Strongly correlated ground states and exotic quasiparticle excitations in low-dimensional systems are central research topics in the solid-state research community. The present work develops a layered material and explores the physical properties. Single crystals of $3R\text{−}{\mathrm{Na}}_2\mathrm{MnTe}{\mathrm{O}}_6$ were synthesized via a flux method. Single-crystal x-ray diffraction and transmission electron microscopy reveal a crystal structure with ABC-type stacking and an $R\text{−}3$ space group, which establishes this material as a stacking polytype to previously reported $2H\text{−}{\mathrm{Na}}_2\mathrm{MnTe}{\mathrm{O}}_6$. Magnetic- and heat-capacity measurements demonstrate dominant antiferromagnetic interactions, the absence of long-range magnetic order down to 0.5 K, and field-dependent short-range magnetic correlations. A structural transition at $\sim 23$ K observed in dielectric measurements may be related to displacements of the Na positions. Our results demonstrate that $3R\text{−}{\mathrm{Na}}_2\mathrm{MnTe}{\mathrm{O}}_6$ displays low-dimensional magnetism, disordered structure and spins, and the system displays a rich structure variety.

Layer-by-layer stacking of two-dimensional materials results in van der Waals heterostructures (vdWH) of nanometer thickness and clean interfaces that often exhibit exceptional properties and present novel challenges. We perform first-principles calculations using density functional theory to examine the electronic properties of vdWH consisting of graphene (Gr) and semiconducting transition metal dichalcogenide (TMD) monolayers at several twist angles. We describe in detail our methodology for the creation of simulation cells which are almost free of strain due to lattice mismatch (less than 1%) and for unfolding the electronic bands of the vdWH in a way that allows for a straightforward comparison with the electronic structure of the constituent monolayers. The weak interlayer interactions in Gr/${\mathrm{MoS}}_2$ and Gr/${\mathrm{WS}}_2$ heterobilayers leave the Gr and TMD band structures almost unaffected but move the Fermi level closer to the TMD conduction band minimum. Careful examination of transitions from direct to indirect band gaps for some twist angles reveals that these are due to very small strain remaining in the simulation cells. In ${\mathrm{WS}}_2/{\mathrm{MoS}}_2$ and ${\mathrm{WSe}}_2/{\mathrm{MoSe}}_2$ heterobilayers, interlayer interactions do not affect the conduction band minimum at $\mathrm{K}$ but lead to eigenenergy splitting and eigenstate hybridization at the valence band around the $\mathrm{\Gamma }$ point, which is very sensitive to interlayer distance and determines whether the valence band maximum is at the $\mathrm{\Gamma }$ or $\mathrm{K}$ point. Our results unveil a small redshift of the intralayer electronic transitions of TMDs when interacting with either Gr or other TMD monolayers.

We report experimental studies of spin-wave excitations in individual 22-nm-thick ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ nanotubes with diameters of about 150 nm. We apply Brillouin light-scattering spectroscopy under microwave irradiation, and we resolve sets of discrete resonances in the center of nanotubes ranging from 2.5 to 12.5 GHz. Comparing to a recent theoretical work and micromagnetic simulations, we identify different characteristic eigenmodes depending on the axial, mixed, or vortex configuration. The mixed and vortex states give rise to modes with helical phase profiles substantiating an unusual nature of confined modes attributed to nonreciprocal spin waves. Our findings provide microscopic insight into realistic tubular spin-wave nanocavities and magnetochiral effects for three-dimensional nanomagnonics.

