Local charge and spin currents are evaluated from the solutions of fully relativistic quantum mechanical scattering calculations for systems that include temperature-induced lattice and spin disorder as well as intrinsic alloy disorder. This makes it possible to determine material-specific spin transport parameters at finite temperatures. Illustrations are given for a number of important materials and parameters at 300 K. The spin-flip diffusion length $l_{\mathrm{sf}}$ of Pt is determined from the exponential decay of a spin current injected into a long length of thermally disordered Pt; we find $l_{\mathrm{sf}}^{\mathrm{Pt}}=5.3\pm 0.4\mathrm{nm}$. For the ferromagnetic substitutional disordered alloy permalloy (Py), we inject currents that are fully polarized parallel and antiparallel to the magnetization and calculate $l_{\mathrm{sf}}$ from the exponential decay of their difference; we find $l_{\mathrm{sf}}^{\mathrm{Py}}=2.8\pm 0.1\mathrm{nm}$. The transport polarization $\beta$ is found from the asymptotic polarization of a charge current in a long length of Py to be $\beta =0.75\pm 0.01$. The spin Hall angle ${\mathrm{\Theta }}_{\mathrm{sH}}$ is determined from the transverse spin current induced by the passage of a longitudinal charge current in thermally disordered Pt; our best estimate is ${\mathrm{\Theta }}_{\mathrm{sH}}^{\mathrm{Pt}}=4.5\pm 1\%$ corresponding to the experimental room-temperature bulk resistivity $\rho =10.8\mu \mathrm{\Omega }\mathrm{cm}$.

We calculate the thermoelectric response coefficients of three-dimensional Dirac or Weyl semimetals as a function of magnetic field, temperature, and Fermi energy. We focus in particular on the thermoelectric Hall coefficient ${\alpha }_{xy}$ and the Seebeck coefficient $S_{xx}$, which are well-defined even in the dissipationless limit. We contrast the behaviors of ${\alpha }_{xy}$ and $S_{xx}$ with those of traditional Schrödinger particle systems, such as doped semiconductors. Strikingly, we find that for Dirac materials ${\alpha }_{xy}$ acquires a constant, quantized value at sufficiently large magnetic field, which is independent of the magnetic field or the Fermi energy, and this leads to unprecedented growth in the thermopower and the thermoelectric figure of merit. We further show that even relatively small fields, such that ${\omega }_c\tau \sim 1$ (where ${\omega }_c$ is the cyclotron frequency and $\tau$ is the scattering time), are sufficient to produce a more than $100\%$ increase in the figure of merit.

We observe enhanced second harmonic generation in monolayer graphene in the presence of an ultrastrong terahertz field pulse of picosecond duration with a peak amplitude of 250 kV/cm. This is a strongly nonperturbative regime of light-matter interaction in which particles get accelerated to energies exceeding the initial Fermi energy of 0.2 eV over a timescale of a few femtoseconds. The second harmonic current is generated as electrons drift through the region of momenta corresponding to interband transition resonance at an optical frequency. The resulting strongly asymmetric distortion of carrier distribution in momentum space gives rise to an enhanced electric-dipole nonlinear response at the second harmonic. We develop an approximate analytic theory of this effect which accurately predicts observed intensity and polarization of the second harmonic signal.

We study the surface of a three-dimensional spin chiral ${\mathrm{Z}}_2$ topological insulator (class CII), demonstrating the possibility of its localization. This arises through an interplay of interaction and statistically symmetric disorder that confines the gapless fermionic degrees of freedom to a network of one-dimensional helical domain walls that can be localized. We identify two distinct regimes of this gapless insulating phase, a “clogged” regime wherein the network localization is induced by its junctions between otherwise metallic helical domain walls, and a “fully localized” regime of localized domain walls. The experimental signatures of these regimes are also discussed.

Anticrossing behavior between magnons in the noncollinear chiral magnet ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ and a two-mode $X$-band microwave resonator was studied in the temperature range 5–100 K. In the field-induced ferrimagnetic phase, we observed a strong-coupling regime between magnons and two microwave cavity modes with a cooperativity reaching 3600. In the conical phase, cavity modes are dispersively coupled to a fundamental helimagnon mode, and we demonstrate that the magnetic phase diagram of ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ can be reconstructed from the measurements of the cavity resonance frequency. In the helical phase, a hybridized state of a higher-order helimagnon mode and a cavity mode—a helimagnon polariton—was found. Our results reveal a class of magnetic systems where strong coupling of microwave photons to nontrivial spin textures can be observed.

