First-principles simulations are conducted to predict that ferroelectric nitride perovskite $\mathrm{LaW}{\mathrm{N}}_3$ not only exhibits large spin splittings (2.7 eV Å) but also possesses unique spin textures for some of its conduction levels. Such spin splittings around the $\mathrm{\Gamma }$ and $L$ points cannot be interpreted as a typical mixture of Rashba or Dresselhaus configurations but rather require the development of four-band $\mathit{k}•\mathit{p}$ models with high-order terms. We further identify that, for some bands, spin splittings can be greatly contributed by the pure orbital degree of freedom (PODF), a unique character of our four-band Hamiltonian compared to the traditional two-band version. The concept of PODF-enhanced spin splittings paves a way for designing materials with large spin splittings. Moreover, the energy levels possessing such large splittings and complex spin textures can be brought close to the conduction-band minimum by applying epitaxial strain.

We develop a first-principles approach for the treatment of vibronic interactions in solids that overcomes the main limitations of state-of-the-art electron-phonon coupling formalisms. In particular, anharmonic effects in the nuclear dynamics are accounted for to all orders via ab initio molecular dynamics simulations. This nonperturbative, self-consistent approach evaluates the response of the wave functions along the computed anharmonic trajectory; thus, it fully considers the coupling between nuclear and electronic degrees of freedom. We validate and demonstrate the merits of the concept by calculating temperature-dependent, momentum-resolved spectral functions for silicon and the cubic perovskite ${\mathrm{SrTiO}}_3$, a strongly anharmonic material featuring soft modes. In the latter case, our approach reveals that anharmonicity and higher-order vibronic couplings contribute substantially to the electronic structure at finite temperatures, noticeably affecting band gaps and effective masses and hence macroscopic properties such as transport coefficients.

The stability of the nonmodulated martensitic phase, the austenitic Fermi surface, and the phonon dispersion relations for ferromagnetic ${\mathrm{Ni}}_2\mathrm{MnGa}$ are studied using density functional theory. Exchange-correlation effects are considered with various degrees of precision, starting from the simplest local spin density approximation (LSDA), then adding corrections within the generalized gradient approximation (GGA), and finally, including the meta-GGA corrections within the strongly constrained and appropriately normed (SCAN) functional. We discuss a simple procedure to reduce a possible overestimation of magnetization and underestimation of nesting vector in SCAN by parametrically decreasing self-interaction corrections.

Single-layer transition metal dichalcogenides (TMDCs) can adopt two distinct structures corresponding to different coordination of the metal atoms. TMDCs adopting the $T$-type structure exhibit a rich and diverse set of phenomena, including charge density waves (CDWs) in a $\sqrt{13}\times \sqrt{13}$ supercell pattern in ${\mathrm{TaS}}_2$ and ${\mathrm{TaSe}}_2$, and a possible excitonic insulating phase in ${\mathrm{TiSe}}_2$. These properties make the $T$-TMDCs desirable components of layered heterostructure devices. In order to predict the emergent properties of combinations of different layered materials, one needs simple and accurate models for the constituent layers which can take into account potential effects of lattice mismatch, relaxation, strain, and structural distortion. Previous studies have developed ab initio tight-binding Hamiltonians for $H$-type TMDCs [S. Fang et al., Phys. Rev. B 98, 075106 (2018)]. Here we extend this work to include $T$-type TMDCs. We demonstrate the capabilities and limitations of our model using three example systems: a one-dimensional sinusoidal ripple, which represents a longitudinal acoustic phonon; the $2\times 2$ CDW in ${\mathrm{TiSe}}_2$; and the $\sqrt{13}\times \sqrt{13}$ CDW in ${\mathrm{TaS}}_2$. Using the technique of band unfolding we compare the electronic structure of the distorted crystals to the pristine band structure and find our tight-binding model reproduces many features revealed by direct density functional theory calculations, provided the magnitude of the distortions remains in the linear regime. This model of the strain response of single layers is a necessary ingredient for the construction of models of van der Waals heterostructures with multiple layers, because the deformation and strain from mechanical relaxations in a twisted bilayer have important effects on the electronic structure.

