We study theoretically the effective spin Hall properties of a composite consisting of two materials with and without spin-orbit (SO) coupling. In particular, we assume that SO material represents a system of grains in a matrix with no SO. We calculate the effective spin Hall angle and the effective spin diffusion length of the mixture. Our main qualitative finding is that, when the bare spin diffusion length is much smaller than the radius of the grain, the effective spin diffusion length is strongly enhanced, well beyond the “geometrical” factor. The physical origin of this additional enhancement is that, with small diffusion length, the spin current mostly flows around the grain without suffering much loss. We also demonstrate that the voltage, created by a spin current, is sensitive to a very weak magnetic field directed along the spin current, and even reverses sign in a certain domain of fields. The origin of this sensitivity is that the spin precession, caused by magnetic field, takes place outside the grains where SO is absent.

Using high energy synchrotron x-ray radiation combined with electrostatic levitation, in situ structural analysis of a bulk metallic glass forming liquid is performed from above the liquidus temperature down to the glass transition. The data indicate a liquid-liquid transition (LLT) in the deeply undercooled state at $T/T_g\sim 1.2$ which manifests as a maximum in the heat capacity and an abrupt shift in the first peak position of the total structure factor in the absence of a pronounced density change. Analysis of the corresponding real-space data shows that the LLT involves changes in short- and medium-range order. The structural changes on the length scale of medium-range order imply a fragile-strong transition in agreement with experimental viscosity data.

We present a comprehensive $1/S$ study of the field-induced dynamical properties of the nearest-neighbor $\mathit{XXZ}$ antiferromagnet on a honeycomb lattice using the formalism of nonlinear spin-wave theory developed for this model. The external magnetic field controls spin frustration in the system and induces noncollinearity of the spin structure, which is essential for the two-magnon decay processes. Our results include an intriguing field-evolution of the regions of the Brillouin zone wherein decays of spin excitations are prominent, a detailed classification of the decay channels involving magnons from both excitation branches, and a thorough analysis of the singularities in the magnon spectra due to coupling to the two-magnon continuum, all of which are illustrated for several field and anisotropy values. We highlight a number of features related to either the non-Bravais nature of the lattice or the existence of the Dirac-like points in the spectrum. In addition, the asymptotic behavior of the decay rates near high-symmetry points is analyzed in detail. The inelastic neutron-scattering spin-spin structure factor is obtained in the leading $1/S$ order and is shown to exhibit qualitatively distinct fingerprints of the decay-induced magnon dynamics such as quasiparticle peaks broadened by decays and strong spectral weight redistribution.

We study the complex spin-state switching mechanism in a bistable molecular crystal undergoing a stepwise conversion from high-spin to low-spin states at thermal equilibrium as well as during the relaxation process from the photoinduced metastable state. We experimentally evidence that such steps are associated with complex types of long-range and short-range ordering phenomena, resulting from the occupational modulation of bistable molecular magnetic states. The conversion is then described by using two order parameters: the totally symmetric average high spin fraction and the symmetry breaking ordering parameter. The use of the anisotropic next-nearest-neighbor Ising (ANNNI) model allows us to describe the microscopic origin of the ordering, and Monte Carlo simulations reproduce the observed stepwise thermal phase transition as well as the stepwise relaxation from the photoinduced high-spin state to the low-spin state.

