Monoclinic ${\mathrm{Ga}}_2{\mathrm{O}}_3$ and ${\left({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\right)}_2{\mathrm{O}}_3$ alloys are wide-band-gap semiconductors with promising applications in power electronics. Although the physical properties of monoclinic ${\mathrm{Ga}}_2{\mathrm{O}}_3$ ($\beta$ phase) have been extensively explored, information is lacking about other phases ($\alpha , \gamma , \kappa$) of ${\mathrm{Ga}}_2{\mathrm{O}}_3$ and related alloys. Here we use density functional theory to assess the phase stability of different polymorphs of ${\mathrm{Ga}}_2{\mathrm{O}}_3$ and ${\left({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\right)}_2{\mathrm{O}}_3$ alloys at both zero and finite temperatures. Due to the preference of Al for the octahedral site, the $\gamma$ and $\kappa$ phases of ${\left({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\right)}_2{\mathrm{O}}_3$ exhibit a minimum enthalpy of formation at 62.5% and 50% Al concentrations, respectively. Relative to the other phases, the enthalpy of formation of the $\gamma$ phase is the highest over the entire range of alloy compositions. We examined the effect of strain arising from pseudomorphic growth of (010)-oriented ${\left({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\right)}_2{\mathrm{O}}_3$ films on $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$ substrates and found that the alloys become less stable. At finite temperature we found that the lattice vibrations tend to stabilize the $\kappa$ phase and destabilize the $\alpha$ and $\gamma$ phases, referenced to the monoclinic phase. This can be attributed to the greater phonon density of states of the $\kappa$ phase at low frequencies. A unique configurational entropy that is present in the $\gamma$ phase due to the cation vacancy disorder plays a substantial contribution in stabilizing the $\gamma$ phase at finite temperature, particularly at 50% Al concentration. Our study provides a comprehensive overview of stability of different phases of ${\left({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\right)}_2{\mathrm{O}}_3$, offering insights into the driving forces for polymorph formation that should prove useful for improved control over phase-pure growth of these important alloys.

We use a convolutional, recurrent neural network approach to learn morphological evolution driven by surface diffusion. To this aim we first produce a training set using phase field simulations. Intentionally, we insert in such a set only relatively simple, isolated shapes. After proper data augmentation, training and validation, the model is shown to correctly predict also the evolution of previously unobserved morphologies and to have learned the correct scaling of the evolution time with size. Importantly, we quantify prediction uncertainties based on a bootstrap-aggregation procedure. The latter proved to be fundamental in pointing out high uncertainties when applying the model to more complex initial conditions (e.g., leading to the splitting of high aspect-ratio individual structures). The automatic smart augmentation of the training set and design of a hybrid simulation method are discussed.

We present a theoretical study of the excitonic band edge states applied to the more commonly studied inorganic perovskite nanocrystals: ${\mathrm{CsPbBr}}_3$. We highlight the key role played by the electron-hole exchange interaction, including long-range and short range terms, in the positioning and splitting of dark and bright exciton sublevels, and discuss the influence of dielectric confinement, crystal field and shape anisotropy effects on the excitonic fine structure. The electron-hole exchange interaction splits the four excitonic states into three bright excitonic states at higher energy and a singlet dark state at lower energy. We find that, whatever the crystal phase, the bright-dark splitting is weakly sensitive to shape anisotropy. However, a strong enhancement, of about 50%, is obtained at the maximum of the dielectric contrast. For two crystalline symmetries, we particularly analyze how the bright exciton triplet energies vary, taking into account the states polarizations and considering the directions along which shape elongation/contraction applies. The main effect of the dielectric contrast is to increase significantly the exciton energies, for all the configurations. For a cubic crystal—${\mathrm{O}}_h$ symmetry—the bright exciton triplet degeneracy can be partially or fully lifted with anisotropic shape along one or two directions, respectively. For a tetragonal crystal—${\mathrm{D}}_{4h}$ symmetry—the partial triplet degeneracy can be completely lifted by a single distortion from the cubic shape along a direction perpendicular to the tetragonal axis but a partial degeneracy is maintained for small distortions along the tetragonal axis. The amplitude of bright exciton splittings are basically driven by the shape anisotropy and is almost insensitive to the dielectric contrast. On the basis of this model, we discuss recent results on ${\mathrm{CsPbBr}}_3$ nanocrystals photoluminescence.

