Monte Carlo simulations were used to study the assembly of model binary mixtures whose pure components have either distinct or similar crystal order symmetry. Specifically, we simulated mixtures of hard disks with either squares or hexagons, where the components have size ratios that optimize their co-assembly into compositionally disordered solids. For the disks + squares mixture, along with the enhanced regions of solid miscibility, we report a continuous-looking transition from the disklike to the squarelike behavior that occurs through a region that seamlessly bridges the regions of hexatic phase of disks and the tetratic phase of squares, which we term the mosaic ($M$) region. For the equimolar composition, this $M$ region is bound by the isotropic phase at low pressures and by the hexatic-tetratic (two-phase) macrophase segregated region above a critical transition pressure. Our analysis showed that the $M$ region lies in the vicinity of the critical point, manifesting local compositional fluctuations that give rise to microphase segregated regions of interspersed square-rich fourfold and rhombic lattice symmetry and disk-rich sixfold clusters that coexist across the system. The $M$ behavior is characterized by a short-ranged translational order and an algebraic decay of the correlation functions for sixfold and fourfold orientational order. A finite size scaling analysis was used to evaluate the dependence of the local compositional susceptibility with system size to extract the critical exponent associated with the approach of the critical point. For the disks + hexagons mixture, a fully mixed hexatic phase was observed for all compositions.

Van der Waals heterostructures provide a versatile tool to not only protect or control, but also enhance the properties of a 2D material. We use ab initio calculations and semianalytical models to find strategies which boost the mobility of a current-carrying two-dimensional (2D) semiconductor within a heterostructure. Free-carrier screening from a metallic “screener” layer remotely suppresses electron-phonon interactions in the current-carrying layer. This concept is most effective in 2D semiconductors whose scattering is dominated by screenable electron-phonon interactions, and in particular, the Fröhlich coupling to polar-optical phonons. Such materials are common and characterized by overall low mobilities in the small doping limit and much higher ones when the 2D material is doped enough for electron-phonon interactions to be screened by its own free carriers. We use GaSe as a prototype and place it in a heterostructure with doped graphene as the “screener” layer and boron nitride as a separator. We develop an approach to determine the electrostatic response of any heterostructure by combining the responses of the individual layers computed within density functional perturbation theory. Remote screening from graphene can suppress the long-wavelength Fröhlich interaction, leading to a consistently high mobility around $500–600{\mathrm{cm}}^2$/V s for carrier densities in GaSe from ${10}^{11}$ to ${10}^{13}{\mathrm{cm}}^{-2}$. Notably, the low-doping mobility is enhanced by a factor 2.5. This remote free-carrier screening is more efficient than more conventional manipulation of the dielectric environment, and it is most effective when the separator (boron nitride) is thin.

We present a structural and density-functional theory study of the interface of the quasi-twin-free grown three-dimensional topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ on Ge(111). Aberration-corrected scanning transmission electron microscopy and electron energy-loss spectroscopy in combination with first-principles calculations show that the weak van der Waals adhesion between the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ quintuple layer and Ge can be overcome by forming an additional Te layer at their interface. The first-principles calculations of the formation energy of the additional Te layer show it to be energetically favorable as a result of the strong hybridization between the Te and Ge.

Lanthanum hydride (${\mathrm{LaH}}_{10}$) with a sodalitelike clathrate structure was experimentally synthesized to exhibit a near-room-temperature superconductivity under megabar pressures. Based on first-principles density-functional theory calculations, we reveal that the metal framework of La atoms has excess electrons at interstitial regions. Such anionic electrons are easily captured to form a stable clathrate structure of H cages. We thus propose that the charge transfer from La to H atoms is mostly driven by the electride property of the La framework. Furthermore, the interaction between La atom and H cage induces a delocalization of La $5p$ semicore states to hybridize with the H $1s$ state. Consequently, the bonding nature of ${\mathrm{LaH}}_{10}$ is characterized as a mixture of ionic and covalent bonding between La atom and H cage. Our findings demonstrate that anionic and semicore electrons play important roles in stabilizing clathrate H cages in ${\mathrm{LaH}}_{10}$, which can be broadly applicable to other compressed rare-earth hydrides with clathrate structures.

We obtain parameters for nonorthogonal and orthogonal tight-binding (TB) models from two-atomic molecules for all combinations of elements of period 1 to 6 and group 3 to 18 of the periodic table. The TB bond parameters for 1711 homoatomic and heteroatomic dimers show clear chemical trends. In particular, using our parameters we compare to the rectangular $d$-band model, the reduced $sp$ TB model, as well as canonical TB models for $sp$- and $d$-valent systems, which have long been used to gain qualitative insight into the interatomic bond. The transferability of our dimer-based TB bond parameters to bulk systems is discussed exemplarily for the bulk ground-state structures of Mo and Si. Our dimer-based TB bond parameters provide a well-defined and promising starting point for developing refined TB parametrizations and for making the insight of TB available for guiding materials design across the periodic table.