The influence of magnetic dipole interactions (MDIs) on the magnetic properties of local-moment Heisenberg spin systems is investigated. A general formulation is presented for calculating the eigenvalues $\lambda$ and eigenvectors $\widehat{\mu }$ of the MDI tensor of the magnetic dipoles in a line (one dimension, 1D), within a circle (2D) or a sphere (3D) of radius $r$ surrounding a given moment ${\vec{\mu }}_i$ for given magnetic propagation vectors $\mathbf{k}$ for collinear and coplanar noncollinear magnetic structures on both Bravais and non-Bravais spin lattices. Results are calculated for collinear ordering on 1D chains, 2D square and simple-hexagonal (triangular) Bravais lattices, 2D honeycomb and kagomé non-Bravais lattices, and 3D cubic Bravais lattices. The $\lambda$ and $\widehat{\mu }$ values are compared with previously reported results. Calculations for collinear ordering on 3D simple tetragonal, body-centered tetragonal, and stacked triangular and honeycomb lattices are presented for $c/a$ ratios from 0.5 to 3 in both graphical and tabular form to facilitate comparison of experimentally determined easy axes of ordering on these Bravais lattices with the predictions for MDIs. Comparisons with the easy axes measured for several illustrative collinear antiferromagnets (AFMs) are given. The calculations are extended to the cycloidal noncollinear ${120}^{\circ }$ AFM ordering on the triangular lattice where $\lambda$ is found to be the same as for collinear AFM ordering with the same k. The angular orientation of the ordered moments in the noncollinear coplanar AFM structure of ${\mathrm{GdB}}_4$ with a distorted stacked 3D Shastry-Sutherland spin-lattice geometry is calculated and found to be in disagreement with experimental observations, indicating the presence of another source of anisotropy. Similar calculations for the undistorted 2D and stacked 3D Shastry-Sutherland lattices are reported. The thermodynamics of dipolar magnets are calculated using the Weiss molecular field theory for quantum spins, including the magnetic transition temperature $T_{\mathrm{m}}$ and the ordered moment, magnetic heat capacity, and anisotropic magnetic susceptibility $\chi$ versus temperature $T$. The anisotropic Weiss temperature ${\theta }_{\mathrm{p}}$ in the Curie-Weiss law for $T>T_{\mathrm{m}}$ is calculated. A quantitative study of the competition between FM and AFM ordering on cubic Bravais lattices versus the demagnetization factor in the absence of FM domain effects is presented. The contributions of Heisenberg exchange interactions and of the MDIs to $T_{\mathrm{m}}$ and to ${\theta }_{\mathrm{p}}$ are found to be additive, which simplifies analysis of experimental data. Some properties in the magnetically-ordered state versus $T$ are presented, including the ordered moment and magnetic heat capacity and, for AFMs, the dipolar anisotropy of the free energy and the perpendicular critical field. The anisotropic $\chi$ for dipolar AFMs is calculated both above and below the Néel temperature $T_{\mathrm{N}}$ and the results are illustrated for a simple tetragonal lattice with $c/a>1, c/a=1$ (cubic), and $c/a<1$, where a change in sign of the $\chi$ anisotropy is found at $c/a=1$. Finally, following the early work of Keffer [Phys. Rev. 87, 608 (1952)], the dipolar anisotropy of $\chi$ above $T_{\mathrm{N}}=69$ K of the prototype collinear Heisenberg-exchange-coupled tetragonal compound ${\mathrm{MnF}}_2$ is calculated and found to be in excellent agreement with experimental single-crystal literature data above 130 K, where the smoothly increasing deviation of the experimental data from the theory on cooling from 130 K to $T_{\mathrm{N}}$ is deduced to arise from dynamic short-range collinear $c$-axis AFM ordering in this temperature range driven by the exchange interactions.

We theoretically study thermally activated phase slips in superfluid spin transport in easy-plane magnetic wires within the stochastic Landau-Lifshitz-Gilbert phenomenology, which runs parallel to the Langer-Ambegaokar-McCumber-Halperin theory for thermal resistances in superconducting wires. To that end, we start by obtaining the exact solutions for free-energy minima and saddle points. We provide an analytical expression for the phase-slip rate in the zero spin-current limit, which involves a detailed analysis of spin fluctuations at the extrema of the free energy. An experimental setup for a magnetoelectric circuit is proposed, in which thermal phase slips can be inferred by measuring nonlocal magnetoresistance.

The current-voltage characteristics of a voltage-biased Josephson junction in the low conductance regime of an ultra-low temperature scanning tunneling microscope (STM) is dominated by sequential charge tunneling. Using $P\left(E\right)$ theory we show that the Josephson coupling energy, experimentally determined in this regime, is in good agreement with the critical current $I_0$ calculated from the Ambegaokar-Baratoff formula. In this way, we can determine the critical current values of a Josephson junction in an STM. Furthermore, we experimentally determine a range of validity for $P\left(E\right)$ theory, which is in accordance with theoretical predictions. In this way, we establish an optimal parameter range, in which Josephson STM can be performed.

