accepted that the identification of the\nnature of spectral features is a highly non-trivial task. In this article we\npropose a general procedure for tracing the origin of satellites of different\ntypes within modern ab initio calculations. As an illustration, we analyze the\nternary transition metal oxides SrVO$_3$ and SrMoO$_3$, which are drosophila\ncompounds for the coexistence of Hubbard and plasmonic satellites, reconciling\nprevious seemingly contradictory findings in an unexpected manner.", "category": "cond-mat_str-el" }, { "text": "Quantum oscillations in the anomalous phase in Sr3Ru2O7: We report measurements of quantum oscillations detected in the putative\nnematic phase of Sr3Ru2O7. Significant improvements in sample purity enabled\nthe resolution of small amplitude dHvA oscillations between two first order\nmetamagnetic transitions delimiting the phase. Two distinct frequencies were\nobserved, and their amplitudes follow the normal Lifshitz-Kosevich profile. The\nFermi surface sheets seem to correspond to a subset of those detected outside\nthe phase. Variations of the dHvA frequencies are explained in terms of a\nchemical potential shift produced by reaching a peak in the density of states,\nand an anomalous field dependence of the oscillatory amplitude provides\ninformation on domains.", "category": "cond-mat_str-el" }, { "text": "Multipole Ordering and Fluctuations in f-Electron Systems: We investigate effects of multipole moments in f-electron systems both from\nphenomenological and microscopic viewpoints. First, we discuss significant\neffects of octupole moment on the magnetic susceptibility in a paramagnetic\nphase. It is found that even within mean-field approximation, the magnetic\nsusceptibility deviates from the Curie-Weiss law due to interactions between\ndipole and octupole moments. Next, we proceed to a microscopic theory for\nmultipole ordering on the basis of a j-j coupling scheme. After brief\nexplanation of a method to derive multipole interactions from the $f$-electron\nmodel, we discuss several multipole ordered phases depending on lattice\nstructure. Finally, we show our new development of the microscopic approach to\nthe evaluation of multipole response functions. We apply fluctuation exchange\napproximation to the f-electron model, and evaluate multipole response\nfunctions.", "category": "cond-mat_str-el" }, { "text": "Thermal conductivity of anisotropic and frustrated spin-1/2 chains: We analyze the thermal conductivity of anisotropic and frustrated spin-1/2\nchains using analytical and numerical techniques. This includes mean-field\ntheory based on the Jordan-Wigner transformation, bosonization, and exact\ndiagonalization of systems with N<=18 sites. We present results for the\ntemperature dependence of the zero-frequency weight of the conductivity for\nseveral values of the anisotropy \\Delta. In the gapless regime, we show that\nthe mean-field theory compares well to known results and that the\nlow-temperature limit is correctly described by bosonization. In the\nantiferromagnetic and ferromagnetic gapped regime, we analyze the temperature\ndependence of the thermal conductivity numerically. The convergence of the\nfinite-size data is remarkably good in the ferromagnetic case. Finally, we\napply our numerical method and mean-field theory to the frustrated chain where\nwe find a good agreement of these two approaches on finite systems. Our\nnumerical data do not yield evidence for a diverging thermal conductivity in\nthe thermodynamic limit in case of the antiferromagnetic gapped regime of the\nfrustrated chain.", "category": "cond-mat_str-el" }, { "text": "Orbital Localization and Delocalization Effects in the U 5f^2\n Configuration: Impurity Problem: Anderson models, based on quantum chemical studies of the molecule of\nU(C_8H_8)_2, are applied to investigate the problem of an U impurity in a\nmetal. The special point here is that the U 5f-orbitals are divided into two\nsubsets: an almost completely localized set and a considerably delocalized one.\nDue to the crystal field, both localized and delocalized U 5f-orbitals affect\nthe low-energy physics. A numerical renormalization group study shows that\nevery fixed point is characterized by a residual local spin and a phase shift.\nThe latter changes between 0 and \\pi/2, which indicates the competition between\ntwo different fixed points. Such a competition between the different local\nspins at the fixed points reflects itself in the impurity magnetic\nsusceptibility at high temperatures. These different features cannot be\nobtained if the special characters of U 5f-orbitals are neglected.", "category": "cond-mat_str-el" }, { "text": "Kinetic theory of the non-local electrodynamic response in anisotropic\n metals: skin effect in 2D systems: The electrodynamic response of ultra-pure materials at low temperatures\nbecomes spatially non-local. This non-locality gives rise to phenomena such as\nhydrodynamic flow in transport and the anomalous skin effect in optics. In\nsystems characterized by an anisotropic electronic dispersion, the non-local\ndynamics becomes dependent on the relative orientation of the sample with\nrespect to the applied field, in ways that go beyond the usual, homogeneous\nresponse. Such orientational dependence should manifest itself not only in\ntransport experiments, as recently observed, but also in optical spectroscopy.\nIn this paper we develop a kinetic theory for the distribution function and the\ntransverse conductivity of two- and three-dimensional Fermi systems with\nanisotropic electronic dispersion. By expanding the collision integral into the\neigenbasis of a collision operator, we include momentum-relaxing scattering as\nwell as momentum-conserving collisions. We examine the isotropic 2D case as a\nreference, as well as anisotropic hexagonal and square Fermi-surface shapes. We\napply our theory to the quantitative calculation of the skin depth and the\nsurface impedance, in all regimes of skin effect. We find qualitative\ndifferences between the frequency dependence of the impedance in isotropic and\nanisotropic systems. Such differences are shown to persist even for more\ncomplex 2D Fermi surfaces, including the ''supercircle'' geometry and an\nexperimental parametrization for PdCoO$_2$, which deviate from an ideal\npolygonal shape. We study the orientational dependence of skin effect due to\nFermi-surface anisotropy, thus providing guidance for the experimental study of\nnon-local optical effects.", "category": "cond-mat_str-el" }, { "text": "Low rank Green's function representations applied to dynamical\n mean-field theory: Several recent works have introduced highly compact representations of\nsingle-particle Green's functions in the imaginary time and Matsubara frequency\ndomains, as well as efficient interpolation grids used to recover the\nrepresentations. In particular, the intermediate representation with sparse\nsampling and the discrete Lehmann representation (DLR) make use of low-rank\ncompression techniques to obtain optimal approximations with controllable\naccuracy. We consider the use of the DLR in dynamical mean-field theory (DMFT)\ncalculations, and in particular, show that the standard full Matsubara\nfrequency grid can be replaced by the compact grid of DLR Matsubara frequency\nnodes. We test the performance of the method for a DMFT calculation of\nSr$_2$RuO$_4$ at temperature $50$K using a continuous-time quantum Monte Carlo\nimpurity solver, and demonstrate that Matsubara frequency quantities can be\nrepresented on a grid of only $36$ nodes with no reduction in accuracy, or\nincrease in the number of self-consistent iterations, despite the presence of\nsignificant Monte Carlo noise.", "category": "cond-mat_str-el" }, { "text": "Are multiphase competition & order-by-disorder the keys to understanding\n Yb2Ti2O7?: If magnetic frustration is most commonly known for undermining long-range\norder, as famously illustrated by spin liquids, the ability of matter to\ndevelop new collective mechanisms in order to fight frustration is no less\nfascinating, providing an avenue for the exploration and discovery of\nunconventional properties of matter. Here we study an ideal minimal model of\nsuch mechanisms which, incidentally, pertains to the perplexing quantum spin\nice candidate Yb2Ti2O7. Specifically, we explain how thermal and quantum\nfluctuations, optimized by order-by-disorder selection, conspire to expand the\nstability region of an accidentally degenerate continuous symmetry U(1)\nmanifold against the classical splayed ferromagnetic ground state that is\ndisplayed by the sister compound Yb2Sn2O7. The resulting competition gives rise\nto multiple phase transitions, in striking similitude with recent experiments\non Yb2Ti2O7 [Lhotel et al., Phys. Rev. B 89 224419 (2014)]. Considering the\neffective Hamiltonian determined for Yb2Ti2O7, we provide, by combining a gamut\nof numerical techniques, compelling evidence that such multiphase competition\nis the long-sought missing key to understanding the intrinsic properties of\nthis material. As a corollary, our work offers a pertinent illustration of the\ninfluence of chemical pressure in rare-earth pyrochlores.", "category": "cond-mat_str-el" }, { "text": "From itinerant to local-moment antiferromagnetism in Kondo lattices:\n Adiabatic continuity vs. quantum phase transitions: Motivated by both experimental and theoretical activities, we discuss the\nfate of Kondo screening and possible quantum phase transitions in\nantiferromagnetically ordered phases of Kondo lattices. While transitions with\ntopological changes of the Fermi surface may occur, we demonstrate that an\nentirely continuous evolution from itinerant to local-moment antiferromagnetism\n(i.e. from strong to negligible Kondo screening) is possible as well. This\nsituation is in contrast to that in a non-symmetry-broken situation where a\nquantum phase transition towards an exotic metallic spin-liquid state\nnecessarily accompanies the disappearance of Kondo screening. We discuss\ncriteria for the existence of topological transitions in the antiferromagnetic\nphase, as well as implications for theoretical scenarios and for current\nexperiments.", "category": "cond-mat_str-el" }, { "text": "Hund electronic correlation in La$_3$Ni$_2$O$_7$ under high pressure: By means of density functional theory plus dynamical mean-field theory\n(DFT+DMFT), we investigate the correlated electronic structures of\nLa$_3$Ni$_2$O$_7$ under high pressure. Our calculations show that\nLa$_3$Ni$_2$O$_7$ is a multi-orbital Hund metal. Both the 3$d_{z^2}$ and\n3$d_{x^2 - y^2}$ orbitals of Ni are close to be half filled and contribute the\nbands across the Fermi level. Band renormalization and orbital selective\nelectronic correlation are observed. Through imaginary-time correlation\nfunctions, the discovery of high-spin configuration, spin-frozen phase, and\nspin-orbital separation shows that the system is in a frozen moment phase at\nhigh temperatures above 290 K and is a Fermi liquid at low temperatures, which\nis further comfirmed by the calculated spin, orbital, and charge\nsusceptibilities under high temperatures. Our study uncovers Hundness in\nLa$_3$Ni$_2$O$_7$ under high pressure.", "category": "cond-mat_str-el" }, { "text": "Screened hybrid functional applied to 3d^0-->3d^8 transition-metal\n perovskites LaMO3 (M=Sc-Cu): influence of the exchange mixing parameter on\n the structural, electronic and magnetic properties: We assess the performance of the Heyd-Scuseria-Ernzerhof (HSE) screened\nhybrid density functional scheme applied to the perovskite family LaMO3\n(M=Sc-Cu) and discuss the role of the mixing parameter alpha (which determines\nthe fraction of exact Hartree-Fock exchange included in the density functional\ntheory (DFT) exchange-correlation functional) on the structural, electronic,\nand magnetic properties. The physical complexity of this class of compounds,\nmanifested by the largely varying electronic characters\n(band/Mott-Hubbard/charge-transfer insulators and metals), magnetic orderings,\nstructural distortions (cooperative Jahn-Teller like instabilities), as well as\nby the strong competition between localization/delocalization effects\nassociated with the gradual filling of the t_2g and e_g orbitals, symbolize a\ncritical and challenging case for theory. Our results indicates that HSE is\nable to provide a consistent picture of the complex physical scenario\nencountered across the LaMO3 series and significantly improve the standard DFT\ndescription. The only exceptions are the correlated paramagnetic metals LaNiO3\nand LaCuO3, which are found to be treated better within DFT. By fitting the\nground state properties with respect to alpha we have constructed a set of\n'optimum' values of alpha from LaScO3 to LaCuO3: it is found that the 'optimum'\nmixing parameter decreases with increasing filling of the d manifold (LaScO3:\n0.25; LaTiO3 & LaVO3: 0.10-0.15; LaCrO3, LaMnO3, and LaFeO3: 0.15; LaCoO3:\n0.05; LaNiO3 & LaCuO3: 0). This trend can be nicely correlated with the\nmodulation of the screening and dielectric properties across the LaMO3 series,\nthus providing a physical justification to the empirical fitting procedure.", "category": "cond-mat_str-el" }, { "text": "Angular dependence of Hall effect and magnetoresistance in\n SrRuO$_3$-SrIrO$_3$ heterostructures: Perovskite SrRuO$_3$ is a prototypical itinerant ferromagnet which allows\ninterface engineering of its electronic and magnetic properties. We report\nsynthesis and investigation of atomically flat artificial multilayers of\nSrRuO$_3$ with the spin-orbit semimetal SrIrO$_3$ in combination with\nband-structure calculations with a Hubbard $U$ term and topological analysis.\nThey reveal an electronic reconstruction and emergence of flat Ru-4d$_{xz}$\nbands near the interface, ferromagnetic interlayer coupling and negative\nBerry-curvature contribution to the anomalous Hall effect. We analyze the Hall\neffect and magnetoresistance measurements as a function of the field angle from\nout of plane towards in-plane orientation (either parallel or perpendicular to\nthe current direction) by a two-channel model. The magnetic easy direction is\ntilted by about $20^\\circ$ from the sample normal for low magnetic fields,\nrotating towards the out-of-plane direction by increasing fields. Fully\nstrained epitaxial growth enables a strong anisotropy of magnetoresistance. An\nadditional Hall effect contribution, not accounted for by the two-channel model\nis compatible with stable skyrmions only up to a critical angle of roughly\n$45^\\circ$ from the sample normal. Within about $20^\\circ$ from the thin film\nplane an additional peak-like contribution to the Hall effect suggests the\nformation of a non-trivial spin structure.", "category": "cond-mat_str-el" }, { "text": "Anisotropic magnetoresistance and piezoelectric effect in GaAs Hall\n samples: In this work, we argue that an anisotropic interaction potential may\nstabilize anisotropic liquid phases of electrons even in a strong magnetic\nfield regime where normally one expects to see only isotropic quantum Hall or\nisotropic Fermi liquid states. We use this approach to support a theoretical\nframework that envisions the possibility of an anisotropic liquid crystalline\nstate of electrons in the lowest Landau level. In particular, we argue that an\nanisotropic liquid state of electrons may stabilize in the lowest Landau level\nclose to the liquid-solid transition region at filling factor $\\nu=1/6$ for a\ngiven anisotropic Coulomb interaction potential. Quantum Monte Carlo\nsimulations for a liquid crystalline state with broken rotational symmetry\nindicate stability of liquid crystalline order consistent with the existence of\nan anisotropic liquid state of electrons stabilized by anisotropy at filling\nfactor $\\nu=1/6$ of the lowest Landau level.", "category": "cond-mat_str-el" }, { "text": "Magnetic Order in Laser-Irradiated Kagome Antiferromagnets: Dispersionless \"zero energy mode'' is one of the hallmarks of frustrated\nkagome antiferromagnets (KAFMs). It points to extensive classically degenerate\nground-states. The \"zero energy mode'' can be observed experimentally when\nlifted to a flat mode at finite energy by a strong intrinsic magnetic\nanisotropy. In this letter, we study the effects of irradiation of laser light\non the KAFMs. We adopt the magnon picture without loss of generality. It is\nshown that circularly or linearly polarized light lifts the \"zero energy\nmode'', stabilizes magnetic order, and induces energy gaps in the KAFMs. We\nfind that the circularly polarized light-induced anisotropies have similar\nfeatures as the intrinsic in-plane and out-of-plane Dzyaloshinskii-Moriya\ninteraction in KAFMs. The former stabilizes long-range magnetic order and the\nlatter induces spin canting out-of-plane with nonzero scalar spin chirality.\nThe Floquet thermal Hall effect shows that the synthetic magnetic excitation\nmodes in the case of circularly polarized light are topological, whereas those\nof linearly polarized light are not.", "category": "cond-mat_str-el" }, { "text": "Inducing topological order in a honeycomb lattice: We explore the possibility of inducing a topological insulator phase in a\nhoneycomb lattice lacking spin-orbit interaction using a metallic (or Fermi\ngas) environment. The lattice and the metallic environment interact through a\ndensity-density interaction without particle tunneling, and integrating out the\nmetallic environment produces a honeycomb sheet with in-plane oscillating\nlong-ranged interactions. We find the ground state of the interacting system in\na variational mean-field method and show that the Fermi wave vector, kF, of the\nmetal determines which phase occurs in the honeycomb lattice sheet. This is\nanalogous to the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism in which the\nmetal's kF determines the interaction profile as a function of the distance.\nTuning kF and the interaction strength may lead to a variety of ordered phases,\nincluding a topological insulator and anomalous quantum-hall states with\ncomplex next-nearest-neighbor hopping, as in the Haldane and the Kane-Mele\nmodel. We estimate the required range of parameters needed for the topological\nstate and find that the Fermi vector of the metallic gate should be of the\norder of 3Pi/8a (with a being the graphene lattice constant). The net coupling\nbetween the layers, which includes screening in the metal, should be of the\norder of the honeycomb lattice bandwidth. This configuration should be most\neasily realized in a cold-atoms setting with two interacting Fermionic species.", "category": "cond-mat_str-el" }, { "text": "Spectral signatures of the Luttinger liquid to charge-density-wave\n transition: Electron- and phonon spectral functions of the one-dimensional,\nspinless-fermion Holstein model at half filling are calculated in the four\ndistinct regimes of the phase diagram, corresponding to an attractive or\nrepulsive Luttinger liquid at weak electron-phonon coupling, and a band- or\npolaronic insulator at strong coupling. The results obtained by means of kernel\npolynomial and systematic cluster approaches reveal substantially different\nphysics in these regimes and further indicate that the size of the phonon\nfrequency significantly affects the nature of the quantum Peierls phase\ntransition.", "category": "cond-mat_str-el" }, { "text": "Ground-State Phase Diagram of (1/2,1/2,1) Mixed Diamond Chains: The ground-state phases of mixed diamond chains with ($S, \\tau^{(1)},\n\\tau^{(2)})=(1/2,1/2,1)$, where $S$ is the magnitude of vertex spins, and\n$\\tau^{(1)}$ and $\\tau^{(2)}$ are those of apical spins, are investigated. The\ntwo apical spins in each unit cell are coupled by an exchange coupling\n$\\lambda$. The vertex spins are coupled with the top and bottom apical spins by\nexchange couplings $1+\\delta$ and $1-\\delta$, respectively. Although this model\nhas an infinite number of local conservation laws for $\\delta=0$, they are lost\nfor finite $\\delta$. The ground-state phase diagram is determined using the\nnumerical exact diagonalization and DMRG method in addition to the analytical\napproximations in various limiting cases. The phase diagram consists of a\nnonmagnetic phase and several kinds of ferrimagnetic phases. We find two\ndifferent ferrimagnetic phases without spontaneous translational symmetry\nbreakdown. It is also found that the quantized ferrimagnetic phases with large\nspatial periodicities present for $\\delta=0$ are easily destroyed by small\n$\\delta$ and replaced by a partial ferrimagnetic phase. The nonmagnetic phase\nis considered to be a gapless Tomonaga-Luttinger liquid phase based on the\nrecently extended Lieb-Schultz-Mattis theorem to the site-reflection invariant\nspin chains and numerical diagonalization results.", "category": "cond-mat_str-el" }, { "text": "The free energy of anisotropic quantum spin systems: Functional integral\n representation: In this work, we propose a method for calculating the free energy of\nanisotropic quantum spin systems. We use the Hubbard-Stratonovich\ntransformation to express the partition function of a generic bilinear\nsuper-exchange Hamiltonian in terms of a functional integral over classical\ntime-dependent fields. In the general case the result is presented as an\ninfinite series. The series may be summed up in the case of Ising-type models.\nFor any ordered state we derive a compact expression for the contribution of\nGaussian spin fluctuations to the free energy.", "category": "cond-mat_str-el" }, { "text": "The electronic structure of the high-symmetry perovskite iridate Ba2IrO4: We report angle-resolved photoemission (ARPES) measurements, density\nfunctional and model tight-binding calculations on Ba$_2$IrO$_4$ (Ba-214), an\nantiferromagnetic ($T_N=230$ K) insulator. Ba-214 does not exhibit the\nrotational distortion of the IrO$_6$ octahedra that is present in its sister\ncompound Sr$_2$IrO$_4$ (Sr-214), and is therefore an attractive reference\nmaterial to study the electronic structure of layered iridates. We find that\nthe band structures of Ba-214 and Sr-214 are qualitatively similar, hinting at\nthe predominant role of the spin-orbit interaction in these materials.\nTemperature-dependent ARPES data show that the energy gap persists well above\n$T_N$, and favour a Mott over a Slater scenario for this compound.", "category": "cond-mat_str-el" }, { "text": "Field induced tricritical behavior in the S=1/2 quasi one-dimensional\n frustrated Ising antiferromagnet: The results of extensive histogram cluster heat-bath Monte Carlo simulations\non the critical behavior of the quasi-one dimensional Ising antiferromagnet on\na stacked triangular lattice are presented. A small applied field is shown to\ninduce a crossover from XY universality to mean-field tricritical behavior.\nExperimental estimates of critical exponents suggest that these two types of\nphase transitions are observed in S=1 CsNiCl$_3$ and $S=1/2$ CsCoBr$_3$,\nrespectively. The present results demonstrate that this difference can be\nexplained by an unusual staggered magnetic field arising from quantum exchange\nmixing previously proposed to account for spin excitations in $S=1/2$\nquasi-one-dimensional Ising antiferromagnets.", "category": "cond-mat_str-el" }, { "text": "Diagrammatic quantum Monte Carlo study of an acoustic lattice polaron: We present the first approximation free diagrammatic Monte Carlo study of a\nlattice polaron interacting with an acoustic phonon branch through the\ndeformation potential. Weak and strong coupling regimes are separated by a\nself-trapping region where quantum resonance between various possible lattice\ndeformations is seen in the ground state properties, spectral function, and\noptical conductivity. The unique feature of such polaron is the interplay\nbetween long- and short wavelength acoustic vibrations creating a composite\nphonon cloud and leading to persistent self-trapping due to the existence of\nmultiple quasi-stable states. This results in a spectral response whose\nstructure is much more complex than in any of the previously considered polaron\nmodels.", "category": "cond-mat_str-el" }, { "text": "Quantum Skyrmion Lattices in Heisenberg Ferromagnets: Skyrmions are topological magnetic textures that can arise in\nnon-centrosymmetric ferromagnetic materials. In most systems experimentally\ninvestigated to date, skyrmions emerge as classical objects. However, the\ndiscovery of skyrmions with nanometer length scales has sparked interest in\ntheir quantum properties. Here, we simulate the ground states of\ntwo-dimensional spin-$1/2$ Heisenberg lattices with Dzyaloshinskii-Moriya\ninteractions and discover a broad region in the zero-temperature phase diagram\nwhich hosts quantum skyrmion lattices. We argue that the quantum skyrmion\nlattice phase can be detected experimentally in the magnetization profile via\nlocal magnetic polarization measurements as well as in the spin structure\nfactor measurable via neutron scattering experiments. Finally, we explore the\nresulting quantum skyrmion state, analyze its real-space polarization profile\nand show that it is a non-classical state featuring entanglement between\nquasiparticle and environment mainly localized near the boundary spins of the\nskyrmion.", "category": "cond-mat_str-el" }, { "text": "LDA+DMFT approach to resonant inelastic x-ray scattering in correlated\n materials: We present a computational study of $L$-edge resonant inelastic x-ray\nscattering (RIXS) in correlated 3$d$ transition-metal oxides using an $ab$\n$initio$ method based on local density approximation + dynamical mean-field\ntheory (DMFT). The present method, building on Anderson impurity model with an\noptimized continuum bath within DMFT, is an extension of the cluster model to\ninclude unbound electron-hole pair excitations as well as material-specific\ncharge-transfer excitations with less empirical parameters. We find a good\nagreement with available experimental data. The relationship between correlated\nbands and fluorescence-like feature in the RIXS spectra is discussed.", "category": "cond-mat_str-el" }, { "text": "Magnetic model for A2CuP2O7 (A = Na, Li) revisited: 1D versus 2D\n behavior: We report magnetization measurements, full-potential band structure\ncalculations, and microscopic modeling for the spin-1/2 Heisenberg magnets\nA2CuP2O7 (A = Na, Li). Based on a quantitative evaluation of the leading\nexchange integrals and the subsequent quantum Monte-Carlo simulations, we\npropose a quasi-one-dimensional magnetic model for both compounds, in contrast\nto earlier studies that conjectured on the two-dimensional scenario. The\none-dimensional nature of A2CuP2O7 is unambiguously verified by magnetization\nisotherms measured in fields up to 50 T. The saturation fields of about 40 T\nfor both Li and Na compounds are in excellent agreement with the intrachain\nexchange J1 ~ 27 K extracted from the magnetic susceptibility data. The\nproposed magnetic structure entails spin chains with the dominating\nantiferromagnetic nearest-neighbor interaction J1 and two inequivalent,\nnonfrustrated antiferromagnetic interchain couplings of about 0.01*J1 each. A\npossible long-range magnetic ordering is discussed in comparison with the\navailable experimental information.", "category": "cond-mat_str-el" }, { "text": "Topological Blocking in Quantum Quench Dynamics: We study the non-equilibrium dynamics of quenching through a quantum critical\npoint in topological systems, focusing on one of their defining features:\nground state degeneracies and associated topological sectors. We present the\nnotion of 'topological blocking', experienced by the dynamics due to a mismatch\nin degeneracies between two phases and we argue that the dynamic evolution of\nthe quench depends strongly on the topological sector being probed. We\ndemonstrate this interplay between quench and topology in models stemming from\ntwo extensively studied systems, the transverse Ising chain and the Kitaev\nhoneycomb model. Through non-local maps of each of these systems, we\neffectively study spinless fermionic $p$-wave paired superconductors. Confining\nthe systems to ring and toroidal geometries, respectively, enables us to\ncleanly address degeneracies, subtle issues of fermion occupation and parity,\nand mismatches between topological sectors. We show that various features of\nthe quench, which are related to Kibble-Zurek physics, are sensitive to the\ntopological sector being probed, in particular, the overlap between the\ntime-evolved initial ground state and an appropriate low-energy state of the\nfinal Hamiltonian. While most of our study is confined to translationally\ninvariant systems, where momentum is a convenient quantum number, we briefly\nconsider the effect of disorder and illustrate how this can influence the\nquench in a qualitatively different way depending on the topological sector\nconsidered.", "category": "cond-mat_str-el" }, { "text": "Orbital polarons in the metal-insulator transition of manganites: The metal-insulator transition in manganites is strongly influenced by the\nconcentration of holes present in the system. Based upon an orbitally\ndegenerate Mott-Hubbard model we analyze two possible localization scenarios to\naccount for this doping dependence: First, we rule out that the transition is\ninitiated by a disorder-order crossover in the orbital sector, showing that its\neffect on charge itineracy is only small. Second, we introduce the idea of\norbital polarons originating from a strong polarization of orbitals in the\nvicinity of holes. Considering this direct coupling between charge and orbital\ndegree of freedom in addition to lattice effects we are able to explain well\nthe phase diagram of manganites for low and intermediate hole concentrations.", "category": "cond-mat_str-el" }, { "text": "Quantized gravitational responses and the sign problem: It is believed that not all quantum systems can be simulated efficiently\nusing classical computational resources. This notion is supported by the fact\nthat in quantum Monte Carlo (QMC) simulations for a large number of important\nproblems it is not known how to express the partition function in a sign-free\nmanner. The answer to the question --- whether there is an fundamental\nobstruction to such a sign-free representation in generic quantum systems ---\nremains unclear. Here, focussing on systems with bosonic degrees of freedom, we\nshow that quantized gravitational responses appear as obstructions to local\nsign-free QMC. In condensed matter physics settings these responses, such as\nthermal Hall conductance, are associated with fractional quantum Hall effects.\nWe show that similar arguments hold also in the case of spontaneously broken\ntime-reversal (TR) symmetry such as in the chiral phase of a perturbed quantum\nKagome antiferromagnet. The connection between quantized gravitational\nresponses and the sign problem is also clearly manifested in certain vertex\nmodels, where TR symmetry is preserved.", "category": "cond-mat_str-el" }, { "text": "Unconventional many-body phase transitions in a non-Hermitian Ising\n chain: We study many-body phase transitions in a one-dimensional ferromagnetic\ntransversed field Ising model with an imaginary field and show that the system\nexhibits three phase transitions: one second-order phase transition and two\n$\\mathcal{PT}$ phase transitions. The second-order phase transition occurring\nin the ground state is investigated via biorthogonal and self-normal\nentanglement entropy, for which we develop an approach to perform finite-size\nscaling theory to extract the central charge for small systems. Compared with\nthe second-order phase transition, the first $\\mathcal{PT}$ transition is\ncharacterized by the appearance of an exceptional point in the full energy\nspectrum, while the second $\\mathcal{PT}$ transition only occurs in specific\nexcited states. Furthermore, we interestingly show that both of exceptional\npoints are second-order in terms of scalings of imaginary parts of the energy.\nThis work provides an exact solution for unconventional many-body phase\ntransitions in non-Hermitian systems.", "category": "cond-mat_str-el" }, { "text": "Matrix product states approaches to operator spreading in ergodic\n quantum systems: We review different tensor network approaches to study the spreading of\noperators in generic nonintegrable quantum systems. As a common ground to all\nmethods, we quantify this spreading by means of the Frobenius norm of the\ncommutator of a spreading operator with a local operator, which is usually\nreferred to as the out of time order correlation (OTOC) function. We compare\ntwo approaches based on matrix-product states in the Schr\\\"odinger picture: the\ntime dependent block decimation (TEBD) and the time dependent variational\nprinciple (TDVP), as well as TEBD based on matrix-product operators directly in\nthe Heisenberg picture. The results of all methods are compared to numerically\nexact results using Krylov space exact time evolution. We find that for the\nSchr\\\"odinger picture the TDVP algorithm performs better than the TEBD\nalgorithm. Moreover the tails of the OTOC are accurately obtained both by TDVP\nMPS and TEBD MPO. They are in very good agreement with exact results at short\ntimes, and appear to be converged in bond dimension even at longer times.\nHowever the growth and saturation regimes are not well captured by both\nmethods.", "category": "cond-mat_str-el" }, { "text": "Charge collective modes in strongly correlated electron systems with\n long range interactions: Elucidating the impact of strong electronic correlations on the collective\nmodes of metallic systems has been of longstanding interest, mainly due to the\ninadequacy of the random phase approximation (RPA) in the strongly correlated\nregime. In his regard, we analyze the charge excitation spectrum of a Hubbard\nmodel on the face centered cubic lattice, extended with long range\ninteractions, in different coupling regimes ranging from uncorrelated to the\nmetal-to-insulator transition at half filling. We argue that the slave boson\nrepresentation introduced by Kotliar and Ruckenstein, when formulated in radial\ngauge, constitutes a suitable framework to carry out this endeavor, and we\ncompare its results to conventional RPA as a benchmark. We focus on the\ninfluence of the local and long range couplings on the particle-hole excitation\ncontinuum and the quantum collective phenomena generically comprised in our\nspectra, and find numerous qualitative and quantitative discrepancies between\nour method and standard RPA in the intermediate-to-strong coupling regime. At\nthe onset of the Mott transition, the plasmon gap is found to vanish,\nsupporting a quasiparticle description of the mode.", "category": "cond-mat_str-el" }, { "text": "Relativistic and thermal effects on the magnon spectrum of a\n ferromagnetic monolayer: A spin model including magnetic anisotropy terms and Dzyaloshinsky-Moriya\ninteractions is studied for the case of a ferromagnetic monolayer with C2v\nsymmetry like Fe/W(110). Using the quasiclassical stochastic\nLandau-Lifshitz-Gilbert equations, the magnon spectrum of the system is derived\nusing linear response theory. The Dzyaloshinsky-Moriya interaction leads to\nasymmetry in the spectrum, while the anisotropy terms induce a gap. It is shown\nthat in the presence of lattice defects, both the Dzyaloshinsky-Moriya\ninteractions and the two-site anisotropy lead to a softening of the magnon\nenergies. Two methods are developed to investigate the magnon spectrum at\nfinite temperatures. The theoretical results are compared to atomistic spin\ndynamics simulations and a good agreement is found between them.", "category": "cond-mat_str-el" }, { "text": "Metal-Insulator-Like Behavior in Semimetallic Bismuth and Graphite: When high quality bismuth or graphite crystals are placed in a magnetic field\ndirected along the c-axis (trigonal axis for bismuth) and the temperature is\nlowered, the resistance increases as it does in an insulator but then\nsaturates. We show that the combination of unusual features specific to\nsemimetals, i.e., low carrier density, small effective mass, high purity, and\nan equal number of electrons and holes (compensation), gives rise to a unique\nordering and spacing of three characteristic energy scales, which not only is\nspecific to semimetals but which concomitantly provides a wide window for the\nobservation of apparent field induced metal-insulator behavior. Using\nmagnetotransport and Hall measurements, the details of this unusual behavior\nare captured with a conventional multi-band model, thus confirming the\noccupation by semimetals of a unique niche between conventional metals and\nsemiconductors.", "category": "cond-mat_str-el" }, { "text": "Real Space Coulomb Interaction: A Pairing Glue for FeAs Superconductors: In this paper we present a real space pairing glue for the iron-based layered\nsuperconductors. It is shown that two static electrons embedded symmetrically\ninto two adjacent Fe plaquettes of the superconductor can be bounded due to the\nCoulombic interaction. The pairing mechanism favors the existence of the\npseudogap in the underdoped FeAs superconductors. A criterion is introduced to\ndistinguish whether or not the pseudogap can open in a material.", "category": "cond-mat_str-el" }, { "text": "Anomalies in bosonic SPT edge theories: connection to F-symbols and a\n method for calculation: We describe a systematic procedure for determining the identity of a 2D\nbosonic symmetry protected topological (SPT) phase from the properties of its\nedge excitations. Our approach applies to general bosonic SPT phases with\neither unitary or antiunitary symmetries, and with either continuous or\ndiscrete symmetry groups, with the only restriction being that the symmetries\nmust be on-site. Concretely, our procedure takes a bosonic SPT edge theory as\ninput, and produces an element $\\omega$ of the cohomology group $H^3(G,\nU_T(1))$. This element $\\omega \\in H^3(G, U_T(1))$ can be interpreted as either\na label for the bulk 2D SPT phase or a label for the anomaly carried by the SPT\nedge theory. The basic idea behind our approach is to compute the $F$-symbol\nassociated with domain walls in a symmetry broken edge theory; this domain wall\n$F$-symbol is precisely the anomaly we wish to compute. We demonstrate our\napproach with several SPT edge theories including both lattice models and\ncontinuum field theories.", "category": "cond-mat_str-el" }, { "text": "From the double-exchange Hamiltonian to the $t-J$ model: Classical spins: From the double-exchange Hamiltonian with classical localized spins in the\nlimit of large but finit Hund exchange coupling we obtain the $t-J$ model (with\nclassical localized spins).", "category": "cond-mat_str-el" }, { "text": "Theory of Twist Liquids: Gauging an Anyonic Symmetry: Topological phases in (2+1)-dimensions are frequently equipped with global\nsymmetries, like conjugation, bilayer or electric-magnetic duality, that\nrelabel anyons without affecting the topological structures. Twist defects are\nstatic point-like objects that permute the labels of orbiting anyons. Gauging\nthese symmetries by quantizing defects into dynamical excitations leads to a\nwide class of more exotic topological phases referred as twist liquids, which\nare generically non-Abelian. We formulate a general gauging framework,\ncharacterize the anyon structure of twist liquids and provide solvable lattice\nmodels that capture the gauging phase transitions. We explicitly demonstrate\nthe gauging of the $\\mathbb{Z}_2$-symmetric toric code, $SO(2N)_1$ and\n$SU(3)_1$ state as well as the $S_3$-symmetric $SO(8)_1$ state and a\nnon-Abelian chiral state we call the \"4-Potts\" state.", "category": "cond-mat_str-el" }, { "text": "Thermal drag in spin ladders coupled to phonons: We study the spin-phonon drag effect in the magnetothermal transport of\nspin-1/2 two-leg ladders coupled to lattice degrees of freedom. Using a bond\noperator description for the triplon excitations of the spin ladder and\nmagnetoelastic coupling to acoustic phonons, we employ the time convolutionless\nprojection operator method to derive expressions for the diagonal and\noff-diagonal thermal conductivities of the coupled two-component triplon-phonon\nsystem. We find that for magnetoelastic coupling strengths and diagonal\nscattering rates relevant to copper-oxide spin-ladders the drag heat\nconductivity can be of similar magnitude as the diagonal triplon heat\nconductivity. Moreover, we show that the drag and diagonal conductivities\ndisplay very similar overall temperature dependences. Finally, the drag\nconductivity is shown to be rather susceptible to external magnetic fields.", "category": "cond-mat_str-el" }, { "text": "Twofold van Hove singularity and origin of charge order in topological\n kagome superconductor CsV3Sb5: The layered vanadium antimonides AV3Sb5 (A = K, Rb, Cs) are a recently\ndiscovered family of topological kagome metals with a rich phenomenology of\nstrongly correlated electronic phases including charge order and\nsuperconductivity. Understanding how the singularities inherent to the kagome\nelectronic structure are linked to the observed many-body phases is a topic of\ngreat interest and relevance. Here, we combine angle-resolved photoemission\nspectroscopy and density functional theory to reveal multiple kagome-derived\nvan Hove singularities (vHs) coexisting near the Fermi level of CsV3Sb5 and\nanalyze their contribution to electronic symmetry breaking. Intriguingly, the\nvHs in CsV3Sb5 have two distinct flavors - p-type and m-type - which originate\nfrom their pure and mixed sublattice characters, respectively. This twofold vHs\nis unique property of the kagome lattice, and its flavor critically determines\nthe pairing symmetry and ground states emerging in AV3Sb5 series. We establish\nthat, among the multiple vHs in CsV3Sb5, the m-type vHs of the dxz/dyz kagome\nband and the p-type vHs of the dxy/dx2-y2 kagome band cross the Fermi level to\nset the stage for electronic symmetry breaking. The former band exhibits\npronounced Fermi surface nesting, while the latter contributes via higher-order\nvHs. Our work reveals the essential role of kagome-derived vHs for the\ncollective phenomena realized in the AV3Sb5 family, paving the way to a deeper\nunderstanding of strongly correlated topological kagome systems.", "category": "cond-mat_str-el" }, { "text": "A Gate-tunable Polarized Phase of Two-Dimensional Electrons at the\n LaAlO3/SrTiO3 Interface: Controlling the coupling between localized spins and itinerant electrons can\nlead to exotic magnetic states. A novel system featuring local magnetic moments\nand extended 2D electrons is the interface between LaAlO3 and SrTiO3. The\nmagnetism of the interface, however, was observed to be insensitive to the\npresence of these electrons and is believed to arise solely from extrinsic\nsources like oxygen vacancies and strain. Here we show the existence of\nunconventional electronic phases in the LaAlO3/SrTiO3 system pointing to an\nunderlying tunable coupling between itinerant electrons and localized moments.\nUsing anisotropic magnetoresistance and anomalous Hall effect measurements in a\nunique in-plane configuration, we identify two distinct phases in the space of\ncarrier density and magnetic field. At high densities and fields, the\nelectronic system is strongly polarized and shows a response, which is highly\nanisotropic along the crystalline directions. Surprisingly, below a\ndensity-dependent critical field, the polarization and anisotropy vanish\nwhereas the resistivity sharply rises. The unprecedented vanishing of the easy\naxes below a critical field is in sharp contrast with other coupled magnetic\nsystems and indicates strong coupling with the moments that depends on the\nsymmetry of the itinerant electrons. The observed interplay between the two\nphases indicates the nature of magnetism at the LaAlO3/SrTiO3 interface as both\nhaving an intrinsic origin and being tunable.", "category": "cond-mat_str-el" }, { "text": "Strong enhancement of magnetic order from bulk to stretched monolayer\n FeSe as Hund's metals: Despite of the importance of magnetism in possible relation to other key\nproperties in iron-based superconductors, its understanding is still far from\ncomplete especially for FeSe systems. On one hand, the origin of the absence of\nmagnetic orders in bulk FeSe is yet to be clarified. On the other hand, it is\nstill not clear how close monolayer FeSe on SrTiO$_3$, with the highest\ntransition temperature among iron-based superconductors, is to a magnetic\ninstability. Here we investigate magnetic properties of bulk and monolayer FeSe\nusing dynamical mean-field theory combined with density-functional theory. We\nfind that suppressed magnetic order in bulk FeSe is associated with the\nreduction of inter-orbital charge fluctuations, an effect of Hund's coupling,\nenhanced by a larger crystal field splitting. Meanwhile, spatial isolation of\nFe atoms in expanded monolayer FeSe leads into a strong magnetic order, which\nis completely destroyed by a small electron doping. Our work provides a\ncomprehensive understanding of the magnetic order in iron-based superconductors\nand other general multi-orbital correlated systems as Hund's metals.", "category": "cond-mat_str-el" }, { "text": "How does a quadratic term in the energy dispersion modify the\n single-particle Green's function of the Tomonaga-Luttinger model?: We calculate the effect of a quadratic term in the energy dispersion on the\nlow-energy behavior of the Green's function of the spinless Tomonaga-Luttinger\nmodel (TLM). Assuming that for small wave-vectors q = k - k_F the fermionic\nexcitation energy relative to the Fermi energy is v_F q + q^2 / (2m), we\nexplicitly calculate the single-particle Green's function for finite but small\nvalues of lambda = q_c /(2k_F). Here k_F is the Fermi wave-vector, q_c is the\nmaximal momentum transfered by the interaction, and v_F = k_F / m is the Fermi\nvelocity. Assuming equal forward scattering couplings g_2 = g_4, we find that\nthe dominant effect of the quadratic term in the energy dispersion is a\nrenormalization of the anomalous dimension. In particular, at weak coupling the\nanomalous dimension is tilde{gamma} = gamma (1 - 2 lambda^2 gamma), where gamma\nis the anomalous dimension of the TLM. We also show how to treat the change of\nthe chemical potential due to the interactions within the functional\nbosonization approach in arbitrary dimensions.", "category": "cond-mat_str-el" }, { "text": "Dynamical Mean-Field Theory for Doped Antiferromagnets: We have generalized the dynamical mean-field theory to study the doping\ndependence of the crossover from antiferromagnetic to short-range order\nmodelled by an incommensurate spin density wave in the Hubbard model. The local\nselfenergy which includes spin fluctuations gives quasiparticle weights and\nspectral properties in good agreement with quantum Monte Carlo and exact\ndiagonalization data in two dimensions. The spectra at finite doping are\ncharacterized by a Mott-Hubbard `gap' accompanied by a pseudogap induced by the\nlocal spin order.", "category": "cond-mat_str-el" }, { "text": "Doped carrier formulation of the t-J model: the projection constraint\n and the effective Kondo-Heisenberg lattice representation: We show that the recently proposed doped carrier Hamiltonian formulation of\nthe t-J model should be complemented with the constraint that projects out the\nunphysical states. With this new important ingredient, the previously used and\nseemingly different spin-fermion representations of the t-J model are shown to\nbe gauge related to each other. This new constraint can be treated in a\ncontrolled way close to half-filling suggesting that the doped carrier\nrepresentation provides an appropriate theoretical framework to address the t-J\nmodel in this region. This constraint also suggests that the t-J model can be\nmapped onto a Kondo-Heisenberg lattice model. Such a mapping highlights\nimportant physical similarities between the quasi two-dimensional heavy\nfermions and the high-T$_c$ superconductors. Finally we discuss the physical\nimplications of our model representation relating in particular the small\nversus large Fermi surface crossover to the closure of the lattice spin gap.", "category": "cond-mat_str-el" }, { "text": "Orbital-selective Mott transitions in two-band Hubbard models: The anisotropic two-orbital Hubbard model is investigated at low temperatures\nusing high-precision quantum Monte Carlo (QMC) simulations within dynamical\nmean-field theory (DMFT). We demonstrate that two distinct orbital-selective\nMott transitions (OSMTs) occur for a bandwidth ratio of 2 even without\nspin-flip contributions to the Hund exchange, and we quantify numerical errors\nin earlier QMC data which had obscured the second transition. The limit of\nsmall inter-orbital coupling is introduced via a new generalized Hamiltonian\nand studied using QMC and Potthoff's self-energy functional method, yielding\ninsight into the nature of the OSMTs and the non-Fermi-liquid OSM phase and\nopening the possibility for a new quantum-critical point.", "category": "cond-mat_str-el" }, { "text": "Topological and trivial magnetic oscillations in nodal loop semimetals: Nodal loop semimetals are close descendants of Weyl semimetals and possess a\ntopologically dressed band structure. We argue by combining the conventional\ntheory of magnetic oscillation with topological arguments that nodal loop\nsemimetals host coexisting topological and trivial magnetic oscillations. These\noriginate from mapping the topological properties of the extremal Fermi surface\ncross sections onto the physics of two dimensional semi Dirac systems, stemming\nfrom merging two massless Dirac cones. By tuning the chemical potential and the\ndirection of magnetic field, a sharp transition is identified separating purely\ntrivial oscillations, arising from the Landau levels of a normal two\ndimensional (2D) electron gas, to a phase where oscillations of topological and\ntrivial origin coexist, originating from 2D massless Dirac and semi Dirac\npoints, respectively. These could in principle be directly identified in\ncurrent experiments.", "category": "cond-mat_str-el" }, { "text": "Quantum adiabatic theorem for chemical reactions and systems with\n time-dependent orthogonalization: A general quantum adiabatic theorem with and without the time-dependent\northogonalization is proven, which can be applied to understand the origin of\nactivation energies in chemical reactions. Further proofs are also developed\nfor the oscillating Schwinger Hamiltonian to establish the relationship between\nthe internal (due to time-dependent eigenfunctions) and external (due to\ntime-dependent Hamiltonian) time scales. We prove that this relationship needs\nto be taken as an independent quantum adiabatic approximation criterion. We\ngive four examples, including logical expositions based on the spin-1/2\ntwo-level system to address the gapped and gapless (due to energy level\ncrossings) systems, as well as to understand how does this theorem allows one\nto study dynamical systems such as chemical reactions.", "category": "cond-mat_str-el" }, { "text": "The Falicov-Kimball model in external magnetic field: orbital effects: We study thermodynamic properties of the two-dimensional (2D) Falicov-Kimball\nmodel in the presence of external magnetic field perpendicular to the lattice.\nThe field is taken into account by the Peierls substitution in the hopping\nterm. In the non-interacting case the field dependent energy spectrum forms the\nfamous Hofstadter butterfly. Our results indicate that for arbitrary nonzero\ninteraction strength and arbitrary magnetic field there is a gap in the energy\nspectrum at sufficiently low temperature. The gap vanishes with increase of\ntemperature for weak coupling, however, it persists at high temperatures if the\ncoupling is strong enough. Numerical results have been obtained with the help\nof Monte Carlo technique based on a modified Metropolis algorithm.", "category": "cond-mat_str-el" }, { "text": "Excitations and relaxation dynamics in multiferroic GeV4S8 studied by\n THz and dielectric spectroscopy: We report on THz time-domain spectroscopy on multiferroic GeV4S8, which\nundergoes orbital ordering at a Jahn-Teller transition at 30.5 K and exhibits\nantiferromagnetic order below 14.6 K. The THz experiments are complemented by\ndielectric experiments at audio and radio frequencies. We identify a low-lying\nexcitation close to 15 cm-1, which is only weakly temperature dependent and\nprobably corresponds to a molecular excitation within the electronic level\nscheme of the V4 clusters. In addition, we detect complex temperature-dependent\nbehavior of a low-lying phononic excitation, closely linked to the onset of\norbitally-driven ferroelectricity. In the high-temperature cubic phase, which\nis paramagnetic and orbitally disordered, this excitation is of relaxational\ncharacter, becomes an overdamped Lorentzian mode in the orbitally ordered phase\nbelow the Jahn-Teller transition, and finally appears as well-defined phonon\nexcitation in the antiferromagnetic state. Abrupt changes in real and imaginary\nparts of the complex dielectric permittivity show that orbital ordering appears\nvia a structural phase transition with strong first-order character and that\nthe onset of antiferromagnetic order is accompanied by significant structural\nchanges, which are of first-order character, too. Dielectric spectroscopy\ndocuments that, at low frequencies, significant dipolar relaxations are present\nin the orbitally ordered, paramagnetic phase only. In contrast to the closely\nrelated GaV4S8, this relaxation dynamics that most likely mirrors coupled\norbital and polar fluctuations does not seem to be related to the dynamic\nprocesses detected in the THz regime.", "category": "cond-mat_str-el" }, { "text": "Electronic Liquid Crystalline Phases in a Spin-Orbit Coupled\n Two-Dimensional Electron Gas: We argue that the ground state of a two-dimensional electron gas with Rashba\nspin-orbit coupling realizes one of several possible liquid crystalline or\nWigner crystalline phases in the low-density limit, even for short-range\nrepulsive electron-electron interactions (which decay with distance with a\npower larger than 2). Depending on specifics of the interactions, preferred\nground-states include an anisotropic Wigner crystal with an increasingly\nanisotropic unit cell as the density decreases, a striped or electron smectic\nphase, and a ferromagnetic phase which strongly breaks the lattice point-group\nsymmetry, i.e. exhibits nematic order. Melting of the anisotropic Wigner\ncrystal or the smectic phase by thermal or quantum fluctuations can gives rise\nto a non-magnetic nematic phase which preserves time-reversal symmetry.", "category": "cond-mat_str-el" }, { "text": "Spin-polarization coupling in multiferroic transition-metal oxides: A systematic microscopic theory of magnetically induced ferroelectricity and\nlattice modulation is presented for all electron configurations of\nMott-insulating transition-metal oxides. Various mechanisms of polarization are\nidentified in terms of a strong-coupling perturbation theory. Especially, the\nspin-orbit interaction acting on the ligand p orbitals is shown to give the\nferroelectric polarization of the spin-current form, which plays a crucial role\nparticularly in eg systems. Semiquantitative agreements with the multiferroic\nTbMnO3 are obtained. Predictions for X-ray and neutron scattering experiments\nare proposed to clarify the microscopic mechanism of the spin-polarization\ncoupling in different materials.", "category": "cond-mat_str-el" }, { "text": "Determination of intrinsic ferroelectric polarization in lossy improper\n ferroelectric systems: We measured the intrinsic hysteretic polarization in lossy improper and\nnanoferroelectric systems where the nonhysteretic polarization and leakage are\nlarge and the relaxation takes place over a broader time scale. We used\ndifferent measurement protocols such as standard single triangular voltage\npulse, a pulse train of PUND (Positive Up Negative Down), and an even more\ncomplicated pulse train of fourteen voltage pulses and compared the results\nobtained. We show that a protocol which sends a train of fourteen pulses is\nmore appropriate for extracting relaxed (i.e., time scale independent) and\nintrinsic remanent polarization for these samples. We also point out that it is\npossible to select and design an appropriate measurement protocol depending on\nthe magnitude of polarization and leakage of the system.", "category": "cond-mat_str-el" }, { "text": "Spin Waves in Antiferromagnetic Spin Chains with Long Range Interactions: We study antiferromagnetic spin chains with unfrustrated long-range\ninteractions that decays as a power law with exponent $\\beta$, using the spin\nwave approximation. We find for sufficiently large spin $S$, the Neel order is\nstable at T=0 for $\\beta < 3$, and survive up to a finite Neel temperature for\n$\\beta < 2$, validating the spin-wave approach in these regimes. We estimate\nthe critical values of $S$ and $T$ for the Neel order to be stable. The spin\nwave spectra are found to be gapless but have non-linear momentum dependence at\nlong wave length, which is responsible for the suppression of quantum and\nthermal fluctuations and stabilizing the Neel state. We also show that for\n$\\beta\\le 1$ and for a large but finite-size system size $L$, the excitation\ngap of the system approaches zero slower than $L^{-1}$, a behavior that is in\ncontrast to the Lieb-Schulz-Mattis theorem.", "category": "cond-mat_str-el" }, { "text": "Spin-orbit physics of j=1/2 Mott insulators on the triangular lattice: The Heisenberg-Kitaev (HK) model on the triangular lattice is conceptually\ninteresting for its interplay of geometric and exchange frustration. HK models\nare also thought to capture the essential physics of the spin-orbital\nentanglement in effective $j=1/2$ Mott insulators studied in the context of\nvarious 5d transition metal oxides. Here we argue that the recently synthesized\nBa$_3$IrTi$_2$O$_9$ is a prime candidate for a microscopic realization of the\ntriangular HK model. We establish that an infinitesimal Kitaev exchange\ndestabilizes the 120$^\\circ$ order of the quantum Heisenberg model and results\nin the formation of an extended $\\mathbb{Z}_2$-vortex crystal phase in the\nparameter regime most likely relevant to the real material. Using a combination\nof analytical and numerical techniques we map out the entire phase diagram of\nthe model, which further includes various ordered phases as well as an extended\nnematic phase around the antiferromagnetic Kitaev point.", "category": "cond-mat_str-el" }, { "text": "Details of Sample Dependence and Transport Properties of URu2Si2: Resistivity and specific heat measurements were performed in the low carrier\nunconventional superconductor URu2Si2 on various samples with very different\nqualities. The superconducting transition temperature (TSC) and the hidden\norder transition temperature (THO) of these crystals were evaluated as a\nfunction of the residual resistivity ratio (RRR). In high quality single\ncrystals the resistivity does not seem to follow a T2 dependence above TSC,\nindicating that the Fermi liquid regime is restricted to low temperatures.\nHowever, an analysis of the isothermal longitudinal magnetoresistivity points\nout that the T2 dependence may be \"spoiled\" by residual inhomogeneous\nsuperconducting contribution. We discuss a possible scenario concerning the\ndistribution of TSC related with the fact that the hidden order phase is very\nsensitive to the pressure inhomogeneity.", "category": "cond-mat_str-el" }, { "text": "Interaction Correction of Conductivity Near a Ferromagnetic Quantum\n Critical Point: We calculate the temperature dependence of conductivity due to interaction\ncorrection for a disordered itinerant electron system close to a ferromagnetic\nquantum critical point which occurs due to a spin density wave instability. In\nthe quantum critical regime, the crossover between diffusive and ballistic\ntransport occurs at a temperature $T^{\\ast}=1/[\\tau \\gamma (E_{F}\\tau)^{2}]$,\nwhere $\\gamma$ is the parameter associated with the Landau damping of the spin\nfluctuations, $\\tau$ is the impurity scattering time, and $E_{F}$ is the Fermi\nenergy. For a generic choice of parameters, $T^{\\ast}$ is few orders of\nmagnitude smaller than the usual crossover scale $1/\\tau$. In the ballistic\nquantum critical regime, the conductivity has a $T^{(d-1)/3}$ temperature\ndependence, where $d$ is the dimensionality of the system. In the diffusive\nquantum critical regime we get $T^{1/4}$ dependence in three dimensions, and\n$\\ln^2 T$ dependence in two dimensions. Away from the quantum critical regime\nwe recover the standard results for a good metal.", "category": "cond-mat_str-el" }, { "text": "Quantum Wire Hybridized with a Single-Level Impurity: We have studied low-temperature properties of interacting electrons in a\none-dimensional quantum wire (Luttinger liquid) side-hybridized with a\nsingle-level impurity. The hybridization induces a back-scattering of electrons\nin the wire which strongly affects its low energy properties. Using a one-loop\nrenormalization group approach valid for a weak electron-electron interaction,\nwe have calculated a transmission coefficient through the wire,\n$\\mathcal{T}(\\varepsilon)$, and a local density of states, $\\nu(\\varepsilon)$\nat low energies $\\varepsilon $. In particular, we have found that the\nantiresonance in $\\mathcal{T}(\\varepsilon)$ has a generalized Breit-Wigner\nshape with the effective width $\\Gamma(\\varepsilon)$ which diverges at the\nFermi level.", "category": "cond-mat_str-el" }, { "text": "Kondo Effect and Persistent Currents in a Mesoscopic Ring: Numerically\n Exact Results: We study the persistent current circulating along a mesoscopic ring with a\ndot side-coupled to it when threaded by a magnetic field. A cluster including\nthe dot and its vicinity is diagonalized and embedded into the rest of the\nsystem. The result is numerically exact. We show that a ring of any size can be\nin the Kondo regime, although for small sizes it depends upon the magnetic\nflux. In the Kondo regime, the current can be a smooth or a strongly dependent\nfunction of the gate potential according to the structure of occupation of the\nhighest energetic electrons of the system.", "category": "cond-mat_str-el" }, { "text": "Nonlinear spectroscopy of collective modes in excitonic insulator: The nonlinear optical response of an excitonic insulator coupled to lattice\ndegrees of freedom is shown to depend in strong and characteristic ways on\nwhether the insulating behavior originates primarily from electron-electron or\nelectron-lattice interactions. Linear response optical signatures of the\nmassive phase mode and the amplitude (Higgs) mode are identified. Upon\nnonlinear excitation resonant to the phase mode, a new in-gap mode at twice the\nphase mode frequency is induced, leading to a huge second harmonic response.\nExcitation of in-gap phonon modes leads to different and much smaller effects.\nA Landau-Ginzburg theory analysis explain these different behavior and reveals\nthat a parametric resonance of the strongly excited phase mode is the origin of\nthe photo-induced mode in the electron-dominant case. The difference in the\nnonlinear optical response serve as a measure of the dominant mechanism of the\nordered phase.", "category": "cond-mat_str-el" }, { "text": "The Chern-Simons Invariant in the Berry Phase of a Two by Two\n Hamiltonian: The positive (negaive)-energy eigen vectors of the two by two Hamiltonian\n$H=\\v{r}\\cdot\\vec{\\s}$ where $\\vec{\\s}$ are the Pauli matrices and $\\v{r}$ is a\n3-vector, form a U(1) fiber bundle when $\\v{r}$ sweeps over a manifold $\\cM$ in\nthe three dimensional parameter space of $\\v{r}$ . For appropriately chosen\nbase space $\\cM$ the resulting fiber bundle can have non-trivial topology. For\nexample when $\\cM=S^2\\equiv\\{\\v{r}; |\\v{r}|=1\\}$ the corresponding bundle has a\nnon-zero Chern number, which is the indicator that it is topologically\nnon-trivial. In this paper we construct a two by two Hamiltonian whose eigen\nbundle shows a more subtle topological non-triviality over\n$\\cM=R^3\\bigcup\\{\\infty\\}$, the stereographic projection of $S^3$. This\nnon-triviality is characterized by a non-zero Chern-Simons invariant.", "category": "cond-mat_str-el" }, { "text": "Energetics of Domain Walls in the 2D t-J model: Using the density matrix renormalization group, we calculate the energy of a\ndomain wall in the 2D t-J model as a function of the linear hole density\n\\rho_\\ell, as well as the interaction energy between walls, for J/t=0.35. Based\non these results, we conclude that the ground state always has domain walls for\ndopings 0 < x < 0.3. For x < 0.125, the system has (1,0) domain walls with\n\\rho_\\ell ~ 0.5, while for 0.125 < x < 0.17, the system has a possibly\nphase-separated mixture of walls with \\rho_\\ell ~ 0.5 and \\rho_\\ell =1. For x >\n0.17, there are only walls with \\rho_\\ell =1. For \\rho_\\ell = 1, diagonal (1,1)\ndomain walls have very nearly the same energy as (1,0) domain walls.", "category": "cond-mat_str-el" }, { "text": "A photo-induced strange metal with electron and hole quasi-particles: Photo-doping of Mott insulators or correlated metals can create an unusual\nmetallic state which simultaneously hosts hole-like and electron-like\nparticles. We study the dynamics of this state up to long times, as it passes\nits kinetic energy to the environment. When the system cools down, it crosses\nover from a bad metal into a resilient quasiparticle regime, in which\nquasiparticle bands are formed with separate Fermi levels for electrons and\nholes, but quasiparticles do not yet satisfy the Fermi liquid paradigm.\nSubsequently, the transfer of energy to the environment slows down\nsignificantly, and the system does not reach the Fermi liquid state even on the\ntimescale of picoseconds. The transient photo-doped strange metal exhibits\nunusual properties of relevance for ultrafast charge and heat transport: In\nparticular, there can be an asymmetry in the properties of electrons and holes,\nand strong correlations between electrons and holes, as seen in the spectral\nproperties.", "category": "cond-mat_str-el" }, { "text": "Spin excitations in metallic kagome lattice FeSn and CoSn: In two-dimensional (2D) metallic kagome lattice materials, destructive\ninterference of electronic hopping pathways around the kagome bracket can\nproduce nearly localized electrons, and thus electronic bands that are flat in\nmomentum space. When ferromagnetic order breaks the degeneracy of the\nelectronic bands and splits them into the spin-up majority and spin-down\nminority electronic bands, quasiparticle excitations between the spin-up and\nspin-down flat bands should form a narrow localized spin-excitation Stoner\ncontinuum coexisting with well-defined spin waves in the long wavelengths. Here\nwe report inelastic neutron scattering studies of spin excitations in 2D\nmetallic Kagome lattice antiferromagnetic FeSn and paramagnetic CoSn, where\nangle resolved photoemission spectroscopy experiments found spin-polarized and\nnonpolarized flat bands, respectively, below the Fermi level. Although our\ninitial measurements on FeSn indeed reveal well-defined spin waves extending\nwell above 140 meV coexisting with a flat excitation at 170 meV, subsequent\nexperiments on CoSn indicate that the flat mode actually arises mostly from\nhydrocarbon scattering of the CYTOP-M commonly used to glue the samples to\naluminum holder. Therefore, our results established the evolution of spin\nexcitations in FeSn and CoSn, and identified an anomalous flat mode that has\nbeen overlooked by the neutron scattering community for the past 20 years.", "category": "cond-mat_str-el" }, { "text": "The emergence of charged collective modes from a large N extrapolation\n of the Hubbard model: We consider a symplectic extrapolation of the Hubbard model of N fold\nreplicated electrons and solve this model exactly in two special cases, at\nN=infinity in the bosonic sector and for any N on a dimer of two points. At\nN=infinity we find a multiplet of collective modes that contains neutral spin\nfluctuations and charged pair fluctuations that are degenerate with each other\nat zero doping. Our solution of the symplectic model on a dimer of two points\nfor any N interpolates smoothly between N=1 and N=infinity without any visible\ndiscontinuity. These results suggest that the inclusion of charged pairing\nmodes in weakly doped antiferromagnets is essential and that an expansion about\nthe N=infinity limit is appropriate in this context.", "category": "cond-mat_str-el" }, { "text": "Heavy holes: precursor to superconductivity in antiferromagnetic CeIn3: Numerous phenomenological parallels have been drawn between f- and d-\nelectron systems in an attempt to understand their display of unconventional\nsuperconductivity. The microscopics of how electrons evolve from participation\nin large moment antiferromagnetism to superconductivity in these systems,\nhowever, remains a mystery. Knowing the origin of Cooper paired electrons in\nmomentum space is a crucial prerequisite for understanding the pairing\nmechanism. Of especial interest are pressure-induced superconductors CeIn3 and\nCeRhIn5 in which disparate magnetic and superconducting orders apparently\ncoexist - arising from within the same f-electron degrees of freedom. Here we\npresent ambient pressure quantum oscillation measurements on CeIn3 that\ncrucially identify the electronic structure - potentially similar to high\ntemperature superconductors. Heavy pockets of f-character are revealed in\nCeIn3, undergoing an unexpected effective mass divergence well before the\nantiferromagnetic critical field. We thus uncover the softening of a branch of\nquasiparticle excitations located away from the traditional spin-fluctuation\ndominated antiferromagnetic quantum critical point. The observed Fermi surface\nof dispersive f-electrons in CeIn3 could potentially explain the emergence of\nCooper pairs from within a strong moment antiferromagnet.", "category": "cond-mat_str-el" }, { "text": "On planar fermions with quartic interaction at finite temperature and\n density: We study the breaking of parity symmetry in the 2+1 Gross-Neveu model at\nfinite temperature with chemical potential $\\mu$, in the presence of an\nexternal magnetic field. We find that the requirement of gauge invariance,\nwhich is considered mandatory in the presence of gauge fields, breaks parity at\nany finite temperature and provides for dynamical mass generation, preventing\nsymmetry restoration for any non-vanishing $\\mu$. The dynamical mass becomes\nnegligibly small as temperature is raised. We comment on the relevance of our\nobservation for the gap generation of nodal quasi-particles in the pseudo-gap\nphase of high $T_c$ superconductors.", "category": "cond-mat_str-el" }, { "text": "Composition and field tuned magnetism and superconductivity in\n Nd1-xCexCoIn5: The Nd1-xCexCoIn5 alloys evolve from local moment magnetism (x = 0) to heavy\nfermion superconductivity (x =1). Magnetic order is observed over a broad range\nof x. For a substantial range of x (0.83 <= x <= 0.95) in the temperature -\ncomposition phase diagram we find that superconductivity may coexist with spin\n- density wave magnetic order at the Fermi surface. We show that a delicate\nbalance betwen superconducting and magnetic instabilities can be reversibly\ntuned by both the Ce/Nd ratio and magnetic field, offering a new and unique\nmodel electronic system.", "category": "cond-mat_str-el" }, { "text": "Boundary effects in the critical scaling of entanglement entropy in 1D\n systems: We present exact diagonalization and density matrix renormalization group\nresults for the entanglement entropy of critical spin-1/2 XXZ chains. We find\nthat open boundary conditions induce an alternating term in both the energy\ndensity and the entanglement entropy which are approximately proportional,\ndecaying away from the boundary with a power-law. The power varies with\nanisotropy along the XXZ critical line and is corrected by a logarithmic\nfactor, which we calculate analytically, at the isotropic point. A heuristic\nresonating valence bond explanation is suggested.", "category": "cond-mat_str-el" }, { "text": "The effects of k-dependent self-energy in the electronic structure of\n correlated materials: It is known from self-energy calculations in the electron gas and sp\nmaterials based on the GW approximation that a typical quasiparticle\nrenormalization factor (Z factor) is approximately 0.7-0.8. Band narrowing in\nelectron gas at rs = 4 due to correlation effects, however, is only\napproximately 10%, significantly smaller than the Z factor would suggest. The\nband narrowing is determined by the frequency-dependent self-energy, giving the\nZ factor, and the momentum-dependent or nonlocal self-energy. The results for\nthe electron gas point to a strong cancellation between the effects of\nfrequency- and momentum-dependent self-energy. It is often assumed that for\nsystems with a nar- row band the self-energy is local. In this work we show\nthat even for narrow-band materials, such as SrVO3, the nonlocal self-energy is\nimportant.", "category": "cond-mat_str-el" }, { "text": "Absence of static stripes in the two-dimensional $t{-}J$ model by an\n accurate and systematic quantum Monte Carlo approach: We examine the two-dimensional $t{-}J$ model by using variational approach\ncombined with well established quantum Monte Carlo techniques [S. Sorella {\\it\net al.