We report on the epitaxial film growth and characterization of ${\mathrm{CoV}}_2{\mathrm{O}}_4$, a near-itinerant spinel vanadate, grown on (001) ${\mathrm{SrTiO}}_3$. The symmetry lowering of the unit cell from cubic in the bulk to orthorhombic in the films results in dramatic differences in the magnetic anisotropy compared to bulk, as determined from structural and magnetic characterization. Bulk cubic ${\mathrm{CoV}}_2{\mathrm{O}}_4$ has been found to defy predictions by showing orbital degeneracy seemingly lasting to very low temperatures, with only small anomalies in magnetization and neutron experiments signaling a possible spin/orbital glass transition at $T=90$ K. In epitaxial thin films presented in this paper, structurally tuning the ${\mathrm{CoV}}_2{\mathrm{O}}_4$ away from cubic symmetry leads to a completely different low temperature noncollinear ground state. Via magnetization and neutron scattering measurements we show that the 90-K transition is associated with a major spin reorientation away from the ferrimagnetic easy axis [001] to the [110] direction. Furthermore, the V spins cant away from this direction with extracted perpendicular moments providing evidence of a larger canting angle compared to bulk. This result indicates that compressive strain pushes the system deeper into the insulating state, i.e., away from the localized-itinerant crossover regime.

Lithium garnet-type oxide ${\mathrm{Li}}_7{\mathrm{La}}_3{\mathrm{Zr}}_2{\mathrm{O}}_{12}$ (LLZO) is one of the most promising solid electrolyte materials. In this study, the Li conduction properties of Nb-doped LLZO, ${\mathrm{Li}}_{6.75}{\mathrm{La}}_3\left({\mathrm{Zr}}_{1.75}{\mathrm{Nb}}_{0.25}\right){\mathrm{O}}_{12}$, have been investigated by molecular dynamics simulations with the machine learning potential. It is shown that Nb-doped LLZO holds a high-conductivity cubic structure at low temperatures. Two kinds of Li occupation sites, $24d$ and $96h$, are correctly predicted, which form the three-dimensional network of the Li migration pathway. The Li conductivity at 298 K and the activation energy are obtained as ${\sigma }_{\mathrm{Li}}=5.0\times {10}^{-3}\mathrm{S}/\mathrm{cm}$ and $E_a=223\mathrm{meV}$, respectively, which are in reasonably good agreement with the experimental data. The free energy difference between the $24d$ and $96h$ sites is expected to be negligibly small. This most likely contributes a high Li conductivity in the Nb-doped LLZO. The Nb substitution for Zr is accompanied with the Li vacancy formation due to the charge neutrality condition, which stabilizes the cubic phase and enhance the Li conduction. The Nb substitution, however, also has a negative effect. The supervalency of the ${\mathrm{Nb}}^{5+}$ ion destabilizes the Li atoms at the surrounding $96h$ sites. The Li conductivity in Nb-doped LLZO is determined by trading off between these effects.

Recent progress in the power conversion efficiency of $\mathrm{Cu}\left(\mathrm{In},\mathrm{Ga}\right)\mathrm{S}{\mathrm{e}}_2$ thin film solar cells has been achieved by an alkali postdeposition treatment. This treatment has been shown to change the surface composition and structure as well as the bulk properties. To investigate the relative importance of those two effects we study the impact of the treatment on Cu-rich and Cu-poor $\mathrm{CuInS}{\mathrm{e}}_2$, which show a different influence of interface recombination without the treatment. We develop a potassium postdeposition treatment that can be applied to Cu-rich material, where an additional etching step is necessary. The same postdeposition treatment with etching step is applied to Cu-poor material. In both cases we observe an increase of the power conversion efficiency and open circuit voltage. Comparing the increase in open circuit voltage to the increase in quasi-Fermi level splitting indicates that the improvement in Cu-poor solar cells is mostly due to changes in the bulk, whereas in Cu-rich solar cells both the bulk and the interface are improved. The improvement of the interface is corroborated by temperature dependent current-voltage characteristics, which show that the dominating recombination path in Cu-rich solar cells moves from the interface to the bulk after treatment and by admittance spectroscopy, which shows that the treatment removes a 200 meV deep defect. Photoluminescence spectroscopy shows that even in Cu-rich material the alkali treatment creates a Cu-poor surface, which in this case cannot be created by diffusion of Cu into the bulk, but is grown during the treatment.

