We present a theoretical description and numerical simulations of the superconducting transition in hybrid structures including strong spin-orbit interactions. The spin-orbit coupling is taken to be of Rashba type for concreteness, and we allow for an arbitrary magnitude of the spin-orbit strength as well as an arbitrary thickness of the spin-orbit coupled layer. This allows us to make contact with the experimentally relevant case of enhanced interfacial spin-orbit coupling via atomically thin heavy metal layers. We consider both interfacial spin-orbit coupling induced by inversion asymmetry in an S/F junction, as well as in-plane spin-orbit coupling in the ferromagnetic region of an S/F/S and an S/F structure. Both the pair amplitudes, local density of states, and critical temperature show dependency on the Rashba strength and, importantly, the orientation of the exchange field. In general, spin-orbit coupling increases the critical temperature of a proximity system where a magnetic field is present, and enhances the superconducting gap in the density of states. We perform a theoretical derivation which explains these results by the appearance of long-ranged singlet correlations. Our results suggest that $T_c$ in ballistic spin-orbit coupled superconducting structures may be tuned by using only a single ferromagnetic layer.

We show how to define a dynamical topological invariant for one-dimensional two-band topological systems after a quantum quench. By analyzing general two-band models of topological insulators, we demonstrate that the reduced momentum-time manifold can be viewed as a series of submanifolds $S^2$, and thus we are able to define a dynamical topological invariant on each of the spheres. We also unveil the intrinsic relation between the dynamical topological invariant and the difference in the topological invariant of the initial and final static Hamiltonian. By considering some concrete examples, we illustrate the calculation of the dynamical topological invariant and its geometrical meaning explicitly.

We report high power spin transfer studies in open magnetic geometries by measuring the spin conductance between two nearby Pt wires deposited on top of an epitaxial yttrium iron garnet thin film. Spin transport is provided by propagating spin waves that are generated and detected by direct and inverse spin Hall effects. We observe a crossover in spin conductance from a linear transport dominated by exchange magnons (low current regime) to a nonlinear transport dominated by magnetostatic magnons (high current regime). The latter are low-damping magnetic excitations, located near the spectral bottom of the magnon manifold, with a sensitivity to the applied magnetic field. This picture is supported by microfocus Brillouin light-scattering spectroscopy. Our findings could be used for the development of controllable spin conductors by variation of relatively weak magnetic fields.

The chemical reaction products of molecular hydrogen $\left({\mathrm{H}}_2\right)$ with selenium (Se) are studied by synchrotron x-ray diffraction (XRD) and Raman spectroscopy at high pressures. We find that a common ${\mathrm{H}}_2\mathrm{Se}$ is synthesized at 0.3 GPa using laser heating. Upon compression at 300 K, a crystal of the theoretically predicted Cccm ${\mathrm{H}}_3\mathrm{Se}$ has been grown at 4.6 GPa. At room temperature, ${\mathrm{H}}_3\mathrm{Se}$ shows a reversible phase decomposition after laser irradiation above 8.6 GPa, but remains stable up to 21 GPa. However, at 170 K Cccm ${\mathrm{H}}_3\mathrm{Se}$ persists up to 39.5 GPa based on XRD measurements, while low-temperature Raman spectra weaken and broaden above 23.1 GPa. At these conditions, the sample is visually nontransparent and shiny suggesting that metallization occurred.

The magnetopolaron effect on shallow-impurity states in semiconductors is investigated when subjected simultaneously to a magnetic field and an intense terahertz laser field within the Faraday configuration. We use a time-dependent nonperturbative theory to describe electron interactions. The externally applied fields are exactly included via a laser-dressed interaction potential. Through a variational approach we evaluate the binding energy of the shallow-impurity states. We find that the interaction strength of the laser-dressed Coulomb potential can not only be enhanced but also weakened by varying the two external fields. In this way, the binding energy can be tuned by the external fields and red- or blue-shifted with respect to the static binding energy. In the nonresonant polaron region, a magnetopolaron correction that includes the effects of photon process is observed. In the resonant polaron region, moreover, the resonant magnetopolaron effect accompanied by the emission and absorption of a single photon is distinctly observed. This can be modulated to be far away from the reststrahlen band. The intriguing findings of this paper can be observed experimentally and, in turn, provide a way to measure the strength of the electron-phonon interaction.

