Traditional models of interfacial phonon scattering, including the acoustic mismatch model and diffuse mismatch model, take into account the bulk properties of the material surrounding the interface, but not the atomic structure and properties of the interface itself. Here, we derive a theoretical formalism for the phonon scattering at a dislocation grid, or two interpenetrating orthogonal arrays of dislocations, as this is the most stable structure of both the symmetric twist boundary and semicoherent heterointerface. With this approach, we are able to separately examine the contribution to thermal resistance due to the step-function change in acoustic properties and due to interfacial dislocation strain fields, which induces diffractive scattering. Both low-angle Si-Si twist boundaries and the Si-Ge heterointerfaces are considered here and compared to previous experimental and simulation results. This work indicates that scattering from misfit dislocation strain fields doubles the thermal boundary resistance of Si-Ge heterointerfaces compared to scattering due to acoustic mismatch alone. Scattering from grain boundary dislocation strain fields is predicted to dominate the thermal boundary resistance of Si-Si twist boundaries. This physical treatment can guide the thermal design of devices by quantifying the relative importance of interfacial strain fields, which can be engineered via fabrication and processing methods, versus acoustic mismatch, which is fixed for a given interface. Additionally, this approach captures experimental and simulation trends such as the dependence of thermal boundary resistance on the grain boundary angle and interfacial strain energy.

Long-range spin transport in magnetic systems can be achieved by means of exchange-mediated spin textures with robust topological winding, a phenomenon referred to as spin superfluidity. Its experimental signatures have been discussed in antiferromagnets, which are nearly free of dipolar interaction. However, in ferromagnets, which possess non-negligible dipole fields, realization of such spin transport has remained a challenge. Using micromagnetic simulations, we investigate coherent exchange-mediated spin transport in extended thin ferromagnetic films. We uncover a two-fluid state in which the long-range spin transport by spin textures coexists with spin waves, as well as a soliton-screened spin transport regime at high spin injection biases. Both states are associated with distinct spin texture reconstructions near the spin injection region and sustain spin transport over large distances.

We use particle-tracking velocimetry to study Eulerian and Lagrangian second-order statistics of superfluid ${}^4\mathrm{He}$ grid turbulence. The Eulerian energy spectra at scales larger than the mean distance between quantum vortex lines behave classically with close to Kolmogorov-1941 scaling and are almost isotropic. The Lagrangian second-order structure functions and frequency power spectra, measured at scales comparable with the intervortex distance, demonstrate a sharp transition from nearly classical behavior to a regime dominated by the motion of quantum vortex lines. Employing the homogeneity of the flow, we verify a set of relations that connect various second-order statistical objects that stress different aspects of turbulent behavior, allowing a multifaceted analysis. We use the two-way bridge relations between Eulerian energy spectra and second-order structure functions to reconstruct the energy spectrum from the known second-order velocity structure function and vice versa. The Lagrangian frequency spectrum reconstructed from the measured Eulerian spectrum using the Eulerian-Lagrangian bridge differs from the measured Lagrangian spectrum in the quasiclassical range, which calls for further investigation.

The resonant-state expansion (RSE) provides a precise and computationally cheap tool to find resonant states in complex systems using the optical modes of a simpler system as a basis. We apply the RSE to a photonic crystal slab in order to identify and analyze its bound states in the continuum (BICs). We show that the RSE is a useful and reliable method for not only finding the BICs but also for differentiating between accidental and symmetry-protected BICs, as well as for understanding their formation from the basis modes and evolution with structural and material parameters of the system. The high efficiency of the RSE allows us to track the properties of BICs and other high-quality optical modes, covering the full parameter space of the system in a reasonable time frame.

Recently, the concept of a rotation anomaly in the surface of $C_n$ rotation ($n=2,4,6$) protected topological crystalline insulators (TCIs) was proposed. However, verified material realizations of the rotation anomaly are still rare. Furthermore, owing to fruitful crystallographic symmetries, in addition to the rotation protected surface states, other kinds of topological boundary states could also emerge on suitable surfaces or hinges in one material, which is promising for manipulation in device applications. In this work, we identify a $C_2$ rotation anomaly in 24 TCIs and furthermore ascertain the positions of coexisting topological boundary states by first-principles calculations. Of these TCIs, using BiSe in space group 164 to illustrate, we demonstrate the mirror and $C_2$ rotation protected surface states in specific surfaces as well as the $C_2$ protected hinge states between two side surfaces. The predicated TCI materials with the $C_2$ rotation anomaly have diverse structures and chemical compositions and thus provide a good reference for future experimental studies.

