We discuss the transport properties of a quantum spin-Hall insulator with sizable Rashba spin-orbit coupling in a disk geometry. The presence of topologically protected helical edge states allows for the control and manipulation of spin polarized currents: When ferromagnetic leads are coupled to the quantum spin-Hall device, the ballistic conductance is modulated by the Rashba strength. Therefore, by tuning the Rashba interaction via an all-electric gating, it is possible to control the spin polarization of injected electrons.

For quantum fermion problems, many accurate solvers are limited by the temperature regime in which they can be usefully applied. The Mermin theorem implies the uniqueness of an effective potential from which both the exact density and free energy at a target temperature can be found, via a calculation at a different, reference temperature. We derive exact expressions for both the potential and the free energy in such a calculation and introduce three controllable approximations that reduce the cost of such calculations. We illustrate the effective potential and its free energy, and test the approximations, on the asymmetric two-site Hubbard model at finite temperature.

Understanding the fine interplay between spin and orbital degrees of freedom in the surface states of topological insulators is the key to the development of next-generation spintronics devices. So far, the majority of studies have focused on the helical spin texture of the topologically protected surface state (TSS) and little attention has been devoted to the spin texture of the other surface states: the bulk-originated quantum well states (QWSs). This paper presents such study in a prototypical topological insulator, ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. By using spin- and angle-resolved photoemission spectroscopy, we reveal a full momentum-dependent spin texture of the QWSs and their response to light polarization. We find that these states share a similar spin-orbital texture with the TSS. This paper shows an intimate correlation between the surface and bulk states, which was overseen before, and provides relevant information to understand how the surface states evolve with the bulk states.

We consider theoretically a conical Ag tip coupled to surface states of a three-dimensional topological insulator (3D TI) such as ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Generation of propagating surface plasmon polaritons (SPPs) in a metal cone by a laser field produces an intense optical field near the tip of the cone and high concentration of hot carriers in the metal near the surface of the topological insulator. Such hot carriers are transferred through a Schottky contact between tip and TI to the chiral spin surface states of TI, resulting in finite spin polarization and spin current at the surface of TI. SPPs in the metal cone are excited with a THz laser with energy in the optical band gap of 3D TI to avoid photoexcitations in the bulk of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. We also propose a detail scheme for experimental measurements of such currents. Our results can find applications in spintronics.

We investigate quantum transport in a bilayer ${\mathrm{WSe}}_2$ in the presence of an external potential $V$ and interlayer coupling. The dc and ac conductivities are evaluated in the framework of linear response theory. As functions of the Fermi energy, the dc ones exhibit a nonmonotonic behavior with a flat region in the band gap. A deep local minimum is visible in spin-Hall conductivity near the top of the valence band as a consequence of a large valence band splitting. On the other hand, the ac spin- and valley-Hall conductivities, as functions of the frequency, show two opposite peaks near the top of the valence band. The spin-Hall conductivity exhibits a minimum value at $V=0$ whereas the valley-Hall conductivity decreases monotonically upon varying $V$ from negative to positive values. Furthermore, we evaluate the power absorption spectrum and assess its dependence on the spin and valley degrees of freedom. The results are pertinent to the development of electrically controlled spintronic and valleytronic devices based on bilayer ${\mathrm{WSe}}_2$ and other group VI semiconductors.

Topological materials which possess topologically protected surface states have attracted much attention in recent years. In this work, we study the critical current of superconductor/inversion symmetry breaking topological material/superconductor junctions. We found surprisingly that, in topological materials with broken inversion symmetry, the magnitude of the critical Josephson currents $|I_c^{+}\left(B\right)|$ at fixed magnetic field $B$ is not the same for critical currents $|I_c^{-}\left(B\right)|$ flowing in the opposite direction. Moreover, the critical currents violate the $|I_c^{\pm }\left(B\right)|=|I_c^{\pm }(-B)|$ relation and give rise to asymmetric Fraunhofer patterns. We call this phenomenon asymmetric Josephson effect (AJE). AJE can be used to detect inversion symmetry breaking in topological materials such as in quantum spin Hall systems and Weyl semimetals.

Electronic states of the Bi/InAs(110)-$(2\times 1)$ surface and its spin-polarized structure are revealed by angle-resolved photoelectron spectroscopy (ARPES), spin-resolved ARPES, and density-functional-theory calculation. The surface electronic state showed quasi-one-dimensional (Q1D) dispersion curves and a nearly metallic character; the top of the holelike band is just below the Fermi level. The size of the Rashba parameter ${\alpha }_{\mathrm{R}}$ reached a much larger value ($\sim 5.5$ eVÅ) than for previously reported 1D systems. The present result will provide a fertile playground for further studies of the exotic electronic phenomena in 1D or Q1D systems with the spin-split electronic states as well as for advanced spintronic devices.

