[ { "text": "Discovery of ferromagnetism with large magnetic anisotropy in ZrMnP and\n HfMnP: ZrMnP and HfMnP single crystals are grown by a self-flux growth technique and\nstructural as well as temperature dependent magnetic and transport properties\nare studied. Both compounds have an orthorhombic crystal structure. ZrMnP and\nHfMnP are ferromagnetic with Curie temperatures around $370$~K and $320$~K\nrespectively. The spontaneous magnetizations of ZrMnP and HfMnP are determined\nto be $1.9$~$\\mu_\\textrm{B}$/f.u. and $2.1$~$\\mu_\\textrm{B}$/f.u. respectively\nat $50$~K. The magnetocaloric effect of ZrMnP in term of entropy change\n($\\Delta S$) is estimated to be $-6.7$ kJm$^{-3}$K$^{-1}$ around $369$~K. The\neasy axis of magnetization is [100] for both compounds, with a small anisotropy\nrelative to the [010] axis. At $50$~K, the anisotropy field along the [001]\naxis is $\\sim4.6$~T for ZrMnP and $\\sim10$~T for HfMnP. Such large magnetic\nanisotropy is remarkable considering the absence of rare-earth elements in\nthese compounds. The first principle calculation correctly predicts the\nmagnetization and hard axis orientation for both compounds, and predicts the\nexperimental HfMnP anisotropy field within 25 percent. More importantly, our\ncalculations suggest that the large magnetic anisotropy comes primarily from\nthe Mn atoms suggesting that similarly large anisotropies may be found in other\n3d transition metal compounds.", "category": "cond-mat_mtrl-sci" }, { "text": "Folding Energetics in Thin-Film Diaphragms: We perform experiments on thin-film diaphragms to show that the folding\npatterns of anisotropically compressed diaphragms are strikingly different from\nthose of isotropically compressed ones. We then use a simple von Karman model\nto relate the overall features of these folding patterns to the underlying\nenergetics. We show that the differences between the isotropic and anisotropic\ncases can be traced back to fundamental changes in the energy structure of the\ndiaphragms. Finally, we point out that the energy structure of thin-film\ndiaphragms is similar to that of many other systems in physics and engineering,\ninto which our study may provide interesting insights.", "category": "cond-mat_mtrl-sci" }, { "text": "Phase Stability in 3d-5d (NiPt and CuAu) and 3d-4d (NiPd and CuAg)\n Systems: We show the differences in the stability of 3d-5d (NiPt and CuAu) and 3d-4d\n(NiPd and CuAg) alloys arise mainly due to relativistic corrections. The\nmagnetic properties of disordered NiPd and NiPt alloys also differ due to these\ncorrections which lead to increase in the separation between s-d bands of 5d\nelements in these alloys. For the magnetic case we analyze the results in terms\nof splitting of majority and minority spin d-band centers of the 3d elements.\nWe further examine the effect of relativistic corrections to the pair energies\nand order disorder transition temperatures in these alloys. The magnetic\nmoments and Curie temperatures have also been studied along with the short\nrange ordering/segregation effects in NiPt/NiPd alloys.", "category": "cond-mat_mtrl-sci" }, { "text": "Thermoelectric power factor of Bi-Sb-Te and Bi-Te-Se alloys and doping\n strategy: First-principles study: By performing first principles calculations combined with Boltzmann transport\nequations, we calculate the thermoelectric power factor (PF) of Bi-Sb-Te and\nBi-Te-Se ternary alloys as a function of alloy composition ratio, carrier\nconcentration, and temperature. The point defect formation energy calculations\nalso perform to search potential n-type dopant candidates in ternaries.", "category": "cond-mat_mtrl-sci" }, { "text": "Strain engineering of two-dimensional piezo-photocatalytic materials for\n hydrogen production: Low-dimensional transition metal dichalcogenides (TMDC) exhibit great\nphotocatalytic performance and tunability. In this work, using first-principles\nsimulations based on density functional theory (DFT), we demonstrate that\nexternal electric bias can be employed to further improve the photocatalytic\nhydrogen production efficiency of the six AB$_{2}$ (A=Mo, W and B=S, Se, Te)\nTMDC monolayers by exploiting their piezoelectric response. In particular, when\nsubjected to a proper amount of electrically induced tensile biaxial strain,\nmost TMDC monolayers turn into potentially ideal photocatalyst towards the\nhydrogen evolution reaction (HER). The beneficial effects of introducing\ntensile biaxial strain on the TMDC monolayers are not limited to the reduction\nof the band gap and proper adjustment of the band edge positions, but also to\nthe modification of the H adsorption free energy in such a way that the HER\nreaction is noticeably favored.", "category": "cond-mat_mtrl-sci" }, { "text": "Room Temperature Magnetocaloric Effect in Ni-Mn-In: We have studied the effect of magnetic field on a non-stoichiometric Heusler\nalloy Ni$_{50}$Mn$_{35}$In$_{15}$ that undergoes a martensitic as well as a\nmagnetic transition near room temperature. Temperature dependent magnetization\nmeasurements demonstrate the influence of magnetic field on the structural\nphase transition temperature. From the study of magnetization as a function of\napplied field, we show the occurrence of inverse-magnetocaloric effect\nassociated with this magneto-structural transition. The magnetic entropy change\nattains a value as high as 25 J/kg-K (at 5 T field) at room temperature as the\nalloy transforms from the austenitic to martensitic phase with a concomitant\nmagnetic ordering.", "category": "cond-mat_mtrl-sci" }, { "text": "Strain-induced dynamic control over the population of quantum emitters\n in two-dimensional materials: The discovery of quantum emitters in two-dimensional materials has triggered\na surge of research to assess their suitability for quantum photonics. While\ntheir microscopic origin is still the subject of intense studies, ordered\narrays of quantum emitters are routinely fabricated using static\nstrain-gradients, which are used to drive excitons toward localized regions of\nthe 2D crystals where quantum-light-emission takes place. However, the\npossibility of using strain in a dynamic fashion to control the appearance of\nindividual quantum emitters has never been explored so far. In this work, we\ntackle this challenge by introducing a novel hybrid semiconductor-piezoelectric\ndevice in which WSe2 monolayers are integrated onto piezoelectric pillars\ndelivering both static and dynamic strains. Static strains are first used to\ninduce the formation of quantum emitters, whose emission shows photon\nanti-bunching. Their excitonic population and emission energy are then\nreversibly controlled via the application of a voltage to the piezoelectric\npillar. Numerical simulations combined with drift-diffusion equations show that\nthese effects are due to a strain-induced modification of the\nconfining-potential landscape, which in turn leads to a net redistribution of\nexcitons among the different quantum emitters. Our work provides relevant\ninsights into the role of strain in the formation of quantum emitters in 2D\nmaterials and suggests a method to switch them on and off on demand.", "category": "cond-mat_mtrl-sci" }, { "text": "Coercivity and random interfacial exchange coupling in CoPt/Co films: Hard-soft bilayers are analogous to prototype exchange-biased ferromagnetic\n-antiferromagnetic systems as the minor loop of the soft layer is biased by the\nhard and furthermore they offer bias layer tunability. In sputtered CoPt/Co\nhard-soft bilayers we demonstrate that the exchange bias field shows a linear\ndependence on the hard layer magnetization, while the coercivity shows a\nquadratic dependence. Analysis of the minor hysteresis loop features supported\nby Monte-Carlo simulations provide clear evidence that the coercivity of the\nsoft layer is mainly determined by the tunable randomness of the domain state\nof the hard layer.", "category": "cond-mat_mtrl-sci" }, { "text": "Thermodynamics of point defects and diffusion mechanisms in B2-ordered\n compounds: The point defect thermodynamics in a general family of binary compounds,\nincluding B2 compounds as a specific representative, are classified by way of\ntwo non-trivial energy parameters. The scheme is applied to published ab initio\ndefect formation energies, and the variety of resulting phenomena is\ndemonstrated. Further, by introducing model assumptions the consequences for\nthe active diffusion mechanisms are deduced. It is shown that particularly for\nthe off-stoichiometric case, the assumed prevalence of either the six-jump\ncycle or the triple-defect mechanism has to be reconsidered, as a number of\nqualitatively different mechanisms emerge as likely candidates for the dominant\neffect. Two of those, the 4+2-jump cycles and the waltzing-step mechanism, are\nintroduced here.", "category": "cond-mat_mtrl-sci" }, { "text": "Tunable Optoelectronic Properties of Triply-Bonded Carbon Molecules with\n Linear and Graphyne Substructures: In this paper we present a detailed computational study of the electronic\nstructure and optical properties of triply-bonded hydrocarbons with linear, and\ngraphyne substructures, with the aim of identifying their potential in\nopto-electronic device applications. For the purpose, we employed a correlated\nelectron methodology based upon the Pariser-Parr-Pople model Hamiltonian,\ncoupled with the configuration interaction (CI) approach, and studied\nstructures containing up to 42 carbon atoms. Our calculations, based upon\nlarge-scale CI expansions, reveal that the linear structures have intense\noptical absorption at the HOMO-LUMO gap, while the graphyne ones have those at\nhigher energies. Thus, the opto-electronic properties depend on the topology of\nthe {graphyne substructures, suggesting that they can be tuned by means of\nstructural modifications. Our results are in very good agreement with the\navailable experimental data.", "category": "cond-mat_mtrl-sci" }, { "text": "Nano-scale oxygen octahedral tilting in\n 0.90(Bi1/2Na1/2)TiO3-0.05(Bi1/2K1/2)TiO3-0.05BaTiO3 lead-free perovskite\n piezoelectric ceramics: The oxygen octahedral tilted domains in\n0.90(Bi1/2Na1/2)TiO3-0.5(Bi1/2K1/2)TiO3-0.5BaTiO3 lead-free perovskite\npiezoelectric ceramic have been studied by transmission electron microscopy\n(TEM). Selected-area electron diffraction patterns shows the 1/2ooo and 1/2ooe\nreflections, indicating the presence of antiphase (a-a-a-) and in-phase\n(aoaoc+) octahedral tilting, respectively. The morphology and distributions of\nthese tilted domains are shown in the centered dark-field images. Further, the\nBragg-filtered high-resolution TEM image reveals that the size of the in-phase\ntilted domains varies from 1 to 8 nm across. The ceramic contains the mixture\nof non-tilted and variants of the antiphase and in-phase tilted domains.", "category": "cond-mat_mtrl-sci" }, { "text": "A Graphene-Carbon Nanotube Hybrid Material for Photovoltaic Applications: Large area graphene sheets grown by chemical vapor deposition can potentially\nbe employed as a transparent electrode in photovoltaics if their sheet\nresistance can be significantly lowered, without any loss in transparency.\nHere, we report the fabrication of a graphene-conducting-carbon-nanotube (CCNT)\nhybrid material with a sheet resistance considerably lower than neat graphene,\nand with the requisite small reduction in transparency. Graphene is deposited\non top of a a self-assembled CCNT monolayer which creates parallel conducting\npaths on the graphene surface. The hybrid thereby circumvents electron\nscattering due to defects in the graphene sheet, and reduces the sheet\nresistance by a factor of two. The resistance can be further reduced by\nchemically doping the hybrid. Moreover, the chemically doped hybrid is more\nstable than a standalone chemically doped graphene sheet, as the CCNT network\nenhances the dopant binding. In order to understand the results, we develop a\n2D resistance network model in which we couple the CCNT layer to the graphene\nsheet and demonstrate the model accounts quantitatively for the resistance\ndecrease. Our results show that a graphene-CCNT hybrid system has high\npotential for use as a transparent electrode with high transparency and low\nsheet resistance.", "category": "cond-mat_mtrl-sci" }, { "text": "Nonvolatile Electric-Field Control of Inversion Symmetry: In condensed-matter systems, competition between ground states at phase\nboundaries can lead to significant changes in material properties under\nexternal stimuli, particularly when these ground states have different crystal\nsymmetries. A key scientific and technological challenge is to stabilize and\ncontrol coexistence of symmetry-distinct phases with external stimuli. Using\nBiFeO3 (BFO) layers confined between layers of the dielectric TbScO3 as a model\nsystem, we stabilize the mixed-phase coexistence of centrosymmetric and\nnon-centrosymmetric BFO phases with antipolar, insulating and polar,\nsemiconducting behavior, respectively at room temperature. Application of\nin-plane electric (polar) fields can both remove and introduce centrosymmetry\nfrom the system resulting in reversible, nonvolatile interconversion between\nthe two phases. This interconversion between the centrosymmetric insulating and\nnon-centrosymmetric semiconducting phases coincides with simultaneous changes\nin the non-linear optical response of over three orders of magnitude, a change\nin resistivity of over five orders of magnitude, and a change in the polar\norder. Our work establishes a materials platform allowing for novel\ncross-functional devices which take advantage of changes in optical,\nelectrical, and ferroic responses.", "category": "cond-mat_mtrl-sci" }, { "text": "Bulk-sensitive photoemission spectroscopy of A_2FeMoO_6 double\n perovskites (A=Sr, Ba): Electronic structures of Sr_2FeMoO_6 (SFMO) and Ba_2FeMoO_6 (BFMO) double\nperovskites have been investigated using the Fe 2p->3d resonant photoemission\nspectroscopy (PES) and the Cooper minimum in the Mo 4d photoionization cross\nsection. The states close to the Fermi level are found to have strongly mixed\nMo-Fe t_{2g} character, suggesting that the Fe valence is far from pure 3+. The\nFe 2p_{3/2} XAS spectra indicate the mixed-valent Fe^{3+}-Fe^{2+}\nconfigurations, and the larger Fe^{2+} component for BFMO than for SFMO,\nsuggesting a kind of double exchange interaction. The valence-band PES spectra\nreveal good agreement with the LSDA+U calculation.", "category": "cond-mat_mtrl-sci" }, { "text": "Thermal Conductivity and Phonon Transport in Suspended Few-Layer\n Hexagonal Boron Nitride: The thermal conductivity of suspended few-layer hexagonal boron nitride\n(h-BN) was measured using a micro-bridge device with built-in resistance\nthermometers. Based on the measured thermal resistance values of 11-12 atomic\nlayer h-BN samples with suspended length ranging between 3 and 7.5 um, the\nroom-temperature thermal conductivity of a 11-layer sample was found to be\nabout 360 Wm-1K-1, approaching the basal plane value reported for bulk h-BN.\nThe presence of a polymer residue layer on the sample surface was found to\ndecrease the thermal conductivity of a 5-layer h-BN sample to be about 250\nWm-1K-1 at 300 K. Thermal conductivities for both the 5 layer and the 11 layer\nsamples are suppressed at low temperatures, suggesting increasing scattering of\nlow frequency phonons in thin h-BN samples by polymer residue.", "category": "cond-mat_mtrl-sci" }, { "text": "Comparison of exact-exchange calculations for solids in\n current-spin-density- and spin-density-functional theory: The relative merits of current-spin-density- and spin-density-functional\ntheory are investigated for solids treated within the exact-exchange-only\napproximation. Spin-orbit splittings and orbital magnetic moments are\ndetermined at zero external magnetic field. We find that for magnetic (Fe, Co\nand Ni) and non-magnetic (Si and Ge) solids, the exact-exchange\ncurrent-spin-density functional approach does not significantly improve the\naccuracy of the corresponding spin-density functional results.", "category": "cond-mat_mtrl-sci" }, { "text": "The roles of adhesion, internal heat generation and elevated\n temperatures in normally loaded, sliding rough surfaces: The thermal effects of plastic and frictional heat generation and elevated\ntemperature were examined along with the role of adhesion in the context of\ngalling wear, using a representative crystal plasticity, normally loaded,\nsliding surface model. Galling frequency behaviour was predicted for 316L\nsteel. Deformation of the surfaces was dominated by the surface geometry, with\nno significant effect due to variations in frictional models. Plastic and\nfrictional heating were found to have a minimal effect on the deformation of\nthe surface, with the rapid conduction of heat preventing any highly localised\nheating. There was no corresponding effect on the predicted galling frequency\nresponse.\n Isothermal, elevated temperature conditions caused a decrease in galling\nresistance, driven by the temperature sensitivity of the critical resolved\nshear stress. The extent of deformation, as quantified by the area of\nplastically deformed material and plastic reach, increased with temperature.\nComparisons were made with literature results for several surface amplitude and\nwavelength conditions. Model results compared favourably with those in the\nliterature. However, the reduction in predicted galling resistance with\nelevated temperature for a fixed surface was not as severe as observations in\nthe literature, suggesting other mechanisms (e.g. phase transformations,\nsurface coatings and oxides) are likely important.", "category": "cond-mat_mtrl-sci" }, { "text": "Multiscale Kinetic Monte-Carlo for Simulating Epitaxial Growth: We present a fast Monte-Carlo algorithm for simulating epitaxial surface\ngrowth, based on the continuous-time Monte-Carlo algorithm of Bortz, Kalos and\nLebowitz. When simulating realistic growth regimes, much computational time is\nconsumed by the relatively fast dynamics of the adatoms. Continuum and\ncontinuum-discrete hybrid methods have been developed to approach this issue;\nhowever in many situations, the density of adatoms is too low to efficiently\nand accurately simulate as a continuum. To solve the problem of fast adatom\ndynamics, we allow adatoms to take larger steps, effectively reducing the\nnumber of transitions required. We achieve nearly a factor of ten speed up, for\ngrowth at moderate temperatures and large D/F.", "category": "cond-mat_mtrl-sci" }, { "text": "Undercooling growth and magnetic characterization of ferromagnetic shape\n memory alloy Ni2FeGa single crystals: Ni2FeGa single crystals have been grown in undercooling conditions provided\nby a glass-purification method. It has been found that trace amounts of gamma\nphase embededin the single crystalline matrix preferentially orients in the\n<100> orientation along the growth direction. This gamma phase generates\ndirectional residual stress and results in an anisotropic two-way shape memory\neffect. Large strains of -2.5% in the [001] and 1.5% in the [010] directions\nhave been observed. This trace gamma phase also improves the ductility of the\nmaterial and thus the crystals could be plastically deformed at room\ntemperature in the parent phase. The <110> and <111> orientations in Ni2FeGa\nalloy were identified as the easy and hard magnetization directions,\nrespectively, in the parent phase by using low field M-T measurements.", "category": "cond-mat_mtrl-sci" }, { "text": "Effect of edge vacancies on localized states in semi-infinite zigzag\n graphene sheet: The effect of vacancies on the robustness of zero-energy edge electronic\nstates in zigzag-type graphene layer is studied at different concentrations and\ndistributions of defects. All calculations are performed by using the Green's\nfunction method and the tight-binding approximation. It is found that the\narrangement of defects plays a crucial role in the destruction of the edge\nstates. We have specified a critical distance between edge vacancies when their\nmutual influence becomes significant and affects markedly the density of\nelectronic states at graphene edge.", "category": "cond-mat_mtrl-sci" }, { "text": "Ba(Zn,Co)2As2: a II-II-V Diluted Ferromagnetic Semiconductor with N-type\n Carriers: Diluted ferromagnetic semiconductors (DMSs) that combine the properties of\nsemiconductors with ferromagnetism have potential application in spin-sensitive\nelectronics (spintronics) devices. The search for DMS materials exploded after\nthe observation of ferromagnetic ordering in III-V (Ga,Mn)As films. Recently, a\nseries of DMS compounds isostructural to iron-based superconductors have been\nreported. Among them, the highest Curie temperature $T_C$ of 230 K has been\nachieved in (Ba,K)(Zn,Mn)$_2$As$_2$. However, most DMSs, including (Ga,Mn)As,\nare p-type, i.e., the carriers that mediate ferromagnetism are holes. For\npractical applications, DMS with n-type carriers are also advantageous. Here we\nreport the successful synthesis of a II-II-V diluted ferromagnetic\nsemiconductor with n-type carriers, Ba(Zn,Co)$_2$As$_2$. Magnetization\nmeasurements show that the ferromagnetic transition occurs up to $T_{C} \\sim$\n45 K. Hall effect and Seebeck effect measurements jointly confirm that the\ndominant carriers are electrons. Through muon spin relaxation ($\\mu$SR), a\nvolume sensitive magnetic probe, we have also confirmed that the ferromagnetism\nin Ba(Zn,Co)$_2$As$_2$ is intrinsic and the internal field is static.", "category": "cond-mat_mtrl-sci" }, { "text": "High-throughput search for triplet point defects with narrow emission\n lines in 2D materials: We employ a first-principles computational workflow to screen for optically\naccessible, high-spin point defects in wide band gap two-dimensional (2D)\ncrystals. Starting from an initial set of 5388 point defects, comprising both\nintrinsic and extrinsic, single and double defects in ten previously\nsynthesised 2D host materials, we identify 596 defects with a triplet ground\nstate. For these defects, we calculate the defect formation energy, the\nhyperfine (HF) coupling, and the zero-field splitting (ZFS) tensors. For 39\ntriplet transitions exhibiting particularly low Huang-Rhys factors, we\ncalculate the full photo-luminescence (PL) spectrum. Our approach reveals many\nnew spin defects with narrow PL line shapes and emission frequencies covering a\nbroad spectral range. Most of the defects are hosted in hexagonal BN, which we\nascribe to its high stiffness, but some are also found in MgI2, MoS2, MgBr2 and\nCaI2. As specific examples, we propose the defects vSMoS0 and NiSMoS0 in MoS2\nas interesting candidates with potential applications to magnetic field sensors\nand quantum information technology. All the data will be made available in the\nopen access database QPOD.", "category": "cond-mat_mtrl-sci" }, { "text": "Ab initio vibrational free energies including anharmonicity for\n multicomponent alloys: A density-functional-theory based approach to efficiently compute numerically\nexact vibrational free energies - including anharmonicity - for chemically\ncomplex multicomponent alloys is developed. It is based on a combination of\nthermodynamic integration and a machine-learning potential. We demonstrate the\nperformance of the approach by computing the anharmonic free energy of the\nprototypical five-component VNbMoTaW refractory high entropy alloy.", "category": "cond-mat_mtrl-sci" }, { "text": "Imaging Antiferromagnetic Domains in Nickel-oxide Thin Films by\n Magneto-optical Voigt Effect: Recent demonstrations of electrical detection and manipulation of\nantiferromagnets (AFMs) have opened new opportunities towards robust and\nultrafast spintronics devices. However, it is difficult to establish the\nconnection between the spin-transport behavior and the microscopic AFM domain\nstates due to the lack of the real-time AFM domain imaging technique under the\nelectric field. Here we report a significant Voigt rotation up to 60 mdeg in\nthin NiO(001) films at room temperature. Such large Voigt rotation allows us to\ndirectly observe AFM domains in thin-film NiO by utilizing a wide-field optical\nmicroscope. Further complementary XMLD-PEEM measurement confirms that the Voigt\ncontrast originates from the NiO AFM order. We examine the domain pattern\nevolution at a wide range of temperature and with the application of external\nmagnetic field. Comparing to large-scale-facility techniques such as the X-ray\nphotoemission electron microscopy, the use with a wide-field, tabletop optical\nimaging method enables straightforward access to domain configurations of\nsingle-layer AFMs.", "category": "cond-mat_mtrl-sci" }, { "text": "The role of the catalytic particle temperature gradient for SWNT growth\n from small particles: The Vapour-Liquid-Solid (VLS) model, which often includes a temperature\ngradient (TG) across the catalytic metal particle, is often used to describe\nthe nucleation and growth of carbon nanostructures. Although the TG may be\nimportant for the growth of carbon species from large metal particles,\nmolecular dynamics simulations show that it is not required for single-walled\ncarbon nanotube growth from small catalytic particles.", "category": "cond-mat_mtrl-sci" }, { "text": "Giant optical anisotropy in cylindrical self-assembled InAs/GaAs quantum\n rings: Using a single-particle atomistic pseudopotential method followed by a\nmany-particle configuration interaction method, we investigate the geometry,\nelectronic structure and optical transitions of a self-assembled InAs/GaAs\nquantum ring (QR), changing its shape continously from a lens-shaped quantum\ndot (QD) to a nearly one dimensional ring. We find that the biaxial strain in\nthe ring is strongly asymmetric in the plane perpendicular to the QR growth\ndirection, leading to giant optical anisotropy.", "category": "cond-mat_mtrl-sci" }, { "text": "Quantum Phase Transitions in Ba(1-x)CaxFe12O19: The ground state of BaFe12O19 (BFO) is controversial as three different\nquantum states, namely quantum paraelectric, frustrated antiferroelectric and\nquantum electric dipole liquid (QEDL), have been proposed. We have investigated\nthe quantum critical behavior of BFO as a function of chemical pressure (a\nnon-thermal variable) generated by smaller isovalent ion Ca2+ at the Ba2+ site.\nAnalysis of synchrotron x-ray diffraction data confirms that Ca2+ substitution\ngenerates positive chemical pressure. Our dielectric measurements reveal that\nCa2+ substitution drives BFO away from its quantum critical point (QCP) and\nstabilizes a quantum electric dipolar glass state whose dielectric peak\ntemperature (Tc) increases with increasing Ca2+ content as Tc ~ (x-xc)1/2, a\ncanonical signature of quantum phase transitions. Our dielectric measurements\nreveal that pure BFO is slightly away from its QCP with a Tc of 2.91 K.\nSpecific heat measurements reveal excess specific heat of non-Debye and\nnon-magnetic origin with linear temperature dependence below Tc which could be\ndue to QEDL state of BFO.", "category": "cond-mat_mtrl-sci" }, { "text": "Toy nanoindentation model and incipient plasticity: A toy model of two dimensional nanoindentation in finite crystals is\nproposed. The crystal is described by periodized discrete elasticity whereas\nthe indenter is a rigid strain field of triangular shape representing a hard\nknife-like indenter. Analysis of the model shows that there are a number of\ndiscontinuities in the load vs penetration depth plot which correspond to the\ncreation of dislocation loops. The stress vs depth bifurcation diagram of the\nmodel reveals multistable stationary solutions that appear as the\ndislocation-free branch of solutions develops turning points for increasing\nstress. Dynamical simulations show that an increment of the applied load leads\nto nucleation of dislocation loops below the nanoindenter tip. Such\ndislocations travel inside the bulk of the crystal and accommodate at a certain\ndepth in the sample. In agreement with experiments, hysteresis is observed if\nthe stress is decreased after the first dislocation loop is created. Critical\nstress values for loop creation and their final location at equilibrium are\ncalculated.", "category": "cond-mat_mtrl-sci" }, { "text": "NMR in magnetic molecular rings and clusters (I): In the last years there has been a great interest in magnetic systems formed\nby a cluster of transition metal ions covalently bonded via superexchange\nbridges, embedded in a large organic molecule. Following the synthesis and the\nstructural and magnetic characterization of these magnetic molecules by\nchemists, the physicists realized the great interest of these systems as a\npractical realization of zero-dimensional model magnetic systems. In fact the\nmagnetic molecules can be synthesized in crystalline form whereby each molecule\nis magnetically independent since the intramolecular exchange interaction among\nthe transition metal ions is dominant over the weak intermolecular, usually\ndipolar, magnetic interaction. We have undertaken a systematic NMR\ninvestigation of molecular nanomagnets since back in 1996. The present review\ntries to give an account of the main results obtained so far and of the many\nexciting projects that still lie ahead. The work was done through a continuous\nvery fruitful collaboration among three NMR laboratories: at the University of\nPavia, Italy, at Iowa State University and Ames Laboratory, Ames, IA, USA and\nat Hokkaido University, Sapporo, Japan with occasional very useful\ncollaborations with the high field NMR lab. in Grenoble, France. None of the\nwork could be done without the precious collaboration and help of our\ncolleagues in chemistry at the University of Florence and of Modena, Italy and\nat Ames Laboratory in USA who synthesized and characterized the samples used in\nthe NMR work.", "category": "cond-mat_mtrl-sci" }, { "text": "Morphology of supported polymer electrolyte ultra-thin films: a\n numerical study: Morphology of polymer electrolytes membranes (PEM), e.g., Nafion, inside PEM\nfuel cell catalyst layers has significant impact on the electrochemical\nactivity and transport phenomena that determine cell performance. In those\nregions, Nafion can be found as an ultra-thin film, coating the catalyst and\nthe catalyst support surfaces. The impact of the hydrophilic/hydrophobic\ncharacter of these surfaces on the structural formation of the films has not\nbeen sufficiently explored yet. Here, we report about Molecular Dynamics\nsimulation investigation of the substrate effects on the ionomer ultra-thin\nfilm morphology at different hydration levels. We use a mean-field-like model\nwe introduced in previous publications for the interaction of the hydrated\nNafion ionomer with a substrate, characterized by a tunable degree of\nhydrophilicity. We show that the affinity of the substrate with water plays a\ncrucial role in the molecular rearrangement of the ionomer film, resulting in\ncompletely different morphologies. Detailed structural description in different\nregions of the film shows evidences of strongly heterogeneous behavior. A\nqualitative discussion of the implications of our observations on the PEMFC\ncatalyst layer performance is finally proposed.", "category": "cond-mat_mtrl-sci" }, { "text": "Third order perturbed modified Heisenberg Hamiltonian of fcc structured\n ferromagnetic films with seventy spin layers: Magnetic properties of fcc structured ferromagnetic films with the number of\nspin layers up to seventy was described using third order perturbed Heisenberg\nHamiltonian. The variation of magnetic easy direction, magnetic energies in\neasy and hard directions, magnetic anisotropy energy and the angle between easy\nand hard directions was investigated by varying the number of spin layers. Spin\nexchange interaction, magnetic dipole interaction, second and fourth order\nmagnetic anisotropies, in and out of plane applied magnetic fields,\ndemagnetization factor and stress induced anisotropy were considered in the\nmodel. Because magnetic dipole interaction and demagnetization factor represent\nmicroscopic and macroscopic properties of the sample, respectively, both these\nterms were incorporated in our theoretical model. Although our model is a\nsemi-classical model, some discrete variations of angle of easy axis were\nobserved. Our theoretical data qualitatively agree with experimental data of Fe\nand Ni ferromagnetic films.", "category": "cond-mat_mtrl-sci" }, { "text": "Local structural ordering determines the mechanical damage tolerance of\n amorphous grain boundary complexions: Amorphous grain boundary complexions act as toughening features within a\nmicrostructure because they can absorb dislocations more efficiently than\ntraditional grain boundaries. This toughening effect should be a strong\nfunction of the local internal structure of the complexion, which has recently\nbeen shown to be determined by grain boundary crystallography. To test this\nhypothesis, molecular dynamics are used here to simulate dislocation absorption\nand damage nucleation for complexions with different distributions of\nstructural short-range order. The complexion with a more disordered structure\naway from the dislocation absorption site is actually found to better resist\ncrack nucleation, as damage tolerance requires delocalized deformation and the\noperation of shear-transformation zones through the complexion thickness. The\nmore damage tolerant complexion accommodates plastic strain efficiently within\nthe entire complexion, providing the key mechanistic insight that local\npatterning and asymmetry of structural short-range order controls the\ntoughening effect of amorphous complexions.", "category": "cond-mat_mtrl-sci" }, { "text": "Segregation-induced phase transformations in grain boundaries: Phase transformations in metallic grain boundaries (GBs) present significant\nfundamental interest in the context of thermodynamics of low-dimensional\nphysical systems. We report on atomistic computer simulations of the Cu-Ag\nsystem that provide direct evidence that GB phase transformations in a\nsingle-component GB can continue to exist in a binary alloy. This gives rise to\nsegregation-induced phase transformations with varying chemical composition at\na fixed temperature. Furthermore, for such transformations we propose an\napproach to calculations of free energy differences between different GB phases\nby thermodynamic integration along a segregation isotherm. This opens the\npossibility of developing quantitative thermodynamics of GB phases, their\ntransformations to each other, and critical phenomena in the future.", "category": "cond-mat_mtrl-sci" }, { "text": "The growth of ZnO crystals from the melt: The peculiar properties of zinc oxide (ZnO) make this material interesting\nfor very different applications like light emitting diodes, lasers, and\npiezoelectric transducers. Most of these applications are based on epitaxial\nZnO layers grown on suitable substrates, preferably bulk ZnO. Unfortunately the\nthermochemical properties of ZnO make the growth of single crystals difficult:\nthe triple point 1975 deg C., 1.06 bar and the high oxygen fugacity at the\nmelting point p_O2 = 0.35 bar lead to the prevailing opinion that ZnO crystals\nfor technical applications can only be grown either by a hydrothermal method or\nfrom \"cold crucibles\" of solid ZnO. Both methods are known to have significant\ndrawbacks. Our thermodynamic calculations and crystal growth experiments show,\nthat in contrast to widely accepted assumptions, ZnO can be molten in metallic\ncrucibles, if an atmosphere with \"self adjusting\" p_O2 is used. This new result\nis believed to offer new perspectives for ZnO crystal growth by established\nstandard techniques like the Bridgman method.", "category": "cond-mat_mtrl-sci" }, { "text": "Complete mapping of magnetic anisotropy for prototype Ising van der\n Waals FePS$_3$: Several Ising-type magnetic van der Waals (vdW) materials exhibit stable\nmagnetic ground states. Despite these clear experimental demonstrations, a\ncomplete theoretical and microscopic understanding of their magnetic anisotropy\nis still lacking. In particular, the validity limit of identifying their\none-dimensional (1-D) Ising nature has remained uninvestigated in a\nquantitative way. Here we performed the complete mapping of magnetic anisotropy\nfor a prototypical Ising vdW magnet FePS$_3$ for the first time. Combining\ntorque magnetometry measurements with their magnetostatic model analysis and\nthe relativistic density functional total energy calculations, we successfully\nconstructed the three-dimensional (3-D) mappings of the magnetic anisotropy in\nterms of magnetic torque and energy. The results not only quantitatively\nconfirm that the easy axis is perpendicular to the $ab$ plane, but also reveal\nthe anisotropies within the $ab$, $ac$, and $bc$ planes. Our approach can be\napplied to the detailed quantitative study of magnetism in vdW materials.", "category": "cond-mat_mtrl-sci" }, { "text": "Spin Angular Momentum Transfer and Plasmogalvanic Phenomena: We introduce the continuity equation for the electromagnetic spin angular\nmomentum (SAM) in matter and discuss the torque associated with the SAM\ntransfer in terms of effective spin forces acting in a material. In plasmonic\nmetal, these spin forces result in plasmogalvanic phenomenon which is pinning\nthe plasmon-induced electromotive force to atomically-thin layer at the metal\ninterface.", "category": "cond-mat_mtrl-sci" }, { "text": "Anomalous Hall effect in a two-dimensional electron gas with spin-orbit\n interaction: We discuss the mechanism of anomalous Hall effect related to the contribution\nof electron states below the Fermi surface (induced by the Berry phase in\nmomentum space). Our main calculations are made within a model of\ntwo-dimensional electron gas with spin-orbit interaction of the Rashba type,\ntaking into account the scattering from impurities. We demonstrate that such an\n\"intrinsic\" mechanism can dominate but there is a competition with the\nimpurity-scattering mechanism, related to the contribution of states in the\nvicinity of Fermi surface. We also show that the contribution to the Hall\nconductivity from electron states close to the Fermi surface has the intrinsic\nproperties as well.", "category": "cond-mat_mtrl-sci" }, { "text": "Local Structural Evidence for Strong Electronic Correlations in\n LiRh$_2$O$_4$ Spinel: The local structure of the spinel LiRh$_2$O$_4$ has been studied using atomic\npair distribution function (PDF) analysis of powder x-ray diffraction data.\nThis measurement is sensitive to the presence of short Rh-Rh bonds that form\ndue to dimerization of Rh$^{4+}$ ions on the pyrochlore sublattice, independent\nof the existence of long range order. We show that structural dimers exist in\nthe low-temperature phase, as previously supposed, with a bond shortening of\n$\\Delta r \\sim 0.15$ \\AA . The dimers persist up to 350 K, well above the\ninsulator-metal transition, with $\\Delta r$ decreasing in magnitude on warming.\nSuch behavior is inconsistent with the Fermi surface nesting-driven Peierls\ntransition model. Instead, we argue that LiRh$_2$O$_4$ should properly be\ndescribed as a strongly correlated system.", "category": "cond-mat_mtrl-sci" }, { "text": "Hybrid-functional and quasi-particle calculations of band structures of\n Mg2Si, Mg2Ge, and Mg2Sn: We perform hybrid functional and quasi-particle band structure calculations\nwith spin-orbit interaction to investigate the band structures of Mg2Si, Mg2Ge,\nand Mg2Sn. For all Mg2X materials, where X = Si, Ge, and Sn, the\ncharacteristics of band edge states, i.e., band and valley degeneracies, and\norbital characters, are found to be conserved, independent of the computational\nschemes such as density functional generalized gradient approximation, hybrid\nfunctionals, or quasi-particle calculations. However, the magnitude of the\ncalculated band gap varies significantly with the computational schemes. Within\ndensity-functional calculations, the one-particle band gaps of Mg2Si, Mg2Ge,\nand Mg2Sn are 0.191, 0.090, and -0.346 eV, respectively, and thus severely\nunderestimated compared to the experimental gaps, due to the band gap error in\nthe density functional theory and the significant relativistic effect on the\nlow-energy band structures. By employing hybrid-functional calculations with a\n35% fraction of the exact Hartree-Fock exchange energy (HSE-35%), we overcame\nthe negative band gap issue in Mg2Sn. Finally, in quasi-particle calculations\non top of the HSE-35% Hamiltonians, we obtained band gaps of 0.835, 0.759, and\n0.244 eV for Mg2Si, Mg2Ge, and Mg2Sn, respectively, consistent with the\nexperimental band gaps of 0.77, 0.74, and 0.36 eV, respectively.", "category": "cond-mat_mtrl-sci" }, { "text": "Scaling of alloy interfacial properties under compositional strain: Complex morphologies and microstructures that emerge during materials growth\nand solidification are often determined by both equilibrium and kinetic\nproperties of the interface and their crystalline anisotropies. However limited\nknowledge is available for the alloying and particularly the compositionally\ngenerated elastic effects on these interface characteristics. Here we\nsystematically investigate such compositional effects on the interfacial\nproperties of an alloy model system based on the phase-field-crystal analysis,\nincluding the solid-liquid interfacial free energy, kinetic coefficient, and\nlattice pinning strength. Scaling relations for these interfacial quantities\nover various ranges of material parameters are identified and predicted. Our\nresults indicate the important effects of couplings among mesoscopic and\nmicroscopic length scales of alloy structure and concentration, and also the\ninfluence of compressive and tensile interface stresses induced by composition\nvariations. The approach developed here provides an efficient way to\nsystematically identify these key material properties beyond the traditional\natomistic and continuum methods.", "category": "cond-mat_mtrl-sci" }, { "text": "Computer simulations of defects in perovskite KNbO3 crystals: An ab initio LMTO approach and semi-empirical quantum chemical INDO method\nhave been used for supercell calculations of basic point defects - F-type\ncenters and hole polarons bound to cation vacancy - in partly covalent\nperovskite KNbO3. We predict the existence of both one-site and two-site\n(molecular) polarons with close absorption energies (~ 1 eV). The relevant\nexperimental data are discussed and interpreted.", "category": "cond-mat_mtrl-sci" }, { "text": "Disclinations in the geometric theory of defects: In the geometric theory of defects, media with a spin structure, for example,\nferromagnet, is considered as a manifold with given Riemann--Cartan geometry.\nWe consider the case with the Euclidean metric corresponding to the absence of\nelastic deformations but with nontrivial ${\\mathbb S}{\\mathbb O}(3)$-connection\nwhich produces nontrivial curvature and torsion tensors. We show that the 't\nHooft--Polyakov monopole has physical interpretation in solid state physics\ndescribing media with continuous distribution of dislocations and\ndisclinations. The Chern--Simons action is used for the description of single\ndisclinations. Two examples of point disclinations are considered: spherically\nsymmetric point \"hedgehog\" disclination and the point disclination for which\nthe $n$-field has a fixed value at infinity and essential singularity at the\norigin. The example of linear disclinations with the Franc vector divisible by\n$2\\pi$ is considered.", "category": "cond-mat_mtrl-sci" }, { "text": "Nanostructured Immunosensors. Application to the detection of\n Progesterone: A novel nanostructured electrochemical immunsensor for the determination of\nprogesterone is reported. The approach combines the properties of gold\nnanoparticles with the use of a graphite-Teflon composite electrode matrix,\ninto which gold nanoparticles are incorporated by simple physical inclusion.\nThe antibody anti-progesterone was directly attached to the electrode surface.\nThe immunosensor functioning is based on competitive assay between progesterone\nand alkaline phosphatase-labelled progesterone. Monitoring of the affinity\nreaction was accomplished by the electrochemical oxidation of 1-naphtol.\nModification of the graphite -Teflon electrode matrix with gold nanoparticles\nimproves substantially the electrooxidation response of 1-naphtol. Using a\ndetection potential of +0.3V, a detection limit for progesterone of 0.84 ng\nml-1 was obtained. Analysis of seven milk samples spiked at a 3.5 ng ml-1\nprogesterone concentration level yielded a mean recovery of 101+6%. Detection\nof the antigen-antibody reaction with a graphite - Teflon - colloidal - gold -\nTyrosinase electrode, using phenylphosphate as alkaline phosphatase substrate\nto generate phenol, which is subsequently reduced at -0.1 V at the composite\nelectrode, produced a high improvement in the sensitivity for progesterone\ndetection", "category": "cond-mat_mtrl-sci" }, { "text": "Fabrication of diamond diffraction gratings for experiments with intense\n hard x-rays: The demands on optical components to tolerate high radiation dose and\nmanipulate hard x-ray beams that can fit the experiment requirements, are\nconstantly increasing due to the advancements in the available x-ray sources.\nHere we have successfully fabricated the transmission type gratings using\ndiamond, with structure sizes ranging from few tens of nanometres up to\nmicrometres, and aspect ratio of up to 20. The efficiencies of the gratings\nwere measured over a wide range of photon energies and their radiation\ntolerance was confirmed using the most intense x-ray source in the world. The\nfidelity of these grating structures was confirmed by the quality of the\nmeasured experimental results.", "category": "cond-mat_mtrl-sci" }, { "text": "Room temperature Epitaxial Stabilization of a Tetragonal Phase in ARuO3\n (A=Ca,Sr) Thin Films: We demonstrate that SrRuO3 and CaRuO3 thin films undergo a room temperature\nstructural phase transition driven by the substrate imposed epitaxial biaxial\nstrain. As tensile strain increases, ARuO3 (A=Ca, Sr) films transform from the\northorhombic phase which is usually observed in bulk SrRuO3 and CaRuO3 at room\ntemperature, into a tetragonal phase which in bulk samples is only stable at\nhigher temperatures. More importantly, we show that the observed phenomenon\nstrongly affects the electronic and magnetic properties of ARuO3 thin films\nthat are grown on different single crystal substrates which in turn offers the\npossibility to tune these properties.", "category": "cond-mat_mtrl-sci" }, { "text": "Graphene oxyhydride catalysts in view of spin radical chemistry: The article discusses carbocatalysis provided with amorphous carbons. The\ndiscussion is conducted from the standpoint of the spin chemistry of graphene\nmolecules, in the framework of which the amorphous carbocatalysts are a\nconglomerate of graphene-oxynitrothiohydride stable radicals presenting the\nbasic structural units (BSUs) of the species. The chemical activity of the BSUs\natoms is reliably determined computationally, which allows mapping the\ndistribution of active sites in these molecular catalysts. The presented maps\nreliably evidence the BSUs radicalization provided with carbon atoms only, the\nnon-terminated edge part of which presents a set of active cites. Spin mapping\nof carbocatalysts active cites is suggested as the first step towards the spin\ncarbocatalysis of the species.", "category": "cond-mat_mtrl-sci" }, { "text": "Observation of Magnetopiezoelectric Effect in Antiferromagnetic Metal\n EuMnBi2: We have experimentally studied a magnetopiezoelectric effect predicted\nrecently for magnetic metals with low crystal symmetries. In EuMnBi2 with\nantiferromagnetic Mn moments at 77 K, dynamic displacements emerge along the\n$a$ direction upon application of ac electric fields in the $c$ direction, and\nincrease in proportion to the applied electric fields. Such displacements are\nnot observed along the $c$ direction of EuMnBi2 or EuZnBi2 with nonmagnetic Zn\nions. As temperature increases from 77 K, the displacement signals decrease and\ndisappear at about 200 K, above which electric conduction changes from coherent\nto incoherent. These results demonstrate the emergence of the\nmagnetopiezoelectric effect in a magnetic metal lacking inversion and\ntime-reversal symmetries.", "category": "cond-mat_mtrl-sci" }, { "text": "Structural defects induced by Fe-ion implantation in TiO2: X-ray photoelectron spectroscopy (XPS) and resonant x-ray emission\nspectroscopy (RXES) measurements of pellet and thin film forms of TiO$_2$ with\nimplanted Fe ions are presented and discussed. The findings indicate that\nFe-implantation in a TiO$_2$ pellet sample induces heterovalent cation\nsubstitution (Fe$^{2+}\\rightarrow$ Ti$^{4+}$) beneath the surface region. But\nin thin film samples, the clustering of Fe atoms is primarily detected. In\naddition to this, significant amounts of secondary phases of Fe$^{3+}$ are\ndetected on the surface of all doped samples due to oxygen exposure. These\nexperimental findings are compared with density functional theory (DFT)\ncalculations of formation energies for different configurations of structural\ndefects in the implanted TiO$_2$:Fe system. According to our calculations, the\nclustering of Fe-atoms in TiO$_2$:Fe thin films can be attributed to the\nformation of combined substitutional and interstitial defects. Further, the\ndifferences due to Fe doping in pellet and thin film samples can ultimately be\nattributed to different surface to volume ratios.", "category": "cond-mat_mtrl-sci" }, { "text": "Dynamic friction force in a carbon peapod oscillator: We investigate a new generation of fullerene nano-oscillators: a\nsingle-walled carbon nanotube with one buckyball inside with an operating\nfrequency in the tens-of-gigahertz range. A quantitative characterization of\nenergy dissipation channels in the peapod pair has been performed via molecular\ndynamics simulation. Edge effects are found to the dominant cause of dynamic\nfriction in the carbon-peapod oscillators. A comparative study on energy\ndissipation also reveals significant impact of temperature and impulse velocity\non the frictional force.", "category": "cond-mat_mtrl-sci" }, { "text": "The structure of amorphous two-dimensional materials: Elemental\n monolayer amorphous carbon versus binary monolayer amorphous boron nitride: The structure of amorphous materials has been debated since the 1930's as a\nbinary question: amorphous materials are either Zachariasen continuous random\nnetworks (Z-CRNs) or Z-CRNs containing crystallites. It was recently\ndemonstrated, however, that amorphous diamond can be synthesized in either\nform. Here we address the question of the structure of single-atom-thick\namorphous monolayers. We reanalyze the results of prior simulations for\namorphous graphene and report kinetic Monte Carlo simulations based on\nalternative algorithms. We find that crystallite-containing Z-CRN is the\nfavored structure of elemental amorphous graphene, as recently fabricated,\nwhereas the most likely structure of binary monolayer amorphous BN is\naltogether different than either of the two long-debated options: it is a\ncompositionally disordered \"pseudo-CRN\" comprising a mix of B-N and\nnoncanonical B-B and N-N bonds and containing \"pseudocrystallites\", namely\nhoneycomb regions made of noncanonical hexagons. Implications for other\nnon-elemental 2D and bulk amorphous materials are discussed.", "category": "cond-mat_mtrl-sci" }, { "text": "The decoupled DFT-$\\frac{1}{2}$ method for defect excitation energies: The DFT-$\\frac{1}{2}$ method is a band gap correction with GW precision at a\nDFT computational cost. The method was also extended to correct the gap between\ndefect levels, allowing for the calculation of optical transitions. However,\nthis method fails when the atomic character of the occupied and unoccupied\ndefect levels are similar as we illustrate by two examples, the tetrahedral\nhydrogen interstitial and the negatively charged vacancy in diamond. We solve\nthis problem by decoupling the effect of the occupied and unoccupied defect\nlevels and call this the decoupled DFT-$\\frac{1}{2}$ method for defects.", "category": "cond-mat_mtrl-sci" }, { "text": "Direct-written polymer field-effect transistors operating at 20 MHz: Printed polymer electronics has held for long the promise of revolutionizing\ntechnology by delivering distributed, flexible, lightweight and cost-effective\napplications for wearables, healthcare, diagnostic, automation and portable\ndevices. While impressive progresses have been registered in terms of organic\nsemiconductors mobility, field-effect transistors (FET), the basic building\nblock of any circuit, are still showing limited speed of operation, thus\nlimiting their real applicability. So far, attempts with organic FET to achieve\nthe tens of MHz regime, a threshold for many applications comprising the\ndriving of high resolution displays, have relied on the adoption of\nsophisticated lithographic techniques and/or complex architectures, undermining\nthe whole concept. In this work we demonstrate polymer FETs which can operate\nup to 20 MHz and are fabricated by means only of scalable printing techniques\nand direct-writing methods with a completely mask-less procedure. This is\nachieved by combining a fs-laser process for the sintering of high resolution\nmetal electrodes, thus easily achieving micron-scale channels with reduced\nparasitism down to 0.19 pF mm-1, and a large area coating technique of a high\nmobility polymer semiconductor, according to a simple and scalable process\nflow.", "category": "cond-mat_mtrl-sci" }, { "text": "Magnetic spin excitations in Mn doped GaAs : A model study: We provide a quantitative theoretical model study of the dynamical magnetic\nproperties of optimally annealed Ga$_{1-x}$Mn$_x$As. This model has already\nbeen shown to reproduce accurately the Curie temperatures for\nGa$_{1-x}$Mn$_x$As. Here we show that the calculated spin stiffness are in\nexcellent agreement with those which were obtained from ab-initio based\nstudies. In addition, an overall good agreement is also found with available\nexperimental data. We have also evaluated the magnon density of states and the\ntypical density of states from which the \"mobility edge\", separating the\nextended from localized magnon states, was determined. The power of the model\nlies in its ability to be generalized for a broad class of diluted magnetic\nsemiconductor materials, thus it bridges the gap between first principle\ncalculations and model based studies.", "category": "cond-mat_mtrl-sci" }, { "text": "Superintermolecular orbitals in the C$_{60}$/pentacene complex: We report a group of unusually big molecular orbitals in the C60/pentacene\ncomplex. Our first-principles density functional calculation shows that these\norbitals are very delocalized and cover both C60 and pentacene, which we call\nsuperintermolecular orbitals or SIMOs. Their spatial extension can reach 1 nm\nor larger. Optically, SIMOs are dark. Different from ordinary unoccupied\nmolecular orbitals, SIMOs have a very weak Coulomb and exchange interaction.\nTheir energy levels are very similar to the native superatomic molecular\norbitals in C60, and can be approximately characterized by orbital angular\nmomentum quantum numbers. They have a distinctive spatial preference. These\nfeatures fit the key characters of charge-generation states that channel\ninitially-bound electrons and holes into free charge carriers. Thus, our\nfinding is important for C60/pentacene photovoltaics.", "category": "cond-mat_mtrl-sci" }, { "text": "Elastic and magnetic effects on the infrared phonon spectra of MnF2: We measured the temperature dependent infrared reflectivity spectra of MnF2\nbetween 4 K and room temperature. We show that the phonon spectrum undergoes a\nstrong renormalization at TN. The ab-initio calculation we performed on this\ncompound accurately predict the magnitude and the direction of the phonon\nparameters changes across the antiferromagnetic transition, showing that they\nare mainly induced by the magnetic order. In this material, we found that the\ndielectric constant is mostly from phonon origin. The large change in the\nlattice parameters with temperature seen by X-ray diffraction as well as the\nA2u phonon softening below TN indicate that magnetic order induced distortions\nin MnF2 are compatible with the ferroelectric instabilities observed in TiO2,\nFeF2 and other rutile-type fluorides. This study also shows the anomalous\ntemperature evolution of the lower energy Eu mode in the paramagnetic phase,\nwhich can be compared to that of the B1g one seen by Raman spectroscopy in many\nisostructural materials. This was interpreted as being a precursor of a phase\ntransition from rutile to CaCl2 structure which was observed under pressure in\nZnF2.", "category": "cond-mat_mtrl-sci" }, { "text": "An Efficient DFT Solver for Nanoscale Simulations and Beyond: We present the One-orbital Ensemble Self-Consistent Field (OE-SCF) method, an\n{alternative} orbital-free DFT solver that extends the applicability of DFT to\nsystem sizes beyond the nanoscale while retaining the accuracy required to be\npredictive. OE-SCF is an iterative solver where the (typically computationally\nexpensive) Pauli potential is treated as an external potential and updated\nafter each iteration. Because only up to a dozen iterations are needed to reach\nconvergence, OE-SCF dramatically outperforms current orbital-free DFT solvers.\nEmploying merely a single CPU, we carried out the largest ab initio simulation\nfor silicon-based materials to date. OE-SCF is able to converge the energy of\nbulk-cut Si nanoparticles as a function of their diameter up to 16 nm, for the\nfirst time reproducing known empirical results. We model polarization and\ninterface charge transfer when a Si slab is sandwiched between two metal slabs\nwhere lattice matching mandates a very large slab size. Additionally, OE-SCF\nopens the door to adopt even more accurate functionals in orbital-free DFT\nsimulations while still tackling systems sizes beyond the nanoscale.", "category": "cond-mat_mtrl-sci" }, { "text": "Topological surface states of Bi2Se3 with the coexistence of Se\n vacancies: Although topological surface states are known to be robust against\nnon-magnetic surface perturbations, their band dispersions and spatial\ndistributions are still sensitive to the surface defects. Take Bi2Se3 as an\nexample, we demonstrated that Se vacancies modifies the surface band structures\nconsiderably. When large numbers of Se vacancies exist on the surface,\ntopological surface states may sink down from the first to second quintuple\nlayer and get separated from the vacancies. We simulated STM images to\ndistinguish the surfaces with Se- and Bi-terminations.", "category": "cond-mat_mtrl-sci" }, { "text": "Langevin spin dynamics based on ab initio calculations: numerical\n schemes and applications: A method is proposed to study the finite-temperature behaviour of small\nmagnetic clusters based on solving the stochastic Landau-Lifshitz-Gilbert\nequations, where the effective magnetic field is calculated directly during the\nsolution of the dynamical equations from first principles instead of relying on\nan effective spin Hamiltonian. Different numerical solvers are discussed in the\ncase of a one-dimensional Heisenberg chain with nearest-neighbour interactions.\nWe performed detailed investigations for a monatomic chain of ten Co atoms on\ntop of Au(001) surface. We found a spiral-like ground state of the spins due to\nDzyaloshinsky-Moriya interactions, while the finite-temperature magnetic\nbehaviour of the system was well described by a nearest-neighbour Heisenberg\nmodel including easy-axis anisotropy.", "category": "cond-mat_mtrl-sci" }, { "text": "Electric-field control of magnetic domain wall motion and local\n magnetization reversal: Spintronic devices currently rely on magnetic switching or controlled motion\nof domain walls by an external magnetic field or spin-polarized current.\nAchieving the same degree of magnetic controllability using an electric field\nhas potential advantages including enhanced functionality and low power\nconsumption. Here, we report on an approach to electrically control local\nmagnetic properties, including the writing and erasure of regular ferromagnetic\ndomain patterns and the motion of magnetic domain walls, in multiferroic\nCoFe-BaTiO3 heterostructures. Our method is based on recurrent strain transfer\nfrom ferroelastic domains in ferroelectric media to continuous magnetostrictive\nfilms with negligible magnetocrystalline anisotropy. Optical polarization\nmicroscopy of both ferromagnetic and ferroelectric domain structures reveals\nthat domain correlations and strong inter-ferroic domain wall pinning persist\nin an applied electric field. This leads to an unprecedented electric\ncontrollability over the ferromagnetic microstructure, an accomplishment that\nproduces giant magnetoelectric coupling effects and opens the way to\nmultiferroic spintronic devices.", "category": "cond-mat_mtrl-sci" }, { "text": "The Robustness of Cluster Expansion: Assessing the Roles of Relaxation\n and Numerical Error: Cluster expansion (CE) is effective in modeling the stability of metallic\nalloys, but sometimes cluster expansions fail. Failures are often attributed to\natomic relaxation in the DFT-calculated data, but there is no metric for\nquantifying the degree of relaxation. Additionally, numerical errors can also\nbe responsible for slow CE convergence. We studied over one hundred different\nHamiltonians and identified a heuristic, based on a normalized mean-squared\ndisplacement of atomic positions in a crystal, to determine if the effects of\nrelaxation in CE data are too severe to build a reliable CE model. Using this\nheuristic, CE practitioners can determine a priori whether or not an alloy\nsystem can be reliably expanded in the cluster basis. We also examined the\nerror distributions of the fitting data. We find no clear relationship between\nthe type of error distribution and CE prediction ability, but there are clear\ncorrelations between CE formalism reliability, model complexity, and the number\nof significant terms in the model. Our results show that the \\emph{size} of the\nerrors is much more important than their distribution.", "category": "cond-mat_mtrl-sci" }, { "text": "2D-MoS2 with Narrowest Excitonic Linewidths Grown by Flow-Less Direct\n Heating of Bulk Powders: Developing techniques for high-quality synthesis of mono and few-layered 2D\nmaterials with lowered complexity and cost continues to remain an important\ngoal, both for accelerating fundamental research and for applications\ndevelopment. We present the simplest conceivable technique to synthesize\nmicrometer-scale single-crystal triangular monolayers of MoS2, i.e. by direct\nheating of bulk MoS2 powder onto proximally-placed substrates. Room-temperature\nexcitonic linewidth values of our samples are narrower and more uniform than\nthose of 2D-MoS2 obtained by most other techniques reported in literature, and\ncomparable to those of ultraflat h-BN-capped mechanically exfoliated samples,\nindicative of their high quality. Feature-rich Raman spectra absent in samples\ngrown or obtained by most other techniques, also stand out as a testament of\nthe high quality of our samples. A contact-growth mode facilitates direct\ngrowth of crystallographically-strained circular samples, which allows us to\ndirectly compare the optoelectronic properties of flat vs. strained growth from\nthe same growth runs. Our method allows, for the first time, to quantitatively\ncompare the impact of strain on excitonic and Raman peak positions on\nidentically-synthesized 2D-MoS2. Strain leads to average Red-shifts of ~ 30 meV\nin the A-exciton position, and ~ 2 cm-1 in the E12g Raman peak in these\nsamples. Our findings open-up several new possibilities that expand 2D material\nresearch. By eliminating the need for carrier gas flow, mechanical motion or\nchemical reactions, our method can be either miniaturized for substantially\nlow-cost, high-quality scientific research or potentially scaled-up for\nmass-production of 2D crystals for commercial purposes. Moreover, we believe\nthis technique can also be extended to other transition metal dichalcogenides\nand other layered materials.", "category": "cond-mat_mtrl-sci" }, { "text": "Topological semimetal phases in a family of monolayer X3YZ6 (X=Nb,Ta,\n Y=Si,Ge,Sn, Z=S,Se,Te) with abundant nodal lines and nodes: The electronic and topological properties of single-layer X3YZ6 (X=Nb,Ta,\nY=Si,Ge,Sn, Z=S,Se,Te) materials have been studied with the aid of first\nprinciples calculations. This kind of materials belong to topological\nsemimetals (TMs) with abundant nodal lines and nodes. Considering their similar\nproperties, we focus on the analysis of Ta3SnTe6 and Ta3SiSe6. The present of\nspin-orbit coupling (SOC) leads to the transition from type-I nodal lines to\nDirac points as well as the disappear of type-II Dirac points. The\nthree-dimensional (3D) band diagrams reproduce vividly the characteristics of\nnodes and nodal lines. The appearance of the flat bands in (110) edge states\nfurther confirm their nontrivial topological properties. We also explore the\nrelationship among different nodal lines (nodes), crystal symmetry and SOC. The\ntype-I nodal lines are protected by Mz and My symmetry in the absent of SOC.\nSymmetry breaking leads to band splitting even in the presence of SOC. The\nsingle-layer X3YZ6 can be used as candidates for two-dimensional (2D) TMs and\nprovide a platform for further study of interesting physical phenomena.", "category": "cond-mat_mtrl-sci" }, { "text": "Electronic structure of above-room-temperature van der Waals ferromagnet\n Fe$_3$GaTe$_2$: Fe$_3$GaTe$_2$, a recently discovered van der Waals ferromagnet, demonstrates\nintrinsic ferromagnetism above room temperature, necessitating a comprehensive\ninvestigation of the microscopic origins of its high Curie temperature\n($\\textit{T}$$_C$). In this study, we reveal the electronic structure of\nFe$_3$GaTe$_2$ in its ferromagnetic ground state using angle-resolved\nphotoemission spectroscopy and density functional theory calculations. Our\nresults establish a consistent correspondence between the measured band\nstructure and theoretical calculations, underscoring the significant\ncontributions of the Heisenberg exchange interaction ($\\textit{J}$$_{ex}$) and\nmagnetic anisotropy energy to the development of the high-$\\textit{T}$$_C$\nferromagnetic ordering in Fe$_3$GaTe$_2$. Intriguingly, we observe substantial\nmodifications to these crucial driving factors through doping, which we\nattribute to alterations in multiple spin-splitting bands near the Fermi level.\nThese findings provide valuable insights into the underlying electronic\nstructure and its correlation with the emergence of high-$\\textit{T}$$_C$\nferromagnetic ordering in Fe$_3$GaTe$_2$.", "category": "cond-mat_mtrl-sci" }, { "text": "Harmonic vibrational excitations in graded elastic networks: transition\n from phonons to gradons: We have identified a new type of transition from extended to localized\nvibrational states in one-dimensional graded elastic chains of coupled harmonic\noscillators, in which the vibrating masses or nearest-coupling force constants\nvary linearly along the chain. We found that the delocalization transition\noccurs at the maximum frequency of the corresponding homogeneous chain, which\nis in a continuous single band. Although each state in the localized phase,\ncalled gradon, can be regarded as an impurity localized mode, the localization\nprofile is clearly distinct from usual impurity modes or the Anderson localized\nmodes. We also argue how gradons may affect the macroscopic properties of\ngraded systems. Our results can provide insights into many analogous systems\nwith graded characters.", "category": "cond-mat_mtrl-sci" }, { "text": "Switching ferromagnetic spins by an ultrafast laser pulse: Emergence of\n giant optical spin-orbit torque: Faster magnetic recording technology is indispensable to massive data storage\nand big data sciences. {All-optical spin switching offers a possible solution},\nbut at present it is limited to a handful of expensive and complex rare-earth\nferrimagnets. The spin switching in more abundant ferromagnets may\nsignificantly expand the scope of all-optical spin switching. Here by studying\n40,000 ferromagnetic spins, we show that it is the optical spin-orbit torque\nthat determines the course of spin switching in both ferromagnets and\nferrimagnets. Spin switching occurs only if the effective spin angular momentum\nof each constituent in an alloy exceeds a critical value. Because of the strong\nexchange coupling, the spin switches much faster in ferromagnets than\nweakly-coupled ferrimagnets. This establishes a paradigm for all-optical spin\nswitching. The resultant magnetic field (65 T) is so big that it will\nsignificantly reduce high current in spintronics, thus representing the\nbeginning of photospintronics.", "category": "cond-mat_mtrl-sci" }, { "text": "Graphene Nanoribbon based T Junctions: Graphene nanoribbons (GNRs) based T junctions were designed and studied in\nthis paper. These junctions were made up of shoulders (zigzag GNRs) joined with\nstems (armchair GNRs). We demonstrated the intrinsic transport properties and\neffective boron (or nitrogen) doping of the junctions by using first principles\nquantum transport simulation. Several interesting results were found: i) The\nI-V characteristics of the pure-carbon T junctions were shown to obey Ohm law\nand the electrical conductivity of the junction depends on the height of the\nstem sensitively. ii) boron (or nitrogen) doping on the stems doesnt change the\nOhm law of the T junctions, but the result is opposite when doping process\noccurs at the shoulders. This feature could make such quasi-2D T junction a\npossible candidate for nanoscale junction devices in a 2D network of\nnanoelectronic devices in which conducting pathways can be controlled.", "category": "cond-mat_mtrl-sci" }, { "text": "Anisotropic Gilbert damping in perovskite La$_{0.7}$Sr$_{0.3}$MnO$_{3}$\n thin film: The viscous Gilbert damping parameter governing magnetization dynamics is of\nprimary importance for various spintronics applications. Although, the damping\nconstant is believed to be anisotropic by theories. It is commonly treated as a\nscalar due to lack of experimental evidence. Here, we present an elaborate\nangle dependent broadband ferromagnetic resonance study of high quality\nepitaxial La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ films. Extrinsic effects are suppressed\nand we show convincing evidence of anisotropic damping with twofold symmetry at\nroom temperature. The observed anisotropic relaxation is attributed to the\nmagnetization orientation dependence of the band structure. In addition, we\ndemonstrated that such anisotropy can be tailored by manipulating the stain.\nThis work provides new insights to understand the mechanism of magnetization\nrelaxation.", "category": "cond-mat_mtrl-sci" }, { "text": "Structure and dynamics of the fullerene polymer Li4 C60 studied with\n neutron scattering: The two-dimensional polymer structure and lattice dynamics of the superionic\nconductor Li4 C60 are investigated by neutron diffraction and spectroscopy. The\npeculiar bonding architecture of this compound is definitely confirmed through\nthe precise localisation of the carbon atoms involved in the intermolecular\nbonds. The spectral features of this phase are revealed through ab-initio\nlattice dynamics calculations and inelastic neutron scattering experiments. The\nneutron observables are found to be in very good agreement with the simulations\nwhich predict a partial charge transfer from the Li atoms to the C60 cage. The\nabsence of a well defined band associated to one category of the Li atoms in\nthe experimental spectrum suggests that this species is not ordered even at the\nlowest temperatures. The calculations predict an unstable Li sublattice at a\ntemperature of 200 K, that we relate to the large ionic diffusivity of this\nsystem. This specificity is discussed in terms of coupling between the low\nfrequency optic modes of the Li ions to the soft structure of the polymer.", "category": "cond-mat_mtrl-sci" }, { "text": "Direct dry transfer of chemical vapor deposition graphene to polymeric\n substrates: We demonstrate the direct dry transfer of large area Chemical Vapor\nDeposition graphene to several polymers (low density polyethylene, high density\npolyethylene, polystyrene, polylactide acid and\npoly(vinylidenefluoride-co-trifluoroethylene) by means of only moderate heat\nand pressure, and the later mechanical peeling of the original graphene\nsubstrate. Simulations of the graphene-polymer interactions, rheological tests\nand graphene transfer at various experimental conditions show that controlling\nthe graphene-polymer interface is the key to controlling graphene transfer.\nRaman spectroscopy and Optical Microscopy were used to identify and quantify\ngraphene transferred to the polymer substrates. The results showed that the\namount of graphene transferred to the polymer, from no-graphene to full\ngraphene transfers, can be achieved by fine tuning the transfer conditions. As\na result of the direct dry transfer technique, the graphene-polymer adhesion\nbeing stronger than graphene to Si/SiO2 wafer.", "category": "cond-mat_mtrl-sci" }, { "text": "Tantalum STJ for Photon Counting Detectors: Superconducting Tunnel Junctions (STJ's) are currently being developed as\nphoton detectors for a wide range of applications. Interest comes from their\nability to cumulate photon counting with chromaticity (i.e. energy resolution)\nfrom the near infrared (2 $\\mu$m) to the X-rays wavelengths and good quantum\nefficiency up to 80%. Resolving power can exceed 10 in the visible wavelength\nrange. Our main goal is to use STJ's for astronomical observations at low light\nlevel in the near infrared. This paper put the emphasis on two main points: the\nimprovement of the tantalum absorber epitaxy and the development of a new\nversion of the fabrication process for making Ta/Al-AlOx-Al/Ta photon counting\nSTJ's. The main features of this process are that pixels have aligned\nelectrodes and vias patterned through a protecting SiO2 layer. These vias are\nthen used to contact the top electrode layer. We use a double thin aluminum\ntrapping layer on top of a 150 nm thick Ta absorber grown epitaxially. Photon\ncounting experiments with Ta junction array are presented at \\lambda = 0.78\n$\\mu$m. Digital filtering methods are used to compute the photon counting data\nin order to minimize the effects of noise.", "category": "cond-mat_mtrl-sci" }, { "text": "Discovery of Weyl nodal lines in a single-layer ferromagnet: Two-dimensional (2D) materials have attracted great attention and spurred\nrapid development in both fundamental research and device applications. The\nsearch for exotic physical properties, such as magnetic and topological order,\nin 2D materials could enable the realization of novel quantum devices and is\ntherefore at the forefront of materials science. Here, we report the discovery\nof two-fold degenerate Weyl nodal lines in a 2D ferromagnetic material, a\nsingle-layer gadolinium-silver compound, based on combined angle-resolved\nphotoemission spectroscopy measurements and theoretical calculations. These\nWeyl nodal lines are symmetry protected and thus robust against external\nperturbations. The coexistence of magnetic and topological order in a 2D\nmaterial is likely to inform ongoing efforts to devise and realize novel\nnanospintronic devices.", "category": "cond-mat_mtrl-sci" }, { "text": "Optimizing Floquet engineering for non-equilibrium steady states with\n gradient-based methods: Non-equilibrium steady states are created when a periodically driven quantum\nsystem is also incoherently interacting with an environment -- as it is the\ncase in most realistic situations. The notion of Floquet engineering refers to\nthe manipulation of the properties of systems under periodic perturbations.\nAlthough it more frequently refers to the coherent states of isolated systems\n(or to the transient phase for states that are weakly coupled to the\nenvironment), it may sometimes be of more interest to consider the final steady\nstates that are reached after decoherence and dissipation take place. In this\nwork, we propose a computational method to find the multicolor periodic\nperturbations that lead to the final steady states that are optimal with\nrespect to a given predefined metric, such as for example the maximization of\nthe temporal average value of some observable. We exemplify the concept using a\nsimple model for the nitrogen-vacancy center in diamond: the goal in this case\nis to find the driving periodic magnetic field that maximizes a time-averaged\nspin component. We show that, for example, this technique permits to prepare\nstates whose spin values are forbidden in thermal equilibrium at any\ntemperature.", "category": "cond-mat_mtrl-sci" }, { "text": "Tuning carrier density and phase transitions in oxide semiconductors\n using focused ion beams: We demonstrate spatial modification of the optical properties of thin-film\nmetal oxides, zinc oxide and vanadium dioxide as representatives, using a\ncommercial focused ion beam (FIB) system. Using a Ga+ FIB and thermal\nannealing, we demonstrated variable doping of a band semiconductor, zinc oxide\n(ZnO), achieving carrier concentrations from 10^18 cm-3 to 10^20 cm-3. Using\nthe same FIB without subsequent thermal annealing, we defect-engineered a\ncorrelated semiconductor, vanadium dioxide (VO2), locally modifying its\ninsulator-to-metal transition (IMT) temperature by range of ~25 degrees C. Such\narea-selective modification of metal oxides by direct writing using a FIB\nprovides a simple, mask-less route to the fabrication of optical structures,\nespecially when multiple or continuous levels of doping or defect density are\nrequired.", "category": "cond-mat_mtrl-sci" }, { "text": "High-order harmonic generation in solid $\\rm \\bf C_{60}$: High harmonic generation (HHG) has unleashed the power of strong laser\nphysics in solids. Here we investigate HHG from a large system, solid C$_{60}$,\nwith 240 valence electrons engaging harmonic generation at each crystal\nmomentum, the first of this kind. We employ the density functional theory and\nthe time-dependent Liouville equation of the density matrix to compute HHG\nsignals. We find that under a moderately strong laser pulse, HHG signals reach\n15th order, consistent with the experimental results from C$_{60}$ plasma. The\nhelicity dependence in solid C$_{60}$ is weak, due to the high symmetry. In\ncontrast to the general belief, HHG is unsuitable for band structure mapping in\nC$_{60}$. However, we find a window of opportunity using a long wavelength,\nwhere harmonics are generated through multiple-photon excitation. In\nparticular, the 5th order harmonic energies closely follow the transition\nenergy dispersion between the valence and conduction bands. This finding is\nexpected to motivate future experimental investigations.", "category": "cond-mat_mtrl-sci" }, { "text": "Controlling the Electrical Properties of Undoped and Ta-doped TiO2\n Polycrystalline Films via Ultra-Fast Annealing Treatments: We present a study on the crystallization process of undoped and Ta doped\nTiO2 amorphous thin films. In particular, the effect of ultra-fast annealing\ntreatments in environments characterized by different oxygen concentrations is\ninvestigated via in-situ resistance measurements. The accurate examination of\nthe key parameters involved in this process allows us to reduce the time needed\nto obtain highly conducting and transparent polycrystalline thin films\n(resistivity about $6 \\times 10^{-4}$ {\\Omega}cm, mean transmittance in the\nvisible range about $81\\%$) to just 5 minutes (with respect to the 180 minutes\nrequired for a standard vacuum annealing treatment) in nitrogen atmosphere (20\nppm oxygen concentration) at ambient pressure. Experimental evidence of\nsuperficial oxygen incorporation in the thin films and its detrimental role for\nthe conductivity are obtained by employing different concentrations of\ntraceable 18O isotopes during ultra-fast annealing treatments. The results are\ndiscussed in view of the possible implementation of the ultra-fast annealing\nprocess for TiO2-based transparent conducting oxides as well as electron\nselective layers in solar cell devices; taking advantage of the high control of\nthe ultra-fast crystallization processes which has been achieved, these two\nfunctional layers are shown to be obtainable from the crystallization of a\nsingle homogeneous thin film.", "category": "cond-mat_mtrl-sci" }, { "text": "Oxidation tuning of ferroic transitions in Gd$_2$C monolayer: Tuning of ferroic phases provides great opportunities for material\nfunctionalities, especially in two-dimensional materials. Here, a $4f$\nrare-earth carbide Gd$_2$C monolayer is predicted to be ferromagnetic metal\nwith large magnetization, inherited from its bulk property. Based on\nfirst-principles calculations, we propose a strategy that the surface\npassivation can effectively tune its ferroicity, namely switching among\nferromagnetic, antiferromagnetic, and ferroelectric phases. Metal-insulator\ntransition also occurs accompanying these ferroic transitions. Our calculation\nalso suggests that the magneto-optic Kerr effect and second harmonic generation\nare effective methods to monitor these phase transitions.", "category": "cond-mat_mtrl-sci" }, { "text": "Nonthermal effects in solids after swift heavy ion impact: This contribution is a brief introduction to nonthermal effects related to\nmodifications of the interatomic potential upon ultrafast excitation of the\nelectronic system of solids, primarily focusing on the swift heavy ion track\nproblem. We clarify the difference between the exchange of the kinetic energy\nof electrons (and holes) scattering on the lattice (electron-phonon coupling,\n\"thermal effects\") and the relaxation of the nonequilibrium potential energy of\na solid (\"nonthermal effects\"). We discuss that at different degrees of\nelectronic excitation, the modification of the interatomic potential may result\nin various phase transitions without an increase of the atomic temperature,\ni.e., at room temperature (nonthermal melting, formation of the superionic\nstate), or in atomic acceleration causing \"nonthermal heating\" of the target\natoms. Examples of theoretically predicted various effects are given, supported\nby known experimental observations.", "category": "cond-mat_mtrl-sci" }, { "text": "Ab initio study of proton-exchanged LiNbO3(I): Structural,\n thermodynamic, dielectric, and optical properties: Using first principles calculations, we study the ground-state structure of\nbulk proton-exchanged lithium niobate, which is also called hydrogen niobate\nand is widely used in waveguides. Thermodynamics helps to establish the most\nfavorable nonpolar surface as well as the water-deficient and water-rich phases\nunder different ambient conditions, which we refer to as \"dehydrated\" and\n\"rehydrated\" phases, respectively. We compute the low-frequency dielectric\nresponse and the optical refractive indices of hydrogen niobate in different\nphases. The dielectric constant is greatly enhanced compared to lithium\nniobate. At shorter wavelengths, the refractive indices vary between each phase\nand have a sharp contrast to lithium niobate. Our study characterizes the\nstructures and thermal instabilities of this compound and reveals its excellent\ndielectric and optical properties, which can be important in the future\napplication in waveguides.", "category": "cond-mat_mtrl-sci" }, { "text": "Spin-orbit Hamiltonian for organic crystals from first principles\n electronic structure and Wannier functions: Spin-orbit coupling in organic crystals is responsible for many\nspin-relaxation phenomena, going from spin diffusion to intersystem crossing.\nWith the goal of constructing effective spin-orbit Hamiltonians to be used in\nmultiscale approaches to the thermodynamical properties of organic crystals, we\npresent a method that combines density functional theory with the construction\nof Wannier functions. In particular we show that the spin-orbit Hamiltonian\nconstructed over maximally localised Wannier functions can be computed by\ndirect evaluation of the spin-orbit matrix elements over the Wannier functions\nconstructed in absence of spin-orbit interaction. This eliminates the prob- lem\nof computing the Wannier functions for almost degenerate bands, a problem\nalways present with the spin-orbit-split bands of organic crystals. Examples of\nthe method are presented for isolated molecules, for mono-dimensional chains of\nPb and C atoms and for triarylamine-based one-dimansional single crystals.", "category": "cond-mat_mtrl-sci" }, { "text": "Moulding flexural waves in elastic plates lying atop a Faqir's bed of\n nails: Platonic crystals (PCs) are the elastic plate analogue of the photonic\ncrystals widely used in optics, and are thin structured elastic plates along\nwhich flexural waves cannot propagate within certain stop band frequency\nintervals. The practical importance of PCs is twofold: these can be used either\nin the design of microstructured acoustic metamaterials or as an approximate\nmodel for surface elastic waves propagating in meter scale seismic\nmetamaterials. Here, we make use of the band spectrum of PCs created with very\nsmall clamped holes, the nails of the title, to achieve surface wave reflectors\nat very large wavelengths, a flat lens, an endoscope, a directive antenna near\nstop band frequencies and cloaking from Dirac cones. The point pinned, Faqir,\nplate is particularly appealing as there is an exact dispersion relation\navailable so the origin of these phenomena can be explained and interpreted\nusing Fourier series and high frequency homogenization.", "category": "cond-mat_mtrl-sci" }, { "text": "Prediction of grain boundary structure and energy by machine learning: Grain boundaries dramatically affect the properties of polycrystalline\nmaterials because of differences in atomic configuration. To fully understand\nthe relationship between grain boundaries and materials properties, systematic\nstudies of the grain boundary atomic structure are crucial. However, such\nstudies are limited by the extensive computation necessary to determine the\nstructure of a single grain boundary. If the structure could be predicted with\nmore efficient computation, the understanding of the grain boundary would be\naccelerated significantly. Here, we predict grain boundary structures and\nenergies using a machine-learning technique. Training data for non-linear\nregression of four symmetric-tilt grain boundaries of copper were used. The\nresults of the regression analysis were used to predict 12 other grain boundary\nstructures. The method accurately predicts both the structures and energies of\ngrain boundaries. The method presented in this study is very general and can be\nutilized in understanding many complex interfaces.", "category": "cond-mat_mtrl-sci" }, { "text": "Cooperative gas adsorption without a phase transition in metal-organic\n frameworks: Cooperative adsorption of gases by porous frameworks permits more efficient\nuptake and removal than does the more usual non-cooperative (Langmuir-type)\nadsorption. Cooperativity, signaled by a step-like isotherm, is usually\nattributed to a phase transition of the framework. However, the class of\nmetal-organic frameworks mmen-M$_2$(dobpdc) exhibit cooperative adsorption of\nCO2 but show no evidence of a phase transition. Here we show how cooperativity\nemerges in these frameworks in the absence of a phase transition. We use a\ncombination of quantum and statistical mechanics to show that cooperativity\nresults from a sharp but finite increase, with pressure, of the mean length of\nchains of CO2 molecules that polymerize within the framework. Our study\nprovides microscopic understanding of the emergent features of cooperative\nbinding, including the position, slope and height of the isotherm step, and\nindicates how to optimize gas storage and separation in these materials.", "category": "cond-mat_mtrl-sci" }, { "text": "Characterization and control of ZnGeN2 cation lattice ordering: ZnGeN2 and other heterovalent ternary semiconductors have important potential\napplications in optoelectronics, but ordering of the cation sublattice, which\ncan affect the band gap, lattice parameters, and phonons, is not yet well\nunderstood. Here the effects of growth and processing conditions on the\nordering of the ZnGeN2 cation sublattice were investigated using x-ray\ndiffraction and Raman spectroscopy. Polycrystalline ZnGeN2 was grown by\nexposing solid Ge to Zn and NH3 vapors at temperatures between 758 degree C and\n914 degree C. Crystallites tended to be rod-shaped, with growth rates higher\nalong the c-axis. The degree of ordering, from disordered, wurtzite-like x-ray\ndiffraction spectra to orthorhombic, with space group Pna21, increased with\nincreasing growth temperature, as evidenced by the appearance of superstructure\npeaks and peak splittings in the diffraction patterns. Annealing disordered,\nlow-temperature-grown ZnGeN2 at 850 degree C resulted in increased cation\nordering. Growth of ZnGeN2 on a liquid Sn-Ge-Zn alloy at 758 degree C showed an\nincrease in the tendency for cation ordering at a lower growth temperature, and\nresulted in hexagonal platelet-shaped crystals. The trends shown here may help\nto guide understanding of the synthesis and characterization of other\nheterovalent ternary nitride semiconductors as well as ZnGeN2.", "category": "cond-mat_mtrl-sci" }, { "text": "Superheating and solid-liquid phase coexistence in nanoparticles with\n non-melting surfaces: We present a phenomenological model of melting in nanoparticles with facets\nthat are only partially wet by their liquid phase. We show that in this model,\nas the solid nanoparticle seeks to avoid coexistence with the liquid, the\nmicrocanonical melting temperature can exceed the bulk melting point, and that\nthe onset of coexistence is a first-order transition. We show that these\nresults are consistent with molecular dynamics simulations of aluminum\nnanoparticles which remain solid above the bulk melting temperature.", "category": "cond-mat_mtrl-sci" }, { "text": "Giant optical birefringence of semiconductor nanowire metamaterials: Semiconductor nanowires exhibit large polarization anisotropy for the\nabsorption and emission of light, making them ideal building blocks for novel\nphotonic metamaterials. Here, we demonstrate that a high density of aligned\nnanowires exhibits giant optical birefringence, a collective phenomenon\nobservable uniquely for collections of wires. The nanowire material was grown\non gallium phosphide (GaP) (111) in the form of vertically standing GaP\nnanowires. We obtain the largest optical birefringence to date, with a\ndifference between the in-plane and out-of-plane refractive indices of 0.80 and\na relative birefringence of 43%. These values exceed by a factor of 75 the\nnatural birefringence of quartz and a by more than a factor of two the highest\nvalues reported so far in other artificial materials. By exploiting the\nspecific crystallographic growth directions of the nanowires on the substrate,\nwe further demonstrate full control over the orientation of the optical\nbirefringence effect in the metamaterial.", "category": "cond-mat_mtrl-sci" }, { "text": "Imaging the stick-slip peeling of an adhesive tape under a constant load: Using a high speed camera, we study the peeling dynamics of an adhesive tape\nunder a constant load with a special focus on the so-called stick-slip regime\nof the peeling. It is the first time that the very fast motion of the peeling\npoint is imaged. The speed of the camera, up to 16000 fps, allows us to observe\nand quantify the details of the peeling point motion during the stick and slip\nphases: stick and slip velocities, durations and amplitudes. First, in contrast\nwith previous observations, the stick-slip regime appears to be only transient\nin the force controlled peeling. Additionally, we discover that the stick and\nslip phases have similar durations and that at high mean peeling velocity, the\nslip phase actually lasts longer than the stick phase. Depending on the mean\npeeling velocity, we also observe that the velocity change between stick and\nslip phase ranges from a rather sudden to a smooth transition. These new\nobservations can help to discriminate between the various assumptions used in\ntheoretical models for describing the complex peeling of an adhesive tape. The\npresent imaging technique opens the door for an extensive study of the velocity\ncontrolled stick-slip peeling of an adhesive tape that will allow to understand\nthe statistical complexity of the stick-slip in a stationary case.", "category": "cond-mat_mtrl-sci" }, { "text": "Glancing-Incidence Focussed Ion Beam Milling: A Coherent X-ray\n Diffraction Study of 3D Nano-scale Lattice Strains and Crystal Defects: This study presents a detailed examination of the lattice distortions\nintroduced by glancing incidence Focussed Ion Beam (FIB) milling. Using\nnon-destructive multi-reflection Bragg coherent X-ray diffraction we probe\ndamage formation in an initially pristine gold micro-crystal following several\nstages of FIB milling. These experiments allow access to the full lattice\nstrain tensor in the micro-crystal with ~25 nm 3D spatial resolution, enabling\na nano-scale analysis of residual lattice strains and defects formed. Our\nresults show that 30 keV glancing incidence milling produces fewer large\ndefects than normal incidence milling at the same energy. However the resulting\nresidual lattice strains have similar magnitude and extend up to ~50 nm into\nthe sample. At the edges of the milled surface, where the ion-beam tails impact\nthe sample at near-normal incidence, large dislocation loops with a range of\nburgers vectors are formed. Further glancing incidence FIB polishing with 5 keV\nion energy removes these dislocation loops and reduces the lattice strains\ncaused by higher energy FIB milling. However, even at the lower ion energy,\ndamage-induced lattice strains are present within a ~20 nm thick surface layer.\nThese results highlight the need for careful consideration and management of\nFIB damage. They also show that low-energy FIB-milling is an effective tool for\nremoving FIB-milling induced lattice strains. This is important for the\npreparation of micro-mechanical test specimens and strain microscopy samples.", "category": "cond-mat_mtrl-sci" }, { "text": "Hybrid exchange-correlation functional for accurate prediction of the\n electronic and structural properties of ferroelectric oxides: Using a linear combination of atomic orbitals approach, we report a\nsystematic comparison of various Density Functional Theory (DFT) and hybrid\nexchange-correlation functionals for the prediction of the electronic and\nstructural properties of prototypical ferroelectric oxides. It is found that\nnone of the available functionals is able to provide, at the same time,\naccurate electronic and structural properties of the cubic and tetragonal\nphases of BaTiO$_3$ and PbTiO$_3$. Some, although not all, usual DFT\nfunctionals predict the structure with acceptable accuracy, but always\nunderestimate the electronic band gaps. Conversely, common hybrid functionals\nyield an improved description of the band gaps, but overestimate the volume and\natomic distortions associated to ferroelectricity, giving rise to an\nunacceptably large $c/a$ ratio for the tetragonal phases of both compounds.\nThis super-tetragonality is found to be induced mainly by the exchange energy\ncorresponding to the Generalized Gradient Approximation (GGA) and, to a lesser\nextent, by the exact exchange term of the hybrid functional. We thus propose an\nalternative functional that mixes exact exchange with the recently proposed GGA\nof Wu and Cohen [Phys. Rev. B 73, 235116 (2006)] which, for solids, improves\nover the treatment of exchange of the most usual GGA's. The new functional\nrenders an accurate description of both the structural and electronic\nproperties of typical ferroelectric oxides.", "category": "cond-mat_mtrl-sci" }, { "text": "Modelling the Nonlinear Response of Fibre-reinforced Bending Fluidic\n Actuators: Soft actuators are receiving increasing attention from the engineering\ncommunity, not only in research but even for industrial applications. Among\nsoft actuators, fibre-reinforced Bending Fluidic Actuators (BFAs) became very\npopular thanks to features such as robustness and easy design and fabrication.\nHowever, an accurate modelling of these smart structures, taking into account\nall the nonlinearities involved, is a challenging task. In this effort, we\npropose an analytical mechanical model to capture the quasi-static response of\nfibre-reinforced BFAs. The model is fully 3D and for the first time includes\nthe effect of the pressure on the lateral surface of the chamber as well as the\nnon-constant torque produced by the pressure at the tip. The presented model\ncan be used for design and control, while providing information about the\nmechanics of these complex actuators.", "category": "cond-mat_mtrl-sci" }, { "text": "Spatially Heterogeneous Dynamics in a Metallic Glass Forming Liquid\n Imaged by Electron Correlation Microscopy: Supercooled liquids exhibit spatial heterogeneity in the dynamics of their\nfluctuating atomic arrangements. The length and time scales of the\nheterogeneous dynamics are central to the glass transition and influence\nnucleation and growth of crystals from the liquid. We report direct\nexperimental visualization of the spatially heterogeneous dynamics as a\nfunction of temperature in the supercooled liquid state of a Pt-based metallic\nglass, using electron correlation microscopy with sub-nanometer resolution. An\nexperimental four point space-time intensity correlation function demonstrates\na growing dynamic correlation length, $\\xi$, upon cooling of the liquid toward\nthe glass transition temperature. $\\xi$ as a function of the relaxation time\n$\\tau$ data are in the good agreement with the Adam-Gibbs, inhomogeneous mode\ncoupling theory and random first order transition theory of the glass\ntransition. The same experiments demonstrate the existence of a nanometer\nthickness near surface layer with order of magnitude shorter relaxation time\nthan inside the bulk.", "category": "cond-mat_mtrl-sci" }, { "text": "Gigahertz Dielectric Polarization of Single-atom Niobium Substituted in\n Graphitic layers: We have synthesized two Nb@C composites with an order of magnitude difference\nin the density of single-atom niobium substituted into graphitic layers. The\nconcentration and sites of single-atom Nb are identified using\naberration-corrected scanning transmission electron microscopy and density\nfunctional theory. Comparing the complex permittivity spectra show that the\nrepresentative dielectric resonance at ~16 GHz originates from the intrinsic\npolarization of single-atom Nb sites, confirmed by theoretical simulations. The\nsingle-atom dielectric resonance represents the physical limit of the\nelectromagnetic response of condensed matter, and thus might open up a new\navenue for designing electromagnetic wave absorption materials. Single-atom\nresonance also has important implications in understanding the correlation\nbetween the macroscopic dielectric behaviors and the atomic-scale structural\norigin.", "category": "cond-mat_mtrl-sci" }, { "text": "Failure time in the fiber-bundle model with thermal noise and disorder: The average time for the onset of macroscopic fractures is analytically and\nnumerically investigated in the fiber-bundle model with quenched disorder and\nthermal noise under a constant load. We find an implicit exact expression for\nthe failure time in the low-temperature limit that is accurately confirmed by\ndirect simulations. The effect of the disorder is to lower the energy barrier.", "category": "cond-mat_mtrl-sci" }, { "text": "Electron-hole versus exciton delocalization in conjugated polymers: the\n role of topology: There is currently a great need for solid state lasers that emit in the\ninfrared. Whether or not conjugated polymers that emit in the IR can be\nsynthesized is an interesting theoretical challenge. We show that the\nrequirement for such a material is that the exciton delocalization in the\nsystem be large, such that the optical gap is small. We develop a theory of\nexciton delocalization in conjugated polymers, and show that the extent of this\ncan be predicted from the topology of the conjugated polymer in question. We\ndetermine the precise structural characteristics that would be necessary for\nlight emission in the IR.", "category": "cond-mat_mtrl-sci" }, { "text": "Structural Transitions at Ionic Liquids Interfaces: Recent advances in experimental and computational techniques have allowed for\nan accurate description of the adsorption of ionic liquids on metallic\nelectrodes. It is now well established that they adopt a multi-layered\nstructure, and that the composition of the layers changes with the potential of\nthe electrode. In some cases, potential-driven ordering transitions in the\nfirst adsorbed layer have been observed in experiments probing the interface on\nthe molecular scale or by molecular simulations. This perspective gives an\noverview of the current understanding of such transitions and of their\npotential impact on the physical and (electro)chemical processes at the\ninterface. In particular, peaks in the differential capacitance, slow dynamics\nat the interface and changes in the reactivity have been reported in\nelectrochemical studies. Interfaces between ionic liquids and metallic\nelectrodes are also highly relevant for their friction properties, the\nvoltage-dependence of which opens the way to exciting applications.", "category": "cond-mat_mtrl-sci" }, { "text": "First principles calculations of the interface properties of\n amorphous-Al2O3/MoS2 under non-strain and biaxial strain conditions: Al2O3 is a potential dielectric material for metal-oxide-semiconductor (MOS)\ndevices. Al2O3 films deposited on semiconductors usually exhibit amorphous due\nto lattice mismatch. Compared to two-dimensional graphene, MoS2 is a typical\nsemiconductor, therefore, it has more extensive application. The\namorphous-Al2O3/MoS2 (a-Al2O3/MoS2) interface has attracted people's attention\nbecause of its unique properties. In this paper, the interface behaviors of\na-Al2O3/MoS2 under non-strain and biaxial strain are investigated by first\nprinciples calculations based on density functional theory (DFT). First of all,\nthe generation process of a-Al2O3 sample is described, which is calculated by\nmolecular dynamics and geometric optimization. Then, we introduce the band\nalignment method, and calculate band offset of a-Al2O3/MoS2 interface. It is\nfound that the valence band offset (VBO) and conduction band offset (CBO)\nchange with the number of MoS2 layers. The dependence of leakage current on the\nband offset is also illustrated. At last, the band structure of monolayer MoS2\nunder biaxial strain is discussed. The biaxial strain is set in the range from\n-6% to 6% with the interval of 2%. Impact of the biaxial strain on the band\nalignment is investigated.", "category": "cond-mat_mtrl-sci" }, { "text": "Magneto-transport and magneto-optical properties of ferromagnetic\n (III,Mn)V semicondcutors: a review: Rapid developments in material research of metallic ferromagnetic (III,Mn)V\nsemiconductors over the past few years have brought a much better understanding\nof these complex materials. We review here some of the main developments and\ncurrent understanding of the bulk properties of these systems within the\nmetallic regime, focusing principally on the magneto-transport and\nmagneto-optical properties. Although several theoretical approaches are\nreviewed, the bulk of the review uses the effective Hamiltonian approach, which\nhas proven useful in describing many of these properties namely in (Ga,Mn)As\nand (In,Mn)As. The model assumes a ferromagnetic coupling between Mn d-shell\nlocal moments mediated by holes in the semiconductor valence band.", "category": "cond-mat_mtrl-sci" }, { "text": "Theory of structural response to macroscopic electric fields in\n ferroelectric systems: We have developed and implemented a formalism for computing the structural\nresponse of a periodic insulating system to a homogeneous static electric field\nwithin density-functional perturbation theory (DFPT). We consider the\nthermodynamic potentials E(R,eta,e) and F(R,eta,e) whose minimization with\nrespect to the internal structural parameters R and unit cell strain eta yields\nthe equilibrium structure at fixed electric field e and polarization P,\nrespectively. First-order expansion of E(R,eta,e) in e leads to a useful\napproximation in which R(P) and eta(P) can be obtained by simply minimizing the\nzero-field internal energy with respect to structural coordinates subject to\nthe constraint of a fixed spontaneous polarization P. To facilitate this\nminimization, we formulate a modified DFPT scheme such that the computed\nderivatives of the polarization are consistent with the discretized form of the\nBerry-phase expression. We then describe the application of this approach to\nseveral problems associated with bulk and short-period superlattice structures\nof ferroelectric materials such as BaTiO3 and PbTiO3. These include the effects\nof compositionally broken inversion symmetry, the equilibrium structure for\nhigh values of polarization, field-induced structural phase transitions, and\nthe lattice contributions to the linear and the non-linear dielectric\nconstants.", "category": "cond-mat_mtrl-sci" }, { "text": "Anomalies in non-stoichiometric uranium dioxide induced by pseudo-phase\n transition of point defects: A uniform distribution of point defects in an otherwise perfect\ncrystallographic structure usually describes a unique pseudo phase of that\nstate of a non-stoichiometric material. With off-stoichiometric uranium dioxide\nas a prototype, we show that analogous to a conventional phase transition,\nthese pseudo phases also will transform from one state into another via\nchanging the predominant defect species when external conditions of pressure,\ntemperature, or chemical composition are varied. This exotic transition is\nnumerically observed along shock Hugoniots and isothermal compression curves in\nUO2 with first-principles calculations. At low temperatures, it leads to\nanomalies (or quasi-discontinuities) in thermodynamic properties and electronic\nstructures. In particular, the anomaly is pronounced in both shock temperature\nand the specific heat at constant pressure. With increasing of the temperature,\nhowever, it transforms gradually to a smooth cross-over, and becomes less\ndiscernible. The underlying physical mechanism and characteristics of this type\nof transition are encoded in the Gibbs free energy, and are elucidated clearly\nby analyzing the correlation with the variation of defect populations as a\nfunction of pressure and temperature. The opportunities and challenges for a\npossible experimental observation of this phase change are also discussed.", "category": "cond-mat_mtrl-sci" }, { "text": "Theory of self-diffusion in GaAs: Ab initio molecular dynamics simulations are employed to investigate the\ndominant migration mechanism of the gallium vacancy in gaas as well as to\nassess its free energy of formation and the rate constant of gallium\nself-diffusion. our analysis suggests that the vacancy migrates by second\nnearest neighbour hops. the calculated self-diffusion constant is in good\nagreement with the experimental value obtained in ^69 GaAs/ ^71 GaAs isotope\nheterostructures and at significant variance with that obtained earlier from\ninterdiffusion experiments in GaAlAs/GaAs-heterostructures.", "category": "cond-mat_mtrl-sci" }, { "text": "Superconductivity in intercalated buckled two-dimensional materials:\n KGe$_2$: Germanene has emerged as a novel two-dimensional material with various\ninteresting properties and applications. Here we report the possibility of\nsuperconductivity in a stable potassium intercalated germanene compound,\nKGe$_2$, with a transition temperature $T_c \\sim 11$ K, and an electron-phonon\ncoupling of 1.9. Applying a 5\\% tensile strain, which reduces the buckling\nheight by 4.5\\%, leads to the reduction of the electron-phonon coupling by 11\\%\nand a slight increase in $T_c \\sim 12$ K. That is, strong electron-phonon\ncoupling results from the buckled structure of the germanene layers. Despite\nbeing an intercalated van der Waals material similar to intercalated graphite\nsuperconductors, it does not possess an occupied interlayer state.", "category": "cond-mat_mtrl-sci" }, { "text": "Oxygen vacancies in strained SrTiO$_{3}$ thin films: formation enthalpy\n and manipulation: We report the enthalpy of oxygen vacancy formation in thin films of\nelectron-doped SrTiO$_{3}$, under different degrees of epitaxial stress. We\ndemonstrate that both compressive and tensile strain decrease this energy at a\nvery similar rate, and promote the formation of stable doubly ionized oxygen\nvacancies. Moreover, we also show that unintentional cationic vacancies\nintroduced under typical growth conditions, produce a characteristic rotation\npattern of TiO$_6$ octahedra. The local concentration of oxygen vacancies can\nbe modulated by an electric field with an AFM tip, changing not only the local\nelectrical potential, but also producing a non-volatile mechanical response\nwhose sign (up/down) can be reversed by the electric field.", "category": "cond-mat_mtrl-sci" }, { "text": "Water adsorption on the P-rich GaP(100) surface: Optical spectroscopy\n from first principles: The contact of water with semiconductors typically changes its surface\nelectronic structure by oxidation or corrosion processes. A detailed knowledge\n- or even control of - the surface structure is highly desirable, as it impacts\nthe performance of opto-electronic devices from gas-sensing to energy\nconversion applications. It is also a prerequisite for density functional\ntheory-based modelling of the electronic structure in contact with an\nelectrolyte. The P-rich GaP(100) surface is extraordinary with respect to its\ncontact with gas-phase water, as it undergoes a surface reordering, but does\nnot oxidise. We investigate the underlying changes of the surface in contact\nwith water by means of theoretically derived reflection anisotropy spectroscopy\n(RAS). A comparison of our results with experiment reveals that a water-induced\nhydrogen-rich phase on the surface is compatible with the boundary conditions\nfrom experiment, reproducing the optical spectra. We discuss potential reaction\npaths that comprise a water-enhanced hydrogen mobility on the surface. Our\nresults also show that computational RAS - required for the interpretation of\nexperimental signatures - is feasible for GaP in contact with water double\nlayers. Here, RAS is sensitive to surface electric fields, which are an\nimportant ingredient of the Helmholtz-layer. This paves the way for future\ninvestigations of RAS at the semiconductor-electrolyte interface.", "category": "cond-mat_mtrl-sci" }, { "text": "Chemical and nuclear catalysis driven by localized anharmonic vibrations: In many-body nonlinear systems with sufficient anharmonicity, a special kind\nof lattice vibrations, namely, Localized Anharmonic Vibrations (LAV) can be\nexcited either thermally or by external triggering, in which the amplitude of\natomic oscillations greatly exceeds that of harmonic oscillations (phonons)\nthat determine the system temperature. Coherency and persistence of LAV may\nhave drastic effect on chemical and nuclear reaction rates due to time-periodic\nmodulation of reaction sites. One example is a strong acceleration of chemical\nreaction rates driven by thermally-activated \"jumps\" over the reaction barrier\ndue to the time-periodic modulation of the barrier height in the LAV vicinity.\nAt sufficiently low temperatures, the reaction rate is controlled by quantum\ntunneling through the barrier rather than by classical jumping over it. A giant\nincrease of sub-barrier transparency was demonstrated for a parabolic potential\nwell with the time-periodic eigenfrequency, when the modulation frequency\nexceeds the eigenfrequency by a factor of ~2 (parametric regime). Such regime\ncan be realized for a hydrogen or deuterium atom in metal hydrides/deuterides,\nsuch as NiH or PdD, in the vicinity of LAV. We present an analytical solution\nof the Schrodinger equation for a nonstationary harmonic oscillator, analyze\nthe parametric regime in details and discuss its applications to the tunnel\neffect and to D-D fusion in PdD lattice. We obtain simple analytical\nexpressions for the increase of amplitude and energy of zero-point oscillations\n(ZPO) induced by the parametric modulation. Based on that, we demonstrate a\ndrastic increase of the D-D fusion rate with in-creasing number of modulation\nperiods evaluated in the framework of Schwinger model, which takes into account\nsuppression of the Coulomb barrier due to lattice vibrations.", "category": "cond-mat_mtrl-sci" }, { "text": "Particle-hole cumulant approach for inelastic losses in x-ray spectra: Inelastic losses in core level x-ray spectra arise from many-body\nexcitations, leading to broadening and damping as well as satellite peaks in\nx-ray photoemission (XPS) and x-ray absorption (XAS) spectra. Here we present a\npractical approach for calculating these losses based on a cumulant\nrepresentation of the particle-hole Green's function, a quasi-boson\napproximation, and a partition of the cumulant into extrinsic, intrinsic and\ninterference terms. The intrinsic losses are calculated using real-time,\ntime-dependent density functional theory while the extrinsic losses are\nobtained from the GW approximation of the photo-electron self-energy and the\ninterference terms are approximated. These effects are included in the spectra\nusing a convolution with an energy dependent particle-hole spectral function.\nThe approach elucidates the nature of the spectral functions in XPS and XAS and\nexplains the significant cancellation between extrinsic and intrinsic losses.\nEdge-singularity effects in metals are also accounted for. Illustrative results\nare presented for the XPS and XAS for both weakly and more correlated systems.", "category": "cond-mat_mtrl-sci" }, { "text": "The spontaneous exchange bias effect in La2-xCaxCoMnO6 series: Structural, electronic and magnetic properties of polycrystalline\nLa2-xCaxCoMnO6 (0 $\\leq$ x $\\leq$ 0.75) compounds are investigated by x-ray\ndiffraction and magnetometry. All the samples have an orthorhombic structure\nand show a slight decrease in the unit cell with Ca-doping.\nTemperature-dependent magnetization measurements reveal a complex magnetic\nbehavior with two ferromagnetic transitions. These transitions are ascribed to\nCo2+--Mn4+ and Co3+--Mn3+ couplings and suggest the presence of additional\nantiferromagnetic couplings in these disordered compounds. Field-dependent\nmagnetization curves, measured after cooling the samples in a zero external\nmagnetic field, reveal the spontaneous exchange bias (SEB) effect for the\nCa-doped samples. The strengthening of the uncompensated magnetic coupling at\nthe interfaces, caused by the increased antiferromagnetic phase, explains the\nincrease of SEB with increasing the Ca-content.", "category": "cond-mat_mtrl-sci" }, { "text": "Light and electric field control of ferromagnetism in magnetic quantum\n structures: A strong influence of illumination and electric bias on the Curie temperature\nand saturation value of the magnetization is demonstrated for semiconductor\nstructures containing a modulation-doped p-type Cd0.96Mn0.04Te quantum well\nplaced in various built-in electric fields. It is shown that both light beam\nand bias voltage generate an isothermal and reversible cross-over between the\nparamagnetic and ferromagnetic phases, in the way that is predetermined by the\nstructure design. The observed behavior is in quantitative agreement with the\nexpectations for systems, in which ferromagnetic interactions are mediated by\nthe weakly disordered two-dimensional hole liquid.", "category": "cond-mat_mtrl-sci" }, { "text": "Spin Hall effect emerging from a chiral magnetic lattice without\n spin-orbit coupling: The spin Hall effect (SHE), which converts a charge current into a transverse\nspin current, has long been believed to be a phenomenon induced by the\nspin--orbit coupling. Here, we propose an alternative mechanism to realize the\nintrinsic SHE through a chiral magnetic structure that breaks the spin rotation\nsymmetry. No spin--orbit coupling is needed even when the scalar spin chirality\nvanishes, different from the case of the topological Hall effect. In known\nchiral antiferromagnetic compounds Mn$_3X$ ($X=$ Ga, Ge, and Sn), for example,\nwe indeed obtain large spin Hall conductivities based on \\textit{ab initio}\ncalculations. Apart further developing the conceptual understanding of the SHE,\nour work suggests an alternative strategy to design spin Hall materials without\ninvolving heavy elements, which may be advantageous for technological\napplications.", "category": "cond-mat_mtrl-sci" }, { "text": "Large magnetocaloric effect in the kagome ferromagnet\n Li$_9$Cr$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$: Single-crystal growth, magnetic properties, and magnetocaloric effect of the\n$S = 3/2$ kagome ferromagnet Li$_9$Cr$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$\n(trigonal, space group: $P\\bar{3}c1$) are reported. Magnetization data suggest\ndominant ferromagnetic intra-plane coupling with a weak anisotropy and the\nonset of ferromagnetic ordering at $T_{\\rm C} \\simeq 2.6$ K. Microscopic\nanalysis reveals a very small ratio of interlayer to intralayer ferromagnetic\ncouplings ($J_{\\perp}/J \\simeq 0.02$). Electron spin resonance data suggest the\npresence of short-range correlations above $T_{\\rm C}$ and confirms\nquasi-two-dimensional character of the spin system. A large magnetocaloric\neffect characterized by isothermal entropy change of $-\\Delta S_{\\rm m}\\simeq\n31$ J kg$^{-1}$ K$^{-1}$ and adiabatic temperature change of $-\\Delta T_{\\rm\nad}\\simeq 9$ K upon a field sweep of 7 T is observed around $T_{\\rm C}$. This\nleads to a large relative cooling power of $RCP \\simeq 284$ J kg$^{-1}$. The\nlarge magnetocaloric effect, together with negligible hysteresis render\nLi$_9$Cr$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$ a promising material for magnetic\nrefrigeration at low temperatures. The magnetocrystalline anisotropy constant\n$K \\simeq -7.42 \\times 10^4$ erg cm$^{-3}$ implies that the compound is an\neasy-plane type ferromagnet with the hard axis normal to the $ab$-plane,\nconsistent with the magnetization data.", "category": "cond-mat_mtrl-sci" }, { "text": "$(111)$ surface states of SnTe: The characterization and applications of topological insulators depend\ncritically on their protected surface states, which, however, can be obscured\nby the presence of trivial dangling bond states. Our first principle\ncalculations show that this is the case for the pristine $(111)$ surface of\nSnTe. Yet, the predicted surface states unfold when the dangling bond states\nare passivated in proper chemisorption. We further extract the anisotropic\nFermi velocities, penetration lengths and anisotropic spin textures of the\nunfolded $\\bar\\Gamma$- and $\\bar M$-surface states, which are consistent with\nthe theory in http://dx.doi.org/10.1103/PhysRevB.86.081303 Phys. Rev. B 86,\n081303 (R). More importantly, this chemisorption scheme provides an external\ncontrol of the relative energies of different Dirac nodes, which is\nparticularly desirable in multi-valley transport.", "category": "cond-mat_mtrl-sci" }, { "text": "Photo-induced persistent inversion of germanium in a 200-nm-deep surface\n region: The controlled manipulation of the charge carrier concentration in nanometer\nthin layers is the basis of current semiconductor technology and of fundamental\nimportance for device applications. Here we show that it is possible to induce\na persistent inversion from n- to p-type in a 200-nm-thick surface layer of a\ngermanium wafer by illumination with white and blue light. We induce the\ninversion with a half-life of ~12 hours at a temperature of 220 K which\ndisappears above 280 K. The photo-induced inversion is absent for a sample with\na 20-nm-thick gold capping layer providing a Schottky barrier at the interface.\nThis indicates that charge accumulation at the surface is essential to explain\nthe observed inversion. The contactless change of carrier concentration is\npotentially interesting for device applications in opto-electronics where the\ngate electrode and gate oxide could be replaced by the semiconductor surface.", "category": "cond-mat_mtrl-sci" }, { "text": "Effect of uniaxial stress on low-frequency dispersion of dielectric\n constant in high-resistivity GaSe crystals: Low-frequency dielectric spectra of high-resistivity GaSe layered crystals\nhave been studied on the samples clamped between two insulating parallel plates\nat frequencies up to 100 kHz. The measurements have been carried out at\ndifferent uniaxial stresses up to $2.4\\times10^5$ Pa applied along the c-axis\nnormal to crystal layer's plane. It is revealed that the dielectric spectra of\nhigh-resistivity GaSe layered crystals with insulating plates obey a universal\npower law ${\\sim}\\omega^{n-1}$, where ${\\omega}$ is the angular frequency and\n$n\\approx 0.8$, earlier observed on high-resistivity GaSe crystals with\nindium-soldered contacts. The same type of spectra on the crystals with\ndifferent types of contacts (insulating and ohmic) confirms the bulk character\nof the observed polarization caused by hopping charge carriers. It is shown\nthat the frequency-dependent dielectric constant increases linearly with the\nuniaxial stress characterized by the coefficient\n${\\Delta}{\\epsilon}/({\\epsilon}{\\Delta}{p})=8{\\times}10^{-7}$ Pa$^{-1}$. A\nslight increase of power $1-n$ with the stress is observed, that leads to a\nstronger dielectric dispersion. The strong stress dependence of the\nlow-frequency dielectric constant in high-resistivity GaSe crystals may be\nreferred to the presence of the formations of elementary dipoles, rotations of\nwhich correspond to hops of localized charge carriers.", "category": "cond-mat_mtrl-sci" }, { "text": "On the Structure of ${\\rm ZnI_2}$: A new structure for ${\\rm ZnI_2}$ is proposed which it exists in tetragonal\nstate. In this structure the ${\\rm ZnI_2}$ molecule exists in a nonlinear array\nand forms the basis of the tetragonal unit cell with one basis per unit cell.\nThe structural analysis based on the reflections listed in ASTM 30-1479 shows\nthat the proposed structure is correct.", "category": "cond-mat_mtrl-sci" }, { "text": "Coherence properties of infrared thermal emission from heated metallic\n nanowires: Coherence properties of the infrared thermal radiation from individual heated\nnanowires are investigated as function of nanowire dimensions. Interfering the\nthermally induced radiation from a heated nanowire with its image in a nearby\nmoveable mirror, well-defined fringes are observed. From the fringe visibility,\nthe coherence length of the thermal emission radiation from the narrowest\nnanowires was estimated to be at least 20 um which is much larger than expected\nfrom a classical blackbody radiator. A significant increase in coherence and\nemission efficiency is observed for smaller nanowires.", "category": "cond-mat_mtrl-sci" }, { "text": "Structural and electronic properties of solid molecular hydrogen from\n many-electron theories: We study the structural and electronic properties of phase III of solid\nhydrogen using accurate many-electron theories and compare to state-of-the-art\nexperimental findings. The atomic structures of phase III modelled by C2/c-24\ncrystals are fully optimized on the level of second-order perturbation theory,\ndemonstrating that previously employed structures optimized on the level of\napproximate density functionals exhibit errors in the H$_2$ bond lengths that\ncause significant discrepancies in the computed quasi particle band gaps and\nvibrational frequencies compared to experiment. Using the newly optimized\natomic structures, we study the band gap closure and change in vibrational\nfrequencies as a function of pressure. Our findings are in good agreement with\nrecent experimental observations and may prove useful in resolving\nlong-standing discrepancies between experimental estimates of metallization\npressures possibly caused by disagreeing pressure calibrations.", "category": "cond-mat_mtrl-sci" }, { "text": "Observation of Coexisting Dirac Bands and Moir\u00e9 Flat Bands in\n Magic-Angle Twisted Trilayer Graphene: Moir\\'e superlattices that consist of two or more layers of two-dimensional\nmaterials stacked together with a small twist angle have emerged as a tunable\nplatform to realize various correlated and topological phases, such as Mott\ninsulators, unconventional uperconductivity and quantum anomalous Hall effect.\nRecently, the magic-angle twisted trilayer graphene (MATTG) has shown both\nrobust superconductivity similar to magic-angle twisted bilayer graphene\n(MATBG) and other unique properties, including the Pauli-limit violating and\nre-entrant superconductivity. These rich properties are deeply rooted in its\nelectronic structure under the influence of distinct moir\\'e potential and\nmirror symmetry. Here, combining nanometer-scale spatially resolved\nangle-resolved photoemission spectroscopy (nano-ARPES) and scanning tunneling\nmicroscopy/spectroscopy (STM/STS), we systematically measure the yet unexplored\nband structure of MATTG near charge neutrality. Our measurements reveal the\ncoexistence of the distinct dispersive Dirac band with the emergent moir\\'e\nflat band, showing nice agreement with the theoretical calculations. These\nresults serve as a stepstone for further understanding of the unconventional\nsuperconductivity in MATTG.", "category": "cond-mat_mtrl-sci" }, { "text": "Femtosecond Demagnetization and Hot Hole Relaxation in Ferromagnetic\n GaMnAs: We have studied ultrafast photoinduced demagnetization in GaMnAs via\ntwo-color time-resolved magneto-optical Kerr spectroscopy. Below-bandgap\nmidinfrared pump pulses strongly excite the valence band, while near-infrared\nprobe pulses reveal sub-picosecond demagnetization that is followed by an\nultrafast ($\\sim$1 ps) partial recovery of the Kerr signal. Through comparison\nwith InMnAs, we attribute the signal recovery to an ultrafast energy relaxation\nof holes. We propose that the dynamical polarization of holes through $p$-$d$\nscattering is the source of the observed probe signal. These results support\nthe physical picture of femtosecond demagnetization proposed earlier for\nInMnAs, identifying the critical roles of both energy and spin relaxation of\nhot holes.", "category": "cond-mat_mtrl-sci" }, { "text": "Portable implementation of a quantum thermal bath for molecular dynamics\n simulations: Recently, Dammak and coworkers (H. Dammak, Y. Chalopin, M. Laroche, M.\nHayoun, and J.J. Greffet. Quantumthermal bath for molecular dynamics\nsimulation. Phys. Rev. Lett., 103:190601, 2009.) proposed that the quantum\nstatistics of vibrations in condensed systems at low temperature could be\nsimulated by running molecular dynamics simulations in the presence of a\ncolored noise with an appropriate power spectral density. In the present\ncontribution, we show how this method can be implemented in a flexible manner\nand at a low computational cost by synthesizing the corresponding noise 'on the\nfly'. The proposed algorithm is tested for a simple harmonic chain as well as\nfor a more realistic model of aluminium crystal. The energy and Debye-Waller\nfactor are shown to be in good agreement with those obtained from harmonic\napproximations based on the phonon spectrum of the systems. The limitations of\nthe method associated with anharmonic effects are also briefly discussed. Some\nperspectives for disordered materials and heat transfer are considered.", "category": "cond-mat_mtrl-sci" }, { "text": "Effect of Pressure on Electrical and optical Properties of Metal Doped\n TiO$_2$: A comparative study of the electrical and optical properties has been done on\n3d-doped TiO$_2$. Ti$_{1-x}$M$_x$O$_2$ (M= Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn)\npowder and its corresponding pellets, with doping concentration $x= 0.05$. The\nsamples were prepared using the solid-state route. Optical and electrical\nmeasurements have been performed for all prepared samples and interestingly, it\nis observed that due to external pressure (i.e. strain) both the properties\nchange significantly. A rigorous theoretical calculation has also been carried\nout to verify the experimental band gap obtained from optical absorption\nspectroscopy. In case of pellet sample band gap decreases as compared to the\npowder sample due to variation of pressure inside the structures. Role of\ndoping has also been investigated both in pellet and powder forms and we found\nthat the band gap decreases as the atomic number of dopants increases. A\ncross-over behavior is seen in pellet samples on doping with Ni, Cu and Zn\n(i.e. band gap increases with an increase in the atomic number of dopant).