We examine vacancy-assisted diffusion in the 57 random, equimolar alloys that can be formed from Cu, Ag, Au, Ni, Pd, and Pt based on the well-tested embedded atom method functions of Foiles, Baskes, and Daw [Phys. Rev. B 33, 7983 (1986)]. We address the suggestion [Yeh et al., Adv. Eng. Mater. 6, 299 (2004)] that increasing the number of constituents causes diffusion to be “sluggish” in random, equimolar alloys. Using molecular dynamics (MD) simulations of random alloys with a single vacancy, combined with calculations of vacancy formation, we extract vacancy-assisted diffusivities in each alloy. After developing and applying several possible criteria for evaluating “sluggishness,” we find that only a small minority (from 1 to 8, depending on how sluggishness is defined) of the alloys exhibit sluggish diffusion whereas in the large majority of alloys diffusion is faster and in quite a few cases ought to be considered vigorous (that is, faster than in any of the constituents). We correlate diffusivity with a combination of the mean of the constituent diffusivity and a simple function of lattice mismatch. We conclude that simply increasing the number of constituents in such alloys does not systematically alter the diffusion, but that instead lattice mismatch plays a primary factor; sluggish diffusion is more likely to occur in a window of small lattice mismatch (1–3%) even in binary alloys. Quantitatively, our calculated diffusivities correlate with a combination of (1) rule of mixtures of the diffusivities of the constituents, and (2) a simple function of the lattice mismatch; this accounts for the large majority of our calculated diffusivities to within a factor of 2 (over a range of three orders of magnitude). We also find that while lattice mismatch on the order of 1–3% is necessary for sluggish diffusion, it is not sufficient.

Monolayer transition-metal dichalcogenides (TMDCs) have been extensively studied due to their wide range of physical properties and applications. It has been demonstrated that the electron beam in a transmission electron microscope (TEM) or scanning TEM (STEM) generates chalcogen vacancies that agglomerate into dispersed linelike or holelike defects. Here we employ a STEM electron beam and demonstrate that, in $\mathrm{W}{\mathrm{Se}}_2$, beam-induced chalcogen vacancies initially form certain dispersed multivacancy structures, as seen in TMDCs in prior work. However, with suitable control of the STEM focused electron beam, these multivacancies gradually evolve into a dense network of ten-, 12-, 14-, and 16-member ring round holes, whereas the same process leads predominantly to chalcogen-vacancy line defects in other trigonal-prismatic TMDCs. Density functional theory calculations find that the underlying atomic-scale processes lead preferentially to defect structures that lower the total energy so that we are able to track the formation of the observed multivacancy complexes, which then lead to the formation of dense large round holes in $\mathrm{W}{\mathrm{Se}}_2$. The same processes in $\mathrm{W}{\mathrm{S}}_2$, however, are energetically unfavorable, while linear multivacancy defects are preferred, as observed. The demonstrated control of the formation of unique high-density round holes in $\mathrm{W}{\mathrm{Se}}_2$ has potential for applications such as atomic and molecular sieving.

Using time-resolved optical Kerr rotation, we measure the low-temperature valley dynamics of resident electrons and holes in exfoliated ${\mathrm{WSe}}_2$ monolayers as a systematic function of carrier density. In an effort to reconcile the many disparate timescales of carrier valley dynamics in monolayer semiconductors reported to date, we directly compare the doping-dependent valley relaxation in two electrostatically gated ${\mathrm{WSe}}_2$ monolayers having different dielectric environments. In a fully encapsulated structure ($\mathrm{hBN}/{\mathrm{WSe}}_2/\mathrm{hBN}$, where hBN is hexagonal boron nitride), valley relaxation is found to be monoexponential. The valley relaxation time ${\tau }_v$ is quite long $(\sim 10\mu \mathrm{s})$ at low carrier densities, but decreases rapidly to less than 100 ns at high electron or hole densities $\gtrsim 2\times {10}^{12}{\mathrm{cm}}^{-2}$. In contrast, in a partially encapsulated ${\mathrm{WSe}}_2$ monolayer placed directly on silicon dioxide $(\mathrm{hBN}/{\mathrm{WSe}}_2/{\mathrm{SiO}}_2)$, carrier valley relaxation is multiexponential at low carrier densities. The difference is attributed to environmental disorder from the ${\mathrm{SiO}}_2$ substrate. Unexpectedly, very small out-of-plane magnetic fields can increase ${\tau }_v$, especially in the $\mathrm{hBN}/{\mathrm{WSe}}_2/{\mathrm{SiO}}_2$ structure, suggesting that localized states induced by disorder can play an important role in depolarizing spins and mediating the valley relaxation of resident carriers in monolayer transition-metal dichalcogenide semiconductors.

