Two-dimensional (2D) magnetic transition metal dichalcogenides have attracted great interest in various fields, including information storage, logic devices, and high-frequency detectors. However, the currently reported 2D magnets are limited by their low Curie temperatures. Based on the density-functional theory calculation, we predicted the metastable $H$ phase $\mathrm{V}{\mathrm{Te}}_2\left(\mathrm{H}\text{−}\mathrm{V}{\mathrm{Te}}_2\right)$ is a promising room-temperature 2D in-plane ferromagnetic semiconductor without charge-density wave (CDW) transition. To achieve the $H\text{−}\mathrm{V}{\mathrm{Te}}_2$ from the $T$ phase or CDW-phase $\mathrm{V}{\mathrm{Te}}_2$, electron doping is proposed as a feasible strategy in practice. Moreover, the significant modulation effect of external strain fields on the magnetocrystalline anisotropy energy and magnetic ordering of monolayer $H\text{−}\mathrm{V}{\mathrm{Te}}_2$ are systematically studied. The electron correlation included from a $\mathrm{Hubbard}\text{−}U$ term has a profound influence on the ground state of magnetic order of $H\text{−}\mathrm{V}{\mathrm{Te}}_2$, as well as the direction of the easy-magnetization axis. These results not only provide an in-depth understanding of the modulation mechanism of the magnetic $H\text{−}\mathrm{V}{\mathrm{Te}}_2$, but also shed light on the potential applications of $H\text{−}\mathrm{V}{\mathrm{Te}}_2$ in spintronics and sensors.

We study transport phenomena through a ballistic ferromagnet-superconductor-ferromagnet (F/S/F) junction, comparing the case in which the ferromagnetic order in the two F layers is of the standard Stoner type with the case where it is driven by a spin mass mismatch (SMM). It is shown that the two mechanisms lead to a different behavior in the charge and the spin conductances, especially when compared to the corresponding nonsuperconducting ferromagnet-normal-ferromagnet (F/N/F) junctions. In particular, when the injected current is perpendicular to the barrier, for high barrier transparency and large magnetization of the F layers, the large mass mismatch gives rise to an enhancement of both low-bias charge and spin conductances of the F/S/F junction, which is not observed in the equal-mass case. When all the allowed injection directions are considered, the low-bias enhancement of the charge conductance for SMM leads still holds for high-barrier transparency and large magnetization of the F layers. However, in the case of nontransparent interfaces, spin transport with SMM ferromagnets exhibits an opposite sign response with respect to the Stoner case at high biases for all magnetization values, also manifesting a significant amplification induced by superconductivity at the gap edge. The above-mentioned differences can be exploited to probe the nature of the electronic mechanism underlying the establishment of the ferromagnetic order in a given material.

Experimental evidence suggests that superconductivity in ${\mathrm{SrTiO}}_3$ is mediated by a soft transverse ferroelectric mode which, according to conventional theories, has negligible coupling with electrons. A phenomenological Rashba-type coupling has been proposed on symmetry arguments but a microscopic derivation is lacking. Here, we fill this gap and obtain a linear coupling directly from a minimal microscopic model of the electronic structure. We find that the effective electron-electron pairing interaction has a strong momentum dependence. This yields an unusual situation in which the leading $s$-wave channel is followed by a subleading $p$-wave state which shows a stronger pairing instability than the $d$-wave state. The bare Rashba coupling constant is estimated for the lowest band of doped ${\mathrm{SrTiO}}_3$ with the aid of first-principles computations. Extrapolating the estimation to the multiband regime we show that it can explain bulk superconductivity. For densities where the Meissner effect has not been observed an extra softening due to inhomogeneities is needed to explain the zero resistance state.