Ferrimagnetic TbFe or TbFeCo amorphous alloy thin films have been grown by coevaporation in ultrahigh vacuum. They exhibit an out-of-plane magnetic anisotropy up to their Curie temperature with a nucleation and propagation reversal mechanism suitable for current induced domain wall motion. Rutherford backscattering experiments confirmed a fine control of the Tb depth-integrated composition within the evaporation process. However, a large set of experimental techniques were used to evidence an interface related contribution in such thin films as compared to much thicker samples. In particular, scanning transmission electron microscopy experiments evidence a depth dependent composition and perturbed top and bottom interfaces with preferential oxidation and diffusion of terbium. Despite that, amorphous and homogeneous alloy film remains in a bulklike part. The composition of that bulklike part of the magnetic layer, labeled as effective composition, is biased when compared with the depth-integrated composition. The magnetic properties of the film are mostly dictated by this effective composition, which we show changes with different top and bottom interfaces.

Searching for novel materials involves identifying potential candidates and selecting those that have desirable properties and facile synthesis. It is relatively easy to generate large numbers of potential candidates, for instance, by computational searches or elemental substitution. The identification of synthesizable compounds, however, is a needle-in-a-haystack problem. Conventionally, the screening is based on a convex hull construction, which identifies structures stabilized by a particular thermodynamic constraint, such as pressure, chosen based on prior experimental evidence or intuition. We introduce a generalized convex hull framework that instead relies on data-driven coordinates, and represents the full structural diversity of the candidate compounds in an unbiased way. Its probabilistic construction addresses the inevitable uncertainty in input structure data and provides a superior measure of stability compared to the input (free) energies, that can, for instance, also be used to assist experimental crystal structure determination. It efficiently identifies candidates with high probabilities of being synthesizable and suggests the relevant experimentally realizable constraints, thereby providing a much needed starting point for the determination of viable synthetic pathways.

Perovskite structure ${\mathrm{SrMnO}}_3$ is a rare example of a multiferroic material where strain tuning and/or cation substitution could lead to coinciding magnetic and ferroelectric ordering temperatures, which would then promise strong magnetoelectric coupling effects. Here, we establish the temperature- and strain-dependent ferroic phase diagram of ${\mathrm{SrMnO}}_3$ using first-principles-based effective Hamiltonians. All parameters of these Hamiltonians are calculated using density functional theory, i.e., no fitting to experimental data is required. Temperature-dependent properties are then obtained from Monte Carlo and molecular dynamics simulations. We observe a sequence of several magnetic transitions under increasing tensile strain, with a moderate variation of the corresponding critical temperatures. In contrast, the ferroelectric Curie temperature increases strongly after its onset around $2.5\%$ strain, and indeed crosses the magnetic transition temperature just above $3\%$ strain. Our results indicate pronounced magnetoelectric coupling, manifested in dramatic changes of the magnetic ordering temperatures and different magnetic ground states as function of the ferroelectric distortion. In addition, coexisting ferroelectric and ferromagnetic order is obtained for strains above $4\%$. Our calculated phase diagram suggests the possibility to control the magnetic properties of ${\mathrm{SrMnO}}_3$ through an applied electric field, significantly altering the magnetic transition temperatures, or even inducing transitions between different magnetic states.

