Showing papers in "Physical Review Materials in 2021"

Superconductivity in the Z 2 kagome metal KV 3 Sb 5

Abstract: Here we report the observation of bulk superconductivity in single crystals of the two-dimensional kagome metal ${\mathrm{KV}}_{3}{\mathrm{Sb}}_{5}$. Magnetic susceptibility, resistivity, and heat capacity measurements reveal superconductivity below ${T}_{c}=0.93\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, and density functional theory (DFT) calculations further characterize the normal state as a ${\mathbb{Z}}_{2}$ topological metal. Our results demonstrate that the recent observation of superconductivity within the related kagome metal ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ is likely a common feature across the $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ ($A$: K, Rb, Cs) family of compounds and establishes them as a rich arena for studying the interplay between bulk superconductivity, topological surface states, and likely electronic density wave order in an exfoliable kagome lattice.

Prediction of low-Z collinear and noncollinear antiferromagnetic compounds having momentum-dependent spin splitting even without spin-orbit coupling

Abstract: Antiferromagnetic order offers a non-relativistic route to create spin splitting and spin polarization effects that do not rely on heavy element compounds (with strong spin-orbit coupling, SOC), and could exist even in centrosymmetric crystals. Cases enabling such non-relativistic, SOC-unrelated spin polarization effect are classified as SST-4 (SST-4A and SST-4B) of all seven possible spin splitting prototypes derived in this paper based on magnetic symmetry analysis. The authors uncovered 422 magnetic space groups (160 centrosymmetric and 262 non-centrosymmetric) and 201 candidate antiferromagnets that belong to the SST-4 category. DFT calculations for collinear and noncollinear cases are provided as basis for guiding future experiments.

Nodeless kagome superconductivity in LaRu 3 Si 2

Abstract: We report muon spin rotation (μSR) experiments together with first-principles calculations on microscopic properties of superconductivity in the kagome superconductor LaRu3Si2 with Tc≃ 7K. Below Tc, μSR reveals type-II superconductivity with a single s-wave gap, which is robust against hydrostatic pressure up to 2 GPa. We find that the calculated normal state band structure features a kagome flat band, and Dirac as well as van Hove points formed by the Ru-dz2 orbitals near the Fermi level. We also find that electron-phonon coupling alone can only reproduce a small fraction of Tc from calculations, which suggests other factors in enhancing Tc such as the correlation effect from the kagome flat band, the van Hove point on the kagome lattice, and the high density of states from narrow kagome bands. Our experiments and calculations taken together point to nodeless moderate coupling kagome superconductivity in LaRu3Si2.

Ferroelectricity in boron-substituted aluminum nitride thin films

Abstract: This manuscript demonstrates ferroelectricity in B-substituted AlN thin films and a complementary set of first-principles calculations to understand their structure-property relationships. ${\mathrm{Al}}_{1--x}{\mathrm{B}}_{x}\mathrm{N}$ films are grown by dual-cathode reactive magnetron sputtering on $(110)\mathrm{W}/(001){\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ substrates at 300\ifmmode^\circ\else\textdegree\fi{}C at compositions spanning $x=0$ to $x=0.20$. X-ray diffraction studies indicate a decrease in both the $c$ and $a$ lattice parameters with increasing B concentration, resulting in a decrease in unit cell volume and a constant $c$/$a$ axial ratio of 1.60 over this composition range. Films with $0.02\ensuremath{\le}x\ensuremath{\le}0.15$ display ferroelectric switching with remanent polarizations exceeding $125\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{--2}$ while maintaining band gap energies of $g5.2\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. The large band gap allows low frequency hysteresis measurement (200 Hz) with modest leakage contributions. At B concentrations of $xg0.15$, $c$-axis orientation deteriorates and ferroelectric behavior is degraded. Density-functional theory calculations corroborate the structural observations and provide predictions for the wurtzite $u$ parameter, polarization reversal magnitudes, and composition-dependent coercive fields.