Bilayer topological insulator/ferromagnet (TI/FM) heterostructures are promising for spintronic applications due to their low switching energy and therefore power efficiency. Until recently, the reactivity of TIs with FM films was overlooked in the spin-orbit-torque literature, even though there are reports that it is energetically favorable for TIs to react with transition metals and form interfacial layers. In this study, we fabricated a TI/FM heterostructure comprised of molecular beam epitaxy grown ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ and dc sputtered ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$. Broadband ferromagnetic resonance revealed spin-pumping evident by the significant enhancement in Gilbert damping, which is likely a signature of the topological surface states or the presence of large spin-orbit coupling in the adjacent ${\mathrm{Sb}}_2{\mathrm{Te}}_3$. With low-temperature magnetometry, an exchange bias is observed that indicates an exchange interaction between an antiferromagnet (AFM) and an adjacent FM. Cross-section high-angle annular dark-field scanning transmission electron microscopy characterization of the ${\mathrm{Sb}}_2{\mathrm{Te}}_3\text{−}\mathrm{N}{\mathrm{i}}_{80}{\mathrm{Fe}}_{20}$ bilayer revealed a complex interface showing diffusion of Fe and Ni into the ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ film yielding the formation of a $\mathrm{Fe}{\mathrm{Te}}_2 1T\text{−}\mathrm{type}$ structural phase. Furthermore, density functional theory calculations revealed that the $\mathrm{Fe}{\mathrm{Te}}_2 1T\text{−}\mathrm{phase}$ has an AFM ground state. Due to experimental limitations in the electron-energy-loss spectroscopy measurements, the precise chemistry of the interfacial phase could not be determined, therefore it is possible that the $\mathrm{Fe}{\mathrm{Te}}_2 1T$ and/or an intermixed $\left({\mathrm{Fe}}_{1–x}{\mathrm{Ni}}_x\right){\mathrm{Te}}_2 1T$ is the AFM interfacial phase contributing to exchange bias in the system. This work emphasizes the chemical complexity of TI/FM interfaces that host novel, metastable magnetic topological phases and require more in-depth studies of other similar interfaces.

In this paper, we explored metastable phases showing large intrinsic spin Hall conductivity (SHC) in bulk tungsten (W) using a crystal structure search scheme based on an evolutionary algorithm (EA) and first-principles calculations. W has been known to have the most stable phase with the body-centered-cubic (bcc) structure $\left(\alpha \text{−}\mathrm{W}\right)$ and the metastable phase with a cubic $Pm\overline{3}n$ structure $\left(\beta \text{−}\mathrm{W}\right)$, the latter of which shows larger SHC in $\beta \text{−}\mathrm{W}$. Through the EA search, we obtained 15 metastable structures in addition to $\alpha \text{−}\mathrm{W}$ and $\beta \text{−}\mathrm{W}$ and found that orthorhombic $Fddd$ and $Cmcm$ structures show larger SHC at the Fermi level. The $Fddd$ structure is formed by adding a fourfold helical distortion along the [110] direction to bcc and the $Cmcm$ structure is obtained by further distortion of $Fddd$. Although the energies are higher by about 300 meV/atom than that of bcc, the SHC values at the Fermi level are 1710 (${\hslash /e)\left(\mathrm{\Omega }\mathrm{cm}\right)}^{-1}$ for $Fddd$ and 1573 (${\hslash /e)\left(\mathrm{\Omega }\mathrm{cm}\right)}^{-1}$ for $Cmcm$, which are almost twice as large as that of $\alpha \text{−}\mathrm{W}$ and exceed 1455 (${\hslash /e)\left(\mathrm{\Omega }\mathrm{cm}\right)}^{-1}$ calculated for $\beta \text{−}\mathrm{W}$.

Selective area growth of nanomembranes (NMs), nanofins, and planar nanowires (NWs) can pave the way for monolithic integration of III-V photonics with Si electronics and enable fabrication of scalable NW networks required for fundamental studies in low-temperature transport physics. Herein, we present an attempt to develop the kinetic growth theory of III-V NMs and related nanostructures in selective area epitaxy. The growth process is assumed to be driven by surface diffusion of group III adatoms. The populations of adatoms diffusing on the mask surface, NM sidewalls, and the top are described by three diffusion equations linked by six boundary conditions. The resulting growth equation provides the NM height as a function of time, slit width and pitch, and deposition conditions. The width and pitch dependences of the NM growth rate are found to be qualitatively different for different directions of the adatom diffusion fluxes (from the mask surface onto the NM or in the opposite direction). A good correlation of the model with the data on the growth kinetics of GaAs NMs by selective area molecular beam epitaxy is demonstrated.