Heteroepitaxy offers a new type of control mechanism for the crystal structure, the electronic correlations, and thus the functional properties of transition-metal oxides. Here we combine electrical transport measurements, high-resolution scanning transmission electron microscopy (STEM), and density functional theory (DFT) to investigate the evolution of the metal-to-insulator transition (MIT) in $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ films as a function of film thickness and $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3$ substrate crystallographic orientation. We find that for two different substrate facets, orthorhombic (101) and (011), modifications of the $\mathrm{Ni}{\mathrm{O}}_6$ octahedral network are key for tuning the transition temperature $T_{\text{MIT}}$ over a wide temperature range. A comparison of films of identical thickness reveals that growth on [101]-oriented substrates generally results in a higher $T_{\text{MIT}}$, which can be attributed to an enhanced bond disproportionation as revealed by the $\text{DFT}+U$ calculations, and a tendency of [011]-oriented films to formation of structural defects and stabilization of nonequilibrium phases. Our results provide insights into the structure-property relationship of a correlated electron system and its evolution at microscopic length scales and give new perspectives for the epitaxial control of macroscopic phases in metal-oxide heterostructures.

Understanding the role of disorder, and the correlations that exist within it, is one of the defining challenges in contemporary materials science. However, there are few material systems, devoid of other complex interactions, that can be used to systematically study the effects of crystallographic conflict on correlated disorder. Here, we report extensive diffuse x-ray scattering studies on the epitaxially stabilized alloy ${\text{U}}_{1-x}{\text{Mo}}_x$, showing that a new form of intrinsically tuneable correlated disorder arises from a mismatch between the preferred symmetry of a crystallographic basis and the lattice upon which it is arranged. Furthermore, combining grazing incidence inelastic x-ray scattering and state-of-the-art ab initio molecular dynamics simulations, we discover strong disorder-phonon coupling. This breaks global symmetry and dramatically suppresses phonon lifetimes compared to alloying alone, providing an additional design strategy for phonon engineering. These findings have implications wherever crystallographic conflict can be accommodated, and they may be exploited in the development of future functional materials.

We report on the enantiopure single-crystal growth using the Czochralski method and the magnetic properties of the Nd-based monoaxial chiral magnet ${\mathrm{NdPt}}_2\mathrm{B}$ based on electrical resistivity, specific heat, and magnetization measurements. A peak in the magnetic susceptibility and a $\lambda$-type anomaly in the specific heat are observed at $T_{\mathrm{N}1}=19.5\mathrm{K}$. Successively, a first-order magnetic transition at $T_{\mathrm{m}}=9.5\mathrm{K}$ is observed in the electrical resistivity, specific heat, and magnetic susceptibility. In addition, we observed a new magnetic phase transition, which has previously not been reported, at $T_{\mathrm{N}2}=11\mathrm{K}$ between $T_{\mathrm{N}1}$ and $T_{\mathrm{m}}$. We observed multiple plateaus in the low-temperature magnetic phase and constructed a nontrivial magnetic phase diagram, which results from the competitive interactions in this system. Furthermore, we observed an increase and decrease in the in-plane anisotropy in the transverse magnetoresistance, which is related to the change in the magnetic structure.

