We study thermoelectric transport through a coherent molecular conductor connected to two electron and two phonon baths using the nonequilibrium Green's function method. We focus on the mutual drag between electron and phonon transport as a result of ‘momentum’ transfer, which happens only when there are at least two phonon degrees of freedom. After deriving expressions for the linear drag coefficients, obeying the Onsager relation, we further investigate their effect on nonequilibrium transport. We show that the drag effect is closely related to two other phenomena: (1) adiabatic charge pumping through a coherent conductor; (2) the current-induced nonconservative and effective magnetic forces on phonons.

Multiscale plasmonic systems (e.g., extended metallic nanostructures with subnanometer inter-distances) play a key role in the development of next-generation nanophotonic devices. An accurate modeling of the optical interactions in these systems requires an accurate description of both quantum effects and far-field properties. Classical electromagnetism can only describe the latter, while time-dependent density functional theory (TD-DFT) can provide a full first-principles quantum treatment. However, TD-DFT becomes computationally prohibitive for sizes that exceed few nanometers, which are instead very important for most applications. In this article, we introduce a method based on the quantum hydrodynamic theory (QHT) that includes nonlocal contributions of the kinetic energy and the correct asymptotic description of the electron density. We show that our QHT method can predict both plasmon energy and spill-out effects in metal nanoparticles in excellent agreement with TD-DFT predictions, thus allowing reliable and efficient calculations of both quantum and far-field properties in multiscale plasmonic systems.

We introduce a system of stacked two-dimensional electron- and hole-gas layers with Rashba spin-orbit interaction and show that the tunnel coupling between the layers induces a strong three-dimensional (3D) topological insulator phase. At each of the two-dimensional bulk boundaries we find the spectrum consisting of a single anisotropic Dirac cone, which we show by analytical and numerical calculations. Our setup has a unit cell consisting of four tunnel coupled Rashba layers and presents a synthetic strong 3D topological insulator and is distinguished by its rather high experimental feasibility.

In a ferroelectric helimagnetic phase of a spin-driven multiferroic, ${\mathrm{CuFeO}}_2$, we find irreversibly additive evolution of electric polarization $P$ induced by sweeping magnetic field $H$ under an applied electric field $E$, despite a large coercive electric field in the phase. From the unpolarized neutron diffraction experiments with in situ $P$ measurements under applied $E$, we reveal that increment of $P$ is achieved by the variation of an incommensurate magnetic modulation wave number $q$ of the helical magnetic ordering in $H$ sweeping regardless of increasing or decreasing $H$. Combining this result with the $H$ dependence of the magnetic diffraction intensity and a result of off-bench $P$ measurements, we conclude that the $H$ evolution of $P$ is caused by a change in a (ferroelectric) helicity domain volume fraction by driving the helicity domain wall (DW). Taking into account the results of further detailed $P$ measurements, we provide a speculation for microscopic helicity DW motion. The present study demonstrates the magnetoelectric cross correlation in driving a multiferroic DW: we can activate the frozen ferroelectric DW by means of $H$ sweeping. This is also an achievement of driving an antiferromagnetic DW, which is difficult in conventional antiferromagnets in principle.

We study the effects of $\delta$-function periodic kicks on the Floquet energy-momentum dispersion in graphene. We find that a rich variety of dispersions can appear depending on the parameters of the kicking: at certain points in the Brillouin zone, the dispersion can become linear but anisotropic, linear in one direction and quadratic in the perpendicular direction, gapped with a quadratic dispersion, or completely flat (called dynamical localization). We show all these results analytically and demonstrate them numerically through the dynamics of wave packets propagating in graphene. We propose experimental methods for producing these effects.

We have studied element-selective magnetic properties of the hetero- and homometallic metallacrowns $\mathrm{Cu}{\left(\mathrm{II}\right)}_2[12-{\mathrm{MC}}_{YN\left(Shi\right)}-4]$ ($Y=\text{Cu}$, Fe, in short ${\mathrm{CuCu}}_4$ and ${\mathrm{CuFe}}_4$). These metallacrowns comprise four Fe or Cu ions surrounding a central Cu ion. Using x-ray magnetic circular dichroism we have probed local symmetries, electronic configuration, orbital and spin magnetic moments of the magnetic ions. The ratio between the Cu and Fe moment of $-0.11$ is independent of temperature in the range of 15 K to 90 K. The Cu moment shows antiparallel to the Fe moment. For ${\mathrm{CuCu}}_4$ we confirm the predicted $S=1/2$ ground state. The comparison of the spectral dependence indicates a localization of the magnetic moment at the central Cu ion. The orbital to spin moment ratio is 0.03 for the Fe ion while a larger ratio of 0.12 was measured for the Cu ion in both compounds. The experimental results are compared to a mean field model and to an isotropic Heisenberg model.

