Ferroelectric and ferroelastic domains have been predicted to enhance metal halide perovskite (MHP) solar cell performance. While the formation of such domains can be modified by temperature, pressure, or strain, established methods lack spatial control at the level of single domains. Here, we induce the formation of ferroelastic domains in ${\mathrm{CsPbBr}}_3$ nanowires at room temperature using an atomic force microscope (AFM) tip and visualize the domains using nanofocused x-ray diffraction with a 60 nm beam. Regions scanned with a low AFM tip force show orthorhombic 004 reflections along the nanowire axis, while regions exposed to higher forces exhibit 220 reflections. The applied stress locally changes the crystal structure, leading to lattice tilts that define ferroelastic domains, which spread spatially and terminate at {112}-type domain walls. The ability to induce individual ferroelastic domains within MHPs using AFM gives new possibilities for device design and fundamental experimental studies.

Although electrostatic gating with liquid electrolytes has been thoroughly investigated to enhance electrical transport in two-dimensional (2D) materials, solid electrolyte alternatives are now actively being researched to overcome the limitations of liquid dielectrics. Here, we report direct growth of few-layer ($3-4L$) molybdenum disulfide (${\mathrm{MoS}}_2$), a prototypical 2D transition metal dichalcogenide (TMD), on lithium-ion solid electrolyte substrate by chemical vapor deposition (CVD), and demonstrate a transfer-free device fabrication method. The growth resulted in 5–10 μm sized triangular ${\mathrm{MoS}}_2$ single crystals as confirmed by Raman spectroscopy, x-ray photoelectron spectroscopy, and scanning electron microscopy. Field-effect transistors (FETs) fabricated on the as-grown few-layer crystals show near-ideal gating performance with room temperature subthreshold swings around 65 mV/decade while maintaining an ON/OFF ratio around ${10}^5$. Field-effect mobility in the range of $42–49\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{–1}{\mathrm{s}}^{–1}$ and current densities as high as 120 μA/μm with 0.5 μm channel length has been achieved, back-gated by the solid electrolyte. This is the highest reported mobility among comparable FETs on as-grown single/few-layer CVD ${\mathrm{MoS}}_2$. This growth and transfer-free device fabrication method on solid electrolyte substrates can be applied to other 2D TMDs for studying advanced thin-film transistors and interesting physics, and is amenable to diverse surface science experiments, otherwise difficult to realize with liquid electrolytes.

The electronic structure of $3^{\circ }$-twisted bilayer graphene (TBG) is studied by angle-resolved photoelectron spectroscopy (ARPES). Sub-mm-sized TBG prepared by direct bonding in a high vacuum enabled us to use conventional ARPES band mapping with synchrotron light. The results indicate that strong interlayer coupling makes a moiré potential for the Dirac electrons and significantly modifies the graphene bands around the $\overline{K}$ points such as the band splitting and electron velocity reduction. The observed electronic structure is consistently reproduced by tight-binding calculations combined with a band unfolding method.

The electrode materials of Na-ion batteries must exhibit fast ion transport, a property that is often negatively impacted when mobile cations order. Many layered Na (and K) intercalation compounds adopt the P3 crystal structure, which tends to stabilize cation-vacancy orderings over a wide range of Na concentrations. The ordered phases have been predicted to consist of domains that are periodically separated by antiphase boundaries (APBs) to accommodate variations in Na concentration. The mechanisms and rates with which Na diffuses within such ordered phases of P3 are not understood. We have conducted a first-principles study of Na diffusion in the P3 structure, using ${\mathrm{Na}}_x{\mathrm{CoO}}_2$ as a model system. We find that most simple nearest-neighbor hops in Na-vacancy ordered phases in P3 result in unstable configurations and are therefore forbidden. We identify an alternative mechanism of Na transport that is mediated by APB migration via the formation and expansion of kinks along domain boundaries. The limiting kinetic barriers for this mechanism are quite low (0.03 to 0.30 eV). Vacancy defects are not required, and furthermore have formation energies that are higher than those of the APB kink defects. Our results suggest that APB migration is a fundamental diffusion mechanism in the P3 structure.