We investigate the excited state electronic structure of the model phase transition system In/Si(111) using femtosecond time- and angle-resolved photoemission spectroscopy (trARPES). An extreme ultraviolet 500 kHz laser source at 21.7 eV is utilized both to map the energy of excited states above the Fermi level and follow the momentum-resolved population dynamics on a femtosecond timescale. Excited-state band mapping is used to characterize the normally unoccupied electronic structure above the Fermi level in both structural phases of In/Si(111): the metallic (4 x 1) and the gapped (8 x 2) phases. The extracted band positions are compared withband-structure calculations utilizing density functional theory within both the local density approximation and GW approximations (single-particle Green's function (G) + screened Coulomb interaction (W)). While good overall agreement is found between the GW-calculated band structure and experiment, deviations in specific momentum regions may indicate the importance of excitonic effects not accounted for at this level of approximation. To probe the dynamics of these excited states, their momentum-resolved transient population dynamics are extracted with trARPES. The transient intensities are compared to a simulated spectral function modeled by a state population employing a transient elevated electronic temperature as determined experimentally. This allows the momentum-resolved population dynamics to be quantitatively reproduced, revealing important insights into the transfer of energy from the electronic system to the lattice. In particular, a comparison between the magnitude and relaxation time of the transient electronic temperature observed by trARPES with those of the lattice as probed in previous ultrafast electron diffraction studies implies a highly nonthermal phonon distribution at the surface following photo-excitation. This suggests that the energy from the initially excited electronic system is initially transferred to high-energy optical phonon modes followed by cooling and thermalization of the photo-excited system by much slower phonon-phonon coupling.

We report on transport signatures of eight distinct bubble phases in the $N=3$ Landau level of an ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}\text{/}{\mathrm{Al}}_{0.24}{\mathrm{Ga}}_{0.76}\mathrm{As}$ quantum well with $x=0.0015$. These phases occur near partial filling factors ${\nu }^{★}\approx 0.2\left(0.8\right)$ and ${\nu }^{★}\approx 0.3\left(0.7\right)$ and have $M=2$ and $M=3$ electrons (holes) per bubble, respectively. We speculate that a small amount of alloy disorder in our sample helps to distinguish these broken symmetry states in low-temperature transport measurements.

Understanding the optical responses of nanostructures with high spatial resolution is paramount in photonic engineering. The excitation of resonant optical-frequency geometric modes in oxide nanoparticles is explored using monochromated electron energy-loss spectroscopy in a scanning transmission electron microscope. These geometric or cavity modes are found to produce a progression of resonance peaks within the bandgap regions of the electron energy-loss spectra of ${\mathrm{CeO}}_2, {\mathrm{TiO}}_2$, and MgO nanoparticles. Complementary simulations of the electron probe combined with analytic Mie analysis are performed to interpret the complex spectral features and to understand their underlying physical origins. The factors that influence the energies, shapes, and strengths of these modes are also investigated and their dependence upon nanoparticle size, geometry, refractive index, aggregation, impact parameter, and electron kinetic energy are elucidated. Taken together, this work demonstrates the unique ability of fast electron spectroscopy to determine the photonic density of states in individual and complex assemblies of dielectric nanoparticles.

We study the RKKY interaction between two magnetic impurities located on the surface of a three-dimensional Dirac semimetal with two Dirac nodes in the band structure. By taking into account both bulk and surface contributions to the exchange interaction between the localized spins, we demonstrate that the surface contribution in general dominates the bulk one at distances larger than the inverse node separation due to a weaker power-law decay. We find a strong anisotropy of the surface term with respect to the spins being aligned along the node separation axis or perpendicular to it. In the many impurity dilute regime, this implies formation of quasi-one-dimensional magnetic stripes orthogonal to the node axis. We also discuss the effects of a surface spin-mixing term coupling electrons from spin-degenerate Fermi arcs.

Diverse interlayer tunability of physical properties of two-dimensional layers mostly lies in the covalent-like quasibonding that is significant in electronic structures but rather weak for energetics. Such characteristics result in various stacking orders that are energetically comparable but may significantly differ in terms of electronic structures, e.g., magnetism. Inspired by several recent experiments showing interlayer antiferromagnetically coupled ${\mathrm{CrI}}_3$ bilayers, we carried out first-principles calculations for ${\mathrm{CrI}}_3$ bilayers. We found that the antiferromagnetic coupling results from a different stacking order with the $C2/m$ space group symmetry, rather than the graphene-like one with $R\overline{3}$ as previously believed. Moreover, we demonstrated that the intra- and interlayer couplings in ${\mathrm{CrI}}_3$ bilayer are governed by two different mechanisms, namely ferromagnetic superexchange and direct-exchange interactions, which are largely decoupled because of their significant difference in strength at the strong- and weak-interaction limits. This allows the much weaker interlayer magnetic coupling to be more feasibly tuned by stacking orders solely. Given the fact that interlayer magnetic properties can be altered by changing crystal structure with different stacking orders, our work opens a paradigm for tuning interlayer magnetic properties with the freedom of stacking order in two-dimensional layered materials.

