Based on crystal structural search and ab initio calculations, we report pressure-induced structural and electronic phase transitions for the semiconductor indium tellurium iodine (InTeI). We have found two structures belonging to the same space group of InTeI at high pressure (labeled as $P4/nmm\text{−}\mathrm{I}$ and $P4/nmm\text{−}\mathrm{II}$, respectively), where $P4/nmm\text{−}\mathrm{II}$ is metastable. Our calculation results show that the ambient $P{2}_1/c$ phase transforms to a tetragonal $P4/nmm\text{−}\mathrm{I}$ phase at about 15 GPa, characterized by the appearance of metallization and superconductivity with $T_{\mathrm{c}}$ of around 7 K. Besides, band-structure calculations suggest that the InTeI system undergoes a pressure-induced electronic phase transition from a direct to indirect band gap in the $P{2}_1/c$ phase. On the application of atomic substitution, the superconducting temperature $T_{\mathrm{c}}$ of InTeI can be further raised to around 9.7 K if the iodine atoms are replaced by the lighter chlorine atoms in the $P4/nmm\text{−}\mathrm{I}$ phase. The results pave the way for applying of Indium tellurohalide in optoelectronic devices and demonstrate a method that adjusts electronic properties by pressure or atomic substitution.

The unique bulk crossing points in the band structure of topological Weyl semimetals have been shown to enhance nonlinear optical and optoelectronic properties, even for light at optical wavelengths. Such pronounced nonlinear optical effects have been studied in type-I Weyl semimetals, yet few studies have quantified these effects in type-II Weyl semimetals. We here present an optical experimental study with polarization resolution of two nonlinear optical effects in the type-II Weyl semimetal $T_{\mathrm{d}}\text{−}\mathrm{W}{\mathrm{Te}}_2$. We begin by investigating the bulk optical second-harmonic response of this material using the rotational anisotropy of the second-harmonic generation. This technique allows us to simultaneously investigate the dependence of the second-harmonic response on the symmetry of the crystal and to quantify the size of that response, which we compare to other nonlinear crystals. We then use polarized time-resolved optical reflectivity spectroscopy to identify the nonlinear optical effect of impulsive stimulated Raman scattering as the origin of the coherent oscillations of the 0.25-THz shear mode. We find that the strength of this response in $T_{\mathrm{d}}\text{−}\mathrm{W}{\mathrm{Te}}_2$ is enhanced compared with the observation of other modes excited through the displacive excitation of coherent phonons. Notably, both the second-harmonic response and the coherent excitations of the phonons demonstrate strong anisotropy, displaying a clear dependence on the polarization of the light used in the experiments. We use point-symmetry analyses of the material to guide our understanding of these observations and perform fluence-dependent measurements to investigate the electron-phonon coupling.

The ruthenium content of half-metallic ${\mathrm{Mn}}_2{\mathrm{Ru}}_x\mathrm{Ga}$ thin films, with a biaxially strained inverse Heusler structure, controls the ferrimagnetism that determines their magnetic and electronic properties. An extensive study of ${\mathrm{Mn}}_y{\mathrm{Ru}}_x\mathrm{Ga}$ films on MgO (100) substrates with $1.8\le y\le 2.6$ and $x=0.5, 0.7 \mathrm{or} 0.9$, including crystallographic, magnetic order, magneto-transport, and spin polarization, is undertaken to map specific composition-dependent properties in this versatile ternary system. A comparison of experimental densities obtained from x-ray reflectivity with calculated densities indicates full-site occupancy for all compositions, which implies chemical disorder. All moments lie on a Slater-Pauling plot with slope 1 and all except $x=0.5$, $y=2.2$ exhibit magnetic compensation at $T_{\text{comp}}$ below 500 K. The coercivity near $T_{\text{comp}}$ exceeds 10 T. Increasing the Mn or Ru content raises $T_{\text{comp}}$, but increasing Ru also decreases the spin polarization determined by point contact Andreev reflection. Molecular-field theory is used to model the temperature dependence of the net ferrimagnetic moment and three principal exchange coefficients are deduced. Marked differences in the shape of anomalous Hall and net magnetization hysteresis loops are explained by substantial canting of the small net moment by up to ${40}^{\circ }$ relative to the $c$ axis in zero field, which is a result of slight noncollinearity of the ${\mathrm{Mn}}^{4c}$ sublattice moments due to competing intrasublattice exchange interactions arising from antisite disorder and excess Mn in the unit cell. Consequences are reduced-spin polarization and an enhanced intrinsic contribution to the anomalous Hall effect. The systematic investigation of the physical properties as a function of $x$ and $y$ will guide the selection of compositions to meet the requirements for magnonic and spintronic MRG-based devices.