We present an atomistic insight into the processes leading to the formation of graphene on SiC(0001) surfaces by resorting to first-principles molecular dynamics empowered by free-energy sampling methods. Based on the experimental surface, consisting of terraces bordered by a sequence of steps, we find that Si atoms are dislodged from step edges and migrate toward more stable sites on the terrace, leaving behind C atoms carrying unsaturated chemical bonds. Our investigations reveal that subsequent Si atoms removal acts as a trigger to the formation of stable C-C bonds among these unsaturated C sites. This process eventually leads to the formation of C clusters which merge into larger structures with the typical pattern of graphene flakes. Specifically, a ${\mathrm{C}}_6$ ring formed during our simulations, assumes the typical hexagonal structure of graphene, becoming a precursor of larger graphene nanostructures. The characterization of the mechanisms and related free-energy landscapes provide an insight into the fundamental processes responsible for the realization of ordered C-based building blocks of graphene on the SiC(0001) surface.

Monolayer $\mathrm{Na}\mathrm{Cr}{\mathrm{X}}_2$ (X=Te,Se,S) was theoretically proposed to be a two-dimensional intrinsic ferromagnetic semiconductor, although its physical properties have not been thoroughly investigated in bulk single crystals. We report the single-crystal growth, structural, magnetic, and electronic transport properties of $\mathrm{Na}\mathrm{Cr}{\left({\mathrm{Te}}_{1-x}{\mathrm{Se}}_x\right)}_2$ ($0⩽x⩽1$) and $\mathrm{Na}\mathrm{Cr}{\mathrm{S}}_2$. For $\mathrm{Na}\mathrm{Cr}{\left({\mathrm{Te}}_{1-x}{\mathrm{Se}}_x\right)}_2$, the strong perpendicular magnetic anisotropy of $\mathrm{Na}\mathrm{Cr}{\mathrm{Te}}_2$ can be gradually tuned to be a nearly isotropic one by Se doping. Meanwhile, a systematic change in the conductivity with increasing $x$ is observed, displaying a doping-induced metal-insulator-like transition. Under magnetic field larger than 30 kOe, both $\mathrm{Na}\mathrm{Cr}{\mathrm{Te}}_2$ and $\mathrm{Na}\mathrm{Cr}{\mathrm{Se}}_2$ can be polarized to a ferromagnetic state. However, for $\mathrm{Na}\mathrm{Cr}{\mathrm{S}}_2$, robust antiferromagnetism is observed up to 70 kOe, and two field-induced metamagnetic transitions are identified along $H\parallel ab$. These intriguing properties together with the potential to be exfoliated down to few-layer thickness make $\mathrm{Na}\mathrm{Cr}{\mathrm{X}}_2$ (X=Te,Se,S) promising for exploring spintronic applications.