Doping-dependent character and possible magnetic ordering of NdNiO 2

Abstract: The novel nickelate superconductors of infinite-layer type feature challenging electronic pecularities in the normal-state phase diagram with doping. Distinct many-body behavior and different dispersion regimes of the entangled ${\mathrm{Ni}\text{\ensuremath{-}}{d}_{{z}^{2}}$, Ni-${d}_{{x}^{2}\ensuremath{-}{y}^{2}}}$ orbital sector give rise to highly rich physics, which is here studied for the case of the $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_{2}$ system. An analysis based on advanced realistic dynamical mean-field theory unveils that the superconducting hole-doped region is the meeting place of a (self-)doped Mott insulator from the underdoped side, and a bad Hund metal from the overdoped side. Fermi-level crossing of the Ni-${d}_{{z}^{2}}$ flat-band ties both regimes together to form a singular arena for unconventional superconductivity. We furthermore shed light on the intriguing problem of elusive magnetism in infinite-layer nickelates. Antiferromagnetic (AFM) order with small Ni moments is shown to be a vital competitor at low temperature. At stoichiometry, C-AFM order with ferromagnetic spin-alignment along the $c$-axis benefits from a conceivable coexistence with Kondo(-lattice) screening.

Activating electrocatalytic hydrogen evolution performance of two-dimensional M Si 2 N 4 ( M = Mo , W ) : A theoretical prediction

Abstract: Very recently, a type of two-dimensional $(2D)$ crystals, $M{\mathrm{Si}}_{2}{\mathrm{N}}_{4}(M=\mathrm{Mo},\mathrm{W})$, has been successfully synthesized in experiment [Y.-L. Hong et al., Science 369, 670 (2020)]. Here, using first-principles calculations, we systematically investigate the potential of two-dimensional $(2D)$ $\mathrm{M}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ as hydrogen evolution reaction (HER) catalysts, and propose two effective ways to trigger their HER activity. Our computations reveal that while the metal vacancies cannot activate the inertia planes for HER, $2D$ $\mathrm{M}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ with $N$ vacancy exhibits excellent HER performance. Especially for $2D$ $\mathrm{W}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$, the introduction of $N$ vacancy can yield an ideal value of hydrogen adsorption Gibbs free energy $(\mathrm{\ensuremath{\Delta}}{G}_{{\mathrm{H}}^{*}}=\ensuremath{-}0.02\phantom{\rule{0.16em}{0ex}}\mathrm{eV})$, even superior to that of Pt $(\mathrm{\ensuremath{\Delta}}{G}_{{\mathrm{H}}^{*}}=\ensuremath{-}0.09\phantom{\rule{0.16em}{0ex}}\mathrm{eV})$. Moreover, by high-throughput screening of $3d$, $4d$, and $5d$ transition metals, we find that introducing V/Fe/Nb/Tc/Ta atom into $2D$ $\mathrm{M}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ can significantly trigger their HER activity. The underlying physics are discussed in detail. Our work not only highlights a family of potential HER electrocatalysts, but also provides feasible strategies for triggering their HER activity.

Valley relaxation of resident electrons and holes in a monolayer semiconductor: Dependence on carrier density and the role of substrate-induced disorder

Abstract: Analogous to the keen interest in electron, hole, and exciton spin relaxation during the early days of semiconductor spintronics, measurements of valley relaxation in monolayer transition-metal dichalcogenide (TMD) semiconductors such as WSe2 are currently a focus of attention for potential applications in valleytronics. For many notional valleytronic devices, the important parameter is the intrinsic valley relaxation time of the resident electrons and holes that exist in n-type and p-type TMD monolayers. Using optical methods, the authors determine these timescales as a systematic function of carrier density, and study the (important) role of the underlying substrate. Microsecond-long valley relaxation of carriers is revealed at low densities.

Transversal flexoelectric coefficient for nanostructures at finite deformations from first principles

Abstract: We present a formulation for calculating the transversal flexoelectric coefficient of nanostructures at finite deformations from first principles. Specifically, we introduce the concept of radial polarization to make the coefficient a well-defined quantity for uniform bending deformations. We use the framework to calculate the flexoelectric coefficient for group IV atomic monolayers using density functional theory. We find that graphene's coefficient is significantly larger than previously reported, with a charge transfer mechanism that differs from other members of its group.

All-electron periodic G 0 W 0 implementation with numerical atomic orbital basis functions: Algorithm and benchmarks