We report a study on the electrical properties of 19-nm-thick yttrium iron garnet (YIG) films grown by liquid phase epitaxy on gadolinium gallium garnet single crystal. The electrical conductivity and Hall coefficient are measured in the high-temperature range [300,400] K using a Van der Pauw four-point probe technique. We find that the electrical resistivity decreases exponentially with increasing temperature following an activated behavior corresponding to a band gap of $E_g\approx 2$ eV. It drops to values about $5\times {10}^3\mathrm{\Omega }\text{cm}$ at $T=400$ K, thus indicating that epitaxial YIG ultrathin films behave as large gap semiconductors. We also infer the Hall mobility, which is found to be positive ($p$ type) at 5 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\sec }^{-1}$ and almost independent of temperature. We discuss the consequence for nonlocal spin transport experiments performed on YIG at room temperature and demonstrate the existence of electrical offset voltages to be disentangled from pure spin effects.

We study spin switching effects in finite-size superconducting multivalve structures. We examine ${\mathrm{F}}_1{\mathrm{F}}_2{\mathrm{SF}}_3$ and ${\mathrm{F}}_1{\mathrm{F}}_2{\mathrm{SF}}_3{\mathrm{F}}_4$ hybrids where a singlet superconductor (S) layer is sandwiched among ferromagnet (F) layers with differing thicknesses and magnetization orientations. Our results reveal a considerable number of experimentally viable spin-valve configurations that lead to on-off switching of the superconducting state. For S widths on the order of the superconducting coherence length ${\xi }_0$, noncollinear magnetization orientations in adjacent F layers with multiple spin axes leads to a rich variety of triplet spin-valve effects. Motivated by recent experiments, we focus on samples where the magnetizations in the ${\mathrm{F}}_1$ and ${\mathrm{F}}_4$ layers exist in a fully spin-polarized half-metallic phase, and calculate the superconducting transition temperature, spatially and energy resolved density of states, and the spin-singlet and spin-triplet superconducting correlations. Our findings demonstrate that superconductivity in these devices can be completely switched on or off over a wide range of magnetization misalignment angles due to the generation of equal-spin and opposite-spin triplet pairings.

The impact of superconducting correlations on localized electronic states is important for a wide range of experiments in fundamental and applied superconductivity. This includes scanning tunneling microscopy of atomic impurities at the surface of superconductors, as well as superconducting-ion-chip spectroscopy of neutral ions and Rydberg states. Moreover, atomlike centers close to the surface are currently believed to be the main source of noise and decoherence in qubits based on superconducting devices. The proximity effect is known to dress atomic orbitals in Cooper-pair-like states known as Yu-Shiba-Rusinov (YSR) states, but the impact of superconductivity on the measured orbital splittings and optical-noise transitions is not known. Here we study the interplay between orbital degeneracy and particle-number admixture in atomic states, beyond the usual classical spin approximation. We model the atom as a generalized Anderson model interacting with a conventional $s$-wave superconductor. In the limit of zero on-site Coulomb repulsion ($U=0$), we obtain YSR subgap energy levels that are identical to the ones obtained from the classical spin model. When $\mathrm{\Delta }$ is large and $U>0$, the YSR spectra are no longer quasiparticle-like, and the highly degenerate orbital subspaces are split according to their spin, orbital, and number-parity symmetry. We show that $U>0$ activates additional poles in the atomic Green's function, suggesting an alternative explanation for the peak splittings recently observed in scanning tunneling microscopy of orbitally-degenerate impurities in superconductors. We describe optical excitation and absorption of photons by YSR states, showing that many additional optical channels open up in comparison to the nonsuperconducting case. Conversely, the additional dissipation channels imply increased electromagnetic noise due to impurities in superconducting devices.

We identify field theories that describe the surfaces of three-dimensional bosonic point group symmetry protected topological (pgSPT) phases. The anomalous nature of the surface field theories is revealed via a dimensional reduction argument. Specifically, we study three different surface field theories. The first field theory is quantum electrodynamics in three space-time dimensions (QED3) with four flavors of fermions. We show this theory can describe the surfaces of a majority of bosonic pgSPT phases protected by a single mirror reflection, or by $C_{nv}$ point group symmetry for $n=2,3,4,6$. The second field theory is a variant of QED3 with charge-1 and charge-3 Dirac fermions. This field theory can describe the surface of a reflection symmetric pgSPT phase built by placing an $E_8$ state on the mirror plane. The third field theory is an $\mathrm{O}\left(4\right)$ nonlinear sigma model with a topological theta term at $\theta =\pi$, or, equivalently, a noncompact ${\mathrm{CP}}^1$ model. Using a coupled wire construction, we show this is a surface theory for bosonic pgSPT phases with $\mathrm{U}\left(1\right)\times {\mathbb{Z}}_2^P$ symmetry. For the latter two field theories, we discuss the connection to gapped surfaces with topological order. Moreover, we conjecture that the latter two field theories can describe surfaces of more general bosonic pgSPT phases with $C_{nv}$ point group symmetry.

