Achieving on-demand control of the valley degree of freedom is essential for valley-based information science and technology. Optical and magnetic control of the valley degree of freedom in monolayer transition-metal dichalcogenide (TMD) semiconductors has been studied extensively. However, electrical control of the valley polarization has remained a challenge. Here we demonstrate switching of the valley polarization in monolayer $\mathrm{WS}{\mathrm{e}}_2$ by electrical gating. This is achieved by coupling a $\mathrm{WS}{\mathrm{e}}_2$ monolayer to a two-dimensional (2D) layered magnetic insulator $\mathrm{Cr}{\mathrm{I}}_3$. The valley degeneracy in $\mathrm{WS}{\mathrm{e}}_2$ is lifted by the magnetic proximity effect. The valley polarization is switched through gate control of the interlayer spin-flip transition in 2D $\mathrm{Cr}{\mathrm{I}}_3$, which switches the magnetization of the $\mathrm{Cr}{\mathrm{I}}_3$ layer adjacent to the $\mathrm{WS}{\mathrm{e}}_2$ layer. The effect is manifested by a sign change in the photoluminescence handedness of $\mathrm{WS}{\mathrm{e}}_2$. Our results provide the basis for high-speed and energy-efficient gate control of the valley degree of freedom in monolayer TMD semiconductors.

Understanding and controlling microscopic heat transfer mechanisms in solids is critical to material design in numerous technological applications. Yet, the current understanding of thermal transport in semiconductors and insulators is limited by the difficulty in directly measuring individual phonon lifetimes and mean free paths, and studying their dependence on the microscopic state of the material. Here we report our measurements of microscopic phonon scattering rates in $\mathrm{Ag}\mathrm{Bi}{\mathrm{Se}}_2$, which exhibits a controllable, reversible change directly linked to microstructure evolution near a reversible structural phase transition, that directly impacts the thermal conductivity. We demonstrate a steplike doubling of phonon scattering rates resultant from the cation disordering at the structural transition. To rationalize the neutron scattering data, we leverage a stepwise approach to account for alterations to the thermal conductivity that are imparted by distinct scattering mechanisms. These results highlight the potential of tunable microstructures housed in a stable crystal matrix to provide a practical route to tailor phonon scattering to optimize thermal transport properties.

Antiferromagnets offer spintronic device characteristics unparalleled in ferromagnets owing to their lack of stray fields, THz spin dynamics, and rich materials landscape. Microscopic imaging of antiferromagnetic domains is one of the key prerequisites for understanding physical principles of the device operation. However, adapting common magnetometry techniques to the dipolar-field-free antiferromagnets has been a major challenge. Here we demonstrate in a collinear antiferromagnet a thermoelectric detection method by combining the magneto-Seebeck effect with local heat gradients generated by scanning far-field or near-field techniques. In a 20-nm epilayer of uniaxial CuMnAs we observe reversible ${180}^{\circ }$ switching of the Néel vector via domain wall displacement, controlled by the polarity of the current pulses. We also image polarity-dependent ${90}^{\circ }$ switching of the Néel vector in a thicker biaxial film, and domain shattering induced at higher pulse amplitudes. The antiferromagnetic domain maps obtained by our laboratory technique are compared to measurements by the established synchrotron-based technique of x-ray photoemission electron microscopy using x-ray magnetic linear dichroism.

Topological semimetals have been attracting great interest for their superb potentials. While many theoretical and experimental investigations have been performed for topological semimetals, their materials platform is still in demand. Here, we report a highly tunable materials system for topological semimetal, indium (In)-doped $\mathrm{P}{\mathrm{b}}_{1-x}\mathrm{S}{\mathrm{n}}_x\mathrm{Te}$. By exploring the crystals with varying Pb/Sn ratios and In doping levels, a phase formation with low carrier concentration, high mobility, and large anomalous Hall effect is found for a finite area of the composition between topological crystalline insulator and normal insulator at ambient pressure. Furthermore, the in-plane anomalous Hall effect as a hallmark of Berry-curvature generation is also observed at low temperatures, where optical second-harmonic generation reveals the breaking of inversion symmetry. These results show that there is a finite range of topological semimetal phase in the PbTe-SnTe binary alloy, providing a promising materials platform to investigate the versatile nature of topological semimetals.