Abstract: We present an all-electron, periodic ${G}_{\text{0}}{W}_{\text{0}}$ implementation within the numerical atomic orbital (NAO) basis framework. A localized variant of the resolution-of-the-identity (RI) approximation is employed to significantly reduce the computational cost of evaluating and storing the two-electron Coulomb repulsion integrals. We demonstrate that the error arising from localized RI approximation can be reduced to an insignificant level by enhancing the set of auxiliary basis functions, used to expand the products of two single-particle NAOs. An efficient algorithm is introduced to deal with the Coulomb singularity in the Brillouin zone sampling that is suitable for the NAO framework. We perform systematic convergence tests and identify a set of computational parameters, which can serve as the default choice for most practical purposes. Benchmark calculations are carried out for a set of prototypical semiconductors and insulators, and compared to independent reference values obtained from an independent ${G}_{\text{0}}{W}_{\text{0}}$ implementation based on linearized augmented plane waves (LAPWs) plus high-energy localized orbitals (HLOs) basis set, as well as experimental results. With a moderate (FHI-aims tier 2) NAO basis set, our ${G}_{\text{0}}{W}_{\text{0}}$ calculations produce band gaps that typically lie in between the standard LAPW and the $\mathrm{LAPW}+\mathrm{HLO}$ results. Complementing tier 2 with highly localized Slater-type orbitals (STOs), we find that the obtained band gaps show an overall convergence towards the $\mathrm{LAPW}+\mathrm{HLO}$ results. The algorithms and techniques developed in this work pave the way for efficient implementations of correlated methods within the NAO framework.

Probing configurational disorder in ZnGeN 2 using cluster-based Monte Carlo

Abstract: ${\mathrm{ZnGeN}}_{2}$ is sought as a semiconductor with comparable lattice constant to GaN and tunable band gap for integration in optoelectronic devices. Configurational disorder on the cation sublattice of ${\mathrm{ZnGeN}}_{2}$ can strongly modify the electronic structure compared to the ordered material, and both ordered and disordered forms of ${\mathrm{ZnGeN}}_{2}$ are candidates for light-emitting diodes and other emitters. The nonisovalent character of the disordered species (${\mathrm{Zn}}^{2+}$ and ${\mathrm{Ge}}^{4+}$) subjects the cation ordering to strong short-range order effects. To model these effects, we use Monte Carlo (MC) simulations utilizing a cluster expansion to approximate formation enthalpy. Representative disordered configurations in 1024-atom supercells are relaxed in density functional theory calculations. From the MC structures, we extract a short-range order parameter (the N-cation coordination motif), and two long-range order parameters (Bragg-Williams and stretching parameters), and examine their correlations. We perform a thermodynamic integration to determine the mixing entropy and free energy. ${\mathrm{ZnGeN}}_{2}$ exhibits a first-order phase transition with pronounced discontinuities in enthalpy and entropy, as well as in the structural order parameters. We discuss the relationship between the effective temperature used in the MC simulation and the growth temperatures in experiment in relation to the crossover from the nonequilibrium to the equilibrium growth regime. This work expands on current models of site disorder in ${\mathrm{ZnGeN}}_{2}$ and provides atomic structure models with a systematic variation of the degree of cation disorder.

Band structure of MoSTe Janus nanotubes

Abstract: Nanotubes generated by rolling up transition metal dichalcogenide Janus monolayers are a new class of low-dimensional materials, which are expected to display unique electronic properties compared to their parent two- and three-dimensional structures. Here, we investigate the band structure of $1H$-MoSTe Janus armchair and zigzag nanotubes, which were recently hypothesized to be stable as single-walled structures with radii of only a few nanometers. We first investigate the most stable nanotube sizes and assess the influence of quantum confinement and curvature on the band structures, showing that these are heavily modified by curvature while confinement effects are negligible. The curvature dependence is then further studied by analyzing the band gap dependence on the nanotube radius, where band gap changes as large as 0.5 eV are observed. By investigating the band edge positions and orbital projected density of states for different tube sizes, we find that this high sensitivity is mainly attributed to the Mo $d$ states in the conduction band.

Extending Shannon's ionic radii database using machine learning

Abstract: The authors extend the ionic radii database of Shannon's seminal work using machine learning regression. The developed consolidated table will allow prediction of material properties with high accuracy by considering the definite ionic radius value based on the oxidation state and coordination number. The work is relevant to the evolving material informatics field and has applications in many related fields.

Strength of effective Coulomb interaction in two-dimensional transition-metal halides M X 2 and M X 3 ( M = Ti , V, Cr, Mn, Fe, Co, Ni; X = Cl , Br, I)