\nElectrical resistivity measurements have been carried out for both pellet and\npowder samples and it is found that in the case of strained samples the value\nof resistivity is smaller while in the case of strain-free samples it is quite\nlarge. We believe that the present study suggests a novel approach for tuning\nthe electrical and optical properties of semiconducting oxides either from\ndoping or from applied pressure (or strain).", "category": "cond-mat_mtrl-sci" }, { "text": "Generalized interface models for transport phenomena: unusual scale\n effects in composite nanomaterials: The effective transport properties of heterogeneous nanoscale materials and\nstructures are affected by several geometrical and physical factors. Among them\nthe presence of imperfect interfaces plays a central role being often at the\norigin of the scale effects. To describe real contacts between different phases\nsome classical schemes have been introduced in literature, namely the low and\nthe high conducting interface models. Here, we introduce a generalized\nformalism, which is able to take into account the properties of both previous\nschemes and, at the same time, it implements more complex behaviors, already\nobserved in recent investigations. We apply our models to the calculation of\nthe effective conductivity in a paradigmatic structure composed of a dispersion\nof particles. In particular we describe the conductivity dependence upon the\nsize of the inclusions finding an unusual non-monotone scale effect with a\npronounced peak at a given particle size. We introduce some intrinsic length\nscales governing the universal scaling laws.", "category": "cond-mat_mtrl-sci" }, { "text": "Effects of macroscopic-polarization built-in electrostatic fields in\n III-V nitrides multi-quantum-wells: Huge built-in electric fields have been predicted to exist in wurtzite III-V\nnitrides thin films and multilayers. Such fields originate from heterointerface\ndiscontinuities of the macroscopic bulk polarization of the nitrides. Here we\ndiscuss the background theory, the role of spontaneous polarization in this\ncontext, and the practical implications of built-in polarization fields in\nnitride nanostructures. To support our arguments, we present detailed\nself-consistent tight-binding simulations of typical nitride QW structures in\nwhich polarization effects are dominant.", "category": "cond-mat_mtrl-sci" }, { "text": "Magnetic Skyrmionic Polarons: We study a two-dimensional electron gas exchanged-coupled to a system of\nclassical magnetic ions. For large Rashba spin-orbit coupling a single electron\ncan become self-trapped in a skyrmion spin texture self-induced in the magnetic\nions system. This new quasiparticle carries electrical and topological charge\nas well as a large spin, and we named it as magnetic skyrmionic polaron. We\nstudy the range of parameters; temperature, exchange coupling, Rashba coupling\nand magnetic field, for which the magnetic skyrmionic polaron is the\nfundamental state in the system. The dynamics of this quasiparticle is studied\nusing the collective coordinate approximation, and we obtain that in presence\nof an electric field the new quasiparticle shows, because the chirality of the\nskyrmion, a Hall effect. Finally we argue that the magnetic skyrmionic polarons\ncan be found in large Rashba spin-orbit coupling semiconductors as GeMnTe.", "category": "cond-mat_mtrl-sci" }, { "text": "Complex structures of dense lithium: electronic origin: Lithium - the lightest alkali metal - exhibits unexpected structures and\nelectronic behaviour at high pressures. As the heavier alkalis, Li is bcc at\nambient pressure and transforms first to fcc (at 7.5 GPa). The post-fcc\nhigh-pressure form Li-cI16 (at 40-60 GPa) is similar to Na-cI16 and related to\nmore complex structures of heavy alkalis Rb-oC52 and Cs-oC84. The other high\npressure phases for Li (oC88, oC40, oC24) found at pressures up to 130 GPa are\nspecific the only to Li. The different route of Li high-pressure structures\ncorrelates with its special electronic configuration containing the only 3\nelectrons (at 1s and 2s levels). Crystal structures for Li are analyzed within\nthe model of Fermi sphere - Brillouin zone interactions. Stability of post-fcc\nstructures for Li can be supported by Hume-Rothery arguments when new Brillouin\nzone plains appear close to the Fermi level producing pseudogaps near the Fermi\nlevel and decreasing the crystal energy. The filling of Brillouin-Jones zones\nby electron states for a given structure defines the physical properties as\noptical reflectivity, electrical resistivity and superconductivity. To\nunderstand complexity of structural and physical properties of Li above 60 GPa\nis necessary to assume the valence electrons band overlap with the upper core\nelectrons and increase the valence electron count under compression.", "category": "cond-mat_mtrl-sci" }, { "text": "Deciphering Cryptic Behavior in Bimetallic Transition Metal Complexes\n with Machine Learning: The rational tailoring of transition metal complexes is necessary to address\noutstanding challenges in energy utilization and storage. Heterobimetallic\ntransition metal complexes that exhibit metal-metal bonding in stacked \"double\ndecker\" ligand structures are an emerging, attractive platform for catalysis,\nbut their properties are challenging to predict prior to laborious synthetic\nefforts. We demonstrate an alternative, data-driven approach to uncovering\nstructure-property relationships for rational bimetallic complex design. We\ntailor graph-based representations of the metal-local environment for these\nheterobimetallic complexes for use in training of multiple linear regression\nand kernel ridge regression (KRR) models. Focusing on oxidation potentials, we\nobtain a set of 28 experimentally characterized complexes to develop a multiple\nlinear regression model. On this training set, we achieve good accuracy (mean\nabsolute error, MAE, of 0.25 V) and preserve transferability to unseen\nexperimental data with a new ligand structure. We trained a KRR model on a\nsubset of 330 structurally characterized heterobimetallics to predict the\ndegree of metal-metal bonding. This KRR model predicts relative metal-metal\nbond lengths in the test set to within 5%, and analysis of key features reveals\nthe fundamental atomic contributions (e.g., the valence electron configuration)\nthat most strongly influence the behavior of complexes. Our work provides\nguidance for rational bimetallic design, suggesting that properties including\nthe formal shortness ratio should be transferable from one period to another.", "category": "cond-mat_mtrl-sci" }, { "text": "Band gap engineering by Bi intercalation of graphene on Ir(111): We report on the structural and electronic properties of a single bismuth\nlayer intercalated underneath a graphene layer grown on an Ir(111) single\ncrystal. Scanning tunneling microscopy (STM) reveals a hexagonal surface\nstructure and a dislocation network upon Bi intercalation, which we attribute\nto a $\\sqrt{3}\\times\\sqrt{3}R30{\\deg}$ Bi structure on the underlying Ir(111)\nsurface. Ab-initio calculations show that this Bi structure is the most\nenergetically favorable, and also illustrate that STM measurements are most\nsensitive to C atoms in close proximity to intercalated Bi atoms. Additionally,\nBi intercalation induces a band gap ($E_g=0.42\\,$eV) at the Dirac point of\ngraphene and an overall n-doping ($\\sim 0.39\\,$eV), as seen in angular-resolved\nphotoemission spectroscopy. We attribute the emergence of the band gap to the\ndislocation network which forms favorably along certain parts of the moir\\'e\nstructure induced by the graphene/Ir(111) interface.", "category": "cond-mat_mtrl-sci" }, { "text": "Towards High-Performance Two-Dimensional Black Phosphorus Optoelectronic\n Devices: the Role of Metal Contacts: The metal contacts on 2D black phosphorus field-effect transistor and\nphotodetectors are studied. The metal work functions can significantly impact\nthe Schottky barrier at the metal-semiconductor contact in black phosphorus\ndevices. Higher metal work functions lead to larger output hole currents in\np-type transistors, while ambipolar characteristics can be observed with lower\nwork function metals. Photodetectors with record high photoresponsivity (223\nmA/W) are demonstrated on black phosphorus through contact-engineering.", "category": "cond-mat_mtrl-sci" }, { "text": "Phonon Dispersion Effects and the Thermal Conductivity Reduction in\n GaAs/AlAs Superlattices: The experimentally observed order-of-magnitude reduction in the thermal\nconductivity along the growth axis of (GaAs)_n/(AlAs)_n (or n x n)\nsuperlattices is investigated theoretically for (2x2), (3x3) and (6x6)\nstructures using an accurate model of the lattice dynamics. The modification of\nthe phonon dispersion relation due to the superlattice geometry leads to\nflattening of the phonon branches and hence to lower phonon velocities. This\neffect is shown to account for a factor-of-three reduction in the thermal\nconductivity with respect to bulk GaAs along the growth direction; the\nremainder is attributable to a reduction in the phonon lifetime. The\ndispersion-related reduction is relatively insensitive to temperature (100 < T\n< 300K) and n. The phonon lifetime reduction is largest for the (2x2)\nstructures and consistent with greater interface scattering. The thermal\nconductivity reduction is shown to be appreciably more sensitive to GaAs/AlAs\nforce constant differences than to those associated with molecular masses.", "category": "cond-mat_mtrl-sci" }, { "text": "Bayesian calibration of interatomic potentials for binary alloys: Developing reliable interatomic potential models with quantified predictive\naccuracy is crucial for atomistic simulations. Commonly used potentials, such\nas those constructed through the embedded atom method (EAM), are derived from\nsemi-empirical considerations and contain unknown parameters that must be\nfitted based on training data. In the present work, we investigate Bayesian\ncalibration as a means of fitting EAM potentials for binary alloys. The\nBayesian setting naturally assimilates probabilistic assertions about uncertain\nquantities. In this way, uncertainties about model parameters and model errors\ncan be updated by conditioning on the training data and then carried through to\nprediction. We apply these techniques to investigate an EAM potential for a\nfamily of gold-copper systems in which the training data correspond to\ndensity-functional theory values for lattice parameters, mixing enthalpies, and\nvarious elastic constants. Through the use of predictive distributions, we\ndemonstrate the limitations of the potential and highlight the importance of\nstatistical formulations for model error.", "category": "cond-mat_mtrl-sci" }, { "text": "A regression-based feature selection study of the Curie temperature of\n transition-metal rare-earth compounds: prediction and understanding: The Curie temperature ($T_C$) of binary alloy compounds consisting of 3$d$\ntransition-metal and 4$f$ rare-earth elements is analyzed by a machine learning\ntechnique. We first demonstrate that nonlinear regression can accurately\nreproduce $T_C$ of the compounds. The prediction accuracy for $T_C$ is\nmaximized when five to ten descriptors are selected, with the rare-earth\nconcentration being the most relevant. We then discuss an attempt to utilize a\nregression-based model selection technique to learn the relation between the\ndescriptors and the actuation mechanism of the corresponding physical\nphenomenon, i.e., $T_C$ in the present case.", "category": "cond-mat_mtrl-sci" }, { "text": "Atomic defects and dopants in ternary Z-phase transition-metal nitrides\n CrMN with M=V, Nb, Ta investigated with density functional theory: A density functional theory study of atomic defects and dopants in ternary\nZ-phase transition-metal nitrides CrMN with M=V, Nb, or Ta is presented.\nVarious defect formation energies of native point defects and of substitutional\natoms of other metal elements which are abundant in the steel as well, are\nevaluated. The dependence thereof on the thermodynamic environment, i.e. the\nchemical conditions of a growing Z-phase precipitate, is studied and different\ngrowth scenarios are compared. The results obtained may help to relate results\nof experimental atomic-scale analysis, by atom probe tomography or transmission\nelectron microscopy, to the theoretical modeling of the formation process of\nthe Z phase from binary transition metal nitrides.", "category": "cond-mat_mtrl-sci" }, { "text": "Electrical transport and percolation in magnetoresistive manganite /\n insulating oxide composites: case of La0.7Ca0.3MnO3 / Mn3O4: We report the results of electrical resistivity measurements carried out on\nwell-sintered La0.7Ca0.3MnO3 / Mn3O4 composite samples with almost constant\ncomposition of the magnetoresistive manganite phase (La0.7Ca0.3MnO3). A\npercolation threshold (fc) occurs when the La0.7Ca0.3MnO3 volume fraction is ~\n0.19. The dependence of the electrical resistivity as a function of\nLa0.7Ca0.3MnO3 volume fraction (fLCMO) can be described by percolation-like\nphenomenological equations. Fitting the conducting regime (fLCMO > fc) by the\npercolation power law returns a critical exponent t value of 2.0 +/- 0.2 at\nroom temperature and 2.6 +/-0.2 at 5 K. The increase of t is ascribed to the\ninfluence of the grain boundaries on the electrical conduction process at low\ntemperature.", "category": "cond-mat_mtrl-sci" }, { "text": "Instability of the rhodium magnetic moment as origin of the metamagnetic\n phase transition in alpha-FeRh: Based on ab initio total energy calculations we show that two magnetic states\nof rhodium atoms together with competing ferromagnetic and antiferromagnetic\nexchange interactions are responsible for a temperature induced metamagnetic\nphase transition, which experimentally is observed for stoichiometric\nalpha-FeRh. A first-principle spin-based model allows to reproduce this\nfirst-order metamagnetic transition by means of Monte Carlo simulations.\nFurther inclusion of spacial variation of exchange parameters leads to a\nrealistic description of the experimental magneto-volume effects in alpha-FeRh.", "category": "cond-mat_mtrl-sci" }, { "text": "Incipient triple point for adsorbed xenon monolayers: Pt(111) versus\n graphite substrates: Simulation evidence of an incipient triple point is reported for xenon\nsubmonolayers adsorbed on the (111) surface of platinum. This is in stark\ncontrast to the \"normal\" triple point found in simulations and experiments for\nxenon on the basal plane surface of graphite. The motions of the atoms in the\nsurface plane are treated with standard 2D \"NVE\" molecular dynamics simulations\nusing modern interactions. The simulation evidence strongly suggests an\nincipient triple point in the 120-150 K range for adsorption on the Pt (111)\nsurface while the adsorption on graphite shows a normal triple point at about\n100 K.", "category": "cond-mat_mtrl-sci" }, { "text": "Magnetization dynamics and damping due to electron-phonon scattering in\n a ferrimagnetic exchange model: We present a microscopic calculation of magnetization damping for a magnetic\n\"toy model.\" The magnetic system consists of itinerant carriers coupled\nantiferromagnetically to a dispersionless band of localized spins, and the\nmagnetization damping is due to coupling of the itinerant carriers to a phonon\nbath in the presence of spin-orbit coupling. Using a mean-field approximation\nfor the kinetic exchange model and assuming the spin-orbit coupling to be of\nthe Rashba form, we derive Boltzmann scattering integrals for the distributions\nand spin coherences in the case of an antiferromagnetic exchange splitting,\nincluding a careful analysis of the connection between lifetime broadening and\nthe magnetic gap. For the Elliott-Yafet type itinerant spin dynamics we extract\ndephasing and magnetization times T_1 and T_2 from initial conditions\ncorresponding to a tilt of the magnetization vector, and draw a comparison to\nphenomenological equations such as the Landau-Lifshitz or the Gilbert damping.\nWe also analyze magnetization precession and damping for this system including\nan anisotropy field and find a carrier mediated dephasing of the localized spin\nvia the mean-field coupling.", "category": "cond-mat_mtrl-sci" }, { "text": "Theoretical estimates for flat voids coalescence by internal necking: Coalescence of voids by internal necking is in most cases the last\nmicroscopic event related to ductile fracture and corresponds to a localized\nplastic flow between adjacent voids. Macroscopic load associated to the onset\nof coalescence is classically estimated based on limit analysis. However, a\nrigorous upper-bound mathematical expression for the limitload required for\nflat voids coalescence that remains finite for penny-shaped voids/cracks is\nstill unavailable. Therefore, based on limit analysis, theoretical upper-bound\nestimates - both integral expression and closed-form formula - are obtained for\nthe limit-load of cylindrical flat voids in cylindrical unit-cell subjected to\nboundary conditions allowing the assessment of coalescence, for axisymmetric\nstress state. These estimates, leading to finite limit-loads for pennyshaped\ncracks, are shown to be in very good agreement with numerical limit analysis,\nfor both cylindrical and spheroidal voids. Approximate formula is also given\nfor coalescence under combined tension and shear loading. These coalescence\ncriteria can thus be used to predict onset of coalescence of voids by internal\nnecking in ductile fracture modelling.", "category": "cond-mat_mtrl-sci" }, { "text": "Coherent generation of symmetry-forbidden phonons by light-induced\n electron-phonon interactions in magnetite: Symmetry breaking across phase transitions often causes changes in selection\nrules and emergence of optical modes which can be detected via spectroscopic\ntechniques or generated coherently in pump-probe experiments. In second-order\nor weakly first-order transitions, fluctuations of the order parameter are\npresent above the ordering temperature, giving rise to intriguing precursor\nphenomena, such as critical opalescence. Here, we demonstrate that in magnetite\n(Fe$_3$O$_4$) light excitation couples to the critical fluctuations of the\ncharge order and coherently generates structural modes of the ordered phase\nabove the critical temperature of the Verwey transition. Our findings are\nobtained by detecting coherent oscillations of the optical constants through\nultrafast broadband spectroscopy and analyzing their dependence on temperature.\nTo unveil the coupling between the structural modes and the electronic\nexcitations, at the origin of the Verwey transition, we combine our results\nfrom pump-probe experiments with spontaneous Raman scattering data and\ntheoretical calculations of both the phonon dispersion curves and the optical\nconstants. Our methodology represents an effective tool to study the real-time\ndynamics of critical fluctuations across phase transitions.", "category": "cond-mat_mtrl-sci" }, { "text": "Thermoelectric properties of semiconducting materials with parabolic and\n pudding-mold band structures: We theoretically investigate the thermoelectric properties of semiconducting\n(gapped) materials by varying the degrees of polynomials in their energy\ndispersion relations, in which either the valence or conduction energy\ndispersion depends on the wave vector raised to the power of two, four, and\nsix. The thermoelectric transport coefficients such as the Seebeck coefficient,\nelectrical conductivity, and thermal conductivity are calculated within the\nlinearized Boltzmann transport theory combined with the relaxation time\napproximation. We consider various effects such as band gaps, dimensionalities,\nand dispersion powers to understand the conditions that can give the optimal\nthermoelectric efficiency or figure of merit ($ZT$). Our calculations show that\nthe so-called pudding-mold band structure produces larger electrical and\nthermal conductivities than the parabolic band, but no significant difference\nis found in the Seebeck coefficients of the pudding-mold and parabolic bands.\nFurthermore, we find that a high $ZT$ can be obtained by tuning the band gap of\nthe material to an optimum value simultaneously with breaking the band\nsymmetry. The largest $ZT$ is found in a combination of two-contrasting\npolynomial powers in the dispersion relations of valence and conduction bands.\nThis band asymmetry also shifts the charge neutrality away from the undoped\nlevel and allows optimal $ZT$ to be located at a smaller chemical potential.\nWith some reasonable values of thermal conductivity parameters, the maximum\n$ZT$ for the bulk systems can be larger than 1, while for one-dimensional\nsystems it can even reach almost 4. We expect this work to trigger\nhigh-throughput calculations for screening of potential thermoelectric\nmaterials combining various polynomial powers in the energy dispersion\nrelations of semiconductors.", "category": "cond-mat_mtrl-sci" }, { "text": "Spin transport in a magnetic insulator with zero effective damping: Applications based on spin currents strongly profit from the control and\nreduction of their effective damping and their transport properties. We here\nexperimentally observe magnon mediated transport of spin (angular) momentum\nthrough a 13.4 nm thin yttrium iron garnet film with full control of the\nmagnetic damping via spin-orbit torque. Above a critical spin-orbit torque, the\nfully compensated damping manifests itself as an increase of magnon\nconductivity by almost two orders of magnitude. We compare our results to\ntheoretical expectations based on recently predicted current induced magnon\ncondensates and discuss other possible origins of the observed critical\nbehaviour.", "category": "cond-mat_mtrl-sci" }, { "text": "Gate-Voltage Tunability of Plasmons in Single and Multi-layer Graphene\n Structures: Analytical Description and Concepts for Terahertz Devices: The strong light-matter interaction in graphene over a broad frequency range\nhas opened up a plethora of photonics applications of graphene. The goal of\nthis paper is to present the voltage tunability of plasmons in gated single-\nand multi-layer graphene structures. Device concepts for plasmonic\ninterconnects and antennas and their performance for THz communication are\npresented. For the first time, the role of gate voltage and the thickness of\nthe gate dielectric on the characteristics of plasmon propagation in graphene\nare quantified by accounting for both the interface trap capacitance and the\nquantum capacitance. The gate voltage serves as a powerful knob to tweak the\ncarrier concentration and allows building electrically reconfigurable terahertz\ndevices. By optimizing the gate voltage to maximize the plasmon propagation\nlength in a gated multi-layer graphene geometry, we derive simple scaling\ntrends that give intuitive insight into device modeling and design.", "category": "cond-mat_mtrl-sci" }, { "text": "DFT Studies of 2D Materials Inspired by Lie Algebras: Inspired by the root systems of Lie algebras of rank 2, we propose a\nmathematical method to engineer new 2D materials with double periodic\nstructures tessellating the plane. Concretely, we investigate two geometries\nrelaying on squares and hexagons exhibiting D4 D4 and D6 D6 dihedral group\ninvariances, respectively. Due to lack of empirical verifications of such\ndouble configurations, we provide a numerical investigation by help of the open\nsource quantum espresso. Motivated by hybrid structures of graphene, silicene,\ngermanene, we investigate two models involving D4 D4 and D6 D6 dihedral\nsymmetries which we refer to as Si4Ge4 and Si6C6 compounds, respectively. For\nsimplicities, we study only the opto-electronic physical properties by applying\nan electromagnetic source propagating in linear and isotropic mediums. We\nbelieve that the Lie algebra inspiration of such 2D material studies, via\ndensity functional theory techniques, could open new roads to think about\nhigher dimensional cases by implementing generalized Cartan matrices.", "category": "cond-mat_mtrl-sci" }, { "text": "Reconciling the ionic and covalent pictures in rare-earth nickelates: The properties of AMO3 perovskite oxides, where M is a 3d transition metal,\ndepend strongly on the level of covalency between the metal d and oxygen p\norbitals. With their complex spin orders and metal-insulator transition,\nrare-earth nickelates verge between dominantly ionic and covalent characters.\nAccordingly, the nature of their ground state is highly debated. Here, we\nreconcile the ionic and covalent visions of the insulating state of nickelates.\nThrough first-principles calculations, we show that it is reminiscent of the\nionic charge disproportionation picture (with strictly low-spin 4+ and\nhigh-spin 2+ Ni sites) while exhibiting strong covalence effects with oxygen\nelectrons shifted toward the depleted Ni cations, mimicking a configuration\nwith identical Ni sites. Our results further hint at strategies to control\nelectronic and magnetic phases of transition metal oxide perovskites.", "category": "cond-mat_mtrl-sci" }, { "text": "A systematic study of magnetodynamic properties at finite temperatures\n in doped permalloy from first principles calculations: By means of first principles calculations, we have systematically\ninvestigated how the magnetodynamic properties Gilbert damping, magnetization\nand exchange stiffness are affected when permalloy (Py)\n(Fe$_{0.19}$Ni$_{0.81}$) is doped with 4d or 5d transition metal impurities. We\nfind that the trends in the Gilbert damping can be understood from relatively\nfew basic parameters such as the density of states at the Fermi level, the\nspin-orbit coupling and the impurity concentration. % The temperature\ndependence of the Gilbert damping is found to be very weak which we relate to\nthe lack of intraband transitions in alloys. % Doping with $4d$ elements has no\nmajor impact on the studied Gilbert damping, apart from diluting the host.\nHowever, the $5d$ elements have a profound effect on the damping and allows it\nto be tuned over a large interval while maintaining the magnetization and\nexchange stiffness. % As regards spin stiffness, doping with early transition\nmetals results in considerable softening, whereas late transition metals have a\nminor impact. % Our result agree well with earlier calculations where\navailable. In comparison to experiments, the computed Gilbert damping appears\nslightly underestimated while the spin stiffness show good general agreement.", "category": "cond-mat_mtrl-sci" }, { "text": "Influence of different exchange-correlation potentials on twisted\n structures of bilayer XS2 (X= Mo, Cr): In this work, we employ the LDA, GGA and GGA with four vdW corrections to\nstudy crystal and electronic structures of bilayer transition metal\ndichalcogenides (TMDs) with different twist angles. We find the GGA interlayer\ndistance of bilayer MoS2 has good agreement with experimental value while vdW\ncorrection method still needs to be further improved. Our results indicate the\nGGA interlayer distances of bilayer XS2 (X= Mo, Cr) with twist angles are\nsmaller than that of normal bilayer, which is the opposite in the LDA case. The\nGGA results show that reduced bandgap is due to the reduction of interlayer\ndistance and, flat valley and conductivity bands appear owing to twist angle.\nOur study not only supports valuable information for application possibility of\ntwisted two-dimensional (2D) materials but also stimulates more related\nresearch.", "category": "cond-mat_mtrl-sci" }, { "text": "Atomic multiplet and charge-transfer screening effects in 1$s$ and 2$p$\n core-level X-ray photoelectron spectra of early 3$d$ transition-metal oxides: We present a comparative analysis of 1$s$ and 2$p$ core-level hard X-ray\nphotoelectron spectroscopy (HAXPES) spectra in metallic VO$_2$ and CrO$_2$.\nEven though the V 1$s$ and 2$p$ spectra in VO$_2$ display similar line shapes\nexcept the absence or presence of a spin-orbit coupling splitting, the Cr 1$s$\nand 2$p$ spectra exhibit distinct main-line shapes. The experimental HAXPES\nspectra are analyzed by the Anderson impurity model based on the density\nfunctional theory + dynamical mean-field theory and a conventional MO$_6$\ncluster model. We elucidate the complex interplay between formation of the\nintra-atomic multiplet and charge transfer effect on the chemical bonding\nfollowed by the 1$s$ and 2$p$ core electron excitations. We demonstrate the\nadvantage of the 1$s$ excitations to the routinely-employed 2$p$ excitations\nfor distinguishing between metal-ligand and metal-metal charge transfer\ncontributions in early 3$d$ transition-metal oxides.", "category": "cond-mat_mtrl-sci" }, { "text": "Thermal expansion and pressure effect in MnWO4: MnWO4 has attracted attention because of its ferroelectric property induced\nby frustrated helical spin order. Strong spin-lattice interaction is necessary\nto explain ferroelectricity associated with this type of magnetic order.We have\nconducted thermal expansion measurements along the a, b, c axes revealing the\nexistence of strong anisotropic lattice anomalies at T1=7.8 K, the temperature\nof the magnetic lock-in transition into a commensurate low-temperature\n(reentrant paraelectric) phase. The effect of hydrostatic pressure up to 1.8\nGPa on the FE phase is investigated by measuring the dielectric constant and\nthe FE polarization. The low- temperature commensurate and paraelectric phase\nis stabilized and the stability range of the ferroelectric phase is diminished\nunder pressure.", "category": "cond-mat_mtrl-sci" }, { "text": "Monte Carlo and kinetic Monte Carlo methods: This article reviews the basic computational techniques for carrying out\nmulti-scale simulations using statistical methods, with the focus on\nsimulations of epitaxial growth. First, the statistical-physics background\nbehind Monte Carlo simulations is briefly described. The kinetic Monte Carlo\n(kMC) method is introduced as an extension of the more wide-spread\nthermodynamic Monte Carlo methods, and algorithms for kMC simulations,\nincluding parallel ones, are discussed in some detail. The step from the\natomistic picture to the more coarse-grained description of Monte Carlo\nsimulations is exemplified for the case of surface diffusion. Here, the aim is\nthe derivation of rate constants from knowledge about the underlying atomic\nprocesses. Both the simple approach of Transition State Theory, as well as more\nrecent approaches using accelerated molecular dynamics are reviewed. Finally, I\naddress the point that simplifications often need to be introduced in practical\nMonte Carlo simulations in order to reduce the complexity of 'real' atomic\nprocesses. Different 'flavors' of kMC simulations and the potential pitfalls\nrelated to the reduction of complexity are presented in the context of\nsimulations of epitaxial growth.", "category": "cond-mat_mtrl-sci" }, { "text": "Carrier Trapping by Oxygen Impurities in Molybdenum Diselenide: Understanding defect effect on carrier dynamics is essential for both\nfundamental physics and potential applications of transition metal\ndichalcogenides. Here, the phenomenon of oxygen impurities trapping\nphoto-excited carriers has been studied with ultrafast pump-probe spectroscopy.\nOxygen impurities are intentionally created in exfoliated multilayer MoSe2 with\nAr+ plasma irradiation and air exposure. After plasma treatment, the signal of\ntransient absorption first increases and then decreases, which is a signature\nof defect capturing carriers. With larger density of oxygen defects, the\ntrapping effect becomes more prominent. The trapping defect densities are\nestimated from the transient absorption signal, and its increasing trend in the\nlonger-irradiated sample agrees with the results from X-ray photoelectron\nspectroscopy. First principle calculations with density functional theory\nreveal that oxygen atoms occupying Mo vacancies create mid-gap defect states,\nwhich are responsible for the carrier trapping. Our findings shed light on the\nimportant role of oxygen defects as carrier trappers in transition metal\ndichalcogenides, and facilitates defect engineering in relevant material and\ndevice applications.", "category": "cond-mat_mtrl-sci" }, { "text": "Ambient temperature pressure driven alkane dehydrogenation by palladium\n metal: Dehydrogenation of alkanes is of increasing importance in fulfilling global\ndemand for olefins and offers a potential source of carbon-neutral hydrogen as\na co-product. Currently commercial dehydrogenation processes occur at\nhigh-temperatures (500-900$^\\circ$C) which is energy intensive and results in\nside reactions and rapid coking of the catalysts. In addition the hydrogen\nproduced is often burned to maintain temperature and to inhibit the back\nreaction. Here we demonstrate pressure as a parameter to enable novel chemical\ncatalytic processes and demonstrate ambient-temperature dehydrogenation of\nalkanes by palladium at 50-100 MPa pressures, with both hydrogen gas and\nolefins recovered on decompression. This reaction follows a fundamentally\ndifferent path to current commercial high-temperature low-pressure\ndehydrogenation processes with the palladium catalyst reversibly forming a\nhydride intermediate.", "category": "cond-mat_mtrl-sci" }, { "text": "A phase field model combined with genetic algorithm for polycrystalline\n hafnium zirconium oxide ferroelectrics: Ferroelectric hafnium zirconium oxide (HZO) thin films show significant\npromise for applications in ferroelectric random-access memory, ferroelectric\nfield-effect transistors, and ferroelectric tunneling junctions. However, there\nare shortcomings in understanding ferroelectric switching, which is crucial in\nthe operation of these devices. Here a computational model based on phase field\nmethod is developed to simulate the switching behavior of polycrystalline HZO\nthin films. Furthermore, we introduce a novel approach to optimize the\neffective Landau coefficients describing the free energy of HZO by combining\nthe phase field model with a genetic algorithm. We validate the model by\naccurately simulating switching curves for HZO thin films with different\nferroelectric phase fractions. The simulated domain dynamics during switching\nalso shows amazing similarity to the available experimental observations. The\npresent work also provides fundamental insights into enhancing the\nferroelectricity in HZO thin films by controlling grain morphology and\ncrystalline texture. It can potentially be extended to improve the\nferroelectric properties of other hafnia based thin films.", "category": "cond-mat_mtrl-sci" }, { "text": "Molecular Motion on Semiconductor Surface via Tip-enhanced Multiple\n Excitation: In a low-temperature study with a scanning tunneling microscope (STM), the\nirreducible lateral motion of a CO molecule adsorbed on a Si(001) surface\nshowed a hyperlinear dependence on the tunneling current. This dependence\nimplies that the adsorbate displacement is caused by multiple excitations of\nadsorbate vibration modes, a situation thus far observed only at metal\nsurfaces. The local vibronic temperature at the atomic scale on the surface\nheated by ohmic inelastic scattering of tunneling electrons indicates that\nthere is an activation barrier of 0.11 eV for the irreversible motion of CO, in\nagreement with the adiabatic potential obtained from first-principles\ncalculation. The highly efficient local heating is caused by a mid-gap state at\nthe surface induced by the electric field of the STM tip.", "category": "cond-mat_mtrl-sci" }, { "text": "First-principles prediction of oxygen octahedral rotations in\n perovskite-structure EuTiO3: We present a systematic first-principles study of the structural and\nvibrational properties of perovskite-structure EuTiO3. Our calculated phonon\nspectrum of the high-symmetry cubic structural prototype shows strong M- and\nR-point instabilities, indicating a tendency to symmetry-lowering structural\ndeformations composed of rotations and tilts of the oxygen octahedra.\nSubsequent explicit study of 14 different octahedral tilt-patterns showed that\nthe I4/mcm, Imma, and R\\bar{3}c structures, all with antiferrodistortive\nrotations of the octahedra, have significantly lower total energy than the\nprototype Pm\\bar{3}m structure. We discuss the dynamical stability of these\nstructures, and the influence of the antiferrodistortive structural distortions\non the vibrational, optical, and magnetic properties of EuTiO3, in the context\nof recent unexplained experimental observations.", "category": "cond-mat_mtrl-sci" }, { "text": "Band restructuring of ordered/disordered blue TiO2 for visible\n photocatalyst: Black TiO2 with/without noble metal has been proposed for visible\nphotocatalyst, still leaving poor catalyst efficiency. Alternatively,\nphase-mixed TiO2 such as anatase and rutile has been commonly used for visible\ncatalysts with the inevitable inclusion of noble metal. Here, we perform a\nnoble metal-free visible photocatalyst blue TiO2 with type-II band-aligned\nordered anatase/disordered rutile structure, via phase-selective reduction with\nalkali metals. The changed band alignment in this heterostructure was\nidentified by absorption and ultraviolet photoemission spectroscopy, which was\nfurther confirmed by transient charge separation. The band alignment of type-I\nand type-II was clearly restructured by converting from ordered to disordered\nphase with a prolonged reduction period and as followed light absorbance\nenhancement also observed. Initiated type-I in a pristine sample, the type-II\nwas organized from disordered rutile phase in 3-day Li-reduction. The type-II\ndisordered rutile TiO2 heterostructure exhibits a remarkable photocatalytic\nperformance by 55 times higher than conventional P25 TiO2 in solar-light driven\nhydrogen evolution reaction owing to an efficient electron and hole separation\nof type-II heterojunction. Furthermore, this restructured heterojunction\ntype-II TiO2 demanded 10 times less Pt amount as a co-catalyst for the\ncomparable photocatalytic performance, compared to Pt decorated type-I pristine\nanatase/rutile phase-mixed TiO2.", "category": "cond-mat_mtrl-sci" }, { "text": "Doping graphene with metal contacts: Making devices with graphene necessarily involves making contacts with\nmetals. We use density functional theory to study how graphene is doped by\nadsorption on metal substrates and find that weak bonding on Al, Ag, Cu, Au and\nPt, while preserving its unique electronic structure, can still shift the Fermi\nlevel with respect to the conical point by $\\sim 0.5$ eV. At equilibrium\nseparations, the crossover from $p$-type to $n$-type doping occurs for a metal\nwork function of $\\sim 5.4$ eV, a value much larger than the graphene work\nfunction of 4.5 eV. The numerical results for the Fermi level shift in graphene\nare described very well by a simple analytical model which characterizes the\nmetal solely in terms of its work function, greatly extending their\napplicability.", "category": "cond-mat_mtrl-sci" }, { "text": "Giant magnetostriction in Tb-doped Fe83Ga17 melt-spun ribbons: Giant magnetostriction is achieved in the slightly Tb-doped Fe83Ga17\nmelt-spun ribbons. The tested average perpendicular magnetostriction is -886\nppm along the melt-spun ribbon direction in the Fe82.89Ga16.88Tb0.23 alloy. The\ncalculated parallel magnetostriction is 1772 ppm, more than 4 times as large as\nthat of binary Fe83Ga17 alloy. The enhanced magnetostriction should be\nattributed to a small amount of Tb solution into the A2 matrix phase during\nrapid solidification. The localized strong magnetocrystalline anisotropy of Tb\nelement is suggested to cause the giant magnetostriction.", "category": "cond-mat_mtrl-sci" }, { "text": "Magnetic phase diagram of Ce2Fe17: Rare-earth-based permanent-magnet materials rich in iron have relatively low\nferromagnetic ordering temperatures. This is believed to be due to the presence\nof antiferromagnetic exchange interactions, besides the ferromagnetic\ninteractions responsible for the magnetic order. The magnetic properties of\nCe2Fe17 are anomalous. Instead of ferromagnetic, it is antiferromagnetic, and\ninstead of one ordering temperature, it shows two, at the Neel temperature TN ~\n208 K and at TT ~ 124 K. Ce2Fe17, doped by 0.5% Ta, also shows two ordering\ntemperatures, one to an antiferromagnetic phase, at TN ~ 214 K, and one to a\nferromagnetic phase, at T0 ~ 75 K. In order to clarify this behavior,\nsingle-crystalline samples were prepared by solution growth, and characterized\nby electron microscopy, single crystal x-ray diffraction, temperature-dependent\nspecific heat, and magnetic field and temperature-dependent electrical\nresistivity and magnetization. From these measurements, magnetic H-T phase\ndiagrams were determined for both Ta-doped Ce2Fe17 and undoped Ce2Fe17. These\nphase diagrams can be very well described in terms of a theory that gives\nmagnetic phase diagrams of systems with competing antiferro- and\nferromagnetism.", "category": "cond-mat_mtrl-sci" }, { "text": "Impact of Sb degrees of freedom on the charge density wave phase diagram\n of the kagome metal CsV$_3$Sb$_5$: Elucidating the microscopic mechanisms responsible for the charge density\nwave (CDW) instability of the AV$_3$Sb$_5$ (A=Cs, K, Rb) family of kagome\nmetals is critical for understanding their unique properties, including\nsuperconductivity. In these compounds, distinct CDW phases with wave-vectors at\nthe $M$ and $L$ points are energetically favorable, opening the possibility of\ntuning the type of CDW order by appropriate external parameters. Here, we shed\nlight on the CDW landscape of CsV$_3$Sb$_5$ via a combination of\nfirst-principles calculations and phenomenology, which consists of extracting\nthe coefficients of the CDW Landau free-energy expansion from density\nfunctional theory. We find that while the main structural distortions of the\nkagome lattice in the staggered tri-hexagonal CDW phase are along the\nnearest-neighbor V-V bonds, distortions associated with the Sb ions play a\ndefining role in the energy gain in this and all other CDW states. Moreover,\nthe coupling between ionic displacements from different unit cells is small,\nthus explaining the existence of multiple CDW instabilities with different\nmodulations along the c-axis. We also investigate how pressure and temperature\nimpact the CDW phase of CsV$_3$Sb$_5$. Increasing pressure does not change the\nstaggered tri-hexagonal CDW ground state, even though the $M$-point CDW\ninstability disappears before the $L$-point one, a behavior that we attribute\nto the large nonlinear coupling between the order parameters. Upon changing the\ntemperature, we find a narrow regime in which another transition can take\nplace, toward a tri-hexagonal Star-of-David CDW phase. We discuss the\nimplications of our results by comparing them with experiments on this\ncompound.", "category": "cond-mat_mtrl-sci" }, { "text": "Shock and Release Temperatures in Molybdenum: Shock and release temperatures in Mo were calculated, taking account of\nheating from plastic flow predicted using the Steinberg-Guinan model. Plastic\nflow was calculated self-consistently with the shock jump conditions: this is\nnecessary for a rigorous estimate of the locus of shock states accessible. The\ntemperatures obtained were significantly higher than predicted assuming ideal\nhydrodynamic loading. The temperatures were compared with surface emission\nspectrometry measurements for Mo shocked to around 60GPa and then released into\nvacuum or into a LiF window. Shock loading was induced by the impact of a\nplanar projectile, accelerated by high explosive or in a gas gun. Surface\nvelocimetry showed an elastic wave at the start of release from the shocked\nstate; the amplitude of the elastic wave matched the prediction to around 10%,\nindicating that the predicted flow stress in the shocked state was reasonable.\nThe measured temperatures were consistent with the simulations, indicating that\nthe fraction of plastic work converted to heat was in the range 70-100% for\nthese loading conditions.", "category": "cond-mat_mtrl-sci" }, { "text": "Powder Diffraction Data and Mesomorphic Properties for 4-Butyloxyphenyl\n 4'-Decyloxybenzoate: Unit cell parameters obtained from X-ray powder diffraction data are\npresented for the crystalline phase of a liquid crystal 4-butyloxyphenyl\n4'-decyloxybenzoate: a = 23.098 (4) {\\AA}, b = 5.974 (6) {\\AA}, c = 12.357 (10)\n{\\AA}, \\b{eta} = 121.53 (8){\\deg}, unit-cell volume V = 1453.56 {\\AA}3.\nTemperature dependent X-ray diffraction data confirmed the existence of smectic\nA and smectic C mesophases and a more ordered, tilted crystalline smectic\nphase. Possibility of existence of previously reported smectic B phase as well\nas another crystalline phase was refuted.", "category": "cond-mat_mtrl-sci" }, { "text": "Investigation of re-entrant relaxor behaviour in lead cobalt niobate\n ceramic: The temperature dependent dielectric properties revealed re-entrant relaxor\nbehaviour (Tm ~130 K and 210 K for 1 kHz) below a high temperature diffused\nphase transition, Tc ~270 K in lead cobalt niobate (PCN). Multiple\npositive/negative magnetodielectric effect and deviation from straight line at\n~130 K is observed in temperature dependence of inverse susceptibility, which\ndepicts origin of frustration. Microstructure examination depicts closely\npacked grains with grain size ~8-10 microm and XRD pattern revealed single\nphase pseudo cubic crystal structure having Pm3m symmetry with lattice constant\n~4.0496(2) {\\AA}. Rietveld Refinement on XRD data yields larger value of\nthermal parameters, implying Pb and O are disordered along <111> and <110>\ndirections respectively. Observation of A1g (780 cm-1) mode in Raman\nspectroscopy and F-spot in SAED pattern along <110> unit axis in TEM suggests\npresence of nano scale 1:1 Co and Nb non-stoichiometric chemical ordering\n(CORs), akin to lead magnesium niobate (PMN). K-edge XANES spectra reveals the\npresence of cobalt in two oxidation states (Co2+ and Co3+); whereas, niobium\nexists in Nb3+ state. Therefore, these local-average structural properties\nsuggest chemical, structural and spatial heterogeneities. Such multiple\nheterogeneities are believed to play a crucial role in producing re-entrant\nrelaxor behaviour.", "category": "cond-mat_mtrl-sci" }, { "text": "The Effects of Vacancy and Oxidation on Black Phosphorus Nanoresonators: Black phosphorene is not stable at ambient conditions, so atomic defects and\noxidation effects are unavoidable in black phosphorus samples in the\nexperiment. The effects of these defects on the performance of the black\nphosphorus nanoresonators are still unclear. Here, we perform classical\nmolecular dynamics to investigate the effects of the vacancy and oxidation on\nsingle-layer black phosphorus nanoresonators at different temperatures. We find\nthat the vacancy causes strong reduction in the quality factor of the\nnanoresonators, while the oxidation has weaker effect on the nanoresonators.\nMore specifically, a 2% concentration of randomly distributed single vacancies\nis able to reduce the quality factor by about 80% and 40% at 4.2K and 50K,\nrespectively. We also find that the quality factor of the nanoresonator is not\nsensitive to the distribution pattern of the vacancy defects.", "category": "cond-mat_mtrl-sci" }, { "text": "Hamiltonian Transformation for Band Structure Calculations: First-principles electronic band structure calculations are essential for\nunderstanding periodic systems in condensed matter physics and materials\nscience. We propose an accurate and parameter-free method, called Hamiltonian\ntransformation (HT), to calculate band structures in both density functional\ntheory (DFT) and post-DFT calculations with plane-wave basis sets. The cost of\nHT is independent of the choice of the density functional and scales as\n$\\mathcal{O}(N_e^3N_k\\log N_k)$, where $N_e$ and $N_k$ are the number of\nelectrons and the number of $\\mathbf{k}$-points. Compared to the widely used\nWannier interpolation (WI), HT adopts an eigenvalue transformation to construct\na spatial localized representation of the spectrally truncated Hamiltonian. HT\nalso uses a non-iterative algorithm to change the basis sets to circumvent the\nconstruction of the maximally localized Wannier functions. As a result, HT can\nsignificantly outperform WI in terms of the accuracy of the band structure\ncalculation. We also find that the eigenvalue transformation can be of\nindependent interest, and can be used to improve the accuracy of the WI for\nsystems with entangled bands.", "category": "cond-mat_mtrl-sci" }, { "text": "Multiferroic Properties of Nanocrystalline BaTiO3: Some of the Multiferroics [1] form a rare class of materials that exhibit\nmagnetoelectric coupling arising from the coexistence of ferromagnetism and\nferroelectricity, with potential for many technological applications.[2,3] Over\nthe last decade, an active research on multiferroics has resulted in the\nidentification of a few routes that lead to multiferroicity in bulk\nmaterials.[4-6] While ferroelectricity in a classic ferroelectric such as\nBaTiO3 is expected to diminish with the reducing particle size,[7,8]\nferromagnetism cannot occur in its bulk form.[9] Here, we use a combination of\nexperiment and first-principles simulations to demonstrate that multiferroic\nnature emerges in intermediate size nanocrystalline BaTiO3, ferromagnetism\narising from the oxygen vacancies at the surface and ferroelectricity from the\ncore. A strong coupling between a surface polar phonon and spin is shown to\nresult in a magnetocapacitance effect observed at room temperature, which can\nopen up possibilities of new electro-magneto-mechanical devices at the\nnano-scale.", "category": "cond-mat_mtrl-sci" }, { "text": "Resonant Raman of OH/OD vibrations and photoluminescence studies in\n LiTaO3 thin film: Resonant Raman spectra (RRS) of O-H and O-D vibration and libration modes,\ntheir combinations and higher harmonics have been observed in LiTaO3\npolycrystalline thin films. RRS peaks are superimposed on photoluminescence\n(PL) spectrum. Monochromatic light from a xenon lamp is used as excitation\nsource. PL spectrum shows two broad peaks, first near the band gap in UV\n(4.4-4.8eV) and another in the sub band gap region (< 4.0 eV). Band gap PL\nalong with RRS peaks are reported for the first time. Photoluminescence\nexcitation spectrum (PLE) shows a peak at 4.8 eV. Peak positions and full width\nat half maximum (FWHM) of RRS peaks depend upon the excitation energy.\nDispersions of the fundamental and the third harmonic of the stretching mode of\nO-H with excitation energy are about 800 cm-1/eV and 2000 cm-1/eV respectively.\nThis dispersion is much higher than reported in any other material.", "category": "cond-mat_mtrl-sci" }, { "text": "Quantitative Temperature Dependence of Longitudinal Spin Seebeck Effect\n at High Temperatures: This article reports temperature-dependent measurements of longitudinal spin\nSeebeck effects (LSSEs) in Pt/Y$_3$Fe$_5$O$_{12}$ (YIG)/Pt systems in a high\ntemperature range from room temperature to above the Curie temperature of YIG.\nThe experimental results show that the magnitude of the LSSE voltage in the\nPt/YIG/Pt systems rapidly decreases with increasing the temperature and\ndisappears above the Curie temperature. The critical exponent of the LSSE\nvoltage in the Pt/YIG/Pt systems at the Curie temperature was estimated to be\n3, which is much greater than that for the magnetization curve of YIG. This\ndifference highlights the fact that the mechanism of the LSSE cannot be\nexplained in terms of simple static magnetic properties in YIG.", "category": "cond-mat_mtrl-sci" }, { "text": "Antiferromagnetic Spin Orientation and Magnetic Domain Structure in\n Epitaxially Grown MnN Studied using Optical Second Harmonic Generation: MnN is a centrosymmetric collinear antiferromagnet belonging to the\ntransition metal nitride family with a high Neel temperature, a low anisotropy\nfield, and a large magnetic moment per Mn atom. Despite several recent\nexperimental and theoretical studies, the spin symmetry (magnetic point group)\nand magnetic domain structure of the material remain unknown. In this work, we\nuse optical second harmonic generation (SHG) to study the magnetic structure of\nthin epitaxially-grown single-crystal (001) MnN films. Our work shows that spin\nmoments in MnN are tilted away from the [001] direction and the components of\nthe spin moments in the (001) plane are aligned along one of the two possible\nin-plane symmetry axes ([100] or [110]) resulting in a magnetic point group\nsymmetry of 2/m1'. Our work rules out magnetic point group symmetries 4/mmm1'\nand mmm1' that have been previously discussed in the literature. Four different\nspin domains consistent with the 2/m1' magnetic point group symmetry are\npossible in MnN. A statistical model based on the observed variations in the\npolarization-dependent intensity of the second harmonic signal collected over\nlarge sample areas puts an upper bound of 0.65 microns on the mean domain size.\nOur results show that SHG can be used to probe the magnetic order in metallic\nantiferromagnets. This work is expected to contribute to the recent efforts in\nusing antiferromagnets for spintronic applications.", "category": "cond-mat_mtrl-sci" }, { "text": "Preventing corona effects: multi-phosphonic acid poly(ethylene glycol)\n copolymers for stable stealth iron oxide nanoparticles: When disperse in biological fluids, engineered nanoparticles are selectively\ncoated with proteins, resulting in the formation of a protein corona. It is\nsuggested that the protein corona is critical in regulating the conditions of\nentry into the cytoplasm of living cells. Recent reports describe this\nphenomenon as ubiquitous and independent of the nature of the particle. For\nnanomedicine applications however, there is a need to design advanced and\ncost-effective coatings that are resistant to protein adsorption and that\nincrease the biodistribution in vivo. In this study, phosphonic acid\npoly(ethylene glycol) copolymers were synthesized and used to coat iron oxide\nparticles. The copolymer composition was optimized to provide simple and\nscalable protocols as well as long-term stability in culture media. It is shown\nthat polymers with multiple phosphonic acid functionalities and PEG chains\noutperform other types of coating, including ligands, polyelectrolytes and\ncarboxylic acid functionalized PEG. PEGylated particles exhibit moreover\nexceptional low cellular uptake, of the order of 100 femtograms of iron per\ncell. The present approach demonstrates that the surface chemistry of\nengineered particles is a key parameter in the interactions with cells. It also\nopens up new avenues for the efficient functionalization of inorganic surfaces.", "category": "cond-mat_mtrl-sci" }, { "text": "Symmetrical laws of structure of helicoidally-like biopolymers in the\n framework of algebraic topology. II. \u03b1-helix and DNA structures: In the framework of algebraic topology the closed sequence of 4-dimensional\npolyhedra (algebraic polytopes) was defined. This sequence is started by the\npolytope {240}, discovered by Coxeter, and is determined by the second\ncoordination sphere of 8-dimensional lattice E8. The second polytope of\nsequence allows to determine a topologically stable rod substructure that\nappears during multiplication by a non-crystallographic axis 40/11 of the\nstarting union of 4 tetrahedra with common vertex. When positioning the\nappropriate atoms tin positions of special symmetry of the staring 4\ntetrahedra, such helicoid determines an {\\alpha}-helix. The third polytope of\nsequence allows to determine the helicoidally-like union of rods with 12-fold\naxis, which can be compare with Z-DNA structures. This model is defined as a\nlocal lattice rod packing, contained within a surface of helicoidally similar\ntype, which ensures its topological stability, as well as possibility for it to\nbe transformed into other forms of DNA structures. Formation of such structures\ncorresponds to lifting a configuration degeneracy, and the stability of a state\n- to existence of a point of bifurcation. Furthermore, in the case of DNA\nstructures, a second \"security check\" possibly takes place in the form of local\nlattice (periodic) property using the lattices other than the main ones.", "category": "cond-mat_mtrl-sci" }, { "text": "Limitations for the determination of piezoelectric constants with\n piezoresponse force microscopy: At first sight piezoresponse force microscopy (PFM) seems an ideal technique\nfor the determination of piezoelectric coefficients (PCs), thus making use of\nits ultra-high vertical resolution (<0.1 pm/V). Christman et al. \\cite{Chr98}\nfirst used PFM for this purpose. Their measurements, however, yielded only\nreasonable results of unsatisfactory accuracy, amongst others caused by an\nincorrect calibration of the setup. In this contribution a reliable calibration\nprocedure is given followed by a careful analysis of the encounted difficulties\ndetermining PCs with PFM. We point out different approaches for their solution\nand expose why, without an extensive effort, those difficulties can not be\ncircumvented.", "category": "cond-mat_mtrl-sci" }, { "text": "Study of the elastocaloric effect and mechanical behavior for the NiTi\n shape memory alloys: The NiTi shape memory alloy exhibited excellent superelastic property and\nelastocaloric effect. Large temperature changes of 30 K upon loading and -19 K\nupon unloading were obtained at room temperature, which were higher than those\nof the other NiTi-based materials and among the highest values reported in the\nelastocaloric materials. The asymmetry of the measured temperature changes\nbetween loading and unloading process was ascribed to the friction dissipation.\nThe large temperature changes originated from the large entropy change during\nthe stress-induced martensite transformation (MT) and the reverse MT. A large\ncoefficient-of-performance of the material (COPmater) of 11.7 was obtained,\nwhich decreased with increasing the applied strain. These results are very\nattractive in the present solid-state cooling which is potential to replace the\nvapor compression refrigeration technologies.", "category": "cond-mat_mtrl-sci" }, { "text": "Highly anisotropic two-dimensional metal in monolayer MoOCl$_2$: Anisotropy is a general feature in materials. Strong anisotropy could lead to\ninteresting physical properties and useful applications. Here, based on\nfirst-principles calculations and theoretical analysis, we predict a stable\ntwo-dimensional (2D) material---the monolayer MoOCl$_2$, and show that it\npossesses intriguing properties related to its high anisotropy. Monolayer\nMoOCl$_2$ can be readily exfoliated from the van der Waals layered bulk, which\nhas already been synthesized. We show that a high in-plane anisotropy manifests\nin the structural, phononic, mechanical, electronic, and optical properties of\nmonolayer MoOCl$_2$. The material is a metal with highly anisotropic Fermi\nsurfaces, giving rise to open orbits at the Fermi level, which can be probed in\nmagneto-transport. Remarkably, the combination of high anisotropy and metallic\ncharacter makes monolayer MoOCl$_2$ an almost ideal hyperbolic material. It has\ntwo very wide hyperbolic frequency windows from 0.41 eV (99 THz) to 2.90 eV\n(701 THz), and from 3.63 eV (878 THz) to 5.54 eV (1340 THz). The former window\nhas a large overlap with the visible spectrum, and the dissipation for most\npart of this window is very small. The window can be further tuned by the\napplied strain, such that at a chosen frequency, a transition between elliptic\nand hyperbolic character can be induced by strain. Our work discovers a highly\nanisotropic 2D metal with extraordinary properties, which holds great potential\nfor electronic and optical applications.", "category": "cond-mat_mtrl-sci" }, { "text": "Monte Carlo simulation of GaAs(001) homoepitaxy: By carrying out Monte Carlo simulations based on the two-species atomic-scale\nkinetic growth model of GaAs(001) homoepitaxy and comparing the results with\nscanning tunneling microscope images, we show that initial growing islands\nundergo the structural transformation before adopting the proper beta2(2x4)\nreconstruction.", "category": "cond-mat_mtrl-sci" }, { "text": "Giant interfacial perpendicular magnetic anisotropy in MgO/CoFe/capping\n layer structures: Magnetic tunnel junction (MTJ) based on CoFeB/MgO/CoFeB structures is of\ngreat interest due to its application in the spin-transfer-torque magnetic\nrandom access memory (STT-MRAM). Large interfacial perpendicular magnetic\nanisotropy (PMA) is required to achieve high thermal stability. Here we use\nfirst-principles calculations to investigate the magnetic anisotropy energy\n(MAE) of MgO/CoFe/capping layer structures, where the capping materials include\n5d metals Hf, Ta, Re, Os, Ir, Pt, Au and 6p metals Tl, Pb, Bi. We demonstrate\nthat it is feasible to enhance PMA by using proper capping materials.\nRelatively large PMA is found in the structures with capping materials of Hf,\nTa, Os, Ir and Pb. More importantly, the MgO/CoFe/Bi structure gives rise to\ngiant PMA (6.09 mJ/m2), which is about three times larger than that of the\nMgO/CoFe/Ta structure. The origin of the MAE is elucidated by examining the\ncontributions to MAE from each atomic layer and orbital. These findings provide\na comprehensive understanding of the PMA and point towards the possibility to\nachieve advanced-node STT-MRAM with high thermal stability.", "category": "cond-mat_mtrl-sci" }, { "text": "The local atomic quasicrystal structure of the icosahedral Mg25Y11Zn64\n alloy: A local and medium range atomic structure model for the face centred\nicosahedral (fci) Mg25Y11Zn64 alloy has been established in a sphere of r = 27\nA. The model was refined by least squares techniques using the atomic pair\ndistribution (PDF) function obtained from synchrotron powder diffraction. Three\nhierarchies of the atomic arrangement can be found: (i) five types of local\ncoordination polyhedra for the single atoms, four of which are of Frank-Kasper\ntype. In turn, they (ii) form a three-shell (Bergman) cluster containing 104\natoms, which is condensed sharing its outer shell with its neighbouring\nclusters and (iii) a cluster connecting scheme corresponding to a\nthree-dimensional tiling leaving space for few glue atoms. Inside adjacent\nclusters, Y8-cubes are tilted with respect to each other and thus allow for\noverall icosahedral symmetry. It is shown that the title compound is\nessentially isomorphic to its holmium analogue. Therefore fci-Mg-Y-Zn can be\nseen as the representative structure type for the other rare earth analogues\nfci-Mg-Zn-RE (RE = Dy, Er, Ho, Tb) reported in the literature.", "category": "cond-mat_mtrl-sci" }, { "text": "Microscale simulation of adhesive and cohesive failure in rough\n interfaces: Multi-material lightweight designs, e.g. the combination of aluminum with\nfiber-reinforced composites, are a key feature for the development of\ninnovative and resource-efficient products. The connection properties of such\nbi-material interfaces are influenced by the geometric structure on different\nlength scales. In this article a modeling strategy is presented to study the\nfailure behavior of rough interfaces within a computational homogenization\nscheme. We study different local phenomena and their effects on the overall\ninterface characteristics, e.g. the surface roughness and different local\nfailure types as cohesive failure of the bulk material and adhesive failure of\nthe local interface. Since there is a large separation in the length scales of\nthe surface roughness, which is in the micrometer range, and conventional\nstructural components, we employ a numerical homogenization approach to extract\neffective traction-separation laws to derive effective interface parameters.\nAdhesive interface failure is modeled by cohesive elements based on a\ntraction-separation law and cohesive failure of the bulk material is described\nby an elastic-plastic model with progressive damage evolution.", "category": "cond-mat_mtrl-sci" }, { "text": "Encoding Complexity within Supramolecular Analogues of Frustrated\n Magnets: At the heart of systems chemistry lies the idea that supramolecular\ninteractions can give rise to complex and unexpected collective states that\nemerge on a fundamentally different lengthscale to that of the interactions\nthemselves. While in certain cases - e.g. the self-assembly of virus-like\npolyhedral cages from coordination building blocks - it is possible to control\nemergence in a systematic manner, the development of general approaches remains\na fundamental challenge in the field. In the conceptually-related domain of\nfrustrated magnetism - where collective states give rise to exotic physics of\nrelevance to data storage and spintronics - the task of predicting emergent\nbehaviour is simplified through control over the geometry and form of the\nmagnetic interactions from which complexity arises. Seeking to combine\napproaches from these two fields, we study here the solid phases of inorganic\npolymer chains assembled from non-magnetic gold(I)/silver(I) cations and\ncyanide anions. We show the periodic inter-chain potential encodes a\nsupramolecular interaction that can be tuned to mimic different magnetic\ninteractions between XY spins (\"spin rotors\"). Because the chains pack on a\ntriangular lattice, the crystal structures of gold(I)/silver(I) cyanides can be\ninterpreted in terms of the phase behaviour of triangular XY magnets. Complex\nmagnetic states predicted for this family - including hidden quadrupolar order\nand emergent spin-vortex quasiparticles - are realised for the first time in\nthe structural chemistry of these cyanide polymers. In this way we demonstrate\nboth how simple inorganic materials might behave as structural analogues of\notherwise-unrealisable \"toy\" spin models, and also how a theoretical\nunderstanding of those models might be used to predict and control emergent\nphenomena in chemical systems.", "category": "cond-mat_mtrl-sci" }, { "text": "Yield criterion and finite strain behavior of random porous isotropic\n materials: The mechanical response of isotropic elastoplastic materials containing\nrandom distributions of initially spherical voids is investigated\ncomputationally based on Fast Fourier Transform simulations. Numerical\nlimit-analysis simulations at constant stress triaxiality allow to determine\nthe yield surfaces, leading in particular to the determination of a\nRepresentative Volume Element size for the onset of coalescence / inhomogeneous\nyielding. Moreover, two different coalescence regimes are observed that differ\nby the presence of shearing. The yield surfaces are found to be consistent with\nthe combination of two models proposed in the literature, a GTN-type model\ncalibrated for homogeneous yielding of random porous materials and an\ninhomogeneous yielding model accounting for both coalescence with or without\nshear. Finite strain simulations performed for different hardening moduli and\nstress triaxialities under axisymmetric loading conditions confirm the\nexistence of a RVE up to the onset of inhomogeneous yielding. Coalescence\nstrains are found to be significantly smaller for random porous materials than\nfor periodic distribution of voids. A homogenized model is finally proposed\nthat reproduces quantitatively the behavior of isotropic elastoplastic\nmaterials containing random distributions of voids under finite strains.", "category": "cond-mat_mtrl-sci" }, { "text": "Weak antilocalization in a noncentrosymmetric CaAgBi single crystal: We report on the single crystal growth and transport properties of a\ntopological semimetal CaAgBi which crystallises in the hexagonal $ABC-$type\nstructure with the non-centrosymmetric space group $\\mathit{P6_3mc}$ (No. 186).\nThe transverse magnetoresistance measurements with current in the basal plane\nof the hexagonal crystal structure reveal a value of about 30 % for I //\n[10-10] direction and about 50 % for I // [1-210] direction at 10 K in an\napplied magnetic field of 14 T. The magnetoresistance shows a cusp-like\nbehavior in the low magnetic-field region, suggesting the presence of weak\nantilocalization effect for temperatures less than 100 K. The Hall measurements\nreveal that predominant charge carriers are $p$ type exhibiting a linear\nbehavior for fields up to 14 T and can be explained based on the single band\nmodel. The magnetoconductance of CaAgBi is analysed based on the modified\nHikami-Larkin-Nagaoka (HLN) model. Our first-principles calculations within a\ndensity-functional theory framework reveal that CaAgBi supports a topological\nDirac semimetal state with Dirac points located on the rotational axis slightly\nabove the Fermi level and are protected by $C_{6v}$ point-group symmetry. The\nFermi surface consists of both the electron and hole pockets. However, the size\nof hole pockets is much larger than electron pockets suggesting the dominant\n$p$ type carriers in accord with our experimental results.", "category": "cond-mat_mtrl-sci" }, { "text": "Systematical, experimental investigations on LiMgZ (Z= P, As, Sb) wide\n band gap semiconductors: This work reports on the experimental investigation of the wide band gap\ncompounds LiMgZ (Z = P, As, Sb), which are promising candidates for\nopto-electronics and anode materials for Lithium batteries. The compounds\ncrystallize in the cubic (C1_b) MgAgAs structure (space group F-43m). The\npolycrystalline samples were synthesized by solid state reaction methods. X-ray\nand neutron diffraction measurements show a homogeneous, single-phased samples.\nThe electronic properties were studied using the direct current (DC) method.\nAdditionally UV-VIS diffuse reflectance spectra were recorded in order to\ninvestigate the band gap nature. The measurements show that all compounds\nexhibit semiconducting behavior with direct band gaps of 1.0 eV to 2.3 eV\ndepending on the Z element. A decrease of the peak widths in the static 7Li\nnuclear magnetic resonance (NMR) spectra with increasing temperature was\nobserved, which can directly be related to an increase of Li ion mobility.", "category": "cond-mat_mtrl-sci" }, { "text": "High entropy van der Waals materials (Review article): By breaking the restrictions on traditional alloying strategy, the high\nentropy concept has promoted the exploration of the central area of phase\nspace, thus broadening the horizon of alloy exploitation. This review\nhighlights the marriage of the high entropy concept and van der Waals systems\nto form a new family of materials category, namely the high entropy van der\nWaals materials (HEX, HE = high entropy, X= anion clusters) and describe the\ncurrent issues and next challenges. The design strategy for HEX has integrated\nthe local feature (e.g., composition, spin, and valence states) of structural\nunits in high entropy materials and the holistic degrees of freedom (e.g.,\nstacking, twisting, and intercalating species) in van der Waals materials, and\nhas been successfully employed for the discovery of high entropy\ndichalcogenides, phosphorus tri-chalcogenides, halogens, and MXene. The rich\ncombination and random distribution of the multiple metallic constituents on\nthe nearly-regular 2D lattice give rise to a flexible platform to study the\ncorrelation features behind a range of selected physical properties, e.g.,\nsuperconductivity, magnetism, and metal-insulator transition. The deliberate\ndesign of structural units and their stacking configuration can also create\nnovel catalysts to enhance their performance in a bunch of chemical reactions.", "category": "cond-mat_mtrl-sci" }, { "text": "Synthesis and Local Probe Gating of a Monolayer Metal-Organic Framework: Achieving large-area uniform two-dimensional (2D) metal-organic frameworks\n(MOFs) and controlling their electronic properties on inert surfaces is a big\nstep towards future applications in electronic devices. Here we successfully\nfabricated a 2D monolayer Cu-dicyanoanthracene (DCA) MOF with long-range order\non an epitaxial graphene surface. Its structural and electronic properties are\nstudied by low-temperature scanning tunneling microscopy (STM) and spectroscopy\n(STS) complemented by density-functional theory (DFT) calculations. We\ndemonstrate access to multiple molecular charge states in the 2D MOF using\ntip-induced local electric fields. We expect that a similar strategy could be\napplied to fabricate and characterize 2D MOFs with exotic, engineered\nelectronic states.", "category": "cond-mat_mtrl-sci" }, { "text": "Giant Anisotropy of Spin-Orbit Splitting at the Bismuth Surface: We investigate the bismuth (111) surface by means of time and angle resolved\nphotoelectron spectroscopy. The parallel detection of the surface states below\nand above the Fermi level reveals a giant anisotropy of the Spin-Orbit (SO)\nspitting. These strong deviations from the Rashba-like coupling cannot be\ntreated in $\\textbf{k}\\cdot \\textbf{p}$ perturbation theory. Instead, first\nprinciple calculations could accurately reproduce the experimental dispersion\nof the electronic states. Our analysis shows that the giant anisotropy of the\nSO splitting is due to a large out-of plane buckling of the spin and orbital\ntexture.", "category": "cond-mat_mtrl-sci" }, { "text": "Conditions for the formation of pure birnessite during the oxidation of\n Mn(II) cations in aqueous alkaline medium: Birnessite was synthetized through redox reaction by mixing MnO4-, Mn2+ and\nOH- solutions. The Mn(VII): Mn(II) ratio of 0.33 was chosen and three methods\nwere used consisting in a quick mixing under vigorous stirring of two of the\nthree reagents and then on the dropwise addition of the third one. The obtained\nsolids were characterized by XRD, FTIR and XPS spectroscopies. Their average\noxidation states were determined from ICP and CEC measurements while their\nsurface properties were investigated by XPS. This study provides an increased\nunderstanding of the importance of dissolved oxygen in the formation of\nbirnessite and hausmannite and shows the ways to obtain pure birnessite. The\nrole of counter-ion ie. Na+ or K+ was also examined.", "category": "cond-mat_mtrl-sci" }, { "text": "A General Framework for Liquid Marbles: Liquid marbles refer to liquid droplets that are covered with a layer of\nnon-wetting particles. They are observed in nature and have practical\nsignificance. However, a generalized framework for analyzing liquid marbles as\nthey inflate or deflate is unavailable. The present study fills this gap by\ndeveloping an analytical framework based on liquid-particle and\nparticle-particle interactions. We demonstrate that the potential final states\nof evaporating liquid marbles are characterized by one of the following: (I)\nconstant surface area, (II) particle ejection, or (III) multilayering. Based on\nthese insights, a single-parameter evaporation model for liquid marbles is\ndeveloped. Model predictions are in excellent agreement with experimental\nevaporation data for water liquid marbles of particle sizes ranging from 7\nnanometers to 300 micrometers (over four orders of magnitude) and chemical\ncompositions ranging from hydrophilic to superhydrophobic. These findings lay\nthe groundwork for the rational design of liquid marble applications.", "category": "cond-mat_mtrl-sci" }, { "text": "Interfacial-Redox-Induced Tuning of Superconductivity in\n YBa$_{2}$Cu$_{3}$O$_{7-\u03b4}$: Solid state ionic approaches for modifying ion distributions in getter/oxide\nheterostructures offer exciting potentials to control material properties. Here\nwe report a simple, scalable approach allowing for total control of the\nsuperconducting transition in optimally doped YBa$_{2}$Cu$_{3}$O$_{7-{\\delta}}$\n(YBCO) films via a chemically-driven ionic migration mechanism. Using a thin Gd\ncapping layer of up to 20 nm deposited onto 100 nm thick epitaxial YBCO films,\noxygen is found to leach from deep within the YBCO. Progressive reduction of\nthe superconducting transition is observed, with complete suppression possible\nfor a sufficiently thick Gd layer. These effects arise from the combined impact\nof redox-driven electron doping and modification of the YBCO microstructure due\nto oxygen migration and depletion. This work demonstrates an effective ionic\ncontrol of superconductivity in oxides, an interface induced effect that goes\nwell into the quasi-bulk regime, opening up possibilities for electric field\nmanipulation.", "category": "cond-mat_mtrl-sci" }, { "text": "Correlation of microdistortions with misfit volumes in High Entropy\n Alloys: The yield strengths of High Entropy Alloys have recently been correlated with\nmeasured picometer-scale atomic distortions. Here, the root mean square\nmicrodistortion in a multicomponent alloy is shown to be nearly proportional to\nthe misfit-volume parameter that enters into a predictive model of solute\nstrengthening. Analysis of two model ternary alloy families, face-centered\ncubic Cr-Fe-Ni and body-centered cubic Nb-Mo-V, demonstrates the correlation\nover a wide composition space. The reported correlation of yield strength with\nmicrodistortion is thus a consequence of the correlation between\nmicrodistortion and misfit parameter and the derived dependence of yield\nstrength on the misfit parameter.", "category": "cond-mat_mtrl-sci" }, { "text": "Stress transmission in planar disordered solid foams: Stress transmission in planar open-cell cellular solids is analysed using a\nrecent theory developed for marginally rigid granular assemblies. This is made\npossible by constructing a one-to-one mapping between the two systems. General\ntrivalent networks are mapped onto assemblies of rough grains, while networks\nwhere Plateau rules are observed, are mapped onto assemblies of smooth grains.\nThe constitutive part of the stress transmission equations couples the stress\ndirectly to the local rotational disorder of the cellular structure via a new\nfabric tensor. An intriguing consequence of the analysis is that the stress\nfield can be determined in terms of the microstructure alone independent of\nstress-strain information. This redefines the problem of structure-property\nrelationship in these materials and poses questions on the relations between\nthis formalism and elasticity theory. The deviation of the stress transmission\nequations from those of conventional solids has been interpreted in the context\nof granular assemblies as a new state of solid matter and the relevance of this\ninterpretation to the state of matter of cellular solids is discussed.", "category": "cond-mat_mtrl-sci" }, { "text": "Induced Giant Piezoelectricity in Centrosymmetric Oxides: Piezoelectrics are materials that linearly deform in response to an applied\nelectric field. As a fundamental prerequisite, piezoelectric material must\npossess a non centrosymmetric crystal structure. For more than a century, this\nremains the major obstacle for finding new piezoelectric materials. We\ncircumvent this limitation by breaking the crystallographic symmetry, and\ninducing large and sustainable piezoelectric effects in centrosymmetric\nmaterials by electric field induced rearrangement of oxygen vacancies\nSurprisingly, the results show the generation of extraordinarily large\npiezoelectric responses d33 ~200,000 pm/V), in cubic fluorite Gd-doped CeO2-x\nfilms, which is two orders of magnitude larger than in the presently best known\nlead based piezoelectric relaxor ferroelectric oxide. These findings open\nopportunities to design new piezoelectric materials from environmentally\nfriendly centrosymmetric ones.", "category": "cond-mat_mtrl-sci" }, { "text": "Interface collisions with diffusive mass transport: We report on a linear Langevin model that describes the evolution of the\nroughness of two interfaces that move towards each other and are coupled by a\ndiffusion field. This model aims at describing the closing of the gap between\ntwo two-dimensional material domains during growth, and the subsequent\nformation of a rough grain boundary. We assume that deposition occurs in the\ngap between the two domains and that the growth units diffuse and may attach to\nthe edges of the domains. These units can also detach from edges, diffuse, and\nre-attach elsewhere. For slow growth, the edge roughness increases monotonously\nand then saturates at some equilibrium value. For fast growth, the roughness\nexhibits a maximum just before the collision between the two interfaces, which\nis followed by a minimum. The peak of the roughness can be dominated by\nstatistical fluctuations or by edge instabilities. A phase diagram with three\nregimes is obtained: slow growth without peak, peak dominated by statistical\nfluctuations, and peak dominated by instabilities. These results reproduce the\nmain features observed in Kinetic Monte Carlo simulations.", "category": "cond-mat_mtrl-sci" }, { "text": "Scaling of the thermally induced sign inversion of longitudinal spin\n Seebeck effect in a compensated ferrimagnet: Role of magnetic anisotropy: We report on a systematic investigation of the longitudinal spin Seebeck\neffect (LSSE) in a GGG(Gd3Ga5O12)/GdIG(Gd3Fe5O12)/Pt film series exhibiting an\nin-plane magnetic easy axis with a compensation temperature (T_Comp) that\ndecreases from 270 to 220 K when decreasing GdIG film thickness from 272 to 31\nnm, respectively. For all the films, the LSSE signal flips its sign below\nT_Comp. We demonstrate a universal scaling behavior of the temperature\ndependence of LSSE signal for our GdIG films around their respective T_Comp.\nAdditionally, we demonstrate LSSE in a 31 nm GdIG film grown on a\nlattice-mismatched GSGG (Gd3Sc2Ga3O12) substrate that exhibits an out-of-plane\nmagnetic easy axis at room temperature. However, this sample reveals a spin\nreorientation transition where the magnetic easy axis changes its orientation\nto in-plane at low temperatures. We observed a clear distinction in the LSSE\nsignal for the GSGG/GdIG(31 nm)/Pt heterostructure, relative to\nGGG/GdIG(31nm)/Pt showing an in-plane magnetic easy axis. Our findings\nunderscore a strong correlation between the LSSE signal and the orientation of\nmagnetic easy axis in compensated ferrimagnets and opens the possibility to\ntune LSSE through effective anisotropy.", "category": "cond-mat_mtrl-sci" }, { "text": "Polarization Morphology and Electrocaloric Response of Strained\n Ferroelectric Core-Shell Nanorods and Nanowires: Using Landau-Ginzburg-Devonshire (LGD) approach we proposed the analytical\ndescription of the Vegard strains influence on the spontaneous polarization and\nelectrocaloric response in ferroelectric core-shell nanorods. The nanorod core\npresents a defect-free single-crystalline ferroelectric material, and the\nVegard strains are induced by elastic defects in the ultra-thin shell. The\nfinite element modeling (FEM) based on the LGD approach reveals transitions of\ndomain structure morphology induced by the Vegard strains in the BaTiO3\nnanorods. Namely, tensile Vegard strains induce and support the single-domain\nstate in the central part of the nanorod, while the curled domain structures\nappear near the unscreened or partially screened ends of the rod. The\nvortex-like domains propagate toward the central part of the rod and fill it\nentirely, when the rod is covered by a shell with compressive Vegard strains\nabove some critical value. The critical value depends on the nanorod sizes,\naspect ratio, and screening conditions at its ends. Both analytical theory and\nFEM predict that the tensile Vegard strains in the shell increase the nanorod\npolarization, lattice tetragonality, and electrocaloric response well-above the\nvalues corresponding to the bulk material. The physical reason of the increase\nis the strong electrostriction coupling between the mismatch-type elastic\nstrains induced in the core by the Vegard strains in the shell. Comparison with\nthe earlier XRD data confirmed an increase of tetragonality ratio in tensiled\nBaTiO3 nanorods compared to the bulk material. Obtained analytical expressions,\nwhich are suitable for the description of strain-induced changes in a wide\nclass of multiaxial ferroelectric core-shell nanorods and nanowires, can be\nuseful for strain engineering of advanced ferroelectric nanomaterials for\nelectrocaloric applications and negative capacitance elements.", "category": "cond-mat_mtrl-sci" }, { "text": "Ultrafast Epitaxial Growth of Metre-Sized Single-Crystal Graphene on\n Industrial Cu Foil: A foundation of the modern technology that uses single-crystal silicon has\nbeen the growth of high-quality single-crystal Si ingots with diameters up to\n12 inches or larger. For many applications of graphene, large-area high-quality\n(ideally of single-crystal) material will be enabling. Since the first growth\non copper foil a decade ago, inch-sized single-crystal graphene has been\nachieved. We present here the growth, in 20 minutes, of a graphene film of 5 x\n50 cm2 dimension with > 99% ultra-highly oriented grains. This growth was\nachieved by: (i) synthesis of sub-metre-sized single-crystal Cu(111) foil as\nsubstrate; (ii) epitaxial growth of graphene islands on the Cu(111) surface;\n(iii) seamless merging of such graphene islands into a graphene film with high\nsingle crystallinity and (iv) the ultrafast growth of graphene film. These\nachievements were realized by a temperature-driven annealing technique to\nproduce single-crystal Cu(111) from industrial polycrystalline Cu foil and the\nmarvellous effects of a continuous oxygen supply from an adjacent oxide. The\nas-synthesized graphene film, with very few misoriented grains (if any), has a\nmobility up to ~ 23,000 cm2V-1s-1 at 4 K and room temperature sheet resistance\nof ~ 230 ohm/square. It is very likely that this approach can be scaled up to\nachieve exceptionally large and high-quality graphene films with single\ncrystallinity, and thus realize various industrial-level applications at a low\ncost.", "category": "cond-mat_mtrl-sci" }, { "text": "First-principles methodology for studying magnetotransport in narrow-gap\n semiconductors: an application to Zirconium Pentatelluride ZrTe5: The origin of anomalous resistivity peak and accompanied sign reversal of\nHall resistivity of ZrTe$_5$ has been under debate for a long time. Although\nvarious theoretical models have been proposed to account for these intriguing\ntransport properties, a systematic study from first principles view is still\nlacking. In this work, we present a first principles calculation combined with\nBoltzmann transport theory to investigate the transport properties in\nnarrow-gap semiconductors at different temperatures and doping densities within\nthe relaxation time approximation. Regarding the sensitive\ntemperature-dependent chemical potential and relaxation time of semiconductors,\nwe take proper approximation to simulate these two variables, and then\ncomprehensively study the transport properties of ZrTe$_5$ both in the absence\nand presence of an applied magnetic field. Without introducing topological\nphases and correlation interactions, we qualitatively reproduced crucial\nfeatures observed in experiments, including zero-field resistivity anomaly,\nnonlinear Hall resistivity with sign reversal, and non-saturating\nmagnetoresistance at high temperatures. Our calculation allows a systematic\ninterpretation of the observed properties in terms of multi-carrier and Fermi\nsurface geometry. Our method can be extended to other narrow-gap semiconductors\nand further pave the way to explore interesting and novel transport properties\nof this field.", "category": "cond-mat_mtrl-sci" }, { "text": "Lattice expansion and non-collinear to collinear ferrimagnetic order in\n MnCr$_2$O$_4$ nanoparticle: We report magnetic behaviour of MnCr$_2$O$_4$, which belongs to a special\nclass of spinel, known as chromite. Bulk MnCr$_2$O$_4$ shows a sequence of\nmagnetic states, which follows paramagnetic (PM) to collinear ferrimagnetic\n(FM) state below T$_C$ $\\sim$ 45 K and collinear FM state to non-collinear FM\nstate below T$_S$ $\\sim$ 18 K. The non-collinear spin structure has been\nmodified on decreasing the particle size, and magnetic transition at T$_S$\ndecreases in nanoparticle samples. However, ferrimagnetic order is still\ndominating in nanoparticles, except the observation of superparamagnetic like\nblocking and decrease of spontaneous magnetization for nanoparticle. This may,\naccording to the core-shell model of ferrimagnetic nanoparticle, be the surface\ndisorder effect of nanoparticle. The system also show the increase of T$_C$ in\nnanoparticle samples, which is not consistent with the core-shell model. The\nanalysis of the M(T) data, applying spin wave theory, has shown an unusual\nBloch exponent value 3.35 for bulk MnCr$_2$O$_4$, which decreases and\napproaches to 1.5, a typical value for any standard ferromagnet, with\ndecreasing the particle size. MnCr$_2$O$_4$ has shown a few more unusual\nbehaviour. For example, lattice expansion in nanoparticle samples. The present\nwork demonstrates the correlation between a systematic increase of lattice\nparameter and the gradual decrease of B site non-collinear spin structure in\nthe light of magnetism of MnCr$_2$O$_4$ nanoparticles.", "category": "cond-mat_mtrl-sci" }, { "text": "Strongly Constrained and Appropriately Normed Semilocal Density\n Functional: The ground-state energy, electron density, and related properties of ordinary\nmatter can be computed efficiently when the exchange-correlation energy as a\nfunctional of the density is approximated semilocally. We propose the first\nmeta-GGA (meta-generalized gradient approximation) that is fully constrained,\nobeying all 17 known exact constraints that a meta-GGA can. It is also exact or\nnearly exact for a set of appropriate norms, including rare-gas atoms and\nnonbonded interactions. This SCAN (strongly constrained and appropriately\nnormed) meta-GGA achieves remarkable accuracy for systems where the exact\nexchange-correlation hole is localized near its electron, and especially for\nlattice constants and weak interactions.", "category": "cond-mat_mtrl-sci" }, { "text": "New apparatus for DTA at 2000 bar: thermodynamic studies on Au, Ag, Al\n and HTSC oxides: A new DTA (Differential Thermal Analysis) device was designed and installed\nin a Hot Isostatic Pressure (HIP) furnace in order to perform high-pressure\nthermodynamic investigations up to 2 kbar and 1200C. Thermal analysis can be\ncarried out in inert or oxidising atmosphere up to p(O2) = 400 bar. The\ncalibration of the DTA apparatus under pressure was successfully performed\nusing the melting temperature (Tm) of pure metals (Au, Ag and Al) as standard\ncalibration references. The thermal properties of these metals have been\nstudied under pressure. The values of DV (volume variation between liquid and\nsolid at Tm), ROsm (density of the solid at Tm) and ALPHAm (linear thermal\nexpansion coefficient at Tm) have been extracted. A very good agreement was\nfound with the existing literature and new data were added. This HP-DTA\napparatus is very useful for studying the thermodynamics of those systems where\none or more volatile elements are present, such as high TC superconducting\noxides. DTA measurements have been performed on Bi,Pb(2223) tapes up to 2 kbar\nunder reduced oxygen partial pressure (p(O2) = 0.07 bar). The reaction leading\nto the formation of the 2223 phase was found to occur at higher temperatures\nwhen applying pressure: the reaction DTA peak shifted by 49C at 2 kbar compared\nto the reaction at 1 bar. This temperature shift is due to the higher stability\nof the Pb-rich precursor phases under pressure, as the high isostatic pressure\nprevents Pb from evaporating.", "category": "cond-mat_mtrl-sci" }, { "text": "A scalable parallel Monte Carlo algorithm for atomistic simulations of\n precipitation in alloys: We present an extension of the semi-grandcanonical (SGC) ensemble that we\nrefer to as the variance-constrained semi-grandcanonical (VC-SGC) ensemble. It\nallows for transmutation Monte Carlo simulations of multicomponent systems in\nmultiphase regions of the phase diagram and lends itself to scalable\nsimulations on massively parallel platforms. By combining transmutation moves\nwith molecular dynamics steps structural relaxations and thermal vibrations in\nrealistic alloys can be taken into account. In this way, we construct a robust\nand efficient simulation technique that is ideally suited for large-scale\nsimulations of precipitation in multicomponent systems in the presence of\nstructural disorder. To illustrate the algorithm introduced in this work, we\nstudy the precipitation of Cu in nanocrystalline Fe.", "category": "cond-mat_mtrl-sci" }, { "text": "Deep Learning and Crystal Plasticity: A Preconditioning Approach for\n Accurate Orientation Evolution Prediction: Efficient and precise prediction of plasticity by data-driven models relies\non appropriate data preparation and a well-designed model. Here we introduce an\nunsupervised machine learning-based data preparation method to maximize the\ntrainability of crystal orientation evolution data during deformation. For\nTaylor model crystal plasticity data, the preconditioning procedure improves\nthe test score of an artificial neural network from 0.831 to 0.999, while\ndecreasing the training iterations by an order of magnitude. The efficacy of\nthe approach was further improved with a recurrent neural network. Electron\nbackscattered (EBSD) lab measurements of crystal rotation during rolling were\ncompared with the results of the surrogate model, and despite error introduced\nby Taylor model simplifying assumptions, very reasonable agreement between the\nsurrogate model and experiment was observed. Our method is foundational for\nfurther data-driven studies, enabling the efficient and precise prediction of\ntexture evolution from experimental and simulated crystal plasticity results.", "category": "cond-mat_mtrl-sci" }, { "text": "Fourier analysis of the IR response of van der Waals materials: In this letter, we report on an analytical technique for optical\ninvestigations of semitransparent samples. By Fourier transforming optical\nspectra with Fabry-Perot resonances we extract information about sample\nthickness and its discrete variations. Moreover, this information is used to\nrecover optical spectra devoid of Fabry-Perot fringes, which simplifies optical\nmodelling, and can reveal previously concealed spectral features. To illustrate\nits use, we apply our technique to a Si wafer as well as six different\ncleavable layered materials, including topological insulators, thermoelectrics,\nand magnetic insulators. In the layered materials, we find strong evidence of\nlarge step edges and thickness inhomogeneity, and cannot conclusively exclude\nthe presence of voids in the bulk of cleaved samples. This could strongly\naffect the interpretation of transport and optical data of crystals with\ntopologically protected surfaces states.", "category": "cond-mat_mtrl-sci" }, { "text": "Impact of electron solvation on ice structures at the molecular scale: We determine the impact of electron solvation on D$_2$O structures adsorbed\non Cu(111) with low temperature scanning tunneling microscopy, two-photon\nphotoemission, and ab initio theory. UV photons generating solvated electrons\nlead not only to transient, but also to permanent structural changes through\nthe rearrangement of individual molecules. The persistent changes occur near\nsites with a high density of dangling OH groups that facilitate electron\nsolvation. We conclude that energy dissipation during solvation triggers\npermanent molecular rearrangement via vibrational excitation.", "category": "cond-mat_mtrl-sci" }, { "text": "Bandgap of two-dimensional materials: Thorough assessment of modern\n exchange-correlation functionals: The density functional theory (DFT) approximations that are the most accurate\nfor the calculation of band gap of bulk materials are hybrid functionals like\nHSE06, the MBJ potential, and the GLLB-SC potential. More recently, generalized\ngradient approximations (GGA), like HLE16, or meta-GGAs, like (m)TASK, have\nproven to be also quite accurate for the band gap. Here, the focus is on 2D\nmaterials and the goal is to provide a broad overview of the performance of DFT\nfunctionals by considering a large test set of 298 2D systems. The present work\nis an extension of our recent studies [Rauch et al., Phys. Rev. B 101, 245163\n(2020) and Patra et al., J. Phys. Chem. C 125, 11206 (2021)]. Due to the lack\nof experimental results for the band gap of 2D systems, $G_{0}W_{0}$ results\nwere taken as reference. It is shown that the GLLB-SC potential and mTASK\nfunctional provide the band gaps that are the closest to $G_{0}W_{0}$.\nFollowing closely, the local MBJ potential has a pretty good accuracy that is\nsimilar to the accuracy of the more expensive hybrid functional HSE06.", "category": "cond-mat_mtrl-sci" }, { "text": "Electronic structure of Ba(Zn0.875Mn0.125)2As2 studied by angle-resolved\n photoemission spectroscopy: Electronic structure of single crystalline\nBa(Zn$_{0.875}$Mn$_{0.125}$)$_{2}$As$_{2}$, parent compound of the recently\nfounded high-temperature ferromagnetic semiconductor, was studied by\nhigh-resolution photoemission spectroscopy (ARPES). Through systematically\nphoton energy and polarization dependent measurements, the energy bands along\nthe out-of-plane and in-plane directions were experimentally determined. Except\nthe localized states of Mn, the measured band dispersions agree very well with\nthe first-principle calculations of undoped BaZn$_{2}$As$_{2}$. A new feature\nrelated to Mn 3d states was identified at the binding energies of about -1.6 eV\nbesides the previously observed feature at about -3.3 eV. We suggest that the\nhybridization between Mn and As orbitals strongly enhanced the density of\nstates around -1.6 eV. Although our resolution is much better compared with\nprevious soft X-ray photoemission experiments, no clear hybridization gap\nbetween Mn 3d states and the valence bands proposed by previous model\ncalculations was detected.", "category": "cond-mat_mtrl-sci" }, { "text": "Reaction rate approach to dipolar relaxation in alkali halides:\n Adiabaticity versus classical, activated-tunneling, and quantal dipoles: This paper is aimed at presenting a simple vibronic model for describing the\ndipolar reorientation in crystals by means of reaction rate theory. The\nHamiltonian of an isolated dipole is simplified so as to render the problem\nsolvable. Depending on the crossover splitting the dipoles may reorientate\nadiabatically with a high electron-transfer expectancy or exhibit low\nreorientation rates due to low expectancy. An important quantity to distinguish\nbetween adiabatic dipoles behaving classically and ones reorientating by means\nof quantum-mechanical tunneling is Christov's characteristic temperature which\nis found to relate to the barrier height and crossover splitting. ITC data on\nimpurity-vacancy dipoles in Eu-doped alkali halides are reanalyzed.", "category": "cond-mat_mtrl-sci" }, { "text": "Prediction of three-fold fermions in a nearly-ideal Dirac semimetal\n BaAgAs: Materials with triply-degenerate nodal points in their low-energy electronic\nspectrum produce crystalline-symmetry-enforced three-fold fermions, which\nconceptually lie between the two-fold Weyl and four-fold Dirac fermions. Here\nwe show how a silver-based Dirac semimetal BaAgAs realizes three-fold fermions\nthrough our first-principles calculations combined with a low-energy effective\n$\\mathbf{k.p}$ model Hamiltonian analysis. BaAgAs is shown to harbor\ntriply-degenerate nodal points, which lie on its $C_{3}$ rotation axis, and are\nprotected by the $C_{6v}$($C_2\\otimes C_{3v}$) point-group symmetry in the\nabsence of spin-orbit coupling (SOC) effects. When the SOC is turned on, BaAgAs\ntransitions into a nearly-ideal Dirac semimetal state with a pair of Dirac\nnodes lying on the $C_{3}$ rotation axis. We show that breaking inversion\nsymmetry in the BaAgAs$_{1-x}$P$_x$ alloy yields a clean and tunable three-fold\nfermion semimetal. Systematic relaxation of other symmetries in BaAgAs\ngenerates a series of other topological phases. BaAgAs materials thus provide\nan ideal platform for exploring tunable topological properties associated with\na variety of different fermionic excitations.", "category": "cond-mat_mtrl-sci" }, { "text": "Low-field microwave-free sensors using dipolar spin relaxation of\n quartet spin states in silicon carbide: Paramagnetic defects and nuclear spins are the major sources of magnetic\nfield-dependent spin relaxation in point defect quantum bits. The detection of\nrelated optical signals has led to the development of advanced relaxometry\napplications with high spatial resolution. The nearly degenerate quartet ground\nstate of the silicon vacancy qubit in silicon carbide (SiC) is of special\ninterest in this respect, as it gives rise to relaxation rate extrema at\nvanishing magnetic field values and emits in the first near-infra-red\ntransmission window of biological tissues, providing an opportunity for\ndeveloping novel sensing applications for medicine and biology. However, the\nrelaxation dynamics of the silicon vacancy center in SiC have not yet been\nfully explored. In this paper, we present results from a comprehensive\ntheoretical investigation of the dipolar spin relaxation of the quartet spin\nstates in various local spin environments. We discuss the underlying physics\nand quantify the magnetic field and spin bath dependent relaxation time $T_1$.\nUsing these findings we demonstrate that the silicon vacancy qubit in SiC can\nimplement microwave-free low magnetic field quantum sensors of great potential.", "category": "cond-mat_mtrl-sci" }, { "text": "Field-free spin-orbit torque switching through domain wall motion: Deterministic current-induced spin-orbit torque (SOT) switching of\nmagnetization in a heavy transition metal/ferromagnetic metal/oxide magnetic\nheterostructure with the ferromagnetic layer being perpendicularly-magnetized\ntypically requires an externally-applied in-plane field to break the switching\nsymmetry. We show that by inserting an in-plane magnetized ferromagnetic layer\nCoFeB underneath the conventional W/CoFeB/MgO SOT heterostructure,\ndeterministic SOT switching of the perpendicularly-magnetized top CoFeB layer\ncan be realized without the need of in-plane bias field. Kerr imaging study\nfurther unveils that the observed switching is mainly dominated by domain\nnucleation and domain wall motion, which might limit the potentiality of using\nthis type of multilayer stack design for nanoscale SOT-MRAM application.\nComparison of the experimental switching behavior with micromagnetic\nsimulations reveals that the deterministic switching in our devices cannot be\nexplained by the stray field contribution of the in-plane magnetized layer, and\nthe roughness-caused N\\'eel coupling effect might play a more important role in\nachieving the observed field-free deterministic switching.", "category": "cond-mat_mtrl-sci" }, { "text": "Tuning valleys and wave functions of van der Waals heterostructures by\n varying the number of layers: A first-principles study: In van der Waals heterostructures of two-dimensional transition-metal\ndichalcogenides (2D TMDCs) electron and hole states are spatially localized in\ndifferent layers forming long-lived interlayer excitons. Here, we have\ninvestigated, from first principles, the influence of additional electron or\nhole layers on the electronic properties of a MoS2/WSe2 heterobilayer (HBL),\nwhich is a direct band gap material. Additional layers modify the interlayer\nhybridization, mostly affecting the quasiparticle energy and real-space extend\nof hole states at the G and electron states at the Q valleys. For a sufficient\nnumber of additional layers, the band edges move from K to Q or G,\nrespectively. Adding electron layers to the HBL leads to more delocalized Q\nstates, while G states do not extend much beyond the HBL, even when more hole\nlayers are added. These results suggest a simple and yet powerful way to tune\nband edges and the real-space extend of the electron and hole wave function in\nTMDC heterostructures, strongly affecting the lifetime and dynamics of\ninterlayer excitons.", "category": "cond-mat_mtrl-sci" }, { "text": "Computational analysis of short-range interactions between an edge\n dislocation and an array of equally-spaced identical shearable or\n non-shearable precipitates: The interaction between dislocations and precipitates plays an important role\nin the mechanical behavior of alloys. To provide more insight into the physics\nof this interaction, this research analyzes short-range interactions of an edge\ndislocation with an array of equally-spaced identical precipitates. We use a\nmodified dislocation dynamics approach accounting for penetrable and\nimpenetrable precipitates. This research quantifies the effects of precipitate\nresistance on the geometry of the dislocation-precipitation interaction and the\nlocal distribution of plastic strain near a precipitate. The results show that\na precipitate with a higher resistance causes an increase in the maximum value\nof dislocation curvature during the bypass. In addition, a higher level of\nprecipitate resistance leads to a lower level of plastic deformation. Moreover,\nwe observed a high plastic strain gradient at the interface of non-shearable\nprecipitates.", "category": "cond-mat_mtrl-sci" }, { "text": "Cooperatively Modulating Magnetic Anisotropy and Colossal\n Magnetoresistance via Atomic-Scale Buffer Layers in Highly Strained\n La0.7Sr0.3MnO3 Films: Simultaneous control of magnetic anisotropy and magnetoresistance, especially\nwith atomic scale precision, remains a pivotal challenge for realizing advanced\nspintronic functionalities. Here we demonstrate cooperative continuous control\nover both magnetoresistance and magnetic anisotropy in highly strained\nLa0.7Sr0.3MnO3 (LSMO) thin films. By inserting varying perovskite buffer\nlayers, compressively strained LSMO films transition from a ferromagnetic\ninsulator with out-of-plane magnetic anisotropy to a metallic state with\nin-plane anisotropy. Atomic-scale buffer layer insertion enables remarkably\nacute, precise control to sharply modulate this magnetic phase transformation.\nA gigantic 10,000% modulation of the colossal magnetoresistance (CMR) and an\nexceptionally sharp transition from out-of-plane to in-plane magnetic\nanisotropy are attained in just a few contiguous layers. These atomic-scale\ncorrelations among electronic, magnetic, and structural order parameters yield\nflexible multifunctional control promising for next-generation oxide\nspintronics.", "category": "cond-mat_mtrl-sci" }, { "text": "Impact of Cr doping on the structure, optical and magnetic properties of\n nanocrystalline ZnO particles: The role of Cr incorporation into the ZnO were probed through investigations\ninto the structural, optical and magnetic properties. Zn1-xCrxO with x = 0,\n0.01, 0.03 and 0.05, nanoparticles were prepared by solution combustion method.\nPowder x-ray diffraction (XRD) results reveal, all the synthesized samples are\nin single hexagonal wurtzite crystal structures, indicating that Cr3+ ions\nsubstitute the Zn2+ ions without altering the structure. The crystallite size\nand microstrain were calculated using the Willamson-Hall method and found to be\n36 +- 2 nm for ZnO and it reduced with the increase of Cr dopant concentration\nto 20 +- 2 nm for Zn0.95Cr0.05O. Transmission electron microscopy (TEM)\nrevealed that the particle size were 48 +- 2 nm, 29 +- 2 nm and 25 +- 2 nm for\nthe Zn1-xCrxO with x = 0, 0.03 and 0.05, respectively. TEM morphology indicated\nparticles are agglomerated in the doped samples. The band-gap decreases\nslightly from 3.305 +- 0.003 eV to 3.292 +- 0.003 eV with increase of Cr\ncontent from x = 0 to 0.05, respectively. Photoluminescence measurements\nrevealed the presence of defects in the samples, associated with zinc vacancies\nand singly ionized oxygen vacancy. The field-dependent magnetization\nmeasurements of ZnO and Cr-doped ZnO were carried out using a vibrating sample\nmagnetometer (VSM) at 300 K. All the samples exhibits ferromagnetic behavior.\nThis long-range ferromagnetism ordering observed in ZnO is explained based on\nbound magnetic polaron (BMP) mechanism. The singly ionized oxygen vacancies\nplaying a crucial role in observed room temperature ferromagnetism (RTFM) in\nZnO. There is a sufficient amount of BMPs formed in Cr doped ZnO because of the\ndefects present in these samples. Therefore, the overlapping of BMPs results in\nthe RTFM. However, the antiferromagnetic coupling at a higher doping\nconcentration of Cr, weakens the observed RTFM.", "category": "cond-mat_mtrl-sci" }, { "text": "Room temperature ferromagnetic-like behavior in Mn-implanted and\n post-annealed InAs layers deposited by Molecular Beam Epitaxy: We report on the magnetic and structural properties of Ar and Mn implanted\nInAs epitaxial films grown on GaAs (100) by Molecular Beam Epitaxy (MBE) and\nthe effect of Rapid Thermal Annealing (RTA) for 30 seconds at 750C. Channeling\nParticle Induced X- ray Emission (PIXE) experiments reveal that after Mn\nimplantation almost all Mn atoms are subsbtitutional in the In-site of the InAs\nlattice, like in a diluted magnetic semiconductor (DMS). All of these samples\nshow diamagnetic behavior. But, after RTA treatment the Mn-InAs films exhibit\nroom-temperature magnetism. According to PIXE measurements the Mn atoms are no\nlonger substitutional. When the same set of experiments were performed with As\nas implantation ion all of the layers present diamagnetism without exception.\nThis indicates that the appearance of room-temperature ferromagnetic-like\nbehavior in the Mn-InAs-RTA layer is not related to lattice disorder produce\nduring implantation, but to a Mn reaction produced after a short thermal\ntreatment. X-ray diffraction patterns (XRD) and Rutherford Back Scattering\n(RBS) measurements evidence the segregation of an oxygen deficient-MnO2 phase\n(nominally MnO1.94) in the Mn-InAs-RTA epitaxial layers which might be on the\norigin of room temperature ferromagnetic-like response observed.", "category": "cond-mat_mtrl-sci" }, { "text": "Deposition and photoluminescence of zinc gallium oxide thin films with\n varied stoichiometry made by reactive magnetron co-sputtering: This paper reports on the deposition and photoluminescence of amorphous and\ncrystalline thin films of zinc gallium oxide with Ga:Zn atomic ratio varied\nbetween 0.3 and 5.7. The films are prepared by reactive direct current\nmagnetron co-sputtering from liquid/solid gallium/zinc targets onto fused\nquartz substrates; the temperature of the substrate is varied from room\ntemperature (RT) to 800{\\deg}C. The sputtering process is effectively\ncontrolled by fixing the sputtering power of one of the targets and controlling\nthe power of the other target by plasma optical emission spectroscopy. The\nmethod, in conjunction with oxygen flow adjustment, enables the production of\nnear-stoichiometric films at any temperature used. The composition analysis\nsuggests a few at.% oxygen deficiency in the films. The resulting deposition\nrate is at least an order of magnitude higher compared to the commonly used\nradio-frequency sputtering from a ceramic ZnO:Ga2O3 target. Deposited onto\nunheated substrates, the films with Ga:Zn {\\approx} 2 are X-ray amorphous.\nWell-defined X-ray diffraction peaks of spinel ZnGa2O4 start to appear at a\nsubstrate temperature of 300{\\deg}C. The surface of the as-deposited films is\ndense and exhibits a fine-featured structure observed in electron microscopy\nimages. Increasing the deposition temperature from RT to 800{\\deg}C eliminates\ndefects and improves crystallinity, which for the films with Ga:Zn ratio close\nto 2 results in an increase in the optical band gap from 4.6 eV to 5.1 eV. Room\ntemperature photoluminescence established the main peak at 3.1 eV (400 nm); a\nsimilar peak in Ga2O3 is ascribed to oxygen-vacancy related transitions. A\nprominent feature around 2.9 eV (428 nm) is attributed to self-activation\ncenter of the octahedral Ga-O groups in the spinel lattice of ZnGa2O4. It was\nfound that photoluminescence from ZnGa2O4 depends significantly on the ratio\nGa:Zn.", "category": "cond-mat_mtrl-sci" }, { "text": "Inelastic electron tunneling spectroscopy of local \"spin accumulation\"\n devices: We investigate the origin of purported \"spin accumulation\" signals observed\nin local \"three-terminal\" (3T) measurements of ferromagnet/insulator/n-Si\ntunnel junctions using inelastic electron tunneling spectroscopy (IETS).\nVoltage bias and magnetic field dependences of the IET spectra were found to\naccount for the dominant contribution to 3T magnetoresistance signals, thus\nindicating that it arises from inelastic tunneling through impurities and\ndefects at junction interfaces and within the barrier, rather than from spin\naccumulation due to pure elastic tunneling into bulk Si as has been previously\nassumed.", "category": "cond-mat_mtrl-sci" }, { "text": "Magnetic and structural properties of Co2FeAl thin films grown on Si\n substrate: The correlation between magnetic and structural properties of Co_{2} FeAl\n(CFA) thin films of different thickness (10 nmT_{PM}>T_M$,\n$T_C>T_M$ without premartensitic transition, $T_C\\approx T_M$, and $T_C1.41 the samples undergo structural relaxations through adaptive\nnanotwinning. For all tetragonal structures, we observe a significant increase\nof the magnetocrystalline anisotropy constant K1, which reaches a maximum of\nK1=-2.4*10^5 Jm^-3 at room temperature around c/a_bct=1.33 and is thus even\nlarger than for binary Fe70Pd30 and the prototype Ni-Mn-Ga magnetic shape\nmemory system. Since K1 represents the driving force for variant reorientation\nin magnetic shape memory systems, we conclude that Fe-Pd-Cu alloys offer a\npromising route towards microactuators applications with significantly improved\nwork output.", "category": "cond-mat_mtrl-sci" }, { "text": "Perpendicular magnetic anisotropy and spin glass-like behavior in\n molecular beam epitaxy grown chromium telluride thin films: Reflection high energy electron diffraction (RHEED), scanning tunneling\nmicroscopy (STM), vibrating sample magnetometry and other physical property\nmeasurements are used to investigate the structure, morphology, magnetic and\nmagneto-transport properties of (001)-oriented Cr$_2$Te$_3$ thin films grown on\nAl$_2$O$_3$(0001) and Si(111)-(7$\\times$7) surfaces by molecular beam epitaxy\n(MBE). Streaky RHEED patterns indicate flat smooth film growth on both\nsubstrates. STM studies show the hexagonal arrangements of surface atoms.\nDetermination of the lattice parameter from atomically resolved STM image is\nconsistent with the bulk crystal structures. Magnetic measurements show the\nfilm is ferromagnetic having the Curie temperature of about 180 K, and a spin\nglass-like behavior was observed below 35 K. Magneto-transport measurements\nshow the metallic nature of the film with a perpendicular magnetic anisotropy\nalong the $c$-axis.", "category": "cond-mat_mtrl-sci" }, { "text": "Comment on \"Hysteretic transition between states of a filled hexagonal\n magnetic dipole cluster\": In the paper \"Andrew D.P. Smith, Peter T. Haugen, Boyd F. Edwards: Hysteretic\ntransition between states of a filled hexagonal magnetic dipole cluster,\nJournal of Magnetism and Magnetic Materials 549 (2022): 168991\" a hysteretic\ntransition between two stable arrangements of a cluster of seven dipoles is\npresented. The relative strength of the center dipole in a hexagonal\narrangement serves as the bifurcation parameter. The authors clearly\ndemonstrate the existence of two instabilities accompanied by discontinuous\njumps of the dipole arrangement, but leave the question about the nature of\nthese instabilities unanswered. This comment clarifies the nature of the two\ninstabilities: the first one is a symmetry-breaking sub-critical bifurcation\nwith parabolic scaling of the magnetic potential energy difference between the\ntwo branches, and the second one is a fold with its characteristic scaling in\nthe form of a semi-cubic parabola.", "category": "cond-mat_mtrl-sci" }, { "text": "Bulk and Lattice Properties for Rigid Carbon Nanotubes Materials: We use an atom-atom potential between carbon atoms to obtain an interaction\npotential between nanotubes (assumed rigid), thereby calculating the cohesive\nenergy of a bunch of nanotubes in hexagonal two dimensional packing. The model\nproposed is quite similar to our earlier work on fullerenes and organic\nmolecular crystals. The results for inter-nanotube distances, energy per unit\nlength, bulk modulus and phonons for inter-nanotube vibrations are obtained and\ncompared with available data from measurements and other available\ncalculations. We also model formation of multi-wall nanotubes. We find the\nresults for various calculated quantities agreeing very well with measured\nstructural parameters and other calculations. The reversible energy stored on\ncompression of the bunch of nanotubes on application of pressure up to 30 Kbar\ncalculated in this rigid molecule model is overestimated by about 30% when\ncompared with measured results, signifying the appreciable flexibility of tubes\nat high pressures. The model is considered very suitable for incorporating\nflexible nanotubes in bunches of single and multi-wall nanotube materials of\nvarious types.", "category": "cond-mat_mtrl-sci" }, { "text": "Self-Assembled Triply Periodic Minimal Surfaces as moulds for Photonic\n Band Gap Materials: We propose systems with structures defined by self-assembled triply periodic\nminimal surfaces (STPMS) as candidates for photonic bandgap materials. To\nsupport our proposal we have calculated the photonic bands for different STPMS\nand we have found that, at least, the double diamond and gyroid structures\npresent full photonic bandgaps. Given the great variety of systems which\ncrystalize in these structures, the diversity of possible materials that form\nthem and the range of lattice constants they present, the construction of\nphotonic bandgap materials with gaps in the visible range may be presently\nwithin reach.", "category": "cond-mat_mtrl-sci" }, { "text": "Imaging of Spin Dynamics in Closure Domain and Vortex Structures: Time-resolved Kerr microscopy is used to study the excitations of individual\nmicron- scale ferromagnetic thin film elements in their remnant state. Thin (18\nnm) square elements with edge dimensions between 1 and 10 $\\mu$m form closure\ndomain structures with 90 degree Neel walls between domains. We identify two\nclasses of excitations in these systems. The first corresponds to precession of\nthe magnetization about the local demagnetizing field in each quadrant, while\nthe second excitation is localized in the domain walls. Two modes are also\nidentified in ferromagnetic disks with thicknesses of 60 nm and diameters from\n2 $\\mu$m down to 500 nm. The equilibrium state of each disk is a vortex with a\nsingularity at the center. As in the squares, the higher frequency mode is due\nto precession about the internal field, but in this case the lower frequency\nmode corresponds to gyrotropic motion of the entire vortex. These results\ndemonstrate clearly the existence of well-defined excitations in\ninhomogeneously magnetized microstructures.", "category": "cond-mat_mtrl-sci" }, { "text": "Limits to crystallization pressure: Crystallization pressure drives deformation and damage in monuments,\nbuildings and the Earth's crust. Even though the phenomenon has been known for\n170 years there is no agreement between theoretical calculations of the maximum\nattainable pressure and that found experimentally. We have therefore developed\na novel experimental technique to image the nano-confined crystallization\nprocess while controlling the pressure and applied it to calcite. The results\nshow that displacement by crystallization pressure is arrested at pressures\nwell below the thermodynamic limit. We use existing molecular dynamics\nsimulations and atomic force microscopy data to construct a robust model of the\ndisjoining pressure in this system and thereby calculate the absolute distance\nbetween the surfaces. Based on the high resolution experiments and modelling we\nformulate a novel mechanism for the transition between damage and adhesion by\ncrystallization that may find application in Earth and materials sciences and\nin conservation of cultural heritage.", "category": "cond-mat_mtrl-sci" }, { "text": "Giant magnetocaloric effect in exchange-frustrated GdCrTiO5\n antiferromagnet: We report the effect of exchange frustration on the magnetocaloric properties\nof GdCrTiO$_5$ compound. Due to the highly exchange-frustrated nature of\nmagnetic interaction, in GdCrTiO$_5$, the long-range antiferromagnetic ordering\noccurs at much lower temperature $T_N$=0.9 K and the magnetic cooling power\nenhances dramatically relative to that observed in several geometrically\nfrustrated systems. Below 5 K, isothermal magnetic entropy change (-$\\Delta\nS_{\\rm m}$) is found to be 36 J kg$^{-1}$ K$^{-1}$, for a field change ($\\Delta\nH$) of 7 T. Further, -$\\Delta S_{\\rm m}$ does not decrease from its maximum\nvalue with decreasing in $T$ down to very low temperatures and is reversible in\nnature. The adiabatic temperature change, $\\Delta T_{\\rm ad}$, is 15 K for\n$\\Delta H$=7 T. These magnetocaloric parameters are significantly larger than\nthat reported for several potential magnetic refrigerants, even for small and\nmoderate field changes. The present study not only suggests that GdCrTiO$_5$\ncould be considered as a potential magnetic refrigerant at cryogenic\ntemperatures but also promotes further studies on the role of exchange\nfrustration on magnetocaloric effect. In contrast, only the role of geometrical\nfrustration on magnetocaloric effect has been previously reported theoretically\nand experimentally investigated on very few systems.", "category": "cond-mat_mtrl-sci" }, { "text": "Structures and velocities of noisy ferroelectric domain walls: Ferroelectric domain wall motion is fundamental to the switching properties\nof ferroelectric devices and is influenced by a wide range of factors including\nspatial disorder within the material and thermal noise. We build a\nLandau-Ginzburg-Devonshire (LGD) model of 180${}^{\\circ}$ ferroelectric domain\nwall motion that explicitly takes into account the presence of both spatial and\ntemporal disorder. We demonstrate both creep flow and linear flow regimes of\nthe domain wall dynamics by solving the LGD equations in a Galilean frame\nmoving with the wall velocity $v$. Thermal noise plays a key role in the wall\ndepinning process at small fields $E$. We study the scaling of the velocity $v$\nwith the applied DC electric field $E$ and show that noise strongly affects\ndomain wall velocities. We also show that the domain wall widens significantly\nin the presence of thermal noise, especially as the material temperature $T$\napproaches the critical temperature $T_c$. These calculations therefore point\nto the potential of noise and disorder to become control factors for the\nswitching properties of ferroelectric materials, for example for advancement of\nmicroelectronic applications.", "category": "cond-mat_mtrl-sci" }, { "text": "Fascinating interplay between Charge Density Wave Order and magnetic\n field in Non-magnetic Rare-Earth Tritelluride LaTe$_{3}$: Charge density wave (CDW) states in solids bear an intimate connection to\nunderlying fermiology. Modification of the latter by a suitable perturbation\nprovides an attractive handle to unearth novel CDW states. Here, we combine\nextensive magnetotransport experiments and first-principles electronic\nstructure calculations on a non-magnetic tritelluride LaTe$_{3}$ single crystal\nto uncover phenomena rare in CDW systems: $(i)$ hump-like feature in the\ntemperature dependence of resistivity at low temperature under application of\nmagnetic field, which moves to higher temperature with increasing field\nstrength, $(ii)$ highly anisotropic large transverse magnetoresistance (MR)\nupon rotation of magnetic field about current parallel to crystallographic\nc-axis, (iii) anomalously large positive MR with spike-like peaks at\ncharacteristic angles when the angle between current and field is varied in the\nbc-plane, (iv) extreme sensitivity of the angular variation of MR on field and\ntemperature. Moreover, our Hall measurement reveals remarkably high carrier\nmobility $\\sim$ 33000 cm$^{2}$/Vs, which is comparable to that observed in some\ntopological semimetals. These novel observations find a comprehensive\nexplication in our density functional theory (DFT) and dynamical mean field\ntheory (DMFT) calculations that capture field-induced electronic structure\nmodification in LaTe$_{3}$. The band structure theory together with transport\ncalculations suggest the possibility of a second field-induced CDW transition\nfrom the field-reconstructed Fermi surface, which qualitatively explains the\nhump in temperature dependence of resistivity at low temperature. Thus, our\nstudy exposes the novel manifestations of the interplay between CDW order and\nfield-induced electronic structure modifications in LaTe$_{3}$, and establishes\na new route to tune CDW states by perturbations like magnetic field.", "category": "cond-mat_mtrl-sci" }, { "text": "Spatial decomposition of magnetic anisotropy in magnets: application for\n doped Fe16N2: We propose a scheme of decomposition of the total relativistic energy in\nsolids to intra- and interatomic contributions. The method is based on a\nvariation of the speed of light from its value in relativistic theory to\ninfinity (a non-relativistic limit). As an illustration of the method, we\ntested such decomposition in the case of a spin-orbit interaction variation for\ndecomposition of the magnetic anisotropy energy (MAE) in CoPt. We further\nstudied the {\\alpha}''-Fe16N2 magnet doped by Bi, Sb, Co and Pt atoms. It has\nbeen found that the addition of Pt atoms can enhance the MAE by as large as\nfive times while Bi and Sb substitutions double the total MAE. Using the\nproposed technique we demonstrate the spatial distribution of these\nenhancements. Our studies also suggest that Sb, Pt and Co substitutions could\nbe synthesized by experiments.", "category": "cond-mat_mtrl-sci" }, { "text": "The electronic transport properties and microstructure of carbon\n nanofiber/epoxy composites: Carbon nanofibres (CNF) were dispersed into an epoxy resin using a\ncombination of ultrasonication and mechanical mixing. The electronic transport\nproperties of the resulting composites were investigated by means of impedance\nspectroscopy. It was found that a very low critical weight fraction (pc = 0.064\nwt %) which may be taken to correspond to the formation of a tunneling\nconductive network inside the matrix. The insulator-to-conductor transition\nregion spanned about one order of magnitude from 0.1 to 1 wt %. Far from the\ntransition, the conductivity increased by two orders of magnitude. This\nincrease and the low value of the conductivity were explained in terms of the\npresence of an epoxy film at the contact between CNF. A simple model based on\nthe CNF-CNF contact network inside the matrix was proposed in order to evaluate\nthe thickness of that film.", "category": "cond-mat_mtrl-sci" }, { "text": "Quest for Dr. Yia-Chung Chang's Calculations about the superlattice\n phonon band structures: Quest for Dr. Yia-Chung Chang's Calculations about the superlattice phonon\nband structures", "category": "cond-mat_mtrl-sci" }, { "text": "Crystal growth and metallic ferromagnetism induced by electron doping in\n FeSb$_2$: In order to study the metallic ferromagnetism induced by electron doping in\nthe narrow-gab semiconductor FeSb$_2$, single crystals of FeSb$_2$,\nFe$_{1-x}$Co$_x$Sb$_2$ ($0 \\le x \\le 0.5$) and FeSb$_{2-y}$Te$_y$ ($0 \\le y \\le\n0.4$), were grown by a simplified self-flux method. From powder x-ray\ndiffraction (XRD) patterns, wavelength-dispersive x-ray spectroscopy (WDX) and\nx-ray Laue diffraction, pure and doped high-quality single crystals, within the\nselected solubility range, show only the orthorhombic $Pnnm$ structure of\nFeSb$_2$ with a monotonic change in lattice parameters with increasing the\ndoping level. In consistence with the model of nearly ferromagnetic small-gap\nsemiconductor, the energy gap of FeSb$_2$ Pauli paramagnet gradually collapses\nby electron doping before it closes at about $x$ or $y$ = 0.15 and subsequent\nitinerant electron anisotropic ferromagnetic states are observed with higher\ndoping levels. A magnetic phase diagram is established and discussed in view of\nproposed theoretical scenarios.", "category": "cond-mat_mtrl-sci" }, { "text": "In Situ X-Ray Radiography and Tomography Observations of the\n Solidification of Alumina Particles Suspensions. Part II: Steady State: This paper investigates the behaviour of colloidal suspensions of alumina\nparticles during directional solidification, by in situ high-resolution\nobservations using X-ray radiography and tomography. This second part is\nfocussed on the evolution of ice crystals during steady state growth (in terms\nof interface velocity) and on the particles redistribution taking place in this\nregime. In particular, it is shown that diffusion cannot determine the\nconcentration profile and the particles redistribution in this regime of\ninterface velocities (20-40 microns/s); constitutional supercooling arguments\ncannot be invoked to interpret particles redistribution. Particles are\nredistributed by a direct interaction with the moving solidification interface.\nSeveral parameters controlling the particles redistribution were identified,\nnamely the interface velocity, the particle size, the shape of the ice crystals\nand the orientation relationships between the crystals and the temperature\ngradient.", "category": "cond-mat_mtrl-sci" }, { "text": "Density dependent local structures in InTe phase-change materials: Chalcogenide phase-change materials (PCMs) based random access memory (PCRAM)\nis one of the leading candidates for the development of non-volatile memory and\nneuro-inspired computing technologies. Recent work shows Indium to be an\nimportant alloying element for PCRAM, while a thorough understanding of the\nparent compound InTe, in particular, its amorphous phase, is still lacking. In\nthis work, we carry out ab initio simulations and chemical bonding analyses on\namorphous and various crystalline polymorphs of InTe. We reveal that the local\ngeometries are highly density dependent in amorphous structures, forming\nIn-centered tetrahedral motifs under ambient conditions but defective\noctahedral motifs under pressure, which stems from the bonding characters of\nits crystalline polymorphs. In addition, our ab initio molecular dynamics\nsimulations predict rapid crystallization capability of InTe under pressure. At\nlast, we make a suggestion for better use of Indium and propose an \"active\"\ndevice design to utilize both thermal and mechanical effects for phase-change\napplications.", "category": "cond-mat_mtrl-sci" }, { "text": "Effects of biaxial strain on the improper multiferroicity in h-LuFeO3\n films: Elastic strain is potentially an important approach in tuning the properties\nof the improperly multiferroic hexagonal ferrites, the details of which have\nhowever been elusive due to the experimental difficulties. Employing the method\nof restrained thermal expansion, we have studied the effect of isothermal\nbiaxial strain in the basal plane of h-LuFeO3 (001) films. The results indicate\nthat a compressive biaxial strain significantly enhances the ferrodistortion,\nand the effect is larger at higher temperatures. The compressive biaxial strain\nand the enhanced ferrodistortion together, cause an increase in the electric\npolarization and a reduction in the canting of the weak ferromagnetic moments\nin h-LuFeO3, according to our first principle calculations. These findings are\nimportant for understanding the strain effect as well as the coupling between\nthe lattice and the improper multiferroicity in h-LuFeO3. The experimental\nelucidation of the strain effect in h-LuFeO3 films also suggests that the\nrestrained thermal expansion can be a viable method to unravel the strain\neffect in many other epitaxial thin film materials.", "category": "cond-mat_mtrl-sci" }, { "text": "Electronic and structural properties of vacancies and hydrogen\n adsorbates on trilayer graphene: Using ab initio calculations, we study the electronic and structural\nproperties of vacancies and hydrogen adsorbates on trilayer graphene. Those\ndefects are found to share similar low-energy electronic features, since they\nboth remove a pz electron from the honeycomb lattice and induce a defect level\nnear the Fermi energy. However, a vacancy also leaves unpaired $\\sigma $\nelectrons on the lattice, which lead to important structural differences and\nalso contribute to magnetism. We explore both ABA and ABC stackings and compare\nproperties such as formation energies, magnetic moments, spin density and the\nlocal density of states (LDOS) of the defect levels. These properties show a\nstrong sensitivity to the layer in which the defect is placed and smaller\nsensitivities to sublattice placing and stacking type. Finally, for the ABC\ntrilayer, we also study how these states behave in the presence of an external\nfield, which opens a tunable gap in the band structure of the non-defective\nsystem. The pz defect states show a strong hybridization with band states as\nthe field increases, with reduction and eventually loss of magnetization, and a\nnon-magnetic, midgap-like state is found when the defect is at the middle\nlayer.", "category": "cond-mat_mtrl-sci" }, { "text": "Concentration behavior of liquidus temperatures and undercooling of\n Al-Cu-Co at normal pressure: Differential thermal analysis has been conducted for the Al-Cu-Co alloys with\nthe composition range of 15 at.