We show that the pressure-induced metal-insulator transition (MIT) in ${\mathrm{LaMnO}}_3$ is fundamentally different from the Mott-Hubbard transition and is percolative in nature, with the measured resistivity obeying the percolation scaling laws. Using the Gutzwiller method to treat correlation effects in a model Hamiltonian that includes both Coulomb and Jahn-Teller interactions, we show, one, that the MIT is driven by a competition between electronic correlation and the electron-lattice interaction, an issue that has been long debated, and two, that with compressed volume, the system has a tendency towards phase separation into insulating and metallic regions, consisting, respectively, of Jahn-Teller distorted and undistorted octahedra. This tendency manifests itself in a mixed phase of intermixed insulating and metallic regions in the experiment. Conduction in the mixed phase occurs by percolation and the MIT occurs when the metallic volume fraction, steadily increasing with pressure, exceeds the percolation threshold $v_c\approx 0.29$. Measured high-pressure resistivity follows the percolation scaling laws quite well, and the temperature dependence follows the Efros-Shklovskii variable-range hopping behavior for granular materials.

Using synchrotron and neutron diffraction measurements, we find a low-temperature orthorhombic phase in vanadium spinel ${\mathrm{FeV}}_2{\mathrm{O}}_4$. The orbital order of ${\mathrm{V}}^{3+}$ ions with tetragonal normal modes occurs at 68 K, and this leads to an appearance of the pseudotetragonal phase at a noncollinear ferrimagnetic transition temperature. Below the magnetic transition temperature, unconventional behavior of the orbital state of ${\mathrm{Fe}}^{2+}$ ions accompanied by the emergence of the orthorhombic phase was observed by using the normal mode analysis. We have also studied the structural properties of orbitally diluted materials. The orthorhombic phase, which is significantly affected by the other ions, is intrinsic in ${\mathrm{FeV}}_2{\mathrm{O}}_4$. We suggest the orthorhombic phase is strongly related with the double orbital states of ${\mathrm{Fe}}^{2+}$ and ${\mathrm{V}}^{3+}$ ions.

The interplay between thermal transport properties and topological aspects is investigated in a spin-triplet chiral $p$-wave superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ with strong two dimensionality. We show that thermal Hall conductivity is well described by a temperature linear term and an exponential term in the low-temperature region. While the former term is directly proportional to the so-called Chern number, the latter is associated with the superconducting gap amplitude of the $\gamma$ band. We also demonstrate that the coefficient of the exponential term changes sign around the Lifshitz transition. Our obtained result may enable us to easily access the physical quantities and topological properties of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ in detail.

We present a scheme for calculating electronic transport within the propagation approach to time-dependent density functional theory. Our scheme is based on solving the time-dependent Kohn-Sham equations on grids in real space and real time for a finite system. We use absorbing and antiabsorbing boundaries for simulating the coupling to a source and a drain. The boundaries are designed to minimize the effects of quantum-mechanical reflections and electrical polarization build-up, which are the major obstacles when calculating transport by applying an external bias to a finite system. We show that the scheme can readily be applied to real molecules by calculating the current through a conjugated molecule as a function of time. By comparing to literature results for the conjugated molecule and to analytic results for a one-dimensional model system we demonstrate the reliability of the concept.

Weyl semimetals (WSMs) are topologically protected three-dimensional materials whose low-energy excitations are linearly dispersing massless Dirac fermions, possessing a nontrivial Berry curvature. Using semiclassical Boltzmann dynamics in the relaxation time approximation for a lattice model of time-reversal (TR) symmetry broken WSMs, we compute both magnetic field dependent and anomalous contributions to the Nernst coefficient. In addition to the magnetic field dependent Nernst response, which is present in both Dirac and Weyl semimetals, we show that, contrary to previous reports, the TR-broken WSM also has an anomalous Nernst response due to a nonvanishing Berry curvature. We also compute the thermal conductivities of a WSM in the Nernst ($\nabla T\perp \mathbf{B}$) and the longitudinal ($\nabla T\parallel \mathbf{B}$) setup and confirm from our lattice model that in the parallel setup, the Wiedemann-Franz law is violated between the longitudinal thermal and electrical conductivities due to the chiral anomaly.

Using ab initio calculations, we investigate the effect of hydrostatic pressure on the electronic structure of ${\mathrm{LaMnO}}_3$ (111) bilayers sandwiched between ${\mathrm{LaAlO}}_3$. In the ideal heterostructure we observe Dirac cones at the Fermi energy. However, octahedral tiltings open a band gap and thus destroy the Dirac nature. We show that the effect of the tiltings can be suppressed by hydrostatic pressure from 40 GPa to 60 GPa. At higher pressure further phase transitions are encountered.