Verdazyl-based complexes, $\left[\mathrm{Zn}\right({\mathrm{hfac})}_2\left]\right(m\text{−}\mathrm{Py}\text{−}{\mathrm{V})}_2·{\mathrm{CH}}_3\mathrm{CN}$ and $\left[\mathrm{Cu}\right({\mathrm{hfac})}_2\left]\right[m\text{−}\mathrm{Py}\text{−}\mathrm{V}\text{−}({p\text{−}\mathrm{F})}_2{]}_2$, were successfully synthesized. For $\left[\mathrm{Zn}\right({\mathrm{hfac})}_2](m\text{−}\mathrm{Py}\text{−}{\mathrm{V})}_2·{\mathrm{CH}}_3\mathrm{CN}$, molecular orbital (MO) calculations indicated the formation of a spin-$\frac{1}{2}$ antiferromagnetic (AFM) chain with fourfold magnetic periodicity consisting of three types of exchange interactions. We quantitatively explained the magnetic properties based on the expected spin model. For $\left[\mathrm{Cu}\right({\mathrm{hfac})}_2\left]\right[m\text{−}\mathrm{Py}\text{−}\mathrm{V}\text{−}({p\text{−}\mathrm{F})}_2{]}_2$, MO calculations indicated the formation of a spin-$\frac{1}{2}$ spin lattice, topologically equivalent to distorted octagons arranged two-dimensionally. We reproduced the observed magnetic properties assuming a spin-$\frac{1}{2}$ AFM dimer of radical spins, with residual Cu spins exhibiting paramagnetic behavior. Furthermore, we evaluated the anisotropic $g$ values from the electron spin resonance absorption spectra, confirming that the low-temperature magnetic properties are mainly attributed to the Cu spins. These results demonstrate the realization of unconventional spin systems composed of organic radicals and transition metals and propose an effective method for spin arrangement design.

Inspired by the unique physical properties of two-dimensional (2D) V-based Janus dichloride monolayers with intrinsic ferromagnetism and high Curie temperature $T_c$, we investigate the electronic structure, spin-valley polarization, and magnetic anisotropy of a Janus 2H-VSeS monolayer in detail using first-principles calculations. The results show that the Janus 2H-VSeS monolayer exhibits a large valley polarization of 105 meV, high $T_c$ of 278 K, and in-plane magnetocrystalline anisotropy contributed by the $d_{x^2-y^2}/d_{xy}$ orbitals of V atoms. The biaxial strain ($-8\%<\varepsilon <8\%$) can effectively tune the magnetic moments of the V atom, valley polarization $\mathrm{\Delta }E, T_c$, and magnetocrystalline anisotropy energy of the Janus 2H-VSeS monolayer. The corresponding $\mathrm{\Delta }E$ and $T_c$ are adjusted from 72 to 106.8 meV and from 180 to 340 K, respectively. Due to the broken space- and time-reversal symmetries, opposite valley charge carriers carry opposite Berry curvatures, which leads to prominent anomalous Hall conductivity at the $K$ and $K^{\prime }$ valleys. The maximum modulation of Berry curvature can reach to 45% and 9.5% by applying the biaxial strain and charge carrier doping, respectively. The stable in-plane magnetocrystalline anisotropy and robust spontaneous valley polarization make the ferromagnetic Janus 2H-VSeS monolayer a promising material for achieving spintronic and valleytronic devices.

Single crystals of ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$ have been successfully grown using hydroxides flux. ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$ crystallize in the ${\mathrm{K}}_4\mathrm{Cd}{\mathrm{Cl}}_6\text{-type}$ structure with the space group $R\text{−}3c$ (no. 167). ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$ are electrically insulating with estimated activation gaps of 0.68 and 0.80 eV, respectively. ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$ show paramagnetic behavior down to 2 K. In this work, the observed effective moments, ${\mu }_{\mathrm{eff}}$, for ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ single crystals are $0.31{\mu }_{\mathrm{B}}$ for $H\perp c$ and $0.28{\mu }_{\mathrm{B}}$ for $H\left|\right|c$, which are much smaller than that of $0.49{\mu }_{\mathrm{B}}$ previously reported for the polycrystalline ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ samples. For ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$ single crystals, a much larger value of ${\mu }_{\mathrm{eff}}=0.57{\mu }_{\mathrm{B}}$ is observed as compared with ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ single crystals. The x-ray absorption spectroscopy and low-temperature specific heat data indicate that the Ir in ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ has an almost pure ${\mathrm{Ir}}^{5+}$ valence state, while the Ir in ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$ is slightly lower than +5. The estimated low limits of magnetic impurity ${\mathrm{Ir}}^{4+}$ are about ∼1.7% and ∼9.2% for ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$, respectively. These magnetic impurities are likely to fully explain the observed ${\mu }_{\mathrm{eff}}$ values for ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$ single crystals, supporting the $J=0$ ground state of ${\mathrm{Ir}}^{5+}$ in ${\mathrm{Sr}}_3\mathrm{NaIr}{\mathrm{O}}_6$ and ${\mathrm{Sr}}_3\mathrm{AgIr}{\mathrm{O}}_6$.