We analyze planar electromagnetic waves confined by a slab waveguide formed by two perfect electrical conductors. Remarkably, two-dimensional (2D) Maxwell equations describing transverse electromagnetic modes in such waveguides are exactly mapped onto equations for acoustic waves in fluids or gases. We show that interfaces between two slab waveguides with opposite-sign permeabilities support 1D edge modes, analogous to surface acoustic plasmons at interfaces with opposite-sign mass densities. We analyze this type of edge mode for the cases of isotropic media and anisotropic media with tensor permeabilities (including hyperbolic media). We also take into account “non-Hermitian” edge modes with imaginary frequencies or/and propagation constants. Our theoretical predictions are feasible for optical and microwave experiments involving 2D metamaterials.

A striking feature of time-reversal symmetry (TRS) protected topological insulators (TIs) is that they are characterized by a half integer quantum Hall effect on the boundary when the surface states are gapped by time-reversal breaking perturbations. While TRS-protected TIs have become increasingly under control, magnetic analogs are still a largely unexplored territory with novel rich structures. In particular, magnetic topological insulators can also host a quantized axion term in the presence of lattice symmetries. Since these symmetries are naturally broken on the boundary, the surface states can develop a gap without external manipulation. In this paper, we combine theoretical analysis, density-functional calculations and experimental evidence to reveal intrinsic axion insulating behavior in ${\mathrm{MnBi}}_6{\mathrm{Te}}_{10}$. The quantized axion term arises from the simplest possible mechanism in the antiferromagnetic regime where it is protected by inversion symmetry and the product of a fractional translation and TRS. The anticipated gapping of the Dirac surface state at the edge is subsequently experimentally established using angle resolved photoemission spectroscopy (ARPES). As a result, this system provides the magnetic analog of the simplest TRS-protected TI with a single, gapped Dirac cone at the surface.

Using a two-color time-resolved pump-probe spectroscopy scheme, we studied the generation and detection of longitudinal coherent acoustic phonons, generated by ultrafast laser pulses in a semiconductor heterostructure. Our structure was a $p$-doped, 100-nm-thick ferromagnetic GaMnAs layer grown on a GaAs substrate. By probing the transient reflectivity in the time domain, we observed a strong dependence of the coherent phonon's amplitude on the external magnetic field. Our theoretical model relates this dependence to the increase in the detectability of coherent phonons in the presence of external magnetic fields. This enhancement comes from the formation of Landau levels in the absorption spectrum and leads to large changes in the real and imaginary parts of the dielectric function with strain. When the probe laser energy is close to an allowed Landau-level transition, the detectability of the coherent phonons can be significantly enhanced. Our results suggest that not only can one increase the detection of coherent phonons with magnetic fields, one can also enhance the generation by tuning the wavelength of the pump laser pulse to coincide with a Landau-level resonance.

We study ballistic transport across a time-dependent barrier present on the surface of a three-dimensional topological insulator. We show that such a barrier can be implemented for Dirac electrons on the surface of a three-dimensional topological insulator by a combination of a proximate magnetic material and linearly polarized electromagnetic radiation. We find that the differential conductance of the system can be tuned by varying the frequency and amplitude of the radiation and the energy of an electron incident on the barrier, providing us optical control on the conductance of such junctions. We first study a $\delta$-function barrier which shows a number of interesting features such as sharp peaks and dips in the transmission at certain angles of incidence. Approximate methods for studying the limits of small and large frequencies are presented. We then study a barrier with a finite width. This gives rise to some features which are not present for a $\delta$-function barrier, such as regions of both large conductance (resonances) and small conductance (dips) at certain values of the system parameters. We present a Floquet perturbation theory which can explain all these features. Finally, we use a semiclassical approach to study transmission across a time-dependent barrier and show how this can qualitatively explain some of the results found in the earlier analysis. We discuss experiments which can test our theory.