Abstract: We calculate the strength of the effective on-site Coulomb interaction (Hubbard $U$) in two-dimensional transition-metal (TM) dihalides $M{X}_{2}$ and trihalides $M{X}_{3}$ ($M=\mathrm{Ti}$, V, Cr, Mn, Fe, Co, Ni; $X=\mathrm{Cl}$, Br, I) from first principles using the constrained random-phase approximation. The correlated subspaces are formed from ${t}_{2g}$ or ${e}_{g}$ bands at the Fermi energy. Elimination of the efficient screening taking place in these narrow bands gives rise to sizable interaction parameters $U$ between the localized ${t}_{2g}$ (${e}_{g}$) electrons. Due to this large Coulomb interaction, we find $U/Wg1$ (with the bandwidth $W$) in most TM halides, making them strongly correlated materials. Among the metallic TM halides in the paramagnetic state, the correlation strength $U/W$ reaches a maximum in $\mathrm{Ni}{X}_{2}$ and $\mathrm{Cr}{X}_{3}$ with values much larger than the corresponding values in elementary TMs and other TM compounds. Based on the Stoner model and the calculated $U$ and $J$ values, we discuss the tendency of the electron spins to order ferromagnetically.

Active terahertz spin state and optical chirality in liquid crystal chiral metasurface

Abstract: Dynamic control of photonic spin state and chirality plays a vital role in various applications, such as polarization control, polarization-sensitive imaging, and biosensing. Here, we present a scheme for the flexible and dynamic manipulation of terahertz spin state conversion and optical chirality by combining two achiral structures: an asymmetric metasurface and a layer of anisotropic liquid crystal. The proposed asymmetric metasurface can realize the polarization conversion effect. For the circularly polarized incidence, it exhibits the asymmetric transmission of the spin-flipped states but no spin-locked optical chirality since its geometry is mirror symmetric along with the wave propagation. The introduction of the liquid crystal makes the composite metasurface not only exhibit the spin state conversion but also spin-locked chirality and spin-flipped chirality on account of breaking mirror symmetry, which realizes an electrically active terahertz chiral device. The experimental results show that the asymmetric transmission of the terahertz spin states can be dynamically manipulated, resulting in a large controllable range 83.8% to \ensuremath{-}30.7% of spin-locked circular dichroism at 0.76 THz and \ensuremath{-}98.2% to 44.7% of spin-flipped circular dichroism at 0.73 THz. This work paves the way for the development of terahertz meta devices capable of enabling active photonic spin state and chirality manipulation.

Exploring diamondlike lattice thermal conductivity crystals via feature-based transfer learning

Abstract: Ultrahigh lattice thermal conductivity materials hold great importance since they play a critical role in the thermal management of electronic and optical devices. Models using machine learning can search for materials with outstanding higher-order properties like thermal conductivity. However, the lack of sufficient data to train a model is a serious hurdle. Herein we show that big data can complement small data for accurate predictions when lower-order feature properties available in big data are selected properly and applied to transfer learning. The connection between the crystal information and thermal conductivity is directly built with a neural network by transferring descriptors acquired through a pretrained model for the feature property. Successful transfer learning shows the ability of extrapolative prediction and reveals descriptors for lattice anharmonicity. The resulting model is employed to screen over $60\phantom{\rule{0.16em}{0ex}}000$ compounds to identify novel crystals that can serve as alternatives to diamond. Even though most materials in the top list are superhard materials, we reveal that superhard property does not necessarily lead to high lattice thermal conductivity. Large hardness means high elastic constants and group velocity of phonons in the linear dispersion regime, but the lattice thermal conductivity is determined also by other important factors such as the phonon relaxation time. What is more, the average or maximum dipole polarizability and the van der Waals radius are revealed to be the leading descriptors among those that can also be qualitatively related to anharmonicity.

Topological surface states of Mn Bi 2 Te 4 at finite temperatures and at domain walls

Abstract: $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ has recently been the subject of intensive study, due to the prediction of axion insulator, Weyl semimetal, and quantum anomalous Hall insulator phases, depending on the structure and magnetic ordering. Experimental results have confirmed some aspects of this picture, but several experiments have seen zero-gap surfaces states at low temperature, in conflict with expectations. In this work, we develop a first-principles-based tight-binding model that allows for arbitrary control of the local spin direction and spin-orbit coupling, enabling us to accurately treat large unit-cells. Using this model, we examine the behavior of the topological surface state as a function of temperature, finding a gap closure only above the N\'eel temperature. In addition, we examine the effect of magnetic domains on the electronic structure, and we find that the domain wall zero-gap states extend over many unit-cells. These domain wall states can appear similar to the high-temperature topological surface state when many domain sizes are averaged, potentially reconciling theoretical results with experiments.