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

We investigate the impact of mechanical strains and a perpendicular electric field on the electronic and magnetic ground-state properties of single-layer ${\mathrm{CrI}}_3$ using density functional theory. We propose a minimal spin model Hamiltonian, consisting of symmetric isotropic exchange interactions, magnetic anisotropy energy, and Dzyaloshinskii-Moriya (DM) interactions, to capture most pertinent magnetic properties of the system. We compute the mechanical strain and electric field dependence of various spin-spin interactions. Our results show that both the amplitudes and signs of the exchange interactions can be engineered by means of strain, while the electric field affects only their amplitudes. However, strain and electric fields affect both the directions and amplitudes of the DM vectors. The amplitude of the magnetic anisotropy energy can also be substantially modified by an applied strain. We show that in comparison with an electric field, strain engineering can be more efficiently used to manipulate the magnetic and electronic properties of the system. Notably, such systematic tuning of the spin interactions is essential for the engineering of room-temperature spintronic nanodevices.

Recent advances in complex oxide heterostructures have realized extraordinary control over oxygen vacancies ($V_{\mathrm{O}}$), including strain-tuned $V_{\mathrm{O}}$ order, and electric-field-controlled transformations between perovskite and $V_{\mathrm{O}}$-ordered structures. Perovskite cobaltites such as ${\mathrm{La}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Co}{\mathrm{O}}_{3-\delta }$ provide a prime example, recent work demonstrating that strain engineering of $V_{\mathrm{O}}$ ordering induces large ($\sim {10}^7\mathrm{erg}/\mathrm{c}{\mathrm{m}}^3$) perpendicular magnetic anisotropy. Here we show that $V_{\mathrm{O}}$-ordered epitaxial ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Co}{\mathrm{O}}_{3-\delta }$ films exhibit not only strong magnetic anisotropy, but also a giant form of anisotropic magnetoresistance (AMR). This has magnetic field, temperature, and angular dependencies in quantitative accord with conventional AMR, but with AMR ratios up to an extraordinary 40.3%, 20 times enhanced over bulk cobaltites, and ∼10–100 times larger than typical transition metals. This giant AMR has no strong dependence on heteroepitaxial strain (between −2.1% and +1.8%) or thickness, and is instead ascribed to symmetry lowering associated with $V_{\mathrm{O}}$ ordering. The AMR ratios thus obtained are among the largest reported in the over 160-year history of this phenomenon, despite the absence of heavy elements.

The cesium copper halides ${\mathrm{CsCu}}_2{X}_3$ $(X=\mathrm{Cl}, \mathrm{Br}, \mathrm{and} \mathrm{I})$ are a class of all-inorganic perovskites with interesting and potentially useful optical properties, characterized by distinct excitonic features. We present a computational study of the optical absorption spectra of ${\mathrm{CsCu}}_2{X}_3$, comparing time-dependent density-functional theory (TDDFT) and the Bethe-Salpeter equation (BSE), using $GW$ quasiparticle band structures as input. The TDDFT calculations are carried out using several types of global hybrid exchange-correlation functionals. It is found that an admixture of nonlocal exchange determined by the dielectric constant produces optical spectra in excellent agreement with the BSE. Thus, hybrid TDDFT emerges as a promising first-principles approach for excitonic effects in solids.

Using first principles calculations we show that several alkali lead halides potentially present collective ferroelectric polarization. This should occur at least at the nanoscale; it could be detected macroscopically provided it is not concealed by lattice vibrations in the temperature range of stability of the cubic perovskite phase. For potassium lead halides and for alkali lead fluorides, remarkably, the ferroelectric behavior turns hyperferroelectric, suggesting a more robust ferroelectric polarization in spite of depolarization potentials induced by charge accumulation on surfaces or interfaces.