% Co and 10 to 30 at.% Cu, and 25 at.% Co and\n2.5 to 20 at.% Co. The features of the formation of solid phases have been\nstudied during the crystallization in a crucible in the conditions of slow\ncooling (rate of cooling to 1 K s-1) at normal pressure. On the state diagram\nof the Al-Cu-Co system with 15 at.% Co and 25 at.% Cu the concentration\nsections have been built, which allows to determine the concentration ranges\nfrom which different phases are formed during the first stage of\ncrystallization. Along the boundaries of different phase regions, extrema are\nobserved on the liquidus line. The observed extrema on the liquidus lines and\nconcentration dependences of undercooling are associated with change in the\nchemical short-range order at the considered concentrations both in the liquid\nand solid states.", "category": "cond-mat_mtrl-sci" }, { "text": "Coherent control of photomagnetic back-switching by double-pump laser\n pulses: The control of nonthermal, all-optical magnetization switching under the\nregime with an independent state of laser polarization opens up new\nopportunities for ultrafast magnetic recording. Here, we investigate the\nphoto-magnetic back-switching capabilities of the write and erase magnetic\ndomain pattern using double-pump pulse excitations in an iron garnet film with\npure cubic magnetocrystalline symmetry. It is essential to note that forward\nand backward magnetization switching is achievable in two distinctive\nscenarios: using identical linearly polarized laser pulses and with pulses\nhaving orthogonal polarization planes. By observing the switch of magnetization\nat domains independent of the initial state, one can nonthermally toggle the\nmagnetization, equivalent to XOR logical operation, at frequencies reaching up\nto 50 GHz.", "category": "cond-mat_mtrl-sci" }, { "text": "An oxide thermal rectifier: We have experimentally demonstrated thermal rectification as bulk effect.\nAccording to a theoretical design of a thermal rectifier, we have prepared an\noxide thermal rectifier made of two cobalt oxides with different thermal\nconductivities, and have made an experimental system to detect the thermal\nrectification. The rectifying coefficient of the device is found to be 1.43,\nwhich is in good agreement with the numerical calculation.", "category": "cond-mat_mtrl-sci" }, { "text": "Connectivity of the Icosahedral Network and a Dramatically Growing\n Static Length Scale in Cu-Zr Binary Metallic Glasses: We report on and characterize, via molecular dynamics (MD) studies, the\nevolution of the structure of Cu50Zr50 and Cu64Zr36 metallic glasses (MGs) as\ntemperature is varied. Interestingly, a percolating icosahedral network appears\nin the Cu64Zr36 system as it is supercooled. This leads us to introduce a\nstatic length scale, which grows dramatically as this three dimensional system\napproaches the glass transition. Amidst interpenetrating connections,\nnon-interpenetrating connections between icosahedra are shown to become\nprevalent upon supercooling and to greatly enhance the connectivity of the MG's\nicosahedral network. Additionally, we characterize the chemical compositions of\nthe icosahedral networks and their components. These findings demonstrate the\nimportance of non-interpenetrating connections for facilitating extensive\nstructural networks in Cu-Zr MGs, which in turn drive dynamical slowing in\nthese materials.", "category": "cond-mat_mtrl-sci" }, { "text": "Microscopic mechanism of high-temperature ferromagnetism in Fe, Mn, and\n Cr-doped InSb, InAs, and GaSb magnetic semiconductors: In recent experiments, high Curie temperatures Tc above room temperature were\nreported in ferromagnetic semiconductors Fe-doped GaSb and InSb, while low Tc\nbetween 20 K to 90 K were observed in some other semiconductors with the same\ncrystal structure, including Fe-doped InAs and Mn-doped GaSb, InSb, and InAs.\nHere we study systematically the origin of high temperature ferromagnetism in\nFe, Mn, Cr-doped GaSb, InSb, and InAs magnetic semiconductors by combining the\nmethods of density functional theory and quantum Monte Carlo. In the diluted\nimpurity limit, the calculations show that the impurities Fe, Mn, and Cr have\nsimilar magnetic correlations in the same semiconductors. Our results suggest\nthat high (low) Tc obtained in these experiments mainly comes from high (low)\nimpurity concentrations. In addition, our calculations predict the\nferromagnetic semiconductors of Cr-doped InSb, InAs, and GaSb that may have\npossibly high Tc. Our results show that the origin of high Tc in (Ga,Fe)Sb and\n(In,Fe)Sb is not due to the carrier induced mechanism because Fe3+ does not\nintroduce carriers.", "category": "cond-mat_mtrl-sci" }, { "text": "Visualization of reaction chemistry in W-KClO4-BaCrO4 delay mixtures via\n a Sestak-Berggren model based isoconversional method: The combustion delay mixture of tungsten (W), potassium perchlorate (KClO4),\nand barium chromate (BaCrO4), also known as the WKB mixture, has long been\nconsidered to be an integral part of military-grade ammunition. Despite its\nlong history, however, their progressive reaction dynamics remains a question\nmark, especially due to the complex nature of their combustion reaction. As\nopposed to a one-step oxidation commonly observed in conventional combustions,\nthe WKB mixture is associated with a multibody reaction between its solid-state\ncomponents. To this end, the emergence of three combustion peaks, which we\ncorresponded with disparate chemical reactions, was observed using\nthermogravimetric analysis on two separate WKB mixtures with differing mixture\nratios. We applied the stepwise isoconversional method on each of the peaks to\nmatch the combustion chemistry it represents to the Sestak-Berggren model and\ncomputed the conceptual activation energy. Further plotting the logarithmic\npre-exponential factor as a function of the reaction progress, we demonstrate a\nmethod of using the plot as an intuitive tool to understand the dynamics of\nindividual reactions that compose multi-step chemical reactions. Our study\nprovides a systematic approach in visualizing the reaction chemistry, thereby\nstrengthening the analytical arsenal against reaction dynamics of combustion\ncompounds in general.", "category": "cond-mat_mtrl-sci" }, { "text": "Dipolar-stabilized first and second-order antiskyrmions in ferrimagnetic\n multilayers: Skyrmions and antiskyrmions are topologically protected spin structures with\nopposite topological charge. Particularly in coexisting phases, these two types\nof magnetic quasi-particles may show fascinating physics and potential for\nspintronic devices. While skyrmions are observed in a wide range of materials,\nuntil now antiskyrmions were exclusive to materials with D2d symmetry. In this\nwork, we show first and second-order antiskyrmions stabilized by magnetic\ndipole-dipole interaction in Fe/Gd-based multilayers. We modify the magnetic\nproperties of the multilayers by Ir insertion layers. Using Lorentz\ntransmission electron microscopy imaging, we observe coexisting antiskyrmions,\nBloch skyrmions, and type-2 bubbles and determine the range of material\nproperties and magnetic fields where the different spin objects form and\ndissipate. We perform micromagnetic simulations to obtain more insight into the\nstudied system and conclude that the reduction of saturation magnetization and\nuniaxial anisotropy leads to the existence of this zoo of different spin\nobjects and that they are primarily stabilized by dipolar interaction.", "category": "cond-mat_mtrl-sci" }, { "text": "Effect of zirconium doping on mechanical properties of $W_{1-x}Zr_xB_2$\n on the base of ab initio calculations and magnetron sputtered films: Potentially superhard $W_{1-x}Zr_xB_2$ polymorph hP6-P6$_3$/mmc-$WB_2$ with\nzirconium doping in the range of x=0.0-0.25 was thoroughly analyzed within the\nframework of first-principles density functional theory from the structural and\nmechanical point of view. The obtained results were subsequently compared with\nproperties of material deposited by magnetron sputtering method. All predicted\nstructures are mechanically and thermodynamically stable. Due to theoretical\ncalculations zirconium doping reduces hardness and fracture toughness $K_{IC}$\nof $WB_2$. Deposited films are characterized by greater hardness $H_v$ but\nlower fracture toughness $K_{IC}$. The results of experiments show that not\nonly solid solution hardening is responsible for strengthening of predicted new\nmaterial but also change of microstructure, Hall-Petch effect and boron\nvacancies.", "category": "cond-mat_mtrl-sci" }, { "text": "Melting of hexane monolayers adsorbed on graphite: the role of domains\n and defect formation: We present the first large-scale molecular dynamics simulations of hexane on\ngraphite that completely reproduces all experimental features of the melting\ntransition. The canonical ensemble simulations required and used the most\nrealistic model of the system: (i) fully atomistic representation of hexane;\n(ii) explicit site-by-site interaction with carbon atoms in graphite; (iii)\nCHARMM force field with carefully chosen adjustable parameters of non-bonded\ninteraction; (iv) numerous $\\ge$ 100 ns runs, requiring a total computation\ntime of ca. 10 CPU-years. This has allowed us to determine correctly the\nmechanism of the transition: molecular reorientation within lamellae without\nperturbation of the overall adsorbed film structure. We observe that the melted\nphase has a dynamically reorienting domain-type structure whose orientations\nreflect that of graphite.", "category": "cond-mat_mtrl-sci" }, { "text": "Understanding electronic excited states in BiFeO$_3$ via ab initio\n calculations and symmetry analysis: BiFeO$_3$ is a technologically relevant multiferroic perovskite featuring\nferroelectricity and antiferromagnetism. Its lattice, magnetic, and\nferroelectric degrees of freedoms are coupled to its optically active\nexcitations and thus hold the potential to be reversible probed and controlled\nby light. In this work, we combine ab initio density functional and many-body\nperturbation theory methods with an extensive symmetry and atomic-orbital\nanalysis to describe and understand the electronic excited states spectrum and\nits imprint on the optical absorption spectrum with quantitative accuracy and\nqualitative insights. We find that the optical absorption spectrum of BiFeO$_3$\ncontain several strongly bound and spatially localized electronic transitions\nin which the spin-degree of freedom is almost fully flipped. With our analysis\nwe thoroughly characterize these localized spin-flip transitions in terms of\nthe unusual crystal field splitting of Fe-$3d$ single-electron orbitals. Our\nsymmetry analysis further allows us to thoroughly explain how the spin content\nand the energetic fine structure of these strongly bound excitons are dictated\nby the interplay between crystal symmetry, electron-hole attraction, and the\nspin-orbit coupling.", "category": "cond-mat_mtrl-sci" }, { "text": "First-order Reversal Curve Analysis of Phase Transitions in\n Electrochemical Adsorption: A New Experimental Technique Suggested by\n Computer Simulations: The first-order reversal curve (FORC) method for analysis of systems\nundergoing hysteresis is applied to dynamical models of electrochemical\nadsorption. In this setting, the method can not only differentiate between\ndiscontinuous and continuous phase transitions, but can also quite accurately\nrecover equilibrium behavior from dynamic analysis for systems with a\ncontinuous phase transition. Discontinuous and continuous phase transitions in\na two-dimensional lattice-gas model are compared using the FORC method. The\nFORC diagram for a discontinuous phase transition is characterized by a\nnegative (unstable) region separating two positive (stable) regions, while such\na negative region does not exist for continuous phase transitions. Experimental\ndata for FORC analysis could easily be obtained by simple reprogramming of a\npotentiostat designed for cyclic-voltammetry experiments.", "category": "cond-mat_mtrl-sci" }, { "text": "Computational exfoliation of atomically thin 1D materials with\n application to Majorana bound states: We introduce a computational database with calculated structural,\nthermodynamic, electronic, magnetic, and optical properties of 820\none-dimensional materials. The materials are systematically selected and\nexfoliated from experimental databases of crystal structures based on a\ndimensionality scoring parameter. The database is furthermore expanded by\nchemical element substitution in the materials. The materials are investigated\nin both their bulk form and as isolated one-dimensional components. We discuss\nthe methodology behind the database, give an overview of some of the calculated\nproperties, and look at patterns and correlations in the data. The database is\nfurthermore applied in computational screening to identify materials, which\ncould exhibit Majorana bound states.", "category": "cond-mat_mtrl-sci" }, { "text": "Wave impedance matrices for cylindrically anisotropic radially\n inhomogeneous elastic solids: Impedance matrices are obtained for radially inhomogeneous structures using\nthe Stroh-like system of six first order differential equations for the time\nharmonic displacement-traction 6-vector. Particular attention is paid to the\nnewly identified solid-cylinder impedance matrix ${\\mathbf Z} (r)$ appropriate\nto cylinders with material at $r=0$, and its limiting value at that point, the\nsolid-cylinder impedance matrix ${\\mathbf Z}_0$. We show that ${\\mathbf Z}_0$\nis a fundamental material property depending only on the elastic moduli and the\nazimuthal order $n$, that ${\\mathbf Z} (r)$ is Hermitian and ${\\mathbf Z}_0$ is\nnegative semi-definite. Explicit solutions for ${\\mathbf Z}_0$ are presented\nfor monoclinic and higher material symmetry, and the special cases of $n=0$ and\n1 are treated in detail. Two methods are proposed for finding ${\\mathbf Z}\n(r)$, one based on the Frobenius series solution and the other using a\ndifferential Riccati equation with ${\\mathbf Z}_0$ as initial value. %in a\nconsistent manner as the solution of an algebraic Riccati equation. The\nradiation impedance matrix is defined and shown to be non-Hermitian. These\nimpedance matrices enable concise and efficient formulations of dispersion\nequations for wave guides, and solutions of scattering and related wave\nproblems in cylinders.", "category": "cond-mat_mtrl-sci" }, { "text": "Micromagnetic simulations of spinel ferrite particles: This paper presents the results of simulations of the magnetization field\n{\\it ac} response (at $2$ to $12$ GHz) of various submicron ferrite particles\n(cylindrical dots). The ferrites in the present simulations have the spinel\nstructure, expressed here by M$_{1-n}$Zn$_{n}$Fe$_2$O$_4$ (where M stands for a\ndivalent metal), and the parameters chosen were the following: (a) for $n=0$: M\n= \\{ Fe, Mn, Co, Ni, Mg, Cu \\}; (b) for $n=0.1$: M = \\{ Fe, Mg \\} (mixed\nferrites). These runs represent full 3D micromagnetic (one-particle) ferrite\nsimulations. We find evidences of confined spin waves in all simulations, as\nwell as a complex behavior nearby the main resonance peak in the case of the M\n= \\{ Mg, Cu \\} ferrites. A comparison of the $n=0$ and $n=0.1$ cases for fixed\nM reveals a significant change in the spectra in M = Mg ferrites, but only a\nminor change in the M = Fe case. An additional larger scale simulation of a $3$\nby $3$ particle array was performed using similar conditions of the Fe$_3$O$_4$\n(magnetite; $n=0$, M = Fe) one-particle simulation. We find that the main\nresonance peak of the Fe$_3$O$_4$ one-particle simulation is disfigured in the\ncorresponding 3 by 3 particle simulation, indicating the extent to which\ndipolar interactions are able to affect the main resonance peak in that\nmagnetic compound.", "category": "cond-mat_mtrl-sci" }, { "text": "Picosecond acoustic excitation driven ultrafast magnetization dynamics\n in dielectric Bi-substituted yttrium iron garnet: Using femtosecond optical pulses, we have investigated the ultrafast\nmagnetization dynamics induced in a dielectric film of bismuth-substituted\nyttrium iron garnet (Bi-YIG) buried below a thick Cu/Pt metallic bilayer. We\nshow that exciting the sample from Pt surface launches an acoustic strain pulse\npropagating into the garnet film. We discovered that this strain pulse induces\na coherent magnetization precession in the Bi-YIG at the frequency of the\nferromagnetic resonance. The observed phenomena can be explain by\nstrain-induced changes of magnetocristalline anisotropy via the inverse\nmagnetostriction effect. These findings open new perspectives toward the\ncontrol of the magnetization in magnetic garnets embedded in complex\nheterostructure devices.", "category": "cond-mat_mtrl-sci" }, { "text": "Effect of thermal annealing on the heat transfer properties of reduced\n graphite oxide flakes: a nanoscale characterization via scanning thermal\n microscopy: This paper reports on the thermal properties of reduced graphite oxide (RGO)\nflakes, studied by means of scanning thermal microscopy (SThM). This technique\nwas demonstrated to allow thermal characterization of the flakes with a spatial\nresolution of the order of a few tens of nanometers, while recording nanoscale\ntopography at the same time. Several individual RGO flakes were analyzed by\nSThM, both as obtained after conventional thermal reduction and after a\nsubsequent annealing at 1700{\\deg}C. Significant differences in the thermal\nmaps were observed between pristine and annealed flakes, reflecting higher heat\ndissipation on annealed RGO flakes compared with pristine ones. This result was\ncorrelated with the reduction of RGO structure defectiveness. In particular, a\nsubstantial reduction of oxidized groups and sp3 carbons upon annealing was\nproven by X-ray photoelectron and Raman spectroscopies, while the increase of\ncrystalline order was demonstrated by X-ray diffraction, in terms of higher\ncorrelation lengths both along and perpendicular to the graphene planes.\nResults presented in this paper provide experimental evidence for the\nqualitative correlation between the defectiveness of graphene-related materials\nand their thermal conductivity, which is clearly crucial for the exploitation\nof these materials into thermally conductive nanocomposites.", "category": "cond-mat_mtrl-sci" }, { "text": "Chiral Spin Bobbers in Exchange-Coupled Hard-Soft Magnetic Bilayers: The spin structure of exchange-coupled MnBi:Co-Fe bilayers is investigated by\nX-ray magnetic circular dichroism (XMCD), polarized neutron reflectometry\n(PNR), and micromagnetic simu-lations. The purpose of the present research is\ntwo-fold. First, the current search for new permanent-magnet materials includes\nhard-soft nanocomposites, and the analysis of coercivity mechanisms in these\nstructures is an important aspect of this quest. Second, topological\nmicro-magnetic structures such as skyrmions have recently become of intense\nfundamental and applied research, for example in the context of spin-based\nelectronics. We find that the magnetization reversal of the MnBi:Co-Fe bilayer\nstructure involves a curling-type twisting of the magnetization in the film\nplane. This curling in the exchange-coupled hard-soft magnetic bilayers is\nreminiscent of chiral spin structures known as bobbers and, in fact,\nestablishes a new type of skyrmionic spin structure.", "category": "cond-mat_mtrl-sci" }, { "text": "A method to computationally screen for tunable properties of crystalline\n alloys: Conventionally, high-throughput computational materials searches start from\nan input set of bulk compounds extracted from material databases, and this set\nis screened for candidate materials for specific applications. In contrast,\nmany functional materials, and especially semiconductors, are heavily\nengineered alloys or solid solutions of multiple compounds rather than a single\nbulk compound. To improve our ability to design functional materials, in this\nwork we propose a framework and open-source code to automatically construct\npossible \"alloy pairs\" and \"alloy systems\" and detect \"alloy members\" from a\nset of existing, experimental or calculated ordered compounds, without\nrequiring any additional metadata beyond their crystal structure. We provide\nanalysis tools to estimate stability across each alloy. As a demonstration, we\napply this framework to all inorganic materials in the Materials Project\ndatabase to create a new database of over 600,000 unique alloy pair entries\nthat can then be used in materials discovery studies to search for materials\nwith tunable properties. This new database has been incorporated into the\nMaterials Project website and linked with corresponding material identifiers\nfor any user to query and explore. Using an example of screening for p-type\ntransparent conducting materials, we demonstrate how using this methodology\nreveals candidate material systems that might otherwise have been excluded by a\ntraditional screening. This work lays a foundation from which materials\ndatabases can go beyond stoichiometric compounds, and approach a more realistic\ndescription of compositionally tunable materials.", "category": "cond-mat_mtrl-sci" }, { "text": "Low-temperature thermal expansion of rock-salt ZnO: Lattice parameter of metastable high-pressure phase of zinc oxide, rock-salt\nZnO was measured in the 10-300 K temperature range using synchrotron X-ray\npowder diffraction. No phase transition was observed down to 10 K. The lattice\nparameter of rock-salt ZnO was found to increase from 4.266 {\\AA} in the 10-80\nK range up to 4.2752(3) {\\AA} at 298 K, while the volume thermal expansion\ncoefficient increases from slight negative values below 40 K up to\n4.77\\times10^-5 K^-1 at 298 K.", "category": "cond-mat_mtrl-sci" }, { "text": "Impact of lattice rotation on dislocation motion: We introduce a phenomenological theory of dislocation motion appropriate for\ntwo dimensional lattices. A coarse grained description is proposed that\ninvolves as primitive variables local lattice rotation and Burgers vector\ndensities along distinguished slip systems of the lattice. We then use symmetry\nconsiderations to propose phenomenological equations for both defect energies\nand their dissipative motion. As a consequence, the model includes explicit\ndependences on the local state of lattice orientation, and allows for\ndifferential defect mobilities along distinguished directions. Defect densities\nand lattice rotation need to determined self consistently and we show specific\nresults for both square and hexagonal lattices. Within linear response,\ndissipative equations of motion for the defect densities are derived which\ncontain defect mobilities that depend nonlocally on defect distribution.", "category": "cond-mat_mtrl-sci" }, { "text": "Vanadium Dioxide: Metal-Insulator Transition, Electrical Switching and\n Oscillations. A Review of State of the Art and Recent Progress: Vanadium dioxide is currently considered as one of the most promising\nmetarials for oxide elcteronics. Both planar and sandwich thin-film MOM devices\nbased on VO2 exhibit electrical switching with an S-shaped I-V characteristic,\nand this switching effect is associated with the metal-insulator transition\n(MIT). In an electrical circuit containing such a switching device, relaxation\noscillations are observed if the load line intersects the I-V curve at a unique\npoint in NDR region. All these effects are potentially prospective for\ndesigning various devices of oxide electronics, particularly, elements of\ndynamical neural networks based on coupled oscillators.", "category": "cond-mat_mtrl-sci" }, { "text": "Anionic nickel and nitrogen effects in the chiral antiferromagnetic\n antiperovskite Mn$_3$NiN: Magnetic antiperovskites, holding chiral noncollinear antiferromagnetic\nordering, have shown remarkable properties that cover from negative thermal\nexpansion to anomalous Hall effect. Nevertheless, details on the electronic\nstructure related to the oxidation states and the octahedral center's site\neffect are still scarce. Here, we show a theoretical study, based on\nfirst-principles calculations in the framework of the density-functional\ntheory, DFT, on the electronic details associated with the nitrogen site effect\ninto the structural, electronic, magnetic, and topological degrees of freedom.\nThus, we show that the nitrogen-vacancy increases the values of the anomalous\nHall conductivity and retains the chiral $\\Gamma_{4g}$ antiferromagnetic\nordering. Moreover, we reveal, based on the Bader charges and the electronic\nstructure analysis, the negative and positive oxidation states in the Ni and Mn\nsites, respectively. The latter is in agreement with the expected\n$A_3^{\\alpha+}B^{\\beta-}X^{\\delta-}$ oxidation states to satisfy the charge\nneutrality in the antiperovskites, but rare for transition metals. Finally, we\nextrapolate our findings on the oxidation states to several Mn$_3B$N compounds\nshowing that the antiperovskite structure is an ideal platform to encounter\nnegative oxidation states in metals sitting at the corner $B$-site.", "category": "cond-mat_mtrl-sci" }, { "text": "Ion Intercalation in Lanthanum Strontium Ferrite for Aqueous\n Electrochemical Energy Storage Devices: Ion intercalation of perovskite oxides in liquid electrolytes is a very\npromising method for controlling their functional properties while storing\ncharge, which opens the potential application in different energy and\ninformation technologies. Although the role of defect chemistry in the oxygen\nintercalation in a gaseous environment is well established, the mechanism of\nion intercalation in liquid electrolytes at room temperature is poorly\nunderstood. In this study, the defect chemistry during ion intercalation of\nLa0.5Sr0.5FeO3-{\\delta} thin films in alkaline electrolytes is studied. Oxygen\nand proton intercalation into the LSF perovskite structure is observed at\nmoderate electrochemical potentials (0.5 V to -0.4 V), giving rise to a change\nin the oxidation state of Fe (as a charge compensation mechanism). The\nvariation of the concentration of holes as a function of the intercalation\npotential was characterized by in-situ ellipsometry and the concentration of\nelectron holes was indirectly quantified for different electrochemical\npotentials. Finally, a dilute defect chemistry model that describes the\nvariation of defect species during ionic intercalation was developed.", "category": "cond-mat_mtrl-sci" }, { "text": "Lattice Dynamics Calculations based on Density-functional Perturbation\n Theory in Real Space: A real-space formalism for density-functional perturbation theory (DFPT) is\nderived and applied for the computation of harmonic vibrational properties in\nmolecules and solids. The practical implementation using numeric atom-centered\norbitals as basis functions is demonstrated exemplarily for the all-electron\nFritz Haber Institute ab initio molecular simulations (FHI-aims) package. The\nconvergence of the calculations with respect to numerical parameters is\ncarefully investigated and a systematic comparison with finite-difference\napproaches is performed both for finite (molecules) and extended (periodic)\nsystems. Finally, the scaling tests and scalability tests on massively parallel\ncomputer systems demonstrate the computational efficiency.", "category": "cond-mat_mtrl-sci" }, { "text": "Two-dimensional Graphene Heterojunctions: the Tunable Mechanical\n Properties: We report the mechanical properties of different two-dimensional carbon\nheterojunctions (HJs) made from graphene and various stable graphene\nallotropes, including {\\alpha}-, {\\beta}-, {\\gamma}- and 6612-graphyne (GY),\nand graphdiyne (GDY). It is found that all HJs exhibit a brittle behaviour\nexcept the one with {\\alpha}-GY, which however shows a hardening process due to\nthe formation of triple carbon rings. Such hardening process has greatly\ndeferred the failure of the structure. The yielding of the HJs is usually\ninitiated at the interface between graphene and graphene allotropes, and\nmonoatomic carbon rings are normally formed after yielding. By varying the\nlocations of graphene (either in the middle or at the two ends of the HJs),\nsimilar mechanical properties have been obtained, suggesting insignificant\nimpacts from location of graphene allotropes. Whereas, changing the types and\npercentages of the graphene allotropes, the HJs exhibit vastly different\nmechanical properties. In general, with the increasing graphene percentage, the\nyield strain decreases and the effective Young's modulus increases. Meanwhile,\nthe yield stress appears irrelevant with the graphene percentage. This study\nprovides a fundamental understanding of the tensile properties of the\nheterojunctions that are crucial for the design and engineering of their\nmechanical properties, in order to facilitate their emerging future\napplications in nanoscale devices, such as flexible/stretchable electronics.", "category": "cond-mat_mtrl-sci" }, { "text": "Phase-Modulated Elastic Properties of Two-Dimensional Magnetic FeTe:\n Hexagonal and Tetragonal Polymorphs: Two-dimensional (2D) layered magnets, such as iron chalcogenides, have\nemerged these years as a new family of unconventional superconductor and\nprovided the key insights to understand the phonon-electron interaction and\npairing mechanism. Their mechanical properties are of strategic importance for\nthe potential applications in spintronics and optoelectronics. However, there\nis still lack of efficient approach to tune the elastic modulus despite the\nextensive studies. Herein, we report the modulated elastic modulus of 2D\nmagnetic FeTe and its thickness-dependence via phase engineering. The grown 2D\nFeTe by chemical vapor deposition can present various polymorphs, i.e.\ntetragonal FeTe (t-FeTe, antiferromagnetic) and hexagonal FeTe (h-FeTe,\nferromagnetic). The measured Young's modulus of t-FeTe by nanoindentation\nmethod showed an obvious thickness-dependence, from 290.9+-9.2 to 113.0+-8.7\nGPa when the thicknesses increased from 13.2 to 42.5 nm, respectively. In\ncomparison, the elastic modulus of h-FeTe remains unchanged. Our results could\nshed light on the efficient modulation of mechanical properties of 2D magnetic\nmaterials and pave the avenues for their practical applications in nanodevices.", "category": "cond-mat_mtrl-sci" }, { "text": "Thermoelectric probe of defect state induced by ionic liquid gating in\n vanadium dioxide: Thermoelectric measurements detect the asymmetry between the density of\nstates above and below the chemical potential in a material. It provides\ninsights into small variations in the density of states near the chemical\npotential, complementing electron transport measurements. Here, combined\nresistance and thermoelectric power measurements are performed on vanadium\ndioxide (VO2), a prototypical correlated electron material, under ionic-liquid\n(IL) gating. With IL gating, charge transport below the\nmetal-to-insulator-transition (MIT) temperature remains in the thermally\nactivated regime, while the Seebeck coefficient exhibits an apparent transition\nfrom semiconducting to metallic behavior. The contrasting behavior indicates\nchanges in electronic structure upon IL gating, due to the formation of oxygen\ndefect states. The experimental results are corroborated by numerical\nsimulations based on a model density of states incorporating a gating induced\ndefect band. This study reveals thermoelectric measurements to be a convenient\nand sensitive probe for the role of defect states induced by IL gating in\nsuppressing the MIT in VO2, which remains benign in charge transport\nmeasurements, and possibly for studying defect sates in other materials.", "category": "cond-mat_mtrl-sci" }, { "text": "Intermediate anomalous Hall states induced by noncollinear spin\n structure in magnetic topological insulator MnBi2Te4: The combination of topology and magnetism is attractive to produce exotic\nquantum matters, such as the quantum anomalous Hall state, axion insulators and\nthe magnetic Weyl semimetals. MnBi2Te4, as an intrinsic magnetic topological\ninsulator, provides a platform for the realization of various topological\nphases. Here we report the intermediate Hall steps in the magnetic hysteresis\nof MnBi2Te4, where four distinguishable magnetic memory states at zero magnetic\nfield are revealed. The gate and temperature dependence of the magnetic\nintermediate states indicates the noncollinear spin structure in MnBi2Te4,\nwhich can be attributed to the Dzyaloshinskii-Moriya interaction as the\ncoexistence of strong spin-orbit coupling and local inversion symmetry breaking\non the surface. Moreover, these multiple magnetic memory states can be\nprogrammatically switched among each other through applying designed pulses of\nmagnetic field. Our results provide new insights of the influence of bulk\ntopology on the magnetic states, and the multiple memory states should be\npromising for spintronic devices.", "category": "cond-mat_mtrl-sci" }, { "text": "Ferroelectric field effect of the bulk heterojunction in polymer solar\n cells: A ferroelectric field effect in the bulk heterojunction was found when an\nexternal electric field (EEF) was applied on the active layer of polymer solar\ncells (PSCs) during the annealing process of the active layer spin-coated with\npoly (3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM).\nFor one direction field, the short circuit current density of PSCs was improved\nfrom 7.2 to 8.0 mA/cm2, the power conversion efficiency increased from 2.4 to\n2.8%, and the incident photon-to-current conversion efficiency increased from\n42 to 49% corresponding to the different EEF magnitude. For an opposite\ndirection field, the applied EEF brought a minus effect on the performance\nmentioned above. EEF treatment can orientate molecular ordering of the polymer,\nand change the morphology of the active layer. The authors suggest a\nexplanation that the ferroelectric field has been built in the active layer,\nand therefore it plays a key role in PSCs system. A needle-like surface\nmorphology of the active film was also discussed.", "category": "cond-mat_mtrl-sci" }, { "text": "Giant enhanced optical nonlinearity of colloidal nanocrystals with a\n graded-index host: The effective linear and third-order nonlinear optical properties of metallic\ncolloidal crystal immersed in a graded-index host fluid are investigated\ntheoretically. The local electric fields are extracted self-consistently based\non the layer-to-layer interactions, which are readily given by the Lekner\nsummation method. The resultant optical absorption and nonlinearity enhancement\nshow a series of sharp peaks, which merge in a broadened resonant band. The\nsharp peaks become a continuous band for increasing packing density and number\nof layers. We believe that the sharp peaks arise from the in-plane dipolar\ninteractions and the surface plasmon resonance, whereas the continuous band is\ndue to the presence of the gradient in the host refractive index. These results\nhave not been observed in homogeneous and randomly-dispersed colloids, and thus\nwould be of great interest in optical nanomaterial engineering.", "category": "cond-mat_mtrl-sci" }, { "text": "Left handed materials: We review recent progress in the studies of left handed materials.", "category": "cond-mat_mtrl-sci" }, { "text": "On the preparation and NMR spectroscopic characterization of potassium\n aluminium tetrahydride KAlH4: Potassium aluminium tetrahydride KAlH4 of high phase purity (space group Pnma\n(62)) was synthesized via a mechanochemical route. The thus obtained material\nwas studied by 27Al and 39K MAS NMR spectroscopy. For both nuclei precise data\nfor the isotropic chemical shift and the quadrupole coupling at T=295 K were\nderived (27Al: delta_iso=(107.6+-0.2) ppm, C_Q = (1.29+-0.02) MHz and eta =\n0.64+-0.02; 39K: delta_iso=(6.1+-0.2) ppm, C_Q = (0.562+-0.005) MHz and eta =\n0.74+-0.02). The straightforward NMR spectroscopic approach applied here should\nalso work for other complex aluminium hydrides and for many other materials\ncontaining half-integer nuclei experiencing small to medium-sized quadrupole\ncouplings.", "category": "cond-mat_mtrl-sci" }, { "text": "Acoustic phonon scattering in a low density, high mobility AlGaN/GaN\n field effect transistor: We report on the temperature dependence of the mobility, $\\mu$, of the\ntwo-dimensional electron gas in a variable density AlGaN/GaN field effect\ntransistor, with carrier densities ranging from 0.4$\\times10^{12}$ cm$^{-2}$ to\n3.0$\\times10^{12}$ cm$^{-2}$ and a peak mobility of 80,000 cm$^{2}$/Vs. Between\n20 K and 50 K we observe a linear dependence $\\mu_{ac}^{-1} = \\alpha$T\nindicating that acoustic phonon scattering dominates the temperature dependence\nof the mobility, with $\\alpha$ being a monotonically increasing function of\ndecreasing 2D electron density. This behavior is contrary to predictions of\nscattering in a degenerate electron gas, but consistent with calculations which\naccount for thermal broadening and the temperature dependence of the electron\nscreening. Our data imply a deformation potential D = 12-15 eV.", "category": "cond-mat_mtrl-sci" }, { "text": "Energetics of the oxidation and opening of a carbon nanotube: We apply first principles calculations to study the opening of single-wall\ncarbon nanotubes (SWNT's) by oxidation. We show that an oxygen rim can\nstabilize the edge of the open tube. The sublimation of CO$_2$ molecules from\nthe rim with the subsequent closing of the tube changes from endothermic to\nexothermic as the tube radius increases, within the range of experimental\nfeasible radii. We also obtain the energies for opening the tube at the cap and\nat the wall, the latter being significantly less favorable.", "category": "cond-mat_mtrl-sci" }, { "text": "Stress-Induced Phase Transitions in Nanoscale CuInP$_2$S$_6$: Using Landau-Ginsburg-Devonshire approach and available experimental results\nwe reconstruct the thermodynamic potential of the layered ferroelectric\nCuInP$_2$S$_6$ (CIPS), which is expected to be applicable a wide range of\ntemperatures and applied pressures. The analysis of temperature dependences of\nthe dielectric permittivity and lattice constants for different applied\npressures unexpectedly reveals the critically important role of the nonlinear\nelectrostriction in this material. With the nonlinear electrostriction included\nwe calculated temperature and pressure phase diagrams and spontaneous\npolarization of bulk CIPS. Using the coefficients of the reconstructed\nfour-well thermodynamic potential, we study the strain-induced phase\ntransitions in thin epitaxial CIPS films, as well as the stress-induced phase\ntransitions in CIPS nanoparticles, which shape varies from prolate needles to\noblate disks. We reveal the strong influence of the mismatch strain, elastic\nstress and shape anisotropy on the polar properties and phase diagrams of\nnanoscale CIPS. Also, we derived analytical expressions, which allow the\nelastic control of the nanoscale CIPS polar properties. Hence obtained results\ncan be of particular interest for the strain-engineering of nanoscale layered\nnanoferroelectrics.", "category": "cond-mat_mtrl-sci" }, { "text": "A multimodal operando neutron study of the phase evolution in a graphite\n electrode: Obtaining a complete picture of local processes still poses a significant\nchallenge in battery research. Here we demonstrate an in-situ combination of\nmultimodal neutron imaging with neutron diffraction for spatially resolved\noperando observations of the lithiation-delithiation of a graphite electrode in\na Li-ion battery cell. Throughout the lithiation-delithiation process we image\nthe Li distribution based on the local beam attenuation. Simultaneously, we\nobserve the development of the lithiated graphite phases as a function of\ncycling time and electrode thickness and integral throughout its volume by\ndiffraction contrast imaging and diffraction, respectively. While the\nconventional imaging data allows to observe the Li uptake in graphite already\nduring the formation of the solid electrolyte interphase, diffraction indicates\nthe onset and development of the Li insertion/extraction globally, which\nsupports the local structural transformation observations by diffraction\ncontrast imaging.", "category": "cond-mat_mtrl-sci" }, { "text": "Pressure-induced Lifshitz transition in NbP: Raman, x-ray diffraction,\n electrical transport and density functional theory: We report high pressure Raman, synchrotron x-ray diffraction and electrical\ntransport studies on Weyl semimetals NbP and TaP along with first-principles\ndensity functional theoretical (DFT) analysis. The frequencies of first-order\nRaman modes of NbP harden with increasing pressure and exhibit a slope change\nat P$_c$ $\\sim$ 9 GPa, and its resistivity exhibits a minimum at P$_c$. The\npressure-dependent volume of NbP exhibits a change in its bulk modulus from 207\nGPa to 243 GPa at P$_c$. Using DFT calculations, we show that these anomalies\nare associated with pressure induced Lifshitz transition which involves\nappearance of electron and hole pockets in its electronic structure. In\ncontrast, results of Raman and synchrotron x-ray diffraction experiments on TaP\nand DFT calculations show that TaP is quite robust under pressure and does not\nundergo any phase transition.", "category": "cond-mat_mtrl-sci" }, { "text": "Moir\u00e9 pattern formation in epitaxial growth on a covalent substrate:\n Sb on InSb(111)A: Structural moir\\'e superstructures arising from two competing lattices may\nlead to unexpected electronic behavior, such as superconductivity or Mottness.\nMost investigated moir\\'e heterostructures are based on van der Waals (vdW)\nmaterials, as strong interface interactions typically lead to the formation of\nstrained films or regular surface reconstructions. Here we successfully\nsynthesize ultrathin Sb films, that are predicted to show thickness-dependent\ntopological properties, on semi-insulating InSb(111)A. Despite the covalent\nnature of the substrate surface, we prove by scanning transmission electron\nmicroscopy (STEM) that already the first layer of Sb atoms grows completely\nunstrained, while azimuthally aligned. Rather than compensating the lattice\nmismatch of -6.4% by structural modifications, the Sb films form a pronounced\nmoir\\'e pattern as we evidence by scanning tunneling microscopy (STM)\ntopography up to film thicknesses of several bilayers. Our model calculations\nbased on density functional theory (DFT) assign the moir\\'e pattern to a\nperiodic surface corrugation. In agreement with DFT predictions, irrespective\nof the moir\\'e modulation, the topological surface state known on thick Sb film\nis experimentally confirmed to persist down to low film thicknesses, and the\nDirac point shifts towards lower binding energies with decreasing Sb thickness.", "category": "cond-mat_mtrl-sci" }, { "text": "An estimate for thermal diffusivity in highly irradiated tungsten using\n Molecular Dynamics simulation: The changing thermal conductivity of an irradiated material is among the\nprincipal design considerations for any nuclear reactor, but at present few\nmodels are capable of predicting these changes starting from an arbitrary\natomistic model. Here we present a simple model for computing the thermal\ndiffusivity of tungsten, based on the conductivity of the perfect crystal and\nresistivity per Frenkel pair, and dividing a simulation into perfect and\nathermal regions statistically. This is applied to highly irradiated\nmicrostructures simulated with Molecular Dynamics. A comparison to experiment\nshows that simulations closely track observed thermal diffusivity over a range\nof doses from the dilute limit of a few Frenkel pairs to the high dose\nsaturation limit at 3 displacements per atom (dpa).", "category": "cond-mat_mtrl-sci" }, { "text": "Effect of spin-orbit interaction on the excitonic effects in\n single-layer, double-layer, and bulk MoS2: We present converged ab-initio calculations of the optical absorption spectra\nof single-layer, bi-layer, and bulk MoS$_2$. Both the quasiparticle-energy\ncalculations (on the level of the GW approximation) and the calculation of the\nabsorption spectra (on the level of the Bethe-Salpeter equation) explicitly\ninclude spin-orbit coupling, using the full spinorial Kohn-Sham wave-functions\nas input. Without excitonic effects, the absorption spectra would have the form\nof a step-function, corresponding to the joint-density of states of a parabolic\nband-dispersion in 2D. This profile is deformed by a pronounced bound excitonic\npeak below the continuum onset. The peak is split by spin-orbit interaction in\nthe case of single-layer and (mostly) by inter-layer interaction in the case of\ndouble-layer and bulk MoS$_2$. The resulting absorption spectra are thus very\nsimilar in the three cases but the interpretation of the spectra is different.\nDifferences in the spectra can be seen around 3 eV where the spectra of single\nand double-layer are dominated by a strongly bound exciton.", "category": "cond-mat_mtrl-sci" }, { "text": "Spatio-Temporal Electron Propagation Dynamics in Au/Fe/MgO(001) in\n nonequilibrium: Revealing Single Scattering Events and the Ballistic Limit: Understanding the microscopic spatio-temporal dynamics of nonequilibrium\ncharge carriers in heterosystems promises optimization of process and device\ndesign towards desired energy transfer. Hot electron transport is governed by\nscattering with other electrons, defects, and bosonic excitations. Analysis of\nthe energy dependence of scattering pathways and identification of diffusive,\nsuper-diffusive, and ballistic transport regimes are current challenges. We\ndetermine in femtosecond time-resolved two-photon photoelectron emission\nspectroscopy the energy-dependent change of the electron propagation time\nthrough epitaxial Au/Fe(001) heteostructures as a function of Au layer\nthickness for energies of 0.5 to \\unit[2.0]{eV} above the Fermi energy. We\ndescribe the laser-induced nonequilibrium electron excitation and injection\nacross the Fe/Au interface using real-time time-dependent density functional\ntheory and analyze the electron propagation through the Au layer by microscopic\nelectron transport simulations. We identify ballistic transport of minority\nelectrons at energies with a nascent, optically excited electron population\nwhich is determined by the combination of photon energy and the specific\nelectronic structure of the material. At lower energy, super-diffusive\ntransport with 1 to 4 scattering events dominates. The effective electron\nvelocity accelerates from 0.3 to \\unit[1]{nm/fs} with an increase in the Au\nlayer thickness from 10 to 100~nm. This phenomenon is explained by electron\ntransport that becomes preferentially aligned with the interface normal for\nthicker Au layers, which facilitates electron momentum / energy selection by\nchoice of the propagation layer thickness.", "category": "cond-mat_mtrl-sci" }, { "text": "Properties of heavy rare-gases adlayers on graphene substrates: We investigated properties of heavy rare-gases, Ne, Ar, Kr, Xe and Rn,\nadsorbed on graphene substrates using molecular dynamics. We gathered evidences\nof commensurate solids for Ne and Kr adlayers, one of them is given by a\ntypical behavior of the nearest neighbor distance of the adatoms. The specific\nheat and the melting temperature were calculated and both indicate continuous\nmelting for all heavy noble-gases studied. We also determined the distance\nbetween the adlayer and the substrate.", "category": "cond-mat_mtrl-sci" }, { "text": "Intrinsic interfacial van der Waals monolayers and their effect on the\n high-temperature superconductor FeSe/SrTiO$_3$: The sensitive dependence of monolayer materials on their environment often\ngives rise to unexpected properties. It was recently demonstrated that\nmonolayer FeSe on a SrTiO$_3$ substrate exhibits a much higher superconducting\ncritical temperature T$_C$ than the bulk material. Here, we examine the\ninterfacial structure of FeSe / SrTiO$_3$ and the effect of an interfacial\nTi$_{1+x}$O$_2$ layer on the increased T$_C$ using a combination of scanning\ntransmission electron microscopy and density functional theory. We find\nTi$_{1+x}$O$_2$ forms its own quasi-two-dimensional layer, bonding to both the\nsubstrate and the FeSe film by van der Waals interactions. The excess Ti in\nthis layer electron-dopes the FeSe monolayer in agreement with experimental\nobservations. Moreover, the interfacial layer introduces symmetry-breaking\ndistortions in the FeSe film that favor a T$_C$ increase. These results suggest\nthat this common substrate may be functionalized to modify the electronic\nstructure of a variety of thin films and monolayers.", "category": "cond-mat_mtrl-sci" }, { "text": "A meta-analysis of the mechanical properties of ice-templated ceramics\n and metals: Ice templating, also known as freeze casting, is a popular shaping route for\nmacroporous materials. Over the past 15 years, it has been widely applied to\nvarious classes of materials, and in particular ceramics. Many formulation and\nprocess parameters, often interdependent, affect the outcome. It is thus\ndifficult to understand the various relationships between these parameters from\nisolated studies where only a few of these parameters have been investigated.\nWe report here the results of a meta analysis of the structural and mechanical\nproperties of ice templated materials from an exhaustive collection of records.\nWe use these results to identify which parameters are the most critical to\ncontrol the structure and properties, and to derive guidelines to optimize the\nmechanical response of ice templated materials. We hope these results will be a\nhelpful guide to anyone interested in such materials.", "category": "cond-mat_mtrl-sci" }, { "text": "Tunable surface configuration of skyrmion lattices in cubic helimagnets: In bulk helimagnets, the presence of magnetic skyrmion lattices is always\naccompanied by a periodic stress field due to the intrinsic magnetoelastic\ncoupling. The release of this nontrivial stress field at the surface causes a\nperiodic displacement field, which characterizes a novel particle-like property\nof skyrmion: its surface configuration. Here, we derive the analytical solution\nof this displacement field for semi-infinite cubic helimagnets when skyrmions\nare present. For MnSi, we show that the skyrmion lattices have a bumpy surface\nconfiguration characterized by periodically arranged peaks with a\ncharacteristic height of about 10$^{-13}$ m. The pattern of the peaks can be\ncontrolled by varying the strength of the applied magnetic field. Moreover, we\nprove that the surface configuration varies together with the motion and\ndeformation of the skyrmion lattices. As a result, the surface configuration\ncan be tuned by application of electric current, mechanical loads, as well as\nany other form of external field which has an effect on the skyrmions.", "category": "cond-mat_mtrl-sci" }, { "text": "Spin splitting and strain in epitaxial monolayer WSe$_2$ on graphene: We present the electronic and structural properties of monolayer WSe$_{2}$\ngrown by pulsed-laser deposition on monolayer graphene (MLG) on SiC. The spin\nsplitting in the WSe$_{2}$ valence band at $\\overline{\\mathrm{K}}$ was\n$\\Delta_\\mathrm{SO}=0.469\\pm0.008$ eV by angle-resolved photoemission\nspectroscopy (ARPES). Synchrotron-based grazing-incidence in-plane X-ray\ndiffraction (XRD) revealed the in-plane lattice constant of monolayer WSe$_{2}$\nto be $a_\\mathrm{WSe_2}=3.2757\\pm0.0008 \\mathrm{\\r{A}}$. This indicates a\nlattice compression of -0.19 % from bulk WSe$_{2}$. By using experimentally\ndetermined graphene lattice constant ($a_\\mathrm{MLG}=2.4575\\pm0.0007\n\\mathrm{\\r{A}}$), we found that a 3$\\times$3 unit cell of the slightly\ncompressed WSe$_{2}$ is perfectly commensurate with a 4$\\times$4 graphene\nlattice with a mismatch below 0.03 %, which could explain why the monolayer\nWSe$_{2}$ is compressed on MLG. From XRD and first-principles calculations,\nhowever, we conclude that the observed size of strain is negligibly small to\naccount for a discrepancy in $\\Delta_\\mathrm{SO}$ found between exfoliated and\nepitaxial monolayers in earlier ARPES. In addition, angle-resolved, ultraviolet\nand X-ray photoelectron spectroscopy shed light on the band alignment between\nWSe$_{2}$ and MLG/SiC and indicate electron transfer from graphene to the\nWSe$_{2}$ monolayer. As further revealed by atomic force microscopy, the\nWSe$_{2}$ island size depends on the number of carbon layers on top of the SiC\nsubstrate. This suggests that the epitaxy of WSe$_{2}$ favors the weak van der\nWaals interactions with graphene while it is perturbed by the influence of the\nSiC substrate and its carbon buffer layer.", "category": "cond-mat_mtrl-sci" }, { "text": "Emergence of Type-I and Type-II Dirac line nodes in penta-octa-graphene: Carbon allotropes have a large family of materials with varieties of crystal\nstructures and properties and can realize different topological phases. Using\nfirst principles calculations, we predict a new two-dimensional (2D) carbon\nallotrope, namely penta-octa-graphene, which consists of pentagonal and\noctagonal carbon rings. We find that penta-octa-graphene can host both type-I\nand type-II Dirac line nodes (DLNs). The band inversion between conduction and\nvalence bands forms the type-I DLNs and the two highest valence bands form the\ntype-II DLNs. We find that the type-I DLNs are robust to the biaxial strain and\nthe type-II DLNs can be driven to type-I when applying over 3 $\\%$ biaxial\nstretching strain. A lattice model based on the $\\pi$ orbitals of carbons is\nderived to understand the coexistence mechanism of type-I and type-II DLNs in\npenta-octa-graphene. Possible realizations and characterizations of this\npenta-octa-graphene in the experiment are also discussed. Our findings shed new\nlight on the study of the coexistence of multiple topological states in the 2D\ncarbon allotropes.", "category": "cond-mat_mtrl-sci" }, { "text": "Thickness of the air-water interface from first-principles\n simulation-based hydrogen bond dynamics: The thickness of the air-water interface is determined by interface hydrogen\nbond (HB) dynamics. By density functional theory-based molecular dynamics\n(DFTMD) simulations, two extreme cases of the interface HB dynamics are\nobtained: one underestimates the HB breaking rate constant and the other\noverestimates it. The interface HB dynamics in these two cases tends to be the\nsame as the thickness of the air-water interface increases to 4 Angstroms. The\ninterface thickness is determined when the interface HB dynamics under the two\ncases is converged.", "category": "cond-mat_mtrl-sci" }, { "text": "Chemical trends of substitutional transition metal dopants in diamond:\n an ab initio study: The electronic and magnetic properties of neutral substitutional\ntransition-metal dopants in dia- mond are calculated within density functional\ntheory using the generalized gradient approximation to the exchange-correlation\npotential. Ti and Fe are nonmagnetic, whereas the ground state of V, Cr and Mn\nare magnetic with a spin entirely localized on the magnetic ion. For Co, Ni,\nand Cu, the ground state is magnetic with the spin distributed over the\ntransition-metal ion and the nearest-neighbor carbon atoms; furthermore a bound\nstate is found in the gap that originates from the hybridization of the\n3d-derived level of the dopant and the 2p-derived dangling bonds of the\nnearest-neighbor carbons. A p{d hybridization model is developed in order to\ndescribe the origin of the magnetic interaction. This model predicts high-spin\nto low-spin transitions for Ni and Cu under compressive strain.", "category": "cond-mat_mtrl-sci" }, { "text": "Brillouin zone spin filtering mechanism of enhanced TMR and correlation\n effects in Co(0001)/h-BN/Co(0001) magnetic tunnel junction: The 'Brillouin zone spin filtering' mechanism of enhanced tunneling\nmagnetoresistance (TMR) is described for magnetic tunnel junctions (MTJ) and\nstudied on an example of the MTJ with hcp Co electrodes and hexagonal BN (h-BN)\nspacer. Our calculations based on local density approximation of density\nfunctional theory (LDA-DFT) for Co(0001)/h-BN/Co(0001) MTJ predict high TMR in\nthis device due to Brillouin zone filtering mechanism. Owning to the specific\ncomplex band structure of the h-BN the spin-dependent tunneling conductance of\nthe system is ultra-sensitive to small variations of the Fermi energy position\ninside the BN band gap. Doping of the BN and, consequentially, changing the\nFermi energy position could lead to variation of the TMR by several orders of\nmagnitude. We show also that taking into account correlation effects on beyond\nDFT level is required to accurately describe position of the Fermi level and\nthus transport propertied of the system. Our study suggests that new MTJ based\non hcp Co-Pt or Co-Pd disordered alloy electrodes and p-doped hexagonal BN\nspacer is a promising candidate for the spin-transfer torque magnetoresistive\nrandom-access memory (STT-MRAM).", "category": "cond-mat_mtrl-sci" }, { "text": "Effect of Layer-Stacking on the Electronic Structure of Graphene\n Nanoribbons: The evolution of electronic structure of graphene nanoribbons (GNRs) as a\nfunction of the number of layers stacked together is investigated using\n\\textit{ab initio} density functional theory (DFT) including interlayer van der\nWaals interactions. Multilayer armchair GNRs (AGNRs), similar to single-layer\nAGNRs, exhibit three classes of band gaps depending on their width. In zigzag\nGNRs (ZGNRs), the geometry relaxation resulting from interlayer interactions\nplays a crucial role in determining the magnetic polarization and the band\nstructure. The antiferromagnetic (AF) interlayer coupling is more stable\ncompared to the ferromagnetic (FM) interlayer coupling. ZGNRs with the AF\nin-layer and AF interlayer coupling have a finite band gap while ZGNRs with the\nFM in-layer and AF interlayer coupling do not have a band gap. The ground state\nof the bi-layer ZGNR is non-magnetic with a small but finite band gap. The\nmagnetic ordering is less stable in multilayer ZGNRs compared to single-layer\nZGNRs. The quasipartcle GW corrections are smaller for bilayer GNRs compared to\nsingle-layer GNRs because of the reduced Coulomb effects in bilayer GNRs\ncompared to single-layer GNRs.", "category": "cond-mat_mtrl-sci" }, { "text": "Raman thresholds and rigid to floppy transitions in calcium silicate\n glasses: Alkaline earth silicate glasses $xCaO-(1-x)SiO_2$ exhibit a well marked\nthreshold in Raman lineshapes which can be related to the onset of network\nrigidity as the concentration of calcium oxide $x$ is decreased. The present\nresults are analyzed by constraint counting algorithms and more deeply\ncharacterized by a size increasing cluster approximation that allows to perform\nMaxwell mechanical constraint counting beyond the usual mean-field treatment.\nThis permits to discuss under which structural conditions an elastic\nintermediate phase can be obtained.", "category": "cond-mat_mtrl-sci" }, { "text": "Theoretical assessment on the possibility of constraining point defect\n energetics by pseudo-phase transition pressures: Making use of the energetics and equations of state of defective uranium\ndioxide that calculated with first-principles method, we demonstrate a\npossibility of constraining the formation energy of point defects by measuring\nthe transition pressures of the corresponding pseudo-phase of defects. The\nmechanically stable range of fluorite structure of UO2, which dictates the\nmaximum possible pressure of relevant pseudo-phase transitions, gives rise to\ndefect formation energies that span a wide band and overlap with the existing\nexperimental estimates. We reveal that the knowledge about pseudo-phase\nboundaries can not only provide important information of energetics that is\nhelpful for reducing the scattering in current estimates, but also be valuable\nfor guiding theoretical assessments, even to validate or disprove a theory. In\norder to take defect interactions into account and to extrapolate the physical\nquantities at finite stoichiometry deviations to that near the stoichiometry,\nwe develop a general formalism to describe the thermodynamics of a defective\nsystem. We also show that it is possible to include interactions among defects\nin a simple expression of point defect model (PDM) by introducing an auxiliary\nconstant mean-field. This generalization of the simple PDM leads to great\nversatility that allows one to study nonlinear effects of stoichiometry\ndeviation on materials' behavior. It is a powerful tool to extract the defect\nenergetics from finite defect concentrations to the dilute limit. Besides\nthese, the full content of the theoretical formalism and some relevant and\ninteresting issues, including reentrant pseudo-transition, multi-defect\ncoexistence, charged defects, and possible consequence of instantaneous\ndefective response in a quantum crystal, are explored and discussed.", "category": "cond-mat_mtrl-sci" }, { "text": "One million percent tunnel magnetoresistance in a magnetic van der Waals\n heterostructure: We report the observation of a very large negative magnetoresistance effect\nin a van der Waals tunnel junction incorporating a thin magnetic semiconductor,\nCrI3, as the active layer. At constant voltage bias, current increases by\nnearly one million percent upon application of a 2 Tesla field. The effect\narises from a change between antiparallel to parallel alignment of spins across\nthe different CrI3 layers. Our results elucidate the nature of the magnetic\nstate in ultrathin CrI3 and present new opportunities for spintronics based on\ntwo-dimensional materials.", "category": "cond-mat_mtrl-sci" }, { "text": "Van der Waals Stacking Induced Topological Phase Transition in Layered\n Ternary Transition Metal Chalcogenides: Novel materials with nontrivial electronic and photonic band topology are\ncrucial for realizing novel devices with low power consumption and heat\ndissipation, and quantum computing free of decoherence. Here using\nfirst-principles approach, we predict a class of ternary transition metal\nchalcogenides (TTMC) MM'Te$_4$ exhibits dual topological characteristics:\nquantum spin Hall (QSH) insulators in their 2D monolayers and topological Weyl\nsemimetals in their 3D noncentrosymmetric crystals upon van der Waals (vdW)\nstacking. Remarkably, we find that one can create and annihilate Weyl fermions,\nand realize the transition between Type-I and Type-II Weyl fermions by tuning\nvdW interlayer spacing. Our calculations show that they possess excellent\nthermodynamic stability and weak interlayer binding, implying their great\npotentials for experimental synthesis, direct exfoliation and vdW\nheterostacking. Moreover, their ternary nature will offer more tunability for\nelectronic structure by controlling different stoichiometry and valence\ncharges. Our findings provide an ideal materials platform for realizing QSH\neffect and exploring topological phase transition, and will open up a variety\nof new opportunities for two-dimensional materials and topological materials\nresearch.", "category": "cond-mat_mtrl-sci" }, { "text": "Unconventional Fermi surface spin textures in the Bi_xPb_{1-x}/Ag(111)\n surface alloy: The Fermi and Rashba energies of surface states in the Bi_xPb_{1-x}/Ag(111)\nalloy can be tuned simultaneously by changing the composition parameter x. We\nreport on unconventional Fermi surface spin textures observed by spin and\nangle-resolved photoemission spectroscopy {that are correlated with a\ntopological transition of the Fermi surface occurring at x=0.5. We show that\nthe surface states remain fully spin polarized upon alloying and that the spin\npolarization vectors are approximately tangential to the constant energy\ncontours. We discuss the implications of the topological transition for the\ntransport of spin.", "category": "cond-mat_mtrl-sci" }, { "text": "Low In solubility and band offsets in the small-$x$\n $\u03b2$-Ga$_2$O$_3$/(Ga$_{1-x}$In$_x$)$_2$O$_3$ system: Based on first-principles calculations, we show that the maximum reachable\nconcentration $x$ in the (Ga$_{1-x}$In$_x$)$_2$O$_3$ alloy in the low-$x$\nregime (i.e. In solubility in $\\beta$-Ga$_2$O$_3$) is around 10%. We then\ncalculate the band alignment at the (100) interface between $\\beta$-Ga$_2$O$_3$\nand (Ga$_{1-x}$In$_x$)$_2$O$_3$ at 12%, the nearest computationally treatable\nconcentration. The alignment is strongly strain-dependent: it is of type-B\nstaggered when the alloy is epitaxial on Ga$_2$O$_3$, and type-A straddling in\na free-standing superlattice. Our results suggest a limited range of\napplicability of low-In-content GaInO alloys.", "category": "cond-mat_mtrl-sci" }, { "text": "Spin-wave stiffness and micromagnetic exchange interactions expressed by\n means of the KKR Green function approach: We represent an approach to calculate micromagnetic model parameters such as\nthe tensor of exchange stiffness, Dzyaloshinskii-Moriya interaction (DMI) as\nwell as spin-wave stiffness. The scheme is based on the fully relativistic\nKorringa-Kohn-Rostoker Green function (KKR-GF) technique and can be seen as a\nrelativistic extension of the work of Lichtenstein {\\em et al.} The expression\nfor $D^{z\\alpha}$ elements of DMI differ from the expressions for $D^{x\\alpha}$\nand $D^{y\\alpha}$ elements as the former are derived via second-order\nperturbation term of the energy caused by spin-spiral while the latter are\nassociated with the first-order term. Corresponding numerical results are\ncompared with those obtained using other schemes reported in the literature.", "category": "cond-mat_mtrl-sci" }, { "text": "DC Resistance Degradation of SrTiO$_3$: The Role of Virtual-Cathode\n Needles and Oxygen Bubbles: This study of highly accelerated lifetime tests of SrTiO$_3$, a model\nsemiconducting oxide, is motivated by the interest in reliable multilayer\nceramic capacitors and resistance-switching thin-film devices. Our analytical\nsolution to oxygen-vacancy migration under a DC voltage -- the cause of\nresistance degradation in SrTiO$_3$ -- agrees with previous numerical\nsolutions. However, all solutions fail to explain why degradation kinetics\nfeature a very strong voltage dependence, which we attribute to the nucleation\nand growth of cathode-initiated fast-conducting needles. While they have no\ncolor contrast in SrTiO$_3$ single crystals and are nominally invisible,\nneedles presence in DC-degraded samples -- in silicone oil and in air -- was\nunambiguously revealed by in-situ hot-stage photography. Observations in\nsilicone oil and thermodynamic considerations of voltage boundary conditions\nfurther revealed a cooccurrence of copious oxygen bubbling and the onset of\nfinal accelerating degradation, suggesting sudden oxygen loss is a precursor of\nfinal failure. Remarkably, both undoped and Fe-doped SrTiO$_3$ can emit\nelectroluminescence at higher current densities, thus providing a vivid\nindicator of resistance degradation and a metal-to-insulator resistance\ntransition during cooling. The implications of these findings to thin ceramic\nand thin film SrTiO$_3$ devices are discussed, along with connections to\nsimilar findings in likewise degraded fast-ion yttria-stabilized zirconia.", "category": "cond-mat_mtrl-sci" }, { "text": "Insights into the structural symmetry of single-crystal YCrO$_3$ from\n synchrotron X-ray diffraction: We report on the crystallographic information such as lattice parameters,\natomic positions, bond lengths and angles, and local crystalline distortion\nsize and mode of single-crystal YCrO$_3$ compound by a high-resolution\nsynchrotron X-ray diffraction study. The data was collected at 120 K (below\n$T_\\textrm{N} \\sim$ 141.5 K), 300 K (within [$T_\\textrm{N}$, $T_\\textrm{C}$]),\nand 500 K (above $T_\\textrm{C} \\sim$ 473 K). Taking advantages of high\nintensity and brilliance of synchrotron X-rays, we are able to refine collected\npatterns with the noncentrosymmetric monoclinic structural model ($P12_11$, No.\n4) that was proposed previously but detailed structural parameters have not\ndetermined yet. Meanwhile, we calculated patterns with the centrosymmetric\northorhombic space group (\\emph{Pmnb}, No. 62) for a controlled study. Lattice\nconstants \\emph{a}, \\emph{b}, and \\emph{c} as well as unit-cell volume almost\nincrease linearly upon warming. We observed more dispersive distributions of\nbond length and angle and local distortion strength with the $P12_11$ space\ngroup. This indicates that (i) The local distortion mode of Cr2O$_6$ at 120 K\ncorrelates the formation of the canted antiferromagnetic order by Cr1-Cr2 spin\ninteractions mainly through intermediate of O3 and O4 ions. (ii) The\nstrain-balanced Cr1-O3(O4) and Cr2-O5(O5) bonds as well as the local distortion\nmodes of Cr1O$_6$ and Cr2O$_6$ octohedra at 300 K may be a microscopic origin\nof the previously-reported dielectric anomaly. Our study demonstrates that\nlocal crystalline distortion is a key factor for the formation of ferroelectric\norder and provides a complete set of crystallography for a full understanding\nof the interesting magnetic and quasi-ferroelectric properties of YCrO$_3$\ncompound.", "category": "cond-mat_mtrl-sci" }, { "text": "Prediction of Chlorine and Fluorine Crystal Structures at High Pressure\n Using Symmetry Driven Structure Search with Geometric Constraints: The high-pressure properties of fluorine and chlorine are not yet well\nunderstood because both are highly reactive and volatile elements, which has\nmade conducting diamond anvil cell and x-ray diffraction experiments a\nchallenge. Here we use ab initio methods to search for stable crystal\nstructures of both elements at megabar pressures. We demonstrate how symmetry\nand geometric constraints can be combined to efficiently generate crystal\nstructures that are composed of diatomic molecules. Our algorithm extends the\nsymmetry driven structure search method [Phys. Rev. B 98 (2018) 174107] by\nadding constraints for the bond length and the number of atoms in a molecule,\nwhile still maintaining generality. As a method of validation, we have tested\nour approach for dense hydrogen and reproduced the known molecular structures\nof Cmca-12 and Cmca-4. We apply our algorithm to study chlorine and fluorine in\nthe pressure range from 10--4000 GPa while considering crystal structures with\nup to 40 atoms per unit cell. We predict chlorine to follow the same series of\nphase transformations as elemental iodine from Cmca to Immm to Fm$\\bar{3}$m,\nbut at substantially higher pressures. We predict fluorine to transition from a\nC2/c to an Cmca structure at 70 GPa, to a novel orthorhombic and metallic\nstructure with P$4_2$/mmc symmetry at 2500 GPa, and finally into its cubic\nanalogue form with Pm$\\bar{3}$n symmetry at 3000 GPa.", "category": "cond-mat_mtrl-sci" }, { "text": "Combining experiments on luminescent centres in hexagonal boron nitride\n with the polaron model and ab initio methods towards the identification of\n their microscopic origin: The two-dimensional material hexagonal boron nitride (hBN) hosts luminescent\ncentres with emission energies of 2 eV which exhibit pronounced phonon\nsidebands. We investigate the microscopic origin of these luminescent centres\nby combining ab initio calculations with non-perturbative open quantum system\ntheory to study the emission and absorption properties of 26 defect\ntransitions. Comparing the calculated line shapes with experiments we narrow\ndown the microscopic origin to three carbon-based defects: $\\mathrm{C_2C_B}$,\n$\\mathrm{C_2C_N}$, and $\\mathrm{V_NC_B}$. The theoretical method developed\nenables us to calculate so-called photoluminescence excitation (PLE) maps,\nwhich show excellent agreement with our experiments. The latter resolves\nhigher-order phonon transitions, thereby confirming both the vibronic structure\nof the optical transition and the phonon-assisted excitation mechanism with a\nphonon energy 170 meV. We believe that the presented experiments and\npolaron-based method accurately describe luminescent centres in hBN and will\nhelp to identify their microscopic origin.", "category": "cond-mat_mtrl-sci" }, { "text": "High-order harmonic generation in solid $\\rm \\bf C_{60}$: High harmonic generation (HHG) has unleashed the power of strong laser\nphysics in solids. Here we investigate HHG from a large system, solid C$_{60}$,\nwith 240 valence electrons engaging harmonic generation at each crystal\nmomentum, the first of this kind. We employ the density functional theory and\nthe time-dependent Liouville equation of the density matrix to compute HHG\nsignals. We find that under a moderately strong laser pulse, HHG signals reach\n15th order, consistent with the experimental results from C$_{60}$ plasma. The\nhelicity dependence in solid C$_{60}$ is weak, due to the high symmetry. In\ncontrast to the general belief, HHG is unsuitable for band structure mapping in\nC$_{60}$. However, we find a window of opportunity using a long wavelength,\nwhere harmonics are generated through multiple-photon excitation. In\nparticular, the 5th order harmonic energies closely follow the transition\nenergy dispersion between the valence and conduction bands. This finding is\nexpected to motivate future experimental investigations.", "category": "cond-mat_mtrl-sci" }, { "text": "Excitonic Photoluminescence properties of nanocrystalline GaSb and\n Ga0.62In0.38Sb embedded in silica films: The GaSb and Ga0.62In0.38Sb nanocrystals were embedded in the SiO2 films by\nradio-frequency magnetron co-sputtering and were grown on GaSb and Si\nsubstrates at different temperatures. We present results on the 10K excitonic\nphotoluminescence (PL) properties of nanocrystalline GaSb and Ga0.62In0.38Sb as\na function of their size. The measurements show that the PL of the GaSb and\nGa0.62In0.38Sb nanocrystallites follows the quantum confinement model very\nclosely. By using deconvolution of PL spectra, origins of structures in\nphotoluminescence were identified.", "category": "cond-mat_mtrl-sci" }, { "text": "Self-assembly of Nanometer-scale Magnetic Dots with Narrow Size\n Distributions on an Insulating Substrate: The self-assembly of iron dots on the insulating surface of NaCl(001) is\ninvestigated experimentally and theoretically. Under proper growth conditions,\nnanometer-scale magnetic iron dots with remarkably narrow size distributions\ncan be achieved in the absence of a wetting layer Furthermore, both the\nvertical and lateral sizes of the dots can be tuned with the iron dosage\nwithout introducing apparent size broadening, even though the clustering is\nclearly in the strong coarsening regime. These observations are interpreted\nusing a phenomenological mean-field theory, in which a coverage-dependent\noptimal dot size is selected by strain-mediated dot-dot interactions.", "category": "cond-mat_mtrl-sci" }, { "text": "Electronic structure and optical properties of lightweight metal\n hydrides: We study the electronic structures and dielectric functions of the simple\nhydrides LiH, NaH, MgH2 and AlH3, and the complex hydrides Li3AlH6, Na3AlH6,\nLiAlH4, NaAlH4 and Mg(AlH4)2, using first principles density functional theory\nand GW calculations. All these compounds are large gap insulators with GW\nsingle particle band gaps varying from 3.5 eV in AlH3 to 6.5 eV in the MAlH4\ncompounds. The valence bands are dominated by the hydrogen atoms, whereas the\nconduction bands have mixed contributions from the hydrogens and the metal\ncations. The electronic structure of the aluminium compounds is determined\nmainly by aluminium hydride complexes and their mutual interactions. Despite\nconsiderable differences between the band structures and the band gaps of the\nvarious compounds, their optical responses are qualitatively similar. In most\nof the spectra the optical absorption rises sharply above 6 eV and has a strong\npeak around 8 eV. The quantitative differences in the optical spectra are\ninterpreted in terms of the structure and the electronic structure of the\ncompounds.", "category": "cond-mat_mtrl-sci" }, { "text": "3D versus 2D domain wall interaction in ideal and rough nanowires: The interaction between transverse magnetic domain walls (TDWs) in planar\n(2D) and cylindrical (3D) nanowires is examined using micromagnetic\nsimulations. We show that in perfect and surface deformed wires the free TDWs\nbehave differently, as the 3D TDWs combine into metastable states with average\nlifetimes of 300ns depending on roughness, while the 2D TDWs do not due to 2D\nshape anisotropy. When the 2D and 3D TDWs are pinned at artificial\nconstrictions, they behave similarly as they interact mainly through the\ndipolar field. This magnetostatic interaction is well described by the point\ncharge model with multipole expansion. In surface deformed wires with\nartificial constrictions, the interaction becomes more complex as the depinning\nfield decreases and dynamical pinning can lead to local resonances. This can\nstrongly influence the control of TDWs in DW-based devices.", "category": "cond-mat_mtrl-sci" }, { "text": "Effective density of states map of undoped microcrystalline Si films: a\n combined experimental and numerical simulation approach: The phototransport properties of plasma deposited highly crystalline undoped\nhydrogenated microcrystalline silicon films were studied by measuring the\nsteady state photoconductivity (SSPC) as a function of temperature and light\nintensity. The films possessing different thicknesses and microstructures had\nbeen well characterized by various microstructural probes. Microcrystalline Si\nfilms possessing dissimilar microstructural attributes were found to exhibit\ndifferent phototransport behaviors. We have employed numerical modeling of SSPC\nto corroborate and further elucidate the experimental results. Our study\nindicates that the different phototransport behaviors are linked to different\nfeatures of the proposed density of states maps of the material which are\ndifferent for microcrystalline Si films having different types of\nmicrostructure.", "category": "cond-mat_mtrl-sci" }, { "text": "Domains and ferroelectric switching pathways in Ca$_3$Ti$_2$O$_7$ from\n first principles: Hybrid improper ferroelectricity, where an electrical polarization can be\ninduced via a trilinear coupling to two non-polar structural distortions of\ndifferent symmetry, has recently been experimentally demonstrated for the first\ntime in the $n$=2 Ruddlesden-Popper compound Ca$_3$Ti$_2$O$_7$. In this paper\nwe use group theoretic methods and first-principles calculations to identify\npossible ferroelectric switching pathways in Ca$_3$Ti$_2$O$_7$. We identify\nlow-energy paths that reverse the polarization direction by switching via an\northorhombic twin domain, or via an antipolar structure. We also introduce a\nchemically intuitive set of local order parameters to give insight into how\nthese paths are relevant to switching nucleated at domain walls. Our findings\nsuggest that switching may proceed via more than one mechanism in this\nmaterial.", "category": "cond-mat_mtrl-sci" }, { "text": "Can CF(3)-functionalized La@C(60) be isolated experimentally and become\n superconducting?: Superconducting behavior even under harsh ambient conditions is expected to\noccur in La@C(60) if it could be isolated from the primary metallofullerene\nsoot when functionalized by CF(3) radicals. We use ab initio density functional\ntheory calculations to compare the stability and electronic structure of C(60)\nand the La@C(60) endohedral metallofullerene to their counterparts\nfunctionalized by CF(3). We found that CF(3) radicals favor binding to C(60)\nand La@C(60), and have identified the most stable isomers. Structures with an\neven number m of radicals are energetically preferred for C(60) and structures\nwith odd m for La@C(60) due to the extra charge on the fullerene. This is\nconsistent with a wide HOMO-LUMO gap in La@C(60)(CF(3))(m) with odd m, causing\nextra stabilization in the closed-shell electronic configuration. CF(3)\nradicals are both stabilizing agents and molecular separators in a metallic\ncrystal, which could increase the critical temperature for superconductivity.", "category": "cond-mat_mtrl-sci" }, { "text": "Temperature Dependence of the Energy Levels of Methylammonium Lead\n Iodide Perovskite from First Principles: Environmental effects and intrinsic energy-loss processes lead to\nfluctuations in the operational temperature of solar cells, which can\nprofoundly influence their power conversion efficiency. Here we determine from\nfirst principles the effects of temperature on the band gap and band edges of\nthe hybrid pervoskite CH$_3$NH$_3$PbI$_3$ by accounting for electron-phonon\ncoupling and thermal expansion. From $290$ to $380$ K, the computed band gap\nchange of $40$ meV coincides with the experimental change of $30$-$40$ meV. The\ncalculation of electron-phonon coupling in CH$_3$NH$_3$PbI$_3$ is particularly\nintricate, as the commonly used Allen-Heine-Cardona theory overestimates the\nband gap change with temperature, and excellent agreement with experiment is\nonly obtained when including high-order terms in the electron-phonon\ninteraction. We also find that spin-orbit coupling enhances the electron-phonon\ncoupling strength, but that the inclusion of nonlocal correlations using hybrid\nfunctionals has little effect. We reach similar conclusions in the metal-halide\nperovskite CsPbI$_3$. Our results unambiguously confirm for the first time the\nimportance of high-order terms in the electron-phonon coupling by direct\ncomparison with experiment.", "category": "cond-mat_mtrl-sci" }, { "text": "Electronic structure and enhanced visible light absorption of N,\n B-codoped TiO2: We present the GGA+U calculations to investigate the electronic structure and\nvisible light absorption of the N, B-codoped anatase TiO2. The NsBi\n(substitutional N, interstitial B) codoped TiO2 produces significant Ti 3d and\nN 2p mid-gap states when the distance of N and B atoms is far, and the NiBi\n(interstitial N and B) and NsBs (substitutional N and B) codoped TiO2 prefer to\nform localized p states at 0.3-1.2 eV above the valence band maximum. Further,\nthe optical band edges of the three codoped systems shift slightly to the\nvisible region, but only the far distance NsBi codoped TiO2 shows an obvious\nvisible optical transition. These results indicate that the NsBi codoped TiO2\nhas a dominant contribution to the visible absorption of the N, B-codoped TiO2.", "category": "cond-mat_mtrl-sci" }, { "text": "Human-in-the-loop: The future of Machine Learning in Automated Electron\n Microscopy: Machine learning methods are progressively gaining acceptance in the electron\nmicroscopy community for de-noising, semantic segmentation, and dimensionality\nreduction of data post-acquisition. The introduction of the APIs by major\ninstrument manufacturers now allows the deployment of ML workflows in\nmicroscopes, not only for data analytics but also for real-time decision-making\nand feedback for microscope operation. However, the number of use cases for\nreal-time ML remains remarkably small. Here, we discuss some considerations in\ndesigning ML-based active experiments and pose that the likely strategy for the\nnext several years will be human-in-the-loop automated experiments (hAE). In\nthis paradigm, the ML learning agent directly controls beam position and image\nand spectroscopy acquisition functions, and human operator monitors experiment\nprogression in real- and feature space of the system and tunes the policies of\nthe ML agent to steer the experiment towards specific objectives.", "category": "cond-mat_mtrl-sci" }, { "text": "Is Cement a Glassy Material?: The nature of Calcium--Silicate--Hydrate (C--S--H), the binding phase of\ncement, remains a controversial question. In particular, contrary to the former\ncrystalline model, it was recently proposed that its nanoscale structure was\nactually amorphous. To elucidate this issue, we analyzed the structure of a\nrealistic simulation of C--S--H, and compared the latter to crystalline\ntobermorite, a natural analogue to cement, and to an artificial ideal glass.\nResults clearly support that C--S--H is amorphous. However, its structure shows\nan intermediate degree of order, retaining some characteristics of the crystal\nwhile acquiring an overall glass-like disorder. Thanks to a detailed\nquantification of order and disorder, we show that its amorphous state mainly\narises from its hydration.", "category": "cond-mat_mtrl-sci" }, { "text": "Computationally-driven, high throughput identification of CaTe and\n Li$_\\textrm{3}$Sb as promising candidates for high mobility $p$-type\n transparent conducting materials: High-performance $p$-type transparent conducting materials (TCMs) must\nexhibit a rare combination of properties including high mobility, transparency\nand $p$-type dopability. The development of high-mobility/conductivity $p$-type\nTCMs is necessary for many applications such as solar cells, or transparent\nelectronic devices. Oxides have been traditionally considered as the most\npromising chemical space to dig out novel $p$-type TCMs. However, non-oxides\nmight perform better than traditional $p$-type TCMs (oxides) in terms of\nmobility. We report on a high-throughput (HT) computational search for\nnon-oxide $p$-type TCMs from a large dataset of more than 30,000 compounds\nwhich identified CaTe and Li$_\\textrm{3}$Sb as very good candidates for\nhigh-mobility $p$-type TCMs. From our calculations, both compounds are expected\nto be $p$-type dopable: intrinsically for Li$_\\textrm{3}$Sb while CaTe would\nrequire extrinsic doping. Using electron-phonon computations, we estimate hole\nmobilities at room-temperature to be about 20 and 70 cm$^2$/Vs for CaTe and\nLi$_\\textrm{3}$Sb, respectively. The computed hole mobility for\nLi$_\\textrm{3}$Sb is quite exceptional and comparable with the electron\nmobility in the best $n$-type TCMs.", "category": "cond-mat_mtrl-sci" }, { "text": "Preparation of poly(sodium acrylate-co-acrylamide) superabsorbent\n copolymer via alkaline hydrolysis of acrylamide using microwave irradiation: In this paper we present a new one-pot synthesis method of\npoly(acrylate-co-acrylamide) superabsorbent polymer via partial alkaline\nhydrolysis of acrylamide using microwave irradiation. This method allows to\nhydrolysis, polymerization and gelation to take place in one pot during a very\nshort reaction time (90 s), and with no need to operate under inert atmosphere.\nThe degree of hydrolysis of the gel was determined by a back titration method.\nThe gel is compact and has a water absorbency of 1031 g/g while the\ncorresponding copolymer prepared from polymerization of sodium acrylate and\nacrylamide, using microwave irradiation and under the same experimental\nconditions, has a water absorbency of only 658g/g. This difference in water\nabsorbency is discussed. FTIR spectroscopy was used to verify the hydrolysis\nand the formation of sodium acrylate. Scanning electron microscopy (SEM) showed\nthat the synthesized hydrogel has a macroporous structure. The influence of the\nenvironmental parameters on water absorbency such as the pH and the ionic force\nwas also investigated.", "category": "cond-mat_mtrl-sci" }, { "text": "Electronic and optical properties of the fully and partially inverse\n CoFe$_{2}$O$_{4}$ spinel from first principles calculations including\n many-body effects: Using density functional theory (DFT) calculations and state-of-the-art\nmany-body perturbation theory, we investigate the electronic and optical\nproperties of the inverse spinel CoFe$_{2}$O$_{4}$, a common anode material for\nphotocatalytic water splitting. Starting with different exchange-correlation\nfunctionals, at the independent particle level we obtain a direct band gap of\n1.38~eV (PBE+$U$) and 1.69 eV (SCAN+$U$), whereas HSE06 renders an indirect\nband gap of 2.02~eV. Including quasiparticle effects within $G_{0}W_{0}$, a\nlarger and indirect band gap is obtained for all functionals: 1.78~eV\n(PBE+$U$), 1.95~eV (SCAN+$U$) and 2.17~eV (HSE06), higher than the independent\nparticle (IP) band gap. Excitonic effects, taken into account by solving the\nBethe-Salpeter equation (BSE) lead to a redshift of the optical band gap to\n1.50 (SCAN+$U$) and 1.61~eV (HSE06), in good agreement with the reported\nexperimental values. The lowest optical transitions in the visible range,\nidentified by means of oscillator strength, are at 2.0, 3.5, and 5.0~eV,\nconsistent with experimental observations. We also explored the effect of the\ndegree of inversion: the band gap is found to decrease from 1.69 ($x=1$) to\n1.45 ($x=0.5$), and 1.19~eV ($x=0)$ within the IP approximation with SCAN+$U$.\nThis trend is reversed after the inclusion of excitonic effects, resulting in a\nband gap of 1.50, 1.57, and 1.64~eV for $x$ = 1.0, 0.5, and 0.0, respectively.\nThe oscillator strength analysis of the BSE calculations indicates that both\n$x$ = 0.0 and $x$ = 0.5 exhibit transitions below 1~eV with extremely small\noscillator strengths that are absent in the inverse spinel. This corroborates\nprevious suggestions that these transitions are due to the presence of\nCo$^{2+}$ cations at the tetrahedral sites.", "category": "cond-mat_mtrl-sci" }, { "text": "Statistical Analysis of Contacts to Synthetic Monolayer MoS2: Two-dimensional (2D) semiconductors are promising candidates for scaled\ntransistors because they are immune to mobility degradation at the monolayer\nlimit. However, sub-10 nm scaling of 2D semiconductors, such as MoS2, is\nlimited by the contact resistance. In this work, we show for the first time a\nstatistical study of Au contacts to chemical vapor deposited monolayer MoS2\nusing transmission line model (TLM) structures, before and after dielectric\nencapsulation. We report contact resistance values as low as 330 ohm-um, which\nis the lowest value reported to date. We further study the effect of Al2O3\nencapsulation on variability in contact resistance and other device metrics.\nFinally, we note some deviations in the TLM model for short-channel devices in\nthe back-gated configuration and discuss possible modifications to improve the\nmodel accuracy.", "category": "cond-mat_mtrl-sci" }, { "text": "Thermodynamic driving force in the formation of hexagonal-diamond Si and\n Ge nanowires: The metastable hexagonal-diamond phase of Si and Ge (and of SiGe alloys)\ndisplays superior optical properties with respect to the cubic-diamond one. The\nlatter is the most stable and popular one: growing hexagonal-diamond Si or Ge\nwithout working at extreme conditions proved not to be trivial. Recently,\nhowever, the possibility of growing hexagonal-diamond group-IV nanowires has\nbeen demonstrated, attracting attention on such systems. Based on\nfirst-principle calculations we show that the surface energy of the typical\nfacets exposed in Si and Ge nanowires is lower in the hexagonal-diamond phase\nthan in cubic ones. By exploiting a synergic approach based also on a recent\nstate-of-the-art interatomic potential and on a simple geometrical model, we\ninvestigate the relative stability of nanowires in the two phases up to few\ntens of nm in radius, highlighting the surface-related driving force and\ndiscussing its relevance in recent experiments. We also explore the stability\nof Si and Ge core-shell nanowires with hexagonal cores (made of GaP for Si\nnanowires, of GaAs for Ge nanowires). In this case, the stability of the\nhexagonal shell over the cubic one is also favored by the energy cost\nassociated with the interface linking the two phases. Interestingly, our\ncalculations indicate a critical radius of the hexagonal shell much lower than\nthe one reported in recent experiments, indicating the presence of a large\nkinetic barrier allowing for the enlargement of the wire in a metastable phase.", "category": "cond-mat_mtrl-sci" }, { "text": "Extended Lagrangian formulation of time-reversible Born-Oppenheimer\n molecular dynamics for higher-order symplectic integration: A Lagrangian generalization of time-reversible Born-Oppenheimer molecular\ndynamics [Niklasson et al., Phys. Rev. Lett. vol. 97, 123001 (2006)] is\nproposed. The Lagrangian includes extended electronic degrees of freedom as\nauxiliary dynamical variables in addition to the nuclear coordinates and\nmomenta. While the nuclear degrees of freedom propagate on the Born-Oppenheimer\npotential energy surface, the extended auxiliary electronic degrees of freedom\nevolve as a harmonic oscillator centered around the adiabatic propagation of\nthe self-consistent ground state. The formulation enables the application of\nhigher-order symplectic or geometric integration schemes that are stable and\nenergy conserving even under incomplete self-consistency convergence. It is\ndemonstrated how the extended Born-Oppenheimer molecular dynamics improves the\naccuracy by over an order of magnitude compared to previous formulations at the\nsame level of computational cost.", "category": "cond-mat_mtrl-sci" }, { "text": "Interfacial and Surface Magnetism in Epitaxial NiCo2O4(001)/MgAl2O4\n Films: NiCo2O4 (NCO) films grown on MgAl2O4 (001) substrates have been studied using\nmagnetometry, x-ray magnetic circular dichroism (XMCD) based on x-ray\nabsorption spectroscopy, and spin-polarized inverse photoemission spectroscopy\n(SPIPES) with various thickness down to 1.6 nm. The magnetic behavior can be\nunderstood in terms of a layer of optimal NCO and an interfacial layer (1.2+/-\n0.1 nm), with a small canting of magnetization at the surface. The thickness\ndependence of the optimal layer can be described by the finite-scaling theory\nwith a critical exponent consistent with the high perpendicular magnetic\nanisotropy. The interfacial layer couples antiferromagnetically to the optimal\nlayer, generating exchange-spring styled magnetic hysteresis in the thinnest\nfilms. The non-optimal and measurement-speed-dependent magnetic properties of\nthe interfacial layer suggest substantial interfacial diffusion.", "category": "cond-mat_mtrl-sci" }, { "text": "The strain-induced transitions of the piezoelectric, pyroelectric and\n electrocaloric properties of the CuInP$_2$S$_6$ films: The low-dimensional ferroelectrics, ferrielectrics and antiferroelectrics are\nof urgent scientific interest due to their unusual polar, piezoelectric,\nelectrocaloric and pyroelectric properties. The strain engineering and strain\ncontrol of the ferroelectric properties of layered 2D Van der Waals materials,\nsuch as CuInP$_2$(S,Se)$_6$ monolayers, thin films and nanoflakes, are of\nfundamental interest and especially promising for their advanced applications\nin nanoscale nonvolatile memories, energy conversion and storage, nano-coolers\nand sensors. Here, we study the polar, piezoelectric, electrocaloric and\npyroelectric properties of thin strained films of a ferrielectric\nCuInP$_2$S$_6$ covered by semiconducting electrodes and reveal an unusually\nstrong effect of a mismatch strain on these properties. In particular, the sign\nof the mismatch strain and its magnitude determine the complicated behavior of\npiezoelectric, electrocaloric and pyroelectric responses. The strain effect on\nthese properties is opposite, i.e., \"anomalous\", in comparison with many other\nferroelectric films, for which the out-of-plane remanent polarization,\npiezoelectric, electrocaloric and pyroelectric responses increase strongly for\ntensile strains and decrease or vanish for compressive strains.", "category": "cond-mat_mtrl-sci" }, { "text": "Pentacene islands grown on ultra-thin SiO2: Ultra-thin oxide (UTO) films were grown on Si(111) in ultrahigh vacuum at\nroom temperature and characterized by scanning tunneling microscopy. The\nultra-thin oxide films were then used as substrates for room temperature growth\nof pentacene. The apparent height of the first layer is 1.57 +/- 0.05 nm,\nindicating standing up pentacene grains in the thin-film phase were formed.\nPentacene is molecularly resolved in the second and subsequent molecular\nlayers. The measured in-plane unit cell for the pentacene (001) plane (ab\nplane) is a=0.76+/-0.01 nm, b=0.59+/-0.01 nm, and gamma=87.5+/-0.4 degrees. The\nfilms are unperturbed by the UTO's short-range spatial variation in tunneling\nprobability, and reduce its corresponding effective roughness and correlation\nexponent with increasing thickness. The pentacene surface morphology follows\nthat of the UTO substrate, preserving step structure, the long range surface\nrms roughness of ~0.1 nm, and the structural correlation exponent of ~1.", "category": "cond-mat_mtrl-sci" }, { "text": "The Structure of Eu-III: Previous x-ray diffraction studies have reported Eu to transform from the hcp\nstructure to a new phase, Eu-III, at 18 GPa. Using x-ray powder diffraction we\nhave determined that Eu remains hcp up to 33 GPa, and that the extra peaks that\nappear at 18 GPa are from an impurity phase with space group R-3c . Above 33\nGPa the diffraction pattern becomes very much more complex, signalling a\ntransition to a phase with a distorted hcp structure.", "category": "cond-mat_mtrl-sci" }, { "text": "Nitrogen-vacancy centers created by N$^+$ ion implantation through\n screening SiO$_2$ layers on diamond: We report on an ion implantation technique utilizing a screening mask made of\nSiO$_2$ to control both the depth profile and the dose. By appropriately\nselecting the thickness of the screening layer, this method fully suppresses\nthe ion channeling, brings the location of the highest NV density to the\nsurface, and effectively reduces the dose by more than three orders of\nmagnitude. With a standard ion implantation system operating at the energy of\n10 keV and the dose of 10$^{11}$ cm$^2$ and without an additional etching\nprocess, we create single NV centers close to the surface with coherence times\nof a few tens of $\\mu$s.", "category": "cond-mat_mtrl-sci" }, { "text": "Tunable magnon-magnon coupling in synthetic antiferromagnets: In this work, we study magnon-magnon coupling in synthetic antiferromagnets\n(SyAFs) using microwave spectroscopy at room temperature. Two distinct\nspin-wave modes are clearly observed and are hybridised at degeneracy points.\nWe provide a phenomenological model that captures the coupling phenomena and\nexperimentally demonstrate that the coupling strength is controlled by the\nout-of-plane tilt angle as well as the interlayer exchange field. We\nnumerically show that a spin-current mediated damping in SyAFs plays a role in\ninfluencing the coupling strength.", "category": "cond-mat_mtrl-sci" }, { "text": "Transverse circular photogalvanic effect associated with\n Lorentz-violating Weyl fermions: Nonlinear optical responses of quantum materials have recently undergone\ndramatic developments to unveil nontrivial geometry and topology. A remarkable\nexample is the quantized longitudinal circular photogalvanic effect (CPGE)\nassociated with the Chern number of Weyl fermions, while the physics of\ntransverse CPGE in Weyl semimetals remains exclusive. Here, we show that the\ntransverse CPGE of Lorentz invariant Weyl fermions is forced to be zero. We\nfind that the transverse photocurrents of Weyl fermions are associated not only\nwith the Chern numbers but also with the degree of Lorentz-symmetry breaking in\ncondensed matter systems. Based on the generic two-band model analysis, we\nprovide a new powerful equation to calculate the transverse CPGE based on the\ntilting and warping terms of Weyl fermions. Our results are more capable in\ndesigning large transverse CPGE of Weyl semimetals in experiments and are\napplied to more than tens of Weyl materials to estimate their photocurrents.\nOur method paves the way to study the CPGE of massless or massive\nquasiparticles to design next-generation quantum optoelectronics.", "category": "cond-mat_mtrl-sci" }, { "text": "Directional carrier transport in micrometer-thick gallium oxide films\n for high-performance deep-ultraviolet photodetection: Incorporating emerging ultrawide bandgap semiconductors with a\nmetal-semiconductor-metal (MSM) architecture is highly desired for\ndeep-ultraviolet (DUV) photodetection. However, synthesis-induced defects in\nsemiconductors complicate the rational design of MSM DUV photodetectors due to\ntheir dual role as carrier donors and trap centers, leading to a commonly\nobserved trade-off between responsivity and response time. Here, we demonstrate\na simultaneous improvement of these two parameters in {\\epsilon}-Ga2O3 MSM\nphotodetectors by establishing a low-defect diffusion barrier for directional\ncarrier transport. Specifically, using a micrometer thickness far exceeding its\neffective light absorption depth, the {\\epsilon}-Ga2O3 MSM photodetector\nachieves over 18-fold enhancement of responsivity and simultaneous reduction of\nthe response time, which exhibits a state-of-the-art photo-to-dark current\nratio near 10^8, a superior responsivity of >1300 A/W, an ultrahigh detectivity\nof >10^16 Jones and a decay time of 123 ms. Combined depth-profile\nspectroscopic and microscopic analysis reveals the existence of a broad\ndefective region near the lattice-mismatched interface followed by a more\ndefect-free dark region, while the latter one serves as a diffusion barrier to\nassist frontward carrier transport for substantially enhancing the\nphotodetector performance. This work reveals the critical role of the\nsemiconductor defect profile in tuning carrier transport for fabricating\nhigh-performance MSM DUV photodetectors.", "category": "cond-mat_mtrl-sci" }, { "text": "Tunable polarization components and electric field induced\n crystallization in polyvinylidenefluoride (PVDF); a piezo polymer: Polyvinylidenefluoride (PVDF) a semicrystalline pieozoelectric polymer was\nsynthesized with varying process conditions and its ferroelectric domain\norientations were studied using piezoresponse force microscope (PFM). PVDF thin\nfilms fabricated using tape casting technique with precursor solutions of\nvarying viscosities reveal that the polarization components transform from a\ndominant planar component to an out-of-plane polarization components with\nincrease in viscosity. Interestingly the planar components possessed a head to\nhead or tail to tail kind of paired domains separated by a distance of ~\n380-400nm. The electrostatic energies computed by numerically solving the\nelectrostatic equilibrium equation for the electrically inhomogeneous system\nare in good correlation with the experiments. On increment of electric field,\nthe domains were observed to grow in size and shape which indicates amorphous\nto crystalline transformation in the case of PVDF. Such transformation was\nevident from x-ray diffraction studies performed in-situ in the presence of an\napplied electric field.", "category": "cond-mat_mtrl-sci" }, { "text": "Understanding and tuning magnetism in layered Ising-type antiferromagnet\n FePSe3 for potential 2D magnet: Recent development in two-dimensional (2D) magnetic materials have motivated\nthe search for new van der Waals magnetic materials, especially Ising-type\nmagnets with strong magnetic anisotropy. Fe-based MPX3 (M = transition metal, X\n= chalcogen) compounds such as FePS3 and FePSe3 both exhibit an Ising-type\nmagnetic order, but FePSe3 receives much less attention compared to FePS3. This\nwork focuses on establishing the strategy to engineer magnetic anisotropy and\nexchange interactions in this less-explored compound. Through chalcogen and\nmetal substitutions, the magnetic anisotropy is found to be immune against S\nsubstitution for Se whereas tunable only with heavy Mn substitution for Fe. In\nparticular, Mn substitution leads to a continuous rotation of magnetic moments\nfrom the out-of-plane direction towards in-plane. Furthermore, the magnetic\nordering temperature displays non-monotonic doping dependence for both\nchalcogen and metal substitutions but due to different mechanisms. These\nfindings provide deeper insight into the Ising-type magnetism in this important\nvan der Waals material, shedding light on the study of other Ising-type\nmagnetic systems as well as discovering novel 2D magnets for potential\napplications in spintronics.", "category": "cond-mat_mtrl-sci" }, { "text": "Diffraction at GaAs/Fe$_{3}$Si core/shell nanowires: the formation of\n nanofacets: GaAs/Fe$_{3}$Si core/shell nanowire structures were fabricated by\nmolecular-beam epitaxy on oxidized Si(111) substrates and investigated by\nsynchrotron x-ray diffraction. The surfaces of the Fe$_3$Si shells exhibit\nnanofacets. These facets consist of well pronounced Fe$_3$Si{111} planes.\nDensity functional theory reveals that the Si-terminated Fe$_3$Si{111} surface\nhas the lowest energy in agreement with the experimental findings. We can\nanalyze the x-ray diffuse scattering and diffraction of the ensemble of\nnanowires avoiding the signal of the substrate and poly-crystalline films\nlocated between the wires. Fe$_3$Si nanofacets cause streaks in the x-ray\nreciprocal space map rotated by an azimuthal angle of 30{\\deg} compared with\nthose of bare GaAs nanowires. In the corresponding TEM micrograph the facets\nare revealed only if the incident electron beam is oriented along\n[1$\\overline{1}$0] in accordance with the x-ray results. Additional maxima in\nthe x-ray scans indicate the onset of chemical reactions between Fe$_{3}$Si\nshells and GaAs cores occurring at increased growth temperatures.", "category": "cond-mat_mtrl-sci" }, { "text": "Plasmonic electromagnetically-induced transparency in symmetric\n structures: A broken symmetry is generally believed to be a prerequisite of plasmonic\nelectromagnetically-induced transparency (EIT), since the asymmetry renders the\nexcitation of the otherwise forbidden dark mode possible. Nevertheless,\naccording to the picture of magnetic-plasmon resonance (MPR) mediated plasmonic\nEIT, we show that the plasmonic EIT can be achieved even in the symmetric\nstructures based on the second-order MPR. This sharpens our understanding of\nthe existing concept, but also a profound insight into the plasmonic coherent\ninterference in the near-field zone.", "category": "cond-mat_mtrl-sci" }, { "text": "Reversible phase transformation and doubly-charged anions at the surface\n of simple cubic RbC60: The simple cubic phase of a RbC60 thin film has been studied using\nphotoelectron spectroscopy. The simple cubic-to-dimer transition is found to be\nreversible at the film surface. A sharp Fermi edge is observed and a lower\nlimit of 0.5 eV is found for the surface Hubbard U, pointing to a\nstrongly-correlated metallic character of thin-film simple cubic RbC60. A\nmolecular charge state is identified in the valence band and core level\nphotoemission spectra which arises from C602- anions and contributes to the\nspectral intensity at the Fermi level.", "category": "cond-mat_mtrl-sci" }, { "text": "Quantum anomalous Hall effect in two-dimensional magnetic insulator\n heterojunctions: Recent years have witnessed tremendous success in the discovery of\ntopological states of matter. Particularly, sophisticated theoretical methods\nin time-reversal-invariant topological phases have been developed, leading to\nthe comprehensive search of crystal database and the prediction of thousands of\nnew topological materials. In contrast, the discovery of magnetic topological\nphases that break time reversal is still limited to several exemplary materials\nbecause the coexistence of magnetism and topological electronic band structure\nis rare in a single compound. To overcome this challenge, we propose an\nalternative approach to realize the quantum anomalous Hall (QAH) effect, a\ntypical example of magnetic topological phase, via engineering two-dimensional\n(2D) magnetic van der Waals heterojunctions. Instead of a single magnetic\ntopological material, we search for the combinations of two 2D (typically\ntrivial) magnetic insulator compounds with specific band alignment so that they\ncan together form a type-III heterojunction with topologically non-trivial band\nstructure. By combining the data-driven materials search, first principles\ncalculations, and the symmetry-based analytical models, we identify 8 type-III\nheterojunctions consisting of 2D ferromagnetic insulator materials from a\nfamily of 2D monolayer MXY compounds (M = metal atoms, X = S, Se, Te, Y = F,\nCl, Br, I) as a set of candidates for the QAH effect. In particular, we\ndirectly calculate the topological invariant (Chern number) and chiral edge\nstates in the MnNF/MnNCl heterojunction with ferromagnetic stacking. This work\nillustrates how data-driven material science can be combined with\nsymmetry-based physical principles to guide the search for novel\nheterojunction-based quantum materials hosting the QAH effect and other exotic\nquantum states in general.", "category": "cond-mat_mtrl-sci" }, { "text": "Dual-band metacomposites containing hybrid Fe and Co-based ferromagnetic\n microwires: We investigated the microwave properties of polymer based metacomposites\ncontaining hybridized parallel Fe- and Co-based microwire arrays. A dual-band\nleft-handed feature was observed in the frequency bands of 1.5 to 5.5 GHz and 9\nto 17 GHz, indicated by two transmission windows associated with ferromagnetic\nresonance of Fe-based microwires and long range dipolar resonance between the\nwire arrays. The plasma frequency after hybridization is significantly\nincreased due to the enhanced effective diameter through the wire-wire\ninteractions between the Fe- and Co- microwire couples. These results offer\nessential perspectives in designing the multi-band metamaterial for microwave\napplications such as sensors and cloaking devices.", "category": "cond-mat_mtrl-sci" }, { "text": "Molecular Tuning of the Magnetic Response in Organic Semiconductors: The tunability of high-mobility organic semi-conductors (OSCs) holds great\npromise for molecular spintronics. In this study, we show this extreme\nvariability - and therefore potential tunability - of the molecular\ngyromagnetic coupling (\"g-\") tensor with respect to the geometric and\nelectronic structure in a much studied class of OSCs. Composed of a structural\ntheme of phenyl- and chalcogenophene (group XVI element containing,\nfive-membered) rings and alkyl functional groups, this class forms the basis of\nseveral intensely studied high-mobility polymers and molecular OSCs. We show\nhow in this class the g-tensor shifts, $\\Delta g$, are determined by the\neffective molecular spin-orbit coupling (SOC), defined by the overlap of the\natomic spin-density and the heavy atoms in the polymers. We explain the\ndramatic variations in SOC with molecular geometry, chemical composition,\nfunctionalization, and charge life-time using a first-principles theoretical\nmodel based on atomic spin populations. Our approach gives a guide to tuning\nthe magnetic response of these OSCs by chemical synthesis.", "category": "cond-mat_mtrl-sci" }, { "text": "Intrinsic anomalous Hall effect in Ni-substituted magnetic Weyl\n semimetal Co3Sn2S2: Topological materials have recently attracted considerable attention among\nmaterials scientists as their properties are predicted to be protected against\nperturbations such as lattice distortion and chemical substitution. However,\nany experimental proof of such robustness is still lacking. In this study, we\nexperimentally demonstrate that the topological properties of the ferromagnetic\nkagome compound Co3Sn2S2 are preserved upon Ni substitution. We systematically\nvary the Ni content in Co3Sn2S2 single crystals and study their magnetic and\nanomalous transport properties. For the intermediate Ni substitution, we\nobserve a remarkable increase in the coercive field while still maintaining\nsignificant anomalous Hall conductivity. The large anomalous Hall conductivity\nof these compounds is intrinsic, consistent with first-principle calculations,\nwhich proves its topological origin. Our results can guide further studies on\nthe chemical tuning of topological materials for better understanding.", "category": "cond-mat_mtrl-sci" }, { "text": "On the impact of capillarity for strength at the nanoscale: The interior of nanoscale crystals experiences stress that compensates the\ncapillary forces and that can be large, in the order of 1 GPa. Various studies\nhave speculated on whether and how this surface-induced stress affects the\nstability and plasticity of small crystals. Yet, experiments have so far failed\nto discriminate between the surface contribution and other, bulk-related size\neffects. In order to clarify the issue, we study the variation of the flow\nstress of a nanomaterial while distinctly different variations of the two\ncapillary parameters surface tension and surface stress are imposed under\ncontrol of an applied electric potential. Our theory qualifies the suggested\nimpact of $\\textit{surface stress}$ as not forceful and instead predicts a\nsignificant contribution of the surface energy, as measured by the\n$\\textit{surface tension}$. The predictions for the combined potential- and\nsize dependence of the flow stress are quantitatively supported by the\nexperiment. Previous suggestions, favoring the surface stress as the relevant\ncapillary parameter, are not consistent with the experiment.", "category": "cond-mat_mtrl-sci" }, { "text": "Spin-phonon coupling in BaFe12O19 M-type hexaferrite: The spin-phonon coupling in magnetic materials is due to the modulation of\nthe exchange integral by lattice vibrations. BaFe12O19 M-type hexaferrite,\nwhich is the most used magnetic material as permanent magnet, transforms into\nferromagnet at high temperatures, but no spin-phonon coupling was previously\nobserved at this transition. In this letter, we investigated the\ntemperature-dependent Raman spectra of polycrystalline BaFe12O19 M-type\nhexaferrite from room temperature up to 780 K to probe spin-phonon coupling at\nthe ferrimagnetic transition. An anomaly was observed in the position of the\nphonon attributed to the Fe(4)O6 octahedra, evidencing the presence of a\nspin-phonon coupling in BaM in the ferrimagnetic transition at 720 K. The\nresults also confirmed the spin-phonon coupling is different for each phonon\neven when they couple with the same spin configuration.", "category": "cond-mat_mtrl-sci" }, { "text": "Multiferroic RMnO3 thin films: Multiferroic materials have received an astonishing attention in the last\ndecades due to expectations that potential coupling between distinct ferroic\norders could inspire new applications and new device concepts. As a result, a\nnew knowledge on coupling mechanisms and materials science has dramatically\nemerged. Multiferroic RMnO3 perovskites are central to this progress providing\na suitable platform to tailor spin-spin and spin-lattice interactions. With\nviews towards applications, development of thin films of multiferroic materials\nhave also progressed enormously and nowadays thin film manganites are available\nwith properties mimicking those of bulk compounds. Here we review achievements\non the growth and characterization of magnetic and ferroelectric properties of\nhexagonal and orthorhombic RMnO3 epitaxial thin films, discuss some challenging\nissues and we suggest some guidelines for future research and developments.", "category": "cond-mat_mtrl-sci" }, { "text": "Cooling rate dependence of the antiferromagnetic domain structure of a\n single crystalline charge ordered manganite: The low temperature phase of single crystals of Nd$_{0.5}$Ca$_{0.5}$MnO$_3$\nand Gd$_{0.5}$Ca$_{0.5}$MnO$_3$ manganites is investigated by squid\nmagnetometry. Nd$_{0.5}$Ca$_{0.5}$MnO$_3$ undergoes a charge-ordering\ntransition at $T_{CO}$=245K, and a long range CE-type antiferromagnetic state\nis established at $T_N$=145K. The dc-magnetization shows a cooling rate\ndependence below $T_N$, associated with a weak spontaneous moment. The\nassociated excess magnetization is related to uncompensated spins in the\n CE-type antiferromagnetic structure, and to the presence in this state of\nfully orbital ordered regions separated by orbital domain walls. The observed\ncooling rate dependence is interpreted to be a consequence of the rearrangement\nof the orbital domain state induced by the large structural changes occurring\nupon cooling.", "category": "cond-mat_mtrl-sci" }, { "text": "Electrodynamics of magnetoelectric media and magnetoelectric fields: The relationship between magnetoelectricity and electromagnetism is a subject\nof a strong interest and numerous discussions in microwave and optical wave\nphysics and material sciences. The definition of the energy and momentum of the\nelectromagnetic (EM) field in a magnetoelectric (ME) medium is not a trivial\nproblem. The question of whether electromagnetism and magnetoelectricity can\ncoexist without an extension of Maxwell theory arises when we study the effects\nof EM energy propagation and consider group velocity of the waves in a ME\nmedium. The energy balance equation reveals unusual topological structure of\nfields in ME materials. Together with certain constraints on the constitutive\nparameters of a medium, definite constraints on the local field structure\nshould be imposed. Analyzing the EM phenomena inside a ME material, we should\nanswer the question: what kind of the near fields arising from a sample of such\na material can we measure? Visualization of the ME states requires an\nexperimental technique that is based on an effective coupling to the violation\nof spatial as well as temporal inversion symmetry. To observe the ME energy in\na subwavelength region, it is necessary to assume the existence of first\nprinciple near fields, the ME fields. These are non Maxwellian near fields with\nspecific properties of violation of spatial and temporal inversion symmetry. A\nparticular interest to the ME fields arises in studies of metamaterials with\nartificial atoms ME elements.", "category": "cond-mat_mtrl-sci" }, { "text": "Strain-induced stabilization of Al functionalization in graphene oxide\n nanosheet for enhanced NH3 storage: Strain effects on the stabilization of Al ad-atom on graphene\noxide(GO)nanosheet as well as its implications for NH3 storage have been\ninvestigated using first-principles calculations.The binding energy of Al\nad-atom on GO is found to be a false indicator of its stability.Tensile strain\nis found to be very effective in stabilizing the Al ad-atom on GO.It\nstrengthens the C-O bonds through an enhanced charge transfer from C to O\natoms. Interestingly,C-O bond strength is found to be the correct index for\nAl's stability.Optimally strained Al-functionalized GO binds up to 6 NH3\nmolecules,while it binds no NH3 molecule in unstrained condition.", "category": "cond-mat_mtrl-sci" }, { "text": "A novel coupled RPL/OSL system to understand the dynamics of the\n metastable states: Metastable states form by charge (electron and hole) capture in defects in a\nsolid. They play an important role in dosimetry, information storage, and many\nmedical and industrial applications of photonics. Despite many decades of\nresearch, the exact mechanisms resulting in luminescence signals such as\noptically/thermally stimulated luminescence (OSL or TL) or long persistent\nluminescence through charge transfer across the metastable states remain poorly\nunderstood. Our lack of understanding owes to the fact that such luminescence\nsignals arise from a convolution of several steps such as charge (de)trapping,\ntransport and recombination, which are not possible to track individually. Here\nwe present a novel coupled RPL(radio-photoluminescence)/OSL system based on an\nelectron trap in a ubiquitous, natural, geophotonic mineral called feldspar\n(aluminosilicate). RPL/OSL allows understanding the dynamics of the trapped\nelectrons and trapped holes individually. We elucidate for the first time trap\ndistribution, thermal eviction, and radiation-induced growth of trapped\nelectron and holes. The new methods and insights provided here are crucial for\nnext generation model-based applications of luminescence dating in Earth and\nenvironmental sciences, e.g. thermochronometry and photochronometry.", "category": "cond-mat_mtrl-sci" }, { "text": "The nano-structural inhomogeneity of dynamic hydrogen bond network of\n water: In the present study, water is considered as a dynamic network between\nmolecules at distances not exceeding 3.2 angstroms. The instantaneous\nconfigurations obtained by using the molecular dynamics method have been\nsequentially analyzed, the mutual orientation of each molecule with its\nneighboring molecules has been studied and the interaction energy of each pair\nof neighbor molecules has been calculated. The majority of mutual orientation\nangles between molecules lie in the interval [10, 30] degrees. It has been\nshown that more than 85% of the molecular pairs in each instantaneous\nconfiguration form H-bonds and the H-bond network includes all water molecules\nin the temperature range 233-293 K. The number of H-bonds fluctuates near the\nmean value and increases with decreasing temperature, and the energy of the\nvast majority of such bonds is much higher than the thermal energy. The\ninteraction energy of 80% of the H-bonding molecular pairs lies in the interval\n[-7; -4] kcal/mol. The interaction energy of pairs that do not satisfy the\nH-bond angle criterion lies in the interval [-5; 4] kcal/mol, and the number of\nsuch bonds does not exceed 15% and decreases with decreasing temperature. For\nthe first time it was found that in each instantaneous configuration the H-bond\nnetwork contains built-in nanometric structural heterogeneities formed by\nshorter H-bonds. The fraction of molecules involved in the structural\nheterogeneities increases from 40% to 60% with a temperature decrease from 293\nK to 233 K. These heterogeneities have a finite lifetime, but are constantly\npresent in the water. The number of large heterogeneities (containing more than\n20 molecules) increases with decreasing temperature, and the number of small\nstructural heterogeneities (less than 20) decreases.", "category": "cond-mat_mtrl-sci" }, { "text": "Digital Twins solve the mystery of Raman spectra of parental and reduced\n graphene oxides: A still amazing identity of the D-G doublet Raman spectra of parental and\nreduced graphene oxides is considered from the digital twins viewpoint. About\nthirty DTs, presenting different aspects of the GO structure and properties,\nwere virtually synthesized using atomic spin-density algorithm, which allowed\nreliably displaying reasons for this extraordinary spectral feature. In both\ncases, it was established that the D-G doublets owe their origin to the sp3-sp2\nC-C stretchings, respectively. This outwardly similar community of the doublets\norigin of GO and rGO is thoroughly analyzed to reveal different grounds of the\nfeature in the two cases. Multilayer packing of individual rGO molecules in\nstacks, in the first case, and spin-influenced prohibition of the 100%\noxidative reaction, the termination of which is accompanied with a particular\nset of highly ordered by length sp3- and sp2 C-C bonds, protecting the carbon\ncarcass from destruction caused by the stress induced sp2-to-sp3\ntransformation, in the second, are the main reasons. The DT concept has been\nrealized on the basis of virtual vibrational spectrometer HF Spectrodyn.", "category": "cond-mat_mtrl-sci" }, { "text": "Efficient prediction of grain boundary energies from atomistic\n simulations via sequential design: Data based materials science is the new promise to accelerate materials\ndesign. Especially in computational materials science, data generation can\neasily be automatized. Usually, the focus is on processing and evaluating the\ndata to derive rules or to discover new materials, while less attention is\nbeing paid on the strategy to generate the data. In this work, we show that by\na sequential design of experiment scheme, the process of generating and\nlearning from the data can be combined to discover the relevant sections of the\nparameter space. Our example is the energy of grain boundaries as a function of\ntheir geometric degrees of freedom, calculated via atomistic simulations. The\nsampling of this grain boundary energy space, or even subspaces of it,\nrepresents a challenge due to the presence of deep cusps of the energy, which\nare located at irregular intervals of the geometric parameters. Existing\napproaches to sample grain boundary energy subspaces therefore either need a\nhuge amount of datapoints or a~priori knowledge of the positions of these\ncusps. We combine statistical methods with atomistic simulations and a\nsequential sampling technique and compare this strategy to a regular sampling\ntechnique. We thereby demonstrate that this sequential design is able to sample\na subspace with a minimal amount of points while finding unknown cusps\nautomatically.", "category": "cond-mat_mtrl-sci" }, { "text": "First-principles Engineering of Charged Defects for Two-dimensional\n Quantum Technologies: Charged defects in 2D materials have emerging applications in quantum\ntechnologies such as quantum emitters and quantum computation. Advancement of\nthese technologies requires rational design of ideal defect centers, demanding\nreliable computation methods for quantitatively accurate prediction of defect\nproperties. We present an accurate, parameter-free and efficient procedure to\nevaluate quasiparticle defect states and thermodynamic charge transition levels\nof defects in 2D materials. Importantly, we solve critical issues that stem\nfrom the strongly anisotropic screening in 2D materials, that have so far\nprecluded accurate prediction of charge transition levels in these materials.\nUsing this procedure, we investigate various defects in monolayer hexagonal\nboron nitride (h-BN) for their charge transition levels, stable spin states and\noptical excitations. We identify $C_BN_V$ (nitrogen vacancy adjacent to carbon\nsubstitution of boron) to be the most promising defect candidate for scalable\nquantum bit and emitter applications.", "category": "cond-mat_mtrl-sci" }, { "text": "Accurate polarization within a unified Wannier function formalism: We present an alternative formalism for calculating the maximally localized\nWannier functions in crystalline solids, obtaining an expression which is\nextremely simple and general. In particular, our scheme is exactly invariant\nunder Brillouin zone folding, and therefore it extends trivially to the\nGamma-point case. We study the convergence properties of the Wannier functions,\ntheir quadratic spread and centers as obtained by our simplified technique. We\nshow how this convergence can be drastically improved by a simple and\ninexpensive ``refinement'' step, which allows for very efficient and accurate\ncalculations of the polarization in zero external field.", "category": "cond-mat_mtrl-sci" }, { "text": "Self-Organized Graphene/Graphite Structures Obtained Directly on Paper: We experimentally investigated the properties of graphite layers produced by\nan easy and non-conventional method of repeatedly rubbing conventional random\nstacked graphite bulk against insulating and semiconductor substrates. The\npatterned structure composed of rubbed-off and transferred layers exhibits\nproperties of a solid-state material with through-thickness anisotropy of\ncarrier mobility reaching ~10^3 cm^2/V*sec at the surface. The surface of the\nstructure demonstrates quality of more ordered and optically oriented mono or\nfew layer graphene shaped by self-organization process due to friction.\nEnhanced photoconductivity originating from modification of continuous and\nlinear valence and conduction bands caused by interaction between 4 graphene\nlayers made possible obtaining Raman spectra at near infrared excitation\nwavelength of 976 nm.", "category": "cond-mat_mtrl-sci" }, { "text": "Very low bias stress in n-type organic single crystal transistors: Bias stress effects in n-channel organic field-effect transistors (OFETs) are\ninvestigated using PDIF-CN2 single-crystal devices with Cytop gate dielectric,\nboth under vacuum and in ambient. We find that the amount of bias stress is\nvery small as compared to all (p-channel) OFETs reported in the literature.