Heteroepitaxy enables the engineering of novel properties, which do not exist in a single material. Two principal growth modes are identified for material combinations with a large lattice mismatch, Volmer-Weber, and Stranski-Krastanov. Both lead to the formation of three-dimensional islands, hampering the growth of flat defect-free thin films. This limits the number of viable material combinations. Here, we report a distinct growth mode found in molecular beam epitaxy of PbTe on InP initiated by pregrowth surface treatments. Early nucleation forms islands analogous to the Volmer-Weber growth mode, but film closure exhibits a flat surface with atomic terracing. Remarkably, despite multiple distinct crystal orientations found in the initial islands, the final film is single crystalline. This is possible due to a reorientation process occurring during island coalescence, facilitating high quality heteroepitaxy despite the large lattice mismatch, difference in crystal structures, and diverging thermal expansion coefficients of PbTe and InP. This growth mode offers a new strategy for the heteroepitaxy of dissimilar materials and expands the realm of possible material combinations.

The elastic properties of tungsten, a ubiquitous material in future energy systems, are investigated up to its melting temperature by means of a data-driven approach. The proposed workflow combines machine learning of the force field and enhanced sampling of the crystalline structure. While the machine learning force field achieves the accuracy of ab initio calculations, its implementation in sampling methods is often limited due to its high computational cost, which is commonly a few orders of magnitude larger than that of traditional potentials. To overcome this limitation, we propose a fast and robust Bayesian sampling scheme aiming at estimating the fully anharmonic free energy of crystalline solids with the help of an improved adaptive biasing force method. This method performs a thermodynamic integration from a harmonic reference system, wherein zero frequencies associated with the periodic boundaries are screened off. The proposed sampling method drastically improves the speed of convergence and overall accuracy. We demonstrate the efficiency of the improved method by calculating the second order derivatives of the free energy, such as the elastic constants, which are performed almost 100 times faster than with the standard methods. The proposed method enables the prediction of the elastic properties of tungsten in the range of temperatures that cannot be investigated experimentally, from 2100 K to the melting temperature. The accuracy and numerical efficiency of the proposed strategy open up many avenues for the reliable prediction of finite-temperature properties of materials, such as the relative stability of structural defects and elastic constants.

We investigate electrical transport and magnetic properties of the single-crystalline, Mn-based kagomé magnets $R{\mathrm{Mn}}_6{\mathrm{Ge}}_6$ ($R$=Tb-Lu). The compounds mostly feature a series of magnetic orderings of $R$ and Mn moments at different temperatures and metamagnetic transition (MMT) under an external magnetic field. We classify the MMT into three types, i.e., from skewed spiral to a structure containing decoupled $R$ and Mn moments, from antiferromagnetic to flat spiral Mn moments, and from flat spiral to a ferrimagnetic state of $R$ and Mn moments. The former two induce a small anomalous Hall effect (AHE), whereas the latter gives rise to a large one. We conclude that ferromagnetic ordered Mn moments in the kagomé lattice lead to a significant AHE, while other magnetic structures of $R$ and Mn moments do not. The AHE in ${\mathrm{TbMn}}_6{\mathrm{Ge}}_6$ generally has an intrinsic origin, while no clear sign of intrinsic AHE is found in other compounds.

We study the electronic properties of the glide set of dislocations in diamond from first principles using hybrid exchange correlation functionals and find that the atomic-scale dislocation core states give rise to a prototypical one-dimensional (1D) band structure, i.e., natural quantum wires. The position and character of the core states varies strongly with local structure, where mixed dislocations with dangling bonds exhibit a 1D metallic band with a characteristic 1D density of states ($1/\sqrt{E}$). This 1D Fermi gas is spatially localized to a single atomic diameter orbital chain along the dislocation core. When the dangling bonds within the core are reconstructed, the 1D metallic band disappears. In contrast, pure edge dislocations in diamond reveal a 1D semiconductor with a direct band gap of 3.21 eV. These calculations provide a possible explanation to the long-standing observation of a blue luminescence band correlated with dislocations in diamond. This opens the door to using dislocations as 1D quantum phases with functional (electronic and optical) properties arising from the atomic-scale core states.