}, \\prl {\\bf 88}, 117002 (2002)] that are used to improve systematically\nthe accuracy of the variational ansatz. Contrary to recent density-matrix\nrenormalization group and projected entangled-pair state calculations [P.\nCorboz {\\it et al.}, \\prb {\\bf 84}, 041108(R) (2011)], a uniform phase is found\nfor $J/t=0.4$, even when the calculation is biased with an ansatz that\nexplicitly contains stripe order. Moreover, in the small hole doping regime,\ni.e., $\\delta \\lesssim 0.1$, our results support the coexistence of\nantiferromagnetism and superconductivity.", "category": "cond-mat_str-el" }, { "text": "Magnetic Field Effect on Crossover Temperature from Non-Fermi Liquid to\n Fermi Liquid Behavior in f^2-Impurity Systems with Crystalline-Electric-Field\n Singlet State Competing with Kondo-Yosida Singlet State: We investigate the magnetic field dependence of the physical properties of\nf^2-configuration systems with a crystalline-electric field (CEF) singlet\nground state, which gives rise to a non- Fermi liquid (NFL) fixed point due to\nthe competition between the Kondo-Yosida singlet and CEF singlet states. On the\nbasis of the numerical renormalization group method, we find that the magnetic\nfield breaks this NFL fixed point via two mechanisms: one causing the\npolarization of f-electrons and the other giving the \"channel\" anisotropy.\nThese two mechanisms induce a difference in the magnetic field dependence of\nthe characteristic temperature T_F^{*}(H), the crossover temperature from NFL\nto Fermi-liquid behavior. While the polarization of f-electrons gives\nT_F^{*}(H) \\propto H^x (x\\sim2.0), the \"channel\" anisotropy gives the\nH-independent T_F^{*}(H). These two mechanisms cross over continuously at\napproximately the crossover magnetic field H_c, where an anomalous H-dependence\nof T_F^{*}(H) appears. Such T_F^{*}(H) well reproduces the NFL behaviors\nobserved in Th_{1-x}U_xRu_2Si_2. We also find that the H-dependence of the\nresistivity and the magnetic susceptibility are in good agreement with the\nexperimental results of this material. These results suggest that the NFL\nbehaviors observed in Th_{1-x}U_xRu_2Si_2 can be understood if this material is\nlocated in the CEF singlet side near the critical phase boundary between the\ntwo singlet states.", "category": "cond-mat_str-el" }, { "text": "Reply to \"Comment on `Orbital-selective Mott transitions in the\n anisotropic two-band Hubbard model at finite temperatures'\": In a Comment [cond-mat/0506138] on our recent e-print [cond-mat/0505106]\nLiebsch claimed \"excellent correspondence\" between our high-precision quantum\nMonte-Carlo (QMC) data for the anisotropic two-band Hubbard model with Ising\ntype exchange couplings and his earlier QMC results. Liebsch also claimed that\nthe sequence of two orbital-selective Mott transitions, identified by us in\nthis model, had already been reported in his earlier work. Here we demonstrate\nthat both claims are incorrect. We establish that Liebsch's previous QMC\nestimates for the quasiparticle weight Z have relative errors exceeding 100%\nnear transitions and cannot be used to infer the existence of a second Mott\ntransition (for U_{c2}~2.5). We further show that Liebsch's attribution of our\nfindings to his own earlier work is disproved by the published record.\nConsequently, the Comment is unwarranted; all results and formulations of our\ne-print remain valid.", "category": "cond-mat_str-el" }, { "text": "Crystalline Solutions of Kohn-Sham Equations in the Fractional Quantum\n Hall Regime: A Kohn-Sham density functional approach has recently been developed for the\nfractional quantum Hall effect, which maps the strongly interacting electrons\ninto a system of weakly interacting composite fermions subject to an exchange\ncorrelation potential as well as a density dependent gauge field that mimics\nthe \"flux quanta\" bound to composite fermions. To get a feel for the role of\nvarious terms, we study the behavior of the self-consistent solution as a\nfunction of the strength of the exchange correlation potential, which is varied\nthrough an {\\it ad hoc} multiplicative factor. We find that a crystal phase is\nstabilized when the exchange correlation interaction is sufficiently strong\nrelative to the composite-fermion cyclotron energy. Various properties of this\ncrystal are examined.", "category": "cond-mat_str-el" }, { "text": "The spin-1 ladder : A bosonization study: We construct a field-theoretic description of two coupled spin-1 Heisenberg\nchains, starting with the known representation of a single spin-1 chain in\nterms of Majorana fermions (or Ising models). After reexamining the\nbosonization rules for two Ising models, taking particular care of order and\ndisorder operators, we obtain a bosonic description of the spin-1 ladder. From\nrenormalization-group and mean-field arguments, we conclude that, for a small\ninterchain coupling, the spin-1 ladder is approximately described by three\ndecoupled, two-frequency sine-Gordon models. We then predict that, starting\nwith decoupled chains, the spin gap decreases linearly with interchain\ncoupling, both in the ferromagnetic and antiferromagnetic directions. Finally,\nwe discuss the possibility of an incommensurate phase in the spin-1 zigzag\nchain.", "category": "cond-mat_str-el" }, { "text": "Effect of Interdots Electronic Repulsion in the Majorana Signature for a\n Double Dot Interferometer: We investigate theoretically the features of the Majorana hallmark in the\npresence of Coulomb repulsion between two quantum dots describing a spinless\nAharonov-Bohm-like interferometer, where one of the dots is strongly coupled to\na Kitaev wire within the topological phase. Such a system has been originally\nproposed without Coulomb interaction in J. of Appl. Phys. 116, 173701 (2014).\nOur findings reveal that for dots in resonance, the ratio between the strength\nof Coulomb repulsion and the dot-wire coupling changes the width of the\nMajorana zero-bias peak for both Fano regimes studied, indicating thus that the\nelectronic interdots correlation influences the Majorana state lifetime in the\ndot hybridized with the wire. Moreover, for the off-resonance case, the swap\nbetween the energy levels of the dots also modifies the width of the Majorana\npeak, which does not happen for the noninteracting case. The results obtained\nhere can guide experimentalists that pursuit a way of revealing Majorana\nsignatures.", "category": "cond-mat_str-el" }, { "text": "Reply to Millis et al. on \"A Tale of Two Theories: Quantum Griffiths\n Effects in Metallic Systems\": In a recent paper (cond-mat/0411197) we showed the equivalence of two\nseemingly contradictory theories on Griffiths-McCoy singularities (GMS) in\nmetallic antiferromagnets close to a quantum critical point (QCP). In a recent\ncomment, Millis {\\it et al.} (cond-mat/0411738) argue that in heavy-fermion\nmaterials the electronic damping is large leading to the freezing of locally\nmagnetically ordered droplets at high temperatures. In this reply we show that\nthis erroneous conclusion is based on a treatment of the problem of disorder\nclose to a QCP which is not self-consistent. We argue that a self-consistent\ntreatment of the ordered droplets must lead to weak damping and to a large\nregion of GMS behavior, in agreement with the our ealier results.", "category": "cond-mat_str-el" }, { "text": "Transport in a classical model of an one-dimensional Mott insulator:\n Influence of conservation laws: We study numerically how conservation laws affect the optical conductivity\nsigma(w) of a slightly doped one-dimensional Mott insulator. We investigate a\nregime where the average distance between charge excitations is large compared\nto their thermal de Broglie wave length and a classical description is\npossible. Due to conservation laws, the dc-conductivity is infinite and the\nDrude weight D is finite even at finite temperatures. Our numerical results\ntest and confirm exact theoretical predictions for D both for integrable and\nnon-integrable models. Small deviations from integrability induce slowly\ndecaying modes and, consequently, low-frequency peaks in sigma(w) which can be\ndescribed by a memory matrix approach.", "category": "cond-mat_str-el" }, { "text": "Magnetism and berry phase manipulation in an emergent structure of\n perovskite ruthenate by (111) strain engineering: The interplay among symmetry of lattices, electronic correlations, and Berry\nphase of the Bloch states in solids has led to fascinating quantum phases of\nmatter. A prototypical system is the magnetic Weyl candidate SrRuO3, where\ndesigning and creating electronic and topological properties on artificial\nlattice geometry is highly demanded yet remains elusive. Here, we establish an\nemergent trigonal structure of SrRuO3 by means of heteroepitaxial strain\nengineering along the [111] crystallographic axis. Distinctive from bulk, the\ntrigonal SrRuO3 exhibits a peculiar XY-type ferromagnetic ground state, with\nthe coexistence of high-mobility holes likely from linear Weyl bands and\nlow-mobility electrons from normal quadratic bands as carriers. The presence of\nWeyl nodes are further corroborated by capturing intrinsic anomalous Hall\neffect, acting as momentum-space sources of Berry curvatures. The experimental\nobservations are consistent with our first-principles calculations, shedding\nlight on the detailed band topology of trigonal SrRuO3 with multiple pairs of\nWeyl nodes near the Fermi level. Our findings signify the essence of magnetism\nand Berry phase manipulation via lattice design and pave the way towards\nunveiling nontrivial correlated topological phenomena.", "category": "cond-mat_str-el" }, { "text": "Superconducting properties of Pr-based Filled skutterudite\n PrRu$_4$As$_{12}$: We report on systematic study of superconducting characteristics and Pr\ncrystalline-electric-field (CEF) levels of filled-skutterudite \\pra ($T_{\\rm\nc}$ = 2.33 K). The temperature dependences of the upper critical field $H_{\\rm\nc2}$ and the Ginzburg-Landau (Maki) parameter $\\kappa_2$ suggest an s-wave\nclean-limit superconductivity. The electronic specific heat coefficient $\\gamma\n\\sim 95$ mJ/K$^2$mol, being $\\sim 1.5$ times larger than that for \\lra,\nindicates $4f$-originating quasiparticle mass enhancement. Magnetic\nsusceptibility $\\chi(T)$ indicates that the CEF ground state is a $\\Gamma_1$\nsinglet and a $\\Gamma_4^{(1)}$ triplet first excited state lies at $\\Delta_{\\rm\nCEF}\\sim 30$ K above. Systematic comparison among \\pos, \\prs, \\pra and La-based\nreference compounds suggests that inelastic exchange- and\naspherical-charge-scatterings of conduction electrons from CEF-split $4f$\nlevels play an essential role for the quasiparticle mass enhancement and the\nvalue of $T_{\\rm c}$ in the Pr-based filled skutterudites.", "category": "cond-mat_str-el" }, { "text": "Transfer of spectral weight across the gap of Sr2IrO4 induced by La\n doping: We study with Angle Resolved PhotoElectron Spectroscopy (ARPES) the evolution\nof the electronic structure of Sr2IrO4, when holes or electrons are introduced,\nthrough Rh or La substitutions. At low dopings, the added carriers occupy the\nfirst available states, at bottom or top of the gap, revealing an anisotropic\ngap of 0.7eV in good agreement with STM measurements. At further doping, we\nobserve a reduction of the gap and a transfer of spectral weight across the\ngap, although the quasiparticle weight remains very small. We discuss the\norigin of the in-gap spectral weight as a local distribution of gap values.", "category": "cond-mat_str-el" }, { "text": "Conversion of glassy antiferromagnetic-insulating phase to equilibrium\n ferromagnetic-metallic phase by devitrification and recrystallization in Al\n substituted Pr${_{0.5}}$Ca$_{0.5}$MnO${_3}$: We show that Pr${_{0.5}}$Ca$_{0.5}$MnO${_3}$ with 2.5% Al substitution and\nLa${_{0.5}}$Ca$_{0.5}$MnO${_3}$ (LCMO) exhibit qualitatively similar and\nvisibly anomalous M-H curves at low temperature. Magnetic field causes a broad\nfirst-order but irreversible antiferromagnetic (AF)-insulating (I) to\nferromagnetic (FM)-metallic (M) transition in both and gives rise to soft FM\nstate. However, the low temperature equilibrium state of\nPr$_{0.5}$Ca$_{0.5}$Mn$_{0.975}$Al$_{0.025}$O$_3$ (PCMAO) is FM-M whereas that\nof LCMO is AF-I. In both the systems the respective equilibrium phase coexists\nwith the other phase with contrasting order, which is not in equilibrium, and\nthe cooling field can tune the fractions of the coexisting phases. It is shown\nearlier that the coexisting FM-M phase behaves like `magnetic glass' in LCMO.\nHere we show from specially designed measurement protocols that the AF-I phase\nof PCMAO has all the characteristics of magnetic glassy states. It devitrifies\non heating and also recrystallizes to equilibrium FM-M phase after annealing.\nThis glass-like AF-I phase also shows similar intriguing feature observed in\nFM-M magnetic glassy state of LCMO that when the starting coexisting fraction\nof glass is larger, successive annealing results in larger fraction of\nequilibrium phase. This similarity between two manganite systems with\ncontrasting magnetic orders of respective glassy and equilibrium phases points\ntoward a possible universality.", "category": "cond-mat_str-el" }, { "text": "Comment on arXiv:0811.1575 entitled \"Quantum phase transitions in the\n Hubbard model on triangular lattice\" by T. Yoshioka, A. Koga and N. Kawakami: We show that the phase boundary between the paramagnetic metal and the\nnonmagnetic Mott insulator for the Hubbard model on a triangular lattice\nobtained by Yoshioka et al. in arXiv:0811.1575 does not correctly represent\nthat of the thermodynamic limit but is an artifact of the 6 by 6 lattice they\nrely on. After the system size extrapolation, the phase boundary is located at\nU/t=5.2 as proposed by Morita et al., J. Phys. Soc. Jpn. 71 (2008) 2109 and in\ncontrast to Yoshioka et al. Here, U is the onsite Coulomb repulsion and t is\nthe nearest-neighbor transfer.", "category": "cond-mat_str-el" }, { "text": "Magnetic order in Tb$_2$Sn$_2$O$_7$ under high pressure: from ordered\n spin ice to spin liquid and antiferromagnetic order: We have studied the Tb$_2$Sn$_2$O$_7$ frustrated magnet by neutron\ndiffraction under isotropic pressure of 4.6 GPa, combined with uniaxial\npressure of 0.3 GPa, in the temperature range 0.06 K$<$T$<$100 K. Magnetic\norder persists under pressure but the ordered spin ice structure stabilized at\nambient pressure below 1.3 K partly transforms into an antiferromagnetic one.\nThe long range ordered moment at 0.06 K is reduced under pressure, which is\ninterpreted by a pressure induced enhancement of the spin liquid fluctuations.\nAbove the ordering transition, short range spin correlations are affected by\npressure, and ferromagnetic correlations are suppressed. The influence of\npressure on the ground state is discussed considering both isotropic and stress\neffects.", "category": "cond-mat_str-el" }, { "text": "Abrupt disappearance and reemergence of the SU(2) and SU(4) Kondo\n effects due to population inversion: The interplay of almost degenerate levels in quantum dots and molecular\njunctions with possibly different couplings to the reservoirs has lead to many\nobservable phenomena, such as the Fano effect, transmission phase slips and the\nSU(4) Kondo effect. Here we predict a dramatic repeated disappearance and\nreemergence of the SU(4) and anomalous SU(2) Kondo effects with increasing gate\nvoltage. This phenomenon is attributed to the level occupation switching which\nhas been previously invoked to explain the universal transmission phase slips\nin the conductance through a quantum dot. We use analytical arguments and\nnumerical renormalization group calculations to explain the observations and\ndiscuss their experimental relevance and dependence on the physical parameters.", "category": "cond-mat_str-el" }, { "text": "Emergence of a 2d macro-spin liquid in a highly frustrated 3d quantum\n magnet: The classical Ising model on the frustrated 3d swedenborgite lattice has\ndisordered spin liquid ground states for all ratios of inter- and intra-planar\ncouplings. Quantum fluctuations due to a transverse field give rise to several\nexotic quantum phenomena. In the limit of weakly coupled Kagom\\'e layers we\nfind a 3d version of disorder by disorder. For large out-of-plane couplings 1d\nmacro-spins are formed which realize a disordered macro-spin liquid on an\nemerging triangular lattice. Signatures of this dimensional reduction are also\nfound in critical exponents of the quantum phase transition out of the fully\npolarized phase into the macro-spin liquid displaying quantum criticality\ntypical for 2d quantum systems.", "category": "cond-mat_str-el" }, { "text": "On conservation of the of crystal lattice symmetry in transition at\n Curie point in exchange magnets: We show that symmetry of the crystal lattice of exchange magnets (containing\nonly 3d magneto-active elements) does not change at the Curie point; only the\nmagnetic symmetry of the crystal is decreasing in the transition point. In the\nnon-exchange magnets (containing only rare-earth magneto-active elements), on\nthe contrary, both the magnetic and crystal-chemical symmetry decrease at the\nCurie point. There is isotropic magnetic phase in exchange magnets; and their\nmagnetic symmetry is described by color groups of magnetic symmetry of P-type.\nNon-exchange magnets do not have isotropic phase; their symmetry is described\nby color groups of magnetic symmetry of Q-type.", "category": "cond-mat_str-el" }, { "text": "A fractionalised \"$\\mathbb{Z}_2$\" classical Heisenberg spin liquid: Quantum spin systems are by now known to exhibit a large number of different\nclasses of spin liquid phases. By contrast, for \\textit{classical} Heisenberg\nmodels, only one kind of fractionalised spin liquid phase, the so-called\nCoulomb or $U(1)$ spin liquid, has until recently been identified: this\nexhibits algebraic spin correlations and impurity moments, `orphan spins',\nwhose size is a fraction of that of the underlying microscopic degrees of\nfreedom. Here, we present two Heisenberg models exhibiting fractionalisation in\ncombination with exponentially decaying correlations. These can be thought of\nas a classical continuous spin version of a $\\mathbb{Z}_2$ spin liquid. Our\nwork suggests a systematic search and classification of classical spin liquids\nas a worthwhile endeavour.", "category": "cond-mat_str-el" }, { "text": "Soft magnon contributions to dielectric constant in spiral magnets with\n domain walls: Competing magnetic exchange interactions often result in non-collinear\nmagnetic states, such as spin spirals, which break the inversion symmetry and\ninduce ferroelectric polarization. The resulting strong interactions between\nmagnetic and dielectric degrees of freedom lead to a technologically important\npossibility to control magnetic order by electric fields and to electromagnons,\nmagnetic excitations that can be excited by an electric dipole of the\nelectromagnetic field. Here we study the effects of chiral domain walls on\nmagnetoelectric properties of spiral magnets. We use a quasi-1D model\nHamiltonian with competing Heisenberg exchange interactions, leading to a spin\nspiral, and Dzyaloshinskii-Moriya interactions, that couple spins and electric\ndipoles and mix magnon and phonon excitations. The results suggest that low\nfrequency dielectric anomalies in spiral magnets, such as TbMnO3 and MnWO4, may\noriginate from hybrid magnon - polar phonon excitations associated with domain\nwalls.", "category": "cond-mat_str-el" }, { "text": "Basal-Plane Nonlinear Susceptibility: A Direct Probe of the Single-Ion\n Physics in URu2Si2: The microscopic nature of the hidden order state in URu2Si2 is dependent on\nthe low-energy configurations of the uranium ions, and there is currently no\nconsensus on whether it is predominantly 5f^2 or 5f^3. Here we show that\nmeasurement of the basal-plane nonlinear susceptibility can resolve this issue;\nits sign at low-temperatures is a distinguishing factor. We calculate the\nlinear and nonlinear susceptibilities for specific 5f^2 and 5f^3 crystal-field\nschemes that are consistent with current experiment. Because of its dual\nmagnetic and orbital character, a \\Gamma_5 magnetic non-Kramers doublet\nground-state of the U ion can be identified by $\\chi_1^c(T) \\propto\n\\chi_3^\\perp(T)$ where we have determined the constant of proportionality for\nURu2Si2.", "category": "cond-mat_str-el" }, { "text": "Effects of Disorder on the Pressure-Induced Mott Transition in\n $\u03ba$-BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl: We present a study of the influence of disorder on the Mott metal-insulator\ntransition for the organic charge-transfer salt\n$\\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl. To this end, disorder was introduced\ninto the system in a controlled way by exposing the single crystals to x-ray\nirradiation. The crystals were then fine-tuned across the Mott transition by\nthe application of continuously controllable He-gas pressure at low\ntemperatures. Measurements of the thermal expansion and resistance show that\nthe first-order character of the Mott transition prevails for low irradiation\ndoses achieved by irradiation times up to 100 h. For these crystals with a\nmoderate degree of disorder, we find a first-order transition line which ends\nin a second-order critical endpoint, akin to the pristine crystals. Compared to\nthe latter, however, we observe a significant reduction of both, the critical\npressure $p_c$ and the critical temperature $T_c$. This result is consistent\nwith the theoretically-predicted formation of a soft Coulomb gap in the\npresence of strong correlations and small disorder. Furthermore, we\ndemonstrate, similar to the observation for the pristine sample, that the Mott\ntransition after 50 h of irradiation is accompanied by sizable lattice effects,\nthe critical behavior of which can be well described by mean-field theory. Our\nresults demonstrate that the character of the Mott transition remains\nessentially unchanged at a low disorder level. However, after an irradiation\ntime of 150 h, no clear signatures of a discontinuous metal-insulator\ntransition could be revealed anymore. These results suggest that, above a\ncertain disorder level, the metal-insulator transition becomes a smeared\nfirst-order transition with some residual hysteresis.", "category": "cond-mat_str-el" }, { "text": "Lattice dynamics and electronic excitations in a large family of lacunar\n spinels with a breathing pyrochlore lattice structure: Reproducing the electronic structure of AM$_4$X$_8$ lacunar spinels with a\nbreathing pyrochlore lattice is a great theoretical challenge due to the\ninterplay of various factors. The character of the M$_4$X$_4$ cluster orbitals\nis critically influenced by the Jahn-Teller instability, the spin-orbit\ninteraction, and also by the magnetic state of the clusters. Consequently, to\nreproduce the narrow-gap semiconducting nature of these moderately correlated\nmaterials requires advanced approaches, since the strength of the inter-cluster\nhopping is strongly affected by the character of the cluster orbitals. In order\nto provide a solid experimental basis for theoretical studies, we performed\nbroadband optical spectroscopy on a large set of lacunar spinels, with\nsystematically changing ions at the A and M sites as well as the ligand (A=Ga,\nGe, Al; M=V, Mo, Nb, Ta; X=S, Se). Our study covers the range of phonon\nexcitations and also electronic transitions near the gap edge. In the phonon\nexcitation spectrum a limited subset of the symmetry allowed modes is observed\nin the cubic state, with a few additional modes emerging upon the\nsymmetry-lowering structural transition. All the infrared active modes are\nassigned to vibrations of the ligands and ions at the A sites, with no obvious\ncontribution from the M-site ions. Concerning the electronic states, we found\nthat all compounds are narrow-gap semiconductors ($E_\\mathrm{g} = 130 -\n350\\,$meV) already in their room-temperature cubic state and their structural\ntransitions induce weak, if any, changes in the band gap. The gap value is\ndecreased when substituting S with Se and also when replacing $3d$ ions by $4d$\nor $5d$ ions at the M sites.", "category": "cond-mat_str-el" }, { "text": "Ordered spin-ice state in the geometrically frustrated\n metallic-ferromagnet Sm2Mo2O7: The recent discovery of Spin-ice is a spectacular example of non-coplanar\nspin arrangements that can arise in the pyrochlore A2B2O7 structure. We present\nmagnetic and thermodynamic studies on the metallic-ferromagnet pyrochlore\nSm2Mo2O7. Our studies, carried out on oriented crystals, suggest that the Sm\nspins have an ordered spin-ice ground state below about T* = 15 K. The\ntemperature- and field-evolution of the ordered spin-ice state are governed by\nan antiferromagnetic coupling between the Sm and Mo spins. We propose that as a\nconsequence of a robust feature of this coupling, the tetrahedra aligned with\nthe external field adopt a \"1-in, 3-out\" spin structure as opposed to \"3-in,\n1-out\" in dipolar spin ices, as the field exceeds a critical value.", "category": "cond-mat_str-el" }, { "text": "Magnetic field-tuned quantum critical point in CeAuSb_2: Transport, magnetic and thermal properties at high magnetic fields (H) and\nlow temperatures (T) of the heavy fermion compound CeAuSb_2 are reported. At\nH=0 this layered system exhibits antiferromagnetic order below T_N = 6 K.\nApplying B along the inter-plane direction, leads to a continuous suppression\nof T_N and a quantum critical point at H_c ~ 5.4 T. Although it exhibits Fermi\nliquid behavior within the Neel phase, in the paramagnetic state the\nfluctuations associated with H_c give rise to unconventional behavior in the\nresistivity (sub-linear in T) and to a TlnT dependence in the magnetic\ncontribution to the specific heat. For H > H_c and low T the electrical\nresistivity exhibits an unusual T^3-dependence.", "category": "cond-mat_str-el" }, { "text": "Cobalt spin state above the valence and spin-state transition in\n (Pr0.7Sm0.3)0.7Ca0.3CoO3: (Pr0.7Sm0.3)0.7Ca0.3CoO3 belongs to a class of cobalt oxides undergoing a\nfirst-order transition (T* close to 90 K) associated to a coupled change in the\nvalence and spin-state degrees of freedom. The Curie-Weiss regime present\naround room temperature (T >> T*) was analysed in detail to address the\ncontroversial issue of the cobalt spin states above the transition. This\nmagnetic investigation indicates that the Co4+ are in an intermediate\nspin-state, while the Co3+ are in a mixed state combining low-spin and\nhigh-spin states. These results are discussed with respect to the literature on\nrelated compounds and recent results of x-ray absorption spectroscopy.", "category": "cond-mat_str-el" }, { "text": "The evolution of the Non-Fermi Liquid behavior of BaVS$_3$ under high\n pressure: Temperature, pressure, and magnetic field dependencies of the resistivity of\nBaVS$_3$ were measured above the critical pressure of $p_{cr}$=2 GPa, which is\nassociated with the zero temperature insulator-to-metal (MI) transition. The\nresistivity exhibits the $T^n$ temperature dependence below $T_g\\approx$15 K,\nwith $n$ of 1.5 at $p_{cr}$, which increases continuously with pressure towards\n2. This is interpreted as a crossover from non-Fermi (NFL) to Fermi-liquid (FL)\nbehavior. Although the spin configuration of the $e_g$ electrons influences the\ncharge propagation, the NFL behavior is attributed to the pseudogap that\nappears in the single particle spectrum of the $d_z^2$ electrons related to\nlarge quasi-one dimensional (Q-1d) 2$k_F$-CDW fluctuations. The non-monotonic\nmagnetic field dependence of $\\Delta$$\\rho$/$\\rho$ reveals a characteristic\nfield $B_0\\approx$12 T attributed to the full suppression of the pseudogap.", "category": "cond-mat_str-el" }, { "text": "Nontrivial ferrimagnetism of the Heisenberg model on the Union Jack\n strip lattice: We study the ground-state properties of the S=1/2 antiferromagnetic\nHeisenberg model on the Union Jack strip lattice by using the\nexact-diagonalization and density matrix renormalization group methods. We\nconfirm a region of the intermediate-magnetization state between the Neel-like\nspin liquid state and the conventional ferrimagnetic state of Lieb-Mattis type.\nIn the intermediate-state, we find that the spontaneous magnetization changes\ngradually with respect to the strength of the inner interaction. In addition,\nthe local magnetization clearly shows an incommensurate modulation with\nlong-distance periodicity in the intermediate-magnetization state. These\ncharacteristic behaviors lead to the conclusion that the\nintermediate-magnetization state is the non-Lieb-Mattis ferrimagnetic one. We\nalso discuss the relationship between the ground-state properties of the S=1/2\nantiferromagnetic Heisenberg model on the original Union Jack lattice and those\non our strip lattice.", "category": "cond-mat_str-el" }, { "text": "Destruction of the Kondo effect in a multi-channel Bose-Fermi Kondo\n model: We consider the SU(N) x SU(kappa N) generalization of the spin-isotropic\nBose-Fermi Kondo model in the limit of large N. There are three fixed points\ncorresponding to a multi-channel non-Fermi liquid phase, a local spin-liquid\nphase, and a Kondo-destroying quantum critical point (QCP). We show that the\nQCP has strong similarities with its counterpart in the single-channel model,\neven though the Kondo phase is very different from the latter. We also discuss\nthe evolution of the dynamical scaling properties away from the QCP.", "category": "cond-mat_str-el" }, { "text": "Susceptibilities of Sr(Cu_(1-x)Zn_x)_2O_3 Studied by Quantum Monte Carlo\n Simulation: The effects of non-magnetic impurities randomly doped into a two-leg\nHeisenberg spin ladder are investigated. Using the continuous time quantum\nMonte Carlo loop algorithm we calculate the uniform and staggered\nsusceptibilities of such a system. The obtained uniform susceptibility is well\ndescribed in terms of an effective model of weakly interacting local moments\ninduced by non-magnetic impurities for a 1% doping case, but not for higher\nconcentrations. The staggered susceptibility however is significantly enhanced\nover that in the effective model already at 1% doping. Using a mean field\napproximation for the interladder coupling, we explain qualitatively the phase\ndiagram of Sr(Cu_{1-x}Zn_x)_2O_3.", "category": "cond-mat_str-el" }, { "text": "Quantum aspects of \"hydrodynamic\" transport from weak electron-impurity\n scattering: Recent experimental observations of apparently hydrodynamic electronic\ntransport have generated much excitement. However, the understanding of the\nobserved non-local transport (whirlpool) effects and parabolic\n(Poiseuille-like) current profiles has largely been motivated by a\nphenomenological analogy to classical fluids. This is due to difficulty in\nincorporating strong correlations in quantum mechanical calculation of\ntransport, which has been the primary angle for interpreting the apparently\nhydrodynamic transport. Here we demonstrate that even free fermion systems, in\nthe presence of (inevitable) disorder, exhibit non-local conductivity effects\nsuch as those observed in experiment because of the fermionic system's\nlong-range entangled nature. On the basis of explicit calculations of the\nconductivity at finite wavevector, $\\sigma({\\bf q})$, for selected weakly\ndisordered free fermion systems, we propose experimental strategies for\ndemonstrating distinctive quantum effects in non-local transport at odds with\nthe expectations of classical kinetic theory. Our results imply that the\nobservation of whirlpools or other \"hydrodynamic\" effects does not guarantee\nthe dominance of electron-electron scattering over electron-impurity\nscattering.", "category": "cond-mat_str-el" }, { "text": "Melting of excitonic insulator phase by an intense terahertz pulse in\n Ta$_2$NiSe$_5$: In this study, the optical response to a terahertz pulse was investigated in\nthe transition metal chalcogenide Ta$_2$NiSe$_5$, a candidate excitonic\ninsulator. First, by irradiating a terahertz pulse with a relatively weak\nelectric field (0.3 MV/cm), the spectral changes in reflectivity near the\nabsorption edge due to third-order optical nonlinearity were measured and the\nabsorption peak characteristic of the excitonic phase just below the interband\ntransition was identified. Next, by irradiating a strong terahertz pulse with a\nstrong electric field of 1.65 MV/cm, the absorption of the excitonic phase was\nfound to be reduced, and a Drude-like response appeared in the mid-infrared\nregion. These responses can be interpreted as carrier generation by exciton\ndissociation induced by the electric field, resulting in the partial melting of\nthe excitonic phase and metallization. The presence of a distinct threshold\nelectric field for carrier generation indicates exciton dissociation via\nquantum-tunnelling processes. The spectral change due to metallization by the\nelectric field is significantly different from that due to the strong optical\nexcitation across the gap, which can be explained by the different melting\nmechanisms of the excitonic phase in the two types of excitations.", "category": "cond-mat_str-el" }, { "text": "Semiquantitative theory for high-field low-temperature properties of a\n distorted diamond spin chain: We consider the antiferromagnetic Heisenberg model on a distorted diamond\nchain and use the localized-magnon picture adapted to a distorted geometry to\ndiscuss some of its high-field low-temperature properties. More specifically,\nin our study we assume that the partition function for a slightly distorted\ngeometry has the same form as for ideal geometry, though with slightly\ndispersive one-magnon energies. We also discuss the relevance of such a\ndescription to azurite.", "category": "cond-mat_str-el" }, { "text": "Magnetic Order in Laser-Irradiated Kagome Antiferromagnets: Dispersionless \"zero energy mode'' is one of the hallmarks of frustrated\nkagome antiferromagnets (KAFMs). It points to extensive classically degenerate\nground-states. The \"zero energy mode'' can be observed experimentally when\nlifted to a flat mode at finite energy by a strong intrinsic magnetic\nanisotropy. In this letter, we study the effects of irradiation of laser light\non the KAFMs. We adopt the magnon picture without loss of generality. It is\nshown that circularly or linearly polarized light lifts the \"zero energy\nmode'', stabilizes magnetic order, and induces energy gaps in the KAFMs. We\nfind that the circularly polarized light-induced anisotropies have similar\nfeatures as the intrinsic in-plane and out-of-plane Dzyaloshinskii-Moriya\ninteraction in KAFMs. The former stabilizes long-range magnetic order and the\nlatter induces spin canting out-of-plane with nonzero scalar spin chirality.\nThe Floquet thermal Hall effect shows that the synthetic magnetic excitation\nmodes in the case of circularly polarized light are topological, whereas those\nof linearly polarized light are not.", "category": "cond-mat_str-el" }, { "text": "Disentangling a quantum antiferromagnet with resonant inelastic X-ray\n scattering: Low-dimensional copper oxide lattices naturally manifest electronic states\nwith strong short range quantum entanglement, which are known to lead to\nremarkable emergent material properties. However the nanometer scale many-body\nwavefunction is challenging to measure or manipulate in a simple way. In this\nstudy, X-ray induced $dd$ electronic transitions are used to suppress spin\nentanglement across a single lattice site in the spin-1/2 antiferromagnetic\nchain compound SrCuO$_2$, revealing a class of cuprate magnetic excitations\nthat result from breaking the spin chain. These measurements are the first to\nemploy two closely spaced X-ray resonances of copper (M$_2$ and M$_3$) as a\nform of natural 2-slit interferometer to distinguish between different types of\nelectronic transition and resolve how they influence the dynamics of nearby\nspin-entangled electrons.", "category": "cond-mat_str-el" }, { "text": "In situ controllable magnetic phases in doped twisted bilayer\n transition-metal dichalcogenides: We study the electronic structure of hole-doped transition metal\ndichalcogenides for small twist-angels, where the onsite repulsion is extremely\nstrong. Using unbiased diagrammatic Monte Carlo simulations, we find evidence\nfor magnetic correlations which are driven by delocalization and can be\ncontrolled in situ via the dielectric environment. For weak spin-orbit\ncoupling, we find that the moderately doped system becomes anti-ferromagnetic,\nwhilst the regime of strong spin-orbit coupling features ferromagnetic\ncorrelations. We show that this behavior is accurately predicted by an\nanalytical theory based on moment expansion of the Hamiltonian, and analysis of\ncorresponding particle trajectories.", "category": "cond-mat_str-el" }, { "text": "Spin excitations in the kagome-lattice metallic antiferromagnet\n Fe$_{0.89}$Co$_{0.11}$Sn: Kagome-lattice materials have attracted tremendous interest due to the broad\nprospect for seeking superconductivity, quantum spin liquid states, and\ntopological electronic structures. Among them, the transition-metal kagome\nlattices are high-profile objects for the combination of topological\nproperties, rich magnetism, and multiple-orbital physics. Here we report an\ninelastic neutron scattering study on the spin dynamics of a kagome-lattice\nantiferromagnetic metal Fe$_{0.89}$Co$_{0.11}$Sn. Although the magnetic\nexcitations can be observed up to $\\sim$250 meV, well-defined spin waves are\nonly identified below $\\sim$90 meV and can be modeled using Heisenberg exchange\nwith ferromagnetic in-plane nearest-neighbor coupling $J_1$, in-plane\nnext-nearest-neighbor coupling $J_2$, and antiferromagnetic (AFM) interlayer\ncoupling $J_c$ under linear spin-wave theory. Above $\\sim$90 meV, the spin\nwaves enter the itinerant Stoner continuum and become highly damped\nparticle-hole excitations. At the K point of the Brillouin zone, we reveal a\npossible band crossing of the spin wave, which indicates a potential Dirac\nmagnon. Our results uncover the evolution of the spin excitations from the\nplanar AFM state to the axial AFM state in Fe$_{0.89}$Co$_{0.11}$Sn, solve the\nmagnetic Hamiltonian for both states, and confirm the significant influence of\nthe itinerant magnetism on the spin excitations.", "category": "cond-mat_str-el" }, { "text": "Peculiar behavior of the electrical resistivity of MnSi at the\n ferromagnetic phase transition: The electrical resistivity of a single crystal of MnSi was measured across\nits ferromagnetic phase transition line at ambient and high pressures. Sharp\npeaks of the temperature coefficient of resistivity characterize the transition\nline. Analysis of these data shows that at pressures to ~0.35 GPa these peaks\nhave fine structure, revealing a shoulder at ~ 0.5 K above the peak. It is\nsymptomatic that this structure disappears at pressures higher than ~0.35 GPa,\nwhich was identified earlier as a tricritical point", "category": "cond-mat_str-el" }, { "text": "Ground state of the frustrated Hubbard model within DMFT: energetics of\n Mott insulator and metal from ePT and QMC: We present a new method, ePT, for extrapolating few known coefficients of a\nperturbative expansion. Controlled by comparisons with numerically exact\nquantum Monte Carlo (QMC) results, 10th order strong-coupling perturbation\ntheory (PT) for the Hubbard model on the Bethe lattice is reliably extrapolated\nto infinite order. Within dynamical mean-field theory (DMFT), we obtain\ncontinuous estimates of energy E and double occupancy D with unprecedented\nprecision O(10^{-5}) for the Mott insulator above its stability edge\nU_{c1}=4.78 as well as critical exponents. In addition, we derive corresponding\nprecise estimates for E and D in the metallic ground state from extensive\nlow-temperature QMC simulations using a fit to weak-coupling PT while enforcing\nthermodynamic consistency.", "category": "cond-mat_str-el" }, { "text": "La$_2$O$_3$Fe$_2$Se$_2$, a Mott insulator on the brink of\n orbital-selective metalization: La$_2$O$_3$Fe$_2$Se$_2$ can be explained in terms of Mott localization in\nsharp contrast with the metallic behavior of FeSe and other parent parent\ncompounds of iron superconductors. We demonstrate that the key ingredient that\nmakes La$_2$O$_3$Fe$_2$Se$_2$ a Mott insulator, rather than a correlated metal\ndominated by the Hund's coupling is the enhanced crystal-field splitting,\naccompanied by a smaller orbital-resolved kinetic energy. The strong deviation\nfrom orbital degeneracy introduced by the crystal-field splitting also pushes\nthis materials close to an orbital-selective Mott transition. We predict that\neither doping or uniaxial external pressure can drive the material into an\norbital-selective Mott state, where only one or few orbitals are metallized\nwhile the others remain insulating.", "category": "cond-mat_str-el" }, { "text": "MIEZE Neutron Spin-Echo Spectroscopy of Strongly Correlated Electron\n Systems: Recent progress in neutron spin-echo spectroscopy by means of longitudinal\nModulation of IntEnsity with Zero Effort (MIEZE) is reviewed. Key technical\ncharacteristics are summarized which highlight that the parameter range\naccessible in momentum and energy, as well as its limitations, are extremely\nwell understood and controlled. Typical experimental data comprising\nquasi-elastic and inelastic scattering are presented, featuring magneto-elastic\ncoupling and crystal field excitations in Ho2Ti2O7, the skyrmion lattice to\nparamagnetic transition under applied magnetic field in MnSi, ferromagnetic\ncriticality and spin waves in Fe. In addition bench marking studies of the\nmolecular dynamics in H2O are reported. Taken together, the advantages of MIEZE\nspectroscopy in studies at small and intermediate momentum transfers comprise\nan exceptionally wide dynamic range of over seven orders of magnitude, the\ncapability to perform straight forward studies on depolarizing samples or under\ndepolarizing sample environments, as well as on incoherently scattering\nmaterials.", "category": "cond-mat_str-el" }, { "text": "Vibrational edge modes in intrinsically heterogeneous doped transition\n metal oxides: By applying an unrestricted Hartree-Fock and a Random Phase approximations to\na multiband Peierls-Hubbard Hamiltonian, we study the phonon mode structure in\nmodels of transition metal oxides in the presence of intrinsic nanoscale\ninhomogeneities induced by hole doping. We identify low frequency $local$\nvibrational modes pinned to the sharp interfaces between regions of distinct\nelectronic structure (doped and undoped) and separated in frequency from the\nband of extended phonons. A characteristic of these ``edge'' modes is that\ntheir energy is essentially insensitive to the doping level. We discuss the\nexperimental manifestations of these modes in inelastic neutron scattering, and\nalso in spin and charge excitation spectra.", "category": "cond-mat_str-el" }, { "text": "Electronic structure of the kagome staircase compounds Ni3V2O8 and\n Co3V2O8: The electronic structure of the kagome staircase compounds, Ni3V2O8 and\nCo3V2O8, has been investigated using soft x-ray absorption, soft x-ray\nemission, and resonant inelastic x-ray scattering (RIXS). Comparison between\nthe two compounds, and with first principles band structure calculations and\ncrystal-field multiplet models, provide unique insight into the electronic\nstructure of the two materials. Whereas the location of the narrow (Ni,Co) d\nbands is predicted to be close to EF, we experimentally find they lie deeper in\nthe occupied O 2p and unoccupied V 3d manifolds, and determine their energy via\nmeasured charge-transfer excitations. Additionally, we find evidence for a dd\nexcitation at 1.5 eV in Ni3V2O8, suggesting the V d states may be weakly\noccupied in this compound, contrary to Co3V2O8. Good agreement is found between\nthe crystal-field dd excitations observed in the experiment and predicted by\natomic multiplet theory.", "category": "cond-mat_str-el" }, { "text": "Composite Dirac liquids: parent states for symmetric surface topological\n order: We introduce exotic gapless states---`composite Dirac liquids'---that can\nappear at a strongly interacting surface of a three-dimensional electronic\ntopological insulator. Composite Dirac liquids exhibit a gap to all charge\nexcitations but nevertheless feature a single massless Dirac cone built from\nemergent electrically neutral fermions. These states thus comprise electrical\ninsulators that, interestingly, retain thermal properties similar to those of\nthe non-interacting topological insulator surface. A variety of novel fully\ngapped phases naturally descend from composite Dirac liquids. Most remarkably,\nwe show that gapping the neutral fermions via Cooper pairing---which crucially\ndoes not violate charge conservation---yields symmetric non-Abelian\ntopologically ordered surface phases captured in several recent works. Other\n(Abelian) topological orders emerge upon alternatively gapping the neutral\nDirac cone with magnetism. We establish a hierarchical relationship between\nthese descendant phases and expose an appealing connection to paired states of\ncomposite Fermi liquids arising in the half-filled Landau level of\ntwo-dimensional electron gases. To controllably access these states we exploit\na quasi-1D deformation of the original electronic Dirac cone that enables us to\nanalytically address the fate of the strongly interacting surface. The\nalgorithm we develop applies quite broadly and further allows the construction\nof symmetric surface topological orders for recently introduced bosonic\ntopological insulators.", "category": "cond-mat_str-el" }, { "text": "Optical studies of gap, hopping energies and the Anderson-Hubbard\n parameter in the zigzag-chain compound SrCuO2: We have investigated the electronic structure of the zigzag ladder (chain)\ncompound SrCuO2 combining polarized optical absorption, reflection,\nphotoreflectance and pseudo-dielectric function measurements with the model\ncalculations. These measurements yield an energy gap of 1.42 eV (1.77 eV) at\n300 K along (perpendicular) to the Cu-O chains. We have found that the lowest\nenergy gap, the correlation gap, is temperature independent. The electronic\nstructure of this oxide is calculated using both the\nlocal-spin-density-approximation with gradient correction method, and the\ntight-binding theory for the correlated electrons. The calculated density of\nelectronic states for non-correlated and correlated electrons shows\nquasi-one-dimensional character. The correlation gap values of 1.42 eV\n(indirect transition) and 1.88 eV (direct transition) have been calculated with\nthe electron hopping parameters t = 0.30 eV (along a chain), t_yz = 0.12 eV\n(between chains) and the Anderson-Hubbard repulsion on copper sites U= 2.0 eV.\nWe concluded that SrCuO_2 belongs to the correlated-gap insulators.", "category": "cond-mat_str-el" }, { "text": "Entanglement in extended Hubbard models and quantum phase transitions: The role of two-point and multipartite entanglement at quantum phase\ntransitions (QPTs) in correlated electron systems is investigated. We consider\na bond-charge extended Hubbard model exactly solvable in one dimension which\ndisplays various QPTs, with two (qubit) as well as more (qudit) on-site degrees\nof freedom involved. The analysis is carried out by means of appropriate\nmeasures of bipartite/multipartite quantum correlations. It is found that all\ntransitions ascribed to two-point correlations are characterized by an\nentanglement range which diverges at the transition points. The exponent\ncoincides with that of the correlation length at the transitions. We introduce\nthe correlation ratio, namely, the ratio of quantum mutual information and\nsingle-site entanglement. We show that at T=0, it captures the relative role of\ntwo-point and multipartite quantum correlations at transition points,\ngeneralizing to qudit systems the entanglement ratio. Moreover, a finite value\nof quantum mutual information between infinitely distant sites is seen to\nquantify the presence of off-diagonal long-range order induced by multipartite\nentanglement.", "category": "cond-mat_str-el" }, { "text": "Quantum versus thermal fluctuations in the fcc antiferromagnet:\n alternative routes to order by disorder: In frustrated magnetic systems with competing interactions fluctuations can\nlift the residual accidental degeneracy. We argue that the state selection may\nhave different outcomes for quantum and thermal order by disorder. As an\nexample, we consider the semiclassical Heisenberg fcc antiferromagnet with only\nthe nearest-neighbor interactions. Zero-point oscillations select the type 3\ncollinear antiferromagnetic state at T=0. Thermal fluctuations favor instead\nthe type 1 antiferromagnetic structure. The opposite tendencies result in a\nfinite-temperature transition between the two collinear states. Competition\nbetween effects of quantum and thermal order by disorder is a general\nphenomenon and is also realized in the J1-J2 square-lattice antiferromagnet at\nthe critical point J2 = 0.5 J1.", "category": "cond-mat_str-el" }, { "text": "Dynamical signatures of topological order in the driven-dissipative\n Kitaev chain: We investigate the effects of dissipation and driving on topological order in\nsuperconducting nanowires. Rather than studying the non-equilibrium steady\nstate, we propose a method to classify and detect dynamical signatures of\ntopological order in open quantum systems. Bulk winding numbers for the\nLindblad generator $\\hat{\\mathcal{L}}$ of the dissipative Kitaev chain are\nfound to be linked to the presence of Majorana edge master modes -- localized\neigenmodes of $\\hat{\\mathcal{L}}$. Despite decaying in time, these modes\nprovide dynamical fingerprints of the topological phases of the closed system,\nwhich are now separated by intermediate regions where winding numbers are\nill-defined and the bulk-boundary correspondence breaks down. Combining these\ntechniques with the Floquet formalism reveals higher winding numbers and\ndifferent types of edge modes under periodic driving. Finally, we link the\npresence of edge modes to a steady state current.", "category": "cond-mat_str-el" }, { "text": "Incommensurate spin Luttinger liquid phase in a model for the\n spin-Peierls materials TiOBr and TiOCl: In the present work we aim to characterize the lattice configurations and the\nmagnetic behavior in the incommensurate phase of spin-Peierls systems. This\nphase emerges when the magnetic exchange interaction is coupled to the\ndistortions of an underlying triangular lattice and has its experimental\nrealization in the quasi-one dimensional compound family TiOX (X = Cl, Br).\nWith a simple model of spin-1/2 chains inserted in a planar triangular geometry\nwhich couples them elastically, we are able to obtain the\nuniform-incommensurate and incommensurate-dimerized phase transitions seen in\nthese compounds. Moreover, we follow the evolution of the wave-vector of the\ndistortions with temperature inside the incommensurate phase. Finally, we\npredict gapless spin excitations for the intermediate phase of TiOX compounds\nalong with incommensurate spin-spin correlations. This exotic Luttinger\nliquid-like behavior could be observed in future experiments.", "category": "cond-mat_str-el" }, { "text": "Interplay of electronic structure and unusual development in crystal\n structure of YbAuIn and Yb$_{3}$AuGe$_{2}$In$_{3}$: First-principles calculations within the DFT are employed to investigate the\nrelationship between the electronic structure and the unexpected features of\nthe hexagonal cell parameters of YbAuIn and Yb$_{3}$AuGe$_{2}$In$_{3}$.\nCalculations indicate that YbAuIn is an intermediate valent system with one Yb\n4\\textit{f} state pinned to the Fermi level, while Yb$_{3}$AuGe$_{2}$In$_{3}$\nis closer to integer valency with all Yb 4\\textit{f} states occupied.\nStructural relaxations performed on LaAuIn and LuAuIn analogs reveal that\nexpansion of the \\textit{c}-parameter in Yb$_{3}$AuGe$_{2}$In$_{3}$ is\nattributable to larger size of the divalent Yb compared with intermediate\nvalent Yb.", "category": "cond-mat_str-el" }, { "text": "Weak-field induced nonmagnetic state in a Co-based honeycomb: Layered honeycomb magnets are of interest as potential realizations of the\nKitaev quantum spin liquid (KQSL), a quantum state with long-range spin\nentanglement and an exactly solvable Hamiltonian. Conventional magnetically\nordered states are present for all currently known candidate materials,\nhowever, because non-Kitaev terms in the Hamiltonians obscure the Kitaev\nphysics. Current experimental studies of the KQSL are focused on 4d- or\n5d-transition-metal-based honeycombs, in which strong spin-orbit coupling can\nbe expected, yielding Kitaev interaction that dominate in an applied magnetic\nfield. In contrast, for 3d-based layered honeycomb magnets, spin orbit coupling\nis weak and thus Kitaev-physics should be substantially less accessible. Here\nwe report our studies on BaCo2(AsO4)2, for which we find that the magnetic\norder associated with the non-Kitaev interactions can be fully suppressed by a\nrelatively low magnetic field, yielding a non-magnetic material and implying\nthe presence of strong magnetic frustration and weak non-Kitaev interactions.", "category": "cond-mat_str-el" }, { "text": "Observation of giant circular dichroism induced by electronic chirality: Chiral phases of matter, characterized by a definite handedness, abound in\nnature, ranging from the crystal structure of quartz to spiraling spin states\nin helical magnets. In $1T$-TiSe$_2$ a source of chirality has been proposed\nthat stands apart from these classical examples as it arises from combined\nelectronic charge and quantum orbital fluctuations. This may allow its\nchirality to be accessed and manipulated without imposing either structural or\nmagnetic handedness. However, direct bulk evidence that broken inversion\nsymmetry and chirality are intrinsic to TiSe$_2$ remains elusive. Here,\nemploying resonant elastic scattering of x-rays, we reveal the presence of\ngiant circular dichroism up to $\\sim$ 40$\\%$ at forbidden Bragg peaks that\nemerge at the charge and orbital ordering transition. The dichroism varies\ndramatically with incident energy and azimuthal angle. Comparison to calculated\nscattering intensities unambiguously traces its origin to bulk chiral\nelectronic order in ${\\mathrm{TiSe}}_2$ and establishes resonant elastic x-ray\nscattering as a sensitive probe to electronic chirality.", "category": "cond-mat_str-el" }, { "text": "Quantum Many-Body Scars from Einstein-Podolsky-Rosen States in Bilayer\n Systems: Quantum many-body scar states are special eigenstates of nonintegrable models\nwith distinctive entanglement features that give rise to infinitely long-lived\ncoherent dynamics under quantum quenches from certain initial states. We\nelaborate on a construction of quantum many-body scar states in which they\nemerge from Einstein-Podolsky-Rosen (EPR) states in systems with two layers,\nwherein the two layers are maximally entangled. We apply this construction to\nspin systems as well as systems of itinerant fermions and bosons and\ndemonstrate how symmetries can be harnessed to enhance its versatility. We show\nthat several well-known examples of quantum many-body scars, including the\ntower of states in the spin-1 XY model and the $\\eta$-pairing states in the\nFermi-Hubbard model, can be understood within this formalism. We also\ndemonstrate how an {\\it infinite} tower of many-body scar states can emerge in\nbilayer Bose-Hubbard models with charge conservation.", "category": "cond-mat_str-el" }, { "text": "Low-energy theory of the Nambu-Goldstone modes of an ultracold $^6Li-$\n $^{40}K$ mixture in an optical lattice: A low-energy theory of the Nambu-Goldstone excitation spectrum and the\ncorresponding speed of sound of an interacting Fermi mixture of Lithium-6 and\nPotassium-40 atoms in a two-dimensional optical lattice at finite temperatures\nwith the Fulde-Ferrell order parameter has been formulated. It is assumed that\nthe two-species interacting Fermi gas is described by the one-band Hubbard\nHamiltonian with an attractive on-site interaction. The discussion is\nrestricted to the BCS side of the Feshbach resonance where the Fermi atoms\nexhibit superfluidity. The quartic on-site interaction is decoupled via a\nHubbard-Stratonovich transformation by introducing a four-component boson field\nwhich mediates the Hubbard interaction. A functional integral technique and a\nLegendre transform are used to give a systematic derivation of the\nSchwinger-Dyson equations for the generalized single-particle Green's function\nand the Bethe-Salpeter equation for the two-particle Green's function and the\nassociated collective modes. The numerical solution of the Bethe-Salpeter\nequation in the generalized random phase approximation shows that there exist\ntwo distinct sound velocities in the long-wavelength limit. In addition to the\nlong-wavelength mode (Goldstone mode), the two-species Fermi gas has a\nsuperfluid phase revealed by two rotonlike minima in the asymmetric\ncollective-mode energy.", "category": "cond-mat_str-el" }, { "text": "Low-energy optical properties of the non-magnetic kagome metal\n CsV$_3$Sb$_5$: Temperature-dependent reflectivity measurements on the kagome metal\nCsV$_3$Sb$_5$ in a broad frequency range of $50-20000$ cm$^{-1}$ down to $T$=10\nK are reported. The charge-density wave (CDW) formed below $T_{\\rm CDW}$ = 94 K\nmanifests itself in a prominent spectral-weight transfer from low to higher\nenergy regions. The CDW gap of 60-75 meV is observed at the lowest temperature\nand shows significant deviations from an isotropic BCS-type mean-field\nbehavior. Absorption peaks appear at frequencies as low as 200 cm$^{-1}$ and\ncan be identified with interband transitions according to density-functional\ncalculations. The change in the interband absorption compared to KV$_3$Sb$_5$\nreflects the inversion of band saddle points between the K and Cs compounds.\nAdditionally, a broader and strongly temperature-dependent absorption feature\nis observed below 1000 cm$^{-1}$ and assigned to a displaced Drude peak. It\nreflects localization effects on charge carriers.", "category": "cond-mat_str-el" }, { "text": "Elastic Instabilities within Antiferromagnetically Ordered Phase in the\n Orbitally-Frustrated Spinel GeCo$_2$O$_4$: Ultrasound velocity measurements of the orbitally-frustrated GeCo$_2$O$_4$\nreveal unusual elastic instabilities due to the phonon-spin coupling within the\nantiferromagnetic phase. Shear moduli exhibit anomalies arising from the\ncoupling to short-range ferromagnetic excitations. Diplike anomalies in the\nmagnetic-field dependence of elastic moduli reveal magnetic-field-induced\norbital order-order transitions. These results strongly suggest the presence of\ngeometrical orbital frustration which causes novel orbital phenomena within the\nantiferromagnetic phase.", "category": "cond-mat_str-el" }, { "text": "Spin diffusion in the low-dimensional molecular quantum Heisenberg\n antiferromagnet Cu(pyz)(NO$_{3}$)$_{2}$ detected with implanted muons: We present the results of muon-spin relaxation measurements of spin\nexcitations in the one-dimensional quantum Heisenberg antiferromagnet\nCu(pyz)(NO$_{3}$)$_{2}$. Using density-functional theory we propose muon sites\nand assess the degree of perturbation the muon probe causes on the system. We\nidentify a site involving the muon forming a hydroxyl-type bond with an oxygen\non the nitrate group that is sensitive to the characteristic spin dynamics of\nthe system. Our measurements of the spin dynamics show that in the temperature\nrange $T_{\\mathrm{N}}J$ and that in the related two-dimensional\nsystem Cu(pyz)$_2$(ClO$_4$)$_{2}$.", "category": "cond-mat_str-el" }, { "text": "A Model for the Schottky Anomaly in Metallic $Nd_{2-y}Ce_{y}CuO_{4}$: We present a simple model for the doped compound $Nd_{2-y}Ce_{y}CuO_{4}$, in\norder to explain some recent experimental results on the latter. Within a\nHartree-Fock context, we start from an impurity Anderson-like model and\nconsider the magnetic splitting of the $Nd$-$4f$ ground state Kramers doublet\ndue to exchange interactions with the ordered $Cu$ moments. Our results are in\nvery good agreement with the experimental data, yielding a Schottky anomaly\npeak for the specific heat that reduces its amplitude, broadens and shifts to\nlower temperatures, upon $Ce$ doping. For overdoped compounds at low\ntemperatures, the specific heat behaves linearly and the magnetic\nsusceptibility is constant. A smooth transition from this Fermi liquid like\nbehavior ocurrs as temperature is increased and at high temperatures the\nsusceptibility exhibits a Curie-like behavior. Finally, we discuss some\nimprovements our model is amenable to incorporate.", "category": "cond-mat_str-el" }, { "text": "Renormalization of f-levels away from the Fermi energy in electron\n excitation spectroscopies: Density functional results of\n Nd$_{2-x}$Ce$_x$CuO$_4$: Relaxation energies for photoemission, when an occupied electronic state is\nexcited, and for inverse photoemission, when an empty state is filled, are\ncalculated within the density functional theory with application to\nNd$_{2-x}$Ce$_x$CuO$_4$. The associated relaxation energies are obtained by\ncomputing differences in total energies between the ground state and an excited\nstate in which one hole or electron is added into the system. The relaxation\nenergies of f-electrons are found to be of the order of several eV's,\nindicating that f-bands will appear substantially away from the Fermi energy\n($E_F$) in their spectroscopic images, even if these bands lie near $E_F$. Our\nanalysis explains why it would be difficult to observe f electrons at the $E_F$\neven in the absence of strong electronic correlations.", "category": "cond-mat_str-el" }, { "text": "The J_1-J_2 model revisited : Phenomenology of CuGeO_3: We present a mean field solution of the antiferromagnetic Heisenberg chain\nwith nearest (J_1) and next to nearest neighbor (J_2) interactions. This\nsolution provides a way to estimate the effects of frustration. We calculate\nthe temperature-dependent spin-wave velocity, v_s(T) and discuss the\npossibility to determine the magnitude of frustration J_2/J_1 present in quasi\n1D compounds from measurements of v_s(T). We compute the thermodynamic\nsusceptibility at finite temperatures and compare it with the observed\nsusceptibility of the spin-Peierls compound CuGeO_3. We also use the method to\nstudy the two-magnon Raman continuum observed in CuGeO_3 above the spin-Peierls\ntransition.", "category": "cond-mat_str-el" }, { "text": "Block magnetic excitations in the orbitally selective Mott insulator\n BaFe2Se3: Iron pnictides and selenides display a variety of unusual magnetic phases\noriginating from the interplay between electronic, orbital, and lattice degrees\nof freedom. Using powder inelastic neutron scattering on the two-leg ladder\nBaFe2Se3, we fully characterize the static and dynamic spin correlations\nassociated with the Fe4 block state, an exotic magnetic ground state observed\nin this low-dimensional magnet and in Rb0.89Fe1.58Se2. All the magnetic\nexcitations of the Fe4 block state predicted by an effective Heisenberg model\nwith localized spins are observed below 300 meV and quantitatively reproduced.\nHowever, the data only account for 16 mub^2 per Fe2+, approximatively 2/3 of\nthe total spectral weight expected for localized S=2 moments. Our results\nhighlight how orbital degrees of freedom in iron-based magnets can conspire to\nstabilize an exotic magnetic state.", "category": "cond-mat_str-el" }, { "text": "Scaling approach for the time-dependent Kondo model: We present a new nonperturbative method to deal with the time-dependent\nquantum many-body problem, which is an extension of Wegner's flow equations to\ntime-dependent Hamiltonians. The formalism provides a scaling procedure for the\nset of time-dependent interaction constants. We apply these ideas to a Kondo\nmodel with a ferromagnetic exchange coupling switched on over a time scale\n$\\tau$. We show that the asymptotic expectation value of the impurity spin\ninterpolates continuously between its quenched and adiabatic value.", "category": "cond-mat_str-el" }, { "text": "Spin polarized STM spectra of Dirac Fermions on the surface of a\n topological insulator: We provide a theory for the tunneling conductance $G(V)$ of Dirac Fermions on\nthe surface of a topological insulator as measured by a spin-polarized scanning\ntunneling microscope tip for low bias voltages $V$. We show that $G(V)$\nexhibits an unconventional dependence on the direction of magnetization of the\ntip and can be used to measure the magnitude of the local out-of-plane spin\norientation of the Dirac Fermions on the surface. We also demonstrate that if\nthe in-plane rotational symmetry on the surface of the topological insulator is\nbroken by an external field, then $G(V)$ acquires a dependence on the azimuthal\nangle of the magnetization of the tip. We explain the role of the Dirac\nFermions in this unconventional behavior and suggest experiments to test our\ntheory.", "category": "cond-mat_str-el" }, { "text": "On the Field-Induced Gap in Cu Benzoate and Other S=1/2 Antiferromagnets: Recent experiments on the S=1/2 antiferromagnetic chain compound, Cu\nbenzoate, discovered an unexpected gap scaling as approximately the 2/3 power\nof an applied magnetic field. A theory of this gap, based on an effective\nstaggered field, orthogonal to the applied uniform field, resulting from a\nstaggered gyromagnetic tensor and a Dzyaloshinskii-Moriya interaction, leading\nto a sine-Gordon quantum field theory, has been developed. Here we discuss many\naspects of this subject in considerable detail, including a review of the S=1/2\nchain in a uniform field, a spin-wave theory analysis of the uniform plus\nstaggered field problem, exact amplitudes for the scaling of gap, staggered\nsusceptibility and staggered magnetization with field or temperature,\nintensities of soliton and breather peaks in the structure function and field\nand temperature dependence of the total susceptibility.", "category": "cond-mat_str-el" }, { "text": "Theory of Ca$_{10}$Cr$_7$O$_{28}$ as a bilayer breathing-kagome magnet:\n Classical thermodynamics and semi-classical dynamics: Ca$_{10}$Cr$_7$O$_{28}$ is a novel spin-$1/2$ magnet exhibiting spin liquid\nbehaviour which sets it apart from any previously studied model or material.\nHowever, understanding Ca$_{10}$Cr$_7$O$_{28}$ presents a significant\nchallenge, because the low symmetry of the crystal structure leads to very\ncomplex interactions, with up to seven inequivalent coupling parameters in the\nunit cell. Here we explore the origin of the spin-liquid behaviour in\nCa$_{10}$Cr$_7$O$_{28}$, starting from the simplest microscopic model\nconsistent with experiment - a Heisenberg model on a single bilayer of the\nbreathing-kagome (BBK) lattice. We use a combination of classical Monte Carlo\n(MC) simulation and (semi-)classical Molecular Dynamics (MD) simulation to\nexplore the thermodynamic and dynamic properties of this model, and compare\nthese with experimental results for Ca$_{10}$Cr$_7$O$_{28}$. We uncover\nqualitatively different behaviours on different timescales, and argue that the\nground state of Ca$_{10}$Cr$_7$O$_{28}$ is born out of a slowly-fluctuating\n\"spiral spin liquid\", while faster fluctuations echo the U(1) spin liquid found\nin the kagome antiferromagnet. We also identify key differences between\nlongitudinal and transverse spin excitations in applied magnetic field, and\nargue that these are a distinguishing feature of the spin liquid in the BBK\nmodel.", "category": "cond-mat_str-el" }, { "text": "Magnetization, thermoelectric, and pressure studies of the magnetic\n field-induced metal to insulator transition in tau phase organic conductors: We have investigated the magnetic field-induced metal-insulator transition in\nthe tau-phase organic conductors, which occurs in fields above 35 T, and below\n14 K, by magnetization, thermoelectric, and pressure dependent transport\nmethods. Our results show that the transition is a bulk thermodynamic process\nwhere a magnetic field-dependent gap opens upon entry into the insulating\nstate. We argue that the transition involves a magnetic field-induced change in\nthe electronic structure.", "category": "cond-mat_str-el" }, { "text": "Efficient representation of long-range interactions in tensor network\n algorithms: We describe a practical and efficient approach to represent physically\nrealistic long-range interactions in two-dimensional tensor network algorithms\nvia projected entangled-pair operators (PEPOs). We express the long-range\ninteraction as a linear combination of correlation functions of an auxiliary\nsystem with only nearest-neighbor interactions. To obtain a smooth and radially\nisotropic interaction across all length scales, we map the physical lattice to\nan auxiliary lattice of expanded size. Our construction yields a long-range\nPEPO as a sum of ancillary PEPOs, each of small, constant bond dimension. This\nrepresentation enables efficient numerical simulations with long-range\ninteractions using projected entangled pair states.", "category": "cond-mat_str-el" }, { "text": "Coexisting localized and itinerant gapless excitations in a quantum spin\n liquid candidate 1T-TaS$_2$: To reveal the nature of elementary excitations in a quantum spin liquid\n(QSL), we measured low temperature thermal conductivity and specific heat of\n1T-TaS$_2$, a QSL candidate material with frustrated triangular lattice of\nspin-1/2. The nonzero temperature linear specific heat coefficient $\\gamma$ and\nthe finite residual linear term of the thermal conductivity in the zero\ntemperature limit $\\kappa_0/T=\\kappa/T(T\\rightarrow 0)$ are clearly resolved.\nThis demonstrates the presence of highly mobile gapless excitations, which is\nconsistent with fractionalized spinon excitations that form a Fermi surface.\nRemarkably, an external magnetic field strongly suppresses $\\gamma$, whereas it\nenhances $\\kappa_0/T$. This unusual contrasting behavior in the field\ndependence of specific heat and thermal conductivity can be accounted for by\nthe presence of two types of gapless excitations with itinerant and localized\ncharacters, as recently predicted theoretically (I. Kimchi et al.,\narXiv:1803.00013 (2018)). This unique feature of 1T-TaS$_2$ provides new\ninsights into the influence of quenched disorder on the QSL.", "category": "cond-mat_str-el" }, { "text": "Fermi liquid identities for the Infinite U Anderson Model: We show how the electron gas methods of Luttinger, Ward and Nozi\\`eres can be\napplied to the infinite U Anderson impurity model within a Schwinger boson\ntreatment. Working to all orders in a 1/N expansion, we show how the Friedel\nLangreth relationship, the Yamada-Yosida-Yoshimori and the Shiba-Korringa\nrelations can be derived, under the assumption that the spinon and holon fields\nare gapped. One of the remarkable features of this treatment, is that the\nLandau amplitudes depend on the exchange of low energy virtual spinons and\nholons. We end the paper with a discussion on the extension of our approach to\nthe lattice, where the spinon-holon is expected to close at a quantum critical\npoint.", "category": "cond-mat_str-el" }, { "text": "Spin-Wave Theory for the Scalar Chiral Phase in the Multiple-Spin\n Exchange Model on a Triangular Lattice: We study the effects of quantum fluctuations on a non-coplanar tetrahedral\nspin structure, which has a scalar chiral order, in the spin-1/2 multiple-spin\nexchange model with up to the six-spin exchange interactions on a triangular\nlattice. We find that, in the linear spin-wave approximation, the tetrahedral\nstructure survives the quantum fluctuations because spin waves do not soften in\nthe whole parameter region of the tetrahedral-structure phase evaluated for the\nclassical system. In the quantum corrections to the ground-state energy,\nsublattice magnetization, and scalar chirality, the effects of the quantum\nfluctuations are small for the ferromagnetic nearest-neighbor interactions and\nfor the strong five-spin interactions. The six-spin interactions have little\neffect on the quantum corrections in the tetrahedral-structure phase. This\ncalculation also corrects an error in the previously reported value of scalar\nchirality for the spin-1/2 multiple-spin exchange model with up to the\nfour-spin exchange interactions.", "category": "cond-mat_str-el" }, { "text": "Magnetic hard-axis ordering near ferromagnetic quantum criticality: We investigate the interplay of quantum fluctuations and magnetic\nanisotropies in metallic ferromagnets. Our central result is that fluctuations\nclose to a quantum critical point can drive the moments to point along a\nmagnetic hard axis. As a proof of concept, we show this behavior explicitly for\na generic two-band model with local Coulomb and Hund's interactions, and a\nspin-orbit-induced easy plane anisotropy. The phase diagram is calculated\nwithin the fermionic quantum order-by-disorder approach, which is based on a\nself-consistent free energy expansion around a magnetically ordered state with\nunspecified orientation. Quantum fluctuations render the transition of the\neasy-plane ferromagnet first-order below a tricritical point. At even lower\ntemperatures, directionally dependent transverse fluctuations dominate the\nmagnetic anisotropy and the moments flip to lie along the magnetic hard axis.