We performed electronic transport measurements on one-dimensional InAs quantum wires. In sufficiently disordered wires, transport is dominated by Coulomb blockade, and the conductance can be well described by tunneling through a quantum dot embedded between two one-dimensional Luttinger liquid wires. In contrast to previous experiments in other material systems, in our system the conductance difference between peak to valley decreases with decreasing temperature for several consecutive peaks. This phenomenon is theoretically expected to occur only for strongly interacting systems with small Luttinger interaction parameter $g<1/2$; we find for our InAs wires a value of $g\approx 0.4$. Possible mechanisms leading to these strong correlations are discussed.

We have investigated the lattice instability and the topological property of the electronic structure in PbPo in comparison with other IV-VI semiconductors, SnTe and PbTe. In the conventional exchange-correlation schemes of the density functional theory, the fcc structure of PbPo tends to be unstable in the presence of spin-orbit coupling (SOC) under [111] distortion so as to have a ferroelectric instability. This feature is revealed in the calculated phonon dispersion of PbPo by the phonon softening instability at $\mathbf{k}=\mathrm{\Gamma }$. But, in the modified Becke-Johnson (mBJ) potential scheme, we have shown that the tendency for SOC-driven ferroelectric instability is suppressed, and the fcc structure becomes stabilized. We have demonstrated that PbPo is a semiconductor having a band inversion at $\mathbf{k}=L$, which leads to a topological crystalline insulator (TCI) phase of PbPo, and the TCI and the ferroelectric states coexist for moderate [111] distortions.

Many promising nanophotonics endeavors hinge upon the unique plasmonic properties of nanometallic structures with narrow nonmetallic gaps, which support superconcentrated bonding modes that singularly redshift with decreasing separations. In this Rapid Communication, we present a descriptive physical picture, complemented by elementary asymptotic formulas, of a nonlocal mechanism for plasmon redshift saturation at subnanometric gap widths. Thus, by considering the electron-charge and field distributions in the close vicinity of the metal-vacuum interface, we show that nonlocality is asymptotically manifested as an effective potential discontinuity. For bonding modes in the near-contact limit, the latter discontinuity is shown to be effectively equivalent to a widening of the gap. As a consequence, the resonance-frequency near-contact asymptotics are a renormalization of the corresponding local ones. Specifically, the renormalization furnishes an asymptotic plasmon-frequency lower bound that scales with the $1/4$ power of the Fermi wavelength. We demonstrate these remarkable features in the prototypical cases of nanowire and nanosphere dimers, showing agreement between our elementary expressions and previously reported numerical computations.

We propose that the dynamical axion field can be realized in a magnetic topological insulator superlattice or a topological paramagnetic insulator. The magnetic fluctuations of these systems produce a pseudoscalar field which has an axionic coupling to the electromagnetic field, and thus it gives a condensed-matter realization of the axion electrodynamics. Compared to the previously proposed dynamical axion materials where a long-range antiferromagnetic order is required, the systems proposed here have the advantage that only a uniform magnetization or a paramagnetic state is needed for the dynamic axion. We further propose several experiments to detect such a dynamical axion field.

The electronic and topological properties of heterovalent and heterocrystalline $\alpha$-Sn/CdTe(001) quantum wells (QWs) are studied in dependence on the thickness of $\alpha$-Sn by means of ab initio calculations. We calculate the topological ${\mathbb{Z}}_2$ invariants of the respective bulk crystals, which identify $\alpha$-Sn as strong three-dimensional (3D) topological insulators (TIs), whereas CdTe is a trivial insulator. We predict the existence of two-dimensional (2D) topological interface states between both materials and show that a topological phase transition from a trivial insulating phase into the quantum spin Hall (QSH) phase in the QW structures occurs at much higher thicknesses than in the HgTe case. The QSH effect is characterized by the localization, dispersion, and spin polarization of the topological interface states. We address the distinction of the 3D and 2D TI characters of the studied QW structures, which is inevitable for an understanding of the underlying quantum state of matter. The 3D TI nature is characterized by two-dimensional topological interface states, while the 2D phase exhibits one-dimensional edge states. The two different state characteristics are often intermixed in the discussion of the topology of 2D QW structures, especially, the comparison of ab initio calculations and experimental transport studies.