Based on first-principles calculations, we performed a systematic study of the energetic stability, structural characterization, and electronic properties of the fully oxidized $A_{2}B$ electrenes, with the following combinations: (i) $A$ = Ca, Sr, and Ba for $B$ = N; (ii) $A$ = Sr and Ba for $B$ = P; and ${\mathrm{Y}}_2\mathrm{C}$ and ${\mathrm{Ba}}_2\mathrm{As}$. We have considered one side oxidation of single layer electrenes (O/$A_{2}B$), and two side oxidation of bilayer electrenes (O/${\left(A_{2}B\right)}_2$/O). We show that the hexagonal lattice of the pristine host is no longer the ground state structure in the (fully) oxidized systems. Our total energy results reveal an exothermic structural transition from hexagonal to tetragonal ($\mathrm{h}\to \mathrm{t}$) geometry, resulting in layered tetragonal structures [(${A\mathrm{O}AB)}^{\mathrm{t}}$ and ${\left(A\text{O}{\left(AB\right)}_{2}A\text{O}\right)}^{\mathrm{t}}$]. Phonon spectra calculations and molecular dynamic simulations show that the O/$A_{2}B$ and O/${\left(A_{2}B\right)}_2$/O systems, with $A$ = Ba, Ca, Sr, and $B$ = N, become dynamically and structurally stable upon such a $\mathrm{h}\to \mathrm{t}$ transition. Further structural characterizations were performed based on simulations of the near edge x-ray absorption spectroscopy at the nitrogen $K$ edge. Finally, the electronic band structure and transport calculations reveal the formation of half-metallic bands spreading out through the $A\mathrm{N}$ layers, which in turn are shielded by oxide $A\mathrm{O}$ sheets. These findings indicate that ($A\mathrm{O}A\mathrm{N}$)${}^{\mathrm{t}}$ and ${\left(A\mathrm{O}{\left(A\mathrm{N}\right)}_{2}A\mathrm{O}\right)}^{\mathrm{t}}$ are quite interesting platforms for application in spintronics; since the half-metallic channels along the $A\mathrm{N}$ or ${\left(A\text{N}\right)}_2$ layers (core) are protected against the environment conditions by the oxidized $A\text{O}$ sheets (cover shells).

Since double resonance Raman (DRR) spectra are laser-energy dependent, the first-principles calculations of DRR for two-dimensional materials are challenging. Here, the DRR spectrum of monolayer $\mathrm{Mo}{\mathrm{Te}}_2$ is calculated by home-made program, in which we combine ab initio density functional theory calculations with the electron-phonon Wannier (EPW) method. Within the fourth-order perturbation theory, we are able to quantify not only the electron-photon matrix elements within the dipole approximation, but also the electron-phonon matrix elements using the Wannier functions. The reasonable agreement between the calculated and experimental Raman spectra is achieved, in which we reproduce some distinctive features of transition metal dichalcogenides (TMDCs) from graphene (for example, the dominant intervalley process involving an electron or a hole). Furthermore, we perform an analysis of the possible DRR modes over the Brillouin zone, highlighting the role of low-symmetry points. Raman tensors for some DRR modes are given by first-principles calculations from which laser polarization dependence is obtained.

Thermal transport in a quasiballistic regime is determined not only by the local temperature $T\left(r\right)$ or its gradient $\mathbf{\nabla }T\left(r\right)$ but also by the temperature distribution at neighboring points. For an accurate description of nonlocal effects on thermal transport, we employ the thermal distributor $\mathrm{\Theta }(r,r^{\prime })$, which provides the temperature response of the system at point $r$ to the heat input at point $r^{\prime }$. We determine the thermal distributors from the linearized Peierls-Boltzmann equation, both with and without the relaxation time approximation, and employ them to describe thermal transport in quasiballistic graphene devices.

It has been known for decades that a magnetic field can deflect phonons as they flow in response to a thermal gradient, producing a thermal Hall effect. Several recent experiments have revealed ratios of the phonon Hall conductivity ${\kappa }_H$ to the phonon longitudinal conductivity ${\kappa }_L$ in oxide dielectrics that are larger than ${10}^{-3}$ when phonon mean-free paths exceed phonon wavelengths. At the same time ${\kappa }_H/{\kappa }_L$ is not strongly temperature dependent. We argue that these two properties together imply a mechanism related to phonon scattering from defects that break time-reversal symmetry, and we show that Lorentz forces acting on charged defects can produce substantial skew-scattering amplitudes and related thermal Hall effects.

We investigate the magnetic excitations of the hyperhoneycomb Kitaev magnet $\beta \text{−}{\text{Li}}_2{\text{IrO}}_3$ by means of inelastic Raman scattering. The spectra exhibit the coexistence of a broad scattering continuum and two sharp low-energy peaks at 2.5 and 3 meV, with a distinctive polarization dependence. While the continuum is suggestive of fractional quasiparticles emerging from a proximate quantum spin liquid phase, the sharp peaks provide the first experimental signature of the “non-Loudon- Fleury” one-magnon scattering processes proposed recently [Yang et al., Phys. Rev. B 104, 144412 (2021)]. The corresponding microscopic mechanism is similar to the one leading to the symmetric off-diagonal exchange interaction $\mathrm{\Gamma }$ (because it involves a combination of both direct and ligand-mediated exchange paths) but is otherwise completely unexpected within the traditional Loudon-Fleury theory of Raman scattering. The present experimental verification therefore calls for a drastic reevaluation of Raman scattering in similar systems with strong spin-orbit coupling and multiple exchange paths.