Defects of various kinds strongly affect local magnetic anisotropy and thus play a crucial role in coercivity in high-performance permanent magnets. In particular, in rare-earth-free permanent magnetic alloys with $L{1}_0$ structure microstructural defects deserve special attention. In this work, we report on the “negative” effect of twin structure, and the “positive” effect of dislocations on the coercivity is clarified in a systematic experimental study of $L{1}_0$-MnAl alloys. We find that the nucleation of magnetization reversal is preferentially activated along the twin boundaries and grows into the twin stripes. This suggests that twin structure reduces the domain wall nucleation field, so that the coercivity decreases by approximately 50% according to the direct comparison of twin-free and twinned magnets. In contrast, dislocations dramatically enhance the coercivity by acting as the pinning center, and a high density of dislocations can modify the dominant coercivity mechanism from nucleation to pinning in severely deformed MnAl magnets. With a decreasing dislocation density, the pinning field remains as a constant while the coercivity reduces monotonously, indicative of the “strong pinning” effect of dislocations on the magnetic domain wall, generating a positive correlation between coercivity and density of dislocation. High coercivity from 424 to 328 kA/m is obtained in deformed and annealed magnets with different densities of dislocations. Thus, the combination of eliminating twin structure and introducing high-density dislocations could overcome the present bottleneck in magnetic performance. This work may inspire avenues for the development of $L{1}_0$ rare-earth-free permanent magnetic alloys, and set up a pathway for accelerating the application process.

There are few known semiconductors exhibiting both strong optical response and large dielectric polarizability. Inorganic materials with large dielectric polarizability tend to be wide-band gap complex oxides. Semiconductors with a strong photoresponse to visible and infrared light tend to be weakly polarizable. Interesting exceptions to these trends are halide perovskites and phase-change chalcogenides. Here we introduce complex chalcogenides in the Ba-Zr-S system in perovskite and Ruddlesden-Popper structures as a family of highly polarizable semiconductors. We report the results of impedance spectroscopy on single crystals that establish $\mathrm{BaZr}{\mathrm{S}}_3$ and ${\mathrm{Ba}}_3{\mathrm{Zr}}_2{\mathrm{S}}_7$ as semiconductors with a low-frequency relative dielectric constant ${\varepsilon }_0$ in the range 50–100 and band gap in the range 1.3–1.8 eV. Our electronic structure calculations indicate that the enhanced dielectric response in perovskite $\mathrm{BaZr}{\mathrm{S}}_3$ versus Ruddlesden-Popper ${\mathrm{Ba}}_3{\mathrm{Zr}}_2{\mathrm{S}}_7$ is primarily due to enhanced IR mode-effective charges and variations in phonon frequencies along 〈001〉; differences in the Born effective charges and the lattice stiffness are of secondary importance. This combination of covalent bonding in crystal structures more common to complex oxides, but comprising sulfur, results in a sizable Fröhlich coupling constant, which suggests that charge carriers are large polarons.

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

The spatial texture of an internal degree of freedom of electrons has profound effects on the properties of materials. Such texture in real space can manifest as an emergent magnetic field (or Berry curvature), which is expected to induce interesting valley/spin-related transport phenomena. Moiré pattern, which emerges as a spatial variation at the interface of two-dimensional atomic crystals, provides a natural platform for investigating such real space Berry curvature effects. Here we study moiré structures formed in homobilayer transition metal dichalcogenides (TMDs) due to twisting, various uniform strain profiles, and their combinations, where electrons can reside in either layer with the layer index serving as an internal degree of freedom. The layer pseudospin exhibits vortex/antivortex textures in the moiré supercell, leading to a giant geometric magnetic field and a scalar potential. Within a geometric picture, the moiré magnetic field is found as the cross product of the gradients of the out-of-plane pseudospin and the in-plane pseudospin orientation, respectively. We discover dual roles of uniform strain: Besides being a cause of the moire atomic texture in the homobilayer, it also contributes a pseudogauge potential that modifies the local phase of interlayer coupling. Consequently, strain can be employed to tune the in-plane pseudospin texture, while interlayer bias tunes the out-of-plane pseudospin, and we show how the moiré magnetic field's spatial profile, intensity, and flux per supercell can be engineered. Through the geometric scalar correction, the landscape of the scalar potential can also be engineered along with the moiré magnetic field, forming distinct effective lattice structures. These properties render TMD moiré structures promising to build tunable flux lattices for transport and topological material applications.