\nWe discuss our findings in the context of recent experiments that show this\nunusual ordering along the magnetic hard direction.", "category": "cond-mat_str-el" }, { "text": "LDA+DMFT Spectral Functions and Effective Electron Mass Enhancement in\n Superconductor LaFePO: In this Letter we report the first LDA+DMFT results (method combining Local\nDensity Approximation with Dynamical Mean-Field Theory) for spectral properties\nof superconductor LaFePO. Calculated {\\bf k}-resolved spectral functions\nreproduce recent angle-resolved photoemission spectroscopy (ARPES) data [D. H.\nLu {\\it et al}., Nature {\\bf 455}, 81 (2008)]. Obtained effective electron mass\nenhancement values $m^{*}/m\\approx$ 1.9 -- 2.2 are in good agreement with\ninfrared and optical studies [M. M. Qazilbash {\\it et al}., Nature Phys. {\\bf\n5}, 647 (2009)], de Haas--van Alphen, electrical resistivity, and electronic\nspecific heat measurements results, that unambiguously evidence for moderate\ncorrelations strength in LaFePO. Similar values of $m^{*}/m$ were found in the\nother Fe-based superconductors with substantially different superconducting\ntransition temperatures. Thus, the dynamical correlation effects are essential\nin the Fe-based superconductors, but the strength of electronic correlations\ndoes not determine the value of superconducting transition temperature.", "category": "cond-mat_str-el" }, { "text": "Emerging frustration effects in ferromagnetic Ce_2[Pd_{1-x}Ag_x]_2In\n alloys: Magnetic and thermal properties of Ferromagnetic (FM)\nCe_{2.15}(Pd_{1-x}Ag_x)_{1.95}In_{0.9} alloys were studied in order to\ndetermine the Quantum Critical Point (QCP) at T_C => 0. The increase of band\nelectrons produced by Pd/Ag substitution depresses T_C(x) from 4.1K down to\nT_C(x=0.5)=1.1K, with a QCP extrapolated to x_{QCP}~ 0.6. Magnetic\nsusceptibility from T>30K indicates an effective moment slightly decreasing\nfrom \\mu_{eff}=2.56\\mu_B to 2.4\\mu_B at x=0.5. These values and the\nparamagnetic temperature \\theta_P~ -10K exclude significant Kondo screening\neffects. The T_C(x) reduction is accompanied by a weakening of the FM\nmagnetization and the emergence of a specific heat C_m(T) anomaly at T*~ 1K,\nwithout signs of magnetism detected from AC-susceptibility. The magnetic\nentropy collected around 4K (i.e. the T_C of the x=0 sample) practically does\nnot change with Ag concentration: S_m(4K)~ 0.8 Rln2, suggesting a progressive\ntransfer of FM degrees of freedom to the non-magnetic (NM) component. No\nantecedent was found concerning any NM anomaly emerging from a FM system at\nsuch temperature. The origin of this anomaly is attributed to an 'entropy\nbottleneck' originated in the nearly divergent power law dependence for T>T*.", "category": "cond-mat_str-el" }, { "text": "Decoupled spin dynamics in the rare-earth orthoferrite YbFeO$_3$:\n Evolution of magnetic excitations through the spin-reorientation transition: In this paper we present a comprehensive study of magnetic dynamics in the\nrare-earth orthoferrite YbFeO$_3$ at temperatures below and above the\nspin-reorientation (SR) transition $T_{\\mathrm{SR}}=7.6$ K, in magnetic fields\napplied along the $a, b$ and $c$ axes. Using single-crystal inelastic neutron\nscattering, we observed that the spectrum of magnetic excitations consists of\ntwo collective modes well separated in energy: 3D gapped magnons with a\nbandwidth of $\\sim$60 meV, associated with the antiferromagnetically (AFM)\nordered Fe subsystem, and quasi-1D AFM fluctuations of $\\sim$1 meV within the\nYb subsystem, with no hybridization of those modes. The spin dynamics of the Fe\nsubsystem changes very little through the SR transition and could be well\ndescribed in the frame of semiclassical linear spin-wave theory. On the other\nhand, the rotation of the net moment of the Fe subsystem at $T_{\\mathrm{SR}}$\ndrastically changes the excitation spectrum of the Yb subsystem, inducing the\ntransition between two regimes with magnon and spinon-like fluctuations. At $T\n< T_{\\mathrm{SR}}$, the Yb spin chains have a well defined field-induced\nferromagnetic (FM) ground state, and the spectrum consists of a sharp\nsingle-magnon mode, a two-magnon bound state, and a two-magnon continuum,\nwhereas at $T > T_{\\mathrm{SR}}$ only a gapped broad spinon-like continuum\ndominates the spectrum. In this work we show that a weak quasi-1D coupling\nwithin the Yb subsystem $J_\\text{Yb-Yb}$, mainly neglected in previous studies,\ncreates unusual quantum spin dynamics on the low energy scales. The results of\nour work may stimulate further experimental search for similar compounds with\nseveral magnetic subsystems and energy scales, where low-energy fluctuations\nand underlying physics could be \"hidden\" by a dominating interaction.", "category": "cond-mat_str-el" }, { "text": "Orbital order in La0.5Sr1.5MnO4: beyond a common local Jahn-Teller\n picture: The standard way to find the orbital occupation of Jahn-Teller (JT) ions is\nto use structural data, with the assumption of a one-to-one correspondence\nbetween the orbital occupation and the associated JT distortion, e.g. in O6\noctahedron. We show, however, that this approach in principle does not work for\nlayered systems. Specifically, using the layered manganite La0.5Sr1.5MnO4 as an\nexample, we found from our x-ray absorption measurements and theoretical\ncalculations, that the type of orbital ordering strongly contradicts the\nstandard local distortion approach for the Mn3+O6 octahedra, and that the\ngenerally ignored long-range crystal field effect and anisotropic hopping\nintegrals are actually crucial to determine the orbital occupation. Our\nfindings may open a pathway to control of the orbital state in multilayer\nsystems and thus of their physical properties.", "category": "cond-mat_str-el" }, { "text": "Realizing the strongly correlated $d$-Mott state in a fermionic cold\n atom optical lattice: We show that a new state of matter, the d-wave Mott-insulator state (d-Mott\nstate) (introduced recently by [H. Yao, W. F. Tsai, and S. A. Kivelson, Phys.\nRev. B 76, 161104 (2007)]), which is characterized by a non-zero expectation\nvalue of a local plaquette operator embedded in an insulating state, can be\nengineered using ultra-cold atomic fermions in two-dimensional double-well\noptical lattices. We characterize and analyze the parameter regime where the\n$d$-Mott state is stable. We predict the testable signatures of the state in\nthe time-of-flight measurements.", "category": "cond-mat_str-el" }, { "text": "Pressure-Induced Antiferromagnetic Dome in the Heavy-Fermion\n $Yb_2Pd_2In_{1-x}Sn_x$ System: In the heavy-fermion system $Yb_2Pd_2In_{1-x}Sn_x$, the interplay of\ncrystal-field splitting, Kondo effect, and Ruderman-Kittel-Kasuya-Yosida\ninteractions leads to complex chemical-, pressure-, and magnetic-field phase\ndiagrams, still to be explored in full detail. By using a series of techniques,\nwe show that even modest changes of parameters other than temperature are\nsufficient to induce multiple quantum-critical transitions in this highly\nsusceptible heavy-fermion family. In particular, we show that, above $\\sim 10$\nkbar, hydrostatic pressure not only induces an antiferromagnetic phase at low\ntemperature, but it likely leads to a reorientation of the Yb magnetic moments\nand/or the competition among different antiferromagnetic configurations.", "category": "cond-mat_str-el" }, { "text": "Topological Phase Transition in the Hofstadter-Hubbard Model: We study the interplay between topological and conventional long range order\nof attractive fermions in a time reversal symmetric Hofstadter lattice using\nquantum Monte Carlo simulations, focussing on the case of one-third flux\nquantum per plaquette. At half-filling, the system is unstable towards s-wave\npairing and charge-density-wave order at infinitesimally small interactions. At\none-third-filling, the noninteracting system is a topological insulator, and a\nnonzero critical interaction strength is needed to drive a transition from the\nquantum spin Hall insulator to a superfluid. We probe the topological signature\nof the phase transition by threading a magnetic flux through a cylinder and\nobserve quantized topological charge pumping.", "category": "cond-mat_str-el" }, { "text": "Photo-induced states in a Mott insulator: We investigate the properties of the metallic state obtained by photo-doping\ncarriers into a Mott insulator. In a strongly interacting system, these\ncarriers have a long life-time, so that they can dissipate their kinetic energy\nto a phonon bath. In the relaxed state, the scattering rate saturates at a\nnon-zero temperature-independent value, and the momentum-resolved spectral\nfunction features broad bands which differ from the well-defined quasi-particle\nbands of a chemically doped system. Our results indicate that a photo-doped\nMott insulator behaves as a bad metal, in which strong scattering between\ndoublons and holes inhibits Fermi-liquid behavior down to low temperature.", "category": "cond-mat_str-el" }, { "text": "Magnetic Order in Laser-Irradiated Kagome Antiferromagnets: Dispersionless \"zero energy mode'' is one of the hallmarks of frustrated\nkagome antiferromagnets (KAFMs). It points to extensive classically degenerate\nground-states. The \"zero energy mode'' can be observed experimentally when\nlifted to a flat mode at finite energy by a strong intrinsic magnetic\nanisotropy. In this letter, we study the effects of irradiation of laser light\non the KAFMs. We adopt the magnon picture without loss of generality. It is\nshown that circularly or linearly polarized light lifts the \"zero energy\nmode'', stabilizes magnetic order, and induces energy gaps in the KAFMs. We\nfind that the circularly polarized light-induced anisotropies have similar\nfeatures as the intrinsic in-plane and out-of-plane Dzyaloshinskii-Moriya\ninteraction in KAFMs. The former stabilizes long-range magnetic order and the\nlatter induces spin canting out-of-plane with nonzero scalar spin chirality.\nThe Floquet thermal Hall effect shows that the synthetic magnetic excitation\nmodes in the case of circularly polarized light are topological, whereas those\nof linearly polarized light are not.", "category": "cond-mat_str-el" }, { "text": "From Quantum Wires to the Chern-Simons Description of the Fractional\n Quantum Hall Effect: We show the explicit connection between two distinct and complementary\napproaches to the fractional quantum Hall system (FQHS): the quantum wires\nformalism and the topological low-energy effective description given in terms\nof an Abelian Chern-Simons theory. The quantum wires approach provides a\ndescription of the FQHS directly in terms of fermions arranged in an array of\none-dimensional coupled wires. In this sense it is usually referred to as a\nmicroscopic description. On the other hand, the effective theory has no\nconnection with the microscopic modes, involving only the emergent topological\ndegrees of freedom embodied in an Abelian Chern-Simons gauge field, which\nsomehow encodes the collective motion of the strongly correlated electrons. The\nbasic strategy pursued in this work is to bosonize the quantum wires system and\nthen consider the continuum limit. By examining the algebra of the bosonic\noperators of the Hamiltonian, we are able to identify the bosonized microscopic\nfields with the components of the field strength (electric and magnetic fields)\nof the emergent gauge field. Thus our study provides a bridge between the\nmicroscopic physical degrees of freedom and the emergent topological ones,\nwithout relying on the bulk-edge correspondence.", "category": "cond-mat_str-el" }, { "text": "Novel quantum criticality due to emergent topological conservation law\n in high-$T_c$ cuprates: We argue that in strongly correlated electron system collective instanton\nexcitations of the phase field (dual to the charge) arise with a great degree\nof stability, governed by gauge flux changes by an integer multiple of $2\\pi$.\nBy unraveling consequences of the non-trivial topology of the charge gauge U(1)\ngroup, we found that the pinning of $\\mu$ and the zero-temperature divergence\nof charge compressibility $\\kappa\\sim\\partial n_e/\\partial\\mu$ defines novel\n\"hidden\" quantum criticality on verge of the Mott transition governed by the\nprotectorate of stable topological numbers rather than Landau paradigm of the\nsymmetry breaking.", "category": "cond-mat_str-el" }, { "text": "Evolution of the 2D antiferromagnetism with temperature and magnetic\n field in multiferroic Ba$_2$CoGe$_2$O$_7$: We report on spherical neutron polarimetry and unpolarized neutron\ndiffraction in zero magnetic field as well as flipping ratio and static\nmagnetization measurements in high magnetic fields on the multiferroic square\nlattice antiferromagnet Ba$_2$CoGe$_2$O$_7$. We found that in zero magnetic\nfield the magnetic space group is $Cm'm2'$ with sublattice magnetization\nparallel to the [100] axis of this orthorhombic setting. The spin canting has\nbeen found to be smaller than $0.2^\\circ$ in the ground state. This assignment\nis in agreement with the field-induced changes of the magnetic domain structure\nbelow 40 mT as resolved by spherical neutron polarimetry. The magnitude of the\nordered moment has been precisely determined. Above the magnetic ordering\ntemperature short-range magnetic fluctuations are observed. Based on the\nhigh-field magnetization data, we refined the parameters of the recently\nproposed microscopic spin model describing the multiferroic phase of\nBa$_2$CoGe$_2$O$_7$.", "category": "cond-mat_str-el" }, { "text": "Flow equation analysis of the anisotropic Kondo model: We use the new method of infinitesimal unitary transformations to calculate\nzero temperature correlation functions in the strong-coupling phase of the\nanisotropic Kondo model. We find the dynamics on all energy scales including\nthe crossover behaviour from weak to strong coupling. The integrable structure\nof the Hamiltonian is not used in our approach. Our method should also be\nuseful in other strong-coupling models since few other analytical methods allow\nthe evaluation of their correlation functions on all energy scales.", "category": "cond-mat_str-el" }, { "text": "Pseudoparticle Description of the 1D Hubbard Model Electronic Transport\n Properties: We extend the pseudoparticle transport description of the Hubbard chain to\nall energy scales. In particular we compute the mean value of the electric\ncurrent transported by any Bethe-ansatz state and the transport masses of the\ncharge carriers. We present numerical results for the optical conductivity of\nthe model at half-filling for values of U/t=3 and 4. We show that these are in\ngood agreement with the pseudoparticle description of the finite-energy\ntransitions involving new pseudoparticle energy bands.", "category": "cond-mat_str-el" }, { "text": "Majorana zero modes in a quantum Ising chain with longer-ranged\n interactions: A one-dimensional Ising model in a transverse field can be mapped onto a\nsystem of spinless fermions with p-wave superconductivity. In the weak-coupling\nBCS regime, it exhibits a zero energy Majorana mode at each end of the chain.\nHere, we consider a variation of the model, which represents a superconductor\nwith longer ranged kinetic energy and pairing amplitudes, as is likely to occur\nin more realistic systems. It possesses a richer zero temperature phase diagram\nand has several quantum phase transitions. From an exact solution of the model\nthese phases can be classified according to the number of Majorana zero modes\nof an open chain: 0, 1, or 2 at each end. The model posseses a multicritical\npoint where phases with 0, 1, and 2 Majorana end modes meet. The number of\nMajorana modes at each end of the chain is identical to the topological winding\nnumber of the Anderson's pseudospin vector that describes the BCS Hamiltonian.\nThe topological classification of the phases requires a unitary time-reversal\nsymmetry to be present. When this symmetry is broken, only the number of\nMajorana end modes modulo 2 can be used to distinguish two phases. In one of\nthe regimes, the wave functions of the two phase shifted Majorana zero modes\ndecays exponentially in space but but in an oscillatory manner. The wavelength\nof oscillation is identical to the asymptotic connected spin-spin correlation\nof the XY-model in a transverse field to which our model is dual.", "category": "cond-mat_str-el" }, { "text": "Energy scales in 4f1 delafossite magnets: crystal-field splittings\n larger than the strength of spin-orbit coupling in KCeO2: Ytterbium-based delafossites with effective S=1/2 moments are investigated\nintensively as candidates for quantum spin-liquid ground states. While the\nsynthesis of related cerium compounds has also been reported,many important\ndetails concerning their crystal, electronic, and magnetic structures are\nunclear. Here we analyze the S=1/2 system KCeO2, combining complementary\ntheoretical methods. The lattice geometry was optimized and the band structure\ninvestigated using density functional theory extended to the level of a GGA+U\ncalculation in order to reproduce the correct insulating behavior. The Ce 4f1\nstates were then analyzed in more detail with the help of ab initio\nwave-function-based computations. Unusually large effective crystal-field\nsplittings of up to 320 meV are predicted, which puts KCeO2 in the strong field\ncoupling regime. Our results reveal a subtle interplay between ligand-cage\nelectrostatics and the trigonal field generated by the extended crystalline\nsurroundings, relevant in the context of recent studies on tuning the nature of\nthe ground-state wave function in 4f triangular-lattice and pyrochlore\ncompounds. It also makes KCeO2 an interesting model system in relation to the\neffect of large crystal-field splittings on the anisotropy of intersite\nexchange in spin-orbit coupled quantum magnets.", "category": "cond-mat_str-el" }, { "text": "Detection of gapped phases of a 1D spin chain with onsite and spatial\n symmetries: We investigate the phase diagram of a quantum spin-1 chain whose Hamiltonian\nis invariant under a global onsite $A_4$, translation and lattice inversion\nsymmetries. We detect different gapped phases characterized by SPT order and\nsymmetry breaking using matrix product state order parameters. We observe a\nrich variety of phases of matter characterized by a combination of symmetry\nbreaking and symmetry fractionalization and also the interplay between the\nonsite and spatial symmetries. Examples of continuous phase transitions\ndirectly between topologically nontrivial SPT phases are also observed.", "category": "cond-mat_str-el" }, { "text": "In-plane magnetic field induced double fan spin structure with\n $\\textit{c}$-axis component in metallic kagome antiferromagnet\n YMn$_{6}$Sn$_{6}$: The geometrical frustration nature of the kagome lattice makes it a great\nhost to flat electronic band, non-trivial topological properties, and novel\nmagnetisms. Metallic kagome antiferromagnet YMn$_{6}$Sn$_{6}$ exhibits the\ntopological Hall effect (THE) when an in-plane magnetic field is applied. THE\nis typically associated with the nanometer-sized non-coplanar spin structure of\nskyrmions in non-centrosymmetric magnets with large Dzyaloshinskii-Moriya\ninteraction. Here we use single crystal neutron diffraction to determine the\nfield/temperature dependence of the magnetic structure in YMn$_{6}$Sn$_{6}$. We\nfind that the observed THE cannot arise from a magnetic skyrmion lattice, but\ninstead from an in-plane field-induced double fan spin structure with\n$\\textit{c}$-axis components (DFC). Our work provides the experimental basis\nfrom which a microscopic theory can be established to understand the observed\nTHE.", "category": "cond-mat_str-el" }, { "text": "Electronic structure of Ce2RhIn8 2D heavy Fermion system studied by\n angle resolved photoemission spectroscopy: We use angle-resolved photoemission spectroscopy to study heavy fermion\nsuperconductor Ce2RhIn8. The Fermi surface is rather complicated and consists\nof several hole and electron pock- ets. We do not observe kz dispersion of\nFermi sheets, which is consistent with 2D character of the electronic\nstructure. Comparison of the ARPES data and band structure calculations points\nto a localized picture of f electrons. Our findings pave the way for\nunderstanding the transport and thermodynamical properties of this material.", "category": "cond-mat_str-el" }, { "text": "Superconductivity, pseudogap, and phase separation in topological flat\n bands: a quantum Monte Carlo study: We study a two-dimensional model of an isolated narrow topological band at\npartial filling with local attractive interactions. Numerically exact quantum\nMonte Carlo calculations show that the ground state is a superconductor with a\ncritical temperature that scales nearly linearly with the interaction strength.\nWe also find a broad pseudogap regime at temperatures above the superconducting\nphase that exhibits strong pairing fluctuations and a tendency towards\nelectronic phase separation; introducing a small nearest neighbor attraction\nsuppresses superconductivity entirely and results in phase separation. We\ndiscuss the possible relevance of superconductivity in this unusual regime to\nthe physics of flat band moir\\'{e} materials, and as a route to designing\nhigher temperature superconductors.", "category": "cond-mat_str-el" }, { "text": "Quantum advantage in the charging process of Sachdev-Ye-Kitaev batteries: The exactly-solvable Sachdev-Ye-Kitaev (SYK) model has recently received\nconsiderable attention in both condensed matter and high energy physics because\nit describes quantum matter without quasiparticles, while being at the same\ntime the holographic dual of a quantum black hole. In this Letter, we examine\nSYK-based charging protocols of quantum batteries with N quantum cells.\nExtensive numerical calculations based on exact diagonalization for N up to 16\nstrongly suggest that the optimal charging power of our SYK quantum batteries\ndisplays a super-extensive scaling with N that stems from genuine quantum\nmechanical effects. While the complexity of the nonequilibrium SYK problem\ninvolved in the charging dynamics prevents us from an analytical proof, we\nbelieve that this Letter offers the first (to the best of our knowledge) strong\nnumerical evidence of a quantum advantage occurring due to the\nmaximally-entangling underlying quantum dynamics.", "category": "cond-mat_str-el" }, { "text": "Relaxor ferroelectric behavior and intrinsic magnetodielectric behavior\n near room temperature in Li2Ni2Mo3O12, a compound with distorted honeycomb\n and spin-chains: Keeping current interests to identify materials with intrinsic\nmagnetodielectric behavior near room temperature and with novel pyroelectric\ncurrent anomalies, we report temperature and magnetic-field dependent behavior\nof complex dielectric permittivity and pyroelectric current for an oxide,\nLi2Ni2Mo3O12, containing magnetic ions with (distorted) honey-comb and chain\narrangement and ordering magnetically below 8 K. The dielectric data reveal the\nexistence of relaxor ferroelectricity behavior in the range 160-240 K and there\nare corresponding Raman mode anomalies as well in that temperature range.\nPyrocurrent behavior is also consistent with this interpretation, with the\npyrocurrent peak-temperature interestingly correlating with the poling\ntemperature. 7Li NMR offer an evidence for crystallographic disorder intrinsic\nto this compound and we therefore conclude that such a disorder is apparently\nresponsible for the randomness of local electric field leading to relaxor\nferroelectric property. Another observation of emphasis is that there is a\nnotable decrease in the dielectric constant with the application of magnetic\nfield to the tune of about -2.4% at 300 K, with the magnitude varying\nmariginally with temperature. Small loss factor values validate intrinsic\nbehavior of the magnetodielectric effect at room temperature.", "category": "cond-mat_str-el" }, { "text": "Theoretical constraints for the magnetic-dimer transition in\n two-dimensional spin models: From general arguments, that are valid for spin models with sufficiently\nshort-range interactions, we derive strong constraints on the excitation\nspectrum across a continuous phase transition at zero temperature between a\nmagnetic and a dimerized phase, that breaks the translational symmetry. From\nthe different symmetries of the two phases, it is possible to predict, at the\nquantum critical point, a branch of gapless excitations, not described by\nstandard semi-classical approaches. By using these arguments, supported by\nintensive numerical calculations, we obtain a rather convincing evidence in\nfavor of a first-order transition from the ferromagnetic to the dimerized phase\nin the two-dimensional spin-half model with four-spin ring-exchange\ninteraction, recently introduced by A.W. Sandvik et al. [Phys. Rev. Lett. 89,\n247201 (2002)].", "category": "cond-mat_str-el" }, { "text": "Quantized Berry Phases for a Local Characterization of Spin Liquids in\n Frustrated Spin Systems: Recently by using quantized Berry phases, a prescription for a local\ncharacterization of gapped topological insulators is given. One requires the\nground state is gapped and is invariant under some anti-unitary operation. A\nspin liquid which is realized as a unique ground state of the Heisenberg spin\nsystem with frustrations is a typical target system, since pairwise exchange\ncouplings are always time-reversal invariants even with frustrations.\n As for a generic Heisenberg model with a finite excitation gap, we locally\nmodify the Hamiltonian by a continuous SU(2) twist only at a specific link and\ndefine the Berry connection by the derivative. Then the Berry phase evaluated\nby the entire many-spin wavefunction is used to define the local topological\norder parameter at the link. We numerically apply this scheme for several spin\nliquids and show its physical validity.", "category": "cond-mat_str-el" }, { "text": "Low frequency Raman response near Ising-nematic quantum critical point:\n a memory matrix approach: Recent Raman scattering experiments have revealed a \"quasi-elastic peak\" in\n$\\mathrm{FeSe_{1-x}S_x}$ near an Ising-nematic quantum critical point (QCP)\n\\cite{zhang17}. Notably, the peak occurs at sub-temperature frequencies, and\nsoftens as $T^{\\alpha}$ when temperature is decreased toward the QCP, with\n$\\alpha>1$. In this work, we present a theoretical analysis of the\nlow-frequency Raman response using a memory matrix approach. We show that such\na quasi-elastic peak is associated with the relaxation of an Ising-nematic\ndeformation of the Fermi surface. Specifically, we find that the peak frequency\nis proportional to $ \\tau^{-1}\\chi^{-1}$, where $\\chi$ is the Ising-nematic\nthermodynamic susceptibility, and $\\tau^{-1}$ is the decay rate of the nematic\ndeformation due to an interplay between impurity scattering and\nelectron-electron scattering mediated by critical Ising-nematic fluctuations.\nWe argue that the critical fluctuations play a crucial role in determining the\nobserved temperature dependence of the frequency of the quasi-elastic peak. At\nfrequencies larger than the temperature, we find that the Raman response is\nproportional to $\\omega^{1/3}$, consistently with earlier predictions\n\\cite{klein18a}.", "category": "cond-mat_str-el" }, { "text": "Electronic correlations and crystal structure distortions in BaBiO3: BaBiO3 is a material where formally Bi4+ ions with the half-filled 6s-states\nform the alternating set of Bi3+ and Bi5+ ions resulting in a charge ordered\ninsulator. The charge ordering is accompanied by the breathing distortion of\nthe BiO6 octahedra (extension and contraction of the Bi-O bond lengths).\nStandard Density Functional Theory (DFT) calculations fail to obtain the\ncrystal structure instability caused by the pure breathing distortions.\nCombining effects of the breathing distortions and tilting of the BiO6\noctahedra allows DFT to reproduce qualitatively experimentally observed\ninsulator with monoclinic crystal structure but gives strongly underestimate\nbreathing distortion parameter and energy gap values. In the present work we\nreexamine the BaBiO3 problem within the GGA+U method using a Wannier functions\nbasis set for the Bi 6s-band. Due to high oxidation state of bismuth in this\nmaterial the Bi 6s-symmetry Wannier function is predominantly extended\nspatially on surrounding oxygen ions and hence differs strongly from a pure\natomic 6s-orbital. That is in sharp contrast to transition metal oxides (with\nexclusion of high oxidation state compounds) where the major part a of d-band\nWannier function is concentrated on metal ion and a pure atomic d-orbital can\nserve as a good approximation. The GGA+U calculation results agree well with\nexperimental data, in particular with experimental crystal structure parameters\nand energy gap values. Moreover, the GGA+U method allows one to reproduce the\ncrystal structure instability due to the pure breathing distortions without\noctahedra tilting.", "category": "cond-mat_str-el" }, { "text": "Quantum critical point in graphene approached in the limit of infinitely\n strong Coulomb interaction: Motivated by the physics of graphene, we consider a model of N species of 2+1\ndimensional four-component massless Dirac fermions interacting through a 3D\ninstantaneous Coulomb interaction. We show that in the limit of infinitely\nstrong Coulomb interaction the system approaches a quantum critical point, at\nleast for sufficiently large fermion degeneracy. In this regime the system\nexhibits invariance under scale transformations in which time and space scale\nby different factors. The elementary excitations are fermions with dispersion\nrelation omega ~ p^z, where the dynamic critical exponent z depends on N. In\nthe limit of large N we find z=1-4/(\\pi^2 N). We argue that due to the\nnumerically large Coulomb coupling, graphene (freely suspended) in vacuum stays\nnear the scale-invariant regime in a large momentum window, before eventually\nflowing to the trivial fixed point at very low momentum scales.", "category": "cond-mat_str-el" }, { "text": "Giant Positive Magnetoresistance and field-induced metal insulator\n transition in Cr2NiGa: We report here the magneto-transport properties of the newly synthesized\nHeusler compound Cr2NiGa which crystallizes in a disordered cubic B2 structure\nbelonging to Pm-3m space group. The sample is found to be paramagnetic down to\n2 K with metallic character. On application of magnetic field, a significantly\nlarge increase in resistivity is observed which corresponds to\nmagnetoresistance as high as 112% at 150 kOe of field at the lowest\ntemperature. Most remarkably, the sample shows negative temperature coefficient\nof resistivity below about 50 K under the application of field gretare than or\nequal to 80 kOe, signifying a field-induced metal to `insulating' transition.\nThe observed magnetoresistance follows Kohler's rule below 20 K indicating the\nvalidity of the semiclassical model of electronic transport in metal with a\nsingle relaxation time. A multi-band model for electronic transport, originally\nproposed for semimetals, is found to be appropriate to describe the\nmagneto-transport behavior of the sample.", "category": "cond-mat_str-el" }, { "text": "Small-angle interband scattering as the origin of the $T^{3/2}$\n resistivity in MnSi: A possible explanation is given for the anomalous $T^{3/2}$ temperature\ndependence of the electrical resistivity of MnSi, which is observed in the\nhigh-pressure paramagnetic state. The unusual Fermi surface of MnSi includes\nlarge open sheets that intersect along the faces of the cubic Brillouin zone.\nClose to these intersections, long-wavelength interband magnetic spin\nfluctuations can scatter electrons from one sheet to the other. The current\nrelaxation rate due to such interband scattering events is not reduced by\nvertex corrections as is that for scattering from intraband ferromagnetic\nfluctuations. Consequently, current relaxation proceeds in a manner similar to\nthat occurring in nearly antiferromagnetic metals, in which low-temperature\n$T^{3/2}$ behavior is well known. It is argued that this type of\nnon-Fermi-liquid behavior can, for a metal with ferromagnetic fluctuations near\nFermi sheet intersections, persist over a much wider temperature range than it\ndoes in nearly antiferromagnetic metals.", "category": "cond-mat_str-el" }, { "text": "Exact results for the entanglement in 1D Hubbard models with spatial\n constraints: We investigate the entanglement in Hubbard models of hardcore bosons in $1D$,\nwith an additional hardcore interaction on nearest neighbouring sites. We\nderive analytical formulas for the bipartite entanglement entropy for any\nnumber of particles and system size, whose ratio determines the system filling.\nAt the thermodynamic limit the entropy diverges logarithmically for all\nfillings, except for half-filling, with the universal prefactor $1/2$ due to\npartial permutational invariance. We show how maximal entanglement can be\nachieved by controlling the interaction range between the particles and the\nfilling which determines the empty space in the system. Our results show how\nentangled quantum phases can be created and controlled, by imposing spatial\nconstraints on states formed in many-body systems of strongly interacting\nparticles.", "category": "cond-mat_str-el" }, { "text": "Lattice and Electronic properties of VO$_2$ with the SCAN(+$U$) approach: Appropriate consideration of the electron correlation is essential to\nreproduce the intriguing metal-insulator transition accompanying the\nPeierls-type structural transition in VO$_2$. In the density functional\ntheory-based approach, this depends on the choice of the exchange-correlation\nfunctional. Here, using a newly developed strongly constrained and\nappropriately norm (SCAN) functional, we investigate the lattice and electronic\nproperties of the metallic rutile phase of VO$_2$ ($R$-VO$_2$) from the\nfirst-principles calculations. We also explored the role of the Coulomb\ncorrelation $U$. By adding $U$, we found that the phonon instability properly\ndescribes the Peierls-type distortions. The orbital-decomposed density of\nstates presents the orbital selective behavior with the SCAN+$U$, which is\nsusceptible to the one-dimensional Peierls distortion. Our results suggest that\neven with the SCAN functional, the explicit inclusion of the Coulomb\ninteraction is necessary to describe the structural transition of VO$_2$.", "category": "cond-mat_str-el" }, { "text": "Double exchange and orbital correlations in electron-doped manganites: A double exchange model for degenerate $e_g$ orbitals with intra- and\ninter-orbital interactions has been studied for the electron doped manganites\nA$_{1-x}$B$_{x}$MnO$_3$ ($x > 0.5$). We show that such a model reproduces the\nobserved phase diagram and orbital ordering in the intermediate bandwidth\nregime and the Jahn-Teller effect, considered to be crucial for the region\n$x<0.5$, does not play a major role in this region. Brink and Khomskii have\nalready pointed this out and stressed the relevance of the anistropic hopping\nacross the degenerate $e_g$ orbitals in the infinite Hund's coupling limit.\n From a more realistic calculation with finite Hund's coupling, we show that\ninclusion of interactions stabilizes the C-phase, the antiferromagnetic\nmetallic A-phase moves closer to $x=0.5$ while the ferromagnetic phase shrinks.\nThis is in agreement with the recent observations of Kajimoto et. al. and\nAkimoto et. al.", "category": "cond-mat_str-el" }, { "text": "Quantum Confinement Transition in a d-wave Superconductor: We study the nature of the zero-temperature phase transition between a d-wave\nsuperconductor and a Mott insulator in two dimensions. In this ``quantum\nconfinement transition'', spin and charge are confined to form the electron in\nthe Mott insulator. Within a dual formulation, direct transitions from d-wave\nsuperconductors at half-filling to insulators with spin-Peierls (as well as\nother) order emerge naturally. The possibility of striped superconductors is\nalso discussed within the dual formulation. The transition is described by\nnodal fermions and bosonic vortices, interacting via a long-ranged statistical\ninteraction modeled by two, coupled Chern-Simons gauge fields, and the critical\nproperties of this model are discussed.", "category": "cond-mat_str-el" }, { "text": "One-dimensional conductance through an arbitrary potential: The finite-size Tomonaga-Luttinger Hamiltonian with an arbitrary potential is\nmapped onto a non-interacting Fermi gas with renormalized potential. This is\ndone by means of flow equations for Hamiltonians and is valid for small\nelectron-electron interaction. This method also yields an alternative\nbosonization formula for the transformed field operator which makes no use of\nKlein factors. The two-terminal conductance can then be evaluated using the\nLandauer formula. We obtain similar results for infinite systems at finite\ntemperature by identifying the flow parameter with the inverse squared\ntemperature in the asymptotic regime. We recover the algebraic behavior of the\nconductance obtained by Kane and Fisher in the limit of low temperatures and\nweak electron-electron interaction, but our results remain valid for arbitrary\nexternal potential.", "category": "cond-mat_str-el" }, { "text": "Emergent Bound States and Impurity Pairs in Chemically Doped\n Shastry-Sutherland System: The search for novel unconventional superconductors is a central topic of\nmodern condensed matter physics. Similar to other Mott insulators,\nShastry-Sutherland (SSL) systems are predicted to become superconducting when\nchemically doped. This makes SrCu2(BO3)2, an experimental realization of SSL\nmodel, a suitable candidate and understanding of the doping effects in it very\nimportant. Here we report doping-induced emergent states in Mg-doped\nSrCu2(BO3)2, which remain stable up to high magnetic fields. Using four\ncomplementary magnetometry techniques and theoretical simulations, a rich\nimpurity-induced phenomenology at high fields is discovered. The results\ndemonstrate a rare example in which even a small doping concentration interacts\nstrongly with both triplets and bound states of triplets, and thus plays a\nsignificant role in the magnetization process even at high magnetic fields.\nMoreover, our findings of the emergence of the very stable impurity pairs\nprovide insights into the anticipated unconventional superconductivity in\nSrCu2(BO3)2 and related materials.", "category": "cond-mat_str-el" }, { "text": "Electronic Correlation Effects on Stabilizing a Perfect Kagome Lattice\n and Ferromagnetic Fluctuation in LaRu$_3$Si$_2$: A perfect Kagome lattice features flat bands that usually lead to strong\nelectronic correlation effects, but how electronic correlation, in turn,\nstabilizes a perfect Kagome lattice has rarely been explored. Here, we study\nsuch effect in a superconducting ($T_c \\sim 7.8$ K) Kagome metal LaRu$_3$Si$_2$\nwith a distorted Kagome plane consisting of pure Ru ions, using density\nfunctional theory plus $U$ and plus dynamical mean-field theory. We find that\nincreasing electronic correlation can stabilize a perfect Kagome lattice and\ninduce substantial ferromagnetic fluctuations in LaRu$_3$Si$_2$. By comparing\nthe calculated magnetic susceptibilities to experimental data, LaRu$_3$Si$_2$\nis found to be on the verge of becoming a perfect Kagome lattice. It thus shows\nmoderate but non-negligible electronic correlations and ferromagnetic\nfluctuations, which are crucial to understanding the experimentally observed\nnon-Fermi-liquid behavior and the pretty high superconducting $T_c$ of\nLaRu$_3$Si$_2$.", "category": "cond-mat_str-el" }, { "text": "Theoretical investigation of the behavior of CuSe2O5 compound in high\n magnetic fields: Based on analytical and numerical approaches, we investigate thermodynamic\nproperties of CuSe2O5 at high magnetic fields which is a candidate for the\nstrong intra-chain interaction in quasi one-dimensional (1D) quantum magnets.\nMagnetic behavior of the system can be described by the 1D spin-1/2 XXZ model\nin the presence of the Dzyaloshinskii-Moriya (DM) interaction. Un- der these\ncircumstances, there is one quantum critical field in this compound. Below the\nquantum critical field the spin chain system is in the gapless Luttinger liquid\n(LL) regime, whereas above it one observes a crossover to the gapped saturation\nmagnetic phase. Indications on the thermodynamic curves confirm the occurrence\nof such a phase transition. The main characteristics of the LL phase are\ngapless and spin-spin correlation functions decay algebraic. The effects of\nzero-temperature quantum phase transition are observed even at rather high\ntemperatures in comparison with the counterpart compounds. In addition, we\ncalculate the Wilson ratio in the model. The Wilson ratio at a fixed\ntemperature remains almost independent of the field in the LL region. In the\nvicinity of the quantum critical field, the Wilson ratio increases and exhibits\nanomalous enhancement.", "category": "cond-mat_str-el" }, { "text": "Control of magnetic interactions between surface adatoms via orbital\n repopulation: We propose a reversible mechanism for switching Heisenberg-type exchange\ninteractions between deposited transition metal adatoms from ferromagnetic to\nantiferromagnetic. Using first-principles calculations, we show that this\nmechanism can be realized for cobalt atoms on the surface of black phosphorus\nby making use of electrically-controlled orbital repopulation, as recently\ndemonstrated by scanning probe techniques [Nat. Commun. 9, 3904 (2018)]. We\nfind that field-induced repopulation not only affects the spin state, but also\ncauses considerable modification of exchange interaction between adatoms,\nincluding its sign. Our model analysis demonstrates that variable\nadatom-substrate hybridization is a key factor responsible for this\nmodification. We perform quantum simulations of inelastic tunneling\ncharacteristics and discuss possible ways to verify the proposed mechanism\nexperimentally.", "category": "cond-mat_str-el" }, { "text": "Neutron Scattering Studies of Spin Fluctuations in High Temperature\n Superconductors: Neutron scattering can provide detailed information about the energy and\nmomentum dependence of the magnetic dynamics of materials provided sufficiently\nlarge single crystals are available. This requirement has limited the number of\nrare earth high temperature superconducting materials that have been studied in\nany detail. However, improvements in crystal growth in recent years has\nresulted in considerable progress in our understanding of the behaviour of the\nmagnetism of the CuO planes in both the superconducting and normal state. This\nreview will focus primarily on the spin fluctuations in La_{2-x}Sr_{x}CuO_{4}\nand YBa_{2}Cu_{3}O_{7-x} since these are the two systems for which the most\ndetailed results are available. Although gaps in our understanding remain,\nthere is now a consistent picture of on the spin fluctuation spectra in both\nsystems as well as the changes induced by the superconducting transition. For\nboth La_{2-x}Sr_{x}CuO_{4} and underdoped YBa_{2}Cu_{3}O_{7-x} the normal state\nresponse is characterised by incommensurate magnetic fluctuations. The low\nenergy excitations are suppressed by the superconducting transition with a\ncorresponding enhancement in the response at higher energies. For\nYBa_{2}Cu_{3}O_{7-x} the superconducting state is accompanied by the rapid\ndevelopment of a commensurate resonant response whose energy varies with T_{c}.\nIn underdoped samples this resonance persists above T_{c}.", "category": "cond-mat_str-el" }, { "text": "Ce$_{2}$Ir$_{3}$Ga$_{5}$ : a new locally non-centrosymmetric heavy\n fermion system: Recently, a new type of unconventional superconductivity with a field-induced\ntransition between two different superconducting (SC) states was discovered in\nthe heavy fermion system CeRh$_{2}$As$_{2}$. This unusual SC state was proposed\nto be based on specific symmetries of the underlying structure, i.e., a\nglobally centrosymmetric layered structure, but where the Ce-layers themselves\nlack inversion symmetry. This new type of SC state has attracted strong\ninterest, prompting the search for further heavy fermion systems crystallizing\nin structures with appropriate symmetries. We report the discovery and the\nstudy of a new Ce-based heavy fermion system with a globally centrosymmetric\nstructure but without inversion symmetry on the Ce-site,\nCe$_{2}$Ir$_{3}$Ga$_{5}$. A single crystal X-ray diffraction study revealed an\northorhombic U$_{2}$Co$_{3}$Si$_{5}$ type structure. Resistivity, specific\nheat, and magnetization measurements indicate a moderate-heavy fermion behavior\nwith a Kondo energy scale of the order of 40 K. Most experimental results\nsuggest the absence of magnetic order, but a tiny anomaly in the specific heat\nopens the possibility for a very weak, itinerant type of ordering.", "category": "cond-mat_str-el" }, { "text": "Ce-L3-XAS study of the temperature dependence of the 4f occupancy in the\n Kondo system Ce2Rh3Al9: We have used temperature dependent x-ray absorption at the Ce-L3 edge to\ninvestigate the recently discovered Kondo compound Ce2Rh3Al9. The systematic\nchanges of the spectral lineshape with decreasing temperature are analyzed and\nfound to be related to a change in the $4f$ occupation number, n_f, as the\nsystem undergoes a transition into a Kondo state. The temperature dependence of\n$n_f$ indicates a characteristic temperature of 150K, which is clearly related\nwith the high temperature anomaly observed in the magnetic susceptibility of\nthe same system. The further anomaly observed in the resistivity of this system\nat low temperature (ca. 20K) has no effect on n_f and is thus not of Kondo\norigin.", "category": "cond-mat_str-el" }, { "text": "Heisenberg Necklace Model in Magnetic Field: Motivated by the experimental realizations of nearly one-dimensional spin-1/2\nHeisenberg model found in chain cuprates SrCuO$_2$ and Sr$_2$CuO$_3$, which\nremain in a quantum-critical Luttinger liquid state down to temperatures that\nare much lower than in-chain exchange coupling, we consider the perturbation to\nthis state caused by interactions with nuclear spins on the same sites. We\nstudy the low-energy sector of the Heisenberg Necklace model and estimate the\neffect of the coupling between the nuclear and the electronic spins on the\noverall spins dynamics and its dependence on the magnetic field. We find that\nthe Necklace model has a characteristic energy scale, $\\Lambda \\sim\nJ^{1/3}(\\gamma I)^{2/3}$, at which the coupling between spins of the necklace\nand the spins of the Heisenberg chain becomes strong. In the strong magnetic\nfield $\\mu_B B > \\Lambda$ the low energy spectrum is gapless, but two gapless\nbosonic modes have different velocities whose ratio at strong fields approaches\na universal number, $\\sqrt 2 +1$. In the case of Sr$_2$CuO$_3$ the energy scale\n$\\Lambda $ is sizable and comparable to the Neel ordering temperature induced\nby the inter-chain coupling, and thus could noticeably modify the low\ntemperature magnon dynamics. We further find that the above energy scale is\ninsensitive to strong magnetic field, $\\mu_B B \\gg \\Lambda \\sim J^{1/3}(\\gamma\nI)^{2/3}$, and therefore the interaction with nuclear spins cannot lead to\nunusually strong magnetic field dependence of the magnon spectrum observed by\nESR in Sr$_2$CuO$_3$, which has been attributed to the magnon interaction with\nthe Higgs mode.", "category": "cond-mat_str-el" }, { "text": "Theory of electronic transport through a triple quantum dot in the\n presence of magnetic field: Theory of electronic transport through a triangular triple quantum dot\nsubject to a perpendicular magnetic field is developed using a tight binding\nmodel. We show that magnetic field allows to engineer degeneracies in the\ntriple quantum dot energy spectrum. The degeneracies lead to zero electronic\ntransmission and sharp dips in the current whenever a pair of degenerate states\nlies between the chemical potential of the two leads. These dips can occur with\na periodicity of one flux quantum if only two levels contribute to the current\nor with half flux quantum if the three levels of the triple dot contribute. The\neffect of strong bias voltage and different lead-to-dot connections on\nAharonov-Bohm oscillations in the conductance is also discussed.", "category": "cond-mat_str-el" }, { "text": "Supersymmetry on the honeycomb lattice: resonating charge stripes,\n superfrustration, and domain walls: We study a model of spinless fermions on the honeycomb lattice with\nnearest-neighbor exclusion and extended repulsive interactions that exhibits\n`lattice supersymmetry' [P. Fendley, K. Schoutens, and J. de Boer, Phys. Rev.\nLett. 90, 120402 (2003)]. Using a combination of exact diagonalization of large\n($N\\leq56$ site) systems, mean-field numerics, and symmetry analysis, we\nestablish a rich phase structure as a function of fermion density, that\nincludes non-Fermi liquid behavior, resonating charge stripes, domain-wall and\nbubble physics, and identify a finite range of fillings with extensive ground\nstate degeneracy and both gapped and gapless spectra. We comment on the\nstability of our results to relaxing the stringent requirements for\nsupersymmetry, and on their possible broader relevance to systems of\nstrongly-correlated electrons with extended repulsive interactions.", "category": "cond-mat_str-el" }, { "text": "Quantum Entanglement as a Diagnostic of Phase Transitions in Disordered\n Fractional Quantum Hall Liquids: We investigate the disorder-driven phase transition from a fractional quantum\nHall state to an Anderson insulator using quantum entanglement methods. We find\nthat the transition is signaled by a sharp increase in the sensitivity of a\nsuitably averaged entanglement entropy with respect to disorder -- the\nmagnitude of its disorder derivative appears to diverge in the thermodynamic\nlimit. We also study the level statistics of the entanglement spectrum as a\nfunction of disorder. However, unlike the dramatic phase-transition signal in\nthe entanglement entropy derivative, we find a gradual reduction of level\nrepulsion only deep in the Anderson insulating phase.", "category": "cond-mat_str-el" }, { "text": "Bethe ansatz description of edge-localization in the open-boundary XXZ\n spin chain: At large values of the anisotropy \\Delta, the open-boundary Heisenberg\nspin-1/2 chain has eigenstates displaying localization at the edges. We present\na Bethe ansatz description of this `edge-locking' phenomenon in the entire\n\\Delta>1 region. We focus on the simplest spin sectors, namely the highly\npolarized sectors with only one or two overturned spins, i.e., one-particle and\ntwo-particle sectors. Edge-locking is associated with pure imaginary solutions\nof the Bethe equations, which are not commonly encountered in periodic chains.\nIn the one-particle case, at large anisotropies there are two eigenstates with\nimaginary Bethe momenta, related to localization at the two edges. For any\nfinite chain size, one of the two solutions become real as the anisotropy is\nlowered below a certain value. For two particles, a richer scenario is\nobserved, with eigenstates having the possibility of both particles locked on\nthe same or different edge, one locked and the other free, and both free either\nas single magnons or as bound composites corresponding to `string' solutions.\nFor finite chains, some of the edge-locked spins get delocalized at certain\nvalues of the anisotropy (`exceptional points'), corresponding to imaginary\nsolutions becoming real. We characterize these phenomena thoroughly by\nproviding analytic expansions of the Bethe momenta for large chains, large\nanisotropy, and near the exceptional points. In the large-chain limit all the\nexceptional points coalesce at the isotropic point (\\Delta=1) and edge-locking\nbecomes stable in the whole \\Delta>1 region.", "category": "cond-mat_str-el" }, { "text": "Disordered and interacting parabolic semimetals in two and three\n dimensions: A clean noninteracting parabolic semimetal is characterized by quadratic band\ntouching between the conduction and the valence bands at isolated diabolic\npoints in the Brillouin zone and describes a fermionic quantum critical system\nwith dynamic exponent z=2. We consider the stability of such a semimetal\nagainst electronic interaction and quenched disorder using a perturbative\nrenormalization group analysis for two and three spatial dimensions. For the\nnoninteracting problem infinitesimally weak disorder leads to an Anderson\ninsulator and a diffusive metal respectively in two and three dimensions. On\nthe other hand, the long range Coulomb interaction causes an excitonic\ninstability for the clean interacting problem towards a broken symmetry ground\nstate in both dimensions. Our weak coupling analysis of the combined effects of\ndisorder and interaction suggests the competition between a broken symmetry and\na disorder controlled metallic or insulating states, but is inadequate for\ndescribing the quantum phase transitions among them. We discuss the relevance\nof our results for bilayer graphene and some 227 iridate compounds, and\nidentify these materials as promising candidates for exploring novel disorder\nand interaction controlled quantum critical phenomena.", "category": "cond-mat_str-el" }, { "text": "Thickness-dependent magnetic properties and strain-induced orbital\n magnetic moment in SrRuO3 thin films: Thin films of the ferromagnetic metal SrRuO3 (SRO) show a varying easy\nmagnetization axis depending on the epitaxial strain and undergo a\nmetal-to-insulator transition with decreasing film thickness. We have\ninvestigated the magnetic properties of SRO thin films with varying thicknesses\nfabricated on SrTiO3(001) substrates by soft x-ray magnetic circular dichroism\n(XMCD) at the Ru M2,3 edge. Results have shown that, with decreasing film\nthickness, the film changes from ferromagnetic to non-magnetic around\n3monolayer thickness, consistent with previous magnetization and\nmagneto-optical Kerr effect measurements. The orbital magnetic moment\nperpendicular to the film was found to be ~ 0.1{\\mu}B/Ru atom, and remained\nnearly unchanged with decreasing film thickness while the spin magnetic moment\ndecreases. Mechanism for the formation of the orbital magnetic moment is\ndiscussed based on the electronic structure of the compressively strained SRO\nfilm.", "category": "cond-mat_str-el" }, { "text": "Expansion of the tetragonal magnetic phase with pressure in the\n iron-arsenide superconductor Ba{1-x}KxFe2As2: In the temperature-concentration phase diagram of most iron-based\nsuperconductors, antiferromagnetic order is gradually suppressed to zero at a\ncritical point, and a dome of superconductivity forms around that point. The\nnature of the magnetic phase and its fluctuations is of fundamental importance\nfor elucidating the pairing mechanism. In Ba{1-x}KxFe2As2 and Ba{1-x}NaxFe2As2,\nit has recently become clear that the usual stripe-like magnetic phase, of\northorhombic symmetry, gives way to a second magnetic phase, of tetragonal\nsymmetry, near the critical point, between x = 0.24 and x = 0.28. Here we\nreport measurements of the electrical resistivity of Ba{1-x}KxFe2As2 under\napplied hydrostatic pressures up to 2.75 GPa, for x = 0.22, 0.24 and 0.28. We\ntrack the onset of the tetragonal magnetic phase using the sharp anomaly it\nproduces in the resistivity. In the temperature-concentration phase diagram of\nBa{1-x}KxFe2As2, we find that pressure greatly expands the tetragonal magnetic\nphase, while the stripe-like phase shrinks. This raises the interesting\npossibility that the fluctuations of the former phase might be involved in the\npairing mechanism responsible for the superconductivity.", "category": "cond-mat_str-el" }, { "text": "Field-Tuned Quantum Effects in a Triangular-Lattice Ising Magnet: We report thermodynamic and neutron scattering measurements of the\ntriangular-lattice quantum Ising magnet TmMgGaO 4 in longitudinal magnetic\nfields. Our experiments reveal a quasi-plateau state induced by quantum\nfluctuations. This state exhibits an unconventional non-monotonic field and\ntemperature dependence of the magnetic order and excitation gap. In the high\nfield regime where the quantum fluctuations are largely suppressed, we observed\na disordered state with coherent magnon-like excitations despite the\nsuppression of the spin excitation intensity. Through detailed semi-classical\ncalculations, we are able to understand these behaviors quantitatively from the\nsubtle competition between quantum fluctuations and frustrated Ising\ninteractions.", "category": "cond-mat_str-el" }, { "text": "Antiferromagnetic and d-wave pairing correlations in the strongly\n interacting two-dimensional Hubbard model from the functional renormalization\n group: Using the dynamical mean-field theory (DMFT) as a `booster-rocket', the\nfunctional renormalization group (fRG) can be upgraded from a weak-coupling\nmethod to a powerful computation tool for strongly interacting fermion systems.\nThe strong local correlations are treated non-perturbatively by the DMFT, while\nthe fRG flow can be formulated such that it is driven exclusively by non-local\ncorrelations, which are more amenable to approximations. We show that the full\nfrequency dependence of the two-particle vertex needs to be taken into account\nin this approach, and demonstrate that this is actually possible -- in spite of\nthe singular frequency dependence of the vertex at strong coupling. We are thus\nable to present the first results obtained from the DMFT-boosted fRG for the\ntwo-dimensional Hubbard model in the strongly interacting regime. We find\nstrong antiferromagnetic correlations from half-filling to 18 percent\nhole-doping, and, at the lowest temperature we can access, a sizable $d$-wave\npairing interaction driven by magnetic correlations at the edge of the\nantiferromagnetic regime.", "category": "cond-mat_str-el" }, { "text": "Spin Precession and Real Time Dynamics in the Kondo Model: A\n Time-Dependent Numerical Renormalization-Group Study: A detailed derivation of the recently proposed time-dependent numerical\nrenormalization-group (TD-NRG) approach to nonequilibrium dynamics in quantum\nimpurity systems is presented. We demonstrate that the method is suitable for\nfermionic as well as bosonic baths. A comparison with exact analytical results\nfor the charge relaxation in the resonant-level model and for dephasing in the\nspin-boson model establishes the accuracy of the method. The real-time dynamics\nof a single spin coupled to both types of baths is investigated. We use the\nTD-NRG to calculate the spin relaxation and spin precession of a single Kondo\nimpurity. The short- and long-time dynamics is studied as a function of\ntemperature in the ferromagnetic and antiferromagnetic regimes. The short-time\ndynamics agrees very well with analytical results obtained at second order in\nthe exchange coupling $J$. In the ferromagnetic regime, the long-time spin\ndecay is described by the scaling variable $x = 2\\rho_F J(T) T t$. In the\nantiferromagnetic regime it is governed for $T < T_K$ by the Kondo time scale\n$1/T_K$. Here $\\rho_F$ is the conduction-electron density of states and $T_K$\nis the Kondo temperature. Results for spin precession are obtained by rotating\nthe external magnetic field from the x axis to the z axis.", "category": "cond-mat_str-el" }, { "text": "Pinball liquid phase from Hund's coupling in frustrated transition metal\n oxides: The interplay of non-local Coulomb repulsion and Hund's coupling in the\nd-orbital manifold in frustrated triangular lattices is analyzed by a mutliband\nextended Hubbard model. We find a rich phase diagram with several competing\nphases, including a robust pinball liquid phase, which is an unconventional\nmetal characterized by threefold charge order, bad metallic behavior and the\nemergence of high spin local moments. Our results naturally explain the\nanomalous charge-ordered metallic state observed in the triangular layered\ncompound AgNiO2. The potential relevance to other triangular transition metal\noxides is discussed.", "category": "cond-mat_str-el" }, { "text": "Spin correlations in Ca3Co2O6: A polarised-neutron diffraction and Monte\n Carlo study: We present polarised-neutron diffraction measurements of the Ising-like\nspin-chain compound Ca3Co2O6 above and below the magnetic ordering temperature\nTN. Below TN, a clear evolution from a single-phase spin-density wave (SDW)\nstructure to a mixture of SDW and commensurate antiferromagnet (CAFM)\nstructures is observed on cooling. For a rapidly-cooled sample, the majority\nphase at low temperature is the SDW, while if the cooling is performed\nsufficiently slowly, then the SDW and the CAFM structure coexist between 1.5\nand 10 K. Above TN, we use Monte Carlo methods to analyse the magnetic diffuse\nscattering data. We show that both intra- and inter-chain correlations persist\nabove TN, but are essentially decoupled. Intra-chain correlations resemble the\nferromagnetic Ising model, while inter-chain correlations resemble the\nfrustrated triangular-lattice antiferromagnet. Using previously-published bulk\nproperty measurements and our neutron diffraction data, we obtain values of the\nferromagnetic and antiferromagnetic exchange interactions and the single-ion\nanisotropy.", "category": "cond-mat_str-el" }, { "text": "Anderson impurity in the one-dimensional Hubbard model on finite size\n systems: An Anderson impurity in a Hubbard model on chains with finite length is\nstudied using the density-matrix renormalization group (DMRG) technique. In the\nfirst place, we analyzed how the reduction of electron density from\nhalf-filling to quarter-filling affects the Kondo resonance in the limit of\nHubbard repulsion U=0. In general, a weak dependence with the electron density\nwas found for the local density of states (LDOS) at the impurity except when\nthe impurity, at half-filling, is close to a mixed valence regime. Next, in the\ncentral part of this paper, we studied the effects of finite Hubbard\ninteraction on the chain at quarter-filling. Our main result is that this\ninteraction drives the impurity into a more defined Kondo regime although\naccompanied in most cases by a reduction of the spectral weight of the impurity\nLDOS. Again, for the impurity in the mixed valence regime, we observed an\ninteresting nonmonotonic behavior. We also concluded that the conductance,\ncomputed for a small finite bias applied to the leads, follows the behavior of\nthe impurity LDOS, as in the case of non-interacting chains. Finally, we\nanalyzed how the Hubbard interaction and the finite chain length affect the\nspin compensation cloud both at zero and at finite temperature, in this case\nusing quantum Monte Carlo techniques.", "category": "cond-mat_str-el" }, { "text": "Modified Curie-Weiss Law for $j_{\\rm eff}$ Magnets: In spin-orbit-coupled magnetic materials, the usually applied Curie-Weiss law\ncan break down. This is due to potentially sharp temperature-dependence of the\nlocal magnetic moments. We therefore propose a modified Curie-Weiss formula\nsuitable for analysis of experimental susceptibility. We show for octahedrally\ncoordinated materials of $d^5$ filling that the Weiss constant obtained from\nthe improved formula is in excellent agreement with the calculated Weiss\nconstant from microscopic exchange interactions. Reanalyzing the measured\nsusceptibility of several Kitaev candidate materials with the modified formula\nresolves apparent discrepancies between various experiments regarding the\nmagnitude and anisotropies of the underlying magnetic couplings.", "category": "cond-mat_str-el" }, { "text": "Low Curie-temperature ferromagnetic phase in SmPt2Cd20 possibly\n accompanied by strong quantum fluctuations: Electrical resistivity, magnetization and specific heat have been measured\nfor single crystals of SmPt$_{2}$Cd$_{20}$. It has been found that\nSmPt$_{2}$Cd$_{20}$ exhibits a ferromagnetic (FM) transition at $T_{\\rm C} =\n0.64$ K, the lowest among cubic compounds. Specific heat divided by temperature\nincreases with decreasing temperature even below $T_{\\rm C}$ and attains 4.5\nJ/mol K$^{2}$ at 0.26 K, implying substantial magnetic quantum fluctuations. An\nanalysis of the magnetic entropy suggests the crystalline-electric-field\nsplitting of the Sm $J = 5/2$ multiplet with a $\\Gamma_{7}$ doublet ground\nstate and a $\\Gamma_8$ quartet excited state (the excitation energy of $\\sim30$\nK). The electrical resistivity shows a power-law behavior with $T^{0.74}$ below\n2 K without showing any noticeable anomaly at $T_{\\rm C}$. SmPt$_{2}$Cd$_{20}$\nis regarded as a rare cubic system that is located in the vicinity of a FM\nquantum critical point.", "category": "cond-mat_str-el" }, { "text": "Spin polarization induced tenfold magneto-resistivity of highly metallic\n 2D holes in a narrow GaAs quantum well: We observe that an in-plane magnetic field ($B_{||}$) can induce an order of\nmagnitude enhancement in the low temperature ($T$) resistivity ($\\rho$) of\nmetallic 2D holes in a narrow (10nm) GaAs quantum well. Moreover, we show the\nfirst observation of saturating behavior of $\\rho(B_{||})$ at high $B_{||}$ in\nGaAs system, which suggests our large positive $\\rho(B_{||})$ is due to the\nspin polarization effect alone. We find that this tenfold increase in\n$\\rho(B_{||})$ even persists deeply into the 2D metallic state with the high\n$B_{||}$ saturating values of $\\rho$ lower than 0.1$\\times$h/e$^2$. The\ndramatic effect of $B_{||}$ we observe on the highly conductive 2D holes (with\n$B$=0 conductivity as high as 75e$^2$/h) sets strong constraint on models for\nthe spin dependent transport in dilute metallic 2D systems.", "category": "cond-mat_str-el" }, { "text": "X-Ray Resonant Scattering as a Direct Probe of Orbital Ordering in\n Transition-Metal Oxides: X-ray resonant scattering at the K-edge of transition metal oxides is shown\nto measure the orbital order parameter, supposed to accompany magnetic ordering\nin some cases. Virtual transitions to the 3d-orbitals are quadrupolar in\ngeneral. In cases with no inversion symmetry, such as V$_2$O$_3$, treated in\ndetail here, a dipole component enhances the resonance. Hence, we argue that\nthe detailed structure of orbital order in V$_2$O$_3$ is experimentally\naccessible.", "category": "cond-mat_str-el" }, { "text": "Far Infrared absorption of non center of mass modes and optical sum rule\n in a few electron quantum dot with Rashba spin-orbit coupling: Spin-orbit interaction in a quantum dot couples far infrared radiation to non\ncenter of mass excitation modes, even for parabolic confinement and dipole\napproximation. The intensities of the absorption peaks satisfy the optical sum\nrule, giving direct information on the total number of electrons inside the\ndot. In the case of a circularly polarized radiation the sum rule is\ninsensitive to the strength of a Rashba spin-orbit coupling due to an electric\nfield orthogonal to the dot plane, but not to other sources of spin-orbit\ninteraction, thus allowing to discriminate between the two.", "category": "cond-mat_str-el" }, { "text": "Unconventional electron states in $\u03b4$-doped SmTiO$_3$: The Mott-insulating distorted perovskite SmTiO$_3$, doped with a single SrO\nlayer in a quantum-well architecture is studied by the combination of density\nfunctional theory with dynamical mean-field theory. A rich correlated\nelectronic structure in line with recent experimental investigations is\nrevealed by the given realistic many-body approach to a large-unit-cell oxide\nheterostructure. Coexistence of conducting and Mott-insulating TiO$_2$ layers\nprone to magnetic order gives rise to multi-orbital electronic transport beyond\nstandard Fermi-liquid theory. First hints towards a pseudogap opening due to\nelectron-electron scattering within a background of ferromagnetic and\nantiferromagnetic fluctuations are detected.", "category": "cond-mat_str-el" }, { "text": "Flavor Degeneracy and Effects of Disorder in Ultracold Atom Systems: Cold atoms in optical lattices offer an exciting new laboratory where quantum\nmany-body phenomena can be realized in a highly controlled way. They can even\nserve as quantum simulators for notoriously difficult problems like\nhigh-temperature superconductivity. This review is focussed on recent\ndevelopments and new results in multi-component systems. Fermionic atoms with\nSU(N) symmetry have exotic superfluid and flavor-ordered ground states. We\ndiscuss symmetry breaking, collective modes and detection issues. Bosonic\nmulti-flavor ensembles allow for engineering of spin Hamiltonians which are\ninteresting from a quantum computation point of view. Finally, we will address\nthe competition of disorder and interaction in optical lattices. We present a\ncomplete phase diagram obtained within dynamical mean-field theory and discuss\nexperimental observability of the Mott and Anderson phases.", "category": "cond-mat_str-el" }, { "text": "Reduced fidelity in topological quantum phase transitions: We study the reduced fidelity between local states of lattice systems\nexhibiting topological order. By exploiting mappings to spin models with\nclassical order, we are able to analytically extract the scaling behavior of\nthe reduced fidelity at the corresponding quantum phase transitions out of the\ntopologically ordered phases. Our results suggest that the reduced fidelity,\nalbeit being a local measure, generically serves as a faithful marker of a\ntopological quantum phase transition.", "category": "cond-mat_str-el" }, { "text": "Development of spin fluctuations under the presence of $d$-wave bond\n order in cuprate superconductors: In cuprate superconductors, superconductivity appears below the CDW\ntransition temperature $T_{CDW}$. However, many-body electronic states under\nthe CDW order are still far from understood. Here, we study the development of\nthe spin fluctuations under the presence of $d$-wave bond order (BO) with\nwavevector $q=(\\pi/2,0),(0,\\pi/2)$, which is derived from the paramagnon\ninterference mechanism in recent theoretical studies. Based on the $4 \\times 1$\nand $4 \\times 4$ cluster Hubbard models, the feedback effects between spin\nsusceptibility and self-energy are calculated self-consistently by using the\nfluctuation-exchange (FLEX) approximation. It is found that the $d$-wave BO\nleads to a sizable suppression of the nuclear magnetic relaxation rate $1/T_1$.\nIn contrast, the reduction in $T_c$ is small, since the static susceptibility\n$\\chi^s(Q_s)$ is affected by the BO just slightly. It is verified that the\n$d$-wave BO scenario is consistent with the experimental electronic properties\nbelow $T_{CDW}$.", "category": "cond-mat_str-el" }, { "text": "A Proposal to Use Neutron Scattering to Measure Scalar Spin Chirality\n Fluctuations in Kagome Lattices: In the theory of quantum spin liquids, gauge fluctuations are emergent\nexcitations at low energy. The gauge magnetic field is proportional to the\nscalar spin chirality, S1.