Dispersionless (flat) quantum states is a paradigm shift in condensed matter physics, serving as a natural platform for emergent electronic phases. In this work, we propose the foundation for the new framework addressing the issue of band flatness from quantum-geometrical perspective involving Wannier functions probes in the real space. The perfectly flat bands serve as ideal objects for classification, ranging from intrinsically distinct examples such as flat electronic bands in argon ice, artificial lattices (Kagome, Lieb), Landau levels, and magic-angle twisted bilayer graphene and its descendants. Through using the quantum geometry concepts and Wannier function analysis we reveal a hidden connection between the flatness of topological bands, their Chern numbers, and quantum-geometrical limit for flat-band orbital cross section, reminiscent of Lifshitz-Onsager quantization.

Spin-polarized scanning tunneling microscopy has been applied to study noncollinear spin textures of a Mn monolayer on a fourfold symmetric W(001) substrate revealing a zero-field spin spiral ground state and two different types of rotational domain walls. With an applied magnetic field of 9 T, we observe a coexistence of the spin spiral and the skyrmion phase, even though a previous theoretical study reported that the phase transition occurs at 18 T. The skyrmions show a roughly hexagonal arrangement despite the square lattice symmetry of the W(001) substrate. Based on a reduced set of energy parameters, we are able to describe the experimental findings and to analyze the topological properties of the rotational domain walls.

The quantized response is one distinguishing feature of a topological system. In non-Hermitian systems, the spectral winding topology yields a quantized steady-state response. By considering two weakly coupled non-Hermitian chains, we discover that the spectral winding topology of one chain can be probed by a steady-state response defined solely on the other chain, even when other important properties, e.g., energetics and entanglement entropy, indicate that eigensolutions are effectively not hybridized between the two chains. This intriguing phenomenon, as carefully investigated in a large parameter space with a varying system size, not only offers a different angle to understand interchain signal propagation in a non-Hermitian setting but also reveals unexpected physics of spectral winding topology vs quantized response.

Spin-wave (SW) spectra have theoretically been studied in a thin film of a ferromagnet (FM) on a substrate from a paramagnet (PM) (an FM above the critical temperature) or from superconductor (SC). A spin-wave propagating in the FM induces the dynamic magnetization and superconducting current in the underlying PM and SC, respectively, which affect the SW propagation by their magnetic fields. As a result of this interaction, the SW spectrum becomes nonreciprocal to depend on the sign of the SW wave-vector $\mathbf{q}$. We show that the nonreciprocal contribution to the SW spectra in FM/PM and FM/SC systems is given by the frequency shift of $\mathrm{\Delta }\omega \left(\mathbf{q}\right)\equiv \omega \left(\mathbf{q}\right)-\omega \left(-\mathbf{q}\right)=a\left(T\right)\left(\mathit{\tau }·\mathbf{q}\right)$ with $\mathbf{\tau }=\left(\mathbf{n}\times \mathbf{M}\right)$ being the toroidal magnetic moment, $\mathbf{M}$ the FM magnetization, $\mathbf{n}$ the unit vector normal to the FM/PM(SC) interface, and $a\left(T\right)$ the temperature-dependent constant of a dipole nature, whose sign depends on the substrate type. As the $\mathrm{\Delta }\omega \left(T\right)$ dependence is strong at temperatures $T$ close to the critical temperature $T_c$ for the FM-PM or normal metal-SC transition, one gets a possibility to control the frequency SW nonreciprocity with temperature variation near $T_c$. The dipolar mechanism we propose for SW frequency nonreciprocity is promising for introducing this property of SW propagation into functional devices.

Manipulation of three-dimensional (3D) topological objects is of both fundamental interest and practical importance in many branches of physics. We show by spin dynamics simulations that the bifurcation of a 3D skyrmion string in a layered frustrated system could be induced by the dampinglike spin-orbit torque. The bifurcation of a skyrmion string happens when the skyrmion string carries a minimal topological charge of $Q=2$. We demonstrate that three types of bifurcations could be realized by applying different current injection geometries, which lead to the transformation from I-shaped skyrmion strings to Y-, X-, and O-shaped ones. Besides, different branches of a bifurcated skyrmion string may merge into an isolated skyrmion string spontaneously. The mechanism of bifurcation should be universal to any skyrmion strings with $Q\ge 2$ in the layered frustrated system and could offer a general approach to manipulate 3D stringlike topological objects for spintronic functions.