The recent reports of superconductivity in ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ heterostructures have reinvigorated interest in potential superconductivity of low-oxidation state nickelates. Synthesis of ${\mathrm{Ni}}^{1+}$-containing compounds is notoriously difficult. In the current work, a combined sol-gel combustion and high-pressure annealing technique was employed to prepare polycrystalline perovskite ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_3$ $(x=0, 0.1, \mathrm{and} 0.2)$. Metal nitrates and metal acetates were used as starting materials, and the latter were found to be superior to the former in terms of safety and reactivity. The ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_3$ compounds were subsequently reduced to ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2$ using calcium hydride in a sealed, evacuated quartz tube. To understand the synthesis pathway, the evolution from $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ to $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_2$ was monitored using in situ synchrotron x-ray diffraction during the reduction process. Electrical transport properties were consistent with an insulator-metal transition occurring between $x=0$ and 0.1 for ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_3$. Superconductivity was not observed in our bulk samples of ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2$. Neutron diffraction experiments at 3 and 300 K were performed on ${\mathrm{Nd}}_{0.9}{\mathrm{Sr}}_{0.1}\mathrm{Ni}{\mathrm{O}}_2$, in which no magnetic Bragg reflections were observed, and the results of structural Rietveld refinement are provided.

As-grown $\mathrm{Ag}{\mathrm{F}}_2$ has an electronic structure remarkably similar to insulating cuprates, but it is extremely electronegative, which makes it hard to handle and dope. Furthermore, buckling of layers reduces magnetic interactions and enhances unwanted self-trapping lattice effects. We argue that epitaxial engineering can solve all these problems. By using a high-throughput approach and first-principle computations, we find a set of candidate substrates which can sustain the chemical aggressiveness of $\mathrm{Ag}{\mathrm{F}}_2$ and at the same time have good lattice parameter matching for heteroepitaxy, enhancing $\mathrm{Ag}{\mathrm{F}}_2$ magnetic and transport properties, and opening the possibility of field-effect carrier injection to achieve a unique generation of high-$T_c$ superconductors. Assuming a magnetic mechanism and extrapolating from cuprates we predict that the superconducting critical temperature of a single layer can reach 195 K.

Electrophoretic deposition (EPD) is an industrially relevant and scalable technique used to form particle deposits from colloidal suspensions. Highly concentrated particle suspensions generally prevent real-time in situ microscopy observations which limit the characterization of EPD films to ex situ, or postprocessed, laboratory techniques. For dynamic systems, such as tunable amorphous photonic crystals (APCs), only reversible deposits are formed during the EPD process. Since reversible deposits cannot be characterized with standard ex situ methods, the particle-particle and particle-field interactions that govern the displayed color and crystallinity of these systems are not well understood. Here, we present in situ small-angle x-ray scattering and UV-Vis techniques for measuring both the structural and optical response of an APC under applied electric fields. We also develop a computational model based on colloidal interactions to explain the observed change in the interparticle spacing of APCs due to the applied electric field which correlates to displayed color. Ultimately, this work provides a new in situ characterization method that could be expanded for other dynamic, tunable colloidal systems.

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

Borides of group-IB and group-IIB metals normally do not exist in the bulk form due to the small electronegativity difference and large mismatch in their atomic sizes. Contrary to these established rules, several metastable two-dimensional structures of copper boride were predicted from ab initio evolutionary searches, identifying that two structures are metallic whereas another one is strikingly a nodal line semimetal. The reduced dimensionality and intralayer coupling are responsible for the emergence of copper borides. This makes copper borides an attractive material for potential electronics applications.