Characterization of room-temperature in-plane magnetization in thin flakes of Cr Te 2 with a single-spin magnetometer

Abstract: We demonstrate room-temperature ferromagnetism with in-plane magnetic anisotropy in thin flakes of the $\mathrm{Cr}{\mathrm{Te}}_{2}$ van der Waals ferromagnet. Using quantitative magnetic imaging with a single-spin magnetometer based on a nitrogen-vacancy defect in diamond, we infer a room-temperature in-plane magnetization in the range of $M\ensuremath{\sim}27$ kA/m for flakes with thicknesses down to 20 nm. In addition, our measurements indicate that the orientation of the magnetization is not determined solely by shape anisotropy in micron-sized $\mathrm{Cr}{\mathrm{Te}}_{2}$ flakes, which suggests the existence of a non-negligible magnetocrystalline anisotropy. These results make $\mathrm{Cr}{\mathrm{Te}}_{2}$ a unique system in the growing family of van der Waals ferromagnets, as it is the only material platform known to date that offers an intrinsic in-plane magnetization and a Curie temperature above 300 K in thin flakes.

Imaging the spin chirality of ferrimagnetic Néel skyrmions stabilized on topological antiferromagnetic Mn 3 Sn

Abstract: Author(s): Xu, T; Chen, Z; Zhou, HA; Wang, Z; Dong, Y; Aballe, L; Foerster, M; Gargiani, P; Valvidares, M; Bracher, DM; Savchenko, T; Kleibert, A; Tomasello, R; Finocchio, G; Je, SG; Im, MY; Muller, DA; Jiang, W | Abstract: Neel skyrmions are generally realized in asymmetric multilayers made of heavy metals (HMs) and ultrathin ferromagnets possessing strong interfacial Dzyaloshinskii-Moriya interactions (iDMIs). Depending on the relative strengths of iDMIs at the interfaces, various types of Neel skyrmions have been suggested, which are typified with characteristically different topological properties and current-driven dynamics. This suggests the importance of a precise quantification of their spin chiralities. In this paper, we explore the possibility of realizing Neel skyrmions in magnetic multilayers without the direct usage of standard HMs. Specifically, through depositing a thin layer of ferrimagnetic (FIM) CoTb layer on top of an antiferromagnetic (AFM) quantum material of composition Mn3Sn, the AFM exchange interaction at the asymmetric interface provides an equivalent iDMI for stabilizing FIM Neel skyrmions. Secondly, through using advanced four-dimensional Lorentz scanning transmission electron microscopy (4D LSTEM), in combination with x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM), we can directly determine the spin chirality of FIM Neel skyrmions. The present findings not only broaden the phase space for chiral interfacial magnetism but also provide a possibility for future applications of heavy-metal-free skyrmionic devices.

Multistability of isolated and hydrogenated Ga-O divacancies in β -Ga 2 O 3

Abstract: The combination of an ultrawide band gap and controllable $n$-type conductivity makes monoclinic gallium sesquioxide a promising material for high-power electronics However, this technological development will require accurate knowledge about the identity and properties of prominent deep-level defects in the material This work explores close-associate Ga-O divacancies Owing to the low symmetry of the crystal structure, divacancies can potentially occur in a plethora of crystallographically inequivalent configurations Hybrid functional calculations were performed to shed light on the relative stability of different divacancy configurations, the energy barriers for transformation between them, and trends in their electrical properties

Nonuniversal structure of point defects in face-centered cubic metals

Abstract: Using ab initio density function theory calculations, we have determined the structure of self-interstitial atom (SIA) defects in the most commonly occurring face-centered cubic (FCC) metals. The most stable SIA defects in Al, Ca, Ni, Cu, Pd, and Ag are the $\ensuremath{\langle}100\ensuremath{\rangle}$ dumbbells whereas octahedral SIA configurations have the lowest energy in Pt, Rh, and Th. The relative stability of defect configurations in Sr, Ir, Au, and Pb is less well defined, and calculations suggest that an SIA defect has the $\ensuremath{\langle}100\ensuremath{\rangle}$ dumbbell structure in Sr and Ir, a $\ensuremath{\langle}110\ensuremath{\rangle}$ crowdion/dumbbell structure in Au, and that it adopts an octahedral configuration in Pb. The occurrence of octahedral and $\ensuremath{\langle}110\ensuremath{\rangle}$ crowdion/dumbbell SIA configurations implies that defects diffuse one-dimensionally. This is fundamentally different from the three-dimensional translation-rotation migration characterizing the mobility of a $\ensuremath{\langle}100\ensuremath{\rangle}$ dumbbell. Elastic fields of point defects are defined by their elastic dipole tensors, which we compute for all the defect configurations. The magnetism of a $\ensuremath{\langle}100\ensuremath{\rangle}$ dumbbell in ferromagnetic nickel appears to have little effect on the structure of the defect. The variation of energy and elastic field of an SIA defect in copper is explored in detail as a function of its structural transformation along the migration pathway.