The overestimation, rather than the usual underestimation, of the electronic band gap at 0 K of boron carbide with the ideally stoichiometric composition of ${\mathrm{B}}_4\mathrm{C}$, represented by ${\mathrm{B}}_{11}{\mathrm{C}}^p$(CBC), in density functional theory calculations is one of the outstanding controversial issues in the field of icosahedral boron-rich solids. Using a first-principles approach, we explore the effect of temperature and configurational disorder on the electronic band gap of ${\mathrm{B}}_4\mathrm{C}$. Ab initio molecular dynamics simulations are performed to account for the effects of vibrational disorder. The results reveal that the volumetric thermal expansion as well as the thermally induced configurational disorder of icosahedral ${\mathrm{C}}^p$ atoms residing in the ${\mathrm{B}}_{11}{\mathrm{C}}^p$ icosahedra have a minimal impact on the band gap of ${\mathrm{B}}_4\mathrm{C}$, while a major decrease of the band gap is caused by explicit atomic displacements, induced by lattice vibrations. At 298 K, the band gap of ${\mathrm{B}}_4\mathrm{C}$ is overestimated, as compared to the experimental value, by approximately 31%. However, configurational disorder induced by introducing a small fraction of ${\mathrm{B}}_{12}$(CBC) and ${\mathrm{B}}_{12}({\mathrm{B}}_4$) into a matrix of ${\mathrm{B}}_{11}{\mathrm{C}}^p$(CBC) to make the composition of boron carbide approximately ${\mathrm{B}}_{4.3}\mathrm{C}$, claimed to be the carbon-rich limit of the material in experiment, leads to a smaller band gap due to the appearance of midgap states. These results can explain at least a part of the previous discrepancies between theory and experiments for the band gap of boron carbide.

Understanding oxide interface-induced effects requires controlled epitaxial growth of films on well-defined substrate surfaces. While conventional film growth on ex situ prepared substrates has proven to be a successful route, the choices of appropriate substrates with atomically defined surfaces are limited. Here, by depositing $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ on $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_4$ (001), we present an alternative method of growing oxide thin films on in situ cleaved surfaces of layered-structured substrates. Cleaving $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_4$ at low temperature in ultrahigh vacuum exposes an atomically flat, solely SrO-terminated surface with up to micrometer-scale terraces. The deposition of $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ spontaneously diminishes the surface $\mathrm{Ru}{\mathrm{O}}_6$ in-plane rotational distortion of the substrate and results in a cubic-like perovskite film structure with (La/Sr)-O layer termination. The interface is atomically sharp without obvious deviation of lattice spacing and chemical valence, except in the first unit cell where Ru-Mn intermixing is observed. These results demonstrate that film growth on a cleaved substrate can be an alternative route to obtain well-defined interfaces and in addition increase the availability of substrates for future oxide films.

Magnetic phase transition contributes to magnetocaloric effects and magnetoelastic coupling, producing significant multifunctions in Ni-based Heusler alloys. In this paper, the peculiar intermediate premartensitic phase during the transition from parent phase to martensite is identified in Ga-free $\mathrm{N}{\mathrm{i}}_{50}\mathrm{M}{\mathrm{n}}_{35.2}\mathrm{I}{\mathrm{n}}_{14.8}$ alloy via Lorentz transmission electron microscopy combined with in situ magnetizing and cooling manipulation. The simultaneous coexistence of three skyrmion configurations at zero field in martensite is directly observed with correlation to the appearance of the intermediate magnetic phase and martensite twinning confinement. The evolution of magnetic domains demonstrates a mechanism to generate skyrmions with magnetization orientation adjusted via magnetic phase transition, which illustrates the associated physical properties.

High mobility, strong spin-orbit coupling, and large Landé $g$ factor make the two-dimensional electron gas (2DEG) in InAs quantum wells grown on nearly lattice-matched GaSb substrates an attractive platform for mesoscopic quantum transport experiments. Successful operation of mesoscopic devices relies on three key properties: electrical isolation from the substrate, ability to fully deplete the 2DEG and control residual sidewall conduction with lithographic gates, and high mobility to ensure ballistic transport over mesoscopic length scales. Simultaneous demonstration of these properties has remained elusive for InAs 2DEGs on GaSb substrates. Here we report on heterostructure design, molecular beam epitaxy growth, and device fabrication that result in high carrier mobility and full 2DEG depletion with minimal residual edge conduction. Our results provide a pathway to fully controlled 2DEG-based InAs mesoscopic devices.

