We report on a theoretical study of electronic and magnetic properties of hydrogen-saturated InSe nanoribbons (H-ZISNs). Based on hybrid-functional first-principles calculations, we find that H-ZISNs exhibit tunable half-metallicity and short-range ferromagnetic order. We first show that $p$-type doping turns narrow H-ZISNs from semimetal to half-metal with spin polarization along the In-terminated edge. This behavior is further analyzed in terms of a two-band tight-binding model, which provides a tractable description of the H-ZISN electronic structure, and serves as a starting point for the determination of magnetic interactions. The dominant exchange interaction determined within the Heisenberg model is found to be ferromagnetic independently of the ribbon width and charge doping. Short-range stability of magnetic order is assessed in terms of the zero-field spin correlation length, which is found to be about 1 nm at liquid nitrogen temperatures. Finally, by calculating spin-dependent transport properties, we find a doping regime in which strongly spin-selective electrical conductivities can be observed. Our findings suggest that H-ZISNs are appealing candidates for the realization of spintronic effects at the nanoscale.

With a goal to improve upon the mechanical properties of the MAX phase, materials of high technological interest, we explore boron substitution in these compounds. Employing first-principles density functional theory (DFT) calculations, combined with continuum modeling to access the core structure of dislocations, we investigate the effect of boron-substitution on plastic deformation properties of a typical MAX phase compound, ${\mathrm{V}}_2\mathrm{AlC}$. Our $T=0$ K results show that, due to the differential nature of chemical bonding between V and B compared to that between V and C, both V-Al and V-B basal slip planes get activated in boron-substituted compounds, compared to only V-Al basal slip in the parent compound. This, in turn, makes the boron compounds significantly more ductile compared to their carbide counterpart ${\mathrm{V}}_2\mathrm{AlC}$. The computation of temperature-dependent free energies of stable and unstable stacking faults further reveals the important and interesting role of thermal fluctuations in the deformation behavior of the boride compounds at elevated temperature. This suggests a change in the microscopic mechanism of plastic deformation upon varying temperature condition in the proposed boron compounds. Our study should motivate future exploration of boron-based MAX compounds.

Graphene on silicon carbide (SiC) bears great potential for future graphene electronic applications because it is available on the wafer scale and its properties can be custom tailored by inserting various atoms into the graphene/SiC interface. It remains unclear, however, how atoms can cross the impermeable graphene layer during this widely used intercalation process. Here we demonstrate that in contrast to the current consensus, graphene layers grown in argon atmosphere on SiC are not homogeneous, but instead are composed of domains of different crystallographic stacking as they have been observed in other systems. We show that these domains are intrinsically formed during growth and that dislocations between domains dominate the (de)intercalation dynamics. Tailoring these dislocation networks, e.g., through substrate engineering, will increase the control over the intercalation process and could open a playground for topological and correlated electron phenomena in two-dimensional superstructures.

Under normal conditions, bulk crystals of ${\mathrm{BiScO}}_3$, ${\mathrm{BiCrO}}_3$, ${\mathrm{BiMnO}}_3$, ${\mathrm{BiFeO}}_3$, and ${\mathrm{BiCoO}}_3$ present three very different variations of the perovskite structure: an antipolar phase, a rhombohedral phase with a large polarization along the space diagonal of the pseudocubic unit cell, and a supertetragonal phase with even larger polarization. With the aim of understanding the causes for this variety, we have used a genetic algorithm to search for minima in the energy surface of these materials. Our results show that the number of these minima is very large when compared to that of typical ferroelectric perovskites like ${\mathrm{BaTiO}}_3$ and ${\mathrm{PbTiO}}_3$, and that a fine energy balance between them results in the large structural differences seen. As byproducts of our search we have identified charge-ordering structures with low energy in ${\mathrm{BiMnO}}_3$, and several phases with energies that are similar to that of the ground state of ${\mathrm{BiCrO}}_3$. We have also found that an inverse supertetragonal phase exists in bulk, likely to be favored in films epitaxially grown at large values of tensile misfit strain.