We theoretically investigate the dynamics of solitons in two sublattice antiferromagnets under external perturbations, focusing on the effect of Dzyaloshinsky-Moriya (DM) interactions. To this end, we construct a micromagnetic field theory for the antiferromagnet in the presence of the external magnetic field, DM interaction, and spin-transfer torque. In particular, we show external magnetic field and spin current couple to Néel vector in a Zeeman-like manner when DM interactions present, which can be used to efficiently drive antiferromagnetic solitons of different dimensions. Besides, we study the effect of straining the local lattice. It can serve as an external handle on the Néel field inertia and thus dynamical properties. Our findings may find applications in antiferromagnetic spintronics.

We report the temperature-dependent magnetic and structural properties of epitaxial ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$ thin films grown on Ge substrates. Utilizing density-functional theory (DFT) calculations and various experimental methods, we reveal mechanisms governing the switching between collinear and noncollinear spin configuration in ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$. The Mn atoms in ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$ occupy two distinct Wyckoff positions with fourfold $\left({\mathrm{Mn}}_1\right)$ and sixfold $\left({\mathrm{Mn}}_2\right)$ multiplicity. The DFT calculations reveal that below a critical distance of approximately 3.002 Å the coupling between ${\mathrm{Mn}}_2$ atoms is antiferromagnetic (AFM) while ferromagnetic (FM) above that critical distance. The FM coupling between ${\mathrm{Mn}}_1$ atoms is weakly affected by the strain. The observed noncollinear spin configuration is due to the coexistence of AFM and FM coupling at low temperatures. The findings give insight in developing strain-controlled spintronic devices.

We formulate a theory of skyrmion and antiskyrmion generation using magnetic field and charge current pulses. We show that the topological defect can be created at an edge of a system with Dzyaloshinskii-Moriya interaction (DMI) as well as at a boundary between regions with different DMI. We consider both perpendicular and in-plane (also known as magnetic bimerons) versions of skyrmions and antiskyrmions. We show that the magnetization twist in the vicinity of an edge or a boundary is described by a kink solution, the presence of which can instigate the generation of topological defects. We study the collective excitations of magnetization analytically and numerically, and demonstrate that under application of magnetic field and charge current pulses the magnon modes localized near boundaries can develop instabilities leading to the formation of skyrmions or antiskyrmions. Due to the skyrmion and antiskyrmion Hall effects, a properly chosen current direction can push the topological defects away from the boundary, thus facilitating their generation.

The recently discovered material ${\mathrm{Cs}}_3{\mathrm{Fe}}_2{\mathrm{Br}}_9$ contains ${\mathrm{Fe}}_2{\mathrm{Br}}_9$ bi-octahedra forming triangular layers with hexagonal stacking along the $c$ axis. In contrast to isostructural Cr-based compounds, the zero-field ground state is not a nonmagnetic $S=0$ singlet-dimer state. Instead, the ${\mathrm{Fe}}_2{\mathrm{Br}}_9$ bi-octahedra host semiclassical $S=\frac{5}{2}{\mathrm{Fe}}^{3+}$ spins with a pronounced easy-axis anisotropy along $c$, and interestingly, the intradimer spins are ordered ferromagnetically. The high degree of magnetic frustration due to (various) competing intradimer and interdimer couplings leads to a surprisingly rich magnetic phase diagram. The zero-field ground state is already reached via an intermediate phase, and the high-field magnetization and thermal expansion data for $H\parallel c$ identify 10 different ordered phases. Among them are phases with constant magnetization of $\frac{1}{3}$, respectively $\frac{1}{2}$, of the saturation value, and several transitions are strongly hysteretic with pronounced length changes, reflecting strong magnetoelastic coupling.