We present an ab initio relativistic $\mathbf{k}·\mathbf{p}$ theory of the effect of magnetic exchange field on the band structure in the gap region of bulk crystals and thin films of three-dimensional layered topological insulators. For the field perpendicular to the layers (along $z$), we reveal unconventional scenarios of the response of the band-gap edges to the magnetization. The modification of the valence and conduction states is considered in terms of their $\mathrm{\Gamma }$-point spin $s^z$ and total angular momentum $J^z$ on the atomic sites where the states are localized. The actual scenario depends on whether $s^z$ and $J^z$ have the same or opposite sign. In particular, the opposite sign for the valence state and the same sign for the conduction state give rise to an unconventional response in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$—both in the bulk crystal and in ultrathin films, which fundamentally distinguishes this topological insulator from ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, where both states have the same sign. To gain a deeper insight into different scenarios in insulators with both inverted and noninverted zero-field band structure, a minimal four-band third-order $\mathbf{k}·\mathbf{p}$ model is constructed from first principles. Within this model, we analyze the field-induced band structure of the insulators and identify Weyl nodes that appear in a magnetic phase and behave differently depending on the scenario. We characterize the topology of the modified band structure by the Chern number $\mathcal{C}$ and find the unconventional response to be accompanied by a large Chern number $\mathcal{C}=\pm 3$.

We have investigated the magnetotransport properties of $\beta \text{-B}{\mathrm{i}}_4{\mathrm{I}}_4$ bulk crystal, which was recently theoretically proposed and experimentally demonstrated to be a topological insulator. At low temperature $T$ and magnetic field $B$ , a series of Shubnikov–De Haas (SdH) oscillations is observed on the magnetoresistivity (MR). Detailed analysis reveals a light cyclotron mass of $0.1m_e$, and the field angle dependence of MR reveals that the SdH oscillations originate from a convex Fermi surface. In the extreme quantum limit (EQL) region, there is a metal-insulator transition occurring soon after the EQL. We perform scaling analysis, and all the isotherms fall onto a universal scaling with a fitted critical exponent $\zeta \approx 6.5$. The enormous value of critical exponent $\zeta$ implies this insulating quantum phase originated from strong electron-electron interactions in high fields. However, in the far end of EQL, both the longitudinal and Hall resistivity increase exponentially with $B$ , and the temperature dependence of the MR reveals an energy gap induced by the high magnetic field, signifying a magnetic freeze-out effect. Our findings indicate that bulk $\beta \text{-B}{\mathrm{i}}_4{\mathrm{I}}_4$ is an excellent candidate for a three-dimensional topological system for exploring EQL physics and relevant exotic quantum phases.

In this paper, we derive symmetry indicator formulas for the filling anomaly on two-dimensional (2D) $C_4$ symmetric square lattices with and without time reversal, inversion symmetry, or their product, in the presence of spin-orbit coupling. We go beyond previous work by considering lattices with atoms occupying multiple Wyckoff positions. We also provide an algorithm using the Smith normal form that systematizes the derivation. The formulas determine the corner charge in 2D atomic or fragile topological insulators, as well as in three-dimensional (3D) insulators and semimetals by studying their 2D slices in momentum space. We apply our results to a 3D tight-binding model on a body-centered tetragonal lattice, whose projection into the 2D plane has two atoms in the unit cell. Our symmetry indicators correctly describe the higher-order hinge states and Fermi arcs in cases where the existing indicators do not apply.