The unique functionalities present at surfaces and interfaces of complex oxides have attracted intense research in the past decade. Yet, the fundamental mechanisms underpinning functionality are often elusive, especially in doped manganites, limiting their implementation in functional electronic devices such as memristors and spin valves. Here, we present a local probe-based study on mixed-terminated $\mathrm{L}{\mathrm{a}}_{5/8}\mathrm{C}{\mathrm{a}}_{3/8}\mathrm{Mn}{\mathrm{O}}_3$ (LCMO) films, and reveal surface metallicity in a thin film grown by pulsed-laser deposition. Using first-principles density-functional theory calculations with Hubbard correction that are more accurate to capture effects of correlation in these systems, we show that for Ca-segregated (001) LCMO surfaces the (La,Ca)O-site terminated surfaces are half metallic due to delocalized Mn-$d$ states populating the Fermi level, whereas the ${\mathrm{MnO}}_2$-site terminated surfaces exhibit a half-metallic or insulating character depending on the type of surface reconstruction. Computations not only explain the current measurements, but also explain other recent surface measurements on LCMO thin films, leading to a coherent picture of how the crucial link between surface segregation and Jahn-Teller couplings in the manganese oxides tune the surface electronic/magnetic structure, thereby pointing to the fine control of transport and magnetism at the conductive oxide surface independent of the bulk.

${\mathrm{Mg}}_2{X}_{1-x}{Y}_x$ thermoelectric materials and their pseudobinary related alloys, for $X,Y$ = Si,Ge,Sn, have been the subject of intense research activity. Studies have revealed their electronic nature, but important properties are still unclear or missing in the case of intermediate solid solutions. Within the CPA-KKR formalism, in the full-relativistic description, we observe stronger deviations from Vegard's law as the lattice constant difference increases. We compute the Bloch spectral density function to map a local-parabolic band structure in the alloys, whenever the disorder-induced broadening is small. We trace nonlinear trends for the indirect and direct band gaps, spin-orbit coupling and the crossing between the low-lying conduction bands observed in the ${\mathrm{Mg}}_2{X}_{1-x}{\mathrm{Sn}}_x$ systems. Our computations show that the broadening of the heavy- and light-hole bands is the smallest, but, in either case, we discuss the trends and anisotropy effects of band- and direction-dependent effective masses.

We examine the effects of strain and cation substitution on the superconducting phase of polar semiconductors near a ferroelectric quantum phase transition with a model that combines a strong coupling theory of superconductors with a standard microscopic framework for displacive polar modes coupled to strain degrees of freedom. Our calculations reveal that the superconducting transition temperature $T_c$ is enhanced by proximity to the ferroelectric instability from the disordered side, while it is generally suppressed in the ordered phase due to its increase in dielectric stiffness and a reduction of critical fluctuations from dipolar induced anisotropies. The condensation of the pairing phonon excitations generates a kink in $T_c$ at a charge density that is generally lower than that of the quantum critical point (QCP) and where both superconducting and ferroelectric orders set in. We apply our model to ${\mathrm{SrTiO}}_3$ and find that the antiadiabatic limit places the kink nearly at its QCP. As the QCP is pushed to higher charge densities with either tuning parameter, we find that the dome narrows and sharpens. Our model is in qualitative and fair quantitative agreement with the recent observation of overlapping ferroelectric-like and superconducting instabilities in n-doped ${\mathrm{Sr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{TiO}}_3$ and strain tuning of $T_c$ in n-doped ${\mathrm{SrTiO}}_3$. We compare our results to previous models invoking order-disorder lattice dynamics to describe the pairing excitations.

Wannier tight-binding models are effective models constructed from first-principles calculations. As such, they bridge a gap between the accuracy of first-principles calculations and the computational simplicity of effective models. In this work, we extend the existing methodology of creating Wannier tight-binding models from first-principles calculations by introducing the symmetrization post-processing step, which enables the production of Wannier-like models that respect the symmetries of the considered crystal. Furthermore, we implement automatic workflows, which allow for producing a large number of tight-binding models for large classes of chemically and structurally similar compounds or materials subject to external influence such as strain. As a particular illustration, these workflows are applied to strained III-V semiconductor materials. These results can be used for further study of topological phase transitions in III-V quantum wells.