The thermoelectric properties of conductors with low electron density can be altered significantly by an applied magnetic field. For example, recent work has shown that Dirac and Weyl semimetals with a single pocket of carriers can exhibit a large enhancement of thermopower when subjected to a sufficiently large field that the system reaches the extreme quantum limit, in which only a single Landau level is occupied. Here we study the magnetothermoelectric properties of compensated semimetals, for which pockets of electron- and hole-type carriers coexist at the Fermi level. We show that, when the compensation is nearly complete, such systems exhibit a huge enhancement of thermopower starting at a much smaller magnetic field, such that ${\omega }_c\tau >1$, and the stringent conditions associated with the extreme quantum limit are not necessary. We discuss our results in light of recent measurements on the compensated Weyl semimetal tantalum phosphide, in which an enormous magnetothermoelectric effect was observed. We also calculate the Nernst coefficient of compensated semimetals, and show that it exhibits a maximum value with increasing magnetic field that is much larger than in the single-band case. In the dissipationless limit, where the Hall angle is large, the thermoelectric response can be described in terms of quantum Hall edge states, and we use this description to generalize previous results to the multiband case.

Starting from the achiral body-centered-cubic Wigner-Seitz cell, a truncated octahedron, we design a chiral simple-cubic metamaterial crystal which exhibits chiral phonons for all phonon propagation directions in three-dimensional (3D) space over a broad frequency range as well as nearly isotropic acoustical activity. The resulting blueprint has simple-cubic crystal symmetry and is radically simpler than previous proposals based on 3D quasicrystalline or 3D triclinic truss lattices, making it amenable to state-of-the-art 3D additive manufacturing. To emphasize this aspect, we present electron micrographs of microstructured metamaterial samples fabricated by using 3D laser nanoprinting.

This work systematically explores 19 unique configurations of the close-associate Ga–O divacancies $\left(V_{\text{Ga}}{V}_{\text{O}}\right)$ in $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$, including their complexes with H impurities, using hybrid functional calculations. Interestingly, most configurations are found to retain the negative-$U$ behavior of $V_{\text{O}}$, as they exhibit a thermodynamic ($-/3-$) charge-state transition level energetically located in the upper part of the band gap, where the $3-$ charge state is associated with the formation of a Ga–Ga dimer. The energy positions of the thermodynamic ($-/3-$) charge-state transition levels divide the divacancy configurations into three different groups, which can be understood from the three possible Ga–Ga dimerizations resulting from the tetrahedral and octahedral Ga sites. The relative formation energies of the different divacancy configurations, and hence the electrical activity of the divacancies, is found to depend on the Fermi-level position, and the energy barriers for transformation between different divacancy configurations are explored from nudged elastic band calculations. Hydrogenation of the divacancies is found to either passivate their negative-$U$ charge-state transition levels or shift them down in Fermi level position, depending on whether the H resides at $V_{\text{O}}$ or forms an O–H bond at $V_{\text{Ga}}$, respectively. Finally, the divacancy is discussed as a potential origin of the so-called $E_2^{*}$ center previously observed by deep-level transient spectroscopy.