Defects have been regarded as playing a critical role in the functionalities of many solid dielectrics. However, the contribution of defects to the specific coupling between strain gradient and electric polarization (i.e., flexoelectricity) has not yet been thoroughly understood. Herein, we selected the typical ferroelectric ${\mathrm{BaTiO}}_3$ (BTO) ceramics and introduced oxygen vacancies and trapped charge defects by using stoichiometric and nonstoichiometric Fe dopants, respectively. Compared with the pure BTO ceramics, the flexoelectric coefficients of stoichiometric Fe-doped BTO ceramics were increased by fivefold (from 9.5 to 65 μC/m) while that of the nonstoichiometric counterparts almost keep stable. The results show that the oxygen vacancies rather than trapped defects make a remarkable contribution to the enhancement of flexoelectricity, and this is explained by the reorientation of the defect dipoles formed by the oxygen vacancies. The result presented in this work not only benefits the understanding of the mechanism of flexoelectricity but also provides a feasible strategy to design flexoelectric materials and related devices with high flexoelectric coefficients.

${\mathrm{Fe}}_2{\left({\mathrm{MoO}}_4\right)}_3$ was synthesized and characterized as a prototype of L-type ferrimagnets (L-FiM) with an ordering temperature $T_{\mathrm{N}1}\sim 12\mathrm{K}$ using magnetic susceptibility $\left(\chi \right)$, specific heat $\left(C_{\mathrm{p}}\right)$, and dielectric $\left({\varepsilon }^{\prime }\right)$ anomaly. Two remarkable findings are magnetic field $\left(H\right)$ induced (i) an additional magnetic phase transition at $T_{\mathrm{N}2}$ below $T_{\mathrm{N}1}$; and (ii) the emergence of flexible ferroelectric polarization $\left(P\right)$ with the tuning parameters $H$ and $T$ below $T_{\mathrm{N}2}$. Thus, the H-T phase diagram for spin-induced type-II multiferroics was established. In contrast to some known multiferroics with a critical field-induced spin-flip $P$ below $T_{\mathrm{N}1}$, the observed multiferroic nature is exotic. The origin of the profound multiferroic nature may be hidden in the complex $T$-and $H$-dependent spin and lattice structures. Consequently, the schematic picture of $H$-induced possible change of lattice symmetry and conical spin structure has been proposed. More experimental and theoretical works providing solid evidence and interpretations have been suggested to explore these peculiar multiferroic phenomena.

We report a comprehensive study of the static susceptibility, high-field magnetization and high-frequency/high-magnetic field electron spin resonance (HF-ESR) spectroscopy of polycrystalline samples of the bismuth cobalt oxyphosphate ${\mathrm{BiCoPO}}_5$. This compound features a peculiar spin system that can be considered as antiferromagnetic (AFM) chains built of pairs of ferromagnetically coupled Co spins and interconnected in all three spatial directions. It was previously shown that ${\mathrm{BiCoPO}}_5$ orders antiferromagnetically at $T_{\mathrm{N}}\approx 10\mathrm{K}$ and this order can be continuously suppressed by magnetic field towards the critical value ${\mu }_0{H}_{\mathrm{c}}\approx 15$ T. In our experiments we find strongly enhanced magnetic moments and spectroscopic $g$ factors as compared to the expected spin-only values, suggesting a strong contribution of orbital magnetism for the ${\mathrm{Co}}^{2+}$ ions. This is quantitatively confirmed by ab initio quantum chemical calculations. Within the AFM ordered phase, we observe a distinct field-induced magnetic phase transition. Its critical field rises to $\sim 6$ T at $T\ll T_{\mathrm{N}}$. The HF-ESR spectra recorded at $T\ll T_{\mathrm{N}}$ are very rich comprising up to six resonance modes possibly of the multimagnonic nature that soften towards the critical region around 6 T. Interestingly, we find that the Co moments are not yet fully polarized at $H_{\mathrm{c}}$ which supports a theoretical proposal identifying $H_{\mathrm{c}}$ as the quantum critical point for the transition of the spin system in ${\mathrm{BiCoPO}}_5$ to the quantum disordered state at stronger fields.