We study the electronic structure and magnetic properties of epitaxial $\left(\mathrm{N}{\mathrm{i}}_{1–x}\mathrm{C}{\mathrm{o}}_x\right)\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_4$(111) layers $\left(x=0–1\right)$ with thicknesses $d=1.7–5.2\mathrm{nm}$ grown on $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$(111)/Si(111) structures, to achieve a high value of inversion parameter $y$ which is the inverse-to-normal spinel-structure ratio, and hence to obtain good magnetic properties even when the thickness is thin enough for electron tunneling as a spin filter. We revealed the crystallographic (octahedral $O_h$ or tetrahedral $T_d)$ sites and the valences of the Fe, Co, and Ni cations using experimental soft x-ray absorption spectroscopy and x-ray magnetic circular dichroism spectra and configuration-interaction cluster-model calculation. In all the $\left(\mathrm{N}{\mathrm{i}}_{1–x}\mathrm{C}{\mathrm{o}}_x\right)\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_4$ layers with $d\sim 4$ nm, all Ni cations occupy the $\mathrm{N}{\mathrm{i}}^{2+}\left(O_h\right)$ site, whereas Co cations occupy the three different $\mathrm{C}{\mathrm{o}}^{2+}\left(O_h\right),\mathrm{C}{\mathrm{o}}^{2+}\left(T_d\right)$, and $\mathrm{C}{\mathrm{o}}^{3+}\left(O_h\right)$ sites with constant occupancies. According to these features, the occupancy of the $\mathrm{F}{\mathrm{e}}^{3+}\left(O_h\right)$ cations decreases and that of the $\mathrm{F}{\mathrm{e}}^{3+}\left(T_d\right)$ cations increases with decreasing $x$. Consequently, we obtained a systematic increase of $y$ with decreasing $x$ and achieved the highest $y$ value of 0.91 for the $\mathrm{NiF}{\mathrm{e}}_2{\mathrm{O}}_4$ layer with $d=3.5\mathrm{nm}$. From the $d$ dependences of $y$ and magnetization in the $d$ range of 1.7–5.2 nm, a magnetically dead layer is present near the $\mathrm{NiF}{\mathrm{e}}_2{\mathrm{O}}_4/\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ interface, but its influence on the magnetization was significantly suppressed compared with the case of $\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4$ layers reported previously [Wakabayashi et al., Phys. Rev. B 96, 104410 (2017)], due to the high site selectivity of the Ni cations. Since our epitaxial $\mathrm{NiF}{\mathrm{e}}_2{\mathrm{O}}_4$ layer with $d=3.5\mathrm{nm}$ has a high $y$ value (0.91) and a reasonably large magnetization, it is expected to exhibit a strong spin-filter effect which can be used for efficient spin injection into Si.

The advancement of transparent electronics, one of the most anticipated technological developments for the future, is currently inhibited by a shortage of high-performance $p$-type semiconductors. Recent demonstration of tin monoxide as a successful transparent $p$-type thin-film transistor and the discovery of its potential for ambipolar doping, suggests that tin monoxide—an environmentally friendly earth-abundant material—could offer a solution to this challenge. With the aim of enhancing the electronic properties, an extensive search for useful dopant elements was performed. Substitutional doping with the family of alkali metals was identified as a successful route to increase the concentration of acceptors in SnO and over ten shallow donors, which, to the best of our knowledge, have not been previously contemplated, were discovered. This work presents a detailed analysis of the most promising $n$-/$p$-type dopants—offering new insights into the design of an ambipolar SnO. If synthesized successfully, such a doped ambipolar oxide could open new avenues for many transparent technologies.

The interfacial contact between active material and solid electrolyte in a composite electrode limits the kinetics of all-solid-state batteries (ASSB). Despite the progress in processing techniques to improve cohesion in composite electrodes, the electrochemical reactions and mechanical stresses developed during battery operation affects interface properties. Here, we propose a one-dimensional radially symmetric analytical model based on the cohesive theory of fracture, to investigate the mechanical stability of interfaces in ASSB microstructures. Using the cohesive-energy approach, we analyze the delamination criterion and derive a stability condition for fracture propagation. Furthermore, we investigate the role of particle size and material properties on delamination, and we explore the effect of delamination on area-specific impedance. We report that delamination is induced when electrode particles undergo a volumetric change of about $7.5\%$ during (de)intercalation. Compliant electrolytes $(E<25\mathrm{GPa})$ are found to accommodate up to $25\%$ of particle volume change and delay the onset of delamination. The study identifies geometric regimes for mechanical stability. Such regimes are based on the relative size of the damage zone with respect to the particle radius. Finally, we demonstrate that delamination can significantly influence the total charge/discharge time if highly conductive electrolytes are employed. Overall, the analyses provide guidelines for engineering electrode-electrolyte interfacial properties by controlling particle size, material stiffness, and adhesive strength and length scale.

The interaction between dislocations and point defects is crucial for many physical properties and phenomena in materials, such as strengthening mechanisms, dislocation bias and void swelling, creep, and impurity segregation around dislocations. Conventional dislocation-point defect interaction models use approximations based on elasticity theory and/or based on assumptions that transition states energies can be deduced from the binding energies of the defects. In this paper, we present the transport properties of point defects near dislocations based on the actual saddle-point configuration of vacancies and self-interstitial atoms as a function of position with respect to screw and edge dislocations in a model system (bcc iron) using the self-evolving atomistic kinetic Monte Carlo. KMC simulations reveal defect dynamics near dislocations are highly anisotropic and correlated, particularly for dumbbells in the compressive field of the edge dislocation, which could result in zones near dislocation cores where dumbbells are less efficiently absorbed compared with vacancies. This study provides accurate saddle-point configurations and energies required to properly describe the dynamics of point defects around dislocations, allowing fundamental insights on the transport mechanisms which are essential for understanding microstructural evolution and mechanical properties of metallic materials.