We explore the frustrated spin-$\frac{1}{2}$ Heisenberg model on the star lattice with antiferromagnetic (AF) couplings inside each triangle and ferromagnetic (FM) intertriangle couplings ($J_e<0$), and calculate its magnetic and thermodynamic properties. We show that the FM couplings do not sabotage the magnetic disordering of the ground state due to the frustration from the AF interactions inside each triangle, but trigger a fully gapped inversion-symmetry-breaking trimerized valence bond crystal (TVBC) with emergent spin-1 degrees of freedom. We discover that with strengthening $J_e$, the system exhibits a universal scaling behavior either with or without a magnetic field $h$: the order parameter, the five critical fields that separate the $J_e\text{−}h$ ground-state phase diagram into six phases, and the excitation gap obtained by low-temperature specific heat, all depend exponentially on $J_e$. Our work implies that the spin-1 VBCs can be stabilized by introducing small FM couplings in the geometrically frustrated spin-$\frac{1}{2}$ systems.

Three-dimensional topological insulators (TIs) are a new class of quantum materials that have insulating energy gaps in bulk form but exhibit robust gapless surface states protected by time-reversal symmetry. It was theoretically predicted and experimentally confirmed that the binary tetradymites $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$, $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$, and $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ are all TIs, while the fourth system out of the 2 × 2 combination, $\mathrm{S}{\mathrm{b}}_2\mathrm{S}{\mathrm{e}}_3$, was prevailingly believed to be topologically trivial due to its relatively weak spin-orbit coupling (SOC). In this paper, we demonstrate through a comparative first-principles study that $\mathrm{S}{\mathrm{b}}_2\mathrm{S}{\mathrm{e}}_3$ is actually also an intrinsic TI, as characterized by its topologically protected surface states and $Z_2$ invariant. The underlying reason is the inclusion of the ubiquitous van der Waals attraction between the quintuple layers, effectively narrowing the bulk band gap, allowing the SOC effect to induce a band inversion around the Fermi level. This paper also points to the importance of the seemingly weak dispersion forces in defining the topological properties of quantum materials.

Nonequilibrium warm dense copper produced by a femtosecond optical laser pulse is investigated with x-ray absorption near-edge spectroscopy. A laser-plasma-based tabletop device is set up to probe a pre-edge structure observed near the $L$ edge with a picosecond resolution. Careful ab initio calculations are used to retrieve the electron temperature evolution up to equilibration with ions. Measurements are well reproduced by two-temperature model calculations based on temperature-dependent parameters, provided that the hydrodynamic expansion is taken into account. At the highest temperatures investigated ($\sim 20000\mathrm{K}$), the last affects significantly the energy balance between electrons and ions.

We studied the formation mechanism of the in-plane nuclear field in single self-assembled ${\mathrm{In}}_{0.75}{\mathrm{Al}}_{0.25}\mathrm{As}/{\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ quantum dots. The Hanle curves with an anomalously large width and hysteretic behavior at the critical transverse magnetic field were observed in many single quantum dots grown in the same sample. In order to explain the anomalies in the Hanle curve indicating the formation of a large nuclear field perpendicular to the photo-injected electron spin polarization, we propose a new model based on the current phenomenological model for dynamic nuclear spin polarization. The model includes the effects of the nuclear quadrupole interaction and the sign inversion between in-plane and out-of-plane components of nuclear g factors, and the model calculations reproduce successfully the characteristics of the observed anomalies in the Hanle curves.

To discuss the properties of metamaterials on physical grounds and to consider them in applications, effective material parameters are usually introduced and assigned to a given metamaterial. In most cases, only weak spatial dispersion is considered. It allows to assign local material properties, e.g., a permittivity and a permeability. However, this turned out to be insufficient. To solve this problem, we study here the effective properties of metamaterials with constitutive relations beyond a local response and take strong spatial dispersion into account. This research requires two contributions. First, bulk properties in terms of eigenmodes need to be studied. We particularly investigate the isofrequency surfaces of their dispersion relation are investigated and compared to those of an actual metamaterial. The significant improvement to effectively describe it provides evidence for the necessity to use nonlocal material laws in the effective description of metamaterials. Second, to be able to capitalize on such constitutive relations, also interface conditions need to be known. They are derived in this contribution for our form of the nonlocality using a generalized (weak) formulation of Maxwell's equations. Based on such interface conditions, Fresnel expressions are obtained that predict the amplitude of the reflected and transmitted plane wave upon illuminating a slab of such a nonlocal metamaterial. This all together offers the necessary means for the in-depth analysis of metamaterials characterized by strong spatial dispersion. The general formulation we choose here renders our approach applicable to a wide class of metamaterials.