(S2xS3). It is therefore highly desirable to measure\nthe fluctuation spectrum of the scalar spin chirality. We show that in the\nKagome lattice with a Dzyaloshinskii-Moriya term, the fluctuation in Sz which\nis readily measured by neutron scattering contains a piece which is\nproportional to the chirality fluctuation.", "category": "cond-mat_str-el" }, { "text": "Emergent Potts order in the kagom\u00e9 $J_1-J_3$ Heisenberg model: Motivated by the physical properties of Vesignieite\nBaCu$_3$V$_2$O$_8$(OH)$_2$, we study the $J_1-J_3$ Heisenberg model on the\nkagom\\'e lattice, that is proposed to describe this compound for $J_1<0$ and\n$J_3\\gg|J_1|$. The nature of the classical ground state and the possible phase\ntransitions are investigated through analytical calculations and parallel\ntempering Monte Carlo simulations. For $J_1<0$ and\n$J_3>\\frac{1+\\sqrt{5}}4|J_1|$, the ground states are not all related by an\nHamiltonian symmetry. Order appears at low temperature via the order by\ndisorder mechanism, favoring colinear configurations and leading to an emergent\n$q=4$ Potts parameter. This gives rise to a finite temperature phase\ntransition. Effect of quantum fluctuations are studied through linear spin wave\napproximation and high temperature expansions of the $S=1/2$ model. For $J_3$\nbetween $\\frac14|J_1|$ and $\\frac{1+\\sqrt{5}}4|J_1|$, the ground state goes\nthrough a succession of semi-spiral states, possibly giving rise to multiple\nphase transitions at low temperatures.", "category": "cond-mat_str-el" }, { "text": "Theory of inelastic light scattering in spin-1 systems: resonant regimes\n and detection of quadrupolar order: Motivated by the lack of an obvious spectroscopic probe to investigate\nnon-conventional order such as quadrupolar orders in spin S>1/2 systems, we\npresent a theoretical approach to inelastic light scattering for spin-1 quantum\nmagnets in the context of a two-band Hubbard model. In contrast to the S=1/2\ncase, where the only type of local excited state is a doubly occupied state of\nenergy $U$, several local excited states with occupation up to 4 electrons are\npresent. As a consequence, we show that two distinct resonating scattering\nregimes can be accessed depending on the incident photon energy. For\n$\\hbar\\omega_{in}\\lesssim U$, the standard Loudon-Fleury operator remains the\nleading term of the expansion as in the spin-1/2 case. For\n$\\hbar\\omega_{in}\\lesssim4U$, a second resonant regime is found with a leading\nterm that takes the form of a biquadratic coupling $\\sim({\\bf S}_{i}\\cdot{\\bf\nS}_{j)^{2}$. Consequences for the Raman spectra of S=1 magnets with magnetic or\nquadrupolar order are discussed. Raman scattering appears to be a powerful\nprobe of quadrupolar order.", "category": "cond-mat_str-el" }, { "text": "lattice-symmetries: A package for working with quantum many-body bases: Exact diagonalization (ED) is one of the most reliable and established\nnumerical methods of quantum many-body theory. The main limiting factor of the\nmethod is the exponential scaling of Hilbert space dimension with system size.\nFortunately, by symmetry considerations the effective dimension can be reduced\nby multiple orders of magnitude. Here, we present lattice-symmetries, a package\nfor working with such symmetry-adapted quantum many-body bases and operators.\nIt supports bases for spin-1/2 particles with arbitrary user-defined symmetries\nand generic 1-, 2-, 3-, and 4-point operators. As an example application we\ndiscuss SpinED program which allows to easily diagonalize clusters of at least\n42 sites on a single node thus making large-scale ED easily accessible to\npeople with no background in numerical methods and computational physics.", "category": "cond-mat_str-el" }, { "text": "Anyonic braiding via quench dynamics in fractional quantum Hall liquids: In a Laughlin fractional quantum Hall state, one- and two-quasihole states\ncan be obtained by diagonalizing the many-body Hamiltonian with a trapping\npotential or, for larger systems, from the linear combination of the edge Jack\npolynomials. The quasihole states live entirely in the subspace of the\nlowest-energy branch in the energy spectrum with a fixed number of orbits, or a\nhard-wall confinement. The reduction in the Hilbert space dimension facilitates\nthe study of time evolution of the quasihole states after, say, the removal of\nthe trapping potential. We explore the quench dynamics under a harmonic\nexternal potential, which rotates the quasiholes in the droplet, and discuss\nthe effect of long-range interaction and more realistic confinement. Accurate\nevaluation of the mutual statistics phase of anyons for a wide range of anyon\nseparation can be achieved from the Berry-phase calculation.", "category": "cond-mat_str-el" }, { "text": "Metallic mean-field stripes, incommensurability and chemical potential\n in cuprates: We perform a systematic slave-boson mean-field analysis of the three-band\nmodel for cuprates with first-principle parameters. Contrary to widespread\nbelieve based on earlier mean-field computations low doping stripes have a\nlinear density close to 1/2 added hole per lattice constant. We find a\ndimensional crossover from 1D to 2D at doping $\\sim 0.1$ followed by a breaking\nof particle-hole symmetry around doping 1/8 as doping increases. Our results\nexplain in a simple way the behavior of the chemical potential, the magnetic\nincommensurability, and transport experiments as a function of doping. Bond\ncentered and site-centered stripes become degenerate for small overdoping.", "category": "cond-mat_str-el" }, { "text": "Localized excitation in the hybridization gap in YbAl3: The intermediate valence compound YbAl3 exhibits a broad magnetic excitation\nwith characteristic energy E1 ~ 50meV, of order of the Kondo energy (TK ~\n600-700K). In the low temperature (T < Tcoh ~ 40K) Fermi liquid state, however,\na new magnetic excitation arises at E2 ~ 33meV, which lies in the hybridization\ngap that exists in this compound. We show, using inelastic neutron scattering\non a single-crystal sample, that while the scattering at energies near E1 has\nthe momentum (Q-) dependence expected for interband scattering across the\nindirect gap, the scattering near E2 is independent of Q. This suggests that it\narises from a spatially-localized excitation in the hybridization gap.", "category": "cond-mat_str-el" }, { "text": "Theory of Low-Temperature Hall Effect in Stripe--Ordered Cuprates: We investigate the effect of static anti-phase stripe order on the weak-field\nHall effect of electrons on a two-dimensional square lattice with electron\ndispersion appropriate to the high T$_c$ cuprates. We first consider the cases\nwhere the magnitudes of the spin and charge stripe potentials are smaller than\nor of the same order as the bandwidth of the two-dimensional electrons, so that\nthe electronic properties are not too strongly one-dimensional. In a model with\nonly spin stripe potential, and at carrier concentrations appropriate to\nhole-doped cuprates, increasing the stripe scattering potential from zero leads\nto an increase in $R_H$, followed by a sign change. If the scattering amplitude\nis yet further increased, a second sign change occurs. The results are in\nsemiquantitative agreement with data. In a charge-stripe-potential-only model,\n$R_H$ increases as the charge stripe scattering strength increases, with no\nsign change occurring. In a model with both spin and charge stripe potentials,\n$R_H$ may be enhanced or may change sign, depending on the strengths of the two\nscattering potentials. We also consider the case in which the magnitudes of the\nstripe potentials are much larger than the bandwidth, where analytical results\ncan be obtained. In this limit, the system is quasi-one-dimensional, while\n$R_H$ remains finite and its sign is determined by the carrier density and the\nelectron band parameters.", "category": "cond-mat_str-el" }, { "text": "Magnetic phase diagram of a spin S=1/2 antiferromagnetic two-leg ladder\n in the presence of modulated along legs Dzyaloshinskii-Moriya interaction: We study the ground-state magnetic phase diagram of a spin S=1/2\nantiferromagnetic two-leg ladder in the presence of period two lattice units\nmodulated, Dzyaloshinskii-Moriya (DM) interaction along the legs. We consider\nthe case of collinear DM vectors and strong rung exchange and magnetic field.\nIn this limit we map the initial ladder model onto the effective spin\n$\\sigma=1/2$ XXZ chain and study the latter using the continuum-limit\nbosonization approach. We identified four quantum phase transitions and\ncorresponding critical magnetic fields, which mark transitions from the spin\ngapped regimes into the gapless quantum spin-liquid regimes. In the gapped\nphases the magnetization curve of the system shows plateaus at magnetisation\nM=0 and to its saturation value per rung M=1. We have shown that the very\npresence of alternating DM interaction leads to opening of a gap in the\nexcitation spectrum at magnetization M=0.5. The width of the magnetization\nplateau at M=0.5, is determined by the associated with the dynamical generation\nof a gap in the spectrum is calculated and is shown that its length scales as\n$(D_{0}D_{1}/J^{2})^{\\alpha}$ where $D_{0},D_{1}$ are uniform and staggered\ncomponents of the DM term, $J$ is the intraleg exchange and $\\alpha \\leq 3/4$\nand weakly depends on the DM couplings.", "category": "cond-mat_str-el" }, { "text": "Low-dimensional quantum magnetism in Cu(NCS)$_2$: A molecular framework\n material: Low-dimensional magnetic materials with spin-$\\frac{1}{2}$ moments can host a\nrange of exotic magnetic phenomena due to the intrinsic importance of quantum\nfluctuations to their behavior. Here, we report the structure, magnetic\nstructure and magnetic properties of copper(II) thiocyanate, Cu(NCS)$_2$, a\none-dimensional coordination polymer which displays low-dimensional quantum\nmagnetism. Magnetic susceptibility, electron paramagnetic resonance (EPR)\nspectroscopy, $^{13}$C magic-angle spinning nuclear magnetic resonance (MASNMR)\nspectroscopy, and density functional theory (DFT) investigations indicate that\nCu(NCS)$_2$ behaves as a two-dimensional array of weakly coupled\nantiferromagnetic spin chains ($J_2 = 133(1)$ K, $\\alpha = J_1/J_2 = 0.08$).\nPowder neutron-diffraction measurements confirm that Cu(NCS)$_2$ orders as a\ncommensurate antiferromagnet below $T_\\mathrm{N} = 12$ K, with a strongly\nreduced ordered moment (0.3 $\\mu_\\mathrm{B}$) due to quantum fluctuations.", "category": "cond-mat_str-el" }, { "text": "Comment on ``Exact bosonization for an interacting Fermi gas in\n arbitrary dimensions'': This a comment on arXiv:0907.3243v2. We demonstrate that the method proposed\nby Efetov {\\it et. al.} is just a reformulation of the Blankenbeckler,\nScalapino, and Sugar approach and thus it contains exactly the same sign\nproblem, including the dependence of the sign on the smoothness of the paths.", "category": "cond-mat_str-el" }, { "text": "Analysis of the Knight shift data on Li and Zn substituted YBCO: The Knight shift data on Li and Zn substituted YBa$_2$Cu$_3$O$_{6+x}$ are\nanalysed using an itinerant model with short-range antiferromagnetic\ncorrelations. The model parameters, which are determined by fitting the\nexperimental data on the transverse nuclear relaxation rate $T_2^{-1}$ of pure\nYBa$_2$Cu$_3$O$_{6+x}$, are used to calculate the Knight shifts for various\nnuclei around a nonmagnetic impurity located in the CuO$_2$ planes. The\ncalculations are carried out for Li and Zn impurities substituted into\noptimally doped and underdoped YBa$_2$Cu$_3$O$_{6+x}$. The results are compared\nwith the $^7$Li and $^{89}$Y Knight shift measurements on these materials.", "category": "cond-mat_str-el" }, { "text": "Spin and charge density waves in the Lieb lattice: We study the mean-field phase diagram of the two-dimensional (2D) Hubbard\nmodel in the Lieb lattice allowing for spin and charge density waves. Previous\nstudies of this diagram have shown that the mean-field magnetization\nsurprisingly deviates from the value predicted by Lieb's theorem\n\\cite{Lieb1989} as the on-site repulsive Coulomb interaction ($U$) becomes\nsmaller \\cite{Gouveia2015}. Here, we show that in order for Lieb's theorem to\nbe satisfied, a more complex mean-field approach should be followed in the case\nof bipartite lattices or other lattices whose unit cells contain more than two\ntypes of atoms. In the case of the Lieb lattice, we show that, by allowing the\nsystem to modulate the magnetization and charge density between sublattices,\nthe difference in the absolute values of the magnetization of the sublattices,\n$m_{\\text{Lieb}}$, at half-filling, saturates at the exact value $1/2$ for any\nvalue of $U$, as predicted by Lieb. Additionally, Lieb's relation,\n$m_{\\text{Lieb}}=1/2$, is verified approximately for large $U$, in the $n \\in\n[2/3,4/3]$ range. This range includes not only the ferromagnetic region of the\nphase diagram of the Lieb lattice (see Ref.~\\onlinecite{Gouveia2015}), but also\nthe adjacent spiral regions. In fact, in this lattice, below or at\nhalf-filling, $m_{\\text{Lieb}}$ is simply the filling of the quasi-flat bands\nin the mean-field energy dispersion both for large and small $U$.", "category": "cond-mat_str-el" }, { "text": "Quantum criticality and non-Fermi-liquid behavior in a two-level\n two-lead quantum dot: Analytical and continuous-time quantum Monte Carlo methods are used to\ninvestigate the possibility of occupation switching and quantum criticality in\na model of two quantum impurities coupled to two leads. A general discussion of\npotential occupancy-switching related quantum critical points is given, and a\ndetailed analysis is made of a specific model which has been recently\ndiscussed. For spinless electrons, no phase transition is found. For electrons\nwith spin, a critical value of the interaction strength separates a weak\ncoupling regime in which all properties vary smoothly with parameters from a\nstrong coupling phase in which occupation numbers vary discontinuously as level\nenergies are changed. The discontinuity point is characterized by\nnon-Fermi-liquid behavior. Results for self-energies and correlation functions\nare given. Phase diagrams are presented.", "category": "cond-mat_str-el" }, { "text": "Spin Drag and Spin-Charge Separation in Cold Fermi Gases: Low-energy spin and charge excitations of one-dimensional interacting\nfermions are completely decoupled and propagate with different velocities.\nThese modes however can decay due to several possible mechanisms. In this paper\nwe expose a new facet of spin-charge separation: not only the speeds but also\nthe damping rates of spin and charge excitations are different. While the\npropagation of long-wavelength charge excitations is essentially ballistic,\nspin propagation is intrinsically damped and diffusive. We suggest that cold\nFermi gases trapped inside a tight atomic waveguide offer the opportunity to\nmeasure the spin-drag relaxation rate that controls the broadening of a spin\npacket.", "category": "cond-mat_str-el" }, { "text": "Relationship between ferroelectricity and Dzyaloshinskii-Moriya\n interaction in multiferroics and the effect of bond-bending: We studied the microscopic mechanism of multiferroics, in particular with the\n\"spin current\" model (Hosho Katsura, Naoto Nagaosa and Aleander V. Balatsky,\nPhys. Rev. Lett. 95, 057205 (2005)). Starting from a system with helical spin\nconfiguration, we solved for the forms of the electron wave functions and\nanalyzed their characteristics. The relation between ferroelectricity and\nDzyaloshinskii-Moriya interaction (I. Dzyaloshinskii, J. Phys. Chem. Solids 4,\n241 (1958) and T. Moriya, Phys. Rev. 120, 91 (1960)) is clearly established.\nThere is also a simple relation between the electric polarization and the wave\nvector of magnetic orders. Finally, we show that the bond-bending exists in\ntransition metal oxides can enhance ferroelectricity.", "category": "cond-mat_str-el" }, { "text": "Quasiparticle lifetime behaviour in a simplified self-consistent\n T-matrix treatment of the attractive Hubbard model in 2D: The attractive Hubbard model on a 2-D square lattice is studied at low\nelectronic densities using the ladder approximation for the pair\nsusceptibility. This model includes (i) the short coherence lengths known to\nexist experimentally in the cuprate superconductors, and (ii) two-particle\nbound states that correspond to electron pairs. We study the quasiparticle\nlifetimes in both non self-consistent and self-consistent theories, the latter\nincluding interactions between the pairs. We find that if we include the\ninteractions between pairs the quasiparticle lifetimes vary approximately\nlinearly with the inverse temperature, consistent with experiment.", "category": "cond-mat_str-el" }, { "text": "Spin-Wave and Electromagnon Dispersions in Multiferroic MnWO4 as\n Observed by Neutron Spectroscopy: Isotropic Heisenberg Exchange versus\n Anisotropic Dzyaloshinskii-Moriya Interaction: High resolution inelastic neutron scattering reveals that the elementary\nmagnetic excitations in multiferroic MnWO4 consist of low energy dispersive\nelectromagnons in addition to the well-known spin-wave excitations. The latter\ncan well be modeled by a Heisenberg Hamiltonian with magnetic exchange coupling\nextending to the 12th nearest neighbor. They exhibit a spin-wave gap of 0.61(1)\nmeV. Two electromagnon branches appear at lower energies of 0.07(1) meV and\n0.45(1) meV at the zone center. They reflect the dynamic magnetoelectric\ncoupling and persist in both, the collinear magnetic and paraelectric AF1\nphase, and the spin spiral ferroelectric AF2 phase. These excitations are\nassociated with the Dzyaloshinskii-Moriya exchange interaction, which is\nsignificant due to the rather large spin-orbit coupling.", "category": "cond-mat_str-el" }, { "text": "Polaronic signatures in the optical properties of\n Nd$_{2-x}$Ce$_x$CuO$_4$: We investigate the temperature and doping dependence of the optical\nconductivity $\\sigma(\\omega)$ of Nd$_{2-x}$Ce$_x$CuO$_4$ in terms of\nmagnetic/lattice polaron formation. We employ dynamical mean-field theory in\nthe context of the Holstein-t-J model where an exact analytical solution is\navailable in the limit of infinite connectivity. We show that the pseudogap\nfeatures in the optical conductivity of this compound can be associated to the\nformation of lattice polarons assisted by the magnetic interaction.", "category": "cond-mat_str-el" }, { "text": "Symmetry-protected topological phases of alkaline-earth cold fermionic\n atoms in one dimension: We investigate the existence of symmetry-protected topological phases in\none-dimensional alkaline-earth cold fermionic atoms with general half-integer\nnuclear spin I at half filling. In this respect, some orbital degrees of\nfreedom are required. They can be introduced by considering either the\nmetastable excited state of alkaline-earth atoms or the p-band of the optical\nlattice. Using complementary techniques, we show that SU(2) Haldane topological\nphases are stabilised from these orbital degrees of freedom. On top of these\nphases, we find the emergence of topological phases with enlarged SU(2I+1)\nsymmetry which depend only on the nuclear spin degrees of freedom. The main\nphysical properties of the latter phases are further studied using a\nmatrix-product state approach. On the one hand, we find that these phases are\nsymmetry-protected topological phases, with respect to inversion symmetry, when\nI=1/2,5/2,9/2,..., which is directly relevant to ytterbium and strontium cold\nfermions. On the other hand, for the other values of I(=half-odd integer),\nthese topological phases are stabilised only in the presence of exact\nSU(2I+1)-symmetry.", "category": "cond-mat_str-el" }, { "text": "On the stability of topological order in tensor network states: We construct a tensor network representation of the 3d toric code ground\nstate that is stable to a generating set of uniform local tensor perturbations,\nincluding those that do not map to local operators on the physical Hilbert\nspace. The stability is established by mapping the phase diagram of the\nperturbed tensor network to that of the 3d Ising gauge theory, which has a\nnon-zero finite temperature transition. More generally, we find that the\nstability of a topological tensor network state is determined by the form of\nits virtual symmetries and the topological excitations created by virtual\noperators that break those symmetries. In particular, a dual representation of\nthe 3d toric code ground state, as well as representations of the X-cube and\ncubic code ground states, for which point-like excitations are created by such\noperators, are found to be unstable.", "category": "cond-mat_str-el" }, { "text": "Extended DFT+U+V method with on-site and inter-site electronic\n interactions: In this article we introduce a generalization of the popular DFT+U method\nbased on the extended Hubbard model that includes on-site and inter-site\nelectronic interactions. The novel corrective Hamiltonian is designed to study\nsystems for which electrons are not completely localized on atomic states\n(according to the general scheme of Mott localization) and hybridization\nbetween orbitals from different sites plays an important role. The application\nof the extended functional to archetypal Mott - charge-transfer (NiO) and\ncovalently bonded insulators (Si and GaAs) demonstrates its accuracy and\nversatility and the possibility to obtain a unifying and equally accurate\ndescription for a broad range of very diverse systems.", "category": "cond-mat_str-el" }, { "text": "Possible interaction driven topological phases in (111) bilayers of\n LaNiO3: We use the variational mean-field approach to systematically study the phase\ndiagram of a bilayer heterostructure of the correlated transition metal oxide\nLaNiO3, grown along the (111) direction. The Ni 3+ ions with d7 (or eg1)\nconfiguration form a buckled honeycomb lattice. We show that as a function of\nthe strength of the on-site interactions, various topological phases emerge. In\nthe presence of a reasonable size of the Hund's coupling, as the correlation is\ntuned from intermediate to strong, the following sequence of phases is found:\n(1) a Dirac half-semimetal phase, (2) a quantum anomalous hall insulator (QAHI)\nphase with Chern number one, and (3) a ferromagnetic nematic phase breaking the\nlattice point group symmetry. The spin-orbit couplings and magnetism are both\ndynamically generated in the QAHI phase.", "category": "cond-mat_str-el" }, { "text": "Nine classes of integrable boundary conditions for the eight-state\n supersymmetric fermion model: Nine classes of integrable boundary conditions for the eight-state\nsupersymmetric model of strongly correlated fermions are presented. The\nboundary systems are solved by using the coordinate Bethe ansatz method and the\nBethe ansatz equations for all nine cases are given.", "category": "cond-mat_str-el" }, { "text": "Extension of the spin-1/2 frustrated square lattice model: the case of\n layered vanadium phosphates: We study the influence of the spin lattice distortion on the properties of\nfrustrated magnetic systems and consider the applicability of the spin-1/2\nfrustrated square lattice model to materials lacking tetragonal symmetry. We\nfocus on the case of layered vanadium phosphates AA'VO(PO4)2 (AA' = Pb2, SrZn,\nBaZn, and BaCd). To provide a proper microscopic description of these\ncompounds, we use extensive band structure calculations for real materials and\nmodel structures and supplement this analysis with simulations of thermodynamic\nproperties, thus facilitating a direct comparison with the experimental data.\nDue to the reduced symmetry, the realistic spin model of layered vanadium\nphosphates AA'VO(PO4)2 includes four inequivalent exchange couplings: J1 and\nJ1' between nearest-neighbors and J2 and J2' between next-nearest-neighbors.\nThe estimates of individual exchange couplings suggest different regimes, from\nJ1'/J1 and J2'/J2 close to 1 in BaCdVO(PO4)2, a nearly regular frustrated\nsquare lattice, to J1'/J1 ~ 0.7 and J2'/J2 ~ 0.4 in SrZnVO(PO4)2, a frustrated\nsquare lattice with sizable distortion. The underlying structural differences\nare analyzed, and the key factors causing the distortion of the spin lattice in\nlayered vanadium compounds are discussed. We propose possible routes for\nfinding new frustrated square lattice materials among complex vanadium oxides.\nFull diagonalization simulations of thermodynamic properties indicate the\nsimilarity of the extended model to the regular one with averaged couplings. In\ncase of moderate frustration and moderate distortion, valid for all the\nAA'VO(PO4)2 compounds reported so far, the distorted spin lattice can be\nconsidered as a regular square lattice with the couplings (J1+J1')/2 between\nnearest-neighbors and (J2+J2')/2 between next-nearest-neighbors.", "category": "cond-mat_str-el" }, { "text": "Effects of quantum impurity spins on the magnetic properties of zigzag\n and linear spin chains: We investigated the magnetic ground state and low-energy excitations of the\nspin chains compounds SrCuO$_{2}$ (zigzag chains) and Sr$_{2}$CuO$_{3}$ (linear\nchains) in the presence of quantum impurities induced by lightly doping ($\\leq\n1 \\%$) with Zn$^{2+}$ ($S = 0$), Co$^{2+}$ ($S =1/2$) and Ni$^{2+}$ ($S = 1$)\nimpurities at the Cu$^{2+}$ site. We show that the ground states and the nature\nof low-lying excitations (i.e., gapped or gapless) depend on the spin state and\nsymmetry of the defects. For Ni doped chains a spin gap is observed but for Zn\nand Co doping the excitations remain gapless. Co-doped chains exhibit magnetic\norder with critical temperatures significantly enhanced compared to those of\nthe pristine compounds. In the specific case of 1 \\% Co impurities, the linear\nchains exhibit long-range order below 11 K, while the zigzag chain is\ncharacterized by a quasi-long range ordered phase below 6 K with correlation\nlengths of about 12\\textit{a} and 40\\textit{c} units along the crystal axes\n\\textit{a} and \\textit{c}, respectively. The different magnetic behaviours of\nthese two compounds with comparable intra- and interchain couplings underpin\nthe role of spin frustration in the zigzag chains.", "category": "cond-mat_str-el" }, { "text": "Unraveling Orbital Correlations via Magnetic Resonant Inelastic X-ray\n Scattering: Although orbital degrees of freedom are a factor of fundamental importance in\nstrongly correlated transition metal compounds, orbital correlations and\ndynamics remain very difficult to access, in particular by neutron scattering.\nVia a direct calculation of scattering amplitudes we show that instead magnetic\nresonant inelastic x-ray scattering (RIXS) does reveal orbital correlations. In\ncontrast to neutron scattering, the intensity of the magnetic excitations in\nRIXS depends very sensitively on both the symmetry of the orbitals that spins\noccupy, and on photon polarizations. We show in detail how this effect allows\nmagnetic RIXS to distinguish between alternating orbital ordered and\nferro-orbital (or orbital liquid) states.", "category": "cond-mat_str-el" }, { "text": "Emergent SU(3) symmetry in random spin-1 chains: We show that generic SU(2)-invariant random spin-1 chains have phases with an\nemergent SU(3) symmetry. We map out the full zero-temperature phase diagram and\nidentify two different phases: (i) a conventional random singlet phase (RSP) of\nstrongly bound spin pairs (SU(3) \"mesons\") and (ii) an unconventional RSP of\nbound SU(3) \"baryons\", which are formed, in the great majority, by spin trios\nlocated at random positions. The emergent SU(3) symmetry dictates that\nsusceptibilities and correlation functions of both dipolar and quadrupolar spin\noperators have the same asymptotic behavior.", "category": "cond-mat_str-el" }, { "text": "Comment on \"Critical spin dynamics of the 2D quantum Heisenberg\n antiferromagnets: Sr2CuO2Cl2 and Sr2Cu3O4Cl2\": We compare the neutron measurements of Kim et al. (cond-mat/0012239) on\ntwo-dimensional, S=1/2 antiferromagnets with the continuum quasiclassical\ntheory of S. Sachdev and O.A. Starykh (cond-mat/9904354). The damping of the\nlowest energy spin excitations is characterized by a dimensionless number whose\ntemperature dependence was predicted to be determined entirely by that of the\nuniform spin susceptibility. Theory and experiment are consistent with each\nother.", "category": "cond-mat_str-el" }, { "text": "Energy band of graphene ribbons under the tensile force: According to the tight-binding approximation, we investigate the electronic\nstructures of graphene ribbons with zigzag shaped edges (ZGRs) and armchair\nshaped edges (AGRs) drawn by the tensile force, and obtain the analytic\nrelations between the energy bands of pi-electrons in ZGR, AGR and the tensile\nforce based on only considering the nearest-neighbor interaction and the\nhydrogen-like atomic wave function is considered as pi-electron wave function.\nImportantly, we find the tensile force can open an energy gap at the K point\nfor ZGR and AGR, and the force perpendicular to the zigzag edges can open\nenergy gap more easily besides the gap values of ZGR and AGR at the K point\nboth increase as the tensile force increases.", "category": "cond-mat_str-el" }, { "text": "Dimensionality Control of d-orbital Occupation in Oxide Superlattices: Manipulating the orbital state in a strongly correlated electron system is of\nfundamental and technological importance for exploring and developing novel\nelectronic phases. Here, we report an unambiguous demonstration of orbital\noccupancy control between t2g and eg multiplets in quasi-twodimensional\ntransition metal oxide superlattices (SLs) composed of a Mott insulator LaCoO3\nand a band insulator LaAlO3. As the LaCoO3 sublayer thickness approaches its\nfundamental limit (i.e. one unit-cell-thick), the electronic state of the SLs\nchanged from a Mott insulator, in which both t2g and eg orbitals are partially\nfilled, to a band insulator by completely filling (emptying) the t2g (eg)\norbitals. We found the reduction of dimensionality has a profound effect on the\nelectronic structure evolution, which is, whereas, insensitive to the epitaxial\nstrain. The remarkable orbital controllability shown here offers a promising\npathway for novel applications such as catalysis and photovoltaics, where the\nenergy of d level is an essential parameter.", "category": "cond-mat_str-el" }, { "text": "Pseudofermion ferromagnetism in the Kondo lattices: a mean-field\n approach: Ground state ferromagnetism of the Kondo lattices is investigated within\nslave fermion approach by Coleman and Andrei within a mean-field approximation\nin the effective hybridization model. Conditions for formation of both\nsaturated (half-metallic) and non-saturated magnetic state are obtained for\nvarious lattices. A description in terms of universal functions which depend\nonly on bare electron density of states (DOS) is presented. A crucial role of\nthe energy dependence of the bare DOS (especially, of DOS peaks) for the\nsmall-moment ferromagnetism formation is demonstrated.", "category": "cond-mat_str-el" }, { "text": "Ferromagnetism and Fermi-surface transition in the periodic Anderson\n model: Second-order phase transition without symmetry breaking: We study ferromagnetism in the periodic Anderson model with and without a\nmagnetic field by the Gutzwiller theory. We find three ferromagnetic phases: a\nweak ferromagnetic phase (FM0), a half-metallic phase without Fermi surface for\nthe majority spin (FM1), and a ferromagnetic phase with almost completely\npolarized f-electrons (FM2). The Fermi surface changes from the large\nFermi-surface in the paramagnetic state to the small Fermi-surface in FM2. We\nalso find that the transitions between the ferromagnetic phases can be\nsecond-order phase transitions in spite of the absence of symmetry breaking.\nWhile we cannot define an order parameter for such transitions in an ordinary\nway, the topology of the Fermi surface characterizes the transitions, i.e.,\nthey are Lifshitz transitions.", "category": "cond-mat_str-el" }, { "text": "Steady-state superconductivity in electronic materials with repulsive\n interactions: We study the effect of laser driving on a minimal model for a hexagonal\ntwo-dimensional material with broken inversion symmetry. Through the\napplication of circularly polarised light and coupling to a thermal free\nelectron bath, the system is driven into a nonequilibrium steady state with\nasymmetric, nonthermal carrier populations in the two valleys. We show that, in\nthis steady state, interband superconducting correlations between electrons can\ndevelop independent of the sign of the electron-electron interactions. We\ndiscuss how our results apply, for example, to transition metal\ndichalcogenides. This work opens the door to technological applications of\nsuperconductivity in a range of materials that were hitherto precluded from it.", "category": "cond-mat_str-el" }, { "text": "Correlation-driven electronic nematicity in the Dirac semimetal BaNiS2: In BaNiS2 a Dirac nodal-line band structure exists within a two-dimensional\nNi square lattice system, in which significant electronic correlation effects\nare anticipated. Using scanning tunneling microscopy, we discover signs of\ncorrelated-electron behavior, namely electronic nematicity appearing as a pair\nof C2-symmetry striped patterns in the local density-of-states at ~60 meV above\nthe Fermi energy. In observations of quasiparticle interference, as well as\nidentifying scattering between Dirac cones, we find that the striped patterns\nin real space stem from a lifting of degeneracy among electron pockets at the\nBrillouin zone boundary. We infer a momentum-dependent energy shift with d-form\nfactor, which we model numerically within a density wave equation framework\nthat considers spin-fluctuation-driven nematicity. This suggests an unusual\nmechanism driving the nematic instability, stemming from only a small\nperturbation to the Fermi surface, in a system with very low density of states\nat the Fermi energy. The Dirac points lie at nodes of the d-form factor, and\nare almost unaffected by it. These results highlight BaNiS2 as a unique\nmaterial in which Dirac electrons and symmetry-breaking electronic correlations\ncoexist.", "category": "cond-mat_str-el" }, { "text": "Quantum chaos on a critical Fermi surface: We compute parameters characterizing many-body quantum chaos for a critical\nFermi surface without quasiparticle excitations. We examine a theory of $N$\nspecies of fermions at non-zero density coupled to a $U(1)$ gauge field in two\nspatial dimensions, and determine the Lyapunov rate and the butterfly velocity\nin an extended random-phase approximation. The thermal diffusivity is found to\nbe universally related to these chaos parameters i.e. the relationship is\nindependent of $N$, the gauge coupling constant, the Fermi velocity, the Fermi\nsurface curvature, and high energy details.", "category": "cond-mat_str-el" }, { "text": "Magnetic critical properties and basal-plane anisotropy of Sr$_2$IrO$_4$: The anisotropic magnetic properties of Sr$_2$IrO$_4$ are investigated, using\nlongitudinal and torque magnetometry. The critical scaling across $T_c$ of the\nlongitudinal magnetization is the one expected for the 2D XY universality\nclass. Modeling the torque for a magnetic field in the basal-plane, and taking\ninto account all in-plane and out-of-plane magnetic couplings, we derive the\neffective 4-fold anisotropy $K_4 \\approx$ 1 10$^5$ erg mole$^{-1}$. Although\nlarger than for the cuprates, it is found too small to account for a\nsignificant departure from the isotropic 2D XY model. The in-plane torque also\nallows us to put an upper bound for the anisotropy of a field-induced shift of\nthe antiferromagnetic ordering temperature.", "category": "cond-mat_str-el" }, { "text": "Angular dependence of the Hall effect of lsmo films: We find that the Hall effect resistivity ($\\rho_{xy}$) of thin films of\n\\lsmo\\ varies as a function of the angle $\\theta$ between the applied magnetic\nfield and the film normal as $\\rho_{xy}=a\\cos \\theta + b\\cos 3\\theta$, where\n$|b|$ increases with increasing temperature and decreases with increasing\nmagnetic field. We find that the angular dependence of the longitudinal\nresistivity and the magnetization cannot fully explain the surprising term $b$,\nsuggesting it is a manifestation of an intrinsic transport property.", "category": "cond-mat_str-el" }, { "text": "Spin Density and Non-Collinear Magnetization in Frustrated Pyrochlore\n \\tbti from Polarized Neutron Scattering: We used a local susceptibility approach in extensive polarized neutron\ndiffraction studies of the spin liquid \\tbti. For a magnetic field applied\nalong the [110] and [111] directions, we found that, at high temperature, all\nTb moments are collinear and parallel to the field. With decreasing\ntemperature, the Tb moments reorient from the field direction to their local\nanisotropy axes. For the [110] field direction, the field induced magnetic\nstructure at 10 K is spin ice-like, but with two types of Tb moments of very\ndifferent magnitudes. For a field along [111], the magnetic structure resembles\nthe so-called \"one in-three out\" found in spin ices, with the difference that\nall Tb moments have an additional component along the [111] direction due to\nthe magnetic field. The temperature evolution of the local susceptibilities\nclearly demonstrates a progressive change from Heisenberg to Ising behavior of\nthe Tb moments when lowering the temperature, which appears to be a crystal\nfield effect.", "category": "cond-mat_str-el" }, { "text": "Microscopic characterization of the magnetic properties of the itinerant\n antiferromagnet La2Ni7 by 139La NMR/NQR measurements: 139La nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR)\nmeasurements have been performed to investigate the magnetic properties of the\nitinerant magnet La2Ni7 which shows a series of antiferromagnetic (AFM) phase\ntransitions at $T_{N1}$=61 K, $T_{N2}$=56 K, and $T_{N3}$=42 K under zero\nmagnetic field. Two distinct La NMR signals were observed due to the two\ncrystallographically inequivalent La sites in La2Ni7 (La1 and La2 in the La2Ni4\nand the LaNi5 sub-units of the La2Ni7 unit cell, respectively). From the 139La\nNQR spectrum in the AFM state below $T_{N3}$, the AFM state was revealed to be\na commensurate state where Ni ordered moments align along the crystalline c\naxis. Owing to the two different La sites, we were able to estimate the average\nvalues of the Ni ordered moments ($\\sim$0.09-0.10 $\\mu_{B}$/Ni and\n$\\sim$0.17$\\mu_{B}$/Ni around La1 and La2, respectively) from 139La NMR\nspectrum measurements in the AFM state below $T_{N3}$, suggesting a non-uniform\ndistribution of the Ni-ordered moments in the AFM state. In contrast, a more\nuniform distribution of the Ni-ordered moments in the saturated paramagnetic\nstate induced by the application of high magnetic fields is observed. The\ntemperature dependence of the sublattice magnetization measured by the internal\nfield at the La2 site in the AFM state was reproduced by a local moment model\nbetter than the self-consistent renormalization (SCR) theory for weak itinerant\nantiferromagnets. Given the small Ni-ordered moments in the magnetically\nordered state, our results suggest that La2Ni7 has characteristics of both\nitinerant and localized natures in its magnetism. With this in mind, it is\nnoteworthy that the temperature dependence of nuclear spin-relaxation rates in\nthe paramagnetic state above $T_{N1}$ measured at zero magnetic field can be\nexplained qualitatively by both the SCR theory and the local-moment model.", "category": "cond-mat_str-el" }, { "text": "Metamagnetic transition in the two $f$ orbitals Kondo lattice model: In this work, we study the effects of a transverse magnetic field in a Kondo\nlattice model with two $f$ orbitals interacting with the conduction electrons.\nThe $f$ electrons that are present on the same site interact through Hund's\ncoupling, while on neighboring sites they interact through intersite exchange.\nWe consider here that part of $f$ electrons are localized (orbital 1) while\nanother part (orbital 2) are delocalized, as it is frequent in uranium systems.\nThen, only electrons in the localized orbital 1 interact through exchange\ninteraction with the neighboring ones, while electrons in orbital 2 are coupled\nwith conduction electrons through a Kondo interaction. We obtain a solution\nwhere ferromagnetism and Kondo effect coexist for small values of an applied\ntransverse magnetic field for $T\\rightarrow0$. Increasing the transverse field,\ntwo situations can be obtained when Kondo coupling vanishes: first, a\nmetamagnetic transition occurs just before or at the same time of the fully\npolarized state, and second, a metamagnetic transition occurs when the spins\nare already pointing out along the magnetic field.", "category": "cond-mat_str-el" }, { "text": "Renormalization Group Potential for Quasi-One-Dimensional Correlated\n Systems: We studied the correlated quasi-one-dimensional systems by one-loop\nrenormalization group techniques in weak coupling. In contrast to conventional\ng-ology approach, we formulate the theory in terms of bilinear currents and\nobtain all possible interaction vertices. Furthermore, the one-loop\nrenormalization group equations are derived by operator product expansions of\nthese currents at short length scale. It is rather remarkable that these\ncoupled non-linear equations, after appropriate rescaling, can be casted into\npotential flows. The existence of what we nicknamed \"RG potential\" provides a\nnatural explanation of the emergent symmetry enhancement in ladder systems.\nFurther implications arisen from the RG potential are also discussed at the\nend.", "category": "cond-mat_str-el" }, { "text": "Zero frequency divergence and gauge phase factor in the optical response\n theory: The static current-current correlation leads to the definitional zero\nfrequency divergence (ZFD) in the optical susceptibilities. Previous\ncomputations have shown nonequivalent results between two gauges (${\\bf p\\cdot\nA}$ and ${\\bf E \\cdot r}$) under the exact same unperturbed wave functions. We\nreveal that those problems are caused by the improper treatment of the\ntime-dependent gauge phase factor in the optical response theory. The gauge\nphase factor, which is conventionally ignored by the theory, is important in\nsolving ZFD and obtaining the equivalent results between these two gauges. The\nHamiltonians with these two gauges are not necessary equivalent unless the\ngauge phase factor is properly considered in the wavefunctions. Both\nSu-Shrieffer-Heeger (SSH) and Takayama-Lin-Liu-Maki (TLM) models of\ntrans-polyacetylene serve as our illustrative examples to study the linear\nsusceptibility $\\chi^{(1)}$ through both current-current and dipole-dipole\ncorrelations. Previous improper results of the $\\chi^{(1)}$ calculations and\ndistribution functions with both gauges are discussed. The importance of gauge\nphase factor to solve the ZFD problem is emphasized based on SSH and TLM\nmodels. As a conclusion, the reason why dipole-dipole correlation favors over\ncurrent-current correlation in the practical computations is explained.", "category": "cond-mat_str-el" }, { "text": "Majorana Edge States for Z2 Topological Orders of the Wen-plaquette\n Model and the Toric-code Model: In this paper we study the symmetry protected Majorana edge states for the Z2\ntopological order of the Wen-plaquette model and the toric-code model and\ncalculate the dispersion of the Majorana edge states. For the system with\ntranslational symmetry, the Majorana edge states are gapless and have the nodal\npoints at k=0 and k=pi. For the edge states of the toric-code model without\ntranslational symmetry, the edge modes become gapped.", "category": "cond-mat_str-el" }, { "text": "Reply to 'Comment on \"Dynamic correlations of the spinless Coulomb\n Luttinger liquid [Phys. Rev. B 65, 125109 (2002)]\"': We show that the criticism of our paper [Phys. Rev. B 65, 125109 (2002)] by\nWang, Millis, and Das Sarma [cond-mat/0206203] is based on a trivial\nmathematical mistake they have committed.", "category": "cond-mat_str-el" }, { "text": "Symmetry projected Jastrow mean field wavefunction in variational Monte\n Carlo: We extend our low-scaling variational Monte Carlo (VMC) algorithm to optimize\nthe symmetry projected Jastrow mean field (SJMF) wavefunctions. These\nwavefunctions consist of a symmetry-projected product of a Jastrow and a\ngeneral broken-symmetry mean field reference. Examples include Jastrow\nantisymmetrized geminal power (JAGP), Jastrow-Pfaffians, and resonating valence\nbond (RVB) states among others, all of which can be treated with our algorithm.\nWe will demonstrate using benchmark systems including the nitrogen molecule, a\nchain of hydrogen atoms, and the 2-D Hubbard model that a significant amount of\ncorrelation can be obtained by optimizing the energy of the SJMF wavefunction.\nThis can be achieved at a relatively small cost when multiple symmetries\nincluding spin, particle number, and complex conjugation are simultaneously\nbroken and projected. We also show that reduced density matrices can be\ncalculated using the optimized wavefunctions, which allows us to calculate\nother observables such as correlation functions and will enable us to embed the\nVMC algorithm in a complete active space self-consistent field (CASSCF)\ncalculation.", "category": "cond-mat_str-el" }, { "text": "Matrix-product-state method with a dynamical local basis optimization\n for bosonic systems out of equilibrium: We present a method for simulating the time evolution of one-dimensional\ncorrelated electron-phonon systems which combines the time-evolving block\ndecimation algorithm with a dynamical optimization of the local basis. This\napproach can reduce the computational cost by orders of magnitude when boson\nfluctuations are large. The method is demonstrated on the nonequilibrium\nHolstein polaron by comparison with exact simulations in a limited functional\nspace and on the scattering of an electronic wave packet by local phonon modes.\nOur study of the scattering problem reveals a rich physics including transient\nself-trapping and dissipation.", "category": "cond-mat_str-el" }, { "text": "Floquet second-order topological insulators in non-Hermitian systems: Second-order topological insulator (SOTI) is featured with the presence of\n$(d-2)$-dimensional boundary states in $d$-dimension systems. The\nnon-Hermiticity induced breakdown of bulk-boundary correspondence (BBC) and the\nperiodic driving on systems generally obscure the description of non-Hermitian\nSOTI. To prompt the applications of SOTIs, we explore the role of periodic\ndriving in controllably creating exotic non-Hermitian SOTIs both for 2D and 3D\nsystems. A scheme to retrieve the BBC and a complete description to SOTIs via\nthe bulk topology of such nonequilibrium systems are proposed. It is found that\nrich exotic non-Hermitian SOTIs with a widely tunable number of 2D corner\nstates and 3D hinge states and a coexistence of the first- and second-order\ntopological insulators are induced by the periodic driving. Enriching the\nfamily of topological phases, our result may inspire the exploration to apply\nSOTIs via tuning the number of corner/hinge states by the periodic driving.", "category": "cond-mat_str-el" }, { "text": "Stability of skyrmions in perturbed ferromagnetic chiral magnets: Magnetic skyrmions, topological spin textures observed in chiral magnets,\nhave attracted huge interest due to their applications in the field of\nspintronics. In this work we study the stability of circular isolated skyrmions\nin ferromagnetic chiral magnets under the influence of different perturbations\nand external fields. To this end we develop a general systematic procedure\nbased in a harmonic expansion series of the skyrmion boundary which allows the\nidentifycation of the breakdown of the skyrmion circular shape on each\ninstability channel independently. We apply our approach to a few\nrepresentative spin models with actual interest in order to obtain the zero\ntemperature phase diagram, where isolated skyrmions emerge as metaestable\nstates. The results presented in this paper are in agreement with properties of\nisolated skyrmions observed in recent experiments opening the possibility of\nextending the analysis to more complex situations.", "category": "cond-mat_str-el" }, { "text": "Floquet multi-Weyl points in crossing-nodal-line semimetals: Weyl points with monopole charge $\\pm 1$ have been extensively studied,\nhowever, real materials of multi-Weyl points, whose monopole charges are higher\nthan $1$, have yet to be found. In this Rapid Communication, we show that\nnodal-line semimetals with nontrivial line connectivity provide natural\nplatforms for realizing Floquet multi-Weyl points. In particular, we show that\ndriving crossing nodal lines by circularly polarized light generates\ndouble-Weyl points. Furthermore, we show that monopole combination and\nannihilation can be observed in crossing-nodal-line semimetals and nodal-chain\nsemimetals. These proposals can be experimentally verified in pump-probe\nangle-resolved photoemission spectroscopy.", "category": "cond-mat_str-el" }, { "text": "The possibility of measuring intrinsic electronic correlations in\n graphene using a d-wave contact Josephson junction: While not widely recognized, electronic correlations might play an important\nrole in graphene. Indeed, Pauling's resonance valence bond (RVB) theory for the\npp-bonded planar organic molecules, of which graphene is the infinite\nextension, already established the importance of the nearest neighbor\nspin-singlet bond (SB) state in these materials. However, despite the recent\ngrowth of interest in graphene, there is still no quantitative estimate of the\neffects of Coulomb repulsion in either undoped or doped graphene. Here we use a\ntight-binding Bogoliubov-de Gennes (TB BdG) formalism to show that in\nunconventional d-wave contact graphene Josephson junctions the intrinsic SB\ncorrelations are strongly enhanced. We show on a striking effect of the SB\ncorrelations in both proximity effect and Josephson current as well as\nestablishing a 1/(T-T_c) functional dependence for the superconducting decay\nlength. Here T_c is the superconducting transition temperature for the\nintrinsic SB correlations, which depends on both the effects of Coulomb\nrepulsion and the doping level. We therefore propose that d-wave contact\ngraphene Josephson junctions will provide a promising experimental system for\nthe measurement of the effective strength of intrinsic SB correlations in\ngraphene.", "category": "cond-mat_str-el" }, { "text": "Bending and Breaking of Stripes in a Charge-Ordered Manganite: In complex electronic materials, coupling between electrons and the atomic\nlattice gives rise to remarkable phenomena, including colossal\nmagnetoresistance and metal-insulator transitions. Charge-ordered phases are a\nprototypical manifestation of charge-lattice coupling, in which the atomic\nlattice undergoes periodic lattice displacements (PLDs). Here we directly map\nthe picometer scale PLDs at individual atomic columns in the room temperature\ncharge-ordered manganite Bi$_{0.35}$Sr$_{0.18}$Ca$_{0.47}$MnO$_3$ using\naberration corrected scanning transmission electron microscopy (STEM). We\nmeasure transverse, displacive lattice modulations of the cations, distinct\nfrom existing manganite charge-order models. We reveal locally unidirectional\nstriped PLD domains as small as $\\sim$5 nm, despite apparent bidirectionality\nover larger length scales. Further, we observe a direct link between disorder\nin one lattice modulation, in the form of dislocations and shear deformations,\nand nascent order in the perpendicular modulation. By examining the defects and\nsymmetries of PLDs near the charge-ordering phase transition, we directly\nvisualize the local competition underpinning spatial heterogeneity in a complex\noxide.", "category": "cond-mat_str-el" }, { "text": "Valence Bond Phases in $S=1/2$ Kane-Mele-Heisenberg Model: The phase diagram of Kane-Mele-Heisenberg (KMH) model in classical\nlimit~\\cite{zare}, contains disordered regions in the coupling space, as the\nresult of to competition among different terms in the Hamiltonian, leading to\nfrustration in finding a unique ground state. In this work we explore the\nnature of these phase in the quantum limit, for a $S=1/2$. Employing exact\ndiagonalization (ED) in $S_z$ and nearest neighbor valence bond (NNVB) bases,\nbond and plaquette valence bond mean field theories, We show that the\ndisordered regions are divided into ordered quantum states in the form of\nplaquette valence bond crystal(PVBC) and staggered dimerized (SD) phases.", "category": "cond-mat_str-el" }, { "text": "Correlation effects in partially ionized mass asymmetric electron-hole\n plasmas: The effects of strong Coulomb correlations in dense three-dimensional\nelectron-hole plasmas are studied by means of unbiased direct path integral\nMonte Carlo simulations. The formation and dissociation of bound states, such\nas excitons and bi-excitons is analyzed and the density-temperature region of\ntheir appearance is identified. At high density, the Mott transition to the\nfully ionized metallic state (electron-hole liquid) is detected. Particular\nattention is paid to the influence of the hole to electron mass ratio $M$ on\nthe properties of the plasma. Above a critical value of about M=80 formation of\na hole Coulomb crystal was recently verified [Phys. Rev. Lett. {\\bf 95}, 235006\n(2005)] which is supported by additional results. Results are related to the\nexcitonic phase diagram of intermediate valent Tm[Se,Te], where large values of\n$M$ have been observed experimentally.", "category": "cond-mat_str-el" }, { "text": "The ALPS project release 1.3: open source software for strongly\n correlated systems: We present release 1.3 of the ALPS (Algorithms and Libraries for Physics\nSimulations) project, an international open source software project to develop\nlibraries and application programs for the simulation of strongly correlated\nquantum lattice models such as quantum magnets, lattice bosons, and strongly\ncorrelated fermion systems. Development is centered on common XML and binary\ndata formats, on libraries to simplify and speed up code development, and on\nfull-featured simulation programs. The programs enable non-experts to start\ncarrying out numerical simulations by providing basic implementations of the\nimportant algorithms for quantum lattice models: classical and quantum Monte\nCarlo (QMC) using non-local updates, extended ensemble simulations, exact and\nfull diagonalization (ED), as well as the density matrix renormalization group\n(DMRG). Changes in the new release include a DMRG program for interacting\nmodels, support for translation symmetries in the diagonalization programs, the\nability to define custom measurement operators, and support for inhomogeneous\nsystems, such as lattice models with traps. The software is available from our\nweb server at http://alps.comp-phys.org/ .", "category": "cond-mat_str-el" }, { "text": "Magnetic excitations in hole-doped Sr2IrO4: A comparison with\n electron-doped cuprates: We have studied the evolution of magnetic and orbital excitations as a\nfunction of hole-doping in single crystal samples of Sr2Ir(1-x)Rh(x)O4 (0.07 <\nx < 0.42) using high resolution Ir L3-edge resonant inelastic x-ray scattering\n(RIXS). Within the antiferromagnetically ordered region of the phase diagram (x\n< 0.17) we observe highly dispersive magnon and spin-orbit exciton modes.\nInterestingly, both the magnon gap energy and the magnon bandwidth appear to\nincrease as a function of doping, resulting in a hardening of the magnon mode\nwith increasing hole doping. As a result, the observed spin dynamics of\nhole-doped iridates more closely resemble those of the electron-doped, rather\nthan hole-doped, cuprates. Within the paramagnetic region of the phase diagram\n(0.17 < x < 0.42) the low-lying magnon mode disappears, and we find no evidence\nof spin fluctuations in this regime. In addition, we observe that the orbital\nexcitations become essentially dispersionless in the paramagnetic phase,\nindicating that magnetic order plays a crucial role in the propagation of the\nspin-orbit exciton.", "category": "cond-mat_str-el" }, { "text": "The Thermoelectric Effect and Its Natural Heavy Fermion Explanation in\n Twisted Bilayer and Trilayer Graphene: We study the interacting transport properties of twisted bilayer graphene\n(TBG) using the topological heavy-fermion (THF) model. In the THF model, TBG\ncomprises localized, correlated $f$-electrons and itinerant, dispersive\n$c$-electrons. We focus on the Seebeck coefficient, which quantifies the\nvoltage difference arising from a temperature gradient. We find that the TBG's\nSeebeck coefficient shows unconventional (strongly-interacting) traits:\nnegative values with sawtooth oscillations at positive fillings, contrasting\ntypical band-theory expectations. This behavior is naturally attributed to the\npresence of heavy (correlated, short-lived $f$-electrons) and light\n(dispersive, long-lived $c$-electrons) electronic bands. Their longer lifetime\nand stronger dispersion lead to a dominant transport contribution from the\n$c$-electrons. At positive integer fillings, the correlated TBG insulators\nfeature $c$- ($f$-)electron bands on the electron (hole) doping side, leading\nto an overall negative Seebeck coefficient. Additionally, sawtooth oscillations\noccur around each integer filling due to gap openings. Our results highlight\nthe essential importance of electron correlations in understanding the\ntransport properties of TBG and, in particular, of the lifetime asymmetry\nbetween the two fermionic species (naturally captured by the THF model). Our\nfindings are corroborated by new experiments in both twisted bilayer and\ntrilayer graphene, and show the natural presence of strongly-correlated heavy\nand light carriers in the system.", "category": "cond-mat_str-el" }, { "text": "Emergent soft-gap Anderson models at quantum criticality in a lattice\n Hamiltonian within dynamical mean field theory: Local quantum criticality in itinerant fermion systems has been extensively\ninvestigated through the soft-gap Anderson impurity model, wherein a localized,\ncorrelated impurity, hybridizes with a broad conduction band with a singular,\n$|\\omega|^r$, density of states. However, lattice models hosting quantum\ncritical points (QCPs), do not appear to have such a spectrum emerging at the\nQCP. In this work, we report the emergence of such a singular form of the\ndensity of states in a three-orbital lattice model, within dynamical mean field\ntheory, precisely at a quantum critical point, separating a gapless, Fermi\nliquid, metallic phase from a gapped, Mott insulating phase. A\ntemperature-dependent exponent, $\\alpha$, defined using the corresponding\nMatsubara self-energy, is found to vary from $+1$ deep in the FL regime, to\n$-1$ in the Mott insulator regime. Interestingly, we find that $\\alpha$ becomes\ntemperature independent, and hence isosbestic, precisely at the QCP. The\nisosbestic exponent is shown to lead to an emergent soft-gap spectrum,\n$|\\omega|^r$ at the QCP, where $r = |\\alpha_{\\rm iso}|$. We discuss the\nimplications of our findings for non-Fermi liquid behaviour in the quantum\ncritical region of the phase diagram.", "category": "cond-mat_str-el" }, { "text": "Calculating ground state properties of correlated fermionic systems with\n BCS trial wave functions in Slater determinant path-integral approaches: We introduce an efficient and numerically stable technique to make use of a\nBCS trial wave function in the computation of correlation functions of strongly\ncorrelated quantum fermion systems. The technique is applicable to any\nprojection approach involving paths of independent-fermion propagators, for\nexample in mean-field or auxiliary-field quantum Monte Carlo (AFQMC)\ncalculations. Within AFQMC, in the absence of the sign problem, the methodology\nallows the use of a BCS reference state which can greatly reduce the required\nimaginary time of projection, and improves Monte Carlo sampling efficiency and\nstatistical accuracy for systems where pairing correlations are important. When\nthe sign problem is present, the approach provides a powerful generalization of\nthe constrained-path AFQMC technique which usually uses Slater determinant\ntrial wave functions. As a demonstration of the capability of the methodology,\nwe present benchmark results for the attractive Hubbard model, both\nspin-balanced (no sign problem) and with a finite spin polarization (with sign\nproblem).", "category": "cond-mat_str-el" }, { "text": "Demonstration of a robust pseudogap in a three-dimensional correlated\n electronic system: We outline a partial-fractions decomposition method for determining the\none-particle spectral function and single-particle density of states of a\ncorrelated electronic system on a finite lattice in the non self-consistent\nT-matrix approximation to arbitrary numerical accuracy, and demonstrate the\napplication of these ideas to the attractive Hubbard model. We then demonstrate\nthe effectiveness of a finite-size scaling ansatz which allows for the\nextraction of quantities of interest in the thermodynamic limit from this\nmethod. In this approximation, in one or two dimensions, for any finite lattice\nor in the thermodynamic limit, a pseudogap is present and its energy diverges\nas Tc is approached from above; this is an unphysical manifestation of using an\napproximation that predicts a spurious phase transition in one or two\ndimensions. However, in three dimensions one expects the transition predicted\nby this approximation to represent a true continuous phase transition, and in\nthe thermodynamic limit any pseudogap predicted by this formulation will remain\nfinite. We have applied our method to the attractive Hubbard model on a\nthree-dimensional simple cubic lattice, and find that for intermediate coupling\na prominent pseudogap is found in the single-particle density of states, and\nthis gap persists over a large temperature range. In addition, we also show\nthat for weak coupling a pseudogap is also present. The pseudogap energy at the\ntransition temperature is almost a factor of three larger than the T=0 BCS gap\nfor intermediate coupling, whereas for weak coupling the pseudogap and BCS gap\nenergies are essentially equal.", "category": "cond-mat_str-el" }, { "text": "Fermi surface reconstruction by a charge-density-wave with finite\n correlation length: Even a small amplitude charge-density-wave (CDW) can reconstruct a Fermi\nsurface, giving rise to new quantum oscillation frequencies. Here, we\ninvestigate quantum oscillations when the CDW has a finite correlation length\n$\\xi$ -- a case relevant to the hole-doped cuprates. By considering the Berry\nphase induced by a spatially varying CDW phase, we derive an effective Dingle\nfactor that depends exponentially on the ratio of the cyclotron orbit radius,\n$R_c$, to $\\xi$. In the context of YBCO, we conclude that the values of $\\xi$\nreported to date for bidirectional CDW order are, prima facie, too short to\naccount for the observed Fermi surface reconstruction; on the other hand, the\nvalues of $\\xi$ for the unidirectional CDW are just long enough.", "category": "cond-mat_str-el" }, { "text": "A look at the crossover region between BCS superconductivity and Bose\n Einstein Condensation: Pair fluctuation theory has been used to study the crossover from the weak\ncoupling BCS theory to the strong coupling Bose Einstein Condensation. The\neffect of fluctuations has been studied over the whole crossover regime. It has\nbeen shown that the pair fluctuations are enhanced considerably in both two and\nthree dimensions and hence mean field theory is inadequate to study the\nphysical properties in this regime. A self consistent scheme for calculating\nthe pair susceptibility is given.", "category": "cond-mat_str-el" }, { "text": "Magnetic field dependence of the many-electron states in a magnetic\n quantum dot: The ferromagnetic-antiferromagnetic transition: The electron-electron correlations in a many-electron (Ne = 1, 2,..., 5)\nquantum dot confined by a parabolic potential is investigated in the presence\nof a single magnetic ion and a perpendicular magnetic field. We obtained the\nenergy spectrum and calculated the addition energy which exhibits cusps as\nfunction of the magnetic field. The vortex properties of the many-particle wave\nfunction of the ground state are studied and for large magnetic fields are\nrelated to composite fermions. The position of the impurity influences strongly\nthe spin pair correlation function when the external field is large. In small\napplied magnetic field, the spin exchange energy together with the Zeeman terms\nleads to a ferromagnetic-antiferromagnetic(FM-AFM) transition. When the\nmagnetic ion is shifted away from the center of the quantum dot a remarkable\nre-entrant AFM-FM-AFM transition is found as function of the strength of the\nCoulomb interaction. Thermodynamic quantities as the heat capacity, the\nmagnetization, and the susceptibility are also studied. Cusps in the energy\nlevels show up as peaks in the heat capacity and the susceptibility.", "category": "cond-mat_str-el" }, { "text": "Thermodynamic investigations in the precursor region of FeGe: High-resolution DC magnetization and AC-specific heat data of the cubic\nhelimagnet FeGe have been measured as function of temperature and magnetic\nfield. The magnetization data as well as the isothermal susceptibility data\nconfirm the complexity of the magnetic phase diagram in the vicinity of the\nonset of long-rang magnetic order (Tc = 278.5 K) and the existence of a\nsegmented A-phase region. Moreover, these data revealed independent and clear\nindications of phase boundaries and crossovers within the A-phase region.\nTogether with the anomalies in the specific-heat data around Tc and at small\nmagnetic fields (H < 600 Oe) a complex magnetic phase diagram of FeGe is\nobtained.", "category": "cond-mat_str-el" }, { "text": "Formation of energy gap in higher dimensional spin-orbital liquids: A Schwinger boson mean field theory is developed for spin liquids in a\nsymmetric spin-orbital model in higher dimensions. Spin, orbital and coupled\nspin-orbital operators are treated equally. We evaluate the dynamic correlation\nfunctions and collective excitations spectra. As the collective excitations\nhave a finite energy gap, we conclude that the ground state is a spin-orbital\nliquid with a two-fold degeneracy, which breaks the discrete spin-orbital\nsymmetry. Possible relevence of this spin liquid state to several realistic\nsystems, such as CaV$_4$V$_9$ and Na$_2$Sb$_2$Ti$_2$O, are discussed.", "category": "cond-mat_str-el" }, { "text": "Emergent Anisotropic Non-Fermi Liquid at a Topological Phase Transition\n in Three Dimensions: Understanding correlation effects in topological phases and their transitions\nis a cutting-edge area of research in recent condensed matter physics. We study\ntopological quantum phase transitions (TQPTs) between double-Weyl semimetals\n(DWSMs) and insulators, and argue that a novel class of quantum criticality\nappears at the TQPT characterized by emergent anisotropic non-Fermi liquid\nbehaviors, in which the interplay between the Coulomb interaction and\nelectronic critical modes induces not only anisotropic renormalization of the\nCoulomb interaction but also strongly correlated electronic excitation in three\nspatial dimensions. Using the standard renormalization group methods, large\n$N_f$ theory and the $\\epsilon= 4-d$ method with fermion flavor number $N_f$\nand spatial dimension $d$, we obtain the anomalous dimensions of electrons\n($\\eta_f=0.366/N_f $) in large $N_f$ theory and the associated anisotropic\nscaling relations of various physical observables. Our results may be observed\nin candidate materials for DWSMs such as HgCr$_2$Se$_4$ or SrSi$_2$ when the\nsystem undergoes a TQPT.", "category": "cond-mat_str-el" }, { "text": "Dynamical Effects from Anomaly: Modified Electrodynamics in Weyl\n Semimetal: We discuss the modified quantum electrodynamics from a time-reversal-breaking\nWeyl semimetal coupled with a $U(1)$ gauge (electromagnetic) field. A key role\nis played by the soft dispersion of the photons in a particular direction, say\n$\\hat{z}$, due to the Hall conductivity of the Weyl semimetal. Due to the soft\nphoton, the fermion velocity in $\\hat{z}$ is logarithmically reduced under\nrenormalization group flow, together with the fine structure constant.\nMeanwhile, fermions acquire a finite lifetime from spontaneous emission of the\nsoft photon, namely the Cherenkov radiation. At low energy $E$, the inverse of\nthe fermion lifetime scales as $\\tau^{-1}\\sim E/{\\rm PolyLog}(E)$. Therefore,\neven though fermion quasiparticles are eventually well-defined at very low\nenergy, over a wide intermediate energy window the Weyl semimetal behaves like\na marginal Fermi liquid. Phenomenologically, our results are more relevant for\nemergent Weyl semimetals, where the fermions and photons all emerge from\nstrongly correlated lattice systems. Possible experimental implications are\ndiscussed.", "category": "cond-mat_str-el" }, { "text": "Model for the Magnetic Order and Pairing Channels in Fe Pnictide\n Superconductors: A two-orbital model for Fe-pnictide superconductors is investigated using\ncomputational techniques on two-dimensional square clusters. The hopping\namplitudes are derived from orbital overlap integrals, or by band structure\nfits, and the spin frustrating effect of the plaquette-diagonal Fe-Fe hopping\nis remarked. A spin 'striped' state is stable in a broad range of couplings in\nthe undoped regime, in agreement with neutron scattering. Adding two electrons\nto the undoped ground state of a small cluster, the dominant pairing operators\nare found. Depending on parameters, two pairing operators were identified: they\ninvolve inter-xz-yz orbital combinations forming spin singlets or triplets,\ntransforming according to the B_2g and A_2g representations of the D_4h group,\nrespectively.", "category": "cond-mat_str-el" }, { "text": "Light-induced magnetization driven by interorbital charge motion in a\n spin-orbit assisted Mott insulator alpha-RuCl3: In a honeycomb-lattice spin-orbit assisted Mott insulator {\\alpha}-RuCl3, an\nultrafast magnetization is induced by circularly polarized excitation below the\nMott gap. Photo-carriers play an important role, which are generated by turning\ndown the synergy of the on-site Coulomb interaction and the spin-orbit\ninteraction realizing the insulator state. An ultrafast 6- fs measurement of\nphoto-carrier dynamics and a quantum mechanical analysis clarify the mechanism,\naccording to which the magnetization emerges from a coherent charge motion\nbetween different t2g orbitals (dyz-dxz-dxy) of Ru3+ ions. This ultrafast\nmagnetization is weakened in the antiferromagnetic (AF) phase, which is\nopposite to the general tendency that the inverse Faraday effect is larger in\nAF compounds than in paramagnetic ones. This temperature dependence indicates\nthat the interorbital charge motion is affected by pseudo-spin rotational\nsymmetry breaking in the AF phase.", "category": "cond-mat_str-el" }, { "text": "Wigner-molecule supercrystal in transition-metal dichalcogenide moir\u00e9\n superlattices: Lessons from the bottom-up approach: The few-body problem for $N=4$ fermionic charge carriers in a double-well\nmoir\\'{e} quantum dot (MQD), representing the first step in a bottom-up\nstrategy to investigate formation of molecular supercrystals in transition\nmetal dichalcogenide (TMD) moir\\'e superlattices with integral fillings, $\\nu >\n1$, is solved exactly by employing large-scale exact-diagonalization via full\nconfiguration interaction (FCI) computations. A comparative analysis with the\nmean-field solutions of the often used spin-and-space unrestricted Hartree Fock\n(sS-UHF) demonstrates the limitations of the UHF method (by itself) to provide\na proper description of the influence of the interdot Coulomb interaction. In\nparticular, it is explicitly shown for $\\nu=2$ that the exact charge densities\n(CDs) within each MQD retain the ring-like shape characteristic (for a wide\nrange of relevant parameters) of a fully isolated MQD, as was found for sliding\nWigner molecules (WMs). This deeply quantum-mechanical behavior contrasts\nsharply with the UHF CDs that portray solely orientationally pinned and well\nlocalized dumbbell dimers. An improved CD, which agrees with the FCI-calculated\none, derived from the restoration of the sS-UHF broken parity symmetries is\nfurther introduced, suggesting a beyond-mean-field methodological roadmap for\ncorrecting the sS-UHF results. It is conjectured that the conclusions for the\n$\\nu=2$ moir\\'e TMD superlattice case extend to all cases with integral\nfillings that are associated with sliding WMs in isolated MQDs. The case of\n$\\nu=3$, associated with a pinned WM in isolated MQDs, is an exception.", "category": "cond-mat_str-el" }, { "text": "Excitation Spectra and Thermodynamic Response of Segmented Heisenberg\n Spin Chains: The spectral and thermodynamic response of segmented quantum spin chains is\nanalyzed using a combination of numerical techniques and finite-size scaling\narguments. Various distributions of segment lengths are considered, including\nthe two extreme cases of quenched and annealed averages. As the impurity\nconcentration is increased, it is found that (i) the integrated spectral weight\nis rapidly reduced, (ii) a pseudo-gap feature opens up at small frequencies,\nand (iii) at larger frequencies a discrete peak structure emerges, dominated by\nthe contributions of the smallest cluster segments. The corresponding\nlow-temperature thermodynamic response has a divergent contribution due to the\nodd-site clusters and a sub-dominant exponentially activated component due to\nthe even-site segments whose finite-size gap is responsible for the spectral\nweight suppression at small frequencies. Based on simple scaling arguments,\napproximate low-temperature expressions are derived for the uniform\nsusceptibility and the heat capacity. These are shown to be in good agreement\nwith numerical solutions of the Bethe ansatz equations for ensembles of\nopen-end chains.", "category": "cond-mat_str-el" }, { "text": "Itinerant Quantum Critical Point with Fermion Pockets and Hot Spots: Metallic quantum criticality is among the central theme in the understanding\nof correlated electronic systems, and converging results between analytical and\nnumerical approaches are still under calling. In this work, we develop\nstate-of-art large scale quantum Monte Carlo simulation technique and\nsystematically investigate the itinerant quantum critical point on a 2D square\nlattice with antiferromagnetic spin fluctuations at wavevector\n$\\mathbf{Q}=(\\pi,\\pi)$ -- a problem that resembles the Fermi surface setup and\nlow-energy antiferromagnetic fluctuations in high-Tc cuprates and other\ncritical metals, which might be relevant to their non-Fermi-liquid behaviors.