Zirconium trihydride and, for comparison, trideuteride with hexagonal close-packed (hcp) metal lattices were synthesized at a hydrogen/deuterium pressure of 9 GPa and a temperature of 873 K using toroid-type high-pressure chambers. After a rapid cooling (quenching) to 100 K and lowering the pressure to atmospheric, the quenched sample was removed from the chamber and studied by hot extraction analysis, powder x-ray diffraction, and ac magnetic susceptometry. The hot extraction showed that thermal decomposition of both hcp-${\mathrm{ZrH}}_3$ and hcp-${\mathrm{ZrD}}_3$ in vacuum begins after heating at a rate of 10 K/min to ∼200 K and almost ceases at 270 K, leaving tetragonal $\varepsilon \text{−}{\mathrm{ZrH}}_2/\varepsilon \text{−}{\mathrm{ZrD}}_2$ as a result. The x-ray diffraction study at 85 K gave lattice parameters $a=3.443\left(2\right)\AA$ and $c=5.964\left(4\right)\AA$ for hcp-${\mathrm{ZrH}}_3$ and somewhat smaller values of $a=3.430\left(5\right)\AA$ and $c=5.947\left(5\right)\AA$ for hcp-${\mathrm{ZrD}}_3$. The measurements of magnetic susceptibility revealed superconductivity with an onset at 11.6(1) K for hcp-${\mathrm{ZrH}}_3$ and 9.5(2) K for hcp-${\mathrm{ZrD}}_3$.

The orthorhombic compound ${\mathrm{AuSn}}_4$ is compositionally similar to the Dirac node arc semimetal ${\mathrm{PtSn}}_4$. ${\mathrm{AuSn}}_4$ is, contrary to ${\mathrm{PtSn}}_4$, superconducting with a critical temperature of $T_c=2.35$ K. Recent measurements present indications for quasi-two-dimensional superconducting behavior in ${\mathrm{AuSn}}_4$. Here we present measurements of the superconducting density of states and the band structure of ${\mathrm{AuSn}}_4$ through scanning tunneling microscopy and angular resolved photoemission spectroscopy (ARPES). The superconducting gap values in different portions of the Fermi surface are spread around ${\mathrm{\Delta }}_0=0.4$ meV, which is close to but somewhat larger than $\mathrm{\Delta }=1.76k_B{T}_c$ expected from BCS theory. We observe superconducting features in the tunneling conductance at the surface up to temperatures about 20% larger than bulk $T_c$. The band structure calculated with density functional theory follows well the results of ARPES. The crystal structure presents two possible stackings of Sn layers, giving two nearly degenerate polytypes. This makes ${\mathrm{AuSn}}_4$ a rather unique case with a three-dimensional electronic band structure but properties ressembling those of low-dimensional layered compounds.

Phase boundary indicates the conditions of transition between phase regions, which is a key constituent of a phase diagram. We propose an approach to determine the phase boundary and its uncertainty in a phase-composition map based on the data from high-throughput experimentation. Bayesian logistics regression combined with the domain knowledge of phase diagrams was applied in the approach. For a typical ternary isothermal section, both the linear and nonlinear phase boundaries as well as the vertices of a ternary tie triangle were modeled to quantify the uncertainty with consideration of a couple of affecting factors such as data-point density, noise in the data, data coverage, etc. The effectiveness of the present approach was demonstrated by the Fe-Cr-Ni isothermal section data from database and the Fe-Co-Ni composition-phase map data by experiment. Moreover, the uncertainty of the phase boundary in the ternary system can be further reduced by incorporating the available data from the subbinary systems. The data-driven nature of the developed approach can further guide the efficient implementation of high-throughput experiments and provide confidence measures for decision-making in materials design and further Computer Coupling of Phase Diagrams and Thermochemistry (CALPHAD) method modeling.

