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.

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.