We present a compositional and structural investigation of silicene, germanene, and stanene bilayers from first principles. Due to the staggering of the individual layers, several stacking patterns are possible, most of which are not available to the bilayer graphene. This structural variety, in conjunction with the presence of the spin-orbit coupling, unveils a diversity of the electronic properties, with the appearance of distinct band features, including orbital hybridization and band inversion. We show that for particular cases, the intrinsic spin Hall response exhibits signatures of nontrivial electronic band topology, making these structures promising candidates to probe Dirac-like physics.

In this paper, we use a multiscale approach to describe a realistic model of a permanent magnet based on MnAl $\tau$-phase and elucidate how the antiphase boundary defects present in this material affect the energy product. We show how the extrinsic properties of a microstructure depend on the intrinsic properties of a structure with defects by performing micromagnetic simulations. For an accurate estimation of the energy product of a realistic permanent magnet based on the MnAl $\tau$-phase with antiphase boundaries, we quantify exchange interaction strength across the antiphase boundary defect with a simple approach derived from first-principles calculations. These two types of calculations, performed at different scales, are linked via atomistic spin-dynamics simulations.

We demonstrate a route to prepare thin films of the ferromagnetic insulator europium monoxide. Key is a redox-controlled interface reaction between metallic Eu and the substrate ${\mathrm{SrTiO}}_3$ as the supplier of oxygen. The process allows tuning the electronic, magnetic, and structural properties of the EuO films. Furthermore, we apply this technique to various oxidic substrates and demonstrate the universality and limits of a redox-controlled EuO film synthesis.

Ionizing events can lead to panoply of irradiation effects, and in silicon carbide (SiC), they drastically modify the defect production rate or the initial density. To better understand this phenomenon, 6H-SiC single crystals were first predamaged using low-velocity 100-keV $\mathrm{F}{\mathrm{e}}^{+}$ ions at three fluences in the range of ${10}^{14}\mathrm{c}{\mathrm{m}}^{-2}$ to induce three different initial disorder levels peaking at values between ∼0.8 and 1 (1 corresponding to full amorphization). Crystals were then submitted to swift heavy ion irradiation in the ${10}^{13}\mathrm{c}{\mathrm{m}}^{-2}$ fluence range at both low (∼100 K) and high (∼770 K) temperature. Rutherford backscattering spectrometry in channeling conditions revealed that swift ions allow annealing part of the initial damage, the recovery efficiency increasing with the irradiation temperature and reaching 75% in initially severely disordered crystals. This temperature effect has been qualitatively predicted by molecular dynamics simulations. Transmission electron microscopy allowed imaging both the recovery and the difference in the microstructure of the layers irradiated at low or high temperature. Recovery cross sections are found to lie in the range of a few square nanometers, consistent with previously reported values. A scenario for a general, two-step annealing mechanism referred to as an ionization-activated, thermally assisted defect-annealing (IATADA) process is proposed. This mechanism rationalizes the diverse descriptions reported so far in the literature.

We report the high-pressure, high-temperature synthesis of $\mathrm{BiV}{\mathrm{O}}_3$, a cubic perovskite that thus far has remained inaccessible under ambient pressure conditions. We created this material at $\sim 25$ GPa and 1500 K in a laser-heated diamond-anvil cell and recovered it to ambient pressure and temperature. Our synthetic approach circumvents the oxidative chemistry that, at ambient pressures, previously rendered the cubic $\mathrm{BiV}{\mathrm{O}}_3$ perovskite inaccessible. We find through density-functional theory calculations that this material is a unique metallic and antiferromagnetic transition-metal oxide.

A machine-learning approach that we term the “stochastic replica voting machine” (SRVM) algorithm is presented and applied to a binary and a three-class classification problem in materials science. Here, we employ SRVM to predict candidate compounds capable of forming stable perovskites and double perovskites and further classify binary ($AB$) solids. The results of our binary and ternary classifications compared well to those obtained by SVM and neural network algorithms.