\nSystem sizes of $60\\times 60 \\times 320$ ($L \\times L \\times L_\\tau$) are\ncomfortably accessed, and the quantum critical scaling behaviors are revealed\nwith unprecedingly high precision. We found that the antiferromagnetic spin\nfluctuations introduce effective interactions among fermions and the fermions\nin return render the bare bosonic critical point into a new universality,\ndifferent from both the bare Ising universality class and the\nHertz-Mills-Moriya RPA prediction. At the quantum critical point, a finite\nanomalous dimension $\\eta\\sim 0.125$ is observed in the bosonic propagator, and\nfermions at hot spots evolve into a non-Fermi-liquid. In the\nantiferromagnetically ordered metallic phase, fermion pockets are observed as\nenergy gap opens up at the hot spots. These results bridge the recent\ntheoretical and numerical developments in metallic quantum criticality and can\nbe served as the stepping stone towards final understanding of the 2D\ncorrelated fermions interacting with gapless critical excitations.", "category": "cond-mat_str-el" }, { "text": "First-order melting of a weak spin-orbit Mott insulator into a\n correlated metal: The electronic phase diagram of the weak spin-orbit Mott insulator\n(Sr(1-x)Lax)3Ir2O7 is determined via an exhaustive experimental study. Upon\ndoping electrons via La substitution, an immediate collapse in resistivity\noccurs along with a narrow regime of nanoscale phase separation comprised of\nantiferromagnetic, insulating regions and paramagnetic, metallic puddles\npersisting until x~0.04. Continued electron doping results in an abrupt,\nfirst-order phase boundary where the Neel state is suppressed and a homogenous,\ncorrelated, metallic state appears with an enhanced spin susceptibility and\nlocal moments. As the metallic state is stabilized, a weak structural\ndistortion develops and suggests a competing instability with the parent\nspin-orbit Mott state.", "category": "cond-mat_str-el" }, { "text": "Local Potential Functional Embedding Theory: A Self-Consistent Flavor of\n Density Functional Theory for Lattices without Density Functionals: The recently proposed Householder transformed density-matrix functional\nembedding theory (Ht-DMFET) [Sekaran et al., Phys. Rev. B 104, 035121 (2021)],\nwhich is equivalent to (but formally simpler than) density matrix embedding\ntheory (DMET) in the non-interacting case, is revisited from the perspective of\ndensity-functional theory (DFT). An in-principle-exact density-functional\nversion of Ht-DMFET is derived for the one-dimensional Hubbard lattice with a\nsingle embedded impurity. On the basis of well-identified density-functional\napproximations, a local potential functional embedding theory (LPFET) is\nformulated and implemented. Even though LPFET performs better than Ht-DMFET in\nthe low-density regime, in particular when electron correlation is strong, both\nmethods are unable to describe the density-driven Mott-Hubbard transition, as\nexpected. These results combined with our formally exact density-functional\nembedding theory reveal that a single statically embedded impurity can in\nprinciple describe the gap opening, provided that the complementary correlation\npotential (that describes the interaction of the embedding cluster with its\nenvironment, which is simply neglected in both Ht-DMFET and LPFET) exhibits a\nderivative discontinuity (DD) at half filling. The extension of LPFET to\nmultiple impurities (which would enable to circumvent the modeling of DDs) and\nits generalization to quantum chemical Hamiltonians are left for future work.", "category": "cond-mat_str-el" }, { "text": "Detection of long-range entanglement in gapped quantum spin liquids by\n local measurements: Topological order, reflected in long range patterns of entanglement, is\nquantified by the topological entanglement entropy (TEE) $\\gamma$. We show that\nfor gapped quantum spin liquids (QSL) it is possible to extract $\\gamma$ using\ntwo-spin local correlators. We demonstrate our method for the gapped\n$\\mathbb{Z}_2$ Kitaev spin liquid on a honeycomb lattice with anisotropic\ninteractions. We show that the $\\gamma = \\log 2$ for $\\mathbb{Z}_2$ topological\norder can be simply extracted from local two-spin correlators across two\ndifferent bonds, with an accuracy comparable or higher than the Kitaev-Preskill\nconstruction. This implies that the different superselection sectors of\n$\\mathbb{Z}_2$ gauge theory determined by global Wilson loop operators can be\nfully reflected locally in the matter majorana sector.", "category": "cond-mat_str-el" }, { "text": "Ring Exchange Mechanism for Triplet Superconductivity in a Two-Chain\n Hubbard Model: Possible Relevance to Bechgaard Salts: The density-matrix renormalization group method is used to study the ground\nstate of the two-chain zigzag-bond Hubbard model at quarter filling. We show\nthat, with a proper choice of the signs of hopping integrals, the ring exchange\nmechanism yields ferromagnetic spin correlations between interchain neighboring\nsites, and produces the attractive interaction between electrons as well as the\nlong-range pair correlations in the spin-triplet channel, thereby leading the\nsystem to triplet superconductivity. We argue that this novel mechanism may\nhave possible relevance to observed superconductivity in Bechgaard salts.", "category": "cond-mat_str-el" }, { "text": "Geometrical quadrupolar frustration in DyB$_4$: Physical properties of DyB$_4$ have been studied by magnetization, specific\nheat, and ultrasonic measurements. The magnetic entropy change and the\nultrasonic properties in the intermediate phase II indicate that the degeneracy\nof internal degrees of freedom is not fully lifted in spite of the formation of\nmagnetic order. The ultrasonic attenuation and the huge softening of $C_{44}$\nin phase II suggests existence of electric-quadrupolar (orbital) fluctuations\nof the 4$f$-electron. These unusual properties originate from the geometrical\nquadrupolar frustration.", "category": "cond-mat_str-el" }, { "text": "Signatures of a topological Weyl loop in Co$_3$Sn$_2$S$_2$: The search for novel topological phases of matter in quantum magnets has\nemerged as a frontier of condensed matter physics. Here we use state-of-the-art\nangle-resolved photoemission spectroscopy (ARPES) to investigate single\ncrystals of Co$_3$Sn$_2$S$_2$ in its ferromagnetic phase. We report for the\nfirst time signatures of a topological Weyl loop. From fundamental symmetry\nconsiderations, this magnetic Weyl loop is expected to be gapless if spin-orbit\ncoupling (SOC) is strictly zero but gapped, with possible Weyl points, under\nfinite SOC. We point out that high-resolution ARPES results to date cannot\nunambiguously resolve the SOC gap anywhere along the Weyl loop, leaving open\nthe possibility that Co$_3$Sn$_2$S$_2$ hosts zero Weyl points or some non-zero\nnumber of Weyl points. On the surface of our samples, we further observe a\npossible Fermi arc, but we are unable to clearly verify its topological nature\nusing the established counting criteria. As a result, we argue that from the\npoint of view of photoemission spectroscopy the presence of Weyl points and\nFermi arcs in Co$_3$Sn$_2$S$_2$ remains ambiguous. Our results have\nimplications for ongoing investigations of Co$_3$Sn$_2$S$_2$ and other\ntopological magnets.", "category": "cond-mat_str-el" }, { "text": "Asymptotic Freedom and Large Spin Antiferromagnetic Chains: Building on the mapping of large-$S$ spin chains onto the O($3$) nonlinear\n$\\sigma$ model with coupling constant $2/S$, and on general properties of that\nmodel (asymptotic freedom, implying that perturbation theory is valid at high\nenergy, and Elitzur's conjecture that rotationally invariant quantities are\ninfrared finite in perturbation theory), we use the Holstein-Primakoff\nrepresentation to derive analytic expressions for the equal-time and dynamical\nspin-spin correlations valid at distances smaller than $S^{-1} \\exp(\\pi S)$ or\nat energies larger than $J S^2 \\exp(-\\pi S)$, where $J$ is the Heisenberg\nexchange coupling. This is supported by comparing the static correlations with\nquantum Monte Carlo simulations for $S = 5/2$.", "category": "cond-mat_str-el" }, { "text": "Imaginary part of Hall conductivity in tilted doped Weyl semimetal with\n both broken time reversal and inversion symmetry: We consider a Weyl semimetal (WSM) with finite doping and tilt within a\ncontinuum model Hamiltonian with both broken time reversal and inversion\nsymmetry. We calculate the absorptive part of the anomalous AC Hall\nconductivity as a function of photon energy ($\\Omega$) for both type I and type\nII Weyl semimetal. For a given Weyl node, changing the sign of its chirality or\nof its tilt changes the sign of its contribution to the absorptive Hall\nconductivity with no change in magnitude. For a noncentrosymmetric system we\nfind that there are ranges of photon energies for which only the positive or\nonly the negative chirality node contributes to the imaginary (absorptive) part\nof the Hall conductivity. There are also other photon energies where both\nchirality contribute and there can be other ranges of $\\Omega$ where there is\nno absorption associated with the AC Hall conductivity in type I and regions\nwhere it is instead constant for type II. We comment on implications for the\nabsorption of circular polarized light.", "category": "cond-mat_str-el" }, { "text": "Orbital Polarization in Strained LaNiO$_{3}$: Structural Distortions and\n Correlation Effects: Transition-metal heterostructures offer the fascinating possibility of\ncontrolling orbital degrees of freedom via strain. Here, we investigate\ntheoretically the degree of orbital polarization that can be induced by\nepitaxial strain in LaNiO$_3$ films. Using combined electronic structure and\ndynamical mean-field theory methods we take into account both structural\ndistortions and electron correlations and discuss their relative influence. We\nconfirm that Hund's rule coupling tends to decrease the polarization and point\nout that this applies to both the $d^8\\underline{L}$ and $d^7$ local\nconfigurations of the Ni ions. Our calculations are in good agreement with\nrecent experiments, which revealed sizable orbital polarization under tensile\nstrain. We discuss why full orbital polarization is hard to achieve in this\nspecific system and emphasize the general limitations that must be overcome to\nachieve this goal.", "category": "cond-mat_str-el" }, { "text": "Comment on \"Origin of Giant Optical Nonlinearity in\n Charge-Transfer--Mott Insulators: A New Paradigm for Nonlinear Optics\": Comment on Phys. Rev. Lett. 86, 2086 (2001)", "category": "cond-mat_str-el" }, { "text": "Ineffectiveness of the Dzyaloshinskii-Moriya interaction in the\n dynamical quantum phase transition in the ITF model: Quantum phase transition occurs at a quantum critical value of a control\nparameter such as the magnetic field in the Ising model in a transverse\nmagnetic field (ITF). Recently, it is shown that ramping across the quantum\ncritical point generates non-analytic behaviors in the time evolution of a\nclosed quantum system in the thermodynamic limit at zero temperature. The\nmentioned phenomenon is called the dynamical quantum phase transition (DQPT).\nHere, we consider the one-dimensional (1D) ITF model with added the\nDzyaloshinskii-Moriya interaction (DMI). Using the fermionization technique,\nthe Hamiltonian is exactly diagonalized. Although the DM interaction induces\nchiral phase in the ground state phase diagram of the model, the study of the\nrate function of the return probability has proven that the DMI does not affect\nin the DQPT. We conclude accordingly that the ramping across the quantum\ncritical point is not a necessary and sufficient condition for DQPT.", "category": "cond-mat_str-el" }, { "text": "Strong correlation induced charge localization in antiferromagnets: The fate of an injected hole in a Mott antiferromagnet is an outstanding\nissue of strongly correlated physics. It provides important insights into doped\nMott insulators closely related to high-temperature superconductivity in\ncuprates. Here, we report a systematic numerical study based on the density\nmatrix renormalization group (DMRG). It reveals a remarkable novelty and\nsurprise for the single hole's motion in otherwise well-understood Mott\ninsulators. Specifically, we find that the charge of the hole is self-localized\nby a novel quantum interference mechanism purely of strong correlation origin,\nin contrast to Anderson localization due to disorders. The common belief of\nquasiparticle picture is invalidated by the charge localization concomitant\nwith spin-charge separation: the spin of the doped hole is found to remain a\nmobile object. Our findings unveil a new paradigm for doped Mott insulators\nthat emerges already in the simplest single hole case.", "category": "cond-mat_str-el" }, { "text": "Hartree-Fock study of the moir\u00e9 Hubbard model for twisted bilayer\n transition metal dichalcogenides: Twisted bilayer transition metal dichalcogenides have emerged as important\nmodel systems for the investigation of correlated electron physics because\ntheir interaction strength, carrier concentration, band structure, and\ninversion symmetry breaking are controllable by device fabrication, twist\nangle, and most importantly, gate voltage, which can be varied in situ. The low\nenergy physics of some of these materials has been shown to be described by a\n\"moir\\'e Hubbard model\" generalized from the usual Hubbard model by the\naddition of strong, tunable spin orbit coupling and inversion symmetry\nbreaking. In this work, we use a Hartree-Fock approximation to reach a\ncomprehensive understanding of the moir\\'e Hubbard model on the mean field\nlevel. We determine the magnetic and metal-insulator phase diagrams, and assess\nthe effects of spin orbit coupling, inversion symmetry breaking, and the\ntunable van Hove singularity. We also consider the spin and orbital effects of\napplied magnetic fields. This work provides guidance for experiments and sets\nthe stage for beyond mean-field calculations.", "category": "cond-mat_str-el" }, { "text": "Vortex creation and control in the Kitaev spin liquid by local bond\n modulations: The Kitaev model realizes a quantum spin liquid where the spin excitations\nare fractionalized into itinerant Majorana fermions and localized\n$\\mathbb{Z}_2$ vortices. Quantum entanglement between the fractional\nexcitations can be utilized for decoherence-free topological quantum\ncomputation. Of particular interest is the anyonic statistics realized by\nbraiding the vortex excitations under a magnetic field. Despite the promising\npotential, the practical methodology for creation and control of the vortex\nexcitations remains elusive thus far. Here we theoretically propose how one can\ncreate and move the vortices in the Kitaev spin liquid. We find that the\nvortices are induced by a local modulation of the exchange interaction;\nespecially, the local Dzyaloshinskii-Moriya (symmetric off-diagonal)\ninteraction can create vortices most efficiently in the (anti)ferromagnetic\nKitaev model, as it effectively flips the sign of the Kitaev interaction. We\ntest this idea by performing the {\\it ab initio} calculation for a candidate\nmaterial $\\alpha$-RuCl$_3$ through the manipulation of the ligand positions\nthat breaks the inversion symmetry and induces the local Dzyaloshinskii-Moriya\ninteraction. We also demonstrate a braiding of vortices by adiabatically and\nsuccessively changing the local bond modulations.", "category": "cond-mat_str-el" }, { "text": "Reply to Comment on \"Magnetotransport signatures of a single nodal\n electron pocket constructed from Fermi arcs\": In a recent manuscript, we showed how an electron pocket in the shape of a\ndiamond with concave sides could potentially explain changes in sign of the\nHall coefficient R_H in the underdoped high-Tc cuprates as a function of\nmagnetic field and temperature. For simplicity, this Fermi surface is assumed\nto be constructed from arcs of a circle connected at vertices which is an idea\nborrowed from Banik and Overhauser. Such a diamond-shaped pocket is proposed to\nbe the product of biaxial charge-density wave order, which was subsequently\nconfirmed in x-ray scattering experiments. Since those x-ray scattering\nexperiments were performed, the biaxial Fermi surface reconstruction scheme has\ngarnered widespread support in the scientific literature. It has been shown to\naccurately account for the cross-section of the Fermi surface pocket observed\nin quantum oscillation measurements, the sign and behavior of the Hall\ncoefficient, the size of the high magnetic field electronic contribution to the\nheat capacity and more recently the form of the angle-dependent\nmagnetoresistance.In their comment, Chakravarty and Wang raise several\nimportant questions relating to the validity of the Hall coefficient we\ncalculated for such a diamond-shaped Fermi surface pocket. These questions\nconcern specifically (1) whether a change in sign of the Hall coefficient R_H\nwith magnetic field and temperature is dependent on a `special' form for the\nrounding of the vertices, (2) whether a pocket of such a geometry can produce\nquantum oscillations in R_H in the absence of other Fermi surface sections and\n(3) whether a reconstructed Fermi surface consisting of a single pocket is less\n`natural' than one consisting of multiple pockets. Below we consider each of\nthese in turn.", "category": "cond-mat_str-el" }, { "text": "Supersymmetric Approach to Heavy-Fermion Systems: We present a new supersymmetric approach to the Kondo lattice model in order\nto describe simultaneously the quasiparticle excitations and the low-energy\nmagnetic fluctuations in heavy-Fermion systems. This approach mixes the\nfermionic and the bosonic representation of the spin following the standard\nrules of superalgebra. Our results show the formation of a bosonic band within\nthe hybridization gap reflecting the spin collective modes. The density of\nstates at the Fermi level is strongly renormalized while the Fermi surface sum\nrule includes $n_{c}+1$ states. The dynamical susceptibility is made of a Fermi\nliquid superimposed on a localized magnetism contribution.", "category": "cond-mat_str-el" }, { "text": "Electrically controllable magnetic order in the bilayer Hubbard model on\n honeycomb lattice --- a determinant quantum Monte Carlo study: Layered antiferromagnetic spin density wave (LAF) state is one of the\nplausible ground states of charge neutral Bernal stacked bilayer graphene. In\nthis paper, we use determinant quantum Monte Carlo method to study the effect\nof the electric field on the magnetic order in bilayer Hubbard model on a\nhoneycomb lattice. Our results qualitatively support the LAF ground state found\nin the mean field theory. The obtained magnetic moments, however, are much\nsmaller than what are estimated in the mean field theory. As electric field\nincreases, the magnetic order parameter rapidly decreases.", "category": "cond-mat_str-el" }, { "text": "Thermal stability and irreversibility of skyrmion-lattice phases in\n Cu$_2$OSeO$_3$: Small angle neutron scattering measurements have been performed to study the\nthermodynamic stability of skyrmion-lattice phases in Cu$_2$OSeO$_3$. We found\nthat the two distinct skyrmion-lattice phases [SkX(1) and SkX(2) phases] can be\nstabilized through different thermal histories; by cooling from the\nparamagnetic phase under finite magnetic field, the SkX(2) phase is selected.\nOn the other hand, the 30$^{\\circ}$-rotated SkX(1) phase becomes dominant by\nheating the sample from the ordered conical phase under finite field. This\ndifference in stabilization is surprisingly similar to the irreversibility\nobserved in spin glasses. The zero-field cooling results in the co-existence of\nthe two phases. It is further found that once one of the skyrmion-lattice\nphases is formed, it is hardly destabilized. This indicates unusual thermal\nstability of the two skyrmion-lattice phases originating from an unexpectedly\nlarge energy barrier between them.", "category": "cond-mat_str-el" }, { "text": "Disorder effects in spiral spin liquids: Long-range spin textures,\n Friedel-like oscillations, and spiral spin glasses: Spiral spin liquids are correlated states of matter in which a frustrated\nmagnetic system evades order by fluctuating between a set of (nearly)\ndegenerate spin spirals. Here, we investigate the response of spiral spin\nliquids to quenched disorder in a $J_1$-$J_2$ honeycomb-lattice Heisenberg\nmodel. At the single-impurity level, we identify different\norder-by-quenched-disorder phenomena and analyze the ensuing spin textures. In\nparticular, we show that the latter generally display Friedel-like\noscillations, which encode direct information about the spiral contour, i.e.,\nthe classical ground-state manifold. At finite defect concentrations, we\nperform extensive numerical simulations and characterize the resulting phases\nat zero temperature. As a result, we find that the competition between\nincompatible order-by-quenched-disorder mechanisms can lead to spiral spin\nglass states already at low to moderate disorder. Finally, we discuss\nextensions of our conclusions to nonzero temperatures and higher-dimensional\nsystems, as well as their applications to experiments.", "category": "cond-mat_str-el" }, { "text": "The Composite Particle-Hole Spinor of the Lowest Landau Level: We propose to form a two-component effective field theory from L = (L_ce +\nL_ch)/2, where L_ce is the Lagrangian of composite electrons with a\nChern-Simons term, and L_ch is the particle-hole conjugate of L_ce - the\nLagrangian of composite holes. In the theory, the two-component fermion field\nphi is a composite particle-hole spinor coupled to an emergent effective gauge\nfield in the presence of a background electromagnetic field. The Chern-Simons\nterms for both the composite electrons and composite holes are exactly\ncancelled out, and a 1/2 pseudospin degree of freedom, which responses to the\nemergent gauge field the same way as the real spin to the electromagnetic\nfield, emerges automatically. Furthermore, the composite particle-hole spinor\ntheory has exactly the same form as the non-relativistic limit of the massless\nDirac composite fermion theory after expanded to the four-component form and\nwith a mass term added.", "category": "cond-mat_str-el" }, { "text": "Phase transitions in chiral magnets from Monte Carlo simulations: Motivated by the unusual temperature dependence of the specific heat in MnSi,\ncomprising a combination of a sharp first-order feature accompanied by a broad\nhump, we study the extended Heisenberg model with competing exchange $J$ and\nanisotropic Dzyaloshinskii-Moriya $D$ interactions in a broad range of ratio\n$D/J$. Utilizing classical Monte Carlo simulations we find an evolution of the\ntemperature dependence of the specific heat and magnetic susceptibility with\nvariation of $D/J$. Combined with an analysis of the Bragg intensity patterns,\nwe clearly demonstrate that the observed puzzling hump in the specific heat of\nMnSi originates from smearing out of the virtual ferromagnetic second order\nphase transition by helical fluctuations, which manifest themselves in the\ntransient multiple spiral state. These fluctuations finally condense into the\nhelical ordered phase via a first order phase transition as is indicated by the\nspecific heat peak. Thus the model demonstrates a crossover from a second-order\nto a first-order transition with increasing $D/J$. Upon further increasing\n$D/J$ another crossover from a first-order to a second-order transition takes\nplace in the system. Moreover, the results of the calculations clearly indicate\nthat these competing interactions are the primary factor responsible for the\nappearance of first order phase transitions in helical magnets with the\nDzyaloshinskii-Moriya (DM) interaction.", "category": "cond-mat_str-el" }, { "text": "Magnetic structure determination of Ca$_3$LiOsO$_6$ using neutron and\n x-ray scattering: We present a neutron and x-ray scattering investigation of Ca$_3$LiOsO$_6$, a\nmaterial predicted to host magnetic ordering solely through an extended\nsuperexchange pathway involving two anions, an interaction mechanism that has\nundergone relatively little investigation. This contrasts with the ubiquitous\nsuperexchange interaction mechanism involving a single anion that has well\ndefined and long standing rules. Despite the apparent 1D nature and triangular\nunits of magnetic osmium ions the onset of magnetic correlations has been\nobserved at a high temperature of 117 K in bulk measurements. We experimentally\ndetermine the magnetically ordered structure and show it to be long range and\nthree dimensional. Our results support the model of extended superexchange\ninteraction.", "category": "cond-mat_str-el" }, { "text": "Spin polarons in the t-J model in an unconstrained representation: The report discusses the slave-fermion representations of the t-J model and\ndescribes another representation, in which fermions and bosons are completely\ncommuting and in which the properties of fermions are directly related to the\nproperties of physical holes. For a study of the system in the new\nrepresentation at half-filling, interaction of fermions with two magnons is\ntreated in mean-field theory. The obtained effective model, in comparison to\nthat of the usual slave-fermion representation, has an additional bare hole\ndispersion due to the hole moving by using quantum spin fluctuations present in\nthe undoped antiferromagnetic ground state. The single-hole Green's function at\nhalf-filling is then found numerically using the self-consistent Born\napproximation. For all studied quantities good or excellent agreement with\nnumerical data is observed in the entire parameter range, noticeably better\nthan in the studies with the slave-fermion representation. Using the same\neffective model, the two-hole problem is also studied by solving numerically\nthe Bethe-Salpeter equation with noncrossing diagrams.", "category": "cond-mat_str-el" }, { "text": "DMRG Approach to Optimizing Two-Dimensional Tensor Networks: Tensor network algorithms have been remarkably successful solving a variety\nof problems in quantum many-body physics. However, algorithms to optimize\ntwo-dimensional tensor networks known as PEPS lack many of the aspects that\nmake the seminal density matrix renormalization group (DMRG) algorithm so\npowerful for optimizing one-dimensional tensor networks known as matrix product\nstates. We implement a framework for optimizing two-dimensional PEPS tensor\nnetworks which includes all of steps that make DMRG so successful for\noptimizing one-dimension tensor networks. We present results for several 2D\nspin models and discuss possible extensions and applications.", "category": "cond-mat_str-el" }, { "text": "Results on the symmetries of integrable fermionic models on chains: We investigate integrable fermionic models within the scheme of the graded\nQuantum Inverse Scattering Method, and prove that any symmetry imposed on the\nsolution of the Yang-Baxter Equation reflects on the constants of motion of the\nmodel; generalizations with respect to known results are discussed. This\ntheorem is shown to be very effective when combined with the Polynomial\n$\\Rc$-matrix Technique (PRT): we apply both of them to the study of the\nextended Hubbard models, for which we find all the subcases enjoying several\nkinds of (super)symmetries. In particular, we derive a geometrical construction\nexpressing any $gl(2,1)$-invariant model as a linear combination of EKS and\nU-supersymmetric models. Furtherly, we use the PRT to obtain 32 integrable\n$so(4)$-invariant models. By joint use of the Sutherland's Species technique\nand $\\eta$-pairs construction we propose a general method to derive their\nphysical features, and we provide some explicit results.", "category": "cond-mat_str-el" }, { "text": "Multiple supersonic phase fronts launched at a complex-oxide\n hetero-interface: Selective optical excitation of a substrate lattice can drive phase changes\nacross hetero-interfaces. This phenomenon is a non-equilibrium analogue of\nstatic strain control in heterostructures and may lead to new applications in\noptically controlled phase change devices. Here, we make use of time-resolved\nnon-resonant and resonant x-ray diffraction to clarify the underlying physics,\nand to separate different microscopic degrees of freedom in space and time. We\nmeasure the dynamics of the lattice and that of the charge disproportionation\nin NdNiO3, when an insulator-metal transition is driven by coherent lattice\ndistortions in the LaAlO3 substrate. We find that charge redistribution\npropagates at supersonic speeds from the interface into the NdNiO3 film,\nfollowed by a sonic lattice wave. When combined with measurements of magnetic\ndisordering and of the metal-insulator transition, these results establish a\nhierarchy of events for ultrafast control at complex oxide hetero-interfaces.", "category": "cond-mat_str-el" }, { "text": "Monte Carlo modeling the phase diagram of magnets with the\n Dzyaloshinskii - Moriya interaction: We use classical Monte Carlo calculations to model the high-pressure behavior\nof the phase transition in the helical magnets. We vary values of the exchange\ninteraction constant J and the Dzyaloshinskii-Moriya interaction constant D,\nwhich is equivalent to changing spin-spin distances, as occurs in real systems\nunder pressure. The system under study is self-similar at D/ J = constant, and\nits properties are defined by the single variable J / T , where T is\ntemperature. The existence of the first order phase transition critically\ndepends on the ratio D / J. A variation of J strongly affects the phase\ntransition temperature and width of the fluctuation region (the hump) as\nfollows from the system self-similarity. The high-pressure behavior of the spin\nsystem depends on the evolution of the interaction constants J and D on\ncompression. Our calculations are relevant to the high pressure phase diagrams\nof helical magnets MnSi and Cu2OSeO3.", "category": "cond-mat_str-el" }, { "text": "Fermionic Monte Carlo study of a realistic model of twisted bilayer\n graphene: The rich phenomenology of twisted bilayer graphene (TBG) near the magic angle\nis believed to arise from electron correlations in topological flat bands. An\nunbiased approach to this problem is highly desirable, but also particularly\nchallenging, given the multiple electron flavors, the topological obstruction\nto defining tight binding models and the long-ranged Coulomb interactions.\nWhile numerical simulations of realistic models have thus far been confined to\nzero temperature, typically excluding some spin or valley species, analytic\nprogress has relied on fixed point models away from the realistic limit. Here\nwe present for the first time unbiased Monte Carlo simulations of realistic\nmodels of magic angle TBG at charge-neutrality. We establish the absence of a\nsign problem for this model in a momentum space approach, and describe a\ncomputationally tractable formulation that applies even on breaking chiral\nsymmetry and including band dispersion. Our results include (i) the emergence\nof an insulating Kramers inter-valley coherent ground state in competition with\na correlated semi-metal phase, (ii) detailed temperature evolution of order\nparameters and electronic spectral functions which reveal a `pseudogap' regime,\nin which gap features are established at a higher temperature than the onset of\norder and (iii) predictions for electronic tunneling spectra and their\nevolution with temperature. Our results pave the way towards uncovering the\nphysics of magic angle graphene through exact simulations of over a hundred\nelectrons across a wide temperature range.", "category": "cond-mat_str-el" }, { "text": "Inelastic neutron scattering studies of the quantum frustrated magnet\n clinoatacamite, $\u03b3$-Cu2(OD)3Cl, a proposed valence bond solid (VBS): The frustrated magnet clinoatacamite, $\\gamma$-Cu$_2$(OH)$_3$Cl, is\nattracting a lot of interest after suggestions that at low temperature it forms\nan exotic quantum state termed a Valence Bond Solid (VBS) made from dimerised\nCu$^{2+}$ ($S=1/2$) spins.\\cite{Lee_clinoatacamite} Key to the arguments\nsurrounding this proposal were suggestions that the kagom\\'e planes in the\nmagnetic pyrochlore lattice of clinoatacamite are only weakly coupled, causing\nthe system to behave as a quasi-2-dimensional magnet. This was reasoned from\nthe near 95$^\\circ$ angles made at the bridging oxygens that mediate exchange\nbetween the Cu ions that link the kagom\\'e planes.\n Recent work pointed out that this exchange model is inappropriate for\n$\\gamma$-Cu$_2$(OH)$_3$Cl, where the oxygen is present as a\n$\\mu_3$-OH.\\cite{Wills_JPC} Further, it used symmetry calculations and neutron\npowder diffraction to show that the low temperature magnetic structure ($T<6$\nK) was canted and involved significant spin ordering on all the Cu$^{2+}$\nspins, which is incompatible with the interpretation of simultaneous VBS and\nN\\'eel ordering. Correspondingly, clinoatacamite is best considered a distorted\npyrochlore magnet. In this report we show detailed inelastic neutron scattering\nspectra and revisit the responses of this frustrated quantum magnet.", "category": "cond-mat_str-el" }, { "text": "Modified kagome physics in the natural spin-1/2 kagome lattice systems -\n kapellasite Cu3Zn(OH)6Cl2 and haydeeite Cu3Mg(OH)6Cl2: The recently discovered natural minerals Cu3Zn(OH)6Cl2 and Cu3Mg(OH)6Cl2 are\nspin 1/2 systems with an ideal kagome geometry. Based on electronic structure\ncalculations, we develop a realistic model which includes couplings across the\nkagome hexagons beyond the original kagome model that are intrinsic in real\nkagome materials. Exact diagonalization studies for the derived model reveal a\nstrong impact of these couplings on the magnetic ground state. Our predictions\ncould be compared to and supplied with neutron scattering, thermodynamic and\nNMR data.", "category": "cond-mat_str-el" }, { "text": "Freezing out of a low-energy bulk spin exciton in SmB6: The Kondo insulator SmB6 is purported to develop into a robust topological\ninsulator at low temperature. Yet there are several puzzling and unexplained\nphysical properties of the insulating bulk. It has been proposed that bulk spin\nexcitons may be the source of these anomalies and may also adversely affect the\ntopologically-protected metallic surface states. Here, we report muon spin\nrotation measurements of SmB6 that show thermally-activated behavior for the\ntemperature dependences of the transverse-field (TF) relaxation rate below 20 K\nand muon Knight shift below 5-6 K. Our data are consistent with the freezing\nout of a bulk low-energy (~ 1 meV) spin exciton concurrent with the appearance\nof metallic surface conductivity. Furthermore, our results support the idea\nthat spin excitons play some role in the anomalous low-temperature bulk\nproperties of SmB6.", "category": "cond-mat_str-el" }, { "text": "Rigorous Wilsonian Renormalization Group for impurity models with a\n spectral gap: The Anderson impurity model (AIM) has long served as a cornerstone in the\nstudy of correlated electron systems. While numerical renormalization group\n(RG) offers great flexibility for metallic reservoirs, it becomes impossible in\nan unbiased way when a spectral gap $\\Delta$ opens up in the tunneling density\nof states. The only known exception is provided by the superconducting bath. In\nthis paper, we lift these limitations by a novel numerical RG procedure that\nemploys a discretization of the gapped tunneling densities of states into\npatches which accumulate at the gap edges. This reveals an unusual double\nscaling which is a shared behavior by the superconducting and the scalar gapped\nAIMs. Moreover, it requires a special iterative diagonalization procedure with\nan alternating scheme for discarding states only every second iteration. The\ndiscretization and the diagonalization scheme form together, what we refer to\nas, the log-gap numerical RG. It is successfully applied to the superconducting\nand to the scalar gapped AIM. Consequently, it reveals that both models belong\nto the same RG equivalence class which manifests physically in common\nsinglet-doublet quantum phase transitions accompanied by in-gap bound states of\ngiven parities. While superconducting AIM is mainly used for benchmarking of\nthe log-gap numerical RG, we also rigorously confirm the phenomenon of in-gap\nstates escaping into the continuum, which was recently indirectly considered in\nRef. [1]. The gapped AIM is then tackled in a first ever exact numerical RG\napproach and confirms quantitatively assertions based on models with auxiliary\nmetallic leads [2-5]. Moreover, it reveals that calculations performed in Refs.\n[6-8] are of strictly approximate nature.", "category": "cond-mat_str-el" }, { "text": "Quantization of fractional corner charge in $C_n$-symmetric higher-order\n topological crystalline insulators: In the presence of crystalline symmetries, certain topological insulators\npresent a filling anomaly: a mismatch between the number of electrons in an\nenergy band and the number of electrons required for charge neutrality. In this\npaper, we show that a filling anomaly can arise when corners are introduced in\n$C_n$-symmetric crystalline insulators with vanishing polarization, having as\nconsequence the existence of corner-localized charges quantized in multiples of\n$\\frac{e}{n}$. We characterize the existence of this charge systematically and\nbuild topological indices that relate the symmetry representations of the\noccupied energy bands of a crystal to the quanta of fractional charge robustly\nlocalized at its corners. When an additional chiral symmetry is present,\n$\\frac{e}{2}$ corner charges are accompanied by zero-energy corner-localized\nstates. We show the application of our indices in a number of atomic and\nfragile topological insulators and discuss the role of fractional charges bound\nto disclinations as bulk probes for these crystalline phases.", "category": "cond-mat_str-el" }, { "text": "Unconventional density wave in CeCoIn_5?: Very recently large Nernst effect and Seebeck effect were observed above the\nsuperconducting transition temperature 2.3K in a heavy fermion superconductor\nCeCoIn_5. We shall interpret this large Nernst effect in terms of\nunconventional density wave (UDW), which appears around T=18K. Also the\ntemperature dependence of the Seebeck coefficient below T=18K is described in\nterms of UDW. Another hallmark for UDW is the angular dependent\nmagnetoresistance, which should be readily accessible experimentally.", "category": "cond-mat_str-el" }, { "text": "Magnetic-Field-Independent Ultrasonic Dispersions in the Magnetically\n Robust Heavy Fermion System SmOs4Sb12: Elastic properties of the filled skutterudite compound SmOs$_4$Sb$_{12}$ have\nbeen investigated by ultrasonic measurements. The elastic constant\n$C_{11}(\\omega)$ shows two ultrasonic dispersions at $\\sim$15 K and $\\sim$53 K\nfor frequencies $\\omega$ between 33 and 316 MHz, which follow a Debye-type\nformula with Arrhenius-type temperature-dependent relaxation times, and remain\nunchanged even with applied magnetic fields up to 10 T. The corresponding\nactivation energies were estimated to be $E_2$ = 105 K and $E_1$ = 409 K,\nrespectively. The latter, $E_1$, is the highest value reported so far in the\nSb-based filled skutterudites. The presence of magnetically robust ultrasonic\ndispersions in SmOs$_4$Sb$_{12}$ implies a possibility that an emergence of a\nmagnetically insensitive heavy fermion state in this system is associated with\na novel local charge degree of freedom which causes the ultrasonic dispersion.", "category": "cond-mat_str-el" }, { "text": "Dynamical spin susceptibility in La2CuO4 studied by resonant inelastic\n x-ray scattering: Resonant inelastic X-ray scattering (RIXS) is a powerful probe of elementary\nexcitations in solids. It is now widely applied to study magnetic excitations.\nHowever, its complex cross-section means that RIXS has been more difficult to\ninterpret than inelastic neutron scattering (INS). Here we report\nhigh-resolution RIXS measurements of magnetic excitations of La2CuO4, the\nantiferromagnetic parent of one system of high-temperature superconductors. At\nhigh energies (~2 eV), the RIXS spectra show angular-dependent dd orbital\nexcitations which are found to be in good agreement with single-site multiplet\ncalculations. At lower energies (<0.3 eV), we show that the\nwavevector-dependent RIXS intensities are proportional to the product of the\nsingle-ion spin-flip cross section and the dynamical susceptibility of the\nspin-wave excitations. When the spin-flip crosssection is dividing out, the\nRIXS magnon intensities show a remarkable resemblance to INS data. Our results\nshow that RIXS is a quantitative probe the dynamical spin susceptibility in\ncuprate and therefore should be used for quantitative investigation of other\ncorrelated electron materials.", "category": "cond-mat_str-el" }, { "text": "Contiguous 3d and 4f magnetism: towards strongly correlated 3d electrons\n in YbFe2Al10: We present magnetization, specific heat, and 27Al NMR investigations on\nYbFe2Al10 over a wide range in temperature and magnetic field. The magnetic\nsusceptibility at low temperatures is strongly enhanced at weak magnetic\nfields, accompanied by a ln(T0/T) divergence of the low-T specific heat\ncoefficient in zero field, which indicates a ground state of correlated\nelectrons. From our hard X-ray photo emission spectroscopy (HAXPES) study, the\nYb valence at 50 K is evaluated to be 2.38. The system displays valence\nfluctuating behavior in the low to intermediate temperature range, whereas\nabove 400 K, Yb3+ carries a full and stable moment, and Fe carries a moment of\nabout 3.1 mB. The enhanced value of the Sommerfeld Wilson ratio and the dynamic\nscaling of spin-lattice relaxation rate divided by T [27(1/T1T)] with static\nsusceptibility suggests admixed ferromagnetic correlations. 27(1/T1T)\nsimultaneously tracks the valence fluctuations from the 4f -Yb ions in the high\ntemperature range and field dependent antiferromagnetic correlations among\npartially Kondo screened Fe 3d moments at low temperature, the latter evolve\nout of an Yb 4f admixed conduction band.", "category": "cond-mat_str-el" }, { "text": "Dynamics of a bond-disordered $S=1$ quantum magnet near $z=1$\n criticality: Neutron scattering is used to study\nNiCl$_{2-2x}$Br$_{2x}\\cdot$4SC(NH$_2$)$_2$, $x=0.06$, a bond-disordered\nmodification of the well-known gapped $S=1$ antiferromagnetic quantum spin\nsystem NiCl$_{2}\\cdot$4SC(NH$_2$)$_2$. The magnetic excitation spectrum\nthroughout Brillouin zone is mapped out at $T=60$ mK using high-resolution\ntime-of-flight spectroscopy. It is found that the dispersion of spin excitation\nis renormalized, as compared to that in the parent compound. The lifetime of\nexcitations near the bottom of the band is substantially decreased. No\nlocalized states are found below the gap energy $\\Delta\\simeq0.2$ meV. At the\nsame time, localized zero wave vector states are detected above the top of the\nband. The results are consistent with a more or less continuous random\ndistribution of bond strengths, and a discrete, possibly bimodal, distribution\nof single-ion anisotropies in the disordered material.", "category": "cond-mat_str-el" }, { "text": "Competing Correlated Insulators in multi-orbital systems coupled to\n phonons: We study the interplay between electron-electron interaction and a\nJahn-Teller phonon coupling in a two-orbital Hubbard model. We demonstrate that\nthe e-ph interaction coexists with the Mott localization driven by the Hubbard\nrepulsion U, but it competes with the Hund's coupling J. This interplay leads\nto two spectacularly different Mott insulators, a standard high-spin Mott\ninsulator with frozen phonons which is stable when the Hund's coupling\nprevails, and a low-spin Mott-bipolaronic insulator favoured by phonons, where\nthe characteristic features of Mott insulators and bipolarons coexist. The two\nphases are separated by a sharp boundary along which an intriguing intermediate\nsolution emerges as a kind of compromise between the two solutions.", "category": "cond-mat_str-el" }, { "text": "Orbital Disordering and metal-insulator transition with hole-doping in\n perovskite-type vanadium oxides: Filling-control metal-insulator transitions (MITs) and related electronic\nphase diagrams have been investigated for hole-doped vanadium oxides,\nPr_{1-x}Ca_xVO_3, Nd_{1-x}Sr_xVO_3 and Y_{1-x}Ca_xVO_3, with perovskite\nstructure. The increase of the doping level x causes the melting of the G-type\n(and C-type) orbital order, prior to or concomitantly with the MIT, due partly\nto the doped-hole motion and partly to the ramdom potential arising from the\nquenched disorder. In particular, the G-type spin- and C-type orbital-ordered\nphase present in Y_{1-x}Ca_xVO_3 disappears immediately upon hole doping,\naround x=0.02. On the other hand, the critical doping level x for MIT is\ngoverned by the electron-correlation strength of the undoped parent compound.", "category": "cond-mat_str-el" }, { "text": "Quantum criticality and the formation of a putative electronic liquid\n crystal in Sr3Ru2O7: We present a brief review of the physical properties of Sr3Ru2O7, in which\nthe approach to a magnetic-field-tuned quantum critical point is cut off by the\nformation of a novel phase with transport characteristics consistent with those\nof a nematic electronic liquid crystal. Our goal is to summarize the physics\nthat led to that conclusion being drawn, describing the key experiments and\ndiscussing the theoretical approaches that have been adopted. Throughout the\nreview we also attempt to highlight observations that are not yet understood,\nand to discuss the future challenges that will need to be addressed by both\nexperiment and theory.", "category": "cond-mat_str-el" }, { "text": "Gapless edges of 2d topological orders and enriched monoidal categories: In this work, we give a precise mathematical description of a fully chiral\ngapless edge of a 2d topological order (without symmetry). We show that the\nobservables on the 1+1D world sheet of such an edge consist of a family of\ntopological edge excitations, boundary CFT's and walls between boundary CFT's.\nThese observables can be described by a chiral algebra and an enriched monoidal\ncategory. This mathematical description automatically includes that of gapped\nedges as special cases. Therefore, it gives a unified framework to study both\ngapped and gapless edges. Moreover, the boundary-bulk duality also holds for\ngapless edges. More precisely, the unitary modular tensor category that\ndescribes the 2d bulk phase is exactly the Drinfeld center of the enriched\nmonoidal category that describes the gapless/gapped edge. We propose a\nclassification of all gapped and fully chiral gapless edges of a given bulk\nphase. In the end, we explain how modular-invariant bulk conformal field\ntheories naturally emerge on certain gapless walls between two trivial phases.", "category": "cond-mat_str-el" }, { "text": "An anomalous butterfly-shaped magnetoresistance loop in an alloy,\n Tb4LuSi3: Magnetic-field (H) induced first-order magnetic transition and the assiciated\nelectronic phase-separation phenomena are active topics of research in\nmagnetism. Magnetoresistance (MR) is a key property to probe these phenomena\nand, in literature, a butterfly-shaped MR loop has been noted while cycling the\nfield, with the envelope curve lying below the virgin curve in MR versus H\nplots of such materials. Here, we report an opposite behavior of MR loop for an\nalloy, Tb4LuSi3, at low temperatures (<<20 K) in the magnetically ordered\nstate. Such an anomalous curve reveals unexpected domination of higher\nresistive high-field phase in electronic conduction, unlike in other materials\nwhere conducion is naturally by low-resistive high-field phase that follows\nfirst-order transition. The observed features reveal an unusual electronic\nphase separation, namely involving high-resistive high-field phase and\nlow-resistive virgin phase.", "category": "cond-mat_str-el" }, { "text": "Spin-fluctuation mechanism of superconductivity in cuprates: The theory of superconductivity within the t-J model, as relevant for\ncuprates, is developed. It is based on the equations of motion for projected\nfermionic operators and the mode-coupling approximation for the self-energy\nmatrix. The dynamical spin susceptibility at various doping is considered as an\ninput, extracted from experiments. The analysis shows that the\nsuperconductivity onset is dominated by the spin-fluctuation contribution. We\nshow that T_c is limited by the spin-fluctuation scale $\\Gamma$ and shows a\npronounced dependence on the next-nearest-neighbor hopping t'. The latter can\noffer an explanation for the variation of T_c among different families of\ncuprates.", "category": "cond-mat_str-el" }, { "text": "Quantum and thermal effects in the double exchange ferromagnet: The physics of the ferromagnetic phase of the ``double exchange'' model has\nbeen widely discussed in the context of the CMR manganites. Usually, the double\nexchange ferromagnet is treated is classically, by mapping it onto an effective\nHeisenberg model. However this mapping does not permit a correct treatment of\nquantum or thermal fluctuation effects, and the results obtained lack many of\nthe interesting features seen in experiments on the manganites. Here we outline\na new analytic approach to systematically evaluating quantum and thermal\ncorrections to the magnetic and electronic properties of the double exchange\nferromagnet.", "category": "cond-mat_str-el" }, { "text": "Interacting Anisotropic Dirac Fermions in Strained Graphene and Related\n Systems: We study the role of long-range electron-electron interactions in a system of\ntwo-dimensional anisotropic Dirac fermions, which naturally appear in\nuniaxially strained graphene, graphene in external potentials, some strongly\nanisotropic topological insulators, and engineered anisotropic graphene\nstructures. We find that while for small interactions and anisotropy the system\nrestores the conventional isotropic Dirac liquid behavior, strong enough\nanisotropy can lead to the formation of a quasi-one dimensional electronic\nphase with dominant charge order (anisotropic excitonic insulator).", "category": "cond-mat_str-el" }, { "text": "Elastic Properties and Magnetic Phase Diagrams of Dense Kondo Compound\n Ce0.75La0.25B6: We have investigated the elastic properties of the cubic dense Kondo compound\nCe0.75La0.25B6 by means of ultrasonic measurements. We have obtained magnetic\nfields vs temperatures (H-T) phase diagrams under magnetic fields along the\ncrystallographic [001], [110] and [111] axes. An ordered phase IV showing the\nelastic softening of c44 locates in low temperature region between 1.6 and 1.1\nK below 0.7 T in all field directions. The phase IV shows an isotropic nature\nwith regard to the field directions, while the antiferro-magnetic phase III\nshows an anisotropic character. A remarkable softening of c44 and a spontaneous\ntrigonal distortion εyz+εzx+εxy recently reported by Akatsu et\nal. [J. Phys. Soc. Jpn. 72 (2003) 205] in the phase IV favor a ferro-quadrupole\n(FQ) moment of Oyz+Ozx+Oxy induced by an octupole ordering.", "category": "cond-mat_str-el" }, { "text": "Magnetism and fine electronic structure of UPd2Al3 and NpPd2Al3: We claim the existence of the f3 (U3+) configuration in UPd2Al3. It is in\nagreement with inelastic-neutron-scattering (INS) excitations and is consistent\nwith the trivalent neptunium configuration in NpPd2Al3. We have derived set of\nCEF parameters for the U3+ state that reproduces the INS excitations and\ntemperature dependence of the heat capacity. On basis of the crystal-field\ntheory, extended to Quantum Atomistic Solid State Theory we argue that the\nmagnetic moment of the uranium moment amounts at 0 K to 1.7-1.8 muB.\n Keywords: Crystalline Electric Field, Heavy fermion magnetism, UPd2Al3,\nNpPd2Al3\n PACS: 71.70.E, 75.10.D", "category": "cond-mat_str-el" }, { "text": "Temperature-dependent $f$-electron evolution in CeCoIn$_5$ via a\n comparative infrared study with LaCoIn$_5$: We investigated CeCoIn$_5$ and LaCoIn$_5$ single crystals, which have the\nsame HoCoGa$_5$-type tetragonal crystal structure, using infrared spectroscopy.\nHowever, while CeCoIn$_5$ has 4$f$ electrons, LaCoIn$_5$ does not. By comparing\nthese two material systems, we extracted the temperature-dependent electronic\nevolution of the $f$ electrons of CeCoIn$_5$. We observed that the differences\ncaused by the $f$ electrons are more obvious in low-energy optical spectra at\nlow temperatures. We introduced a complex optical resistivity and obtained a\nmagnetic optical resistivity from the difference in the optical resistivity\nspectra of the two material systems. From the temperature-dependent average\nmagnetic resistivity, we found that the onset temperature of the Kondo effect\nis much higher than the known onset temperature of Kondo scattering ($\\simeq$\n200 K) of CeCoIn$_5$. Based on momentum-dependent hybridization, the periodic\nAnderson model, and a maximum entropy approach, we obtained the hybridization\ngap distribution function of CeCoIn$_5$ and found that the resulting gap\ndistribution function of CeCoIn$_5$ was mainly composed of two (small and\nlarge) components (or gaps). We assigned the small and large gaps to the\nin-plane and out-of-plane hybridization gaps, respectively. We expect that our\nresults will provide useful information for understanding the\ntemperature-dependent electronic evolution of $f$-electron systems near Fermi\nlevel.", "category": "cond-mat_str-el" }, { "text": "Vortices in the presence of a nonmagnetic atom impurity in 2D XY\n ferromagnets: Using a model of nonmagnetic impurity potential, we have examined the\nbehavior of planar vortex solutions in the classical two-dimensional XY\nferromagnets in the presence of a spin vacancy localized out of the vortex\ncore. Our results show that a spinless atom impurity gives rise to an effective\npotential that repels the vortex structure.", "category": "cond-mat_str-el" }, { "text": "Antiferromagnetic spin-1/2 chains in (NO)Cu(NO3)3: a microscopic study: We report on the microscopic model of the recently synthesized\none-dimensional quantum magnet (NO)Cu(NO3)3. Applying density functional theory\nband structure calculations, we obtain a leading antiferromagnetic exchange\ncoupling J ~ 200 K, which runs via NO3 groups forming spin chains along the b\ndirection. Much weaker couplings J' ~ 2 K link the chains into layers in a\nnon-frustrated manner. Our calculations do not support the earlier conjecture\non an anisotropic frustrated square lattice physics in (NO)Cu(NO3)3. In\ncontrast, the model of uniform spin chains leads to a remarkably good fit of\nthe experimental magnetic susceptibility data, although the low-temperature\nfeatures of the intrinsic magnetic susceptibility measured by electron spin\nresonance might call for extension of the model. We outline possible\nexperiments to observe the suggested long-range magnetic ordering in\n(NO)Cu(NO3)3 and briefly compare this compound to other spin-1/2 uniform-chain\nsystems.", "category": "cond-mat_str-el" }, { "text": "Anisotropic pseudogap in the half-filling 2-d Hubbard model at finite T: We have studied the pseudogap formation in the single-particle spectra of the\nhalf-filling two-dimensional Hubbard model. Using a Green's function with the\none-loop self-energy correction of the spin and charge fluctuations, we have\nnumerically calculated the self-energy, the spectral function, and the density\nof states in the weak-coupling regime at finite temperature. Pseudogap\nformations have been observed in both the density of states and the spectral\nfunction at the Fermi level. The pseudogap in the spectral function is\nexplained by the non-Fermi-liquid-like nature of the self-energy. The anomalous\nbehavior in the self-energy is caused by both the strong antiferromagnetic spin\nfluctuation and the nesting condition on the non-interacting Fermi surface. In\nthe present approximation, we find a logarithmic singularity in the integrand\nof the imaginary part of the self-energy. Concerning the energy dependence of\nthe spectral function and the self-energy, two theorems are proved. They give a\nnecessary condition in the self-energy to produce the pseudogap at the Fermi\nlevel. The pseudogap in the spectral function is highly momentum dependent on\nthe Fermi surface. It opens initially in the $(\\pm \\pi,0)$, $(0,\\pm \\pi)$\nregions as the normal state pseudogap observed in the high-$T_c$\nsuperconductors and if the interaction is increased, it spreads to other Fermi\nsurface sectors. The anisotropy of the pseudogap is produced by the low-energy\nenhancement of the spin excitation around ${\\bf Q}=(\\pi,\\pi)$ and the flatness\nof the band dispersion around the saddle point.", "category": "cond-mat_str-el" }, { "text": "Fermionic superfluidity: From high Tc superconductors to ultracold Fermi\n gases: We present a pairing fluctuation theory which self-consistently incorporates\nfinite momentum pair excitations in the context of BCS--Bose-Einstein\ncondensation (BEC) crossover, and we apply this theory to high $T_c$\nsuperconductors and ultracold Fermi gases. There are strong similarities\nbetween Fermi gases in the unitary regime and high Tc superconductors. Here we\naddress key issues of common interest, especially the pseudogap. In the Fermi\ngases we summarize recent experiments including various phase diagrams (with\nand without population imbalance), as well as evidence for a pseudogap in\nthermodynamic and other experiments.", "category": "cond-mat_str-el" }, { "text": "Renormalized perturbation calculations for the single impurity Anderson\n model: We illustrate the renormalized perturbation expansion method by applying it\nto a single impurity Anderson model. Previously, we have shown that this\napproach gives the {\\it exact} leading order results for the specific heat,\nspin and charge susceptibilities and leading order temperature dependence of\nthe resistivity for this model in the Fermi-liquid regime, when carried out to\nsecond order in the renormalized interaction $\\tilde U$. Here we consider the\neffects of higher order quasi-particle scattering and calculate the third order\ncontributions to the $H^3$ term in the impurity magnetization for the symmetric\nmodel in a weak magnetic field $H$. The result is asymptotically exact in the\nweak coupling regime, and is very close to the exact Bethe ansatz result in the\nKondo regime. We also calculate the quasi-particle density of states in a\nmagnetic field, which is of interest in relation to recent experimental work on\nquantum dots.", "category": "cond-mat_str-el" }, { "text": "Phonon-induced disorder in dynamics of optically pumped metals from\n non-linear electron-phonon coupling: The non-equilibrium dynamics of matter excited by light may produce\nelectronic phases that do not exist in equilibrium, such as laser-induced\nhigh-transition-temperature superconductivity. Here we simulate the dynamics of\na metal driven at initial time $t=0$ by a spatially uniform pump that excites\ndipole-active vibrational modes which couple quadratically to electrons. We\nstudy in detail the evolution of electronic and vibrational observables and\ntheir coherences. We provide evidence for enhancement of local electronic\ncorrelations, including double occupancy, accompanied by rapid loss of spatial\nstructure, which we interpret as a signature of emergent effective disorder in\nthe dynamics. This effective disorder, which arises in absence of quenched\nrandomness, dominates the electronic dynamics as the system evolves towards a\ncorrelated electron-phonon long-time state, possibly explaining why transient\nsuperconductivity is not observed. The pumped electron-phonon systems studied\nhere, which are governed by non-linear coupling, exhibit a much more\nsubstantial dynamical response than linearly coupled models relevant in\nequilibrium, thus presenting a pathway to new modalities for out-of-equilibrium\nphases. Our results provide a basis within which to understand correlation\ndynamics in current pump-probe experiments of vibrationally coupled electrons,\nhighlight the importance of the evolution of phase coherence, and demonstrate\nthat pumped electron-phonon systems provide a means of approximately realizing\nrecently proposed scenarios of dynamically induced disorder in\ntranslation-invariant systems.", "category": "cond-mat_str-el" }, { "text": "Complex pressure-temperature structural phase diagram of honeycomb\n iridate Cu$_2$IrO$_3$: $\\mathrm{Cu_2IrO_3}$ is among the newest layered honeycomb iridates and a\npromising candidate to harbor a Kitaev quantum spin liquid state. Here, we\ninvestigate the pressure and temperature dependence of its structure through a\ncombination of powder x-ray diffraction and x-ray absorption fine structure\nmeasurements, as well as $ab$-$initio$ evolutionary structure search. At\nambient pressure, we revise the previously proposed $C2/c$ solution with a\nrelated but notably more stable $P2_1/c$ structure. Pressures below 8 GPa drive\nthe formation of Ir-Ir dimers at both ambient and low temperatures, similar to\nthe case of $\\mathrm{Li_2IrO_3}$. At higher pressures, the structural evolution\ndramatically depends on temperature. A large discontinuous reduction of the Ir\nhoneycomb interplanar distance is observed around 15 GPa at room temperature,\nlikely driven by a collapse of the O-Cu-O dumbbells. At 15 K, pressures beyond\n20 GPa first lead to an intermediate phase featuring a continuous reduction of\nthe interplanar distance, which then collapses at 30 GPa across yet another\nphase transition. However, the resulting structure around 40 GPa is not the\nsame at room and low temperatures. Remarkably, the reduction in interplanar\ndistance leads to an apparent healing of the stacking faults at room\ntemperature, but not at 15 K. Possible implications on the evolution of\nelectronic structure of $\\mathrm{Cu_2IrO_3}$ with pressure are discussed.", "category": "cond-mat_str-el" }, { "text": "Magnetic transition and spin fluctuations in the unconventional\n antiferromagnetic compound Yb3Pt4: Muon spin rotation and relaxation measurements have been carried out on the\nunconventional antiferromagnet Yb_3Pt_4. Oscillations are observed below T_N =\n2.22(1) K, consistent with the antiferromagnetic (AFM) Neel temperature\nobserved in bulk experiments. In agreement with neutron diffraction experiments\nthe oscillation frequency omega_ mu(T) follows a S = 1/2 mean-field temperature\ndependence, yielding a quasistatic local field 1.71(2) kOe at T = 0. A crude\nestimate gives an ordered moment of ~0.66 mu_B at T = 0, comparable to 0.81\nmu_B from neutron diffraction. As T approaches T_N from above the dynamic\nrelaxation rate lambda_d exhibits no critical slowing down, consistent with a\nmean-field transition. In the AFM phase a T-linear fit to lambda_d(T),\nappropriate to a Fermi liquid, yields highly enhanced values of lambda_d/T and\nthe Korringa constant K_ mu^2 T/lambda_d, with K_ mu the estimated muon Knight\nshift. A strong suppression of lambda_d by applied field is observed in the AFM\nphase. These properties are consistent with the observed large\nSommerfeld-Wilson and Kadowaki-Woods ratios in Yb_3Pt_4 (although the data do\nnot discriminate between Fermi-liquid and non-Fermi-liquid states), and suggest\nstrong enhancement of q ~ 0 spin correlations between large-Fermi-volume band\nquasiparticles in the AFM phase of Yb_3Pt_4.", "category": "cond-mat_str-el" }, { "text": "Quantum kagome antiferromagnet: ZnCu3(OH)6Cl2: Herbertsmithite, ZnCu3(OH)6Cl2, is the prototype candidate for a spin liquid\nbehavior on a geometrically perfect kagome lattice. Its discovery and the\nabsence of any evidence for spin-freezing down to the lowest probed temperature\nto-date enable one to explore the properties of kagome-related physics in an\nunprecedented temperature range. We review its properties and discuss some open\nissues. A tentative comparison to models is also performed.", "category": "cond-mat_str-el" }, { "text": "Construction of Localized Basis for Dynamical Mean Field Theory: Many-body Hamiltonians obtained from first principles generally include all\npossible non-local interactions. But in dynamical mean field theory the\nnon-local interactions are ignored, and only the effects of the local\ninteractions are taken into account. The truncation of the non-local\ninteractions is a basis dependent approximation. We propose a criterion to\nconstruct an appropriate localized basis in which the truncation can be carried\nout. This involves finding a basis in which a functional given by the sum of\nthe squares of the local interactions with appropriate weight factors is\nmaximized under unitary transformations of basis. We argue that such a\nlocalized basis is suitable for the application of dynamical mean field theory\nfor calculating material properties from first principles. We propose an\nalgorithm which can be used for constructing the localized basis. We test our\ncriterion on a toy model and find it satisfactory.", "category": "cond-mat_str-el" }, { "text": "Structural and dynamical study of moment localization in\n beta-Mn(1-x)In(x): We have used neutron scattering and muon spin relaxation (muSR) to\ninvestigate the structural and magnetic properties of the beta-phase of\nelemental manganese doped with dilute amounts of indium. beta-Mn is an example\nof a topologically frustrated antiferromagnetically correlated metal - but\nwhich remains paramagnetic at all temperatures. The addition of In to beta-Mn\nresults in a vast volume expansion of the lattice, and would therefore be\nexpected to have a major effect on the stability and localization of the Mn\nmoment - as observed in, for example, Ru and Al doped beta-Mn alloys. We find\nthat In doping in beta-Mn results in a short-range ordered spin-glass like\nground state, similar to that of Al-doped beta-Mn but with residual low\nfrequency spin fluctuations. This is in contrast to Ru doping which results in\nthe stabilization of a long-range ordered Mn moment", "category": "cond-mat_str-el" }, { "text": "Terahertz parametric amplification as a reporter of exciton condensate\n dynamics: Condensates are a hallmark of emergence in quantum materials with\nsuperconductors and charge density wave as prominent examples. An excitonic\ninsulator (EI) is an intriguing addition to this library, exhibiting\nspontaneous condensation of electron-hole pairs. However, condensate\nobservables can be obscured through parasitic coupling to the lattice.\nTime-resolved terahertz (THz) spectroscopy can disentangle such obscurants\nthrough measurement of the quantum dynamics. We target $Ta_{2}NiSe_{5}$, a\nputative room-temperature EI where electron-lattice coupling dominates the\nstructural transition ($T_{c}$=326 K), hindering identification of excitonic\ncorrelations. A pronounced increase in the THz reflectivity manifests following\nphotoexcitation and exhibits a BEC-like temperature dependence. This occurs\nwell below the $T_{c}$, suggesting a novel approach to monitor exciton\ncondensate dynamics. Nonetheless, dynamic condensate-phonon coupling remains as\nevidenced by peaks in the enhanced reflectivity spectrum at select\ninfrared-active phonon frequencies. This indicates that parametric reflectivity\nenhancement arises from phonon squeezing, validated using Fresnel-Floquet\ntheory and density functional calculations. Our results highlight that coherent\ndynamics can drive parametric stimulated emission with concomitant\npossibilities, including entangled THz photon generation.", "category": "cond-mat_str-el" }, { "text": "Scaling of THz-conductivity at metal-insulator transition in doped\n manganites: Magnetic field and temperature dependence of the Terahertz conductivity and\npermittivity of the colossal magnetoresistance manganite\nPr_{0.65}Ca_{0.28}Sr_{0.07}MnO_3 (PCSMO) is investigated approaching the\nmetal-to-insulator transition (MIT) from the insulating side. In the\ncharge-ordered state of PCSMO both conductivity and dielectric permittivity\nincrease as function of magnetic field and temperature. Universal scaling\nrelationships between the changes in permittivity and conductivity are observed\nin a broad range of temperatures and magnetic fields. Similar scaling is also\nseen in La_{1-x}Sr_xMnO_3 for different doping levels. The observed\nproportionality points towards the importance of pure ac-conductivity and\nphononic energy scale at MIT in manganites.", "category": "cond-mat_str-el" }, { "text": "Ground-state selection and spin-liquid behaviour in the classical\n Heisenberg model on the breathing pyrochlore lattice: Magnetic pyrochlore oxides, including the spin ice materials, have proved to\nbe a rich field for the study of geometrical frustration in 3 dimensions.