$3d\text{−}5d$ perovskite oxides, $AB{\mathrm{O}}_3$ (where $A$ and $B$ are $3d$ or $5d$ elements), form polar surfaces in the (001)-stacked thin films. As a result, the polar-polar $\left(001\right)$ interface between two $AB{\mathrm{O}}_3$ insulators could create polar discontinuity potentially producing a two-dimensional electron gas of higher density and stronger spatial localization compared to the widely studied polar-nonpolar oxide interfaces, such as $\left(001\right) \mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$. Here, as a model system, we explore the interface between polar $\left(001\right) \mathrm{TbSc}{\mathrm{O}}_3$ and polar $\left(001\right) \mathrm{KTa}{\mathrm{O}}_3$ using first-principles density functional theory. We find that the intermixed interface ${\mathrm{Ta}}_{0.75}{\mathrm{Sc}}_{0.25}{\mathrm{O}}_2/{\mathrm{Tb}}_{0.75}{\mathrm{K}}_{0.25}\mathrm{O}$ maintaining the bulk perovskite charge stacking (e.g., $...+1/-1/+1...$) is insulating and has a lower energy than the metallic interface $\mathrm{TbO}/\mathrm{Ta}{\mathrm{O}}_2$ breaking such stacking. This intermixed interface is, however, prone to the formation of oxygen vacancies which make it conducting. We emphasize that the driving force for the formation of the two-dimensional electron gas (2DEG) here is not a built-in electric field stemming from the polar discontinuity but the interface stoichiometry. We find that the ratio of oxygen vacancy concentration is a factor of 30 times larger at the interface than in bulk KTO at room temperature. The oxygen vacancy-induced 2DEG resides on the $\text{Ta-}5d$ electronic orbitals with $d_{xy}$ and $d_{\mathrm{xz}/\mathrm{yz}}$ occupation dominating overall charge density near and far away from the interface.

We report a giant magnetic-field effect on optical second-harmonic generation (SHG) in a noncentrosymmetric ferrimagnet ${\mathrm{Eu}}_2\mathrm{Mn}{\mathrm{Si}}_2{\mathrm{O}}_7$. The SHG intensity changes at visible photon energy of 2.8 eV by more than 7000% upon the reversal of magnetic-field direction. Such pronounced modulation of the nonlinear optical activity results from an interference between crystallographic and magnetically induced nonlinear polarization. The amplitude of these nonlinear optical susceptibilities is controlled by the spin alignment and the light incident angle, enabling to optimize the magnetic switching of SHG in detail.

In this paper a systematic examination of graphene/hexagonal boron nitride (g/hBN) bilayers is presented, through a recently developed two-dimensional phase field crystal model that incorporates out-of-plane deformations. The system parameters are determined by closely matching the stacking energies and heights of g/hBN bilayers to those obtained from existing quantum-mechanical density functional theory calculations. Out-of-plane deformations are shown to reduce the energies of inversion domain boundaries in hBN, and the coupling between graphene and hBN layers leads to a bilayer defect configuration consisting of an inversion boundary in hBN and a domain wall in graphene. Simulations of twisted bilayers reveal the structure, energy, and elastic properties of the corresponding moiré patterns and show a crossover as the misorientation angle between the layers increases from a well-defined hexagonal network of domain boundaries and junctions to smeared-out patterns. The transition occurs when the thickness of domain walls approaches the size of the moiré patterns and coincides with the peaks in the average von Mises and volumetric stresses of the bilayer.

The search for topological superconductivity has motivated investigations into materials that combine topological and superconducting properties. The half-Heusler compound YPtBi appears to be such a material; however, experiments have thus far been limited to bulk single crystals, drastically limiting the scope of available experiments. This has made it impossible to investigate the potential topological nature of the superconductivity in this material. Experiments to access details about the superconducting state require sophisticated lithographic structures, typically based on thin films. Here we report on the establishment of high-crystalline-quality epitaxial thin films of YPtBi(111), grown using molecular beam epitaxy on ${\mathrm{Al}}_2{\mathrm{O}}_3$(0001) substrates. A robust superconducting state is observed, with both critical temperature and critical field consistent with that previously reported for bulk crystals. Moreover, we find that $\mathrm{Al}{\mathrm{O}}_x$ capping sufficiently protects the sample surface from degradation to allow for proper lithography. Our results pave a path towards the development of advanced lithographic structures, which will allow the exploration of the potentially topological nature of superconductivity in YPtBi.