In this paper, the structural, electrical, and spin properties of Janus ${\mathrm{Si}}_{2}XY$ ($X,Y=\text{P}$, As, Sb, Bi) structures are studied using first-principles calculations. Elastic constants and phonon dispersion prove that all these structures are stable. The band structures of six compounds are studied and the electrical properties are compared. The charge density has been studied to discover the charge distribution in the conduction and valence bands. These compounds exhibit Rashba spin splitting at the $\mathrm{\Gamma }$ and $M$ valleys in the conduction band. Biaxial in-plane strain is used to control the electrical and spin properties. The conduction band minimum can be tuned between the $\mathrm{\Gamma }, K$, and $M$ valleys by applying strain. The Rashba spin splitting around the $\mathrm{\Gamma }$ and $M$ valleys in the conduction bands of these structures can be improved by strain and have the potential for applications in spintronic devices.

In the diffraction of visible light by a dielectric microcylinder, packages of evanescent waves always arise. However, single plane-wave incidence corresponds to rather small impact of evanescent waves outside the cylinder. In this paper, we theoretically show that a pair of plane waves impinging a glass microcylinder under certain conditions may correspond to much higher impact of the evanescent waves. Namely, the interference of the evanescent waves with the propagating ones results in the suppression of the electromagnetic field in an area with very small cross section. This area is located in free space at a substantial distance from the rear side of the microcylinder and along its axis. It may serve a linear optical trap for cold atoms and ions.

In the present study, we investigated magnetodielectric, magnetoelectric (ME), and angular-dependent polarization in single-crystal ${\mathrm{Fe}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$. The magnetodielectric effects in ${\varepsilon }_x$ ($x//\left[100\right]$), ${\varepsilon }_y$ ($y//\left[120\right]$), and ${\varepsilon }_z$ ($z//\left[001\right]$) are found to be significant only around $T_{\mathrm{N}}\approx 95\mathrm{K}$ when magnetic fields are applied along three orthogonal x-, y- ($y//\left[120\right]$), and $z$ directions. The finite polarization $P_x, P_y$, and $P_z$ of 70, 100, and $30\mu \mathrm{C}/{\mathrm{m}}^2$, respectively, can be induced in the antiferromagnetic phase when a finite magnetic field up to 9 T is applied along the three orthogonal directions. The angular-dependent polarization measurements verify the dominating linear ME effects below $T_{\mathrm{N}}$. From the above experimental results, a linear ME tensor $a_{ij}$ with all nine nonzero components can be inferred, demonstrating a much lower magnetic point group of $-{\mathbf{1}}^{\prime }$ for the canted antiferromagnetic configuration.

We extend the quasiparticle self-consistent approach beyond the $GW$ approximation by using a range-separated vertex function. The developed approach yields band gaps, dielectric constants, and band positions with an accuracy similar to highest-level electronic-structure calculations without exceeding the cost of regular quasiparticle self-consistent $GW$. We introduce an exchange-correlation kernel that accounts for the vertex over the full spatial range. In the long range it complies with the Ward identity, while it is approximated through the adiabatic local density functional in the short range. In this approach, the renormalization factor is balanced and the higher-order diagrams are effectively taken into account.

We present an experimental and theoretical study of the anomalous Hall effect (AHE) in MnAs epilayers grown over GaAs, with the aim to identify the intrinsic contribution to the AHE, which can be accurately evaluated using ab initio electronic structure calculations. Our magnetotransport measurements show a quadratic behavior of the Hall resistivity with longitudinal resistivity, characteristic of scattering-independent processes, thus enabling the comparison with our ab initio calculations. The calculated Berry phase contribution to the AHE is in quantitative agreement with the measured AHE in these epilayers. Moreover, the predicted anisotropic dependence of the experimental AHE on the magnetization is well reproduced.

It is widely recognized that a physical system can respond to a periodic driving significantly only when the driving frequency matches the normal mode frequency of the system, which leads to resonance. Off-resonant phenomena are rarely considered because of the difficulty to realize strong coupling between physical systems and off-resonant waves. Here we examine the response of a magnetic system to squeezed light and surprisingly find that the magnons are maximally excited when the effective driving frequency is several orders of magnitude larger than the resonant frequency. The generated magnons are squeezed, which brings the advantage of tunable squeezing through an external magnetic field. Furthermore, we demonstrate that such off-resonant quasiparticle excitation is universal in all the hybrid systems in which the coherent and parametric interaction of bosons exists and that it is purely a quantum effect, which is rooted in the quantum fluctuations of particles in the squeezed vacuum. Our findings may provide an unconventional route to study off-resonant phenomena and may further benefit the use of hybrid matter-light systems in continuous-variable quantum information.