\nRecently, a new family of magnetic oxides has been synthesised in which the\nhalf of the tetrahedra in the pyrochlore lattice are inflated relative to the\nother half, making an alternating array of small and large tetrahedra. These\n\"breathing pyrochlore\" materials such as LiGaCr4O8, LiInCr4O8 and Ba3Yb2Zn5O11\nprovide new opportunities in the study of frustrated magnetism. Here we provide\nan analytic theory for the ground state phase diagram and spin correlations for\nthe minimal model of magnetism in breathing pyrochlores: a classical nearest\nneighbour Heisenberg model with different exchange coefficients for the two\nspecies of tetrahedra. We find that the phase diagram comprises a Coulombic\nspin liquid phase, a conventional ferromagnetic phase and an unusual\nantiferromagnetic phase with lines of soft modes in reciprocal space,\nstabilised by an order-by-disorder mechanism. We obtain a theory of the spin\ncorrelations in this model using the Self Consistent Gaussian Approximation\n(SCGA) which enables us to discuss the development of correlations in breathing\npyrochlores as a function of temperature, and we quantitatively characterise\nthe thermal crossover from the limit of isolated tetrahedra to the strongly\ncorrelated limit of the problem. We compare the results of our analysis with\nthe results of recent neutron scattering experiments on LiInCr4O8.", "category": "cond-mat_str-el" }, { "text": "My Random Walks in Anderson's Garden: Anderson's Garden is a drawing presented to Philip W. Anderson on the eve of\nhis 60th birthday celebration, in 1983. This cartoon (Fig. 1), whose author is\nunknown, succinctly depicts some of Anderson's pre-1983 works, as a blooming\ngarden. As an avid reader of Anderson's papers, random walk in Anderson's\ngarden had become a part of my routine since graduate school days. This was of\nimmense help and prepared me for a wonderful collaboration with the gardener\nhimself, on the resonating valence bond (RVB) theory of High Tc cuprates and\nquantum spin liquids, at Princeton. The result was bountiful - the first (RVB\nmean field) theory for i) quantum spin liquids, ii) emergent fermi surfaces in\nMott insulators and iii) superconductivity in doped Mott insulators. Beyond\nmean field theory - i) emergent gauge fields, ii) Ginzbuerg Landau theory with\nRVB gauge fields, iii) prediction of superconducting dome, iv) an early\nidentification and study of a non-fermi liquid normal state of cuprates and so\non. Here I narrate this story, years of my gardening attempts and end with a\nbrief summary of my theoretical efforts to extend RVB theory of\nsuperconductivity to encompass the recently observed very high Tc ~ 203 K\nsuperconductivity in molecular solid H2S at high pressures ~ 200 GPa.", "category": "cond-mat_str-el" }, { "text": "Magnetic-field induced triplet superconductivity in the Hubbard model on\n a triangular lattice: We propose theoretically that a magnetic field can realize spin-triplet\nsuperconductivity in repulsively interacting electron systems having strong\nferromagnetic spin fluctuations. We confirm the general idea for the\nlow-density Hubbard model on a triangular lattice, whose Fermi surface consists\nof disconnected pieces, by calculating the pairing susceptibility in a moderate\nmagnetic field with the quantum Monte-Carlo method combined with the dynamical\ncluster approximation.", "category": "cond-mat_str-el" }, { "text": "Phonon Thermal Transport of URu2Si2: Broken Translational Symmetry and\n Strong-Coupling of the Hidden Order to the Lattice: A dramatic increase in the total thermal conductivity (k) is observed in the\nHidden Order (HO) state of single crystal URu2Si2. Through measurements of the\nthermal Hall conductivity, we explicitly show that the electronic contribution\nto k is extremely small, so that this large increase in k is dominated by\nphonon conduction. An itinerant BCS/mean-field model describes this behavior\nwell: the increase in kappa is associated with the opening of a large energy\ngap at the Fermi Surface, thereby decreasing electron-phonon scattering. Our\nanalysis implies that the Hidden Order parameter is strongly coupled to the\nlattice, suggestive of a broken symmetry involving charge degrees of freedom.", "category": "cond-mat_str-el" }, { "text": "Charge Stripe in an Antiferromagnet: 1d Band of Composite Excitations: With the help of analytical and numerical studies of the $t$-$J_z$ model we\nargue that the charge stripe in an antiferromagnetic insulator should be\nunderstood as a system of holon-spin-polaron excitations condensed at the\nself-induced antiphase domain wall. The structure of such a charge excitation\nis studied in detail with numerical and analytical results for various\nquantities being in a very close agreement. An analytical picture of these\nexcitations occupying an effective 1D stripe band is also in a very good accord\nwith numerical data. The emerging concept advocates the primary role of the\nkinetic energy in favoring the stripe as a ground state. A comparative analysis\nsuggests the effect of pairing and collective meandering on the energetics of\nthe stripe formation to be secondary.", "category": "cond-mat_str-el" }, { "text": "Josephson current through the SYK model: We calculate the equilibrium Josephson current through a disordered\ninteracting quantum dot described by a Sachdev-Ye-Kitaev model contacted by two\nBCS superconductors. We show that, at zero temperature and at the conformal\nlimit, i.e. in the strong interacting limit, the Josephson current is\nsuppressed by $U$, the strength of the interaction, as $\\ln(U)/U$ and becomes\nuniversal, namely it gets independent on the superconducting pairing. At finite\ntemperature $T$, instead, it depends on the ratio between the gap $\\Delta$ and\nthe temperature and goes as $\\Delta^2/T^2$ for sufficiently large temperatures.\nA proximity effect exists but the self-energy corrections induced by the\ncoupling with the superconducting leads seem subleading as compared to the\nself-energy due to the interaction for large number of particles.", "category": "cond-mat_str-el" }, { "text": "Lattice source for charge and spin inhomogeneity in 2D perovskite\n cuprates: In the work we highlight the structural features of 2D perovskite cuprates\n(tilted CuO$_6$ octahedra with different orientation with respect to spacer\nrocksalt layers), where sources of charge and spin inhomogeneity can be hidden.\nWe used the impurity Anderson model with the Jahn-Teller(JT) local cells to\nshow the charge inhomogeneity arises at any low doping concentration $x$, but\ndisappears when the doping level exceeds threshold concentration $x_c$, and the\nlower the magnitudes $x_c$, the more JT region square. It is expected that\nspontaneous chiral symmetry breaking in the dynamic JT state of the stripe\nCuO$_2$ layer as a whole can lead to the appearance of the goldstone phonon\nmode. As consequence, the giant thermal Hall effect could be observed in the 2D\nperovskite cuprates with CuO$_6$ octahedra, rather than with CuO$_4$ squares,\ne.g. in Tl-based $n$ layer cuprates or cuprates based on the infinite-layer\nCaCuO$_2$ structure.", "category": "cond-mat_str-el" }, { "text": "Connecting high-field quantum oscillations to zero-field electron\n spectral functions in the underdoped cuprates: The central puzzle of the cuprate superconductors at low hole density is the\nnature of the pseudogap regime. It has a number of seemingly distinct\nexperimental signatures: a suppression of the paramagnetic spin susceptibility\nat high temperatures, low energy electronic excitations that extend over arcs\nin the Brillouin zone, X-ray detection of charge density wave order at\nintermediate temperatures, and quantum oscillations at high magnetic fields and\nlow temperatures. We show that a model of competing charge density wave and\nsuperconducting orders provides a unified description of the intermediate and\nlow temperature regimes. We treat quantum oscillations at high field beyond\nsemiclassical approximations, and find clear and robust signatures of an\nelectron pocket compatible with existing observations; we also predict\noscillations due to additional hole pockets. In the zero field and intermediate\ntemperature regime, we compute the electronic spectrum in the presence of\nthermally fluctuating charge density and superconducting orders. Our results\nare compatible with experimental trends.", "category": "cond-mat_str-el" }, { "text": "Minimalist approach to the classification of symmetry protected\n topological phases: A number of proposals with differing predictions (e.g. Borel group\ncohomology, oriented cobordism, group supercohomology, spin cobordism, etc.)\nhave been made for the classification of symmetry protected topological (SPT)\nphases. Here we treat various proposals on an equal footing and present\nrigorous, general results that are independent of which proposal is correct. We\ndo so by formulating a minimalist Generalized Cohomology Hypothesis, which is\nsatisfied by existing proposals and captures essential aspects of SPT\nclassification. From this Hypothesis alone, formulas relating classifications\nin different dimensions and/or protected by different symmetry groups are\nderived. Our formalism is expected to work for fermionic as well as bosonic\nphases, Floquet as well as stationary phases, and spatial as well as on-site\nsymmetries. As an application, we predict that the complete classification of\n3-dimensional bosonic SPT phases with space group symmetry $G$ is $H^4_{\\rm\nBorel}\\left(G;U(1)\\right) \\oplus H^1_{\\rm group}\\left(G;\\mathbb Z\\right)$,\nwhere the $H^1$ term classifies phases beyond the Borel group cohomology\nproposal.", "category": "cond-mat_str-el" }, { "text": "Realistic Modeling of Complex Oxide Materials: Since electronic and magnetic properties of many transition-metal oxides can\nbe efficiently controlled by external factors such as the temperature,\npressure, electric or magnetic field, they are regarded as promising materials\nfor various applications. From the viewpoint of electronic structure, these\nphenomena are frequently related to the behavior of a small group of states\nclose to the Fermi level. The basic idea of this project is to construct a\nlow-energy model for the states near the Fermi level on the basis of\nfirst-principles density functional theory, and to study this model by modern\nmany-body techniques. After a brief review of the method, the abilities of this\napproach will be illustrated on a number of examples, including multiferroic\nmanganites and spin-orbital-lattice coupled phenomena in RVO3 (R being the\nthree-valent element).", "category": "cond-mat_str-el" }, { "text": "Intrasubband and Intersubband Electron Relaxation in Semiconductor\n Quantum Wire Structures: We calculate the intersubband and intrasubband many-body inelastic Coulomb\nscattering rates due to electron-electron interaction in two-subband\nsemiconductor quantum wire structures. We analyze our relaxation rates in terms\nof contributions from inter- and intrasubband charge-density excitations\nseparately. We show that the intersubband (intrasubband) charge-density\nexcitations are primarily responsible for intersubband (intrasubband) inelastic\nscattering. We identify the contributions to the inelastic scattering rate\ncoming from the emission of the single-particle and the collective excitations\nindividually. We obtain the lifetime of hot electrons injected in each subband\nas a function of the total charge density in the wire.", "category": "cond-mat_str-el" }, { "text": "Decoherence of charge density waves in beam splitters for interacting\n quantum wires: Simple intersections between one-dimensional channels can act as coherent\nbeam splitters for non-interacting electrons. Here we examine how coherent\nsplitting at such intersections is affected by inter-particle interactions, in\nthe special case of an intersection of topological edge states. We derive an\neffective impurity model which represents the edge-state intersection within\nLuttinger liquid theory at low energy. For Luttinger K = 1 / 2 , we compute the\nexact time-dependent expectation values of the charge density as well as the\ndensity-density correlation functions. In general a single incoming charge\ndensity wave packet will split into four outgoing wave packets with\ntransmission and reflection coefficients depending on the strengths of the\ntunnelling processes between the wires at the junction. We find that when\nmultiple charge density wave packets from different directions pass through the\nintersection at the same time, reflection and splitting of the packets depend\non the relative phases of the waves. Active use of this phase-dependent\nsplitting of wave packets may make Luttinger interferometry possible. We also\nfind that coherent incident packets generally suffer partial decoherence from\nthe intersection, with some of their initially coherent signal being\ntransferred into correlated quantum noise. In an extreme case four incident\ncoherent wave packets can be transformed entirely into density-density\ncorrelations, with the charge density itself having zero expectation value\neverywhere in the final state.", "category": "cond-mat_str-el" }, { "text": "Influence of phonon-assisted tunneling on the linear thermoelectric\n transport through molecular quantum dots: We investigate the effect of vibrational degrees of freedom on the linear\nthermoelectric transport through a single-level quantum dot described by the\nspinless Anderson-Holstein impurity model. To study the effects of strong\nelectron-phonon coupling, we use the nonperturbative numerical renormalization\ngroup approach. We also compare our results, at weak to intermediate coupling,\nwith those obtained by employing the functional renormalization group method,\nfinding good agreement in this parameter regime. When applying a gate voltage\nat finite temperatures, the inelastic scattering processes, induced by\nphonon-assisted tunneling, result in an interesting interplay between\nelectrical and thermal transport. We explore different parameter regimes and\nidentify situations for which the thermoelectric power as well as the\ndimensionless figure of merit are significantly enhanced via a Mahan-Sofo type\nof mechanism. We show, in particular, that this occurs at strong\nelectron-phonon coupling and in the antiadiabatic regime.", "category": "cond-mat_str-el" }, { "text": "Muon spin rotation and neutron scattering study of the\n non-centrosymmetric tetragonal compound CeAuAl3: We have investigated the non-centrosymmetric tetragonal heavy-fermion\ncompound CeAuAl3 using muon spin rotation (muSR), neutron diffraction (ND) and\ninelastic neutron scattering (INS) measurements. We have also revisited the\nmagnetic, transport and thermal properties. The magnetic susceptibility reveals\nan antiferromagnetic transition at 1.1 K with a possibility of another magnetic\ntransition near 0.18 K. The heat capacity shows a sharp lambda-type anomaly at\n1.1 K in zero-filed, which broadens and moves to higher temperature in applied\nmagnetic field. Our zero-field muSR and ND measurements confirm the existence\nof a long-range magnetic ground state below 1.2 K. Further the ND study reveals\nan incommensurate magnetic ordering with a magnetic propagation vector k = (0,\n0, 0.52) and a spiral structure of Ce moments coupled ferromagnetically within\nthe ab-plane. Our INS study reveals the presence of two well-defined crystal\nelectric field (CEF) excitations at 5.1 meV and 24.6 meV in the paramagnetic\nphase of CeAuAl3 which can be explained on the basis of the CEF theory.\nFurthermore, low energy quasi-elastic excitations show a Gaussian line shape\nbelow 30 K compared to a Lorentzian line shape above 30 K, indicating a\nslowdown of spin fluctuation below 30 K. We have estimated a Kondo temperature\nof TK=3.5 K from the quasi-elastic linewidth, which is in good agreement with\nthat estimated from the heat capacity. This study also indicates the absence of\nany CEF-phonon coupling unlike that observed in isostructural CeCuAl3. The CEF\nparameters, energy level scheme and their wave functions obtained from the\nanalysis of INS data explain satisfactorily the single crystal susceptibility\nin the presence of two-ion anisotropic exchange interaction in CeAuAl3.", "category": "cond-mat_str-el" }, { "text": "Magnetic tunneling induced Weyl node annihilation in TaP: Weyl nodes are topological objects in three-dimensional metals. Their\ntopological property can be revealed by studying the high-field transport\nproperties of a Weyl semimetal. While the energy of the lowest Landau band\n(LLB) of a conventional Fermi pocket always increases with magnetic field due\nto the zero point energy, the LLB of Weyl cones remains at zero energy unless a\nstrong magnetic field couples the Weyl fermions of opposite chirality. In the\nWeyl semimetal TaP, we achieve such a magnetic coupling between the\nelectron-like Fermi pockets arising from the W1 Weyl fermions. As a result,\ntheir LLBs move above chemical potential, leading to a sharp sign reversal in\nthe Hall resistivity at a specific magnetic field corresponding to the W1 Weyl\nnode separation. By contrast, despite having almost identical carrier density,\nthe annihilation is unobserved for the hole-like pockets because the W2 Weyl\nnodes are much further separated. These key findings, corroborated by other\nsystematic analyses, reveal the nontrivial topology of Weyl fermions in\nhigh-field measurements.", "category": "cond-mat_str-el" }, { "text": "NaCo2(SeO3)2(OH): Competing Magnetic Ground States of a New Sawtooth\n Structure with 3d7 Co2+ Ions: While certain magnetic sublattices have long been known theoretically to give\nrise to emergent physics via competing magnetic interactions and quantum\neffects, finding such configurations in real materials is often deeply\nchallenging. Here we report the synthesis and characterization of a new such\nmaterial, NaCo2(SeO3)2(OH) which crystallizes with a highly frustrated\nsublattice of sawtooth Co2+ chains. Single crystals of NaCo2(SeO3)2(OH) were\nsynthesized using a low-temperature hydrothermal method. X-ray single crystal\nstructure analysis reveals that the material crystallizes in orthorhombic space\ngroup of Pnma (no. 62). Its crystal structure exhibits one-dimensional chains\nof corner-sharing isosceles triangles that are made of two crystallographically\ndistinct 3d7 Co2+ sites (Co(1) and Co(2)). The chains run along the b-axis and\nare interconnected via [SeO3] groups to form a three-dimensional structure\nmediating super-exchange interactions. The temperature dependent magnetization\ndata show a ferromagnetic-like (FM) transition at 11 K (T1) followed by an\nantiferromagnetic (AFM) transition at about 6 K (T2). Neutron-powder\ndiffraction measurements reveal that at T1 = 11 K only Co(2) site orders\nmagnetically, forming ferromagnetic zigzag chains along the b-axis. Below T2 =\n6 K, both Co(1) and Co(2) sites order in an nearly orthogonal configuration,\nwith Co(1) moments lying inside the plane of the sawtooth chain while Co(2)\nmoments cant out of the plane. The canting of the magnetic moments leads to a\nnet ferromagnetic component along b-axis, parallel to the chain direction. The\nordered moments are fully compensated in the ac-plane.", "category": "cond-mat_str-el" }, { "text": "Van der Waals Schottky barriers as interface probes of the correlation\n between chemical potential shifts and charge density wave formation in\n 1T-TiSe$_2$ and 2H-NbSe$_2$: Layered transition metal dichalcogenide (TMD) materials, i.e. 1T-TiSe$_2$ and\n2H-NbSe$_2$, harbor a second order charge density wave (CDW) transition where\nphonons play a key role for the periodic modulations of conduction electron\ndensities and associated lattice distortions. We systematically study the\ntransport and capacitance characteristics over a wide temperature range of\nSchottky barriers formed by intimately contacting freshly exfoliated flakes of\n1T-TiSe$_2$ and 2H-NbSe$_2$ to \\textit{n}-type GaAs semiconductor substrates.\nThe extracted temperature-dependent parameters (zero-bias barrier height,\nideality and built-in potential) reflect changes at the TMD/GaAs interface\ninduced by CDW formation for both TMD materials. The measured built-in\npotential reveals chemical-potential shifts relating to CDW formation. With\ndecreasing temperature a peak in the chemical-potential shifts during CDW\nevolution indicates a competition between electron energy re-distributions and\na combination of lattice strain energies and Coulomb interactions. These\nmodulations of chemical potential in CDW systems, such as 1T-TiSe$_2$ and\n2H-NbSe$_2$ harboring second-order phase transitions, reflect a corresponding\nconversion from short to long-range order.", "category": "cond-mat_str-el" }, { "text": "Quantum renormalization of entanglement in an antisymmetric anisotropic\n and bond-alternating spin system: The quantum renormalization group method is applied to study the quantum\ncriticality and entanglement entropy of the ground state of the Ising chain in\nthe presence of antisymmetric anisotropic couplings and alternating exchange\ninteractions. The quantum phase transitions can be characterized by the\ndiscontinuity in the second derivative of the energy of renormalized ground\nstate. The phase diagram is obtained by the critical boundary line. The first\nderivative of entanglement entropy also diverges at the same critical points\nafter enough iterations of the renormalization of coupling constants. The\nantisymmetric anisotropy and alternating interaction can enhance the\nrenormalized entanglement via the creation of quantum fluctuations. The scaling\nbehavior of the derivative of the entropy around the critical points manifest\nthe logarithm dependence on the size of the spin system.", "category": "cond-mat_str-el" }, { "text": "Photo-induced Tomonaga-Luttinger-like liquid in a Mott insulator: Photo-induced metallic states in a Mott insulator are studied for the\nhalf-filled, one-dimensional Hubbard model with the time-dependent density\nmatrix renormalization group. An irradiation of strong AC field is found to\ncreate a linear dispersion in the optical spectrum (current-current\ncorrelation) in the nonequilibrium steady state reminiscent of the\nTomonaga-Luttinger liquid for the doped Mott insulator in equilibrium. The spin\nspectrum in nonequilibrium retains the des Cloizeaux-Pearson mode with the spin\nvelocity differing from the charge velocity. The mechanism of the\nphotocarrier-doping, along with the renormalization in the charge velocity, is\nanalyzed in terms of an effective Dirac model.", "category": "cond-mat_str-el" }, { "text": "Interaction-induced adiabatic cooling and antiferromagnetism of cold\n fermions in optical lattices: We propose an interaction-induced cooling mechanism for two-component cold\nfermions in an optical lattice. It is based on an increase of the ``spin''\nentropy upon localisation, an analogue of the Pomeranchuk effect in liquid\nHelium 3. We discuss its application to the experimental realisation of the\nantiferromagnetic phase. We illustrate our arguments with Dynamical Mean-Field\nTheory calculations.", "category": "cond-mat_str-el" }, { "text": "Magnon-Hole Scattering and Charge Order in $Sr_{14-x}Ca_xCu_{24}O_{41}$: The magnon thermal conductivity $\\kappa_{\\mathrm{mag}}$ of the hole doped\nspin ladders in $\\rm Sr_{14-x}Ca_xCu_{24}O_{41}$ has been investigated at low\ndoping levels $x$. The analysis of $\\kappa_{\\mathrm{mag}}$ reveals a strong\ndoping and temperature dependence of the magnon mean free path\n$l_{\\mathrm{mag}}$ which is a local probe for the interaction of magnons with\nthe doped holes in the ladders. In particular, this novel approach to studying\ncharge degrees of freedom via spin excitations shows that charge ordering of\nthe holes in the ladders leads to a freezing out of magnon-hole scattering\nprocesses.", "category": "cond-mat_str-el" }, { "text": "Negative local resistance caused by viscous electron backflow in\n graphene: Graphene hosts a unique electron system in which electron-phonon scattering\nis extremely weak but electron-electron collisions are sufficiently frequent to\nprovide local equilibrium above liquid nitrogen temperature. Under these\nconditions, electrons can behave as a viscous liquid and exhibit hydrodynamic\nphenomena similar to classical liquids. Here we report strong evidence for this\ntransport regime. We find that doped graphene exhibits an anomalous (negative)\nvoltage drop near current injection contacts, which is attributed to the\nformation of submicrometer-size whirlpools in the electron flow. The viscosity\nof graphene's electron liquid is found to be ~0.1 m$^2$ /s, an order of\nmagnitude larger than that of honey, in agreement with many-body theory. Our\nwork shows a possibility to study electron hydrodynamics using high quality\ngraphene.", "category": "cond-mat_str-el" }, { "text": "Scattering rate, transport and specific heat in a metal close to a\n quantum critical point : emergence of a robust Fermi liquid picture ?: We calculate the low temperature one-particle scattering rate and the\nspecific heat in a weakly disordered metal close to a quantum critical point.\nTo lowest order in the fluctuation potential, we obtain typical Fermi-liquid\nresults proportional to T^2 and T respectively, with prefactors which diverge\nas a power law of the control parameter upon approaching the critical point.\nThe Kadowaki-Woods ratio is shown to be independent of the control parameter\nonly for the case of 3-D FM fluctuations. Our work is relevant for experiments\non CeCoIn$_5$ and Sr_3Ru_2O_7.", "category": "cond-mat_str-el" }, { "text": "Nonresonant B1g Raman scattering in the Hubbard model: The numerically exact solution for nonresonant B1g Raman scattering is\npresented for the half-filled Hubbard model in infinite dimensions. This\nsolution illustrates the modifications of the Raman response (in a system tuned\nthrough the quantum-critical point of a metal-insulator transition) due to\nFermi-liquid properties in the metallic phase. In the insulating phase, we\nrecover the predicted universal behavior, while we find the Raman response is\nquite anomalous on the metallic side of the transition. Our calculated results\nare similar to those measured in FeSi, SmB6, and underdoped cuprates.", "category": "cond-mat_str-el" }, { "text": "Spontaneous Magnetization of an Ideal Ferromagnet: Beyond Dyson's\n Analysis: Using the low-energy effective field theory for magnons, we systematically\nevaluate the partition function of the O(3) ferromagnet up to three loops.\nDyson, in his pioneering microscopic analysis of the Heisenberg model, showed\nthat the spin-wave interaction starts manifesting itself in the low-temperature\nexpansion of the spontaneous magnetization of an ideal ferromagnet only at\norder $T^4$. Although several authors tried to go beyond Dyson's result, to the\nbest of our knowledge, a fully systematic and rigorous investigation of higher\norder terms induced by the spin-wave interaction, has never been achieved. As\nwe demonstrate in the present paper, it is straightforward to evaluate the\npartition function of an ideal ferromagnet beyond Dyson's analysis, using\neffective Lagrangian techniques. In particular, we show that the\nnext-to-leading contribution to the spontaneous magnetization resulting from\nthe spin-wave interaction already sets in at order $T^{9/2}$ -- in contrast to\nall claims that have appeared before in the literature. Remarkably, the\ncorresponding coefficient is completely determined by the leading-order\neffective Lagrangian and is thus independent of the anisotropies of the cubic\nlattice. We also consider even higher-order corrections and thereby solve --\nonce and for all -- the question of how the spin-wave interaction in an ideal\nferromagnet manifests itself in the spontaneous magnetization beyond the Dyson\nterm.", "category": "cond-mat_str-el" }, { "text": "Systematic improvement of the Momentum Average approximation for the\n Green's function of a Holstein polaron: We show how to systematically improve the Momentum Average (MA)approximation\nfor the Green's function of a Holstein polaron, bysystematically improving the\naccuracy of the self-energy diagrams in such a way that they can still all be\nsummed efficiently. This allows us to fix some of the problems of the MA\napproximation, e.g. we now find the expected polaron+phonon continuum at the\ncorrect location, and a momentum-dependent self-energy. The quantitative\nagreement with numerical data is further improved, as expected since the number\nof exactly satisfied spectral weight sum rules is increased. The corrections\nare found to be larger in lower dimensional systems.", "category": "cond-mat_str-el" }, { "text": "Strong in-plane anisotropy in the electronic structure of fixed-valence\n $\u03b2$-LuAlB$_4$: The origin of intrinsic quantum criticality in the heavy-fermion\nsuperconductor $\\beta$-YbAlB$_4$ has been attributed to strong Yb valence\nfluctuations and its peculiar crystal structure. Here, we assess these\ncontributions individually by studying the isostructural but fixed-valence\ncompound $\\beta$-LuAlB$_4$. Quantum oscillation measurements and DFT\ncalculations reveal a Fermi surface markedly different from that of\n$\\beta$-YbAlB$_4$, consistent with a `large' Fermi surface there. We also find\nan unexpected in-plane anisotropy of the electronic structure, in contrast to\nthe isotropic Kondo hybridization in $\\beta$-YbAlB$_4$.", "category": "cond-mat_str-el" }, { "text": "Photoinduced Structural Phase Transitions in Polyacene: There exist two types of structural instability in polyacene: double bonds in\na cis pattern and those in a trans pattern. They are isoenergetic but\nspectroscopically distinct. We demonstrate optical characterization and\nmanipulation of Peierls-distorted polyacene employing both correlated and\nuncorrelated Hamiltonians. We clarify the phase boundaries of the cis- and\ntrans-distorted isomers, elucidate their optical-conductivity spectra, and then\nexplore their photoresponses. There occurs a photoinduced transformation in the\npolyacene structure, but it is one-way switching: The trans configuration is\nwell convertible into the cis one, whereas the reverse conversion is much less\nfeasible. Even the weakest light irradiation can cause a transition of\nuncorrelated electrons, while correlated electrons have a transition threshold\nagainst light irradiation.", "category": "cond-mat_str-el" }, { "text": "Emergent particle-hole symmetry in the half-filled Landau level: We provide an effective description of a particle-hole symmetric state of\nelectrons in a half-filled Landau level, starting from the traditional approach\npioneered by Halperin, Lee and Read. Specifically, we study a system consisting\nof alternating quasi-one-dimensional strips of composite Fermi liquid (CFL) and\ncomposite hole liquid (CHL), both of which break particle-hole symmetry. When\nthe CFL and CHL strips are identical in size, the resulting state is manifestly\ninvariant under the combined action of a particle-hole transformation with\nrespect to a single Landau level (which interchanges the CFL and CHL) and\ntranslation by one unit, equal to the strip width, in the direction transverse\nto the strips. At distances long compared to the strip width, we demonstrate\nthat the system is described by a Dirac fermion coupled to an emergent gauge\nfield, with an anti-unitary particle-hole symmetry, as recently proposed by\nSon.", "category": "cond-mat_str-el" }, { "text": "Reduction of ordered moment and Neel temperature of quasi\n one-dimensional antiferromagnets Sr2CuO3 and Ca2CuO3: We report elastic neutron diffraction and muon spin relaxation (muSR)\nmeasurements of the quasi one-dimensional antiferromagnets Sr2CuO3 and Ca2CuO3,\nwhich have extraordinarily reduced TN/J ratios. We observe almost\nresolution-limited antiferromagnetic Bragg reflections in Sr2CuO3 and obtain a\nreduced ordered moment size of ~0.06 \\muB. We find that the ratio of ordered\nmoment size \\mu(Ca2CuO3)/\\mu(Sr2CuO3)=1.5(1) roughly scales with their Neel\ntemperatures, which suggests that the ordered moment size of quasi\none-dimensional antiferromagnets decreases continuously in the limit of\nvanishing inter-chain interactions.", "category": "cond-mat_str-el" }, { "text": "Random singlet-like phase of disordered Hubbard chains: Local moment formation is ubiquitous in disordered semiconductors such as\nSi:P, where it is observed both in the metallic and the insulating regimes.\nHere, we focus on local moment behavior in disordered insulators, which arises\nfrom short-ranged, repulsive electron-electron interactions. Using density\nmatrix renormalization group and strong-disorder renormalization group methods,\nwe study paradigmatic models of interacting insulators: one dimensional Hubbard\nchains with quenched randomness. In chains with either random fermion hoppings\nor random chemical potentials, both at and away from half-filling, we find\nexponential decay of charge and fermion 2-point correlations but power-law\ndecay of spin correlations that are indicative of the random singlet phase. The\nnumerical results can be understood qualitatively by appealing to the\nlarge-interaction limit of the Hubbard chain, in which a remarkably simple\npicture emerges.", "category": "cond-mat_str-el" }, { "text": "Paramagnetism in the kagome compounds\n (Zn,Mg,Cd)Cu$_{3}$(OH)$_{6}$Cl$_{2}$: Frustrated magnetism on the kagome lattice has been a fertile ground for rich\nand fascinating physics, ranging from experimental evidence of a spin liquid to\ntheoretical predictions of exotic superconductivity. Among experimentally\nrealized spin-$\\frac{1}{2}$ kagome magnets, herbertsmithite, kapellasite, and\nhaydeeite [(Zn,Mg)Cu$_{3}$(OH)$_{6}$Cl$_{2}$] are all well described by a\nthree-parameter Heisenberg model, but they exhibit distinctly different\nphysics. We address the problem using a pseudofermion functional\nrenormalization-group approach and analyze the low-energy physics in the\nexperimentally accessible parameter range. Our analysis places kapellasite and\nhaydeeite near the boundaries between magnetically ordered and disordered\nphases, implying that slight modifications could dramatically affect their\nmagnetic properties. Inspired by this, we perform \\textit{ab initio} density\nfunctional theory calculations of (Zn,Mg,Cd)Cu$_{3}$ (OH)$_{6}$Cl$_{2}$ at\nvarious pressures. Our results suggest that by varying pressure and composition\none can traverse a paramagnetic regime between different magnetically ordered\nphases.", "category": "cond-mat_str-el" }, { "text": "Local magnetic moment formation and Kondo screening in the half filled\n two dimensional single band Hubbard model: We study formation of local magnetic moments in strongly correlated Hubbard\nmodel within dynamical mean field theory and associate peculiarities of\ntemperature dependence of local charge $\\chi_c$ and spin $\\chi_s$\nsusceptibilities with different stages of local moment formation. Local maximum\nof temperature dependence of the charge susceptibility $\\chi_c$ is associated\nwith beginning of local magnetic moments formation, while the minimum of the\nsusceptibility $\\chi_c$ and double occupation, as well as low temperature\nboundary of the plateau of effective local magnetic moment $\\mu_{\\rm\neff}^2=T\\chi_s$ temperature dependence are connected with full formation of\nlocal moments. We also obtain interaction dependence of the Kondo temperature\n$T_K$, which is compared to the fingerprint criterion of Phys. Rev. Lett. 126,\n056403 (2021). Near the Mott transition the two criteria coincide, while\nfurther away from Mott transition the fingerprint criterion somewhat\noverestimates Kondo temperature. The relation of the observed features to the\nbehavior of eigenvectors/eigenvalues of fermionic frequency-resolved charge\nsusceptibility and divergences of irreducible vertices is discussed.", "category": "cond-mat_str-el" }, { "text": "Orbital superexchange and crystal field simultaneously at play in YVO3:\n resonant inelastic x-ray scattering at the V L edge and the O K edge: We report on the observation of orbital excitations in YVO3 by means of\nresonant inelastic x-ray scattering (RIXS) at energies across the vanadium L3\nand oxygen K absorption edges. Due to the excellent experimental resolution we\nare able to resolve the intra-t2g excitations at 0.1-0.2 eV, 1.07 eV, and 1.28\neV, the lowest excitations from the t2g into the eg levels at 1.86 eV, and\nfurther excitations above 2.2 eV. For the intra-t2g excitations at 0.1-0.2 eV,\nthe RIXS peaks show small shifts of the order of 10-40 meV as a function of\ntemperature and of about 13-20 meV as a function of the transferred momentum\nq||a. We argue that the latter reflects a finite dispersion of the orbital\nexcitations. For incident energies tuned to the oxygen K edge, RIXS is more\nsensitive to intersite excitations. We observe excitations across the\nMott-Hubbard gap and find an additional feature at 0.4 eV which we attribute to\ntwo-orbiton scattering, i.e., an exchange of orbitals between adjacent sites.\nAltogether, these results indicate that both superexchange interactions and the\ncoupling to the lattice are important for a quantitative understanding of the\norbital excitations in YVO3.", "category": "cond-mat_str-el" }, { "text": "Spin-triplet f-wave pairing due to three-site cyclic-exchange\n ferromagnetic interactions: Ferromagnetiam and superconductivity in a two-dimensional triangular-lattice\nHubbard model are studied using the density-matrix renormalization group\nmethod. We propose a mechanism of the {\\it f}-wave spin-triplet pairing derived\nfrom the three-site cyclic-exchange ferromagnetic interactions. We point out\nthat a triangular network of hopping integrals, which is required for the\nthree-site cyclic hopping processes, is contained in the (possibly)\nspin-triplet superconducting systems, such as Bechgaard salts (TMTSF)$_2$X,\ncobalt oxide Na$_{0.35}$CoO$_2$$\\cdot$1.3H$_2$O, and layered perovskite\nSr$_2$RuO$_4$.", "category": "cond-mat_str-el" }, { "text": "Orbital Polarization in Strained LaNiO$_{3}$: Structural Distortions and\n Correlation Effects: Transition-metal heterostructures offer the fascinating possibility of\ncontrolling orbital degrees of freedom via strain. Here, we investigate\ntheoretically the degree of orbital polarization that can be induced by\nepitaxial strain in LaNiO$_3$ films. Using combined electronic structure and\ndynamical mean-field theory methods we take into account both structural\ndistortions and electron correlations and discuss their relative influence. We\nconfirm that Hund's rule coupling tends to decrease the polarization and point\nout that this applies to both the $d^8\\underline{L}$ and $d^7$ local\nconfigurations of the Ni ions. Our calculations are in good agreement with\nrecent experiments, which revealed sizable orbital polarization under tensile\nstrain. We discuss why full orbital polarization is hard to achieve in this\nspecific system and emphasize the general limitations that must be overcome to\nachieve this goal.", "category": "cond-mat_str-el" }, { "text": "Electronic correlations and magnetic interactions in infinite-layer\n NdNiO$_2$: The large antiferromagnetic exchange coupling in the parent high-$T_{\\rm c}$\ncuprate superconductors is believed to play a crucial role in pairing the\nsuperconducting carriers. The recent observation of superconductivity in\nhole-doped infinite-layer (IL-) NdNiO$_2$ brings to the fore the relevance of\nmagnetic coupling in high-$T_{\\rm c}$ superconductors, particularly because no\nmagnetic ordering is observed in the undoped IL-NdNiO$_2$ unlike in parent\ncopper oxides. Here, we investigate the electronic structure and the nature of\nmagnetic exchange in IL-NdNiO$_2$ using state-of-the-art many-body quantum\nchemistry methods. From a systematic comparison of the electronic and magnetic\nproperties with isostructural cuprate IL-CaCuO$_2$, we find that the on-site\ndynamical correlations are significantly stronger in IL-NdNiO$_2$ compared to\nthe cuprate analog. These dynamical correlations play a critical role in the\nmagnetic exchange resulting in an unexpectedly large antiferromagnetic nearest\nneighbor isotropic $J$ of 77 meV between the Ni$^{1+}$ ions within the\n$ab$-plane. While we find many similarities in the electronic structure between\nthe nickelate and the cuprate, the role of electronic correlations is\nprofoundly different in the two. We further discuss the implications of our\nfindings in understanding the origin of superconductivity in nickelates.", "category": "cond-mat_str-el" }, { "text": "Computation of dynamical correlation functions of Heisenberg chains: the\n gapless anisotropic regime: We compute all dynamical spin-spin correlation functions for the spin-1/2\n$XXZ$ anisotropic Heisenberg model in the gapless antiferromagnetic regime,\nusing numerical sums of exact determinant representations for form factors of\nspin operators on the lattice. Contributions from intermediate states\ncontaining many particles and string (bound) states are included. We present\nmodified determinant representations for the form factors valid in the general\ncase with string solutions to the Bethe equations. Our results are such that\nthe available sum rules are saturated to high precision. We Fourier transform\nour results back to real space, allowing us in particular to make a comparison\nwith known exact formulas for equal-time correlation functions for small\nseparations in zero field, and with predictions for the zero-field asymptotics\nfrom conformal field theory.", "category": "cond-mat_str-el" }, { "text": "de Haas-van Alphen oscillations in quasi-two-dimensional underdoped\n cuprate superconductors in the canonical ensemble: We calculate the de Haas-van Alphen (dHvA) effect waveform using the\ncanonical ensemble for different Fermi surface scenarios applicable to the\nunderdoped cuprate superconductor YBa2Cu3O6.5, in which quantum oscillations\nhave recently been observed. The harmonic content of the dHvA waveform of the\nprincipal F ~ 500 T frequency is consistent with the existence of a second\nthermodynamically dominant section of Fermi surface that acts primarily as a\ncharge reservoir. Oscillations in the charge density to and from this reservoir\nare shown to potentially contribute to the observed large quantum oscillations\nin the Hall resistance.", "category": "cond-mat_str-el" }, { "text": "Magnon-phonon interactions in magnetic insulators: We address the theory of magnon-phonon interactions and compute the\ncorresponding quasi-particle and transport lifetimes in magnetic insulators\nwith focus on yttrium iron garnet at intermediate temperatures from anisotropy-\nand exchange-mediated magnon-phonon interactions, the latter being derived from\nthe volume dependence of the Curie temperature. We find in general weak effects\nof phonon scattering on magnon transport and the Gilbert damping of the\nmacrospin Kittel mode. The magnon transport lifetime differs from the\nquasi-particle lifetime at shorter wavelengths.", "category": "cond-mat_str-el" }, { "text": "Construction of Variational Matrix Product States for the Heisenberg\n Spin-1 Chain: We propose a simple variational wave function that captures the correct\nground state energy of the spin-1 Heisenberg chain model to within 0.04\\%. The\nwave function is written in the matrix product state (MPS) form with the bond\ndimension $D=8$, and characterized by three fugacity parameters. The proposed\nMPS generalizes the Affleck-Kennedy-Lieb-Tasaki (AKLT) state by dressing it\nwith dimers, trimers, and general $q$-dimers. The fugacity parameters control\nthe number and the average size of the $q$-mers. Furthermore, the $D=8$\nvariational MPS state captures the ground states of the entire family of\nbilinear-biquadratic Hamiltonian belonging to the Haldane phase to high\naccuracy. The 2-4-2 degeneracy structure in the entanglement spectrum of our\nMPS state is found to match well with the results of density matrix\nrenormalization group (DMRG) calculation, which is computationally much\nheavier. Spin-spin correlation functions also find excellent fit with those\nobtained by DMRG.", "category": "cond-mat_str-el" }, { "text": "A Simple Treatment of Metal-Insulator Transition: Effects of Degeneracy,\n Temperature and Applied Magnetic Field: A simple slave-boson representation combined with the Hartree-Fock\napproximation for the Hund's rule coupling is introduced for a doubly\ndegenerate narrow band, which bears a direct relation to that introduced\npreviously in the nondegenerate case. Namely, one keeps the fermion\nrepresentation of the spin operator to recover properly the energy of fermionic\nquasiparticles in the presence of an applied magnetic field. A simple\ntwo-parameter mean-field analysis of the metamagnetism is provided, with the\nemphasis on the role of the Hund's rule coupling. We also analyse the\nappearance of the spin-split effective masses in the applied field and for\nnonhalf-filled-band situation. The Mott-Hubbard boundary is determined at\nnonzero temperature (T>0); it shifts towards lower interactions with increasing\nT and the field signalling the precursory localization effects, explicitly\nexhibited in the behavior of the magnetic susceptibility calculated in the\nAppendix. We also formulate a more general two-parameter rotationally invariant\napproach for an arbitrary degeneracy d of equivalent orbitals and show that the\nMott-Hubbard transition at zero temperature and at any integer filling n>1 is\nalways discontinuous. A brief overview of experimental situation is also made.", "category": "cond-mat_str-el" }, { "text": "Prototypical many-body signatures in transport properties of\n semiconductors: We devise a methodology for charge, heat, and entropy transport driven by\ncarriers with finite lifetimes. Combining numerical simulations with analytical\nexpressions for low temperatures, we establish a comprehensive and\nthermodynamically consistent phenomenology for transport properties in\nsemiconductors. We demonstrate that the scattering rate (inverse lifetime) is a\nrelevant energy scale: It causes the emergence of several characteristic\nfeatures in each transport observable. The theory is capable to reproduce --\nwith only a minimal input electronic structure -- the full temperature profiles\nmeasured in correlated narrow-gap semiconductors. In particular, we account for\nthe previously elusive low-$T$ saturation of the resistivity and the Hall\ncoefficient, as well as the (linear) vanishing of the Seebeck and Nernst\ncoefficient in systems, such as FeSb$_2$, FeAs$_2$, RuSb$_2$ and FeGa$_3$.", "category": "cond-mat_str-el" }, { "text": "Coexisting charge-ordered states with distinct driving mechanisms in\n monolayer VSe$_2$: Thinning crystalline materials to two dimensions (2D) creates a rich\nplayground for electronic phases, including charge, spin, superconducting, and\ntopological order. Bulk materials hosting charge density waves (CDWs), when\nreduced to ultrathin films, have shown CDW enhancement and tunability. However,\ncharge order confined to only 2D remains elusive. Here we report a distinct\ncharge ordered state emerging in the monolayer limit of $1T$-VSe$_2$.\nSystematic scanning tunneling microscopy experiments reveal that bilayer\nVSe$_2$ largely retains the bulk electronic structure, hosting a\ntri-directional CDW. However, monolayer VSe$_2$ -- consistently across distinct\nsubstrates -- exhibits a dimensional crossover, hosting two CDWs with distinct\nwavelengths and transition temperatures. Electronic structure calculations\nreveal that while one CDW is bulk-like and arises from the well-known Peierls\nmechanism, the other is decidedly unconventional. The observed CDW-lattice\ndecoupling and the emergence of a flat band suggest that the new CDW could\narise from enhanced electron-electron interactions in the 2D limit. These\nfindings establish monolayer-VSe$_2$ as a host of coexisting charge orders with\ndistinct origins, and enable the tailoring of electronic phenomena via emergent\ninteractions in 2D materials.", "category": "cond-mat_str-el" }, { "text": "Double-Exchange Model on Triangle Chain: We study ground state properties of the double-exchange model on triangle\nchain in the classical limit on $t_{2g}$ spins. The ground state is determined\nby a competition among the kinetic energy of the $e_g$ electron, the\nantiferromagnetic exchange energy between the $t_{2g}$ spins, and frustration\ndue to a geometric structure of the lattice. The phase diagrams are obtained\nnumerically for two kinds of the models which differ only in the transfer\nintegral being real or complex. The properties of the states are understood\nfrom the viewpoint of the spin-induced Peierls instability. The results suggest\nthe existence of a chiral glass phase which is characterized by a local spin\nchirality and a continuous degeneracy.", "category": "cond-mat_str-el" }, { "text": "Bose condensation in a model microcavity: We study the equilibrium properties of a system of dipole-active excitons\ncoupled to a single photon mode at fixed total excitation. Treating the\npresence or absence of a trapped exciton as a two-level system produces a model\nthat is exactly soluble. It gives a simple description of the physics of\npolariton condensation in optical cavities beyond the low-density bosonic\nregime.", "category": "cond-mat_str-el" }, { "text": "Incommensurate magnetic orders and topological Hall effect in the\n square-net centrosymmetric EuGa$_2$Al$_2$ system: Neutron diffraction on the centrosymmetric square-net magnet EuGa$_2$Al$_2$\nreveals multiple incommensurate magnetic states (AFM1,2,3) in zero field. In\napplied field, a new magnetic phase (A) is identified from magnetization and\ntransport measurements, bounded by two of the $\\mu_0H$~=~0 incommensurate\nmagnetic phases (AFM1,helical and AFM3, cycloidal) with different moment\norientations. Moreover, magneto-transport measurements indicate the presence of\na topological Hall effect, with maximum values centered in the A phase.\nTogether, these results render EuGa$_2$Al$_2$ a material with non-coplanar or\ntopological spin texture in applied field. X-ray diffraction reveals an\nout-of-plane (OOP) charge density wave (CDW) below $T_{CDW} \\sim$ 50 K while\nthe magnetic propagation vector lies in plane below $T_N$ = 19.5 K. Together\nthese data point to a new route to realizing in-plane non-collinear spin\ntextures through an OOP CDW. In turn, these non-collinear spin textures may be\nunstable against the formation of topological spin textures in an applied\nfield.", "category": "cond-mat_str-el" }, { "text": "Magnetic excitations in the low-temperature ferroelectric phase of\n multiferroic YMn2O5 using inelastic neutron scattering: We studied magnetic excitations in a low-temperature ferroelectric phase of\nthe multiferroic YMn2O5 using inelastic neutron scattering (INS). We identify\nlow-energy magnon modes and establish a correspondence between the magnon peaks\nobserved by INS and electromagnon peaks observed in optical absorption [1].\nFurthermore, we explain the microscopic mechanism, which results in the\nlowest-energy electromagnon peak, by comparing the inelastic neutron spectral\nweight with the polarization in the commensurate ferroelectric phase.", "category": "cond-mat_str-el" }, { "text": "Spin waves in the spiral phase of a doped antiferromagnet: a\n strong-coupling approach: We study spin fluctuations in the spiral phase of the two-dimensional Hubbard\nmodel at low doping on the basis of the spin-particle-hole coherent-state path\nintegral. In the strong correlation limit, we obtain an analytical expression\nof the spin-wave excitations over the entire Brillouin zone except in the\nvicinity of ${\\bf q}=0$. We discuss the validity of the Hartree-Fock and\nrandom-phase approximations in the strong-coupling limit, and compare our\nresults with previous numerical and analytical calculations. Although the\nspiral phase is unstable, as shown by a negative mean-field compressibility and\nthe presence of imaginary spin-fluctuation modes, we expect the\nshort-wavelength fluctuation modes (with real energies) to survive in the\nactual ground-state of the system.", "category": "cond-mat_str-el" }, { "text": "Bosonic Short Range Entangled states Beyond Group Cohomology\n classification: We explore and construct a class of bosonic short range entangled (BSRE)\nstates in all $4k+2$ spatial dimensions, which are higher dimensional\ngeneralizations of the well-known Kitaev's $E_8$ state in 2d. These BSRE states\nshare the following properties: (1) their bulk is fully gapped and\nnondegenerate; (2) their $(4k+1)d$ boundary is described by a \"self-dual\"\nrank-$2k$ antisymmetric tensor gauge field, and it is guaranteed to be gapless\nwithout assuming any symmetry; (3) their $(4k+1)d$ boundary has intrinsic\ngravitational anomaly once coupled to the gravitational field; (4) their bulk\nis described by an effective Chern-Simons field theory with rank-$(2k+1)$\nantisymmetric tensor fields, whose $K^{IJ}$ matrix is identical to that of the\n$E_8$ state in $2d$; (5) The existence of these BSRE states lead to various\nbosonic symmetry protected topological (BSPT) states as their descendants in\nother dimensions; (6) These BSRE states can be constructed by confining\nfermionic degrees of freedom from 8 copies of $(4k+2)d$ SRE states with\nfermionic $2k-$branes; (7) After compactifying the $(4k+2)d$ BSRE state on a\nclosed $4k$ dimensional manifold, depending on the topology of the compact $4k$\nmanifold, the system could reduce to nontrivial $2d$ BSRE states.", "category": "cond-mat_str-el" }, { "text": "Topological charge pumping in excitonic insulators: We show that in excitonic insulators with $s$-wave electron-hole pairing, an\napplied electric field (either pulsed or static) can induce a $p$-wave\ncomponent to the order parameter, and further drive it to rotate in the $s+ip$\nplane, realizing a Thouless charge pump. In one dimension, each cycle of\nrotation pumps exactly two electrons across the sample. Higher dimensional\nsystems can be viewed as a stack of one dimensional chains in momentum space in\nwhich each chain crossing the fermi surface contributes a channel of charge\npumping. Physics beyond the adiabatic limit, including in particular\ndissipative effects is discussed.", "category": "cond-mat_str-el" }, { "text": "Emergent topological spin structures in a centrosymmetric cubic\n perovskite: The skyrmion crystal (SkX) characterized by a multiple-q helical spin\nmodulation has been reported as a unique topological state that competes with\nthe single-q helimagnetic order in non-centrosymmetric materials. Here we\nreport the discovery of a rich variety of multiple-q helimagnetic spin\nstructures in the centrosymmetric cubic perovskite SrFeO3. On the basis of\nneutron diffraction measurements, we have identified two types of robust\nmultiple-q topological spin structures that appear in the absence of external\nmagnetic fields: an anisotropic double-q spin spiral and an isotropic\nquadruple-q spiral hosting a three-dimensional lattice of hedgehog\nsingularities. The present system not only diversifies the family of SkX host\nmaterials, but furthermore provides an experimental missing link between\ncentrosymmetric lattices and topological helimagnetic order. It also offers\nperspectives for integration of SkXs into oxide electronic devices.", "category": "cond-mat_str-el" }, { "text": "Unfrustrating the t-J Model: d-wave BCS Superconductivity in the\n $t'$-$J_z$-$V$ Model: The t-J model is believed to be a minimal model that may be capable of\ndescribing the low-energy physics of the cuprate superconductors. However,\nalthough the t-J model is simple in appearance, obtaining a detailed\nunderstanding of its phase diagram has proved to be challenging. We are\ntherefore motivated to study modifications to the t-J model such that its phase\ndiagram and mechanism for d-wave superconductivity can be understood\nanalytically without making uncontrolled approximations. The modified model we\nconsider is a $t'$-$J_z$-$V$ model on a square lattice, which has a\nsecond-nearest-neighbor hopping $t'$ (instead of a nearest-neighbor hopping\n$t$), an Ising (instead of Heisenberg) antiferromagnetic coupling $J_z$, and a\nnearest-neighbor repulsion $V$. In a certain strongly interacting limit, the\nground state is an antiferromagnetic superconductor that can be described\nexactly by a Hamiltonian where the only interaction is a nearest-neighbor\nattraction. BCS theory can then be applied with arbitrary analytical control,\nfrom which nodeless d-wave or s-wave superconductivity can result.", "category": "cond-mat_str-el" }, { "text": "On the Problem of the Staggered Field in CuGeO3 Doped with Magnetic\n Impurities: The magnitude of the staggered field is calculated from the EPR data for\nCuGeO3 doped with Co and Fe magnetic impurities. It is found that this\nparameter demonstrate an anomalous temperature and magnetic field dependence\nprobably due to (i) the specific mechanism of the staggered field generation in\ndoped CuGeO3 and (ii) a competition between antiferromagnetic interchain\nexchange and staggered Zeeman energy.", "category": "cond-mat_str-el" }, { "text": "Emergent gravity in graphene: We reconsider monolayer graphene in the presence of elastic deformations. It\nis described by the tight - binding model with varying hopping parameters. We\ndemonstrate, that the fermionic quasiparticles propagate in the emergent 2D\nWeitzenbock geometry and in the presence of the emergent U(1) gauge field. Both\nemergent geometry and the gauge field are defined by the elastic deformation of\ngraphene.", "category": "cond-mat_str-el" }, { "text": "High magnetic field phase diagram and weak FM breaking in\n (Ni0.93Co0.07)3V2O8: We present magnetostriction and thermal expansion measurements on\nmultiferroic (Ni0.93Co0.07)3V2O8. The high field phase diagrams up to 33 T\nalong the a, b and c directions are built. For H//a, as the magnetic field\nincreases, two intermediate phases appear between the incommensurate phase and\nthe paramagnetic phase at about 7 K, and then a magnetically induced phase\nappears above the paramagnetic phase. For H//b,thermal expansion measurement\nindicates a mutation in the spin lattice coupling of the high field phases. The\ninterlaced phase boundary suggests a mixed state in the optical high field\nphase. For H//c, an intermediate phase between the commensurate phase and the\nincommensurate phase is detected. A nonlinear boundary between the intermediate\nphase and the low temperature incommensurate phase, and a clear boundary\nbetween the commensurate phase and the paramagnetic phase are found. These\nresults indicate that doping Co2+ breaks the weak ferromagnetic moment of the\ncommensurate phase, which exists in the parent compound Ni3V2O8 and\n(Ni0.9Co0.1)3V2O8. This nonlinear influence reflects complicated spin\nmodulation in Ni3V2O8 by doping Co2+.", "category": "cond-mat_str-el" }, { "text": "QMC study of the chiral Heisenberg Gross-Neveu universality class: We investigate a quantum criticality of an antiferromagnetic phase transition\nin the Hubbard model on a square lattice with a $d$-wave pairing field by\nlarge-scale auxiliary-field quantum Monte Carlo simulations. Since the $d$-wave\npairing filed induces Dirac cones in the non-interacting single-particle\nspectrum, the quantum criticality should correspond to the chiral Heisenberg\nuniversality class in terms of the Gross-Neveu theory, which is the same as\nthose expected in the Hubbard model on the honeycomb lattice, despite the unit\ncells being different (e.g., they contain one and two sites, respectively). We\nshow that both the two phase transitions, expected to occur on the square and\non the honeycomb lattices, indeed have the same quantum criticality. We also\nargue that details of the models, i.e., the way of counting the total number\n$N$ of fermion components and the anisotropy of the Dirac cones, do not change\nthe critical exponents.", "category": "cond-mat_str-el" }, { "text": "Ferromagnetic fluctuation and possible triplet superconductivity in\n Na_xCoO_2*yH_2O: Fluctuation-exchange study of multi-orbital Hubbard model: Spin and charge fluctuations and superconductivity in a recently discovered\nsuperconductor Na_xCoO_2*yH_2O are studied based on a multi-orbital Hubbard\nmodel. Tight-binding parameters are determined to reproduce the LDA band\ndispersions with the Fermi surface, which consist of a large cylindrical one\naround the Gamma-point and six hole pockets near the K-points. By applying the\nfluctuation-exchange (FLEX) approximation, we show that the Hund's-rule\ncoupling between the Co t2g orbitals causes ferromagnetic (FM) spin\nfluctuation. Triplet f_{y(y^2-3x^2)}-wave and p-wave pairings are favored by\nthis FM fluctuation on the hole-pocket band. We propose that, in\nNa_xCoO_2*yH_2O, the Co t2g orbitals and inter-orbital Hund's-rule coupling\nplay important roles on the triplet pairing, and this compound can be a first\nexample of the triplet superconductor in which the orbital degrees of freedom\nplay substantial roles.", "category": "cond-mat_str-el" }, { "text": "Cascade of field-induced magnetic transitions in a frustrated\n antiferromagnetic metal: Frustrated magnets can exhibit many novel forms of order when exposed to high\nmagnetic fields, however, much less is known about materials where frustration\noccurs in the presence of itinerant electrons. Here we report thermodynamic and\ntransport measurements on micron-sized single crystals of the\ntriangular-lattice metallic antiferromagnet 2H-AgNiO2, in magnetic fields of up\nto 90 T and temperatures down to 0.35 K. We observe a cascade of magnetic phase\ntransitions at 13.5 20, 28 and 39T in fields applied along the easy axis, and\nwe combine magnetic torque, specific heat and transport data to construct the\nfield-temperature phase diagram. The results are discussed in the context of a\nfrustrated easy-axis Heisenberg model for the localized moments where\nintermediate applied magnetic fields are predicted to stabilize a magnetic\nsupersolid phase. Deviations in the measured phase diagram from this model\npredictions are attributed to the role played by the itinerant electrons.", "category": "cond-mat_str-el" }, { "text": "Resonant inelastic X-ray scattering response of the Kitaev honeycomb\n model: We calculate the resonant inelastic X-ray scattering (RIXS) response of the\nKitaev honeycomb model, an exactly solvable quantum-spin-liquid model with\nfractionalized Majorana and flux excitations. We find that the fundamental RIXS\nchannels, the spin-conserving (SC) and the non-spin-conserving (NSC) ones, do\nnot interfere and give completely different responses. SC-RIXS picks up\nexclusively the Majorana sector with a pronounced momentum dispersion, whereas\nNSC-RIXS also creates immobile fluxes, thereby rendering the response only\nweakly momentum dependent, as in the spin structure factor measured by\ninelastic neutron scattering. RIXS can therefore pick up the fractionalized\nexcitations of the Kitaev spin liquid separately, making it a sensitive probe\nto detect spin-liquid character in potential material incarnations of the\nKitaev honeycomb model.", "category": "cond-mat_str-el" }, { "text": "Parasitic small-moment-antiferromagnetism and non-linear coupling of\n hidden order and antiferromagnetism in URu2Si2 observed by Larmor diffraction: We report simultaneous measurements of the distribution of lattice constants\nand the antiferromagnetic moment in high-purity URu2Si2, using both Larmor and\nconventional neutron diffraction, as a function of temperature and pressure up\nto 18 kbar. We establish that the tiny moment in the hidden order (HO) state is\npurely parasitic and quantitatively originates from the distribution of lattice\nconstants. Moreover, the HO and large-moment antiferromagnetism (LMAF) at high\npressure are separated by a line of first-order phase transitions, which ends\nin a bicritical point. Thus the HO and LMAF are coupled non-linearly and must\nhave different symmetry, as expected of the HO being, e.g., incommensurate\norbital currents, helicity order, or multipolar order.", "category": "cond-mat_str-el" }, { "text": "Antiferromagnetism in semiconducting SrMn2Sb2 and BaMn2Sb2 single\n crystals: Crystals of SrMn2Sb2 and BaMn2Sb2 were grown using Sn flux and characterized\nby powder and single-crystal x-ray diffraction, respectively, and by\nsingle-crystal electrical resistivity rho, heat capacity Cp, and magnetic\nsusceptibility chi measurements versus temperature T, and magnetization versus\nfield M(H) isotherm measurements. SrMn2Sb2 adopts the trigonal CaAl2Si2-type\nstructure whereas BaMn2Sb2 crystallizes in the tetragonal ThCr2Si2-type\nstructure. The rho(T) data indicate semiconducting behaviors for both compounds\nwith activation energies of 0.35 eV for SrMn2Sb2 and 0.16 eV for BaMn2Sb2. The\nchi(T) and Cp(T) data reveal antiferromagnetic (AFM) ordering at TN = 110 K for\nSrMn2Sb2 and 450~K for BaMn2Sb2. The anisotropic chi(T < TN) data also show\nthat the ordered moments in SrMn2Sb2 are aligned in the hexagonal ab plane\nwhereas the ordered moments in BaMn2Sb2 are aligned collinearly along the\ntetragonal c axis. The ab-plane M(H) data for SrMn2Sb2 exhibit a continuous\nmetamagnetic transition at low fields 0 < H < 1 T, whereas BaMn2Sb2 exhibits no\nmetamagnetic transitions up to 5.5 T. The chi(T) data for both compounds and\nthe Cp(T) data for SrMn2Sb2 and BaMn2Sb2 indicate strong dynamic short-range\nAFM correlations above their respective TN up to at least 900 K within a\nlocal-moment picture, corresponding to quasi-two-dimensional magnetic behavior.\nThe present results and a survey of the literature for Mn pnictides with the\nCaAl2Si2 and ThCr2Si2 crystal structures show that the TN values for the\nCaAl2Si2-type compounds are much smaller than those for the ThCr2Si2-type\nmaterials.", "category": "cond-mat_str-el" }, { "text": "Quantum Criticality of Topological Phase Transitions in 3D Interacting\n Electronic Systems: Topological phase transitions in condensed matters accompany emerging\nsingularities of the electronic wave function, often manifested by gap-closing\npoints in the momentum space. In conventional topological insulators in three\ndimensions (3D), the low energy theory near the gap-closing point can be\ndescribed by relativistic Dirac fermions coupled to the long range Coulomb\ninteraction, hence the quantum critical point of topological phase transitions\nprovides a promising platform to test the novel predictions of quantum\nelectrodynamics. Here we show that a new class of quantum critical phenomena\nemanates in topological materials breaking either the inversion symmetry or the\ntime-reversal symmetry. At the quantum critical point, the theory is described\nby the emerging low energy fermions, dubbed the anisotropic Weyl fermions,\nwhich show both the relativistic and Newtonian dynamics simultaneously. The\ninterplay between the anisotropic dispersion and the Coulomb interaction brings\nabout a new screening phenomena distinct from the conventional Thomas-Fermi\nscreening in metals and logarithmic screening in Dirac fermions.", "category": "cond-mat_str-el" }, { "text": "Thermal DMRG for highly frustrated quantum spin chains: a user\n perspective: Thermal DMRG is investigated with emphasis of employability in molecular\nmagnetism studies. To this end magnetic observables at finite temperature are\nevaluated for two one-dimensional quantum spin systems: a Heisenberg chain with\nnearest-neighbor antiferromagnetic interaction and a frustrated sawtooth\n(delta) chain. It is found that thermal DMRG indeed accurately approximates\nmagnetic observables for the chain as well as for the sawtooth chain, but in\nthe latter case only for sufficiently high temperatures. We speculate that the\nreason is due to the peculiar structure of the low-energy spectrum of the\nsawtooth chain induced by frustration.", "category": "cond-mat_str-el" }, { "text": "Thermodynamic and information-theoretic description of the Mott\n transition in the two-dimensional Hubbard model: At the Mott transition, electron-electron interaction changes a metal, in\nwhich electrons are itinerant, to an insulator, in which electrons are\nlocalized. This phenomenon is central to quantum materials. Here we contribute\nto its understanding by studying the two-dimensional Hubbard model at finite\ntemperature with plaquette cellular dynamical mean-field theory. We provide an\nexhaustive thermodynamic description of the correlation-driven Mott transition\nof the half-filled model by calculating pressure, charge compressibility,\nentropy, kinetic energy, potential energy and free energy across the\nfirst-order Mott transition and its high-temperature crossover (Widom line).\nThe entropy is extracted from the Gibbs-Duhem relation and shows complex\nbehavior near the transition, marked by discontinuous jumps at the first-order\nboundary, singular behavior at the Mott endpoint and inflections marking sharp\nvariations in the supercritical region. The free energy allows us to identify\nthe thermodynamic phase boundary, to discuss phases stability and\nmetastability, and to touch upon nucleation and spinodal decomposition\nmechanisms for the transition. We complement this thermodynamic description of\nthe Mott transition by an information-theoretic description. We achieve this by\ncalculating the local entropy, which is a measure of entanglement, and the\nsingle-site total mutual information, which quantifies quantum and classical\ncorrelations. These information-theoretic measures exhibit characteristic\nbehaviors that allow us to identify the first-order coexistence regions, the\nMott critical endpoint and the crossovers along the Widom line in the\nsupercritical region.", "category": "cond-mat_str-el" }, { "text": "How Hidden Orders Generate Gaps in 1D Fermionic Systems: We demonstrate that hidden long range order is always present in the gapped\nphases of interacting fermionic systems on one dimensional lattices. It is\ncaptured by correlation functions of appropriate nonlocal charge and/or spin\noperators, which remain asymptotically finite. The corresponding microscopic\norders are classified. The results are confirmed by DMRG numerical simulation\nof the phase diagram of the extended Hubbard model, and of a Haldane insulator\nphase.", "category": "cond-mat_str-el" }, { "text": "Evidence for Anisotropic Kondo Behavior in Ce0.8La0.2Al3: We have performed an inelastic neutron scattering study of the low energy\nspin dynamics of the heavy fermion compound Ce0.8La0.2Al3 as a function of\ntemperature and external pressure up to 5 kbar. At temperatures below 3 K, the\nmagnetic response transforms from a quasi-elastic form, common to many heavy\nfermion systems, to a single well-defined inelastic peak, which is extremely\nsensitive to external pressure. The scaling of the spin dynamics and the\nthermodynamic properties are in agreement with the predictions of the\nanisotropic Kondo model.", "category": "cond-mat_str-el" }, { "text": "Diagrammatic theory for Anderson Impurity Model. Stationary property of\n the thermodynamic potential: A diagrammatic theory around atomic limit is proposed for normal state of\nAnderson Impurity Model. The new diagram method is based on the ordinary Wick's\ntheorem for conduction electrons and a generalized Wick's theorem for gtrongly\ncorrelated impurity electrons. This last theorem coincides with the definition\nof Kubo cumulants. For the mean value of the evolution operator a linked\ncluster theorem is proved and a Dyson's type equations for one-particle\npropagators are established. The main element of these equations is the\ncorrelation function which contains the spin, charge and pairing fluctuations\nof the system. The thermodynamic potential of the system is expressed through\none-particle renormalized Green's functions and the corelation function. The\nstationary property of the thermodynamic potential is established with respect\nto the changes of correlation function.", "category": "cond-mat_str-el" }, { "text": "Competing Ground States of a Peierls-Hubbard Nanotube: Motivated by iodo platinum complexes assembled within a quadratic-prism\nlattice, [Pt(C$_2$H$_8$N$_2$)(C$_{10}$H$_8$N$_2$)I]$_4$(NO$_3$)$_8$, we\ninvestigate the ground-state properties of a Peierls-Hubbard four-legged tube.\nMaking a group-theoretical analysis, we systematically reveal a variety of\nvalence arrangements, including half-metallic charge-density-wave states.