Cathodoluminescence (CL) spectra were measured to determine the characteristics of luminescence bands and carrier dynamics in $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$ bulk single crystals. The CL emission was found to be dominated by a broad UV emission peaked at 3.40 eV, which exhibits strong quenching with increasing temperature; however, its spectral shape and energy position remain virtually unchanged. We observed a superlinear increase of CL intensity with excitation density; this kinetics of carrier recombination can be explained in terms of carrier trapping and charge transfer at Fe impurity centers. The temperature-dependent properties of this UV band are consistent with weakly bound electrons in self-trapped excitons with an activation energy of $48\pm 10\mathrm{meV}$. In addition to the self-trapped exciton emission, a blue luminescence (BL) band is shown to be related to a donor-like defect, which increases significantly in concentration after hydrogen plasma annealing. The point defect responsible for the BL, likely an oxygen vacancy, is strongly coupled to the lattice exhibiting a Huang-Rhys factor of $\sim 7.3$.

Low-cost chemical engineering of two-dimensional layered hybrid halide perovskite structures allows for the design of hybrid semiconductor quantum wells with tailored room-temperature excitonic optical absorption, emission, and charge carrier transport properties. Here density functional theory and the Bethe-Salpeter equation are used to predict the electronic structure and optical response of layered perovskites with two representative single-ring conjugated organic spacers, ammonium-propyl-imidazole (API) and 2-phenethylammonium (PEA). The inorganic perovskite quantum well properties are further tuned by analyzing the effect of halogen ($X$ = I, Br, Cl) substitution. We found that visible light absorption occurs primarily within the perovskite layer and that UV light absorption induces partial electron-hole separation between layers. In addition, a strong exciton binding energy and influence on absorption spectrum is found by solving the Bethe-Salpeter equation. Our results suggest that further engineering is necessary beyond the single-ring limit, by introducing more conjugated rings and/or heavier nuclei into the organic spacer. This is a promising future direction to achieve photoinduced charge separation and more generally hybrid heterostructures with attractive optoelectronic properties.

Semimetallic ${\mathrm{MoTe}}_2$ has recently generated enormous attention due to its topological properties, large magnetoresistance, superconductivity, suitability for homojunction phase patterning, and, in particular, metal-insulator transition of thin layers, possibly indicating a quantum spin hall state. These observations prove the potential of ${\mathrm{MoTe}}_2$ for thin film applications and call for systematic investigations of the thickness dependent electronic structure. Here we apply angle-resolved photoemission spectroscopy supported by band structure calculations to elucidate the electronic structure of exfoliated $1{\mathrm{T}}^{\prime }\text{−}{\mathrm{MoTe}}_2$. Electron and hole pockets of the inverted conduction and valence bands near $\mathrm{\Gamma }$ are resolved down to the monolayer. The Fermi level of exfoliated $1{\mathrm{T}}^{\prime }\text{−}{\mathrm{MoTe}}_2$ monolayers lays within the electron pockets indicating intrinsic $n$-type doping. $E_F$ shifts downward with increasing thickness consistent with a surface driven mechanism. Our study provides insight on the electronic properties of semimetallic $1{\mathrm{T}}^{\prime }\text{−}{\mathrm{MoTe}}_2$ as an indispensable ingredient for future thin film functionalization.

Helimagnetic materials are identified as promising for novel spintronic applications. Since helical spin order is manifested as a compromise of competing magnetic exchange interactions, its emergence is limited by unique constraints imposed by the crystalline lattice and the interaction geometries as exemplified by the multiferroic perovskite manganites with large orthorhombic distortion. Here we show that a simple cubic perovskite ${\mathrm{SrCoO}}_3$ with room-temperature ferromagnetism has the potential to host helimagnetic order upon isotropic lattice expansion. Increasing the Ba content $x$ in ${\mathrm{Sr}}_{1-x}{\mathrm{Ba}}_x{\mathrm{CoO}}_3$ continuously expands the cubic lattice, eventually suppressing the ferromagnetic order near $x=0.4$ where helimagnetic correlations are observed as incommensurate diffuse magnetic scattering by neutron-diffraction measurements. The emergence of helimagnetism is semiquantitatively reproduced by first-principles calculations, leading to the conjecture that a simple cubic lattice with strong $d\text{−}p$ hybridization can exhibit a variety of novel magnetic phases originating from competing exchange interactions.