We propose and demonstrate a compact acoustic retroreflector that reroutes incident signals back toward the source with minimal scattering. Gradient refractive index acoustic metamaterials, based on Archimedean spiral structures, are designed to fulfill the required refractive index profile. The experiments show that the compact acoustic retroreflector, whose radius is only approximately one wavelength, works in an incident angular range up to 120° over a bandwidth of about 27% of the central frequency. Such compact acoustic retroreflectors can be potentially applied in pulse-echo-based acoustic detection and communication, such as unmanned aerial vehicle sonar systems, robotic ranging detectors, and ultrasonic imaging systems.

To overcome the limitations of thermodynamic defect models, we introduce a multilevel kinetic approach to simulate the activation of $p$ doping in CdTe by group V elements using arsenic as a “model” dopant. On the lowest level, we calculate thermodynamic and kinetic parameters of point defects, complexes, and reactions from first principles. On the intermediate level, we use these parameters to calculate the kinetic rates of defect reactions. Finally, we simulate the time evolution of defects and free carriers. Our results show the importance of kinetic factors in defect chemistry models. We reveal the primary arsenic activation pathway to be a fast reaction in which the tellurium atoms get kicked out and replaced on the regular anion sites by interstitial arsenic species. We discover the important role of ($\mathrm{A}{\mathrm{s}}_{\mathrm{i}}\mathrm{A}{\mathrm{s}}_{\mathrm{Te}}$) and ($\mathrm{A}{\mathrm{s}}_{\mathrm{i}}\mathrm{A}{\mathrm{s}}_{\mathrm{i}}$) complexes that arise during activation anneal to form kinetically stabilized transient states that not only compensate the doping but also can produce deep recombination levels. We expect that our modeling approach and the gained insight into the atomic processes behind the doping formation will advance the defect chemistry modeling of electronic properties of materials.

Two-dimensional (2D) emergent fermions and negative Poisson's ratio in 2D materials are fascinating subjects of research. Here, based on first-principles calculations and theoretical analysis, we predict that the hexacoordinated ${\mathrm{Mg}}_2\mathrm{C}$ monolayer hosts both exotic properties. We analyze its phonon spectrum, reveal the Raman-active modes, and show that it has small in-plane stiffness constants. Particularly, under the tensile strain in the zigzag direction, the ${\mathrm{Mg}}_2\mathrm{C}$ monolayer shows an intrinsic negative Poisson's ratio $\sim -0.023$, stemming from its unique puckered hinge structure. The material is metallic at its equilibrium state. A moderate biaxial strain can induce a metal-semimetal-semiconductor phase transition, during which several types of 2D unconventional fermions emerge, including the anisotropic Dirac fermions around 12 tilted Dirac points in the metallic phase, the 2D double Weyl fermions in the semimetal phase where the conduction and valence bands touch quadratically at a single Fermi point, and the 2D pseudospin-1 fermions at the critical point of the semimetal-semiconductor phase transition where three bands cross at a single point on the Fermi level. In addition, uniaxial strains along the high-symmetry directions break the threefold rotational symmetry and reduce the number of Dirac points. Interestingly, it also generates 2D type-II Dirac points. We construct effective models to characterize the properties of these fermions. Our result reveals ${\mathrm{Mg}}_2\mathrm{C}$ monolayer as an intriguing platform for the study of 2D unconventional fermions, and also suggests its great potential for nanoscale device applications.

The antiferromagnetic-to-ferromagnetic phase transition in B2-ordered FeRh is imaged in laterally confined nanopatterned islands using photoemission electron microscopy with x-ray magnetic circular dichroism contrast. The resulting magnetic images directly detail the progression in the shape and size of the FM phase domains during heating and cooling through the transition. In 5-$\mu \mathrm{m}$-square islands this domain development during heating is shown to proceed in three distinct modes—nucleation, growth, and merging—each with subsequently greater energy costs. In 0.5-$\mu \mathrm{m}$ islands, which are smaller than the typical final domain size, the growth mode is stunted and the transition temperature is found to be reduced by 20 K. The modification to the transition temperature is found by high-resolution scanning transmission electron microscopy to be due to a 100-nm chemically disordered edge grain present as a result of ion implantation damage during the patterning. FeRh has unique possibilities for magnetic memory applications; the inevitable changes to its magnetic properties due to subtractive nanofabrication will need to be addressed in future work in order to progress from sheet films to suitable patterned devices.