\nQuantum and thermal phase competition is numerically demonstrated with\nparticular emphasis on doping-induced successive insulator-to-metal transitions\nwith conductivity increasing stepwise.", "category": "cond-mat_str-el" }, { "text": "Phase Thermalization: from Fermi Liquid to Incoherent Metal: When a system consists of a large subsystem (bath) and a small one (probe),\nthermalization implies induction of temperature of the bath onto the probe. If\nboth the bath and the probe are described by same microscopic Hamiltonian,\nthermalization further entails that the probe imbibes the phase of the bath. We\nrefer to this phenomenon as phase thermalization. However, it is not clear\nwhether this phenomenon is realizable when the probe and the bath are described\nby different microscopic Hamiltonians. We show phase thermalization is possible\neven when the microscopic Hamiltonians differ significantly. We provide an\nexplicit example, where the probe is a Fermi liquid realized by a Majorana\nchain with $n \\gg 1$ fermions per site interacting through random hopping and\nthe bath is an incoherent metal described by another Majorana chain with $N >\nn$ fermions per site interacting through arbitrarily long range random\nfour-fermion interaction. In deep infrared, the probe turns into an incoherent\nmetal, with Lyapunov spectrum and diffusion coefficient identical to the bath.", "category": "cond-mat_str-el" }, { "text": "Dimensional-Crossover-Driven Mott Insulators in SrVO3 Ultrathin Films: High-quality epitaxial SrVO3 (SVO) thin films of various thicknesses were\ngrown on (001)-oriented LSAT substrates by pulsed electron-beam deposition\ntechnique. Thick SVO films (~25 nm) exhibited metallic behavior with the\nelectrical resistivity following the T2 law corresponding to a Fermi liquid\nsystem. We observed a temperature driven metal-insulator transition (MIT) in\nSVO ultrathin films with thicknesses below 6.5 nm, the transition temperature\nTMIT was found to be at 50 K for the 6.5 nm film, 120 K for the 5.7 nm film and\n205 K for the 3 nm film. The emergence of the observed MIT can be attributed to\nthe dimensional crossover from a three-dimensional metal to a two-dimensional\nMott insulator, as the resulting reduction in the effective bandwidth W opens a\nband gap at the Fermi level. The magneto-transport study of the SVO ultrathin\nfilms also confirmed the observed MIT is due to the electron-electron\ninteractions other than localization.", "category": "cond-mat_str-el" }, { "text": "Fermi- to non-Fermi-liquid crossover and Kondo transition in\n two-dimensional Cu$_{2/3}$V$_{1/3}$V$_2$S$_4$: By means of a specific heat ($C$) and electrical resistivity ($\\varrho$)\nstudy, we give evidence of a pronounced Fermi liquid (FL) behavior with sizable\nmass renormalization, $m^{\\ast}/m = 30$, up to unusually high temperatures\n$\\sim$70 K in the layered system Cu$_{2/3}$V$_{1/3}$V$_2$S$_4$. At low\ntemperature, a marked upturn of both $C$ and $\\varrho$ is suppressed by\nmagnetic field, which suggests a picture of Kondo coupling between conduction\nelectrons in the VS$_2$ layers and impurity spins of the V$^{3+}$ ions located\nbetween layers. This picture opens the possibility of controlling electronic\ncorrelations and the FL to non-FL crossover in simple layered materials. For\ninstance, we envisage that the coupling between layers provided by the impurity\nspins may realize a two-channel Kondo state.", "category": "cond-mat_str-el" }, { "text": "An effect of Sm vacancies on the hybridization gap in topological Kondo\n insulator candidate SmB$_6$: A necessary element for the predicted topological state in Kondo insulator\nSmB$_6$ is the hybridization gap which opens in this compound at low\ntemperatures. In this work, we present a comparative study of the in-gap\ndensity of states due to Sm vacancies by Raman scattering spectroscopy and heat\ncapacity for samples where the number of Sm vacancies is equal to or below 1 %.\nWe demonstrate that hybridization gap is very sensitive to the presence of Sm\nvacancies. At the amount of vacancies above 1 % the gap fills in with impurity\nstates and low temperature heat capacity is enhanced.", "category": "cond-mat_str-el" }, { "text": "Exact solution of the topological symplectic Kondo problem: The Kondo effect is an archetypical phenomenon in the physics of strongly\ncorrelated electron systems. Recent attention has focused on the application of\nKondo physics to quantum information science by exploiting overscreened Kondo\nimpurities with residual anyon-like impurity entropy. While this physics was\nproposed in the fine-tuned multi-channel Kondo setup or in the Majorana-based\ntopological Kondo effect, we here study the Kondo effect with symplectic\nsymmetry Sp(2k) and present details about the implementation which importantly\nonly involves conventional s-wave superconductivity coupled to an array of\nresonant levels and neither requires perfect channel symmetry nor Majorana\nfermions. We carefully discuss the role of perturbations and show that a global\nZeeman drives the system to a 2-channel SU(k) fixed point. Exact results for\nthe residual entropy, specific heat, and magnetization are derived using the\nthermodynamic Bethe Ansatz for Sp(2k). This solution not only proves the\nexistence of a quantum critical ground state with anyon-like Hilbert space\ndimension but also a particularly weak non-Fermi liquid behavior at\ncriticality. We interpret the weakness of non-analyticities as a manifestation\nof suppressed density of states at the impurity causing only a very weak\nconnection of putative anyons and conduction electrons. Given this weak\nconnection, the simplicity of the design, and the stability of the effect, we\nconjecture that the symplectic Kondo effect may be particularly suitable for\nquantum information applications.", "category": "cond-mat_str-el" }, { "text": "Manifestation of topological behaviors in interacting Weyl systems:\n one-body verse two-body correlations: Understanding correlation effects in topological phases of matter is at the\nforefront of current research in condensed matter physics. Here we try to\nclarify some subtleties in studying topological behaviors of interacting Weyl\nsemimetals. It is well-known that there exist two topological invariants\ndefined to identify their topological character. One is the many-body Chern\nnumber, which can be directly linked to the Hall conductivity and thus to the\ntwo-particle correlations. The other is the topological index constructed from\nthe single-particle Green's functions. Because the information of Green's\nfunctions is easier to be achieved than the many-body wavefunctions, usually\nonly the latter is employed in the literature. However, the approach based on\nthe single-particle Green's function can break down in the strongly correlated\nphase. For illustration, an exactly solvable two-orbital model with\nmomentum-local two-body interactions is discussed, in which both topological\ninvariants can be calculated analytically. We find that the topological index\ncalculated from the Green's function formalism can be nonzero even for a\nnon-topological strongly correlated phase with vanishing many-body Chern\nnumber. In addition, we stress that the physical surface states implied by\nnonzero many-body Chern numbers should be the edge modes of particle-hole\ncollective excitations, rather than those of quasiparticle nature derived from\nthe Green's function formalism. Our observations thus demonstrate the\nlimitation of the validity of Green's function formalism in the investigations\nof interacting topological materials.", "category": "cond-mat_str-el" }, { "text": "Revised crystal structure and electronic properties of high dielectric\n Ba(Fe$_{1/2}$Nb$_{1/2}$)O$_{3}$ ceramics: Ba(Fe$_{1/2}$Nb$_{1/2}$)O$_3$ (BFN) ceramics are considered to be promising\nfor technological applications owing to their high dielectric constant over a\nwide range of temperatures. However, there exists considerable discrepancy over\nthe structural details. We address this discrepancy through a combined x-ray\ndiffraction at room temperature and neutron powder diffraction measurements in\nthe range from 5K up to room temperature, supplemented by a comparative\nanalysis of the earlier reported structures. Our study reveals a cubic\nstructure with space group Pm$\\bar{3}$m at all measured temperatures. Further,\nthe x-ray near edge structure and the extended x-ray absorption fine structure\nstudies on the local environment of the Fe ions is consistent with the cubic\nsymmetry. An appropriate value of $U$ for DFT+$U$ calculations is obtained by\ncomparison with x-ray absorption spectroscopy, which agrees well with the\nearlier reported electronic properties.", "category": "cond-mat_str-el" }, { "text": "Capturing long range correlations in two-dimensional quantum lattice\n systems using correlator product states: We study the suitability of correlator product states for describing\nground-state properties of two-dimensional spin models. Our ansatz for the\nmany-body wave function takes the form of either plaquette or bond correlator\nproduct states and the energy is optimized by varying the correlators using\nMonte Carlo minimization. For the Ising model we find that plaquette\ncorrelators are best for estimating the energy while bond correlators capture\nthe expected long-range correlations and critical behavior of the system more\nfaithfully. For the antiferromagnetic Heisenberg model, however, plaquettes\noutperform bond correlators at describing both local and long-range\ncorrelations because of the substantially larger number of local parameters\nthey contain. These observations have quantitative implications for the\napplication of correlator product states to other more complex systems, and\ngive important heuristic insights: in particular the necessity of carefully\ntailoring the choice of correlators to the system considered, its interactions\nand symmetries.", "category": "cond-mat_str-el" }, { "text": "Resistivity and Thermal Conductivity of an Organic Insulator\n beta'-EtMe3Sb[Pd(dmit)2]2: A finite residual linear term in the thermal conductivity at zero temperature\nin insulating magnets indicates the presence of gapless excitations of\nitinerant quasiparticles, which has been observed in some candidate materials\nof quantum spin liquids (QSLs). In the organic triangular insulator\nbeta'-EtMe3Sb[Pd(dmit)2]2, a QSL candidate material, the low-temperature\nthermal conductivity depends on the cooling process and the finite residual\nterm is observed only in samples with large thermal conductivity. Moreover, the\ncooling rate dependence is largely sample dependent. Here we find that, while\nthe low-temperature thermal conductivity significantly depends on the cooling\nrate, the high-temperature resistivity is almost perfectly independent of the\ncooling rate. These results indicate that in the samples with the finite\nresidual term, the mean free path of the quasiparticles that carry the heat at\nlow temperatures is governed by disorders, whose characteristic length scale of\nthe distribution is much longer than the electron mean free path that\ndetermines the high-temperature resistivity. This explains why recent X-ray\ndiffraction and nuclear magnetic resonance measurements show no cooling rate\ndependence. Naturally, these measurements are unsuitable for detecting\ndisorders of the length scale relevant for the thermal conductivity, just as\nthey cannot determine the residual resistivity of metals. Present results\nindicate that very careful experiments are needed when discussing itinerant\nspin excitations in beta'-EtMe3Sb[Pd(dmit)2]2.", "category": "cond-mat_str-el" }, { "text": "Restricted Boltzmann Machines for Quantum States with Nonabelian or\n Anyonic Symmetries: Although artificial neural networks have recently been proven to provide a\npromising new framework for constructing quantum many-body wave functions, the\nparameterization of a quantum wavefunction with nonabelian symmetries in terms\nof a Boltzmann machine inherently leads to biased results due to the basis\ndependence. We demonstrate that this problem can be overcome by sampling in the\nbasis of irreducible representations instead of spins, for which the\ncorresponding ansatz respects the nonabelian symmetries of the system. We apply\nour methodology to find the ground states of the one-dimensional\nantiferromagnetic Heisenberg (AFH) model with spin-half and spin-1 degrees of\nfreedom, and obtain a substantially higher accuracy than when using the\n$s_z$-basis as input to the neural network. The proposed ansatz can target\nexcited states, which is illustrated by calculating the energy gap of the AFH\nmodel. We also generalize the framework to the case of anyonic spin chains.", "category": "cond-mat_str-el" }, { "text": "Kondo effect and STM spectra through ferromagnetic nanoclusters: Motivated by recent scanning tunneling microscope (STM) experiments on cobalt\nclusters adsorbed on single wall metallic nanotubes [Odom {\\em et al.}, Science\n{\\bf 290}, 1549 (2000)], we study theoretically the size dependence of STM\nspectra and spin-flip scattering of electrons from finite size ferromagnetic\nclusters adsorbed on metallic surfaces. We study two models of nanometer size\nferromagnets: (i) An itinerant model with delocalized s, p and d electrons and\n(ii) a local moment model with both localized d-level spins and delocalized\ncluster electrons. The effective exchange coupling between the spin of the\ncluster and the conduction electrons of the metallic substrate depends on the\nspecific details of the single particle density of states on the cluster. The\ncalculated Kondo coupling is inversely proportional to the total spin of the\nferromagnetic cluster in both models and thus the Kondo temperature is rapidly\nsuppressed as the size of the cluster increases. Mesoscopic fluctuations in the\ncharging energies and magnetization of nanoclusters can lead to large\nfluctuations in the Kondo temperatures and a very asymmetric voltage dependence\nof the STM spectra. We compare our results to the experiments.", "category": "cond-mat_str-el" }, { "text": "Resonant Nernst effect in the metallic and field-induced spin density\n wave states of (TMTSF)2ClO4: We examine an unusual phenomenon where, in tilted magnetic fields near magic\nangles parallel to crystallographic planes, a \"giant\" resonant Nernst signal\nhas been observed by Wu et al.[Phys. Rev. Lett. 91 56601(2003)] in the metallic\nstate of an organic conducting Bechgaard salt. We show that this effect appears\nto be a general feature of these materials, and is also present in the field\ninduced spin density wave phase with even larger amplitude. Our results place\nnew restrictions on models that treat the metallic state as an unconventional\ndensity wave or as a state with finite Cooper pairing.", "category": "cond-mat_str-el" }, { "text": "Simple mechanisms that impede the Berry phase identification from\n magneto-oscillations: The phase of quantum magneto-oscillations is often associated with the Berry\nphase and is widely used to argue in favor of topological nontriviality of the\nsystem (Berry phase $2\\pi n+\\pi$). Nevertheless, the experimentally determined\nvalue may deviate from $2\\pi n+\\pi$ arbitrarily, therefore more care should be\nmade analyzing the phase of magneto-oscillations to distinguish trivial systems\nfrom nontrivial. In this paper we suggest two simple mechanisms dramatically\naffecting the experimentally observed value of the phase in three-dimensional\ntopological insulators: (i) magnetic field dependence of the chemical\npotential, and (ii) possible nonuniformity of the system. These mechanisms are\nnot limited to topological insulators and can be extended to other\ntopologically trivial and non-trivial systems.", "category": "cond-mat_str-el" }, { "text": "Investigation of Ultrafast Demagnetization and Gilbert Damping and their\n Correlation in Different Ferromagnetic Thin Films Grown Under Identical\n Conditions: Following the demonstration of laser-induced ultrafast demagnetization in\nferromagnetic nickel, several theoretical and phenomenological propositions\nhave sought to uncover its underlying physics. In this work we revisit the\nthree temperature model (3TM) and the microscopic three temperature model\n(M3TM) to perform a comparative analysis of ultrafast demagnetization in\n20-nm-thick cobalt, nickel and permalloy thin films measured using an\nall-optical pump-probe technique. In addition to the ultrafast dynamics at the\nfemtosecond timescales, the nanosecond magnetization precession and damping are\nrecorded at various pump excitation fluences revealing a fluence-dependent\nenhancement in both the demagnetization times and the damping factors. We\nconfirm that the Curie temperature to magnetic moment ratio of a given system\nacts as a figure of merit for the demagnetization time, while the\ndemagnetization times and damping factors show an apparent sensitivity to the\ndensity of states at the Fermi level for a given system. Further, from\nnumerical simulations of the ultrafast demagnetization based on both the 3TM\nand the M3TM, we extract the reservoir coupling parameters that best reproduce\nthe experimental data and estimate the value of the spin flip scattering\nprobability for each system. We discuss how the fluence-dependence of\ninter-reservoir coupling parameters so extracted may reflect a role played by\nnonthermal electrons in the magnetization dynamics at low laser fluences.", "category": "cond-mat_str-el" }, { "text": "High-density two-dimensional small polaron gas in a delta-doped Mott\n insulator: Heterointerfaces in complex oxide systems open new arenas in which to test\nmodels of strongly correlated material, explore the role of dimensionality in\nmetal-insulator-transitions (MITs) and small polaron formation. Close to the\nquantum critical point Mott MITs depend on band filling controlled by random\ndisordered substitutional doping. Delta-doped Mott insulators are potentially\nfree of random disorder and introduce a new arena in which to explore the\neffect of electron correlations and dimensionality. Epitaxial films of the\nprototypical Mott insulator GdTiO3 are delta-doped by substituting a single\n(GdO)+1 plane with a monolayer of charge neutral SrO to produce a\ntwo-dimensional system with high planar doping density. Unlike metallic SrTiO3\nquantum wells in GdTiO3 the single SrO delta-doped layer exhibits thermally\nactivated DC and optical conductivity that agree in a quantitative manner with\npredictions of small polaron transport but with an extremely high\ntwo-dimensional density of polarons, ~ 7E14 cm-2", "category": "cond-mat_str-el" }, { "text": "Dissipative Majorana quantum wires: In this paper, we formulate and quantitatively examine the effect of\ndissipation on topological systems. We use a specific model of Kitaev quantum\nwire with an onsite Ohmic dissipation, and perform a numerically exact quantum\nMonte Carlo simulation to investigate this interacting open quantum system with\na strong system-bath (SB) coupling beyond the scope of Born-Markovian\napproximation. We concentrate on the effect of dissipation on the topological\nfeatures of the system (e.g. the Majorana edge mode) at zero temperature, and\nfind that even though the topological phase is robust against weak SB couplings\nas it is supposed to be, it will eventually be destroyed by sufficiently strong\ndissipations via either a continuous quantum phase transition or a crossover\ndepending on the symmetry of the system. The dissipation-driven quantum\ncriticality is also discussed. In addition, using the framework of Abelian\nbosonization, we provide an analytical description of the interplay between\npairing, dissipation and interaction in our model.", "category": "cond-mat_str-el" }, { "text": "Spin model for the Honeycomb $\\rm NiPS_3$: In the Van der Waal material $\\rm NiPS_3$, Ni atoms have spin S=1 and realize\na honeycomb lattice. Six sulfur atoms surround each Ni and split their d\nmanifold into three filled and two unfilled bands. Aimed to determine the spin\nHamiltonian of $\\rm NiPS_3$, we study its exchange mechanisms using a two-band\nhalf-filled Hubbard model. Hopping between d orbitals is mediated by p orbitals\nof sulfur and gives rise to bilinear and biquadratic spin couplings in the\nlimit of strong electronic correlations. The microscopic model exposed a\nferromagnetic biquadratic spin interaction $\\rm K_1$ allowing the completion of\na minimal $\\rm J_1-J_3-K_1$ spin Hamiltonian for $\\rm NiPS_3$. In bulk, a\nferromagnetic first nearest neighbor $\\rm J_1$ and a more significant\nantiferromagnetic third nearest neighbor spin coupling $\\rm J_3$ agreed with\nthe literature, while in monolayer $\\rm J_1$ is positive and very small in\ncomparison. Using a variational scheme we found that a zig-zag\nantiferromagnetic order is the ground state of bulk samples. The zig-zag\npattern is adjacent to commensurate and incommensurate spin spirals, which\ncould hint at the puzzling results reported in $\\rm NiPS_3$ monolayers.", "category": "cond-mat_str-el" }, { "text": "Determination of Fermi surface by charge density correlations: The Fermi surface topology in the two-dimensional Hubbard model is\nparticularly relevant for the high-temperature superconductors, whereas its\ntheoretical research encounters with the difficulty of the analytical\ncontinuation problem. To this end, we proposed the concept of the\nmomentum-dependent compressibility, defined as the variation of the momentum\ndistribution function with respect to the chemical potential. The surface\ndetermined by the maximum of the momentum-dependent compressibility is nearly\nidentical to the Fermi surface in the weakly and intermediate coupling regions\naccording to our numerical results. In the correlated region, this surface also\nexhibits pocket and arc features, just like the Fermi surface in\nhigh-temperature superconductors. Therefore, for theoretical studies, this\nsurface can be used as an alternative to determine the underlying Fermi\nsurface. Considering that the momentum-dependent compressibility is closely\nrelated to the charge density correlations, our work also shows a connection\nbetween the Fermi surface topology and the charge density fluctuations.", "category": "cond-mat_str-el" }, { "text": "Duality and ground-state phase diagram for the quantum XYZ model with\n arbitrary spin $s$ in one spatial dimension: Five duality transformations are unveiled for the quantum XYZ model with\narbitrary spin $s$ in one spatial dimension. The presence of these duality\ntransformations drastically reduces the entire ground-state phase diagram to\ntwo {\\it finite} regimes - the principal regimes, with all the other ten\nregimes dual to them. Combining with the determination of critical points from\nthe conventional order parameter approach and/or the fidelity approach to\nquantum phase transitions, we are able to map out the ground-state phase\ndiagram for the quantum XYZ model with arbitrary spin $s$. This is explicitly\ndemonstrated for $s=1/2,1,3/2$ and 2. As it turns out, all the critical points,\nwith central charge $c=1$, are self-dual under a respective duality\ntransformation for half-integer as well as integer spin $s$. However, in the\nlatter case, the presence of the so-called symmetry protected topological\nphase, i.e., the Haldane phase, results in extra lines of critical points with\ncentral charge $c=1/2$, which is not self-dual under any duality\ntransformation.", "category": "cond-mat_str-el" }, { "text": "High-pressure versus isoelectronic doping effect on the honeycomb\n iridate Na$_2$IrO$_3$: We study the effect of isoelectronic doping and external pressure in tuning\nthe ground state of the honeycomb iridate Na$_2$IrO$_3$ by combining optical\nspectroscopy with synchrotron x-ray diffraction measurements on single\ncrystals. The obtained optical conductivity of Na$_2$IrO$_3$ is discussed in\nterms of a Mott insulating picture versus the formation of quasimolecular\norbitals and in terms of Kitaev-interactions. With increasing Li content $x$,\n(Na$_{1-x}$Li$_x$)$_2$IrO$_3$ moves deeper into the Mott insulating regime and\nthere are indications that up to a doping level of 24\\% the compound comes\ncloser to the Kitaev-limit. The optical conductivity spectrum of single\ncrystalline $\\alpha$-Li$_2$IrO$_3$ does not follow the trends observed for the\nseries up to $x=0.24$. There are strong indications that $\\alpha$-Li$_2$IrO$_3$\nis less close to the Kitaev-limit compared to Na$_2$IrO$_3$ and closer to the\nquasimolecular orbital picture. Except for the pressure-induced hardening of\nthe phonon modes, the optical properties of Na$_2$IrO$_3$ seem to be robust\nagainst external pressure. Possible explanations of the unexpected evolution of\nthe optical conductivity with isolectronic doping and the drastic change\nbetween $x=0.24$ and $x=1$ are given by comparing the pressure-induced changes\nof lattice parameters and the optical conductivity with the corresponding\nchanges induced by doping.", "category": "cond-mat_str-el" }, { "text": "Quantum correlations in the spin-1/2 Heisenberg XXZ chain with modulated\n Dzyaloshinskii-Moriya interaction: We study a one-dimensional spin-1/2 XXZ Heisenberg model with alternating\nDzyaloshinskii- Moriya interaction, using the numerical Lanczos method.\nRecently, the ground state (GS) phase diagram of this model has been\nestablished using the bosonization approach and extensive density matrix\nrenormalization group computations. Four quantum phases - saturated\nferromagnetic (FM), Luttinger liquid (LL), and two (C1 and C2) gapped phases\nwith composite structure of GS order, characterized by the coexistence of\nlong-range alternating dimer, chirality and antiferromagnetic order have been\nidentified. Here we reexamine the same problem using the exact diagonalization\nLanczos method for chains up to N = 26 sites and explicitly detect positions of\nquantum critical points (QCP) by investigating the quantum correlations as the\nentanglement and the quantum discord (QD). It is shown that the entanglement\nquantified by concurrence and the first derivative of the QD are able to reveal\nbesides the standard FM QCP also the Berezinskii-Kosterlitz-Thouless (BKT)\nphase transition point between the LL and the gapped C1 phase and the Ising\ntype critical point separating the C1 and C2 phases.", "category": "cond-mat_str-el" }, { "text": "Quantum magnetism on the Cairo pentagonal lattice: We present an extensive analytical and numerical study of the\nantiferromagnetic Heisenberg model on the Cairo pentagonal lattice, the dual of\nthe Shastry-Sutherland lattice with a close realization in the S=5/2 compound\nBi2Fe4O9. We consider a model with two exchange couplings suggested by the\nsymmetry of the lattice, and investigate the nature of the ground state as a\nfunction of their ratio x and the spin S. After establishing the classical\nphase diagram we switch on quantum mechanics in a gradual way that highlights\nthe different role of quantum fluctuations on the two inequivalent sites of the\nlattice. The most important findings for S=1/2 include: (i) a surprising\ninterplay between a collinear and a four-sublattice orthogonal phase due to an\nunderlying order-by-disorder mechanism at small x (related to an emergent J1-J2\neffective model with J2 >> J1), and (ii) a non-magnetic and possibly\nspin-nematic phase with d-wave symmetry at intermediate x.", "category": "cond-mat_str-el" }, { "text": "Gaussian state approximation of quantum many-body scars: Quantum many-body scars are atypical, highly nonthermal eigenstates of\nkinetically constrained systems embedded in a sea of thermal eigenstates. These\nspecial eigenstates are characterized, for example, by a bipartite entanglement\nentropy that scales as most logarithmically with subsystem size. We use\nnumerical optimization techniques to investigate if quantum many-body scars of\nthe experimentally relevant PXP model are well approximated by Gaussian states.\nThese states are described by a number of parameters that scales quadratically\nwith system size, thereby having a much lower complexity than generic quantum\nmany-body states. We find that this is a good description for the quantum\nmany-body scars away from the center of the spectrum.", "category": "cond-mat_str-el" }, { "text": "A phason disordered two dimensional quantum antiferromagnet: We examine a novel type of disorder in quantum antiferromagnets. Our model\nconsists of localized spins with antiferromagnetic exchanges on a bipartite\nquasiperiodic structure, which is geometrically disordered in such a way that\nno frustration is introduced. In the limit of zero disorder, the structure is\nthe perfect Penrose rhombus tiling. This tiling is progressively disordered by\naugmenting the number of random \"phason flips\" or local tile-reshuffling\noperations. The ground state remains N\\'eel ordered, and we have studied its\nproperties as a function of increasing disorder using linear spin wave theory\nand quantum Monte Carlo. We find that the ground state energy decreases,\nindicating enhanced quantum fluctuations with increasing disorder. The magnon\nspectrum is progressively smoothed, and the effective spin wave velocity of low\nenergy magnons increases with disorder. For large disorder, the ground state\nenergy as well as the average staggered magnetization tend towards limiting\nvalues characteristic of this type of randomized tilings.", "category": "cond-mat_str-el" }, { "text": "Patterning of two-dimensional electron systems in SrTiO3 based\n heterostructures using a CeO2 template: Two-dimensional electron systems found at the interface of SrTiO3-based oxide\nheterostructures often display anisotropic electric transport whose origin is\ncurrently under debate. To characterize transport along specific\ncrystallographic directions, we developed a hard-mask patterning routine based\non an amorphous CeO2 template layer. The technique allows preparing\nwell-defined microbridges by conventional ultraviolet photolithography which,\nin comparison to standard techniques such as ion- or wet-chemical etching, does\nnot induce any degradation of interfacial conductance. The patterning scheme is\ndescribed in details and the successful production of microbridges based on\namorphous Al2O3-SrTiO3 heterostructures is demonstrated. Significant\nanisotropic transport is observed for T < 30 K which is mainly related to\nimpurity/defect scattering of charge carriers in these heterostructures.", "category": "cond-mat_str-el" }, { "text": "Odd-Parity Triplet Pair Induced by Hund's Rule Coupling: We discuss microscopic aspects of odd-parity triplet pair in orbital\ndegenerate systems. From the concept of off-diagonal long-range order, a pair\nstate is unambiguously defined as the eigenstate with the maximum eigenvalue of\npair correlation function. Performing this scheme by a numerical technique, we\nclarify that the odd-parity triplet pair occurs as an out-of-phase combination\nof local triplets induced by Hund's rule coupling for the lattice including two\nsites in the unit cell.", "category": "cond-mat_str-el" }, { "text": "Intrinsic Structural Disorder and the Magnetic Ground State in Bulk\n EuTiO3: The magnetic properties of single-crystal EuTiO3 are suggestive of nanoscale\ndisorder below its cubic-tetragonal phase transition. We demonstrate that\nelectric field cooling acts to restore monocrystallinity, thus confirming that\nemergent structural disorder is an intrinsic low-temperature property of this\nmaterial. Using torque magnetometry, we deduce that tetragonal EuTiO3 enters an\neasy-axis antiferromagnetic phase at 5.6 K, with a first-order transition to an\neasy-plane ground state below 3 K. Our data is reproduced by a 3D anisotropic\nHeisenberg spin model.", "category": "cond-mat_str-el" }, { "text": "Superfluid-Insulator transition of quantum Hall domain walls in bilayer\n graphene: We consider the zero-filled quantum-Hall ferromagnetic state of bilayer\ngraphene subject to a kink-like perpendicular electric field, which generates\ndomain walls in the electronic state and low-energy collective modes confined\nto move along them. In particular, it is shown that two pairs of collective\nhelical modes are formed at opposite sides of the kink, each pair consisting of\nmodes with identical helicities. We derive an effective field-theoretical model\nof these modes in terms of two weakly coupled anisotropic quantum spin-ladders,\nwith parameters tunable through control of the electric and magnetic fields.\nThis yields a rich phase diagram, where due to the helical nature of the modes,\ndistinct phases possess very different charge conduction properties. Most\nnotably, this system can potentially exhibit a transition from a superfluid to\nan insulating phase.", "category": "cond-mat_str-el" }, { "text": "Probabilistic Simulation of Quantum Circuits with the Transformer: The fundamental question of how to best simulate quantum systems using\nconventional computational resources lies at the forefront of condensed matter\nand quantum computation. It impacts both our understanding of quantum materials\nand our ability to emulate quantum circuits. Here we present an exact\nformulation of quantum dynamics via factorized generalized measurements which\nmaps quantum states to probability distributions with the advantage that local\nunitary dynamics and quantum channels map to local quasi-stochastic matrices.\nThis representation provides a general framework for using state-of-the-art\nprobabilistic models in machine learning for the simulation of quantum\nmany-body dynamics. Using this framework, we have developed a practical\nalgorithm to simulate quantum circuits with the Transformer, a powerful ansatz\nresponsible for the most recent breakthroughs in natural language processing.\nWe demonstrate our approach by simulating circuits which build GHZ and linear\ngraph states of up to 60 qubits, as well as a variational quantum eigensolver\ncircuit for preparing the ground state of the transverse field Ising model on\nsix qubits. Our methodology constitutes a modern machine learning approach to\nthe simulation of quantum physics with applicability both to quantum circuits\nas well as other quantum many-body systems.", "category": "cond-mat_str-el" }, { "text": "Tripartite entangled plaquette state in a cluster magnet: Using large-scale quantum Monte Carlo simulations we show that a spin-$1/2$\nXXZ model on a two-dimensional anisotropic Kagome lattice exhibits a tripartite\nentangled plaquette state that preserves all of the Hamiltonian symmetries. It\nis connected via phase boundaries to a ferromagnet and a valence-bond solid\nthat break U(1) and lattice translation symmetries, respectively. We study the\nphase diagram of the model in detail, in particular the transitions to the\ntripartite entangled plaquette state, which are consistent with conventional\norder-disorder transitions. Our results can be interpreted as a description of\nthe charge sector dynamics of a Hubbard model applied to the description of the\nspin liquid candidate ${\\mathrm{LiZn}}_{2}{\\mathrm{Mo}}_{3}{\\mathrm{O}}_{8}$,\nas well as a model of strongly correlated bosonic atoms loaded onto highly\ntunable {\\it trimerized} optical Kagome lattices.", "category": "cond-mat_str-el" }, { "text": "Magnetic behavior, Griffiths phase and magneto-transport study in 3$d$\n based nano-crystalline double perovskite Pr$_2$CoMnO$_6$: Double perovskite (DP) oxide material receive extensive research interest due\nto exciting physical properties with potential technological application. 3$d$\nbased DP oxides are promising for exciting physics like magnetodielectric,\nferroelectric, Griffith phase etc., specially Co/Mn DPs are gaining much\nresearch interest. In this paper we present the study of magnetic phase and\ntransport properties in nano-crystalline Pr$_2$CoMnO$_6$ a 3$d$ based double\nperovskite compound. This material shows a paramagnetic (PM) to ferromagnetic\n(FM) phase transition below 173 K marked by a rapid increase in magnetic moment\ndue to spin ordering. We found divergence in inverse magnetic susceptibility\n($\\chi$$^{-1}$) from Curie weiss behavior around 206 K which indicates the\nevolution of Griffiths phase before actual PM-FM transition. We found that the\nGriffiths phase suppressed with increasing applied magnetic filed. For the\nunderstanding of charge transport in this material we have measured temperature\ndependent electrical resistivity. Pr$_2$CoMnO$_6$ is a strong insulator where\nresistivity increase abruptly below magnetic phase transition. To understand\nthe effect of magnetic field on transport behavior we have also measured the\nmagnetoresistance (MR) at different temperatures. Sample shows the negative MR\nwith maximum value $\\sim$22 $\\%$ under applied magnetic field of 50 kOe at 125\nK. MR follows quadratic field dependency above $T_c$ however below $T_c$ the MR\nshows deviation from this field dependency at low field.", "category": "cond-mat_str-el" }, { "text": "Electrical and thermal transport in van der Waals magnets\n 2H-M$_x$TaS$_2$ (M = Mn, Co): We report a detailed study of electrical and thermal transport properties in\n2H-M$_x$TaS$_2$ (M = Mn, Co) magnets where M atoms are intercalated in the van\nder Waals gap. The intercalation induces ferromagentism with an easy-plane\nanisotropy in 2H-Mn$_x$TaS$_2$, but ferromagnetism with a strong uniaxial\nanisotropy in 2H-Co$_{0.22}$TaS$_2$, which finally evolves into a\nthree-dimensional antiferromagnetism in 2H-Co$_{0.34}$TaS$_2$.\nTemperature-dependent electrical resistivity shows metallic behavior for all\nsamples. Thermopower is negative in the whole temperature range for\n2H-Co$_x$TaS$_2$, whereas the sign changes from negative to positive with\nincreasing Mn for 2H-Mn$_x$TaS$_2$. The diffusive thermoelectric response\ndominates in both high- and low-temperature ranges for all samples. A clear\nkink in electrical resistivity, a weak anomaly in thermal conductivity, as well\nas a slope change in thermopower were observed at the magnetic transitions for\n2H-Mn$_{0.28}$TaS$_2$ ($T_\\textrm{c}$ $\\approx$ 82 K) and 2H-Co$_{0.34}$TaS$_2$\n($T_\\textrm{N}$ $\\approx$ 36 K), respectively, albeit weaker for lower $x$\ncrystals. Co-intercalation promoted ferromagnetic to antiferromagnetic\ntransition is further confirmed by the Hall resistivity; the sign change of the\nordinary Hall coefficient indicates a multi-band behavior in 2H-Co$_x$TaS$_2$.", "category": "cond-mat_str-el" }, { "text": "Matrix Product State applications for the ALPS project: The density-matrix renormalization group method has become a standard\ncomputational approach to the low-energy physics as well as dynamics of\nlow-dimensional quantum systems. In this paper, we present a new set of\napplications, available as part of the ALPS package, that provide an efficient\nand flexible implementation of these methods based on a matrix-product state\n(MPS) representation. Our applications implement, within the same framework,\nalgorithms to variationally find the ground state and low-lying excited states\nas well as simulate the time evolution of arbitrary one-dimensional and\ntwo-dimensional models. Implementing the conservation of quantum numbers for\ngeneric Abelian symmetries, we achieve performance competitive with the best\ncodes in the community. Example results are provided for (i) a model of\nitinerant fermions in one dimension and (ii) a model of quantum magnetism.", "category": "cond-mat_str-el" }, { "text": "Doping Effects on the two-dimensional Spin Dimer Compound\n $SrCu_2(BO_3)_2$: A series of compounds M$_{0.1}$Sr$_{0.9}$Cu$_2$(BO$_3$)$_2$ with Sr\nsubstituted by M=Al, La, Na and Y were prepared by solid state reaction. XRD\nanalysis showed that these doping compounds are isostructural to\nSrCu$_2$(BO$_3$)$_2$. The magnetic susceptibility from 1.9K to 300K in an\napplied magnetic field of 1.0T and the specific heat from 1.9K to 25K in\napplied fields up to 14T were measured. The spin gap is deduced from the low\ntemperature susceptibility as well as the specific heat. It is found that the\nspin gap is strongly suppressed by magnetic fields. No superconductivity is\nobserved in all four samples.", "category": "cond-mat_str-el" }, { "text": "Magnetism in Kitaev Quantum Spin Liquid Candidate RuBr$_3$: The present studies show that long-range magnetic order takes place in\nRuBr$_3$ at $\\approx$ 34 K. The observations of clear oscillations in the muon\ntime spectra demonstrate the presence of well-defined internal fields at the\nmuon sites. The magnetic ordering appears to be very robust and static\nsuggesting a more conventional nature of magnetic ordering in the RuBr$_3$\nsystem at zero field. Present investigations prove that in RuBr$_3$ the Kitaev\ninteractions are likely to be weakened at zero field in comparison to the\n$\\alpha$-RuCl$_3$ system. This proves that it is possible to tune the Kitaev\ninteractions by replacing Cl with heavier halogen elements such as Br.", "category": "cond-mat_str-el" }, { "text": "Combining complex and radial slave boson fields within the\n Kotliar-Ruckenstein representation of correlated impurities: The gauge symmetry group of any slave boson representation allows to gauge\naway the phase of bosonic fields. One benefit of this radial field formulation\nis the elimination of spurious Bose condensations when saddle-point\napproximation is performed. Within the Kotliar-Ruckenstein representation,\nthree of the four bosonic fields can be radial while the last one has to remain\ncomplex. In this work, we present the procedure to carry out the functional\nintegration involving constrained fermionic fields, complex bosonic fields, and\nradial bosonic fields. The correctness of the representation is verified by\nexactly evaluating the partition function and the Green's function of the\nHubbard model in the atomic limit.", "category": "cond-mat_str-el" }, { "text": "Microscopic model realization of $\\boldsymbol{d}$-wave pseudospin\n current order in Sr$_{\\boldsymbol{2}}$IrO$_{\\boldsymbol{4}}$: The $d$-wave pseudospin current order ($d$PSCO) with staggered circulating\npseudospin current has been proposed as the hidden electronic order to describe\nthe unexpected breaking of spatial symmetries in stoichiometric\nSr$_{2}$IrO$_{4}$ and the unconventional pseudogap phenomena in electron doped\nSr$_{2}$IrO$_{4}$. However, a microscopic model for the emergence of $d$PSCO is\nstill lacking. The nearest neighbor Coulomb repulsion $V$, which is expected to\nbe significant in Sr$_{2}$IrO$_{4}$ due to the large spatial extension of the\nIr $5d$ orbitals, is capable of driving $d$PSCO on the mean-field level, albeit\nthe latter is energetically degenerate to the staggered flux phase with\ncirculating charge current. We find the in-plane anisotropy $\\Gamma_2$ in the\neffective superexchange interaction between $J_\\text{eff}={1\\over 2}$\npseudospins, originating from the cooperative interplay between Hund's rule\ncoupling and spin-orbit coupling of Ir $5d$ electrons, is able to lift the\ndegeneracy and stabilize the pseudospin currents. The effective single-orbital\nmodel of $J_\\text{eff}={1\\over 2}$ electrons, including onsite Coulomb\nrepulsion $U$, nearest neighbor Coulomb repulsion $V$, and the in-plane\nanisotropy $\\Gamma_2$, is then studied. We obtain the mean-field ground states,\nanalyze their properties, and determine the phase diagram of stoichiometric\nSr$_{2}$IrO$_{4}$ in the plane spanned by $U$ and $V$ at a fixed $\\Gamma_2$. We\ndemonstrate the realization of $d$PSCO, and its competition and coexistence\nwith antiferromagnetism. Remarkably, we find the coexistence of $d$PSCO and\nantiferromagnetism naturally leads to spin bond nematicity, with the spin\ndirections of these three orders forming nontrivial chirality. Furthermore, we\nshow that the emergence of the coexistent state and its chirality can be tuned\nby carrier doping.", "category": "cond-mat_str-el" }, { "text": "Valley dependent many-body effects in 2D semiconductors: We calculate the valley degeneracy ($g_v$) dependence of the many-body\nrenormalization of quasiparticle properties in multivalley 2D semiconductor\nstructures due to the Coulomb interaction between the carriers. Quite\nunexpectedly, the $g_v$ dependence of many-body effects is nontrivial and\nnon-generic, and depends qualitatively on the specific Fermi liquid property\nunder consideration. While the interacting 2D compressibility manifests\nmonotonically increasing many-body renormalization with increasing $g_v$, the\n2D spin susceptibility exhibits an interesting non-monotonic $g_v$ dependence\nwith the susceptibility increasing (decreasing) with $g_v$ for smaller (larger)\nvalues of $g_v$ with the renormalization effect peaking around $g_v\\sim 1-2$.\nOur theoretical results provide a clear conceptual understanding of recent\nvalley-dependent 2D susceptibility measurements in AlAs quantum wells.", "category": "cond-mat_str-el" }, { "text": "Destruction of long-range order in non-collinear two-dimensional\n antiferromagnets by random-bond disorder: We consider frustrated Heisenberg antiferromagnets, whose clean-limit ground\nstate is characterized by non-collinear long-range order with non-zero vector\nchirality, and study the effects of quenched bond disorder, i.e., random\nexchange couplings. A single bond defect is known to induce a dipolar texture\nin the spin background independent of microscopic details. Using general\nanalytical arguments as well as large-scale simulations for the classical\ntriangular-lattice Heisenberg model, we show that any finite concentration of\nsuch defects destroys long-range order for spatial dimension $d\\leq 2$, in\nfavor of a glassy state whose correlation length in $d=2$ is exponentially\nlarge for small randomness. Our results are relevant for a wide range of\nlayered frustrated magnets.", "category": "cond-mat_str-el" }, { "text": "On the dangers of partial diagrammatic summations: Benchmarks for the\n two-dimensional Hubbard model in the weak-coupling regime: We study the two-dimensional Hubbard model in the weak-coupling regime and\ncompare the self-energy obtained from various approximate diagrammatic schemes\nto the result of diagrammatic Monte Carlo simulations, which sum up all\nweak-coupling diagrams up to a given order. While dynamical mean-field theory\nprovides a good approximation for the local part of the self-energy, including\nits frequency dependence, the partial summation of bubble and/or ladder\ndiagrams typically yields worse results than second order perturbation theory.\nEven widely used self-consistent schemes such as GW or the fluctuation-exchange\napproximation (FLEX) are found to be unreliable. Combining the dynamical\nmean-field self-energy with the nonlocal component of GW in GW+DMFT yields\nimproved results for the local self-energy and nonlocal self-energies of the\ncorrect order of magnitude, but here, too, a more reliable scheme is obtained\nby restricting the nonlocal contribution to the second order diagram. FLEX+DMFT\nis found to give accurate results in the low-density regime, but even worse\nresults than FLEX near half-filling.", "category": "cond-mat_str-el" }, { "text": "Hall resistance in quantum Hall metals due to Pancharatnam phase\n retardation and energy level spacing: We derive the trial Hall resistance formula for the quantum Hall metals to\naddress both the integer and fractional quantum Hall effects. Within the\ndegenerate Landau levels, Zeeman splitting and level crossings in the presence\nof changing magnetic-field strength determine the Pancharatnam phase\nretardation, including the phase acceleration or deceleration, which are\nrelated to the changes in the phase and group momenta of a wavefunction. We\ndiscuss the relevant physical postulates with respect to Pancharatnam phase\nretardation to qualitatively reproduce the measured Hall resistance's zigzag\ncurve for both the integer and the fractional filling factors. Along the way,\nwe give out some hints to falsify our postulates with experiments.", "category": "cond-mat_str-el" }, { "text": "Kondo effect due to a hydrogen impurity in graphene: a multichannel\n Kondo problem with diverging hybridization: We consider the Kondo effect arising from a hydrogen impurity in graphene. As\na first approximation, the strong covalent bond to a carbon atom removes that\ncarbon atom without breaking the $C_{3}$ rotation symmetry, and we only retain\nthe Hubbard interaction on the three nearest neighbors of the removed carbon\natom which then behave as magnetic impurities. These three impurity spins are\ncoupled to three conduction channels with definite helicity, two of which\nsupport a diverging local density of states (LDOS) $\\propto 1/ [ | \\omega \\ |\n\\ln ^{2}( \\Lambda /| \\omega \\ | \\ ) \\ ] $ near the Dirac point $\\omega\n\\rightarrow 0$ even though the bulk density of states vanishes linearly. We\nstudy the resulting 3-impurity multi-channel Kondo model using the numerical\nrenormalization group method. For weak potential scattering, the ground state\nof the Kondo model is a particle-hole symmetric spin-$1/2$ doublet, with\nferromagnetic coupling between the three impurity spins; for moderate potential\nscattering, the ground state becomes a particle-hole asymmetric spin singlet,\nwith antiferromagnetic coupling between the three impurity spins. This behavior\nis inherited by the Anderson model containing the hydrogen impurity and all\nfour carbon atoms in its vicinity.", "category": "cond-mat_str-el" }, { "text": "Spin Correlations in Quantum Wires: We consider theoretically spin correlations in an 1D quantum wire with\nRashba-Dresselhaus spin-orbit interaction (RDI). The correlations of\nnon-interacting electrons display electron-spin resonance at a frequency\nproportional to the RDI coupling. Interacting electrons on varying the\ndirection of external magnetic field transit from the state of Luttinger liquid\n(LL) to the spin density wave (SDW) state. We show that the two-time total spin\ncorrelations of these states are significantly different. In the LL the\nprojection of total spin to the direction of the RDI induced field is conserved\nand the corresponding correlator is equal to zero. The correlators of two\ncomponents perpendicular to the RDI field display a sharp ESR driven by RDI\ninduced intrinsic field. In contrast, in the SDW state the longitudinal\nprojection of spin dominates, whereas the transverse components are suppressed.\nThis prediction indicates a simple way for experimental diagnostic of the SDW\nin a quantum wire.", "category": "cond-mat_str-el" }, { "text": "Coulomb correlation effects in LaOFeAs: LDA+DMFT(QMC) study: Effects of Coulomb correlation on LaOFeAs electronic structure have been\ninvestigated by LDA+DMFT(QMC) method. The calculation results show that LaOFeAs\nis in the regime of intermediate correlation strength with significant part of\nthe spectral density moved from the Fermi energy to Hubbard bands. However the\nsystem is not on the edge of metal insulator-transition because increase of the\nCoulomb interaction parameter value from $U$=4.0 eV to $U$=5.0 eV did not\nresult in insulator state. Correlations affect different d-orbitals not in the\nsame way. $t_{2g}$ states ($xz,yz$ and $x^2-y^2$ orbitals) have higher energy\ndue to crystal filed splitting and are nearly half-filled. Their spectral\nfunctions have pseudogap with Fermi energy position on the higher sub-band\nslope. Lower energy $e_g$ set of d-orbitals ($3z^2-r^2$ and $xy$) have\nsignificantly larger occupancy values with typically metallic spectral\nfunctions.", "category": "cond-mat_str-el" }, { "text": "Spin-frame field theory of a three-sublattice antiferromagnet: We present a nonlinear field theory of a three-sublattice hexagonal\nantiferromagnet. The order parameter is the spin frame, an orthogonal triplet\nof vectors related to sublattice magnetizations and spin chirality. The\nexchange energy, quadratic in spin-frame gradients, has three coupling\nconstants, only two of which manifest themselves in the bulk. As a result, the\nthree spin-wave velocities satisfy a universal relation. Vortices generally\nhave an elliptical shape with the eccentricity determined by the Lam\\'e\nparameters.", "category": "cond-mat_str-el" }, { "text": "Transport properties of Metallic Ruthenates: a DFT+DMFT investigation: We present a systematical theoretical study on the transport properties of an\narchetypal family of Hund's metals, Sr$_2$RuO$_4$,Sr$_3$Ru$_2$O$_7$, SrRuO$_3$\nand CaRuO$_3$, within the combination of first principles density functional\ntheory and dynamical mean field theory. The agreement between theory and\nexperiments for optical conductivity and resistivity is good, which indicates\nthat electron-electron scattering dominates the transport of ruthenates. We\ndemonstrate that in the single-site dynamical mean field approach the transport\nproperties of Hund's metals fall into the scenario of \"resilient\nquasiparticles\". We explains why the single layered compound Sr$_2$RuO$_4$ has\na relative weak correlation with respect to its siblings, which corroborates\nits good metallicity.", "category": "cond-mat_str-el" }, { "text": "Fermi liquid state and enhanced electron correlations in the new iron\n pnictide CaFe$_4$As$_3$: The newly discovered CaFe$_4$As$_3$ system displays low-temperature Fermi\nliquid behavior, with enhanced electron-electron correlations. At high\ntemperatures, the magnetic susceptibility shows Curie-Weiss behavior, with a\nlarge temperature-independent contribution. Antiferromagnetic ordering is\nobserved below T$_N$ = (88.0 $\\pm$ 1.0) K, possibly via a spin density wave\n(SDW) transition. A remarkably sharp drop in resistivity occurs below T$_2$ =\n(26.4 $\\pm$ 1.0) K, correlated with a similarly abrupt increase in the\nsusceptibility, but no visible feature in the specific heat. The electronic\nspecific heat coefficient $\\gamma$ at low temperatures is close to 0.02 J\nmol$^{-1}_{Fe}$ K$^{-2}$, but a higher value for $\\gamma$ ($\\sim$0.08 J\nmol$^{-1}_{Fe}$ K$^{-2}$ can be inferred from a linear C$ / $T \\textit{vs.}\nT$^2$ just above T$_2$. The Kadowaki-Woods ratio A$/\\gamma^2$ = 55$*10^{-5}$\n$\\mu \\Omega$cm mol$^2$ K$^2 $mJ$^{-2}$ is nearly two orders of magnitude larger\nthan that of heavy fermions.", "category": "cond-mat_str-el" }, { "text": "A Criterion for Strange Metallicity in the Lorenz Ratio: The Wiedemann-Franz (WF) law, stating that the Lorenz ratio $L =\n\\kappa/(T\\sigma)$ between the thermal and electrical conductivities in a metal\napproaches a universal constant $L_0=\\pi^2 k_B^2/ (3 e^2)$ at low temperatures,\nis often interpreted as a signature of fermionic Landau quasi-particles. In\ncontrast, we show that various models of weakly disordered non-Fermi liquids\nalso obey the WF law at $T \\to 0$. Instead, we propose using the leading\nlow-temperature correction to the WF law, $L(T)-L_0$ (proportional to the\ninelastic scattering rate), to distinguish different types of strange metals.\nAs an example, we demonstrate that in a solvable model of a marginal Fermi\nliquid, $L(T)-L_0\\propto -T$. Using the quantum Boltzmann equation (QBE)\napproach, we find analogous behavior in a class of marginal- and non-Fermi\nliquids with a weakly momentum-dependent inelastic scattering. In contrast, in\na Fermi liquid, $L(T)-L_0$ is proportional to $-T^2$. This holds even when the\nresistivity grows linearly with $T$, due to $T-$linear quasi-elastic scattering\n(as in the case of electron-phonon scattering at temperatures above the Debye\nfrequency). Finally, by exploiting the QBE approach, we demonstrate that the\ntransverse Lorenz ratio, $L_{xy} = \\kappa_{xy}/(T\\sigma_{xy})$, exhibits the\nsame behavior.", "category": "cond-mat_str-el" }, { "text": "Spin-filtering by field dependent resonant tunneling: We consider theoretically transport in a spinfull one-channel interacting\nquantum wire placed in an external magnetic field. For the case of two\npoint-like impurities embedded in the wire, under a small voltage bias the\nspin-polarized current occurs at special points in the parameter space, tunable\nby a single parameter. At sufficiently low temperatures complete\nspin-polarization may be achieved, provided repulsive interaction between\nelectrons is not too strong.", "category": "cond-mat_str-el" }, { "text": "Signatures of a liquid-crystal transition in spin-wave excitations of\n skyrmions: Understanding the spin-wave excitations of chiral magnetic order, such as the\nskyrmion crystal (SkX), is of fundamental interest to confirm such exotic\nmagnetic order. The SkX is realized by competing Dzyaloshinskii-Moriya and\nferromagnetic-exchange interactions with a magnetic field or anisotropy. Here\nwe compute the dynamical spin structure factor, using Monte Carlo and spin\ndynamics simulations, extracting the spin-wave spectrum in the SkX, in the\nvicinity of the paramagnet to SkX transition. Inside the SkX, we find six\nspin-wave modes, which are supplemented by another mode originating from the\nferromagnetic background. Above the critical temperature $T_s$ for the skyrmion\ncrystallization, we find a diffusive regime, reminiscent of the\nliquid-to-crystal transition, revealing that topological spin texture of\nskyrmionic character starts to develop above $T_s$ as the precursor of the SkX.\nWe discuss the opportunities for the detection of the spin waves of the SkX\nusing inelastic-neutron-scattering experiments in manganite-iridate\nheterostructures.", "category": "cond-mat_str-el" }, { "text": "Candidate local parent Hamiltonian for 3/7 fractional quantum Hall\n effect: While a parent Hamiltonian for Laughlin $1/3$ wave function has been long\nknown in terms of the Haldane pseudopotentials, no parent Hamiltonians are\nknown for the lowest-Landau-level projected wave functions of the composite\nfermion theory at $n/(2n+1)$ with $n\\geq2$. If one takes the two lowest Landau\nlevels to be degenerate, the Trugman-Kivelson interaction produces the\nunprojected 2/5 wave function as the unique zero energy solution. If the lowest\nthree Landau levels are assumed to be degenerate, the Trugman-Kivelson\ninteraction produces a large number of zero energy states at $\\nu=3/7$. We\npropose that adding an appropriately constructed three-body interaction yields\nthe unprojected $3/7$ wave function as the unique zero energy solution, and\nreport extensive exact diagonalization studies that provide strong support to\nthis proposal.", "category": "cond-mat_str-el" }, { "text": "On the effective reconstruction of expectation values from ab initio\n quantum embedding: Quantum embedding is an appealing route to fragment a large interacting\nquantum system into several smaller auxiliary `cluster' problems to exploit the\nlocality of the correlated physics. In this work we critically review\napproaches to recombine these fragmented solutions in order to compute\nnon-local expectation values, including the total energy. Starting from the\ndemocratic partitioning of expectation values used in density matrix embedding\ntheory, we motivate and develop a number of alternative approaches, numerically\ndemonstrating their efficiency and improved accuracy as a function of\nincreasing cluster size for both energetics and non-local two-body observables\nin molecular and solid state systems. These approaches consider the\n$N$-representability of the resulting expectation values via an implicit global\nwave~function across the clusters, as well as the importance of including\ncontributions to expectation values spanning multiple fragments simultaneously,\nthereby alleviating the fundamental locality approximation of the embedding. We\nclearly demonstrate the value of these introduced functionals for reliable\nextraction of observables and robust and systematic convergence as the cluster\nsize increases, allowing for significantly smaller clusters to be used for a\ndesired accuracy compared to traditional approaches in ab initio wave~function\nquantum embedding.", "category": "cond-mat_str-el" }, { "text": "On the origin of the quantum-critical transition in the bilayer\n Heisenberg model: The bilayer Heisenberg antiferromagnet is known to exhibit a quantum-critical\ntransition at a particular value of the inter-layer coupling. Using a new type\nof coherent state, appropriate to the special order parameter structure of the\nbilayer, we map the problem onto the quantum non-linear sigma model. It is\nfound that the bare coupling constant diverges at the classical transition of\nChubukov and Morr, so that in any finite dimension the actual transition occurs\ninside the ordered phase of the classical theory.", "category": "cond-mat_str-el" }, { "text": "Interacting Topological Superconductors and possible Origin of $16n$\n Chiral Fermions in the Standard Model: Motivated by the observation that the Standard Model of particle physics\n(plus a right-handed neutrino) has precisely 16 Weyl fermions per generation,\nwe search for $(3+1)$-dimensional chiral fermionic theories and chiral gauge\ntheories that can be regularized on a 3 dimensional spatial lattice when and\nonly when the number of flavors is an integral multiple of 16. All these\nresults are based on the observation that local interactions reduce the\nclassification of certain $(4+1)$-dimensional topological superconductors from\n$\\mathbb{Z}$ to $\\mathbb{Z}_{8}$, which means that one of their\n$(3+1)$-dimensional boundaries can be gapped out by interactions without\nbreaking any symmetry when and only when the number of boundary chiral fermions\nis an integral multiple of $16$.", "category": "cond-mat_str-el" }, { "text": "Short-range antiferromagnetic correlations in the superconducting state\n of filled skutterudite alloys Pr$_{1-x}$Eu$_x$Pt$_4$Ge$_{12}$: Motivated by current research efforts towards exploring the interplay between\nmagnetism and superconductivity in multiband electronic systems, we have\ninvestigated the effects of Eu substitution through thermodynamic measurements\non the superconducting filled skutterudite alloys\nPr$_{1-x}$Eu$_x$Pt$_4$Ge$_{12}$. An increase in Eu concentration leads to a\nsuppression of the superconducting transition temperature consistent with an\nincrease of magnetic entropy due to Eu local moments. While the low-temperature\nheat capacity anomaly is present over the whole doping range, we find that in\nalloys with $x\\leq0.5$ the Schottky peaks in the heat capacity in the\nsuperconducting state appear to be due to Zeeman splitting by an internal\nmagnetic field. Our theoretical modeling suggests that this field is a result\nof the short-range antiferromagnetic correlations between the europium ions.\nFor the samples with $x > 0.5$, the peaks in the heat capacity signal the onset\nof antiferromagnetic (AFM) ordering of the Eu moments.", "category": "cond-mat_str-el" }, { "text": "Temperature-driven hidden 5f itinerant-localized crossover in\n heavy-fermion compound PuIn3: The temperature-dependent evolution pattern of 5f electrons helps to\nelucidate the long-standing itinerant-localized dual nature in plutonium-based\ncompounds. In this work, we investigate the correlated electronic states of\nPuIn3 dependence on temperature by using a combination of the density\nfunctional theory and the dynamical mean-field theory. Not only the\nexperimental photoemission spectroscopy is correctly reproduced, but also a\npossible hidden 5f itinerant-localized crossover is identified. Moreover, it is\nfound that the quasiparticle multiplets from the many-body transitions\ngradually enhance with decreasing temperature, accompanied by the\nhybridizations with 5f electrons and conduction bands. The temperature-induced\nvariation of Fermi surface topology suggests a possible electronic Lifshitz\ntransition and the onset of magnetic order at low temperature. Finally, the\nubiquitous existence orbital selective 5f electron correlation is also\ndiscovered in PuIn3. These illuminating results shall enrich the understanding\non Pu-based compounds and serve as critical predictions for ongoing\nexperimental research.", "category": "cond-mat_str-el" }, { "text": "Flat-band ferromagnetism in a correlated topological insulator on a\n honeycomb lattice: We study the flat-band ferromagnetic phase of a spinfull and time-reversal\nsymmetric Haldane-Hubbard model on a honeycomb lattice within a bosonization\nformalism for flat-band Z$_2$ topological insulators. Such a study extend our\nprevious one [L. S. G. Leite and R. L. Doretto, Phys. Rev. B {\\bf 104}, 155129\n(2021)] concerning the flat-band ferromagnetic phase of a correlated Chern\ninsulator described by a Haldane-Hubbard model. We consider the topological\nHubbard model at $1/4$ filling of its corresponding noninteracting limit and in\nthe nearly flat band limit of its lower free-electronic bands. We show that it\nis possible to define boson operators associated with two distinct spin-flip\nexcitations, one that changes (mixed-lattice excitations) and a second one that\npreserves (same-lattice excitations) the index related with the two triangular\nsublattices. Within the bosonization scheme, the fermionic model is mapped into\nan effective interacting boson model, whose quadratic term is considered at the\nharmonic approximation in order to determine the spin-wave excitation spectrum.\nFor both mixed and same-lattice excitations, we find that the spin-wave\nspectrum is gapped and has two branches, with an energy gap between the lower\nand the upper bands at the $K$ and $K'$ points of the first Brillouin zone.\nSuch a behavior is distinct from the one of the corresponding correlated Chern\ninsulator, whose spin-wave spectrum has a Goldstone mode at the center of the\nfirst Brillouin zone and Dirac points at $K$ and $K'$ points. We also find some\nevidences that the spin-wave bands for the same-lattice excitations might be\ntopologically nontrivial even in the completely flat band limit.", "category": "cond-mat_str-el" }, { "text": "Theory of Half-metallic Ferrimagnetism in Double Perovskites: We present a comprehensive theory of the temperature- and disorder-dependence\nof half-metallic ferrimagnetism in the double perovskite Sr$_2$FeMoO$_6$ (SFMO)\nwith $T_c$ above room temperature. We show that the magnetization $M(T)$ and\nconduction electron polarization $P(T)$ are both proportional to the\nmagnetization $M_S(T)$ of localized Fe spins. We derive and validate an\neffective spin Hamiltonian, amenable to large-scale three-dimensional\nsimulations. We show how $M(T)$ and $T_c$ are affected by disorder, ubiquitous\nin these materials. We suggest a way to enhance $T_c$ in SFMO without\nsacrificing polarization.", "category": "cond-mat_str-el" }, { "text": "Exact band structures for 1D superlattices beyond the tight-binding\n approximation: The band structures describing non-interacting particles in one-dimensional\nsuperlattices of arbitrary periodicity are obtained by an analytical\ndiagonalization of the Hamiltonian without adopting the popular tight-binding\napproximation. The results are compared with those of the tight-binding\napproximation. In this way, a quantitative prediction of the validity and\nfailure of the tight-binding approximation becomes possible. In particular, it\nis demonstrated that in contrast to the prediction of the tight-binding\napproximation the central energy bands do not touch for periodicities $\\tau$ of\nthe lattice where $\\tau=4n$ and $n$ is an integer.", "category": "cond-mat_str-el" }, { "text": "Spin Dynamics at Very Low Temperature in Spin Ice Dy$_2$Ti$_2$O$_7$: We have performed AC susceptibility and DC magnetic relaxation measurements\non the spin ice system Dy$_2$Ti$_2$O$_7$ down to 0.08 K. The relaxation time of\nthe magnetization has been estimated below 2 K down to 0.08 K. The spin\ndynamics of Dy$_2$Ti$_2$O$_7$ is well described by using two relaxation times\n($\\tau_{\\rm S}$ (short time) and $\\tau_{\\rm L}$ (long time)). Both $\\tau_{\\rm\nS}$ and $\\tau_{\\rm L}$ increase on cooling. Assuming the Arrhenius law in the\ntemperature range 0.5-1 K, we obtained an energy barrier of 9 K. Below 0.5 K,\nboth $\\tau_{\\rm S}$ and $\\tau_{\\rm L}$ show a clear deviation from the thermal\nactivated dynamics toward temperature independent relaxation, suggesting a\nquantum dynamics.", "category": "cond-mat_str-el" }, { "text": "Exact solution of electronic transport in semiconductors dominated by\n scattering on polaronic impurities: The scattering of electrons on impurities with internal degrees of freedom is\nbound to produce the signatures of the scatterer's own dynamics and results in\nnontrivial electronic transport properties. Previous studies of polaronic\nimpurities in low-dimensional structures, like molecular junctions and\none-dimensional nanowire models, have shown that perturbative treatments cannot\naccount for a complex energy dependence of the scattering cross section in such\nsystems. Here we derive the exact solution of polaronic impurities shaping the\nelectronic transport in bulk (3D) systems. In the model with a short-ranged\nelectron-phonon interaction, we solve for and sum over all elastic and\ninelastic partial cross sections, abundant in resonant features. The\ntemperature dependence of the charge mobility shows the power-law dependence,\n$\\mu(T)\\propto T^{-\\nu}$, with $\\nu$ being highly sensitive to impurity\nparameters. The latter may explain nonuniversal power-law exponents observed\nexperimentally, e.g. in high-quality organic molecular semiconductors.", "category": "cond-mat_str-el" }, { "text": "Electronic structure and parity effects in correlated nanosystems: We discuss the spectral, transport and magnetic properties of quantum\nnanowires composed of N\\leq 13 atoms and containing either even or odd numbers\nof valence electrons. In our approach we combine Exact Diagonalization and Ab\nInitio calculations (EDABI method). The analysis is performed as a function of\nthe interatomic distance. The momentum distribution differs drastically for\nthose obtained for even N with those for odd N, whereas the Drude weight\nevolves smoothly. A role of boundary conditions is stressed.", "category": "cond-mat_str-el" }, { "text": "Colloquium: Hidden Order, Superconductivity, and Magnetism -- The\n Unsolved Case of URu2Si2: This Colloquium reviews the 25 year quest for understanding the continuous\n(second-order) mean-field-like phase transition occurring at 17.5 K in URu2Si2.