The transport mechanism in titanium dioxide through polarons is an open issue. High-resolution electron energy-loss spectroscopy (HREELS) is in principle of great relevance in such context, provided the fingerprints on the loss spectrum of the charge carriers involved in the material are disclosed. This paper aims at evidencing those fingerprints. Through a suitable parametrization of the dielectric function, a theoretical analysis of EELS excitations in defective ${\mathrm{TiO}}_2$ rutile is developed in the framework of the semiclassical dielectric theory. The focus is put on the interplay between phonons, interband transitions, and defect-related excitations, namely, plasmon and band-gap states. Transport properties are demonstrated to be more efficiently grasped through the screening they induce on phonons than through the existence of a defined surface plasmon peak. While the corresponding imaginary part of the dielectric function only yields a slight broadening and temperature dependence of the quasielastic peak due to the large static dielectric function and electron effective mass, a sizable upward shift in energy and a decrease in intensity of phonons due to the real part are predicted. Band-gap states also screen phonons but with downward shift in energy loss. Due to its large oscillator strength, the high-energy-lying surface phonon at 95 meV is a very sensitive reporter of the combined effects of transport behavior and band-gap states. Finally, it is highlighted that extracting quantitative information out of EELS experiments requires an accurate modeling of the depth profile.

We have investigated the local low-energy excitations in ${\mathrm{CeRu}}_4{\mathrm{Sn}}_6$, a material discussed recently in the framework of strongly correlated Weyl semimetals, by means of Ce $M_5$ resonant inelastic x-ray scattering (RIXS). The availability of both ${}^2\mathrm{F}_{\frac{5}{2}}$ and ${}^2\mathrm{F}_{\frac{7}{2}}$ excitations of the Ce $4f^1$ configuration in the spectra allows for the determination of the crystal-electric field (CEF) parameters that explain quantitatively the high-temperature anisotropy of the magnetic susceptibility. The absence of an azimuthal dependence in the spectra indicates that all CEF states are close to being rotational symmetric. We show further that the non-negligible impact of the ${\check{A}}_6^0$ parameter on the ground state of ${\mathrm{CeRu}}_4{\mathrm{Sn}}_6$ leads to a reduction of the magnetic moment ${\mu }_c$ due to multiplet intermixing. This improves the agreement between CEF calculations and the experimentally determined magnetic susceptibility considerably at low temperatures. Deviations that persist at low temperatures for fields within the tetragonal plane are attributed to the Kondo interaction between $4f$ and conduction electrons. The RIXS results are consistent with inelastic neutron scattering data and are compared to the predictions from ab initio based electronic structure calculations.

Through first-principles calculations, the phonon-limited transport properties of cubic boron-V compounds (BP, BAs, and BSb) are studied. We find that the high optical phonon frequency in these compounds leads to the substantial suppression of polar scattering and the reduction of intervalley transition mediated by large-wave-vector optical phonons, both of which significantly facilitate charge transport. We also discover that BAs simultaneously has a high hole mobility ($2110\mathrm{c}{\mathrm{m}}^2/\mathrm{V}\mathrm{s}$) and electron mobility ($1400\mathrm{c}{\mathrm{m}}^2/\mathrm{V}\mathrm{s}$) at room temperature, which is rare in semiconductors. Our findings present insight into searching high mobility polar semiconductors, and point to BAs as a promising material for electronic and photovoltaic devices in addition to its predicted high thermal conductivity.

We introduce a minimum tight-binding model with only three parameters extracted from graphene and untwisted bilayer graphene. This model reproduces quantitatively the electronic structure of not only these two systems and bulk graphite near the Fermi level, but also that of twisted bilayer graphene including the value of the first magic angle, at which bands at $E_F$ flatten without overlap and two gaps open, one above and one below $E_F$. Our approach also predicts the second and third magic angle. The Hamiltonian is sufficiently transparent and flexible to be adopted to other twisted layered systems.

Parafermion zero modes are generalizations of Majorana modes that underlie comparatively rich non-Abelian-anyon properties. We introduce exact mappings that connect parafermion chains, which can emerge in two-dimensional fractionalized media, to strictly one-dimensional fermionic systems. In particular, we show that parafermion zero modes in the former setting translate into symmetry-enriched Majorana modes that intertwine with a bulk order parameter—yielding braiding and fusion properties that are impossible in standard Majorana platforms. Fusion characteristics of symmetry-enriched Majorana modes are directly inherited from the associated parafermion setup and can be probed via two kinds of anomalous pumping cycles that we construct. Most notably, our mappings relate ${\mathbb{Z}}_4$ parafermions to conventional electrons with time-reversal symmetry. In this case, one of our pumping protocols entails fairly minimal experimental requirements: Cycling a weakly correlated wire between a trivial phase and time-reversal-invariant topological superconducting state produces an edge magnetization with quadrupled periodicity. Our work highlights new avenues for exploring beyond-Majorana physics in experimentally relevant one-dimensional electronic platforms, including proximitized ferromagnetic chains.