Here, we develop a gauge-independent Green's-function approach to characterize the Chern invariants of generic non-Hermitian systems. It is shown that, analogous to the Hermitian case, the Chern number can be expressed as an integral of the system Green's function over a line parallel to the imaginary-frequency axis. The approach introduces in a natural way the “band gaps” of non-Hermitian systems as the strips of the complex-frequency plane wherein the system Green's function is analytical. We apply the developed theory to nonreciprocal electromagnetic continua, showing that the topological properties of gyrotropic materials are strongly robust to the effect of material loss. Furthermore, it is proven that the spectrum of a topological material cavity terminated with opaque-type walls must be gapless. This result suggests that the bulk-edge correspondence remains valid for a class of non-Hermitian systems.

The spin-triplet correlations in superconducting spin valve structures arising in the presence of noncollinear textures of magnetic moment are shown to enhance strongly the electromagnetic proximity effect, i.e., the long-range leakage of the magnetic field from the ferromagnet (F) to the superconducting (S) layer. Both the dirty and clean limits are studied on the basis of the Usadel and Eilenberger theory, correspondingly. Our results suggest a natural explanation for the puzzling enhancement of the spontaneous magnetic fields induced by the noncollinear magnetic structures observed by the muon spin rotation techniques in a wide class of layered S/F systems. We show that the electromagnetic proximity effect causes the shift of the Fraunhofer dependence of the critical current on the external magnetic field in the Josephson junction with one superconducting electrode covered by the ferromagnetic layer. This provides an alternative way to measure both the magnitude and the direction of the spontaneous magnetic field induced in the superconductor. We also demonstrate the possibility of the long-ranged superconductivity control of the magnetic state in ${\mathrm{F}}_1/\mathrm{S}/{\mathrm{F}}_2$ structures.

Operators in ergodic spin chains are found to grow according to hydrodynamical equations of motion. The study of such operator spreading has aided our understanding of many-body quantum chaos in spin chains. Here we initiate the study of “operator spreading” in quantum maps on a torus, systems which do not have a tensor-product Hilbert space or a notion of spatial locality. Using the perturbed Arnold cat map as an example, we analytically compare and contrast the evolutions of functions on classical phase space and quantum operator evolutions, and identify distinct timescales that characterize the dynamics of operators in quantum chaotic maps. Until an Ehrenfest time, the quantum system exhibits classical chaos, i.e., it mimics the behavior of the corresponding classical system. After an operator scrambling time, the operator looks “random” in the initial basis, a characteristic feature of quantum chaos. These timescales can be related to the quasienergy spectrum of the unitary via the spectral form factor. Furthermore, we show examples of “emergent classicality” in quantum problems far away from the classical limit. Finally, we study operator evolution in nonchaotic and mixed quantum maps using the Chirikov standard map as an example.

We consider three-dimensional superconductors in class DIII with a fourfold rotation axis and inversion symmetry. It is shown that such systems can exhibit higher-order topology with helical Majorana hinge modes. In the case of even-parity superconductors, we show that higher-order topological superconductors can be obtained by adding a small pairing with the appropriate $C_4$ symmetry implementation to a topological insulator. We also show that a hybrid case is possible, where Majorana surface cones resulting from nontrivial strong topology coexist with helical hinge modes. We propose a bulk invariant detecting this hybrid scenario, and numerically analyze a tight binding model exhibiting both Majorana cones and hinge modes.

We study classical percolation models in Fock space as proxies for the quantum many-body localization (MBL) transition. Percolation rules are defined for two models of disordered quantum spin chains using their microscopic quantum Hamiltonians and the topologies of the associated Fock-space graphs. The percolation transition is revealed by the statistics of Fock-space cluster sizes, obtained by exact enumeration for finite-sized systems. As a function of disorder strength, the typical cluster size shows a transition from a volume law in Fock space to subvolume law, directly analogous to the behavior of eigenstate participation entropies across the MBL transition. Finite-size scaling analyses for several diagnostics of cluster size statistics yield mutually consistent critical properties. We show further that local observables averaged over Fock-space clusters also carry signatures of the transition, with their behavior across it in direct analogy to that of corresponding eigenstate expectation values across the MBL transition. The Fock-space clusters can be explored under a mapping to kinetically constrained models. Dynamics within this framework likewise show the ergodicity-breaking transition via Monte Carlo averaged local observables and yield critical properties consistent with those obtained from both exact cluster enumeration and analytic results derived in our recent work [arXiv:1812.05115]. This mapping allows access to system sizes two orders of magnitude larger than those accessible in exact enumerations. Simple physical pictures based on freezing of local real-space segments of spins are also presented and shown to give values for the critical disorder strength and correlation length exponent $\nu$ consistent with numerical studies.