X-ray diffraction, thermodynamic measurements, and density-functional band-structure calculations are used to study the magnetic behavior of ${\mathrm{BaCuTe}}_2{\mathrm{O}}_6$, a member of the $A{\mathrm{CuTe}}_2{\mathrm{O}}_6$ structural family that hosts complex three-dimensional frustrated spin networks with possible spin-liquid physics. Temperature-dependent magnetic susceptibility and heat capacity of the Ba compound are well described by the one-dimensional spin-$\frac{1}{2}$ Heisenberg chain model reminiscent of the Sr-analog ${\mathrm{SrCuTe}}_2{\mathrm{O}}_6$. While the intrachain coupling $J/k_{\mathrm{B}}\simeq 37\mathrm{K}$ is reduced compared to 49 K in the Sr compound, the Néel temperature increases from 5.5 K (Sr) to 6.1 K (Ba). Unlike the Sr compound, ${\mathrm{BaCuTe}}_2{\mathrm{O}}_6$ undergoes only one magnetic transition as a function of temperature and shows signatures of weak spin canting. We elucidate the microscopic difference between the Sr and Ba compounds and suggest that one of the interchain couplings changes sign as a result of negative pressure caused by the Sr/Ba substitution. The Néel temperature of ${\mathrm{BaCuTe}}_2{\mathrm{O}}_6$ is remarkably insensitive to the magnetic dilution with ${\mathrm{Zn}}^{2+}$ up to the highest reachable level of about 20 %.

We study semiconductor nanowires coupled to a bilayer of a disordered superconductor and a magnetic insulator, motivated by recent experiments reporting possible Majorana-zero-mode signatures in related architectures. Specifically, we pursue a quasiclassical Usadel equation approach that treats superconductivity in the bilayer self-consistently in the presence of spin-orbit scattering, magnetic-impurity scattering, and Zeeman splitting induced by both the magnetic insulator and a supplemental applied field. Within this framework we explore prospects for engineering topological superconductivity in a nanowire proximate to the bilayer. We find that a magnetic-insulator-induced Zeeman splitting, mediated through the superconductor alone, cannot induce a topological phase since the destruction of superconductivity (i.e., Clogston limit) preempts the required regime in which the nanowire's Zeeman energy exceeds the induced pairing strength. However, this Zeeman splitting does reduce the critical applied field needed to access the topological phase transition, with fields antiparallel to the magnetization of the magnetic insulator having an optimal effect. Finally, we show that magnetic-impurity scattering degrades the topological phase, and spin-orbit scattering, if present in the superconductor, pushes the Clogston limit to higher fields yet simultaneously increases the critical applied field strength.

We demonstrate that the general model of a linearly time-dependent crossing of two energy bands is integrable. Namely, the Hamiltonian of this model has a quadratically time-dependent commuting operator. We apply this property to four-state Landau-Zener (LZ) models that have previously been used to describe the Landau-Stückelberg interferometry experiments with an electron shuttling between two semiconductor quantum dots. The integrability then leads to simple but nontrivial exact relations for the transition probabilities. In addition, the integrability leads to a semiclassical theory that provides analytical approximation for the transition probabilities in these models for all parameter values. The results predict a dynamic phase transition, and show that similarly looking models belong to different topological classes.

We demonstrate using micromagnetic simulations and a theoretical model that a gradient in the saturation magnetization ($M_s$) of a perpendicularly magnetized ferromagnetic film induces a nonreciprocal spin-wave propagation and, consequently, an asymmetric dispersion relation. The $M_s$ gradient adds a linear potential to the spin-wave equation of motion consistent with the presence of a force. We consider a transformation from an inertial reference frame in which the $M_s$ is constant to an accelerated reference frame where the resulting inertial force corresponds to the force from the $M_s$ gradient. As in the Doppler effect, the frequency shift leads to an asymmetric dispersion relation. We show that under certain circumstances, unidirectional propagation of spin waves can be achieved, which is essential for the design of magnonic circuits. Our results become more relevant in light of recent experimental works in which a suitable thermal landscape is used to dynamically modulate the saturation magnetization.