We study Josephson junctions made of semiconducting nanowires with Rashba spin-orbit coupling, where superconducting correlations are induced by the proximity effect. In the presence of a suitably directed magnetic field, the system displays the anomalous Josephson effect: a nonzero supercurrent in the absence of a phase bias between two superconductors. We show that this anomalous current can be increased significantly by tuning the nanowire into the helical regime. In particular, in a short junction, a large anomalous current is a signature for topologically nontrivial superconductivity in the nanowire.

When current flows through a magnetic tunnel junction (MTJ), there is spin accumulation at the electrode-barrier interfaces if the magnetic moments of the two ferromagnetic electrodes are not aligned. Here we report that such nonequilibrium spin accumulation generates its own characteristic low frequency noise (LFN). Past work viewed the LFN in MTJs as an equilibrium effect arising from resistance fluctuations $\left(S_R\right)$ which a passively applied current $\left(I\right)$ converts to measurable voltage fluctuations $\left(S_V=I^2{S}_R\right)$. We treat the LFN associated with spin accumulation as a nonequilibrium effect, and find that the noise power can be fitted in terms of the spin-polarized current by $S_{I}f=aIcoth\left(\frac{I}{b}\right)-ab$, resembling the form of the shot noise for a tunnel junction, but with current now taking the role of the bias voltage, and spin-flip probability taking the role of tunneling probability.

The diffusion properties of a wide range of impurities (transition metals and Al, Si, and P) in ferritic alloys are here investigated by means of a combined ab initio–atomic diffusion theory approach. The flux-coupling mechanisms and the solute-diffusion coefficients are inferred from electronic-structure calculations of solute-defect interactions and microscopic jump frequencies. All properties except the second-nearest-neighbor binding energy are found to have a characteristic bell shape as a function of the $d$-band filling for the $4d$ and $5d$ series, and an M shape for the $3d$ row because of the out-of-trend behavior of Mn. The solute jump frequencies are governed by compressibility, which makes diffusion of large solutes faster, although this effect is partially compensated for by lower attempt frequencies and larger correlations with the vacancy. Diffusion coefficients are predicted in a wide temperature range, far below the experimentally accessible temperatures. In accordance with experiments, Co is found to be a slow diffuser in iron, and the same behavior is predicted for Re, Os, and Ir impurities. Finally, flux-coupling phenomena depend on the iron jump frequencies next to a solute atom, which are mainly controlled by similar electronic interactions to those determining the binding energies. Vacancy drag and solute enrichment at sinks systematically arise below a solute-dependent temperature threshold, directly correlated with the electronic-level interactions at the equilibrium and the saddle-point states. Early transition metals with repulsive second-nearest-neighbor interactions also diffuse via vacancy drag, although they show a lower temperature threshold than the late metals. This confirms that drag is the most common solute-vacancy coupling mechanism in iron at low temperatures, and this is likely to be confirmed as well for impurity diffusion in other transition metals.

The electronic structures and topological properties of transition metal dipnictides $XP{n}_2$ ($X=\mathrm{Ta}$, Nb; $Pn=\mathrm{P}$, As, Sb) have been systematically studied using first-principles calculations. In addition to small bulk Fermi surfaces, the band anticrossing features near the Fermi level can be identified from band structures without spin-orbit coupling, leading to nodal lines in all these compounds. Inclusion of spin-orbit coupling gaps out these nodal lines, leaving only a pair of disentangled electron/hole bands crossing the Fermi level. Therefore, the low-energy physics can be in general captured by the corresponding two-band model with several isolated small Fermi pockets. Detailed analysis of the Fermi surfaces suggests that the arsenides and ${\mathrm{NbSb}}_2$ are nearly compensated semimetals while the phosphorides and ${\mathrm{TaSb}}_2$ are not. Based on the calculated band parities, the electron and hole bands are found to be weakly topological nontrivial, giving rise to surface states. As an example, we presented the surface-direction-dependent band structure of the surfaces states in ${\mathrm{TaSb}}_2$.

Valley asymmetry of the electron spectrum in transition-metal dichalcogenides (TMDs) originates from the spin-orbit coupling. Presence of spin-orbit fields of opposite signs for electrons in $K$ and $K^{\prime }$ valleys in combination with possibility of intervalley scattering result in a nontrivial spin dynamics. This dynamics is reflected in the dependence of nonlocal resistance on external magnetic field (the Hanle curve). We calculate theoretically the Hanle shape in TMDs. It appears that, unlike conventional materials without valley asymmetry, the Hanle shape in TMDs is different for normal and parallel orientations of the external field. For normal orientation, it has two peaks for slow intervalley scattering, while for fast intervalley scattering, the shape is usual. For parallel orientation, the Hanle curve exhibits a cusp at zero field. This cusp is a signature of a slow-decaying valley-asymmetric mode of the spin dynamics.