Assuming a vacancy-mediated diffusion, a continuum model for electromigration-driven transport of an embedded atom cluster across a surface terrace of a phase-separating ${\mathrm{A}}_x{\mathrm{B}}_{1-x}$ surface alloy, such as fcc AgPt(111), is presented. Computations show that the electron wind carries the cluster over hundreds of lattice spacings and in the set direction, while the cluster grows and its purity improves during the drift. Impacts of the current density, the diffusion anisotropy, the magnitude, and sign of the ratio of the effective charges $q_A/q_B$, and the jump frequencies ratio ${\mathrm{\Gamma }}_A/{\mathrm{\Gamma }}_B$ on the cluster's drift speed, drift direction, purity, and shape are demonstrated.

We have used highly accurate quantum Monte Carlo methods to determine the chemical structure and electronic band gaps of monolayer GeSe. Two-dimensional (2D) monolayer GeSe has received a great deal of attention due to its unique thermoelectric, electronic, and optoelectronic properties with a wide range of potential applications. Density functional theory (DFT) methods have usually been applied to obtain optical and structural properties of bulk and 2D GeSe. For the monolayer, DFT typically yields a larger band-gap energy than for bulk GeSe but cannot conclusively determine if the monolayer has a direct or indirect gap. Moreover, the DFT-optimized lattice parameters and atomic coordinates for monolayer GeSe depend strongly on the choice of approximation for the exchange-correlation functional, which makes the ideal structure—and its electronic properties—unclear. In order to obtain accurate lattice parameters and atomic coordinates for the monolayer, we use a surrogate Hessian-based parallel line search within diffusion Monte Carlo to fully optimize the GeSe monolayer structure. The DMC-optimized structure is different from those obtained using DFT, as are calculated band gaps. The potential energy surface has a shallow minimum at the optimal structure. This, combined with the sensitivity of the electronic structure to strain, suggests that the optical properties of monolayer GeSe are highly tunable by strain.

The impact of kiloelectronvolt ${\mathrm{He}}^{+}$ and kiloelectronvolt ${\mathrm{Ne}}^{+}$ ion irradiation on the magnetic properties of rare earth-3$d$ transition metal amorphous alloys was investigated. The studies were performed for 20-nm-thick ${\mathrm{Gd}}_x{\mathrm{Fe}}_{100-x}$ and ${\mathrm{Tb}}_x{\mathrm{Fe}}_{100-x}$ films with perpendicular magnetic anisotropy, irradiated with doses ranging from $5\times {10}^{13}$ to $5\times {10}^{15}\mathrm{ions}\mathrm{per}\mathrm{c}{\mathrm{m}}^2$. We demonstrate that this approach can be used to engineer magnetic properties such as magnetization, magnetic anisotropy, and compensation temperature in both alloys and to control the spin-flop reorientation transition in GdFe. We find that fine-tuning of the compensation point is possible without losing perpendicular magnetic anisotropy by appropriate selection of ion energy and irradiation dose. The experiments were supported by atomistic simulations revealing that the observed changes can be attributed to selective oxidation of rare earth atoms. Oxidation of 1% of atoms in the lattice gives reasonable agreement to the experimental results for low and medium ion doses.

Perpendicular magnetic anisotropy (PMA) in magnetic thin films is a fundamental key feature in the design of spintronic devices. As one of magnetic Weyl semimetals, ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ has been studied for its large anomalous Hall effect (AHE), uniaxial crystalline magnetic anisotropy, and half metallicity. In this study, we investigated the effect of off-stoichiometric composition on the PMA and AHE of ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_x$ thin films fabricated by the sputtering technique. The prepared thin films have off-stoichiometric S compositions $x$ of 1.54 (S poor) and 3.27 (S rich) as well as the nearly stoichiometric one of 2.02. In addition to the ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ phase, the segregated Co metal is found to contribute to the measured magnetization in the S-poor and S-rich films. The coercive field of perpendicular magnetization in all the films is much larger than that in the ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ bulk crystals despite the fact that effective perpendicular magnetic anisotropy constants ($K_{\mathrm{u}}^{\mathrm{eff}}$) between the prepared films are significantly different. In addition, the $K_{\mathrm{u}}^{\mathrm{eff}}$ values of two samples with $x=2.02$ and 2.22 are comparable to those of the bulk crystals. In contrast to the isotropic magnetization behavior in the S-rich film, the S-poor film holds the PMA feature. This result means that the PMA is more robust in the S-poor film than in the S-rich film. For the electrical transport properties, a large tangent of Hall angle of about 0.2 is observed for both the nearly stoichiometric and the S-poor films. This large tangent of Hall angle demonstrates that the Weyl feature of ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ phase is well maintained even in the S-poor thin films as well as the nearly stoichiometric films although the amount of Co segregation in both S-poor and S-rich films is similar. Our findings on the influence of off-stoichiometry on the PMA and AHE are beneficial to design magnetic devices incorporated with the Weyl features of ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$.