We present an interacting model for the electronic and magnetic behaviors of a strained $\left(001\right)$ atomic layer of ${\mathrm{Sr}}_2{\mathrm{FeMoO}}_6,$ which shows room-temperature ferrimagnetism and magnetoresistance with potential spintronics application in the bulk. We find that the strong spin-orbit coupling in the molybdenum $4d$ shell gives rise to a robust ferrimagnetic state with an emergent spin-polarized electronic structure consisting of flatbands and four massive or massless Dirac dispersions. Based on the spin-wave theory, we demonstrate that the magnetic order remains intact for a wide range of doping, leading to the possibility of exploring flatband physics, such as Wigner crystallization in electron-doped ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{FeMoO}}_6.$

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

${\mathrm{Fe}}_{5-x}{\mathrm{GeTe}}_2$ is a van der Waals material with one of the highest reported bulk Curie temperatures, $T_{\mathrm{C}}\approx 310\mathrm{K}$. In this study, theoretical calculations and experiments are utilized to demonstrate that the magnetic ground state is highly sensitive to local atomic arrangements and the interlayer stacking. Cobalt substitution is found to be an effective way to manipulate the magnetic properties while also increasing the ordering temperature. In particular, cobalt substitution up to $\approx 30\%$ enhances $T_{\mathrm{C}}$ and changes the magnetic anisotropy, while $\approx 50\%$ cobalt substitution yields an antiferromagnetic state. Single crystal x-ray diffraction evidences a structural change upon increasing the cobalt concentration, with a rhombohedral cell observed in the parent material and a primitive cell observed for $\approx 46\%$ cobalt content relative to iron. First-principles calculations demonstrate that it is a combination of high cobalt content and the concomitant change to primitive layer stacking that produces antiferromagnetic order. These results illustrate the sensitivity of magnetism in ${\mathrm{Fe}}_{5-x}{\mathrm{GeTe}}_2$ to composition and structure, and emphasize the important role of local structural order-disorder and layer stacking in cleavable magnetic materials.

While it is known that alloy components can segregate to grain boundaries (GBs) and the atomic mobility in GBs greatly exceeds the atomic mobility in the lattice, little is known about the effect of GB segregation on GB diffusion. Atomistic computer simulations offer a means of gaining insights into the segregation-diffusion relationship by computing the GB diffusion coefficients of the alloy components as a function of their segregated amounts. In such simulations, thermodynamically equilibrium GB segregation is prepared by a semigrand canonical Monte Carlo method, followed by calculation of the diffusion coefficients of all alloy components by molecular dynamics. As a demonstration, the proposed methodology is applied to a GB is the Cu-Ag system. The GB diffusivities obtained exhibit nontrivial composition dependencies that can be explained by site blocking, site competition, and the onset of GB disordering due to the premelting effect.

The magnetic order and the spin dynamics in the antiferromagnetic entropy-stabilized oxide (${\mathrm{Mg}}_{0.2}{\mathrm{Co}}_{0.2}{\mathrm{Ni}}_{0.2}{\mathrm{Cu}}_{0.2}{\mathrm{Zn}}_{0.2}$)O (MgO-ESO) have been studied using muon spin relaxation ($\mu \mathrm{SR}$) and inelastic neutron scattering. We find that antiferromagnetic order develops gradually in the sample volume as it is cooled below 140 K, becoming fully ordered around 100 K. The spin dynamics show a critical slowing down in the vicinity of the transition, and the magnetic order parameter grows continuously in the ordered state. These results indicate that the antiferromagnetic transition is continuous but proceeds with a Gaussian distribution of ordering temperatures. The magnetic contribution to the specific heat determined from inelastic neutron scattering likewise shows a broad feature centered around 120 K. High-resolution inelastic neutron scattering further reveals an initially gapped spectrum at low temperature which sees an increase in a quasielastic contribution upon heating until the ordering temperature.

Recent discoveries of two-dimensional (2D) superconductors have uncovered various aspects of physical properties including vortex matter. In this paper, we report transport properties and a dynamical phase diagram at zero magnetic field in ion-gated $\mathrm{Mo}{\mathrm{S}}_2$. In addition to the universal jump in the current-voltage characteristic showing unambiguous evidence of the Berezinskii-Kosterlitz-Thouless transition, we observed multiple peaks in the temperature and current derivative of the electrical resistance, based on which a dynamical phase diagram in the current-temperature plane was constructed. We found current-induced dynamical states of vortex-antivortex pairs, containing that with the phase slip line. Also, we present a global phase diagram of vortices in gated $\mathrm{Mo}{\mathrm{S}}_2$ which captures the nature of vortex matter of clean 2D superconductors.