Tuning of electronic structure, magnetic phase, and transition temperature in two-dimensional Cr-based Janus MXenes

Abstract: Two-dimensional (2D) transition metal carbides and nitrides, called as MXenes, displaying astonishing properties are emerged as a new class of 2D layered materials. In this study, we have studied the structural, electronic, and magnetic properties of 2D bare ${M}_{2}\mathrm{C}$ and Janus Cr-based ${MM}^{\ensuremath{'}}\mathrm{C}$ metal carbides $(M\ensuremath{ e}{M}^{\ensuremath{'}}=\mathrm{Cr}, \mathrm{Ti}, \mathrm{Sc}, \mathrm{V})$ based on density functional theory. We found that 2D Janus ${MM}^{\ensuremath{'}}\mathrm{C}$ MXenes are dynamically and thermally stable and show unique electronic and magnetic properties which are not seen in their individual bare cases. Our calculated electronic band structures indicate that the Janus ${MM}^{\ensuremath{'}}\mathrm{C}$ MXenes have Dirac-type band dispersion as seen in stanene with tiny band gaps. In addition, all considered bare ${M}_{2}\mathrm{C}$ MXenes show metallic property. However, Janus CrScC has dilute magnetic semiconducting property, CrVC shows half-metallicity, and CrTiC shows metallic property which can easily turn to half-metallicity by external effects. Our extensive density functional theory and Monte Carlo calculations indicate that Janus ${MM}^{\ensuremath{'}}\mathrm{C}$ MXenes also show different magnetic properties than their bare cases. While ${\mathrm{Cr}}_{2}\mathrm{C}$ and ${\mathrm{Sc}}_{2}\mathrm{C}$ have ferromagnetic spin orientations with ${T}_{C}=2000$ and 226 K Curie temperatures, respectively, CrScC shows ferrimagnetic character with antiferromagnetic N\'eel spin orientation and has ${T}_{N}=1120$ K magnetic phase transition temperature between ferrimagnetic and paramagnetic phases. Finally, external magnetic field effects on each crystal have also been elucidated in detail. The numerical outcomes reveal that Janus Cr-based ${MM}^{\ensuremath{'}}\mathrm{C}$ metal carbides have some unusual and interesting hysteresis characters. Our investigations presented here are not only interesting from a theoretical perspective, but also they show that Janus ${MM}^{\ensuremath{'}}\mathrm{C}$ MXenes are promising candidates for future spintronic applications, which should encourage their synthesis.

Highly sensitive spin-flop transition in antiferromagnetic van der Waals material M P S 3 ( M = Ni and Mn)

Abstract: Recent developments in two-dimensional (2D) magnetism have motivated the search for van der Waals (vdW) magnetic materials to explore magnetic phenomena in the 2D limit. Metal thiophosphates, $M\mathrm{P}{X}_{3}$, are a class of magnetic vdW materials with antiferromagnetic ordering persisting down to the atomically thin limit. The magnetism in this material family has been found to be highly dependent on the choice of transition metal $M$. In this work, we have synthesized the intermediate compounds ${\mathrm{Ni}}_{1\text{\ensuremath{-}}x}{\mathrm{Mn}}_{x}\mathrm{P}{\mathrm{S}}_{3}$ ($0\ensuremath{\le}x\ensuremath{\le}1$) and investigated their magnetic properties. Our study reveals that the variation of Ni and Mn content in ${\mathrm{Ni}}_{1\text{\ensuremath{-}}x}{\mathrm{Mn}}_{x}\mathrm{P}{\mathrm{S}}_{3}$ can efficiently tune the spin-flop transition, likely due to the modulation of the magnetic anisotropy. Such effective tuning offers a promising candidate to engineer 2D magnetism for future device applications.

Direct observation of local chemical ordering in a few nanometer range in CoCrNi medium-entropy alloy by atom probe tomography and its impact on mechanical properties

Abstract: Local chemical ordering in CoCrNi medium-entropy alloy (MEA) was directly observed by the use of atom probe tomography. It was found that the densities of Cr, Co, and Ni were almost the same along the [111] direction, while those along the [001] direction were modulated to take a slightly enhanced and depleted value alternately within approximately 10 atomic layers corresponding to about 2 nm. The degree of modulation of Co and Ni was stronger than that of Cr. It was suggested that Cr-rich {001} atomic layers and (Ni + Co)-rich {001} layers tended to align mutually in the face-centered-cubic CoCrNi solid solution alloy. The mechanical properties of the MEA was found not to be affected by the presence of the local chemical ordering.