Spintronic thin films were shown to be promising high-performance broadband terahertz (THz) emitters. For spintronic ferromagnet/heavy metal heterostructures, THz emission arises from three mechanisms: the magnetic dipolar radiation due to the ultrafast demagnetization, the transient current radiation due to spin-charge conversion, and anomalous Hall effect. Here, we demonstrate that the three mechanisms show distinct dependences on the thickness of the ferromagnetic layer. With the insertion of an ultrathin ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer between the ferromagnetic and heavy metal layer, the spin-charge conversion is suppressed when the ${\mathrm{Al}}_2{\mathrm{O}}_3$ thickness exceeds 1 nm. Our results agree with the theoretical predictions based on a simple spin-diffusion model.

Suppression of the occurrence of remanent vortices is necessary to improve the quality factor of superconducting resonators. In particular, the flux-expulsion dynamics in Nb during cooling has become of major interest to researchers focusing on superconducting cavities. To study the vortex states and their behavior in high-purity cavity-grade Nb, we used a magneto-optical imaging technique to perform real-space observations of the magnetic field distributions during the field-cooling and field-scanning processes. In the field-cooling process, the distributions were observed to undergo phase separation into vortex and Meissner regions, as would be expected in an intermediate mixed state (IMS). The vortex regions in the IMS, such as vortex bundles, tend to be larger in higher fields, in contrast to the Meissner regions, which experience shrinkage. In the field-scanning process, domelike field profiles, which indicate a geometrical barrier with very weak bulk pinning, were observed. The existence of the IMS suggests that cavity-grade Nb is in a type-II/1 superconductor regime, in which attractive interaction between vortices at a length scale of the penetration depth is crucial for the behavior of vortices.

We derive a series of quantitative bulk-boundary correspondences for 3D bosonic and fermionic symmetry-protected topological (SPT) phases under the assumption that the surface is gapped, symmetric, and topologically ordered, i.e., a symmetry-enriched topological (SET) state. We consider those SPT phases that are protected by the mirror symmetry and continuous symmetries that form a group of U(1), SU(2), or SO(3). In particular, the fermionic cases correspond to a crystalline version of 3D topological insulators and topological superconductors in the famous tenfold-way classification, with the time-reversal symmetry replaced by the mirror symmetry and with strong interaction taken into account. For surface SETs, the most general interplay between symmetries and anyon excitations is considered. Based on the previously proposed dimensional reduction and folding approaches, we rederive the classification of bulk SPT phases and define a complete set of bulk topological invariants for every symmetry group under consideration and then derive explicit expressions of the bulk invariants in terms of surface topological properties (such as topological spin, quantum dimension) and symmetry properties (such as mirror fractionalization, fractional charge or spin). These expressions are our quantitative bulk-boundary correspondences. Meanwhile, the bulk topological invariants can be interpreted as anomaly indicators for the surface SETs which carry 't Hooft anomalies of the associated symmetries whenever the bulk is topologically nontrivial. Hence, the quantitative bulk-boundary correspondences provide an easy way to compute the 't Hooft anomalies of the surface SETs. Moreover, our anomaly indicators are complete. Our derivations of the bulk-boundary correspondences and anomaly indicators are explicit and physically transparent. The anomaly indicators obtained in this work can be straightforwardly translated to their time-reversal counterparts that apply to the usual topological insulators and topological superconductors, due to a known correspondence between mirror and time-reversal topological phases.

Recent works have provided evidence that an axial anomaly can arise in Weyl semimetals. If this is the case, then the electromagnetic response of Weyl semimetals should be governed by the equations of axion electrodynamics. These equations capture both the chiral magnetic and anomalous Hall effects in the limit of linear response, while at higher orders their solutions can provide detectable electromagnetic signatures of the anomaly. In this work, we consider three versions of axion electrodynamics that have been proposed in the Weyl semimetal literature. These versions differ in the form of the chiral magnetic term and in whether or not the axion is treated as a dynamical field. In each case, we look for solutions to these equations for simple sample geometries subject to applied external fields. We find that in the case of a linear chiral magnetic term generated by a nondynamical axion, self-consistent solutions can generally be obtained. In this case, the magnetic field inside of the Weyl semimetal can be magnified significantly, providing a testable signature for experiments. Self-consistent solutions can also be obtained for dynamical axions, but only in cases where the chiral magnetic term vanishes identically. Finally, for a nonlinear form of the chiral magnetic term frequently considered in the literature, we find that there are no self-consistent solutions aside from a few special cases.