\nStressing the PDIF-CN2 devices by applying 80 V to the gate for up to a week\nresults in a decrease of the source drain current of only ~1% under vacuum and\n~10% in air. This remarkable stability of the devices leads to characteristic\ntime constants, extracted by fitting the data with a stretched exponential -\nthat are \\tau ~ 2\\cdot10^9 s in air and \\tau ~ 5\\cdot10^9 s in vacuum -\napproximately two orders of magnitude larger than the best values reported\npreviously for p-channel OFETs.", "category": "cond-mat_mtrl-sci" }, { "text": "Microstructural Effects of Chemical Island Templating in Patterned\n Matrix-Pillar Oxide Nanocomposites: The ability to pattern the location of pillars in epitaxial matrix-pillar\nnanocomposites is a key challenge to develop future technologies using these\nintriguing materials. One such model system employs a ferrimagnetic\nCoFe$_{2}$O$_{4}$ (CFO) pillar embedded in a ferroelectric BiFeO$_{3}$ (BFO)\nmatrix, which has been proposed as a possible memory or logic system. These\ncomposites self-assemble spontaneously with pillars forming through nucleation\nat a random location when grown via physical vapor deposition. Recent results\nhave shown that if an island of the pillar material is pre-patterned on the\nsubstrate, it is possible to control the nucleation process and determine the\nlocations where pillars form. In this work, we employ electron microscopy and\nx-ray diffraction to examine the chemical composition and microstructure of\npatterned CFO-BFO nanocomposites. Cross-sectional transmission electron\nmicroscopy is used to examine the nucleation effects at the interface between\nthe template island and resulting pillar.Evidence of grain boundaries and\nlattice tilting in the templated pillars is also presented and attributed to\nthe microstructure of the seed island.", "category": "cond-mat_mtrl-sci" }, { "text": "Submolecular resolution by variation of IETS amplitude and its relation\n to AFM/STM signal: Here we show scanning tunnelling microscopy (STM), non-contact atomic force\nmicroscopy (AFM) and inelastic electron tunnelling spectroscopy (IETS)\nmeasurements on organic molecule with a CO- terminated tip at 5K. The\nhigh-resolution contrast observed simultaneously in all channels unam-\nbiguously demonstrates the common imaging mechanism in STM/AFM/IETS, related to\nthe lateral bending of the CO-functionalized tip. The IETS spectroscopy reveals\nthat the submolecular con- trast at 5K consists of both renormalization of\nvibrational frequency and variation of the amplitude of IETS signal. This\nfinding is also corroborated by first principles simulations. We extend accord-\ningly the probe-particle AFM/STM/IETS model to include these two main\ningredients necessary to reproduce the high-resolution IETS contrast. We also\nemploy the first principles simulations to get more insight into different\nresponse of frustrated translation and rotational modes of CO-tip during\nimaging.", "category": "cond-mat_mtrl-sci" }, { "text": "Dielectric response of BaTiO3 electronic states under AC fields via\n microsecond time-resolved X-ray absorption spectroscopy: For the first time, the dielectric response of a BaTiO3 thin film under an AC\nelectric field is investigated using time-resolved X-ray absorption\nspectroscopy at the Ti K-edge to clarify correlated contributions of each\nconstituent atom on the electronic states. Intensities of the pre-edge eg peak\nand shoulder structure just below the main edge increase with an increase in\nthe amplitude of the applied electric field, whereas that of the main peak\ndecreases in an opposite manner. Based on the multiple scattering theory, the\nincrease and decrease of the eg and main peaks are simulated for different Ti\noff-center displacements. Our results indicate that these spectral features\nreflect the inter- and intra-atomic hybridization of Ti 3d with O 2p and Ti 4p,\nrespectively. In contrast, the shoulder structure is not affected by changes in\nthe Ti off-center displacement but is susceptible to the effect of the corner\nsite Ba ions. This is the first experimental verification of the dynamic\nelectronic contribution of Ba to polarization reversal.", "category": "cond-mat_mtrl-sci" }, { "text": "Sliding induced multiple polarization states in two-dimensional\n ferroelectrics: When the atomic layers in a non-centrosymmetric van der Waals structure slide\nagainst each other, the interfacial charge transfer results in a reversal of\nthe structures spontaneous polarization. This phenomenon is known as sliding\nferroelectricity and it is markedly different from conventional ferroelectric\nswitching mechanisms relying on ion displacement. Here, we present layer\ndependence as a new dimension to control sliding ferroelectricity. By\nfabricating 3R MoS2 of various thicknesses into dual-gate field-effect\ntransistors, we obtain anomalous intermediate polarization states in multilayer\n3R MoS2. Using results from ab initio density functional theory calculations,\nwe propose a generalized model to describe the ferroelectric switching process\nin multilayer 3R MoS2 and to explain the formation of these intermediate\npolarization states. This work reveals the critical roles that layer number and\ninterlayer dipole coupling play in sliding ferroelectricity and presents a new\nstrategy for the design of novel sliding ferroelectric devices.", "category": "cond-mat_mtrl-sci" }, { "text": "Breakdown-limited endurance in HZO FeFETs: mechanism and improvement\n under bipolar stress: Breakdown is one of main failure mechanisms that limit write endurance of\nferroelectric devices using hafnium oxide-based ferroelectric materials. In\nthis study, we investigate the gate current and breakdown characteristics of\nHf0.5Zr0.5O2/Si ferroelectric field-effect transistors (FeFETs) by using\ncarrier separation measurements to analyze electron and hole leakage currents\nduring time-dependent dielectric breakdown (TDDB) tests. Rapidly increasing\nsubstrate hole currents and stress-induced leakage current (SILC)-like electron\ncurrents can be observed before the breakdown of the ferroelectric gate\ninsulator of FeFETs. This apparent degradation under voltage stress is\nrecovered and the time-to-breakdown is significantly improved by interrupting\nthe TDDB test with gate voltage pulses with the opposite polarity, suggesting\nthat defect redistribution, rather than defect generation, is responsible for\nthe trigger of hard breakdown.", "category": "cond-mat_mtrl-sci" }, { "text": "Anomalous electronic conductance in quasicrystals: Generic quantum interference effects occuring in 1D-quasicrystals are\nreviewed with emphasis on the joint effect of phason disorder on electronic\nlocalization and propagation modes. In close conjunction with properties of\nreal materials, the contributions of quantum interferences in several regimes\nclose to the metal insulator transition are outlined.", "category": "cond-mat_mtrl-sci" }, { "text": "Van der Waals density functionals applied to solids: The van der Waals density functional (vdW-DF) of Dion et al. [Phys. Rev.\nLett. 92, 246401 (2004)] is a promising approach for including dispersion in\napproximate density functional theory exchange-correlation functionals. Indeed,\nan improved description of systems held by dispersion forces has been\ndemonstrated in the literature. However, despite many applications, standard\ngeneral tests on a broad range of materials are lacking. Here we calculate the\nlattice constants, bulk moduli, and atomization energies for a range of solids\nusing the original vdW-DF and several of its offspring. We find that the\noriginal vdW-DF overestimates lattice constants in a similar manner to how it\noverestimates binding distances for gas phase dimers. However, some of the\nmodified vdW functionals lead to average errors which are similar to those of\nPBE or better. Likewise, atomization energies that are slightly better than\nfrom PBE are obtained from the modified vdW-DFs. Although the tests reported\nhere are for \"hard\" solids, not normally materials for which dispersion forces\nare thought to be important, we find a systematic improvement in cohesive\nproperties for the alkali metals and alkali halides when non-local correlations\nare accounted for.", "category": "cond-mat_mtrl-sci" }, { "text": "A New Kind of Atlas of Zeolite Building Blocks: We have analysed structural motifs in the Deem database of hypothetical\nzeolites, to investigate whether the structural diversity found in this\ndatabase can be well-represented by classical descriptors such as distances,\nangles, and ring sizes, or whether a more general representation of atomic\nstructure, furnished by the smooth overlap of atomic positions (SOAP) method,\nis required to capture accurately structure-property relations. We assessed the\nquality of each descriptor by machine-learning the molar energy and volume for\neach hypothetical framework in the dataset. We have found that SOAP with a\ncutoff-length of 6 \\AA, which goes beyond near-neighbor tetrahedra, best\ndescribes the structural diversity in the Deem database by capturing relevant\ninter-atomic correlations. Kernel principal component analysis shows that SOAP\nmaintains its superior performance even when reducing its dimensionality to\nthose of the classical descriptors, and that the first three kernel principal\ncomponents capture the main variability in the data set, allowing a 3D point\ncloud visualization of local environments in the Deem database. This ``cloud\natlas\" of local environments was found to show good correlations with the\ncontribution of a given motif to the density and stability of its parent\nframework. Local volume and energy maps constructed from the\nSOAP/machine-learning analyses provide new images of zeolites that reveal\nsmooth variations of local volumes and energies across a given framework, and\ncorrelations between local volume and energy in a given framework.", "category": "cond-mat_mtrl-sci" }, { "text": "Comments on All-electron self-consistent GW in the Matsubara-time\n domain: Implementation and benchmarks of semiconductors and insulators: Chu et al. recently reported extensive results of local density approximation\n(LDA) and of four (4) different Green function and dressed Coulomb (GW)\napproximation calculations of electronic properties of several semiconductors\nand insulators [Phys. Rev. B 93, 125210 (2016)].", "category": "cond-mat_mtrl-sci" }, { "text": "Two types of magnetic bubbles in MnNiGa observed via Lorentz microscopy: Magnetic bubbles are remarkable spin structures that developed in uniaxial\nmagnets with strong magnetocrystalline anisotropy. Several contradictory\nreports have been published concerning the magnetic bubble structure in a\nmetallic magnet MnNiGa: Biskyrmions or type-II bubbles. Lorentz microscopy in\npolycrystalline MnNiGa was used to explain the magnetic bubble structure.\nDepending on the connection between the magnetic easy axis and the observation\nplane, two types of magnetic bubbles were formed. Magnetic bubbles with\n180{\\deg} domains were formed if the easy axis was away from the direction\nperpendicular to the observation plane. The contrast of biskyrmion is\nreproduced by this form of a magnetic bubble. When the easy axis was\napproximately perpendicular to the observing plane, type-II bubbles were\nobserved in the same specimen. The findings will fill a knowledge gap between\nprior reports on magnetic bubbles in MnNiGa.", "category": "cond-mat_mtrl-sci" }, { "text": "Ultralow Work Function of the Electride Sr$_3$CrN$_3$: Electrides have valence electrons that occupy free space in the crystal\nstructure, making them easier to extract. This feature can be used in catalysis\nfor important reactions that usually requires a high-temperature and\nhigh-pressure environments, such as ammonia synthesis. In this paper, we use\ndensity functional theory to investigate the behaviour of interstitial\nelectrons of the 1-dimensional electride Sr$_3$CrN$_3$. We find that the bulk\nexcess electron density persists on introduction of surface terminations, that\nthe crystal termination perpendicular to the 1D free-electron channel is highly\nstable and we confirm an extremely low work function with hybrid functional\nmethods. Our results indicate that Sr$_3$CrN$_3$ is a potentially important\nnovel catalyst, with accessible, directional and extractable free electron\ndensity.", "category": "cond-mat_mtrl-sci" }, { "text": "Can single crystal X-ray diffraction determine a structure uniquely?: The diffraction technique is widely used in the determination of crystal\nstructures and is one of the bases for the modern science and technology. All\nrelated structure determination methods are based on the assumption that\nperfect single crystal X-ray diffraction (SXRD) can determine a structure\nuniquely. But as the structure factor phases are lost in SXRD and even more\ninformation is lost in powder X-ray diffraction (PXRD), this assumption is\nstill questionable. In this work, we found that structures with certain\ncharacteristic can have its sister structure with exactly the same PXRD or even\nSXRD pattern. A computer program is developed to search the ICSD database, and\nabout 1000 structures were identified to have this characteristic. The original\nstructure and its sister structures can have different space groups,\ntopologies, crystal systems etc. and some may even have multiple sisters. Our\nstudies indicate that special caution is needed since a structure with\nreasonable atomic positions and perfect match of experimental diffraction\nintensities could still be wrong.", "category": "cond-mat_mtrl-sci" }, { "text": "Next-generation non-local van der Waals density functional: The fundamental ideas for a non-local density functional theory -- capable of\nreliably capturing van der Waals interaction -- were already conceived in the\n1990's. In 2004, a seminal paper introduced the first practical non-local\nexchange-correlation functional called vdW-DF, which has become widely\nsuccessful and laid the foundation for much further research. However, since\nthen, the functional form of vdW-DF has remained unchanged. Several successful\nmodifications paired the original functional with different (local) exchange\nfunctionals to improve performance and the successor vdW-DF2 also updated one\ninternal parameter. Bringing together different insights from almost two\ndecades of development and testing, we present the next-generation non-local\ncorrelation functional called vdW-DF3, in which we change the functional form\nwhile staying true to the original design philosophy. Although many popular\nfunctionals show good performance around the binding separation of van der\nWaals complexes, they often result in significant errors at larger separations.\nWith vdW-DF3, we address this problem by taking advantage of a recently\nuncovered and largely unconstrained degree of freedom within the vdW-DF\nframework that can be constrained through empirical input, making our\nfunctional semi-empirical. For two different parameterizations, we benchmark\nvdW-DF3 against a large set of well-studied test cases and compare our results\nwith the most popular functionals, finding good performance in general for a\nwide array of systems and a significant improvement in accuracy at larger\nseparations. Finally, we discuss the achievable performance within the current\nvdW-DF framework, the flexibility in functional design offered by vdW-DF3, as\nwell as possible future directions for non-local van der Waals density\nfunctional theory.", "category": "cond-mat_mtrl-sci" }, { "text": "Effect of acetylene links on electronic and optical properties of\n semiconducting graphynes: The family of graphynes, novel two-dimensional semiconductors with various\nand fascinating chemical and physical properties, has attracted great interest\nfrom both science and industry. Currently, the focus of graphynes is on\ngraphdiyne, or graphyne-2. In this work, we systematically study the effect of\nacetylene, i.e., carbon-carbon triple bond, links on the electronic and optical\nproperties of a series of graphynes (graphyne-n, where n = 1-5, the number of\nacetylene bonds) using the ab initio calculations. We find an even-odd pattern,\ni.e., n = 1, 3, 5 and n = 2, 4 having different features, which has not be\ndiscovered in studying graphyne or graphdyine only. It is found that as the\nnumber of acetylene bonds increases, the electron effective mass increases\ncontinuously in the low energy range because of the flatter conduction band\ninduced by the longer acetylene links. Meanwhile, longer acetylene links result\nin larger redshift of the imaginary part of the dielectric function, loss\nfunction, and extinction coefficient. In this work, we propose an effective\nmethod to tune and manipulate both the electronic and optical properties of\ngraphynes for the applications in optoelectronic devices and photo-chemical\ncatalysis.", "category": "cond-mat_mtrl-sci" }, { "text": "Quasi-Two-Dimensional Extraordinary Hall Effect: Quasi-two-dimensional transport is investigated in a system consisting of one\nferromagnetic layer placed between two insulating layers. Using the mechanism\nof skew-scattering to describe the Extraordinary Hall Effect (EHE) and\ncalculating the conductivity tensor, we compare the quasi- two-dimensional Hall\nresistance with the resistance of a massive sample. In this study a new\nmechanism of EHE (geometric mechanism of EHE) due to non-ideal interfaces and\nvolume defects is also proposed.", "category": "cond-mat_mtrl-sci" }, { "text": "Thermal Conductivity and Mechanical Properties of Nitrogenated Holey\n Graphene: Nitrogenated holey graphene (NHG), a two-dimensional graphene-derived\nmaterial with a C2N stoichiometry and evenly distributed holes and nitrogen\natoms in its basal plane, has recently been synthesized. We performed first\nprinciples calculations and molecular dynamics simulations to investigate\nmechanical and heat transport properties of this novel two-dimensional material\nat various temperatures. First principles calculations based on density\nfunctional theory yield an elastic modulus of 400 +/- 5 GPa at 0 K, 10% larger\nthan predicted by molecular dynamics simulations at low temperatures. We\nobserved an overall decreasing trend in elastic modulus and tensile strength as\ntemperature increases. At room temperature, we found that NHG can present a\nremarkable elastic modulus of 335 +/- 5 GPa and tensile strength of 60 GPa. We\nalso investigated the thermal conductivity of NHG via non-equilibrium molecular\ndynamics simulations. At 300 K an intrinsic thermal conductivity of 64.8 W/m-K\nwas found, with an effective phonon mean free path of 34.0 nm, both of which\nare smaller than respective values for graphene, and decrease with temperature.\nOur modeling-based predictions should serve as guide to experiments concerning\nphysical properties of this novel material.", "category": "cond-mat_mtrl-sci" }, { "text": "Complete characterization of the macroscopic deformations of periodic\n unimode metamaterials of rigid bars and pivots: A complete characterization is given of the possible macroscopic deformations\nof periodic nonlinear affine unimode metamaterials constructed from rigid bars\nand pivots. The materials are affine in the sense that their macroscopic\ndeformations can only be affine deformations: on a local level the deformation\nmay vary from cell to cell. Unimode means that macroscopically the material can\nonly deform along a one dimensional trajectory in the six dimensional space of\ninvariants describing the deformation (excluding translations and rotations).\nWe show by explicit construction that any continuous trajectory is realizable\nto an arbitrarily high degree of approximation provided at all points along the\ntrajectory the geometry does not collapse to a lower dimensional one. In\nparticular, we present two and three dimensional dilational materials having an\narbitrarily large flexibility window. These are perfect auxetic materials for\nwhich a dilation is the only easy mode of deformation. They are free to dilate\nto arbitrarily large strain with zero bulk modulus.", "category": "cond-mat_mtrl-sci" }, { "text": "The rise of Single-Atom Catalysts: In recent years, single-atom catalysts attracted lots of attention because of\ntheir high catalytic activity, selectivity, stability, maximum atom\nutilization, exceptional performance, and low cost. Single-atom catalyst\ncontains isolated individual atom which are coordinated with the surface atoms\nof support such as a metal oxide or 2d - materials. In this review article, we\npresent the advancement in single-atom catalysis in recent years with a focus\non the various synthesis methods and their application in catalytic reactions.\nWe also demonstrate the reaction mechanism of a single-atom catalyst for\ndifferent catalytic reactions from theoretical aspects using density functional\ntheory.", "category": "cond-mat_mtrl-sci" }, { "text": "GPU Acceleration of Large-Scale Full-Frequency GW Calculations: Many-body perturbation theory is a powerful method to simulate electronic\nexcitations in molecules and materials starting from the output of density\nfunctional theory calculations. By implementing the theory efficiently so as to\nrun at scale on the latest leadership high-performance computing systems it is\npossible to extend the scope of GW calculations. We present a GPU acceleration\nstudy of the full-frequency GW method as implemented in the WEST code.\nExcellent performance is achieved through the use of (i) optimized GPU\nlibraries, e.g., cuFFT and cuBLAS, (ii) a hierarchical parallelization strategy\nthat minimizes CPU-CPU, CPU-GPU, and GPU-GPU data transfer operations, (iii)\nnonblocking MPI communications that overlap with GPU computations, and (iv)\nmixed-precision in selected portions of the code. A series of performance\nbenchmarks have been carried out on leadership high-performance computing\nsystems, showing a substantial speedup of the GPU-accelerated version of WEST\nwith respect to its CPU version. Good strong and weak scaling is demonstrated\nusing up to 25920 GPUs. Finally, we showcase the capability of the GPU version\nof WEST for large-scale, full-frequency GW calculations of realistic systems,\ne.g., a nanostructure, an interface, and a defect, comprising up to 10368\nvalence electrons.", "category": "cond-mat_mtrl-sci" }, { "text": "Hydrogen trapping and embrittlement in high-strength Al-alloys: Ever more stringent regulations on greenhouse gas emissions from\ntransportation motivate efforts to revisit materials used for vehicles.\nHigh-strength Al-alloys often used in aircrafts could help reduce the weight of\nautomobiles, but are susceptible to environmental degradation. Hydrogen (H)\n\"embrittlement\" is often pointed as the main culprit, however, the mechanisms\nunderpinning failure are elusive: atomic-scale analysis of H inside an alloy\nremains a challenge, and this prevents deploying alloy design strategies to\nenhance the materials' durability. Here we successfully performed near-atomic\nscale analysis of H trapped in second-phase particles and at grain boundaries\nin a high-strength 7xxx Al-alloy. We used these observations to guide atomistic\nab-initio calculations which show that the co-segregation of alloying elements\nand H favours grain boundary decohesion, while the strong partitioning of H\ninto the second-phases removes solute H from the matrix, hence preventing\nH-embrittlement. Our insights further advance the mechanistic understanding of\nH-assisted embrittlement in Al-alloys, emphasizing the role of H-traps in\nretarding cracking and guiding new alloy design.", "category": "cond-mat_mtrl-sci" }, { "text": "Magnetic behavior of a spin-1 dimer: model system for homodinuclear\n nickel (II) complexes: Magnetic behavior of a spin-1 Heisenberg dimer is analysed in dependence on\nboth uniaxial single-ion anisotropy and XXZ exchange anisotropy in a zero- as\nwell as non-zero longitudinal magnetic field. A complete set of eigenfunctions\nand eigenvalues of the total Hamiltonian is presented together with an exact\nanalytical expression for the Gibbs free energy, longitudinal magnetization,\nlongitudinal and transverse susceptibility. The obtained theoretical results\nare compared with the relevant experimental data of\n[Ni2(Medpt)2(ox)(H2O)2](ClO4)2.2H2O (Medpt = methyl-bis(3-aminopropyl)amine).", "category": "cond-mat_mtrl-sci" }, { "text": "Electronic and energetic properties of Ge(110) pentagons: The electronic and energetic properties of the elementary building block,\ni.e. a five-membered atom ring (pentagon), of the Ge(110) surface was studied\nby scanning tunneling microscopy and spectroscopy at room temperature. The\nGe(110) surface is composed of three types of domains: two ordered domains\n((16x2) and c(8x10)) and a disordered domain. The elementary building block of\nall three domains is a pentagon. Scanning tunneling spectra recorded on the\n(16x2), c(8x10) and disordered domains are very similar and reveal three\nwell-defined electronic states. Two electronic states are located 1.1 eV and\n0.3 eV below the Fermi level respectively, whereas the third electronic state\nis located 0.4 eV above the Fermi level. The electronic states at -0.3 eV and\n0.4 eV can be ascribed to the pentagons, whilst we tentatively assigned the\nelectronic state at -1.1 eV to a Ge-Ge back bond or trough state. In addition,\nwe have analyzed the straight [1-12] oriented step edges. From the kink density\nand kink-kink distance distributions we extracted the nearest neighbor\ninteraction energy between the pentagons, which exhibit a strong preference to\noccur in twins, as well as the strain relaxation energy along the pentagon-twin\nchains.", "category": "cond-mat_mtrl-sci" }, { "text": "Traversal of pulses through negative ($\\varepsilon$, $\u03bc$) materials: We study the traversal times of electromagnetic pulses across dispersive\nmedia with negative dielectric permittivity ($\\varepsilon$) and magnetic\npermeability ($\\mu$) parameters. First we investigate the transport of optical\npulses through an electrical plasma and a negative refractive index medium\n(NRM) of infinite and semi-infinite extents where no resonant effects come into\nplay. The total delay time of the pulse constitutes of the group delay time and\nthe reshaping delay time as analyzed by Peatross et al \\cite{peatross}. For\nevanescent waves, even with broadband width, the total delay time is negative\nfor an infinite medium whereas it is positive for the semi-infinite case.\nEvidence of the Hartman effect is seen for small propagation distance compared\nto the free space pulse length. The reshaping delay mostly dominates the total\ndelay time in NRM whereas it vanishes when $\\varepsilon(\\omega)=\\mu(\\omega)$.\n Next we present results on the propagation times through a dispersive slab.\nWhile both large bandwidth and large dissipation have similar effects in\nsmoothening out the resonant features that appear due to Fabry-P\\'{e}rot\nresonances, large dissipation can result in very small or even negative\ntraversal times near the resonant frequencies. We investigate the traversal and\nthe Wigner delay times for obliquely incident pulses. The coupling of\nevanescent waves to slab plasmon polariton modes results in large traversal\ntimes at the resonant conditions. We also find that the group velocity mainly\ncontributes to the delay time for pulse propagating across a slab with\nrefractive index (n) = -1. The traversal times are positive and subluminal for\npulses with sufficiently large bandwidths.", "category": "cond-mat_mtrl-sci" }, { "text": "Elastic properties and mechanical stability of bilayer graphene:\n Molecular dynamics simulations: Graphene has become in last decades a paradigmatic example of two-dimensional\nand so-called van-der-Waals layered materials, showing large anisotropy in\ntheir physical properties. Here we study the elastic properties and mechanical\nstability of graphene bilayers in a wide temperature range by molecular\ndynamics simulations. We concentrate on in-plane elastic constants and\ncompression modulus, as well as on the atomic motion in the out-of-plane\ndirection. Special emphasis is placed upon the influence of anharmonicity of\nthe vibrational modes on the physical properties of bilayer graphene. We\nconsider the excess area appearing in the presence of ripples in graphene\nsheets at finite temperatures. The in-plane compression modulus of bilayer\ngraphene is found to decrease for rising temperature, and results to be higher\nthan for monolayer graphene. We analyze the mechanical instability of the\nbilayer caused by an in-plane compressive stress. This defines a spinodal\npressure for the metastability limit of the material, which depends on the\nsystem size. Finite-size effects are described by power laws for the\nout-of-plane mean-square fluctuation, compression modulus, and spinodal\npressure. Further insight into the significance of our results for bilayer\ngraphene is gained from a comparison with data for monolayer graphene and\ngraphite.", "category": "cond-mat_mtrl-sci" }, { "text": "New ultrahigh pressure phases of H2O ice predicted using an adaptive\n genetic algorithm: We propose three new phases of H2O under ultrahigh pressure. Our structural\nsearch was performed using an adaptive genetic algorithm which allows an\nextensive exploration of crystal structure. The new sequence of\npressure-induced transitions beyond ice X at 0 K should be ice X - Pbcm - Pbca\n- Pmc21 - P21 - P21/c phases. Across the Pmc21 - P21 transition, the\ncoordination number of oxygen increases from 4 to 5 with a significant increase\nof density. All stable crystalline phases have nonmetallic band structures up\nto 7 TPa.", "category": "cond-mat_mtrl-sci" }, { "text": "Low temperature investigations and surface treatments of colloidal\n narrowband fluorescent nanodiamonds: We report fluorescence investigations and Raman spectroscopy on colloidal\nnanodiamonds (NDs) obtained via bead assisted sonic disintegration (BASD) of a\npolycrystalline chemical vapor deposition film. The BASD NDs contain in situ\ncreated silicon vacancy (SiV) centers. Whereas many NDs exhibit emission from\nSiV ensembles, we also identify NDs featuring predominant emission from a\nsingle bright SiV center. We demonstrate oxidation of the NDs in air as a tool\nto optimize the crystalline quality of the NDs via removing damaged regions\nresulting in a reduced ensemble linewidth as well as single photon emission\nwith increased purity. We furthermore investigate the temperature dependent\nzero-phonon-line fine-structure of a bright single SiV center as well as the\npolarization properties of its emission and absorption.", "category": "cond-mat_mtrl-sci" }, { "text": "Detecting Chiral Orbital Angular Momentum by Circular Dichroism ARPES: We show, by way of tight-binding and first-principles calculations, that a\none-to-one correspondence between electron's crystal momentum k and non-zero\norbital angular momentum (OAM) is a generic feature of surface bands. The OAM\nforms a chiral structure in momentum space much as its spin counterpart in\nRashba model does, as a consequence of the inherent inversion symmetry breaking\nat the surface but not of spin-orbit interaction. Circular dichroism (CD)\nangle-resolved photoemission (ARPES) experiment is an efficient way to detect\nthis new order, and we derive formulas explicitly relating the CD-ARPES signal\nto the existence of OAM in the band structure. The cases of degenerate p- and\nd-orbital bands are considered.", "category": "cond-mat_mtrl-sci" }, { "text": "Nano-thermodynamics of chemically induced graphene-diamond\n transformation: Nearly two-dimensional diamond, or diamane, is coveted as ultrathin\n$sp^3$-carbon film with unique mechanics and electro-optics. The very thinness\n($~h$) makes it possible for the surface chemistry, e.g. adsorbed atoms, to\nshift the bulk phase thermodynamics in favor of diamond, from multilayer\ngraphene. Thermodynamic theory coupled with atomistic first principles\ncomputations predicts not only the reduction of required pressure\n($p/p_{\\infty}>1-h_0/h$), but also the nucleation barriers, definitive for the\nkinetic feasibility of diamane formation. Moreover, the optimal adsorbent\nchair-pattern on a bilayer graphene results in a cubic diamond lattice, while\nfor thicker precursors the adsorbent boat-structure tends to produce hexagonal\ndiamond (lonsdaleite), if graphene was in AA` stacking to start with. As\nadsorbents, H and F are conducive to diamond formation, while Cl appears\nsterically hindered.", "category": "cond-mat_mtrl-sci" }, { "text": "Microscopic theory of ionic motion in solid electrolytes: We propose a microscopic, first-principles description of the ionic\nconduction in crystals. This formalism allows us to gain new insights into the\nideal characteristics of general ionic conducting materials and, in particular,\nsolid electrolytes. Using \\textit{ab initio} calculations, we show that our\nformalism results in ionic mobilities consistent with experiments for several\nmaterials. Our work opens the possibility of developing solid electrolytes\nbased on fundamental physical principles rather than empirical descriptions of\nthe underlying processes.", "category": "cond-mat_mtrl-sci" }, { "text": "Limitations of ab initio methods to predict the electronic-transport\n properties of two-dimensional materials: The computational example of\n 2H-phase transition metal dichalcogenides: Over the last few years, $ab~initio$ methods have become an increasingly\npopular tool to evaluate intrinsic carrier transport properties in 2D\nmaterials. The lack of experimental information, and the progress made in the\ndevelopment of DFT tools to evaluate electronic band structures, phonon\ndispersions, and electron-phonon scattering matrix-elements, have made them a\nfavored choice. However, a large discrepancy is observed in the literature\namong the $ab~initio$ calculated carrier mobility in 2D materials. Some of the\ndiscrepancies are a result of the physical approximations made in calculating\nthe electron-phonon coupling constants and the carrier mobility. These\napproximations can be avoided by using a sophisticated transport model.\nHowever, despite using appropriate transport models, the uncertainty in the\nreported carrier mobility is still quite large in some materials. The major\ndifferences observed between these refined model calculations are the `flavors'\nof DFT (exchange-correlation functional, pseudopotential, and the effect of\nspin-orbit coupling) used. Here, considering several monolayer 2H-TMDs as\nexamples, we calculate the low- and high-field transport properties using\ndifferent `flavors' of DFT, and calculate a range for the electron mobility\nvalues. We observe that in some materials the values differ by orders of\nmagnitude (For example, in monolayer WS$_{2}$ the electron low-field mobility\nvaries between 37 cm$^{2}$/(V$\\cdot$s) and 767 cm$^{2}$/(V$\\cdot$s)). We\nanalyze critically these discrepancies, and try to understand the limitations\nof the current $ab~initio$ methods in calculating carrier transport properties.", "category": "cond-mat_mtrl-sci" }, { "text": "Magnetic kagome materials RETi3Bi4 family with weak interlayer\n interactions: Kagome materials have attracted a surge of research interest recently,\nespecially for the ones combining with magnetism, and the ones with weak\ninterlayer interactions which can fabricate thin devices. However, kagome\nmaterials combining both characters of magnetism and weak interlayer\ninteractions are rare. Here we investigate a new family of titanium based\nkagome materials RETi3Bi4 (RE = Eu, Gd and Sm). The flakes of nanometer\nthickness of RETi3Bi4 can be obtained by exfoliation due to the weak interlayer\ninteractions. According to magnetic measurements, out-of-plane ferromagnetism,\nout-of-plane anti-ferromagnetism, and in-plane ferromagnetism are formed for RE\n= Eu, Gd, and Sm respectively. The magnetic orders are simple and the\nsaturation magnetizations can be relatively large since the rare earth elements\nsolely provide the magnetic moments. Further by angle-resolved photoemission\nspectroscopy (ARPES) and first-principles calculations, the electronic\nstructures of RETi3Bi4 are investigated. The ARPES results are consistent with\nthe calculations, indicating the bands characteristic with kagome sublattice in\nRETi3Bi4. We expect these materials to be promising candidates for observation\nof the exotic magnetic topological phases and the related topological quantum\ntransport studies.", "category": "cond-mat_mtrl-sci" }, { "text": "Electronic Janus lattice and kagome-like bands in coloring-triangular\n MoTe2 monolayers: Polymorphic structures of transition-metal dichalcogenides (TMDs) host exotic\nelectronic states, like charge density wave and superconductivity. However, the\nnumber of these structures is limited by crystal symmetries, which poses a\nchallenge to achieve tailored lattices and properties both theoretically and\nexperimentally. Here, we report a coloring-triangle (CT) latticed MoTe2\nmonolayer, termed CT-MoTe2, constructed by controllably introducing uniform and\nordered mirror-twin-boundaries into a pristine monolayer in molecular beam\nepitaxy. Low-temperature scanning tunneling microscopy and spectroscopy\n(STM/STS) together with theoretical calculations reveal that the monolayer has\nan electronic Janus lattice, i.e., an energy-dependent atomic-lattice and a\npseudo-Te sublattice, and shares the identical geometry with the Mo5Te8 layer.\nDirac-like and flat electronic bands inherently existing in the CT lattice are\nidentified by two broad and two prominent peaks in STS spectra, respectively,\nand verified with density-functional-theory calculations. Two types of\nintrinsic domain boundaries were observed, in one of which the\nelectronic-Janus-lattice feature maintains, implying potential applications as\nan energy-tunable electron-tunneling barrier in future functional devices.", "category": "cond-mat_mtrl-sci" }, { "text": "Pressure tuning the Fermi-surface topology of the Weyl semimetal NbP: We report on the pressure evolution of the Fermi surface topology of the Weyl\nsemimetal NbP, probed by Shubnikov-de Haas oscillations in the\nmagnetoresistance combined with ab-initio calculations of the band-structure.\nAlthough we observe a drastic effect on the amplitudes of the quantum\noscillations, the frequencies only exhibit a weak pressure dependence up to 2.8\nGPa. The pressure-induce variations in the oscillation frequencies are\nconsistent with our band-structure calculations. Furthermore, we can relate the\nchanges in the amplitudes to small modifications in the shape of the Fermi\nsurface. Our findings evidenced the stability of the electronic band structure\nof NbP and demonstrate the power of combining quantum-oscillation studies and\nband-structure calculations to investigate pressure effects on the\nFermi-surface topology in Weyl semimetals.", "category": "cond-mat_mtrl-sci" }, { "text": "Recent advances in the internal functionalization of carbon nanotubes:\n synthesis, optical, and magnetic resonance studies: The hollow inside of single-wall carbon nanotubes (SWCNT) provides a unique\ndegree of freedom to investigate chemical reactions inside this confined\nenvironment and to study the tube properties. It is reviewed herein, how\nencapsulating fullerenes, magnetic fullerenes, $^{13}$C isotope enriched\nfullerenes and organic solvents inside SWCNTs enables to yield unprecedented\ninsight into their electronic, optical, and interfacial properties and to study\ntheir growth. Encapsulated C$_{60}$ fullerenes are transformed to inner tubes\nby a high temperature annealing. The unique, low defect concentration of inner\ntubes makes them ideal to study the effect of diameter dependent treatments\nsuch as opening and closing of the tubes. The growth of inner tubes is achieved\nfrom $^{13}$C enriched encapsulated organic solvents, which shows that\nfullerenes do not have a distinguished role and it opens new perspectives to\nexplore the in-the-tube chemistry. Encapsulation of magnetic fullerenes, such\nas N@C$_{60}$ and C$_{59}$N is demonstrated using ESR. Growth of inner tubes\nfrom $^{13}$C enriched fullerenes provides a unique isotope engineered\nheteronuclear system, where the outer tubes contain natural carbon and the\ninner walls are controllably $^{13}$C isotope enriched. The material enables to\nidentify the vibrational modes of inner tubes which otherwise strongly overlap\nwith the outer tube modes. The $^{13}$C NMR signal of the material is specific\nfor the small diameter SWCNTs. Temperature and field dependent $^{13}$C $T_1$\nstudies show a uniform metallic-like electronic state for all inner tubes and a\nlow energy, ~3 meV gap is observed that is assigned to a long sought Peierls\ntransition.", "category": "cond-mat_mtrl-sci" }, { "text": "Native point defects in few-layer phosphorene: Using hybrid density functional theory combined with a semiempirical van der\nWaals dispersion correction, we have investigated the structural and electronic\nproperties of vacancies and self-interstitials in defective few-layer\nphosphorene. We find that both a vacancy and a self-interstitial defect are\nmore stable in the outer layer than in the inner layer. The formation energy\nand transition energy of both a vacancy and a self-interstitial P defect\ndecrease with increasing film thickness, mainly due to the upward shift of the\nhost valence band maximum in reference to the vacuum level. Consequently, both\nvacancies and self-interstitials could act as shallow acceptors, and this well\nexplains the experimentally observed p-type conductivity in few-layer\nphosphorene. On the other hand, since these native point defects have moderate\nformation energies and are stable in negatively charged states, they could also\nserve as electron compensating centers in n-type few-layer phosphorene.", "category": "cond-mat_mtrl-sci" }, { "text": "${\\it Ab\\ initio}$ thermodynamic properties of certain compounds in\n Nd-Fe-B system: In this work, we report the results of \\emph{ab initio} calculations of\nthermochemical properties of several compounds in the Fe-Nd, B-Nd and B-Fe-Nd\nsystems. We have performed DFT+U calculations to compute the enthalpy of\nformation of the compounds NdB$_6$, NdB$_4$, Nd$_2$B$_5$, Nd$_2$Fe$_{17}$ and\nNd$_5$Fe$_2$B$_6$. It was found that the values obtained with an effective\nHubbard $U$ correction have better agreement with the experimental data. We\nhave also computed the vibrational contribution to the heat capacity ($C_p$) of\nthe compounds as a function of temperature was computed using the quasharmonic\napproximation. For most of the compounds these properties have not been\nexperimentally determined until now. Hence, the computed \\emph{ab initio}\nthermodynamic properties will serve as useful input for the Gibbs energy model\nparameter assessment using the CALPHAD method.", "category": "cond-mat_mtrl-sci" }, { "text": "Jacutingaite-family: a class of topological materials: Jacutingate, a recently discovered Brazilian naturally occurring mineral, has\nshown to be the first experimental realization of the Kane-Mele topological\nmodel. In this letter we have unveiled a class of materials $M_2NX_3$ ($M$=Ni,\nPt, Pd; $N$=Zn, Cd, Hg; and $X$=S, Se, Te), sharing jacutingaite's key\nfeatures, i.e., high stability, and topological phase. By employing\nfirst-principles calculations we extensively characterize the energetic\nstability of this class while showing a common occurrence of the Kane-Mele\ntopological phase. Here we found Pt-based materials surpassing jacutingaite's\nimpressive topological gap and lower exfoliation barrier while retaining its\nstability.", "category": "cond-mat_mtrl-sci" }, { "text": "Challenges for density functional theory in simulating metal-metal\n singlet bonding: a case study of dimerized VO2: VO2 is renowned for its electric transition from an insulating monoclinic\n(M1) phase characterized by V-V dimerized structures, to a metallic rutile (R)\nphase above 340 Kelvin. This transition is accompanied by a magnetic change:\nthe M1 phase exhibits a non-magnetic spin-singlet state, while the R phase\nexhibits a state with local magnetic moments. Simultaneous simulation of the\nstructural, electric, and magnetic properties of this compound is of\nfundamental importance, but the M1 phase alone has posed a significant\nchallenge to density functional theory (DFT). In this study, we show none of\nthe commonly used DFT functionals, including those combined with on-site\nHubbard U to better treat 3d electrons, can accurately predict the V-V dimer\nlength. The spin-restricted method tends to overestimate the strength of the\nV-V bonds, resulting in a small V-V bond length. Conversely, the\nspin-symmetry-breaking method exhibits the opposite trends. Each\nbond-calculation method underscores one of the two contentious mechanisms,\ni.e., Peierls or Mott, involved in the metal-insulator transition in VO2. To\nelucidate the challenges encountered in DFT, we also employ an effective\nHamiltonian that integrates one-dimensional magnetic sites, thereby revealing\nthe inherent difficulties linked with the DFT computations.", "category": "cond-mat_mtrl-sci" }, { "text": "An ab-initio study on physical properties of Pd2+ incorporated double\n perovskites CaPd3B4O12 (B = Ti, V): Numerous physical properties of CaPd3Ti4O12 (CPTO) and CaPd3V4O12 (CPVO)\ndouble perovskites have been explored based on density functional theory (DFT).\nThe calculated structural parameters fairly agree with the experimental data to\nconfirm their stability. The mechanical stability of these two compounds was\nclearly observed by the Born stability criteria. To rationalize the mechanical\nbehavior, we investigate elastic constants, bulk, shear and Young's modulus,\nPugh's ratio, Poisson's ratio and elastic anisotropy index. The ductility index\nconfirms that both materials are ductile in nature. The electronic band\nstructure of CPTO and CPVO reveals the direct band gap semiconducting in nature\nand metallic characteristics, respectively. The calculated partial density of\nstates indicates the strong hybridization between Pd 4d and O 2p orbital\nelectrons for CPTO and Pd 4d and V 3d O 2p for CPVO. The study of electronic\ncharge density map confirms the coexistence of covalent, ionic and metallic\nbonding for both compounds. Fermi surface calculation of CPVO ensures both\nelectron and hole like surfaces indicating the multiple band nature. In the\nmidst of optical properties, photoconductivity and absorption coefficient of\nboth compounds reveal well qualitative compliance with consequences of band\nstructure computations. Among the thermodynamic properties, the Debye\ntemperature has been calculated to correlate its topical features including\nthermoelectric behavior. The studied thermoelectric transport properties of\nCPTO yielded the Seebeck coefficient (186 microVK-1), power factor (11.9\nmicroWcm-1K-2) and figure of merit (ZT) value of about 0.8 at 800 K indicate\nthat this material could be a promising candidate for thermoelectric device\napplication.", "category": "cond-mat_mtrl-sci" }, { "text": "Study of anharmonicity in Zirconium Hydrides using inelastic neutron\n scattering and ab-initio computer modeling: The anharmonic phenomena in Zirconium Hydrides and Deuterides, including\n{\\epsilon}-ZrH2, {\\gamma}-ZrH, and {\\gamma}-ZrD, have been investigated from\naspects of inelastic neutron scattering (INS) and lattice dynamics calculations\nwithin the framework of density functional theory (DFT). The observed multiple\nsharp peaks below harmonic multi-phonon bands in the experimental spectra of\nall three materials did not show up in the simulated INS spectra based on the\nharmonic approximation, indicating the existence of strong anharmonicity in\nthose materials and the necessity of further explanations. We present a\ndetailed study on the anharmonicity of zirconium hydrides/deuterides by\nexploring the 2D potential energy surface of hydrogen/deuterium atoms, and\nsolving the corresponding 2D single-particle Schrodinger equation to get the\neigenfrequencies. The obtained results well describe the experimental INS\nspectra and show harmonic behavior in the fundamental modes and strong\nanharmonicity at higher energies.", "category": "cond-mat_mtrl-sci" }, { "text": "Spin wavepackets in the Kagome ferromagnet Fe$_3$Sn$_2$: propagation and\n precursors: The propagation of spin waves in magnetically ordered systems has emerged as\na potential means to shuttle quantum information over large distances.\nConventionally, the arrival time of a spin wavepacket at a distance, $d$, is\nassumed to be determined by its group velocity, $v_g$. He we report\ntime-resolved optical measurements of wavepacket propagation in the Kagome\nferromagnet Fe$_3$Sn$_2$ that demonstrate the arrival of spin information at\ntimes significantly less than $d/v_g$. We show that this spin wave \"precursor\"\noriginates from the interaction of light with the unusual spectrum of\nmagnetostatic modes in Fe$_3$Sn$_2$. Related effects may have far-reaching\nconsequences toward realizing long-range, ultrafast spin wave transport in both\nferromagnetic and antiferromagnetic systems.", "category": "cond-mat_mtrl-sci" }, { "text": "Controllable thickness inhomogeneity and Berry-curvature-engineering of\n anomalous Hall effect in SrRuO3 ultrathin films: In quantum matters hosting electron-electron correlation and spin-orbit\ncoupling, spatial inhomogeneities, arising from competing ground states, can be\nessential for determining and understanding topological properties. A prominent\nexample is Hall anomalies observed in SrRuO3 films, which were interpreted in\nterms of either magnetic skyrmion-induced topological Hall effect (THE) or\ninhomogeneous anomalous Hall effect (AHE). To clarify this ambiguity, we\nsystematically investigated the AHE of SrRuO3 ultrathin films with controllable\ninhomogeneities in film thickness (tSRO). By harnessing the step-flow growth of\nSrRuO3 films, we induced microscopically-ordered stripes with one-unit-cell\ndifferences in tSRO. The resultant spatial distribution of momentum-space Berry\ncurvatures enables a two-channel AHE, which shows hump-like anomalies similar\nto the THE and can be continuously engineered via sub-unit-cell control of\ntSRO. In these inhomogeneous SRO films, we microscopically identified a\ntwo-step magnetic switching and stripe-like ferromagnetic domains. These\nfeatures are fingerprints for distinguishing the two-channel AHE from the\nskyrmion-induced THE.", "category": "cond-mat_mtrl-sci" }, { "text": "Simple prediction of immiscible metal alloying based on metastability\n analysis: It has been known that even though two elemental metals, $X$ and $Y$, are\nimmiscible, they can form alloys on surfaces of other metal $Z$. In order to\nunderstand such surface alloying of immiscible metals, we study the energetic\nstability of binary alloys, $XZ$ and $YZ$, in several structures with various\ncoordination numbers (CNs). By analyzing the formation energy modified to\nenhance the subtle energy difference between metastable structures, we find\nthat $XZ$ and $YZ$ with B2-type structure (CN$=$8) become energetically stable\nwhen the $X$ and $Y$ metals form an alloy on the $Z$ metal surface. This is\nconsistent with the experimental results for Pb-Sn alloys on metal surfaces\nsuch as Rh(111) and Ru(0001). Some suitable metal substrates are also predicted\nto form Pb-Sn alloys.", "category": "cond-mat_mtrl-sci" }, { "text": "Melting of graphene: from two to one dimension: The high temperature behaviour of graphene is studied by atomistic\nsimulations based on an accurate interatomic potential for carbon. We find that\nclustering of Stone-Wales defects and formation of octagons are the first steps\nin the process of melting which proceeds via the formation of carbon chains.\nThe molten state forms a three-dimensional network of entangled chains rather\nthan a simple liquid. The melting temperature estimated from the\ntwo-dimensional Lindemann criterion and from extrapolation of our simulation\nfor different heating rates is about 4900 K.", "category": "cond-mat_mtrl-sci" }, { "text": "Exploring conformational energy landscape of glassy disaccharides by\n CPMAS 13C NMR and DFT/GIAO simulations. II. Enhanced molecular flexibility in\n amorphous trehalose: This paper deals with the comparative use of the chemical shift surfaces to\nsimulate experimental 13C CPMAS data on amorphous solid state disaccharides,\npaying particular attention to -1-1 linkage of trehalose, to\n-1,4 linkage between pyranose rings (lactose) and to linkage implying a\nfuranose ring (sucrose). The combination of molecular mechanics with DFT/GIAO\nab-initio methods provides reliable structural information on the\nconformational distribution in the glass. The results are interpreted in terms\nof an enhanced flexibility that trehalose experiences in amorphous solid state\ncompared to the other sugars. An attempt to relate this property to the balance\nbetween intra- and inter-molecular hydrogen bonding network in the glass is\npresented.", "category": "cond-mat_mtrl-sci" }, { "text": "Topological Crystalline Insulator Nanostructures: Topological crystalline insulators are topological insulators whose surface\nstates are protected by the crystalline symmetry, instead of the time reversal\nsymmetry. Similar to the first generation of three-dimensional topological\ninsulators such as Bi2Se3 and Bi2Te3, topological crystalline insulators also\npossess surface states with exotic electronic properties such as spin-momentum\nlocking and Dirac dispersion. Experimentally verified topological crystalline\ninsulators to date are SnTe, Pb1-xSnxSe, and Pb1-xSnxTe. Because topological\nprotection comes from the crystal symmetry, magnetic impurities or in-plane\nmagnetic fields are not expected to open a gap in the surface states in\ntopological crystalline insulators. Additionally, because they are cubic\nstructure instead of layered structure, branched structures or strong coupling\nwith other materials for large proximity effects are possible, which are\ndifficult with layered Bi2Se3 and Bi2Te3. Thus, additional fundamental\nphenomena inaccessible in three-dimensional topological insulators can be\npursued. In this review, topological crystalline insulator SnTe nanostructures\nwill be discussed. For comparison, experimental results based on SnTe thin\nfilms will be covered. Surface state properties of topological crystalline\ninsulators will be discussed briefly.", "category": "cond-mat_mtrl-sci" }, { "text": "Phase locking of vortex based spin transfer oscillators to a microwave\n current: Phase locking experiments on vortex based spin transfer oscillators with an\nexternal microwave current are performed. We present clear evidence of phase\nlocking, frequency pulling, as well as fractional synchronization in this\nsystem, with a minimum peak linewidth of only 3 kHz in the locked state. We\nfind that locking ranges of the order of 1/3 of the oscillator frequency are\neasily achievable because of the large tunability $\\partial f/\\partial I_{dc}$\nobserved in our vortex based systems. Such large locking ranges allow us to\ndemonstrate the simultaneous phase locking of two independent oscillators\nconnected in series with the external source.", "category": "cond-mat_mtrl-sci" }, { "text": "Quasi-equilibrium optical nonlinearities in spin-polarized GaAs: Semiconductor Bloch equations, which microscopically describe the dynamics of\na Coulomb interacting, spin-unpolarized electron-hole plasma, can be solved in\ntwo limits: the coherent and the quasi-equilibrium regime. These equations have\nbeen recently extended to include the spin degree of freedom, and used to\nexplain spin dynamics in the coherent regime. In the quasi-equilibrium limit,\none solves the Bethe-Salpeter equation in a two-band model to describe how\noptical absorption is affected by Coulomb interactions within a\nspin-unpolarized plasma of arbitrary density. In this work, we modified the\nsolution of the Bethe-Salpeter equation to include spin-polarization and light\nholes in a three-band model, which allowed us to account for spin-polarized\nversions of many-body effects in absorption. The calculated absorption\nreproduced the spin-dependent, density-dependent and spectral trends observed\nin bulk GaAs at room temperature, in a recent pump-probe experiment with\ncircularly polarized light. Hence our results may be useful in the microscopic\nmodelling of density-dependent optical nonlinearities in spin-polarized\nsemiconductors.", "category": "cond-mat_mtrl-sci" }, { "text": "Second-phase nucleation on an edge dislocation: A model for nucleation of second phase at or around dislocation in a\ncrystalline solid is considered. The model employs the Ginzburg-Landau theory\nof phase transition comprising the sextic term in order parameter in the Landau\nfree energy. The ground state solution of the linearized time-independent\nGinzburg-Landau equation has been derived, through which the spatial variation\nof the order parameter has been delineated. Moreover, a generic phase diagram\nindicating a tricritical behavior near and away from the dislocation is\ndepicted. The relation between the classical nucleation theory and the\nGinzburg-Landau approach has been discussed, for which the critical formation\nenergy of nucleus is related to the maximal of the Landau potential energy. A\nnumerical example illustrating the application of the model to the case of\nnucleation of hydrides in zirconium alloys is provided.", "category": "cond-mat_mtrl-sci" }, { "text": "Giant thermoelectric figure of merit in multivalley\n high-complexity-factor LaSO: We report a giant thermoelectric figure of merit $ZT$ (up to 6 at 1100 K) in\n$n$-doped lanthanum oxysulphate LaSO. Thermoelectric coefficients are computed\nfrom ab initio bands within Bloch-Boltzmann theory in an energy-, chemical\npotential- and temperature-dependent relaxation time approximation. The lattice\nthermal conductivity is estimated from a model employing the ab initio phonon\nand Gr\\\"uneisen-parameter spectrum. The main source of the large $ZT$ is the\nsignificant power factor which correlates with a large band complexity factor.\nWe also suggest a possible $n$-type dopant for the material based on ab initio\ncalculations.", "category": "cond-mat_mtrl-sci" }, { "text": "Static Hopf Solitons and Knotted Emergent Fields in Solid-State\n Noncentrosymmetric Magnetic Nanostructures: Two-dimensional topological solitons, commonly called Skyrmions, are\nextensively studied in solid-state magnetic nanostructures and promise many\nspintronics applications. However, three-dimensional topological solitons\ndubbed hopfions have not been demonstrated as stable spatially localized\nstructures in solid-state magnetic materials. Here we model the existence of\nsuch static solitons with different Hopf index values in noncentrosymmetric\nsolid magnetic nanostructures with a perpendicular interfacial magnetic\nanisotropy. We show how this surface anisotropy, along with the\nDzyaloshinskii-Moriya interactions and the geometry of nanostructures,\nstabilize hopfions. We demonstrate knots in emergent field lines and computer\nsimulate Lorentz transmission electron microscopy images of such solitonic\nconfigurations to guide their experimental discovery in magnetic solids.", "category": "cond-mat_mtrl-sci" }, { "text": "Creating and modulating electronic states on noble metal surfaces:\n ultrathin Ag islands on Si(111)-7$\\times$7 as a prototype: Various-thickness Ag islands were prepared on Si(111)-7$\\times$7 using the\none-step deposition at a high substrate temperature. An electronic state\ncentered at -0.40$\\sim$-0.15eV versus E$_{Fermi}$, detectable on the surface of\nthe Ag islands thinner than 9 layers, was created by the electronic\nhybridization between Ag and Si at the Ag-Si interface. Scanning tunneling\nmicroscopy/spectroscopy and density functional theory revealed that the\nthickness of Ag islands determined the strength of the hybridization, leading\nto a modulation to the energy and intensity of the state on the surface.", "category": "cond-mat_mtrl-sci" }, { "text": "Ground-state polariton condensation in 2D-GaAs semiconductor\n microcavities: We observe ground-state polariton condensation in a two dimensional GaAs/AlAs\nsemiconductor microcavity under non resonant pulsed optical excitation. We\nresolve the formation of a polariton condensate by studying the spatial,\nangular, coherence, energy and transient dynamics of polariton\nphotoluminescence. For high excitation densities we also observe a transition\nfrom the weak- to the strong-coupling regime in the time-domain and resolve the\nbuild-up of a coherent polariton state.", "category": "cond-mat_mtrl-sci" }, { "text": "Optical Rotatory Dispersion of $\u03b1$-quartz: It is shown that some formulae describing optical rotatory dispersion of\n$\\alpha$-quartz with the aid of two Drude's terms reduce to the combined\nformula containing one Drude's and one Chandrasekhar's term. Comparison of\nvarious formulae describing the experimental data of $\\alpha$-quartz leads to\nthe conclusion that the optical activity of this crystal is due to its crystal\nstructure only, that means the optical activity is not of molecular origin.\nFurther the rotatory strengths are discussed with the regard to coupled\noscillator model and to the structure of $\\alpha$-quartz.", "category": "cond-mat_mtrl-sci" }, { "text": "Theory of volumetric capacitance of an electric double-layer\n supercapacitor: Electric double layer supercapacitors are a fast-rising class of high-power\nenergy storage devices based on porous electrodes immersed in a concentrated\nelectrolyte or ionic liquid. As of yet there is no microscopic theory to\ndescribe their surprisingly large capacitance per unit volume (volumetric\ncapacitance) of ~ 100 F/cm^3, nor is there a good understanding of the\nfundamental limits on volumetric capacitance. In this paper we present a\nnon-mean-field theory of the volumetric capacitance of a supercapacitor that\ncaptures the discrete nature of the ions and the exponential screening of their\nrepulsive interaction by the electrode. We consider analytically and via\nMonte-Carlo simulations the case of an electrode made from a good metal and\nshow that in this case the volumetric capacitance can reach the record values.\nWe also study how the capacitance is reduced when the electrode is an imperfect\nmetal characterized by some finite screening radius. Finally, we argue that a\ncarbon electrode, despite its relatively large linear screening radius, can be\napproximated as a perfect metal because of its strong nonlinear screening. In\nthis way the experimentally-measured capacitance values of ~ 100 F/cm^3 may be\nunderstood.", "category": "cond-mat_mtrl-sci" }, { "text": "A comparison of Monte-Carlo simulations using RESTRAX and McSTAS with\n experiment on IN14: Monte-Carlo simulations of a focusing supermirror guide after the\nmonochromator on the IN14 cold neutron three-axis spectrometer, I.L.L. were\ncarried out using the instrument simulation programs RESTRAX and McSTAS. The\nsimulations were compared to experiment to check their accuracy. Comparisons of\nthe flux ratios over both a 100 mm2 and a 1600 mm2 area at the sample position\ncompare well, and there is very close agreement between simulation and\nexperiment for the energy spread of the incident beam.", "category": "cond-mat_mtrl-sci" }, { "text": "Molecular Beam Epitaxy of a Half-Heusler Topological Superconductor\n Candidate YPtBi: The search for topological superconductivity has motivated investigations\ninto materials that combine topological and superconducting properties. The\nhalf-Heusler compound YPtBi appears to be such a material, however experiments\nhave thus far been limited to bulk single crystals, drastically limiting the\nscope of available experiments. This has made it impossible to investigate the\npotential topological nature of the superconductivity in this material.\nExperiments to access details about the superconducting state require\nsophisticated lithographic structures, typically based on thin films. Here we\nreport on the establishment of high crystalline quality epitaxial thin films of\nYPtBi(111), grown using molecular beam epitaxy on Al2O3(0001) substrates. A\nrobust superconducting state is observed, with both critical temperature and\ncritical field consistent with that previously reported for bulk crystals.\nMoreover we find that AlOx capping sufficiently protects the sample surface\nfrom degradation to allow for proper lithography. Our results pave a path\ntowards the development of advanced lithographic structures, that will allow\nthe exploration of the potentially topological nature of superconductivity in\nYPtBi.", "category": "cond-mat_mtrl-sci" }, { "text": "Sn delta-doping in GaAs: We have prepared a number of GaAs structures delta-doped by Sn using the\nwell-known molecular beam epitaxy growth technique. The samples obtained for a\nwide range of Sn doping densities were characterised by magnetotransport\nexperiments at low temperatures and in high magnetic fields up to 38 T.\nHall-effect and Shubnikov-de Haas measurements show that the electron densities\nreached are higher than for other delta-dopants, like Si and Be. The maximum\ncarrier density determined by the Hall effect equals 8.4x10^13 cm^-2. For all\nsamples several Shubnikov-de Haas frequencies were observed, indicating the\npopulation of multiple subbands. The depopulation fields of the subbands were\ndetermined by measuring the magnetoresistance with the magnetic field in the\nplane of the delta-layer. The experimental results are in good agreement with\nselfconsistent bandstructure calculations. These calculation shows that in the\nsample with the highest electron density also the conduction band at the L\npoint is populated.", "category": "cond-mat_mtrl-sci" }, { "text": "Low-temperature Raman scaterring of PMN-PT close to the morphotropic\n phase boundary: This paper has been withdrawn by the author due to a crucial sign error in\nequation.", "category": "cond-mat_mtrl-sci" }, { "text": "Energy landscape of relaxed amorphous silicon: We analyze the structure of the energy landscape of a well-relaxed 1000-atom\nmodel of amorphous silicon using the activation-relaxation technique (ART\nnouveau). Generating more than 40,000 events starting from a single minimum, we\nfind that activated mechanisms are local in nature, that they are distributed\nuniformly throughout the model and that the activation energy is limited by the\ncost of breaking one bond, independently of the complexity of the mechanism.\nThe overall shape of the activation-energy-barrier distribution is also\ninsensitive to the exact details of the configuration, indicating that\nwell-relaxed configurations see essentially the same environment. These results\nunderscore the localized nature of relaxation in this material.", "category": "cond-mat_mtrl-sci" }, { "text": "Effects of the Position Reversal of Friction Pairs on the Strength of\n Tribocharging and Tribodischarging: The friction-induced charging (i.e., tribocharging) and the following\ndischarging (referred here as tribodischarging) are always believed to have\nnegative effects on the daily life and on the industrial production. Thus, how\nto inhibit the tribocharging and the tribodischarging has caused wide public\nconcern. Because the discharge caused by the electrical breakdown of the\nambient gas is generally accompanied with the generation of light, we\ninvestigated here the tribocharging and the tribodischarging by observing the\nlight emitted during friction. We found that the position reversal of the\nfriction pair has a dramatic impact on the intensity of the tribo-induced\nlight. Experimental results show that an intense light is produced when a\nstationary Al2O3 disk is sliding on a rotating SiO2 disk, but only a weak light\nis observed for the case of a stationary SiO2 disk and a rotating Al2O3 disk.\nThis means that the process of the tribocharging and the tribodischarging can\nbe significantly influenced owing to the change in the relative position of the\nfriction couple. The experimentally measured polarities of the tribo-induced\ncharge on the friction surfaces further indicated that the strong discharging\noccurs when the rotating surface is negatively charged. The reason for the\ndifference in the intensity of the tribocharging and tribodischarging can be\nattributed to the combined effects of the contact potential difference and the\ntemperature gradient between the contacting surfaces on the charge transfer\nwhen friction. Finally, a simple, low cost, yet effective approach, i.e., just\nkeep the friction partner whose surface is tribo-induced negatively charged as\nthe stationary one, can be utilized to suppress the intensity of the\ntribocharging and the tribodischarging. This work may provide potential\napplications in numerous areas of science and engineering and also in the\neveryday life.", "category": "cond-mat_mtrl-sci" }, { "text": "Influence of surface anisotropy on the hysteresis of magnetic\n nanoparticles: We present the results of Monte Carlo simulations of the magnetic properties\nof individual spherical nanoparticles with the aim to explain the role played\nby surface anisotropy on their low temperature magnetization processes. Phase\ndiagrams for the equilibrium configurations have been obtained, showing a\nchange from quasi-uniform magnetization state to a state with hedgehog-like\nstructures at the surface as $k_S$ increases. Through the simulated hysteresis\nloops and the analysis of spin configurations along them, we have identified a\nchange in the magnetization reversal mechanism from quasi-uniform rotation at\nlow $k_S$ values, to a non-uniform switching process at high $k_S$. Results for\nthe dependence of the coercive field and remanence on $k_S$ and particle size\nare also reported.", "category": "cond-mat_mtrl-sci" }, { "text": "A One-Dimensional Coordination Polymer, BBDTA-InCl4; Possible\n Spin-Peierls Transition with High Critical Temperature of 108 K: We have studied the crystal structure and magnetic properties of the organic\nradical cation salt, BBDTA-InCl4. This material formed a one-dimensional\ncoordination polymer, whose structure was characteristic of inorganic\nspin-Peierls materials. Magnetic measurements indicated the spin-Peierls\ntransition like behavior at 108 K, which was higher than those typically\nobserved for the other organic spin-Peierls materials. The structural aspects\nof the lattice distortion from X-ray diffraction measurements at 50 K have been\ndiscussed.", "category": "cond-mat_mtrl-sci" }, { "text": "Twin interaction with $\u03a3$11 tilt grain boundaries in BCC Fe :\n Formation of new grain boundaries: It is well known that the twinning is an important mode of plastic\ndeformation in nanocrystalline materials. As a result, it is expected that the\ntwin can interact with different grain boundaries (GBs) during the plastic\ndeformation. Understanding these twin-GB interactions is crucial for our\nunderstanding of mechanical behavior of materials. In this work, the twin\ninteraction with different $\\Sigma$11 symmetric and asymmetric tilt GBs has\nbeen investigated in BCC Fe using molecular dynamics (MD) simulations. The\nresults indicate that twin nucleate from the crack or GB and, its interaction\nwith $\\Sigma$11 asymmetric tilt GBs leads to the formation of a new GB. This\nnew GB consist of $<$100$>$ Cottrell type immobile dislocations. The detailed\natomistic mechanisms responsible for this new GB formation have been revealed\nusing atomistic simulations. Interestingly, the new GB formation has not been\nobserved in the case of twin interaction with $\\Sigma$11 symmetric tilt GBs.", "category": "cond-mat_mtrl-sci" }, { "text": "Simulation of stress-impedance effects in low magnetostrictive films: A theoretical study of stress-impedance effect based on the solution of\nLandau-Lifsitz-Gilbert equation has been carried out. The results show that\nstress impedance effects depend largely on several extrinsic (external bias\nfield, external frequency) and intrinsic (orientation and magnitude of uniaxial\nanisotropy, damping) parameters.", "category": "cond-mat_mtrl-sci" }, { "text": "Patterns and driving forces of dimensionality-dependent charge density\n waves in 2H-type transition metal dichalcogenides: Two-dimensional (2D) materials have become a fertile playground for the\nexploration and manipulation of novel collective electronic states. Recent\nexperiments have unveiled a variety of robust 2D orders in highly-crystalline\nmaterials ranging from magnetism to ferroelectricity and from superconductivity\nto charge density wave (CDW) instability. The latter, in particular, appears in\ndiverse patterns even within the same family of materials with isoelectronic\nspecies. Furthermore, how they evolve with dimensionality has so far remained\nelusive. Here we propose a general framework that provides a unfied picture of\nCDW ordering in the 2H polytype of four isoelectronic transition metal\ndichalcogenides 2H-MX$_2$ (M=Nb, Ta and X=S, Se). We first show experimentally\nthat whilst NbSe$_2$ exhibits a strongly enhanced CDW order in the 2D limit,\nthe opposite trend exists for TaSe$_2$ and TaS$_2$, with CDW being entirely\nabsent in NbS$_2$ from its bulk to the monolayer. Such distinct behaviours are\nthen demonstrated to be the result of a subtle, yet profound, competition\nbetween three factors: ionic charge transfer, electron-phonon coupling, and the\nspreading extension of the electronic wave functions. Despite its simplicity,\nour approach can, in essence, be applied to other quasi-2D materials to account\nfor their CDW response at different thicknesses, thereby shedding new light on\nthis intriguing quantum phenomenon and its underlying mechanisms.", "category": "cond-mat_mtrl-sci" }, { "text": "Phonon-mediated sticking of electrons at dielectric surfaces: We study phonon-mediated temporary trapping of an electron in\npolarization-induced external surface states (image states) of a dielectric\nsurface. Our approach is based on a quantum-kinetic equation for the occupancy\nof the image states. It allows us to distinguish between prompt and kinetic\nsticking. Because the depth of the image potential is much larger than the\nDebye energy multi-phonon processes are important. Taking two-phonon processes\ninto account in cases where one-phonon processes yield a vanishing transition\nprobability, as it is applicable, for instance, to graphite, we analyze the\nadsorption scenario as a function of potential depth and surface temperature\nand calculate prompt and kinetic sticking coefficients. We find rather small\nsticking coefficients, at most of the order of $10^{-3}$, and a significant\nsuppression of the kinetic sticking coefficient due to a relaxation bottleneck\ninhibiting thermalization of the electron with the surface at short timescales.", "category": "cond-mat_mtrl-sci" }, { "text": "The effect of intrinsic point defects on ferroelectric polarization\n behavior of SrTiO$_3$: The effect of a variety of intrinsic defects and defect clusters in bulk and\nthin films of SrTiO$_3$ on ferroelectric polarization and switching mechanism\nis investigated by means of density-functional-theory (DFT) based calculations\nand the Berry phase approach. Our results show that both the titanium\nTi$_\\mathrm{Sr}^{\\bullet \\bullet}$ and strontium Sr$_\\mathrm{Ti}^{\"}$ antisite\ndefects induce ferroelectric polarization in SrTiO$_3$, with the\nTi$_\\mathrm{Sr}^{\\bullet \\bullet}$ defect causing a more pronounced spontaneous\npolarization and higher activation barriers of polarization reversal than\nSr$_\\mathrm{Ti}^{\"}$. The presence of oxygen vacancies bound to the antisite\ndefects can either enhance or diminish polarization depending on the\nconfiguration of the defect pair, but it always leads to larger activation\nbarriers of polarization switching as compared to the antisite defects with no\noxygen vacancies. We also show that the magnitude of spontaneous polarization\nin SrTiO$_3$ can be tuned by controlling the degree of Sr/Ti nonstroichiometry.\nOther intrinsic point defects such as Frenkel defect pairs and electron small\npolarons also contribute to the emergence of ferroelectric polarization in\nSrTiO$_{3}$.", "category": "cond-mat_mtrl-sci" }, { "text": "Time-, spin-, and angle-resolved photoemission spectroscopy with a 1-MHz\n 10.7-eV pulse laser: We describe a setup of time-, spin-, and angle-resolved photoemission\nspectroscopy (tr-SARPES) employing a 10.7-eV ($\\lambda$=115.6 nm) pulse laser\nat 1-MHz repetition rate as a probe photon source. This equipment effectively\ncombines technologies of a high-power Yb:fiber laser, ultraviolet-driven\nharmonic generation in Xe gas, and a SARPES apparatus equipped with\nvery-low-energy-electron-diffraction (VLEED) spin detectors. A high repetition\nrate (1 MHz) of the probe laser allows experiments with the photoemission\nspace-charge effects significantly reduced, despite a high flux of 10$^{13}$\nphotons/s on the sample. The relatively high photon energy (10.7 eV) also\nbrings the capability of observing a wide momentum range that covers the entire\nBrillouin zone of many materials while ensuring high momentum resolution. The\nexperimental setup overcomes a low efficiency of spin-resolved measurements,\nwhich gets even more severe for the pump-probed unoccupied states, and affords\nfor investigating ultrafast electron and spin dynamics of modern quantum\nmaterials with energy and time resolutions of 25 meV and 360 fs, respectively.", "category": "cond-mat_mtrl-sci" }, { "text": "Understanding correlations in BaZrO3:Structure and dynamics on the\n nano-scale: Barium zirconate BaZrO3 is one of few perovskites that is claimed to retain\nan average cubic structure down to 0K at ambient pressure, while being\nenergetically very close to a tetragonal phase obtained by condensation of a\nsoft phonon mode at the R-point. Previous studies suggest, however, that the\nlocal structure of BaZrO3 may change at low temperature forming nanodomains or\na glass-like phase. Here, we investigate the global and local structure of\nBaZrO3 as a function of temperature and pressure via molecular dynamics\nsimulations using a machine-learned potential with near density functional\ntheory (DFT) accuracy. We show that the softening of the octahedral tilt mode\nat the R-point gives rise to weak diffuse superlattice reflections at low\ntemperatures and ambient pressure, which are also observed experimentally.\nHowever, we do not observe any static nanodomains but rather soft dynamic\nfluctuations of the ZrO6 octahedra with a correlation length of 2 to 3nm over\ntime-scales of about 1ps. This soft dynamic behaviour is the precursor of a\nphase transition and explains the emergence of weak superlattice peaks in\nmeasurements. On the other hand, when increasing the pressure at 300K we find a\nphase transition from the cubic to the tetragonal phase at around 16GPa, also\nin agreement with experimental studies.", "category": "cond-mat_mtrl-sci" }, { "text": "First principles calculations of steady-state voltage-controlled\n magnetism: application to x-ray absorption spectroscopy experiment: Recent x-ray absorption experiments have demonstrated the possibility to\naccurately monitor the magnetism of metallic hetero-structures controlled via a\ntime-independent perturbation caused for example by a static electric field.\nUsing a first-principles, non-equilibrium Green function scheme, we show how\nthe measured dichroic signal for the corresponding steady-state situation can\nbe related to the underlying electronic structure and its response to the\nexternal stimulus. The suggested approach works from the infinitesimal limit of\nlinear response to the regime of strong electric field effects, which is\nrealized in present experimental high sensitivity investigations.", "category": "cond-mat_mtrl-sci" }, { "text": "Orientation before destruction. A multiscale molecular dynamics study: The emergence of ultra-fast X-ray free-electron lasers opens the possibility\nof imaging single molecules in the gas phase at atomic resolution. The main\ndisadvantage of this imaging technique is the unknown orientation of the sample\nexposed to the X-ray beam, making the three dimensional reconstruction not\ntrivial. Induced orientation of molecules prior to X-ray exposure can be highly\nbeneficial, as it significantly reduces the number of collected diffraction\npatterns whilst improving the quality of the reconstructed structure. We\npresent here the possibility of protein orientation using a time-dependent\nexternal electric field. We used ab initio simulations on Trp-cage protein to\nprovide a qualitative estimation of the field strength required to break\nprotein bonds, with 45 V/nm as a breaking point value. Furthermore, we\nsimulated, in a classical molecular dynamics approach, the orientation of\nubiquitin protein by exposing it to different time-dependent electric fields.\nThe protein structure was preserved for all samples at the moment orientation\nwas achieved, which we denote `orientation before destruction'. Moreover, we\nfind that the minimal field strength required to induce orientation within ten\nns of electric field exposure, was of the order of 0.5 V/nm. Our results help\nexplain the process of field orientation of proteins and can support the design\nof instruments for protein orientation.", "category": "cond-mat_mtrl-sci" }, { "text": "Giant electrophononic response in PbTiO$_3$ by strain engineering: We demonstrate theoretically how, by imposing epitaxial strain in a\nferroelectric perovskite, it is possible to achieve a dynamical control of\nphonon propagation by means of external electric fields, which yields a giant\nelectrophononic response, i.e. the dependence of the lattice thermal\nconductivity on external electric fields. Specifically, we study the\nstrain-induced manipulation of the lattice structure and analyze its interplay\nwith the electrophononic response. We show that tensile biaxial strain can\ndrive the system to a regime where the electrical polarization can be\neffortlessly rotated and thus yield giant electrophononic responses that are at\nleast one order of magnitude larger than in the unstrained system. These\nresults derive directly from the almost divergent behavior of the electrical\nsusceptibility at those critical strains that drive the polarization on the\nverge of a spontaneous rotation.", "category": "cond-mat_mtrl-sci" }, { "text": "The Benefits of Trace Cu in Wrought Al-Mg Alloys: The softening and strengthening contributions in pre-deformed and aged\nAl-Mg-Cu alloys containing 3wt.%Mg and 0.5wt.%Cu are evaluated by a combination\nof microscopy, mechanical testing and modelling. A refined phenomenological\nmodel for the work hardening response, accounting for the separate effects of\nrecovery and precipitation, is shown to be suitable for an unambiguous\ndetermination of the precipitation hardening contribution in these alloys.\nSignificantly, it is found that the mechanical response of these alloys is not\nstrongly impacted by Cu content (in the low Cu content regime), pre-deformation\nlevel or aging temperature meaning that the alloys are robust with respect to\nvariations in composition. This is interesting from the perspective of alloy\ndesign concepts based on `recycling friendly' compositions in applications that\ninclude paint-baking.", "category": "cond-mat_mtrl-sci" }, { "text": "Modeling of Nucleation Processes: Nucleation is the onset of a first-order phase transition by which a\nmetastable phase transforms into a more stable one. Such a phase transition\noccurs when an initial system initially in equilibrium is destabilized by the\nchange of an external parameter like the temperature or the pressure. If the\nperturbation is small enough, the system does not become unstable but rather\nstays metastable. In diffusive transformations, the system then evolves through\nthe nucleation, the growth and the coarsening of a second phase. Such a phase\ntransformation is found in a lot of situations in materials science like\ncondensation of liquid droplets from a supersaturated vapor, solidification,\nprecipitation from a supersaturated solid solution, ... The initial stage of\nall these different processes can be well described within the same framework.\nSince its initial formulation in 1927 by Volmer, Weber and Farkas and its\nmodification in 1935 by Becker and D\\\"oring the classical nucleation theory has\nbeen a suitable tool to model the nucleation stage in phase transformations. In\nthis article, we first describe this theory. A kinetic approach, the cluster\ndynamics, can also be used to describe nucleation. This constitutes the second\npart of this article. The links as well as the difference between both\ndescriptions are emphasized. Since its initial formulation, the classical\nnucleation theory has been enriched, so as to take into account the fact that\nclusters other than monomers can migrate and react. It has been also extended\nto multi-component systems. These generalizations of the initial formalism are\nalso presented.", "category": "cond-mat_mtrl-sci" }, { "text": "Brownian Motion of Graphene: We study the Brownian motion (BM) of optically trapped graphene flakes. These\norient orthogonal to the light polarization, due to the optical constants\nanisotropy. We explain the flake dynamics, measure force and torque constants\nand derive a full electromagnetic theory of optical trapping. The understanding\nof two dimensional BM paves the way to light-controlled manipulation and\nall-optical sorting of biological membranes and anisotropic macromolecules.", "category": "cond-mat_mtrl-sci" }, { "text": "Modification of electron states in CdTe absorber due to a buffer layer\n in CdS/CdTe solar cells: By application of the ac admittance spectroscopy method, the defect state\nenergy distributions were determined in CdTe incorporated in thin film solar\ncell structures concluded on ZnO, ZnSe, and ZnS buffer layers. Together with\nthe Mott-Schottky analysis, the results revealed a strong modification of the\ndefect density of states and the concentration of the uncompensated acceptors\nas influenced by the choice of the buffer layer. In the solar cells formed on\nZnSe and ZnS, the Fermi level and the energy position of the dominant deep trap\nlevels were observed to shift closer to the midgap of CdTe suggesting the\nmid-gap states may act as recombination centers and impact the open-circuit\nvoltage and the fill factor of the solar cells. For the deeper states, the\nbroadening parameter was observed to increase indicating fluctuations of the\ncharge on a microscopic scale. Such changes can be attributed to the\ngrain-boundary strain and the modification of the charge trapped at the\ngrain-boundary interface states in polycrystalline CdTe", "category": "cond-mat_mtrl-sci" }, { "text": "Hafnia for analog memristor: Influence of stoichiometry and crystalline\n structure: The highly non-linear switching behavior of hafnia memristor actually hinders\nits wide application in neuromorphic computing. Theoretical understanding into\nits switching mechanism has been focused on the processes of conductive\nfilament generation and rupture, but possible phase transition and\ncrystallization around the region of conductive filaments (CFs) due to the\nvariation of O content have been paid less attention to. In this paper,\nHfO$\\mathrm{_x}$ structural models covering the full stoichiometries from Hf to\nHfO$\\mathrm{_2}$ were established, and the crystal structure evolution during\nthe reduction process of hafnia was obtained through first-principles\ncalculation. The electronic structures and O vacancy migration characteristics\nof these structures were analyzed. A criterion was prescribed to predict the\nmode of abrupt binary switching or gradual conductance modulation according to\nthe structure evolution of the CFs. In particular, factors that influence the\nmerging of tiny conductive channels into strong filaments are intensively\ndiscussed, including the anisotropy of O vacancy migration and the size effect.\nThe feasibility of Mg doping to achieve robust gradual switching is discussed.", "category": "cond-mat_mtrl-sci" }, { "text": "Quantitative characterization of surface topography using spectral\n analysis: Roughness determines many functional properties of surfaces, such as\nadhesion, friction, and (thermal and electrical) contact conductance. Recent\nanalytical models and simulations enable quantitative prediction of these\nproperties from knowledge of the power spectral density (PSD) of the surface\ntopography. The utility of the PSD is that it contains statistical information\nthat is unbiased by the particular scan size and pixel resolution chosen by the\nresearcher. In this article, we first review the mathematical definition of the\nPSD, including the one- and two-dimensional cases, and common variations of\neach. We then discuss strategies for reconstructing an accurate PSD of a\nsurface using topography measurements at different size scales. Finally, we\ndiscuss detecting and mitigating artifacts at the smallest scales, and\ncomputing upper/lower bounds on functional properties obtained from models. We\naccompany our discussion with virtual measurements on computer-generated\nsurfaces. This discussion summarizes how to analyze topography measurements to\nreconstruct a reliable PSD. Analytical models demonstrate the potential for\ntuning functional properties by rationally tailoring surface topography -\nhowever, this potential can only be achieved through the accurate, quantitative\nreconstruction of the power spectral density of real-world surfaces.", "category": "cond-mat_mtrl-sci" }, { "text": "Domain Dynamics in Piezoresponse Force Microscopy: Quantitative\n Deconvolution and Hysteresis Loop Fine Structure: Domain dynamics in the Piezoresponse Force Spectroscopy (PFS) experiment is\nstudied using the combination of local hysteresis loop acquisition with\nsimultaneous domain imaging. The analytical theory for PFS signal from domain\nof arbitrary cross-section is developed and used for the analysis of\nexperimental data on Pb(Zr,Ti)O3 polycrystalline films. The results suggest\nformation of oblate domain at early stage of the domain nucleation and growth,\nconsistent with efficient screening of depolarization field within the\nmaterial. The fine structure of the hysteresis loop is shown to be related to\nthe observed jumps in the domain geometry during domain wall propagation\n(nanoscale Barkhausen jumps), indicative of strong domain-defect interactions.", "category": "cond-mat_mtrl-sci" }, { "text": "Effect of nitrogen introduced at the SiC/SiO$_2$ interface and SiC side\n on the electronic states by first-principles calculation: In this study, using first-principles calculations, we investigate the\nbehavior of electrons at the SiC/SiO$_2$ interface when nitrogen is introduced\nas a dopant within a few nm of the SiC surface. When a highly doped nitrogen\nlayer (5$\\times$10$^{19}$ cm$^{-3}$) is introduced within a few nm of the\nSiC(11$\\bar{2}$0) surface, the electronic state is not significantly affected\nif the doping region is less than 4 nm. However, if the doping region exceeds 4\nnm, the effect of quantum confinement decreases, which increases the electron\ndensity induced in the inversion layer. As for the wave function, even when an\nelectric field is applied, the peak shifts toward the direction in which the\nelectrons are pulled away from the interface. This reduces the effect of\nelectron scattering at the interface and improves electron mobility.", "category": "cond-mat_mtrl-sci" }, { "text": "Selection of strain and fitting schemes for calculating higher-order\n elastic constants: Criteria of selecting strain and fitting schemes are proposed for the\ncalculation of higher-order elastic constants more efficiently, robustly and\naccurately. As demonstrated by the third-order elastic constants (TOECs) of\ndiamond, the proposed method is 3-5 times faster than existing methods, and the\nrange of strain for getting correct TOECs is expanded. In addition, our result\nprovides an evidence for the inaccuracy of some previous experiments caused by\nhigher-order effect, and the difference among experiments and several different\ntheoretical methods is resolved. Finally, we give the recommend TOECs values\nfor diamond.", "category": "cond-mat_mtrl-sci" }, { "text": "Disordered Hyperuniform Solid State Materials: Disordered hyperuniform (DHU) states are recently discovered exotic states of\ncondensed matter. DHU systems are similar to liquids or glasses in that they\nare statistically isotropic and lack conventional long-range translational and\norientational order. On the other hand, they completely suppress normalized\ninfinite-wavelength density fluctuations like crystals, and in this sense\npossess a hidden long-range order. Very recently, there are several exciting\ndiscoveries of disordered hyperuniformity in solid-state materials, including\namorphous carbon nanotubes, amorphous 2D silica, amorphous graphene, defected\ntransition metal dichalcogenides, defected pentagonal 2D materials, and\nmedium/high-entropy alloys. It has been found the DHU states of these materials\noften possess a significantly lower energy than other disorder models, and can\nlead to unique electronic and thermal transport properties, which resulted from\nmechanisms distinct from those identified for their crystalline counterparts.\nFor example, DHU states can enhance electronic transport in 2D amorphous\nsilica; DHU medium/high-entropy alloys realize the Vegard's law, and possess\nenhanced electronic band gaps and thermal transport at low temperatures. These\nunique properties open up many promising potential device applications in\noptoelectronics and thermoelectrics. Here, we provide a focused review on these\nimportant new developments of hyperuniformity in solid-state materials, taking\nan applied and ``materials'' perspective, which complements the existing\nreviews on hyperuniformity in physical systems and photonic materials. Future\ndirections and outlook are also provided, with a focus on the design and\ndiscovery of DHU quantum materials for quantum information science and\nengineering.", "category": "cond-mat_mtrl-sci" }, { "text": "Properties of $(TiZrNbCu)_{1-x}$$Ni_{x}$ Metallic Glasses: Recent studies (J. Alloys Compd. 695 (2017) 2661) of the electronic structure\nand properties of $(TiZrNbCu)_{1-x}$$Ni_{x}$ (x$\\leq$0.25) amorphous high\nentropy alloys (a-HEA) have been extended to x=0.5 in order to compare\nbehaviours of a-HEA and conventional Ni-base metallic glasses (MG). The\namorphous state of all samples was verified by thermal analysis and X-ray\ndiffraction (XRD). XRD indicated a probable change in local atomic\narrangements, i.e. short-range-order (SRO) for x$\\geq$0.35. Simultaneously,\nthermal parameters, such as the first crystallization temperature $T_{x}$ and\nthe liquidus temperature showed a tendency to saturate for x$\\geq$0.35 . The\nsame tendency also appeared in the magnetic susceptibility $\\chi_{exp}$ and the\nlinear term in the low temperature specific heat {\\gamma}. The Debye\ntemperatures and Youngs moduli also tend to saturate for x$\\geq$0.35. These\nunusual changes in SRO and all properties within the amorphous phase seem\ncorrelated with the change of valence electron number (VEC) on increasing x.", "category": "cond-mat_mtrl-sci" }, { "text": "Equivalent circuit representation of hysteresis in solar cells that\n considers interface charge accumulation: Potential cause of hysteresis in\n perovskite solar cells: If charge carriers accumulate in the charge transport layer of a solar cell,\nthen the transient response of the electric field that originates from these\naccumulated charges results in hysteresis in the current-voltage ($J$-$V$)\ncharacteristics. While this mechanism was previously known, a theoretical model\nto explain these $J$-$V$ characteristics has not been considered to date. We\nderived an equivalent circuit from the proposed hysteresis mechanism. By\nsolving the equivalent circuit model, we were able to reproduce some of the\nfeatures of hysteresis in perovskite solar cells.", "category": "cond-mat_mtrl-sci" }, { "text": "Ferrous Metal Matrix Composites Status Scope and Challenges: The present paper is an effort to culminate the status, scopes and challenges\nin the development of ferrous metal matrix composites (FMMCs). The FMMCs are\nold but less in use than the non-ferrous metal matrix composites (NFMMCs), as\nfar as literature and actual applications are concerned. Therefore, this\nstimulates the exploration of the reasons behind the scarcity of literature and\nfield applications of the FMMCs, which must be investigated scientifically. The\npowder metallurgy route is the most used process for fabricating iron and steel\nbased FMMCs by reinforcing particulates. At the same time, the in-situ method\nhas been used for the fabrication and cast iron-based FMMCs. The main\ncharacteristics being considered during the designing and fabrication of FMMCs\nare wear resistance and improved specific mechanical properties. To fabricate\ncheaper and eco-friendly FMMCs, traditionally used costly reinforcements such\nas SiC, WC, TiC, SiO2, TiO2, TiB2 are required to be replaced by inexpensive\nindustrial wastes like red-mud, fly-ashes and grinding swarf. The data\nextracted from the web of science exhibited that the FMMCs have been researched\nless than the NFMMCs. The increasing number of research papers on FMMCs\nindicates a bright future. FMMCs are going to be a favourite topic among\nresearchers and manufacturers. Higher strengths, wear resistance, dimensional\nstability at elevated temperatures, and, most importantly, the lower cost will\nput forward the FMMCs as a stiff competitor of NFMMCs. In developing and mass\nproduction of FMMCs for field applications, challenges like oxidation and\nhigher weight still require special research efforts.", "category": "cond-mat_mtrl-sci" }, { "text": "Universal solvent restructuring induced by colloidal nanoparticles: Colloidal nanoparticles, used for applications from catalysis and energy\napplications to cosmetics, are typically embedded in matrixes or dispersed in\nsolutions. The entire particle surface, which is where reactions are expected\nto occur, is thus exposed. Here we show with x-ray pair distribution function\nanalysis that polar and non-polar solvents universally restructure around\nnanoparticles. Layers of enhanced order exist with a thickness influenced by\nthe molecule size and up to 2 nanometers beyond the nanoparticle surface. These\nresults show that the enhanced reactivity of solvated nanoparticles includes a\ncontribution from a solvation shell of the size of the particle itself.", "category": "cond-mat_mtrl-sci" }, { "text": "Calculation of the specific heat in ultra-thin free-standing silicon\n membranes: The specific heat of ultra-thin free-standing membranes is calculated using\nthe elastic continuum model. We first obtain the dispersion relations of the\ndiscrete set of acoustic modes in the system. The specific heat is then\ncalculated by summing over the discrete out-of-plane wavevector component and\nintegrating over the continuous in-plane wavevector of these waves. In the\nlow-temperature regime (T < 4 K), the flexural polarization is seen to have the\nhighest contribution to the total specific heat. This leads to a linear\ndependence with temperature, resulting in a larger specific heat for the\nmembrane compared to that of the bulk counterpart", "category": "cond-mat_mtrl-sci" }, { "text": "Sensitive electronic correlation effects on electronic properties in\n ferrovalley material Janus FeClF monolayer: The electronic correlation may have essential influence on electronic\nstructures in some materials with special structure and localized orbital\ndistribution. In this work, taking Janus monolayer FeClF as a concrete example,\nthe correlation effects on its electronic structures are investigated by using\ngeneralized gradient approximation plus $U$ (GGA+$U$) approach. For\nperpendicular magnetic anisotropy (PMA), the increasing electron correlation\neffect can induce the ferrovalley (FV) to half-valley-metal (HVM) to quantum\nanomalous Hall (QAH) to HVM to FV transitions. For QAH state, there are a unit\nChern number and a chiral edge state connecting the conduction and valence\nbands. The HVM state is at the boundary of the QAH phase, whose carriers are\nintrinsically 100\\% valley polarized. With the in-plane magnetic anisotropy, no\nspecial QAH states and prominent valley polarization are observed. However, for\nboth out-of-plane and in-plane magnetic anisotropy, sign-reversible Berry\ncurvature can be observed with increasing $U$. It is found that these\nphenomenons are related with the change of $d_{xy}$/$d_{x^2-y^2}$ and $d_{z^2}$\norbital distributions and different magnetocrystalline directions. It is also\nfound that the magnetic anisotropy energy (MAE) and Curie temperature strongly\ndepend on the $U$. With PMA, taking typical $U=$2.5 eV, the electron valley\npolarization can be observed with valley splitting of 109 meV, which can be\nswitched by reversing the magnetization direction. The analysis and results can\nbe readily extended to other nine members of monolayer FeXY (X/Y=F, Cl, Br and\nI) due to sharing the same Fe-dominated low-energy states and electronic\ncorrelations with FeClF monolayer.", "category": "cond-mat_mtrl-sci" }, { "text": "Structure-dependent exchange in the organic magnets Cu(II)Pc and\n Mn(II)Pc: We study exchange couplings in the organic magnets copper(II) phthalocyanine\n(Cu(II)Pc) and manganese(II) phthalocyanine (Mn(II)Pc) by a combination of\nGreen's function perturbation theory and \\textsl{ab initio} density-functional\ntheory (DFT). Based on the indirect exchange model our perturbation-theory\ncalculation of Cu(II)Pc qualitatively agrees with the experimental\nobservations. DFT calculations performed on Cu(II)Pc dimer show a very good\nquantitative agreement with exchange couplings that we extract by using a\nglobal fitting for the magnetization measurements to a spin-1/2 Bonner-Fisher\nmodel. These two methods give us remarkably consistent trends for the exchange\ncouplings in Cu(II)Pc when changing the stacking angles. The situation is more\ncomplex for Mn(II)Pc owing to the competition between super-exchange and\nindirect exchange.", "category": "cond-mat_mtrl-sci" }, { "text": "Can we predict interface dipoles based on molecular properties?: We apply high-throughput DFT calculations and symbolic regression to hybrid\ninorganic/organic interfaces with the intent to extract physically meaningful\ncorrelations between the adsorption-induced work function modifications and the\nproperties of the constituents. We separately investigate two cases:\nHypothetical, free standing self-assembled monolayers with a large intrinsic\ndipole moment, and metal-organic interfaces with a large charge-transfer\ninduced dipole. For the former we find - without notable prior assumptions -\nthe Topping model, as expected from literature. For the latter, highly accurate\ncorrelations are found, which are, however, clearly unphysical.", "category": "cond-mat_mtrl-sci" }, { "text": "Single-Atom Scale Structural Selectivity in Te Nanowires Encapsulated\n inside Ultra-Narrow, Single-Walled Carbon Nanotubes: Extreme nanowires (ENs) represent the ultimate class of crystals: They are\nthe smallest possible periodic materials. With atom-wide motifs repeated in one\ndimension (1D), they offer a privileged perspective into the Physics and\nChemistry of low-dimensional systems. Single-walled carbon nanotubes (SWCNTs)\nprovide ideal environments for the creation of such materials. Here we present\na comprehensive study of Te ENs encapsulated inside ultra- narrow SWCNTs with\ndiameters between 0.7 nm and 1.1 nm. We combine state-of-the-art imaging\ntechniques and 1D-adapted ab initio structure prediction to treat both\nconfinement and periodicity effects. The studied Te ENs adopt a variety of\nstructures, exhibiting a true 1D realisation of a Peierls structural distortion\nand transition from metallic to insulating behaviour as a function of\nencapsulating diameter. We analyse the mechanical stability of the encapsulated\nENs and show that nanoconfinement is not only a useful means to produce ENs,\nbut may actually be necessary, in some cases, to prevent them from\ndisintegrating. The ability to control functional properties of these ENs with\nconfinement has numerous applications in future device technologies, and we\nanticipate that our study will set the basic paradigm to be adopted in the\ncharacterisation and understanding of such systems.", "category": "cond-mat_mtrl-sci" }, { "text": "Two-dimensional modeling of the self-limiting oxidation in silicon and\n tungsten nanowires: Self-limiting oxidation of nanowires has been previously described as a\nreaction- or diffusion-controlled process. In this letter, the concept of\nfinite reactive region is introduced into a diffusion-controlled model, based\nupon which a two-dimensional cylindrical kinetics model is developed for the\noxidation of silicon nanowires and is extended for tungsten. In the model,\ndiffusivity is affected by the expansive oxidation reaction induced stress. The\ndependency of the oxidation upon curvature and temperature is modeled. Good\nagreement between the model predictions and available experimental data is\nobtained. The developed model serves to quantify the oxidation in\ntwo-dimensional nanostructures and is expected to facilitate their fabrication\nvia thermal oxidation techniques. https://doi.org/10.1016/j.taml.2016.08.002", "category": "cond-mat_mtrl-sci" }, { "text": "Raman and Far Infrared Synchrotron Nanospectroscopy of Layered\n Crystalline Talc: Vibrational Properties, Interlayer Coupling and Symmetry\n Crossover: Talc is an insulating layered material that is stable at ambient conditions\nand has high-quality basal cleavage, which is a major advantage for its use in\nvan der Waals heterostructures. Here, we use near-field synchrotron infrared\nnanospectroscopy, Raman spectroscopy, and first-principles calculations to\ninvestigate the structural and vibrational properties of talc crystals, ranging\nfrom monolayer to bulk, in the 300-750 cm-1 and <60 cm-1 spectral windows. We\nobserve a symmetry crossover from mono to bilayer talc samples, attributed to\nthe stacking of adjacent layers. The in-plane lattice parameters and\nfrequencies of intralayer modes of talc display weak dependence with the number\nof layers, consistent with a weak interlayer interaction. On the other hand,\nthe low-frequency (<60 cm-1) rigid-layer (interlayer) modes of talc are\nsuitable to identify the number of layers in ultrathin talc samples, besides\nrevealing strong in-plane and out-of-plane anisotropy in the interlayer force\nconstants and related elastic stiffnesses of single crystals. The shear and\nbreathing force constants of talc are found to be 66% and 28%, respectively,\nlower than those of graphite, making talc an excellent lubricant that can be\neasily exfoliated. Our results broaden the understanding of the structural and\nvibrational properties of talc at the nanoscale regime and serve as a guide for\nfuture ultrathin heterostructures applications.", "category": "cond-mat_mtrl-sci" }, { "text": "Nearly triple nodal point topological phase in half-metallic GdN: Recent developments in topological semimetals open a way to realize\nrelativistic dispersions in condensed matter systems. One recently studied type\nof topological feature is the \"triple nodal point\" where three bands become\ndegenerate. In contrast to Weyl and Dirac nodes, triple nodal points, which are\nprotected by a rotational symmetry, have nodal lines attached, so that a\ncharacterization in terms of a chirality is not possible. Previous studies of\ntriple nodal points considered nonmagnetic systems, although an artificial\nZeeman splitting was used to probe the topological nature. Here instead we\ntreat a ferromagnetic material, half-metallic GdN, where the splitting of the\ntriple nodal points comes from the spin-orbit coupling. The size of the\nsplitting ranges from 15 to 150 meV depending on the magnetization orientation,\nenabling a transition between a Weyl-point phase and a \"nearly triple nodal\npoint\" phase that exhibits very similar surface spectra and transport\nproperties compared to a true triple-node system. The rich topological surface\nstates, manipulable via the orientation of the magnetization, make\nhalf-metallic GdN a promising platform for future investigations and\napplications.", "category": "cond-mat_mtrl-sci" }, { "text": "Will Zigzag Graphene Nanoribbon Turn to Half Metal under Electric Field?: At B3LYP level of theory, we predict that the half-metallicity in zigzag edge\ngraphene nanoribbon (ZGNR) can be realized when an external electric field is\napplied across the ribbon. The critical electric field to induce the\nhalf-metallicity decreases with the increase of the ribbon width. Both the spin\npolarization and half-metallicity are removed when the edge state electrons\nfully transferred from one side to the other under very strong electric field.\nThe electric field range under which ZGNR remain half-metallic increases with\nthe ribbon width. Our study demonstrates a rich field-induced spin polarization\nbehavior, which may leads to some important applications in spinstronics.", "category": "cond-mat_mtrl-sci" }, { "text": "Promising photovoltaic efficiency of a layered silicon oxide crystal\n Si$_{3}$O: Computational searching and screening of new functional materials exploiting\nearth abundant elements can accelerate developments of their energy\napplications. Based on a state-of-the-art materials search algorithm and ab\ninitio calculations, we demonstrate a recently suggested stable silicon oxide\nwith a layered structure (Si$_{3}$O) as an ideal photovoltaic material. With\nmany-body first-principles approaches, the monolayer and layered bulk of\nSi$_{3}$O show direct quasiparticle gaps of 1.85 eV and 1.25 eV, respectively,\nwhile an optical gap of about 1.2 eV is nearly independent of the number of\nlayers. Spectroscopic limited maximum efficiency (SLME) is estimated to be 27%\nfor a thickness of 0.5 {\\mu}m, making it a promising candidate for solar energy\napplications.", "category": "cond-mat_mtrl-sci" }, { "text": "Viscoelastic Constitutive Artificial Neural Networks (vCANNs) $-$ a\n framework for data-driven anisotropic nonlinear finite viscoelasticity: The constitutive behavior of polymeric materials is often modeled by finite\nlinear viscoelastic (FLV) or quasi-linear viscoelastic (QLV) models. These\npopular models are simplifications that typically cannot accurately capture the\nnonlinear viscoelastic behavior of materials. For example, the success of\nattempts to capture strain rate-dependent behavior has been limited so far. To\novercome this problem, we introduce viscoelastic Constitutive Artificial Neural\nNetworks (vCANNs), a novel physics-informed machine learning framework for\nanisotropic nonlinear viscoelasticity at finite strains. vCANNs rely on the\nconcept of generalized Maxwell models enhanced with nonlinear strain\n(rate)-dependent properties represented by neural networks. The flexibility of\nvCANNs enables them to automatically identify accurate and sparse constitutive\nmodels of a broad range of materials. To test vCANNs, we trained them on\nstress-strain data from Polyvinyl Butyral, the electro-active polymers VHB 4910\nand 4905, and a biological tissue, the rectus abdominis muscle. Different\nloading conditions were considered, including relaxation tests, cyclic\ntension-compression tests, and blast loads. We demonstrate that vCANNs can\nlearn to capture the behavior of all these materials accurately and\ncomputationally efficiently without human guidance.", "category": "cond-mat_mtrl-sci" }, { "text": "Na9Bi5Os3O24: A Unique Diamagnetic Oxide Featuring a Pronouncedly\n Jahn-Teller Compressed Octahedral Coordination of Osmium(VI): The Jahn-Teller theorem constitutes one of the most popular and stringent\nconcepts, applicable to all fields of chemistry. In open shell transition\nelements chemistry and physics, 3d4, 3d9, and 3d7(low-spin) configurations in\noctahedral complexes serve as particular illustrative and firm examples, where\na striking change (distortion) in local geometry is associated to a substantial\nreduction of electronic energy. However, there has been a lasting debate, about\nthe fact that the octahedra are found to exclusively elongate, (at least for eg\nelectrons). Against this background, the title compound displays two marked\nfeatures, (1) the octahedron of oxygen atoms around Os6+ (d2) is drastically\ncompressed, in contrast to the standard JT expectations, and (2) the splitting\nof the t2g set induced by this compression is extreme, such that a diamagnetic\nground state results. What we see is obviously a Jahn-Teller distortion\nresulting in a compression of the respective octahedron and acting on the t2g\nset of orbitals. Both these issues are unprecedented. Noteworthy, the splitting\ninto a lower dxy (hosting two d electrons with opposite spin) and two higher\ndxz and dyz orbitals is so large that for the first time ever the Hund's\ncoupling for t2g electrons is overcome. We show that these effects are not\nforced by structural frustration, the structure offers sufficient space for Os\nto shift the apical oxygen atoms to a standard distance. Local electronic\neffects appear to be responsible, instead. The relevance of these findings is\nfar reaching, since they provide insights in the hierarchy of perturbations\ndefining ground states of open shell electronic systems. The system studied\nhere, offers substantially more structural and compositional degrees of\nfreedom, such that a configuration could form that enables Os6+ to adopt its\napparently genuine diamagnetic ground state.", "category": "cond-mat_mtrl-sci" }, { "text": "Bulk and surface properties of the Ruddlesden-Popper oxynitride\n Sr$_2$TaO$_3$N: Oxynitrides with the perovskite structure are promising candidates for\nphotocatalysis under visible light due to their appropriate optical and\nelectronic properties. Recently, layered perovskites have attracted attention\nfor their improved performance with respect to the bulk perovskites in\nphotocatalytic water splitting. In this paper, we investigate the structural\nand electronic properties of the layered Ruddlesden-Popper oxynitride\nSr$_2$TaO$_3$N and its (001) surfaces using density functional theory (DFT)\ncalculations. We find that the energetically favoured configuration of the bulk\nhas an in-plane \\textit{cis} anion order and exhibits rotations of the TaO$_6$\noctahedra. Furthermore, we show that the TaON-terminated (001) surface\nsuppresses exciton recombination due to higher-energy surface states, giving a\npotential explanation for the good photocatalytic performance.", "category": "cond-mat_mtrl-sci" }, { "text": "Understanding atom probe's analytical performance for iron oxides using\n correlation histograms and ab initio calculations: Field evaporation from ionic or covalently bonded materials often leads to\nthe emission of molecular ions. The metastability of these molecular ions,\nparticularly under the influence of the intense electrostatic field (1010\nVm-1), makes them prone to dissociation with or without an exchange of energy\namongst them. These processes can affect the analytical performance of atom\nprobe tomography (APT). For instance, neutral species formed through\ndissociation may not be detected at all or with a time of flight no longer\nrelated to their mass, causing their loss from the analysis. Here, we evaluated\nthe changes in the measured composition of FeO, Fe2O3 and Fe3O4 across a wide\nrange of analysis conditions. Possible dissociation reactions are predicted by\ndensity-functional theory (DFT) calculations considering the spin states of the\nmolecules. The energetically favoured reactions are traced on to the multi-hit\nion correlation histograms, to confirm their existence within experiments,\nusing an automated Python-based routine. The detected reactions are carefully\nanalysed to reflect upon the influence of these neutrals from dissociation\nreactions on the performance of APT for analysing iron oxides.", "category": "cond-mat_mtrl-sci" }, { "text": "Sensitivity of the MnTe valence band to orientation of magnetic moments: An effective model of the hexagonal (NiAs-structure) manganese telluride\nvalence band in the vicinity of the A-point of the Brillouin zone is derived.\nIt is shown that while for the usual antiferromagnetic order (magnetic moments\nin the basal plane) band splitting at A is small, their out-of-plane rotation\nenhances the splitting dramatically (to about 0.5 eV). We propose extensions of\nrecent experiments (Moseley et al., Phys. Rev. Materials 6, 014404) where such\ninversion of magnetocrystalline anisotropy has been observed in Li-doped MnTe,\nto confirm this unusual sensitivity of a semiconductor band structure to\nmagnetic order.", "category": "cond-mat_mtrl-sci" }, { "text": "Parent grain reconstruction from partially or fully transformed\n microstructures in MTEX: A versatile generic framework for parent grain reconstruction from fully or\npartially transformed child microstructures was integrated into the open-source\ncrystallographic toolbox MTEX. The framework extends traditional parent grain\nreconstruction, phase transformation and variant analysis to all parent-child\ncrystal symmetry combinations. The inherent versatility of the universally\napplicable parent grain reconstruction methods, and the ability to conduct\nin-depth variant analysis are showcased via example workflows that can be\nprogrammatically modified by users to suit their specific applications. This is\nhighlighted by three applications namely, $\\alpha$-to-$\\gamma$ reconstruction\nin a lath martensitic steel, $\\alpha$-to-$\\beta$ reconstruction in a Ti alloy,\nand a two-step reconstruction from $\\alpha$-to-$\\varepsilon$-to-$\\gamma$ in a\ntwinning and transformation -induced plasticity steel. Advanced orientation\nrelationship discovery and analysis options, including variant analysis, is\ndemonstrated via the add-on function library, ORTools.", "category": "cond-mat_mtrl-sci" }, { "text": "Novel superhard structures of high-pressure C-N compounds: Through machine learning force field accelerated structure search combined\nwith first-principles calculations, we have studied the structures of new C-N\ncompounds with different stoichiometric ratios, and found twelve new superhard\nC-N compounds, the energies of these structures are similar to c-C3N4 , which\nis possibly synthesized by high pressure experiment, the XRD of Pa-3(C4N)\nP3(C4N) and C2/m(C2N) are consistent with previous experimental data and can be\nused as the structural candidate. According to the macro hardness model, they\nare all superhard structures, with Vickers hardness over 40GPa, even, the\nhardness of Pa-3 (C4N) as high as 82.2GPa, and Pa-3 (C4N) combines high tensile\nand shear resistance. Compared with the hardness calculated by macro hardness\nmodel and bond resistance model, we obtained the relationship between the\nhardness and chemical concentration of C-N compounds under the two models,\nbesides that, we also calculated the fracture toughness of these structures.\nAccording to Niu's model, P2_1/c(C4N) has the best fracture toughness, which is\nhigher than WC in calculation, This also indicated the superior mechanical\nproperties of the novel C-N compounds. Moreover, for nitrogen-rich structures,\nthey have the potential to be used as high energy density, the energy density\nof Pa-3(CN3), P-3c1 (CN4), and I-42d (CN4) are 7.076kJ/g, 7.742kJ/g and\n8.045kJ/g, which is close or higher than CL-20, therefore, the C-N compounds\nsynthesized under high pressure have great potential as ideally superhard\nmaterials and high energy density materials(HEDMs).", "category": "cond-mat_mtrl-sci" }, { "text": "Fabrication of ice-templated tubes by rotational freezing:\n microstructure, strength, and permeability: We demonstrate a facile and scalable technique, rotational freezing, to\nproduce porous tubular ceramic supports with radially aligned porosity. The\nmethod is based on a conventional ice-templating process in a rotatory mold and\ndemonstrated here with yttria-stabilized zirconia (YSZ). We investigated the\neffects of solid loading, freezing temperature, and volume of the slurry on the\nmicrostructure, strength (o-ring test and four-point bending), and air\npermeability. The results show that pore volume and pore size can be controlled\nby the solid loading and freezing temperature respectively, and overall tube\nthickness can be adjusted by the volume of slurry initially poured into the\nmold. Decreasing pore size and pore volume increases the mechanical properties\nbut decreases the air permeability. These tubes could be particularly\ninteresting as tubular membrane supports such as oxygen transport membranes.", "category": "cond-mat_mtrl-sci" }, { "text": "Giant anomalous Hall effect from spin-chirality scattering in a chiral\n magnet: The electrical Hall effect can be significantly enhanced through the\ninterplay of the conduction electrons with magnetism, which is known as the\nanomalous Hall effect (AHE). Whereas the mechanism related to band topology has\nbeen intensively studied towards energy efficient electronics, those related to\nelectron scattering have received limited attention. Here we report the\nobservation of giant AHE of electron-scattering origin in a chiral magnet MnGe\nthin film. The Hall conductivity and Hall angle respectively reach 40,000\n{\\Omega}-1cm-1 and 18 % in the ferromagnetic region, exceeding the conventional\nlimits of AHE of intrinsic and extrinsic origins, respectively. A possible\norigin of the large AHE is attributed to a new type of skew-scattering via\nthermally-excited spin-clusters with scalar spin chirality, which is\ncorroborated by the temperature-magnetic-field profile of the AHE being\nsensitive to the film-thickness or magneto-crystalline anisotropy. Our results\nmay open up a new platform to explore giant AHE responses in various systems,\nincluding frustrated magnets and thin-film heterostructures.", "category": "cond-mat_mtrl-sci" }, { "text": "Transport between metals and magnetic insulators: We derive the Onsager response matrix of fluctuation-mediated spin-collinear\ntransport through a ferromagnetic insulator and normal metal interface driven\nby a temperature difference, spin accumulation, or magnetic field. We predict\nmagnon-squeezing spin currents, magnetic field-induced cooling (magnon Peltier\neffect), temperature induced magnetization (thermal magnetic field) as well as\nuniversal spin Seebeck/Peltier coefficients.", "category": "cond-mat_mtrl-sci" }, { "text": "Cesium Enhances Long-Term Stability of Lead Bromide Perovskite-Based\n Solar Cells: Direct comparison between perovskite-structured hybrid organic-inorganic -\nmethyl ammonium lead bromide (MAPbBr3) and all-inorganic cesium lead bromide\n(CsPbBr3), allows identifying possible fundamental differences in their\nstructural, thermal and electronic characteristics. Both materials possess a\nsimilar direct optical band-gap, but CsPbBr3 demonstrates a higher thermal\nstability than MAPbBr3. In order to compare device properties we fabricated\nsolar cells, with similarly synthesized MAPbBr3 or CsPbBr3, over mesoporous\ntitania scaffolds. Both cell types demonstrated comparable photovoltaic\nperformances under AM1.5 illumination, reaching power conversion efficiencies\nof ~6 % with a poly-aryl amine-based derivative as hole transport material.\nFurther analysis shows that Cs-based devices are as efficient as, and more\nstable than methyl ammonium-based ones, after aging (storing the cells for 2\nweeks in a dry (relative humidity 15-20%) air atmosphere in the dark) for 2\nweeks, under constant illumination (at maximum power), and under electron beam\nirradiation.", "category": "cond-mat_mtrl-sci" }, { "text": "Quantum Composites with the Functionality Defined by the\n Charge-Density-Wave Phase Transitions: We demonstrate a unique class of advanced materials - quantum composites\nbased on polymers with fillers comprised of a van der Waals quantum material\nthat reveals multiple charge-density-wave quantum condensate phases. Materials\nthat exhibit quantum phenomena are typically crystalline, pure, and have few\ndefects because disorder destroys the coherence of the electrons and phonons,\nleading to collapses of the quantum states. We succeeded in preserving the\nmacroscopic charge-density-wave phases of filler particles after multiple\ncomposite processing steps. The prepared composites manifest strong\ncharge-density-wave phenomena even above room temperature. The dielectric\nconstant experiences more than two orders of magnitude enhancement while the\nmaterial maintains its electrically insulating properties, opening a venue for\nadvanced applications in energy storage and electronics. The results present a\nconceptually different approach for engineering the properties of materials,\nextending the application domain for van der Waals materials.", "category": "cond-mat_mtrl-sci" }, { "text": "Thermal conductivity of perovskite KTaO3 and PbTiO3 from first\n principles: The low thermal conductivity of piezoelectric perovskites is a challenge for\nhigh power transducer applications. We report first principles calculations of\nthe thermal conductivity of ferroelectric PbTiO$_3$ and the cubic nearly\nferroelectric perovskite KTaO$_3$. The calculated thermal conductivity of\nPbTiO$_3$ is much lower than that of KTaO$_3$ in accord with experiment.\nAnalysis of the results shows that the reason for the low thermal conductivity\nof PbTiO$_3$ is the presence of low frequency optical phonons associated with\nthe polar modes. These are less dispersive in PbTiO$_3$, leading to a large\nthree phonon scattering phase space. These differences between the two\nmaterials are associated with the $A$-site driven ferroelectricity of PbTiO$_3$\nin contrast to the $B$-site driven near ferroelectricity of KTaO$_3$. The\nresults are discussed in the context of modification of the thermal\nconductivity of electroactive materials.", "category": "cond-mat_mtrl-sci" }, { "text": "Enhanced Born Charge and Proximity to Ferroelectricity in Thallium\n Halides: Electronic structure and lattice dynamics calculations on thallium halides\nshow that the Born effective charges in these compounds are more than twice\nlarger than the nominal ionic charges. This is a result of cross-band-gap\nhybridization between Tl-p and halogen-p states. The large Born charges cause\ngiant splitting between longitudinal and transverse optic phonon modes,\nbringing the lattice close to ferroelectric instability. Our calculations\nindeed show spontaneous lattice polarization upon lattice expansion starting at\n2%. It is remarkable that the apparently ionic thallium halides with a simple\ncubic CsCl structure and large differences in electronegativity between cations\nand anions can be very close to ferroelectricity. This can lead to effective\nscreening of defects and impurities that would otherwise be strong carrier\ntraps and may therefore contribute to the relatively good carrier transport\nproperties in TlBr radiation detectors.", "category": "cond-mat_mtrl-sci" }, { "text": "Acousto-electric Characteristics of Periodically Poled Ferroelectric\n Plate: A multidomain two-dimensional periodically poled ferroelectric plate vibrator\nis reported for the first time. The theoretical calculations, computer\nsimulations by the Finite Element Method and experimental data from the lithium\ntantalite samples reveal a domain acousto-electric resonance. A polarization\ninversion in a y-rotated cut of a ferroelectric chip is firstly done. The\nacousto-electric characteristics of the vibrator are calculated and measured.", "category": "cond-mat_mtrl-sci" }, { "text": "Chemical gradients across phase boundaries between martensite and\n austenite in steel studied by atom probe tomography and simulation: Partitioning at phase boundaries of complex steels is important for their\nproperties. We present atom probe tomography results across martensite /\naustenite interfaces in a precipitation-hardened maraging TRIP steel (12.2 Mn,\n1.9 Ni, 0.6 Mo, 1.2 Ti, 0.3 Al; at.%). The system reveals compositional changes\nat the phase boundaries: Mn and Ni are enriched while Ti, Al, Mo, and Fe are\ndepleted. More specific, we observe up to 27 at.% Mn in a 20 nm layer at the\nphase boundary. This is explained by the large difference in diffusivity\nbetween martensite and austenite. The high diffusivity in martensite leads to a\nMn-flux towards the retained austenite. The low diffusivity in the austenite\ndoes not allow accommodation of this flux. Consequently, the austenite grows\nwith a Mn-composition given by local equilibrium. The interpretation is based\non DICTRA and mixed-mode diffusion calculations (using a finite interface\nmobility).", "category": "cond-mat_mtrl-sci" }, { "text": "Ferroelectricity induced by acentric spin-density waves in YMn$_2$O$_5$: The commensurate and incommensurate magnetic structures of the\nmagnetoelectric system YMn$_{2}$O$_{5}$, as determined from neutron\ndiffraction, were found to be spin-density waves lacking a global center of\nsymmetry. We propose a model, based on a simple magneto-elastic coupling to the\nlattice, which enables us to predict the polarization based entirely on the\nobserved magnetic structure. Our data accurately reproduce the\ntemperature-dependence of the spontaneous polarization, in particular its sign\nreversal at the commensurate-incommensurate transition.", "category": "cond-mat_mtrl-sci" }, { "text": "Effects of CdCl$_2$ treatment on the local electronic properties of\n polycrystalline CdTe measured with photoemission electron microscopy: To investigate the effects of CdCl$_2$ treatment on the local electronic\nproperties of polycrystalline CdTe films, we conducted a photoemission electron\nmicroscopy (PEEM) study of polished surfaces of CdTe films in superstrate\nconfiguration, with and without CdCl$_2$ treatment. From photoemission\nintensity images, we observed the tendency for individual exposed grain\ninteriors to vary in photoemission intensity, regardless of whether or not\nfilms received CdCl$_2$ treatment. Additionally, grain boundaries develop\ncontrast in photoemission intensity images different from grain interiors after\nan air exposure step, similar to observations of activated grain boundaries\nusing scanning Kelvin probe force microscopy studies. These results suggest\nthat work function varies locally, from one grain interior to another, as well\nas between grain boundaries and grain interiors.", "category": "cond-mat_mtrl-sci" }, { "text": "Verification of Wiedemann-Franz law in silver with moderate residual\n resistivity ratio: Electrical and thermal transport were studied in a vacuum-annealed\npolycrystalline silver wire with residual resistivity ratio 200-400, in the\ntemperature range 0.1-1.2K and in magnetic fields up to 5T. Both at zero field\nand at 5T the wire exhibits the Wiedemann-Franz law with the fundamental Lorenz\nnumber, contrary to an earlier report [Gloos, K. et al, Cryogenics 30, 14\n(1990)]. Our result demonstrates that silver is an excellent material for\nthermal links in ultra-low-temperature experiments operating at high magnetic\nfields.", "category": "cond-mat_mtrl-sci" }, { "text": "A numerical strategy for coarse-graining two-dimensional atomistic\n models at finite temperature: the membrane case: We present a numerical strategy to compute ensemble averages of\ncoarse-grained two-dimensional membrane-like models. The approach consists in\ngeneralizing to these two-dimensional models a one-dimensional strategy exposed\nin [Blanc, Le Bris, Legoll, Patz, JNLS 2010], which is based on applying the\nergodic theorem to Markov chains. This may be considered as a first step\ntowards computing the constitutive law associated to such models, in the\nthermodynamic limit.", "category": "cond-mat_mtrl-sci" }, { "text": "A detailed analysis of impact collision ion scattering spectroscopy of\n bismuth selenide: Impact collision ion scattering spectroscopy (ICISS), which is a variation of\nlow energy ion scattering (LEIS) that employs large scattering angles, is\nperformed on Bi2Se3 surfaces prepared by ion bombardment and annealing (IBA).\nICISS angular scans are collected experimentally and simulated numerically\nalong the [120] and [-1 -2 0] azimuths, and the match of the positions of the\nflux peaks shows that the top three atomic layers are bulk-terminated. A newly\nobserved feature is identified as a minimum in the multiple scattering\nbackground when the ion beam incidence is along a low index direction.