Penta carbides: Two-dimensional group-IV semiconductors containing C 2 dimers for nanoelectronics and photocatalytic water splitting

Abstract: The discovery of novel materials with superior tailored properties is highly attractive for nano- and optoelectronics. Inspired by penta graphene, we introduce a novel family of two-dimensional group-IV materials with ${\mathrm{C}}_{2}$ dimers, namely penta carbides. The first-principles calculations reveal that the unitary, binary, and ternary penta carbides have excellent energetic, dynamical, thermal, and mechanical stability with remarkable properties. The unitary penta carbide displays auxetic behavior whereas the binary and ternary penta carbides posses near zero and positive Poisson's ratio. The phonon dispersion curves of penta carbides indicate a remarkable phononic gap, which can be tuned by alloy engineering and hydrogenation. They are semiconductor in nature with band-gap energy ranging from 1.35 and 2.39 eV. Alloying and strain engineering enable the direct modification of atomic bonding and thereby tuning of electronic band gap of penta carbides. Such novel tunable electronic and phononic properties of penta carbides can find applications in the field of nanoelectronics, sensors, and frequency filter applications. The band edge positions of penta carbides (except for Sn-based ones) straddle the redox potentials of water. Remarkably, penta carbides exhibit very high optical absorption in the visible and ultraviolet regions (up to ${10}^{\ensuremath{-}6} {\mathrm{cm}}^{\ensuremath{-}1}$). They have small and anisotropic carrier effective masses, indicating fast carrier transport characteristics and promoting the photo-generated electron-hole separation efficiency. The high specific surface areas, suitable and sizable band gaps, appropriate band edges, small effective carrier masses, and excellent optical absorption capability, all these exotic properties taken together, make penta carbides promising candidates for photocatalytic water splitting.

Stability and bonding nature of clathrate H cages in a near-room-temperature superconductor LaH 10

Abstract: Lanthanum hydride (${\mathrm{LaH}}_{10}$) with a sodalitelike clathrate structure was experimentally synthesized to exhibit a near-room-temperature superconductivity under megabar pressures Based on first-principles density-functional theory calculations, we reveal that the metal framework of La atoms has excess electrons at interstitial regions Such anionic electrons are easily captured to form a stable clathrate structure of H cages We thus propose that the charge transfer from La to H atoms is mostly driven by the electride property of the La framework Furthermore, the interaction between La atom and H cage induces a delocalization of La $5p$ semicore states to hybridize with the H $1s$ state Consequently, the bonding nature of ${\mathrm{LaH}}_{10}$ is characterized as a mixture of ionic and covalent bonding between La atom and H cage Our findings demonstrate that anionic and semicore electrons play important roles in stabilizing clathrate H cages in ${\mathrm{LaH}}_{10}$, which can be broadly applicable to other compressed rare-earth hydrides with clathrate structures

Unveiling the spin-to-charge current conversion signal in the topological insulator Bi 2 Se 3 by means of spin pumping experiments

Abstract: We report an investigation of the spin-to-charge current conversion in sputter-deposited films of the topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ onto single crystalline layers of yttrium iron garnet (YIG-${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$) and polycrystalline films of permalloy ($\mathrm{Py}\text{\ensuremath{-}}{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$). Pure spin current was injected into the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ layer by means of the spin pumping process produced by the spin precession in microwave driven ferromagnetic resonance of the ferromagnetic film. The spin-to-charge current conversion occurring at the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$/ferromagnet interface is attributed to the inverse Rashba-Edelstein effect (IREE). From the data, we verified that the voltage generated by the spin-to-charge current conversion process in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ has the same polarity as the inverse spin Hall effect in Ta, which is opposite to the one in Pt. Also, from the dependence of the voltage on the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ thickness we were able the to calculate the IREE length values and found that $1.2\phantom{\rule{0.16em}{0ex}}\mathrm{pm}\ensuremath{\le}|{\ensuremath{\lambda}}_{\mathrm{IREE}}|\ensuremath{\le}2.2\phantom{\rule{0.16em}{0ex}}\mathrm{pm}$. This result allows us to conclude that indeed the surface states of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ have a dominant role in the spin-to-charge current conversion process, and the mechanism based on the spin diffusion process plays a secondary role.