Most of the searches for Kitaev materials deal with $4d/5d$ magnets with spin-orbit-coupled $J=1/2$ local moments such as iridates and $\alpha \text{−}{\mathrm{RuCl}}_3$. Here we propose the monoclinic ${\mathrm{YbCl}}_3$ with a ${\mathrm{Yb}}^{3+}$ honeycomb lattice for the exploration of Kitaev physics. We perform thermodynamic, $ac$ susceptibility, angle-dependent magnetic torque, and neutron diffraction measurements on ${\mathrm{YbCl}}_3$ single crystal. We find that the ${\mathrm{Yb}}^{3+}$ ion exhibits a Kramers doublet ground state that gives rise to an effective spin $J_{\text{eff}}=1/2$ local moment. The compound exhibits short-range magnetic order below 1.20 K, followed by a long-range Néel-type antiferromagnetic order at 0.60 K, below which the ordered ${\mathrm{Yb}}^{3+}$ spins lie in the $ac$ plane with an angle of 16(11)${}^{\circ }$ away from the $a$ axis. These orders can be suppressed by in-plane and out-of-plane magnetic fields at around 6 and 10 T, respectively. Moreover, the Néel temperature varies nonmonotonically under the out-of-plane magnetic fields, suggesting a reduced spin dimensionality. Together with the strong in-plane magnetic anisotropy and the reduced order moment 0.8(1) ${\mu }_B$ at 0.25 K, all indicate that ${\mathrm{YbCl}}_3$ could be a two-dimensional spin system to proximate the Kitaev physics.

We report results of muon spin relaxation and rotation ($\mu \mathrm{SR}$) experiments on the spin-liquid candidate ${\mathrm{YbMgGaO}}_4$. No static magnetism $\gtrsim 0.003{\mu }_B$ per Yb ion, ordered or disordered, is observed down to 22 mK, a factor of 2 lower in temperature than previous measurements. Persistent (temperature-independent) spin dynamics are observed up to 0.20 K and at least 1 kOe, thus extending previous zero-field $\mu \mathrm{SR}$ results over a substantial region of the $H\text{−}T$ phase diagram. Knight shift measurements in a 10-kOe transverse field reveal two lines with nearly equal amplitudes. Inhomogeneous muon depolarization in a longitudinal field, previously characterized by stretched-exponential relaxation due to spatial inhomogeneity, is fit equally well with two exponentials, also of equal amplitudes. We attribute these results to two interstitial muon sites in the unit cell rather than disorder or other spatial distribution. Further evidence for this attribution is found from agreement between the ratio of the two measured relaxation rates and calculated mean-square local ${\mathrm{Yb}}^{3+}$ dipolar fields at candidate muon sites. Zero-field data can be understood as a combination of two-exponential dynamic relaxation and quasistatic nuclear dipolar fields.

Exchange-coupled structures consisting of ferromagnetic and ferrimagnetic layers become technologically more and more important. We show experimentally the occurrence of completely reversible, hysteresis-free minor loops of ${[\mathrm{Co}\left(0.2\mathrm{nm}\right)/\mathrm{Ni}\left(0.4\mathrm{nm}\right)/\mathrm{Pt}\left(0.6\mathrm{nm}\right)]}_N$ multilayers exchange-coupled to a 20-nm-thick ferrimagnetic ${\mathrm{Tb}}_{28}{\mathrm{Co}}_{14}{\mathrm{Fe}}_{58}$ layer, acting as a hard magnetic pinning layer. Furthermore, we present detailed theoretical investigations by means of micromagnetic simulations and, most importantly, a purely analytical derivation for the condition of the occurrence of full reversibility in magnetization reversal. Hysteresis-free loops always occur if a domain wall is formed during the reversal of the ferromagnetic layer and generates an intrinsic hard-axis bias field that overcomes the magnetic anisotropy field of the ferromagnetic layer. The derived condition further reveals that the magnetic anisotropy and the bulk exchange of both layers, as well as the exchange coupling strength and the thickness of the ferromagnetic layer, play an important role for its reversibility.