\nCalculated scans as a function of scattering angle are employed to identify the\nbehavior of flux peaks to show whether they originate from shadowing, blocking\nor both. This new method for analysis of large-angle LEIS data is shown to be\nuseful for accurately investigating complex surface structures.", "category": "cond-mat_mtrl-sci" }, { "text": "Valley Degree of Freedom in Two-Dimensional van der Waals Materials: Layered materials can possess valleys that are indistinguishable from one\nanother except for the momentum. These valleys are individually addressable in\nmomentum space at the K and K' points in the first Brillouin zone. Such valley\naddressability opens up the possibility of utilizing the momentum state of\nquasi-particles as a completely new paradigm in quantum and classical\ninformation processing. This review focuses on the physics behind valley\npolarization and talks about carriers of valley degree of freedom (VDF) in\nlayered materials. Then we provide a detailed survey of simple spectroscopic\ntechniques commonly utilized to identify and manipulate valley polarization in\nvan der Waals layered materials. Finally, we conclude with the recent\ndevelopments towards the manipulation of VDF for device application and\nassociated challenges.", "category": "cond-mat_mtrl-sci" }, { "text": "Multiferroicity and hydrogen-bond ordering in (C2H5NH3)2CuCl4 featuring\n dominant ferromagnetic interactions: We demonstrate that ethylammonium copper chloride, (C2H5NH3)2CuCl4, a member\nof the hybrid perovskite family is an electrically polar and magnetic compound\nwith dielectric anomaly around the Curie point (247 K). We have found large\nspontaneous electric polarization below this point accompanied with a color\nchange in the sample. The system is also ferroelectric, with large remnant\npolarization (37{\\mu}C/cm2) that is comparable to classical ferroelectric\ncompounds. The results are ascribed to hydrogen-bond ordering of the organic\nchains. The coexistence of ferroelectricity and dominant ferromagnetic\ninteractions allows to relate the sample to a rare group of magnetic\nmultiferroic compounds. In such hybrid perovskites the underlying hydrogen\nbonding of easily tunable organic building blocks in combination with the 3d\ntransition-metal layers offers an emerging pathway to engineer multifuctional\nmultiferroics.", "category": "cond-mat_mtrl-sci" }, { "text": "Theory of domain structure in ferromagnetic phase of diluted magnetic\n semiconductors near the phase transition temperature: We discuss the influence of disorder on domain structure formation in\nferromagnetic phase of diluted magnetic semiconductors (DMS) of p-type. Using\nanalytical arguments we show the existence of critical ratio $\\nu_{\\rm {cr}}$\nof concentration of charge carriers and magnetic ions such that sample critical\nthickness $L_{\\rm{cr}}$ (such that at $L \\nu_{\\rm {cr}}$ the sample is\nmonodomain. This feature makes DMS different from conventional ordered magnets\nas it gives a possibility to control the sample critical thickness and emerging\ndomain structure period by variation of $\\nu $. As concentration of magnetic\nimpurities grows, $\\nu_{\\rm {cr}}\\to \\infty$ restoring conventional behavior of\nordered magnets. Above facts have been revealed by examination of the\ntemperature of transition to inhomogeneous magnetic state (stripe domain\nstructure) in a slab of finite thickness $L$ of p-type DMS. Our analysis is\ncarried out on the base of homogeneous exchange part of magnetic free energy of\nDMS calculated by us earlier [\\prb, {\\bf 67}, 195203 (2003)].", "category": "cond-mat_mtrl-sci" }, { "text": "Cathodoluminescence spectroscopy of monolayer hexagonal boron nitride: Cathodoluminescence (CL) spectroscopy is a powerful technique for studying\nemission properties of optoelectronic materials because CL is free from\nexcitable bandgap limits and from ambiguous signals due to simple light\nscattering and resonant Raman scattering potentially involved in the\nphotoluminescence (PL) spectra. However, direct CL measurements of atomically\nthin two-dimensional materials, such as transition metal dichalcogenides and\nhexagonal boron nitride (hBN), have been difficult due to the small excitation\nvolume that interacts with high-energy electron beams (e-beams). Herein,\ndistinct CL signals from a monolayer hBN, namely mBN, epitaxial film grown on a\nhighly oriented pyrolytic graphite substrate are shown by using a home-made CL\nsystem capable of large-area and surface-sensitive excitation by an e-beam. The\nspatially resolved CL spectra at 13 K exhibited a predominant 5.5-eV emission\nband, which has been ascribed to originate from multilayered aggregates of hBN,\nmarkedly at thicker areas formed on the step edges of the substrate.\nConversely, a faint peak at 6.04 eV was routinely observed from atomically flat\nareas. Since the energy agreed with the PL peak of 6.05 eV at 10 K that has\nbeen assigned as being due to the recombination of phonon-assisted direct\nexcitons of mBN by Elias et al. [Nat. Commun. 10, 2639 (2019)], the CL peak at\n6.04 eV is attributed to originate from the mBN epilayer. The CL results\nsupport the transition from indirect bandgap in bulk hBN to direct bandgap in\nmBN, in analogy with molybdenum disulfide. The results also encourage to\nelucidate emission properties of other low-dimensional materials with reduced\nexcitation volumes by using the present CL configuration.", "category": "cond-mat_mtrl-sci" }, { "text": "Crystallization Mechanism Tuned Phase-Change Materials: Quantum Effect\n on Te-Terminated Boundary: While phase-change materials (PCMs) composed of chalcogenide have different\ncrystallization mechanisms (CM), such as nucleation-dominated Ge2Sb2Te5 (GST)\nand growth-dominated GeTe (GT), revealing the essential reason of CM as well as\nthe tuned properties is still a long-standing issue. Here, we remarkably find\nthe distinct stability of Te-terminated (111) boundaries (TTB) in different\nsystems, which provides a path to understand the difference in CM. It stems\nfrom the quantum effect of molecular orbital theory: the optimal local chemical\ncomposition results in the formation of TTB without dangling bonds (DB) in GST\nbut with DB in GT, where DB destabilizes boundary due to its distorted local\nenvironment mismatching Oh symmetry of p orbitals. Moreover, the inner vacancy\nconcentration in GST is alterable and controlled by TTB, manifested by the\nabsence of cubic-to-hexagonal transition in carbon-doped GST of small grains\nand minimized inner vacancy. Finally, the charge transport property (CTP) is\ncontrolled by boundary via changing the density of charge or hole nearby as\nwell as vacancy. These findings open the door to tune CTP by CM, which is\nnecessary for achieving low-power and ultrafast devices.", "category": "cond-mat_mtrl-sci" }, { "text": "Role of polyhedron unit in distinct photodynamics of zero-dimensional\n organic-inorganic hybrid tin halide compounds: The zero-dimensional (0D) metal halides comprise periodically distributed and\nisolated metal-halide polyhedra, which act as the smallest inorganic quantum\nsystems and can accommodate quasi-localized Frenkel excitons. These excitons\nexhibit unique photophysics including broadband photon emission, huge Stokes\nshift, and long decay lifetime. The polyhedra can have different symmetries due\nto the coordination degree of the metal ions. Little is known about how the\npolyhedron type affects the characteristics of the 0D metal halide crystals. We\nsynthesize and comparatively study three novel kinds of 0D organic-inorganic\nhybrid tin halide compounds. They are efficient light emitters with a highest\nquantum yield of 92.3%. Although they have the same compositional organic\ngroup, the most stable phases are composed of octahedra for the bromide and\niodide but disphenoids (see-saw structures) for the chloride. They separately\nexhibit biexponential and monoexponential luminescence decays due to different\nsymmetries (Ci group for octahedra and C2 group for disphenoids) and\ncorresponding different electronic structures. The chloride has the largest\nabsorption photon energy among the three halides, but it has the smallest\nemission photon energy. A model regarding the unoccupied energy band degeneracy\nis proposed based on the experiments and density functional theory\ncalculations, which explains well the experimental phenomena and reveals the\ncrucial role of polyhedron type in determining the optical properties of the 0D\ntin halide compounds.", "category": "cond-mat_mtrl-sci" }, { "text": "Quantum transport evidence of the boundary states and Lifshitz\n transition in Bi$_4$Br$_4$: The quasi-one-dimensional van der Waals compound Bi$_4$Br$_4$ was recently\nfound to be a promising high-order topological insulator with exotic electronic\nstates. In this paper, we study the electrical transport properties of\nBi$_4$Br$_4$ bulk crystals. Two electron-type samples with different electron\nconcentrations are investigated. Both samples have saturation resistivity\nbehavior in low temperature. In the low-concentration sample, two-dimensional\nquantum oscillations are clearly observed in the magnetoresistance\nmeasurements, which are attributed to the band-bending-induced surface state on\nthe (001) facet. In the high-concentration sample, the angular\nmagnetoresistance exhibits two pairs of symmetrical sharp valleys with an\nangular difference close to the angle between the crystal planes (001) and\n(100). The additional valley can be explained by the contribution of the\nboundary states on the (100) facet. Besides, Hall measurements at low\ntemperatures reveal an anomalous decrease of electron concentration with\nincreasing temperature, which can be explained by the temperature-induced\nLifshitz transition. These results shed light on the abundant surface and\nboundary state transport signals and the temperature-induced Lifshitz\ntransition in Bi$_4$Br$_4$.", "category": "cond-mat_mtrl-sci" }, { "text": "Formation of cBN nanocrystals by He+ implantations of hBN: The structural modifications of polycrystalline hexagonal boron nitride\nimplanted with He+ ion beams at energies between 200 keV and 1.2 MeV to\nfluences of 1.0 \\times 1017 ions \\cdot cm-2 were investigated using micro-Raman\nspectroscopy. The measured Raman spectra show evidence of implantation-induced\nstructural transformations from the hexagonal phase to nanocrystalline cubic\nboron nitride, rhombohedral boron nitride and amorphous boron nitride phases.\nThe first-order Longitudinal-Optical cBN phonon was observed to be downshifted\nand asymmetrically broadened and this was explained using the spatial\ncorrelation model coupled with the high ion implantation-induced defect\ndensity.", "category": "cond-mat_mtrl-sci" }, { "text": "Interlayer Configurations in Twisted Bilayers of Folded Graphene: The folding of monolayer graphene leads to new layered systems, termed\ntwisted bilayer graphene (TBG), generally displaying a certain interlayer\nrotation away from crystallographic alignment. We here present an atomic force\nmicroscopy study on folded graphene, revealing unexpectedly large twist angle\ndependent modulations of ~3 angstrom in interlayer distance. At the TBG\nsurface, we find enhanced friction attributable to superlubricity in between\nincommensurate layers. At the bended edge, the radius of curvature scales with\nthe folded length, congruent to earlier studies on carbon nanotubes.", "category": "cond-mat_mtrl-sci" }, { "text": "ATiO$_{3}$/TiO (A=Pb, Sn) superlattice: bridging ferroelectricity and\n conductivity: We propose to insert TiO layers to perovskite ATiO$_{3}$ to form a\nsuperlattice and use first-principles calculations to investigate its basic\nproperties. Our computational analysis shows that the structure, which consists\nof repeated ATiO_{3} and TiO layers, has strong anisotropic conductivity. The\nstructure immediately suggests a possible control of its conductivity by ion\ndisplacements related to its intrinsic ferroelectricity. In addition, we have\nobtained the structural information of its low-energy phases with the aid of\nphonon calculation and examined their evolution with epitaxial strain. Since\nthe number of possible combinations is huge, we have therefore suggested an\napproach to mix perovskites and simpler metal-oxides to build materials with\nnovel properties.", "category": "cond-mat_mtrl-sci" }, { "text": "Chemical functionalization on planar polysilane and graphane: Two dimensional materials are important for electronics applications. A\nnatural way for electronic structure engineering on two dimensional systems is\non-plane chemical functionalization. Based on density functional theory, we\nstudy the electronic structures of fluorine substituted planar polysilane and\ngraphane. We find that carbon and silicon present very different surface\nchemistry. The indirect energy gap of planar polysilane turns to be direct upon\nfluorine decoration, and the gap width is mainly determined by fluorine\ncoverage regardless of its distribution on the surface. However, electronic\nstructure of fluorine doped graphane is very sensitive to the doping\nconfiguration, due to the competition between antibonding states and\nnearly-free-electron (NFE) states. With specific fluorine distribution pattern,\nzero-dimensional and one-dimensional NFE states can be obtained. We have also\nstudied the chemical modification with -OH or -NH$_2$ group. Carbon seems to be\ntoo small to accommodate big functional groups on its graphane skeleton with a\nhigh concentration.", "category": "cond-mat_mtrl-sci" }, { "text": "Single crystal growth of TIMETAL LCB titanium alloy by a floating zone\n method: The methodology of single crystal growth of metastable $\\beta$-Ti alloy\nTIMETAL LCB in an optical floating zone furnace is presented in this paper.\nChemical compositions of both precursor material and single crystals were\nchecked. It was found that the concentration of base alloying elements did not\nchange significantly during the growth process, while the concentrations of\ninterstitial elements O and N increased. DSC measurement determined that this\nconcentration shift has a slight impact on ongoing phase transformations, as in\nthe single-crystalline material peak associated with $\\alpha$ phase\nprecipitation moves by a few degrees to a lower temperature and peak attributed\nto diffusion controlled growth of $\\omega$ particles shifts to a higher\ntemperature. X-ray reciprocal space maps were measured and their simulation\nshowed that the single crystal has a mosaic structure with mean size of mosaic\nblocks of approximately 60 nm.", "category": "cond-mat_mtrl-sci" }, { "text": "Machine learning driven simulated deposition of carbon films: from\n low-density to diamondlike amorphous carbon: Amorphous carbon (a-C) materials have diverse interesting and useful\nproperties, but the understanding of their atomic-scale structures is still\nincomplete. Here, we report on extensive atomistic simulations of the\ndeposition and growth of a-C films, describing interatomic interactions using a\nmachine learning (ML) based Gaussian Approximation Potential (GAP) model. We\nexpand widely on our initial work [Phys. Rev. Lett. 120, 166101 (2018)] by now\nconsidering a broad range of incident ion energies, thus modeling samples that\nspan the entire range from low-density ($sp^{2}$-rich) to high-density\n($sp^{3}$-rich, \"diamond-like\") amorphous forms of carbon. Two different\nmechanisms are observed in these simulations, depending on the impact energy:\nlow-energy impacts induce $sp$- and $sp^{2}$-dominated growth directly around\nthe impact site, whereas high-energy impacts induce peening. Furthermore, we\npropose and apply a scheme for computing the anisotropic elastic properties of\nthe a-C films. Our work provides fundamental insight into this intriguing class\nof disordered solids, as well as a conceptual and methodological blueprint for\nsimulating the atomic-scale deposition of other materials with ML-driven\nmolecular dynamics.", "category": "cond-mat_mtrl-sci" }, { "text": "Many-body Green's function GW and Bethe-Salpeter study of the optical\n excitations in a paradigmatic model dipeptide: We study within the many-body Green's function GW and Bethe-Salpeter\nformalisms the excitation energies of a paradigmatic model dipeptide, focusing\non the four lowest-lying local and charge-transfer excitations. Our GW\ncalculations are performed at the self-consistent level, updating first the\nquasiparticle energies, and further the single-particle wavefunctions within\nthe static Coulomb-hole plus screened-exchange approximation to the GW\nself-energy operator. Important level crossings, as compared to the starting\nKohn-Sham LDA spectrum, are identified. Our final Bethe-Salpeter singlet\nexcitation energies are found to agree, within 0.07 eV, with CASPT2 reference\ndata, except for one charge-transfer state where the discrepancy can be as\nlarge as 0.5 eV. Our results agree best with LC-BLYP and CAM-B3LYP calculations\nwith enhanced long-range exchange, with a 0.1 eV mean absolute error. This has\nbeen achieved employing a parameter-free formalism applicable to metallic or\ninsulating extended or finite systems.", "category": "cond-mat_mtrl-sci" }, { "text": "The electronic structure of liquid water within density functional\n theory: In the last decade, computational studies of liquid water have mostly\nconcentrated on ground state properties. However recent spectroscopic\nmeasurements have been used to infer the structure of water, and the\ninterpretation of optical and x-ray spectra requires accurate theoretical\nmodels of excited electronic states, not only of the ground state. To this end,\nwe investigate the electronic properties of water at ambient conditions using\nab initio density functional theory within the generalized gradient\napproximation (DFT/GGA), focussing on the unoccupied subspace of Kohn-Sham\neigenstates. We generate long (250 ps) classical trajectories for large\nsupercells, up to 256 molecules, from which uncorrelated configurations of\nwater molecules are extracted for use in DFT/GGA calculations of the electronic\nstructure. We find that the density of occupied states of this molecular liquid\nis well described with 32 molecule supercells using a single k-point (k = 0) to\napproximate integration over the first Brillouin zone. However, the description\nof the density of unoccupied states (u-EDOS) is sensitive to finite size\neffects. Small, 32 molecule supercell calculations, using Gamma-the point\napproximation, yield a spuriously isolated state above the Fermi level.\nNevertheless, the more accurate u-EDOS of large, 256 molecule supercells may be\nreproduced using smaller supercells and increased k-point sampling. This\nindicates that the electronic structure of molecular liquids like water is\nrelatively insensitive to the long-range disorder in the molecular structure.\nThese results have important implications for efficiently increasing the\naccuracy of spectral calculations for water and other molecular liquids.", "category": "cond-mat_mtrl-sci" }, { "text": "Classical potential describes martensitic phase transformations between\n the $\u03b1$, $\u03b2$ and $\u03c9$ titanium phases: A description of the martensitic transformations between the $\\alpha$,\n$\\beta$ and $\\omega$ phases of titanium that includes nucleation and growth\nrequires an accurate classical potential. Optimization of the parameters of a\nmodified embedded atom potential to a database of density-functional\ncalculations yields an accurate and transferable potential as verified by\ncomparison to experimental and density functional data for phonons, surface and\nstacking fault energies and energy barriers for homogeneous martensitic\ntransformations. Molecular dynamics simulations map out the\npressure-temperature phase diagram of titanium. For this potential the\nmartensitic phase transformation between $\\alpha$ and $\\beta$ appears at\nambient pressure and 1200 K, between $\\alpha$ and $\\omega$ at ambient\nconditions, between $\\beta$ and $\\omega$ at 1200 K and pressures above 8 GPa,\nand the triple point occurs at 8GPa and 1200 K. Molecular dynamics explorations\nof the dynamics of the martensitic $\\alpha-\\omega$ transformation show a\nfast-moving interface with a low interfacial energy of 30 meV/\\AA$^2$. The\npotential is applicable to the study of defects and phase transformations of\nTi.", "category": "cond-mat_mtrl-sci" }, { "text": "Sub-ps thermionic electron injection effects on exciton-formation\n dynamics at a van der Waals semiconductor/metal interface: Inorganic van der Waals bonded semiconductors like transition metal\ndichalcogenides are subject of intense research due to their electronic and\noptical properties which are promising for next-generation optoelectronic\ndevices. In this context, understanding the ultrafast carrier dynamics, as well\nas charge and energy transfer at the interface between metals and\nsemiconductors is crucial and yet quite unexplored. Here, we present an\nexperimental study on how thermally induced ultrafast charge carrier injection\naffects the exciton formation dynamics in bulk WS2 by employing a\npump-push-probe scheme, where a pump pulse induces thermionic injection of\nelectrons from the gold substrate into the conduction band of the\nsemiconductor, and another delayed push pulse excites direct transitions in the\nWS2. The transient response shows different dynamics on the sub-ps timescale by\nvarying the delay between pump and push pulses or by changing the pump fluence,\nthus disclosing the important role of ultrafast hot electron injection on the\nexciton formation dynamics. Our findings might have potential impact on\nresearch fields that target the integration of ultrafast optics at the boundary\nof photonics and electronics, as well as in optically-driven CMOS and quantum\ntechnologies.", "category": "cond-mat_mtrl-sci" }, { "text": "Transformation kinetics of alloys under non-isothermal conditions: The overall solid-to-solid phase transformation kinetics under non-isothermal\nconditions has been modeled by means of a differential equation method. The\nmethod requires provisions for expressions of the fraction of the transformed\nphase in equilibrium condition and the relaxation time for transition as\nfunctions of temperature. The thermal history is an input to the model. We have\nused the method to calculate the time/temperature variation of the volume\nfraction of the favored phase in the alpha-to-beta transition in a zirconium\nalloy under heating and cooling, in agreement with experimental results. We\nalso present a formulation that accounts for both additive and non-additive\nphase transformation processes. Moreover, a method based on the concept of path\nintegral, which considers all the possible paths in thermal histories to reach\nthe final state, is suggested.", "category": "cond-mat_mtrl-sci" }, { "text": "Combined SANS and SAXS study of the action of ultrasound on the\n structure of amorphous zirconia gels: In the present work, we have studied for the first time the combined effect\nof both sonication and precipitation pH on the structure of amorphous zirconia\ngels synthesized from zirconium(IV) propoxide. The techniques of small-angle\nneutron and X-ray scattering (SANS and SAXS) and low temperature nitrogen\nadsorption provided the integral data on the changes in the microstructure and\nmesostructure of these materials caused by ultrasonic (US) treatment. Amorphous\nZrO2.xH2O synthesized under ultrasonic treatment was found to possess a very\nstructured surface, characterized by the surface fractal dimension 2.9-3.0,\ncompared to 2.3-2.5 for the non US-assisted synthesis, and it was also found to\npossess a higher specific surface area, while the sizes of the primary\nparticles remain unchanged.", "category": "cond-mat_mtrl-sci" }, { "text": "Thermoelectric properties of marcasite and pyrite FeX$_2$(X=Se,Te): A\n first principle study: Electronic structure and thermoelectric properties of marcasite (m) and\nsynthetic pyrite (p) phases of FeX$_2$ (X=Se,Te) have been investigated using\nfirst principles density functional theory and Boltzmann transport equation.\nThe plane wave pseudopotential approximation was used to study the structural\nproperties and full-potential linear augmented plane wave method was used to\nobtain the electronic structure and thermoelectric properties (thermopower and\npower factor scaled by relaxation time). From total energy calculations we find\nthat m-FeSe$_2$ and m-FeTe$_2$ are stable at ambient conditions and no\nstructural transition from marcasite to pyrite is seen under the application of\nhydrostatic pressure. The calculated ground state structural properties agree\nquite well with available experiments. From the calculated thermoelectric\nproperties, we find that both m and p forms are good candidates for\nthermoelectric applications. However, hole doped m-FeSe$_2$ appears to be the\nbest among all the four systems.", "category": "cond-mat_mtrl-sci" }, { "text": "Production of Gas Phase Zinc Oxide Nanoclusters by Pulsed Laser Ablation: We present experimental results on the photoluminescence (PL) of\ngas-suspended zinc oxide nanoclusters prepared during ablation of sintered ZnO\ntargets by a pulsed ArF laser in the presence of oxygen ambient gas. The PL\nspectra in the UV spectral region correspond to the exciton recombination in\nthe nanoclusters which are crystallized and cooled down to the temperature of\nthe ambient gas in the ablation chamber. The time evolution of the spectra as\nwell as their dependence on the ambient gas pressure are discussed.", "category": "cond-mat_mtrl-sci" }, { "text": "First Principles Study of the Giant Magnetic Anisotropy Energy in Bulk\n Na4IrO4: In 5d transition metal oxides, novel properties arise from the interplay of\nelectron correlations and spin--orbit interactions. Na4IrO4, where 5d\ntransition-metal Ir atom occupies the center of the square-planar coordination\nenvironment, is synthesized. Based on density functional theory, we calculate\nits electronic and magnetic properties. Our numerical results show that the\nIr-5d bands are quite narrow, and the bands around the Fermi level are mainly\ncontributed by d_{xy},d_{yz} and d_{zx} orbitals. The magnetic easy-axis is\nperpendicular to the IrO4 plane, and the magnetic anisotropy energy (MAE) of\nNa4IrO4 is found to be very giant. We estimate the magnetic parameters by\nmapping the calculated total energy for different spin configurations onto a\nspin model. The next nearest neighbor exchange interaction J2 is much larger\nthan other intersite exchange interactions and results in the magnetic ground\nstate configuration. Our study clearly demonstrates that the huge MAE comes\nfrom the single-ion anisotropy rather than the anisotropic interatomic spin\nexchange. This compound has a large spin gap but very narrow spin-wave\ndispersion, due to the large single-ion anisotropy and relatively small\nexchange couplings. Noticing this remarkable magnetic feature originated from\nits highly isolated IrO4 moiety, we also explore the possiblity to further\nenhance the MAE.", "category": "cond-mat_mtrl-sci" }, { "text": "Electronic, transport and optical properties of monolayer $\u03b1$ and\n $\u03b2-$GeSe: A first-principles study: The extraordinary properties and the novel applications of black phosphorene\ninduce the research interest on the monolayer group-IV monochalcogenides. Here\nusing the first-principles calculations, we systematically investigate the\nelectronic, transport and optical properties of monolayer $\\alpha-$ and\n$\\beta-$GeSe, the latter of which was recently experimentally realized. We\nfound that, monolayer $\\alpha-$GeSe is a semiconductor with direct band gap of\n1.6 eV, and $\\beta-$GeSe displays indirect semiconductor with the gap of 2.47\neV, respectively. For monolayer $\\beta-$GeSe, the electronic/hole transport is\nanisotropic with an extremely high electron mobility of 7.84\n$\\times10^4$$cm^2/V\\cdot {s}$ along the zigzag direction, comparable to that of\nblack phosphorene. Furthermore, for $\\beta-$GeSe, robust band gaps nearly\ndisregarding the applied tensile strain along the zigzag direction is observed.\nBoth monolayer $\\alpha-$ and $\\beta-$GeSe exhibit anisotropic optical\nabsorption in the visible spectrum.", "category": "cond-mat_mtrl-sci" }, { "text": "The commensurate phase of multiferroic HoMn2O5 studied by X-ray magnetic\n scattering: The commensurate phase of multiferroic HoMn2O5 was studied by X-ray magnetic\nscattering, both off resonance and in resonant conditions at the Ho-L3 edge.\nBelow 40 K, magnetic ordering at the Ho sites is induced by the main Mn\nmagnetic order parameter, and its temperature dependence is well accounted for\nby a simple Curie-Weiss susceptibility model. A lattice distortion of\nperiodicity twice that of the magnetic order is also evidenced. Azimuthal scans\nconfirm the model of the magnetic structure recently refined from neutron\ndiffraction data for both Mn and Ho sites, indicating that the two sublattices\ninteract via magnetic superexchange.", "category": "cond-mat_mtrl-sci" }, { "text": "Electron emission from plasmonically induced Floquet bands at metal\n surfaces: We explore the possibility of existence of plasmonically generated electronic\nFloquet bands at metal surfaces by studying the gauge transformed\nelectron-surface plasmon interaction in the prepumped plasmonic coherent state\nenvironment. These bands may promote non-Einsteinian electron emission from\nmetal surfaces exposed to primary interactions with strong electromagnetic\nfields. Resonant behaviour and scaling of emission yield with the parent\nelectronic structure and plasmonic state parameters are estimated for Ag(111)\nsurface. Relative yield intensities from non-Einsteinian emission channels in\nphotoelectron spectra offer the means to calibrate the mediating plasmonic\nfields and therefrom ensuing surface Floquet bands.", "category": "cond-mat_mtrl-sci" }, { "text": "A critical study of the elastic properties and stability of Heusler\n compounds: Phase change and tetragonal $X_{2}YZ$ compounds: In the present work, the elastic constants and derived properties of\ntetragonal and cubic Heusler compounds were calculated using the high accuracy\nof the full-potential linearized augmented plane wave (FPLAPW). To find the\ncriteria required for an accurate calculation, the consequences of increasing\nthe numbers of $k$-points and plane waves on the convergence of the calculated\nelastic constants were explored. Once accurate elastic constants were\ncalculated, elastic anisotropies, sound velocities, Debye temperatures,\nmalleability, and other measurable physical properties were determined for the\nstudied systems. The elastic properties suggested metallic bonding with\nintermediate malleability, between brittle and ductile, for the studied Heusler\ncompounds. To address the effect of off-stoichiometry on the mechanical\nproperties, the virtual crystal approximation (VCA) was used to calculate the\nelastic constants. The results indicated that an extreme correlation exists\nbetween the anisotropy ratio and the stoichiometry of the Heusler compounds,\nespecially in the case of Ni$_{2}$MnGa.", "category": "cond-mat_mtrl-sci" }, { "text": "Cubic-scaling iterative solution of the Bethe-Salpeter equation for\n finite systems: The Bethe-Salpeter equation (BSE) is currently the state of the art in the\ndescription of neutral electron excitations in both solids and large finite\nsystems. It is capable of accurately treating charge-transfer excitations that\npresent difficulties for simpler approaches. We present a local basis set\nformulation of the BSE for molecules where the optical spectrum is computed\nwith the iterative Haydock recursion scheme, leading to a low computational\ncomplexity and memory footprint. Using a variant of the algorithm we can go\nbeyond the Tamm-Dancoff approximation (TDA). We rederive the recursion\nrelations for general matrix elements of a resolvent, show how they translate\ninto continued fractions, and study the convergence of the method with the\nnumber of recursion coefficients and the role of different terminators. Due to\nthe locality of the basis functions the computational cost of each iteration\nscales asymptotically as $O(N^3)$ with the number of atoms, while the number of\niterations is typically much lower than the size of the underlying\nelectron-hole basis. In practice we see that , even for systems with thousands\nof orbitals, the runtime will be dominated by the $O(N^2)$ operation of\napplying the Coulomb kernel in the atomic orbital representation", "category": "cond-mat_mtrl-sci" }, { "text": "THz emission from Co/Pt bilayers with varied roughness, crystal\n structure, and interface intermixing: Femtosecond laser excitation of a Co/Pt bilayer results in the efficient\nemission of picosecond THz pulses. Two known mechanisms for generating THz\nemission are spin-polarized currents through a Co/Pt interface, resulting in\nhelicity-independent electric currents in the Pt layer due to the inverse\nspin-Hall effect and helicity-dependent electric currents at the Co/Pt\ninterface due to the inverse spin-orbit torque effect. Here we explore how\nroughness, crystal structure and intermixing at the Co/Pt interface affect the\nefficiency of the THz emission. In particular, we varied the roughness of the\ninterface, in the range of 0.1-0.4 nm, by tuning the deposition pressure\nconditions during the fabrication of the Co/Pt bilayers. To control the\nintermixing at the Co/Pt interface a 1-2 nm thick CoxPt1-x alloy spacer layer\nwas introduced with various compositions of Co and Pt. Finally, the crystal\nstructure of Co was varied from face centered cubic to hexagonal close packed.\nOur study shows that the roughness of the interface is of crucial importance\nfor the efficiency of helicity-dependent THz emission induced by femtosecond\nlaser pulses. However, it is puzzling that intermixing while strongly enhancing\nthe helicity-independent THz emission had no effect on the helicity-dependent\nTHz emission which is suppressed and similar to the smooth interfaces.", "category": "cond-mat_mtrl-sci" }, { "text": "Investigation of thickness dependent composition of boron carbide thin\n films by resonant soft x-ray reflectivity: Boron carbide thin films of different thicknesses deposited by ion beam\nsputtering were studied. The deposited films were characterized by grazing\nincidence hard x-ray reflectivity (GIXR), resonant soft x-ray reflectivity\n(RSXR), x-ray photo electron spectroscopy (XPS), resonant Rutherford\nbackscattering spectrometry (RRBS), and time of flight secondary ion mass\nspectrometry (TOF-SIMS). An in-depth profile of the chemical elements\nconstitute the films is reconstructed based on analysis of reflectivity curves\nmeasured in the vicinity of B K-edge. The composition of films is closely\ndependent on film thickness. Boron to Carbon (B/C) ratio reaches to ~4 as the\nthickness of deposited films increases. The B/C ratio estimated from RSXR\nmeasurements are in agreement with the RRBS measurements. TOF-SIMS data also\nsuggested that decrease in boron content with decrease in film thickness. XPS\nmeasurements confirm the presence of little amount of B atoms on the surface of\nlow thickness film.", "category": "cond-mat_mtrl-sci" }, { "text": "Magnetic ordering and fluctuation in kagome lattice antiferromagnets, Fe\n and Cr jarosites: Jarosite family compounds, KFe_3(OH)_6(SO_4)_2, (abbreviate Fe jarosite), and\nKCr_3(OH)_6(SO_4)_2, (Cr jarosite), are typical examples of the Heisenberg\nantiferromagnet on the kagome lattice and have been investigated by means of\nmagnetization and NMR experiments. The susceptibility of Cr jarosite deviates\nfrom Curie-Weiss law due to the short-range spin correlation below about 150 K\nand shows the magnetic transition at 4.2 K, while Fe jarosite has the\ntransition at 65 K. The susceptibility data fit well with the calculated one on\nthe high temperature expansion for the Heisenberg antiferromagnet on the kagome\nlattice. The values of exchange interaction of Cr jarosite and Fe jarosite are\nderived to be J_Cr = 4.9 K and J_Fe = 23 K, respectively. The 1H-NMR spectra of\nFe jarosite suggest that the ordered spin structure is the q = 0 type with\npositive chirality of the 120 degrees configuration. The transition is caused\nby a weak single-ion type anisotropy. The spin-lattice relaxation rate, 1/T_1,\nof Fe jarosite in the ordered phase decreases sharply with lowering the\ntemperature and can be well explained by the two-magnon process of spin wave\nwith the anisotropy.", "category": "cond-mat_mtrl-sci" }, { "text": "A Temporal Filter to Extract Doped Conducting Polymer Information\n Features from an Electronic Nose: Identifying relevant machine-learning features for multi-sensing platforms is\nboth an applicative limitation to recognize environments and a necessity to\ninterpret the physical relevance of transducers' complementarity in their\ninformation processing. Particularly for long acquisitions, feature extraction\nmust be fully automatized without human intervention and resilient to\nperturbations without increasing significantly the computational cost of a\nclassifier. In this study, we investigate on the relative resistance and\ncurrent modulation of a 24-dimensional conductimetric electronic nose, which\nuses the exponential moving average as a floating reference in a low-cost\ninformation descriptor for environment recognition. In particular, we\nidentified that depending on the structure of a linear classifier, the 'modema'\ndescriptor is optimized for different material sensing elements' contributions\nto classify information patterns. The low-pass filtering optimization leads to\nopposite behaviors between unsupervised and supervised learning: the latter one\nfavors longer integration of the reference, allowing to recognize five\ndifferent classes over 90%, while the first one prefers using the latest events\nas its reference to clusterize patterns by environment nature. Its electronic\nimplementation shall greatly diminish the computational requirements of\nconductimetric electronic noses for on-board environment recognition without\nhuman supervision.", "category": "cond-mat_mtrl-sci" }, { "text": "Electronic structure and properties of pure and doped $\u03b5$-FeSi\n from ab-initio local density theory: Local density calculations of the electronic structure of FeSi,\nFeSi_{1-x}Al_x and Fe_{1-x}Ir_xSi systems in the B20 structure are presented.\nPure FeSi has a semi-conducting gap of 6 mRy at 0 K.\n Effects of temperature (T) in terms of electronic and vibrational excitations\nare included. Various measurable properties, such as magnetic susceptibility\nchi(T), electronic specific heat C(T), thermoelectric power S(T), relative\nvariations in resistivity rho(T), and peak positions in the density-of-states\n(DOS) are calculated. The feedback from vibrational disorder onto the\nelectronic structure is found to be essential for a good description of most\nproperties, although the results for S(T) in undoped FeSi can be described up\nto about 150 K without considerations of disorder. Above this T, only the\nfilling of the gap due to disorder accompanied by exchange enhancement, can\nexplain the large susceptiblity. The overall good agreement with experimental\ndata for most properties in doped and pure FeSi suggests that this system is\nwell described by LDA even at large T. Doped FeSi can be described quite well\nfrom rigid-band shifts of the Fermi energy on the DOS of pure FeSi.\nSpin-polarization in Ir doped FeSi leads to a semi-metallic magnetic state at\nlow T. (Submitted to Phys. Rev. B)", "category": "cond-mat_mtrl-sci" }, { "text": "Factors influencing graphene growth on metal surfaces: Graphene forms from a relatively dense, tightly-bound C-adatom gas, when\nelemental C is deposited on or segregates to the Ru(0001) surface. Nonlinearity\nof the graphene growth rate with C adatom density suggests that growth proceeds\nby addition of C atom clusters to the graphene edge. The generality of this\npicture has now been studied by use of low-energy electron microscopy (LEEM) to\nobserve graphene formation when Ru(0001) and Ir(111) surfaces are exposed to\nethylene. The finding that graphene growth velocities and nucleation rates on\nRu have precisely the same dependence on adatom concentration as for elemental\nC deposition implies that hydrocarbon decomposition only affects graphene\ngrowth through the rate of adatom formation; for ethylene, that rate decreases\nwith increasing adatom concentration and graphene coverage. Initially, graphene\ngrowth on Ir(111) is like that on Ru: the growth velocity is the same nonlinear\nfunction of adatom concentration (albeit with much smaller equilibrium adatom\nconcentrations, as we explain with DFT calculations of adatom formation\nenergies). In the later stages of growth, graphene crystals that are rotated\nrelative to the initial nuclei nucleate and grow. The rotated nuclei grow much\nfaster. This difference suggests first, that the edge-orientation of the\ngraphene sheets relative to the substrate plays an important role in the growth\nmechanism, and second, that attachment of the clusters to the graphene is the\nslowest step in cluster addition, rather than formation of clusters on the\nterraces.", "category": "cond-mat_mtrl-sci" }, { "text": "Determining the Fundamental Failure Modes in Ni-rich Lithium Ion Battery\n Cathodes: Challenges associated with in-service mechanical degradation of Li-ion\nbattery cathodes has prompted a transition from polycrystalline to single\ncrystal cathode materials. Whilst for single crystal materials,\ndislocation-assisted crack formation is assumed to be the dominating failure\nmechanism throughout battery life, there is little direct information about\ntheir mechanical behaviour, and mechanistic understanding remains elusive.\nHere, we demonstrated, using in situ micromechanical testing, direct\nmeasurement of local mechanical properties within LiNi0.8Mn0.1Co0.1O2 single\ncrystalline domains. We elucidated the dislocation slip systems, their critical\nstresses, and how slip facilitate cracking. We then compared single crystal and\npolycrystal deformation behaviour. Our findings answer two fundamental\nquestions critical to understanding cathode degradation: What dislocation slip\nsystems operate in Ni-rich cathode materials? And how does slip cause fracture?\nThis knowledge unlocks our ability to develop tools for lifetime prediction and\nfailure risk assessment, as well as in designing novel cathode materials with\nincreased toughness in-service.", "category": "cond-mat_mtrl-sci" }, { "text": "Experimental exploration of ErB$_2$ and SHAP analysis on a\n machine-learned model of magnetocaloric materials for materials design: Stimulated by a recent report of a giant magnetocaloric effect in HoB$_2$\nfound via machine-learning predictions, we have explored the magnetocaloric\nproperties of a related compound ErB$_2$, that has remained the last\nferromagnetic material among the rare-earth diboride (REB$_2$) family with\nunreported magnetic entropy change |{\\Delta}SM|. The evaluated $|\\Delta S_M|$\nat field change of 5 T in ErB$_2$ turned out to be as high as 26.1 (J kg$^{-1}$\nK$^{-1}$) around the ferromagnetic transition (${T_C}$) of 14 K. In this\nseries, HoB$_2$ is found to be the material with the largest $|\\Delta S_M|$ as\nthe model predicted, while the predicted values showed a deviation with a\nsystematic error compared to the experimental values. Through a coalition\nanalysis using SHAP, we explore how this rare-earth dependence and the\ndeviation in the prediction are deduced in the model. We further discuss how\nSHAP analysis can be useful in clarifying favorable combinations of constituent\natoms through the machine-learned model with compositional descriptors. This\nanalysis helps us to perform materials design with aid of machine learning of\nmaterials data.", "category": "cond-mat_mtrl-sci" }, { "text": "Origin of training effect of exchange bias in Co/CoO due to irreversible\n thermoremanent magnetization of the magnetically diluted antiferromagnet: The irreversible thermoremanent magnetization of a sole, magnetically diluted\nepitaxial antiferromagnetic Co$_{1-y}$O(100) layer is determined by the mean of\nits thermoremanent magnetizations at positive and negative remanence. During\nhysteresis-loop field cycling, thermoremanent magnetization exhibits successive\nreductions, consistent with the training effect (TE) of the exchange bias\nmeasured for the corresponding Co$_{1-y}$O(100)/Co bilayer. The TE of exchange\nbias is shown to have its microscopic origin in the TE of the irreversible\nthermoremanent magnetization of the magnetically diluted AFM.", "category": "cond-mat_mtrl-sci" }, { "text": "Intermittency in aging: The fluctuation-dissipation relation (FDR) is measured on the dielectric\nproperties of a gel (Laponite) and of a polymer glass (polycarbonate). For the\ngel it is found that during the transition from a fluid-like to a solid-like\nstate the fluctuation dissipation theorem is strongly violated. The amplitude\nand the persistence time of this violation are decreasing functions of\nfrequency. Around $1Hz$ it may persist for several hours. A very similar\nbehavior is observed in polycarbonate after a quench below the glass transition\ntemperature. In both cases the origin of this violation is a highly\nintermittent dynamics characterized by large fluctuations. The relevance of\nthese results for recent models of aging are discussed.", "category": "cond-mat_mtrl-sci" }, { "text": "Electronic structure and stability of hydrogen defects in diamond and\n boron doped diamond: A density functional theory study: Isolated hydrogen and hydrogen pairs in bulk diamond matrix have been studied\nusing density functional theory calculations. The electronic structure and\nstability of isolated and paired hydrogen defects are investigated at different\npossible lattice sites in pure diamond and boron doped diamond. Calculations\nrevealed that isolated hydrogen defect is stable at bond center sites for pure\ndiamond and bond center puckered site for boron doped diamond. In case of\nhydrogen pairs, H2 defect (one hydrogen at bond center and second at\nanti-bonding site) is stable for pure diamond, while for boron doped diamond\nB-H2BC complex (one H atom at the B-C bond centered puckered position and the\nother one at the puckered position of one of the C-C bond first neighbor of the\nB atom) is most stable. Multiple hydrogen trapping sites in boron doped diamond\nhas also been studied. Calculated results are discussed and compared with\npreviously reported theoretical results in detailed.", "category": "cond-mat_mtrl-sci" }, { "text": "First-principles based simulations of electronic transmission in\n ReS$_{2}$/WSe$_{2}$ and ReS$_{2}$/MoSe$_{2}$ type-II vdW heterointerfaces: Electronic transmission in monolayer ReS$_{2}$ and ReS$_{2}$ based van der\nWaals (vdW) heterointerfaces are studied here. Since ReS$_{2}$/WSe$_{2}$ and\nReS$_{2}$/MoSe$_{2}$ type-II vdW heterostructures are suitable for near\ninfrared (NIR)/short-wave infrared (SWIR) photodetection, the role of\ninterlayer coupling at the heterointerfaces is examined in this work. Besides,\na detailed theoretical study is presented employing density functional theory\n(DFT) and nonequilibrium Green's function (NEGF) combination to analyse the\ntransmission spectra of the two-port devices with ReS$_{2}$/WSe$_{2}$ and\nReS$_{2}$/MoSe$_{2}$ channels and compare the near-equilibrium conductance\nvalues.Single layer distorted1T ReS$_{2}$ exhibits formation of parallel chains\nof 'Re' - 'Re' bonds, leading to in-plane anisotropy. Owing to this structural\nanisotropy, the charge carrier transport is very much orientation dependent in\nReS$_{2}$. Therefore, this work is further extended to investigate the role of\nclusterized 'Re' atoms in electronic transmission.", "category": "cond-mat_mtrl-sci" }, { "text": "Atomic-Scale Tailoring of Chemisorbed Atomic Oxygen on Epitaxial\n Graphene for Graphene-Based Electronic Devices: Graphene, with its unique band structure, mechanical stability, and high\ncharge mobility, holds great promise for next-generation electronics.\nNevertheless, its zero band gap challenges the control of current flow through\nelectrical gating, consequently limiting its practical applications. Recent\nresearch indicates that atomic oxygen can oxidize epitaxial graphene in a\nvacuum without causing unwanted damage. In this study, we have investigated the\neffects of chemisorbed atomic oxygen on the electronic properties of epitaxial\ngraphene, using scanning tunneling microscopy (STM). Our findings reveal that\noxygen atoms effectively modify the electronic states of graphene, resulting in\na band gap at its Dirac point. Furthermore, we demonstrate that it is possible\nto selectively induce desorption or hopping of oxygen atoms with atomic\nprecision by applying appropriate bias sweeps with an STM tip. These results\nsuggest the potential for atomic-scale tailoring of graphene oxide, enabling\nthe development of graphene-based atomic-scale electronic devices.", "category": "cond-mat_mtrl-sci" }, { "text": "Function follows form: From semiconducting to metallic towards\n superconducting PbS nanowires by faceting the crystal: In the realm of colloidal nanostructures, with its immense capacity for shape\nand dimensionality control, the form is undoubtedly a driving factor for the\ntunability of optical and electrical properties in semiconducting or metallic\nmaterials. However, influencing the fundamental properties is still challenging\nand requires sophisticated surface or dimensionality manipulation. In this\nwork, we present such a modification for the example of colloidal lead sulphide\nnanowires. We show that the electrical properties of lead sulphide\nnanostructures can be altered from semiconducting to metallic with indications\nof superconductivity, by exploiting the flexibility of the colloidal synthesis\nto sculpt the crystal and to form different surface facets. A particular\nmorphology of lead sulphide nanowires has been synthesized through the\nformation of {111} surface facets, which shows metallic and superconducting\nproperties in contrast to other forms of this semiconducting crystal, which\ncontain other surface facets ({100} and {110}). This effect, which has been\ninvestigated with several experimental and theoretical approaches, is\nattributed to the presence of lead rich {111} facets. The insights promote new\nstrategies for tuning the properties of crystals as well as new applications\nfor lead sulphide nanostructures.", "category": "cond-mat_mtrl-sci" }, { "text": "Quantum effects in graphene monolayers: Path-integral simulations: Path-integral molecular dynamics (PIMD) simulations have been carried out to\nstudy the influence of quantum dynamics of carbon atoms on the properties of a\nsingle graphene layer. Finite-temperature properties were analyzed in the range\nfrom 12 to 2000~K, by using the LCBOPII effective potential. To assess the\nmagnitude of quantum effects in structural and thermodynamic properties of\ngraphene, classical molecular dynamics simulations have been also performed.\nParticular emphasis has been laid on the atomic vibrations along the\nout-of-plane direction. Even though quantum effects are present in these\nvibrational modes, we show that at any finite temperature classical-like motion\ndominates over quantum delocalization, provided that the system size is large\nenough. Vibrational modes display an appreciable anharmonicity, as derived from\na comparison between kinetic and potential energy of the carbon atoms. Nuclear\nquantum effects are found to be appreciable in the interatomic distance and\nlayer area at finite temperatures. The thermal expansion coefficient resulting\nfrom PIMD simulations vanishes in the zero-temperature limit, in agreement with\nthe third law of thermodynamics.", "category": "cond-mat_mtrl-sci" }, { "text": "Elastic and thermodynamic properties of the shape-memory alloy AuZn: The current work reports on the elastic shear moduli, internal friction, and\nthe specific heat of the B2 cubic ordered alloy AuZn as a function of\ntemperature. Measurements were made on single-crystal and polycrystalline\nsamples using Resonant Ultrasound Spectroscopy (RUS), semi-adiabatic\ncalorimetry and stress-strain measurements. Our results confirm that this alloy\nexhibits the shape-memory effect and a phase transition at 64.75 K that appears\nto be continuous (second-order) from the specific heat data. It is argued that\nthe combination of equiatomic composition and a low transformation temperature\nconstrain the chemical potential and its derivatives to exhibit behavior that\nlies at the borderline between that of a first-order (discontinuous) and a\ncontinuous phase transition. The acoustic dissipation does not peak at the\ntranstion temperature as expected, but shows a maximum well into the\nlow-temperature phase. The Debye temeprature value of 219 K, obtained from the\nlow-temperature specific heat data is in favorable agreement with that\ndetermined from the acoustic data (207 K) above the transition.", "category": "cond-mat_mtrl-sci" }, { "text": "Photo-Hall, photorefractive and photomagnetoelectric effects in tungsten\n bronzes and related tetragonal ferroelectrics: We present an extensive study of the electric, magnetic and elastic responses\nof tetragonal ferroelectrics under illumination, using a new theory\nintertwining material and wave symmetries. Optical rectification,\nphotomagnetic, photovoltaic and phototoroidal vector responses are worked out\nas functions of the wave vector and wave polarization directions. Second-order\nresponse tensors associated with photoelastic, photomagnetoelectric,\nphotorefractive effects and photoconductivity are described. We discuss in\ndetail the photo-Hall and a new non-linear optical effect in tungsten bronzes.\nFinally, we compare the properties of tetragonal materials with those of\nhexagonal, orthorhombic and trigonal ferroelectrics previously reported in the\nliterature.", "category": "cond-mat_mtrl-sci" }, { "text": "TC++: First-principles calculation code for solids using the\n transcorrelated method: TC++ is a free/libre open-source software of the transcorrelated (TC) method\nfor first-principles calculation of solids. Here, the TC method is one of the\npromising wave-function theories that can be applied to periodic systems with\nreasonable computational cost and satisfactory accuracy. We present our\nimplementation of TC++ including a detailed description of the divergence\ncorrection technique applied to the TC effective interactions. We also present\nthe way to use TC++ and some results of application to simple periodic systems:\nbulk silicon and homogeneous electron gas.", "category": "cond-mat_mtrl-sci" }, { "text": "Strain engineering of epitaxial oxide heterostructures beyond substrate\n limitations: The limitation of commercially available single-crystal substrates and the\nlack of continuous strain tunability preclude the ability to take full\nadvantage of strain engineering for further exploring novel properties and\nexhaustively studying fundamental physics in complex oxides. Here we report an\napproach for imposing continuously tunable, large epitaxial strain in oxide\nheterostructures beyond substrate limitations by inserting an interface layer\nthrough tailoring its gradual strain relaxation. Taking BiFeO3 as a model\nsystem, we demonstrate that the introduction of an ultrathin interface layer\nallows the creation of a desired strain that can induce phase transition and\nstabilize a new metastable super-tetragonal phase as well as morphotropic phase\nboundaries overcoming substrate limitations. Furthermore, continuously tunable\nstrain from tension to compression can be generated by precisely adjusting the\nthickness of the interface layer, leading to the first achievement of\ncontinuous O-R-T phase transition in BiFeO3 on a single substrate. This\nproposed route could be extended to other oxide heterostructures, providing a\nplatform for creating exotic phases and emergent phenomena.", "category": "cond-mat_mtrl-sci" }, { "text": "Nanowire growth and sublimation: CdTe quantum dots in ZnTe nanowires: The role of the sublimation of the compound and of the evaporation of the\nconstituents from the gold nanoparticle during the growth of semiconductor\nnanowires is exemplified with CdTe-ZnTe heterostructures. Operating close to\nthe upper temperature limit strongly reduces the amount of Cd present in the\ngold nanoparticle and the density of adatoms on the nanowire sidewalls. As a\nresult, the growth rate is small and strongly temperature dependent, but a good\ncontrol of the growth conditions allows the incorporation of quantum dots in\nnanowires with sharp interfaces and adjustable shape, and it minimizes the\nradial growth and the subsequent formation of additional CdTe clusters on the\nnanowire sidewalls, as confirmed by photoluminescence. Uncapped CdTe segments\ndissolve into the gold nanoparticle when interrupting the flux, giving rise to\na bulb-like (pendant-droplet) shape attributed to the Kirkendall effect.", "category": "cond-mat_mtrl-sci" }, { "text": "Cyclic Ferroelectric Switching and Quantized Charge Transport in\n CuInP$_2$S$_6$: The van der Waals layered ferroelectric CuInP$_2$S$_6$ has been found to\nexhibit a variety of intriguing properties arising from the fact that the Cu\nions are unusually mobile in this system. While the polarization switching\nmechanism is usually understood to arise from Cu ion motion within the\nmonolayers, a second switching path involving Cu motion across the van der\nWaals gaps has been suggested. In this work, we perform zero-temperature\nfirst-principles calculations on such switching paths, focusing on two types\nthat preserve the periodicity of the primitive unit cell: ``cooperative\" paths\npreserving the system's glide mirror symmetry, and ``sequential\" paths in which\nthe two Cu ions in the unit cell move independently of each other. We find that\nCuInP$_2$S$_6$ features a rich and varied energy landscape, and that sequential\npaths are clearly favored energetically both for cross-gap and through-layer\npaths. Importantly, these segments can be assembled to comprise a globally\ninsulating cycle with the out-of-plane polarization evolving by a quantum as\nthe Cu ions shift to neighboring layers. In this sense, we argue that\nCuInP$_2$S$_6$ embodies the physics of a quantized adiabatic charge pump.", "category": "cond-mat_mtrl-sci" }, { "text": "Dzyaloshinskii-Moriya interaction from unquenched orbital angular\n momentum: Orbitronics is an emerging and fascinating field that explores the\nutilization of the orbital degree of freedom of electrons for information\nprocessing. An increasing number of orbital phenomena are being currently\ndiscovered, with spin-orbit coupling mediating the interplay between orbital\nand spin effects, thus providing a wealth of control mechanisms and device\napplications. In this context, the orbital analog of spin Dzyaloshinskii-Moriya\ninteraction (DMI), i.e. orbital DMI, deserves to be explored in depth, since it\nis believed to be capable of inducing chiral orbital structures. Here, we\nunveil the main features and microscopic mechanisms of the orbital DMI in a\ntwo-dimensional square lattice using a tight-binding model of t2g orbitals in\ncombination with the Berry phase theory. This approach allows us to investigate\nand transparently disentangle the role of inversion symmetry breaking, strength\nof orbital exchange interaction and spin-orbit coupling in shaping the\nproperties of the orbital DMI. By scrutinizing the band-resolved contributions\nwe are able to understand the microscopic mechanisms and guiding principles\nbehind the orbital DMI and its anisotropy in two dimensional magnetic\nmaterials, and uncover a fundamental relation between the orbital DMI and its\nspin counterpart, which is currently explored very intensively. The insights\ngained from our work contribute to advancing our knowledge of orbitalrelated\neffects and their potential applications in spintronics, providing a path for\nfuture research in the field of chiral orbitronics.", "category": "cond-mat_mtrl-sci" }, { "text": "Experimental realization of chiral Landau levels in two-dimensional\n Dirac cone systems with inhomogeneous effective mass: Chiral zeroth Landau levels are topologically protected bulk states that give\nrise to chiral anomaly. Previous discussions on such chiral Landau levels are\nbased on three-dimensional Weyl degeneracies. Their realizations using\ntwo-dimensional Dirac point systems, being more promising for future\napplications, were never reported before. Here we propose a theoretical and\nexperimental scheme for realizing chiral Landau levels in a photonic system. By\nintroducing an inhomogeneous effective mass through breaking local parity\ninversion symmetries, the zeroth-order chiral Landau levels with one-way\npropagation characteristics are experimentally observed. In addition, the\nrobust transport of the chiral zeroth mode against defects in the system is\nexperimentally tested. Our system provides a new pathway for the realization of\nchiral Landau levels in two-dimensional Dirac systems, and may potentially be\napplied in device designs utilizing the transport robustness.", "category": "cond-mat_mtrl-sci" }, { "text": "Designing magnetic properties in CrSBr through hydrostatic pressure and\n ligand substitution: The ability to control magnetic properties of materials is crucial for\nfundamental research and underpins many information technologies. In this\ncontext, two-dimensional materials are a particularly exciting platform due to\ntheir high degree of tunability and ease of implementation into nanoscale\ndevices. Here we report two approaches for manipulating the A-type\nantiferromagnetic properties of the layered semiconductor CrSBr through\nhydrostatic pressure and ligand substitution. Hydrostatic pressure compresses\nthe unit cell, increasing the interlayer exchange energy while lowering the\nN\\'eel temperature. Ligand substitution, realized synthetically through Cl\nalloying, anisotropically compresses the unit cell and suppresses the\nCr-halogen covalency, reducing the magnetocrystalline anisotropy energy and\ndecreasing the N\\'eel temperature. A detailed structural analysis combined with\nfirst-principles calculations reveal that alterations in the magnetic\nproperties are intricately related to changes in direct Cr-Cr exchange\ninteractions and the Cr-anion superexchange pathways. Further, we demonstrate\nthat Cl alloying enables chemical tuning of the interlayer coupling from\nantiferromagnetic to ferromagnetic, which is unique amongst known\ntwo-dimensional magnets. The magnetic tunability, combined with a high ordering\ntemperature, chemical stability, and functional semiconducting properties, make\nCrSBr an ideal candidate for pre- and post-synthetic design of magnetism in\ntwo-dimensional materials.", "category": "cond-mat_mtrl-sci" }, { "text": "Discovering Equations that Govern Experimental Materials Stability under\n Environmental Stress using Scientific Machine Learning: While machine learning (ML) in experimental research has demonstrated\nimpressive predictive capabilities, inductive reasoning and knowledge\nextraction remain elusive tasks, in part because of the difficulty extracting\nfungible knowledge representations from experimental data. In this manuscript,\nwe use ML to infer the underlying dynamical differential equation (DE) from\nexperimental data of degrading organic-inorganic methylammonium lead iodide\n(MAPI) perovskite thin films under environmental stressors (elevated\ntemperature, humidity, and light). We apply a sparse regression algorithm that\nautomatically identifies the differential equation describing the dynamics from\ntime-series data. We find that the underlying DE governing MAPI degradation\nacross a broad temperature range of 35 to 85{\\deg}C is described minimally with\nthree terms (specifically, a second-order polynomial), and not a simple\nsingle-order reaction (i.e. 0th, 1st, or 2nd-order reaction). We demonstrate\nhow computer-derived results can aid the researcher to develop profound\nmechanistic insights. This DE corresponds to the Verhulst logistic function,\nwhich describes reaction kinetics analogous in functional form to autocatalytic\nor self-propagating reactions, suggesting future strategies to suppress MAPI\ndegradation. We examine the robustness of our conclusions to experimental\nluck-of-the-draw variance and Gaussian noise using a combination of experiment\nand simulation, and describe the experimental limits within which this\nmethodology can be applied. Our study demonstrates the application of\nscientific ML in experimental chemical and materials systems, highlighting the\npromise and challenges associated with ML-aided scientific discovery.", "category": "cond-mat_mtrl-sci" }, { "text": "A Bayesian Committee Machine Potential for Organic Nitrogen Compounds: Large-scale computer simulations of chemical atoms are used in a wide range\nof applications, including batteries, drugs, and more. However, there is a\nproblem with efficiency as it takes a long time due to the large amount of\ncalculation. To solve these problems, machine learning interatomic potential\n(ML-IAP) technology is attracting attention as an alternative. ML-IAP not only\nhas high accuracy by faithfully expressing the density functional theory (DFT),\nbut also has the advantage of low computational cost. However, there is a\nproblem that the potential energy changes significantly depending on the\nenvironment of each atom, and expansion to a wide range of compounds within a\nsingle model is still difficult to build in the case of a kernel-based model.\nTo solve this problem, we would like to develop a universal ML-IAP using this\nactive Bayesian Committee Machine (BCM) potential methodology for\ncarbon-nitrogen-hydrogen (CNH) with various compositions. ML models are trained\nand generated through first-principles calculations and molecular dynamics\nsimulations for molecules with only CNH. Using long amine structures to test an\nML model trained only with short chains, the results show excellent consistency\nwith DFT calculations. Consequently, machine learning-based models for organic\nmolecules not only demonstrate the ability to accurately describe various\nphysical properties but also hold promise for investigating a broad spectrum of\ndiverse materials systems.", "category": "cond-mat_mtrl-sci" }, { "text": "A new generation of subnanometer-sized materials reveals a general\n surface polarons property: The recent advent of cutting-edge experimental techniques allows for a\nprecise synthesis of monodisperse subnanometer metal clusters composed by just\na few atoms, and opens new possibilities for subnanometer science. The\ndecoration of titanium dioxide surfaces with the Ag$_{5}$ atomic cluster\nenables the stabilization of surface polarons. A new electron polarization\nphenomenon accompanying surface polaron formation has thus been revealed.", "category": "cond-mat_mtrl-sci" }, { "text": "Current-constraining variational approaches to quantum transport: Presently, the main methods for describing a non-equilibrium\ncharge-transporting steady state are based on time-evolving it from the initial\nzero-current situation. An alternative class of theories would give the\nstatistical non-equilibrium density operator from principles of statistical\nmechanics, in a spirit close to Gibbs ensembles for equilibrium systems,\nleading to a variational principle for the non-equilibrium steady state. We\ndiscuss the existing attempts to achieve this using the maximum entropy\nprinciple based on constraining the average current. We show that the\ncurrent-constrained theories result in a zero induced drop in electrostatic\npotential, so that such ensembles cannot correspond to the time-evolved density\nmatrix, unless left- and right-going scattering states are mutually incoherent.", "category": "cond-mat_mtrl-sci" }, { "text": "Energy exchanges between atoms with a quartz crystal $\u03bc$-balance: We propose an experimental method to fully characterize the energy exchange\nof particles during the physical vapor deposition process of thin surface\nlayers. Our approach is based on the careful observation of perturbations of\nthe oscillation frequency of a Quartz Crystal $\\mu$-balance induced by the\nparticles interaction. With this technique, it is possible to measure the\nmomentum exchange of the atoms during the evaporation process and determine the\nideal evaporation rate for an uniform energy distribution. We are able to\nfollow the desorption dynamics of particles immediately after the first layers\nhave been formed. These results are in close relation to the surface binding\nenergy of the evaporated material, they offer a better control to obtain the\ndesired properties of the thin surface layer. We applied our technique to\ninvestigate the physical vapor evaporation process for diverse elements,\nusually implemented in the development of film surface layers, such as Cu, W,\nAu, Gd and In, and confirm that our results are in agreement with measurements\ndone previously with other techniques such as low-temperature\nphotoluminescence.", "category": "cond-mat_mtrl-sci" }, { "text": "Modeling intercalation chemistry with multi-redox reactions by sparse\n lattice models in disordered rocksalt cathodes: Modern battery materials can contain many elements with substantial site\ndisorder, and their configurational state has been shown to be critical for\ntheir performance. The intercalation voltage profile is a critical parameter to\nevaluate the performance of energy storage. The application of commonly used\ncluster expansion techniques to model the intercalation thermodynamics of such\nsystems from \\textit{ab-initio} is challenged by the combinatorial increase in\nconfigurational degrees of freedom as the number of species grows. Such\nchallenges necessitate efficient generation of lattice models without\nover-fitting and proper sampling of the configurational space under charge\nbalance in ionic systems. In this work, we introduce a combined approach that\naddresses these challenges by (1) constructing a robust cluster-expansion\nHamiltonian using the sparse regression technique, including\n$\\ell_0\\ell_2$-norm regularization and structural hierarchy; and (2)\nimplementing semigrand-canonical Monte Carlo to sample charge-balanced ionic\nconfigurations using the table-exchange method and an ensemble-average\napproach. These techniques are applied to a disordered rocksalt oxyfluoride\nLi$_{1.3-x}$Mn$_{0.4}$Nb$_{0.3}$O$_{1.6}$F$_{0.4}$ (LMNOF) which is part of a\nfamily of promising earth-abundant cathode materials. The simulated voltage\nprofile is found to be in good agreement with experimental data and\nparticularly provides a clear demonstration of the Mn and oxygen contribution\nto the redox potential as a function of Li content.", "category": "cond-mat_mtrl-sci" }, { "text": "How to Simulate Billiards and Similar Systems: An N-component continuous-time dynamic system is considered whose components\nevolve autonomously all the time except for in discrete asynchronous instances\nof pairwise interactions. Examples include chaotically colliding billiard balls\nand combat models. A new efficient serial event-driven algorithm is described\nfor simulating such systems. Rather than maintaining and updating the global\nstate of the system, the algorithm tries to examine only essential events,\ni.e., component interactions. The events are processed in a non-decreasing\norder of time; new interactions are scheduled on the basis of the examined\ninteractions using preintegrated equations of the evolutions of the components.\nIf the components are distributed uniformly enough in the evolution space, so\nthat this space can be subdivided into small sectors such that only O(1)\nsectors and O(1)$components are in the neighborhood of a sector, then the\nalgorithm spends time O (log N) for processing an event which is the\nasymptotical minimum. The algorithm uses a simple strategy for handling data:\nonly two states are maintained for each simulated component. Fast data access\nin this strategy assures the practical efficiency of the algorithm. It works\nnoticeably faster than other algorithms proposed for this model.\n Key phrases: collision detection, dense packing, molecular dynamics, hard\nspheres, granular flow", "category": "cond-mat_mtrl-sci" }, { "text": "Combined single crystal polarized XAFS and XRD at high pressure: probing\n the interplay between lattice distortions and electronic order at multiple\n length scales in high $T_c$ cuprates: Some of the most exotic material properties derive from electronic states\nwith short correlation length (~10-500 {\\AA}), suggesting that the local\nstructural symmetry may play a relevant role in their behavior. Here we discuss\nthe combined use of polarized x-ray absorption fine structure and x-ray\ndiffraction at high pressure as a powerful method to tune and probe structural\nand electronic orders at multiple length scales. Besides addressing some of the\ntechnical challenges associated with such experiments, we illustrate this\napproach with results obtained in the cuprate La$_{1.875}$Ba$_{0.125}$CuO$_4$,\nin which the response of electronic order to pressure can only be understood by\nprobing the structure at the relevant length scales.", "category": "cond-mat_mtrl-sci" }, { "text": "Anomalous Nernst and Hall effects in magnetized platinum and palladium: We study the anomalous Nernst effect (ANE) and anomalous Hall effect (AHE) in\nproximity-induced ferromagnetic palladium and platinum which is widely used in\nspintronics, within the Berry phase formalism based on the relativistic band\nstructure calculations. We find that both the anomalous Hall ($\\sigma_{xy}^A$)\nand Nernst ($\\alpha_{xy}^A$) conductivities can be related to the spin Hall\nconductivity ($\\sigma_{xy}^S$) and band exchange-splitting ($\\Delta_{ex}$) by\nrelations $\\sigma_{xy}^A =\\Delta_{ex}\\frac{e}{\\hbar}\\sigma_{xy}^S(E_F)'$ and\n$\\alpha_{xy}^A =\n-\\frac{\\pi^2}{3}\\frac{k_B^2T\\Delta_{ex}}{\\hbar}\\sigma_{xy}^s(\\mu)\"$,\nrespectively. In particular, these relations would predict that the\n$\\sigma_{xy}^A$ in the magnetized Pt (Pd) would be positive (negative) since\nthe $\\sigma_{xy}^S(E_F)'$ is positive (negative). Furthermore, both\n$\\sigma_{xy}^A$ and $\\alpha_{xy}^A$ are approximately proportional to the\ninduced spin magnetic moment ($m_s$) because the $\\Delta_{ex}$ is a linear\nfunction of $m_s$. Using the reported $m_s$ in the magnetized Pt and Pd, we\npredict that the intrinsic anomalous Nernst conductivity (ANC) in the magnetic\nplatinum and palladium would be gigantic, being up to ten times larger than,\ne.g., iron, while the intrinsic anomalous Hall conductivity (AHC) would also be\nsignificant.", "category": "cond-mat_mtrl-sci" }, { "text": "Multiscale Computation with Interpolating Wavelets: Multiresolution analyses based upon interpolets, interpolating scaling\nfunctions introduced by Deslauriers and Dubuc, are particularly well-suited to\nphysical applications because they allow exact recovery of the multiresolution\nrepresentation of a function from its sample values on a finite set of points\nin space. We present a detailed study of the application of wavelet concepts to\nphysical problems expressed in such bases. The manuscript describes algorithms\nfor the associated transforms which, for properly constructed grids of variable\nresolution, compute correctly without having to introduce extra grid points. We\ndemonstrate that for the application of local homogeneous operators in such\nbases, the non-standard multiply of Beylkin, Coifman and Rokhlin also proceeds\nexactly for inhomogeneous grids of appropriate form. To obtain less stringent\nconditions on the grids, we generalize the non-standard multiply so that\ncommunication may proceed between non-adjacent levels. The manuscript concludes\nwith timing comparisons against naive algorithms and an illustration of the\nscale-independence of the convergence rate of the conjugate gradient solution\nof Poisson's equation using a simple preconditioning, suggesting that this\napproach leads to an O(n) solution of this equation.", "category": "cond-mat_mtrl-sci" }, { "text": "Giant magnetic broadening of ferromagnetic resonance in a GMR\n Co/Ag/Co/Gd quadlayer: Both magnetic-resonance damping and the giant magnetoresistance effect have\nbeen predicted to be strongly affected by the local density of states in thin\nferromagnetic films. We employ the antiferromagnetic coupling between Co and Gd\nto provide a spontaneous change from parallel to antiparallel alignment of two\nCo films. A sharp increase in magnetic damping accompanies the change from\nparallel to antiparallel alignment, analogous to resistivity changes in giant\nmagnetoresistance.", "category": "cond-mat_mtrl-sci" }, { "text": "Comparative study of Mo2Ga2C with superconducting MAX phase Mo2GaC: a\n first-principles calculations: The structural, electronic, optical and thermodynamic properties of Mo2Ga2C\nare investigated using density functional theory (DFT) within the generalized\ngradient approximation (GGA). The optimized crystal structure is obtained and\nthe lattice parameters are compared with available experimental data. The\nelectronic density of states (DOS) is calculated and analyzed. The metallic\nbehavior for the compound is confirmed and the value of DOS at Fermi level is\n4.2 states per unit cell per eV. Technologically important optical parameters\n(e.g., dielectric function, refractive index, absorption coefficient, photo\nconductivity, reflectivity, and loss function) have been calculated for the\nfirst time. The study of dielectric constant (e1) indicates the Drude-like\nbehavior. The absorption and conductivity spectra suggest that the compound is\nmetallic. The reflectance spectrum shows that this compound has the potential\nto be used as a solar reflector. The thermodynamic properties such as the\ntemperature and pressure dependent bulk modulus, Debye temperature, specific\nheats, and thermal expansion coefficient of Mo2Ga2C MAX phase are derived from\nthe quasi-harmonic Debye model with phononic effect also for the first time.\nAnalysis of Tc expression using available parameter values (DOS, Debye\ntemperature, atomic mass etc.) suggests that the compound is less likely to be\nsuperconductor.", "category": "cond-mat_mtrl-sci" }, { "text": "Long Range Magnetic order stabilized by acceptors: Tuning magnetic order in magnetic semiconductors is a long sought goal. A\nproper concentration of acceptors can dramatically suppress local magnetic\norder in favor of the long one. Using Mn and an acceptor codoped LiZnAs as an\nexample, we demonstrate, by first-principles calculation, the emergence of a\nlong-range magnetic order. This intriguing phenomenon can be understood from an\ninterplay between an acceptor-free magnetism and a band coupling magnetism. Our\nobservation thus lays the ground for a precise control of the magnetic order in\nfuture spintronic devices.", "category": "cond-mat_mtrl-sci" }, { "text": "Synthesis, characterization and computational simulation of graphene\n nanoplatelets stabilized in poly(styrene sulfonate) sodium salt: The production of large area interfaces and the use of scalable methods to\nbuild-up designed nanostructures generating advanced functional properties are\nof high interest for many materials science applications. Nevertheless, large\narea coverage remains a major problem for pristine graphene and here we present\na hybrid, composite graphene-like material soluble in water, which can be\nexploited in many areas, such as energy storage, electrodes fabrication,\nselective membranes and biosensing. Graphene oxide (GO) was produced by the\ntraditional Hummers method being further reduced in the presence of\npoly(styrene sulfonate) sodium salt (PSS), thus creating stable reduced\ngraphene oxide (rGO) nanoplateles wrapped by PSS (GPSS). Molecular dynamics\nsimulations were carried out of further clarify the interactions between PSS\nmolecules and rGO nanoplatelets, with calculations supported by FTIR analysis.\nThe intermolecular forces between rGO nanoplatelets and PSS lead to the\nformation of a hybrid material (GPSS) stabilized by van der Waals forces,\nallowing the fabrication of high quality layer-by-layer (LbL) films with\npolyalillamine hydrochloride (PAH). Raman and electrical characterizations\ncorroborated the successful modifications in the electronic structures from GO\nto GPSS after the chemical treatment, resulting in (PAH/GPSS) LbL films four\norders of magnitude more conductive than (PAH/GO).", "category": "cond-mat_mtrl-sci" }, { "text": "Reversible Band Gap Engineering in Carbon Nanotubes by Radial\n Deformation: We present a systematic analysis of the effect of radial deformation on the\natomic and electronic structure of zigzag and armchair single wall carbon\nnanotubes using the first principle plane wave method. The nanotubes were\ndeformed by applying a radial strain, which distorts the circular cross section\nto an elliptical one. The atomic structure of the nanotubes under this strain\nare fully optimized, and the electronic structure is calculated\nself-consistently to determine the response of individual bands to the radial\ndeformation. The band gap of the insulating tube is closed and eventually an\ninsulator-metal transition sets in by the radial strain which is in the elastic\nrange. Using this property a multiple quantum well structure with tunable and\nreversible electronic structure is formed on an individual nanotube and its\nband-lineup is determined from first-principles. The elastic energy due to the\nradial deformation and elastic constants are calculated and compared with\nclassical theories.", "category": "cond-mat_mtrl-sci" }, { "text": "Variational Methods For Phononic Calculations: Three fundamental variational principles used for solving elastodynamic\neigenvalue problems are studied within the context of elastic wave propagation\nin periodic composites (phononics). We study the convergence of the eigenvalue\nproblems resulting from the displacement Rayleigh quotient, the stress Rayleigh\nquotient and the mixed quotient. The convergence rates of the three quotients\nare found to be related to the continuity and differentiability of the density\nand compliance variation over the unit cell. In general, the mixed quotient\nconverges faster than both the displacement Rayleigh and the stress Rayleigh\nquotients, however, there exist special cases where either the displacement\nRayleigh or the stress Rayleigh quotient shows the exact same convergence as\nthe mixed-method. We show that all methods converge faster for smoother\nmaterial property variations, but when density variation is rough, the\ndifference between the mixed quotient and stress Rayleigh quotient is higher\nand similarly, when compliance variation is rough, the difference between the\nmixed quotient and displacement Rayleigh quotient is higher. Since eigenvalue\nproblems such as those considered in this paper tend to be highly\ncomputationally intensive, it is expected that these results will lead to fast\nand efficient algorithms in the areas of phononics and photonics.", "category": "cond-mat_mtrl-sci" }, { "text": "Solution Processed Large-scale Multiferroic Complex Oxide Epitaxy with\n Magnetically Switched Polarization: Complex oxides with tunable structures have many fascinating properties,\nthough high-quality complex oxide epitaxy with precisely controlled composition\nis still out of reach. Here we have successfully developed solution-based\nsingle crystalline epitaxy for multiferroic\n(1-x)BiTi(1-y)/2FeyMg(1-y)/2O3-(x)CaTiO3 (BTFM-CTO) solid solution in large\narea, confirming its ferroelectricity at atomic-scale with a spontaneous\npolarization of 79~89uC/cm2. Careful compositional tuning leads to a bulk\nmagnetization of ~0.07uB/Fe at room temperature, enabling magnetically induced\npolarization switching exhibiting a large magnetoelectric coefficient of\n2.7-3.0X10-7s/m. This work demonstrates the great potential of solution\nprocessing in large-scale complex oxide epitaxy and establishes novel\nroom-temperature magnetoelectric coupling in epitaxial BTFM-CTO film, making it\npossible to explore a much wider space of composition, phase, and structure\nthat can be easily scaled up for industrial applications.", "category": "cond-mat_mtrl-sci" }, { "text": "Tunable Electronic Structure and Magnetic Coupling in Strained\n Two-Dimensional Semiconductor MnPSe3: The electronic structures and magnetic properties of strained monolayer\nMnPSe3 are investigated systematically by first-principles calculations. It is\nfound that the magnetic ground state (GS) of monolayer MnPSe3 can be\nsignificantly affected by biaxial strain engineering, while the semiconducting\ncharacteristics are well preserved. Owing to the sensitivity of the magnetic\ncoupling towards the structural deformation, a biaxial tensile strain about 13%\ncan lead to an antiferromagnetic-ferromagnetic (AFM-FM) transition. The\nunderlying physical mechanism of strain-dependent magnetic stability is mainly\nattributed to the competition effect of direct AFM interaction and indirect FM\nsuperexchange interaction between the nearest-neighbor (NN) two Mn atoms. In\naddition, we find that FM MnPSe3 is an intrinsic half semiconductor with a\nlarge spin exchange splitting in conduction bands, which is crucial for the\nspin-polarized carrier injection and detection. The sensitive interdependence\namong external stimuli, electronic structure and magnetic coupling suggests\nthat monolayer MnPSe3 can be a promising candidate in spintronics.", "category": "cond-mat_mtrl-sci" }, { "text": "CrTe$_2$ as a two-dimensional material for topological magnetism in\n complex heterobilayers: The discovery of two-dimensional (2D) van der Waals magnetic materials and\ntheir heterostructures provided an exciting platform for emerging phenomena\nwith intriguing implications in information technology. Here, based on a\nmultiscale modelling approach that combines first-principles calculations and a\nHeisenberg model, we demonstrate that interfacing a CrTe$_2$ layer with various\nTe-based layers enables the control of the magnetic exchange and\nDzyaloshinskii-Moriya interactions as well as the magnetic anisotropy energy of\nthe whole heterobilayer, and thereby the emergence of topological magnetic\nphases such as skyrmions and antiferromagnetic Neel merons. The latter are\nnovel particles in the world of topological magnetism since they arise in a\nfrustrated Neel magnetic environment and manifest as multiples of intertwined\nhexamer-textures. Our findings pave a promising road for proximity-induced\nengineering of both ferromagnetic and long-sought antiferromagnetic chiral\nobjects in the very same 2D material, which is appealing for new information\ntechnology devices employing quantum materials.", "category": "cond-mat_mtrl-sci" }, { "text": "Two Band Model Interpretation of the p to n Transition in Ternary\n Tetradymite Topological Insulators: The requirement for large bulk resistivity in topological insulators has led\nto the design of complex ternary and quaternary phases with balanced donor and\nacceptor levels. A common feature of the optimized phases is that they lie\nclose to the p to n transition. The tetradymite Bi2Te3_xSex system exhibits\nminimum bulk conductance at the ordered composition Bi2Te2Se. By combining\nlocal and integral measurements of the density of states, we find that the\npoint of minimum electrical conductivity at x=1.0 where carriers change from\nhole-like to electron-like is characterized by conductivity of the mixed type.\nOur experimental findings, which are interpreted within the framework of a two\nband model for the different carrier types, indicate that the mixed state\noriginates from different type of native defects that strongly compensate at\nthe crossover point.", "category": "cond-mat_mtrl-sci" }, { "text": "Investigation of thermal stability of hydrogenated amorphous Si/Ge\n multilayers: Thermal stability of hydrogenated amorphous Si/Ge multilayers has been\ninvestigated by Scanning Electron Microscopy (SEM), Transmission Electron\nMicroscopy (TEM) and Small-Angle X-Ray Diffraction (SAXRD) techniques.\nAmorphous H-Si/Ge multilayers were prepared by RF sputtering with 1.5 and 6\nml/min H2 flow-rate. It is shown by Elastic Recoil Detection Analysis (ERDA)\nthat the hydrogen concentration increased by increasing H2 flow-rate. Annealing\nof the samples was carried out at 400 and 450 oC for several hours. It has been\nobserved that samples prepared with 6 ml/min flow-rate at both annealing\ntemperatures underwent significant structural changes: the surface of the\nsamples was visibly roughened, gas bubbles were formed and craters were\ncreated. The decay of the periodic structure of Si and Ge layers in these types\nof multilayers was faster than in non-hydrogenated samples. Samples prepared\nwith 1.5 ml/min flow-rate have similar behaviour at 450 oC, but at 400 oC the\ndecay of the first order SAXRD peaks was slower than in case of the\nnon-hydrogenated multilayers. Qualitatively the observed behaviour can be\nexplained by the fast desorption of the saturated hydrogen, leading to the\nformation of bubbles and craters at 450 oC, as well as, at 400oC in the sample\nwith lower H-content, by the possible passivation of the dangling bonds\nresulting in a slowing down of the diffusion intermixing.", "category": "cond-mat_mtrl-sci" }, { "text": "Efficient thermal energy harvesting using nanoscale magnetoelectric\n heterostructures: Thermomechanical cycles with a ferroelectric working substance convert heat\nto electrical energy. As shown here, magnetoelectrically coupled\nferroelectric/ferromangtic composites (also called multiferroics) add new\nfunctionalities and allow for an efficient thermal energy harvesting at room\ntemperature by exploiting the pyroelectric effect. By virtue of the\nmagnetoelectric coupling, external electric and magnetic fields can steer the\noperation of these heat engines. Our theoretical predictions are based on a\ncombination of Landau-Khalatnikov-Tani approach (with a\nGinzburg-Landau-Devonshire potential) to simulate the ferroelectric dynamics\ncoupled to the magnetic dynamics. The latter is treated via the\nelectric-polarization-dependent Landau-Lifshitz-Gilbert equation. Performing an\nadapted Olsen cycle we show that a multiferroic working substance is\npotentially much more superior to sole ferroelectrics, as far as thermal energy\nharvesting using pyroelectric effect is concerned. Our proposal holds promise\nnot only for low-energy consuming devices but also for cooling technology.", "category": "cond-mat_mtrl-sci" }, { "text": "Two-dimensional Weyl points and nodal lines in pentagonal materials and\n their optical response: Two-dimensional pentagonal structures based on the Cairo tiling are the basis\nof a family of layered materials with appealing physical properties. In this\nwork we present a theoretical study of the symmetry-based electronic and\noptical properties of these pentagonal materials. We provide a complete\nclassification of the space groups that support pentagonal structures for\nbinary and ternary systems. By means of first-principles calculations, their\nelectronic band structures and the local spin textures in momentum space are\nanalyzed. Our results show that pentagonal structures can be realized in chiral\nand achiral lattices with Weyl nodes pinned at high-symmetry points and nodal\nlines along the Brillouin zone boundary; these degeneracies are protected by\nthe combined action of crystalline and time-reversal symmetries. Additionally,\nwe discuss the linear and nonlinear optical features of some penta-materials,\nsuch as the shift current, which shows an enhancement due to the presence of\nnodal lines and points, and their possible applications.", "category": "cond-mat_mtrl-sci" }, { "text": "Magnetic properties of Quantum Corrals from first principles\n calculations: We present calculations for electronic and magnetic properties of surface\nstates confined by a circular quantum corral built of magnetic adatoms (Fe) on\na Cu(111) surface. We show the oscillations of charge and magnetization\ndensities within the corral and the possibility of the appearance of\nspin--polarized states. In order to classify the peaks in the calculated\ndensity of states with orbital quantum numbers we analyzed the problem in terms\nof a simple quantum mechanical circular well model. This model is also used to\nestimate the behaviour of the magnetization and energy with respect to the\nradius of the circular corral. The calculations are performed fully\nrelativistically using the embedding technique within the\nKorringa-Kohn-Rostoker method.", "category": "cond-mat_mtrl-sci" }, { "text": "Electronic band structure of ultimately thin silicon oxide on Ru(0001): Silicon oxide can be formed in a crystalline form, when prepared on a\nmetallic substrate. It is a candidate support catalyst and possibly the\nultimately-thin version of a dielectric host material for two-dimensional\nmaterials (2D) and heterostructures. We determine the atomic structure and\nchemical bonding of the ultimately thin version of the oxide, epitaxially grown\non Ru(0001). In particular, we establish the existence of two sub-lattices\ndefined by metal-oxygen-silicon bridges involving inequivalent substrate sites.\nWe further discover four electronic bands below Fermi level, at high binding\nenergies, two of them forming a Dirac cone at K point, and two others forming\nsemi-flat bands. While the latter two correspond to hybridized states between\nthe oxide and the metal, the former relate to the topmost silicon-oxygen plane,\nwhich is not directly coupled to the substrate. Our analysis is based on high\nresolution X-ray photoelectron spectroscopy, angle-resolved photoemission\nspectroscopy, scanning tunneling microscopy, and density functional theory\ncalculations.", "category": "cond-mat_mtrl-sci" }, { "text": "TC++: First-principles calculation code for solids using the\n transcorrelated method: TC++ is a free/libre open-source software of the transcorrelated (TC) method\nfor first-principles calculation of solids. Here, the TC method is one of the\npromising wave-function theories that can be applied to periodic systems with\nreasonable computational cost and satisfactory accuracy. We present our\nimplementation of TC++ including a detailed description of the divergence\ncorrection technique applied to the TC effective interactions. We also present\nthe way to use TC++ and some results of application to simple periodic systems:\nbulk silicon and homogeneous electron gas.", "category": "cond-mat_mtrl-sci" }, { "text": "Unveiling the complete dispersion of the giant Rashba split surface\n states of ferroelectric $\u03b1$-GeTe(111) by alkali doping: $\\alpha$-GeTe(111) is a non-centrosymmetric ferroelectric material, for which\na strong spin-orbit interaction gives rise to giant Rashba split states in the\nbulk and at the surface. The detailed dispersions of the surface states inside\nthe bulk band gap remains an open question because they are located in the\nunoccupied part of the electronic structure, making them inaccessible to static\nangle-resolved photoemission spectroscopy. We show that this difficulty can be\novercome via in-situ potassium doping of the surface, leading to a rigid shift\nof 80 meV of the surface states into the occupied states. Thus, we resolve in\ngreat detail their dispersion and highlight their crossing at the\n$\\bar{\\Gamma}$ point, which, in comparison with density functional theory\ncalculations, definitively confirms the Rashba mechanism.", "category": "cond-mat_mtrl-sci" }, { "text": "Electric Field Induced Phase Transition in KDP Crystal Near Curie Point:\n Raman and X-ray Scattering Studies: X-ray scattering measurements are performed in order to verify % that the\nmechanism leading to the DC electric field induced $C_{2v}^{19} \\to\nC_{2v}^{\\neq 19}$ phase transition in KDP crystal at 119 K is the changing of\nthe local sites symmetries of phosphate group from $C_2$ in the $C_{2v}^{19}$\nphase to $C_s$ in the $C_{2v}^{\\neq 19}$ phase. It is shown by analyzing the\nintegrated intensity of the (800) and (080) reflections that under DC electric\nfield the density of oxygen atoms lying on these plane changes indicating that\nphosphate group rotates around the [010] direction relative to the orthorhombic\n$C_{2v}^{19}$ structure. Some Raman results are also discussed.", "category": "cond-mat_mtrl-sci" }, { "text": "Dielectric Engineering of Electronic Correlations in a van der Waals\n Heterostructure: Heterostructures of van der Waals bonded layered materials offer unique means\nto tailor dielectric screening with atomic-layer precision, opening a fertile\nfield of fundamental research. The optical analyses used so far have relied on\ninterband spectroscopy. Here we demonstrate how a capping layer of hexagonal\nboron nitride (hBN) renormalizes the internal structure of excitons in a\nWSe$_2$ monolayer using intraband transitions. Ultrabroadband terahertz probes\nsensitively map out the full complex-valued mid-infrared conductivity of the\nheterostructure after optical injection of $1s$ A excitons. This approach\nallows us to trace the energies and linewidths of the atom-like $1s$-$2p$\ntransition of optically bright and dark excitons as well as the densities of\nthese quasiparticles. The fundamental excitonic resonance red shifts and\nnarrows in the WSe$_2$/hBN heterostructure compared to the bare monolayer.\nFurthermore, the ultrafast temporal evolution of the mid-infrared response\nfunction evidences the formation of optically dark excitons from an initial\nbright population. Our results provide key insight into the effect of non local\nscreening on electron-hole correlations and open new possibilities of\ndielectric engineering of van der Waals heterostructures.", "category": "cond-mat_mtrl-sci" }, { "text": "Aromatic Borozene: Based on our comprehensive theoretical investigation and known experimental\nresults for small boron clusters, we predict the existence of a novel aromatic\ninorganic molecule, B12H6. This molecule, which we refer to as borozene, has\nremarkably similar properties to the well-known benzene. Borozene is planar,\npossesses a large first excitation energy, D3h symmetry, and more importantly\nis aromatic. Furthermore, the calculated anisotropy of the magnetic\nsusceptibility of borozene is three times larger in absolute value than for\nbenzene. Finally, we can show that borozene molecules may be fused together to\ngive larger aromatic compounds with even larger anisotropic susceptibilities.", "category": "cond-mat_mtrl-sci" }, { "text": "Stability and electronic structure of covalently functionalized graphene\n layers: We present exemplary results of extensive studies of mechanical, electronic\nand transport properties of covalent functionalization of graphene monolayers\n(GML) with -NH2. We report new results of ab initio studies of covalent\nfunctionalization of GML with -NH2 groups up to 12.5% concentration. Our\nstudies are performed in the framework of the density functional theory (DFT)\nand non-equilibrium Green's function (NEGF). We discuss the stability\n(adsorption energy), elastic moduli, electronic structure, band gaps, and\neffective electron masses as a function of the density of the adsorbed\nmolecules. We also show the conductance and I(V) characteristic of these\nsystems. Generally, the stability of the functionalized graphene layers\ndecreases with the growing concentration of attachments and we determine the\ncritical density of the molecules that can be chemisorbed on the surface of\nGLs. Because of local deformations of GLs and sp3 rehybridization of the bonds\ninduced by fragments, elastic moduli decrease with increasing number of groups.\nSimultaneously, we observe that the functionalizing molecules stretch the\ngraphenes lattice, the effect being more pronounced for higher concentration of\nadsorbed molecules. We find out that the GLs functionalization leads in many\ncases to the opening of the graphene band gap (up to 0.5302 eV for 12.5%\nconcentration) and can be therefore utilized in graphene devices. The new HOMO\nand LUMO originate mostly from the impurity bands induced by the\nfunctionalization and they exhibit parabolic dispersion with electron effective\nmasses comparable to ones in silicon or gallium nitride.", "category": "cond-mat_mtrl-sci" }, { "text": "Large Itinerant Electron Exchange Coupling in the Magnetic Topological\n Insulator MnBi2Te4: Magnetism in topological materials creates phases exhibiting quantized\ntransport phenomena with applications in spintronics and quantum information.\nThe emergence of such phases relies on strong interaction between localized\nspins and itinerant states comprising the topological bands, and the subsequent\nformation of an exchange gap. However, this interaction has never been measured\nin any intrinsic magnetic topological material. Using a multimodal approach,\nthis exchange interaction is measured in MnBi2Te4, the first realized intrinsic\nmagnetic topological insulator. Interrogating nonequilibrium spin dynamics,\nitinerant bands are found to exhibit a strong exchange coupling to localized Mn\nspins. Momentum-resolved ultrafast electron scattering and magneto-optic\nmeasurements reveal that itinerant spins disorder via electron-phonon\nscattering at picosecond timescales. Localized Mn spins, probed by resonant\nX-ray scattering, disorder concurrently with itinerant spins, despite being\nenergetically decoupled from the initial excitation. Modeling the results using\natomistic simulations, the exchange coupling between localized and itinerant\nspins is estimated to be >100 times larger than superexchange interactions.\nThis implies an exchange gap of >25 meV should occur in the topological surface\nstates. By directly quantifying local-itinerant exchange coupling, this work\nvalidates the materials-by-design strategy of utilizing localized magnetic\norder to create and manipulate magnetic topological phases, from static to\nultrafast timescales.", "category": "cond-mat_mtrl-sci" }, { "text": "Room temperature ferroic orders in Zr and (Zr, Ni) doped SrTiO$_3$: We synthesized strontium titanate SrTiO$_3$ (STO), Zr doped\n$\\text{Sr}_\\text{1-x}\\text{Zr}_\\text{x}\\text{Ti}\\text{O}_3$ and (Zr, Ni)\nco-doped\n$\\text{Sr}_\\text{1-x}\\text{Zr}_\\text{x}\\text{Ti}_\\text{1-y}\\text{Ni}_\\text{y}\\text{O}_3$\nsamples using solid state reaction technique to report their structural,\nelectrical and magnetic properties. The cubic $Pm$-$3m$ phase of the\nsynthesized samples has been confirmed using Rietveld analysis of the powder\nX-ray diffraction pattern. The grain size of the synthesized materials was\nreduced significantly due to Zr doping as well as (Zr, Ni) co-doping in STO.\nThe chemical species of the samples were identified using energy-dispersive\nX-ray spectroscopy. We observed forbidden first order Raman scattering at 148,\n547 and 797 cm$^{-1}$ which may indicate nominal loss of inversion symmetry in\ncubic STO. The absence of absorption at 500 cm$^{-1}$ and within 600-700\ncm$^{-1}$ band in Fourier Transform Infrared spectra corroborates Zr and Ni as\nsubstitutional dopants in our samples. Due to 4% Zr doping in\n$\\text{Sr}_\\text{0.96}\\text{Zr}_\\text{0.04}\\text{Ti}\\text{O}_3$ sample\ndielectric constant, remnant electric polarization, remnant magnetization and\ncoercivity were increased. Notably, in the case of 4% Zr and 10% Ni co-doping\nwe have observed clearly the existence of both FE and FM hysteresis loops in\n$\\text{Sr}_{0.96}\\text{Zr}_{0.04}\\text{Ti}_{0.90}\\text{Ni}_{0.10}\\text{O}_3$\nsample. In this co-doped sample, the remnant magnetization and coercivity were\nincreased by $\\sim$1 and $\\sim$2 orders of magnitude respectively as compared\nto those of undoped STO. The coexistence of FE and FM orders in (Zr, Ni)\nco-doped STO might have the potential for interesting multiferroic\napplications.", "category": "cond-mat_mtrl-sci" }, { "text": "Comment on 'Hysteresis, Switching, and Negative Differential Resistance\n in Molecular Junctions: a Polaron Model', by M. Galperin, M.A. Ratner, and A.\n Nitzan, Nano Lett. 5, 125 (2005): It is shown that the ``hysteresis'' in a polaron model of electron transport\nthrough the molecule found by M.Galperin et al. [Nano Lett. 5, 125 (2005)] is\nan artefact of their ``mean-field'' approximation. The reason is trivial: after\nillegitimate replacement $\\hat{n}^{2}=\\hat{n}n_{0},$ where \\hat{n} is the\nelectron number operator, n_{0} the average molecular level occupation,\nGalperin et al. obtained non-physical dependence of a renormalized molecular\nenergy level on the non-integer mean occupation number n_{0} (i.e. the electron\nself-interaction) and the resulting non-linearity of current. The exact theory\nof correlated polaronic transport through molecular quantum dots (MQDs) that we\nproposed earlier [Phys. Rev. B67, 235312 (2003)] proved that there is no\nhysteresis or switching in current-voltage characteristics of non-degenerate,\nd=1, or double degenerate, d=2, molecular bridges, contrary to the mean-field\nresult. Switching could only appear in multiply degenerate MQDs with d>2 due to\nelectron correlations. Most of the molecular quantum dots are in the regime of\nweak coupling to the electrodes addressed in our formalism.", "category": "cond-mat_mtrl-sci" }, { "text": "Theory of momentum-resolved magnon electron energy loss spectra: The\n case of Yttrium Iron Garnet: We explore the inelastic spectra of electrons impinging in a magnetic system.\nThe methodology here presented is intended to highlight the charge-dependent\ninteraction of the electron beam in a STEM-EELS experiment, and the local\nvector potential generated by the magnetic lattice. This interaction shows an\nintensity $10^{-2}$ smaller than the purely spin interaction, which is taken to\nbe functionally the same as in the inelastic neutron experiment. On the other\nhand, it shows a strong scattering vector dependence ($\\kappa^{-4}$) and a\ndependence with the relative orientation between the probe wavevector and the\nlocal magnetic moments of the solid. We present YIG as a case study due to its\nhigh interest by the community.", "category": "cond-mat_mtrl-sci" }, { "text": "A first-principles study of structural and elastic properties of bulk\n SrRuO$_3$: We present a first-principles investigation of structural and elastic\nproperties of experimentally observed phases of bulk SrRuO$_3$ - namely\northorhombic, tetragonal, and cubic - by applying density functional theory\n(DFT) approximations. At first, we focus our attention on the accuracy of\ncalculated lattice constants in order to find out DFT approaches that best\nrepresent the crystalline structure of SrRuO$_3$, since many important physical\nquantities crucially depend on change in volume. Next, we evaluate\nsingle-crystal elastic constants, mechanical stability, and macroscopic elastic\nparameters trying to at least partially compensate for the existing lack of\ninformation about these fundamental features of SrRuO$_3$. Finally, we analyze\nthe anomalous behavior of low-temperature orthorhombic phase under $C_{44}$\nrelated shear deformation. It turns out that at critical strain values the\nsystem exhibits a distinct deviation from the initial behavior which results in\nan isosymmetric phase transition. Moreover, under $C_{44}$ related shear\ndeformation tetragonal SrRuO3 becomes mechanically unstable raising an open\nquestion of what makes it experimentally observable at high temperatures.", "category": "cond-mat_mtrl-sci" }, { "text": "Vacancy-related color centers in twodimensional silicon carbide\n monolayers: Basic vacancy defects in twodimensional silicon carbide (2D-SiC) are examined\nby means of density functional theory calculations to explore their\nmagneto-optical properties as well as their potential in quantum technologies.\nIn particular, the characteristic hyperfine tensors and optical excited states\nof carbon-vacancy, silicon-vacancy, and carbon antisite-vacancy pair defects in\n2D-SiC are determined that are the key fingerprints of these defects that may\nbe observed in electron paramagnetic resonance and photoluminescence\nexperiments, respectively. Besides the fundamental characterization of the most\nbasic native defects, we show that the negatively charged carbon\nantisite-vacancy defect is a promising candidate for realizing a near-infrared\nsingle-photon quantum emitter with spin doublet ground state, where the\nnegative charge state may be provided by nitrogen doping of 2D-SiC. We find\nthat the neutral carbon-vacancy with spin triplet ground state might be used\nfor quantum sensing with a broad emission in the visible.", "category": "cond-mat_mtrl-sci" }, { "text": "FeRh groundstate and martensitic transformation: Cubic B2 FeRh exhibits a metamagnetic transition [(111) antiferromagnet (AFM)\nto ferromagnet (FM)] around 353 K and remains structurally stable at higher\ntemperatures. However, the calculated zero-Kelvin phonons of AFM FeRh exhibit\nimaginary modes at M-points in the Brillouin zone, indicating a premartensitic\ninstability, which is a precursor to a martensitic transformation at low\ntemperatures. Combining electronic-structure calculations with ab initio\nmolecular dynamics, conjugate gradient relaxation, and the solid-state\nnudged-elastic band (SSNEB) methods, we predict that AFM B2 FeRh becomes\nunstable at ambient pressure and transforms without a barrier to an AFM(111)\northorhombic (martensitic) groundstate below 90K. We also consider competing\nstructures, in particular, a tetragonal AFM(100) phase that is not the global\ngroundstate, as proposed [Phys. Rev. B 94, 180407(R) (2016)], but a constrained\nsolution.", "category": "cond-mat_mtrl-sci" }, { "text": "Shift of Fermi level by substitutional impurity-atom doping in diamond\n and cubic- and hexagonal-boron nitrides II. Generalized Gradient\n Approximation: In succession to the first paper (arXiv 1406.6204v5), the impurity-atom\nconcentrations when the Fermi levels are either at the valence band maximum\n(VBM) or the conduction band minimum (CBM) were identified for diamond, cubic\nboron nitride (cBN), and hexagonal boron nitride (hBN) using the\nKorringa-Kohn-Rostoker (KKR) scheme using the local density approximation\n(LDA). In the present paper, the generalized gradient approximation (GGA) was\nused instead of the LDA for exchange-correlation. The impurity atoms were B and\nN for diamond, Be, Si, and C for cBN, and Be for hBN; these impurity atoms were\nknown in the first paper to form degenerate states by increased impurity-atom\nconcentrations. The impurity-atom concentrations when the Fermi level was\nlocated either at the VBM or the CBM were as follows: (i) the B concentration\nwas 0.27 at.% in B-doped diamond, (ii) the N concentration was 0.25 at.% in\nN-doped diamond, (iii) the concentration of Be substituting B was 0.88 at.% in\ncBN, (iv) the concentration of Si substituting B was 0.06 at.% in cBN, (v) the\nconcentration of C substituting B was 0.07 at.% in cBN, (vi) the concentration\nof C substituting N was 0.88 at.% in cBN, and (vii) the concentration of Be\nsubstituting B was 1.80 at.% in hBN. The values of (iv) and (v) were\nsignificantly smaller than the corresponding values in paper I, but it was\nattributed to the input parameters used in the present paper, hence it was\nconcluded that the computed concentrations were not sensitive to the GGA used.", "category": "cond-mat_mtrl-sci" }, { "text": "Zak's Phase in Non-Symmetric One-Dimensional Crystals: In this work, we derive some analytical properties of Berry's phase in\none-dimensional quantum and classical crystals, also named Zak's phase, when\ncomputed with a Fourier basis. We show that Zak's phase can be divided in two\nterms: a global phase required to make the Bloch wave periodic in the Brillouin\nzone and an internal phase which measures the relative delay of the different\nFourier terms within the Brillouin zone. While the former phase is dependent on\nthe origin of coordinates of the unit cell, the latter is independent of it, so\nthat it can be interpreted as an internal property of the band itself. We show\nthat this internal phase is always zero for a symmetric crystal while it can\ntake any value when this symmetry is broken, showing therefore that it can be\ninterpreted as a measure of the assymetry of the band. Since for a symmetric\ncrystal Zak's phase is entirely determined by the global part, we show that\nthis can be easily calculated by means of the parity of the Fourier terms at\nthe center and edge of the Brillouin zone, being therefore unnecessary the\nintegration of the modes through the unit cell and the entire Brillouin zone.\nWe provide numerical examples analyzing the internal part for both electronic\nand classical waves (acoustic or photonic). We analyze the weakest electronic\npotential capable of presenting asymmetry, as well as the double-Dirac delta\npotential, and in both examples it is found that the internal phase varies\ncontinuously as a function of a symmetry-control parameter, but it is zero when\nthe crystal is symmetric. For classical waves, the layered material is\nanalyzed. Although Zak's phase has been mainly studied in connection with the\nexistence of edge states in finite crystals, we consider that the study of the\ninternal phase can be more relevant to understand bulk properties of quantum\nand classical crystals.", "category": "cond-mat_mtrl-sci" }, { "text": "Examining real-time TDDFT non-equilibrium simulations for the\n calculation of electronic stopping power: In ion irradiation processes, electronic stopping power describes the energy\ntransfer rate from the irradiating ion to the target material's electrons. Due\nto the scarcity and significant uncertainties in experimental electronic\nstopping power data for materials beyond simple solids, there has been growing\ninterest in the use of first-principles theory for calculating electronic\nstopping power. In recent years, advances in high-performance computing have\nopened the door to fully first-principles nonequilibrium simulations based on\nreal-time time-dependent density functional theory (RT-TDDFT). While it has\nbeen demonstrated that the RT-TDDFT approach is capable of predicting\nelectronic stopping power for a wide range of condensed matter systems, there\nhas yet to be an exhaustive examination of the physical and numerical\napproximations involved and their effects on the calculated stopping power. We\ndiscuss the results of such a study for crystalline silicon with protons as\nirradiating ions. We examine the influences of key approximations in RT-TDDFT\nnonequilibrium simulations on the calculated electronic stopping power,\nincluding approximations related to basis sets, finite size effects,\nexchange-correlation approximation, pseudopotentials, and more. Finally, we\npropose a simple and efficient correction scheme to account for the\ncontribution from core-electron excitations to the stopping power, as it was\nfound to be significant for large proton velocities.", "category": "cond-mat_mtrl-sci" }, { "text": "Ultrafast spin dynamics in inhomogeneous systems: a density-matrix\n approach applied to Co/Cu interfaces: Ultrafast spin dynamics on femto- to picosecond timescales is simulated\nwithin a density-operator approach for a Co/Cu bilayer. The electronic\nstructure is represented in a tight-binding form; during the evolution of the\ndensity operator, optical excitation by a femtosecond laser pulse, coupling to\na bosonic bath as well as dephasing are taken into account. Our simulations\ncorroborate the importance of interfaces for ultrafast transport phenomena and\ndemagnetisation processes. Moreover, we establish a reflow from Cu $d$ orbitals\nacross the interface into Co $d$ orbitals, which shows up prominently in the\nmean occupation numbers. On top of this, this refilling manifests itself as a\nminority-spin current proceeding several layers into the Cu region. The present\nstudy suggests that the approach captures essential ultrafast phenomena and\nprovides insight into microscopic processes.", "category": "cond-mat_mtrl-sci" }, { "text": "A Microscopic Model of Ferroelectricity in Stress-free PbTiO3 Ultrathin\n Films: The ground-state polarization of PbTiO3 thin films is studied using a\nmicroscopic effective Hamiltonian with parameters obtained from\nfirst-principles calculations. Under short-circuit electrical boundary\nconditions, (001) films with thickness as low as three unit cells are found to\nhave a perpendicularly polarized ferroelectric ground state with significant\nenhancement of the polarization at the surface.", "category": "cond-mat_mtrl-sci" }, { "text": "nanoNET: Machine Learning Platform for Predicting Nanoparticles\n Distribution in a Polymer Matrix: Polymer nanocomposites (PNCs) offer a broad range of thermophysical\nproperties that are linked to their compositions. However, it is challenging to\nestablish a universal composition-property relation of PNCs due to their\nenormous composition and chemical space. Here, we address this problem and\ndevelop a new method to model the composition-microstructure relation of a PNC\nthrough an intelligent machine learning pipeline named nanoNET. The nanoNET is\na nanoparticles (NPs) distribution predictor, built upon computer vision and\nimage recognition concepts. It integrates unsupervised deep learning and\nregression in a fully automated pipeline. We conduct coarse-grained molecular\ndynamics simulations of PNCs and utilize the data to establish and validate the\nnanoNET. Within this framework, a random forest regression model predicts the\nNPs distribution in a PNC in a latent space. Subsequently, a convolutional\nneural network-based decoder converts the latent space representation to the\nactual radial distribution function (RDF) of NPs in the given PNC. The nanoNET\npredicts NPs distribution in many unknown PNCs very accurately. This method is\nvery generic and can accelerate the design, discovery, and fundamental\nunderstanding of composition-microstructure relations of PNCs and other\nmolecular systems.", "category": "cond-mat_mtrl-sci" }, { "text": "Multi-State, Ultra-thin, BEOL-Compatible AlScN Ferroelectric Diodes: The growth in data generation necessitates efficient data processing\ntechnologies to address the von Neumann bottleneck in conventional computer\narchitecture. Memory-driven computing, which integrates non-volatile memory\n(NVM) devices in a 3D stack, is gaining attention, with CMOS back-end-of-line\n(BEOL) compatible ferroelectric (FE) diodes being ideal due to their\ntwo-terminal design and inherently selector-free nature, facilitating\nhigh-density crossbar arrays. Here, we demonstrate BEOL-compatible,\nhigh-performance FE-diodes scaled to 5, 10, and 20 nm FE\nAl0.72Sc0.28N/Al0.64Sc0.36N films. Through interlayer (IL) engineering, we show\nsubstantial improvements in the ON/OFF ratios (>166 times) and rectification\nratios (>176 times) in these scaled devices. The superlative characteristics\nalso enables 5-bit multi-state operation with a stable retention. We also\nexperimentally and theoretically demonstrate the counterintuitive result that\nthe inclusion of an IL can lead to a decrease in the ferroelectric switching\nvoltage of the device. An in-depth analysis into the device transport\nmechanisms is performed, and our compact model aligns seamlessly with the\nexperimental results. Our results suggest the possibility of using scaled\nAlxSc1-xN FE-diodes for high performance, low-power, embedded NVM.", "category": "cond-mat_mtrl-sci" }, { "text": "Anisotropy of Resonant Inelastic X-Ray Scattering at the K Edge of\n Si:Theoretical Analysis: We investigate theoretically the resonant inelastic x-ray scattering (RIXS)\nat the $K$ edge of Si on the basis of an ab initio calculation. We calculate\nthe RIXS spectra with systematically varying transfered-momenta,\nincident-photon energy and incident-photon polarization. We confirm the\nanisotropy of the experimental spectra by Y. Ma {\\it et al}. (Phys. Rev. Lett.\n74, 478 (1995)), providing a quantitative explanation of the spectra.", "category": "cond-mat_mtrl-sci" }, { "text": "Piezoelectricity in Two-Dimensional Group III Monochalcogenides: We find that several layer-phase group-III monochalcogenides, including GaS,\nGaSe and InSe, are piezoelectric in the monolayer form. First-principles\ncalculations reveal that the piezoelectric coefficients of monolayer GaS, GaSe\nand InSe are on the same order of magnitude as the earlier discovered\ntwo-dimensional piezoelectric materials, such as BN and MoS2 monolayers. Our\nstudy expands the family of two dimensional piezoelectric materials, suggesting\nthat strong piezoelectric response can occur in a wide range of two dimensional\nmaterials with broken inversion symmetry. The co-existence of piezoelectricity\nand superior photo-sensitivity in these two-dimensional semiconductors enables\nthe integration of electromechanical and optical sensors on the same material\nplatform.", "category": "cond-mat_mtrl-sci" }, { "text": "Charge carrier transport and lifetimes in n-type and p-type phosphorene\n as 2D device active materials: an ab initio study: In this work, we provide a detailed analysis of phosphorene performance as\nn-type and p-type active materials. The study is based on first principles\ncalculation of phosphorene electronic structure, and resulting electron and\nhole scattering rates and lifetimes. Emphasis is put on extreme regimes\ncommonly found in semiconductor devices, i.e. high electric fields and heavy\ndoping, where impact ionization and Auger recombination can occur. We found\nthat electron-initiated impact ionization is weaker than the hole-initiated\nprocess, when compared to carrier-phonon interaction rates, suggesting\nresilience to impact ionization initiated breakdown. Moreover, calculated\nminority electron lifetimes are limited by radiative recombination only, not by\nAuger processes, suggesting that phosphorene could achieve good quantum\nefficiencies in optoelectronic devices. The provided scattering rates and\nlifetimes are critical input data for the modeling and understanding of\nphosphorene-based device physics.", "category": "cond-mat_mtrl-sci" }, { "text": "Graphite and Hexagonal Boron-Nitride Possess the Same Interlayer\n Distance. Why?: Graphite and hexagonal boron nitride (h-BN) are two prominent members of the\nfamily of layered materials possessing a hexagonal lattice. While graphite has\nnon-polar homo-nuclear C-C intra-layer bonds, h-BN presents highly polar B-N\nbonds resulting in different optimal stacking modes of the two materials in\nbulk form. Furthermore, the static polarizabilities of the constituent atoms\nconsiderably differ from each other suggesting large differences in the\ndispersive component of the interlayer bonding. Despite these major differences\nboth materials present practically identical interlayer distances. To\nunderstand this finding, a comparative study of the nature of the interlayer\nbonding in both materials is presented. A full lattice sum of the interactions\nbetween the partially charged atomic centers in h-BN results in vanishingly\nsmall monopolar electrostatic contributions to the interlayer binding energy.\nHigher order electrostatic multipoles, exchange, and short-range correlation\ncontributions are found to be very similar in both materials and to almost\ncompletely cancel out by the Pauli repulsions at physically relevant interlayer\ndistances resulting in a marginal effective contribution to the interlayer\nbinding. Further analysis of the dispersive energy term reveals that despite\nthe large differences in the individual atomic polarizabilities the\nhetero-atomic B-N C6 coefficient is very similar to the homo-atomic C-C\ncoefficient in the hexagonal bulk form resulting in very similar dispersive\ncontribution to the interlayer binding. The overall binding energy curves of\nboth materials are thus very similar predicting practically the same interlayer\ndistance and very similar binding energies.", "category": "cond-mat_mtrl-sci" }, { "text": "Observation of Anomalous Hall Effect in Noncollinear Antiferromagnetic\n Mn3Sn Films: Magnetotransport is at the center of the spintronics. Mn3Sn, an\nantiferromagnet that has a noncollinear 120{\\deg} spin order, exhibits large\nanomalous Hall effect (AHE) at room temperature. But such a behavior has been\nremained elusive in Mn3Sn films. Here we report the observation of robust AHE\nup to room temperature in quasi-epitaxial Mn3Sn thin films, prepared by\nmagnetron sputtering. The growth of both (11-20)- and (0001)-oriented Mn3Sn\nfilms provides a unique opportunity for comparing AHE in three different\nmeasurement configurations. When the magnetic field is swept along (0001)\nplane, such as the direction of [01-10] and [2-1-10] the films show\ncomparatively higher anomalous Hall conductivity than its perpendicular\ncounterpart ([0001]), irrespective of their respectively orthogonal current\nalong [0001] or [01-10]. A quite weak ferromagnetic moment of 3 emu/cm^3 is\nobtained in (11-20)-oriented Mn3Sn films, guaranteeing the switching of the\nHall signals with magnetization reversal. Our finding would advance the\nintegration of Mn3Sn in antiferromagnetic spintronics.", "category": "cond-mat_mtrl-sci" }, { "text": "Probing anisotropy in epitaxial Fe/Pt bilayers by spin-orbit torque\n ferromagnetic resonance: We report the generation and detection of spin-orbit torque ferromagnetic\nresonance (STFMR) in micropatterned epitaxial Fe/Pt bilayers grown by molecular\nbeam epitaxy. The magnetic field dependent measurements at an in-plane magnetic\nfield angle of 45 degrees with respect to the microwave-current direction\nreveal the presence of two distinct voltage peaks indicative of a strong\nmagnetic anisotropy. We show that STFMR can be employed to probe the underlying\nmagnetic properties including the anisotropies in the Fe layer. We compare our\nSTFMR results with broadband ferromagnetic resonance spectroscopy of the\nunpatterned bilayer thin films. The experimental STFMR measurements are\ninterpreted using an analytical formalism and further confirmed using\nmicromagnetic modeling, which shed light on the field-dependent magnetization\nalignment in the microstructures responsible for the STFMR rectification. Our\nresults demonstrate a simple and efficient method for determining magnetic\nanisotropies in microstructures by means of rf spectroscopy.", "category": "cond-mat_mtrl-sci" }, { "text": "Peach-Koehler forces within the theory of nonlocal elasticity: We consider dislocations in the framework of Eringen's nonlocal elasticity.\nThe fundamental field equations of nonlocal elasticity are presented. Using\nthese equations, the nonlocal force stresses of a straight screw and a straight\nedge dislocation are given. By the help of these nonlocal stresses, we are able\nto calculate the interaction forces between dislocations (Peach-Koehler\nforces). All classical singularities of the Peach-Koehler forces are\neliminated. The extremum values of the forces are found near the dislocation\nline.", "category": "cond-mat_mtrl-sci" }, { "text": "Modelling of epitaxial graphene functionalization: A new model for graphene, epitaxially grown on silicon carbide is proposed.\nDensity functional theory modelling of epitaxial graphene functionalization by\nhydrogen, fluorine and phenyl groups has been performed with hydrogen and\nfluorine showing a high probability of cluster formation in high adatom\nconcentration. It has also been shown that the clusterization of fluorine\nadatoms provides midgap states in formation due to significant flat distortion\nof graphene. The functionalization of epitaxial graphene using larger species\n(methyl and phenyl groups) renders cluster formation impossible, due to the\nsteric effect and results in uniform coverage with the energy gap opening.", "category": "cond-mat_mtrl-sci" }, { "text": "Conductivity and Dissociation in Metallic Hydrogen: Implications for\n Planetary Interiors: Liquid metallic hydrogen (LMH) was recently produced under static compression\nand high temperatures in bench-top experiments. Here, we report a study of the\noptical reflectance of LMH in the pressure region of 1.4-1.7 Mbar and use the\nDrude free-electron model to determine its optical conductivity. We find static\nelectrical conductivity of metallic hydrogen to be 11,000-15,000 S/cm. A\nsubstantial dissociation fraction is required to best fit the energy dependence\nof the observed reflectance. LMH at our experimental conditions is largely\natomic and degenerate, not primarily molecular. We determine a plasma frequency\nand the optical conductivity. Properties are used to analyze planetary\nstructure of hydrogen rich planets such as Jupiter.", "category": "cond-mat_mtrl-sci" }, { "text": "Continuous Frequency Controllable Nano-electromechanical Systems Based\n on Multiwalled Carbon Nanotubes: We demonstrate a class of model nano-electromechanical systems (NEMS) based\non multiwalled carbon nanotubes (MWNTs) which has longer inner cores coaxially\noscillating inside their respective shorter outer shell holders and can operate\nat continuously controllable frequencies up to the gigahertz range when fuelled\nby AC electric fields. Its additional attributes include much larger\noscillation amplitudes and forces and much lower rates of thermal dissipation\n(Q-factor = 10^5) and air damping (Q-factor = 10^4~10^5) than those of\nnano-beam based NEMS. A crucial feature of the conceived model NEMS is that\nafter having tuned the electric field frequency to any prescribed value within\na permitted range, the NEMS will respond quickly (in sub-nanoseconds) at the\nsame oscillation frequency. These merits, when contrasted with the nano-beam\nresonators developed so far, make it a better potential candidate for the\nongoing miniaturization progress from micro- to nano-electromechanical systems.", "category": "cond-mat_mtrl-sci" }, { "text": "Deep learning and the Schr\u00f6dinger equation: We have trained a deep (convolutional) neural network to predict the\nground-state energy of an electron in four classes of confining two-dimensional\nelectrostatic potentials. On randomly generated potentials, for which there is\nno analytic form for either the potential or the ground-state energy, the\nneural network model was able to predict the ground-state energy to within\nchemical accuracy, with a median absolute error of 1.49 mHa. We also\ninvestigate the performance of the model in predicting other quantities such as\nthe kinetic energy and the first excited-state energy of random potentials.", "category": "cond-mat_mtrl-sci" }, { "text": "Multiscale insight into the Cd1-xZnxTe vibrational-mechanical properties\n -- High-pressure experiments and ab initio calculations: The Cd1-xZnxTe semiconductor alloy is a regular system regarding its\nmacroscopic mechanic properties in that its experimental bulk modulus exhibits\na linear x-dependence, in line with ab initio predictions. Complexity arises at\nthe bond scale, referring to the intricate Cd1-xZnxTe percolation-type Raman\npattern [T. Alhaddad et al., Journal of Applied Physics 133, 065701 (2023)].\nThis offers an appealing benchmark to test various phonon coupling processes at\ndiverse length scales in a compact multi-oscillator assembly, presently tuned\nby pressure. At x around 0, an inter-bond long-range/macro electric coupling\nbetween the matrix and impurity polar phonons is detuned under pressure.