We present a theoretical analysis for chiral sensing using achiral anisotropic metasurfaces. We derive analytically, and verify numerically, simple formulas that provide insight into the sensing mechanism and explain how anisotropic metasurfaces offer additional degrees of freedom with respect to their isotropic counterparts. We demonstrate how to deconvolve the unknown chirality from the background dispersion of the metasurface, and we propose practical measurement schemes for its unambiguous determination. Last, we discuss the key functionalities and benefits of anisotropic metasurfaces, and we provide the design principles towards broadband operation—from near-infrared to near-ultraviolet frequencies—opening the way for highly sensitive nanoscale chiroptical spectroscopy. Our numerical examples are focused on a metasurface design that we recently proposed, which belongs to the wider class of achiral anisotropic metasurfaces.

The surface of a topological insulator hosts Dirac electronic states with the spin-momentum locking, which constrains spin orientation perpendicular to electron momentum. As a result, collective plasma excitations in the interacting Dirac liquid manifest themselves as coupled charge- and spin-waves. Here we demonstrate that the presence of the spin component enables effective coupling between plasma waves and spin waves at interfaces between the surface of a topological insulator and insulating magnet. Moreover, the helical nature of spin-momentum locking textures provides the phase winding in the coupling between the spin and plasma waves that makes the spectrum of hybridized spin-plasma modes to be topologically nontrivial. We also show that such topological modes lead to a large thermal Hall response.

Focusing on equilibrium spin current density tensor, we analyze the spin physics of electrons in single layer graphene subject to a one-dimensional periodic Kronig-Penney potential $u\left(x\right)$ and uniform Rashba spin-orbit coupling of strength $\lambda$. The combination of Dirac theory of massless two-dimensional electrons, Klein paradox, spin-orbit coupling, and the Bloch theorem yields peculiar features relevant to graphene spintronics. For transverse wave number $k_y=0$, we show that: (1) The charge current and the spin density vector vanish. (2) Yet, the components $J_{xy},J_{yx},J_{zy}$ of the equilibrium spin current density tensor are finite, oscillating in space, with amplitudes that increase quadratically for small $\lambda$ and then linearly with $\lambda$. Quite remarkably, $J_{zy}\left(x\right)\ne 0$, that is, the spin is polarized along $z$, perpendicular to the graphene plane. (3) Due to the continuity equation, the space dependence of the spin current density $J_{yx}\left(x\right)$ is associated with a finite spin torque density ${\mathcal{T}}_y\left(x\right)=\frac{\partial J_{yx}\left(x\right)}{\partial x}$.

The detailed analytical and numerical analysis of the electron spectrum, persistent currents, and their densities for an annulus placed in a constant magnetic field (Corbino disk geometry) is presented. We calculate the current density profiles and study their dependence on the inner and outer radii of the annular. We study the evolution of the persistent currents and track their emergence and decay for different limiting cases of such a geometry, starting from a nanodot and ending by a macroscopic circle. Our analytical results for the currents are confirmed by the agreement between the integration of the corresponding current densities and the application of the Byers-Yang formula, when it is applicable. Among other results, we find the general expression for the persistent current in a narrow annulus, which in the one channel approximation reproduces the well-known result for quasi-one-dimensional mesoscopic metallic ring. Moreover, it allows to analyze the multichannel case of a relatively wide annulus. Our study can be used for more accurate treatment and interpretation of the experimental data with measurements of the persistent currents in different doubly connected systems.

The inside of the electrical double layer at perovskite oxide heterointerfaces is examined. Here, we report the local polarization and valence distribution in ${\mathrm{LaNiO}}_3$/${\mathrm{LaMnO}}_3$ and ${\mathrm{LaMnO}}_3$/${\mathrm{LaNiO}}_3$ bilayers on a ${\mathrm{SrTiO}}_3$ (001) substrate. Simultaneous measurements of two aspects of the structure are realized by using Bayesian inference based on resonant- and nonresonant-surface x-ray diffraction data. The results show that the average Mn valences are ${\mathrm{Mn}}^{3.12+}$ and ${\mathrm{Mn}}^{3.19+}$ for the two samples. The intensity of their local electric field is $\sim 1$ GV/m and the direction of the field points from ${\mathrm{LaMnO}}_3$ to ${\mathrm{LaNiO}}_3$.