Optimized structure and electronic band gap of monolayer GeSe from quantum Monte Carlo methods

Abstract: We have used highly accurate quantum Monte Carlo methods to determine the chemical structure and electronic band gaps of monolayer GeSe. Two-dimensional (2D) monolayer GeSe has received a great deal of attention due to its unique thermoelectric, electronic, and optoelectronic properties with a wide range of potential applications. Density functional theory (DFT) methods have usually been applied to obtain optical and structural properties of bulk and 2D GeSe. For the monolayer, DFT typically yields a larger band-gap energy than for bulk GeSe but cannot conclusively determine if the monolayer has a direct or indirect gap. Moreover, the DFT-optimized lattice parameters and atomic coordinates for monolayer GeSe depend strongly on the choice of approximation for the exchange-correlation functional, which makes the ideal structure---and its electronic properties---unclear. In order to obtain accurate lattice parameters and atomic coordinates for the monolayer, we use a surrogate Hessian-based parallel line search within diffusion Monte Carlo to fully optimize the GeSe monolayer structure. The DMC-optimized structure is different from those obtained using DFT, as are calculated band gaps. The potential energy surface has a shallow minimum at the optimal structure. This, combined with the sensitivity of the electronic structure to strain, suggests that the optical properties of monolayer GeSe are highly tunable by strain.

Role of chemical disorder and local ordering on defect evolution in high-entropy alloys

Abstract: High-entropy alloys (HEAs) have stimulated great interest due to their remarkable mechanical and irradiation performance. Experiments suggest that delayed defect evolution in HEAs, compared to conventional metals and dilute alloys, is the main reason for their improved irradiation resistance. However, the mechanism responsible for the observation remains elusive. Here we show that the potential energy landscape of defects under the influence of random arrangement of different species is the reason for the delayed defect evolution. We arrive at the conclusion by investigating the diffusion of defects and defect clusters under three cases: the averaged-atom model, random model, and the model with local short-range ordering. Our results suggest that, compared to the average model, the chemical fluctuation inherent in HEAs can suppress interstitial motion more than vacancy motion. The effects are more pronounced when SRO develops. For defect clusters, the chemical disorder can reduce their jump frequencies significantly and enhance correlation effects, leading to suppressed defect motion. Notably, we find that with SRO, such defect motion can be entirely trapped in local regions. This work demonstrates that chemical fluctuations and SRO are the main reason responsible for the suppressed defect evolution in HEAs, which dictates a promising way to improve the irradiation performance of HEAs through manipulating its chemical disorder states, such as local ordering.

Efficient and transferable machine learning potentials for the simulation of crystal defects in bcc Fe and W

Abstract: Data-driven, or machine learning (ML), approaches have become viable alternatives to semi-empirical methods to construct interatomic potentials, due to their capacity to accurately interpolate and extrapolate from first principles simulations if the training database and descriptor representation of atomic structures are carefully chosen. Here, we present highly accurate interatomic potentials suitable for the study of dislocations, point defects, and their clusters in bcc iron and tungsten, constructed using a linear or quadratic input-output mapping from descriptor space. The proposed quadratic formulation, called Quadratic Noise ML, differs from previous approaches, being strongly preconditioned by the linear solution. The developed potentials are compared to a wide range of existing ML and semi-empirical potentials and are shown to have sufficient accuracy to distinguish changes in the exchange-correlation functional or pseudopotential in the underlying reference data, while retaining excellent transferability. The flexibility of the underlying approach is able to target properties almost unattainable by traditional methods, such as the negative di-vacancy binding energy in W or the shape and the magnitude of the Peierls barrier of the 1/2 screw dislocation in both metals. We also show how the developed potentials can be used to target important observables that require large time-and-space scales unattainable with first-principles methods, though we emphasize the importance of thoughtful database design and degrees of non-linearity of the descriptor space to achieve the appropriate passage of information to large-scale calculations. As a demonstration, we perform direct atomistic calculations of the relative stability of 1/2 dislocations loops and three-dimensional C15 clusters in Fe and find the crossover between the formation energies of the two classes of interstitial defects occurs at around 40 self-interstitial atoms. We also compute the kink-pair formation energy of the 1/2 screw dislocation in Fe and W, finding good agreement with density functional theory informed line tension models that indirectly measure those quantities. Finally, we exploit the excellent finite temperature properties to compute vacancy formation free energies with full anharmonicity in thermal vibrations. The presented potentials thus open up many avenues for systematic investigation of free energy landscape of defects with ab initio accuracy.