It is well known that the conventional lattice flexibility of an epitaxial layer is restricted by its growth template and bulk counterpart. Here, we report that the interface-engineered ${(\mathrm{La},\mathrm{Ca})\mathrm{MnO}}_3$ layer can exhibit an anomalous lattice-tilting pattern, featured by the interaxis angle $\alpha$ that exceeds the range of the lattice flexibility mentioned earlier. By increasing the adjacent ${\mathrm{CaRuO}}_3$ layer thickness, the ${(\mathrm{La},\mathrm{Ca})\mathrm{MnO}}_3$ layer shows a decreasing $\alpha$ down to 89.25 °, which is out of the flexible range between 93.82 ° (from the ${\mathrm{CaRuO}}_3$ template) and 89.86 ° [from the ${(\mathrm{La},\mathrm{Ca})\mathrm{MnO}}_3$ bulk]. The resulting antiparallel lattice tilting makes the ${(\mathrm{La},\mathrm{Ca})\mathrm{MnO}}_3/{\mathrm{CaRuO}}_3$ interface similar to a crystal twinning plane to lower the interfacial energy raised by the structural discontinuity. Also, a monotonic reduction of magnetic coercivity (from 205 to 70 Oe) is observed on decreasing $\alpha$ (from 90 ° to 89.25 °) in ${(\mathrm{La},\mathrm{Ca})\mathrm{MnO}}_3$ layers, providing an additional approach to tunable magnetic properties without changing the epitaxial strain. Our results not only present a new lattice-engineering strategy of using the interface similar to a crystal-twinning plane in designing heterostructures, but also they reveal the application of such a strategy for tunable magnetic properties beyond the epitaxial strain.

In layered Na transition-metal (TM) oxides, TM migration usually occurs at highly charged states and severely deteriorates the capacity and reversibility. Meanwhile, the formation of hybrid phases with the intergrowth of octahedral (O-type) and prismatic (P-type) Na layers also takes place at highly charged states. These hybrid phases are often more stable than simple O3 or P3 stackings. However, little is known about the mechanism and impact of TM migration in these hybrid phases. In this work, a comparative first-principles study is performed to understand the connections between structural changes and Cr migration in layered O3 and hybrid-phased ${\mathrm{NaCrO}}_2$. After Cr migration, the hybrid-phased ${\mathrm{NaCrO}}_2$ suffers from greater layer shrinkage than the O3 phase. Three factors affect the Cr migration energy $E_{\mathrm{mig}}$: the Na concentration, local 3D configurations, and 2D in-plane geometries. Low Na concentrations and certain local 3D configurations facilitate the Cr migration. The Cr migration barriers in both O3 and hybrid-phased ${\mathrm{NaCrO}}_2$ are positively correlated with the $\mathrm{Cr}{E}_{\mathrm{mig}}$. The Cr migration in 17 doped O3 and hybrid-phased ${\mathrm{NaCrO}}_2$ is surveyed. In these doped ${\mathrm{NaCrO}}_2$, a more uniform distribution of the Cr–O bond lengths usually suggests suppressed Cr migration. Optimal dopants for suppressing Cr migration are identified by considering $E_{\mathrm{mig}}$ for both Cr and the dopant. Our comparative study on Cr migration in O3 and hybrid-phased ${\mathrm{NaCrO}}_2$ reveals the significant role of hybrid-phased structures in the development of layered cathode materials.