The over 60 years old Rashba-Dresselhaus effect predicts spin-orbit coupling (SOC) induced momentum-dependent spin splitting and spin polarization in materials with noncentrosymmetric structures. Strong SOC induced effects usually require high-atomic number (Z) elements such as rare-earth elements. It has recently been pointed out [Yuan et al., Phys. Rev. B 102, 014422 (2020)] that antiferromagnets could hold SOC-independent spin splitting and spin polarization. In the present work we develop the spatial and magnetic symmetry conditions enabling such antiferromagnet (AFM)-induced spin splitting, dividing the 1651 magnetic space groups into seven different spin splitting prototypes (SST-1 to SST-7). This analysis places the physics of AFM spin splitting (SST-4) within the broader context of symmetry conditions that enable the more familiar forms of spin splitting, such as ferromagnetic Zeeman effect (SST-5), nonmagnetic no spin splitting (SST-6), and the nonmagnetic Rashba and Dresselhaus effects (SST-7). The AFM-induced spin splitting and spin polarization do not necessarily require breaking of inversion symmetry or the presence of SOC, hence can exist even in centrosymmetric, low-Z light element compounds, considerably broadening the material base for spin manipulations. We use the “inverse design” approach of first formulating the target property (here, spin splitting in low-Z compounds not restricted to low symmetry structures), then derive the enabling physical design principles—the magnetic symmetry conditions—to search realizable compounds that satisfy these a priori design principles. This process uncovers 422 magnetic space groups (160 centrosymmetric and 262 noncentrosymmetric) that could hold AFM-induced, SOC-independent spin splitting and spin polarization. We then search for stable compounds following such enabling symmetries. We investigate the electronic and spin structures of some selected prototype compounds by density functional theory (DFT) and find spin textures that are different than the traditional Rashba-Dresselhaus patterns and exist even in the absence of SOC effect. We provide the DFT results for all antiferromagnetic spin splitting prototypes (SST-1, SST-2, SST-3, SST-4), and concentrate on two limits of SST-4 that are particularly unusual: When spin splitting is momentum dependent (just like the Rashba effect) but is enabled in antiferromagnets even in the absence of SOC in the Hamiltonian. This includes examples of (a) centrosymmetric SST-4A compounds (e.g., orthorhombic $\mathrm{LaMn}{\mathrm{O}}_3$ illustrating collinear AFM, as well as cubic $\mathrm{Ni}{\mathrm{S}}_2$ illustrating noncollinear AFM) and (b) noncentrosymmetric SST-4B compounds (e.g., rhombohedral $\mathrm{MnTi}{\mathrm{O}}_3$ illustrating collinear AFM and hexagonal $\mathrm{ScMn}{\mathrm{O}}_3$ illustrating noncollinear AFM). The symmetry design principles outlined here, along with their transformation into an inverse design material search approach and DFT verification, could open the way to their experimental examination.

This paper is a contribution to the joint Physical Review Applied and Physical Review Materials collection titled Two-Dimensional Materials and Devices.

Atomically thin two-dimensional (2D) transition-metal dichalcogenide (TMD) superconductors enable uniform, flat, and clean van der Waals tunneling interfaces, motivating their integration into conventional superconducting circuits. However, fully superconducting contact must be made between the 2D material and three-dimensional (3D) superconductors to employ the standard microwave drive and readout of qubits in such circuits. We present a method for creating zero-resistance contacts between 2D ${\mathrm{NbSe}}_2$ and 3D aluminum that behave as Josephson junctions (JJs) with large effective areas compared to 3D-3D JJs. The devices formed from 2D TMD superconductors are strongly influenced by the geometry of the flakes themselves as well as the placement of the contacts to bulk 3D superconducting leads. We present a model for the supercurrent flow in a 2D-3D superconducting structure by a numerical solution of the Ginzburg-Landau equations and find good agreement with experiment. These results demonstrate a crucial step towards a new generation of hybrid superconducting quantum circuits.

Photon helicity-dependent photocurrent is measured at zero magnefic field thanks to a device based on an ensemble of (In,Ga)As/GaAs quantum dots that are embedded into a GaAs-based $p\text{−}i\text{−}n$ diode. Our main goal is to take advantage of the long electron spin-relaxation time expected in these nano-objects. In these experiments, no external magnetic field is required thanks to the use of an ultrathin magnetic CoFeB/MgO electrode, presenting perpendicular magnetic anisotropy. We observe a clear asymmetry of the photocurrent measured under respective right and left polarized light that follows the hysteresis of the magnetic layer. The amplitude of this asymmetry at zero magnetic field decreases with increasing temperatures and can be controlled with the bias. Polarization-resolved photoluminescence is detected in parallel while the device is operated as a photodetector. This demonstrates the multifunctional capabilities of the device and gives valuable insights into the spin relaxation of the electrons in the quantum dots.

A basic understanding of the driving forces of ion conduction in solids is critical to the development of new solid-state ion conductors. The physical understanding of ion conduction is limited due to strong deviations from harmonic vibrational dynamics in these systems that are difficult to characterize experimentally and theoretically. We overcome this challenge in superionic AgI by combining THz-frequency Raman polarization-orientation measurements and ab initio molecular dynamics computations. Our findings demonstrate clear signatures of strong coupling between the mobile ions and host lattice that are of importance to the diffusion process. We first derive a dynamic structural model from the Raman measurements that captures the simultaneous crystal-like and fluidlike properties of this fast ion conductor. Then we show and discuss the importance of anharmonic relaxational motion that arises from the iodine host lattice by demonstrating its strong impact on ion conduction in superionic AgI.