\nAbout ten years ago, the term hidden order (HO) was coined and has since been\nutilized to describe the unknown ordered state, whose origin cannot be\ndisclosed by conventional solid-state probes, such as x rays, neutrons, or\nmuons. HO is able to support superconductivity at lower temperatures (Tc ~ 1.5\nK), and when magnetism is developed with increasing pressure both the HO and\nthe superconductivity are destroyed. Other ways of probing the HO are via\nRh-doping and very large magnetic fields. During the last few years a variety\nof advanced techniques have been tested to probe the HO state and their\nattempts will be summarized. A digest of recent theoretical developments is\nalso included. It is the objective of this Colloquium to shed additional light\non the HO state and its associated phases in other materials.", "category": "cond-mat_str-el" }, { "text": "Universal Duality in Luttinger Liquid Coupled to Generic Environment: We study a Luttinger Liquid (LL) coupled to a generic environment consisting\nof bosonic modes with arbitrary density-density and current-current\ninteractions. The LL can be either in the conducting phase and perturbed by a\nweak scatterer or in the insulating phase and perturbed by a weak link. The\nenvironment modes can also be scattered by the imperfection in the system with\narbitrary transmission and reflection amplitudes. We present a general method\nof calculating correlation functions under the presence of the environment and\nprove the duality of exponents describing the scaling of the weak scatterer and\nof the weak link. This duality holds true for a broad class of models and is\nsensitive to neither interaction nor environmental modes details, thus it shows\nup as the universal property. It ensures that the environment cannot generate\nnew stable fixed points of the RG flow. Thus, the LL always flows toward either\nconducting or insulating phase. Phases are separated by a sharp boundary which\nis shifted by the influence of the environment. Our results are relevant, for\nexample, for low-energy transport in (i) an interacting quantum wire or a\ncarbon nanotube where the electrons are coupled to the acoustic phonons\nscattered by the lattice defect; (ii) a mixture of interacting fermionic and\nbosonic cold atoms where the bosonic modes are scattered due to an abrupt local\nchange of the interaction, (iii) mesoscopic electric circuits.", "category": "cond-mat_str-el" }, { "text": "Effective electron-electron interaction in a two dimensional\n paramagnetic system: We analyze the effective electron-electron interaction in a two dimensional\npolarized paramagnetic system. The spin degree of freedom, s, is manifestly\npresent in the expressions of spin dependent local field factors that describe\nthe short range exchange (x) and correlation (c) effects. Starting from the\nexact asymptotic values of the local field correction functions for large and\nsmall momentum at zero frequency we obtain self-consistent expressions across\nthe whole spectrum of momenta. Then, the effective interaction between two\nelectrons with spins s and s' is calculated. We find that the four effective\ninteractions, up-up, up-down, down-up and down-down, are different. We also\nobtain their qualitative dependence on the electronic density and polarization\nand note that these results are independent of the approximation used for the\nlocal field correction functions at intermediate momenta.", "category": "cond-mat_str-el" }, { "text": "Electron-phonon coupling and spin-charge separation in one-dimensional\n Mott insulators: We examine the single-particle excitation spectrum in the one-dimensional\nHubbard-Holstein model at half-filling by performing the dynamical density\nmatrix renormalization group (DDMRG) calculation. The DDMRG results are\ninterpreted as superposition of spectra for a spinless carrier dressed with\nphonons. The superposition is a consequence of robustness of the spin-charge\nseparation against electron-phonon coupling. The separation is in contrast to\nthe coupling between phonon and spin degrees of freedom in two-dimensional\nsystems. We discuss implication of the results of the recent angle-resolved\nphotoemission spectroscopy measurements on SrCuO${}_{2}$.", "category": "cond-mat_str-el" }, { "text": "Anomalous Hall Effect in Graphite: We report on the experimental observation of an anomalous Hall effect (AHE)\nin highly oriented pyrolytic graphite samples. The overall data indicate that\nthe AHE in graphite can be self-consistently understood within the frameworks\nof the magnetic-field-driven excitonic pairing models.", "category": "cond-mat_str-el" }, { "text": "Gapless edge states of BF field theory and translation-symmetric Z2 spin\n liquids: We study possible gapless edge states of translation-symmetric Z2 spin\nliquids. The gapless edge states emerge from dangling Majorana fermions at the\nboundary. We construct a series of mean-field Hamiltonians of Z2 spin liquids\non the square lattice; these models can be obtained by generalization of Wen's\nexactly solvable plaquette model. We also study the details of the edge theory\nof these Z2 spin liquids and find their effective BF theory descriptions. The\neffective BF theories are shown to describe the crystal momenta of the ground\nstates and their degeneracies and to predict the edge theories of these Z2 spin\nliquids. As a byproduct, we obtained a way to classify the BF theories\nreflecting the lattice symmetries. We discuss in closing three-dimensional Z2\nspin liquids with gapless surface states on the cubic lattice.", "category": "cond-mat_str-el" }, { "text": "Quantum phase transitions in d-wave superconductors: Motivated by the strong, low temperature damping of nodal quasiparticles\nobserved in some cuprate superconductors, we study quantum phase transitions in\nd_{x^2-y^2} superconductors with a spin-singlet, zero momentum, fermion\nbilinear order parameter. We present a complete, group-theoretic classification\nof such transitions into 7 distinct cases (including cases with nematic order)\nand analyze fluctuations by the renormalization group. We find that only 2, the\ntransitions to d_{x^2-y^2}+is and d_{x^2-y^2} + i d_{xy} pairing, possess\nstable fixed points with universal damping of nodal quasiparticles; the latter\nleaves the gapped quasiparticles along (1,0), (0,1) essentially undamped.", "category": "cond-mat_str-el" }, { "text": "Renormalized SO(5) symmetry in ladders with next-nearest-neighbor\n hopping: We study the occurrence of SO(5) symmetry in the low-energy sector of\ntwo-chain Hubbard-like systems by analyzing the flow of the running couplings\n($g$-ology) under renormalization group in the weak-interaction limit. It is\nshown that SO(5) is asymptotically restored for low energies for rather general\nparameters of the bare Hamiltonian. This holds also with inclusion of a\nnext-nearest-neighbor hopping which explicitly breaks particle-hole symmetry\nprovided one accounts for a different single-particle weight for the\nquasiparticles of the two bands of the system. The physical significance of\nthis renormalized SO(5) symmetry is discussed.", "category": "cond-mat_str-el" }, { "text": "Fine structures in the spectrum of the open-boundary Heisenberg chain at\n large anisotropies: At large anisotropies, the spectrum of the Heisenberg XXZ spin chain\nseparates into `bands' with energies largely determined by the number of domain\nwalls. The band structure is richer with open boundary conditions: there are\nmore bands and the bands develop intricate fine structures. We characterize and\nexplain these structures and substructures in the open-boundary chain. The fine\nstructures are explained using degenerate perturbation theory. We also present\nsome dynamical consequences of these sub-band structures, through explicit time\nevolution of the wavefunction from initial states motivated by the fine\nstructure analysis.", "category": "cond-mat_str-el" }, { "text": "Smeared quantum phase transition in the dissipative random quantum Ising\n model: We investigate the quantum phase transition in the random transverse-field\nIsing model under the influence of Ohmic dissipation. To this end, we\nnumerically implement a strong-disorder renormalization-group scheme. We find\nthat Ohmic dissipation destroys the quantum critical point and the associated\nquantum Griffiths phase by smearing. Our results quantitatively confirm a\nrecent theory [Phys. Rev. Lett. {\\bf 100}, 240601 (2008)] of smeared quantum\nphase transitions.", "category": "cond-mat_str-el" }, { "text": "Two Anderson impurities in a 2D host with Rashba spin-orbit interaction: We have studied the two-dimensional two-impurity Anderson model with\nadditional Rashba spin-orbit interaction by means of the modified perturbation\ntheory. The impurity Green's functions we have constructed exactly reproduce\nthe first four spectral moments. We discuss the height and the width of the\neven/odd Kondo peaks as functions of the inter-impurity distance and the Rashba\nenergy $E_R$ (the strength of the Rashba spin-orbit interaction). For small\nimpurity separations the Kondo temperature shows a non-monotonic dependence on\n$E_R$ being different in the even and the odd channel. We predict that the\nKondo temperature has only almost linear dependence on $E_R$ and not an\nexponential increase with $E_R$", "category": "cond-mat_str-el" }, { "text": "Origin of infrared peaks in the optical conductivity of ytterbium\n compounds: We have calculated optical conductivity [$\\sigma(\\omega)$] spectra of\nytterbium compounds (YbAl$_3$, YbAl$_2$, YbCu$_2$Si$_2$, YbNi$_2$Ge$_2$,\nYbInCu$_4$, YbRh$_2$Si$_2$, YbIr$_2$Si$_2$, and YbB$_{12}$) based on the direct\ninterband transition derived from first-principle band calculation and compared\nthe results with the experimentally obtained $\\sigma(\\omega)$ spectra. The\nspectral feature of a peak in the middle-infrared region (mid-IR peak) and a\nshoulder structure in the far-infrared region (far-IR shoulder) in the\nexperimental $\\sigma(\\omega)$ spectra can be described by the band calculation\nwith a common renormalization factor. This result indicates that the infrared\nspectra of Yb compounds originate from the interband transition from the Yb\n$4f$ state but that the Yb $4f$ state shifts to the Fermi level with strong\nelectron correlation.", "category": "cond-mat_str-el" }, { "text": "Spin functional renormalization group for dimerized quantum spin systems: We investigate dimerized quantum spin systems using the spin functional\nrenormalization group approach proposed by Krieg and Kopietz [Phys. Rev. B 99,\n060403(R) (2019)] which directly focuses on the physical spin correlation\nfunctions and avoids the representation of the spins in terms of fermionic or\nbosonic auxiliary operators. Starting from decoupled dimers as initial\ncondition for the renormalization group flow equations, we obtain the spectrum\nof the triplet excitations as well as the magnetization in the quantum\nparamagnetic, ferromagnetic, and thermally disordered phases at all\ntemperatures. Moreover, we compute the full phase diagram of a weakly coupled\ndimerized spin system in three dimensions, including the correct mean field\ncritical exponents at the two quantum critical points.", "category": "cond-mat_str-el" }, { "text": "Novel Orbital Ordering induced by Anisotropic Stress in a Manganite Thin\n Film: We performed resonant and nonresonant x-ray diffraction studies of a\nNd0.5Sr0.5MnO3 thin film that exhibits a clear first-order transition. Lattice\nparameters vary drastically at the metal-insulator transition at 170K (=T_MI),\nand superlattice reflections appear below 140K (=T_CO). The electronic\nstructure between T_MI and T_CO is identified as A-type antiferromagnetic with\nthe d_{x2-y2} ferroorbital ordering. Below T_CO, a new type of antiferroorbital\nordering emerges. The accommodation of the large lattice distortion at the\nfirst-order phase transition and the appearance of the novel orbital ordering\nare brought about by the anisotropy in the substrate, a new parameter for the\nphase control.", "category": "cond-mat_str-el" }, { "text": "Anisotropic Magnetic Response in Kondo Lattice with Antiferromagnetic\n Order: Magnetic properties are investigated for the Kondo lattice by using the\ncontinuous time quantum Monte Colro (CT-QMC) and the dynamical mean field\ntheory (DMFT). The DMFT+CT-QMC approach is extended so as to derive the\nanisotropic magnetic response in the antiferromagnetic phase. The longitudinal\nand transverse magnetic susceptibilities are numerically derived in the\nantiferromagnetic phase. For the RKKY regime with a small Kondo coupling, the\ntransverse susceptibility does not decrease below the transition temperature\nwhile the longitudinal susceptibility decreases as expected from the mean field\npicture. In the competing region between the RKKY interaction and the Kondo\neffect, however, both longitudinal and transverse susceptibilities decrease\nbelow the transition temperature. The results obtained naturally explain the\ntemperature dependence of the magnetic susceptibility observed in\nCeT$_2$Al$_{10}$ ($T$=Ru,Os,Fe) family.", "category": "cond-mat_str-el" }, { "text": "Model for the Fractional Quantum Hall Effect problem: A simple one-dimensional model is proposed, in which N spinless repulsively\ninteracting fermions occupy M>N degenerate states. It is argued that the energy\nspectrum and the wavefunctions of this system strongly resemble the spectrum\nand wavefunctions of 2D electrons in the lowest Landau level (the problem of\nthe Fractional Quantum Hall Effect). In particular, Laughlin-type wavefunctions\ndescribe ground states at filling factors v = N/M = 1(2m+1). Within this model\nthe complimentary wavefunction for v = 1-1/(2m + 1) is found explicitly and\nextremely simple ground state wavefunctions for arbitrary odd-denominator\nfilling factors are proposed.", "category": "cond-mat_str-el" }, { "text": "Proximity-induced superconductivity in a 2D Kondo lattice of an\n f-electron-based surface alloy: Realizing hybrids of low-dimensional Kondo lattices and superconducting\nsubstrates leads to fascinating platforms for studying the exciting physics of\nstrongly correlated electron systems with induced superconducting pairing.\nHere, we report a scanning tunneling microscopy and spectroscopy study of a new\ntype of two-dimensional (2D) La-Ce alloy grown epitaxially on a superconducting\nRe(0001) substrate. We observe the characteristic spectroscopic signature of a\nhybridization gap evidencing the coherent spin screening in the 2D Kondo\nlattice realized by the ultrathin La-Ce alloy film on normal conducting\nRe(0001). Upon lowering the temperature below the critical temperature of\nrhenium, a superconducting gap is induced with an in-gap Shiba band arising\nfrom the interaction of residual unscreened magnetic moments with the\nsuperconducting substrate. A positive correlation between the Kondo\nhybridization gap and the binding energy of the subgap Shiba band maximum is\nfound. Our results open up a promising route toward the design of artificial\nsuperconducting Kondo and heavy fermion systems.", "category": "cond-mat_str-el" }, { "text": "Crossover to Fermi-liquid behavior for weakly-coupled Luttinger liquids\n in the anisotropic large-dimension limit: We study the problem of the crossover from one- to higher-dimensional metals\nby considering an array of Luttinger liquids (one-dimensional chains) coupled\nby a weak interchain hopping {\\tp.} We evaluate the exact asymptotic low-energy\nbehavior of the self-energy in the anisotropic infinite-dimension limit. This\nlimit extends the dinamical mean field concept to the case of a chain embedded\nin a self-consistent medium. The system flows to a Fermi-liquid fixed point for\nenergies below the dimensional crossover temperature, and the anomalous\nexponent $\\al$ renormalizes to zero, in the case of equal spin and charge\nvelocities. In particular, the single-particle spectral function shows sharp\nquasiparticle peaks with nonvanishing weight along the whole Fermi surface, in\ncontrast to the lowest-order result. Our result is obtained by carring out a\nresummation of all diagrams of the expansion in \\tp contributing to the\nanisotropic $D\\to\\infty$ limit. This is done by solving, in an almost\ncompletely analytic way, an asymptotically exact recursive equation for the\nrenormalized vertices, within a skeleton expansion. Our outcome shows that\nperturbation expansions in \\tp restricted to lowest orders are unreliable below\nthe crossover temperature. The extension to finite dimensions is discussed.\nThis work extends our recent Letter [Phys. Rev. Lett. {\\bf 83}, 128 (1999)],\nand includes all mathematical details.", "category": "cond-mat_str-el" }, { "text": "Incommensurate magnetic order in Ag$_{2}$NiO$_{2}$: The nature of the magnetic transition of the half-filled triangular\nantiferromagnet Ag$_{2}$NiO$_2$ with $T_{\\rm N}$=56K was studied with positive\nmuon-spin-rotation and relaxation ($\\mu^+$SR) spectroscopy. Zero field\n$\\mu^+$SR measurements indicate the existence of a static internal magnetic\nfield at temperatures below $T_{\\rm N}$. Two components with slightly different\nprecession frequencies and wide internal-field distributions suggest the\nformation of an incommensurate antiferromagnetic order below 56 K. This implies\nthat the antifrerromagnetic interaction is predominant in the NiO$_2$ plane in\ncontrast to the case of the related compound NaNiO$_2$. An additional\ntransition was found at $\\sim$22 K by both $\\mu^+$SR and susceptibility\nmeasurements. It was also clarified that the transition at $\\sim$260 K observed\nin the susceptibility of Ag$_{2}$NiO$_{2}$ is induced by a purely structural\ntransition.", "category": "cond-mat_str-el" }, { "text": "Ferromagnetic diagonal stripe states in the two-dimensional Hubbard\n model with $U\\lesssim\\infty$: We have performed a variational Monte Carlo simulation to study the ground\nstate of a two-dimensional Hubbard model on a square lattice in the strong\ncoupling region. The energy gain of possible inhomogeneous electron states are\ncomputed as a function of $U$ when the hole density $\\epsilon=1/8$ and next\nnearest-neighbor hopping $t'/t=-0.30$. The bond-centered ferromagnetic diagonal\nstripe state is stabilized in the strong coupling region ($U/t\\geq$16), which\nis due to the gain of both kinetic energy and on-site Coulomb interaction\nenergy due to the holon moving over the ferromagnetic domain and the gain of\nkinetic-exchange-interaction energy at the antiferromagnetic domain wall.", "category": "cond-mat_str-el" }, { "text": "The Parent of Misfit-Layered Cobalt Oxides: [Sr2O2]qCoO2: Misfit-layered (ML) cobalt oxides of the general formula of [MmA2Om+2]qCoO2\nhave been proven to be efficient thermoelectric materials as the structure is\ncapable in accommodating the two seemingly contradictory characteristics of\nhigh electrical conductivity and large thermo-electric power. They are also\npotential hosts for other oxymoron-like functions. The known phases all contain\none or two square-planar MO (M = Co, Bi, Pb, Tl, etc.) layers sandwiched\ntogether with AO (A = Ca, Sr, Ba, etc.) planes of square symmetry and CoO2\nlayers of hexagonal symmetry. Here we report realization of the simplest (m =\n0) ML phase forming in the Sr-Co-O system with the cation ratio, Sr/Co = 1.\nAtomic-resolution TEM imaging confirms for the new phase the parent three-layer\ncrystal structure, SrO-SrO-CoO2, which is compatible with the formula of\n[Sr2O2]qCoO2. Electron diffraction reveals that the phase is rather\ncommensurate, i.e. the \"misfit parameter\" q is 0.5. Nevertheless, in terms of\nthe transport-property characteristics the new ML parent is comparable to its\nearlier-established and more complex derivatives.", "category": "cond-mat_str-el" }, { "text": "Model Calculation of Electron-Phonon Couplings in a Dimer with a\n Non-Degenerate Orbital: We evaluate all the electron-phonon couplings derived from the one-body\nelectronic interactions, in both the adiabatic and extreme non-adiabatic limit,\nfor a dimer with a non-degenerate orbital built from atomic wave functions of\nGaussian shape. We find largely different values of the coupling parameters in\nthe two cases, as well as different expressions of the corresponding terms in\nthe Hamiltonian.", "category": "cond-mat_str-el" }, { "text": "Domain switching and exchange bias control by electric field in\n multiferroic conical magnet Mn$_2$GeO$_4$: The electric field effect on magnetism was examined in the multiferroic\nconical magnet Mn$_2$GeO$_4$, which shows a strong coupling between\nferromagnetic and ferroelectric order parameters. The systematic evaluation of\nthe electric polarization in the multiferroic phase below 5.5 K under various\nfield cooling conditions reveals that small magnetic fields of 0.1 T\nsignificantly reduce the required electric fields needed to reach saturation.\nBy applying electric fields during magnetic field dependent hysteresis\nmeasurements of magnetization M and polarization P an electrically controllable\nexchange bias was observed, a phenomenon exceedingly rare in single phase\nmultiferroics. Furthermore, non-reversible electric switching of P and M\ndomains was achieved under specific magnetic field conditions.", "category": "cond-mat_str-el" }, { "text": "Valence Bond Phases in $S=1/2$ Kane-Mele-Heisenberg Model: The phase diagram of Kane-Mele-Heisenberg (KMH) model in classical\nlimit~\\cite{zare}, contains disordered regions in the coupling space, as the\nresult of to competition among different terms in the Hamiltonian, leading to\nfrustration in finding a unique ground state. In this work we explore the\nnature of these phase in the quantum limit, for a $S=1/2$. Employing exact\ndiagonalization (ED) in $S_z$ and nearest neighbor valence bond (NNVB) bases,\nbond and plaquette valence bond mean field theories, We show that the\ndisordered regions are divided into ordered quantum states in the form of\nplaquette valence bond crystal(PVBC) and staggered dimerized (SD) phases.", "category": "cond-mat_str-el" }, { "text": "Correlation-driven insulator-metal transition in near-ideal vanadium\n dioxide films: We use polarization- and temperature-dependent x-ray absorption spectroscopy,\nin combination with photoelectron microscopy, x-ray diffraction and electronic\ntransport measurements, to study the driving force behind the insulator-metal\ntransition in VO2. We show that both the collapse of the insulating gap and the\nconcomitant change in crystal symmetry in homogeneously strained\nsingle-crystalline VO2 films are preceded by the purely-electronic softening of\nCoulomb correlations within V-V singlet dimers. This process starts 7 K (+/-\n0.3 K) below the transition temperature, as conventionally defined by\nelectronic transport and x-ray diffraction measurements, and sets the energy\nscale for driving the near-room-temperature insulator-metal transition in this\ntechnologically-promising material.", "category": "cond-mat_str-el" }, { "text": "Single-molecule-mediated heat current between an electronic and a\n bosonic bath: In molecular devices electronic degrees of freedom are coupled to vibrational\nmodes of the molecule, offering an opportunity to study fundamental aspects of\nthis coupling between at the nanoscale. To this end we consider the\nnonequilibrium heat exchange between a conduction band and a bosonic bath\nmediated by a single molecule. For molecules large enough so that on-site\ninteractions can be dropped we carry out an asymptotically exact calculation of\nthe heat current, governed by the smallness of the electron-phonon coupling,\nand obtain the steady state heat current driven by a finite temperature drop.\nAt low temperatures the heat current is found to have a power-law behavior with\nrespect to the temperature difference with the power depending on the nature of\nthe bosonic bath. At high temperatures, on the other hand, the current is\nlinear in the temperature difference for all types of bosonic baths. The\ncrossover between these behaviors is described. Some of the results are given a\nphysical explanation by comparing to a perturbative Master equation calculation\n(whose limitation we examine).", "category": "cond-mat_str-el" }, { "text": "Strong correlation effects in theoretical STM studies of magnetic\n adatoms: We present a theoretical study for the scanning tunneling microscopy (STM)\nspectra of surface-supported magnetic nanostructures, incorporating strong\ncorrelation effects. As concrete examples, we study Co and Mn adatoms on the\nCu(111) surface, which are expected to represent the opposite limits of the\nKondo physics and local moment behavior, using a combination of density\nfunctional theory and both quantum Monte Carlo and exact diagonalization\nimpurity solvers. We examine in detail the effects of temperature $T$,\ncorrelation strength $U$, and the impurity $d$ electron occupancy $N_d$ on the\nlocal density of states. We also study the effective coherence energy scale,\ni.e., the Kondo temperature $T_K$, which can be extracted from the STM spectra.\nTheoretical STM spectra are computed as a function of the STM tip position\nrelative to each adatom. Because of the multi-orbital nature of the adatoms,\nthe STM spectra are shown to consist of a complicated superposition of orbital\ncontributions, with different orbital symmetries, self-energies and Kondo\ntemperatures. For the Mn adatom, which is close to half-filling, the STM\nspectra are featureless near the Fermi level. On the other hand, the\nquasiparticle peak for Co adatom gives rise to strongly position-dependent Fano\nline-shapes.", "category": "cond-mat_str-el" }, { "text": "Strong electron correlations in the normal state of FeSe0.42Te0.58: We investigate the normal state of the '11' iron-based superconductor\nFeSe0.42Te0.58 by angle resolved photoemission. Our data reveal a highly\nrenormalized quasiparticle dispersion characteristic of a strongly correlated\nmetal. We find sheet dependent effective carrier masses between ~ 3 - 16 m_e\ncorresponding to a mass enhancement over band structure values of m*/m_band ~ 6\n- 20. This is nearly an order of magnitude higher than the renormalization\nreported previously for iron-arsenide superconductors of the '1111' and '122'\nfamilies but fully consistent with the bulk specific heat.", "category": "cond-mat_str-el" }, { "text": "Local Pairing at U-impurities in BCS Superconductors: We analyse here the role electrons on Anderson-U impurities play in\nsuperconductivity in a metal alloy. We find that phonon coupling at impurities\ncounteracts the traditional effects which dominate T_c suppression in the\nnon-magnetic limit. In some cases, we findt hat non-magnetic impurities can\nenhance T_c. In the Kondo limit, a Fermi liquid analysis reveals that it is the\nenhancement in the density of states arising from the Kondo resonancethat\ncounteracts pair-weakening.", "category": "cond-mat_str-el" }, { "text": "Nonmagnetic-magnetic transition and magnetically ordered structure in\n SmS: SmS, a prototypical intermediate valence compound, has been studied by\nperforming high-pressure nuclear magnetic resonance measurements on a\n$^{33}$S-enriched sample. The observation of an additional signal below 15-20 K\nabove a nonmagnetic-magnetic transition pressure $P_{\\rm c2} \\approx 2$ GPa\ngives evidence of a magnetic transition. The absence of a Curie-term in the\nKnight shift near $P_{\\rm c2}$ indicates that the localized character of $4f$\nelectrons is entirely screened and the mechanism of the magnetic ordering is\nnot described within a simple localized model. Simultaneously, the line shape\nin the magnetically ordered state is incompatible with a spin density wave\norder. These suggest that the magnetic order in SmS may require an\nunderstanding beyond the conventional framework for heavy fermions. The fact\nthat hyperfine fields from the ordered moments cancel out at the S site leads\nus to a conclusion that the ordered phase has a type II antiferromagnetic\nstructure.", "category": "cond-mat_str-el" }, { "text": "Selection of factorizable ground state in a frustrated spin tube: Order\n by disorder and hidden ferromagnetism: The interplay between frustration and quantum fluctuation in magnetic systems\nis known to be the origin of many exotic states in condensed matter physics. In\nthis paper, we consider a frustrated four-leg spin tube under a magnetic field.\nThis system is a prototype to study the emergence of a nonmagnetic ground state\nfactorizable into local states and the associated order parameter without\nquantum fluctuation, that appears in a wide variety of frustrated systems. The\none-dimensional nature of the system allows us to apply various techniques: a\npath-integral formulation based on the notion of order by disorder,\nstrong-coupling analysis where magnetic excitations are gapped, and\ndensity-matrix renormalization group. All methods point toward an interesting\nproperty of the ground state in the magnetization plateaus, namely, a quantized\nvalue of relative magnetizations between different sublattices (spin imbalance)\nand an almost perfect factorization of the ground state.", "category": "cond-mat_str-el" }, { "text": "Wave Function and Strange Correlator of Short Range Entangled states: We demonstrate the following conclusion: If $|\\Psi\\rangle$ is a $1d$ or $2d$\nnontrivial short range entangled state, and $|\\Omega \\rangle$ is a trivial\ndisordered state defined on the same Hilbert space, then the following quantity\n(so called strange correlator) $C(r, r^\\prime) = \\frac{\\langle \\Omega|\\phi(r)\n\\phi(r^\\prime) | \\Psi\\rangle}{\\langle \\Omega| \\Psi\\rangle}$ either saturates to\na constant or decays as a power-law in the limit $|r - r^\\prime| \\rightarrow\n+\\infty$, even though both $| \\Omega\\rangle$ and $| \\Psi\\rangle$ are quantum\ndisordered states with short-range correlation. $\\phi(r)$ is some local\noperator in the Hilbert space. This result is obtained based on both field\ntheory analysis, and also an explicit computation of $C(r, r^\\prime)$ for four\ndifferent examples: $1d$ Haldane phase of spin-1 chain, $2d$ quantum spin Hall\ninsulator with a strong Rashba spin-orbit coupling, $2d$ spin-2 AKLT state on\nthe square lattice, and the $2d$ bosonic symmetry protected topological phase\nwith $Z_2$ symmetry. This result can be used as a diagnosis for short range\nentangled states in $1d$ and $2d$. A possible diagnosis for $3d$ short range\nentangled states is also proposed.", "category": "cond-mat_str-el" }, { "text": "Direct determination of the spin structure of Nd$_2$Ir$_2$O$_7$ by means\n of neutron diffraction: We report on the spin structure of the pyrochlore iridate Nd$_2$Ir$_2$O$_7$\nthat could be directly determined by means of powder neutron diffraction. Our\nmagnetic structure refinement unravels a so-called all-in/all-out magnetic\nstructure that appears in both, the Nd and the Ir sublattice. The ordered\nmagnetic moments at 1.8 K amount to 0.34(1) $\\mu_\\mathrm{B}$/Ir$^{4+}$ and\n1.27(1) $\\mu_\\mathrm{B}$/Nd$^{3+}$. The Nd$^{3+}$ moment size at 1.8 K is\nsmaller than that expected for the Nd$^{3+}$ ground state doublet. On the other\nhand, the size of the ordered moments of the Ir$^{4+}$ ions at 1.8 K agrees\nvery well with the value expected for a $J_\\mathrm{eff}$ = 1/2 state based on\nthe presence of strong spin-orbit coupling in this system. Finally, our\nmeasurements reveal a parallel alignment of the Nd$^{3+}$ moments with the net\nmoment of its six nearest neighboring Ir$^{4+}$ ions.", "category": "cond-mat_str-el" }, { "text": "Interesting magnetic properties of Fe$_{1-x}$Co$_x$Si alloys: Solid solution between nonmagnetic narrow gap semiconductor FeSi and\ndiamagnetic semi-metal CoSi gives rise to interesting metallic alloys with\nlong-range helical magnetic ordering, for a wide range of intermediate\nconcentration. We report various interesting magnetic properties of these\nalloys, including low temperature re-entrant spin-glass like behaviour and a\nnovel inverted magnetic hysteresis loop. Role of Dzyaloshinski-Moriya\ninteraction in the magnetic response of these non-centrosymmetric alloys is\ndiscussed.", "category": "cond-mat_str-el" }, { "text": "Incoherent transport in a classical spin liquid: We study the energy and spin transport of the classical spin liquid hosted by\nthe pyrochlore Heisenberg antiferromagnet in the large $S$ limit. Molecular\ndynamics calculation suggests that both the energy and spin diffusion constants\napproach finite limits as the temperature tends to zero. We explain our results\nin terms of an effective disorder model, where the energy/spin-carrying normal\nmodes propagate in a quasi-static disordered spin background. The finite zero\ntemperature limits of the diffusion constants are then naturally understood as\na result of the finite mean free path of the normal modes due to the effective\ndisorder.", "category": "cond-mat_str-el" }, { "text": "Experimentally Realized Correlated Electron Materials: From\n Superconductors to Topological Insulators: Recent discoveries, as well as open questions, in experimentally realized\ncorrelated electron materials are reviewed. In particular, high temperature\nsuperconductivity in the cuprates and in the recently discovered iron\npnictides, possible chiral p-wave superconductivity in strontium ruthenate, the\nsearch for quantum spin liquid behavior in real materials, and new experimental\ndiscoveries in topological insulators are discussed.", "category": "cond-mat_str-el" }, { "text": "Spectral functions in a magnetic field as a probe of spin-charge\n separation in a Luttinger liquid: We show that the single-particle spectral functions in a magnetic field can\nbe used to probe spin-charge separation of a Luttinger liquid. Away from the\nFermi momentum, the magnetic field splits both the spinon peak and holon peak;\nhere the spin-charge separation nature is reflected in the different magnitude\nof the two splittings. At the Fermi momentum, the magnetic field splits the\nzero-field peak into {\\it four} peaks. The feasibility of experimentally\nstudying this effect is discussed.", "category": "cond-mat_str-el" }, { "text": "The electronic structure of epitaxially stabilized 5d perovskite\n Ca_{1-x}Sr_xIrO_3 (x = 0, 0.5, and 1) thin films: the role of strong\n spin-orbit coupling: We have investigated the electronic structure of meta-stable perovskite\nCa1-xSrxIrO3 (x = 0, 0.5, and 1) thin films using transport measurements,\noptical spectroscopy, and first-principles calculations. We artificially\nfabricated the perovskite phase of Ca1-xSrxIrO3, which has a hexagonal or post\nperovskite crystal structure in bulk form, by growing epitaxial thin films on\nperovskite GdScO3 substrates using epi-stabilization technique. The transport\nproperties of the perovskite Ca1-xSrxIrO3 films systematically changed from\nnearly insulating (or semi-metallic) for x = 0 to bad metallic for x = 1. Due\nto the extended wavefunctions, 5d electrons are usually delocalized. However,\nthe strong spin-orbit coupling in Ca1-xSrxIrO3 results in the formation of\neffective total angular momentum Jeff = 1/2 and 3/2 states, which puts\nCa1-xSrxIrO3 in the vicinity of a metal-insulator phase boundary. As a result,\nthe electrical properties of the Ca1-xSrxIrO3 films are found to be sensitive\nto x and strain.", "category": "cond-mat_str-el" }, { "text": "A novel continuous time quantum Monte Carlo solver for dynamical mean\n field theory in the compact Legendre representation: Dynamical mean-field theory (DMFT) is one of the most widely-used methods to\ntreat accurately electron correlation effects in ab-initio real material\ncalculations. Many modern large-scale implementations of DMFT in electronic\nstructure codes involve solving a quantum impurity model with a Continuous-Time\nQuantum Monte Carlo (CT-QMC) solver. The main advantage of CT-QMC is that,\nunlike standard quantum Monte Carlo approaches, it is able to generate the\nlocal Green's functions of the correlated system on an arbitrarily fine\nimaginary time grid, and is free of any systematic errors. In this work, we\nextend a hybrid QMC solver proposed by Khatami et al. and Rost et al. to a\nmulti-orbital context. This has the advantage of enabling impurity solver QMC\ncalculations to scale linearly with inverse temperature and permit its\napplication to d and f band materials. In addition, we present a novel Green's\nfunction processing scheme which generates accurate quasi-continuous imaginary\ntime solutions of the impurity problem which overcome errors inherent to\nstandard QMC approaches. This solver and processing scheme are incorporated\ninto a full DFT+DMFT calculation using the CASTEP DFT code. Benchmark\ncalculations for strontium vanadate properties are presented.", "category": "cond-mat_str-el" }, { "text": "Detecting Quantum Anomalies in Open Systems: Symmetries and quantum anomalies serve as powerful tools for constraining\ncomplicated quantum many-body systems, offering valuable insights into\nlow-energy characteristics based on their ultraviolet structure. Nevertheless,\ntheir applicability has traditionally been confined to closed quantum systems,\nrendering them largely unexplored for open quantum systems described by density\nmatrices. In this work, we introduce a novel and experimentally feasible\napproach to detect quantum anomalies in open systems. Specifically, we claim\nthat, when coupled with an external environment, the mixed 't Hooft anomaly\nbetween spin rotation symmetry and lattice translation symmetry gives\ndistinctive characteristics for half-integer and integer spin chains in\nmeasurements of $\\exp(\\rm{i}\\theta S^z_{\\rm tot})$ as a function of $\\theta$.\nNotably, the half-integer spin chain manifests a topological phenomenon akin to\nthe ``level crossing\" observed in closed systems. To substantiate our\nassertion, we develop a lattice-level spacetime rotation to analyze the\naforementioned measurements. Based on the matrix product density operator and\ntransfer matrix formalism, we analytically establish and numerically\ndemonstrate the unavoidable singular behavior of $\\exp(\\rm{i}\\theta S^z_{\\rm\ntot})$ for half-integer spin chains. Conceptually, our work demonstrates a way\nto discuss notions like ``spectral flow'' and ``flux threading'' in open\nsystems not necessarily with a Hamiltonian.", "category": "cond-mat_str-el" }, { "text": "Generating function for projected entangled-pair states: Diagrammatic summation is a common bottleneck in modern applications of\nprojected entangled-pair states, especially in computing low-energy excitations\nof a two-dimensional quantum many-body system. To solve this problem, here we\nextend the generating function approach for tensor network diagrammatic\nsummation, a scheme previously proposed in the context of matrix product\nstates. Taking the form of a one-particle excitation, we show that the excited\nstate can be computed efficiently in the generating function formalism, which\ncan further be used in evaluating the dynamical structure factor of the system.\nOur benchmark results for the spin-$1/2$ transverse-field Ising model and\nHeisenberg model on the square lattice provide a desirable accuracy, showing\ngood agreement with known results. We then study the spin-$1/2$ $J_1$-$J_2$\nmodel on the same lattice and investigate the dynamical properties of the\nputative gapless spin liquid phase. We conclude with a discussion on\ngeneralizations to multi-particle excitations.", "category": "cond-mat_str-el" }, { "text": "Non-equilibrium conductivity at quantum critical points: Quantum criticality provides an important route to revealing universal\nnon-equilibrium behaviour. A canonical example of a quantum critical point is\nthe Bose-Hubbard model, which we study under the application of an electric\nfield. A Boltzmann transport formalism and $\\epsilon$-expansion are used to\nobtain the non-equilibrium conductivity and current noise. This approach allows\nus to explicitly identify how a universal non-equilibrium steady state is\nmaintained, by identifying the rate-limiting step in balancing Joule heating\nand dissipation to a heat bath. It also reveals that the non-equilibrium\ndistribution function is very far from a thermal distribution.", "category": "cond-mat_str-el" }, { "text": "Charge and orbital order due to cooperative Jahn-Teller effect in\n manganite chains: We derive an effective Hamiltonian that takes into account the quantum nature\nof phonons and models cooperative Jahn-Teller effect in the adiabatic regime\nand at strong electron-phonon coupling in one dimension. Our approach involves\nmapping a strong-coupling problem to a weak-coupling one by using a duality\ntransformation. Subsequently, a sixth-order perturbation theory is employed in\nthe polaronic frame of reference where the small parameter is inversely\n(directly) proportional to the coupling (adiabaticity). We study charge and\norbital order in ferromagnetic manganite chains and address the pronounced\nelectron-hole asymmetry in the observed phase diagram. In particular, at strong\ncoupling, we offer an explanation for the observed density dependence of the\nwavevector of charge modulation, i.e., wavevector is proportional to\n(independent of) electron density on the electron-doped (hole-doped) side of\nthe phase diagram of manganites. We also provide a picture for the charge and\norbital order at special fillings $ \\frac{1}{2}$, $\\frac{1}{3}$, $\\frac{1}{4}$,\nand $ \\frac{1}{5}$; while focusing on the ordering controversy at fillings\n$\\frac{1}{3}$ and $\\frac{1}{4}$, we find that Wigner-crystal arrangement is\npreferred over bi-stripe order.", "category": "cond-mat_str-el" }, { "text": "Thermally activated exchange narrowing of the Gd3+ ESR fine structure in\n a single crystal of Ce1-xGdxFe4P12 (x = 0.001) skutterudite: We report electron spin resonance (ESR) measurements in the Gd3+ doped\nsemiconducting filled skutterudite compound Ce1-xGdxFe4P12 (x = 0.001). As the\ntemperature T varies from T = 150 K to T = 165 K, the Gd3+ ESR fine and\nhyperfine structures coalesce into a broad inhomogeneous single resonance. At T\n= 200 K the line narrows and as T increases further, the resonance becomes\nhomogeneous with a thermal broadening of 1.1(2) Oe/K. These results suggest\nthat the origin of these features may be associated to a subtle interdependence\nof thermally activated mechanisms that combine: i) an increase with T of the\ndensity of activated conduction-carriers across the T-dependent semiconducting\npseudogap; ii) the Gd3+ Korringa relaxation process due to an exchange\ninteraction, J_{fd}S.s, between the Gd3+ localized magnetic moments and the\nthermally activated conduction-carriers and; iii) a relatively weak confining\npotential of the rare-earth ions inside the oversized (Fe2P3)4 cage, which\nallows the rare-earths to become rattler Einstein oscillators above T = 148 K.\nWe argue that the rattling of the Gd3+ ions, via a motional narrowing\nmechanism, also contributes to the coalescence of the ESR fine and hyperfine\nstructure.", "category": "cond-mat_str-el" }, { "text": "Magnetic ordering tendencies in hexagonal boron nitride-bilayer graphene\n moir\u00e9 structures: When hexagonal boron nitride (hBN) and graphene are aligned at zero or small\ntwist angle, a moir\\'e structure is formed due to the small lattice constant\nmismatch between the two structures. In this work, we analyze magnetic ordering\ntendencies, driven by onsite Coulomb interactions, of encapsulated bilayer\ngraphene (BG) forming a moir\\'e structure with one (hBN-BG) or both hBN layers\n(hBN-BG-hBN), using the random phase approximation. The calculations are\nperformed in a fully atomistic Hubbard model that takes into account all\n$\\pi$-electrons of the carbon atoms in one moir\\'e unit cell. We analyze the\ncharge neutral case and find that the dominant magnetic ordering instability is\nuniformly antiferromagnetic. Furthermore, at low temperatures, the critical\nHubbard interaction $U_c$ required to induce magnetic order is slightly larger\nin those systems where the moir\\'e structure has caused a band gap opening in\nthe non-interacting picture, although the difference is less than 6%.\nMean-field calculations are employed to estimate how such an\ninteraction-induced magnetic order may change the observable single-particle\ngap sizes.", "category": "cond-mat_str-el" }, { "text": "Orbital Degree of Freedom and Phase Separation in Ferromagnetic\n Manganites at Finite Temperatures: The spin and orbital phase diagram for perovskite manganites are investigated\nas a function of temperature and hole concentration. The superexchange and\ndouble exchange interactions dominate the ferromagnetic phases in the low and\nhigh concentration regions of doped holes, respectively.The two interactions\nfavor different orbital states each other. Between the phases, two interactions\ncompete with each other and the phase separation appears in the wide range of\ntemperature and hole concentration. The anisotropy of the orbital space causes\ndiscontinuous changes of the orbital state and promotes the phase separation.\nThe relation between the phase separation and the stripe- and sheet-type charge\nsegregation is discussed.", "category": "cond-mat_str-el" }, { "text": "Investigation of the commensurate magnetic structure in heavy fermion\n CePt2In7 using magnetic resonant X-ray diffraction: We investigated the magnetic structure of the heavy fermion compound\nCePt$_2$In$_7$ below $T_N~=5.34(2)$ K using magnetic resonant X-ray diffraction\nat ambient pressure. The magnetic order is characterized by a commensurate\npropagation vector ${k}_{1/2}~=~\\left( \\frac{1}{2} , \\frac{1}{2},\n\\frac{1}{2}\\right)$ with spins lying in the basal plane. Our measurements did\nnot reveal the presence of an incommensurate order propagating along the high\nsymmetry directions in reciprocal space but cannot exclude other incommensurate\nmodulations or weak scattering intensities. The observed commensurate order can\nbe described equivalently by either a single-${k}$ structure or by a\nmulti-${k}$ structure. Furthermore we explain how a commensurate-only ordering\nmay explain the broad distribution of internal fields observed in nuclear\nquadrupolar resonance experiments (Sakai et al. 2011, Phys. Rev. B 83 140408)\nthat was previously attributed to an incommensurate order. We also report\npowder X-ray diffraction showing that the crystallographic structure of\nCePt$_2$In$_7$ changes monotonically with pressure up to $P~=~7.3$ GPa at room\ntemperature. The determined bulk modulus $B_0~=~81.1(3)$ GPa is similar to the\nones of the Ce-115 family. Broad diffraction peaks confirm the presence of\npronounced strain in polycrystalline samples of CePt$_2$In$_7$. We discuss how\nstrain effects can lead to different electronic and magnetic properties between\npolycrystalline and single crystal samples.", "category": "cond-mat_str-el" }, { "text": "Enhanced anisotropic spin fluctuations below tetragonal-to-orthorhombic\n transition in LaFeAs(O_{1-x}F_x) probed by ^{75}As and ^{139}La NMR: $^{75}$As and $^{139}$La NMR results of LaFeAs(O$_{1-x}$F$_x$) ($x$=0, 0.025,\nand 0.04) were reported. Upon F-doping, the tetragonal-to-orthorhombic\nstructural phase transition temperature $T_S$, antiferromagnetic transition\ntemperature $T_N$ and internal magnetic field $\\mu_0H_{\\rm int}$ are gradually\nreduced for $x<0.04$. However, at $x=0.04$, $T_N$ is abruptly suppressed to be\n30 K along with a tiny $\\mu_0H_{\\rm int}$, which is distinct from the\ncontinuous disappearance of the ordered phases in the Ba122 systems of\nBa(Fe,Co)$_2$As$_2$ and BaFe$_2$(As,P)$_2$. The anisotropy of the spin-lattice\nrelaxation rate $T_1^{-1}$, $(T_1)^{-1}_{H\\parallel ab}/(T_1)^{-1}_{H\\parallel\nc}$, in the paramagnetic phase of $x = 0$ and 0.025 is constant ($\\sim 1.5$),\nbut increases abruptly below $T_S$ due to the enhancement of\n$(T_1)^{-1}_{H\\parallel ab}$ by the slowing down of magnetic fluctuations. This\nindicates that the tetragonal-to-orthorhombic structural distortion enhances\nthe anisotropy in the spin space via magnetoelastic coupling and/or spin-orbit\ninteraction.", "category": "cond-mat_str-el" }, { "text": "Gutzwiller variational theory for the Hubbard model with attractive\n interaction: We investigate the electronic and superconducting properties of a negative-U\nHubbard model. For this purpose we evaluate a recently introduced variational\ntheory based on Gutzwiller-correlated BCS wave functions. We find significant\ndifferences between our approach and standard BCS theory, especially for the\nsuperconducting gap. For small values of $|U|$, we derive analytical\nexpressions for the order parameter and the superconducting gap which we\ncompare to exact results from perturbation theory.", "category": "cond-mat_str-el" }, { "text": "Persistent half-metallic ferromagnetism in a (111)-oriented manganite\n superlattice: Heterostructures of mixed-valence manganites are still under intense\nscrutiny, due to the occurrence of exotic quantum phenomena linked to\nelectronic correlation and interfacial composition. For instance, if two\nanti-ferromagnetic insulators as LaMnO$_3$ and SrMnO$_3$ are grown in a\n(001)-oriented superlattice, a half-metallic ferromagnet may form, provided\nthat the thickness is sufficiently small to allow tunneling across interfaces.\nIn this article, we employ electronic structure calculations to show that all\nthe layers of a (111)-oriented LaMnO$_3$|SrMnO$_3$ superlattice retain a\nhalf-metallic ferromagnetic character for a much larger thickness than in its\n(001) counterpart. This behavior is shown to be linked to the charge transfer\nacross the interface, favored by the octahedral connectivity between the\nlayers. This also results in a symmetry-induced quenching of the Jahn-Teller\ndistortions, which are replaced by breathing modes. The latter are coupled to\ncharge and spin oscillations, whose components have a pure e g character. Most\ninterestingly, the magnetization reaches its maximum value inside the LaMnO$_3$\nregion and not at the interface, which is fundamentally different from what\nobserved for the (001) orientation. The analysis of the inter-atomic exchange\ncoupling shows that the magnetic order arises from the double-exchange\nmechanism, despite competing interactions inside the SrMnO$_3$ region. Finally,\nthe van Vleck distortions and the spin oscillations are found to be crucially\naffected by the variation of Hund's exchange and charge doping, which allows us\nto speculate that our system behaves as a Hund's metal, creating an interesting\nconnection between manganites and nickelates.", "category": "cond-mat_str-el" }, { "text": "Band structure approach to the resonant x-ray scattering: We study the resonance behaviour of the forbidden 600 and 222 x-ray Bragg\npeaks in Ge using LDA band structure methods. These Bragg peaks remain\nforbidden in the resonant dipole scattering approximation even taking into\naccount the non local nature of the band states. However they become allowed at\nresonance if the eigenstates of the unoccupied conduction band involve a\nhybridization of p like and d like atomic states. We show that the energy\ndependence of the resonant behaviour, including the phase of the scattering, is\na direct measure of this p-d hybridization.and obtain quantitative agreement\nwith experiment. A simple physical picture involving a product of dipole and\nquadrupolar transition matrix elements explains this behaviour and shows that\nit should be generally true for cases where the resonating atom is not at an\ninversion center. This has strong implications for the description of the\nresonance behavior of x-ray scattering in materials where the resonant atom is\nnot at an inversion center such as V2O3 and in ferro and antiferro electric and\npiezo electric materials in general.", "category": "cond-mat_str-el" }, { "text": "Theory of magnetostriction for multipolar quantum spin ice in pyrochlore\n materials: Multipolar magnetism is an emerging field of quantum materials research. The\nbuilding blocks of multipolar phenomena are magnetic ions with a non-Kramers\ndoublet, where the orbital and spin degrees of freedom are inextricably\nintertwined, leading to unusual spin-orbital entangled states. The detection of\nsuch subtle forms of matter has, however, been difficult due to a limited\nnumber of appropriate experimental tools. In this work, motivated by a recent\nmagnetostriction experiment on Pr$_2$Zr$_2$O$_7$, we theoretically investigate\nhow multipolar quantum spin ice, an elusive three dimensional quantum spin\nliquid, and other multipolar ordered phases in the pyrochlore materials can be\ndetected using magnetostriction. We provide theoretical results based on\nclassical and/or quantum studies of non-Kramers and Kramers magnetic ions, and\ncontrast the behaviors of distinct phases in both systems. Our work paves an\nimportant avenue for future identification of exotic ground states in\nmultipolar systems.", "category": "cond-mat_str-el" }, { "text": "Density Matrix Spectra and Order Parameters in the 1D Extended Hubbard\n Model: Without any knowledge of the symmetry existing in the system, we derive the\nexact forms of the order parameters which show long-range correlation in the\nground state of the one-dimensional extended Hubbard model using a quantum\ninformation approach. Our work demonstrates that the quantum information\napproach can help us to find the explicit form of the order parameter, which\ncannot be derived systematically via traditional methods in the condensed\nmatter theory.", "category": "cond-mat_str-el" }, { "text": "Magnetic-field induced band-structure change in CeBiPt: We report on a field-induced change of the electronic band structure of\nCeBiPt as evidenced by electrical-transport measurements in pulsed magnetic\nfields. Above ~25 T, the charge-carrier concentration increases nearly 30% with\na concomitant disappearance of the Shubnikov-de Haas signal. These features are\nintimately related to the Ce 4f electrons since for the non-4f compound LaBiPt\nthe Fermi surface remains unaffected. Electronic band-structure calculations\npoint to a 4f-polarization-induced change of the Fermi-surface topology.", "category": "cond-mat_str-el" }, { "text": "Double-Pulse Deexcitations in a One-Dimensional Strongly Correlated\n System: We investigate the ultrafast optical response of the one-dimensional\nhalf-filled extended Hubbard model exposed to two successive laser pulses. By\nusing the time-dependent Lanczos method, we find that following the first\npulse, the excitation and deexcitation process between the ground state and\nexcitonic states can be precisely controlled by the relative temporal\ndisplacement of the pulses. The underlying physics can be understood in terms\nof a modified Rabi model. Our simulations clearly demonstrate the\ncontrollability of ultrafast transition between excited and deexcited phases in\nstrongly correlated electron systems.", "category": "cond-mat_str-el" }, { "text": "Multiple Diffusion-Freezing Mechanisms in Molecular Hydrogen Films: Molecular hydrogen is a fascinating candidate for quantum fluid showing\nbosonic and fermionic superfluidity. We have studied diffusion dynamics of thin\nfilms of H$_2$, HD and D$_2$ adsorbed on a glass substrate by measurements of\nelasticity. The elasticity shows multiple anomalies well below bulk triple\npoint. They are attributed to three different diffusion mechanisms of\nadmolecules and their \"freezing\" into localized state: classical thermal\ndiffusion of vacancies, quantum tunneling of vacancies, and diffusion of\nmolecules in the uppermost surface. The surface diffusion is active down to 1\nK, below which the molecules become localized. This suggests that the surface\nlayer of hydrogen films is on the verge of quantum phase transition to\nsuperfluid state.", "category": "cond-mat_str-el" }, { "text": "Phase diagram of a one-dimensional Ising model with an anomalous Z_2\n symmetry: Anomalous global symmetries, which can be realized on the boundary of\nsymmetry-protected topological phases, brings new phases and phase transitions\nto condensed matter physics. In this work, we study a one dimensional model\nwith an anomalous Z2 symmetry, using the density-matrix renormalization group\nmethod. Besides a symmetry-breaking ferromagnetic phase, we find a gapless\nphase described by the SU(2)_1 conformal field theory, despite the existence of\nonly discrete Z_2 symmetry in the Hamiltonian. The phase transition between the\nferromagnetic phase and the gapless phase is continuous and has the same\ncritical scaling as in the gapless phase. Our numerical finding is compatible\nof theoretical constraints on possible phases resulting from the symmetry\nanomaly.", "category": "cond-mat_str-el" }, { "text": "Compensation of Coulomb blocking and energy transfer in the current\n voltage characteristic of molecular conduction junctions: We have studied the influence of both exciton effects and Coulomb repulsion\non current in molecular nanojunctions. We show that dipolar energy-transfer\ninteractions between the sites in the wire can at high voltage compensate\nCoulomb blocking for particular relationships between their values. Tuning this\nrelationship may be achieved by using the effect of plasmonic nanostructure on\ndipolar energy transfer interactions.", "category": "cond-mat_str-el" }, { "text": "Relationship between the ground-state wave function of a magnet and its\n static structure factor: We state and prove two theorems about the ground state of magnetic systems\ndescribed by very general Heisenberg-type models and discuss their implications\nfor magnetic neutron scattering. The first theorem states that two models\ncannot have the same correlator without sharing the corresponding ground\nstates. According to the second theorem, an $N$-qubit wave function cannot\nreproduce the correlators of a given system unless it represents a true ground\nstate of that system. We discuss the implications for neutron scattering\ninverse problems. We argue that the first theorem provides a framework to\nunderstand neutron-based Hamiltonian learning. Furthermore, we propose a\nvariational approach to quantum magnets based on the second theorem where a\nrepresentation of the wave function (held, for instance, in a neural network or\nin the qubit register of a quantum processor) is optimised to fit experimental\nneutron-scattering data directly, without the involvement of a model\nHamiltonian.", "category": "cond-mat_str-el" }, { "text": "Tuning the bond order wave (BOW) phase of half-filled extended Hubbard\n models: Theoretical and computational studies of the quantum phase diagram of the\none-dimensional half-filled extended Hubbard model (EHM) indicate a narrow bond\norder wave (BOW) phase with finite magnetic gap $E_m$ for on-site repulsion $U\n< U^*$, the critical point, and nearest neighbor interaction $V_c \\approx U/2$\nnear the boundary of the charge density wave (CDW) phase. Potentials with more\nextended interactions that retain the EHM symmetry are shown to have a less\ncooperative CDW transition with higher $U^*$ and wider BOW phase. Density\nmatrix renormalization group (DMRG) is used to obtain $E_m$ directly as the\nsinglet-triplet gap, with finite $E_m$ marking the BOW boundary $V_s(U)$. The\nBOW/CDW boundary $V_c(U)$ is obtained from exact finite-size calculations that\nare consistent with previous EHM determinations. The kinetic energy or bond\norder provides a convenient new estimate of $U^*$ based on a metallic point at\n$V_c(U)$ for $U < U^*$. Tuning the BOW phase of half-filled Hubbard models with\ndifferent intersite potentials indicates a ground state with large charge\nfluctuations and magnetic frustration. The possibility of physical realizations\nof a BOW phase is raised for Coulomb interactions.", "category": "cond-mat_str-el" }, { "text": "Entanglement switching via the Kondo effect in triple quantum dots: We consider a triple quantum dot system in a triangular geometry with one of\nthe dots connected to metallic leads. Using Wilson's numerical renormalization\ngroup method, we investigate quantum entanglement and its relation to the\nthermodynamic and transport properties, in the regime where each of the dots is\nsingly occupied on average, but with non-negligible charge fluctuations. It is\nshown that even in the regime of significant charge fluctuations the formation\nof the Kondo singlets induces switching between separable and perfectly\nentangled states. The quantum phase transition between unentangled and\nentangled states is analyzed quantitatively and the corresponding phase diagram\nis explained by exactly solvable spin model.", "category": "cond-mat_str-el" }, { "text": "Observation of re-entrant correlated insulators and interaction driven\n Fermi surface reconstructions at one magnetic flux quantum per moir\u00e9 unit\n cell in magic-angle twisted bilayer graphene: The discovery of flat bands with non-trivial band topology in magic angle\ntwisted bi-layer graphene (MATBG) has provided a unique platform to study\nstrongly correlated phe-nomena including superconductivity, correlated\ninsulators, Chern insulators and magnetism. A fundamental feature of the MATBG,\nso far unexplored, is its high magnetic field Hof-stadter spectrum. Here we\nreport on a detailed magneto-transport study of a MATBG de-vice in external\nmagnetic fields of up to B = 31 T, corresponding to one magnetic flux quan-tum\nper moir\\'e unit cell {\\Phi}0. At {\\Phi}0, we observe a re-entrant correlated\ninsulator at a flat band filling factor of {\\nu} = +2, and interaction-driven\nFermi surface reconstructions at other fillings, which are identified by new\nsets of Landau levels originating from these. These ex-perimental observations\nare supplemented by theoretical work that predicts a new set of 8 well-isolated\nflat bands at {\\Phi}0 , of comparable band width but with different topology\nthan in zero field. Overall, our magneto-transport data reveals a qualitatively\nnew Hofstadter spec-trum in MATBG, which arises due to the strong electronic\ncorrelations in the re-entrant flat bands.", "category": "cond-mat_str-el" }, { "text": "Unified Fock space representation of fractional quantum Hall states: Many bosonic (fermionic) fractional quantum Hall states, such as Laughlin,\nMoore-Read and Read-Rezayi wavefunctions, belong to a special class of\northogonal polynomials: the Jack polynomials (times a Vandermonde determinant).\nThis fundamental observation allows to point out two different recurrence\nrelations for the coefficients of the permanent (Slater) decomposition of the\nbosonic (fermionic) states. Here we provide an explicit Fock space\nrepresentation for these wavefunctions by introducing a two-body squeezing\noperator which represents them as a Jastrow operator applied to reference\nstates, which are in general simple periodic one dimensional patterns.\nRemarkably, this operator representation is the same for bosons and fermions,\nand the different nature of the two recurrence relations is an outcome of\nparticle statistics.", "category": "cond-mat_str-el" }, { "text": "Tensor renormalization group approach to classical dimer models: We analyze classical dimer models on the square and triangular lattice using\na tensor network representation of the dimers. The correlation functions are\nnumerically calculated using the recently developed \"Tensor renormalization\ngroup\" (TRG) technique. The partition function for the dimer problem can be\ncalculated exactly by the Pfaffian method which is used here as a platform for\ncomparing the numerical results. TRG turns out to be a powerful tool for\ndescribing gapped systems with exponentially decaying correlations very\nefficiently due to its fast convergence. This is the case for the dimer model\non the triangular lattice. However, the convergence becomes very slow and\nunstable in case of the square lattice where the model has algebraically\ndecaying correlations. We highlight these aspects with numerical simulations\nand critically appraise the robustness of TRG approach by contrasting the\nresults for small and large system sizes against the exact calculations.\nFurthermore, we benchmark our TRG results with classical Monte Carlo (MC)\nmethod.", "category": "cond-mat_str-el" }, { "text": "The Effect of Intrinsic Quantum Fluctuations on the Phase Diagram of\n Anisotropic Dipolar Magnets: The rare-earth material $\\mathrm{LiHoF_4}$ is believed to be an experimental\nrealization of the celebrated (dipolar) Ising model, and upon the inclusion of\na transverse field $B_x$, an archetypal quantum Ising model. Moreover, by\nsubstituting the magnetic Ho ions by non-magnetic Y ions, disorder can be\nintroduced into the system giving rise to a dipolar disordered magnet and at\nhigh disorders to a spin-glass. Indeed, this material has been scrutinized\nexperimentally, numerically and theoretically over many decades with the aim of\nunderstanding various collective magnetic phenomena. One of the to-date open\nquestions is the discrepancy between the experimental and theoretical $B_x -T$\nphase diagram at low-fields and high temperatures. Here we propose a mechanism,\nbacked by numerical results, that highlights the importance of quantum\nfluctuations induced by the off-diagonal dipolar terms, in determining the\ncritical temperature of anisotropic dipolar magnets in the presence and in the\nabsence of a transverse field. We thus show that the description as a simple\nIsing system is insufficient to quantitatively describe the full phase diagram\nof $\\mathrm{LiHoF_4}$, for the pure as well as for the dilute system.", "category": "cond-mat_str-el" }, { "text": "Cumulant-based calculations of the correlation energy in a molecule: The problem of constructing a guaranteed convergent sequence of corrections\nto the Hartree--Fock ground state energy of a molecule without storing the\nmany-electron wave function is considered. Several methods based on cumulants\nare considered and it is shown that such a sequence is obtained by Lanczos\ntridiagonalization, in which the elements of the tridiagonal matrix are\ncalculated through cumulants.", "category": "cond-mat_str-el" }, { "text": "Numerical Study of the Pairing Correlation of the t-J Type Models: We reported that the pair-pair correlation function of the two-dimensional\nt-J model does not have long-range d-wave superconducting correlations in the\ninteresting parameter range of $J/t \\leq 0.5$. The power-Lanczos method is used\nunder the assumption of monotonic behavior. This assumption has been well\nchecked in the two-dimensional t-J and attractive Hubbard model. Here we\nre-examine this criterion of monotonic behavior of the pairing correlation\nfunction for the one-dimensional and two-leg t-J ladder where other accurate\nnumerical results are available. The method seems to be working well.", "category": "cond-mat_str-el" }, { "text": "Road to zero-field antiferromagnetic skyrmions in a frustrated AFM/FM\n heterostructure: We demonstrate a mechanism of significant reduction, including complete\nelimination, of the external magnetic field required for the stabilization of a\nskyrmion lattice (SkX) phase in a frustrated triangular Heisenberg\nantiferromagnet (AFM) with the Dzyaloshinskii-Moriya interaction. It is\nachieved by coupling of such a AFM plane to a reference ferromagnetic (FM)\nlayer, which generates an effective field cooperating with the external\nmagnetic field. If the FM layer shows some axial single-ion anisotropy then the\neffective field can also be generated in zero external field due to a\nspontaneous FM long-range ordering. Then a sufficiently large interlayer\ncoupling can fully substitute the external magnetic field and the SkX phase in\nthe AFM layer can be stabilized even in zero external field.", "category": "cond-mat_str-el" }, { "text": "Exact treatment of exciton-polaron formation by Diagrammatic Monte Carlo: We develop an approximation-free Diagrammatic Monte Carlo technique to study\nfermionic particles interacting with each other simultaneously through both an\nattractive Coulomb potential and bosonic excitations of the underlying medium.\nExemplarily we apply the method to the long-standing exciton-polaron problem\nand present numerically exact results for the wave function, ground-state\nenergy, binding energy and effective mass of this quasiparticle. Focusing on\nthe electron-hole pair bound-state formation, we discuss various limiting cases\nof a generic exciton-polaron model. The frequently used instantaneous\napproximation to the retarded interaction due to the phonon exchange is found\nto be of very limited applicability. For the case of a light electron and heavy\nhole the system is well approximated by a particle in the field of a static\nattractive impurity.", "category": "cond-mat_str-el" }, { "text": "Magnon crystals and magnetic phases in a Kagom\u00e9-stripe antiferromagnet: In this work we analyze the magnetization properties of an antiferromagnetic\nKagom\\'e stripe lattice, motivated by the recent synthesis of materials\nexhibiting this structure. By employing a variety of techniques that include\nnumerical methods as Density Matrix Renormalization Group and Monte Carlo\nsimulations, as well as analytical techniques, as perturbative low energy\neffective models and exact solutions, we characterize the magnetization process\nand magnetic phase diagram of a Kagom\\'e stripe lattice. The model captures a\nvariety of behaviors present in the two dimensional Kagom\\'e lattice, which are\ndescribed here by analytical models and numerically corroborated. In addition\nto the characterization of semiclassical intermediate plateaus, it is worth\nnoting the determination of an exact magnon crystal phase which breaks the\nunderlying symmetry of the lattice. This magnon crystal phase generalizes\nprevious findings and according to our knowledge is reported here for the first\ntime.", "category": "cond-mat_str-el" }, { "text": "Helical magnetic ordering in Sr(Co1-xNix)2As2: SrCo2As2 is a peculiar itinerant magnetic system that does not order\nmagnetically, but inelastic neutron scattering experiments observe the same\nstripe-type antiferromagnetic (AF) fluctuations found in many of the Fe-based\nsuperconductors along with evidence of magnetic frustration. Here we present\nresults from neutron diffraction measurements on single crystals of\nSr(Co1-xNix)2As2 that show the development of long-range AF order with\nNi-doping. However, the AF order is not stripe-type. Rather, the magnetic\nstructure consists of ferromagnetically-aligned (FM) layers (with moments\nlaying in the layer) that are AF arranged along c with an incommensurate\npropagation vector of (0 0 tau), i.e. a helix. Using high-energy x-ray\ndiffraction, we find no evidence for a temperature-induced structural phase\ntransition that would indicate a collinear AF order. This finding supports a\npicture of competing FM and AF interactions within the square transition-metal\nlayers due to flat-band magnetic instabilities. However, the composition\ndependence of the propagation vector suggests that far more subtle Fermi\nsurface and orbital effects control the interlayer magnetic correlations.", "category": "cond-mat_str-el" }, { "text": "t2g-orbital model on a honeycomb lattice: application to antiferromagnet\n SrRu2O6: Motivated by the recent discovery of high temperature antiferromagnet\nSrRu$_2$O$_6$ and its potential to be the parent of a new superconductor, we\nconstruct a minimal $t_{2g}$-orbital model on a honeycomb lattice to simulate\nits low energy band structure. Local Coulomb interaction is taken into account\nthrough both random phase approximation and mean field theory. Experimentally\nobserved Antiferromagnetic order is obtained in both approximations. In\naddition, our theory predicts that the magnetic moments on three\n$t_{2g}$-orbitals are non-collinear as a result of the strong spin-orbit\ncoupling of Ru atoms.", "category": "cond-mat_str-el" }, { "text": "Concentration dependence in kinetic arrest of first order magnetic\n transition in Ta doped $HfFe_2$: Magnetic behavior of the pseudo-binary alloy $Hf_{1-x}Ta_xFe_2$ has been\nstudied, for which the zero field ferromagnetic (FM) to antiferromagnetic (AFM)\ntransition temperature is tuned near to T=0 K. These alloys show anomalous\nthermomagnetic irreversibility at low temperature due to kinetic arrest of the\nfirst order AFM-FM transition. All the three studied compositions show\nre-entrant transition in zero field cooled warming curve and anomalous\nnon-monotonic variation of upper critical field in isothermal magnetization.\nThe region in H-T space, where these features of kinetic arrest manifest\nthemselves, increases with increasing Ta concentration.", "category": "cond-mat_str-el" }, { "text": "Quantum criticality and confinement in weak Mott insulators: Electrons undergoing a Mott transition may shed their charge but persist as\nneutral excitations of a quantum spin liquid (QSL). We introduce concrete\ntwo-dimensional models exhibiting this exotic behavior as they transition from\nsuperconducting or topological phases into fully charge-localized insulators.\nWe study these Mott transitions and the confinement of neutral fermions at a\nsecond transition into a symmetry-broken phase. In the process, we also derive\ncoupled-wire parent Hamiltonians for a non-Abelian QSL and a $\\mathbb{Z}_4$\nQSL.", "category": "cond-mat_str-el" }, { "text": "Dynamical Properties of small Polarons: On the basis of the two-site polaron problem, which we solve by exact\ndiagonalization, we analyse the spectral properties of polaronic systems in\nview of discerning localized from itinerant polarons and bound polaron pairs\nfrom an ensemble of single polarons. The corresponding experimental techniques\nfor that concern photoemission and inverse photoemission spectroscopy. The\nevolution of the density of states as a function of concentration of charge\ncarriers and strength of the electron-phonon interaction clearly shows the\nopening up of a gap between single polaronic and bi-polaronic states, in\nanalogy to the Hubbard problem for strongly correlated electron systems. The\ncrossover regime between adiabatic and anti-adiabatic small polarons is\ntriggered by two characteristic time scales: the renormalized electron hopping\nrate and the renormalized vibrational frequency becoming equal. This crossover\nregime is then characterized by temporarily alternating self- localization and\ndelocalization of the charge carriers which is accompanied by phase slips in\nthe charge and molecular deformation oscillations and ultimately leads to a\ndephasing between these two dynamical components of the polaron problem. We\nvisualize these features by a study of the temporal evolution of the charge\nredistribution and the change in molecular deformations. The spectral and\ndynamical properties of polarons discussed here are beyond the applicability of\nthe standard Lang Firsov approach to the polaron problem.", "category": "cond-mat_str-el" }, { "text": "Elementary Building Blocks for Cluster Mott Insulators: Mott insulators, in which strong Coulomb interactions fully localize\nelectrons on single atomic sites, play host to an incredibly rich and exciting\narray of strongly correlated physics. One can naturally extend this concept to\ncluster Mott insulators, wherein electrons localize not on single atoms but\nacross clusters of atoms, forming ``molecules in solids''. The resulting\nlocalized degrees of freedom incorporate the full spectrum of electronic\ndegrees of freedom, spin, orbital, and charge. These serve as the building\nblocks for cluster Mott insulators, and understanding them is an important\nfirst step toward understanding the many-body physics that emerges in candidate\ncluster Mott insulators. Here, we focus on elementary building blocks,\nneglecting some of the complexity present in real materials which can often\nobfuscate the underlying principles at play. Through an extensive set of exact\ntheoretical calculations on clusters of varying geometry, number of orbitals,\nand number of electrons, we uncover some of the basic organizing principles of\ncluster Mott phases, particularly when interactions dominate and negate a\nsimple single-particle picture. These include, for example, the identification\nof an additional ``cluster Hund's rule'', of cluster ground states that are\nbest understood from a purely interacting perspective, and of several localized\ndegrees of freedom which are protected by an unusual combination of discrete\nspatial or orbital symmetries. Finally, we discuss the impact of adding\nadditional terms, relevant to material candidates, on the phase diagrams\npresented throughout, as well as the potential next steps in the journey to\nbuilding a more complete picture of cluster Mott insulators.", "category": "cond-mat_str-el" }, { "text": "Phase diagram of the one-dimensional extended attractive Hubbard model\n for large nearest-neighbor repulsion: We consider the extended Hubbard model with attractive on-site interaction U\nand nearest-neighbor repulsions V. We construct an effective Hamiltonian\nH_{eff} for hopping t<