\nInversely, at x around 1, an intra-bond short-range/nano mechanic coupling is\nenforced between the two Zn Te apolar sub-phonons stemming from same and alien\npercolation-type environments. The pressure-induced macro/nano polar/apolar\ncoupling/decoupling processes are compared within a model of two coupled\nelectric/mechanic harmonic oscillators in terms of a compromise between\nproximity to resonance and strength of coupling, impacting the degree of mode\nmixing, with ab initio (apolar case) and analytical (polar case) Raman\ncalculations in support. Notably, the free mechanic coupling at x around 1\nopposes the achievement of a phonon exceptional point, manifesting the\ninhibition of mechanic coupling, earlier evidenced with similar bonds for x\nsmaller than 0.5. Hence, the pressure dependence of a given bond vibration in a\ndisordered alloy basically differs depending on whether the bond is\nmatrix-like, i.e., self-connected in bulk (free coupling), or dispersed, i.e.,\nself-connected in a chain (inhibited coupling). This features pressure-tunable\npercolation-based on-off phonon switches in complex media.", "category": "cond-mat_mtrl-sci" }, { "text": "The deffect effect on electronic conductance in binomially tailored\n quantum wire: The paper considers the effect of the defects on the electronic transmission\nproperties in binomially tailored waveguide quantum wires, in which each Dirac\ndelta function potential strength have been weight on the binomial distribution\nlaw. We have assumed that a single free-electron channel is incident on the\nstructure and the scattering of electrons is solely from the geometric nature\nof the problem. We have used the transfer matrix method to study the electron\ntransmission. We found this novel structure has a good defect tolerance. We\nfound the structure tolerate up to in strength defect and in position defect\nfor the central Dirac delta function in the binomial distribution. Also, we\nfound this structure can tolerate both defect up to in strength and in position\ndislocation", "category": "cond-mat_mtrl-sci" }, { "text": "Phase stability of Au-Li binary systems studied using neural network\n potential: The miscibility of Au and Li exhibits a potential application as an adhesion\nlayer and electrode material in secondary batteries. Here, to explore alloying\nproperties, we constructed a neural network potential (NNP) of Au-Li binary\nsystems based on density functional theory (DFT) calculations. To accelerate\nconstruction of NNPs, we proposed an efficient and inexpensive method of\nstructural dataset generation. The predictions by the constructed NNP on\nlattice parameters and phonon properties agree well with those obtained by DFT\ncalculations. We also investigated the mixing energy of Au$_{1-x}$Li$_{x}$ with\nfine composition grids, showing excellent agreement with DFT verifications. We\nfound the existence of various compositions with structures on and slightly\nabove the convex hull, which can explain the lack of consensus on the Au-Li\nstable phases in previous studies. Moreover, we newly found\nAu$_{0.469}$Li$_{0.531}$ as a stable phase, which has never been reported\nelsewhere. Finally, we examined the alloying process starting from the phase\nseparated structure to the complete mixing phase. We found that when multiple\nadjacent Au atoms dissolved into Li, the alloying of the entire Au/Li interface\nstarted from the dissolved region. This paper demonstrates the applicability of\nNNPs toward miscible phases and provides the understanding of the alloying\nmechanism.", "category": "cond-mat_mtrl-sci" }, { "text": "Hafnia for analog memristor: Influence of stoichiometry and crystalline\n structure: The highly non-linear switching behavior of hafnia memristor actually hinders\nits wide application in neuromorphic computing. Theoretical understanding into\nits switching mechanism has been focused on the processes of conductive\nfilament generation and rupture, but possible phase transition and\ncrystallization around the region of conductive filaments (CFs) due to the\nvariation of O content have been paid less attention to. In this paper,\nHfO$\\mathrm{_x}$ structural models covering the full stoichiometries from Hf to\nHfO$\\mathrm{_2}$ were established, and the crystal structure evolution during\nthe reduction process of hafnia was obtained through first-principles\ncalculation. The electronic structures and O vacancy migration characteristics\nof these structures were analyzed. A criterion was prescribed to predict the\nmode of abrupt binary switching or gradual conductance modulation according to\nthe structure evolution of the CFs. In particular, factors that influence the\nmerging of tiny conductive channels into strong filaments are intensively\ndiscussed, including the anisotropy of O vacancy migration and the size effect.\nThe feasibility of Mg doping to achieve robust gradual switching is discussed.", "category": "cond-mat_mtrl-sci" }, { "text": "Negative Temperature in Spin Dynamics Simulations: A simple and computationally efficient algorithm enables implementing\nnegative temperature values in a spin dynamics simulation. The algorithm uses a\nLangevin spin dynamics thermostat with a negative damping parameter, enabling\nthe thermalization of an arbitrary interacting spin system to the Gibbs energy\ndistribution with a given negative temperature value. Canonical spin dynamics\nsimulations at a negative temperature are as robust as conventional positive\nspin temperature simulations, providing a tool for quantitative dynamic studies\nof the physics of highly excited magnetic states. Two simulation case studies\ndescribing spin systems with antiferromagnetic and ferromagnetic ground states\nare explored. The phase transitions occurring in the negative temperature range\ndo not necessarily exhibit similarities with their positive temperature\ncounterparts. The transition temperatures and the character of spin alignment\nvary depending on the spatial range and strength of spin-spin interactions.", "category": "cond-mat_mtrl-sci" }, { "text": "Size-independent Shear Band Formation in Amorphous Nanowires made from\n Simulated Casting: Molecular dynamics simulations indicate that surfaces strongly influence the\nstrain localization behavior of amorphous nanowires in tension. A sample\npreparation routine that simulates casting was employed to facilitate the\nrelaxation of the sample surface. Samples as short as 15 nm (7.5 nm in\ndiameter) form dominant shear bands during deformation. The elastic energy\nrelease during plastic deformation is sufficient to provide the excess\npotential energy required for the shear band nucleation at rather small sample\nsizes. The results show that shear band formation is almost size-independent\nand is bounded only by its own length scale.", "category": "cond-mat_mtrl-sci" }, { "text": "Semiconducting layered blue phosphorus: A computational study: We investigate a previously unknown phase of phosphorus that shares its\nlayered structure and high stability with the black phosphorus allotrope. We\nfind the in-plane hexagonal structure and bulk layer stacking of this\nstructure, which we call `blue phosphorus', to be related to graphite. Unlike\ngraphite and black phosphorus, blue phosphorus displays a wide fundamental band\ngap and should exfoliate easily to form quasi-2D structures suitable for\nelectronic applications. We study a likely transformation pathway from black to\nblue phosphorus and discuss possible ways to synthesize the new structure.", "category": "cond-mat_mtrl-sci" }, { "text": "Microscopic mechanism of the non-crystalline anisotropic\n magnetoresistance in (Ga,Mn)As: Starting with a microscopic model based on the Kohn-Luttinger Hamiltonian and\nkinetic p-d exchange combined with Boltzmann formula for conductivity we\nidentify the scattering from magnetic Mn combined with the strong spin-orbit\ninteraction of the GaAs valence band as the dominant mechanism of the\nanisotropic magnetoresistance (AMR) in (Ga,Mn)As. This fact allows to construct\na simple analytical model of the AMR consisting of two heavy-hole bands whose\ncharge carriers are scattered on the impurity potential of the Mn atoms. The\nmodel predicts the correct sign of the AMR (resistivity parallel to\nmagnetization is smaller than perpendicular to magnetization) and identifies\nits origin arising from the destructive interference between electric and\nmagnetic part of the scattering potential of magnetic ionized Mn acceptors when\nthe carriers move parallel to the magnetization.", "category": "cond-mat_mtrl-sci" }, { "text": "Mechanism of pressure induced amorphization of SnI4: a combined X-ray\n diffraction -- X-ray absorption spectroscopy study: We have studied the amorphization process of SnI4 up to 26.8GPa with\nunprecedented experimental details by combining Sn and I K edge X-ray\nabsorption spectroscopy and powder X-ray diffraction. Standards and reverse\nMonte Carlo extended X-ray absorption fine structure (EXAFS) refinements\nconfirm that the SnI4 tetrahedron is a fundamental structural unit that is\npreserved through the crystalline phase-I to crystalline phase-II transition\nabout 7 to 10GPa and then in the amorphous phase that appears above 20GPa. Up\nto now unexploited Iodine EXAFS reveals to be extremely informative and\nconfirms the formation of iodine iodine short bonds close to 2.85{\\AA} in the\namorphous phase at 26.8 GPa. A coordination number increase of Sn in the\ncrystalline phase-II appears to be excluded, while the deformation of the\ntetrahedral units proceeds through a flattening that keeps the average I-Sn-I\nangle close to 109.5{\\deg}. Moreover, we put in evidence the impact of pressure\non the Sn near edge structure under competing geometrical and electronic\neffects.", "category": "cond-mat_mtrl-sci" }, { "text": "Plasma-assisted fabrication of monolayer phosphorene and its Raman\n characterization: There have been continuous efforts to seek for novel functional\ntwo-dimensional semiconductors with high performance for future applications in\nnanoelectronics and optoelectronics. In this work, we introduce a successful\nexperimental approach to fabricate monolayer phosphorene by mechanical cleavage\nand the following Ar+ plasma thinning process. The thickness of phosphorene is\nunambiguously determined by optical contrast combined with atomic force\nmicroscope (AFM). Raman spectroscopy is used to characterize the pristine and\nplasma-treated samples. The Raman frequency of A2g mode stiffens, and the\nintensity ratio of A2g to A1g modes shows monotonic discrete increase with the\ndecrease of phosphorene thickness down to monolayer. All those phenomena can be\nused to identify the thickness of this novel two-dimensional semiconductor\nefficiently. This work for monolayer phosphorene fabrication and thickness\ndetermination will facilitates the research of phosphorene.", "category": "cond-mat_mtrl-sci" }, { "text": "Edge chirality determination of graphene by Raman spectroscopy: Raman imaging on the edges of single layer micromechanical cleavage graphene\n(MCG) was carried out. The intensity of disorder-induced Raman feature (D band\nat ~1350 cm-1) was found to be correlated to the edge chirality: it is stronger\nat the armchair edge and weaker at the zigzag edge. This shows that Raman\nspectroscopy is a reliable and practical method to identify the chirality of\ngraphene edge and to help in determination of the crystal orientation. The\ndetermination of graphene chirality is critically important for fundamental\nstudy as well as for applications.", "category": "cond-mat_mtrl-sci" }, { "text": "Hardness of T-carbon: Density functional theory calculations: We revisit and interpret the mechanical properties of the recently proposed\nallotrope of carbon, T-carbon [Sheng \\emph{et al.}, Phys. Rev. Lett.,\n\\textbf{106}, 155703 (2011)], using density functional theory in combination\nwith different empirical hardness models. In contrast with the early estimation\nbased on the Gao's model, which attributes to T-carbon an high Vickers hardness\nof 61 GPa comparable to that of superhard cubic boron nitride (\\emph{c}-BN), we\nfind that T-carbon is not a superhard material, since its Vickers hardenss does\nnot exceed 10 GPa. Besides providing clear evidence for the absence of\nsuperhardenss in T-carbon, we discuss the physical reasons behind the failure\nof Gao's and \\v{S}im$\\rm\\mathring{u}$nek and Vack\\'a\\v{r}'s (SV) models in\npredicting the hardness of T-carbon, residing on their improper treatment of\nthe highly anisotropic distribution of quasi-\\emph{sp}$^3$-like C-C hybrids. A\npossible remedy to the Gao and SV models based on the concept of superatom is\nsuggest, which indeed yields a Vickers hardness of about 8 GPa.", "category": "cond-mat_mtrl-sci" }, { "text": "The Structural Phase Transition of the Relaxor Ferroelectric\n 68%PbMg1/3Nb2/3O3-32%PbTiO3: Neutron scattering techniques have been used to study the relaxor\nferroelectric 0.68PbMg1/3Nb2/3O3-0.32PbTiO3 denoted in this paper as\n0.68PMN-0.32PT. On cooling, these relaxor ferroelectrics have a long-range\nordered ferroelectric phase and the composition is close to that at which the\nferroelectric structure changes from rhombohedral to tetragonal. It was found\nthat above the Burns temperature of about 600K, the transverse optic mode and\nthe transverse acoustic mode are strongly coupled and a model was used to\ndescribe this coupling that gave similar parameters to those obtained for the\ncoupling in PMN. Below the Burns temperature additional quasi-elastic\nscattering was found which increased in intensity as the sample was cooled down\nto the ferroelectric transition temperature but then decreased in intensity.\nThis behaviour is similar to that found in PMN. This scattering is associated\nwith the dynamic polar nano-regions that occur below the Burns temperature. In\naddition to this scattering a strictly elastic resolution limited peak was\nobserved that was much weaker than the corresponding peak in pure PMN and which\ndecreased in intensity on cooling below the ferroelectric phase whereas for\nPMN, which does not have a long-range ordered ferroelectric phase, the\nintensity of this component increased monotonically as the sample was cooled.\nThe results of our study are compared with the recent measurements of Stock et\nal. [PRB 73 064107] who studied 0.4PMN-0.6PT. The results are qualitatively\nconsistent with the random field model developed to describe the scattering\nfrom PMN.", "category": "cond-mat_mtrl-sci" }, { "text": "Optical properties of exciton in two-dimensional transition metal\n dichalcogenide nanobubbles: Strain in two-dimensional (2D) transition metal dichalcogenide (TMD) has led\nto localized states with exciting optical properties, in particular in view of\ndesigning one photon sources. The naturally formed of the MoS2 monolayer\ndeposed on hBN substrate leads to a reduction of the bandgap in the strained\nregion creating a nanobubble. The photogenerated particles are thus confined in\nthe strain-induced potential. Using numerical diagonalization, we simulate the\nspectra of the confined exciton states, their oscillator strengths and\nradiative lifetimes. We show that a single state of the confined exciton is\noptically active, which suggests that the MoS2/hBN nanobubble is a good\ncandidate for the realisation of single-photon sources. Furthermore, the\nexciton binding energy, oscillator strength and radiative lifetime are enhanced\ndue to the confinement effect.", "category": "cond-mat_mtrl-sci" }, { "text": "The Dzyaloshinskii-Moriya interaction is under control: an orchestrated\n flip of the chiral link between structure and magnetism for\n Fe$_{1-x}$Co$_x$Si: Monosilicides of 3d-metals frequently show a chiral magnetic ordering with\nthe absolute configuration defined by the chirality of the crystal structure\nand the sign of the Dzyaloshinskii-Moriya interaction (DMI). Structural and\nmagnetic chiralities are probed here for Fe$_{1-x}$Co$_x$Si series and their\nmutual relationship is found to be dependent on the chemical composition. The\nchirality of crystal structure was previously shown to be governed by crystal\ngrowth, and the value of the DMI is nearly the same for all monosilicides of\nFe, Co and Mn. Our findings indicate that the sign of the DMI in\nFe$_{1-x}$Co$_x$Si is controlled by the Co composition $x$, thus, opening a\nroute towards controlled design of chiral spintronics devices.", "category": "cond-mat_mtrl-sci" }, { "text": "Liberation of slave modes inside domain walls in multiferroic Cu-Cl\n boracite: Domain walls (DWs), the two-dimensional boundaries between symmetry\nequivalent ferroic domains, are actively investigated due to their promise for\nnovel logic and memory devices. Moreover, they can be easily created, erased\nand reshaped at a low energy cost due to their high mobility and large\nelectrical conductivity. Most work so far has been focused on DWs in proper\nferroelectrics, where the primary order parameter, ferroelectric polarization,\ninterpolates between the values in the domains by either reducing to zero (in\nIsing-type DW) or rotating (Bloch type DW). Here we present a new member of DW\nfamily with a complex inner texture of slave order parameters inside the wall\nwhere the primary order parameter reduces to zero. Our first-principles-derived\nmodel predicts the existence of monopolar and toroidal polarization patterns.\nThe results enable large-scale phase field simulations of complex domain\npatterns in boracites and could inspire novel devices based on domain walls in\nimproper ferroelectrics.", "category": "cond-mat_mtrl-sci" }, { "text": "Ferroelectricity in the 1 $\u03bc$C cm$^{-2}$ range induced by canted\n antiferromagnetism in (LaMn$_{3}$)Mn$_{4}$O$_{12}$: Pyroelectric current and magnetoelectric coupling measurements on\npolycrystalline samples of the quadruple perovskite\n(LaMn$_{3}$)Mn$_{4}$O$_{12}$ give evidence of ferroelectricity driven by the\nantiferromagnetic ordering of the $B$-site Mn$^{3+}$ ions at $T_{N,B}$=78 K\nwith record values of remnant electric polarization up to $P$=0.56 $\\mu$C\ncm$^{-2}$. X-ray diffraction measurements indicates an anomalous behavior of\nthe monoclinic $\\beta$ angle at $T_{N,B}$, which suggests that $P$ lies in the\n$ac$-plane, where the moments are collinear, so we conclude that exchange\nstriction is the mechanism of spin-driven ferroelectricity. Polarization values\n$\\sim$3 $\\mu$C cm$^{-2}$ are expected in single crystals, which would open the\navenue towards practical multiferroic applications.", "category": "cond-mat_mtrl-sci" }, { "text": "Structure and optical properties of alpha- and gamma-cerium\n sesquisulfide: Structural and electronic properties of the alpha- and gamma-phases of cerium\nsesquisulfide, Ce2S3, are examined by first-principles calculations using the\nGGA+U extension of density functional theory. The strongly correlated\nf-electrons of Ce are described by a Hubbard-type on-site Coulomb repulsion\nparameter. A single parameter of $U^/prime$=4 eV yields excellent results for\ncrystal structures, band gaps, and thermodynamic stability for both Ce2S3\nallotropes. This approach gives insights in the difference in color of\nbrownish-black alpha-Ce2S3 and dark red gamma-Ce2S3. The calculations predict\nthat both Ce2S3 modifications are insulators with optical gaps of 0.8 eV\n(alpha-phase) and 1.8 eV (gamma-phase). The optical gaps are determined by\ndirect electronic excitations at k=Gamma from localized and occupied Ce\n4f-orbitals into empty Ce 5d-states. The f-states are situated between the\nvalence and conduction bands. The difference of 1 eV between the optical gaps\nof the two Ce2S3 modifications is explained by different coordinations of the\ncerium cations by sulfur anions. For both Ce2S3 modifications the calculations\nyield an effective local magnetic moment of 2.6 $mu_B$ per cerium cation, which\nis in agreement with measurements. The electronic energy of the alpha-phase is\ncomputed to be 6 kJ/mol lower than that of the gamma-phase, which is consistent\nwith the thermodynamic stability of the two allotropes.", "category": "cond-mat_mtrl-sci" }, { "text": "Electronic thermal conductivity at high temperatures: Violation of the\n Wiedemann-Franz law in narrow band metals: We study the electronic part of the thermal conductivity kappa of metals. We\npresent two methods for calculating kappa, a quantum Monte-Carlo (QMC) method\nand a method where the phonons but not the electrons are treated\nsemiclassically (SC). We compare the two methods for a model of alkali-doped\nC60, A3C60, and show that they agree well. We then mainly use the SC method,\nwhich is simpler and easier to interpret. We perform SC calculations for Nb for\nlarge temperatures T and find that kappa increases with T as kappa(T)=a+bT,\nwhere a and b are constants, consistent with a saturation of the mean free\npath, l, and in good agreement with experiment. In contrast, we find that for\nA3C60, kappa(T) decreases with T for very large T. We discuss the reason for\nthis qualitatively in the limit of large T. We give a quantum-mechanical\nexplanation of the saturation of l for Nb and derive the Wiedemann-Franz law in\nthe limit of T much smaller than W, where W is the band width. In contrast, due\nto the small W of A3C60, the assumption T much smaller than W can be violated.\nWe show that this leads to kappa(T) \\sim T^{-3/2} for very large T and a strong\nviolation of the Wiedemann-Franz law.", "category": "cond-mat_mtrl-sci" }, { "text": "Optimal switching of a nanomagnet assisted by microwaves: We develop an efficient and general method for optimizing the microwave field\nthat achieves magnetization switching with a smaller static field. This method\nis based on optimal control and renders an exact solution for the 3D microwave\nfield that triggers the switching of a nanomagnet with a given anisotropy and\nin an oblique static field. Applying this technique to the particular case of\nuniaxial anisotropy, we show that the optimal microwave field, that achieves\nswitching with minimal absorbed energy, is modulated both in frequency and in\nmagnitude. Its role is to drive the magnetization from the metastable\nequilibrium position towards the saddle point and then damping induces the\nrelaxation to the stable equilibrium position. For the pumping to be efficient,\nthe microwave field frequency must match at the early stage of the switching\nprocess the proper precession frequency of the magnetization, which depends on\nthe magnitude and direction of the static field. We investigate the effect of\nthe static field (in amplitude and direction) and of damping on the\ncharacteristics of the microwave field. We have computed the switching curves\nin the presence of the optimal microwave field. The results are in qualitative\nagreement with micro-SQUID experiments on isolated nanoclusters. The strong\ndependence of the microwave field and that of the switching curve on the\ndamping parameter may be useful in probing damping in various nanoclusters.", "category": "cond-mat_mtrl-sci" }, { "text": "Ill-Behaved Convergence of a Model of the Gd3Ga5O12 Garnet\n Antiferromagnet with Truncated Magnetic Dipole-Dipole Interactions: Previous studies have found that calculations which consider long-range\nmagnetic dipolar interactions truncated at a finite cut-off distance Rc predict\nspurious (unphysical) long-range ordered phases for Ising and Heisenberg\nsystems on the pyrochlore lattice. In this paper we show that, similar to these\ntwo cases, calculations that use truncated dipolar interactions to model the\nGd3Ga5O12 garnet antiferromagnet also predict unphysical phases with\nincommensurate ordering wave vector q_ord that is very sensitive to the dipolar\ncut-off distance Rc.", "category": "cond-mat_mtrl-sci" }, { "text": "Magnon Dispersion and Anisotropies in SrCu$_2$(BO$_3$)$_2$: We study the dispersion of the magnons (triplet states) in\nSrCu$_2$(BO$_3$)$_2$ including all symmetry-allowed Dzyaloshinskii-Moriya\ninteractions. We can reduce the complexity of the general Hamiltonian to a new\nsimpler form by appropriate rotations of the spin operators. The resulting\nHamiltonian is studied by both perturbation theory and exact numerical\ndiagonalization on a 32-site cluster. We argue that the dispersion is dominated\nby Dzyaloshinskii-Moriya interactions. We point out which combinations of these\nanisotropies affect the dispersion to linear-order, and extract their\nmagnitudes.", "category": "cond-mat_mtrl-sci" }, { "text": "Designing isoelectronic counterparts to layered group V semiconductors: In analogy to III-V compounds, which have significantly broadened the scope\nof group IV semiconductors, we propose IV-VI compounds as isoelectronic\ncounterparts to layered group V semiconductors. Using {\\em ab initio} density\nfunctional theory, we study yet unrealized structural phases of silicon\nmono-sulfide (SiS). We find the black-phosphorus-like $\\alpha$-SiS to be almost\nequally stable as the blue-phosphorus-like $\\beta$-SiS. Both $\\alpha$-SiS and\n$\\beta$-SiS monolayers display a significant, indirect band gap that depends\nsensitively on the in-layer strain. Unlike 2D semiconductors of group V\nelements with the corresponding nonplanar structure, different SiS allotropes\nshow a strong polarization either within or normal to the layers. We find that\nSiS may form both lateral and vertical heterostructures with phosphorene at a\nvery small energy penalty, offering an unprecedented tunability in structural\nand electronic properties of SiS-P compounds.", "category": "cond-mat_mtrl-sci" }, { "text": "Composites of FeCl3 and TiO2 with Bismaleimide resins: Ferric Chloride-Bismaleimide (FeCl3-BMI) and Titania-Bismaleimide (TiO2-BMI)\ncomposite were synthesized using PVA as a binder. The composite systems were\ndeposited on glass slide as a homogenous coating. XRD and FTIR studies of the\ncomposite system showed its crystalline and structural configuration. A mixed\nphase of TiO2 and BMI as well as short range crystallinity was observed. An\nincrease in crystallinity with temperature was also seen. The percentage of N-H\nsymmetric stretching was also found to increase with temperature.", "category": "cond-mat_mtrl-sci" }, { "text": "Transferring MBE-grown topological insulator films to arbitrary\n substrates and Metal-insulator transition via Dirac gap: Mechanical exfoliation of bulk crystals has been widely used to obtain thin\ntopological insulator (TI) flakes for device fabrication. However, such a\nprocess produces only micro-sized flakes that are highly irregular in shape and\nthickness. In this work, we developed a process to transfer the entire area of\nTI Bi2Se3 thin films grown epitaxially on Al2O3 and SiO2 to arbitrary\nsubstrates, maintaining their pristine morphology and crystallinity. Transport\nmeasurements show that these transferred films have lower carrier\nconcentrations and comparable or higher mobilities than before the transfer.\nFurthermore, using this process we demonstrated a clear metal-insulator\ntransition in an ultrathin Bi2Se3 film by gate-tuning its Fermi level into the\nhybridization gap formed at the Dirac point. The ability to transfer large area\nTI films to any substrate will facilitate fabrication of TI heterostructure\ndevices, which will help explore exotic phenomena such as Majorana fermions and\ntopological magnetoelectricity.", "category": "cond-mat_mtrl-sci" }, { "text": "Langmuir-Blodgett Monolayers of Cationic Dyes in the Presence and\n Absence of Clay Mineral Layers: N,N'-Dioctadecyl Thiacyanine, Octadecyl\n Rhodamine B and Laponite: Langmuir-Blodgett (LB) films of N,N'-dioctadecyl thiacyanine perchlorate (NK)\nand octadecyl rhodamine B chloride (RhB18) and their mixtures in the presence\nand absence of clay mineral layers were investigated by recording surface\npressure - area isotherms and by UV-Vis and fluorescence spectroscopies. The\nisotherms of NK, RhB18 and their mixtures are characteristic for liquid\nexpanded state behaviour with repulsive interactions between the two cationic\ndyes. In the presence of laponite the isotherms show liquid expanded and\ncondensed state behaviour. In laponite dispersions and in monolayers, NK has a\nstrong tendency to aggregate with formation of H- and J- aggregates. The\nabsorption and fluorescence maxima of the monomers in the films are at 435 nm\nand at 480 nm; H-dimer have an absorption maximum around 410 nm and do not\nfluoresce. J-dimers are present in all the films with absorption maximum at 461\nnm and fluorescence at 463 nm. RhB18 is mainly present as monomers in the LB\nfilms with an absorption maximum at 576 nm and fluorescence at 595 nm.\nFluorescence resonance energy transfer from NK to RhB18 has been observed in\nclay dispersions and in films with and without laponite. The optimum condition\nfor NK RhB18 fluorescence energy transfer in the films is 90 mol% NK + 10 mol%\nRhB18.", "category": "cond-mat_mtrl-sci" }, { "text": "Defect formation dynamics during CdTe overlayer growth: The presence of atomic-scale defects at multilayer interfaces significantly\ndegrades performance in CdTe-based photovoltaic technologies. The ability to\naccurately predict and understand defect formation mechanisms during overlayer\ngrowth is, therefore, a rational approach for improving the efficiencies of\nCdTe materials. In this work, we utilize a recently developed CdTe bond-order\npotential (BOP) to enable accurate molecular dynamics (MD) simulations for\npredicting defect formation during multilayer growth. A detailed comparison of\nour MD simulations to high-resolution transmission electron microscopy\nexperiments verifies the accuracy and predictive power of our approach. Our\nsimulations further indicate that island growth can reduce the lattice mismatch\ninduced defects. These results highlight the use of predictive MD simulations\nto gain new insight on defect reduction in CdTe overlayers, which directly\naddresses efforts to improve these materials.", "category": "cond-mat_mtrl-sci" }, { "text": "The Sign of Three: Spin/Charge Density Waves at the Boundaries of\n Transition Metal Dichalcogenides: One-dimensional grain boundaries of two-dimensional semiconducting {\\MX} (M=\nMo,W; X=S,Se) transition metal di-chalcogenides are typically metallic at room\ntemperature. The metallicity has its origin in the lattice polarization, which\nfor these lattices with $D_{3h}$ symmetry is a topological invariant, and leads\nto one-dimenional boundary states inside the band gap. For boundaries\nperpendicular to the polarization direction, these states are necessarily 1/3\noccupied by electrons or holes, making them susceptible to a metal-insulator\ntransition that triples the translation period. Using density-functional-theory\ncalculations we demonstrate the emergence of combined one-dimensional spin\ndensity/charge density waves of that period at the boundary, opening up a small\nband gap of $\\sim 0.1$ eV. This unique electronic structure allows for soliton\nexcitations at the boundary that carry a fractional charge of $\\pm 1/3\\ e$.", "category": "cond-mat_mtrl-sci" }, { "text": "Graphene -- Based Nanocomposites as Highly Efficient Thermal Interface\n Materials: We found that an optimized mixture of graphene and multilayer graphene -\nproduced by the high-yield inexpensive liquid-phase-exfoliation technique - can\nlead to an extremely strong enhancement of the cross-plane thermal conductivity\nK of the composite. The \"laser flash\" measurements revealed a record-high\nenhancement of K by 2300 % in the graphene-based polymer at the filler loading\nfraction f =10 vol. %. It was determined that a relatively high concentration\nof single-layer and bilayer graphene flakes (~10-15%) present simultaneously\nwith thicker multilayers of large lateral size (~ 1 micrometer) were essential\nfor the observed unusual K enhancement. The thermal conductivity of a\ncommercial thermal grease was increased from an initial value of ~5.8 W/mK to\nK=14 W/mK at the small loading f=2%, which preserved all mechanical properties\nof the hybrid. Our modeling results suggest that graphene - multilayer graphene\nnanocomposite used as the thermal interface material outperforms those with\ncarbon nanotubes or metal nanoparticles owing to graphene's aspect ratio and\nlower Kapitza resistance at the graphene - matrix interface.", "category": "cond-mat_mtrl-sci" }, { "text": "Cm2 Scale Synthesis of MoTe2 Thin Films with Large Grains and Layer\n Control David: Owing to the small energy differences between its polymorphs, MoTe2 can\naccess a full spectrum of electronic states, from the 2H semiconducting state\nto the 1T semimetallic state, and from the Td Weyl semimetallic state to the\nsuperconducting state in the 1T and Td phase at low temperature. Thus, it is a\nmodel system for phase transformation studies as well as quantum phenomena such\nas the quantum spin Hall effect and topological superconductivity. Careful\nstudies of MoTe2 and its potential applications require large area MoTe2 thin\nfilms with high crystallinity and thickness control. Here, we present cm2 scale\nsynthesis of 2H MoTe2 thin films with layer control and large grains that span\nseveral microns. Layer control is achieved by controlling the initial thickness\nof the precursor MoOx thin films, which are deposited on sapphire substrates by\natomic layer deposition and subsequently tellurized. Despite the van der Waals\nepitaxy, the precursor-substrate interface is found to critically determine the\nuniformity in thickness and grain size of the resulting MoTe2 films: MoTe2\ngrown on sapphire show uniform films while MoTe2 grown on amorphous SiO2\nsubstrates form islands. This synthesis strategy decouples the layer control\nfrom the variabilities of growth conditions for robust growth results, and is\napplicable to grow other transition metal dichalcogenides with layer control.", "category": "cond-mat_mtrl-sci" }, { "text": "Influence of defects on the critical behaviour at the \\boldmath{105} K\n structural phase transition of SrTiO$_3$: II. The sharp component: The depth dependence of the crystallographic parameters mosaicity, lattice\nparameter variation and integrated reflectivity and of the critical scattering\nabove the 105 K structural phase transition of SrTiO$_3$ have been studied in\nfive different single crystals by means of high resolution triple-crystal\ndiffractometry using 100-120 keV synchrotron radiation. Depth-dependent\nimpedance measurements indicate that the presence of oxygen vacancies is not\nresponsible for the two-length scale phenomenon. It is found that the sharp\ncomponent occurs only in surface near regions of highly perfect single crystals\nand is coupled to an exponential inrease of the crystallographic quantities.\nThe second length scale is absent at a surface where the strain fields are able\nto relax by a macroscopic bending of the lattice planes. The sharp component is\nalso strongly suppressed in crystals of relatively large mosaicity. The\ncombination of long range strain fields in highly perfect samples and the\nvicinity of the surface seem to be necessary conditions for the observation of\nthe sharp component. The critical exponents for the second length scale are in\nsatisfying agreement with scaling laws if the intensity of the critical\nscattering is assumed to be proportional to the square of the Lorentzian\nsusceptibility and not, as usual in the current convention, to a\nLorentzian-squared susceptibility. The critical exponents of the broad\ncomponent are independent of the appearance of the sharp component.", "category": "cond-mat_mtrl-sci" }, { "text": "Possible Kitaev Quantum Spin Liquid State in 2D Materials with S=3/2: Quantum spin liquids (QSLs) form an extremely unusual magnetic state in which\nthe spins are highly correlated and fluctuate coherently down to the lowest\ntemperatures, but without symmetry breaking and without the formation of any\nstatic long-range-ordered magnetism. Such intriguing phenomena are not only of\ngreat fundamental relevance in themselves, but also hold the promise for\nquantum computing and quantum information. Among different types of QSLs, the\nexactly solvable Kitaev model is attracting much attention, with most proposed\ncandidate materials, e.g., RuCl$_3$ and Na$_2$IrO$_3$, having an effective\n$S$=1/2 spin value. Here, via extensive first-principle-based simulations, we\nreport the investigation of the Kitaev physics and possible Kitaev QSL state in\nepitaxially strained Cr-based monolayers, such as CrSiTe$_3$, that rather\npossess a $S$=3/2 spin value. Our study thus extends the playground of Kitaev\nphysics and QSLs to 3$d$ transition metal compounds.", "category": "cond-mat_mtrl-sci" }, { "text": "Bismuth-surfactant-induced growth and structure of InAs/GaAs(110)\n quantum dots: We explore the Bi-surfactant-directed self-assembly and structure of InAs\nquantum dots grown on GaAs(110) by molecular beam epitaxy. The addition of a Bi\nflux during InAs deposition changes the InAs growth mode from two-dimensional\n(2D) Frank-van der Merwe to Stranski-Krastanov, resulting in the formation of\nthree-dimensional (3D) InAs islands on the surface. Furthermore, exposing\nstatic InAs 2D layers to Bi induces a rearrangement of the strained layer into\n3D islands. We explore the effect of varying the InAs thickness and Bi flux for\nthese two growth approaches, observing a critical thickness for 3D island\nformation in both cases. Characterization of (110) InAs quantum dots with\nhigh-resolution transmission electron microscopy reveals that larger islands\ngrown by the Stranski-Krastanov mode are plastically relaxed, while small\nislands grown by the on-demand approach are coherent. Strain relaxation along\nthe [1-10] direction is achieved by 90 degree pure-edge dislocations with\ndislocation lines running along [001]. In contrast, strain relief along [001]\nis by 60 degree misfit dislocations. This behaviour is consistent with\nobservations of planar (In,Ga)As/GaAs(110) layers. These results illustrate how\nsurfactant Bi can provoke and control quantum dot formation where it normally\ndoes not occur.", "category": "cond-mat_mtrl-sci" }, { "text": "Magnetic properties of Sn-substituted Ni-Zn ferrite:synthesized from\n nano-sized powders of NiO, ZnO, Fe2O3 and SnO2: A series of Ni0.6-x/2Zn0.4-x/2SnxFe2O4 (x = 0.0, 0.05, 0.1, 0.15, 0.2 and\n0.3) (NZSFO) ferrite composites have been synthesized from nano powders using\nstandard solid state reaction technique. The spinel cubic structure of the\ninvestigated samples has been observed by the X-ray diffraction (XRD). The\nmagnetic properties such as saturation magnetization (Ms), remanent\nmagnetization (Mr), coercive field (Hc) and Bohr magneton (B) are calculated\nfrom the hysteresis loops. The value of Ms is found to decrease with increasing\nSn content in the samples. This change has been successfully explained by the\nvariation of A-B interaction strength due to Sn substitution in different\nsites. The compositional stability and quality of the prepared ferrite\ncomposites have also been endorsed by the fairly constant initial permeability\n(/) over a wide range of frequency region. The decreasing trend of / with\nincreasing Sn content has been observed. Curie temperature (TC) has found to\nincrease with the increase in Sn content. Wide spread frequency utility zone\nindicates that the NZSFO can be considered as a good candidate for use in\nbroadband pulse transformer and wide band read-write heads for video recording.\nThe abnormal behavior for x = 0.05 has been explained with existing theory.", "category": "cond-mat_mtrl-sci" }, { "text": "The Effect of short-range order on the viscosity and crystallization of\n Al-Mg melts: In this work, using the methods of viscosimetry and thermal analysis, the\nconcentration changes in the values of the supercooling viscosity of Al-Mg\nmelts with Mg content from 2.5 to 95 at.% are studied. It is shown that the\ntemperature dependences of viscosity are well described by an exponential\ndependence. The concentration dependence of viscosity is not monotonous and\nreflects a change in the chemical short-range order in the liquid phase. The\nconcentration dependence of supercooling of Al-Mg melts is determined by the\ntype of solid phase formed during solidification, and also reflects the most\nsignificant changes in the chemical short-range order in the liquid phase at 20\nand 80 at.% Mg. Al-Mg alloys in the concentration ranges: 0-10, 40-50 and\n90-100 at.% Mg are prone to non-equilibrium crystallization, the formation of\nquasi-eutectics and solidification without intermediate intermetallic phases.", "category": "cond-mat_mtrl-sci" }, { "text": "Theoretical investigation on the ferromagnetic two-dimensional scandium\n monochloride sheet that has a high Curie temperature and could be exfoliated\n from a known material: A two-dimensional scandium monochloride sheet was investigated by using\ndensity functional theory. It could be exfoliated from a known bulk material\nwith a cleavage energy slightly lower than that of graphene. The sheet has a\nferromagnetic ground state with a Curie temperature of 100 K. Moreover, the\nsheet becomes a half-metal under hole doping. The Curie temperature increases\nto 250 K with the doping amount of 0.4 per primitive cell, which is close to\nthe ice point. The two-dimensional scandium monochloride sheet should be a good\ncandidate for two-dimensional spintronics.", "category": "cond-mat_mtrl-sci" }, { "text": "A DFT based first-principles investigation of the physical properties of\n Bi2Te2Se topological insulator: A topological insulator possesses a bulk energy gap splitting the lowest\nempty band from the highest occupied electronic band. The electronic states at\nthe surface (or edge in two dimensions), on the other hand, of a topological\ninsulator are gapless and are protected by the time reversal symmetry. Such\nsystems are promising for variety of optoelectronic, superconducting,\nthermoelectric and quantum computation related applications. We have studied\nelastic, mechanical, electronic, optical properties, bonding character and the\nelectronic charge density distribution of ternary Bi2Te2Se topological\ninsulator. The compound under study is mechanically stable and elastically\nanisotropic. The electronic band structure calculations reveal high degree of\nanisotropy in the energy dispersion. Electronic effective mass is high in the\nc-direction compared to that in the ab-plane. The optical constants show\nmoderate level of variation with respect to the polarization of the electric\nfield of the incident radiation. The optical spectra are consistent with the\nelectronic band structure and electronic density of states features. Both\nelectronic band structure and optical constants show clear indications of a\ndirect band gap of 0.610 eV for Bi2Te2Se. It is also found that Bi2Te2Se\npossesses high refractive index at low photon energies in the infrared and\nvisible region. It has low reflectivity in the ultraviolet region. Bi2Te2Se\nabsorbs photons strongly in the ultraviolet energies. All these features make\nBi2Te2Se suitable for diverse class of optoelectronic device applications.", "category": "cond-mat_mtrl-sci" }, { "text": "Colossal room-temperature electrocaloric strength aided by hydrostatic\n pressure in lead-free multiferroic solid solutions: Solid-state cooling applications based on the electrocaloric (EC) effect are\nparticularly promising from a technological point of view due to their downsize\nscalability and natural implementation in circuitry. However, EC effects\ntypically occur far from room temperature, involve materials that contain toxic\nsubstances and require relatively large electric fields ($\\sim 100$-$1000$ kV\ncm$^{-1}$) that cause fateful leakage current and dielectric loss problems.\nHere, we propose a possible solution to these practical issues that consists in\nconcertedly applying hydrostatic pressure and electric fields on lead-free\nmultiferroic materials. We theoretically demonstrate this strategy by\nperforming first-principles simulations on supertetragonal\nBiFe$_{1-x}$Co$_{x}$O$_{3}$ solid solutions (BFCO). It is shown that\nhydrostatic pressure, besides adjusting the occurrence of EC effects to near\nroom temperature, can reduce enormously the intensity of the driving electric\nfields. For pressurized BFCO, we estimate a colossal room-temperature EC\nstrength, defined like the ratio of the adiabatic EC temperature change by the\napplied electric field, of $\\sim 1$ K cm kV$^{-1}$, a value that is several\norders of magnitude larger than those routinely measured in uncompressed\nferroelectrics.", "category": "cond-mat_mtrl-sci" }, { "text": "Review of Theoretical and Computational Methods for 2D Materials\n Exhibiting Charge Density Waves: Two-dimensional (2D) materials that exhibit charge density waves (CDWs) have\ngenerated many research endeavors in the hopes of employing their exotic\nproperties for various quantum-based technologies. Early investigations\nsurrounding CDWs were mostly focused on bulk materials. However, applications\nfor quantum devices have since required devices to be constructed from\nfew-layer material to fully utilize the material's properties. This field has\ngreatly expanded over the decades, warranting a focus on the computational\nefforts surrounding CDWs in 2D materials. In this review, we will cover ground\nin the following relevant, theory-driven subtopics for TaS2 and TaSe2: summary\nof general computational techniques and methods, atomic structures, Raman\nmodes, and effects of confinement and dimensionality. Through understanding how\nthe computational methods have enabled incredible advancements in quantum\nmaterials, one may anticipate the ever-expanding directions available for\ncontinued pursuit as the field brings us through the 21st century.", "category": "cond-mat_mtrl-sci" }, { "text": "Hund's physics and the magnetic ground state of CrOX (X = Cl, Br): To understand the magnetic property of layered van der Waals materials CrOX\n(X = Cl, Br), we performed the detailed first-principles calculations for both\nbulk and monolayer. We found that the charge-only density functional theory\ncombined with the explicit on-site interaction terms (so-called cDFT$+U$) well\nreproduces the experimental magnetic ground state of bulk CrOX, which is not\nthe case for the use of spin-dependent density functional (so-called sDFT$+U$).\nUnlike some of the previous studies, our results show that CrOX monolayers are\nantiferromagnetic as in the bulk. It is also consistent with our magnetic force\nlinear response calculation of exchange couplings $J_{\\rm ex}$. The result of\norbital-decomposed $J_{\\rm ex}$ calculations shows that the Cr\n$t_\\textrm{2g}$-$t_\\textrm{2g}$ component mainly contributes to the\nantiferromagnetic order in both bulk and monolayer. Our result and analysis\nshow that taking the correct Hund's physics into account is of key importance\nto construct the magnetic phase diagram and to describe the electronic\nstructure.", "category": "cond-mat_mtrl-sci" }, { "text": "Data based constitutive modelling of rate independent inelastic effects\n in composite cables using Preisach hysteresis operators: This contribution aims at introducing first steps to develop hysteresis\noperator type inelastic constitutive laws for Cosserat rods for the simulation\nof cables composed of complex interior components. Motivated by the basic\nelements of Cosserat rod theory, we develop a specific approach to constitutive\nmodelling adapted for this application. Afterwards, we describe the\nhysteretical behaviour arising from cyclic bending experiments on cables by\nmeans of the Preisach operator. As shown in pure bending experiments, slender\nstructures such as electric cables behave inelastically, and open hysteresis\nloops arise with noticeable difference between the first load cycle and the\nfollowing ones. The Preisach operator plays an important role in describing the\ninput-output relation in hysteresis behaviours, and it can be expressed as a\nsuperposition of relay operators. Hence, a mathematical formulation of the\nproblem is introduced, and a first attempt is made to determine the hysteresis\nbehaviour that describes the relation between curvature and bending moment.\nTherefore, a suitable kernel function is identified in a way that its\nintegration over the Preisach plane results in the bending moment of the\nspecimen, and a comparison between different kernel functions is performed.", "category": "cond-mat_mtrl-sci" }, { "text": "Vibration Damping of Carbon Nanotube Assembly Materials: Vibration reduction is of great importance in various engineering\napplications, and a material that exhibits good vibration damping along with\nhigh strength and modulus has become more and more vital. Owing to the superior\nmechanical property of carbon nanotube (CNT), new types of vibration damping\nmaterial can be developed. This paper presents recent advancements, including\nour progresses, in the development of high-damping macroscopic CNT assembly\nmaterials, such as forests, gels, films, and fibers. In these assemblies,\nstructural deformation of CNTs, zipping and unzipping at CNT connection nodes,\nstrengthening and welding of the nodes, and sliding between CNTs or CNT bundles\nare playing important roles in determining the viscoelasticity, and elasticity\nas well. Towards the damping enhancement, strategies for micro-structure and\ninterface design are also discussed.", "category": "cond-mat_mtrl-sci" }, { "text": "A Cosserat crystal plasticity and phase field theory for grain boundary\n migration: The microstructure evolution due to thermomechanical treatment of metals can\nlargely be described by viscoplastic deformation, nucleation and grain growth.\nThese processes take place over different length and time scales which present\nsignificant challenges when formulating simulation models. In particular, no\noverall unified field framework exists to model concurrent viscoplastic\ndeformation and recrystallization and grain growth in metal polycrystals. In\nthis work a thermodynamically consistent diffuse interface framework\nincorporating crystal viscoplasticity and grain boundary migration is\nelaborated. The Kobayashi--Warren--Carter (KWC) phase field model is extended\nto incorporate the full mechanical coupling with material and lattice rotations\nand evolution of dislocation densities. The Cosserat crystal plasticity theory\nis shown to be the appropriate framework to formulate the coupling between\nphase field and mechanics with proper distinction between bulk and grain\nboundary behaviour.", "category": "cond-mat_mtrl-sci" }, { "text": "Handedness manipulation of propagating antiferromagnetic magnons: Antiferromagnetic magnons possess a distinctive feature absent in their\nferromagnetic counterparts: the presence of two distinct handedness modes, the\nright-handed (RH) and left-handed (LH) precession modes. The magnon handedness\ndetermines the sign of spin polarization carried by the propagating magnon,\nwhich is indispensable for harnessing the diverse functionalities. However, the\ncontrol of coherently propagating magnon handedness in antiferromagnets has\nremained elusive so far. Here we demonstrate the manipulation and electrical\nreadout of propagating magnon handedness in perpendicularly magnetized\nsynthetic antiferromagnets (SAF). We find that the antiferromagnetic magnon\nhandedness can be directly identified by measuring the inverse spin Hall effect\n(ISHE) voltage, which arises from the spin pumping effect caused by the\npropagating antiferromagnetic magnons in the SAF structure. The RH and LH modes\nof the magnon can be distinguishable particularly when the SAF structure is\nsandwiched by heavy metals with the same sign of spin Hall angle. Moreover, we\nsucceed in controlling the handedness of propagating antiferromagnetic magnons\nby tuning the excitation microwave frequency. This work unveils promising\navenues for harnessing magnon unique properties in antiferromagnet-based\nmagnonic applications.", "category": "cond-mat_mtrl-sci" }, { "text": "Ab Initio Study of the Structural Phase Transition in Cubic Pb_3GeTe_4: In the substitutionally disordered narrow-gap semiconductor Pb_{1-x}Ge_xTe, a\nfinite-temperature cubic-rhombohedral transition appears above a critical\nconcentration $x \\approx 0.005$. As a first step towards a first-principles\ninvestigation of this transition in the disordered system, a (hypothetical)\nordered cubic Pb_3GeTe_4 supercell is studied. First principles\ndensity-functional calculations of total energies and linear response functions\nare performed using the conjugate-gradients method with ab initio\npseudopotentials and a plane-wave basis set. Unstable modes in Pb_3GeTe_4 are\nfound, dominated by off-centering of the Ge ions coupled with displacements of\ntheir neighboring Te ions. A model Hamiltonian for this system is constructed\nusing the lattice Wannier function formalism. The parameters for this\nHamiltonian are determined from first principles. The equilibrium\nthermodynamics of the model system is studied via Metropolis Monte Carlo\nsimulations. The calculated transition temperature, T_c, is approximately 620K\nfor the cubic Pb_3GeTe_4 model, compared to the experimental value of T_c\n\\approx 350K for disordered Pb_{0.75}Ge_{0.25}Te. Generalization of this\nanalysis to the disordered Pb_{1-x}Ge_xTe system is discussed.", "category": "cond-mat_mtrl-sci" }, { "text": "Dislocation scattering in a two-dimensional electron gas: A theory of scattering by charged dislocation lines in a two-dimensional\nelectron gas (2DEG) is developed. The theory is directed towards understanding\ntransport in AlGaN/GaN high-electron-mobility transistors (HEMT), which have a\nlarge number of line dislocations piercing through the 2DEG. The scattering\ntime due to dislocations is derived for a 2DEG in closed form. This work\nidentifies dislocation scattering as a mobility-limiting scattering mechanism\nin 2DEGs with high dislocation densities. The insensitivity of the 2DEG (as\ncompared to bulk) to dislocation scattering is explained by the theory.", "category": "cond-mat_mtrl-sci" }, { "text": "Driving skyrmions with low threshold current density in Pt/CoFeB thin\n film: Magnetic skyrmions are topologically stable spin swirling particle like\nentities which are appealing for next generation spintronic devices. The\nexpected low critical current density for the motion of skyrmions makes them\npotential candidates for future energy efficient electronic devices. Several\nheavy metal/ferromagnetic (HM/FM) systems have been explored in the past decade\nto achieve faster skyrmion velocity at low current densities. In this context,\nwe have studied Pt/CoFeB/MgO heterostructures in which skyrmions have been\nstabilized at room temperature (RT). It has been observed that the shape of the\nskyrmions are perturbed even by the small stray field arising from low moment\nmagnetic tips while performing the magnetic force microscopy (MFM), indicating\npresence of low pinning landscape in the samples. This hypothesis is indeed\nconfirmed by the low threshold current density to drive the skyrmions in our\nsample, at velocities of few 10m/s.", "category": "cond-mat_mtrl-sci" }, { "text": "Interfacial contribution to the dielectric response in semiconducting\n LaBiMn4/3Co2/3O6: Impedance measurements have been performed on a sintered polycrystalline\nsample of the perovskite LaBiMn4/3Co2/3O6. Colossal dielectric permittivity\noften is measured in this class of semiconducting materials as a result of\nextrinsic factors. Our results show that a large offset in the capacitance,\nmeasured on a series of samples with different thickness, is due to the\ninterfacial polarization. This contribution then can be removed from the data,\ncreating a general procedure for dielectric measurements in semiconducting\nsamples.", "category": "cond-mat_mtrl-sci" }, { "text": "Stochastic Continuum Models for High--Entropy Alloys with Short-range\n Order: High entropy alloys (HEAs) are a class of novel materials that exhibit superb\nengineering properties. It has been demonstrated by extensive experiments and\nfirst principles/atomistic simulations that short-range order in the atomic\nlevel randomness strongly influences the properties of HEAs. In this paper, we\nderive stochastic continuum models for HEAs with short-range order from\natomistic models. A proper continuum limit is obtained such that the mean and\nvariance of the atomic level randomness together with the short-range order\ndescribed by a characteristic length are kept in the process from the atomistic\ninteraction model to the continuum equation. The obtained continuum model with\nshort-range order is in the form of an Ornstein--Uhlenbeck (OU) process. This\nvalidates the continuum model based on the OU process adopted\nphenomenologically by Zhang et al. [Acta Mater., 166 (2019), pp. 424--434] for\nHEAs with short-range order. We derive such stochastic continuum models with\nshort-range order for both elasticity in HEAs without defects and HEAs with\ndislocations (line defects). The obtained stochastic continuum models are based\non the energy formulations, whose variations lead to stochastic partial\ndifferential equations.", "category": "cond-mat_mtrl-sci" }, { "text": "Halogenation induced transition of superconductor-to-semiconductor in\n MXene-like MOene with direct band gap and long carrier lifetime: Traditional MXenes with intriguing mechanical and electronic properties,\ntogether with the fertilities of elemental compositions and chemical\ndecorations have aroused much attentions. However, the semiconducting traits\nwith direc band gap are extremetely rare among reported MXenes. Thus,\nbroadening the family of MXene beyond carbides and nitrides with unique\nbehaviors is still an extraordinary and fascinating field.", "category": "cond-mat_mtrl-sci" }, { "text": "Structure and Dielectric Properties of Amorphous High-kappa Oxides:\n HfO2, ZrO2 and their alloys: High-$\\kappa$ metal oxides are a class of materials playing an increasingly\nimportant role in modern device physics and technology. Here we report\ntheoretical investigations of the properties of structural and lattice\ndielectric constants of bulk amorphous metal oxides by a combined approach of\nclassical molecular dynamics (MD) - for structure evolution, and quantum\nmechanical first principles density function theory (DFT) - for electronic\nstructure analysis. Using classical MD based on the Born-Mayer-Buckingham\npotential function within a melt and quench scheme, amorphous structures of\nhigh-$\\kappa$ metal oxides Hf$_{1-x}$Zr$_x$O$_2$ with different values of the\nconcentration $x$, are generated. The coordination numbers and the radial\ndistribution functions of the structures are in good agreement with the\ncorresponding experimental data. We then calculate the lattice dielectric\nconstants of the materials from quantum mechanical first principles, and the\nvalues averaged over an ensemble of samples agree well with the available\nexperimental data, and are very close to the dielectric constants of their\ncubic form.", "category": "cond-mat_mtrl-sci" }, { "text": "Tight-binding molecular-dynamics studies of defects and disorder in\n covalently-bonded materials: Tight-binding (TB) molecular dynamics (MD) has emerged as a powerful method\nfor investigating the atomic-scale structure of materials --- in particular the\ninterplay between structural and electronic properties --- bridging the gap\nbetween empirical methods which, while fast and efficient, lack\ntransferability, and ab initio approaches which, because of excessive\ncomputational workload, suffer from limitations in size and run times. In this\nshort review article, we examine several recent applications of TBMD in the\narea of defects in covalently-bonded semiconductors and the amorphous phases of\nthese materials.", "category": "cond-mat_mtrl-sci" }, { "text": "Resolving diverse oxygen transport pathways across Sr-doped lanthanum\n ferrite and metal-perovskite heterostructures: Perovskite structured transition metal oxides are important technological\nmaterials for catalysis and solid oxide fuel cell applications. Their\nfunctionality often depends on oxygen diffusivity and mobility through complex\noxide heterostructures, which can be significantly impacted by structural and\nchemical modifications, such as doping. Further, when utilized within\nelectrochemical cells, interfacial reactions with other components (e.g. Ni-\nand Cr-based alloy electrodes and interconnects) can influence the perovskite's\nreactivity and ion transport, leading to complex dependencies that are\ndifficult to control in real-world environments. Here we use isotopic tracers\nand atom probe tomography to directly visualize oxygen diffusion and transport\npathways across perovskite and metal-perovskite heterostructures, i.e. (Ni-Cr\ncoated) Sr-doped lanthanum ferrite (LSFO). Annealing in 18O2(g) results in\nelemental and isotopic redistributions through oxygen exchange (OE) in the LSFO\nwhile Ni-Cr undergoes oxidation via multiple mechanisms and transport pathways.\nComplementary density functional theory (DFT) calculations at experimental\nconditions provide rationale for OE reaction mechanisms and reveal a complex\ninterplay of different thermodynamic and kinetic drivers. Our results shed\nlight on the fundamental coupling of defects and oxygen transport in an\nimportant class of catalytic materials.", "category": "cond-mat_mtrl-sci" }, { "text": "Development of local plasticity around voids during tensile deformation: Voids can limit the life of engineering components. This motivates us to\nunderstand local plasticity around voids in a nickel base superalloy combining\nexperiments and simulations. Single crystal samples were deformed in tension\nwith in-situ high angular resolution electron back scatter diffraction to probe\nthe heterogeneous local stress field under load; the reference stress is\ninformed by crystal plasticity finite element simulations. This information is\nused to understand the activation of plastic deformation around the void. Our\ninvestigation indicates that while the resolved shear stress would indicate\nslip activity on multiple slip systems, slip is reduced to specific systems due\nto image forces and forest hardening. This study rationalizes the observed\ndevelopment of plastic deformation around the void, aiding in our understanding\nof component failure and engineering design.", "category": "cond-mat_mtrl-sci" }, { "text": "Magnetism in Graphene Induced by Single-Atom Defects: We study from first principles the magnetism in graphene induced by single\ncarbon atom defects. For two types of defects considered in our study, the\nhydrogen chemisorption defect and the vacancy defect, the itinerant magnetism\ndue to the defect-induced extended states has been observed. Calculated\nmagnetic moments are equal to 1 $\\mu_B$ per hydrogen chemisorption defect and\n1.12$-$1.53 $\\mu_B$ per vacancy defect depending on the defect concentration.\nThe coupling between the magnetic moments is either ferromagnetic or\nantiferromagnetic, depending on whether the defects correspond to the same or\nto different hexagonal sublattices of the graphene lattice, respectively. The\nrelevance of itinerant magnetism in graphene to the high-$T_C$ magnetic\nordering is discussed.", "category": "cond-mat_mtrl-sci" }, { "text": "Reflectometry with registration of secondary radiation at total neutron\n reflection: Neutron reflectometry is a method for measuring of the spatial dependence\n(profile) of the potential interaction between neutron and medium. At interface\nof media the interaction potential is the sum of the elements potentials. For\ndefinition of potentials of separate elements (isotopes) a secondary radiation\nis recorded. Recording channels of secondary radiation are created on\nspectrometer REMUR at pulsed reactor IBR-2 in Dubna (Russia). The results for\ntesting of the channels are reported and perspectives of reflectometry with\nregistration of secondary radiation are discussed", "category": "cond-mat_mtrl-sci" }, { "text": "Pair vs many-body potentials: influence on elastic and plastic behavior\n in nanoindentation: Molecular-dynamics simulation can give atomistic information on the processes\noccurring in nanoindentation experiments. In particular, the nucleation of\ndislocation loops, their growth, interaction and motion can be studied. We\ninvestigate how realistic the interatomic potentials underlying the simulations\nhave to be in order to describe these complex processes. Specifically we\ninvestigate nanoindentation into a Cu single crystal. We compare simulations\nbased on a realistic many-body interaction potential of the\nembedded-atom-method type with two simple pair potentials, a Lennard-Jones and\na Morse potential. We find that qualitatively many aspects of nanoindentation\nare fairly well reproduced by the simple pair potentials: elastic regime,\ncritical stress and indentation depth for yielding, dependence on the crystal\norientation, and even the level of the hardness. The quantitative deficits of\nthe pair potential predictions can be traced back (i) to the fact that the pair\npotentials are unable in principle to model the elastic anisotropy of cubic\ncrystals; (ii) as the major drawback of pair potentials we identify the gross\nunderestimation of the stable stacking fault energy. As a consequence these\npotentials predict the formation of too large dislocation loops, the too rapid\nexpansion of partials, too little cross slip and in consequence a severe\noverestimation of work hardening.", "category": "cond-mat_mtrl-sci" }, { "text": "Nonpolar p-GaN/n-Si heterojunction diode characteristics: A comparison\n between ensemble and single nanowire devices: The electrical and photodiode characteristics of ensemble and single p-GaN\nnanowire and n-Si heterojunction devices were studied. Ideality factor of the\nsingle nanowire p-GaN/n-Si device was found to be about three times lower\ncompared to that of the ensemble nanowire device. Apart from the deep-level\ntraps in p-GaN nanowires, defect states due to inhomogeneity in Mg dopants in\nthe ensemble nanowire device are attributed to the origin of high ideality\nfactor. Photovoltaic mode of ensemble nanowire device showed an improvement in\nthe fill-factors up to 60 percent over the single nanowire device with\nfill-factors up to 30 percent. Reponsivity of the single nanowire device in\nphotoconducting mode was found to be enhanced by five orders, at 470 nm. The\nenhanced photoresponse of the single nanowire device also confirms the\nphotoconduction due to defect states in p-GaN nanowires.", "category": "cond-mat_mtrl-sci" }, { "text": "A novel high-current, high-resolution, low-kinetic-energy electron\n source for inverse photoemission spectroscopy: A high-current electron source for inverse photoemission spectroscopy (IPES)\nis described. The source comprises a thermal cathode electron emission system,\nan electrostatic deflector-monochromator, and a lens system for variable\nkinetic energy (1.6 - 20 eV) at the target. When scaled to the energy\nresolution, the electron current is an order of magnitude higher than that of\npreviously described electron sources developed in the context of electron\nenergy loss spectroscopy. Surprisingly, the experimentally measured energy\nresolution turned out to be significantly better than calculated by standard\nprograms, which include the electron-electron repulsion in the continuum\napproximation. The achieved currents are also significantly higher than\npredicted. We attribute this \"inverse Boersch-effect\" to a mechanism of\nvelocity selection in the forward direction by binary electron-electron\ncollisions.", "category": "cond-mat_mtrl-sci" }, { "text": "On the competition for ultimately stiff and strong architected materials: Advances in manufacturing techniques may now realize virtually any imaginable\nmicrostructures, paving the way for architected materials with properties\nbeyond those found in nature. This has lead to a quest for closing gaps in\nproperty-space by carefully designed metamaterials. Development of mechanical\nmetamaterials has gone from open truss lattice structures to closed plate\nlattice structures with stiffness close to theoretical bounds. However, the\nquest for optimally stiff and strong materials is complex. Plate lattice\nstructures have higher stiffness and (yield) strength but are prone to buckling\nat low volume fractions. Hence here, truss lattice structures may still be\noptimal. To make things more complicated, hollow trusses or structural\nhierarchy bring closed-walled microstructures back in the competition. Based on\nanalytical and numerical studies of common microstructures from the literature,\nwe provide higher order interpolation schemes for their effective stiffness and\n(buckling) strength. Furthermore, we provide a case study based on\nmulti-property Ashby charts for weight-optimal porous beams under bending, that\ndemonstrates the intricate interplay between structure and microarchitecture\nthat plays the key role in the design of ultimate load carrying structures. The\nprovided interpolation schemes may also be used to account for microstructural\nyield and buckling in multiscale design optimization schemes.", "category": "cond-mat_mtrl-sci" }, { "text": "Unipolar and bipolar fatigue in antiferroelectric lead zirconate thin\n films and evidences for switching-induced charge injection inducing fatigue: For the first time, we show that unipolar fatigue does occur in\nantiferroelectric capacitors, confirming the predictions of a previous work\n[Appl. Phys. Lett., 94, 072901 (2009)]. We also show that unipolar fatigue in\nantiferroelectrics is less severe than bipolar fatigue if the driving field is\nof the same magnitude. This phenomenon has been attributed to the\nswitching-induced charge injection, the main cause for polarization fatigue in\nferroelectric and antiferroelectric materials. Other evidences for polarization\nfatigue caused by the switching-induced charge injection from the nearby\nelectrode rather than the charge injection during stable/quasi-stable leakage\ncurrent stage are also discussed.", "category": "cond-mat_mtrl-sci" }, { "text": "The effect of exclusion on nonlinear reaction diffusion system in\n inhomogeneous media: We study a minimal model to understand the formation of clusters on surfaces\nin the presence of surface defects. We consider reaction diffusion model in\nwhich atoms undergoes reactions at the defect centers to form clusters. Volume\nexclusion between particles is introduced through a drift term in the reaction\ndiffusion equation that arises due the repulsive force field produced by the\nclustering atoms. The volume exclusion terms can be derived from master\nequation with a concentration dependent hopping rate. Perturbative analysis is\nperformed for both cross-exclusion and self-exclusion one dimensional system.\nFor two dimension numerical analysis is performed. We have found that the\nclusterization process slows down due to exclusion. As a result the size of the\nclusters reduces. In this model reaction scheme has algebraic nonlinearity and\nplausible mechanism is also given.", "category": "cond-mat_mtrl-sci" }, { "text": "Small-Angle X-ray and neutron scattering from diamond single crystals: Results of Small-Angle Scattering study of diamonds with various types of\npoint and extended defects and different degrees of annealing are presented. It\nis shown that thermal annealing and/or mechanical deformation cause formation\nof nanosized planar and threedimensional defects giving rise to Small-Angle\nScattering. The defects are often facetted by crystallographic planes 111, 100,\n110, 311, 211 common for diamond. The scattering defects likely consist of\nclusters of intrinsic and impurity-related defects; boundaries of mechanical\ntwins also contribute to the SAS signal. There is no clear correlation between\nconcentration of nitrogen impurity and intensity of the scattering.", "category": "cond-mat_mtrl-sci" }, { "text": "Internal stresses analysis on welded joint in Grade 91 steel under creep\n test: synchrotron DRX tests and modelling: The analysis and understanding of creep damage of Grade 91 steel welded\njoints is an important topic in the energy industry. Creep tests on welded\njoints were carried out at 600$^{\\circ}$C, 100MPa and then interrupted at 0%,\n10%, 30%, 50%, 80% of the expected life and after failure. Creep damage is\ncharacterised by cavity bands located exclusively in the core of the sample in\nthe InterCritical Heat Affected Zone (ICHAZ). These samples were tested using\n\\textit{in situ} synchrotron XRD along the welded joint under creep conditions\nfor the different creep life time. The experimental results show a significant\nstrain evolution and creep damage characteristic on the welded joint, with a\nlocal maximum at the Heat Affected Zone (HAZ). Following this, a finite element\ncreep strain analysis was performed for comparison with the experimental\nresults.", "category": "cond-mat_mtrl-sci" }, { "text": "Observation of Large Unidirectional Rashba Magnetoresistance in Ge(111): Relating magnetotransport properties to specific spin textures at surfaces or\ninterfaces is an intense field of research nowadays. Here, we investigate the\nvariation of the electrical resistance of Ge(111) grown epitaxially on\nsemi-insulating Si(111) under the application of an external magnetic field. We\nfind a magnetoresistance term which is linear in current density j and magnetic\nfield B, hence odd in j and B, corresponding to a unidirectional\nmagnetoresistance. At 15 K, for I = 10 $\\mu$A (or j = 0.33 A/m) and B = 1 T, it\nrepresents 0.5 % of the zero field resistance, a much higher value compared to\nprevious reports on unidirectional magnetoresistance. We ascribe the origin of\nthis magnetoresistance to the interplay between the externally applied magnetic\nfield and the current-induced pseudo-magnetic field in the spin-splitted\nsubsurface states of Ge(111). This unidirectional magnetoresistance is\nindependent of the current direction with respect to the Ge crystal axes. It\nprogressively vanishes, either using a negative gate voltage due to carrier\nactivation into the bulk (without spin-splitted bands), or by increasing the\ntemperature due to the Rashba energy splitting of the subsurface states lower\nthan $\\sim$58 k$_B$. The highly developed technologies on semiconductor\nplatforms would allow the rapid optimization of devices based on this\nphenomenon.", "category": "cond-mat_mtrl-sci" }, { "text": "Identifying vacancy complexes in compound semiconductors with positron\n annihilation spectroscopy: a case study of InN: We present a comprehensive study of vacancy and vacancy-impurity complexes in\nInN combining positron annihilation spectroscopy and ab-initio calculations.\nPositron densities and annihilation characteristics of common vacancy-type\ndefects are calculated using density functional theory and the feasibility of\ntheir experimental detection and distinction with positron annihilation methods\nis discussed. The computational results are compared to positron lifetime and\nconventional as well as coincidence Doppler broadening measurements of several\nrepresentative InN samples. The particular dominant vacancy-type positron traps\nare identified and their characteristic positron lifetimes, Doppler ratio\ncurves and lineshape parameters determined. We find that In vacancies and their\ncomplexes with N vacancies or impurities act as efficient positron traps,\ninducing distinct changes in the annihilation parameters compared to the InN\nlattice. Neutral or positively charged N vacancies and pure N vacancy complexes\non the other hand do not trap positrons. The predominantly introduced positron\ntrap in irradiated InN is identified as the isolated In vacancy, while in\nas-grown InN layers In vacancies do not occur isolated but complexed with one\nor more N vacancies. The number of N vacancies per In vacancy in these\ncomplexes is found to increase from the near surface region towards the\nlayer-substrate interface.", "category": "cond-mat_mtrl-sci" }, { "text": "Direct measurements of the magnetocaloric effect in ribbon samples of\n Heusler alloys Ni - Mn - M (M = In, Sn): Direct measurements of the magnetocaloric effect in samples of rapidly\nquenched ribbons of Mn50Ni40In10 and Ni50Mn37Sn13 Heusler alloys, with\npotential applications in magnetic refrigeration technology, are carried out.\nThe measurements were made by a precise method based on the measurement of the\noscillation amplitude of the temperature in the sample while is subjected to a\nmodulated magnetic field. In the studied compositions both direct and inverse\nmagnetocaloric effects associated with magnetic (paramagnet - ferromagnet -\nantiferromagnet) and structural (austenite - martensite) phase transitions are\nfound. Additional inverse magnetocaloric effects of small value are observed\naround the ferromagnetic transitions.", "category": "cond-mat_mtrl-sci" }, { "text": "Energy gap opening in submonolayer lithium on graphene: Local density\n functional and tight-binding calculations: The adsorption of an alkali-metal submonolayer on graphene occupying every\nthird hexagon of the honeycomb lattice in a commensurate\n$(\\sqrt{3}\\times\\sqrt{3})R30^\\circ$ arrangement induces an energy gap in the\nspectrum of graphene. To exemplify this type of band gap, we present \\textit{ab\ninitio} density functional theory calculations of the electronic band structure\nof C$_6$Li. An examination of the lattice geometry of the compound system shows\nthe possibility that the nearest-neighbor hopping amplitudes have alternating\nvalues constructed in a Kekul\\'e-type structure. The band structure of the\ntextured tight-binding model is calculated and shown to reproduce the expected\nband gap as well as other characteristic degeneracy removals in the spectrum of\ngraphene induced by lithium adsorption. More generally we also deduce the\npossibility of energy gap opening in periodic metal on graphene compounds\nC$_x$M if $x$ is a multiple of 3.", "category": "cond-mat_mtrl-sci" }, { "text": "Ionization potentials in the limit of large atomic number: By extrapolating the energies of non-relativistic atoms and their ions with\nup to 3000 electrons within Kohn-Sham density functional theory, we find that\nthe ionization potential remains finite and increases across a row, even as\n$Z\\rightarrow\\infty$. The local density approximation becomes chemically\naccurate (and possibly exact) in some cases. Extended Thomas-Fermi theory\nmatches the shell-average of both the ionization potential and density change.\nExact results are given in the limit of weak electron-electron repulsion.", "category": "cond-mat_mtrl-sci" }, { "text": "Atomistic simulations of ductile failure in a b.c.c. high entropy alloy: Ductile failure is studied in a bcc HfNbTaZr High Entropy Alloy (HEA) with a\npre-existing void. Using molecular dynamics simulations of uniaxial tensile\ntests, we explore the effect of void radius on the elastic modulus and yield\nstress. The elastic modulus scales with porosity as in closed-cell foams. The\ncritical stress for dislocation nucleation as a function of the void radius is\nvery well described by a model designed after pure bcc metals, taking into\naccount a larger core radius for the HEA. Twinning takes place as a\ncomplementary deformation mechanism, and some detwinning occurs at large\nstrain. No solid-solid phase transitions are identified. The concurrent effects\nof element size mismatch and plasticity lead to significant lattice disorder.\nBy comparing our HEA results to pure tantalum simulations, we show that the\ncritical stress for dislocation nucleation and the resulting dislocation\ndensities are much lower than for pure Ta, as expected from lower energy\nbarriers due to chemical complexity", "category": "cond-mat_mtrl-sci" }, { "text": "Color centers in NaCl by hybrid functionals: We present in this work the electronic structure and transition energies\n(both thermodynamic and optical) of Cl vacancies in NaCl by hybrid density\nfunctionals. The underestimated transition energies by the semi-local\nfunctional inherited from the band gap problem are recovered by the PBE0 hybrid\nfunctional through the non-local exact exchange, whose amount is adjusted to\nreproduce the experimental band gap. The hybrid functional also gives a better\naccount of the lattice relaxation for the defect systems arising from the\nreduced self-interaction. On the other hand, the quantitative agreement with\nexperimental vertical transition energy cannot be achieved with hybrid\nfunctionals due to the inaccurate descriptions of the ionization energies of\nthe localized defect and the positions of the band edges.", "category": "cond-mat_mtrl-sci" }, { "text": "Time-dependent phase quantification and local structure analysis of\n hydroxide-activated slag via X-ray total scattering and molecular modeling: Here, an approach to quantify the amorphous-to-disordered/crystalline\ntransformation occurring in NaOH-activated ground granulated blast-furnace slag\n(GGBS) is outlined that combines atomistic modeling with in situ pair\ndistribution function (PDF) analysis. Firstly, by using force-field molecular\ndynamics (MD) simulations, a detailed structural representation is generated\nfor the amorphous GGBS that is in agreement with experimental X-ray scattering\ndata. Use of this structural representation along with literature-derived\nstructures for the reaction products allows for real space X-ray PDF refinement\nof the alkaline activation of GGBS, resulting in the quantification of all\nphases and the degree of reaction (DOR) as a function of reaction time. All\nphases and the DOR are seen to approximately follow a logarithmic-type\ntime-dependent behavior up to 5 months, while at early age (up to 11 hours) the\nDOR is accurately captured by a modified pseudo-single step first-order\nreaction model. Lastly, the evolution of DOR is found to agree with several\nother complementary in situ data containing quantitative reaction information,\nincluding isothermal conduction calorimetry, Fourier transform infrared\nspectroscopy, and quasi-elastic neutron scattering.", "category": "cond-mat_mtrl-sci" }, { "text": "Anomalous Strength Characteristics of Tilt Grain Boundaries in Graphene: Using molecular dynamics simulations and first principles calculations, we\nhave studied the structure and mechanical strength of tilt grain boundaries in\ngraphene sheets that arise during CVD growth of graphene on metal substrates.\nSurprisingly, we find that for tilt boundaries in the vicinity of both the\nzig-zag and arm-chair orientations, large angle boundaries with a higher\ndensity of 5-7 defect pairs are stronger than the low-angle boundaries which\nare comprised of fewer defects per unit length. Interestingly, the trends in\nour results cannot be explained by a continuum Griffith-type fracture mechanics\ncriterion, which predicts the opposite trend due to that fact that it does not\naccount for the critical bonds that are responsible for the failure mechanism.\nWe have identified the highly-strained bonds in the 7-member rings that lead to\nthe failure of the sheets, and we have found that large angle boundaries are\nable to better accommodate the strained 7-rings. Our results provide guidelines\nfor designing growth methods to obtain grain boundary structures that can have\nstrengths close to that of pristine graphene.", "category": "cond-mat_mtrl-sci" }, { "text": "Ytterbium divalency and lattice disorder in near-zero thermal expansion\n YbGaGe: While near-zero thermal expansion (NZTE) in YbGaGe is sensitive to\nstoichiometry and defect concentration, the NZTE mechanism remains elusive. We\npresent x-ray absorption spectra that show unequivocally that Yb is nearly\ndivalent in YbGaGe and the valence does not change with temperature or with\nnominally 1% B or 5% C impurities, ruling out a valence-fluctuation mechanism.\nMoreover, substantial changes occur in the local structure around Yb with B and\nC inclusion. Together with inelastic neutron scattering measurements, these\ndata indicate a strong tendency for the lattice to disorder, providing a\npossible explanation for NZTE in YbGaGe.", "category": "cond-mat_mtrl-sci" }, { "text": "Probing quantum geometry through optical conductivity and magnetic\n circular dichroism: Probing ground-state quantum geometry and topology through optical response\nis not only of fundamental interest, but it can also offer several practical\nadvantages. Here, using first-principles calculations on antiferromagnetic\ntopological insulator MnBi$_2$Te$_4$ thin films, we demonstrate how the\ngeneralized optical weight arising from the absorptive part of the optical\nconductivity can be used to probe the ground state quantum geometry and\ntopology. We show that three septuple layers MnBi$_2$Te$_4$ exhibit an enhanced\nalmost perfect magnetic circular dichroism for a narrow photon energy window in\nthe infrared region. We calculate the quantum weight in a few septuple layers\nMnBi$_2$Te$_4$ and show that it far exceeds the lower bound provided by the\nChern number. Our results suggest that the well-known optical methods are\npowerful tools for probing the ground state quantum geometry and topology.", "category": "cond-mat_mtrl-sci" }, { "text": "Design of efficient vdW thermionic heterostructures from first\n principles: This work is the first step towards understanding thermionic transport\nproperties of graphene/phosphorene/graphene van der Waals heterostructures in\ncontact with gold electrodes by using density functional theory based first\nprinciples calculations combined with real space Green's function formalism. We\nshow that for monolayer phosphorene in the heterostructure, quantum tunneling\ndominates the transport. By adding more phosphorene layers, one can switch from\ntunneling dominated transport to thermionic dominated transport, resulting in\ntransporting more heat per charge carrier, thus, enhancing the cooling\ncoefficient of performance. The thermionic coefficient of performance for the\nproposed device is 18.5 at 600 K corresponding to an equivalent ZT of 0.13,\nwhich is significant for nanoscale devices.", "category": "cond-mat_mtrl-sci" }, { "text": "Diffusion-controlled growth and microstructural evolution of aluminide\n coatings: The diffusion-controlled growth and microstructural evolution at the\ninterface of aluminide coatings and different substrates such as Ni-base\nsuperalloys and steel are reviewed. Quantitative diffusion analysis indicates\nthat the diffusion rates of components in the beta-NiAl phase increases with\nthe addition of Pt. This directly reflects on the growth rate of the\ninterdiffusion zone. The thickness and formation of precipitates between the\nbond coat and the superalloys increase significantly with the Pt addition.\nMainly Fe2Al5 phase grows during hot dip aluminization of steel along with few\nother phases with very thin layer. Chemical vapor deposition process is being\nestablished for a better control of the composition of the Fe-aluminide coating\non steel.", "category": "cond-mat_mtrl-sci" }, { "text": "Diffuse Neutron Scattering Study of a Disordered Complex Perovskite\n Pb(Zn1/3Nb2/3)O3 Crystal: Diffuse scattering around the (110) reciprocal lattice point has been\ninvestigated by elastic neutron scattering in the paraelectric and the relaxor\nphases of the disordered complex perovskite crystal-Pb(Zn1/3Nb2/3)O3(PZN). The\nappearance of a diffuse intensity peak indicates the formation of polar\nnanoregions at temperature T*, approximately 40K above Tc=413K. The analysis of\nthis diffuse scattering indicates that these regions are in the shape of\nellipsoids, more extended in the <111> direction than in the <001> direction.\nThe quantitative analysis provides an estimate of the correlation length, \\xi,\nor size of the regions and shows that \\xi <111>~1.2\\xi < 001>, consistent with\nthe primary or dominant displacement of Pb leading to the low temperature\nrhombohedral phase. Both the appearance of the polar regions at T*and the\nstructural transition at Tc are marked by kinks in the \\xi < 111> curve but not\nin the \\xi < 001> one, also indicating that the primary changes take place in a\n<111> direction at both temperatures.", "category": "cond-mat_mtrl-sci" }, { "text": "Perfect Spin-filtering and Giant Magnetoresistance with Fe-terminated\n Graphene Nanoribbon: Spin-dependent electronic transport properties of Fe-terminated zig-zag\ngraphene nanoribbons (zGNR) have been studied using first-principles transport\nsimulations. The spin configuration of proposed zGNR junction can be controlled\nwith external magnetic field, and the tunneling junction show MR>1000 at small\nbias and is a perfect spin-filter by applying uniform external magnetic filed\nat small bias.", "category": "cond-mat_mtrl-sci" }, { "text": "Noncovalent functionalization of carbon nanotubes and graphene with\n tetraphenylporphyrins: Stability and optical properties from ab-initio\n calculations: The stability, electronic and optical properties of single-walled carbon\nnanotubes (CNTs) and graphene noncovalently functionalized with free-base\ntetraphenylporphyrin (TPP) molecules is addressed by density functional theory\ncalculations, including corrections to dispersive interactions. We study the\nTPP physisorption on 42 CNT species, particularly those with chiral indices\n($n$,$m$), where $5 \\leq n \\leq 12$ and $0\\leq m\\leq n$. Our results show a\nquite strong $\\pi$-$\\pi$ interaction between TPP and the CNT surface, with\nbinding energies ranging from 1.1 to 1.8 eV, where higher energies can be\nassociated with increasing CNT diameters. We also find that the TPP optical\nabsorptions would not be affected by the CNT diameter or chirality. Results for\nthe TPP physisorption on graphene show a remarkable stability with binding\nenergy of 3.2 eV, inducing a small redshift on the $\\pi$-stacked TPP absorption\nbands. The strong graphene-TPP interaction also induces a charge transfer from\nTPP to graphene, indicating a $n$-type doping mechanism without compromising\nthe graphene structure.", "category": "cond-mat_mtrl-sci" }, { "text": "Scanning Tunneling Microscopy of Defect States in the Semiconductor\n Bi$_2$Se$_3$: Scanning tunneling spectroscopy images of Bi$_2$Se$_3$ doped with excess Bi\nreveal electronic defect states with a striking shape resembling clover leaves.\nWith a simple tight-binding model we show that the geometry of the defect\nstates in Bi$_2$Se$_3$ can be directly related to the position of the\noriginating impurities. Only the Bi defects at the Se sites five atomic layers\nbelow the surface are experimentally observed. We show that this effect can be\nexplained by the interplay of defect and surface electronic structure.", "category": "cond-mat_mtrl-sci" }, { "text": "Nonequilibrium molecular dynamics simulation of rapid directional\n solidification: We present the results of non-equilibrium molecular dynamics simulations for\nthe growth of a solid binary alloy from its liquid phase. The regime of high\npulling velocities, $V$, for which there is a progressive transition from\nsolute segregation to solute trapping, is considered. In the segregation\nregime, we recover the exponential form of the concentration profile within the\nliquid phase. Solute trapping is shown to settle in progressively as $V$ is\nincreased and our results are in good agreement with the theoretical\npredictions of Aziz [J. Appl. Phys. {\\bf 53}, 1158 (1981)]. In addition, the\nfluid advection velocity is shown to remain directly proportional to $V$, even\nat the highest velocities considered here ($V\\simeq10$ms$^{-1}$).", "category": "cond-mat_mtrl-sci" }, { "text": "A closer look at how symmetry constraints and the spin-orbit coupling\n shape the electronic structure of Bi(111): Relativistic density-functional-theory calculations of Bi(111) thin films are\nperformed to revisit their band structure and that of macroscopic samples. The\nband structure of a our 39-bilayer film ($\\sim$~15~nm) shows that (1)\n$\\sim$9-nm films are enough to describe that of Bi(111), (2) The two split\nsurface-state metallic branches along the $\\overline{\\Gamma M}$ direction do\nnot overlap with the bulk band at the zone boundary but lie within the\nA7-distortion-induced conduction-valence band gap, and (3) Neither the\nexistence of the metallic surface states nor their observed splitting is\nrelated to inversion \\emph{asymmetry}. Thus, the spin texture observed in such\nstates is not caused by the lifting of the Kramers degeneracy and their\nsplitting is not of the Rashba-type. We instead propose that (1) the large\nsplitting of the metallic branches is a $m_j=\\pm1/2$-$m_j=\\pm3/2$ splitting and\n(2) the spin texture observed for the metallic branches may only occur because\nthe almost unaltered strong covalent bonds retained by Bi(111) surface atoms\ncannot afford magnetic polarization. We emphasize that degeneracy at the\n$M$-point of the SBZ of Bi(111) -- implied by the translational symmetry of the\nsurface -- is satisfied irrespectively of the presence of inversion symmetry\ncenters. We show that the magnetic-moment discontinuity at $M$ does not exist,\nwhich also explains why the measured spin-polarization of the metallic branches\nvanishes near $M$. We induce the Rashba effect on the band structure of Bi(111)\nvia different structural/electronic perturbations to reveal the actual lifting\nof the Kramers degeneracy and find that the magnitude of the perturbation\nimposed on a film correlates with the magnitude of the splitting and the\nlocalization of the Rashba-split states.", "category": "cond-mat_mtrl-sci" }, { "text": "SAMPLE: Surface structure search enabled by coarse graining and\n statistical learning: In this publication we introduce SAMPLE, a structure search approach for\ncommensurate organic monolayers on inorganic substrates. Such monolayers often\nshow rich polymorphism with diverse molecular arrangements in differently\nshaped unit cells. Determining the different commensurate polymorphs from first\nprinciples poses a major challenge due to the large number of possible\nmolecular arrangements. To meet this challenge, SAMPLE employs coarse-grained\nmodeling in combination with Bayesian linear regression to efficiently map the\nminima of the potential energy surface. In addition, it uses ab initio\nthermodynamics to generate phase diagrams. Using the example of naphthalene on\nCu(111), we comprehensively explain the SAMPLE approach and demonstrate its\ncapabilities by comparing the predicted with the experimentally observed\npolymorphs.", "category": "cond-mat_mtrl-sci" }, { "text": "Large Longitudinal Magnetoelectric Coupling in NiFe2O4-BaTiO3 Laminates: In contrast to the Pb-based magnetoelectric laminates (MELs), we find in the\nBaTiO3 and NiFe2O4 laminates (number of layers n = 5-25) that the longitudinal\nmagnetoelectric (ME) voltage coefficient Alpha E33 becomes much larger than the\ntransverse one due to preferential alignment of magnetic moments along the\nNiFe2O4 plane. Moreover, upon decreasing each layer thickness down to 15 um, we\nrealize enhanced Alpha E33 up to 18 mV/ (cm Oe) and systematic increase of the\nME sensitivity in proportion to n to achieve the largest in the Pb-free MELs\n(400*10^-6V/Oe), thereby providing pathways for tailoring ME coupling in\nmass-produced, environment friendly laminates.", "category": "cond-mat_mtrl-sci" }, { "text": "Comments on frequency dependent ac conductivity in polymeric materials\n at low frequency regime: The AC conductivity response in a broad frequency range of disordered\nmaterials is of great interest not only for technological applications, but\nalso from a theoretical point of view. The Jonscher power exponent value, and\nits temperature dependence, is a very important parameter in dielectric data\nanalysis as well as the physical interpretation of conduction mechanisms in\ndisordered materials. In some cases the power exponent of AC conductivity has\nbeen reported to be greater than 1 at the low frequency regime. This fact seems\nto contradict the universal dynamic response. The present work focuses on the\nanalysis of dielectric spectroscopy measurements in polymeric materials, below\n100 MHz. The apparent power exponent n gets values in the range (0,1) and is\ndirectly related to the characteristics of mobile charges at shorter time\nscales, in the case of the occurrence of DC conduction and the slowest\npolarization mechanism that is due to the charge motions within sort length\nscales, in log(epsilon'')-log(frequenvy) plot. The emergence of apparent n\nvalues in the range [1,2], for a relatively narrow frequency range, may be\nattributed to an additional molecular dipolar relaxation contribution at higher\nfrequencies, in log(epsilon'')-log(frequency) plot. The appearance of apparent\nn values in the range (1,2], can be assigned to the existence of a well defined\nminimum between DC conductivity contribution and a molecular dipolar dispersion\nor between two well separated dielectric loss mechanisms, in\nlog(epsilon'')-log(frequency) plots, above the crossover frequency. In these\nlatter cases, the apparent power exponent n is merely related to the\nHavriliak-Negami equation shape parameters of the higher frequencies molecular\ndipolar relaxations.", "category": "cond-mat_mtrl-sci" }, { "text": "Helium-Implantation-Induced Lattice Strains and Defects in Tungsten\n probed by X-ray Micro-diffraction: Tungsten is the main candidate material for plasma-facing armour components\nin future fusion reactors. Bombardment with energetic fusion neutrons causes\ncollision cascade damage and defect formation. Interaction of defects with\nhelium, produced by transmutation and injected from the plasma, modifies defect\nretention and behaviour. Here we investigate the residual lattice strains\ncaused by different doses of helium-ion-implantation into tungsten and\ntungsten-rhenium alloys. Energy and depth-resolved synchrotron X-ray\nmicro-diffraction uniquely permits the measurement of lattice strain with\nsub-micron 3D spatial resolution and ~10-4 strain sensitivity. Increase of\nhelium dose from 300 appm to 3000 appm increases volumetric strain by only ~2.4\ntimes, indicating that defect retention per injected helium atom is ~3 times\nhigher at low helium doses. This suggests that defect retention is not a simple\nfunction of implanted helium dose, but strongly depends on material composition\nand presence of impurities. Conversely, analysis of W-1wt% Re alloy samples and\nof different crystal orientations shows that both the presence of rhenium, and\ncrystal orientation, have comparatively small effect on defect retention. These\ninsights are key for the design of armour components in future reactors where\nit will be essential to account for irradiation-induced dimensional change when\npredicting component lifetime and performance.", "category": "cond-mat_mtrl-sci" }, { "text": "Revealing process and material parameter effects on densification via\n phase-field studies: Sintering is an important processing step in both ceramics and metals\nprocessing. The microstructure resulting from this process determines many\nmaterials properties of interest. Hence the accurate prediction of the\nmicrostructure, depending on processing and materials parameters, is of great\nimportance. The phase-field method offers a way of predicting this\nmicrostructural evolution on a mesoscopic scale. The present paper employs this\nmethod to investigate concurrent densification and grain growth and the\ninfluence of stress on densification. Furthermore, the method is applied to\nsimulate the entire freeze-casting process chain for the first time ever by\nsimulating the freezing and sintering processes separately and passing the\nfrozen microstructure to the present sintering model.", "category": "cond-mat_mtrl-sci" }, { "text": "Substrate Effect on Excitonic Shift and Radiative Lifetime of\n Two-Dimensional Materials: Substrates have strong effects on optoelectronic properties of\ntwo-dimensional (2D) materials, which have emerged as promising platforms for\nexotic physical phenomena and outstanding applications. To reliably interpret\nexperimental results and predict such effects at 2D interfaces, theoretical\nmethods accurately describing electron correlation and electron-hole\ninteraction such as first-principles many-body perturbation theory are\nnecessary. In our previous work [Phys. Rev. B 102, 205113(2020)], we developed\nthe reciprocal-space linear interpolation method that can take into account the\neffects of substrate screening for arbitrarily lattice-mismatched interfaces at\nthe GW level of approximation. In this work, we apply this method to examine\nthe substrate effect on excitonic excitation and recombination of 2D materials\nby solving the Bethe-Salpeter equation. We predict the nonrigid shift of 1s and\n2s excitonic peaks due to substrate screening, in excellent agreements with\nexperiments. We then reveal its underlying physical mechanism through 2D\nhydrogen model and the linear relation between quasiparticle gaps and exciton\nbinding energies when varying the substrate screening. At the end, we calculate\nthe exciton radiative lifetime of monolayer hexagonal boron nitride with\nvarious substrates at zero and room temperature, as well as the one of WS2\nwhere we obtain good agreement with experimental lifetime. Our work answers\nimportant questions of substrate effects on excitonic properties of 2D\ninterfaces.", "category": "cond-mat_mtrl-sci" }, { "text": "Sol-gel synthesis and multiferroic properties of pyrochlore-free\n Pb(Fe0.5Nb0.5)O3 thin films: Lead iron niobate (PbFe0.5Nb0.5O3 - PFN) thin films were synthesized by a\nmodified sol-gel route, which offers the advantage of a rapid, simple and\nnon-toxic reaction method. Polycrystalline perovskite-structured PFN thin films\nwithout pyrochlore phases were obtained on Pt/Ti/SiO2/Si substrates after\nsintering by rapid thermal annealing at 650 {\\deg}C. TEM and AFM images\nconfirmed the excellent quality of the sintered film, while EDS spectroscopy\nrevealed the presence of oxygen vacancies near the film/substrate interface.\nElectric measurements show good dielectric properties and ferroelectric\nbehavior, characterized by typical C-V curves and well-defined P-E\nferroelectric loops at 1 kHz, with remanent polarization values of ~12 uC/cm2.\nThe polarization, however, increases with decreasing frequency, indicating the\npresence of leakage currents. I-V measurements show a significant increase in\nDC-conduction at relatively low fields (around 100 kV/cm). The films display\nferromagnetic behavior at room temperature, with magnetic remanence around 30\nemu/cm3 and a coercive field of 1 kOe. These values are significantly higher\nthan those obtained for PFN powders fabricated by the same sol-gel route, as\nwell as the magnetization values reported in the literature for epitaxial\nfilms.", "category": "cond-mat_mtrl-sci" }, { "text": "Mesoscopic tunneling in strontium titanate: Spatial correlation between atoms can generate a depletion in the energy\ndispersion of acoustic phonons. Two well known examples are rotons in\nsuperfluid helium and the Kohn anomaly in metals. Here we report on the\nobservation of a large softening of the transverse acoustic mode in quantum\nparaelectric SrTiO$_3$ by means of inelastic neutron scattering. In contrast to\nother known cases, this softening occurs at a tiny wave vector implying spatial\ncorrelation extending over a distance as long as 40 lattice parameters. We\nattribute this to the formation of mesoscopic fluctuating domains due to the\ncoupling between local strain and quantum ferroelectric fluctuations. Thus, a\nhallmark of the ground state of insulating SrTiO$_3$ is the emergence of\nhybridized optical-acoustic phonons. Mesoscopic fluctuating domains play a role\nin quantum tunneling, which impedes the emergence of a finite macroscopic\npolarisation.", "category": "cond-mat_mtrl-sci" }, { "text": "Numerical solution of the relativistic single-site scattering problem\n for the Coulomb and the Mathieu potential: For a reliable fully-relativistic Korringa-Kohn-Rostoker Green function\nmethod, an accurate solution of the underlying single-site scattering problem\nis necessary. We present an extensive discussion on numerical solutions of the\nrelated differential equations by means of standard methods for a direct\nsolution and by means of integral equations. Our implementation is tested and\nexemplarily demonstrated for a spherically symmetric treatment of a Coulomb\npotential and for a Mathieu potential to cover the full-potential\nimplementation. For the Coulomb potential we include an analytic discussion of\nthe asymptotic behaviour of irregular scattering solutions close to the origin\n($r\\ll1$).", "category": "cond-mat_mtrl-sci" }, { "text": "Domain-knowledge-aided machine learning method for properties prediction\n of soft magnetic metallic glasses: A machine learning (ML) method aided by domain knowledge was proposed to\npredict saturated magnetization (Bs) and critical diameter (Dmax) of soft\nmagnetic metallic glass (MGs). Two datasets were established based on published\nexperimental works about soft magnetic MGs. A general feature space was\nproposed and proved to be adaptive for ML model training for different\nprediction tasks. It was found that the predictive performance of ML models was\nbetter than traditional physical knowledge-based estimation methods. In\naddition, domain knowledge aided feature selection can greatly reduce the\nnumber of features without significantly reducing the prediction accuracy.\nFinally, binary classification of the critical size of soft magnetic metallic\nglass was studied.", "category": "cond-mat_mtrl-sci" }, { "text": "A surrogate model for studying random field energy release rates in 2D\n brittle fractures: This article proposes a weighted-variational model as an approximated\nsurrogate model to lessen numerical complexity and lower the execution times of\nbrittle fracture simulations. Consequently, Monte Carlo studies of brittle\nfractures become possible when energy release rates are modelled as a random\nfield. In the weighed-variational model, we propose applying a Gaussian random\nfield with a Mat\\'ern covariance function to simulate a non-homogeneous energy\nrelease rate ($G_c$) of a material. Numerical solutions to the\nweighed-variational model, along with the more standard but computationally\ndemanding hybrid phase-field models, are obtained using the FEniCS open-source\nsoftware. The results have indicated that the weighted-variational model is a\ncompetitive surrogate model of the hybrid phase-field method to mimic brittle\nfractures in real structures. This method reduces execution times by 90\\%. We\nconducted a similar study and compared our results with an actual brittle\nfracture laboratory experiment. We present an example where a Monte Carlo study\nis carried out, modeling $G_c$ as a Gaussian Process, obtaining a distribution\nof possible fractures, and load-displacement curves.", "category": "cond-mat_mtrl-sci" }, { "text": "Spin Polarized and Valley Helical Edge Modes in Graphene Nanoribbons: Inspired by recent progress in fabricating precisely zigzag-edged graphene\nnanoribbons and the observation of edge magnetism, we find that spin polarized\nedge modes with well-defined valley index can exist in a bulk energy gap opened\nby a staggered sublattice potential such as that provided by a hexagonal\nBoron-Nitride substrate. Our result is obtained by both tight-binding model and\nfirst principles calculations. These edge modes are helical with respect to the\nvalley degree of freedom, and are robust against scattering, as long as the\ndisorder potential is smooth over atomic scale, resulting from the protection\nof the large momentum separation of the valleys.", "category": "cond-mat_mtrl-sci" }, { "text": "Energy storage properties of ferroelectric nanocomposites: An atomistic effective Hamiltonian technique is used to investigate the\nfinite-temperature energy storage properties of a ferroelectric nanocomposite\nconsisting of an array of BaTiO$_{3}$ nanowires embedded in a SrTiO$_{3}$\nmatrix, for electric field applied along the long axis of the nanowires. We\nfind that the energy density \\textit{versus} temperature curve adopts a\nnonlinear, mostly temperature-independent response when the system exhibits\nphases possessing an out-of-plane polarization and vortices while the energy\ndensity more linearly increases with temperature when the nanocomposite either\nonly possesses vortices (and thus no spontaneous polarization) or is in a\nparaelectric and paratoroidic phase for its equilibrium state. Ultrahigh energy\ndensity up to $\\simeq$140 J/cm$^{3}$ and an ideal 100% efficiency are also\npredicted in this nanocomposite. A phenomenological model, involving a coupling\nbetween polarization and toroidal moment, is further proposed to interpret\nthese energy density results.", "category": "cond-mat_mtrl-sci" }, { "text": "Violation of the `Zero-Force Theorem' in the time-dependent\n Krieger-Li-Iafrate approximation: We demonstrate that the time-dependent Krieger-Li-Iafrate approximation in\ncombination with the exchange-only functional violates the `Zero-Force\nTheorem'. By analyzing the time-dependent dipole moment of Na5 and Na9+, we\nfurthermore show that this can lead to an unphysical self-excitation of the\nsystem depending on the system properties and the excitation strength.\nAnalytical aspects, especially the connection between the `Zero-Force Theorem'\nand the `Generalized-Translation Invariance' of the potential, are discussed.", "category": "cond-mat_mtrl-sci" }, { "text": "The Interplay Between Imprint, Wake-Up Like Effects and Domains in\n Ferroelectric AlScN: This paper investigates wake-up and imprint in ferroelectric AlScN films. The\nstudy employs a series of I-V and P-E measurements with varying electric field\namplitudes and voltage cycles as well as structural investigation via Scanning\nElectron Microscopy to understand the origin and underlying principle of\nwake-up and imprint as well as their relation. It is shown that the material\ncan be considered wake-up free, however inherent imprint and imprint shift in\ncombination with minor loops result in a wake-up like effect. We introduce a\nproposition to explain the influence of initial switching cycles on domains,\ntheir stabilization and corresponding changes in imprint. Unipolar fields and\ntemperature investigations are used to explore the reversibility of imprint and\nways to program it, while partial switching is applied to investigate domain\npropagation and support the aforementioned approach. It is concluded, that\nafter an energetically more demanding domain nucleation, domain wall motion can\nswitch the majority of polarization in Al1-xScxN. As a consequence, the\npresence of initial domains reduces the coercive field in respect to unipolar\nfilms.", "category": "cond-mat_mtrl-sci" }, { "text": "Tunneling magnetoresistance in (La,Pr,Ca)MnO3 nanobridges: The manganite (La,Pr,Ca)MnO3 is well known for its micrometer scale phase\nseparation into coexisting ferromagnetic metallic and antiferromagnetic\ninsulating (AFI) regions. Fabricating bridges with widths smaller than the\nphase separation length scale has allowed us to probe the magnetic properties\nof individual phase separated regions. We observe tunneling magnetoresistance\nacross naturally occurring AFI tunnel barriers separating adjacent\nferromagnetic regions spanning the width of the bridges. Further, near the\nCurie temperature, a magnetic field induced metal-to-insulator transition among\na discrete number of regions within the narrow bridges gives rise to abrupt and\ncolossal low-field magnetoresistance steps at well defined switching fields.", "category": "cond-mat_mtrl-sci" }, { "text": "Evidence of orbit-selective electronic kagome lattice with planar\n flat-band in correlated paramagnetic YCr6Ge6: Electronic properties of kagome lattice have drawn great attention recently.\nIn associate with flat-band induced by destructive interference and Dirac\ncone-type dispersion, abundant exotic phenomena have been theoretically\ndiscussed. The material realization of electronic kagome lattice is a crucial\nstep towards comprehending kagome physics and achieving novel quantum phases.\nHere, combining angle-resolved photoemission spectroscopy, transport\nmeasurements and first-principle calculations, we expose a planar flat-band in\nparamagnetic YCr6Ge6 as a typical signature of electronic kagome lattice. We\nunearth that the planar flat-band arises from the d_(z^2 ) electrons with\nintra-kagome-plane hopping forbidden by destructive interference. On the other\nhand, the destructive interference and flatness of the d_(x^2-y^2 ) and d_xy\nbands are decomposed possibly due to additional in-plane hopping terms, but the\nDirac cone-type dispersion is reserved near chemical potential. We explicitly\nunveil that orbital character plays an essential role to realize electronic\nkagome lattice in bulk materials with transition metal kagome layers.\nParamagnetic YCr6Ge6 provides an opportunity to comprehend intrinsic properties\nof electronic kagome lattice as well as its interplays with spin orbit coupling\nand electronic correlation of Cr-3d electrons, and be free from complications\ninduced by strong local moment of ions in kagome planes.", "category": "cond-mat_mtrl-sci" }, { "text": "Novel time-saving first-principles calculation method for\n electron-transport properties: We present a time-saving simulator within the framework of the density\nfunctional theory to calculate the transport properties of electrons through\nnanostructures suspended between semi-infinite electrodes. By introducing the\nFourier transform and preconditioning conjugate-gradient algorithms into the\nsimulator, a highly efficient performance can be achieved in determining\nscattering wave functions and electron-transport properties of nanostructures\nsuspended between semi-infinite jellium electrodes. To demonstrate the\nperformance of the present algorithms, we study the conductance of metallic\nnanowires and the origin of the oscillatory behavior in the conductance of an\nIr nanowire. It is confirmed that the $s$-$d_{z^2}$ channel of the Ir nanowire\nexhibits the transmission oscillation with a period of two-atom length, which\nis also dominant in the experimentally obtained conductance trace.", "category": "cond-mat_mtrl-sci" }, { "text": "Flexibility of Fluorinated Graphene-Based Materials: The resistivity of different films and structures containing fluorinated\ngraphene (FG) flakes and chemical vapor deposition (CVD) grown graphene of\nvarious fluorination degrees under tensile and compressive strains due to\nbending deformations was studied. Graphene and multilayer graphene films grown\nby means of the CVD method were transferred onto the flexible substrate by\nlaminating and were subjected to fluorination. They demonstrated a weak\nfluorination degree (F/C lower 20%). Compressive strains led to a strong\n(one-two orders of magnitude) decrease in the resistivity in both cases, which\nwas most likely connected with the formation of additional conductive paths\nthrough fluorinated graphene. Tensile strain up to 3% caused by the bending of\nboth types of CVD-grown FG led to a constant value of the resistivity or to an\nirreversible increase in the resistivity under repeated strain cycles. In the\ncase of the structures with the FG thin film printed on polyvinyl alcohol, a\nstable bipolar resistive switching was observed up to 6.5% of the tensile\nstrain (bending radius was 2 mm). The excellent performance of the crossbar\nmemristor structures under tensile strain shows that the FG films and\nstructures created from suspension are especially promising for flexible\nelectronics.", "category": "cond-mat_mtrl-sci" }, { "text": "Synthetic control over polymorph formation in the d-band semiconductor\n system FeS$_2$: Pyrite, also known as fool's gold is the thermodynamic stable polymorph of\nFeS$_2$. It is widely considered as a promising d-band semiconductor for\nvarious applications due to its intriguing physical properties. Marcasite is\nthe other naturally occurring polymorph of FeS$_2$. Measurements on natural\ncrystals have shown that it has similarly promising electronic, mechanical, and\noptical properties as pyrite. However, it has been only scarcely investigated\nso far, because the laboratory-based synthesis of phase-pure samples or\nhigh-quality marcasite single crystal has been a challenge until now. Here, we\nreport the targeted phase formation via hydrothermal synthesis of marcasite and\npyrite. The formation condition and phase purity of the FeS$_2$ polymorphs are\nsystematically studied in the form of a comprehensive synthesis map. We,\nfurthermore, report on a detailed analysis of marcasite single crystal growth\nby a space-separated hydrothermal synthesis. We observe that single phase\nproduct of marcasite forms only on the surface under the involvement of H$_2$S\nand sulphur vapor. The availability of high-quality crystals of marcasite\nallows us to measure the fundamental physical properties, including an allowed\ndirect optical bandgap of 0.76 eV, temperature independent diamagnetism, an\nelectronic transport gap of 0.11 eV, and a room-temperature carrier\nconcentration of 4.14 $\\times$ 10$^{18}$ cm$^{-3}$. X-ray absorption/emission\nspectroscopy are employed to measure the band gap of the two FeS$_2$ phases. We\nfind marcasite has a band gap of 0.73 eV, while pyrite has a band gap of 0.87\neV. Our results indicate that marcasite -- that is now synthetically available\nin a straightforward fashion -- is as equally promising as pyrite as candidate\nfor various semiconductor applications based on earth abundant elements.", "category": "cond-mat_mtrl-sci" }, { "text": "Physical properties of KMgBi single crystals: KMgBi single crystals are grown by using the Bi flux successfully. KMgBi\nshows semiconducting behavior with a metal-semiconductor transition at high\ntemperature region and a resistivity plateau at low temperature region,\nsuggesting KMgBi could be a topological insulator with a very small band gap.\nMoreover, KMgBi exhibits multiband feature with strong temperature dependence\nof carrier concentrations and mobilities.", "category": "cond-mat_mtrl-sci" }, { "text": "A computational study of the configurational and vibrational\n contributions to the thermodynamics of substitutional alloys: the Ni3Al case: We have developed a methodology to study the thermodynamics of order-disorder\ntransformations in n-component substitutional alloys that combines\nnonequilibrium methods, which can efficiently compute free energies, with Monte\nCarlo simulations, in which configurational and vibrational degrees of freedom\nare simultaneously considered on an equal footing basis. Furthermore, by\nappropriately constraining the system, we were able to compute the\ncontributions to the vibrational entropy due to bond proportion, atomic size\nmismatch, and bulk volume effects. We have applied this methodology to\ncalculate configurational and vibrational contributions to the entropy of the\nNi3Al alloy as functions of temperature. We found that the bond proportion\neffect reduces the vibrational entropy at the order-disorder transition, while\nthe size mismatch and the bond proportion effects combined do not change the\nvibrational entropy at the transition. We also found that the volume increase\nat the order-disorder transition causes a vibrational entropy increase of 0.08\nkB/atom, which is significant when compared to the configurational entropy\nincrease of 0.27 kB/atom. Our calculations indicate that the inclusion of\nvibrations reduces in about 30 percent the order-disorder transition\ntemperature determined solely considering the configurational degrees of\nfreedom.", "category": "cond-mat_mtrl-sci" }, { "text": "Magneto-transport characteristics of La1.4Ca1.6Mn2O7 thin film deposited\n by spray pyrolysis: Polycrystalline thin films of double layer manganite La_1.4Ca_1.6Mn_2O_7\n(DLCMO) have been deposited by nebulized spray pyrolysis on single crystal\nLaAlO_3 substrates. These single phase films having grain size in the range\n70-100 nm exhibit ferromagnetic transition at T_C ~ 107K. The short range\nferromagnetic ordering due to in plane spin coherence is evidenced to occur at\na higher temperature around 225 K. Insulator/semiconductor to metal transition\noccurs at a lower temperature T_P ~ 55K. The transport mechanism above T_C is\nof Mott`s variable range hopping type. Below T_C the current-voltage\ncharacteristics show non-linear behaviour that becomes stronger with decreasing\ntemperature. At low temperatures below T_CA ~ 30K a magnetically frustrated\nspin canted state is observed. The DLCMO films exhibit resonable low field\nmagnetoresistance and at 77K the magnetoresistance ratio is ~ 5% at 0.6 kOe and\n\\~ 13% at 3 kOe.", "category": "cond-mat_mtrl-sci" }, { "text": "Defect-induced $4p$-magnetism in layered platinum diselenide: Platinum diselenide (PtSe$_{2}$) is a recently-discovered extrinsic magnet,\nwith its magnetism attributed to the presence of Pt-vacancies. The host\nmaterial to these defects itself displays interesting structural and electronic\nproperties, some of which stem from an unusually strong interaction between its\nlayers. To date, it is not clear how the unique intrinsic properties of\nPtSe$_2$ will affect its induced magnetism. In this theoretical work, we show\nthat the defect-induced magnetism in PtSe$_{2}$ thin films is highly sensitive\nto: (i) the layer-thickness (ii) defect density, and (iii) substrate choice.\nThese different factors dramatically modify all magnetic properties, including\nthe magnitude of local moments, strength of the coupling, and even nature of\nthe coupling between the moments. We further show that the strong inter-layer\ninteractions are key to understanding these effects. A better understanding of\nthe various influences on magnetism, can enable controllable tuning of the\nmagnetic properties in Pt-based dichalcogenides, which can be used to design\nnovel devices for magnetoelectric and magneto-optic applications.", "category": "cond-mat_mtrl-sci" }, { "text": "Extension of the standard Heisenberg Hamiltonian to multispin exchange\n interactions: An extension of the Heisenberg Hamiltonian is discussed, that allows to go\nbeyond the standard bilinear spin Hamiltonian taking into account various\ncontributions due to multispin interactions having both chiral and non-chiral\ncharacter. The parameters of the extended Hamiltonian are calculated from first\nprinciples within the framework of the multiple scattering Green function\nformalism giving access to an explicit representation of these parameters in\nreal space. The discussions are focused on the chiral interactions, i.e.\\\nbiquadratic and three-spin Dzyaloshinskii-Moriya like vector interactions\n$\\vec{\\cal{D}}_{ijij}$ (BDMI) and $\\vec{\\cal{D}}_{ijkj}$ (TDMI), respectively,\nas well as the three-spin chiral interaction (TCI) $J_{ijk}$. Although all\nparameters are driven by spin-orbit coupling (SOC), some differences in their\nproperties are demonstrated by calculations for real materials. In particular\nit is shown that the three-spin chiral interactions $J_{ijk}$ may be topology\nas well as SOC induced, while the TDMI is associated only with the SOC. As the\nmagnitude of the chiral interactions can be quite sizable, they can lead to a\nstabilization of a noncollinear magnetic texture in some materials that is\nabsent when these interactions are neglected.", "category": "cond-mat_mtrl-sci" }, { "text": "Signatures of nonlinear magnetoelectricity in second harmonic spectra of\n SU(2) symmetry broken quantum many-body systems: Quantum mechanical perturbative expressions for second order dynamical\nmagnetoelectric (ME) susceptibilities have been derived and calculated for a\nsmall molecular system using the Hubbard Hamiltonian with SU(2) symmetry\nbreaking in the form of spin-orbit coupling (SOC) or spin-phonon coupling.\nThese susceptibilities will have signatures in second harmonic generation\nspectra. We show that SU(2) symmetry breaking is the key to generate these\nsusceptibilities. We have calculated these ME coefficients by solving the\nHamiltonian for low lying excited states using Lanczos method. Varying the\nHubbard term along with SOC strength, we find spin and charge and both\nspin-charge dominated spectra of dynamical ME coefficients. We have shown that\nintensities of the peaks in the spectra are highest when the magnitudes of\nHubbard term and SOC coupling term are in similar range.", "category": "cond-mat_mtrl-sci" }, { "text": "Ultra fast bit addressing in a magnetic memory matrix with crossed wire\n write line geometry: An ultra fast bit addressing scheme for magnetic random access memories\n(MRAM) in a crossed wire geometry is proposed. In the addressing scheme a word\nof cells is programmed simultaneously by sub nanosecond field pulses making use\nof the magnetization precession of the free layer. Single spin simulations of\nthe free layer dynamics show that the pulse parameters for programming an\narbitrary word of the array can be chosen such that the magnetization of the\ncells to be written performs either a half or a full precessional turn during\napplication of the programming pulse depending on the initial and final\nmagnetization orientation of the addressed cells. Such bit addressing scheme\nleads to a suppression of the magnetization ringing in all cells of the memory\narray thereby allowing ultra high MRAM write clock rates above 1 GHz.", "category": "cond-mat_mtrl-sci" }, { "text": "A first-principles calculation of electron-phonon interactions for the\n $\\text{C}_2\\text{C}_\\text{N}$ and $\\text{V}_\\text{N}\\text{N}_\\text{B}$\n defects in hexagonal boron nitride: Quantum emitters in two-dimensional hexagonal boron nitride (h-BN) have\ngenerated significant interest due to observations of ultra-bright emission\nmade at room temperature. The expectation that solid-state emitters exhibit\nbroad zero-phonon lines at elevated temperatures has been put in question by\nrecent observations of Fourier transform (FT) limited photons emitted from h-BN\nflakes at room temperature. The mechanism responsible for the narrow lines has\nbeen suggested to be a mechanical decoupling from in-plane phonons due to an\nout-of-plane distortion of the emitter's orbitals. All decoupled emitters\nproduce photons that are directed in-plane, suggesting that the dipoles are\noriented perpendicular to the h-BN plane. Motivated by the promise of an\nefficient and scalable source of indistinguishable photons that can operate at\nroom temperature, we have developed an approach using density functional theory\n(DFT) to determine the electron-phonon coupling for defects that have in- and\nout-of-plane transition dipole moments. Our DFT calculations reveal that the\n$\\text{C}_2 \\text{C}_\\text{N}$ defect has an in-plane transition dipole moment,\nand that of the $\\text{V}_\\text{N} \\text{N}_\\text{B}$ defect is perpendicular\nto the plane. We exploit the two-dimensional framework recently implemented in\n\\texttt{QUANTUM ESPRESSO} to determine both the phonon density of states and\nthe electron-phonon matrix elements associated with the h-BN defective\nstructures. We find no indication that an out-of-plane transition dipole is\nsufficient to obtain FT-limited photons at room temperature. Our work also\nprovides direction to future DFT software developments and adds to the growing\nlist of calculations relevant to researchers in the field of solid-state\nquantum information processing.", "category": "cond-mat_mtrl-sci" }, { "text": "First-principles study of the structural energetics of PdTi and PtTi: The structural energetics of PdTi and PtTi have been studied using\nfirst-principles density-functional theory with pseudopotentials and a\nplane-wave basis. We predict that in both materials, the experimentally\nreported orthorhombic $B19$ phase will undergo a low-temperature phase\ntransition to a monoclinic $B19'$ ground state. Within a soft-mode framework,\nwe relate the $B19$ structure to the cubic $B2$ structure, observed at high\ntemperature, and the $B19'$ structure to $B19$ via phonon modes strongly\ncoupled to strain. In contrast to NiTi, the $B19$ structure is extremely close\nto hcp. We draw on the analogy to the bcc-hcp transition to suggest likely\ntransition mechanisms in the present case.", "category": "cond-mat_mtrl-sci" }, { "text": "Structural and magnetic properties of half-heusler alloys NiCrZ (Z = Si,\n P, Ge, As, Te): First principle study: We present a first principle study of new class of high-$T_c$ half-heusler\nferromagnets NiCrZ (Z = Si, P, Ge, As, Te). The structure and magnetic\nproperties are investigated through the calculation of the electronic\nstructure, equilibrium lattice constant, magnetic exchange interaction $J_{ij}$\nand Curie temperature $T_c$. The role of $sp$-elements and the influence of\nlattice expansion/compression are also studied. In alloys having 20 valence\nelectrons, a pseudo-gap of the majority band can be formed at Fermi level.