Combining various two-dimensional materials into van der Waals (vdW) heterostructures has been shown to lead to emergent quantum systems. A heterostructure composed of a vdW topological insulator (TI) such as ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ with a quantum spin liquid (QSL) such as $\alpha \text{−}{\mathrm{RuCl}}_3$ is of great interest for the potential for the chiral Dirac electrons in the TI surface states to interact strongly with the fractionalized fermionic spin excitations in the QSL. We report the heteroepitaxial growth of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ thin films on $\alpha \text{−}{\mathrm{RuCl}}_3$ as well as the characterization of their structural and electrical properties. ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ thin films with an atomically smooth and uniform surface are grown by molecular beam epitaxy. The heterostructure exhibits a preferential epitaxial relationship corresponding to $\left(5\times 5\right)\text{–}{\mathrm{Bi}}_2{\mathrm{Se}}_3/(2\sqrt{3}\times 2\sqrt{3}$) $R{30}^{\circ }\text{–}\alpha \text{−}{\mathrm{RuCl}}_3$ commensurate supercells with a periodicity of 1.2 nm. The formation of the superlattice despite a lattice mismatch as large as 60% is attributed to the vdW heteroepitaxy. Magnetotransport measurements as a function of temperature show ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ films grown on $\alpha \text{−}{\mathrm{RuCl}}_3$ are heavily $n$-doped, $n_e\sim {10}^{14}\mathrm{c}{\mathrm{m}}^{\text{–}2}$, with mobility μ $\sim 450\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{\text{–}1}{\mathrm{s}}^{\text{–}1}$ at low temperatures.

We report high spatial resolution, below 100 nm, scanning nano x-ray diffraction (SnXRD) imaging of incommensurate lattice modulations (ILM) in ${\mathrm{Bi}}_{2.1}{\mathrm{Sr}}_{1.9}\mathrm{CaC}{\mathrm{u}}_{2.0}{\mathrm{O}}_{8+y}$ van der Waals heterostructures of thicknesses down to two unit cells. We reveal the distinct long-range and short-range ILMs in a bulk sample and at the surface. We find that the size and mutual orientation of the puddlelike domains of the ILM are determined by the dimensionality of the system. In the two-unit-cell sample, the wave vectors of the long- and short-range orders become anticorrelated, and the emergent spatial patterns have a directional gradient. These emergent patterns imply static mesoscopic lattice modulation. Our findings open a route for local strain engineering to modulate properties of two-dimensional high-temperature superconductors.

Interatomic potentials constitute the key component of large-scale atomistic simulations of materials. The recently proposed physically informed neural network (PINN) method combines a high-dimensional regression implemented by an artificial neural network with a physics-based bond-order interatomic potential applicable to both metals and nonmetals. In this paper, we present a modified version of the PINN method that accelerates the potential training process and further improves the transferability of PINN potentials to unknown atomic environments. As an application, a modified PINN potential for Al has been developed by training on a large database of electronic structure calculations. The potential reproduces the reference first-principles energies within 2.6 meV per atom and accurately predicts a wide spectrum of physical properties of Al. Such properties include, but are not limited to, lattice dynamics, thermal expansion, energies of point and extended defects, the melting temperature, the structure and dynamic properties of liquid Al, the surface tensions of the liquid surface and the solid-liquid interface, and the nucleation and growth of a grain boundary crack. Computational efficiency of PINN potentials is also discussed.

We develop an open-access database that provides a large array of datasets specialized for magnetic compounds as well as magnetic clusters. Our focus is on rare-earth-free magnets. Available datasets include (i) crystallography, (ii) thermodynamic properties, such as the formation energy, and (iii) magnetic properties that are essential for magnetic-material design. Our database features a large number of stable and metastable structures discovered through our adaptive genetic algorithm (AGA) searches. Many of these AGA structures have better magnetic properties when compared to those of the existing rare-earth-free magnets and the theoretical structures in other databases. Our database places particular emphasis on site-specific magnetic data, which are obtained by high-throughput first-principles calculations. Such site-resolved data are indispensable for machine-learning modeling. We illustrate how our data-intensive methods promote efficiency of the experimental discovery of new magnetic materials. Our database provides massive datasets that will facilitate an efficient computational screening, machine-learning-assisted design, and the experimental fabrication of new promising magnets.