Recently, the ubiquitously applicable phenomenological Dynes formula for the tunneling density of states of superconductors has been explained by assuming that the electrons scatter on a random magnetic field due to impurities. In the original derivation, only the magnitude of the field was taken as random, but the direction of the field was assumed to be fixed. Here we show that this assumption can be relaxed, and therefore the concept of Dynes superconductivity is not restricted to systems with a preferred direction in spin space. We also address the question whether thin Al films in parallel magnetic fields can be described as Dynes superconductors.

We report an ultralow-field frequency-swept electrically detected magnetic resonance (fsEDMR) measurement scheme sensitive to so-called ultrastrong coupling in paramagnetic systems, which arises from comparatively strong driving fields and weak Zeeman interaction with small static fields. We observe multiple-photon transitions in the EDMR spectrum of a 4H-SiC transistor. The multiphoton transitions are a strong function of the linearly polarized driving field and of the static field. The observation of both field-swept EDMR at a constant frequency and fsEDMR demonstrate that the transitions we observe are caused by multiphoton transitions. In the small static field and large driving-field regime, Bloch-Siegert effects cause small changes to the resonant frequency. We observe these Bloch-Siegert shifts in the resonance frequency in the ultralow-field fsEDMR scheme and verify the observation by also measuring the driving field directly using Faraday's law of induction and a sensing coil. Multiphoton transitions are important for quantum engineering applications.

The interaction between spin and nanomechanical degrees of freedom attracts interest from the viewpoint of basic science and device applications. We study the magnon current induced by the torsional oscillation of ferromagnetic nanomechanical cantilever. We find that a finite Dzyaloshinskii-Moriya (DM) interaction emerges by the torsional oscillation, which is described by the spin gauge field, and the DM interaction leads to the detectably large magnon current with frequency the same as that of the torsional oscillation. Our theory paves the way for studying torsional spin-nanomechanical phenomena by using the spin gauge field.

We investigated phonon behavior of hexagonal close-packed titanium under homogeneous shear deformation corresponding to the $\left\{10\overline{1}2\right\}$ twinning mode using first-principles calculations and phonon calculations. By this deformation, we found that a phonon mode located at a point on the Brillouin zone boundary is drastically softened, increasing the shear, and finally it triggers a spontaneous structural transition by breaking the crystal symmetry toward twin from parent.

We study the phase diagram of a spin-1 antiferromagnetic chain with isotropic antiferromagnetic interactions decaying with a power law $\propto r^{-\alpha }$ $(\alpha \ge 1)$ accompanied by modulated single-ion anisotropy. Employing the techniques of the density-matrix renormalization group, the effects of long-range interactions and single-ion anisotropy on a variety of correlations are investigated. In order to check the consistency, the fidelity susceptibilities are evaluated across quantum phase transitions. The quantum critical points are faithfully detected and orders of phase transitions are determined. The correlation-length critical exponent is extracted from scaling functions of the fidelity susceptibility. The presence of long-range interactions leads to a quantitative change of the phase boundaries and reduces the order of the phase transition under certain conditions. A direct first-order transition between the periodic Néel phase and the large-$D$ phase occurs for slowly decaying antiferromagnetic interactions.

We study the effects of random nonmagnetic impurities on superconducting transition temperature $T_c$ in a Cu-doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, for which four types of pair potentials have been proposed. Although all the candidates belong to $s$-wave symmetry, two orbital degrees of freedom in electronic structures enrich the symmetry variety of a Cooper pair such as even-orbital-parity and odd-orbital-parity. We consider realistic electronic structures of Cu-doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ by using a tight-binding Hamiltonian on a hexagonal lattice and consider effects of impurity scatterings through the self-energy of the Green's function within the Born approximation. We find that even-orbital-parity spin-singlet superconductivity is basically robust even in the presence of impurities. The degree of the robustness depends on the electronic structures in the normal state and on the pairing symmetry in orbital space. On the other hand, two odd-orbital-parity spin-triplet order parameters are always fragile in the presence of potential disorder.