\nOtherwise, the half-metallicity and ferromagnetism at temperatures much higher\nthan room temperature are found to be stable in a wide range of lattice\nexpansion. Based on these results, NiCrZ can be expected to be promising\nmaterials for spintronics.", "category": "cond-mat_mtrl-sci" }, { "text": "An Analytical Model to Quantify the Local Lattice Distortion of\n Refractory High Entropy Alloys: Local lattice distortion (LLD) of high entropy alloys (HEAs) especially\nrefractory HEAs, which is different from one lattice site to another,\ndetermines the mechanical properties of HEAs such as yield strength and\nradiation resistance, and is crucial to modulating catalytic activity of HEAs\nvia the atomic strain. In particular, this site-to-site LLD is strongly coupled\nwith the short-range order (SRO) of HEAs. Therefore it is essential to reveal\nthe physical picture of LLD. However, the random distribution of\nmulti-principal constituents of HEAs prohibits the understanding of LLD,\nincluding the determinants of LLD and their coupling rules. Herein, we build\nthe first analytical model to realize the site-to-site prediction of LLD in\nrefractory HEAs, by using the neighbor number ratio of central atoms, the\ncentral-atom radii, the standard deviation of constituent radii and the\nconstituent number, which demonstrates that LLD surprisingly exhibits a similar\nmechanism as the relaxation of metal surfaces. The involved parameters depend\nonly on the radii of constituents and are readily accessible. Moreover, our\nscheme determines not only LLD but also the average lattice distortion, which\nenables us to predict the phase stability and yield strength of HEAs. These\nresults build a novel physical picture of LLD, in particular the quantitative\nrelationship between LLD and SRO, which lay a solid foundation for the further\ntarget-oriented design of HEAs.", "category": "cond-mat_mtrl-sci" }, { "text": "Comprehensive search for buckled honeycomb binary compounds based on\n noble metals (Cu, Ag, and Au): Honeycomb structure has been frequently observed in two-dimensional (2D)\nmaterials. CuAu in the buckled honeycomb (BHC) structure has been synthesized\nrecently, which is the first case of 2D intermetallic compounds. Here, the\ndynamical stability of 2D $AX$ in the BHC structure, where $A=$ Cu, Ag, and Au\nand $X$ is a metallic element in the periodic table, is systematically studied\nby calculating phonon dispersions from first-principles. Among 135 $AX$, more\nthan 50 $AX$ are identified to be dynamically stable. In addition, (i) a\nrelationship between the dynamical stability and the formation energy, (ii) a\ncorrelation of dynamical stability between different constituents $A$, (iii) a\ntrend of lattice parameters, and (iv) electronic and magnetic properties are\ndiscussed. Furthermore, a stable phase of B11-type AuZr is predicted based on\nboth the result (ii) and the stability relationship between 2D and\nthree-dimensional structures. The present findings stimulate future studies\nexploring physics and chemistry of 2D intermetallic compounds.", "category": "cond-mat_mtrl-sci" }, { "text": "Electric Field-Dependent Charge-Carrier Velocity in Semiconducting\n Carbon Nanotubes: Charge transport in semiconducting single-walled nanotubes (SWNTs) with\nSchottky-barrier contacts has been studied at high bias. We observe nearly\nsymmetric ambipolar transport with electron and hole currents significantly\nexceeding 25 micron-ampere, the reported current limit in metallic SWNTs due to\noptical phonon emission. Four simple models for the field-dependent velocity\n(ballistic, current saturation, velocity saturation, and constant mobility) are\nstudied in the unipolar regime; the high-bias behavior is best explained by a\nvelocity saturation model with a saturation velocity of 2 x 10^7 cm/s.", "category": "cond-mat_mtrl-sci" }, { "text": "Fermi Surface Deformation near Charge-Ordering Transition: We study the deformation of a Fermi surface (FS) near charge-ordering (CO)\ntransition. By applying a fluctuation-exchange approximation to the\ntwo-dimensional extended Hubbard model, we show that the FS is largely modified\nby strong charge fluctuations when the wave number of the CO pattern does not\nmatch the nesting vector of the FS in a noninteracting system. We also discuss\nthe enhanced anisotropy in quasiparticle properties in the resultant metallic\nstate.", "category": "cond-mat_mtrl-sci" }, { "text": "Microscopic Property of Amorphous Semiconductor Metal Oxide\n InGaZnO$_{4}$ and Role of O-deficiency: We investigated the microscopic and electronic structures amorphous oxide\nsemiconductors InGaZnO$_{4}$ (a-IGZO) and the role of O-deficiency through the\nfirst-principle calculations. The structure of the amorphous oxide is\ncomplicated by the admixture of many different kinds of substructures, however\nit is surprisingly found that the band tail states, which are well-known to be\npresent in the amorphous semiconductors, are few generated for the conduction\nband minimum (CBM). The electronic structure around CBM is little affected by\nthe disorder and also by the O-deficiency. Free electron carriers can be\ngenerated without a creation of donor-level in the O-deficient amorphous oxide.", "category": "cond-mat_mtrl-sci" }, { "text": "Thermal expansion coefficient and lattice anharmonicity of cubic boron\n arsenide: Recent measurements of an unusual high thermal conductivity of around 1000 W\nm-1 K-1 at room temperature in cubic boron arsenide (BAs) confirm predictions\nfrom theory and suggest potential applications of this semiconductor compound\nfor thermal management applications. Knowledge of the thermal expansion\ncoefficient and Gr\\\"uneisen parameter of a material contributes both to the\nfundamental understanding of its lattice anharmonicity and to assessing its\nutility as a thermal-management material. However, previous theoretical\ncalculations of the thermal expansion coefficient and Gr\\\"uneisen parameter of\nBAs yield inconsistent results. Here we report the linear thermal expansion\ncoefficient of BAs obtained from the X-ray diffraction measurements from 300 K\nto 773 K. The measurement results are in good agreement with our ab initio\ncalculations that account for atomic interactions up to fifth nearest\nneighbours. With the measured thermal expansion coefficient and specific heat,\na Gr\\\"uneisen parameter of BAs of 0.84 +/- 0.09 is obtained at 300 K, in\nexcellent agreement with the value of 0.82 calculated from first principles and\nmuch lower than prior theoretical results. Our results confirm that BAs\nexhibits a better thermal expansion coefficient match with commonly used\nsemiconductors than other high-thermal conductivity materials such as diamond\nand cubic boron nitride.", "category": "cond-mat_mtrl-sci" }, { "text": "Metal-semiconductor (semimetal) superlattices on a graphite sheet with\n vacancies: It has been found that periodically closely spaced vacancies on a graphite\nsheet cause a significant rearrange-ment of its electronic spectrum: metallic\nwaveguides with a high density of states near the Fermi level are formed along\nthe vacancy lines. In the direction perpendicular to these lines, the spectrum\nexhibits a semimetal or semiconductor character with a gap where a vacancy\nminiband is degenerated into impurity levels.", "category": "cond-mat_mtrl-sci" }, { "text": "Theory of electron-plasmon coupling in semiconductors: The ability to manipulate plasmons is driving new developments in\nelectronics, optics, sensing, energy, and medicine. Despite the massive\nmomentum of experimental research in this direction, a predictive\nquantum-mechanical framework for describing electron-plasmon interactions in\nreal materials is still missing. Here, starting from a many-body Green's\nfunction approach, we develop an ab initio approach for investigating\nelectron-plasmon coupling in solids. As a first demonstration of this\nmethodology, we show that electron-plasmon scattering is the primary mechanism\nfor the cooling of hot carriers in doped silicon, it is key to explain measured\nelectron mobilities at high doping, and it leads to a quantum zero-point\nrenormalization of the band gap in agreement with experiment.", "category": "cond-mat_mtrl-sci" }, { "text": "Mesoscopic and Microscopic Phase Segregation in Manganese Perovskites: Mesoscopic (500-2000 Angstrom) and microscopic (5-20 Angstrom) phase\nsegregation with temperature and magnetic field was studied in the model\nmanganite Pr0.7Ca0.3MnO3 by high-resolution neutron diffraction and inelastic\nneutron scattering. Intra-granular strain-driven mesoscopic segregation between\ntwo insulating phases, one of which is charge ordered (CO), sets in below the\nCO temperature in zero field. The CO phase orders antiferromagnetically, while\nthe other insulating phase shows spin-glass behavior. After field-induced\nmetallization, the CO phase coexists with a ferromagnetic metallic phase.", "category": "cond-mat_mtrl-sci" }, { "text": "Ab initio calculations of the structural, electronic and elastic\n properties of the MZN2 (M=Be, Mg; Z=C, Si) chalcopyrite semiconductors: Four ternary semiconductors with the chalcopyrite structure (BeCN2, BeSiN2,\nMgCN2, and MgSiN2) were studied using the first principles methods. The\nstructural, electronic, optical and elastic properties were calculated. All\nthese materials were found to be the indirect band gap semiconductors, with the\ncalculated band gaps in the range from 3.46 eV to 3.88 eV. Comparison of the\ndegree of covalency/ionicity of the chemical bonds in these compounds was\nperformed. Anisotropy of the optical properties of these tetragonal crystals\nwas demonstrated by calculating the real and imaginary parts of the dielectric\nfunction {\\epsilon}. Anisotropy of the elastic properties of these materials\nwas analyzed by plotting the three-dimensional dependences of the Young moduli\nand their two-dimensional cross-sections. It was also shown (at least,\nqualitatively) that there exists a correlation between the optical and elastic\nanisotropy: the most optically anisotropic MgSiN2 is also most elastically\nanisotropic material in the considered group. High hardness (bulk moduli up to\n300 GPa) together with large band gaps may lead to new potential applications\nof these compounds.", "category": "cond-mat_mtrl-sci" }, { "text": "Grains and grain boundaries in highly crystalline monolayer molybdenum\n disulfide: Recent progress in large-area synthesis of monolayer molybdenum disulfide, a\nnew two-dimensional direct-bandgap semiconductor, is paving the way for\napplications in atomically thin electronics. Little is known, however, about\nthe microstructure of this material. Here we have refined chemical vapor\ndeposition synthesis to grow highly crystalline islands of monolayer molybdenum\ndisulfide up to 120 um in size with optical and electrical properties\ncomparable or superior to exfoliated samples. Using transmission electron\nmicroscopy, we correlate lattice orientation, edge morphology, and\ncrystallinity with island shape to demonstrate that triangular islands are\nsingle crystals. The crystals merge to form faceted tilt and mirror boundaries\nthat are stitched together by lines of 8- and 4- membered rings. Density\nfunctional theory reveals localized mid-gap states arising from these 8-4\ndefects. We find that mirror boundaries cause strong photoluminescence\nquenching while tilt boundaries cause strong enhancement. In contrast, the\nboundaries only slightly increase the measured in-plane electrical\nconductivity.", "category": "cond-mat_mtrl-sci" }, { "text": "A quantitative theory of current-induced step bunching on Si(111): We use a one-dimensional step model to study quantitatively the growth of\nstep bunches on Si(111) surfaces induced by a direct heating current.\nParameters in the model are fixed from experimental measurements near 900 deg C\nunder the assumption that there is local mass transport through surface\ndiffusion and that step motion is limited by the attachment rate of adatoms to\nstep edges. The direct heating current is treated as an external driving force\nacting on each adatom. Numerical calculations show both qualitative and\nquantitative agreement with experiment. A force in the step down direction will\ndestabilize the uniform step train towards step bunching. The average size of\nthe step bunches grows with electromigration time as t^beta, with beta = 0.5,\nin agreement with experiment and with an analytical treatment of the steady\nstates. The model is extended to include the effect of direct hopping of\nadatoms between different terraces. Monte-Carlo simulations of a solid-on-solid\nmodel, using physically motivated assumptions about the dynamics of surface\ndiffusion and attachment at step edges, are carried out to study two\ndimensional features that are left out of the present step model and to test\nits validity. These simulations give much better agreement with experiment than\nprevious work. We find a new step bending instability when the driving force is\nalong the step edge direction. This instability causes the formation of step\nbunches and antisteps that is similar to that observed in experiment.", "category": "cond-mat_mtrl-sci" }, { "text": "Second-harmonic phonon spectroscopy of $\u03b1$-quartz: We demonstrate midinfrared second-harmonic generation as a highly sensitive\nphonon spectroscopy technique that we exemplify using $\\alpha$-quartz (SiO$_2$)\nas a model system. A midinfrared free-electron laser provides direct access to\noptical phonon resonances ranging from $350\\ \\mathrm{cm}^{-1}$ to $1400\\\n\\mathrm{cm}^{-1}$. While the extremely wide tunability and high peak fields of\nan free-electron laser promote nonlinear spectroscopic studies---complemented\nby simultaneous linear reflectivity measurements---azimuthal scans reveal\ncrystallographic symmetry information of the sample. Additionally,\ntemperature-dependent measurements show how damping rates increase, phonon\nmodes shift spectrally and in certain cases disappear completely when\napproaching $T_c=846\\ \\mathrm{K}$ where quartz undergoes a structural phase\ntransition from trigonal $\\alpha$-quartz to hexagonal $\\beta$-quartz,\ndemonstrating the technique's potential for studies of phase transitions.", "category": "cond-mat_mtrl-sci" }, { "text": "Insight into Two-Dimensional Borophene: Five-Center Bond and\n Phonon-Mediated Superconductivity: We report a previously unknown monolayer borophene allotrope and we call it\nsuper-B with a flat structure based on the ab initio calculations. It has good\nthermal, dynamical, and mechanical stability compared with many other typical\nborophenes. We find that super-B has a fascinating chemical bond environment\nconsisting of standard sp, sp2 hybridizations, and delocalized five-center\nthree-electron $\\pi$ bond, called $\\pi$(5c-3e). This particular electronic\nstructure plays a pivotal role in stabilizing the super-B chemically. By extra\ndoping, super-B can be transformed into a Dirac material from pristine metal.\nLike graphene, it can also sustain tensile strain smaller than 24%, indicating\nsuperior flexibility. Moreover, due to the small atomic mass and large density\nof states at the Fermi level, super-B has the highest critical temperature Tc\nof 25.3 K in single-element superconductors at ambient conditions. We attribute\nthis high Tc of super-B to the giant anharmonicity of two linear acoustic\nphonon branches and an unusually low optic phonon mode. These predictions\nprovide new insight into the chemical nature of low dimensional boron\nnanostructures and highlight the potential applications of designing flexible\ndevices and high Tc superconductors.", "category": "cond-mat_mtrl-sci" }, { "text": "Density-functional approach to the band gaps of finite and periodic\n two-dimensional systems: We present an approach based on density-functional theory for the calculation\nof fundamental gaps of both finite and periodic two-dimensional (2D) electronic\nsystems. The computational cost of our approach is comparable to that of total\nenergy calculations performed via standard semi-local forms. We achieve this by\nreplacing the 2D local density approximation with a more sophisticated -- yet\ncomputationally simple -- orbital-dependent modeling of the exchange potential\nwithin the procedure by Guandalini et al. [Phys. Rev. B 99, 125140 (2019)]. We\nshowcase promising results for semiconductor 2D quantum dots and artificial\ngraphene systems, where the band structure can be tuned through, e.g., Kekul\\'e\ndistortion.", "category": "cond-mat_mtrl-sci" }, { "text": "Mechanical behavior of high-entropy alloys: A review: High-entropy alloys (HEAs) are materials that consist of equimolar or\nnear-equimolar multiple principal components but tend to form single phases,\nwhich is a new research topic in the field of metallurgy, have attracted\nextensive attention in the past decade. The HEAs families contain the\nface-centered-cubic (fcc), body-centered-cubic (bcc), and\nhexagonal-close-packed (hcp)-structured HEAs. On one hand, mechanical\nproperties, e.g. hardness, strength, ductility, fatigue, and elastic moduli,\nare essential for practical applications of HEAs. Scientists have explored in\nthis direction since the advent of HEAs. On the other hand, the pursuit of high\nstrength and good plasticity is the critical research issue of materials.\nHence, strengthening of HEAs is a crucial issue. Recently, many articles are\nfocusing on the strengthening strategies of HEAs[1-14]. In this chapter, we\nreviewed the recent work on the room-temperature elastic properties and\nmechanical behavior of HEAs, including the mechanisms behind the plastic\ndeformation of HEAs at both low and high temperatures. Furthermore, the present\nwork examined the strengthening strategies of HEAs, e.g. strain hardening,\ngrain-boundary strengthening, solid-solution strengthening, and particle\nstrengthening. The fatigue, creep, and fracture properties were briefly\nintroduced. Lastly, the future scientific issues and challenges of HEAs were\ndiscussed.", "category": "cond-mat_mtrl-sci" }, { "text": "Topological Nature of the Phonon Hall Effect: We provide a topological understanding on phonon Hall effect in dielectrics\nwith Raman spinphonon coupling. A general expression for phonon Hall\nconductivity is obtained in terms of the Berry curvature of band structures. We\nfind a nonmonotonic behavior of phonon Hall conductivity as a function of\nmagnetic field. Moreover, we observe a phase transition in phonon Hall effect,\nwhich corresponds to the sudden change of band topology, characterized by the\naltering of integer Chern numbers. This can be explained by touching and\nsplitting of phonon bands.", "category": "cond-mat_mtrl-sci" }, { "text": "Optical properties of Mn4+ ions in GaN:Mn codoped with Mg acceptors: The optical properties of Mn-Mg codoped epitaxial GaN were studied. Addition\nof Mg acceptors quenches the weak manganese-related photoluminescence (PL) band\nat 1.3 eV in GaN:Mn and a series of sharp PL peaks are observed at 1 eV in\ncodoped epilayers. The change in PL spectra indicates that Mg addition\nstabilizes the Mn4+ charge state by decreasing the Fermi level. The 1 eV PL\npeaks are tentatively attributed to intra center transitions involving Mn4+\nions. Spin allowed 3d-shell 4T2-4T1 transitions and their phonon replicas are\ninvolved. The relative intensities of the sharp peaks are strongly dependent on\nthe excitation wavelength, indicating the optically active Mn4+ centers\ninvolved in the separate peaks are different. The temperature dependence of the\nPL spectrum suggests the presence of at least three distinct Mn4+ complex\ncenters.", "category": "cond-mat_mtrl-sci" }, { "text": "Atoms, dimers, and nanoparticles from orbital-free density-potential\n functional theory: Density-potential functional theory (DPFT) is an alternative formulation of\norbital-free density functional theory that may be suitable for modeling the\nelectronic structure of large systems. To date, DPFT has been applied mainly to\nquantum gases in one- and two dimensional settings. In this work, we study the\nperformance of DPFT when applied to real-life systems: atoms, dimers, and\nnanoparticles. We build on systematic Suzuki-Trotter factorizations of the\nquantum-mechanical propagator and on the Wigner function formalism,\nrespectively, to derive nonlocal as well as semilocal functional approximations\nin complete analogy to their well-established lower-dimensional versions --\nwithout resorting to system-specific approximations or ad-hoc measures of any\nkind. The cost for computing the associated semiclassical ground-state\nsingle-particle density scales (quasi-)linearly with particle number. We\nillustrate that the developed density formulae become relatively more accurate\nfor larger particle numbers, can be improved systematically, are quite\nuniversally applicable, and, hence, may offer alternatives to existing\norbital-free methods for mesoscopic quantum systems.", "category": "cond-mat_mtrl-sci" }, { "text": "Initial stages of the graphite-SiC(0001) interface formation studied by\n photoelectron spectroscopy: Graphitization of the 6H-SiC(0001) surface as a function of annealing\ntemperature has been studied by ARPES, high resolution XPS, and LEED. For the\ninitial stage of graphitization - the 6root3 reconstructed surface - we observe\nsigma-bands characteristic of graphitic sp2-bonded carbon. The pi-bands are\nmodified by the interaction with the substrate. C1s core level spectra indicate\nthat this layer consists of two inequivalent types of carbon atoms. The next\nlayer of graphite (graphene) formed on top of the 6root3 surface at\nTA=1250-1300 degree C has an unperturbed electronic structure. The annealing at\nhigher temperatures results in the formation of a multilayer graphite film. It\nis shown that the atomic arrangement of the interface between graphite and the\nSiC(0001) surface is practically identical to that of the 6root3 reconstructed\nlayer.", "category": "cond-mat_mtrl-sci" }, { "text": "Dirac energy spectrum and inverted band gap in metamorphic InAsSb/InSb\n superlattices: A Dirac-type energy spectrum was demonstrated in gapless ultra-short-period\nmetamorphic InAsSb/InSb superlattices by angle-resolved photoemission\nspectroscopy (ARPES_ measurements. The Fermi velocity value 7.4x10^5 m/s in a\ngapless superlattice with a period of 6.2nm is in a good agreement with the\nresults of magneto-absorption experiments. An \"inverted\" bandgap opens in the\ncenter of the Brillouin zone at higher temperatures and in the SL with a larger\nperiod. The ARPES data indicate the presence of a surface electron accumulation\nlayer", "category": "cond-mat_mtrl-sci" }, { "text": "Photoemission study of the skutterudite compounds\n Co(Sb$_{1-x}$Te$_{x}$)$_3$ and RhSb$_3$: We have studied the electronic structure of the skutterudite compounds\nCo(Sb$_{1-x}$Te$_{x}$)$_3$ (x= 0, 0.02, 0.04) by photoemission spectroscopy.\nValence-band spectra revealed that Sb 5p states are dominant near the Fermi\nlevel and are hybridized with Co 3d states just below it. The spectra of {\\it\np}-type CoSb$_3$ are well reproduced by the band-structure calculation, which\nsuggests that the effect of electron correlations is not strong in CoSb$_3$.\nWhen Te is substituted for Sb and n-type carriers are doped into CoSb$_3$, the\nspectra are shifted to higher binding energies as predicted by the rigid-band\nmodel. From this shift and the free-electron model for the conduction and\nvalence bands, we have estimated the band gap of CoSb$_3$ to be 0.03-0.04 eV,\nwhich is consistent with the result of transport measurements. Photoemission\nspectra of RhSb$_3$ have also been measured and revealed similarities to and\ndifferences from those of CoSb$_3$.", "category": "cond-mat_mtrl-sci" }, { "text": "Pressure induced crossing of the core-levels in 5d metals: Pressure induced interaction between core electrons, the core level crossing\n(CLC) transition has been observed in hcp Os at P~400 GPa [L. Dubrovinksy, et\nal., Nature 525, 226-229 (2015)]. In this work, we carry out a systematic study\nof the influence of pressure on the electronic structure in all metals of the\n5d series (Hf,Ta,W,Re,Os,Ir,Pt,Au) using first-principles electronic structure\ncalculations. We have found that CLC is a general effect for this series of\nmetals. While in Pt it occurs at ~1500 GPa, at a pressure substantially higher\nthan in Os, in Ir it occurs already at 80 GPa. Moreover, we predict that in Re\nthe CLC transition may appear at ambient pressure. We analyze the shifts of the\nCLC transition pressure across the series within the Thomas-Fermi model, and\nshow that the effect has many common features to the atomic collapse in the\nrare-earth elements.", "category": "cond-mat_mtrl-sci" }, { "text": "Ultrafast terahertz field control of the emergent magnetic and\n electronic interactions at oxide interfaces: Ultrafast electric-field control of emergent electronic and magnetic states\nat oxide interfaces offers exciting prospects for the development of new\ngenerations of energy-efficient devices. Here, we demonstrate that the\nelectronic structure and emergent ferromagnetic interfacial state in epitaxial\nLaNiO3/CaMnO3 superlattices can be effectively controlled using intense\nsingle-cycle THz electric-field pulses. We employ a combination of\npolarization-dependent X-ray absorption spectroscopy with magnetic circular\ndichroism and X-ray resonant magnetic reflectivity to measure a detailed\nmagneto-optical profile and thickness of the ferromagnetic interfacial layer.\nThen, we use time-resolved and temperature-dependent magneto-optical Kerr\neffect, along with transient optical reflectivity and transmissivity\nmeasurements, to disentangle multiple correlated electronic and magnetic\nprocesses driven by ultrafast high-field (~1 MV/cm) THz pulses. These processes\ninclude an initial sub-picosecond electronic response, consistent with\nnon-equilibrium Joule heating; a rapid (~270 fs) demagnetization of the\nferromagnetic interfacial layer, driven by THz-field-induced nonequilibrium\nspin-polarized currents; and subsequent multi-picosecond dynamics, possibly\nindicative of a change in the magnetic state of the superlattice due to the\ntransfer of spin angular momentum to the lattice. Our findings shed light on\nthe intricate interplay of electronic and magnetic phenomena in this strongly\ncorrelated material system, suggesting a promising avenue for efficient control\nof two-dimensional ferromagnetic states at oxide interfaces using ultrafast\nelectric-field pulses.", "category": "cond-mat_mtrl-sci" }, { "text": "High-performance Computation of Kubo Formula with Vectorization of\n Batched Linear Algebra Operation: We have proposed a method to accelerate the computation of Kubo formula\noptimized to vector processors. The key concept is parallel evaluation of\nmultiple integration points, enabled by batched linear algebra operations.\nThrough benchmark comparisons between the vector-based NEC SX-Aurora TSUBASA\nand the scalar-based Xeon machines in node performance, we verified that the\nvectorized implementation was speeded up to approximately 2.2 times faster than\nthe baseline. We have also shown that the performance improvement due to\npadding, indicating that avoiding the memory-bank conflict is critically\nimportant in this type of task.", "category": "cond-mat_mtrl-sci" }, { "text": "Experimental demonstration of a generalized Fourier's Law for\n non-diffusive thermal transport: Phonon heat conduction over length scales comparable to their mean free paths\nis a topic of considerable interest for basic science and thermal management\ntechnologies. Although the failure of Fourier's law beyond the diffusive regime\nis well understood, debate exists over the proper physical description of\nthermal transport in the ballistic to diffusive crossover. Here, we derive a\ngeneralized Fourier's law that links the heat flux and temperature fields,\nvalid from ballistic to diffusive regimes and for general geometries, using the\nPeierls-Boltzmann transport equation within the relaxation time approximation.\nThis generalized Fourier's law predicts that thermal conductivity not only\nbecomes nonlocal at length scales smaller than phonon mean free paths, but also\nrequires the inclusion of an inhomogeneous nonlocal source term that has been\npreviously neglected. We demonstrate the validity of this generalized Fourier's\nlaw through direct comparison with time-domain thermoreflectance (TDTR)\nmeasurements in the nondiffusive regime without adjustable parameters.\nFurthermore, we show that interpreting experimental data without this\ngeneralized Fourier's law leads to inaccurate measurement of thermal transport\nproperties.", "category": "cond-mat_mtrl-sci" }, { "text": "High-temperature oxidation and nitridation of substoichiometric\n zirconium carbide in isothermal air: The influence of nitrogen on the oxidation behavior of hot-pressed zirconium\ncarbide was investigated using a flow-tube furnace at temperatures ranging from\n1000 to 1600 {\\deg}C. Mass gain, oxide formation characteristics, and oxide\ntransitions were evaluated at various experimental conditions. Differences in\noxidation behavior across the range of temperatures investigated show both\nkinetic and microstructural dependence with implications pointing to this\nmaterials efficacy in ultra-high temperature applications. Results suggest that\nat temperatures above 1400 {\\deg}C, although oxidation mechanisms remain\ndominant, nitridation and reduction mechanisms may be appreciable enough to\nrequire consideration. Supporting discussions regarding polymorphism and\nmicrostructural influences are outlined.", "category": "cond-mat_mtrl-sci" }, { "text": "Electric field-controlled reversible order-disorder switching of a metal\n tip surface: While it is well established that elevated temperatures can induce surface\nroughening of metal surfaces, the effect of a high electric field on the atomic\nstructure at ambient temperature has not been investigated in detail. Here we\nshow with atomic resolution using in situ transmission electron microscopy how\nintense electric fields induce reversible switching between perfect crystalline\nand disordered phases of gold surfaces at room temperature. Ab initio molecular\ndynamics simulations reveal that the mechanism behind the structural change can\nbe attributed to a vanishing energy cost in forming surface defects in high\nelectric fields. Our results demonstrate how surface processes can be directly\ncontrolled at the atomic scale by an externally applied electric field, which\npromotes an effective decoupling of the topmost surface layers from the\nunderlying bulk. This opens up opportunities for development of active\nnanodevices in e.g. nanophotonics and field-effect transistor technology as\nwell as fundamental research in materials characterization and of yet\nunexplored dynamically-controlled low-dimensional phases of matter.", "category": "cond-mat_mtrl-sci" }, { "text": "Efficient thermo-spin conversion in van der Waals ferromagnet FeGaTe: Recent discovery of 2D van der Waals (vdW) magnetic materials has spurred\nprogress in developing advanced spintronic devices. A central challenge lies in\nenhancing the spin-conversion efficiency for building spin-logic or spin-memory\ndevices. We systematically investigated the anomalous Hall effect and anomalous\nNernst effect in above-room-temperature van der Waals ferromagnet FeGaTe with\nperpendicular anisotropy, uncovering significant spin-conversion effects. The\nanomalous Hall effect demonstrated an efficient electric spin-charge\nconversion, with a notable spin Hall angle of 6 $\\%$ - 10.38 $\\%$. The\ntemperature-dependent behavior of the anomalous Nernst voltage primarily\nresults from the thermo-spin conversion. Uniquely, we have experimentally\nachieved thermo-spin polarization values of over 690 $\\%$ at room temperature\nand extremely large of 4690 $\\%$ at about 93 K. This study illuminates the\npotential of vdW ferromagnets in advancing efficient spin conversion devices.", "category": "cond-mat_mtrl-sci" }, { "text": "Nanostructured Immunosensors. Application to the detection of\n Progesterone: A novel nanostructured electrochemical immunsensor for the determination of\nprogesterone is reported. The approach combines the properties of gold\nnanoparticles with the use of a graphite-Teflon composite electrode matrix,\ninto which gold nanoparticles are incorporated by simple physical inclusion.\nThe antibody anti-progesterone was directly attached to the electrode surface.\nThe immunosensor functioning is based on competitive assay between progesterone\nand alkaline phosphatase-labelled progesterone. Monitoring of the affinity\nreaction was accomplished by the electrochemical oxidation of 1-naphtol.\nModification of the graphite -Teflon electrode matrix with gold nanoparticles\nimproves substantially the electrooxidation response of 1-naphtol. Using a\ndetection potential of +0.3V, a detection limit for progesterone of 0.84 ng\nml-1 was obtained. Analysis of seven milk samples spiked at a 3.5 ng ml-1\nprogesterone concentration level yielded a mean recovery of 101+6%. Detection\nof the antigen-antibody reaction with a graphite - Teflon - colloidal - gold -\nTyrosinase electrode, using phenylphosphate as alkaline phosphatase substrate\nto generate phenol, which is subsequently reduced at -0.1 V at the composite\nelectrode, produced a high improvement in the sensitivity for progesterone\ndetection", "category": "cond-mat_mtrl-sci" }, { "text": "Pitfalls and solutions for perovskite transparent conductors: Transparent conductors-nearly an oxymoron-are in pressing demand, as\nultra-thin-film technologies become ubiquitous commodities. As current\nsolutions rely on non-abundant elements, perovskites such as SrVO3 and SrNbO3\nhave been suggested as next generation transparent conductors. Our ab-initio\ncalculations and analytical insights show, however, that reducing the plasma\nfrequency below the visible spectrum by strong electronic correlations-a\nrecently proposed strategy-unavoidably comes at a price: an enhanced scattering\nand thus a substantial optical absorption above the plasma edge. As a way out\nof this dilemma we identify several perovskite transparent conductors, relying\non hole doping, somewhat larger bandwidths and separations to other bands.", "category": "cond-mat_mtrl-sci" }, { "text": "An Accurate and Transferable Machine-Learning Interatomic Potential for\n Silicon: The development of modern ab initio methods has rapidly increased our\nunderstanding of physics, chemistry and materials science. Unfortunately,\nintensive ab initio calculations are intractable for large and complex systems.\nOn the other hand, empirical force fields are less accurate with poor\ntransferability even though they are efficient to handle large and complex\nsystems. The recent development of machine-learning based neural-network (NN)\nfor local atomic environment representation of density functional theory (DFT)\nhas offered a promising solution to this long-standing challenge. Si is one of\nthe most important elements in science and technology, however, an accurate and\ntransferable interatomic potential for Si is still lacking. Here, we develop a\ngeneralized NN potential for Si, which correctly predicts the Si(111)-(7x7)\nground-state surface reconstruction for the first time and accurately\nreproduces the DFT results in a wide range of complex Si structures. We\nenvision similar developments will be made for a wide range of materials\nsystems in the near future.", "category": "cond-mat_mtrl-sci" }, { "text": "Three-dimensional coordinates of individual atoms in materials revealed\n by electron tomography: Crystallography, the primary method for determining the three-dimensional\n(3D) atomic positions in crystals, has been fundamental to the development of\nmany fields of science. However, the atomic positions obtained from\ncrystallography represent a global average of many unit cells in a crystal.\nHere, we report, for the first time, the determination of the 3D coordinates of\nthousands of individual atoms and a point defect in a material by electron\ntomography with a precision of ~19 picometers, where the crystallinity of the\nmaterial is not assumed. From the coordinates of these individual atoms, we\nmeasure the atomic displacement field and the full strain tensor with a 3D\nresolution of ~1nm^3 and a precision of ~10^-3, which are further verified by\ndensity functional theory calculations and molecular dynamics simulations. The\nability to precisely localize the 3D coordinates of individual atoms in\nmaterials without assuming crystallinity is expected to find important\napplications in materials science, nanoscience, physics and chemistry.", "category": "cond-mat_mtrl-sci" }, { "text": "Tuning the two-dimensional electron gas at the LaAlO3/SrTiO3(001)\n interface by metallic contacts: First principles calculations reveal that adding a metallic overlayer on\nLaAlO3/SrTiO3(001) eliminates the electric field within the polar LaAlO3 film\nand thus suppresses the thickness-dependent insulator-to-metal transition\nobserved in uncovered films. Independent of the LaAlO3 thickness both the\nsurface and the interface are metallic, with an enhanced interface carrier\ndensity relative to LaAlO3/SrTiO3(001) after the metallization transition.\nMoreover, a monolayer thick metallic Ti-contact exhibits a finite magnetic\nmoment and for a thin SrTiO3-substrate induces a spin-polarized 2D electron gas\nat the n-type interface due to confinement effects. A diagram of band alignment\nin M/LaAlO3/SrTiO3(001) and Schottky barriers for M=Ti, Al, and Pt are\nprovided.", "category": "cond-mat_mtrl-sci" }, { "text": "Early stage formation of graphene on the C-face of 6H-SiC: An investigation of the early stage formation of graphene on the C-face of\n6H-SiC is presented. We show that the sublimation of few atomic layers of Si\nout of the SiC substrate is not homogeneous. In good agreement with the results\nof theoretical calculations it starts from defective sites, mainly dislocations\nthat define nearly circular flakes, which have a pyramidal, volcano-like, shape\nwith a center chimney where the original defect was located. At higher\ntemperatures, complete conversion occurs but, again, it is not homogeneous.\nWithin the sample surface the intensity of the Raman G and 2D bands, evidences\nnon-homogeneous thickness.", "category": "cond-mat_mtrl-sci" }, { "text": "3-omega method for thermal properties of thin film multilayers: Short review on the different models for the electro-thermal 3-omega method.\nWe present the deduction of the fundamental relation between the 3-omega\nvoltage with the temperature rise to determine the thermal conductivity. The\nusage of the anisotropy of the films allows a smooth transition between 1D and\n2D models. A comparison between the multilayer methods and analytical solutions\nare presented.", "category": "cond-mat_mtrl-sci" }, { "text": "Efficient Photon Upconverters with Ionic Liquids: This paper presents the development and characterization of photon\nupconverters fabricated with ionic liquids (ILs), which are novel fluids that\nare recently drawing attention due to their unique properties, such as\nnegligible vapor pressures and high thermal stabilities. The upconverters in\nthis study are based on triplet-triplet annihilation (TTA) between excited\npolycyclic aromatic molecules, and TTA requires a fluidic media to allow for\nthe molecules to collide with each other for energy transfer. This process,\nTTA-based photon upconversion (TTA-UC), was therefore mainly accomplished with\norganic solvents previously. It is found that the molecules used for TTA-UC,\nwhich are non-polar or weakly polar, are stably solvated in a certain class of\nILs. The mechanism of the observed solvation is proposed and discussed. The\nupconversion quantum yields (UC-QYs) measured by continuous wave (CW) light\nexcitation reach as high as 10 % with moderate excitation intensity (~ 6\nW/cm2), which is considerably higher than those in previous TTA-UC studies\nperformed with organic solvents (up to ~ 4 % with CW light excitations). It is\nfound that the value of UC-QY starts to saturate as the excitation power\nincreases for all the cases, even within the moderate CW excitation power range\nin this study. An analytical model that describes the UC-QY vs. excitation\npower relationship is derived and compared with the experimental results. The\nagreement between them suggests that the donor-acceptor energy transfer in this\nsystem is highly efficient. Based on these experimental and analytical\nfindings, it is found that efficient energy transfer between the molecules is\npossible in ILs and therefore ILs are not actually viscous media for the\npurpose of TTA-UC.", "category": "cond-mat_mtrl-sci" }, { "text": "Simulation of XANES spectroscopy and the calculation of total energies\n for N-heterocyclic carbenes on Au(111): It has recently been demonstrated that N-heterocyclic carbenes (NHCs) form\nself-assembled monolayers (SAMs) on metal surfaces. Consequently, it is\nimportant to both characterize and understand their binding modes to fully\nexploit NHCs in functional surface systems. To assist with this effort, we have\nperformed {\\it first-principles} total energy calculations for NHCs on Au(111)\nand simulations of X-ray absorption near edge structure (XANES). The NHCs we\nhave considered are N,N-dimethyl-, N,N-diethyl-,\nN,N-diisopropylbenzimidazolylidene ($^B$NHC$^X$, with X=Me, Et, and iPr,\nrespectively) and the bis-$^B$NHC$^X$ complexes with Au derived from these\nmolecules. We present a comprehensive analysis of the energetic stability of\nboth the $^B$NHC$^X$ and the complexes on Au(111) and, for the former, examine\nthe role of the wing group in determining the attachment geometry. Further\nstructural characterization is performed by calculating the nitrogen K-edge\nX-ray absorption spectra. Our simulated XANES results give insight into (i) the\nrelationship between the $^B$NHC$^X$/Au geometry and the N($1s$) $\\rightarrow$\n$\\pi^\\ast/\\sigma^\\ast$, pre-edge/near-edge, absorption intensities, and (ii)\nthe contributions of the molecular deformation and molecule-surface electronic\ninteraction to the XANES spectrum. Our simulations are compared with recent\nexperimental results.", "category": "cond-mat_mtrl-sci" }, { "text": "Numerical Study of Crystal Size Distribution in Polynuclear Growth: The crystal size distribution in polynuclear growth is numerically studied\nusing a coupled map lattice model. The width of the size distribution depends\non c/D, where c is the growth rate at interface sites and $D$ is the diffusion\nconstant. When c/D is sufficiently small, the width W increases linearly with\nc/D and saturates at large c/D. Monodisperse square and cubic crystals are\nobtained respectively on square and cubic lattices when c/D is sufficiently\nsmall for a small kinetic parameter b. The linear dependence of W on c/D in a\nparameter range of small c/D is explained by the eigenfunction for the first\neigenvalue in a two-dimensional model and a mean-field model. For the\nmean-field model, the slope of the linear dependence is evaluated\ntheoretically.", "category": "cond-mat_mtrl-sci" }, { "text": "A survey of energies from pure metals to multi-principal element alloys: In materials science, a wide range of properties of materials are governed by\nvarious types of energies, including thermal, physicochemical, structural, and\nmechanical energies. In 2005, Dr. Frans Spaepen used crystalline\nface-centered-cubic (fcc) copper as an example to discuss a variety of\nphenomena that are associated with energies. Inspired by his pioneering work,\nwe broaden our analysis to include a selection of representative pure metals\nwith fcc, hexagonal close-packed (hcp), and body-centered cubic (bcc)\nstructures. Additionally, we extend our comparison to energies between pure\nmetals and equiatomic binary, ternary, and multi-principal element alloys\n(sometimes also known as high-entropy alloys). Through an extensive collection\nof data and calculations, we compile energy tables that provide a comprehensive\nview of how structure and alloying influence the energy profiles of these\nmetals and alloys. We highlight the significant impact of constituent elements\non the energies of alloys compared to pure metals and reveal a notable\ndisparity in mechanical energies among materials in fcc-, hcp- and\nbcc-structured metals and alloys. Furthermore, we discuss the underlying\nmechanisms behind these patterns and discuss the implications for structural\ntransformations, providing insights into the broader context of these energy\nvariations.", "category": "cond-mat_mtrl-sci" }, { "text": "Possible high-temperature superconductors predicted from electronic\n structure and data-filtering algorithms: We report here the completion of the electronic structure of the majority of\nthe known stoichiometric inorganic compounds, as listed in the International\nCrystal Structure Data-base (ICSD). We make a detailed comparison of the\nelectronic structure, crystal geometry and chemical bonding of cuprate high\ntemperature superconductors, with the calculated over sixty thousand electronic\nstructures. Based on compelling similarities of the electronic structures in\nthe normal state and a data-filtering technique, we propose that high\ntemperature superconductivity is possible for electron- or hole-doping in a\nmuch larger group of materials than previously considered. The indentified\nmaterials are composed of over one hundred layered compounds, most which\nhitherto are untested with respect to their super conducting properties. Of\nparticular interest are the following materials; Ca$_2$(CuBr$_2$O$_2$),\nK$_2$CoF$_4$, Sr$_2$(MoO$_4$) and Sr$_4$V$_3$O$_{10}$, which are discussed in\ndetail.", "category": "cond-mat_mtrl-sci" }, { "text": "Modeling Heterogeneous Materials via Two-Point Correlation Functions:\n II. Algorithmic Details and Applications: In the first part of this series of two papers, we proposed a theoretical\nformalism that enables one to model and categorize heterogeneous materials\n(media) via two-point correlation functions S2 and introduced an efficient\nheterogeneous-medium (re)construction algorithm called the \"lattice-point\"\nalgorithm. Here we discuss the algorithmic details of the lattice-point\nprocedure and an algorithm modification using surface optimization to further\nspeed up the (re)construction process. The importance of the error tolerance,\nwhich indicates to what accuracy the media are (re)constructed, is also\nemphasized and discussed. We apply the algorithm to generate three-dimensional\ndigitized realizations of a Fontainebleau sandstone and a boron\ncarbide/aluminum composite from the two- dimensional tomographic images of\ntheir slices through the materials. To ascertain whether the information\ncontained in S2 is sufficient to capture the salient structural features, we\ncompute the two-point cluster functions of the media, which are superior\nsignatures of the micro-structure because they incorporate the connectedness\ninformation. We also study the reconstruction of a binary laser-speckle pattern\nin two dimensions, in which the algorithm fails to reproduce the pattern\naccurately. We conclude that in general reconstructions using S2 only work well\nfor heterogeneous materials with single-scale structures. However, two-point\ninformation via S2 is not sufficient to accurately model multi-scale media.\nMoreover, we construct realizations of hypothetical materials with desired\nstructural characteristics obtained by manipulating their two-point correlation\nfunctions.", "category": "cond-mat_mtrl-sci" }, { "text": "Thermodynamic properties of a tetramer\n ferro-ferro-antiferro-antiferromagnetic Ising-Heisenberg bond alternating\n chain as a model system for Cu(3-Clpy)$_2$(N$_3$)$_2$: Thermodynamic properties of a tetramer\nferro-ferro-antiferro-antiferromagnetic Ising-Heisenberg bond alternating chain\nare investigated by the use of an exact mapping transformation technique. Exact\nresults for the magnetization, susceptibility and specific heat in the zero as\nwell as nonzero magnetic field are presented and discussed in detail. The\nresults obtained from the mapping are compared with the relevant experimental\ndata of Cu(3-Clpy)$_2$(N$_3$)$_2$ (3-Clpy=3-Chloropyridine).", "category": "cond-mat_mtrl-sci" }, { "text": "Laplacian-level density functionals for the kinetic energy density and\n exchange-correlation energy: We construct a Laplacian-level meta-generalized gradient approximation\n(meta-GGA) for the non-interacting (Kohn-Sham orbital) positive kinetic energy\ndensity $\\tau$ of an electronic ground state of density $n$. This meta-GGA is\ndesigned to recover the fourth-order gradient expansion $\\tau^{GE4}$ in the\nappropiate slowly-varying limit and the von Weizs\\\"{a}cker expression\n$\\tau^{W}=|\\nabla n|^2/(8n)$ in the rapidly-varying limit. It is constrained to\nsatisfy the rigorous lower bound $\\tau^{W}(\\mathbf{r})\\leq\\tau(\\mathbf{r})$.\nOur meta-GGA is typically a strong improvement over the gradient expansion of\n$\\tau$ for atoms, spherical jellium clusters, jellium surfaces, the Airy gas,\nHooke's atom, one-electron Gaussian density, quasi-two dimensional electron\ngas, and nonuniformly-scaled hydrogen atom. We also construct a Laplacian-level\nmeta-GGA for exchange and correlation by employing our approximate $\\tau$ in\nthe Tao, Perdew, Staroverov and Scuseria (TPSS) meta-GGA density functional.\nThe Laplacian-level TPSS gives almost the same exchange-correlation enhancement\nfactors and energies as the full TPSS, suggesting that $\\tau$ and $\\nabla^2 n$\ncarry about the same information beyond that carried by $n$ and $\\nabla n$. Our\nkinetic energy density integrates to an orbital-free kinetic energy functional\nthat is about as accurate as the fourth-order gradient expansion for many real\ndensities (with noticeable improvement in molecular atomization energies), but\nconsiderably more accurate for rapidly-varying ones.", "category": "cond-mat_mtrl-sci" }, { "text": "Quantitative comparison of the magnetic proximity effect in Pt detected\n by XRMR and XMCD: X-ray resonant magnetic reflectivity (XRMR) allows for the simultaneous\nmeasurement of structural, optical and magnetooptic properties and depth\nprofiles of a variety of thin film samples. However, a same-beamtime\nsame-sample systematic quantitative comparison of the magnetic properties\nobserved with XRMR and x-ray magnetic circular dichroism (XMCD) is still\npending. Here, the XRMR results (Pt L$_{3}$ absorption edge) for the magnetic\nproximity effect in Pt deposited on the two different ferromagnetic materials\nFe and Co$_{33}$Fe$_{67}$ are compared with quantitatively analyzed XMCD\nresults. The obtained results are in very good quantitative agreement between\nthe absorption-based (XMCD) and reflectivity-based (XRMR) techniques taking\ninto account an ab initio calculated magnetooptic conversion factor for the\nXRMR analysis. Thus, it is shown that XRMR provides quantitative reliable spin\ndepth profiles important for spintronic and spin caloritronic transport\nphenomena at this type of magnetic interfaces.", "category": "cond-mat_mtrl-sci" }, { "text": "Site-Selective Oxygen Vacancy Formation Derived from the Characteristic\n Crystal Structures of in Sn-Nb complex Oxides: Divalent tin oxides have attracted considerable attention as novel p-type\noxide semiconductors, which are essential for realizing future oxide electronic\ndevices. Recently, p-type Sn2Nb2O7 and SnNb2O6 were developed; however,\nenhanced hole mobility by reducing defect concentrations is required for\npractical use. In this work, we investigate the correlation between the\nformation of oxygen vacancy which may reduce the hole-generation efficiency and\nhole mobility, and the crystal structure in Sn-Nb complex oxides. Extended\nX-ray absorption fine structure spectroscopy and Rietveld analysis of x-ray\ndiffraction revealed the preferential formation of oxygen vacancy at the O site\nbonded to the Sn ions in both the tin niobates. Moreover, a large amount of\noxygen vacancy around the Sn ions were found in the p-type Sn2Nb2O7, thereby\nindicating the effect of oxygen vacancy to the low hole-generation efficiency.\nThe dependence of the formation of oxygen vacancy on the crystal structure can\nbe elucidated from the Sn-O bond strength that is evaluated based on the bond\nvalence sum and Debye temperature. The differences in the bond strengths of the\ntwo Sn-Nb complex oxides are correlated through the steric hindrance of Sn2+\nwith asymmetric electron density distribution. This suggests the importance of\nthe material design with a focus on the local structure around the Sn ions to\nprevent the formation of oxygen vacancy in p-type Sn2+ oxides.", "category": "cond-mat_mtrl-sci" }, { "text": "First-principles prediction of high-entropy-alloy stability: High entropy alloys (HEAs) are multicomponent compounds whose high\nconfigurational entropy allows them to solidify into a single phase, with a\nsimple crystal lattice structure. Some HEA's exhibit desirable properties, such\nas high specific strength, ductility, and corrosion resistance, while\nchallenging the scientist to make confident predictions in the face of multiple\ncompeting phases. We demonstrate phase stability in the multicomponent alloy\nsystem of Cr-Mo-Nb-V, for which some of its binary subsystems are subject to\nphase separation and complex intermetallic-phase formation. Our\nfirst-principles calculation of free energy predicts that the configurational\nentropy stabilizes a single body-centered cubic (BCC) phase from T = 1,700K up\nto melting, while precipitation of a complex intermetallic is favored at lower\ntemperatures. We form the compound experimentally and confirm that it forms as\na single BCC phase from the melt, but that it transforms reversibly at lower\ntemperatures.", "category": "cond-mat_mtrl-sci" }, { "text": "High-temperature cyclic oxidation kinetics and microstructural\n transition mechanisms of Ti-6Al-4V composites reinforced with hybrid\n (TiC+TiB) networks: The microstructural features and high-temperature oxidation resistance of\nhybrid (TiC+TiB) networks reinforced Ti-6Al-4V composites were investigated\nafter fabricated with reaction hot pressing technique. The inhomogeneous\ndistribution of hybrid reinforcers resulted in a sort of stress-induced grain\nrefinement for {\\alpha}-Ti matrix phase, which was further facilitated by\nheterogeneous nucleation upon additive interfaces. HRTEM analyses revealed the\ncrystallographic orientation relation between TiB and alpha-Ti phases as\n(201)TiB//(-1100)alpha-Ti plus [11-2]//[0001] alpha-Ti, while TiC and\n{\\alpha}-Ti phases maintained the interrelation of (-200)TiC//(-2110)\n{\\alpha}-Ti and [001]TiC//[01-10] alpha-Ti. The hybridly reinforced\nTi-6Al-4V/(TiC+TiB) composites displayed superior oxidation resistance to both\nthe sintered matrix alloy and the two composites reinforced solely with TiC or\nTiB addition during the cyclic oxidation at 873, 973 and 1073 K respectively\nfor 100 h. The hybrid reinforcers volume fraction was a more influential factor\nto improve oxidation resistance than the matrix alloy powder size. As\ntemperature rose from 873 to 1073 K, the oxidation kinetics transferred from\nthe nearly parabolic type through qusilinear tendency into the finally linear\nmode. This corresponded to the morphological transition of oxide scales from a\ncontinuous protective film to a partially damaged layer and ended up with the\ncomplete spallation of alternating alumina and rutile multilayers. A\nphenomenological model was proposed to elucidate the growth process of oxides\nscales. The release of thermal stress, the suppression of oxygen diffusion and\nthe fastening of oxide adherence were found as the three major mechanisms to\nenhance the oxidation resistance of hybrid reinforced composites.", "category": "cond-mat_mtrl-sci" }, { "text": "Piezo films with adjustable anisotropic strain for bending actuators\n with tunable bending profiles: We present a method to produce in-plane polarized piezo films with a freely\nadjustable ratio of the strains in orthogonal in-plane directions. They can be\nused in piezo bending actuators with a tunable curvature profile. The strains\nare obtained as mean strains from a periodic polarization pattern produced by a\nsuitable doubly interdigitated electrode structure. This mechanism is\ndemonstrated for several examples using PZT sheets. We further discuss how this\ntuning and the parameters of the electrode layout affect the overall magnitude\nof the displacement.", "category": "cond-mat_mtrl-sci" }, { "text": "Correlating atom probe tomography with X-Ray and electron spectroscopies\n to understand microstructure-activity relationships in electrocatalysts: The search for a new energy paradigm with net-zero carbon emissions requires\nnew technologies for energy generation and storage that are at the crossroad\nbetween engineering, chemistry, physics, surface and materials sciences. To\nkeep pushing the inherent boundaries of device performance and lifetime, we\nneed to step away from a cook-and-look approach and aim to establish the\nscientific ground to guide the design of new materials. This requires strong\nefforts in establishing bridges between microscopy and spectroscopy techniques,\nacross multiple scales. Here, we discuss how the complementarities of X-ray-\nand electron-based spectroscopies and atom probe tomography can be exploited in\nthe study of surfaces and sub-surfaces to understand structure-property\nrelationships in electrocatalysts.", "category": "cond-mat_mtrl-sci" }, { "text": "Peierls Distortion in Two-Dimensional Tight-Binding Model: The Peierls distortions in a two-dimensional electron-lattice system\ndescribed by a Su-Schrieffer-Heeger type model extended to two-dimensions are\nnumerically studied for a square lattice. The electronic band is just\nhalf-filled and the nesting vector is ($\\pi/a$, $\\pi/a$) with $a$ the lattice\nconstant. In contrast to the previous understanding on the Peierls transition\nin two dimensions, the distortions which are determined so as to minimize the\ntotal energy of the system involve not only the Fourier component with the\nnesting wave vector but also many other components with wave vectors parallel\nto the nesting vector. It is found that such unusual distortions contribute to\nthe formation of gap in the electronic energy spectrum by indirectly (in the\nsense of second order perturbation) connecting two states having wave vectors\ndiffering by the nesting vector from each other. Analyses for different system\nsizes and for different electron-lattice coupling constants indicate that the\nexistence of such distortions is not a numerical artifact. It is shown that the\ngap of the electronic energy spectrum is finite everywhere over the Fermi\nsurface.", "category": "cond-mat_mtrl-sci" }, { "text": "Mirror real Chern insulator in two and three dimensions: A real Chern insulator (RCI) featuring a real Chern number and a second-order\nboundary mode appears in a two-dimensional (2D) system with the space-time\ninversion symmetry (PT ). Here, we propose a kind of RCI: mirror real Chern\ninsulator (MRCI) which emerges from the system having additional horizontal\nmirror symmetry Mz. The MRCI generally is characterized by two independent real\nChern numbers, respectively defined in the two mirror subsystems of the system.\nHence, the MRCI may host the second-order boundary modes different from the\nconventional RCI. We show that for spinless systems, the definition of the MRCI\nis straightforward, as PT keeps each mirror subsystem invariant. For the\nspinful systems with both PT and Mz, the real Chern number for the total system\nremain well defined, as MzPT = C2zT , and (C2zT )2= 1. However, since C2zT\nexchanges the two mirror subsystems, the definition of the MRCI in spinful\nsystems requires the help of projective symmetry algebra. We also discuss the\nMRCIs in 3D systems, where the MRCI is defined on certain mirror-invariant 2D\nplanes. Compared with its 2D counterpart, the 3D MRCI can exhibit more abundant\nphysics when the systems have additional nonsymmorphic operators. Several\nconcrete MRCI models including 2D and 3D, spinless and spinful models are\nconstructed to further demonstrate our ideas.", "category": "cond-mat_mtrl-sci" }, { "text": "Simulations on the elastic response of amorphous and nanocomposite\n carbon: Theoretical calculations of the elastic response of carbon composites and\namorphous carbon are reported. The studied composites consist of crystalline\nnanoinclusions, either spherical diamonds or carbon nanotubes, embedded in\namorphous carbon matrices. The elastic constants of the composites were\ncalculated and found to systematically increase as the density increases. The\nelastic recovery under hydrostatic pressure for all structures was also\ninvestigated and was found to be significantly high for both nanocomposite and\namorphous carbon, but decreases as the material becomes more dilute.", "category": "cond-mat_mtrl-sci" }, { "text": "Experimental determination and modelling of volume shrinkage in curing\n thermosets: This work deals with the characterisation and modelling of the curing process\nand its associated volume changes of an epoxy based thermoset resin.\nMeasurements from differential scanning calorimetry (DSC) define the progress\nof the chemical reaction. The related thermochemical volume changes are\nrecorded by an especially constructed experimental setup based on Archimedes\nprinciple. Information on measuring procedure and data processing are provided.\nThis includes investigations on compensation of environmental influences,\nlong-term stability and resolution. With the aim of simulating the adhesives\ncuring process, constitutive models representing the reaction kinetics and\nthermochemical volume changes are presented and the model parameters are\nidentified.", "category": "cond-mat_mtrl-sci" }, { "text": "New type of incommensurate magnetic ordering in Mn3TeO6: The complex metal oxide Mn3TeO6 exhibits a corundum related structure and has\nbeen prepared both in forms of single crystals by chemical transport reactions\nand of polycrystalline powders by a solid state reaction route. The crystal\nstructure and magnetic properties have been investigated using a combination of\nX-ray and neutron powder diffraction, electron microscopy, calorimetric and\nmagnetic measurements. At room temperature this compound adopts a trigonal\nstructure, space group R3 with a = 8.8679(1) {\\AA}, c = 10.6727(2) {\\AA}. A\nlong-range magnetically ordered state is identified below 23 K. An unexpected\nfeature of this magnetic structure is several types of Mn-chains. Under the\naction of the incommensurate magnetic propagation vector k = [0, 0, 0.4302(1)]\nthe unique Mn site is split into two magnetically different orbits. One orbit\nforms a perfect helix with the spiral axis along the c-axis while the other\norbit has a sine wave character along the c-axis.", "category": "cond-mat_mtrl-sci" }, { "text": "Two-dimensional ferroelectrics from high throughput computational\n screening: We report a high throughput computational search for two-dimensional\nferroelectric materials. The starting point is 252 pyroelectric materials from\nthe computational 2D materials database (C2DB) and from these we identify 64\nferroelectric materials by explicitly constructing adiabatic paths connecting\nstates of reversed polarization. In particular we find 49 materials with\nin-plane polarization, 8 materials with out-of-plane polarization and 6\nmaterials with coupled in-plane and out-of-plane polarization. Most of the\nknown 2D ferroelectrics are recovered by the screening and the far majority of\nthe new predicted ferroelectrics are known as bulk van der Waals bonded\ncompounds, which implies that these could be experimentally accessible by\ndirect exfoliation. For roughly 25{\\%} of the materials we find a metastable\nstate in the non-polar structure, which could have important consequences for\nthe thermodynamical properties and may imply a first order transition to the\npolar phase. Finally, we list the magnetic pyroelectrics extracted from the\nC2DB and focus on the case of VAgP$_2$Se$_6$, which exhibits a three-state\nswitchable polarization vector that is strongly coupled to the magnetic\nexcitation spectrum.", "category": "cond-mat_mtrl-sci" }, { "text": "Acoustic Cyclotron Resonance and Giant High Frequency Magnetoacoustic\n Oscillations in Metals with Locally Flattened Fermi Surface: We consider the effect of local flattening on the Fermi surface (FS) of a\nmetal upon geometric oscillations of the velocity and attenuation of ultrasonic\nwaves in the neighborhood of the acoustic cyclotron resonance. It is shown that\nsuch peculiarities of the local geometry of the FS can lead to a significant\nenhancement of both cyclotron resonance and geometric oscillations.\nCharacteristic features of the coupling of ultrasound to shortwave cyclotron\nwaves arising due to the local flattening of the FS are analyzed.\n PACS numbers 71.18.+y; 72.15.Gd; 72.15.-v", "category": "cond-mat_mtrl-sci" }, { "text": "Spin-wave-induced spin torque in Rashba spin-orbit coupling system: We study the effects of Rashba spin-orbit coupling on the spin torque induced\nby spin waves, which are the plane wave dynamics of magnetization. The spin\ntorque is derived from linear response theory, and we calculate the dynamic\nspin torque by considering the impurity-ladder-sum vertex corrections. This\ndynamic spin torque is divided into three terms: a damping term, a $distortion$\nterm, and a correction term for the equation of motion. The $distorting$ torque\ndescribes a phenomenon unique to the Rashba spin-orbit coupling system, where\nthe distorted motion of magnetization precession is subjected to the\nanisotropic force from the Rashba coupling. The oscillation mode of the\nprecession exhibits an elliptical trajectory, and the ellipticity depends on\nthe strength of the nesting effects, which could be reduced by decreasing the\nelectron lifetime.", "category": "cond-mat_mtrl-sci" }, { "text": "Overview of phase-field models for fatigue fracture in a unified\n framework: In the last ten years, the phase-field method has gained much attention as a\nnovel method to simulate fracture due to its straightforward way allowing to\ncover crack initiation and propagation without additional conditions. More\nrecently, it has also been applied to fatigue fracture due to cyclic loading.\nThis publication gives an overview of the main phase-field fatigue models\npublished to date. We present all models in a unified variational framework for\nbest comparability. Subsequently, the models are compared regarding their most\nimportant features. It becomes apparent that they can be classified in mainly\ntwo categories according to the way fatigue is implemented in the model - that\nis as a gradual degradation of the fracture toughness or with an additional\nterm in the crack driving force. We aim to provide a helpful guide for choosing\nthe appropriate model for different applications and for developing existing\nmodels further.", "category": "cond-mat_mtrl-sci" }, { "text": "A First Principles Investigation of Native Interstitial Diffusion in\n Cr2O3: First principles density functional theory (DFT) investigation of native\ninterstitials and the associated self-diffusion mechanisms in {\\alpha}-Cr2O3\nreveals that interstitials are more mobile than vacancies of corresponding\nspecies. Cr interstitials occupy the unoccupied Cr sublattice sites that are\noctahedrally coordinated by 6 O atoms, and O interstitials form a dumbbell\nconfiguration orientated along the [221] direction (diagonal) of the corundum\nlattice. Calculations predict that neutral O interstitials are predominant in\nO-rich conditions and Cr interstitials in +2 and +1 charge states are the\ndominant interstitial defects in Cr-rich conditions. Similar to that of the\nvacancies, the charge transition levels of both O and Cr interstitials are\nlocated deep within the bandgap. Transport calculations reveal a rich variety\nof interstitial diffusion mechanisms that are species, charge, and orientation\ndependent. Cr interstitials diffuse preferably along the diagonal of corundum\nlattice in a two step process via an intermediate defect complex comprising a\nCr interstitial and an adjacent Cr Frenkel defect in the neighboring Cr\nbilayer. This mechanism is similar to that of the vacancy mediated Cr diffusion\nalong the c-axis with intermediate Cr vacancy and Cr Frenkel defect\ncombination. In contrast, O interstitials diffuse via bond switching mechanism.\nO interstitials in -1 and -2 charge states have very high mobility compared to\nneutral O interstitials.", "category": "cond-mat_mtrl-sci" }, { "text": "Study of spin-phonon coupling and magnetic field induced spin\n reorientation in polycrystalline multiferroic $GdFeO_3$: The present work reports the preparation of polycrystalline multiferroic\n$GdFeO_3$ (GdFO) and characterization with x-ray diffraction (XRD),\nmagnetization, temperature dependent Raman spectroscopy, temperature and\nmagnetic field dependent $^{57}Fe$ M$\\ddot{o}$ssbauer spectroscopy\nmeasurements. The sample is found to be phase pure from Rietveld refinement of\nXRD pattern. The M$\\ddot{o}$ssbauer spectra measured in the presence of\nexternal magnetic field show the signatures of field induced spin reorientation\ntransition, which are corroborated by magnetization measurements. From the\ntemperature dependent variation of internal hyperfine field, N$\\grave{e}$el\ntransition temperature ($T_{N,Fe}$) of 672.5$\\pm$0.2 K and critical exponent\n($\\beta$) of 0.333$\\pm$0.003 is obtained. Temperature dependent (300 - 760 K)\nRaman spectroscopy measurements show the signatures of spin-phonon coupling and\nlocal structural re-arrangement across $T_{N,Fe}$.", "category": "cond-mat_mtrl-sci" }, { "text": "Edge Modes and Asymmetric Wave Transport in Topological Lattices:\n Experimental Characterization at Finite Frequencies: Although topological mechanical metamaterials have been extensively studied\nfrom a theoretical perspective, their experimental characterization has been\nlagging. To address this shortcoming, we present a systematic laser-assisted\nexperimental characterization of topological kagome lattices, aimed at\nelucidating their in-plane phononic and topological characteristics. We\nspecifically explore the continuum elasticity limit, which is established when\nthe ideal hinges that appear in the theoretical models are replaced by\nligaments capable of supporting bending deformation, as observed for instance\nin realistic physical lattices. We reveal how the zero-energy floppy edge modes\npredicted for ideal configurations morph into finite-frequency phonon modes\nthat localize at the edges. By probing the lattices with carefully designed\nexcitation signals, we are able to extract and characterize all the features of\na complex low-frequency acoustic regime in which bulk modes and topological\nedge modes overlap and entangle in response. The experiments provide\nunequivocal evidence of the existence of strong asymmetric wave transport\nregimes at finite frequencies.", "category": "cond-mat_mtrl-sci" }, { "text": "Revealing Correlation of Valence State with Nanoporous Structure in\n Cobalt Catalyst Nanoparticles by in Situ Environmental TEM: Simultaneously probing the electronic structure and morphology of materials\nat the nanometer or atomic scale while a chemical reaction proceeds is\nsignificant for understanding the underlying reaction mechanisms and optimizing\na materials design. This is especially important in the study of nanoparticle\ncatalysts, yet such experiments have rarely been achieved. Utilizing an\nenvironmental transmission electron microscope (ETEM) equipped with a\ndifferentially pumped gas cell, we are able to conduct nanoscopic imaging and\nelectron energy loss spectroscopy (EELS) in situ for cobalt catalysts under\nreaction conditions. Analysis revealed quantitative correlation of the cobalt\nvalence states to the particles' nanoporous structures. The in situ experiments\nwere performed on nanoporous cobalt particles coated with silica while a 15\nmTorr hydrogen environment was maintained at various temperatures\n(300-600\\degreeC). When the nanoporous particles were reduced, the valence\nstate changed from cobalt oxide to metallic cobalt and concurrent structural\ncoarsening was observed. In situ mapping of the valence state and the\ncorresponding nanoporous structures allows quantitatively analysis necessary\nfor understanding and improving the mass activity and lifetime of cobalt-based\ncatalysts, i.e., for Fischer-Tropsch synthesis that converts carbon monoxide\nand hydrogen into fuels, and uncovering the catalyst optimization mechanisms.", "category": "cond-mat_mtrl-sci" }, { "text": "First-principles study of PbTiO$_3$ under uniaxial strains and stresses: The behavior of PbTiO$_3$ under uniaxial strains and stresses is investigated\nfrom first-principles calculations within density functional theory. We show\nthat irrespectively of the uniaxial mechanical constraint applied, the system\nkeeps a purely ferroelectric ground-state, with the polarization aligned either\nalong the constraint direction ($FE_z$ phase) or along one of the pseudo-cubic\naxis perpendicular to it ($FE_x$ phase). This contrasts with the cases of\nisotropic or biaxial mechanical constraints for which novel phases combining\nferroelectic and antiferrodistortive motions have been previously reported.\nUnder uniaxial strain, PbTiO$_3$ switched from a $FE_x$ ground state under\ncompressive strain to $FE_z$ ground-state under tensile strain, beyond a\ncritical strain $\\eta_{zz}^c \\approx +1$\\%. Under uniaxial stress, PbTiO$_3$\nexhibits either a $FE_x$ ground state under compression ($\\sigma_{zz} < 0$) or\na $FE_z$ ground state under tension ($\\sigma_{zz} > 0$). Here, however, an\nabrupt jump of the structural parameters is also predicted under both\ncompressive and tensile stresses at critical values $\\sigma_{zz} \\approx$ $+2$\nGPa and $- 8$ GPa. This behavior appears similar to that predicted under\nnegative isotropic pressure and might reveal practically useful to enhance the\npiezoelectric response in nanodevices.", "category": "cond-mat_mtrl-sci" }, { "text": "Surface-state-dominated transport in crystals of the topological\n crystalline insulator In-doped Pb$_{1-x}$Sn$_x$Te: Three-dimensional topological insulators and topological crystalline\ninsulators represent new quantum states of matter, which are predicted to have\ninsulating bulk states and spin-momentum-locked gapless surface states.\nExperimentally, it has proven difficult to achieve the high bulk resistivity\nthat would allow surface states to dominate the transport properties over a\nsubstantial temperature range. Here we report a series of indium-doped\nPb$_{1-x}$Sn$_x$Te compounds that manifest huge bulk resistivities together\nwith strong evidence of topological surface states, based on\nthickness-dependent transport studies and magnetoresistance measurements. For\nthese bulk-insulating materials, the surface states determine the resistivity\nfor temperatures approaching 30 K.", "category": "cond-mat_mtrl-sci" }, { "text": "Hydrogen Compounds of Group-IV Nanosheets: The structural and electronic properties of the hydrides of silicene and\ngermanene have been studied using ab initio calculations. The trend for the M-H\n(M=C, Si, Ge) bond lengths, and corresponding bond energies, is consistent with\nthe atomic size trend, and comparable to those of MH_4 hydrides. Band\nstructures were also obtained for the buckled configuration, which is the\nstable form for both silicene and germanene. Upon hydrogenation, both silicane\n(indirect gap) and germanane (direct gap) are semiconducting.", "category": "cond-mat_mtrl-sci" }, { "text": "Spin-dependent Transparency of Ferromagnet/Superconductor Interfaces: Because the physical interpretation of the spin-polarization of a ferromagnet\ndetermined by point-contact Andreev reflection (PCAR) is non-trivial, we have\ncarried out parameter-free calculations of PCAR spectra based upon a\nscattering-theory formulation of Andreev reflection generalized to\nspin-polarized systems and a tight-binding linear muffin tin orbital method for\ncalculating the corresponding scattering matrices. PCAR is found to measure the\nspin-dependent interface transparency rather than the bulk polarization of the\nferromagnet which is strongly overestimated by free electron model fitting.", "category": "cond-mat_mtrl-sci" }, { "text": "Wave mechanics in media pinned at Bravais lattice points: The propagation of waves through microstructured media with periodically\narranged inclusions has applications in many areas of physics and engineering,\nstretching from photonic crystals through to seismic metamaterials. In the\nhigh-frequency regime, modelling such behaviour is complicated by multiple\nscattering of the resulting short waves between the inclusions. Our aim is to\ndevelop an asymptotic theory for modelling systems with arbitrarily-shaped\ninclusions located on general Bravais lattices. We then consider the limit of\npoint-like inclusions, the advantage being that exact solutions can be obtained\nusing Fourier methods, and go on to derive effective medium equations using\nasymptotic analysis. This approach allows us to explore the underlying reasons\nfor dynamic anisotropy, localisation of waves, and other properties typical of\nsuch systems, and in particular their dependence upon geometry. Solutions of\nthe effective medium equations are compared with the exact solutions, shedding\nfurther light on the underlying physics. We focus on examples that exhibit\ndynamic anisotropy as these demonstrate the capability of the asymptotic theory\nto pick up detailed qualitative and quantitative features.", "category": "cond-mat_mtrl-sci" }, { "text": "Anisotropic magnetoresistance of spin-orbit coupled carriers scattered\n from polarized magnetic impurities: Anisotropic magnetoresistance (AMR) is a relativistic magnetotransport\nphenomenon arising from combined effects of spin-orbit coupling and broken\nsymmetry of a ferromagnetically ordered state of the system. In this work we\nfocus on one realization of the AMR in which spin-orbit coupling enters via\nspecific spin-textures on the carrier Fermi surfaces and ferromagnetism via\nelastic scattering of carriers from polarized magnetic impurities. We report\ndetailed heuristic examination, using model spin-orbit coupled systems, of the\nemergence of positive AMR (maximum resistivity for magnetization along\ncurrent), negative AMR (minimum resistivity for magnetization along current),\nand of the crystalline AMR (resistivity depends on the absolute orientation of\nthe magnetization and current vectors with respect to the crystal axes)\ncomponents. We emphasize potential qualitative differences between pure\nmagnetic and combined electro-magnetic impurity potentials, between short-range\nand long-range impurities, and between spin-1/2 and higher spin-state carriers.\nConclusions based on our heuristic analysis are supported by exact solutions to\nthe integral form of the Boltzmann transport equation in archetypical\ntwo-dimensional electron systems with Rashba and Dresselhaus spin-orbit\ninteractions and in the three-dimensional spherical Kohn-Littinger model. We\ninclude comments on the relation of our microscopic calculations to standard\nphenomenology of the full angular dependence of the AMR, and on the relevance\nof our study to realistic, two-dimensional conduction-band carrier systems and\nto anisotropic transport in the valence band of diluted magnetic\nsemiconductors.", "category": "cond-mat_mtrl-sci" }, { "text": "Coherent control of spontaneous emission of a three-level atom in a\n coherent photonic band gap reservoir: By studying the fluorescence and optical properties of a three-level system,\nwe propose a new point of view on the coherent control of these spectra. With\nthe definite phase difference between the fields of the air band and dielectric\nband in photonic band gap (PBG) reservoirs, the spectra of spontaneous\nemission, absorption, and dispersion exhibit the coherent property and quantum\ninterference effect. This coherent interference depending on the position of\nthe embedded atom and the width of band gap causes the coupling of the\nfree-space light and the PBG light to result in blue shift of spectra and the\nappearance of dark lines and kinks. By coherently controlling the\nposition-dependent dispersion, we can tune the frequency of slow light.", "category": "cond-mat_mtrl-sci" }, { "text": "Strain tunability of perpendicular magnetic anisotropy in van der Waals\n ferromagnets VI3: Layered ferromagnets with high coercivity have special applications in\nnanoscale memory elements in electronic circuits, such as data storage.\nTherefore, searching for new hard ferromagnets and effectively tuning or\nenhancing the coercivity are the hottest topics in layered magnets today. Here,\nwe report a strain tunability of perpendicular magnetic anisotropy in van der\nWaals (vdW) ferromagnets VI3 using magnetic circular dichroism measurements.\nFor an unstrained flake, the M-H curve shows a rectangular-shaped hysteresis\nloop with perpendicular magnetic anisotropy and a large coercivity (up to 1.775\nT at 10 K). Furthermore, the coercivity can be enhanced to a maximum of 2.6 T\nat 10 K under a 2.9% in-plane tensile strain. Our DFT calculations show that\nthe magnetic anisotropy energy (MAE) can be dramatically increased after\napplying an in-plain tensile strain, which contributes to the enhancement of\ncoercivity in the VI3 flake. Meanwhile, the strain tunability on the coercivity\nof CrI3, with a similar crystal structure, is limited. The main reason is the\nstrong spin-orbital coupling in V3+ in VI6 octahedra in comparison with that in\nCr3+. The strain tunability of coercivity in VI3 flakes highlights its\npotential for integration into vdW heterostructures, paving the way toward\nnanoscale spintronic devices and applications in the future.", "category": "cond-mat_mtrl-sci" }, { "text": "Mode-Dependent Damping in Metallic Antiferromagnets Due to\n Inter-Sublattice Spin Pumping: Damping in magnetization dynamics characterizes the dissipation of magnetic\nenergy and is essential for improving the performance of spintronics-based\ndevices. While the damping of ferromagnets has been well studied and can be\nartificially controlled in practice, the damping parameters of\nantiferromagnetic materials are nevertheless little known for their physical\nmechanisms or numerical values. Here we calculate the damping parameters in\nantiferromagnetic dynamics using the generalized scattering theory of\nmagnetization dissipation combined with the first-principles transport\ncomputation. For the PtMn, IrMn, PdMn and FeMn metallic antiferromagnets, the\ndamping coefficient associated with the motion of magnetization ($\\alpha_m$) is\none to three orders of magnitude larger than the other damping coefficient\nassociated with the variation of the N\\'eel order ($\\alpha_n$), in sharp\ncontrast to the assumptions made in the literature.", "category": "cond-mat_mtrl-sci" }, { "text": "The mechanical response of a creased sheet: We investigate the mechanics of thin sheets decorated by non-interacting\ncreases. The system considered here consists in parallel folds connected by\nelastic panels. We show that the mechanical response of the creased structure\nis twofold, depending both on the bending deformation of the panels and the\nhinge-like intrinsic response of the crease. We show that a characteristic\nlength scale, defined by the ratio of bending to hinge energies, governs\nwhether the structure's response consists in angle opening or panel bending\nwhen a small load is applied. The existence of this length scale is a building\nblock for future works on origami mechanics", "category": "cond-mat_mtrl-sci" }, { "text": "Multiferroic and magnetoelectric nature of GaFeO3, AlFeO3 and related\n oxides: GaFeO3, AlFeO3 and related oxides are ferrimagnetic exhibiting\nmagnetodielectric effect. There has been no evidence to date for\nferroelectricity and hence multiferroicity in these oxides. We have\ninvestigated these oxides as well as oxides of the composition\nAl1-x-yGaxFe1+yO3 (x = 0.2, y = 0.2) for possible ferroelectricity by carrying\nout pyroelectric measurements. These measurements establish the occurrence of\nferroelectricity at low temperatures below the N\\`eel temperature in these\noxides. They also exhibit significant magnetoelectric effect. We have tried to\nunderstand the origin of ferroelectricity based on non-centrosymmetric magnetic\nordering and disorder by carrying out first-principles calculations.", "category": "cond-mat_mtrl-sci" }, { "text": "Complete Strain Mapping of Nanosheets of Tantalum Disulfide: Quasi-two-dimensional (quasi-2D) materials hold promise for future\nelectronics because of their unique band structures that result in electronic\nand mechanical properties sensitive to crystal strains in all three dimensions.\nQuantifying crystal strain is a prerequisite to correlating it with the\nperformance of the device, and calls for high resolution but spatially resolved\nrapid characterization methods. Here we show that using fly-scan nano X-ray\ndiffraction we can accomplish a tensile strain sensitivity below 0.001% with a\nspatial resolution of better than 80 nm over a spatial extent of 100 $\\mu$m on\nquasi 2D flakes of 1T-TaS2. Coherent diffraction patterns were collected from a\n$\\sim$ 100 nm thick sheet of 1T-TaS2 by scanning 12keV focused X-ray beam\nacross and rotating the sample. We demonstrate that the strain distribution\naround micron and sub-micron sized 'bubbles' that are present in the sample may\nbe reconstructed from these images. The experiments use state of the art\nsynchrotron instrumentation, and will allow rapid and non-intrusive strain\nmapping of thin film samples and electronic devices based on quasi 2D\nmaterials.", "category": "cond-mat_mtrl-sci" }, { "text": "Highly sensitive NO2 sensors by pulsed laser deposition on graphene: Graphene as a single-atomic-layer material is fully exposed to environment\nand has therefore a great potential for creating of sensitive gas sensors.\nHowever, in order to realize this potential for different polluting gases,\ngraphene has to be functionalized - adsorption centers of different type and\nwith high affinity to target gases have to be created at its surface. In this\npresent work, modification of graphene by small amounts of laser ablated\nmaterials is introduced for this purpose as a versatile and precise tool. The\napproach was demonstrated with two very different materials chosen for pulsed\nlaser deposition (PLD), a metal (Ag) and a dielectric oxide (ZrO2). It was\nshown that the gas response and its recovery rate can be significantly enhanced\nby choosing the PLD target material and deposition conditions. The response to\nNO2 gas in air was amplified up to 40 times in case of PLD-modified graphene in\ncomparison with pristine graphene and reached 7-8% at 40 ppb of NO2 and 20-30%\nat 1 ppm of N2. These results were obtained after PLD in gas environment (5 x\n10-2 mbar oxygen or nitrogen) and atomic areal densities of deposited materials\nof were about 10 15 cm-2. The ultimate level of NO2 detection in air, as\nextrapolated from the experimental data obtained at room temperature under mild\nUV-excitation, was below 1 ppb.", "category": "cond-mat_mtrl-sci" }, { "text": "Impurity-induced transition to a Mott insulator in Sr$_3$Ru$_2$O$_7$: The electrical, magnetic, and structural properties of\nSr$_3$(Ru$_{1-x}$Mn$_x$)$_2$O$_7$ (0 $\\leq x \\leq$ 0.2) are investigated. The\nparent compound Sr$_3$Ru$_2$O$_7$ is a paramagnetic metal, critically close to\nmagnetic order. We have found that, with a Ru-site doping by only a few percent\nof Mn, the ground state is switched from a paramagnetic metal to an\nantiferromagnetic insulator. Optical conductivity measurements show the opening\nof a gap as large as 0.1 eV, indicating that the metal-to-insulator transition\nis driven by the electron correlation. The complex low-temperature\nantiferromagnetic spin arrangement, reminiscent of those observed in some\nnickelates and manganites, suggests a long range orbital order.", "category": "cond-mat_mtrl-sci" }, { "text": "Directional Anisotropy of Crack Propagation Along $\u03a3$3 Grain\n Boundary in BCC Fe: Crack growth behaviour along the coherent twin boundary (CTB), i.e.,\n$\\Sigma$3{112} of BCC Fe is investigated using molecular dynamics (MD)\nsimulations. The growth of an atomistically sharp crack with {112}$<$110$>$\norientation has been examined along the two opposite $<$111$>$ directions of\nCTB under mode-I loading at a constant strain rate. Separate MD simulations\nwere carried out with crack inserted in the left side, right side and middle of\nthe specimen model system. The results indicate that the crack grows\ndifferently along the two opposite $<$111$>$ directions. In case of a crack\ninserted in the left side, the crack grows in ductile manner, while it\npropagates in semi-brittle manner in the case of crack inserted in the right\nside. The directional dependence of crack growth along the CTB is also\nconfirmed by the stress-strain behaviour. This anisotropy in crack growth\nbehaviour has been attributed to the twinning-antitwinning asymmetry of\n1/6$<$111$>$ partial dislocations on {112} planes.", "category": "cond-mat_mtrl-sci" }, { "text": "Refined Geometry and Frozen Phonons in KNbO3: In order to arrive at ultimately accurate results available with the LMTO\nmethod in the local density approximation, the stability of full-potential LMTO\npredictions for off-center displacements in KNbO3, as depending on the choice\nof basis and expansion cutoffs, has been thoroughly investigated. With the\ncalculation setup thus optimized, supercell frozen phonon calculations aimed at\nthe study of the chain-structure instability over the Brillouin zone have been\ndone, and the long-wavelength limit of the LO phonon is discussed.", "category": "cond-mat_mtrl-sci" }, { "text": "Anomalous resistivity upturn in the van der Waals ferromagnet\n Fe$_5$GeTe$_2$: Fe$_5$GeTe$_2$ (n = 3, 4, 5) have recently attracted increasing attention due\nto their two-dimensional van der Waals characteristic and high temperature\nferromagnetism, which make promises for spintronic devices. The Fe(1) split\nsite is one important structural characteristic of Fe$_5$GeTe$_2$ which makes\nit very different from other Fe$_5$GeTe$_2$ (n = 3, 4) systems. The local\natomic disorder and short-range order can be induced by the split site. In this\nwork, the high-quality van der Waals ferromagnet Fe$_5$GeTe$_2$ were grown to\nstudy the low-temperature transport properties. We found a resistivity upturn\nbelow 10 K. The temperature and magnetic field dependence of the resistivity\nare in good agreement with a combination of the theory of disorder-enhanced\nthree-dimensional electron-electron and single-channel Kondo effect. The Kondo\neffect exists only at low magnetic field B < 3 T, while electron-electron\ndominates the appearance for the low-temperature resistivity upturn. We believe\nthat the enhanced three-dimensional electron-electron interaction in this\nsystem is induced by the local atomic structural disorder due to the split site\nof Fe(1). Our results indicate that the split site of Fe plays an important\nrole for the exceptional transport properties.", "category": "cond-mat_mtrl-sci" }, { "text": "Magnetic force microscopy investigation of arrays of nickel nanowires\n and nanotubes: The magnetic properties of arrays of nanowires (NWs) and nanotubes (NTs), 150\nnm in diameter, electrodeposited inside nanoporous polycarbonate membranes are\ninvestigated. The comparison of the nanoscopic magnetic force microscopy (MFM)\nimaging and the macroscopic behavior as measured by alternating gradient force\nmagnetometry (AGFM) is made. It is shown that MFM is a complementary technique\nthat provides an understanding of the magnetization reversal characteristics at\nthe microscopic scale of individual nanostructures. The local hysteresis loops\nhave been extracted by MFM measurements. The influence of the shape of such\nelongated nanostructures on the dipolar coupling and consequently on the\nsquareness of the hysteresis curves is demonstrated. It is shown that the\nnanowires exhibit stronger magnetic interactions than nanotubes. The\nnon-uniformity of the magnetization states is also revealed by combining the\nMFM and AGFM measurements.", "category": "cond-mat_mtrl-sci" }, { "text": "First principles prediction of structural and electronic properties of\n TlxIn(1-x)N alloy: Structural and electronic properties of zinc blende TlxIn(1-x)N alloy have\nbeen evaluated from first principles. The band structures have been obtained\nwithin the density functional theory (DFT), the modified Becke-Johnson (MBJLDA)\napproach for the exchange-correlation potential, and fully relativistic\npseudopotentials. The calculated band-gap dependence on Tl content in this\nhypothetical alloy exhibits a linear behaviour up to the 25 % of thalium\ncontent where its values become close to zero. In turn, the split-off energy at\nthe Gamma point of the Brillouin zone, related to the spin-orbit coupling, is\npredicted to be comparable in value with the band-gap for relatively low\nthalium contents of about 5 %. These findings suggest TlxIn(1-x)N alloy as a\npromising material for optoelectronic applications. Furthermore, the band\nstructure of TlN reveals some specific properties exhibited by topological\ninsulators.", "category": "cond-mat_mtrl-sci" }, { "text": "Observation of sharp metamagnetic transition, Griffiths like phase and\n glassy nature in double perovskite Eu2CoMnO6: In the present investigation, some novel magnetic behaviors exhibited by\ndouble perovskite (DP) Eu2CoMnO6(ECMO) has been reported. XRD analysis of ECMO\nshowed that it has a monoclinic crystal structure (space group P 21/n). A\nsecond-order magnetic phase transition as a sudden jump in the magnetization\ncurve has been observed at 124.5 K. This is related to the paramagnetic to\nferromagnetic/E*-type antiferromagnetic phase transition due to the competing\nCo-O-Mn exchange interactions. A clear low-temperature compensation point\nfollowed by negative magnetization is observed in the zero-field-cooled curve\nof the sample, suggesting the formation of canted ferromagnetic domains or\nantiparallel spins and clusters that are separated by an antiphase boundary.\nThe large bifurcation between the ZFC and FC curves has been observed,\nsuggesting strong spin frustration is present in the system. More\ninterestingly, sharp multiple steps in magnetization are observed in M-H curve\nat 2 K and observed only in the forward field-sweep direction which vanishes on\nincreasing temperature. Moreover, prominent smaller peaks immediately above the\nlong-range ordering temperature are observed suggesting the presence of\npreformed percolating clusters which eventually gives rise to Griffiths like\nphase which is seen in DC as well in AC susceptibility. The real part of AC\nsusceptibility with DC bias shows an unusual sharp peak near TC that broadens\non increasing field strength and splits into two maxima around 750 Oe, which is\nattributed to the presence of critical fluctuations associated with a\ncontinuous transition to the FM state and large magnetic anisotropy in the\nsystem.", "category": "cond-mat_mtrl-sci" }, { "text": "Uncooled bolometer response of a low noise La2/3Sr1/3MnO3 thin film: We report measurements of the optical responses of a La2/3Sr1/3MnO3 (LSMO)\nsample at a wavelength of 533 nm in the 300-400 K range. The 200 nm thick film\nwas grown by pulsed laser deposition on (100) SrTiO3 substrate and showed\nremarkably low noise. At 335 K the temperature coefficient of the resistance of\na 100 micrometers wide 300 micrometers long LSMO line was 0.017 K-1 and the\nnormalized Hooge parameter was 9 e-30 m3, which is among the lowest reported\nvalues. We then measured an optical sensitivity at I = 5 mA of 10.4 V.W-1 and\ncorresponding noise equivalent power (NEP) values of 8.1 e-10 W.Hz-1/2 and 3.3\ne-10 W. Hz-1/2 at 30 Hz and above 1kHz, respectively. Simple considerations on\nbias current conditions and thermal conductance G are finally given for further\nsensitivity improvements using LSMO films. The performances were indeed\ndemonstrated on bulk substrates with G of 10-3 W.K-1. One could expect a NEP\nreduction by three orders of magnitude if a membrane-type geometry was used,\nwhich makes this LSMO device competitive against commercially available\nuncooled bolometers.", "category": "cond-mat_mtrl-sci" }, { "text": "On the mechanism behind the inverse melting in systems with competing\n interactions: Here we present a fundamental comprehension of the microscopic mechanisms\nleading to the emergence of inverse melting transitions by considering a\nthorough mean-field analysis of a variety of minimal models with different\ncompeting interactions. Through analytical and numerical tools we identify the\nspecific connections between the characteristic energy of the homogeneous and\nmodulated phases and the observed reentrant behaviors. In particular, we find\nthat reentrance is appreciable when the characteristic energy cost of the\nhomogeneous and modulated phases are comparable to each other, and for systems\nin which the local order parameter is limited. In the asymptotic limit of high\nenergy cost of the homogeneous phase we obtain analytically that the degree of\nreentrance of the phase diagram decreases exponentially with the ratio of the\ncharacteristic energy cost of homogeneous and modulated phases. We are also\nable to establish theoretical (upper and lower) bounds for the degree of the\nreentrance, according to the nature of the competing interactions. Finally, we\nconfront our mean-field results with Langevin simulations of an effective\ncoarse grained model, confirming the main results regarding the degree of the\nreentrance in the phase diagram. These results shed new light on the many\nsystems undergoing inverse melting transitions, from magnets to colloids and\nvortex matter, by qualitatively improving the understanding of the interplay of\nentropy and energy around the inverse melting points.", "category": "cond-mat_mtrl-sci" }, { "text": "A Predictive Multiphase Model of Silica Aerogels for Building Envelope\n Insulations: This work develops a multiphase thermomechanical model of porous silica\naerogel and implements an uncertainty analysis framework consisting of the\nSobol methods for global sensitivity analyses and Bayesian inference using a\nset of experimental data of silica aerogel. A notable feature of this work is\nimplementing a new noise model within the Bayesian inversion to account for\ndata uncertainty and modeling error. The hyper-parameters in the likelihood\nbalance data misfit and prior contribution to the parameter posteriors and\nprevent their biased estimation. The results indicate that the uncertainty in\nsolid conductivity and elasticity are the most influential parameters affecting\nthe model output variance. Also, the Bayesian inference shows that despite the\nmicrostructural randomness in the thermal measurements, the model captures the\ndata with 2% error. However, the model is inadequate in simulating the\nstress-strain measurements resulting in significant uncertainty in the\ncomputational prediction of a building insulation component.", "category": "cond-mat_mtrl-sci" }, { "text": "Effect of strain on electronic and thermoelectric properties of few\n layers to bulk MoS$_{2}$: The sensitive dependence of electronic and thermoelectric properties of\nMoS$_2$ on the applied strain opens up a variety of applications in the\nemerging area of straintronics. Using first principles based density functional\ntheory calculations, we show that the band gap of few layers of MoS$_2$ can be\ntuned by applying i) normal compressive (NC), ii) biaxial compressive (BC), and\niii) biaxial tensile (BT) strain. A reversible semiconductor to metal\ntransition (S-M transition) is observed under all three types of strain. In the\ncase of NC strain, the threshold strain at which S-M transition occurs\nincreases with increasing number of layers and becomes maximum for the bulk. On\nthe other hand, the threshold strain for S-M transition in both BC and BT\nstrain decreases with the increase in number of layers. The difference in the\nmechanisms for the S-M transition is explained for different types of applied\nstrain. Furthermore, the effect of strain type and number of layers on the\ntransport properties are also studied using Botzmann transport theory. We\noptimize the transport properties as a function of number of layers and applied\nstrain. 3L- and 2L-MoS$_2$ emerge as the most efficient thermoelectric material\nunder NC and BT strain, respectively. The calculated thermopower is large and\ncomparable to some of the best thermoelectric materials. A comparison between\nthe feasibility of these three types of strain is also discussed.", "category": "cond-mat_mtrl-sci" }, { "text": "Plethora of tunable Weyl fermions in kagome magnet Fe3Sn2 thin films: Interplay of magnetism and electronic band topology in unconventional magnets\nenables the creation and fine control of novel electronic phenomena. In this\nwork, we use scanning tunneling microscopy and spectroscopy to study thin films\nof a prototypical kagome magnet Fe3Sn2. Our experiments reveal an unusually\nlarge number of densely-spaced spectroscopic features straddling the Fermi\nlevel. These are consistent with signatures of low-energy Weyl fermions and\nassociated topological Fermi arc surface states predicted by theory. By\nmeasuring their response as a function of magnetic field, we discover a\npronounced evolution in energy tied to the magnetization direction. Electron\nscattering and interference imaging further demonstrates the tunable nature of\na subset of related electronic states. Our experiments provide the first\nvisualization of how in-situ spin reorientation drives changes in the\nelectronic density of states of the Weyl fermion band structure. Combined with\nprevious reports of massive Dirac fermions, flat bands and electronic\nnematicity, our work establishes Fe3Sn2 as a unique platform that harbors an\nextraordinarily wide array of topological and correlated electron phenomena.", "category": "cond-mat_mtrl-sci" }, { "text": "Formation of Core-Shell Precipitates in off-stochiometric Ni-Mn-Sn\n Heusler alloys probed through the induced Sn-moment: The Shell-ferromagnetic effect originates from the segregation process in\noff-stochiometric Ni-Mn-based Heusler. In this work, we investigate the\nprecipitation process of L2$_1$-ordered Ni$_2$MnSn and L1$_0$-ordered NiMn in\noff-stochiometric Ni$_{50}$Mn$_{45}$Sn$_{5}$ during temper annealing, by X-ray\ndiffraction (XRD) and $^{119}$Sn M\\\"ossbauer spectroscopy. While XRD probes\nlong-range ordering of the lattice structure, M\\\"ossbauer spectroscopy probes\nnearest-neighbour interactions, reflected in the induced Sn magnetic moment. As\nshown in this work, the induced magnetic Sn moment can be used as a detector\nfor microscopic structural changes and is, therefore, a powerful tool for\ninvestigating the formation of nano-precipitates. Similar research can be\nperformed in the future, for example, on different pinning type magnets like\nSm-Co or Nd-Fe-B.", "category": "cond-mat_mtrl-sci" }, { "text": "A systematic study of four series of electron-doped rare earth\n manganates, LnxCa1-xMnO3 (Ln=La, Nd, Gd and Y) over the x=0.02-0.25\n composition range: Electrical and magnetic properties of four series of manganates LnxCa1-xMnO3\n(Ln=La, Nd, Gd and Y) have been studied in the electron doped regime\n(x=0.02-0.25) in order to investigate the various inter-dependent phenomena\nsuch as ferromagnetism, phase separation and charge ordering. The general\nbehavior of all the four series of manganates is similar, with some of the\nproperties showing dependence on the average radius of the A-site cations, \nand cation size disorder. Thus, all the compositions show increase in\nmagnetization at 100-120 K (TM) for x, probably due to\nthe increased phase separation induced by site disorder. This is also reflected\nin the larger width of the hysteresis loops at T. In\nthis regime, the electrical resistivity decreases with increasing x, but\nremains low and nearly constant T>TM. The percolative nature of the conduction\nmechanism at Txmax, the\nmaterials become antiferromagnetic and charge-ordered at a temperature TCA,\naccompanied by a marked increase in resistivity. The value of TCA increases\nwith increase in and x (upto x=0.3). Thus, all the four series of\nmanganates are characterized by a phase-separated regime between x=0.02 and\n0.1-0.15 and an antiferromagnetic charge-ordered regime at x>0.1-0.15.", "category": "cond-mat_mtrl-sci" }, { "text": "Electronic stopping for protons and \u03b1 particles from\n first-principles electron dynamics: The case of silicon carbide: We present the first-principles determination of electronic stopping power\nfor protons and {\\alpha} particles in a semiconductor material of great\ntechnological interest: silicon carbide. The calculations are based on\nnonequilibrium simulations of the electronic response to swift ions using\nreal-time, time-dependent density functional theory (RT-TDDFT). We compare the\nresults from this first-principles approach to those of the widely used linear\nresponse formalism and determine the ion velocity regime within which linear\nresponse treatments are appropriate. We also use the nonequilibrium electron\ndensities in our simulations to quantitatively address the longstanding\nquestion of the velocity-dependent effective charge state of projectile ions in\na material, due to its importance in linear response theory. We further examine\nthe validity of the recently proposed centroid path approximation for reducing\nthe computational cost of acquiring stopping power curves from RT-TDDFT\nsimulations.", "category": "cond-mat_mtrl-sci" }, { "text": "Two and one-dimensional honeycomb structures of silicon and germanium: Based on first-principles calculations of structure optimization, phonon\nmodes and finite temperature molecular dynamics, we predict that silicon and\ngermanium have stable, two-dimensional, low-buckled, honeycomb structures.\nSimilar to graphene, they are ambipolar and their charge carriers can behave\nlike a massless Dirac fermions due to their pi- and pi*-bands which are crossed\nlinearly at the Fermi level. In addition to these fundamental properties, bare\nand hydrogen passivated nanoribbons of Si and Ge show remarkable electronic and\nmagnetic properties, which are size and orientation dependent. These properties\noffer interesting alternatives for the engineering of diverse nanodevices.", "category": "cond-mat_mtrl-sci" }, { "text": "Total energy calculation for the metallic hcp phase of Zn in the bulk,\n layered, and quantum dot limits: The structural and electronic properties of the metallic hcp phase of Zn in\nthe bulk, monolayer, bilayer, and quantum dot limits have been studied by using\ntotal energy calculations. From our calculated density of states and electronic\nband structure, in agreement with previous work, bulk hybridization of the\nZn--$4s$, $3p$, and $3d$ orbitals is obtained. Furthermore, we found that this\norbital hybridization is also obtained for the monolayer, bilayer, and quantum\ndot systems. At the same time, we found that the Zn monolayer and bilayer\nsystems show electronic properties characteristic of lamellar systems, while\nthe quantum dot system shows the behavior predicted for a 0D system.", "category": "cond-mat_mtrl-sci" }, { "text": "Possible structural and bond reconstruction in 2D ferromagnetic\n semiconductor VSe2 under uniaxial stress: 2D semiconducting transition metal dichalcogenides have been used to make\nhigh-performance electronic, spintronic, and optoelectronic devices. Recently,\nroom-temperature ferromagnetism and semiconducting property were found in 2D\nVSe$_2$ nanoflakes (mechanically exfoliated onto silicon substrates capped with\na oxide layer) and are attributed to the stable 2H-phase of VSe$_2$ in the 2D\nlimit. Here, our first-principles investigation show that a metastable\nsemiconducting H' phase can be formed from the H VSe2 monolayer and some other\nsimilar when these 2D H-phase materials are under uniaxial stress or uniaxial\nstrain. For the uniaxial stress (uniaxial strain) scheme, the H' phase will\nbecome lower in total energy than the H phase at the transition point. The\ncalculated phonon spectra indicate the dynamical stability of the H' structures\nof VSe$_2$, VS$_2$, and CrS$_2$, and the path of phase switching between the H\nand H' VSe$_2$ phases is calculated. For VSe$_2$, the H' phase has stronger\nferromagnetism and its Currier temperature can be substantially enhanced by\napplying uniaxial stress or strain. Spin-resolved electronic structures, energy\nband edges, and effective carrier masses for both of the H and H' phases can be\nsubstantially changed by the applied uniaxial stress or strain, leading to huge\neffective masses near the band edge of the strained H' phase. Analysis\nindicated that the largest bond length difference between the H' and H phases\ncan reach -19\\% for the Se3-Se3' bond, and there is noticeable covalence for\nthe Se3-Se3' bond, which switches the valence of the nearby V atoms, leading to\nthe enhanced ferromagnetism. Therefore, structural and bond reconstruction can\nbe realized by applying uniaxial stress in 2D ferromagnetic H VSe$_2$ and some\nother similar. These can be useful to seeking more insights and phenomena in\nsuch 2D materials for potential applications.", "category": "cond-mat_mtrl-sci" }, { "text": "Thermoelectric figure of merit of tau-type conductors of several donors: Dimensionless thermoelectric figure of merit $ZT$ is investigated for\ntwo-dimensional organic conductors $\\tau-(EDO-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$,\n$\\tau$-(EDT-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$ and\n$\\tau$-(P-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$ ($y \\le 0.875$), respectively. The\n$ZT$ values were estimated by measuring electrical resistivity, thermopower and\nthermal conductivity simultaneously. The largest $ZT$ is 2.7 $\\times$ 10$^{-2}$\nat 155 K for $\\tau-(EDT-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$, 1.5 $\\times$ 10$^{-2}$\nat 180 K for $\\tau-(EDO-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$ and 5.4 $\\times$\n10$^{-3}$ at 78 K for $\\tau-(P-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$, respectively.\nSubstitution of the donor molecules fixing the counter anion revealed\nEDT-S,S-DMEDT-TTF is the best of the three donors to obtain larger $ZT$.", "category": "cond-mat_mtrl-sci" }, { "text": "Broken translational and rotational symmetries in LiMn1.5Ni0.5O4 spinel: In condensed matter physics broken symmetries and emergence of\nquasi-particles are intimately linked to each other. Whenever a symmetry is\nbroken, it leaves its fingerprints, and that may be observed indirectly via its\ninfluence on the other quasi-particles. Here, we report the strong signature of\nbroken rotational symmetry induced due to long range-ordering of spins in Mn -\nsublattice of LiMn1.5Ni0.5O4 below Tc ~ 113 K reflected with the marked changes\nin the lattice vibrations using Raman scattering. In particular, the majority\nof the observed first-order phonon modes show a sharp shift in frequency in the\nvicinity of long range magnetic-ordering temperature. Phonons exist in a\ncrystalline system because of broken translational symmetry, therefore any\nrenormalization in the phonon-spectrum could be a good gauge for broken\ntranslational symmetry. Anomalous evolution of the few modes associated with\nstretching of Mn/NiO6 octahedra in the intermediate temperature range (~ 60-260\nK) marked the broken translational symmetry attributed to the charge ordering.\nInterestingly same modes also show strong coupling with magnetic degrees of\nfreedom, suggesting that charge-ordering and magnetic transition may be linked\nto each other.", "category": "cond-mat_mtrl-sci" }, { "text": "Demystifying magnetic resonance measurements of the true diffusion\n propagator: In a recent work, a method for the magnetic resonance (MR) measurement of the\ntrue diffusion propagator was introduced, which was subsequently implemented\nand validated for free diffusion on a benchtop MR scanner. Here, we provide a\nbrief theoretical description of the method and discuss various experimental\nregimes.", "category": "cond-mat_mtrl-sci" }, { "text": "Intraband divergences in third order optical response of 2D systems: The existence of large nonlinear optical coefficients is one of the\npreconditions for using nonlinear optical materials in nonlinear optical\ndevices. For a crystal, such large coefficients can be achieved by matching\nphoton energies with resonant energies between different bands, and so the\ndetails of the crystal band structure play an important role. Here we\ndemonstrate that large third-order nonlinearities can also be generally\nobtained by a different strategy: As any of the incident frequencies or the sum\nof any two or three frequencies approaches zero, the doped or excited\npopulations of electronic states lead to divergent contributions in the induced\ncurrent density. We refer to these as intraband divergences, by analogy with\nthe behavior of Drude conductivity in linear response. Physically, such\nresonant processes can be associated with a combination of inraband and\ninterband optical transitions. Current-induced second order nonlinearity,\ncoherent current injection, and jerk currents are all related to such\ndivergences, and we find similar divergences in degenerate four wave mixing and\ncross-phase modulation under certain conditions. These divergences are limited\nby intraband relaxation parameters, and lead to a large optical response from a\nhigh quality sample; we find they are very robust with respect to variations in\nthe details of the band structure. To clearly track all of these effects, we\nanalyze gapped graphene, describing the electrons as massive Dirac fermions;\nunder the relaxation time approximation, we derive analytic expressions for the\nthird order conductivities, and identify the divergences that arise in\ndescribing the associated nonlinear phenomena.", "category": "cond-mat_mtrl-sci" }, { "text": "GaAs(111)A and B in hydrazine sulfide solutions : extreme polarity\n dependence of surface adsorption processes: Chemical bonds formed by hydrazine-sulfide treatment of GaAs(111) were\nstudied by synchrotron photoemission spectroscopy. At the B surface, the top\narsenic atoms are replaced by nitrogen atoms, while GaAs(111)A is covered by\nsulfur, also bonded to underlying gallium, despite the sulfide molar\nconcentration being 103 times smaller than that of the hydrazine. This extreme\ndependence on surface polarity is explained by competitive adsorption processes\nof HS- and OH- anions and of hydrazine molecules, on Ga- adsorption sites,\nwhich have distinct configurations on the A and B surfaces.", "category": "cond-mat_mtrl-sci" }, { "text": "Symmetric Versus Nonsymmetric Structure of the Phosphorus Vacancy on\n InP(110): The atomic and electronic structure of positively charged P vacancies on\nInP(110) surfaces is determined by combining scanning tunneling microscopy,\nphotoelectron spectroscopy, and density-functional theory calculations. The\nvacancy exhibits a nonsymmetric rebonded atomic configuration with a charge\ntransfer level 0.75+-0.1 eV above the valence band maximum. The scanning\ntunneling microscopy (STM) images show only a time average of two degenerate\ngeometries, due to a thermal flip motion between the mirror configurations.\nThis leads to an apparently symmetric STM image, although the ground state\natomic structure is nonsymmetric.", "category": "cond-mat_mtrl-sci" }, { "text": "Discovery of stable surfaces with extreme work functions by\n high-throughput density functional theory and machine learning: The work function is the key surface property that determines how much energy\nis required for an electron to escape the surface of a material. This property\nis crucial for thermionic energy conversion, band alignment in\nheterostructures, and electron emission devices. Here, we present a\nhigh-throughput workflow using density functional theory (DFT) to calculate the\nwork function and cleavage energy of 33,631 slabs (58,332 work functions) that\nwe created from 3,716 bulk materials, including up to ternary compounds. The\nnumber of materials for which we calculated surface properties surpasses the\npreviously largest database, the Materials Project, by a factor of $\\sim$27. On\nthe tail ends of the work function distribution we identify 34 and 56 surfaces\nwith an ultra-low (<2 eV) and ultra-high (>7 eV) work function, respectively.\nFurther, we discover that the $(100)$-Ba-O surface of BaMoO$_3$ and the\n$(001)$-F surface of Ag$_2$F have record-low (1.25 eV) and record-high (9.06\neV) steady-state work functions without requiring coatings, respectively. Based\non this database we develop a physics-based approach to featurize surfaces and\nuse supervised machine learning to predict the work function. We find that\nphysical choice of features improves prediction performance far more than\nchoice of model. Our random forest model achieves a mean absolute test error of\n0.09 eV, which is more than 6 times better than the baseline and comparable to\nthe accuracy of DFT. This surrogate model enables rapid predictions of the work\nfunction ($\\sim 10^5$ faster than DFT) across a vast chemical space and\nfacilitates the discovery of material surfaces with extreme work functions for\nenergy conversion, electronic applications, and contacts in 2-dimensional\ndevices.", "category": "cond-mat_mtrl-sci" }, { "text": "PhySRNet: Physics informed super-resolution network for application in\n computational solid mechanics: Traditional approaches based on finite element analyses have been\nsuccessfully used to predict the macro-scale behavior of heterogeneous\nmaterials (composites, multicomponent alloys, and polycrystals) widely used in\nindustrial applications. However, this necessitates the mesh size to be smaller\nthan the characteristic length scale of the microstructural heterogeneities in\nthe material leading to computationally expensive and time-consuming\ncalculations. The recent advances in deep learning based image super-resolution\n(SR) algorithms open up a promising avenue to tackle this computational\nchallenge by enabling researchers to enhance the spatio-temporal resolution of\ndata obtained from coarse mesh simulations. However, technical challenges still\nremain in developing a high-fidelity SR model for application to computational\nsolid mechanics, especially for materials undergoing large deformation. This\nwork aims at developing a physics-informed deep learning based super-resolution\nframework (PhySRNet) which enables reconstruction of high-resolution\ndeformation fields (displacement and stress) from their low-resolution\ncounterparts without requiring high-resolution labeled data. We design a\nsynthetic case study to illustrate the effectiveness of the proposed framework\nand demonstrate that the super-resolved fields match the accuracy of an\nadvanced numerical solver running at 400 times the coarse mesh resolution while\nsimultaneously satisfying the (highly nonlinear) governing laws. The approach\nopens the door to applying machine learning and traditional numerical\napproaches in tandem to reduce computational complexity accelerate scientific\ndiscovery and engineering design.", "category": "cond-mat_mtrl-sci" }, { "text": "Direct Measurement of the Electronic Structure and band gap nature of\n atomic-layer-thick 2H-MoTe2: The millimeter sized monolayer and bilayer 2H-MoTe2 single crystal samples\nare prepared by a new mechanical exfoliation method. Based on such high-quality\nsamples, we report the first direct electronic structure study on them, using\nstandard high resolution angle-resolved photoemission spectroscopy (ARPES). A\ndirect band gap of 0.924eV is found at K in the rubidium-doped monolayer MoTe2.\nSimilar valence band alignment is also observed in bilayer MoTe2,supporting an\nassumption of a analogous direct gap semiconductor on it. Our measurements\nindicate a rather large band splitting of 212meV at the valence band maximum\n(VBM) in monolayer MoTe2, and the splitting is systematically enlarged with\nlayer stacking, from monolayer to bilayer and to bulk. Meanwhile, our PBE band\ncalculation on these materials show excellent agreement with ARPES results.\nSome fundamental electronic parameters are derived from the experimental and\ncalculated electronic structures. Our findings lay a foundation for further\napplication-related study on monolayer and bilayer MoTe2.", "category": "cond-mat_mtrl-sci" }, { "text": "Magnetoelectric coupling in polycrystalline FeVO4: We report coupling between magnetic and electric orders for antiferromagnetic\npolycrystalline FeVO4 in which magnetism-induced polarization has been recently\nfound in noncollinear antiferromagnetic state below the second\nantiferromagnetic phase transition at TN2=15.7K. In this low symmetry phase\nspace group P-1, the magnetic field dependence of electric polarization\nevidences a clear magnetoelectric coupling in the noncollinear spin-configured\nantiferromagnetic phase. The discontinuity of magnetodielectric effect observed\nat the vicinity of the polar to nonpolar transition evidences competition\nbetween different magnetodielectric couplings in the two different\nantiferromagnetic states. The existence of thermal expansion anomaly near TN2\nand magnetostriction effect support magnetoelastically mediated scenario of the\nobserved magnetoelectric effect.", "category": "cond-mat_mtrl-sci" }, { "text": "Magnetism of two-dimensional defects in Pd: stacking faults, twin\n boundaries and surfaces: Careful first-principles density functional calculations reveal the\nimportance of hexagonal versus cubic stacking of closed packed planes of Pd as\nfar as local magnetic properties are concerned. We find that, contrary to the\nstable face centered cubic phase, which is paramagnetic, the hexagonal\nclose-packed phase of Pd is ferromagnetic with a magnetic moment of 0.35\n$\\mu_{B}$/atom. Our results show that two-dimensional defects with local hcp\nstacking, like twin boundaries and stacking faults, in the otherwise fcc Pd\nstructure, increase the magnetic susceptibility. The (111) surface also\nincreases the magnetic susceptibility and it becomes ferromagnetic in\ncombination with an individual stacking fault or twin boundary close to it. On\nthe contrary, we find that the (100) surface decreases the tendency to\nferromagnetism. The results are consistent with the magnetic moment recently\nobserved in small Pd nanoparticles, with a large surface area and a high\nconcentration of two-dimensional stacking defects.", "category": "cond-mat_mtrl-sci" }, { "text": "Detecting and Directing Single Molecule Binding Events on H-Si(100) with\n Application to Ultra-dense Data Storage: Many new material systems are being explored to enable smaller, more capable\nand energy efficient devices. These bottom up approaches for atomic and\nmolecular electronics, quantum computation, and data storage all rely on a\nwell-developed understanding of materials at the atomic scale. Here, we report\na versatile scanning tunneling microscope (STM) charge characterization\ntechnique, which reduces the influence of the typically perturbative STM tip\nfield, to develop this understanding even further. Using this technique, we can\nnow observe single molecule binding events to atomically defined reactive sites\n(fabricated on a hydrogen-terminated silicon surface) through electronic\ndetection. We then developed a new error correction tool for automated hydrogen\nlithography, directing molecular hydrogen binding events using these sites to\nprecisely repassivate surface dangling bonds (without the use of a scanned\nprobe). We additionally incorporated this molecular repassivation technique as\nthe primary rewriting mechanism in new ultra-dense atomic data storage designs\n(0.88 petabits per in$^{2}$).", "category": "cond-mat_mtrl-sci" }, { "text": "Theory of magnetic domains in uniaxial thin films: For uniaxial easy axis films, properties of magnetic domains are usually\ndescribed within the Kittel model, which assumes that domain walls are much\nthinner than the domains. In this work we present a simple model that includes\na proper description of the magnetostatic energy of domains and domain walls\nand also takes into account the interaction between both surfaces of the film.\nOur model describes the behavior of domain and wall widths as a function of\nfilm thickness, and is especially well suited for the strong stripe phase. We\nprove the existence of a critical value of magneto-crystalline anisotropy above\nwhich stripe domains exist for any film thickness and justify our model by\ncomparison with exact results. The model is in good agreement with experimental\ndata for hcp cobalt.", "category": "cond-mat_mtrl-sci" }, { "text": "Current correlation functions for chemical sensors based on DNA\n decorated carbon nanotube: The current characteristics of DNA decorated carbon nanotubes for different\ngas odors are studied. A simple model of charge transfer between the\nGas-DNA-base complex and single wall carbon nanotube (SWCN) is proposed to\nexplain the current response for different odors. The autocorrelation and\ntwo-point correlation functions are calculated for the current sensitivity\ncurves. These correlation functions together with the current characteristics\nform finger-prints for detection of the odor and DNA sequence.", "category": "cond-mat_mtrl-sci" }, { "text": "Measuring Dislocation Density in Aluminum with Resonant Ultrasound\n Spectroscopy: Dislocations in a material will, when present in enough numbers, change the\nspeed of propagation of elastic waves. Consequently, two material samples,\ndiffering only in dislocation density, will have different elastic constants, a\nquantity that can be measured using Resonant Ultrasound Spectroscopy.\nMeasurements of this effect on aluminum samples are reported. They compare well\nwith the predictions of the theory.", "category": "cond-mat_mtrl-sci" }, { "text": "Novel Cyano-Bridged 4f-3d Coordination Polymers with a Unique 2D\n Topological Architecture and Unusual Magnetic Behavior: Cyano-bridged bimetallic hybrid Prussian Blue one- to three-dimensional\n(1D-3D) coordination polymers based on [M(CN)6]3- (M = Fe, Cr, Mn) have\nattracted great attention because of their rich and interesting structures and\nmagnetic behaviors. The previous study implies that increasing dimensionality\nmay enhance and improve bulk magnetic properties.Our strategy for the rational\nsynthesis of high-dimensional network is to use a suitable combination of\ncyanide groups and other bridging ligands. Here, 2,2'-bipyrimidine (bpym) was\nselected, in preference to 4,4'-bipyrazine and pyrazine, because it is more\ncapable of transmitting magnetic interactions and its bis(chelating)\ncoordination modes facilitate connection between lanthanide ions. Unexpectedly,\ntwo novel coordination polymers [NdM(bpym)(H2O)4(CN)6]3H2O (M = Fe 1, Co 2;)\nwere obtained, which have a unique 2D topological architecture, and exhibit\nunusual magnetic behavior.", "category": "cond-mat_mtrl-sci" }, { "text": "Trends in Ferromagnetism in Mn doped dilute III-V alloys from a density\n functional perspective: Mn doping in dilute III-V alloys has been examined as a route to enhance\nferromagnetic stability. Strong valence band bowing is expected at the dilute\nlimit, implying a strong modification of the ferromagnetic stability upon\nalloying, with even an increase in some cases. Using first principle electronic\nstructure calculations we show that while codoping with a group V anion\nenhances the ferromagnetic stability in some cases when the effects of\nrelaxation of the lattice are not considered, strong impurity scattering in the\nrelaxed structure result in a reduction of the ferromagnetic stability.", "category": "cond-mat_mtrl-sci" }, { "text": "Magnetism, spin-wave relaxation and spiral exchange in a trilayer\n magnetic junction: We study the non-collinear exchange coupling across a trilayer magnetic\njunction consisting of two ferromagnets separated by a thin dilute magnetic\nsemiconductor containing itinerant carriers with finite spin relaxation. It is\nremarkable that, by increasing the spin relaxation, the critical temperature is\nsubstantially enhanced and the shape of the magnetization curve becomes more\nmean-field like. We attribute these interesting changes to the broken\ntime-reversal symmetry which suppresses the oscillatory\nRuderman-Kittel-Kasuya-Yosida interaction. Our argument is further strengthened\nby the emergence of the non-collinear spiral exchange coupling across the\ntrilayer magnetic junction with finite spin relaxation.", "category": "cond-mat_mtrl-sci" }, { "text": "Observation of magnetocapacitance in ferromagnetic nanowires: The authors have investigated magnetic domain wall induced capacitance\nvariation as a tool for the detection of magnetic reversal in magnetic\nnanowires for in-plane (NiFe) and out-of-plane (Co/Pd) magnetization\nconfigurations. The switching fields in the capacitance measurements match with\nthat of the magnetoresistance measurements in the opposite sense. The origin of\nthe magnetocapacitance has been attributed to magnetoresistance. This\nmagnetocapacitance detection technique can be useful for magnetic domain wall\nstudies.", "category": "cond-mat_mtrl-sci" }, { "text": "The First Synchrotron Infrared Beamlines at the ALS: Spectromicroscopy\n and Fast Timing: Two recently commissioned infrared beamlines on the 1.4 bending magnet port\nat the Advanced Light Source, LBNL, are described. Using a synchrotron as an IR\nsource provides three primary advantages: increased brightness, very fast light\npulses, and enhanced far-IR flux. The considerable brightness advantage\nmanifests itself most beneficially when performing spectroscopy on a\nmicroscopic length scale. Beamline (BL) 1.4.3 is a dedicated FTIR\nspectromicroscopy beamline, where a diffraction-limited spot size using the\nsynchrotron source is utilized. BL 1.4.2 consists of a vacuum FTIR bench with a\nwide spectral range and step-scan capability. This BL makes use of the pulsed\nnature of the synchrotron light as well as the far-IR flux. Fast timing is\ndemonstrated by observing the pulses from the electron bunch storage pattern at\nthe ALS. Results from several experiments from both IR beamlines will be\npresented as an overview of the IR research currently being done at the ALS.", "category": "cond-mat_mtrl-sci" }, { "text": "SLKMC-II study of self-diffusion of small Ni clusters on Ni (111)\n surface: We studied self-diffusion of small 2D Ni islands (consisting of up to 10\natoms) on Ni (111) surface using a self-learning kinetic Monte Carlo (SLKMC-II)\nmethod with an improved pattern-recognition scheme that allows inclusion of\nboth fcc and hcp sites in the simulations. In an SLKMC simulation, a database\nholds information about the local neighborhood of an atom and associated\nprocesses that is accumulated on-the-fly as the simulation proceeds. In this\nstudy, these diffusion processes were identified using the drag method, and\ntheir activation barriers calculated using a semi-empirical interaction\npotential based on the embedded-atom method. Although a variety of concerted,\nmulti-atom and single-atom processes were automatically revealed in our\nsimulations, we found that these small islands diffuse primarily via concerted\ndiffusion processes. We report diffusion coefficients for each island size at\nvarious tepmratures, the effective energy barrier for islands of each size and\nthe processes most responsible for diffusion of islands of various sizes,\nincluding concerted and multi-atom processes that are not accessible under\nSLKMC-I or in short time-scale MD simulations.", "category": "cond-mat_mtrl-sci" }, { "text": "Spin-filtering and Disorder Induced Giant Magnetoresistance in Carbon\n Nanotubes: Ab Initio Calculations: Nitrogen-doped carbon nanotubes can provide reactive sites on the\nporphyrin-like defects. It's well known that many porphyrins have transition\nmetal atoms, and we have explored transition metal atoms bonded to those\nporphyrin-like defects in N-doped carbon nanotubes. The electronic structure\nand transport are analyzed by means of a combination of density functional\ntheory and recursive Green's functions methods. The results determined the Heme\nB-like defect (an iron atom bonded to four nitrogens) as the most stable and\nwith a higher polarization current for a single defect. With randomly\npositioned Heme B-defects in a few hundred nanometers long nanotubes the\npolarization reaches near 100% meaning an effective spin filter. A disorder\ninduced magnetoresistance effect is also observed in those long nanotubes,\nvalues as high as 20000% are calculated with non-magnectic eletrodes.", "category": "cond-mat_mtrl-sci" }, { "text": "Ab initio DFT+U study of He atom incorporation into UO2 crystals: We present and discuss results of a density functional theory (DFT) study of\na perfect UO2 crystals and He atoms in octahedral interstitial positions. We\nhave calculated basic bulk crystal properties and He incorporation energies\ninto the low temperature anti-ferromagnetic UO2 phase using several\nexchange-correlation functionals within the spin-polarized local density (LDA)\nand generalized gradient (GGA) approximations. In all these DFT calculations we\nincluded the on-site correlation corrections using the Hubbard model (DFT+U\napproach). We analysed a potential crystalline symmetry reduction and confirmed\nthe presence of the Jahn-Teller effect in a perfect UO2. We discuss also the\nproblem of a conducting electronic state arising when He is placed into a\ntetragonal antiferromagnetic phase of UO2. Consequently, we found a specific\nlattice distortion which allows us to restore the semiconducting state and\nproperly estimate He incorporation energies. Unlike the bulk properties, the He\nincorporation energy strongly depends on several factors, including the\nsupercell size, the use of spin polarization, the exchange-correlation\nfunctionals and on-site correlation corrections. We compare our results for the\nHe incorporation with the previous shell model and ab initio DFT calculations.", "category": "cond-mat_mtrl-sci" }, { "text": "Magnetic nanographite: Hydrogenated nanographite can display spontaneous magnetism. Recently we\nproposed that hydrogenation of nanographite is able to induce finite\nmagnetization. We have performed theoretical investigation of a graphene ribbon\nin which each carbon is bonded to two hydrogen atoms at one edge and to a\nsingle hydrogen atom at another edge. Application of the local-spin-density\napproximation to the calculation of the electronic band-structure of the ribbon\nshows appearance of a spin-polarized flat band at the Fermi energy. Producing\ndifferent numbers of mono-hydrogenated carbons and di-hydrogenated carbons can\ncreate magnetic moments in nanographite.", "category": "cond-mat_mtrl-sci" }, { "text": "Static corrections versus dynamic correlation effects in the valence\n band Compton profile spectra of Ni: We compute the Compton profile of Ni using the Local Density Approximation of\nDensity Functional Theory supplemented with electronic correlations treated at\ndifferent levels. The total/magnetic Compton profiles show not only\nquantitative but also qualitative significant differences depending weather\nHubbard corrections are treated at a mean field +U or in a more sophisticated\ndynamic way. Our aim is to discuss the range and capability of electronic\ncorrelations to modify the kinetic energy along specific spatial directions.