We present a computationally efficient approach to perform systematically convergent real-space all-electron Kohn-Sham density functional theory calculations for solids using an enriched finite element (FE) basis. The enriched FE basis is constructed by augmenting the classical FE basis with atom-centered numerical basis functions, comprising of atomic solutions to the Kohn-Sham problem. Notably, to improve the conditioning, we orthogonalize the enrichment functions with respect to the classical FE basis, without sacrificing the locality of the resultant basis. In addition to improved conditioning, this orthogonalization procedure also renders the overlap matrix block diagonal, greatly simplifying its inversion. Subsequently, we use a Chebyshev polynomial based filtering technique to efficiently compute the occupied eigenspace in each self-consistent field iteration. We demonstrate the accuracy and efficiency of the proposed approach on periodic unit cells and supercells. The benchmark studies show a staggering $130\times$ speedup of the orthogonalized enriched FE basis over the classical FE basis. We also present a comparison of the orthogonalized enriched FE basis with the linearized augmented plane-wave $+$ local orbitals basis, both in terms of accuracy and efficiency. Notably, we demonstrate that the orthogonalized enriched FE basis can handle large system sizes of $\sim 10000$ electrons. Finally, we observe good parallel scalability of our implementation with $92\%$ efficiency at $22\times$ speedup for a system with 620 electrons.

Composite quantum compounds (CQCs) have become an important avenue for the investigation of intercorrelation between two distinct phenomena in physics. Topological superconductors, axion insulators, etc., are a few such CQCs which have recently drawn tremendous attention in the community. Topological nontriviality and Rashba spin physics are two different quantum phenomena but can be intertwined within a CQC platform. In this paper, we present a general symmetry-based mechanism, supported by ab initio calculations, to achieve intertwined giant Rashba splitting and topological nontrivial states simultaneously in a single crystalline system. Such coexistent properties can further be tuned to achieve other rich phenomena. We have achieved Rashba splitting energy ($\mathrm{\Delta }E$) and Rashba coefficient ($\alpha$) values as large as 161 meV and 4.87 eV Å, respectively, in conjunction with the Weyl semimetal phase in ${\mathrm{KSnSb}}_{0.625}{\mathrm{Bi}}_{0.375}$. Interestingly, these values are even larger than the values reported for the widely studied topologically trivial Rashba semiconductor BiTeI. The advantage of our present analysis is that one can achieve various topological phases without compromising the Rashba parameters, within this CQC platform.

The eigenvalues of a parameter-dependent Hamiltonian matrix form a band structure in parameter space. In such $N$-band systems, the quantum geometric tensor (QGT), consisting of the Berry curvature and quantum metric tensors, is usually computed from numerically obtained energy eigenstates. Here, an alternative approach to the QGT based on eigenprojectors and (generalized) Bloch vectors is exposed. It offers more analytical insight than the eigenstate approach. In particular, the full QGT of each band can be obtained without computing eigenstates, using only the Hamiltonian matrix and the respective band energy. Most saliently, the well-known two-band formula for the Berry curvature in terms of the Hamiltonian vector is generalized to arbitrary $N$. The formalism is illustrated using three- and four-band multifold fermion models that have very different geometrical and topological properties despite an identical band structure. From a broader perspective, the methodology used in this work can be applied to compute any physical quantity or to study the quantum dynamics of any observable without the explicit construction of energy eigenstates.

We study theoretically optomechanical interactions in a semiconductor microcavity with an embedded quantum well under optical pumping by a Bessel beam, carrying a nonzero orbital momentum. Due to the transfer of orbital momentum from light to phonons, the microcavity can act as an acoustic circulator: It rotates the propagation direction of the incident phonon by a certain angle clockwise or anticlockwise. Due to the optomechanical heating and cooling effects, the circulator can also function as an acoustic laser emitting sound with nonzero angular momentum. Our calculations demonstrate the potential of semiconductor microcavities for compact integrable optomechanical devices.