Misfit compounds are thermodynamically stable stacks of two-dimensional materials, forming a three-dimensional structure that remains incommensurate in one direction parallel to the layers. As a consequence, no true bonding is expected between the layers, with their interaction being dominated by charge transfer. In contrast to this well-established picture, we show that interlayer coupling can strongly influence the electronic properties of one type of layer in a misfit structure, in a similar way to the creation of modified band structures in an artificial moiré structure between two-dimensional materials. Using angle-resolved photoemission spectroscopy with a micron-scale light focus, we selectively probe the electronic properties of hexagonal ${\mathrm{NbSe}}_2$ and square BiSe layers that terminate the surface of the $({\mathrm{BiSe})}_{1+\delta }{\mathrm{NbSe}}_2$ misfit compound. We show that the band structure in the BiSe layers is strongly affected by the presence of the hexagonal ${\mathrm{NbSe}}_2$ layers, leading to quasi-one-dimensional electronic features. The electronic structure of the ${\mathrm{NbSe}}_2$ layers, on the other hand, is hardly influenced by the presence of the BiSe. Using density functional theory calculations of the unfolded band structures, we argue that the preferred modification of one type of band is mainly due to the atomic and orbital character of the states involved, opening a promising way to design electronic states that exploit the partially incommensurate character of the misfit compounds.

The van der Waals gap of two-dimensional (2D) materials provides natural ion diffusion channels for lithium. However, individual 2D materials (such as graphene, ${\mathrm{MoS}}_2$, and MXene) cannot offer all the properties needed to maximize the performance for energy storage. Heterostructures, by stacking different 2D materials, are promising for combining the advantages and eliminating the disadvantages of individual 2D materials. ${\mathrm{MoS}}_2$-based heterostructures have been prepared for lithium storage, while intercalation and diffusion mechanisms have yet to be elucidated. In the current work, the heterointerface effects on lithium intercalation and diffusion of seven $H$- and distorted $T$ $\left(\mathit{dT}\right)$-${\mathrm{MoS}}_2$-based heterostructures are systematically studied by density functional theory calculations, and highlight the atomistic origin of differences between $H$$\left(\mathit{dT}\right)$-${\mathrm{MoS}}_2$/graphene, $H$$\left(\mathit{dT}\right)$-${\mathrm{MoS}}_2/H$$\left(\mathit{dT}\right)$-${\mathrm{MoS}}_2$, and $H$$\left(\mathit{dT}\right)$-${\mathrm{MoS}}_2$/MXene hetero- and homostructures. From the energetic point of view, the heterointerface makes for an easier phase transition of ${\mathrm{MoS}}_2$ to the $\mathit{dT}$ phase in the ${\mathrm{MoS}}_2$/graphene heterostructure. Then we discuss the general idea of a rational design of the van der Waals gap for efficient lithium storage and diffusion. Our work opens a new avenue for optimizing the ion diffusion properties of 2D material heterostructures by designing the van der Waals channel for ion diffusion.