We examine magnetic domain patterns in symmetric ${[\mathrm{Co}/\mathrm{Ni}]}_M$ and asymmetric ${[\mathrm{Pt}/{(\mathrm{Co}/\mathrm{Ni})}_M/\mathrm{Ir}]}_N$ multi-layers by using Fresnel mode Lorentz transmission electron microscopy (LTEM). In the symmetric multi-layers, where the Dzyaloshinskii–Moriya interaction is expected to be negligible, we observe Bloch-type domain walls with no preferred chirality. In the asymmetric multi-layers, where significant interfacial DMI is present, we observe domain patterns with chiral Néel domain walls, which evolve into sub-100 nm isolated Néel Skyrmions with the application of a perpendicular field at room temperature. The impact of layer thickness and film stack on interfacial magnetic properties is discussed in the context of developing a tunable multi-layer system for future spintronic applications.

We successfully synthesized the verdazyl-based salts ${(m-\mathrm{MePy}-\mathrm{V})}_2{\mathrm{Fe}}_2{\mathrm{OCl}}_6$ and ${[m-\mathrm{MePy}-\mathrm{V}-{(p-\mathrm{Cl})}_2]}_2{\mathrm{Fe}}_2{\mathrm{OCl}}_6·{\left({\mathrm{CH}}_3\mathrm{CN}\right)}_2$ with different ${\mathrm{Fe}}_2{\mathrm{OCl}}_6^{2-}$ structures. The exchange interactions between ${\mathrm{Fe}}^{3+}$ ions in the ${\mathrm{Fe}}_2{\mathrm{OCl}}_6^{2-}$ anions were evaluated to be nearly equivalent and to form an $S=5/2$ antiferromagnetic (AF) dimer in each salt. We obtained the magnetic susceptibility of $S=1/2$ spins on the radical by subtracting the contribution of $S=5/2$ spins in the anion from the observed magnetic susceptibility. For (${m-\mathrm{MePy}-\mathrm{V})}_2{\mathrm{Fe}}_2{\mathrm{OCl}}_6$, ab initio molecular orbital (MO) calculations indicated that the $S=1/2$ AF dimers are weakly connected by ferromagnetic interactions, thereby forming a honeycomb lattice. The experimentally measured magnetic susceptibility and specific heat indicate a corresponding nonmagnetic gapped behavior. For $[m-\mathrm{MePy}-\mathrm{V}-{(p-\mathrm{Cl})}_2{]}_2{\mathrm{Fe}}_2{\mathrm{OCl}}_6·{\left({\mathrm{CH}}_3\mathrm{CN}\right)}_2$, MO calculations indicated that four types of dominant interactions form an unconventional $S=1/2$ honeycomb-based 3D spin model. The magnetic susceptibility and specific heat indicate a phase transition to a 3D order at approximately 4.8 K.

Nanoindentation and electron backscattered diffraction techniques are used to study the dependence of crystallographic orientation on mechanical properties in individual grains of the superelastic polycrystalline Ti-24Nb-4Zr-8Sn alloy. Strain recovery, elastic modulus, and hardness are evaluated from the force-displacement curves using spherical and Berkovich indenters. Experimental data are reported in a standard stereographic triangle covering all possible crystallographic directions of loading in the bcc-β structure of the Ti-24Nb-4Zr-8Sn alloy. Our experiments show that both spherical and Berkovich indenters are suitable for probing anisotropy of elastic modulus whereas spherical indenter is more appropriate for probing anisotropy of superelastic response. The highest indentation strain recovery is measured along $\langle 001{\rangle }_{\beta }$ and the lowest one is observed along $\langle 111{\rangle }_{\beta }$. Our inverse pole figure distribution of indentation strain recoveries is in good agreement with inverse pole figure distribution of compressive lattice distorsions calculated from the crystallographic model of martensitic transformation in β titanium alloys. This established pattern in compression is very different from that seen during tensile deformation, and both our experiments and calculations confirm the strong tension-compression asymmetry of the strain response in superelastic β titanium alloys. The indentation modulus also shows significant crystallographic anisotropy: $E_{\left[001\right]\beta }<E_{\left[101\right]\beta }<E_{\left[111\right]\beta }$. In contrast, orientation dependence is lost when plasticity has set in, and hardness becomes independent of crystallographic orientation due to the fact that multiple directions are prone to generate slip systems in the bcc-β structure.