Time-resolved measurements of materials provide a wealth of information on quasiparticle dynamics, and have been the focus of optical studies for decades. In this paper, we develop a theory for explicitly evaluating time-resolved resonant inelastic x-ray scattering (tr-RIXS). We apply the theory to a noninteracting electronic system and reveal the particle-hole spectrum and its evolution during the pump pulse. With a high-frequency pump, the frequency and amplitude dependence analysis of the spectra agrees well with the steady-state assumptions and Floquet excitations. When the pump frequency is low, the spectrum extracts real-time dynamics of the particle-hole continuum in momentum space. These results verify the correctness of our theory and demonstrate the breadth of physical problems that tr-RIXS could shed light on.

We demonstrate mechanical modulations of the exciton lifetime by using vibrational strain of a gallium arsenide (GaAs) resonator. The strain-induced modulations have anisotropic dependences on the crystal orientation, which reveals the origin of these modulations to be the piezoelectric effect. Numerical analyses based on the tunneling model clarify that the mechanical strain modulates the internal electric field and spatially separates the electrons and holes, leading to nonradiative exciton decomposition. This carrier separation also generates an optomechanical back-action force from the photon to the resonator. Therefore, these results indicate that the mechanical motion can be self-modulated by exciton decays, which enables one to control the thermal noise of the resonators and provides a photon-exciton-phonon interaction in solid-state systems.

The noncentrosymmetric superconductor (NCS) AuBe is investigated using a variety of thermodynamic and resistive probes in magnetic fields of up to 65 T and temperatures down to 0.3 K. Despite the polycrystalline nature of the samples, the observation of a complex series of de Haas–van Alphen (dHvA) oscillations has allowed the calculated band structure for AuBe to be validated. This permits a variety of BCS parameters describing the superconductivity to be estimated, despite the complexity of the measured Fermi surface. In addition, AuBe displays a nonstandard field dependence of the phase of dHvA oscillations associated with a band thought to host unconventional fermions in this chiral lattice. This result demonstrates the power of the dHvA effect to establish the properties of a single band despite the presence of other electronic bands with a larger density of states, even in polycrystalline samples. In common with several other NCSs, we find that the resistive upper critical field exceeds that measured by heat capacity and magnetization by a considerable factor. We suggest that our data exclude mechanisms for such an effect associated with disorder, implying that topologically protected superconducting surface states may be involved.

Two-dimensional magnetic insulators exhibit a plethora of competing ground states, such as ordered (anti)ferromagnets, exotic quantum spin liquid states with topological order and anyonic excitations, and random singlet phases emerging in highly disordered frustrated magnets. Here we show how single-spin qubits, which interact directly with the low-energy excitations of magnetic insulators, can be used as a diagnostic of magnetic ground states. Experimentally tunable parameters, such as qubit level splitting, sample temperature, and qubit-sample distance, can be used to measure spin correlations with energy and wave-vector resolution. Such resolution can be exploited, for instance, to distinguish between fractionalized excitations in spin liquids and spin waves in magnetically ordered states, or to detect anyonic statistics in gapped systems.

A new paradigm of Anderson localization caused by correlations in the long-range hopping along with uncorrelated on-site disorder is considered which requires a more precise formulation of the basic localization-delocalization principles. A new class of random Hamiltonians with translation-invariant hopping integrals is suggested and the localization properties of such models are established both in the coordinate and in the momentum spaces alongside with the corresponding level statistics. Duality of translation-invariant models in the momentum and coordinate space is uncovered and exploited to find a full localization-delocalization phase diagram for such models. The crucial role of the spectral properties of hopping matrix is established and a new matrix inversion trick is suggested to generate a one-parameter family of equivalent localization-delocalization problems. Optimization over the free parameter in such a transformation together with the localization-delocalization principles allows us to establish exact bounds for the localized and ergodic states in long-range hopping models. When applied to the random matrix models with deterministic power-law hopping this transformation allows to confirm localization of states at all values of the exponent in power-law hopping and to prove analytically the symmetry of the exponent in the power-law localized wave functions.