This paper reports on the mechanisms of CuO decomposition and its associated phase transformation into ${\mathrm{Cu}}_2\mathrm{O}$, as a fundamental step of thermite materials reaction, where CuO serves as the oxidizer. The Frenkel pair defects in perfect bulk CuO show extremely high formation energy (>4 eV) indicating that its decomposition initiates at defects/interfaces/surfaces, the latter being sensitive to surface orientation. In contrast to a variety of CuO surfaces [(111), (110), ($10\overline{1}$)] exhibiting three-fold coordinated oxygen atoms, results show that oxygen vacancy formation requires higher disordered surfaces, such as the CuO(001), on which the vacancy formation activation energy is reduced to 1.31 eV. This leads to the exothermic formation of a chemisorbed O-O peroxy-bridge complex at the surface (−1.25 eV adsorption) that thermodynamically moderates the backreaction (vacancy annihilation). Further desorption of molecular oxygen necessitates an activation of 1.53 eV, compatible with CuO decomposition observed experimentally at 600 K (∼second process duration). As a driving mechanism of oxygen release upon decomposition, migrations of the vacancy close to the surface and towards the bulk are determined for a number of crystalline directions and surface orientations and show considerable anisotropy, with two preferential directions: [110] and [001] with 1.35 and 1.77 eV activation, lowered to 1.08 and 1.03 eV activation when approaching the (111) and (001) surfaces, respectively. Finally, the CuO to ${\mathrm{Cu}}_2\mathrm{O}$ phase transformation follows a two-step process: a first structural modification from monoclinic to orthorhombic takes place when CuO has lost 12.5% of its oxygen atoms, followed by a barrierless transition to the cubic phase when 44–48% of the oxygen atoms have been removed, i.e., very close to stoichiometric ${\mathrm{Cu}}_2\mathrm{O}$.

In this work, the high-pressure and high-temperature (HPHT) method was applied to obtain crystalline tin disulfide (${\mathrm{SnS}}_2$). The synthesis was performed in a sulfur-tin-carbon growth system by placing the reaction mixture of sulfur and tin in a graphite capsule. The synthesized ${\mathrm{SnS}}_2$ demonstrated to be a highly crystalline $2H$-polytypic form suitable for mechanical exfoliation of atomically thin films. The crystals reveal strong near-IR photoluminescence attributed to a new luminescence center in ${\mathrm{SnS}}_2$. At 5 K, this center is characterized by a narrow zero-phonon line at 885 nm and phonon sideband whose structure is governed by ${\mathrm{SnS}}_2$ phonon density of states. Based on $\mathit{ab}$ initio calculations, the center is attributed to carbon in the tin sublattice. The side product of the HPHT synthesis is the diamond phase containing optically active tin-vacancy centers (${\mathrm{SnV}}^{-}$).

The emergence of a topological superconducting state in van der Waals heterostructures provides a new platform for exploring novel strategies to control topological superconductors. In particular, impurities in van der Waals heterostructures, generically featuring a moiré pattern, can potentially lead to the unique interplay between atomic and moiré length scales, a feature absent in generic topological superconductors. Here we address the impact of nonmagnetic impurities on a topological moiré superconductor, both in the weak and strong regime, considering both periodic arrays and single impurities in otherwise pristine infinite moiré systems. We demonstrate a fine interplay between impurity-induced modes and the moiré length, leading to radically different spectral and topological properties depending on the relative impurity location and moiré lengths. Our results highlight the key role of impurities in van der Waals heterostructures featuring moiré patterns, revealing the key interplay between length and energy scales in artificial moiré systems.

A comparison is presented between properties of Bi(111) films grown by Stranski-Krastanov epitaxy on Si(111) 7 $\times$ 7 and by van der Waals epitaxy on mica substrates. Thin film morphologies and electronic transport properties of Bi(111) films of variable thickness are investigated for each growth method. Atomic force micrographs for Bi(111) films on mica reveal clearly defined triangular regions consisting of layered steps with height 0.4 nm, corresponding to the Bi(111) bilayer height. Variable-temperature electronic transport measurements show the existence of a quantum confinement-induced energy gap in the film interiors, resulting in a semimetal-to-semiconductor transition. Magnetotransport analysis in a three-carrier model including metallic electrons in surface states and electrons and holes in the films' interiors provides a detailed study of densities, mobilities, and mean-free paths of the three carrier types. Improved electronic transport properties are found in Bi(111) films of higher thickness on mica compared to on Si(111), a likely result of the largely strain-free van der Waals epitaxial growth.