We report on the growth and transport properties of strained thin films of the three-dimensional Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$. Epitaxial heterostructures, consisting of (112)-oriented ${\mathrm{Cd}}_3{\mathrm{As}}_2$ films, are grown on nearly lattice matched $\left({\mathrm{Ga}}_{1-x}{\mathrm{In}}_x\right)\mathrm{Sb}$ buffer layers on (111) GaAs substrates by molecular beam epitaxy. The epitaxial coherency strain breaks the fourfold rotational symmetry, which protects the bulk Dirac nodes in ${\mathrm{Cd}}_3{\mathrm{As}}_2$. All strained films exhibit the quantum Hall effect with most carriers residing in the two-dimensional states, irrespective of the sign of the biaxial stress. The Hall mobility monotonically increases as the biaxial stress is changed from compressive towards tensile. Furthermore, pronounced anisotropy is seen in the transport properties. The results show that the quantum Hall effect, which is quite similar to that of unstrained (112)-oriented films, is independent of the presence of bulk Dirac nodes. Its appearance is consistent with the topological surface states that are a characteristic of the topological ${\mathbb{Z}}_2$ invariant.

Shear-coupled grain boundary (GB) migration has been evidenced as an efficient mechanism of plasticity in the absence of dislocation activity. The GB migration occurs through the nucleation and motion of disconnections. Using molecular simulations, we report a detailed study of the elementary mechanisms occurring during heterogeneous disconnection nucleation. We study the effect of a preexisting sessile disconnection in a symmetric $\mathrm{\Sigma }17\left(410\right)\left[001\right]$ tilt GB on the GB migration mechanism. Shearing this imperfect GB induces its migration and reveals a new GB migration mechanism through the nucleation of a mobile disconnection from the sessile one. Energy barriers and yield stress for the GB migrations are evaluated and compared to the migration of a perfect GB. We show that the migration of the imperfect GB is easier than the perfect one and that a sessile disconnection can operate as a source of disconnection driving the GB migration. This GB migration mechanism has been observed on two other high-angle GBs.

We report an improved reversibility of magnetostriction and inverse magnetocaloric effect (MCE) for the magnetic shape-memory Heusler alloy ${\mathrm{Ni}}_{1.8}{\mathrm{Mn}}_{1.8}{\mathrm{In}}_{0.4}$. We show that the magnetostriction and MCE crucially depends on the geometrical compatibility of the austenite and martensite phases. Detailed information on the compatibility of both phases has been obtained from the transformation matrix calculated from x-ray diffraction data. The uniqueness of the lattice parameters results in an improved reversibility of the magnetostriction and the MCE. In the thermal hysteresis region of the martensitic transformation, the maximum relative length change is 0.3% and the adiabatic temperature change $\mathrm{\Delta }{T}_{\mathrm{ad}}\approx -10$ K in pulsed magnetic fields. Our results reveal that the approach of geometric compatibility will allow one to design materials with reversible magnetostriction and reversible inverse MCE at a first-order magnetostructural phase transition in shape-memory Heusler alloys.

We describe an investigation of microstructure in pure nickel (Ni) after cathodic charging in an aqueous electrolyte. Intergranular corrosion occurs on the sample surface and takes the form of long trenches along coherent twin boundaries (CTBs): sites that have often been considered especially resistant to corrosion. Integrating electron backscatter diffraction and x-ray computed tomography, we show that the trenches are formed by the growth and eventual overlap of isolated conical cavities, which, in turn, are aligned with $\langle 110\rangle$-type directions within CTB planes. The $\langle 110\rangle$-type orientation is consistent with cavity initiation at high-energy, symmetric incoherent twin boundary facets within CTB planes. Our observations show that, far from being always corrosion resistant, CTBs are especially susceptible to intergranular corrosion under some conditions.