\n The second and the fourth order moments of the difference in the Compton\nprofiles are discussed as a function of the strength of local Coulomb\ninteraction $U$.", "category": "cond-mat_mtrl-sci" }, { "text": "Temperature and bias voltage dependence of Co/Pd multilayer-based\n magnetic tunnel junctions with perpendicular magnetic anisotropy: Temperature- and bias voltage-dependent transport measurements of magnetic\ntunnel junctions (MTJs) with perpendicularly magnetized Co/Pd electrodes are\npresented. Magnetization measurements of the Co/Pd multilayers are performed to\ncharacterize the electrodes. The effects of the Co layer thickness in the Co/Pd\nbilayers, the annealing temperature, the Co thickness at the MgO barrier\ninterface, and the number of bilayers on the tunneling magneto resistance (TMR)\neffect are investigated. TMR-ratios of about 11 % at room temperature and 18.5\n% at 13 K are measured and two well-defined switching fields are observed. The\nresults are compared to measurements of MTJs with Co-Fe-B electrodes and\nin-plane anisotropy.", "category": "cond-mat_mtrl-sci" }, { "text": "Low-Energy Electron Diffraction With Energy Invariant Carrier Wave\n Wavenumber Modulated by Exchange-Correlation Interaction: We present low-energy electron diffraction (LEED) as elastic electron-atom\nscattering (EEAS) operating in a target crystal waveguide where a Coulombic\ncarrier wave is wavenumber modulated by exchange-correlation (XC) interaction.\nCarrier potential is designed using a KKR (Korringa-Kohn-Rostoker) muffin-tin\nmodel built on overlapping free atoms. XC potential is constructed using\nSernelius's many-particle theory on electron self-energy. EEAS phase shifts are\nderived from Dirac's differential equations, and four recent LEED\ninvestigations are recalculated: Cu(111)+$( 3\\!\\surd3\\times\\!\\surd3 )\n\\mathrm{R30^\\circ}$-TMB, Ag(111)+$(4\\!\\times \\!4 )$-O, Ag(111)+$(\n7\\!\\times\\!\\surd3 ) \\mathrm{rect}$-$\\mathrm{SO}_4$, Ru(0001)+$(\n\\surd3\\!\\times\\!\\surd3 ) \\mathrm{R30^\\circ}$-C. TMB stands for\n1,3,5-tris(4-mercaptophenyl)-benzene with chemical formula\nC$_{24}$H$_{15}$S$_3$. We are able to report substantially improved reliability\nfactors.", "category": "cond-mat_mtrl-sci" }, { "text": "Tunability of the optical absorption in small silver cluster-polymer\n hybrid systems: We have calculated the absorption characteristics of different hybrid systems\nconsisting of Ag, Ag2 or Ag3 atomic clusters and poly(methacrylic acid) (PMAA)\nusing the time-dependent density-functional theory. The polymer is found to\nhave an extensive structural-dependency on the spectral patterns of the hybrid\nsystems relative to the bare clusters. The absorption spectrum can be `tuned'\nto the visible range for hybrid systems with an odd number of electrons per\nsilver cluster, whereas for hybrid systems comprising an even number of\nelectrons, the leading absorption edge can be shifted up to about 4.5 eV. The\nresults give theoretical support to the experimental observations on the\nabsorption in the visible range in metal cluster-polymer hybrid structures.", "category": "cond-mat_mtrl-sci" }, { "text": "Effect of Gd/Nd doping on the magnetic properties of PrMnO3: A study on temperature dependent magnetic properties of single phase\northorhombic perovskites system associated with space group Pbnm compounds\nPr1-x(Gd/Nd)xMnO3 (x=0.3, 0.5, 0.7) was carried out. A magnetization reversal\nis observed below the Neel temperature (TN), in DC magnetization measurements\n(at 50 Oe) in the doped compounds. This may be due to the antiparallel coupling\nbetween the two magnetic sublattices (|Pr+Gd/Nd | and Mn). With lowering of\ntemperature, the |Pr+ Gd/Nd|) ions begin to polarize under the negative\ninternal field due to canted moment of Mn moments. The hysteresis plot taken at\n50K shows a ferrimagnetic characteristic and the presence of spin canting of\nions in the magnetic sublattices. Arrott plot indicates field induced second\norder paramagnetic to ferrimagnetic (PM-FiM) phase transition in this system.", "category": "cond-mat_mtrl-sci" }, { "text": "Silicon nanowire band gap modification: Band gap modification for small-diameter (1 nm) silicon nanowires resulting\nfrom the use of different species for surface termination is investigated by\ndensity functional theory calculations. Because of quantum confinement,\nsmall-diameter wires exhibit a direct band gap that increases as the wire\ndiameter narrows, irrespective of surface termination. This effect has been\nobserved in previous experimental and theoretical studies for hydrogenated\nwires. For a fixed cross-section, the functional group used to saturate the\nsilicon surface significantly modifies the band gap, resulting in relative\nenergy shifts of up to an electronvolt. The band gap shifts are traced to\ndetails of the hybridization between the silicon valence band and the frontier\norbitals of the terminating group, which is in competition with quantum\nconfinement.", "category": "cond-mat_mtrl-sci" }, { "text": "The quantum spin Hall effect and topological insulators: In topological insulators, spin-orbit coupling and time-reversal symmetry\ncombine to form a novel state of matter predicted to have exotic physical\nproperties.", "category": "cond-mat_mtrl-sci" }, { "text": "Ten-million-atom electronic structure calculations on the K computer\n with a massively parallel order-N theory: A massively parallel order-N electronic structure theory was constructed by\nan interdisciplinary research between physics, applied mathematics and computer\nscience. (1) A high parallel efficiency with ten-million-atom nanomaterials was\nrealized on the K computer with upto 98,304 processor cores. The mathematical\nfoundation is a novel linear algebraic algorithm for the generalized shifted\nlinear equation. The calculation was carried out by our code ' ELSES '\n(www.elses.jp) with modelled (tight-binding-form) systems based on ab initio\ncalculations. (2) A post-calculation analysis method, called pi-orbital\ncrystalline orbital Hamiltonian population (pi-COHP) method, is presented,\nsince the method is ideal for huge electronic structure data distributed among\nmassive nodes. The analysis method is demonstrated in an sp2-sp3 nano-composite\ncarbon solid, with an original visualization software 'VisBAR'. The present\nresearch indicates general aspects of computational physics with current or\nnext-generation supercomputers.", "category": "cond-mat_mtrl-sci" }, { "text": "Charged surfaces and slabs in periodic boundary conditions: Plane wave density functional theory codes generally assume periodicity in\nall three dimensions. This causes difficulties when studying charged systems,\nfor instance energies per unit cell become infinite, and, even after being\nrenormalised by the introduction of a uniform neutralising background, are very\nslow to converge with cell size. The periodicity introduces spurious electric\nfields which decay slowly with cell size and which also slow the convergence of\nother properties relating to the ground state charge density. This paper\npresents a simple self-consistent technique for producing rapid convergence of\nboth energies and charge distribution in the particular geometry of 2D\nperiodicity, as used for studying surfaces.", "category": "cond-mat_mtrl-sci" }, { "text": "Dislocations and cracks in generalized continua: Dislocations play a key role in the understanding of many phenomena in solid\nstate physics, materials science, crystallography and engineering. Dislocations\nare line defects producing distortions and self-stresses in an otherwise\nperfect crystal lattice. In particular, dislocations are the primary carrier of\ncrystal plasticity and in dislocation based fracture mechanics.", "category": "cond-mat_mtrl-sci" }, { "text": "Is there something of the MCT in orientationally disordered crystals ?: Molecular Dynamics simulations have been performed on the orientationally\ndisordered crystal chloroadamantane: a model system where dynamics are almost\ncompletely controlled by rotations. A critical temperature T_c = 225 K as\npredicted by the Mode Coupling Theory can be clearly determined both in the\nalpha and beta dynamical regimes. This investigation also shows the existence\nof a second remarkable dynamical crossover at the temperature T_x > T_c\nconsistent with a previous NMR and MD study [1]. This allows us to confirm\nclearly the existence of a 'landscape-influenced' regime occurring in the\ntemperature range [T_c-T_x] as recently proposed [2,3].", "category": "cond-mat_mtrl-sci" }, { "text": "Dependence of Magnetic Anisotropy and Magnetoresistance of\n Ni81Fe19-Films on Annealing: Permalloy (Py:Ni81Fe19) exhibits an anisotropic magnetoresistance (AMR) which\nis very often used to read magnetic signals from storage devices. Py-films of\nthickness 20nm were prepared by dc-magnetron sputtering in a magnetic field\nonto thermally oxidized Si-wafers and annealed ex situ at temperatures up to\n1000K in order to investigate the dependence of the magnetic anisotropy and the\nAMR on heat treatments. The films exhibit an uniaxial anisotropy after\npreparation which changes during annealing above 520K. The AMR along the former\nmagnetically easy axis as well as the corresponding field sensitivity are\nincreased by a heat treatment around 700K reaching maxima of about 8% and a\nmaximum sensitivity of 1.5%/Oe, respectively. We discuss possible sources for\nthe change in anisotropy, i.e. strain effects, inhomogeneities, and changes of\nthe local atomic order.", "category": "cond-mat_mtrl-sci" }, { "text": "Spin Dependent Lifetimes and Spin-orbit Hybridization Points in Heusler\n Compounds: We present an ab initio calculation of the k and spin-resolved electronic\nlifetimes in the half-metallic Heusler compounds Co(2)MnSi and Co(2)FeSi. We\ndetermine the spin-flip and spin-conserving contributions to the lifetimes and\nstudy in detail the behavior of the lifetimes around states that are strongly\nspin-mixed by spin-orbit coupling. We find that, for non-degenerate bands, the\nspin mixing alone does not determine the energy dependence of the (spin-flip)\nlifetimes. Qualitatively, the lifetimes reflect the lineup of electron and hole\nbands. We predict that different excitation conditions lead to drastically\ndifferent spin-flip dynamics of excited electrons and may even give rise to an\nenhancement of the non-equilibrium spin polarization.", "category": "cond-mat_mtrl-sci" }, { "text": "Spontaneous anomalous Hall effect arising from an unconventional\n compensated magnetic phase in a semiconductor: The anomalous Hall effect, commonly observed in metallic magnets, has been\nestablished to originate from the time-reversal symmetry breaking by an\ninternal macroscopic magnetization in ferromagnets or by a non-collinear\nmagnetic order. Here we observe a spontaneous anomalous Hall signal in the\nabsence of an external magnetic field in an epitaxial film of MnTe, which is a\nsemiconductor with a collinear antiparallel magnetic ordering of Mn moments and\na vanishing net magnetization. The anomalous Hall effect arises from an\nunconventional phase with strong time-reversal symmetry breaking and\nalternating spin polarization in real-space crystal structure and\nmomentum-space electronic structure. The anisotropic crystal environment of\nmagnetic Mn atoms due to the non-magnetic Te atoms is essential for\nestablishing the unconventional phase and generating the anomalous Hall effect.", "category": "cond-mat_mtrl-sci" }, { "text": "Giant interfacial perpendicular magnetic anisotropy in MgO/CoFe/capping\n layer structures: Magnetic tunnel junction (MTJ) based on CoFeB/MgO/CoFeB structures is of\ngreat interest due to its application in the spin-transfer-torque magnetic\nrandom access memory (STT-MRAM). Large interfacial perpendicular magnetic\nanisotropy (PMA) is required to achieve high thermal stability. Here we use\nfirst-principles calculations to investigate the magnetic anisotropy energy\n(MAE) of MgO/CoFe/capping layer structures, where the capping materials include\n5d metals Hf, Ta, Re, Os, Ir, Pt, Au and 6p metals Tl, Pb, Bi. We demonstrate\nthat it is feasible to enhance PMA by using proper capping materials.\nRelatively large PMA is found in the structures with capping materials of Hf,\nTa, Os, Ir and Pb. More importantly, the MgO/CoFe/Bi structure gives rise to\ngiant PMA (6.09 mJ/m2), which is about three times larger than that of the\nMgO/CoFe/Ta structure. The origin of the MAE is elucidated by examining the\ncontributions to MAE from each atomic layer and orbital. These findings provide\na comprehensive understanding of the PMA and point towards the possibility to\nachieve advanced-node STT-MRAM with high thermal stability.", "category": "cond-mat_mtrl-sci" }, { "text": "A fully ab initio approach to inelastic atom-surface scattering: We introduce a fully ab initio theory for inelastic scattering of any atom\nfrom any surface exciting single phonons, and apply the theory to helium\nscattering from Nb(100). The key aspect making our approach general is a direct\nfirst-principles evaluation of the scattering atom-electron vertex. By\ncorrecting misleading results from current state-of-the-art theories, this\nfully ab initio approach will be critical in guiding and interpreting\nexperiments that adopt next-generation, non-destructive atomic beam scattering.", "category": "cond-mat_mtrl-sci" }, { "text": "Metalloboranes from first-principles calculations: A candidate for\n high-density hydrogen storage: Using first principles calculations, we show the high hydrogen storage\ncapacity of a new class of compounds, metalloboranes. Metalloboranes are\ntransition metal (TM) and borane compounds that obey a novel-bonding scheme. We\nhave found that the transition metal atoms can bind up to 10 H2 molecules.", "category": "cond-mat_mtrl-sci" }, { "text": "Modelling the Nonlinear Response of Fibre-reinforced Bending Fluidic\n Actuators: Soft actuators are receiving increasing attention from the engineering\ncommunity, not only in research but even for industrial applications. Among\nsoft actuators, fibre-reinforced Bending Fluidic Actuators (BFAs) became very\npopular thanks to features such as robustness and easy design and fabrication.\nHowever, an accurate modelling of these smart structures, taking into account\nall the nonlinearities involved, is a challenging task. In this effort, we\npropose an analytical mechanical model to capture the quasi-static response of\nfibre-reinforced BFAs. The model is fully 3D and for the first time includes\nthe effect of the pressure on the lateral surface of the chamber as well as the\nnon-constant torque produced by the pressure at the tip. The presented model\ncan be used for design and control, while providing information about the\nmechanics of these complex actuators.", "category": "cond-mat_mtrl-sci" }, { "text": "On microcontinuum field theories: the Eshelby stress tensor and\n incompatibility conditions: We investigate linear theories of incompatible micromorphic elasticity,\nincompatible microstretch elasticity, incompatible micropolar elasticity and\nthe incompatible dilatation theory of elasticity (elasticity with voids). The\nincompatibility conditions and Bianchi identities are derived and discussed.\nThe Eshelby stress tensor (static energy momentum) is calculated for such\ninhomogeneous media with microstructure. Its divergence gives the driving\nforces for dislocations, disclinations, point defects and inhomogeneities which\nare called configurational forces.", "category": "cond-mat_mtrl-sci" }, { "text": "Excitonic Photoluminescence properties of nanocrystalline GaSb and\n Ga0.62In0.38Sb embedded in silica films: The GaSb and Ga0.62In0.38Sb nanocrystals were embedded in the SiO2 films by\nradio-frequency magnetron co-sputtering and were grown on GaSb and Si\nsubstrates at different temperatures. We present results on the 10K excitonic\nphotoluminescence (PL) properties of nanocrystalline GaSb and Ga0.62In0.38Sb as\na function of their size. The measurements show that the PL of the GaSb and\nGa0.62In0.38Sb nanocrystallites follows the quantum confinement model very\nclosely. By using deconvolution of PL spectra, origins of structures in\nphotoluminescence were identified.", "category": "cond-mat_mtrl-sci" }, { "text": "Excitons and Many-Electron Effects in the Optical Response of\n Single-Walled Boron Nitride Nanotubes: We report first-principles calculations of the effects of quasiparticle\nself-energy and electron-hole interaction on the optical properties of\nsingle-walled BN nanotubes. Excitonic effects are shown to be even more\nimportant in BN nanotubes than in carbon nanotubes. Electron-hole interactions\ngive rise to complexes of bright (and dark) excitons, which qualitatively alter\nthe optical response. Excitons with binding energy larger than 2 eV are found\nin the (8,0) BN nanotubes. Moreover, unlike the carbon nanotubes, theory\npredicts that these exciton states are comprised of coherent supposition of\ntransitions from several different subband pairs, giving rise to novel\nbehaviors.", "category": "cond-mat_mtrl-sci" }, { "text": "The roles of adhesion, internal heat generation and elevated\n temperatures in normally loaded, sliding rough surfaces: The thermal effects of plastic and frictional heat generation and elevated\ntemperature were examined along with the role of adhesion in the context of\ngalling wear, using a representative crystal plasticity, normally loaded,\nsliding surface model. Galling frequency behaviour was predicted for 316L\nsteel. Deformation of the surfaces was dominated by the surface geometry, with\nno significant effect due to variations in frictional models. Plastic and\nfrictional heating were found to have a minimal effect on the deformation of\nthe surface, with the rapid conduction of heat preventing any highly localised\nheating. There was no corresponding effect on the predicted galling frequency\nresponse.\n Isothermal, elevated temperature conditions caused a decrease in galling\nresistance, driven by the temperature sensitivity of the critical resolved\nshear stress. The extent of deformation, as quantified by the area of\nplastically deformed material and plastic reach, increased with temperature.\nComparisons were made with literature results for several surface amplitude and\nwavelength conditions. Model results compared favourably with those in the\nliterature. However, the reduction in predicted galling resistance with\nelevated temperature for a fixed surface was not as severe as observations in\nthe literature, suggesting other mechanisms (e.g. phase transformations,\nsurface coatings and oxides) are likely important.", "category": "cond-mat_mtrl-sci" }, { "text": "Buckled honeycomb lattice and unconventional magnetic response: We study the magnetic response of buckled honeycomb-lattice materials. The\nbuckling breaks the sublattice symmetry, enhances the spin-orbit coupling, and\nallows the tuning of a topological quantum phase transition. As a result, there\nare two doubly degenerate spin-valley coupled massive Dirac bands, which\nexhibit an unconventional Hall plateau sequence under strong magnetic fields.\nWe show how to externally control the splitting of anomalous zeroth Landau\nlevels, the prominent Landau level crossing effects, and the polarizations of\nspin, valley, and sublattice degrees of freedom. In particular, we reveal that\nin a p-n junction, spin-resolved fractionally quantized conductance appears in\na two-terminal measurement with a spin-polarized current propagating along the\ninterface. In the low-field regime where the Landau quantization is not\napplicable, we provide a semiclassical description for the anomalous Hall\ntransport. We comment briefly on the effects of electron-electron interactions\nand Zeeman couplings to electron spins and to atomic orbitals.", "category": "cond-mat_mtrl-sci" }, { "text": "Tunable mechanical and thermal properties of ZnS/CdS core/shell\n nanowires: Using all atom molecular dynamics (MD) simulations, we have studied the\nmechanical properties of ZnS/CdS core/shell nanowires. Our results show that\nthe coating of a few atomic layer CdS shell on the ZnS nanowire leads to a\nsignificant change in the stiffness of the core/shell nanowires compared to the\nstiffness of pure ZnS nanowires. The binding energy between the core and shell\nregion decreases due to the lattice mismatch at the core-shell interface. This\nreduction in binding energy plays an important role in determining the\nstiffness of a core/shell nanowire. We have also investigated the effects of\nthe shell on the thermal conductivity and melting behavior of the nanowires.", "category": "cond-mat_mtrl-sci" }, { "text": "Unusually High and Anisotropic Thermal Conductivity in Amorphous Silicon\n Nanostructures: Amorphous Si (a-Si) nanostructures are ubiquitous in numerous electronic and\noptoelectronic devices. Amorphous materials are considered to possess the lower\nlimit to the thermal conductivity (k), which is ~1 W/m-K for a-Si. However,\nrecent work suggested that k of micro-thick a-Si films can be greater than 3\nW/m-K, which is contributed by propagating vibrational modes, referred to as\n\"propagons\". However, precise determination of k in a-Si has been elusive.\nHere, we used novel structures of a-Si nanotubes and suspended a-Si films that\nenabled precise in-plane k measurement within a wide thickness range of 5 nm to\n1.7 um. We showed unexpectedly high in plane k in a-Si nanostructures, reaching\n~3.0 and 5.3 W/m-K at 100 nm and 1.7 um, respectively. Furthermore, the\nmeasured in plane k is significantly higher than the cross-plane k on the same\nfilms. This usually high and anisotropic k in the amorphous Si nanostructures\nmanifests the surprising broad propaganda mean free path distribution, which is\nfound to range from 10 nm to 10 um, in the disordered and atomically isotropic\nstructure. This result provides an unambiguous answer to the century-old\nproblem regarding the mean free path distribution of propagons and also shed\nlight on the design and performance of numerous a-Si based electronic and\noptoelectronics devices.", "category": "cond-mat_mtrl-sci" }, { "text": "Designing of Organic Bridging Linkers of Metal-Organic Frameworks for\n Enhanced Carbon Dioxide Adsorption: The global rate of anthropogenic CO2 emission is rising, which urges the\ndevelopment of efficient carbon capture and storage (CCS) technologies. Among\nthe various CO2 capture methods, adsorption by the linkers of the Metal-Organic\nFrameworks (MOFs) materials has received more interest as excellent CO2\nadsorbents because of their important role in understanding the interaction\nmechanism for CO2 adsorption. Here, we investigate the adsorption of CO2\nmolecules at the center and side positions of several MOF-linkers using\nmolecular cluster models. The interaction between CO2 and the linkers is\napproximated by computing the binding enthalpy ({\\Delta}H) through the first\nprinciples-based Density Functional Theory with Grimmes dispersion correction\n(i.e., B3LYP-D3) and second-order Moller Plesset Theory (MP2). The computed\nvalues of {\\Delta}H indicate the weak nature of CO2 adsorption on the pristine\nlinkers, hence the strategy of lithium decoration is used to see its impact on\nthe binding strength. Among the various linkers tested, CO2 adsorbing at the\nside position of the DFBDC-2 linker has strong adsorption with {\\Delta}H value\nof about -35.32 kJ/mol computed by the B3LYP-D3 method. The Energy\nDecomposition Analysis (EDA) study reveals that among all the energy terms, the\ncontribution of electrostatic and polarization energy terms to the {\\Delta}H\nvalue are the most dominant one. Furthermore, the results of Frontier Molecular\nOrbital Analysis (FMO) revealed that all the linkers remained stable even after\nLi-decoration. The results of our investigations will direct towards the\ndevelopment and synthesis of novel adsorbents with enhanced CO2 adsorption.", "category": "cond-mat_mtrl-sci" }, { "text": "Novel experimental design for high pressure - high temperature\n electrical resistance measurements in a 'Paris-Edinburgh' large volume press: We present a novel experimental design for high sensitivity measurements of\nthe electrical resistance of samples at high pressures (0-6GPa) and high\ntemperatures (300-1000K) in a 'Paris-Edinburgh' type large volume press.\nUniquely, the electrical measurements are carried out directly on a small\nsample, thus greatly increasing the sensitivity of the measurement. The\nsensitivity to even minor changes in electrical resistance can be used to\nclearly identify phase transitions in material samples. Electrical resistance\nmeasurements are relatively simple and rapid to execute and the efficacy of the\npresent experimental design is demonstrated by measuring the electrical\nresistance of Pb, Sn and Bi across a wide domain of temperature-pressure phase\nspace and employing it to identify the loci of phase transitions. Based on\nthese results, the phase diagrams of these elements are reconstructed to high\naccuracy and found to be in excellent agreement with previous studies. In\nparticular, by mapping the locations of several well-studied reference points\nin the phase diagram of Sn and Bi, it is demonstrated that a standard\ncalibration exists for the temperature and pressure, thus eliminating the need\nfor direct or indirect temperature and pressure measurements. The present\ntechnique will allow simple and accurate mapping of phase diagrams under\nextreme conditions and may be of particular importance in advancing studies of\nliquid state anomalies.", "category": "cond-mat_mtrl-sci" }, { "text": "Electronic and optical properties in graphane: We develop the tight-binding model to study electronic and optical properties\nof graphane. The strong sp3 chemical bondings among the carbon and hydrogen\natoms induce a special band structure and thus lead to the rich optical\nexcitations. The absorption spectrum hardly depends on the direction of\nelectric polarization. It ex- hibits a lot of shoulder structures and\nabsorption peaks, which arise from the extreme points and the saddle points of\nthe parabolic bands, respectively. The threshold op- tical excitations, only\nassociated with the 2px and 2py orbitals of the carbon atoms, are revealed in a\nshoulder structure at ?3.5 eV. The first symmetric absorption peak, appearing\nat ~11 eV, corresponds to energy bands due to the considerable hybridiza- tion\nof carbon 2pz orbitals and H 1s orbitals. Also, some absorption peaks at higher\nfrequencies indicate the bonding of 2s and 1s orbitals. These results are in\nsharp contrast to those of the sp2 graphene systems.", "category": "cond-mat_mtrl-sci" }, { "text": "Inherent heating instability of direct microwave sintering process:\n Sample analysis for porous 3Y-ZrO2: Direct microwave heating of 3Y-ZrO 2 is studied at frequency of 2.45 GHz.\nDifferent conditions of input power, sample position and size are tested. For\nthe first time, the experimentally known instability of microwave sintering is\nexplained coupling the effective medium approximation and finite-element\nmethod. We show how the material dielectric permittivity imaginary part which\nincreases with temperature and relative density encourages high hot spot\nphenomena. It is shown that the sample location has a great impact on the", "category": "cond-mat_mtrl-sci" }, { "text": "NiCl3 Monolayer: Dirac Spin-Gapless Semiconductor and Chern Insulator: The great obstacle for practical applications of the quantum anomalous Hall\n(QAH) effect is the lack of suitable QAH materials (Chern insulators) with\nlarge non-trivial band gap, room-temperature magnetic order and high carrier\nmobility. The Nickle chloride (NiCl3) monolayer characteristics are\ninvestigated herein using first-principles calculations. It is reported that\nNiCl3 monolayers constitute a new class of Dirac materials with Dirac\nspin-gapless semiconducting and high-temperature ferromagnetism (~400K). Taking\ninto account the spin-orbit coupling, the NiCl3 monolayer becomes an intrinsic\ninsulator with a large non-trivial band gap of ~24 meV, corresponding to an\noperating temperature as high as ~280K at which the quantum anomalous Hall\neffect could be observed. The calculated large non-trivial gap, high Curie\ntemperature and single-spin Dirac states reported herein for the NiCl3\nmonolayer lead us to propose that this material give a great promise for\npotential realization of a near-room temperature QAH effect and potential\napplications in spintronics. Last but not least the calculated Fermi velocities\nof Dirac fermion of about 4x105 m/s indicate very high mobility in NiCl3\nmonolayers.", "category": "cond-mat_mtrl-sci" }, { "text": "Combined experimental and theoretical investigation of the\n premartensitic transition in Ni$_2$MnGa: Ultraviolet-photoemission (UPS) measurements and supporting specific-heat,\nthermal-expansion, resistivity and magnetic-moment measurements are reported\nfor the magnetic shape-memory alloy Ni$_2$MnGa over the temperature range $100K\n< T < 250K$. All measurements detect clear signatures of the premartensitic\ntransition ($T_\\mathrm{PM}\\sim 247K$) and the martensitic transition\n($T_\\mathrm{M} \\sim 196K$). Temperature-dependent UPS shows a dramatic\ndepletion of states (pseudogap) at $T_\\mathrm{PM}$ located 0.3eV below the\nFermi energy. First-principles electronic structure calculations show that the\npeak observed at 0.3eV in the UPS spectra for $T > T_\\mathrm{PM}$ is due to the\nNi-d minority-spin electrons. Below $T_\\mathrm{M}$ this peak disappears,\nresulting in an enhanced density of states at energies around 0.8eV. This\nenhancement reflects Ni-d and Mn-d electronic contributions to the\nmajority-spin density of states and is accompanied by significant\nreconstruction of the Fermi surface.", "category": "cond-mat_mtrl-sci" }, { "text": "Advanced calculations of x-ray spectroscopies with FEFF10 and Corvus: The real-space Green's function code FEFF has been extensively developed and\nused for calculations of x-ray and related spectra, including x-ray absorption\n(XAS), x-ray emission (XES), inelastic x-ray scattering, and electron energy\nloss spectra (EELS). The code is particularly useful for the analysis and\ninterpretation of the XAS fine-structure (EXAFS) and the near-edge structure\n(XANES) in materials throughout the periodic table. Nevertheless, many\napplications, such as non-equilibrium systems, and the analysis of ultra-fast\npump-probe experiments, require extensions of the code including\nfinite-temperature and auxiliary calculations of structure and vibrational\nproperties. To enable these extensions, we have developed in tandem, a new\nversion FEFF10, and new FEFF based workflows for the Corvus workflow manager,\nwhich allow users to easily augment the capabilities of FEFF10 via auxiliary\ncodes. This coupling facilitates simplified input and automated calculations of\nspectra based on advanced theoretical techniques. The approach is illustrated\nwith examples of high temperature behavior, vibrational properties, many-body\nexcitations in XAS, super-heavy materials, and fits of calculated spectra to\nexperiment.", "category": "cond-mat_mtrl-sci" }, { "text": "Analysis of the linear relationship between asymmetry and magnetic\n moment at the M-edge of 3d transition metals: The magneto-optical response of Fe and Ni during ultrafast demagnetization is\nstudied experimentally and theoretically. We have performed pump-probe\nexperiments in the transverse magneto-optical Kerr effect (T-MOKE) geometry\nusing photon energies that cover the M-absorption edges of Fe and Ni between 40\nto 72 eV. The asymmetry was detected by measuring the reflection of light for\ntwo different orientations of the sample magnetization. Density functional\ntheory (DFT) wasused to calculate the magneto-optical response of different\nmagnetic configurations, representing different types of excitations:\nlong-wavelength magnons, short wavelength magnons, and Stoner excitations. In\nthe case of Fe, we find that the calculated asymmetry is strongly dependent on\nthe specific type of magnetic excitation. Our modelling also reveals that\nduring remagnetization Fe is, to a reasonable approximation, described by\nmagnons, even though small non-linear contributions could indicate some degree\nof Stoner excitations as well. In contrast, we find that the calculated\nasymmetry in Ni is rather insensitive to the type of magnetic excitations.\nHowever, there is a weak non-linearity in the relation between asymmetry and\nthe off-diagonal component of the dielectric tensor, which does not originate\nfrom the modifications of the electronic structure. Our experimental and\ntheoretical results thus emphasize the need of considering a coupling between\nasymmetry and magnetization that may be more complex that a simple linear\nrelationship. This insight is crucial for the microscopic interpretation of\nultrafast magnetization experiments.", "category": "cond-mat_mtrl-sci" }, { "text": "Quantitative Phase Field Model for Electrochemical Systems: Modeling microstructure evolution in electrochemical systems is vital for\nunderstanding the mechanism of various electrochemical processes. In this work,\nwe propose a general phase field framework that is fully variational and thus\nguarantees that the energy decreases upon evolution in an isothermal system.\nThe bulk and interface free energies are decoupled using a grand potential\nformulation to enhance numerical efficiency. The variational definition of the\noverpotential is used, and the reaction kinetics is incorporated into the\nevolution equation for the phase field to correctly capture capillary effects\nand eliminate additional model parameter calibrations. A higher-order kinetic\ncorrection is derived to accurately reproduce general reaction models such as\nthe Butler-Volmer, Marcus, and Marcus-Hush-Chidsey models. Electrostatic\npotentials in the electrode and the electrolyte are considered separately as\nindependent variables, providing additional freedom to capture the interfacial\npotential jump. To handle realistic materials and processing parameters for\npractical applications, a driving force extension method is used to enhance the\ngrid size by three orders of magnitude. Finally, we comprehensively verify our\nphase field model using classical electrochemical theory.", "category": "cond-mat_mtrl-sci" }, { "text": "Real time optical observation and control of atomically thin transition\n metal dichalcogenide synthesis: Understanding the mechanisms involved in chemical vapour deposition (CVD)\nsynthesis of atomically thin transition metal dichalcogenides (TMDCs) requires\nthe precise control of numerous growth parameters. All the proposed mechanisms\nand their relation to the growth conditions are inferred from characterising\nintermediate formations obtained by stopping the growth blindly. To fully\nunderstand the reaction routes that lead to the monolayer formation, real time\nobservation and control of the growth are needed. Here, we demonstrate how a\ncustom-made CVD chamber that allows real time optical monitoring can be\nemployed to study the reaction routes that are critical to the production of\nthe desired layered thin crystals in salt assisted TMDC synthesis. Our real\ntime observations reveal the reaction between the salt and the metallic\nprecursor to form intermediate compounds which lead to the layered crystal\nformation. We identified that both the vapour-solid-solid and\nvapour-liquid-solid growth routes are in an interplay. Furthermore, we\ndemonstrate the role H$_{2}$ plays in the salt-assisted WSe$_{2}$ synthesis.\nFinally, we guided the crystal formation by directing the liquid intermediate\ncompound through pre-patterned channels. The methods presented in this article\ncan be extended to other materials that can be synthesized via CVD.", "category": "cond-mat_mtrl-sci" }, { "text": "Shack-Hartmann wavefront sensing: A new approach to time-resolved\n measurement of stress intensity during dynamic fracture of small brittle\n specimens: The stress intensity factor is important for understanding crack initiation\nand propagation. Because it cannot be measured directly, the characterization\nof the stress intensity factor relies on the measurement of deformation around\na crack tip. Such measurements are challenging for dynamic fracture of brittle\nmaterials where the deformation is small and the crack tip velocity can be high\n(>1 km/s). Digital gradient sensing (DGS) is capable of full-field measurement\nof surface deformation with sub-microsecond temporal resolution, but it is\nlimited to centimeter-scale specimens and has a spatial resolution of only\n$\\sim 1$mm. This limits its ability to measure deformations close to the crack\ntip. Here, we demonstrate the potential of Shack-Hartmann wavefront sensing\n(SHWFS), as an alternative to DGS, for measuring surface deformation during\ndynamic brittle fracture of millimeter-scale specimens. Using an commercial\nglass ceramic as an example material, we demonstrate the capability of SHWFS to\nmeasure the surface slope evolution induced by a propagating crack on\nmillimeter-scale specimens with a micrometer-scale spatial resolution and a\nsub-microsecond temporal resolution. The SHWFS apparatus has the additional\nadvantage of being physically more compact than a typical DGS apparatus. We\nverify our SHWFS measurements by comparing them with analytical predictions and\nphase-field simulations of the surface slope around a crack tip. Then, fitting\nthe surface slope measurements to the asymptotic crack-tip field solution, we\nextract the evolution of the apparent stress intensity factor associated with\nthe propagating crack tip. We conclude by discussing potential future\nenhancements of this technique and how its compactness could enable the\nintegration with other characterization techniques including x-ray\nphase-contrast imaging (XPCI) toward a multi-modal characterization.", "category": "cond-mat_mtrl-sci" }, { "text": "Ultrafast electronic and lattice dynamics in laser-excited crystalline\n bismuth: Femtosecond spectroscopy is applied to study transient electronic and lattice\nprocesses in bismuth. Components with relaxation times of 1 ps, 7 ps and ~ 1 ns\nare detected in the photoinduced reflectivity response of the crystal. To\nfacilitate the assignment of the observed relaxation to the decay of particular\nexcited electronic states we use pump pulses with central wavelengths ranging\nfrom 400 nm to 2.3 mum. Additionally, we examine the variation of parameters of\ncoherent A1g phonons upon the change of excitation and probing conditions. Data\nanalysis reveals a significant wavevector dependence of electron-hole and\nelectron- phonon coupling strength along \\Gamma--T direction of the Brillouin\nzone.", "category": "cond-mat_mtrl-sci" }, { "text": "Random sequential adsorption of tetramers: Adsorption of tetramer built of four identical spheres was studied\nnumerically using the Random Sequential Adsorption (RSA) algorithm. Tetramers\nwere adsorbed on a two dimensional, flat and homogeneous surface. Two different\nmodels of the adsorbate were investigated: a rhomboid and a square one; monomer\ncentres were put on vertices of rhomboids and squares, respectively. Numerical\nsimulations allow to establish the maximal random coverage ratio as well as the\nAvailable Surface Function (ASF), which is crucial for determining kinetics of\nthe adsorption process. These results were compared with data obtained\nexperimentally for KfrA plasmid adsorption. Additionally, the density\nautocorrelation function was measured.", "category": "cond-mat_mtrl-sci" }, { "text": "Hybrid Optical Modes in Hexagonal Crystals: In nanostructure electronic devices, it is well-known that the optical\nlattice waves in the constituent semiconductor crystals have to obey both\nmechanical and electrical boundary conditions at an interface. The theory of\nhybrid optical modes, established for cubic crystals, is here applied to\nhexagonal crystals. In general, the hybrid is a linear combination of a\nlongitudinally-polarized (LO) mode, an interface mode (IF), and an interface TO\nmode. It is noted that the dielectric and elastic anisotropy of these crystals\nadd significant complications to the assessment of the electro-phonon\ninteraction. We point out that, where extreme accuracy is not needed, a cubic\napproximation is available. The crucial role of lattice dispersion is\nemphasised. In the extreme long-wavelength limit, where lattice dispersion is\nunimportant, the polar optical hybrid consists of an LO component plus an IF\ncomponent only. In his case no fields are induced in the barrier, and there are\nno remote-phonon effects.", "category": "cond-mat_mtrl-sci" }, { "text": "Twist-bend heliconical chiral nematic liquid crystal phase of an achiral\n rigid bent-core mesogen: The chiral, heliconical (twist-bend) nematic ground state is reported in an\nachiral, rigid, bent-core mesogen (UD68). Similar to the nematic twist-bend\n(NTB) phase observed in bent molecular dimers, the NTB phase of UD68 forms\nmacroscopic, smectic-like focal-conic textures and exhibits nanoscale, periodic\nmodulation with no associated modulation of the electron density, i.e., without\na detectable lamellar x-ray reflection peak. The NTB helical pitch is pTB ~ 14\nnm. When an electric field is applied normal to the helix axis, a weak\nelectroclinic effect is observed, revealing 50 um-scale left- and right-handed\ndomains in a chiral conglomerate.", "category": "cond-mat_mtrl-sci" }, { "text": "Spin dynamics from a constrained magnetic Tight-Binding model: A dynamics of the precession of coupled atomic moments in the tight-binding\n(TB) approximation is presented. By implementing an angular penalty functional\nin the energy that captures the magnetic effective fields self-consistently,\nthe motion of the orientation of the local magnetic moments is observed faster\nthan the variation of their magnitudes. This allows the computation of the\neffective atomic magnetic fields that are found consistent with the\nHeisenberg's exchange interaction, by comparison with classical atomistic spin\ndynamics on Fe, Co and Ni magnetic clusters.", "category": "cond-mat_mtrl-sci" }, { "text": "Improvement of the poly-3-hexylthiophene transistor performance using\n small molecule contact functionalization: We demonstrate an approach to improve poly-3-hexylthiophene field effect\ntransistors by modifying the gold contacts with monolayer thick\npentacenequinone (PQ) or naphthalene (NL). The effective contact resistance is\nreduced by a factor of two and sixteen for interlayers of PQ and NL,\nrespectively. The observation is attributed to different injection barriers at\nthe metal-organic interface caused by the functionalization and to an\nadditional tunneling barrier enhancing the on/off ratios. This barrier yields\nto activation energies of 37meV (NL) and 104meV (PQ) below 190K, which are\nsmaller than without functionalization, 117meV.", "category": "cond-mat_mtrl-sci" }, { "text": "Drude weight in presence of nonlocal potentials: The nonlocal potential contributes an extra term to the velocity operator; I\nshow here that such term affects the formal expression of the Drude weight in a\nnontrivial way. Notably, the present main result fixes a disturbing discrepancy\nin the Dreyer-Coh-Stengel sum rule [Phys. Rev. Lett. {\\bf 128}, 095901 (2022)].", "category": "cond-mat_mtrl-sci" }, { "text": "Phase Transition of Iron-based Single Crystals at Extreme Strain Rates\n under Dynamic Loadings: Phase transition of iron, as a prototype of martensite phase transition under\ndynamic loadings, exhibits huge diverges in its TP among experiments with\ndifferent pressure medium and loading rates, even in the same initial samples.\nGreat achievements are made in understanding strain or stress dependence of the\nTP under dynamic loadings. However, present understandings on the strain rate\ndependence of the TP are far from clear, even a virgin for extreme high strain\nrates. In this work, large scale NEMD simulations are conducted to study the\neffects of strain rates on the phase transition of iron-based single crystals.\nOur results show that the phase transition is preceded by lattice instabilities\nunder ramp compressions, but present theory, represented by modified Born\ncriteria, cannot correctly predict observed onsets of the instability. Through\nconsidering both strain and strain gradient disturbances, new instability\ncriteria are proposed, which could be generally applied for studying\ninstabilities under either static or dynamic loadings. For the ramp with a\nstrain rate smaller than about 1010s-1, the observed onset of instabilities is\nindeed equal to the one predicted by the new instability criteria under small\ngradient disturbances. The observed onsets deviates from the predicted one at\nlager strain rates because of finite strain gradient effect. Interestingly, the\nstrain rate dependence of the TP also exhibits an obvious change at the same\nstrain rate, i.e., 1010 s-1. When 1010 s-1, a certain power law is obeyed, but\nit is not applicable at larger strain rates. This strain rate effect on the TP\nis well interpreted with nucleation time and the finite strain gradient effect.\nAccording to these basic understandings, the roles of strain rates on\nnucleation and growth of the phase transition are studied.", "category": "cond-mat_mtrl-sci" }, { "text": "Temperature-Modulated Differential Scanning Calorimetry Analysis of\n High-Temperature Silicate Glasses: Differential scanning calorimetry (DSC) is one of the most versatile probes\nfor silicate glasses, allowing determination of, e.g., transition temperatures\n(glass, crystallization, melting) and the temperature dependence of heat\ncapacity. However, complications arise for glasses featuring overlapping\ntransitions and low sensitivity, e.g., arising from SiO2-rich compositions with\nsmall change in heat capacity during glass transition or the low sensitivity of\nthermocouples at high temperature. These challenges might be overcome using\ntemperature-modulated DSC (TM-DSC), which enables separation of overlapping\nsignals and improved sensitivity at the expense of increased measurement\nduration.", "category": "cond-mat_mtrl-sci" }, { "text": "Construction of Multi-Dimensional Functions for Optimization of\n Additive-Manufacturing Process Parameters: The authors present a generic framework for parameter optimization of\nadditive manufacturing (AM) processes, one tailored to a high-throughput\nexperimental methodology (HTEM). Given the large number of parameters, which\nimpact the quality of AM-metallic components, the authors advocate for\npartitioning the AM parameter set into stages (tiers), based on their relative\nimportance, modeling one tier at a time until successful, and then\nsystematically expanding the framework. The authors demonstrate how the\nconstruction of multi-dimensional functions, based on neural networks (NN), can\nbe applied to successfully model relative densities and Rockwell hardness\nobtained from HTEM testing of the Inconel 718 superalloy fabricated, using a\npowder-bed approach. The authors analyze the input data set, assess its\nsuitability for predictions, and show how to optimize the framework for the\nmulti-dimensional functional construction, such as to obtain the highest degree\nof fit with the input data. The novelty of the research work entails the\nversatile and scalable NN framework presented, suitable for use in conjunction\nwith HTEM, for the AM parameter optimization of superalloys, and beyond.", "category": "cond-mat_mtrl-sci" }, { "text": "Large Exotic Spin Torques in Antiferromagnetic Iron Rhodium: Spin torque is a promising tool for driving magnetization dynamics for novel\ncomputing technologies. These torques can be easily produced by spin-orbit\neffects, but for most conventional spin source materials, a high degree of\ncrystal symmetry limits the geometry of the spin torques produced. Magnetic\nordering is one way to reduce the symmetry of a material and allow exotic\ntorques, and antiferromagnets are particularly promising because they are\nrobust against external fields. We present spin torque ferromagnetic resonance\nmeasurements and second harmonic Hall measurements characterizing the spin\ntorques in antiferromagnetic iron rhodium alloy. We report extremely large,\nstrongly temperature-dependent exotic spin torques with a geometry apparently\ndefined by the magnetic ordering direction. We find the spin torque efficiency\nof iron rhodium to be (330$\\pm$150) % at 170 K and (91$\\pm$32) % at room\ntemperature. We support our conclusions with theoretical calculations showing\nhow the antiferromagnetic ordering in iron rhodium gives rise to such exotic\ntorques.", "category": "cond-mat_mtrl-sci" }, { "text": "Oxygen Reduction Activity of Carbon Nitride Supported on Carbon\n Nanotubes: Fuel cells offer an alternative to burning fossil fuels, but use platinum as\na catalyst which is expensive and scarce. Cheap, alternative catalysts could\nenable fuel cells to become serious contenders in the green energy sector. One\npromising class of catalyst for electrochemical oxygen reduction is\niron-containing, nanostructured, nitrogen-doped carbon. The catalytic activity\nof such N-doped carbons has improved vastly over the years bringing industrial\napplications ever closer. Stoichiometric carbon nitride powder has only been\nobserved in recent years. It has nitrogen content up to 57% and as such is an\nextremely interesting material to work with. The electrochemical activity of\ncarbon nitride has already been explored, confirming that iron is not a\nnecessary ingredient for 4-electron oxygen reduction. Here, we synthesize\ncarbon nitride on a carbon nanotube support and subject it to high temperature\ntreatment in an effort to increase the surface area and conductivity. The\nresults lend insight into the mechanism of oxygen reduction and show the\npotential for carbon nanotube-supported carbon nitride to be used as a catalyst\nto replace platinum in fuel cells.", "category": "cond-mat_mtrl-sci" }, { "text": "Fractional Skyrme lines in ferroelectric barium titanate: We predict a topological defect in ferroelectric barium titanate which we\ncall a skyrme line. These are line-like objects characterized by skyrmionic\ntopological charge. As well as configurations with integer charge, the charge\ndensity can split into well-localized fractional parts. We show that under\ncertain conditions the fractional skyrme lines are stable. We discuss a\nmechanism to create fractional topological charge objects and investigate their\nstability.", "category": "cond-mat_mtrl-sci" }, { "text": "Semi-supervised machine learning model for analysis of nanowire\n morphologies from transmission electron microscopy images: In the field of materials science, microscopy is the first and often only\naccessible method for structural characterization. There is a growing interest\nin the development of machine learning methods that can automate the analysis\nand interpretation of microscopy images. Typically training of machine learning\nmodels requires large numbers of images with associated structural labels,\nhowever, manual labeling of images requires domain knowledge and is prone to\nhuman error and subjectivity. To overcome these limitations, we present a\nsemi-supervised transfer learning approach that uses a small number of labeled\nmicroscopy images for training and performs as effectively as methods trained\non significantly larger image datasets. Specifically, we train an image encoder\nwith unlabeled images using self-supervised learning methods and use that\nencoder for transfer learning of different downstream image tasks\n(classification and segmentation) with a minimal number of labeled images for\ntraining. We test the transfer learning ability of two self-supervised learning\nmethods: SimCLR and Barlow-Twins on transmission electron microscopy (TEM)\nimages. We demonstrate in detail how this machine learning workflow applied to\nTEM images of protein nanowires enables automated classification of nanowire\nmorphologies (e.g., single nanowires, nanowire bundles, phase separated) as\nwell as segmentation tasks that can serve as groundwork for quantification of\nnanowire domain sizes and shape analysis. We also extend the application of the\nmachine learning workflow to classification of nanoparticle morphologies and\nidentification of different type of viruses from TEM images.", "category": "cond-mat_mtrl-sci" }, { "text": "Dynamic transverse magnetic susceptibility in the projector\n augmented-wave method. Application to Fe, Ni, and Co: We present a first principles implementation of the dynamic transverse\nmagnetic susceptibility in the framework of linear response time-dependent\ndensity functional theory. The dynamic susceptibility allows one to obtain the\nmagnon dispersion as well as magnon lifetimes for a particular material, which\nstrongly facilitates the interpretation of inelastic neutron scattering\nexperiments as well as other spectroscopic techniques. We apply the method to\nFe, Ni, and Co and perform a thorough convergence analysis with respect the\nbasis set size, $k$-point sampling, spectral smearing and unoccupied bands. In\nparticular, it is shown that while the gap error (acoustic magnon energy at\n$\\mathbf{q}=\\mathbf{0}$) is highly challenging to converge, the spin-wave\nstiffness and the dispersion relation itself are much less sensitive to\nconvergence parameters. Our final results agrees well with experimentally\nextracted magnon dispersion relations except for Ni, where it is well-known\nthat the exchange splitting energy is poorly represented in the local density\napproximation. We also find good agreement with previous first principles\ncalculations and explain how differences in the calculated dispersion relations\ncan arise from subtle differences in computational approaches.", "category": "cond-mat_mtrl-sci" }, { "text": "Thermodynamic dislocation theory: Finite deformations: The present paper extends the thermodynamic dislocation theory initiated by\nLanger, Bouchbinder and Lookman [2010] to non-uniform finite plastic\ndeformations. The equations of motion are derived from the variational equation\ninvolving the free energy density and the positive definite dissipation\nfunction. We also consider the simplified theory by neglecting the excess\ndislocations. For illustration, the problem of finite strain constrained shear\nof single crystals with one active slip system is solved within the proposed\ntheory.", "category": "cond-mat_mtrl-sci" }, { "text": "Prediction of Glass Elasticity from Free Energy Density of Topological\n Constraints: Despite the critical importance of the elastic properties of modern\nmaterials, there is not a singular model that can predict the modulus to an\naccuracy needed for industrial glass design. To address this problem, we\npropose an approach to calculate the elastic modulus based on the free energy\ndensity of topological constraints in the glass-forming network. Our approach\nshows quantitatively accurate agreement with glasses across a variety of\ncompositional families. Moreover, using temperature-dependent constraint\ntheory, the temperature dependence of the modulus can also be predicted. Our\napproach is general and theoretically can be applied to any network glass.", "category": "cond-mat_mtrl-sci" }, { "text": "Expression and interactions of stereo-chemically active lone pairs and\n their relation to structural distortions and thermal conductivity: Stereo-chemically active lone pairs are typically described as an important\nnon-bonding effect, and large interest has centered on understanding the\nderived effect of lone pair expression on physical properties such as the\nthermal conductivity. To manipulate such properties, it is essential to\nunderstand the conditions that lead to lone pair expression and to provide a\nquantitative chemical description. Here we first use density functional theory\ncalculations to establish the presence of stereo-chemically active lone pairs\non antimony in $\\text{MnSb}_{2}\\text{O}_{4}$. The lone pairs are formed through\na similar mechanism to those in binary post-transition metal compounds in an\noxidation state of two less than their main group number, where the degree of\norbital interaction determines the expression of the lone pair. In\n$\\text{MnSb}_{2}\\text{O}_{4}$ the Sb lone pairs interact through a void space\nin the crystal structure, and they minimize their mutual repulsion by\nintroducing a deflection angle. This angle increases significantly with\ndecreasing Sb-Sb distance, thus showing the highly destabilizing nature of the\nlone pair interactions. Analysis of the chemical bonding in the structure shows\nthat it is dominated by polar covalent interactions. A database search of\nrelated ternary chalcogenide structures shows that for structures with a lone\npair the degree of lone pair expression is largely determined by whether the\nantimony-chalcogen units are connected or not, suggesting a cooperative effect.\nIsolated $\\text{SbX}_3$ units have larger X-Sb-X bond angles, and therefore\nweaker lone pair expression than connected units. Since increased lone pair\nexpression is equivalent to an increased orbital interaction (covalent\nbonding), which typically leads to increased heat conduction, this can explain\nthe previously established correlation between larger bond angles and lower\nthermal conductivity.", "category": "cond-mat_mtrl-sci" }, { "text": "Quantum size effect on the dissociation of O2 molecules on ultrathin\n Pb(111) films: Using first-principles calculations, we systematically study the dissociation\nof O$_2$ molecules on different ultrathin Pb(111) films. Based on our previous\nwork revealing the molecular adsorption precursor states for O$_2$, we further\nexplore that why there are two nearly degenerate adsorption states on Pb(111)\nultrathin films, but no precursor adsorption states exist at all on the\nMg(0001) and Al(111) surfaces. And the reason is concluded to be the different\nsurface electronic structures. For the O$_2$ dissociation, we consider both the\nreaction channels from gas-like and molecularly adsorbed O$_2$ molecules. We\nfind that the energy barrier for O$_2$ dissociation from the molecular\nadsorption precursor states is always smaller than from O$_2$ gases. The most\nenergetically favorable dissociation process is found to be the same on\ndifferent Pb(111) films, and the energy barriers are found to be modulated by\nthe quantum size effects of Pb(111) films.", "category": "cond-mat_mtrl-sci" }, { "text": "Ultrahigh ion diffusion in oxide crystal by engineering the interfacial\n transporter channels: The mass storage and removal in solid conductors always played vital role on\nthe technological applications such as modern batteries, permeation membranes\nand neuronal computations, which were seriously lying on the ion diffusion and\nkinetics in bulk lattice. However, the ions transport was kinetically limited\nby the low diffusional process, which made it a challenge to fabricate\napplicable conductors with high electronic and ionic conductivities at room\ntemperature. It was known that at essentially all interfaces, the existed space\ncharge layers could modify the charge transport, storage and transfer\nproperties. Thus, in the current study, we proposed an acid solution/WO3/ITO\nstructure and achieved an ultrafast hydrogen transport in WO3 layer by\ninterfacial job-sharing diffusion. In this sandwich structure, the transport\npathways of the protons and electrons were spatially separated in acid solution\nand ITO layer respectively, resulting the pronounced increasing of effective\nhydrogen diffusion coefficient (Deff) up to 106 times. The experiment and\ntheory simulations also revealed that this accelerated hydrogen transport based\non the interfacial job-sharing diffusion was universal and could be extended to\nother ions and oxide materials as well, which would potentially stimulate\nsystematic studies on ultrafast mixed conductors or faster solid-state\nelectrochemical switching devices in the future.", "category": "cond-mat_mtrl-sci" }, { "text": "Electronic and optical properties of metal-doped TiO$_2$ nanotubes:\n Spintronic and photocatalytic applications: Due to their characteristic geometry, TiO$_2$ nanotubes (TNTs), suitably\ndoped by metal-substitution to enhance their photocatalytic properties, have a\nhigh potential for applications such as clean fuel production. In this context,\nwe present a detailed investigation of the magnetic, electronic, and optical\nproperties of transition-metal doped TNTs, based on hybrid density functional\ntheory. In particular, we focus on the $3d$, the $4d$, as well as selected $5d$\ntransition-metal doped TNTs. Thereby, we are able to explain the enhanced\noptical activity and photocatalytic sensitivity observed in various\nexperiments. We find, for example, that Cr- and W-doped TNTs can be employed\nfor applications like water splitting and carbon dioxide reduction, and for\nspintronic devices. The best candidate for water splitting is Fe-doped TNT, in\nagreement with experimental observations. In addition, our findings provide\nvaluable hints for future experimental studies of the ferromagnetic/spintronic\nbehavior of metal-doped titania nanotubes.", "category": "cond-mat_mtrl-sci" }, { "text": "Modes of Kink Motion on Dislocations in Semiconductors: Analysis is given of the changes of dislocation motion modes with stress and\ntemperature variation. Different regimes of dislocation kink pair formation and\nspreading (motion in the random potential, in the field of random forces, the\nquasi-localization) are considered. Discrepancies are discussed between the\ntheory and experimental data on dislocation velocities.", "category": "cond-mat_mtrl-sci" }, { "text": "Carbon Decorated TiO2 Nanotube Membranes: A Renewable Nanofilter for\n Size- and Charge Selective Enrichment of Proteins: In this work, we design a TiO2 nanomembrane (TiNM) that can be used as a\nnanofilter platform for a selective enrichment of specific proteins. After use\nthe photocatalytic properties of TiO2 allow to decompose unwanted remnant on\nthe substrate and thus make the platform reusable. To construct this platform\nwe fabricate a free-standing TiO2 nanotube array and remove the bottom oxide to\nform a both-end open TiNM. By pyrolysis of the natural tube wall contamination\n(C/TiNM), the walls become decorated with graphitic carbon patches. Owing to\nthe large surface area, the amphiphilic nature and the charge adjustable\ncharacter, this C/TiNM can be used to extract and enrich hydrophobic and\ncharged biomolecules from solutions. Using human serum albumin (HSA) as a model\nprotein as well as protein mixtures, we show that the composite membrane\nexhibits a highly enhanced loading capacity and protein selectivity and is\nreusable after a short UV treatment.", "category": "cond-mat_mtrl-sci" }, { "text": "Large Anomalous Hall Effect at Room Temperature in a Fermi-Level-Tuned\n Kagome Antiferromagnet: The recent discoveries of surperisingly large anomalous Hall effect in chiral\nantiderromagnets have triggered extensive research efforts in various fields,\nranging from topological condensed-matter physics to antiferromagnetic\nspintronics, and energy harvesting technology. However, such AHE-hosting\nantiferromagnetic materials are rare in nature. Herein, we demonstrate that\nMn2.4Ga, a Fermi-level-tuned kagome antiferromagnet, has a large anomalous Hall\nconductivity of about 150 {\\Omega}-1cm-1 at room temperature that surpasses the\nusual high values (i.e.,20-50 {\\Omega}-1cm-1) observed so far in two\noutstanding kagome antiferromagnets, Mn3Sn and Mn3Ge. The spin triangular\nstructure of Mn2.4Ga guarantees a nonzero Berry curvature while generates only\na weak net moment in the kagome plane.Moreover, the anomalous Hall conductivity\nexhibits a sign reversal with the rotation of a small magnetic field, which can\nbe ascribed to the field-controlled chirality of the spin triangular structure.\nOur theoretical calculation indicate that the large AHE in Mn2.4Ga originates\nfrom a significantly enhanced Berry curvature associated wiht the tuning of the\nFermi level close to the Weyl points. These properties, together with the\nability to manipulate moment orientations using a moderate external magnetic\nfield, make Mn2.4Ga extremely exciting for future antiferromagnetic\nspintronics.", "category": "cond-mat_mtrl-sci" }, { "text": "Probing the Electron States and Metal-Insulator Transition Mechanisms in\n Atomically Thin MoS2 Based on Vertical Heterostructures: The metal-insulator transition (MIT) is one of the remarkable electrical\ntransport properties of atomically thin molybdenum disulphide (MoS2). Although\nthe theory of electron-electron interactions has been used in modeling the MIT\nphenomena in MoS2, the underlying mechanism and detailed MIT process still\nremain largely unexplored. Here, we demonstrate that the vertical\nmetal-insulator-semiconductor (MIS) heterostructures built from atomically thin\nMoS2 (monolayers and multilayers) are ideal capacitor structures for probing\nthe electron states in MoS2. The vertical configuration of MIS heterostructures\noffers the added advantage of eliminating the influence of large impedance at\nthe band tails and allows the observation of fully excited electron states near\nthe surface of MoS2 over a wide excitation frequency (100 Hz-1 MHz) and\ntemperature range (2 K- 300 K). By combining capacitance and transport\nmeasurements, we have observed a percolation-type MIT, driven by density\ninhomogeneities of electron states, in the vertical heterostructures built from\nmonolayer and multilayer MoS2. In addition, the valence band of thin MoS2\nlayers and their intrinsic properties such as thickness-dependence screening\nabilities and band gap widths can be easily accessed and precisely determined\nthrough the vertical heterostructures.", "category": "cond-mat_mtrl-sci" }, { "text": "Dependence of magnetic domain patterns on plasma-induced differential\n oxidation of CoPd thin films: We demonstrate the evolution of the micro-patterned magnetic domains in CoPd\nthin films pretreated with e-beam lithography and O2 plasma. During the\ndays-long oxidation, significantly different behaviors of the patterned\nmagnetic domains under magnetization reversal are observed via magneto-optic\nKerr effect microscopy on different days. The evolution of the magnetic\nbehaviors indicate critical changes in the local magnetic anisotropy energies\ndue to the Co oxides that evolve into different oxide forms, which are\ncharacterized by micro-area X-ray absorption spectroscopy and X-ray\nphotoelectron spectroscopy. The coercive field of the area pre-exposed to\nplasma can decrease to a value 10 Oe smaller than that unexposed to plasma,\nwhereas after a longer duration of oxidation the coercive field can instead\nbecome larger in the area pre-exposed to plasma than that unexposed, leading to\nan opposite magnetic pattern. Various forms of oxidation can therefore provide\nan additional dimension for magnetic-domain engineering to the current\nconventional lithographies.", "category": "cond-mat_mtrl-sci" }, { "text": "Tuning the valence and concentration of europium and luminescence\n centers in GaN through co-doping and defect association: Defect physics of europium (Eu) doped GaN is investigated using\nfirst-principles hybrid density-functional defect calculations. This includes\nthe interaction between the rare-earth dopant and native point defects (Ga and\nN vacancies) and other impurities (O, Si, C, H, and Mg) unintentionally present\nor intentionally incorporated into the host material. While the trivalent\nEu$^{3+}$ ion is often found to be predominant when Eu is incorporated at the\nGa site in wurtzite GaN, the divalent Eu$^{2+}$ is also stable and found to be\npredominant in a small range of Fermi-level values in the band-gap region. The\nEu$^{2+}$/Eu$^{3+}$ ratio can be tuned by tuning the position of Fermi level\nand through defect association. We find co-doping with oxygen can facilitate\nthe incorporation of Eu into the lattice. The unassociated Eu$_{\\rm Ga}$ is an\nelectrically and optically active defect center and its behavior is profoundly\nimpacted by local defect--defect interaction. Defect complexes such as Eu$_{\\rm\nGa}$-O$_{\\rm N}$, Eu$_{\\rm Ga}$-Si$_{\\rm Ga}$, Eu$_{\\rm Ga}$-H$_i$, Eu$_{\\rm\nGa}$-Mg$_{\\rm Ga}$, and Eu$_{\\rm Ga}$-O$_{\\rm N}$-Mg$_{\\rm Ga}$ can efficiently\nact as deep carrier traps and mediate energy transfer from the host into the\nEu$^{3+}$ $4f$-electron core which then leads to sharp red intra-$f$\nluminescence. Eu-related defects can also give rise to defect-to-band\nluminescence. The unassociated Eu$_{\\rm Ga}$, for example, is identified as a\npossible source of the broad blue emission observed in n-type,\nEu$^{2+}$-containing GaN. This work calls for a re-assessment of certain\nassumptions regarding specific defect configurations previously made for\nEu-doped GaN and further investigation into the origin of the photoluminescence\nhysteresis observed in (Eu,Mg)-doped samples.", "category": "cond-mat_mtrl-sci" }, { "text": "Ab-initio prediction of the electronic and optical excitations in\n polythiophene: isolated chains versus bulk polymer: We calculate the electronic and optical excitations of polythiophene using\nthe GW approximation for the electronic self-energy, and include excitonic\neffects by solving the electron-hole Bethe-Salpeter equation. Two different\nsituations are studied: excitations on isolated chains and excitations on\nchains in crystalline polythiophene. The dielectric tensor for the crystalline\nsituation is obtained by modeling the polymer chains as polarizable line\nobjects, with a long-wavelength polarizability tensor obtained from the\nab-initio polarizability function of the isolated chain. With this model\ndielectric tensor we construct a screened interaction for the crystalline case,\nincluding both intra- and interchain screening. In the crystalline situation\nboth the quasi-particle band gap and the exciton binding energies are\ndrastically reduced in comparison with the isolated chain. However, the optical\ngap is hardly affected. We expect this result to be relevant for conjugated\npolymers in general.", "category": "cond-mat_mtrl-sci" }, { "text": "Flexomagnetic effect in Mn-based antiperovskites: We report appearance of the net magnetization in Mn-based antiperovskite\ncompounds as a result of the external strain gradient (flexomagnetic effect).\nIn particular, we describe the mechanism of the magnetization induction in the\nMn_{3}GaN at the atomic level in terms of the behavior of the local magnetic\nmoments (LMM) of the Mn atoms. We show that the flexomagnetic effect is linear\nand results from the non-uniformity of the strain, i.e. it is absent not only\nin the ground state but also when the applied external strain is uniform. We\nestimate the flexomagnetic coefficient to be 1.95 mu_{B}*angstrom. We show that\nat the moderate values of the strain gradient (~ 0.1%) the flexomagnetic\ncontribution is the only non-vanishing input to the induced magnetization.", "category": "cond-mat_mtrl-sci" }, { "text": "Quasi-harmonic temperature dependent elastic constants: applications to\n silicon, aluminum, and silver: We present ab-initio calculations of the quasi-harmonic temperature dependent\nelastic constants. The isothermal elastic constants are calculated at each\ntemperature as second derivatives of the Helmholtz free energy with respect to\nstrain and corrected for finite pressure effects. This calculation is repeated\nfor a grid of geometries and the results interpolated at the minimum of the\nHelmholtz free energy. The results are compared with the quasi-static elastic\nconstants. Thermodynamic relationships are used to derive the adiabatic elastic\nconstants that are compared with the experimental measurements. These\napproaches are implemented for cubic solids in the $\\texttt{thermo_pw}$ code\nand are validated by applications to silicon, aluminum, and silver.", "category": "cond-mat_mtrl-sci" }, { "text": "Significant elastic anisotropy in Ti$_{1-x}$Al$_x$N alloys: Strong compositional-dependent elastic properties have been observed\ntheoretically and experimentally in Ti$_{1-x}$Al$_x$ N alloys. The elastic\nconstant, C$_{11}$, changes by more than 50% depending on the Al-content.\nIncreasing the Al-content weakens the average bond strength in the local\noctahedral arrangements resulting in a more compliant material. On the other\nhand, it enhances the directional (covalent) nature of the nearest neighbor\nbonds that results in greater elastic anisotropy and higher sound velocities.\nThe strong dependence of the elastic properties on the Al-content offers new\ninsight into the detailed understanding of the spinodal decomposition and age\nhardening in Ti$_{1-x}$Al$_x$N alloys.", "category": "cond-mat_mtrl-sci" }, { "text": "Flatten the Li-ion Activation in Perfectly Lattice-matched MXene and\n 1T-MoS2 Heterostructures via Chemical Functionalization: MXene and its derivatives have attracted considerable attention for potential\napplication in energy storage like batteries and supercapacitors owing to its\nultrathin metallic structures. However, the complexity of the ionic and\nelectronic dynamics in MXene based hybrids, which are normally needed for\ndevice integration, triggers both challenges and opportunities for its\napplication. In this paper, as a prototype of metallic hybrids of MXene,\nheterostructures consisting of Ti3C2T2 (T= None, O and F atoms) and metallic\nMoS2 (1T phase) are investigated. Through density functional theory, we\ninvestigate the interfacial electronic variation, thermal activation, and anode\nperformance in the lithium-ion battery (LIB) of Ti3C2T2/1T-MoS2. We found that\ndifferent surface atomic groups in MXene can significantly alter the affinity,\nredox reaction and kinetics of Li atoms in the interface of the Ti3C2T2 and\n1T-MoS2. Through examining the three possible pathways of Li by climbing\nimage-nudged elastic band (CI-NEB) and ab-initio molecular dynamics (AIMD)\nsimulation, the diffusion curve becomes significantly flattened from the naked\nto O- and F-terminated Ti3C2 MXene with activation barriers reducing from 0.80\nto 0.22 and 0.29 eV, respectively, and room-temperature diffusion coefficients\nincreasing from 1.20x10-6 to 2.75x10-6, 1.70x10-4 cm2 s-1, respectively. The\nfunctionalization with O or F eliminates the steric hindrance of Li\nintercalation by breaking the strong interaction between two layers and\nprovides additional adsorption sites for Li diffusion in the meantime. Our work\nsuggests that surface functional groups play a significant role in\nTi3C2T2/1T-MoS2 modification and Ti3C2F2/1T-MoS2 with the high diffusion\ncoefficient and theoretical capacity could be a promising anode material for\nLIBs.", "category": "cond-mat_mtrl-sci" }, { "text": "Conductance through atomic point contacts between fcc(100) electrodes of\n gold: Electrical conductance through various nanocontacts between gold electrodes\nis studied by using the density functional theory, scalar-relativistic\npseudopotentials, generalized gradient approximation for the\nexchange-correlation energy and the recursion-transfer-matrix method along with\nchannel decomposition. The nanocontact is modeled with pyramidal fcc(100) tips\nand 1 to 5 gold atoms between the tips. Upon elongation of the contact by\nadding gold atoms between the tips, the conductance at Fermi energy E_F evolves\nfrom G ~ 3 G_0 to G ~ 1 G_0 (G_0 = 2e/h^2). Formation of a true one-atom point\ncontact, with G ~ 1 G_0 and only one open channel, requires at least one atom\nwith coordination number 2 in the wire. Tips that share a common vertex atom or\ntips with touching vertex atoms have three partially open conductance channels\nat E_F, and the symmetries of the channels are governed by the wave functions\nof the tips. The long 5-atom contact develops conductance oscillations and\nconductance gaps in the studied energy range -3 < E-E_F < 5 eV, which reflects\noscillations in the local density of electron states in the 5-atom linear \"gold\nmolecule\" between the electrodes, and a weak coupling of this \"molecule\" to the\ntips.", "category": "cond-mat_mtrl-sci" }, { "text": "Non-Linear Beam Splitter in Bose-Einstein Condensate Interferometers: A beam splitter is an important component of an atomic/optical Mach-Zehnder\ninterferometer. Here we study a Bose Einstein Condensate beam splitter,\nrealized with a double well potential of tunable height. We analyze how the\nsensitivity of a Mach Zehnder interferometer is degraded by the non-linear\nparticle-particle interaction during the splitting dynamics. We distinguish\nthree regimes, Rabi, Josephson and Fock, and associate to them a different\nscaling of the phase sensitivity with the total number of particles.", "category": "cond-mat_mtrl-sci" }, { "text": "High-temperature stability and grain boundary complexion formation in a\n nanocrystalline Cu-Zr alloy: Nanocrystalline Cu-3 at.% Zr powders with ~20 nm average grain size were\ncreated with mechanical alloying and their thermal stability was studied from\n550-950 {\\deg}C. Annealing drove Zr segregation to the grain boundaries, which\nled to the formation of amorphous intergranular complexions at higher\ntemperatures. Grain growth was retarded significantly, with 1 week of annealing\nat 950 {\\deg}C, or 98% of the solidus temperature, only leading to coarsening\nof the average grain size to 54 nm. The enhanced thermal stability can be\nconnected to both a reduction in grain boundary energy with doping as well as\nthe precipitation of ZrC particles. High mechanical strength is retained even\nafter these aggressive heat treatments, showing that complexion engineering may\nbe a viable path toward the fabrication of bulk nanostructured materials with\nexcellent properties.", "category": "cond-mat_mtrl-sci" }, { "text": "Putting Density Functional Theory to the Test in\n Machine-Learning-Accelerated Materials Discovery: Accelerated discovery with machine learning (ML) has begun to provide the\nadvances in efficiency needed to overcome the combinatorial challenge of\ncomputational materials design. Nevertheless, ML-accelerated discovery both\ninherits the biases of training data derived from density functional theory\n(DFT) and leads to many attempted calculations that are doomed to fail. Many\ncompelling functional materials and catalytic processes involve strained\nchemical bonds, open-shell radicals and diradicals, or metal-organic bonds to\nopen-shell transition-metal centers. Although promising targets, these\nmaterials present unique challenges for electronic structure methods and\ncombinatorial challenges for their discovery. In this Perspective, we describe\nthe advances needed in accuracy, efficiency, and approach beyond what is\ntypical in conventional DFT-based ML workflows. These challenges have begun to\nbe addressed through ML models trained to predict the results of multiple\nmethods or the differences between them, enabling quantitative sensitivity\nanalysis. For DFT to be trusted for a given data point in a high-throughput\nscreen, it must pass a series of tests. ML models that predict the likelihood\nof calculation success and detect the presence of strong correlation will\nenable rapid diagnoses and adaptation strategies. These \"decision engines\"\nrepresent the first steps toward autonomous workflows that avoid the need for\nexpert determination of the robustness of DFT-based materials discoveries.", "category": "cond-mat_mtrl-sci" }, { "text": "Controlling the energy of defects and interfaces in the amplitude\n expansion of the phase-field crystal model: One of the major difficulties in employing phase field crystal (PFC) modeling\nand the associated amplitude (APFC) formulation is the ability to tune model\nparameters to match experimental quantities. In this work we address the\nproblem of tuning the defect core and interface energies in the APFC\nformulation. We show that the addition of a single term to the free energy\nfunctional can be used to increase the solid-liquid interface and defect\nenergies in a well-controlled fashion, without any major change to other\nfeatures. The influence of the newly added term is explored in two-dimensional\ntriangular and honeycomb structures as well as bcc and fcc lattices in three\ndimensions. In addition, a finite element method (FEM) is developed for the\nmodel that incorporates a mesh refinement scheme. The combination of the FEM\nand mesh refinement to simulate amplitude expansion with a new energy term\nprovides a method of controlling microscopic features such as defect and\ninterface energies while simultaneously delivering a coarse-grained examination\nof the system.", "category": "cond-mat_mtrl-sci" }, { "text": "Growth-sequence-dependent interface magnetism of SrIrO$_3$ -\n La$_{0.7}$Sr$_{0.3}$MnO$_3$ bilayers: Bilayers of the oxide 3d ferromagnet La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) and\nthe 5d paramagnet SrIrO$_{3}$ (SIO) with large spin-orbit coupling (SOC) have\nbeen investigated regarding the impact of interfacial SOC on magnetic order.\nFor the growth sequence of LSMO on SIO, ferromagnetism is strongly altered and\nlarge out-of-plane-canted anisotropy associated with lacking magnetic\nsaturation up to 4 T has been observed. Thin bilayer films have been grown\ncoherently in both growth sequences on SrTiO$_3$ (001) by pulsed laser\ndeposition and structurally characterized by scanning transmission electron\nmicroscopy (STEM) and x-ray diffraction (XRD). Measurements of magnetization\nand field-dependent Mn L$_{2,3}$ edge x-ray magnetic circular dichroism (XMCD)\nreveal changes of LSMO magnetic order which are strong in LSMO on SIO and weak\nin LSMO underneath of SIO. We attribute the impact of the growth sequence to\nthe interfacial lattice structure/symmetry which is known to influence the\ninterfacial magnetic coupling.", "category": "cond-mat_mtrl-sci" }, { "text": "Synthesis and Characterization of Sodium Iron Antimonate Na2FeSbO5\n One-Dimensional Antiferromagnetic Chain Compound with a Spin Glass Ground\n State: A new oxide, sodium iron antimonate, Na2FeSbO5, was synthesized and\nstructurally characterized, and its static and dynamic magnetic properties were\ncomprehensively studied both experimentally by dc and ac magnetic\nsusceptibility, magnetization, specific heat, electron spin resonance (ESR) and\nMoessbauer measurements, and theoretically by density functional calculations.\nThe resulting single-crystal structure (a = 15.6991(9) A; b = 5.3323 (4) A; c =\n10.8875(6) A; S.G. Pbna) consists of edge-shared SbO6 octahedral chains, which\nalternate with vertex-linked, magnetically active FeO4 tetrahedral chains. The\n57Fe Moessbauer spectra confirmed the presence of high-spin Fe3+ (3d5) ions in\na distorted tetrahedral oxygen coordination. The magnetic susceptibility and\nspecific heat data show the absence of a long-range magnetic ordering in\nNa2FeSbO5 down to 2 K, but ac magnetic susceptibility unambigously demonstrates\nspin-glass-type behavior with a unique two-step freezing at Tf1 about 80 K and\nTf2 about 35 K. Magnetic hyperfine splitting of 57Fe Moessbauer spectra was\nobserved below T* about 104 K (Tf1 < T*). The spectra just below T* (Tf1 < T <\nT*) exhibit a relaxation behavior caused by critical spin fluctuations,\nindicating the existence of short-range correlations. The stochastic model of\nionic spin relaxation was used to account for the shape of the Moessbauer\nspectra below the freezing temperature. A complex slow dynamics is further\nsupported by ESR data revealing two different absorption modes presumably\nrelated to ordered and disordered segments of spin chains. The data imply a\nspin-cluster ground state for Na2FeSbO5.", "category": "cond-mat_mtrl-sci" }, { "text": "Accelerated Design of Chalcogenide Glasses through Interpretable Machine\n Learning for Composition Property Relationships: Chalcogenide glasses possess several outstanding properties that enable\nseveral ground breaking applications, such as optical discs, infrared cameras,\nand thermal imaging systems. Despite the ubiquitous usage of these glasses, the\ncomposition property relationships in these materials remain poorly understood.\nHere, we use a large experimental dataset comprising approx 24000 glass\ncompositions made of 51 distinct elements from the periodic table to develop\nmachine learning models for predicting 12 properties, namely, annealing point,\nbulk modulus, density, Vickers hardness, Littleton point, Youngs modulus, shear\nmodulus, softening point, thermal expansion coefficient, glass transition\ntemperature, liquidus temperature, and refractive index. These models, by far,\nare the largest for chalcogenide glasses. Further, we use SHAP, a game theory\nbased algorithm, to interpret the output of machine learning algorithms by\nanalyzing the contributions of each element towards the models prediction of a\nproperty. This provides a powerful tool for experimentalists to interpret the\nmodels prediction and hence design new glass compositions with targeted\nproperties. Finally, using the models, we develop several glass selection\ncharts that can potentially aid in the rational design of novel chalcogenide\nglasses for various applications.", "category": "cond-mat_mtrl-sci" }, { "text": "Metastability and anharmonicity enhance defect-assisted nonradiative\n recombination in low-symmetry semiconductors: Strong nonradiative recombination has been observed in quasi-one-dimensional\nantimony selenide, which runs counter to the simple intuition that claims high\ndefect tolerance exists in semiconductors with antibonding state in the valence\nband and bonding state in the conduction band. Here we reveal such a defect\nintolerance actually stems from the richness of structural metastability and\nvibrational anharmonicity owing to the low-symmetry atomic structure. Taking\nthe deep defect V$_{\\rm Se}$ as a benchmark, we show the defect with its\nground-state configuration alone does not act as a recombination center.\nInstead, we identify three different configurations with different formation\nenergies, such richness of metastability offers a higher probability to\naccomplish a rapid recombination cycle. Another contributing factor is the\nanharmonicity in the potential energy surfaces that is caused by the large\natomic relaxation, which elevates the total capture coefficient by 2-3 orders\nof magnitude compared with harmonic approximation. Therefore, the unique\nproperties from both crystals and phonons in quasi-one-dimensional system\nenhance the nonradiative recombination, making the traditional intuition of\ndefect tolerance invalid. These results highlight the importance of the correct\nidentification of metastable defects and phonon anharmonicity in the\nnonradiative recombination in low-symmetry semiconductors.", "category": "cond-mat_mtrl-sci" }, { "text": "Spin Modulation in Semiconductor Lasers: We provide an analytic study of the dynamics of semiconductor lasers with\ninjection (pump) of spin-polarized electrons, previously considered in the\nsteady-state regime. Using complementary approaches of quasi-static and small\nsignal analyses, we elucidate how the spin modulation in semiconductor lasers\ncan improve performance, as compared to the conventional (spin-unpolarized)\ncounterparts. We reveal that the spin-polarized injection can lead to an\nenhanced bandwidth and desirable switching properties of spin-lasers.", "category": "cond-mat_mtrl-sci" }, { "text": "Size-dependence of non-empirically tuned DFT starting points for\n $G_0W_0$ applied to $\u03c0$-conjugated molecular chains: $G_0W_0$ calculations for predicting vertical ionization potentials (IPs) and\nelectron affinities of molecules and clusters are known to show a significant\ndependence on the density functional theory (DFT) starting point. A number of\nnon-empirical procedures to find an optimal starting point have been proposed,\ntypically based on tuning the amount of HF exchange in the underlying hybrid\nfunctional specifically for the system at hand. For the case of\n$\\pi$-conjugated molecular chains, these approaches lead to a significantly\ndifferent amount of HF exchange for different oligomer sizes. In this study, we\nanalyze if and how strongly this size dependence affects the ability of\nnon-empirical tuning approaches to predict accurate IPs for $\\pi$-conjugated\nmolecular chains of increasing chain length. To this end, we employ three\ndifferent non-empirical tuning procedures for the $G_0W_0$ starting point to\ncalculate the IP of polyene oligomers up to 22 repeat units and compare the\nresults to highly accurate coupled-cluster calculations. We find that, despite\nits size dependence, using an IP-tuned hybrid functional as a starting point\nfor $G_0W_0$ yields excellent agreement with the reference data for all chain\nlengths.", "category": "cond-mat_mtrl-sci" }, { "text": "Carbon mono and dioxide hydrogenation over pure and metal oxide\n decorated graphene oxide substrates: insight from DFT: Based on first principles density functional theory calculations we explore\nthe energetics of the conversion of carbon mono and dioxide to methane over\ngraphene oxide surfaces. Similar to the recently discovered hydration of\nvarious organic species over this catalyst, the transfer of hydrogen atoms from\nhydroxyl groups of graphene oxide provide a step by step transformation\nhydrogenation of carbon oxides. Estimated yields of modeled reactions at room\ntemperature are about 0.01% for the carbon mono and dioxide. For the modeling\nof graphene oxide/metal oxide composites, calculations in the presence of MO_2\n(where M = V, Cr, Mn, Fe) have been performed. Results of these calculations\ndemonstrate significant decreases of the energy costs and increases of reaction\nyields to 0.07%, which is comparable to the efficiency of these reactions over\nplatinum and ruthenium-based photocatalysts. Increasing the temperature to the\nvalue 100C should provide the total conversion of carbon mono and dioxides.", "category": "cond-mat_mtrl-sci" }, { "text": "Isotropic or anisotropic screening in black phosphorous: can doping tip\n the balance?: Black phosphorus (BP), a layered van der Waals (vdW) crystal, has unique\nin-plane band anisotropy and many resulting anisotropy properties such as the\neffective mass, electron mobility, optical absorption, thermal conductivity and\nplasmonic dispersion. However, whether anisotropic or isotropic charge\nscreening exist in BP remains a controversial issue. Based on first-principles\ncalculations, we study the screening properties in both of single-layer and\nbulk BP, especially concerning the role of doping. Without charge doping, the\nsingle-layer and bulk-phase BP show slight anisotropic screening. Electron and\nhole doping can increase the charge screening of BP and significantly change\nthe relative static dielectric tensor elements along two different in-plane\ndirections. We further study the charge density change induced by potassium (K)\nadatom near the BP surface, under different levels of charge doping. The\ncalculated two-dimensional (2D) charge redistribution patterns also confirm\nthat doping can greatly affect the screening feature and tip the balance\nbetween isotropic and anisotropic screening. We corroborate that screening in\nBP exhibit slight intrinsic anisotropy and doping has significant influence on\nits screening property.", "category": "cond-mat_mtrl-sci" }, { "text": "Effect of wear particles and roughness on nanoscale friction: Frictional contacts lead to the formation of a surface layer called the third\nbody, consisting of wear particles and structures resulting from their\nagglomerates. Its behavior and properties at the nanoscale control the\nmacroscopic tribological performance. It is known that wear particles and\nsurface topography evolve with time and mutually influence one another.\nHowever, the formation of the mature third body is largely uncharted territory\nand the properties of its early stages are unknown. Here we show how a third\nbody initially consisting of particles acting as roller bearings transitions\ninto a shear-band-like state by forming adhesive bridges between the particles.\nUsing large-scale atomistic simulations on a brittle model material, we find\nthat this transition is controlled by the growth and increasing disorganization\nof the particles with increasing sliding distance. Sliding resistance and wear\nrate are at first controlled by the surface roughness, but upon agglomeration\nwear stagnates and friction becomes solely dependent on the real contact area\nin accordance with the plasticity theory of contact by Bowden and Tabor.", "category": "cond-mat_mtrl-sci" }, { "text": "Revealing the ultra-fast domain wall motion in Manganese Gold through\n permalloy capping: Antiferromagnets offer much faster dynamics compared to their ferromagnetic\ncounterparts but their order parameter is extremely difficult to detect and\ncontrol. So far, controlling the N\\'eel order parameter electrically is limited\nto only very few materials where N\\'eel spin-orbit torques are allowed by\nsymmetry. In this work, we show that coupling a thin ferromagnet (permalloy)\nlayer on top of an antiferromagnet (Mn$_2$Au) solves a major roadblock -- the\ncontrolled reading, writing, and manipulation of antiferromagnetic domains. We\nconfirm by atomistic spin dynamics simulations that the domain wall patterns in\nthe Mn$_2$Au are imprinted on the permalloy, therefore allowing for indirect\nimaging of the N\\'eel order parameter. Our simulations show that the coupled\ndomain wall structures in Mn$_2$Au-Py bilayers can be manipulated by either\nacting on the N\\'eel order parameter via N\\'eel spin-orbit torques or by acting\non the magnetisation (the ferromagnetic order parameter) via magnetic fields.\nIn both cases, we predict ultra-high domain wall speeds on the order of 8.5\nkm/s. Thus, employing a thin ferromagnetic layer has the potential to easily\ncontrol the N\\'eel order parameter in antiferromagnets even where N\\'eel\nspin-orbit torques are forbidden by symmetry. The controlled manipulation of\nthe antiferromagnetic order parameter provides a promising basis for the\ndevelopment of high-density storage and efficient computing technologies\nworking in the THz regime.", "category": "cond-mat_mtrl-sci" }, { "text": "The Band-Gap Problem in Semiconductors Revisited: Effects of Core States\n and Many-Body Self-Consistency: A novel picture of the quasiparticle (QP) gap in prototype semiconductors Si\nand Ge emerges from an analysis based on all-electron, self-consistent, GW\ncalculations. The deep-core electrons are shown to play a key role via the\nexchange diagram --if this effect is neglected, Si becomes a semimetal.\nContrary to current lore, the Ge 3d semicore states (e.g., their polarization)\nhave no impact on the GW gap. Self-consistency improves the calculated gaps --a\nfirst clear-cut success story for the Baym-Kadanoff method in the study of\nreal-materials spectroscopy; it also has a significant impact on the QP\nlifetimes. Our results embody a new paradigm for ab initio QP theory.", "category": "cond-mat_mtrl-sci" }, { "text": "Theoretical determination of the Raman spectra of MgSiO3 perovskite and\n post-perovskite at high pressure: We use the density functional perturbation theory to determine for the first\ntime the pressure evolution of the Raman intensities for a mineral, the two\nhigh-pressure structures of MgSiO3 perovskite and post-perovskite. At high\npressures, the Raman powder spectra reveals three main peaks for the perovskite\nstructure and one main peak for the post-perovskite structure. Due to the large\ndifferences in the spectra of the two phases Raman spectroscopy can be used as\na good experimental indication of the phase transition.", "category": "cond-mat_mtrl-sci" }, { "text": "Drastic Changes in Dielectric Function of Silver Under dc Voltage: Significant changes of the relative permittivity of a silver film have been\ndetected using the surface plasmon resonance (SPR) method when a constant\nelectric field is applied to a MDM (metal-dielectric-metal) nanostructure. The\nstructure looks like a capacitor with a 177-nm dielectric corundum film placed\nbetween two silver films 49nm and 37nm thick. The effect manifests itself as a\nnoticeable change of the reflectivity of the structure when the voltage of up\nto 30V is applied to the electrodes. We have a good agreement between the\ntheory and experiment only if we suppose that the optical parameters of anode\nand cathode silver films change differently and the Al_2O_3 film absorbs the\nincident light. The refraction coefficient of the cathode silver layer is shown\nto become zero when the applied voltage is above 16V.", "category": "cond-mat_mtrl-sci" }, { "text": "First-Principles-Based Insight into Electrochemical Reactivity in a\n Cobalt-Carbonate-Hydrate Pseudocapacitor: Cobalt carbonate hydroxide (CCH) is a pseudocapacitive material with\nremarkably high capacitance and cycle stability. Previously, it was reported\nthat CCH pseudocapacitive materials are orthorhombic in nature. Recent\nstructural characterization has revealed that they are hexagonal in nature;\nhowever, their H positions still remain unclear. In this work, we carried out\nfirst-principles simulations to identify the H positions. Through the\nsimulations, we could consider various fundamental deprotonation reactions\ninside the crystal and computationally evaluate the electromotive forces (EMF)\nof the deprotonation ($V_\\mathrm{dp}$). Compared with the experimental\npotential window of the reaction ($< 0.6$ V (vs. saturated calomel electrode\n(SCE))), the computed $V_\\mathrm{dp}$ (vs. SCE) value ($3.05$ V) was beyond the\npotential window, indicating that deprotonation never occurred inside the\ncrystal. This may be attributed to the strongly formed hydrogen-bonds (H-bonds)\nin the crystal, thereby leading to the structural stabilization. We further\ninvestigated the crystal anisotropy in an actual capacitive material by\nconsidering the growth mechanism of the CCH crystal. By associating our X-ray\ndiffraction (XRD) peak simulations with experimental structural analysis, we\nfound that the H-bonds formed between CCH $\\{(\\bar{1}\\bar{1}\\bar{1}),\n(2\\bar{1}\\bar{1}), (2\\bar{1}1)\\}$ planes (approximately parallel to $ab$-plane)\ncan result in 1-D growth (stacked along with $c$-axis).", "category": "cond-mat_mtrl-sci" }, { "text": "Calculation of Gilbert damping and magnetic moment of inertia using\n torque-torque correlation model within ab initio Wannier framework: Magnetization dynamics in magnetic materials are well described by the\nmodified semiclassical Landau-Lifshitz-Gilbert (LLG) equation, which includes\nthe magnetic damping $\\alpha$ and the magnetic moment of inertia $\\mathrm{I}$\ntensors as key parameters. Both parameters are material-specific and physically\nrepresent the time scales of damping of precession and nutation in\nmagnetization dynamics. $\\alpha$ and $\\mathrm{I}$ can be calculated quantum\nmechanically within the framework of the torque-torque correlation model. The\nquantities required for the calculation are torque matrix elements, the real\nand imaginary parts of the Green's function and its derivatives. Here, we\ncalculate these parameters for the elemental magnets such as Fe, Co and Ni in\nan ab initio framework using density functional theory and Wannier functions.\nWe also propose a method to calculate the torque matrix elements within the\nWannier framework. We demonstrate the effectiveness of the method by comparing\nit with the experiments and the previous ab initio and empirical studies and\nshow its potential to improve our understanding of spin dynamics and to\nfacilitate the design of spintronic devices.", "category": "cond-mat_mtrl-sci" }, { "text": "Lattice dynamics study in PbWO4 under high pressure: Room-temperature Raman scattering has been measured in lead tungstate up to\n17 GPa. We report the pressure dependence of all the Raman modes of the\ntetragonal scheelite phase (PbWO4-I, space group I41/a), which is stable at\nambient conditions. Upon compression the Raman spectrum undergoes significant\nchanges around 6.2 GPa due to the onset of a partial structural phase\ntransition to the monoclinic PbWO4-III phase (space group P21/n). Further\nchanges in the spectrum occur at 7.9 GPa, related to a scheelite-to-fergusonite\ntransition. This transition is observed due to the sluggishness and kinetic\nhindrance of the I-to-III transition. Consequently, we found the coexistence of\nthe scheelite, PbWO4-III, and fergusonite phases from 7.9 to 9 GPa, and of the\nlast two phases up to 14.6 GPa. Further to the experiments, we have performed\nab initio lattice dynamics calculations which have greatly helped us in\nassigning the Raman modes of the three phases and discussing their pressure\ndependence.", "category": "cond-mat_mtrl-sci" }, { "text": "Anisotropy effects on the magnetic excitations of a ferromagnetic\n monolayer below and above the Curie temperature: The field-driven reorientation transition of an anisotropic ferromagnetic\nmonolayer is studied within the context of a finite-temperature Green's\nfunction theory. The equilibrium state and the field dependence of the magnon\nenergy gap $E_0$ are calculated for static magnetic field $H$ applied in plane\nalong an easy or a hard axis. In the latter case, the in-plane reorientation of\nthe magnetization is shown to be continuous at T=0, in agreement with free spin\nwave theory, and discontinuous at finite temperature $T>0$, in contrast with\nthe prediction of mean field theory. The discontinuity in the orientation angle\ncreates a jump in the magnon energy gap, and it is the reason why, for $T>0$,\nthe energy does not go to zero at the reorientation field. Above the Curie\ntemperature $T_C$, the magnon energy gap $E_0(H)$ vanishes for H=0 both in the\neasy and in the hard case. As $H$ is increased, the gap is found to increase\nalmost linearly with $H$, but with different slopes depending on the field\norientation. In particular, the slope is smaller when $H$ is along the hard\naxis. Such a magnetic anisotropy of the spin-wave energies is shown to persist\nwell above $T_C$ ($T \\approx 1.2 T_C$).", "category": "cond-mat_mtrl-sci" }, { "text": "Adsorption and Vibrational Spectroscopy of CO on the Surface of MgO from\n Periodic Local Coupled-Cluster Theory: The adsorption of CO on the surface of MgO has long been a model problem in\nsurface chemistry. Here, we report periodic Gaussian-based calculations for\nthis problem using second-order perturbation theory (MP2) and coupled-cluster\ntheory with single and double excitations (CCSD) and perturbative triple\nexcitations [CCSD(T)], with the latter two performed using a recently developed\nextension of the local natural orbital approximation to problems with periodic\nboundary conditions. The low cost of periodic local correlation calculations\nallows us to calculate the full CCSD(T) binding curve of CO approaching the\nsurface of MgO (and thus the adsorption energy) and the two-dimensional\npotential energy surface (PES) as a function of the distance from the surface\nand the CO stretching coordinate. From the PES, we obtain the fundamental\nvibrational frequency of CO on MgO, whose shift from the gas phase value is a\ncommon experimental probe of surface adsorption. We find that CCSD(T) correctly\npredicts a positive frequency shift upon adsorption of\n$+14.7~\\textrm{cm}^{-1}$, in excellent agreement with the experimental shift of\n$+14.3~\\textrm{cm}^{-1}$. We use our CCSD(T) results to assess the accuracy of\nMP2, CCSD, and several density functional theory (DFT) approximations,\nincluding exchange correlation functionals and dispersion corrections. We find\nthat MP2 and CCSD yield reasonable binding energies and frequency shifts,\nwhereas many DFT calculations overestimate the magnitude of the adsorption\nenergy by $5$ -- $15$~kJ/mol and predict a negative frequency shift of about\n$-20~\\textrm{cm}^{-1}$, which we attribute to self-interaction-induced\ndelocalization errors that are mildly ameliorated with hybrid functionals. Our\nfindings highlight the accuracy and computational efficiency of the periodic\nlocal correlation for the simulation of surface chemistry with accurate\nwavefunction methods.", "category": "cond-mat_mtrl-sci" }, { "text": "First-principles approach to dielectric response of graded spherical\n particles: We have studied the effective response of composites of spherical particles\neach having a dielectric profile which varies along the radius of the\nparticles. We developed a first-principles approach to compute the dipole\nmoment of the individual spherical particle and hence the effective dielectric\nresponse of a dilute suspension. The approach has been applied to two model\ndielectric profiles, for which exact solutions are available. Moreover, we used\nthe exact results to validate the results from the differential effective\ndipole approximation, recently developed to treat graded spherical particles of\nan arbitrary dielectric profile. Excellent agreement between the two approaches\nwere obtained. While the focus of this work has been on dielectric responses,\nthe approach is equally applicable to analogous systems such as the\nconductivity and elastic problems.", "category": "cond-mat_mtrl-sci" }, { "text": "Large-Area Two-Dimensional Layered MoTe$_2$ by Physical Vapor Deposition\n and Solid-Phase Crystallization in a Tellurium-Free Atmosphere: Molybdenum ditelluride (MoTe$_2$) has attracted considerable interest for\nnanoelectronic, optoelectronic, spintronic, and valleytronic applications\nbecause of its modest band gap, high field-effect mobility, large\nspin-orbit-coupling splitting, and tunable 1T'/2H phases. However, synthesizing\nlarge-area, high-quality MoTe$_2$ remains challenging. The complicated design\nof gas-phase reactant transport and reaction for chemical vapor deposition or\ntellurization is nontrivial because of the weak bonding energy between Mo and\nTe. Here, we report a new method for depositing MoTe$_2$ that entails using\nphysical vapor deposition followed by a post-annealing process in a Te-free\natmosphere. Both Mo and Te were physically deposited onto the substrate by\nsputtering a MoTe$_2$ target. A composite SiO$_2$ capping layer was designed to\nprevent Te sublimation during the post-annealing process. The post-annealing\nprocess facilitated 1T'-to-2H phase transition and solid-phase crystallization,\nleading to the formation of high-crystallinity few-layer 2H-MoTe$_2$ with a\nfield-effect mobility of ~10 cm$^2$/(V-s), the highest among all nonexfoliated\n2H-MoTe$_2$ currently reported. Furthermore, 2H-MoS$_2$ and Td-WTe$_2$ can be\ndeposited using similar methods. Requiring no transfer or chemical reaction of\nmetal and chalcogen reactants in the gas phase, the proposed method is\npotentially a general yet simple approach for depositing a wide variety of\nlarge-area, high-quality, two-dimensional layered structures.", "category": "cond-mat_mtrl-sci" }, { "text": "Electronic Structures of N-doped Graphene with Native Point Defects: Nitrogen doping in graphene has important implications in graphene-based\ndevices and catalysts. We have performed the density functional theory\ncalculations to study the electronic structures of N-doped graphene with\nvacancies and Stone-Wales defect. Our results show that monovacancies in\ngraphene act as hole dopants and that two substitutional N dopants are needed\nto compensate for the hole introduced by a monovacancy. On the other hand,\ndivacancy does not produce any free carriers. Interestingly, a single N dopant\nat divacancy acts as an acceptor rather than a donor. The interference between\nnative point defect and N dopant strongly modifies the role of N doping\nregarding the free carrier production in the bulk pi bands. For some of the\ndefects and N dopant-defect complexes, localized defect pi states are partially\noccupied. Discussion on the possibility of spin polarization in such cases is\ngiven. We also present qualitative arguments on the electronic structures based\non the local bond picture. We have analyzed the 1s-related x-ray photoemission\nand adsorption spectroscopy spectra of N dopants at vacancies and Stone-Wales\ndefect in connection with the experimental ones. We also discuss characteristic\nscanning tunneling microscope (STM) images originating from the electronic and\nstructural modifications by the N dopant-defect complexes. STM imaging for\nsmall negative bias voltage will provide important information about possible\nactive sites for oxygen reduction reaction.", "category": "cond-mat_mtrl-sci" }, { "text": "Multi-Fields Modulation of Physical Properties of Oxide Thin Films: Oxide thin films exhibit versatile physical properties such as magnetism,\nferroelectricity, piezoelectricity, metal-insulator transition (MIT),\nmultiferroicity, colossal magnetoresistivity, switchable resistivity, etc. More\nimportantly, the exhibited multifunctionality could be tuned by various\nexternal fields, which has enabled demonstration of novel electronic devices.\nIn this article, recent studies of the multi-fields modulation of physical\nproperties in oxide thin films have been reviewed. Some of the key issues and\nprospects about this field are also addressed.", "category": "cond-mat_mtrl-sci" }, { "text": "Communication: Hole localization in Al-doped quartz SiO2 within ab\n initio hybrid-functional DFT: We investigate the long-standing problem of the hole localization at the Al\nimpurity in quartz SiO$_2$, using a relatively recent DFT hybrid-functional\nmethod in which the exchange fraction is obtained \\emph{ab initio}, based on an\nanalogy with the static many-body COHSEX approximation to the electron\nself-energy. As the amount of the admixed exact exchange in hybrid functionals\nhas been shown to be determinant for properly capturing the hole localization,\nthis problem constitutes a prototypical benchmark for the accuracy of the\nmethod, allowing one to assess to what extent self-interaction effects are\navoided. We obtain good results in terms of description of the charge\nlocalization and structural distortion around the Al center, improving with\nrespect to the more popular B3LYP hybrid-functional approach. We also discuss\nthe accuracy of computed hyperfine parameters, by comparison with previous\ncalculations based on other self-interaction-free methods, as well as\nexperimental values. We discuss and rationalize the limitations of our approach\nin computing defect-related excitation energies in low-dielectric-constant\ninsulators.", "category": "cond-mat_mtrl-sci" }, { "text": "Tunable 2D Electron- and 2D Hole States Observed at Fe/SrTiO$_3$\n Interfaces: Oxide electronics provide the key concepts and materials for enhancing\nsilicon-based semiconductor technologies with novel functionalities. However, a\nbasic but key property of semiconductor devices still needs to be unveiled in\nits oxidic counterparts: the ability to set or even switch between two types of\ncarriers - either negatively (n) charged electrons or positively (p) charged\nholes. Here, we provide direct evidence for individually emerging n- or p-type\n2D band dispersions in STO-based heterostructures using resonant photoelectron\nspectroscopy. The key to tuning the carrier character is the oxidation state of\nan adjacent Fe-based interface layer: For Fe and FeO, hole bands emerge in the\nempty band gap region of STO due to hybridization of Ti and Fe-derived states\nacross the interface, while for Fe$_3$O$_4$ overlayers, an 2D electron system\nis formed. Unexpected oxygen vacancy characteristics arise for the hole-type\ninterfaces, which as of yet had been exclusively assigned to the emergence of\n2DESs. In general, this finding opens up the possibility to straightforwardly\nswitch the type of conductivity at STO interfaces by the oxidation state of a\nredox overlayer. This will extend the spectrum of phenomena in oxide\nelectronics, including the realization of combined n/p-type all-oxide\ntransistors or logic gates.", "category": "cond-mat_mtrl-sci" }, { "text": "Cubic-scaling algorithm and self-consistent field for the random-phase\n approximation with second-order screened exchange: The random-phase approximation with second-order screened exchange\n(RPA+SOSEX) is a model of electron correlation energy with two caveats: its\naccuracy depends on an arbitrary choice of mean field, and it scales as\n$\\mathcal{O}(n^5)$ operations and $\\mathcal{O}(n^3)$ memory for $n$ electrons.\nWe derive a new algorithm that reduces its scaling to $\\mathcal{O}(n^3)$\noperations and $\\mathcal{O}(n^2)$ memory using controlled approximations and a\nnew self-consistent field that approximates Brueckner coupled-cluster doubles\n(BCCD) theory with RPA+SOSEX, referred to as Brueckner RPA (BRPA) theory. The\nalgorithm comparably reduces the scaling of second-order\nM$\\mathrm{{\\o}}$ller-Plesset (MP2) perturbation theory with smaller cost\nprefactors than RPA+SOSEX. Within a semiempirical model, we study H$_2$\ndissociation to test accuracy and H$_n$ rings to verify scaling.", "category": "cond-mat_mtrl-sci" }, { "text": "Two Dimensional Ferromagnetic Semiconductor: Monolayer CrGeS$_3$: Recently, two-dimensional ferromagnetic semiconductors have been an important\nclass of materials for many potential applications in spintronic devices. Based\non density functional theory, we systematically explore the magnetic and\nelectronic properties of CrGeS$_3$ with the monolayer structures. The\ncomparison of total energy between different magnetic states ensures the\nferromagnetic ground state of monolayer CrGeS$_3$. It is also shown that\nferromagnetic and semiconducting properties are exhibited in monolayer\nCrGeS$_3$ with the magnetic moment of 3 $\\mu_{B}$ for each Cr atom, donated\nmainly by the intense $dp$$\\sigma$-hybridization of Cr $e_g$-S $p$. There are\nthe bandgap of 0.70 eV of spin-up state in the monolayer structure when 0.77 eV\nin spin-down state. The global gap is 0.34 eV (2.21 eV by using HSE06\nfunctional), which originates from bonding $dp\\sigma$ hybridized states of Cr\n$e_g$-S $p$ and unoccupied Cr $t_{2g}$-Ge $p$ hybridization. Besides, we\nestimate that the monolayer CrGeS$_3$ possesses the Curie temperature of 161 K\nby mean-field theory.", "category": "cond-mat_mtrl-sci" }, { "text": "A new method for measuring excess carrier lifetime in bulk silicon:\n Photoexcited muon spin spectroscopy: We have measured the optically injected excess carrier lifetime in silicon\nusing photoexcited muon spin spectroscopy. Positive muons implanted deep in a\nwafer can interact with the excess carriers and directly probe the bulk carrier\nlifetime whilst minimizing the effect from surface recombination. The method is\nbased on the relaxation rate of muon spin asymmetry, which depends on the\nexcess carrier concentration. The underlying microscopic mechanism has been\nunderstood by simulating the four-state muonium model in Si under illumination.\nWe apply the technique to different injection levels and temperatures, and\ndemonstrate its ability for injection- and temperature-dependent lifetime\nspectroscopy.", "category": "cond-mat_mtrl-sci" }, { "text": "Temperature-dependent stability of polytypes and stacking faults in SiC:\n reconciling theory and experiments: The relative stability of SiC polytypes, changing with temperature, has been\nconsidered a paradox for about thirty years, due to discrepancies between\ntheory and experiments. Based on ab-initio calculations including van der Waals\ncorrections, a temperature-dependent polytypic diagram consistent with the\nexperimental observations is obtained. Results are easily interpreted based on\nthe influence of the hexagonality on both cohesive energy and entropy.\nTemperature-dependent stability of stacking faults is also analyzed and found\nto be in agreement with experimental evidences. Our results suggest that lower\ntemperatures during SiC crystal deposition are advantageous in order to reduce\nubiquitous stacking faults in SiC-based power devices.", "category": "cond-mat_mtrl-sci" }, { "text": "Critical Ruptures: The fracture of materials is a catastrophic phenomenon of considerable\ntechnological and scientific importance. Here, we analysed experiments designed\nfor industrial applications in order to test the concept that, in heterogeneous\nmaterials such as fiber composites, rocks, concrete under compression and\nmaterials with large distributed residual stresses, rupture is a genuine\ncritical point, i.e. the culmination of a self-organization of damage and\ncracking characterized by power law signatures. Specifically, we analyse the\nacoustic emissions recorded during the pressurisation of spherical tanks of\nkevlar or carbon fibers pre-impregnated in a resin matrix wrapped up around a\nthin metallic liner (steel or titanium) fabricated and instrumented by\nA\\'erospatiale-Matra Inc. These experiments are performed as part of a routine\nindustrial procedure which tests the quality of the tanks prior to shipment and\nvaries in nature. We find that the seven acoustic emission recordings of seven\npressure tanks which was brought to rupture exhibit clear acceleration in\nagreement with a power law ``divergence'' expected from the critical point\ntheory. In addition, we find strong evidence of log-periodic corrections that\nquantify the intermittent succession of accelerating bursts and quiescent\nphases of the acoustic emissions on the approach to rupture. An improved model\naccounting for the cross-over from the non-critical to the critical region\nclose to the rupture point exhibits interesting predictive potential.", "category": "cond-mat_mtrl-sci" }, { "text": "Putting Density Functional Theory to the Test in\n Machine-Learning-Accelerated Materials Discovery: Accelerated discovery with machine learning (ML) has begun to provide the\nadvances in efficiency needed to overcome the combinatorial challenge of\ncomputational materials design. Nevertheless, ML-accelerated discovery both\ninherits the biases of training data derived from density functional theory\n(DFT) and leads to many attempted calculations that are doomed to fail. Many\ncompelling functional materials and catalytic processes involve strained\nchemical bonds, open-shell radicals and diradicals, or metal-organic bonds to\nopen-shell transition-metal centers. Although promising targets, these\nmaterials present unique challenges for electronic structure methods and\ncombinatorial challenges for their discovery. In this Perspective, we describe\nthe advances needed in accuracy, efficiency, and approach beyond what is\ntypical in conventional DFT-based ML workflows. These challenges have begun to\nbe addressed through ML models trained to predict the results of multiple\nmethods or the differences between them, enabling quantitative sensitivity\nanalysis. For DFT to be trusted for a given data point in a high-throughput\nscreen, it must pass a series of tests. ML models that predict the likelihood\nof calculation success and detect the presence of strong correlation will\nenable rapid diagnoses and adaptation strategies. These \"decision engines\"\nrepresent the first steps toward autonomous workflows that avoid the need for\nexpert determination of the robustness of DFT-based materials discoveries.", "category": "cond-mat_mtrl-sci" }, { "text": "Novel supercell compounds of layered Bi-Rh-O with $p$-type metallic\n conduction materialized as a thin film form: Layered oxides have been intensively studied due to their high degree of\nfreedom in designing various electromagnetic properties and functionalities.\nWhile Bi-based layered supercell (LSC) compounds\n[Bi$_n$O$_{n+\\delta}$]-[$M$O$_2$] ($M$ = Mn, Mn/Al, Mn/Fe, or Mn/Ni; $n=2, 3$)\nare a group of prospective candidates, all of the reported compounds are\ninsulators. Here, we report on the synthesis of two novel metallic LSC\ncompounds [Bi$_{n}$O$_{n+\\delta}$]-[RhO$_2$] ($n=2, 3$) by pulsed laser\ndeposition and subsequent annealing. With tuning the thickness of the\nsublattice from Bi$_2$O$_{2+\\delta}$ to Bi$_3$O$_{3+\\delta}$, a\ndimensionality-dependent electrical transport is revealed from a conventional\nmetallic transport in $n=2$ to a localized transport in $n=3$. Our successful\ngrowth will be an important step for further exploring novel layered oxide\ncompounds.", "category": "cond-mat_mtrl-sci" }, { "text": "First-principles study of excitonic effects in Raman intensities: The ab initio prediction of Raman intensities for bulk solids usually relies\non the hypothesis that the frequency of the incident laser light is much\nsmaller than the band gap. However, when the photon frequency is a sizeable\nfraction of the energy gap, or higher, resonance effects appear. In the case of\nsilicon, when excitonic effects are neglected, the response of the solid to\nlight increases by nearly three orders of magnitude in the range of frequencies\nbetween the static limit and the gap. When excitonic effects are taken into\naccount, an additional tenfold increase in the intensity is observed. We\ninclude these effects using a finite-difference scheme applied on the\ndielectric function obtained by solving the Bethe-Salpeter equation. Our\nresults for the Raman susceptibility of silicon show stronger agreement with\nexperimental data compared with previous theoretical studies. For the sampling\nof the Brillouin zone, a double-grid technique is proposed, resulting in a\nsignificant reduction in computational effort.", "category": "cond-mat_mtrl-sci" }, { "text": "Exciton-Exciton transitions involving strongly bound excitons: an ab\n initio approach: In pump-probe spectroscopy, two laser pulses are employed to garner dynamical\ninformation from the sample of interest. The pump initiates the optical process\nby exciting a portion of the sample from the electronic ground state to an\naccessible electronic excited state, an exciton. Thereafter, the probe\ninteracts with the already excited sample. The change in the absorbance after\npump provides information on transitions between the excited states and their\ndynamics. In this work we study these exciton-exciton transitions by means of\nan ab initio real time propagation scheme based on dynamical Berry phase\nformulation. The results are then analyzed taking advantage of a Fermi-golden\nrule approach formulated in the excitonic basis-set and in terms of the\nsymmetries of the excitonic states. Using bulk LiF and 2D hBN as two prototype\nmaterials, we discuss the selection rules for transitions involving strongly\nbound excitons, for which the hydrogen model cannot be used.", "category": "cond-mat_mtrl-sci" }, { "text": "First principles investigations in the carbon silicon system of novel\n tetragonal C8 (diamond) and Si8 allotropes and binary Si4C4 phase: Novel extended networks of C8, Si8 and silicon carbide Si4C4 are proposed\nbased on crystal chemistry rationale and optimized structures to ground state\nenergies and derived physical properties within the density functional theory\n(DFT). The two carbon and silicon allotropes and the silicon carbide belong to\nprimitive tetragonal space group P-4m2 Number 115. C8 allotrope structure made\nof corner sharing C4 and Si4 tetrahedra is illustrated by charge density\nprojections exhibiting sp3 like carbon hybridization. From careful symmetry\nanalysis, Symmetry analysis of C8 indicated that it is another representation\nof cubic diamond, space group F-d3m Number 227. C8 is identified as ultra-hard\nwith a similar magnitude of Vickers hardness. The interest in C8 is to serve as\ntemplate to study Si8 and Si-C binary. Si8 allotrope is found soft with HV =13\nGPa alike cubic Si, and Si4C4 is identified with HV =33 GPa close to\nexperimental SiC. All three new phases are mechanically (elastic constants) and\ndynamically (phonons) stable, and their electronic band structures are\ncharacteristic of insulating C8 (diamond) with large band gaps of about 5 eV,\nand semi-conducting Si8 and Si4C4 with band gaps of about 1 eV.", "category": "cond-mat_mtrl-sci" }, { "text": "Persistent Current in Two Coupled Rings: We report the solution of the persistent current in two coupled rings in the\npresence of external magnetic fluxes. We showed that the magnetic fluxes modify\nthe global phase of the electronic wave function for multiple connected\ngeometry formed by the coupled rings. We obtained an exact solution for the\npersistent current and investigated the exact solution numerically. For two\nlarge coupled rings with equal fluxes, we found that the persistent current in\nthe two coupled rings is in fact equal to that in a single ring. This theory\nexplains the experimental results observed in a line of sixteen coupled rings.\n(Phys. Rev. Lett. 86, 3124 (2001).)", "category": "cond-mat_mtrl-sci" }, { "text": "sim-trhepd-rheed -- Open-source simulator of total-reflection\n high-energy positron diffraction (TRHEPD) and reflection high-energy electron\n diffraction (RHEED): The present paper reports sim-trhepd-rheed (STR), an open-source simulator of\ntotal-reflection high-energy positron diffraction (TRHEPD) and reflection\nhigh-energy electron diffraction (RHEED) experiments which are used for\natom-scale surface structure determination of a material. The STR simulator is\nused for the analysis of experimental diffraction data by simulating the\nrocking curve from a given trial surface structure by solving the partial\ndifferential equation of the dynamical quantum diffraction theory for positron\nor electron wavefunctions. Using the obtained surface structure, electronic\nstructure, and other physical quantities can be evaluated through\nfirst-principles calculations. For this purpose, a utility software was also\ndeveloped in order to realize a first principles calculation with the Quantum\nESPRESSO suite.", "category": "cond-mat_mtrl-sci" }, { "text": "Optimal Paths for Spatially Extended Metastable Systems Driven by Noise: The least action principle is exploited as a simulation tool to find the\noptimal dynamic path for spatially extended systems driven by a small noise.\nApplications are presented for thermally activated switching of a\nspatially-extended bistable system as well as the switching dynamics of\nmagnetic thin films. The issue of nucleation versus propagation is discussed\nand the scaling for the number of nucleation events as a function of the\nterminal time and other material parameters is computed.", "category": "cond-mat_mtrl-sci" }, { "text": "WannSymm: A symmetry analysis code for Wannier orbitals: We derived explicit expressions of symmetry operators on Wannier basis, and\nimplemented these operators in WannSymm software. Based on this implementation,\nWannSymm can i) symmetrize the real-space Hamiltonian output from Wannier90\ncode, ii) generate symmetry operators of the little group at a specific\nk-point, and iii) perform symmetry analysis for Wannier band structure. In\ngeneral, symmetrized Hamiltonians yield improved results compared with the\noriginal ones when they are employed for nodal structure searching, surface\nGreen's function calculations, and other model calculations.", "category": "cond-mat_mtrl-sci" }, { "text": "Icosahedral quasicrystal enhanced nucleation in commercially pure Ni\n processed by selective laser melting: This work provides unambiguous evidence for the occurrence of icosahedral\nquasicrystal (iQC) enhanced nucleation during selective laser melting of gas\natomized commercially-pure Ni powders. This solidification mechanism, which has\nonly been recently reported in a few alloys and has to date never been observed\nin pure metals, consists on the solidification of grains of the primary phase\non the facets of iQCs formed due to the presence of icosahedral short range\norder in the liquid. The occurrence of iQC enhanced nucleation has been\ninferred from the observation in the SLM processed pure Ni samples of an excess\nfraction of partially incoherent twin boundaries and of clusters of twinned\ngrain pairs sharing common <110> five-fold symmetry axes. This work further\nevidences that additive manufacturing methods may constitute an invaluable tool\nfor investigating the fundamentals of solidification and for the design of\nunprecedented grain boundary networks.", "category": "cond-mat_mtrl-sci" }, { "text": "Time Constants of Spin-Dependent Recombination Processes: We present experiments to systematically study the time constants of\nspin-dependent recombination processes in semiconductors using pulsed\nelectrically detected magnetic resonance (EDMR). The combination of\ntime-programmed optical excitation and pulsed spin manipulation allows us to\ndirectly measure the recombination time constants of electrons via localized\nspin pairs and the time constant of spin pair formation as a function of the\noptical excitation intensity. Using electron nuclear double resonance, we show\nthat the time constant of spin pair formation is determined by an electron\ncapture process. Based on these time constants we devise a set of rate\nequations to calculate the current transient after a resonant microwave pulse\nand compare the results with experimental data. Finally, we critically discuss\nthe effects of different boxcar integration time intervals typically used to\nanalyze pulsed EDMR experiments on the determination of the time constants. The\nexperiments are performed on phosphorus-doped silicon, where EDMR via spin\npairs formed by phosphorus donors and Si/SiO2 interface dangling bond defects\nis detected.", "category": "cond-mat_mtrl-sci" }, { "text": "Incorporation of Mn in Al$_{x}$Ga$_{1-x}$N probed by x-ray absorption\n and emission spectroscopy, high-resolution microscopy, x-ray diffraction and\n first-principles calculations: Synchrotron radiation x-ray absorption and emission spectroscopy techniques,\ncomplemented by high-resolution transmission electron microscopy methods and\ndensity functional theory calculations are employed to investigate the effect\nof Mn in Al$_{x}$Ga$_{1-x}$N:Mn samples with an Al content up to 100%. The\natomic and electronic structure of Mn is established together with its local\nenvironment and valence state. A dilute alloy without precipitation is obtained\nfor Al$_{x}$Ga$_{1-x}$N:Mn with Al concentrations up to 82%, and the surfactant\nrole of Mn in the epitaxial process is confirmed.", "category": "cond-mat_mtrl-sci" }, { "text": "Depolarizing-Field Effect in Strained Nanoscale Ferroelectric Capacitors\n and Tunnel Junctions: The influence of depolarizing field on the magnitude and stability of a\nuniform polarization in ferroelectric capacitors and tunnel junctions is\nstudied using a nonlinear thermodynamic theory. It is predicted that, in\nheterostructures involving strained epitaxial films and metal electrodes, the\nhomogeneous polarization state may remain stable against transformations into\nthe paraelectric phase and into polydomain states down to the nanoscale. This\nresult supports the possibility of depolarizing-field-related resistive\nswitching in ferroelectric tunnel junctions with dissimilar electrodes. The\nresistance on/off ratio in such junctions is shown to be governed by the\ndifference between the reciprocal capacitances of screening space charges in\nthe electrodes.", "category": "cond-mat_mtrl-sci" } ]