Helium implantation in epitaxial thin films is a way to control the out-of-plane deformation independently from the in-plane strain controlled by epitaxy. In particular, implantation by means of a helium microscope allows for local implantation and patterning down to the nanometer resolution, which is of interest for device applications. We present here a study of bismuth ferrite (${\mathrm{BiFeO}}_3$) films where strain was patterned locally by helium implantation. Our combined Raman, x-ray diffraction, and transmission electron microscopy (TEM) study shows that the implantation causes an elongation of the ${\mathrm{BiFeO}}_3$ unit cell and ultimately a transition towards the so-called supertetragonal polymorph via states with mixed phases. In addition, TEM reveals the onset of amorphization at a threshold dose that does not seem to impede the overall increase in tetragonality. The phase transition from the R-like to T-like ${\mathrm{BiFeO}}_3$ appears as first-order in character, with regions of phase coexistence and abrupt changes in lattice parameters.

Hexagonal germanium in the lonsdaleite structure has a direct band gap, but it is not an efficient light emitter due to the vanishing oscillator strength of electronic transitions at the fundamental gap. Transitions involving the second lowest conduction band are instead at least three orders of magnitude stronger. The inversion of the two lowest conduction bands would therefore make hexagonal germanium ideal for optoelectronic applications. In this work, we investigate the possibility to achieve this band inversion by applying strain. To this end we perform ab initio calculations of the electronic band structure and optical properties of strained hexagonal germanium, using density-functional theory with the modified Becke-Johnson exchange-correlation functional and including spin-orbit interaction. We consider hydrostatic pressure, uniaxial strain along the hexagonal $c$ axis, as well as biaxial strain in planes perpendicular to and containing the hexagonal $c$ axis to simulate the effect of a substrate. We find that the conduction-band inversion, and therefore the transition from a pseudodirect to a direct band gap, is attainable for moderate tensile uniaxial strain parallel to the lonsdaleite $c$ axis.

Memristors with promising applications in nonvolatile memory and unconventional computing have attracted much interest for both materials study and device development. Memristors are not commonly realized in zincblende-like semiconductors that could have optimum lattice matching with Si or GaAs substrates in semiconductor technologies, whereas often based on metal oxides with movable oxygen vacancies. Here, we propose the ordered-vacancy zincblende (OVZ) semiconductors as a type of memristor materials. Based on first-principles calculations on the ${\mathrm{Al}}_2\text{−}X\text{−}{Y}_4$ group of semiconductors, we select ${\mathrm{Al}}_2\mathrm{Cd}{\mathrm{S}}_4$ as the best candidate that is lattice matched to Si, with medium energy barriers of ∼1 eV for vacancy/ion diffusion, comparable to the metal-oxide memristor materials, suggesting that ${\mathrm{Al}}_2\mathrm{Cd}{\mathrm{S}}_4$ could be segregated into ion-rich versus vacancy-rich structures via ion drift under electric operation. We find from defect calculations that both ${\mathrm{V}}_{\mathrm{Cd}}$ and ${\mathrm{Cd}}_i$ are shallow defects, suggesting a bipolar conduction with electron transport dominated. We further find that the electron-rich ${\mathrm{Al}}_2\mathrm{Cd}{\mathrm{S}}_4$ structure can be both electrically conductive and optically transparent, showing potential applications as transparent memristors. Our study therefore opens the way of designing OVZ memristor materials with good compatibility with semiconductor technologies, as well as potentially optimum properties for memristor devices.

Magnetotransport and ferromagnetism in thin films of ${\mathrm{Co}}_2\mathrm{Si}$ nanoclusters are investigated experimentally and theoretically. The nanoclusters are fabricated by an inert-gas condensation-type cluster-deposition method and have an average size of 11.3 nm. Unlike the bulk ${\mathrm{Co}}_2\mathrm{Si}$ that exhibits a very weak net magnetic moment only below 10 K, the nanoclusters exhibit room-temperature ferromagnetism with a substantial saturation magnetization. Key features of the system are its closeness to the Stoner transition, magnetic moments induced by spin polarization starting from surface atoms, and nonuniaxial anisotropy associated with the orthorhombic crystal structure of ${\mathrm{Co}}_2\mathrm{Si}$. A method is introduced to determine the effective anisotropy using the experimental magnetization data of this complex system and its relationship with the two lowest-order nonuniaxial anisotropy constants. On decreasing temperature from 300 K, the nanoclusters show electron-transport properties unusual for a ferromagnetic metal, including an increase of Hall resistivity and a nonmonotonic change of negative magnetoresistance with a peak at around 100 K. The underlying physics is explained on the basis of the large polarization of surface spins and variation in the degree of their misalignments due to temperature-dependent effective anisotropy.