We study the magnetic excitations of the antiferromagnetic XXZ model on two-dimensional triangular lattice. We find there is a magnon valley thermal Hall effect. As far as we know, this effect has been studied on only the ferromagnetic honeycomb lattice. We reveal that the valley thermal Hall conductivity also conforms to the universal behavior of thermal Hall conductivity proposed by Yang et al. [Phys. Rev. Lett. 124, 186602 (2020)]. We derive a low-energy effective theory near the valleys and find that although the low-energy effective dynamic matrix is non-Hermitian, the Berry curvatures are mainly determined by its Hermitian part. We show that our results are general to some extent in the Y phase of the triangular lattice. By an explicit calculation with experimental parameters, we demonstrate that ${\mathrm{Rb}}_4\mathrm{Mn}{\left({\mathrm{MoO}}_4\right)}_3$ is a suitable material to realize the magnon valley thermal Hall effect.

Using first-principles calculations, we predict the two-dimensional (2D) monolayer boron-nitrogen compound ${\mathrm{B}}_3\mathrm{N}$, which shows a honeycomb lattice similar to graphene. Its stability is proved by phonon spectra and ab initio molecular dynamics simulations. Since pristine ${\mathrm{B}}_3\mathrm{N}$ is a metal by band structure calculation, we investigate the electron-phonon coupling and possible phonon-mediated superconductivity. Based on the Eliashberg equation, the calculated electron-phonon coupling strength is about 0.66, and the superconducting transition temperature $T_c$ is 14.1 K, which is comparable to that of borophene. Furthermore, when 0.04 electron/cell doping and 10% biaxial tensile strain are applied, $T_c$ can be increased to 17.4 K. Thus, the predicted ${\mathrm{B}}_3\mathrm{N}$ provides a platform for 2D superconductivity.

We study the electronic properties of a planar semiconductor-superconductor heterostructure, in which a thin ferromagnetic insulator layer lies in between and acts as a spin filtering barrier. We find that in such a system one can simultaneously enhance the strengths of all the three important induced physical quantities, i.e., Rashba spin-orbit coupling, exchange coupling, and superconducting pairing potential, for the hybrid mode by external gating. Our results show the specific advantage of this stacked device geometry compared to conventional devices. We further discuss how to optimize geometrical parameters for the heterostructure and complement our numerical simulations with analytic calculations.

The effect of glide symmetry on the dispersion of acoustic surface waves supported by lines of open-ended holes in acrylic plates is investigated. Two samples are experimentally characterized, each formed of a pair of such parallel lines, one having glide-symmetry. This glide symmetry removes the condition necessary for the supported even and odd character acoustic surface waves to form standing waves at the first Brillouin zone boundary; there is instead one mixed even-odd hybrid mode. This evolves continuously with increasing frequency to a far greater in-plane wave vector than possible without the glide condition. This paper demonstrates that glide symmetry opens a degree of freedom for the design of acoustic metamaterial devices for the control of acoustic energy on a surface.

Sodium niobate (${\mathrm{NaNbO}}_3$ or NN) is described as “the most complex perovskite system,” which exhibits transitions between, as well as coexistence of, several ferroelectrics (FE) and antiferroelectric (AFE) phases at different temperatures. Recently, solid solutions of NN with stabilized AFE phases(s) have gained attention for energy-related applications, such as high-density energy storage and electrocaloric cooling. A better understanding of the atomic mechanisms responsible for AFE/FE phase transitions in ${\mathrm{NaNbO}}_3$ can enable a more rational design of its solid-solution systems with tunable functional properties. Here, we have investigated changes in the average and local atomic structure of NN using a combination of x-ray/neutron diffraction and neutron pair-distribution function (PDF) analyses. The Rietveld refinement of the x-ray/neutron-diffraction patterns indicates a coexistence of the FE $Q$ ($P{2}_{1}ma$) and AFE $P$ ($Pbma$) phases in the temperature range of $300\mathrm{K}\le T\le 615\mathrm{K}$, while PDF analysis indicated that the local structure $\left(r<8\AA \right)$ is better described by a $P{2}_{1}ma$ symmetry. Above 615 K, the average structure transitions to an AFE $R$ phase ($Pmmn$ or $Pnma$), while PDF analysis shows an increased disordering of the octahedral distortions and Na displacements at the local scale. These results indicate that the average P/Q/R phase transitions in NN can be described as a result of complex ordering of distorted octahedral tilts at the nanoscale and off-centered displacements of the Na atoms.