We theoretically investigate the dynamical nuclear spin polarization in a quantum point contact (QPC) at finite magnetic field. We find that when the QPC is tuned to be spin selective, at the conductance of $e^2/h$, a finite bias induces a dipolelike (spatially antisymmetric) nuclear polarization: at the QPC center the polarization is zero, while, for GaAs parameters, the nuclear spins down (up) are induced on the source (drain) side. We predict that the dipolelike polarization pattern can be distinguished from a uniform polarization due to a qualitatively different response of the QPC conductance to the NMR field.

Excitations in (3+1)-dimensional [(3+1)D] topologically ordered phases have very rich structures. (3+1)D topological phases support both pointlike and stringlike excitations, and in particular the loop (closed string) excitations may admit knotted and linked structures. In this work, we ask the following question: How do different types of topological excitations contribute to the entanglement entropy or, alternatively, can we use the entanglement entropy to detect the structure of excitations, and further obtain the information of the underlying topological order? We are mainly interested in (3+1)D topological order that can be realized in Dijkgraaf-Witten (DW) gauge theories, which are labeled by a finite group $G$ and its group 4-cocycle $\omega \in {\mathcal{H}}^4[G;\text{U}\left(1\right)]$ up to group automorphisms. We find that each topological excitation contributes a universal constant $ln{d}_i$ to the entanglement entropy, where $d_i$ is the quantum dimension that depends on both the structure of the excitation and the data $(G,\omega )$. The entanglement entropy of the excitations of the linked/unlinked topology can capture different information of the DW theory $(G,\omega )$. In particular, the entanglement entropy introduced by Hopf-link loop excitations can distinguish certain group 4-cocycles $\omega$ from the others.

We demonstrate that multiple topological transitions can occur, with high sensitivity, by continuous change of the geometry of a simple two-dimensional dielectric-frame photonic crystal consisting of circular air holes. By changing the radii of the holes and/or the distance between them, multiple transitions between normal and topological photonic band gaps (PBGs) can appear. The time-reversal symmetric topological PBGs resemble the quantum spin Hall insulator of electrons and have two counterpropagating edge states. We search for optimal topological transitions, i.e., sharp transitions sensitive to the geometry, and optimal topological PBGs, i.e., large PBGs with a clean spectrum of edge states. Such optimizations reveal that dielectric-frame photonic crystals are promising for optical sensors and unidirectional waveguides.

An efficient and compact approach to the inclusion of dissipative effects in nonequilibrium Green's function (NEGF) simulations of electronic systems is introduced. The algorithm is based on two well-known methods in the literature, first that of the so-called recursive Green's function (RGF) and second that of Büttiker probes. Numerical methods for exact evaluation of the Jacobian are presented by a direct extension to RGF which can be modularly included in any codebase that uses it presently. Then using both physical observations and numerical methods, the computation time of the Büttiker probe Jacobian is improved significantly. An improvement to existing phonon models within Büttiker probes is then demonstrated in the simulation of fully atomistic graphene nanoribbon based field effect transistors in $n-i-n$ and $p-i-n$ operation.

Using exact diagonalization, we study the spin-orbit coupling and interaction-induced mixing between $t_{2g}$ and $e_{g}d$-orbital states in a cubic crystalline environment, as commonly occurs in transition metal oxides. We make a direct comparison with the widely used $t_{2g}$-only or $e_g$-only models, depending on electronic filling. We consider all electron fillings of the $d$ shell and compute the total magnetic moment, the spin, the occupancy of each orbital, and the effective spin-orbit coupling strength (renormalized through interaction effects) in terms of the bare interaction parameters, spin-orbit coupling, and crystal-field splitting, focusing on the parameter ranges relevant to $4d$ and $5d$ transition metal oxides. In various limits, we provide perturbative results consistent with our numerical calculations. We find that the $t_{2g}-e_g$ mixing can be large, with up to 20% occupation of orbitals that are nominally “empty,” which has experimental implications for the interpretation of the branching ratio in experiments, and can impact the effective local moment Hamiltonian used to study magnetic phases and magnetic excitations in transition metal oxides. Our results can aid the theoretical interpretation of experiments on these materials, which often fall in a regime of intermediate coupling with respect to electron-electron interactions.

We report magnetotransport measurements on magnetically doped $({\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ films grown by molecular beam epitaxy. In Hall bar devices, we observe logarithmic dependence of transport coefficients in temperature and bias voltage which can be understood to arise from electron-electron interaction corrections to the conductivity and self-heating. Submicron scale devices exhibit intriguing quantum oscillations at high magnetic fields with dependence on bias voltage. The observed quantum oscillations can be attributed to bulk and surface transport.