We study the stability and electronic structure of previously unexplored two-dimensional (2D) ternary compounds ${\mathrm{BNP}}_2$ and ${\mathrm{C}}_2\mathrm{SiS}$. Using ab initio density functional theory, we have identified four stable allotropes of each ternary compound and confirmed their stability by calculated phonon spectra and molecular dynamics simulations. Whereas all ${\mathrm{BNP}}_2$ allotropes are semiconducting, we find ${\mathrm{C}}_2\mathrm{SiS}$, depending on the allotrope, to be semiconducting or semimetallic. The fundamental band gaps of the semiconducting allotropes we study range from $1.4\mathrm{eV}$ to $2.2\mathrm{eV}$ at the HSE06 level and display carrier mobilities as high as $1.5\times {10}^5{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. Such high mobilities are quite uncommon in semiconductors with so wide band gaps. Structural ridges in the geometry of all allotropes cause a high anisotropy in their mechanical and transport properties, promising a wide range of applications in electronics and optoelectronics.

We present data for epitaxial thin films of the prototypical entropy-stabilized oxide (ESO), ${\mathrm{Mg}}_{0.2}{\mathrm{Ni}}_{0.2}{\mathrm{Co}}_{0.2}{\mathrm{Cu}}_{0.2}{\mathrm{Zn}}_{0.2}\mathrm{O}$, that reveals a systematic trend in lattice parameter and properties as a function of substrate temperature during film growth with negligible changes in microstructure. A larger net Co valence in films grown at substrate temperatures below 350 °C results in a smaller lattice parameter, a smaller optical band gap, and stronger magnetic exchange bias. Observation of this phenomena suggests a complex interplay between thermodynamics and kinetics during ESO synthesis; specifically thermal history, oxygen chemical potential, and entropy. In addition to the compositional degrees of freedom available to ESO systems, subtle nuances in atomic structure at constant metallic element proportions can strongly influence properties, simultaneously complicating physical characterization and providing opportunities for property tuning and development.

The heterostructures composed of an ultrathin ferromagnetic metal (FM) and a material hosting strong spin-orbit (SO) coupling are the principal resource for SO torque and spin-to-charge conversion nonequilibrium effects in spintronics. The key quantity in theoretical description of these effects is nonequilibrium spin density, which can appear on any monolayer of the heterostructure through which the current is flowing as long as the monolayer bands are affected by the native or proximity induced SO coupling. Here we demonstrate how hybridization of wave functions of Co layer and a monolayer of transition metal dichalcogenides (TMDs)—such as semiconducting ${\mathrm{MoSe}}_2$ and ${\mathrm{WSe}}_2$ or metallic ${\mathrm{TaSe}}_2$—can lead to dramatic transmutation of electronic and spin structure of Co within some distance away from its interface with TMD, when compared to the bulk of Co or its surface in contact with a vacuum. This is due to proximity induced SO splitting of Co bands encoded in the spectral functions and spin textures on its monolayers, which we obtain using noncollinear density functional theory (ncDFT) combined with equilibrium Green function (GF) calculations. In fact, SO splitting is present due to structural inversion asymmetry of the bilayer—i.e., just the presence of the interface—even if SO coupling within TMD monolayer is artificially switched off in ncDFT calculations. However, switching it on makes the effects associated with proximity SO coupling within Co layer about five times larger. Injecting spin-unpolarized charge current through SO-proximitized monolayers of Co generates nonequilibrium spin density over them, so that its cross product with the magnetization of Co determines SO torque. The SO torque computed via first-principles quantum transport methodology, which combines ncDFT with nonequilibrium GF calculations, can be used as the screening parameter to identify optimal combination of materials and their interfaces for applications in spintronics. In particular, we identify heterostructure two-monolayer-Co/monolayer-${\mathrm{WSe}}_2$ as the most optimal one, at least in the clean limit.