Defect engineering plays an important role in tailoring the electronic transport properties of van der Waals materials. However, it is usually achieved through tuning the type and concentration of defects, rather than dynamically reconfiguring their spatial distribution. Here, we report temperature-sensitive spatial redistribution of defects in $\mathrm{Pd}{\mathrm{Se}}_2$ thin flakes through scanning tunneling microscopy. We observe that the spatial distribution of Se vacancies in $\mathrm{Pd}{\mathrm{Se}}_2$ flakes exhibits a strong anisotropic characteristic at 80 K, and that this orientation-dependent feature is weakened when temperature is raised. Moreover, we carry out transport measurements on $\mathrm{Pd}{\mathrm{Se}}_2$ thin flakes and show that the anisotropic features of carrier mobility and phase coherent length are also sensitive to temperature. Combining with theoretical analysis, we conclude that temperature-driven defect spatial redistribution could interpret the temperature-sensitive electrical transport behaviors in $\mathrm{Pd}{\mathrm{Se}}_2$ thin flakes. Our work highlights that engineering spatial distribution of defects in the van der Waals materials, which has been overlooked before, may open up an avenue to tailor the physical properties of materials and explore different device functionalities.

We propose a design scheme for potential electrides derived from conventional materials. Starting with rare-earth-based ternary halides, we exclude halogen anions and perform global structure optimization to obtain thermodynamically stable or metastable phases but having an excess of electrons confined inside interstitial cavities. Then, spin-polarized interstitial states are induced by chemical substitution with magnetic lanthanides. To demonstrate the capability of our approach, we test with 11 ternary halides and successfully predict 30 stable and metastable phases of nonmagnetic electrides subject to three different stoichiometric categories, and successively 28 magnetic electrides via chemical substitution with Gd. 56 out of these 58 designed electrides are discovered for the first time. Two electride systems, the monoclinic $A\mathrm{C}$ ($A=$ La, Gd) and the orthorhombic $A_2\mathrm{Ge}$ ($A=$ Y, Gd), are thoroughly studied to exemplify the set of predicted crystals. Interestingly, both systems turn out to be topological nodal line electrides in the absence of spin-orbit coupling and manifest spin-polarized interstitial states in the case of $A=$ Gd. Our work establishes a novel computational approach of functional electrides design and highlights the magnetism and topological phases embedded in electrides.

By considering the valence-electron concentration of $3d$ transition-metal alloys and compounds, we develop $3d$ high-entropy alloy ${\mathrm{Mn}}_{12.1}{\mathrm{Fe}}_{34.2}{\mathrm{Co}}_{33.5}{\mathrm{Ni}}_{12.3}{\mathrm{Cu}}_{7.9}$ with 8.7 electrons per atom, which is identical to that of ${\mathrm{Fe}}_{65}{\mathrm{Ni}}_{35}$ Invar. We carry out x-ray diffraction, scanning electron microscopy, magnetization, thermal expansion, and elastic modulus measurements, by which we show that the HEA alloy indeed carries Invar properties. This is evidenced particularly by the observed spontaneous volume magnetostriction and the lattice softening covering a broad temperature-range around the ferromagnetic Curie temperature.

In computational material design, ionic radius is one of the most important physical parameters used to predict material properties. Motivated by the progress in computational materials science and material informatics, we extend the renowned Shannon's table from 475 ions to 987 ions. Accordingly, a rigorous machine learning (ML) approach is employed to extend the ionic radii table using all possible combinations of oxidation states (OS) and coordination numbers (CN) available in crystallographic repositories. An ionic-radius regression model for Shannon's database is developed as a function of the period number, the valence orbital configuration, OS, CN, and ionization potential. In the Gaussian process regression (GPR) model, the reached $R^2$ accuracy is 99% while the root mean square error of radii is 0.0332 Å. The optimized GPR model is then employed for predicting a new set of ionic radii for uncommon combinations of OS and CN extracted by harnessing crystal structures from materials project databases. The generated data are consolidated with the reputable Shannon's data and are made available online in a database repository.

Two-dimensional (2D) and quasi-2D ferroelectric materials have demonstrated their potential for enabling device miniaturization. The fundamental understanding of the ferroelectric domain structure at atomic scale is limited, however, hindering development of functional device units at the microscopic level. In this paper, we employ a Dion–Jacobson layered oxide $\mathrm{CsBi}{\mathrm{Nb}}_2{\mathrm{O}}_7$ (CBNO), which has been predicted to be ferroelectric with high Curie temperature and large in-plane polarization, as a model system to study its ferroelectric domain structure with atomic scale analysis. We reveal the existence of unit-cell-thick ferroelectric domain size as well as both 180° and 90° domain walls in free-standing CBNO. In an epitaxial CBNO film grown on a $\mathrm{LaAl}{\mathrm{O}}_3$ substrate, we discover multiple stacking structures resembling the Aurivillius and the Ruddlesden–Popper phases. We also analyze the interfacial dislocations and their associated lattice distortion fields in the epitaxial film. This paper may inspire design concepts for ferroelectric devices demanding unit-cell-thick domain size.

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.