Using nonequilibrium Green's functions, we studied numerically the transport properties of a Josephson junction, superconductor-topological insulator-superconductor hybrid system. Our numerical calculation shows first that proximity-induced superconductivity is indeed observed in the edge states of a topological insulator adjoining two superconducting leads and second that the special characteristics of topological insulators endow the edge states with an enhanced proximity effect with a superconductor but do not forbid the bulk states to do the same. In a size-dependent analysis of the local current, it was found that a few residual bulk states can lead to measurable resistance, whereas because these bulk states spread over the whole sample, their contribution to the interference pattern is insignificant when the sample size is in the micrometer range. Based on these numerical results, it is concluded that the apparent disappearance of residual bulk states in the superconducting interference process as described by Hart et al. [Nat. Phys. 10, 638 (2014)] is just due to the effects of size: the contribution of the topological edge states outweighs that of the residual bulk states.

We theoretically reveal the possibility of specular Andreev reflection in a thin film topological insulator normal-superconductor (N/S) junction in the presence of a gate electric field. The probability of specular Andreev reflection increases with the electric field, and electron-hole conversion with unit efficiency happens in a wide experimentally accessible range of the electric field. We show that perfect specular Andreev reflection can occur for all angles of incidence with a particular excitation energy value. In addition, we find that the thermal conductance of the structure displays exponential dependence on the temperature. Our results reveal the potential of the proposed topological insulator thin-film-based N/S structure for the realization of intraband specular Andreev reflection.

We study a class of translational-invariant insulators with discrete rotational symmetry. These insulators have no spin-orbit coupling, and in some cases have no time-reversal symmetry either; i.e., the relevant symmetries are purely crystalline. Nevertheless, topological phases exist which are distinguished by their robust surface modes. Like many well-known topological phases, their band topology is unveiled by the crystalline analog of Berry phases, i.e., parallel transport across certain noncontractible loops in the Brillouin zone. We also identify certain topological phases without any robust surface modes; they are uniquely distinguished by parallel transport along bent loops, whose shapes are determined by the symmetry group. Our findings have experimental implications in cold-atom systems, where the crystalline Berry phase has been directly measured.

Dephasing in terahertz quantum cascade structures is studied within a density matrix formalism. We self-consistently calculate the pure dephasing time from the intrasubband interactions within the upper and lower lasing states. Interface roughness and ionized impurity scattering interactions are included in the calculation. Dephasing times are shown to be consistent with measured spontaneous emission spectra, and the lattice temperature dependence of the device output power is consistent with experiment. The importance of including multiple optical transitions when a lower miniband continuum is present and the resulting multi-longitudinal modes within the waveguide resonant cavity are also shown.

An effective self-consistent model is derived and used to study the dynamics of an electron gas confined in a nonparabolic and anisotropic quantum well. This approach is based on the equations of quantum hydrodynamics, which incorporate quantum and nonlinear effects in an approximate fashion. The effective model consists of a set of six coupled differential equations (dynamical system) for the electric dipole and the size of the electron gas. Using this model we show that: (i) high harmonic generation is related to the appearance of chaos in the phase space, as attested to by related Poincaré sections; (ii) higher order harmonics can be excited efficiently and with relatively weak driving fields by making use of chirped electromagnetic waves.

We study the theoretical conditions to excite a stable self-oscillation in a spin-torque oscillator with an in-plane magnetized free layer and a perpendicularly magnetized pinned layer in the presence of magnetic field pointing in an arbitrary direction. The linearized Landau-Lifshitz-Gilbert (LLG) equation is found to be inapplicable to evaluate the threshold between the stable and self-oscillation states because the critical current density estimated from the linearized equation is considerably larger than that found in the numerical simulation. We derive a theoretical formula of the threshold current density by focusing on the energy gain of the magnetization from the spin torque during a time shorter than a precession period. A good agreement between the derived formula and the numerical simulation is obtained. The condition to stabilize the out-of-plane self-oscillation above the threshold is also discussed.

We present magnetic susceptibility, dielectric constant, high-frequency electron spin resonance, ${}^7\mathrm{Li}$ nuclear magnetic resonance, and zero-field muon spin relaxation measurements of ${\mathrm{LiACr}}_4{\mathrm{O}}_8$ ($\mathrm{A}=\mathrm{Ga}$, In), towards realizing a breathing pyrochlore lattice. Unlike the uniform pyrochlore ${\mathrm{ZnCr}}_2{\mathrm{O}}_4$ lattice, both the In and the Ga compounds feature two-stage symmetry breaking: a magnetostructural phase transition with subsequent antiferromagnetic ordering. We find a disparate symmetry breaking process between the In and the Ga compounds, having different degrees of bond alternation. Our data reveal that the Ga compound with moderate bond alternation shows the concomitant structural and magnetic transition at $T_S=15.2$ K, followed by the magnetic ordering at $T_m=12.9$ K. In contrast, the In compound with strong bond alternation undergoes a thermal crossover at $T^{*}\approx 20.1$ K from a tetramer singlet to a dimer singlet or a correlated paramagnet with a separate weak magnetostructural transition at $T_S=17.6$ K and the second antiferromagnetic ordering at $T_m=13.7$ K. This suggests that the magnetic phases and correlations of the breathing pyrochlore lattice can be determined from the competition between bond alternation and spin-lattice coupling, thus stabilizing long-range magnetic ordering against a nonmagnetic singlet.