Spin state plays a key role in a plethora of physical, biological, and geological phenomena. In octahedral coordination, spin state of $3d$ transition metals can be high spin or low spin, and can be tuned with external stimuli. In a tetrahedral coordination environment, with the smaller crystal field splitting energy, only high spin is considered possible for oxides. Herein we report the realization of low spin state in tetrahedral coordination in a novel oxide, $\mathrm{S}{\mathrm{r}}_9\mathrm{I}{\mathrm{r}}_3{\mathrm{O}}_{17}$. The low spin configuration was confirmed with density functional theory (DFT) calculations, including the effect of spin-orbit coupling (SOC), electron-electron repulsion (U), magnetic data, and crystal chemical consideration. This exotic electronic configuration was realized with the confluence of extended $5d$ orbitals, short bond lengths, and an electron count maximizing the crystal field stabilization energy. The discovery of low spin state for a tetrahedral environment provides a novel platform for spin state manipulation in solid state materials.

The diamond and zinc-blende semiconductors are well-known and have been widely studied for decades. Yet, their electronic structure still surprises with unexpected topological properties of the valence bands. In this joint theoretical and experimental investigation, we demonstrate for the benchmark compounds InSb and GaAs that the electronic structure features topological surface states below the Fermi energy. Our parity analysis shows that the spin-orbit split-off band near the valence band maximum exhibits a strong topologically nontrivial behavior characterized by the ${\mathcal{Z}}_2$ invariants $(1;000)$. The nontrivial character is a consequence of the nonzero spin-orbit coupling and is imposed by the chosen constituents, in contrast to the conventional topological phase transition mechanism which relies on tuning parameters in the system Hamiltonian. Ab initio-based tight-binding calculations resolve topological surface states in the occupied electronic structure of InSb and GaAs, further confirmed experimentally by soft x-ray angle-resolved photoemission from both materials. Our findings are valid for all other materials whose valence bands are adiabatically linked to those of InSb, i.e., many diamond and zinc-blende semiconductors, as well as other related materials, such as half-Heusler compounds.

Magneto-ionic control of magnetism has garnered great interest in recent years due to the large magnetic changes that can be induced using a relatively small voltage. One model structure for this is $\mathrm{Pt}/\mathrm{Co}/\mathrm{Gd}{\mathrm{O}}_x/\mathrm{Au}$, where Co is the magnetic layer and $\mathrm{Gd}{\mathrm{O}}_x$ is the ionic conductor, with the magnetic properties dependent on the oxidation state of Co. While this structure is commonly used, there is limited understanding of the effect of $\mathrm{Gd}{\mathrm{O}}_x$ properties on voltage-induced magnetic changes. In this work, we show that hydration of $\mathrm{G}{\mathrm{d}}_2{\mathrm{O}}_3$ to form $\mathrm{Gd}{\left(\mathrm{OH}\right)}_3$ is crucial for voltage-induced Co oxidation in a $\mathrm{Pt}/\mathrm{Co}/\mathrm{Gd}{\mathrm{O}}_x/\mathrm{Au}$ device. By examining the rate of Co oxidation in nonhydrated and hydrated devices, we conclude that ${\mathrm{H}}_2\mathrm{O}$ in the $\mathrm{Gd}{\mathrm{O}}_x$ layer acts as an oxidant during the voltage-induced Co oxidation process. Co oxidation through this interfacial reaction process is confirmed by in situ x-ray absorption spectroscopy.

Although concrete is the world's most widely used manufactured material, basic questions persist regarding the nature of its binding phase, namely, the calcium-silicate-hydrate (C-S-H) gel. Here, based on reactive molecular dynamics simulations, we address the following fundamental question: Is the chemical composition of C-S-H controlled by the kinetics or thermodynamics of its precipitation? As a major outcome of this study, we demonstrate that the average stoichiometry of the C-S-H gel forming upon the hydration of ordinary portland cement (i.e., Ca/Si = 1.75) is determined by the composition of the C-S-H phase presenting the fastest precipitation kinetics, rather than the highest thermodynamic stability.