We measure by means of in situ magneto-optical Kerr effect the magnetism of ultrathin magnetic films supported on gold and platinum during the capping with molecular layers. We observe both for ${\mathrm{C}}_{60}$ and ${\mathrm{Alq}}_3$ molecules deposited on Co films an enhancement of the out-of-plane magnetic anisotropy, leading to a magnetic hardening for out-of-plane magnetization. Surprisingly, the amplitude of this hardening depends strongly on the underlying substrate. While we find an increase of the coercive field around 100% for Co/Au(111), it can reach almost 600% on Co/Pt(111). Those results are discussed by using both tight-binding and ab initio calculations. It is shown on the case of ${\mathrm{C}}_{60}$ that both the adsorption geometry and the strain in the magnetic layer can be key ingredients to predict the quantitative change of magnetic anisotropy induced by the interface between a magnetic material and a molecular layer. On the contrary, the crystalline stacking of the Co has a minor influence.

We present a formulation for the pole expansion of the scattering matrix of open optical resonators, in which the pole contributions are expressed solely in terms of the resonant states, their wave numbers, and their electromagnetic fields. Particularly, our approach provides an accurate description of the optical scattering matrix without the requirement of a fit for the pole contributions, or the restriction to geometries, or systems with low Ohmic losses. Hence, it is possible to derive the analytic dependence of the scattering matrix on the wave number with low computational effort, which allows for avoiding the artificial frequency discretization of conventional frequency-domain solvers of Maxwell's equations and for finding the optical far- and near-field response based on the physically meaningful resonant states. This is demonstrated for three test systems, including a chiral arrangement of nanoantennas, for which we calculate the absorption and the circular dichroism.

We study a continuum model of dislocation transport in order to investigate the formation of heterogeneous dislocation patterns. We propose a physical mechanism that relates the formation of heterogeneous patterns with a well-defined wavelength to the stress-driven dynamics of dislocation densities that tries to minimize the internal energy while subject to dynamic constraints and a density-dependent, friction-like flow stress. This leads us to an interpretation that resolves the old “energetic vs dynamic” controversy regarding the physical origin of dislocation patterns and emphasizes the hydrodynamic nature of the instability that leads to dislocation patterning, which we identify as an instability of dislocation transport that is not dependent on processes such as dislocation multiplication or annihilation. We demonstrate the robustness of the developed patterning scenario by considering the simplest possible case (plane strain, single slip) in two model versions that consider the same driving stresses but implement the transport of dislocations that controls dislocation density evolution in two very different manners, namely (i) a hydrodynamic formulation that considers transport equations that are continuous in space and time, assuming that the dislocation velocity depends linearly on the local driving stress, and (ii) a stochastic cellular automaton implementation that assumes spatially and temporally discrete transport of discrete “packets” of dislocation density that move according to an extremal dynamics. Despite the differences, we find that the emergent patterns in both models are mutually consistent and in agreement with the prediction of a linear stability analysis of the continuum model. We also show how different types of initial conditions lead to different intermediate evolution scenarios that, however, do not affect the properties of the fully developed patterns.

We revisit the description of ferromagnetic domain wall dynamics through an extended one-dimensional model by allowing flexural distortions of the wall during its motion. This is taken into account by allowing the domain wall center and internal angle to be functions of position in the direction parallel to the wall. In the limit of small applied fields, this model accounts for the nonreciprocity in the propagation of wall modes and their stability in the presence of the Dzyaloshinskii-Moriya interaction and in-plane magnetic field.

Most of the low-energy effective descriptions of spin-orbit driven Mott insulators consider spin-orbit coupling (SOC) as a second-order perturbation to electron-electron interactions. However, when SOC is comparable to anisotropic Hund's coupling, such as in Ir, the validity of this formally weak SOC approach is not a priori known. Depending on the relative strength of SOC and anisotropic Hund's coupling, different descriptions of the multiplet structure should be employed in the weak and strong SOC limits, viz. LS and $jj$ coupling schemes, respectively. We investigate the implications of both the coupling schemes on the low-energy effective $t\text{−}J$ model and calculate the angle-resolved photoemission (ARPES) spectra using self-consistent Born approximation. In particular, we obtain the ARPES spectra of quasi-two-dimensional square-lattice iridate ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ in both weak and strong SOC limits. The differences in the limiting cases are understood in terms of the composition and relative energy splittings of the multiplet structure. Our results indicate that the LS coupling scheme yields better agreement with the experiment, thus providing an indirect evidence for the validity of LS coupling scheme for iridates. We also discuss the implications for other metal ions with strong SOC.