Recently, it was demonstrated that the conductivity of wurzite (wz) ZnO bicrystal samples containing $\left\{0001\right\}$ inversion domain boundaries (IDB) can be massively and reversibly tuned by mechanical loading. As a step towards a detailed microscopic understanding of this effect, we systematically investigate the atomic structure and chemical composition of such IDBs using density functional theory calculations. In total, 92 model geometries that differ in structure and/or chemical composition are constructed, optimized, and compared thermodynamically. The lack of higher symmetries in wz ZnO prohibits a straightforward calculation of individual grain boundary (GB) excess energies. However, we show that, in nonperiodic slab models of wz $\left\{0001\right\}$ IDBs, the additional surface contribution to the total energy may be approximated by that of corresponding zincblende (zb) surfaces; the latter can be obtained by a series of prism calculations. Subtracting these surface energies allows us to construct absolute GB energy diagrams for wz IDBs and compare their thermodynamic stability with other GBs known from the literature. We find that thermodynamically favored IDBs are characterized by fully (4-fold) coordinated atoms and possess relatively low excess energies that range from 45 to $95\mathrm{meV}/{\AA }^2$, depending on the termination (Zn/Zn or O/O) of the IDB and the exchange-correlation functional used in the calculation (LDA, GGA, or $\text{GGA}+U$). The electronic properties of the GB deviate only weakly from those of the bulk and are rather insensitive towards compressive and tensile strains. Our results thus indicate that experimentally observed piezotronic properties of wz bicrystals are not an intrinsic property of the pristine GB itself, but originate, for example, from externally supplied trapped charges, defects, impurities, or dopants. Low-energy structure models identified here may also be transferable to other wz- or zb-type IDBs (e.g., GaN, AlN, SiC, etc.).

We report on results of first-principles electronic structure calculations on disordered $\mathrm{Zn}(\mathrm{Si},\mathrm{Ge},\mathrm{Sn}){\mathrm{N}}_2$ alloys. These calculations on substitutional disordered alloys are carried out using the Korringa-Kohn-Rostoker Green function (KKR-GF) method in combination with the coherent potential approximation (CPA) alloy theory. The band gaps and effective masses as well as the disorder-induced finite lifetime of electronic states at the conduction band minimum and valence band maximum are evaluated by analyzing the Bloch spectral functions. Relativistic effects are found to have a small impact and in particular the influence of the spin-orbit coupling is negligible. The alloys with low Si content show band gaps and effective masses which change almost linearly with the composition. Their relatively small effective mass and long lifetime indicate a high charge carrier mobility.

We consider the plasmon excitations in anisotropic two-dimensional Dirac systems, be it either anisotropic graphene or surfaces of topological insulators. Generalizing the exact density-density response function one finds a plasmon dispersion that is anisotropic already at the lowest frequencies. Asymptotic expressions are obtained for the dispersion in this regime. We show that the plasmon properties of the complete material class of anisotropic Dirac systems are characterized by just two dimensionless material parameters. The strong anisotropy can be used to guide the plasmon modes, introducing new functionalities to the field of Dirac plasmonics.

We investigate the influence of dislocations and twin walls in ${\mathrm{BaTiO}}_3$ on its ferroelectric response and the resulting effect on the perpendicular magnetic anisotropy (PMA) of a strain-coupled ${\left[\mathrm{Co}\setminus \mathrm{Ni}\right]}_{\mathrm{n}}$ film. A dense twinned structure in conjunction with a high dislocation density significantly reduces the converse piezoelectric effect of ${\mathrm{BaTiO}}_3$ by hindering the propagation of newly nucleated domains with an applied electric field. This, in turn, results in a modest reduction of the PMA of the ferromagnetic layer. On the other hand, the ferroelectric polarization reorients from [100] to [001] direction in a dislocation-free ${\mathrm{BaTiO}}_3$, inducing the maximum achievable in-plane compressive strain of 1.1%. A large fraction of this uniaxial strain is transferred to the magnetoelastically coupled ferromagnetic layers whose magnetization switches to in plane via the inverse magnetostriction effect. This work reveals the critical role of the interplay between twin walls and dislocations within a ferroelectric substrate in the performance of multiferroic heterostructures and provides insight into the development of highly energy-efficient magnetoelectric devices.