A lack of spatial inversion symmetry gives rise to a variety of unconventional physics, from noncollinear order and Skyrmion lattice phases in magnetic materials to topologically protected surface states in certain band insulators to mixed-parity pairing states in superconductors. The search for exotic physics in such materials is largely limited by a lack of candidate materials and often by difficulty in obtaining crystals. Here, we report the single-crystal growth and physical properties of the noncentrosymmetric tungsten aluminide cage compounds ${\mathrm{Al}}_4\mathrm{W}$ and ${\mathrm{Al}}_5\mathrm{W}$, alongside related molybdenum aluminides in which spin-orbit coupling should be significantly weaker. All compounds are nonmagnetic metals. Their high conductivities suggest the opportunity to find superconductivity at lower temperatures, while the limits we can place on their transition temperatures suggest that any superconductivity may be expected to exhibit significant parity mixing.

While the microscopic structure of defected solid crystalline materials has significant impact on their physical properties, efficient and accurate determination of a given polycrystalline microstructure remains a challenge. In this paper, we present a highly generalizable and reliable variational method to achieve this goal for two-dimensional crystalline and quasicrystalline materials. The method is benchmarked and optimized successfully using a variety of large-scale systems of defected solids, including periodic structures and quasicrystalline symmetries to quantify their microstructural characteristics, e.g., grain size and lattice misorientation distributions. We find that many microstructural properties show universal features independent of the underlying symmetries.

While nonstoichiometric binary III-V compounds are known to contain group-V antisites, the growth of ternary alloys consisting of two group-V elements might give additional degrees of freedom in the chemical nature of these antisites. Using cross-sectional scanning tunneling microscopy (STM), we investigate low-temperature-grown dilute $\mathrm{GaA}{\mathrm{s}}_{1-x}{\mathrm{P}}_x$ alloys. High concentrations of negatively charged point defects are found. Combined with transmission electron microscopy and pump-probe transient reflectivity, this study shows that the defects have a behavior similar to the group-V antisites. Further analyses with x-ray diffraction point to the preferential incorporation of arsenic antisites, consistent with ab initio calculations, that yield a formation energy 0.83 eV lower than for phosphorus antisites. Although the negative charge carried by the arsenic antisites in the STM images is shown to be induced by the proximity of the STM tip, the arsenic antisites are not randomly distributed in the alloy, providing insight into the evolution of their charge state during the growth.

The topological edge states of the coupled-resonator optical waveguide (CROW) system have been widely researched. Without breaking time-reversal symmetry nor needing to construct complicated optical modes as pseudospins, the CROW system offers characteristics that are effective for exploration of photonic topological effects and has potential for applications in nanodevices. We extend the previous method by using specific coupling conditions to calculate the system's actual band structure, thereby providing a new perspective of this system. Two types of topological properties, one with and the other without an artificial magnetic field, are identified based on the band diagrams. Fractal spectra including the Hofstadter butterfly spectrum are obtained, revealing the abundant physics associated with the CROW system. By connecting the band diagrams with the corresponding structural parameters, we propose a triangular lattice array with reduced losses at the corners and propose a scheme with the potential for use as a reconfigurable multipath platform.

We demonstrate a heuristic optimization algorithm based on the game tree search for multicomponent materials design. The algorithm searches for the largest spin polarization of seven-component Heusler alloys. The algorithm can find the peaks quickly and is more robust against local optima than Bayesian optimization approaches using the expected improvement or upper confidence bound approaches. We also investigate Heusler alloys, including antisite disorder, and we show that ${\left[{\mathrm{Fe}}_{0.9}{\mathrm{Co}}_{0.1}\right]}_2{\mathrm{Cr}}_{0.95}{\mathrm{Mn}}_{0.05}{\mathrm{Si}}_{0.3}{\mathrm{Ge}}_{0.7}$ has the potential to be a high-spin-polarized material with robustness against antisite disorder.