${\mathrm{CsPbI}}_3$ is an inorganic perovskite solar cell material with a band gap appropriate for solar cell applications and photo conversion efficiency greater than 18%. However, this material is also known to undergo many phase transformations within the perovskite structure, starting from a high-temperature cubic phase to a tetragonal phase, leading to a room-temperature orthorhombic phase, with each phase having a different photovoltaic response. The tetragonal phase has been shown to have the highest photoconversion efficiency (18.4%), followed by the cubic phase (17%) and the orthorhombic phase (12.5%). In this work, we use the newly formulated first-principles–based effective Hamiltonian for ${\mathrm{CsPbI}}_3$ to investigate the phase transformation sequence in ${\mathrm{CsPbI}}_3$ thin films. We investigate the effect of film thickness and substrate strain on the phase transformations and the room-temperature phase. The effect of screening is also investigated. The three perovskite phases previously found in bulk by experiments and by computations were also found to exist in the thin film. It is shown that thickness and substrate strain can stabilize photoactive perovskite tetragonal and cubic phases at room temperature, both of which can be used for solar cell applications.

Monolayer 1T’-${\mathrm{WTe}}_2$ has been the first two-dimensional crystal where a quantum spin Hall phase was experimentally observed. In addition, recent experiments and theoretical modeling reported the presence of a robust excitonic insulating phase. While first-principles calculations with hybrid functionals and several measurements at low temperatures suggest the presence of a band gap of the order of 50 meV, experiments could confirm the presence of the helical edge states only up to 100 K. Here, we study with first-principle simulations the temperature effects on the electronic structure of monolayer 1T’-${\mathrm{WTe}}_2$ and consider the contributions of both thermal expansion and electron-phonon coupling. First, we show that thermal expansion is weak but tends to increase the indirect band gap. Then, we calculate the effect of electron-phonon coupling on the band structure with nonperturbative methods and observe a small reduction of the band inversion with increasing temperature. Notably, the topological phase and the presence of a finite gap are found to be particularly robust to thermal effects up to and above room temperature.

We present a study of high-quality $\mathrm{Ba}{\mathrm{Co}}_2{\left(\mathrm{P}{\mathrm{O}}_4\right)}_2$ single crystals via magnetization, heat-capacity, thermal-expansion, and magnetostriction measurements. Sharp anomalies in the thermodynamic properties at $T_N=3.4\mathrm{K}$ reveal a long-range antiferromagnetic order in these single-crystalline samples, which is absent in polycrystalline $\mathrm{Ba}{\mathrm{Co}}_2{\left(\mathrm{P}{\mathrm{O}}_4\right)}_2$. The temperature dependent magnetic susceptibilities for in-plane and out-of-plane magnetic fields are strongly anisotropic and reveal a pronounced easy-plane anisotropy. A Curie-Weiss analysis implies strong orbital magnetism, as it is known from the sister compound $\mathrm{Ba}{\mathrm{Co}}_2{\left(\mathrm{As}{\mathrm{O}}_4\right)}_2$ that is discussed as a potential Kitaev spin-liquid material. When applying in-plane magnetic fields at low temperature, $\mathrm{Ba}{\mathrm{Co}}_2{\left(\mathrm{P}{\mathrm{O}}_4\right)}_2$ is driven to another ordered phase at a critical field ${\mu }_0{H}_{C1}\approx 0.11\mathrm{T}$ and then undergoes a further field-induced transition to a highly polarized paramagnetic phase at ${\mu }_0{H}_{C2}\approx 0.3\mathrm{T}$, which is again similar to the case of $\mathrm{Ba}{\mathrm{Co}}_2{\left(\mathrm{As}{\mathrm{O}}_4\right)}_2$. In addition, our lowest-temperature data reveal that the field-induced transitions in $\mathrm{Ba}{\mathrm{Co}}_2{\left(\mathrm{P}{\mathrm{O}}_4\right)}_2$ become dominated by thermally assisted domain-wall motion.