We have measured the spin injection efficiency and spin lifetime in ${\mathrm{Co}}_2\mathrm{FeSi}/n$-GaAs lateral nonlocal spin valves from 20 to 300 K. We observe large ($\sim 40\mu \mathrm{V}$) spin valve signals at room temperature and injector currents of ${10}^3\mathrm{A}/{\mathrm{cm}}^2$, facilitated by fabricating spin valve separations smaller than the $1\mu \mathrm{m}$ spin diffusion length and applying a forward bias to the detector contact. The spin transport parameters are measured by comparing the injector-detector contact separation dependence of the spin valve signal with a numerical model accounting for spin drift and diffusion. The apparent suppression of the spin injection efficiency at the lowest temperatures reflects a breakdown of the ordinary drift-diffusion model in the regime of large spin accumulation. A theoretical calculation of the D'yakonov-Perel' spin lifetime agrees well with the measured $n$-GaAs spin lifetime over the entire temperature range.

Recently, the spin-selectivity effect of chiral molecules has been attracting extensive and growing interest among the scientific communities. Here, we propose a model Hamiltonian to study spin-dependent electron transport through helicene molecules which are connected by two semi-infinite graphene nanoribbons and try to elucidate a recent experiment of the spin-selectivity effect observed in the helicene molecules. The results indicate that the helicene molecules can present a significant spin-filtering effect in the case of extremely weak spin-orbit coupling, which is three orders of magnitude smaller than the hopping integral. The underlying physics is attributed to intrinsic chiral symmetry of the helicene molecules. When the chirality is switched from the right-handed species to the left-handed species, the spin polarization is reversed exactly. These results are consistent with a recent experiment [V. Kiran et al., Adv. Mater. 28, 1957 (2016)]. In addition, the spin-filtering effect of the helicene molecules is robust against molecular lengths, dephasing strengths, and space position disorder. This theoretical work may motivate further studies on chiral-induced spin selectivity in molecular systems.

A novel topological insulator with tunable edge states, called a quantum spin–quantum anomalous Hall (QSQAH) insulator, is predicted in a heterostructure of a hydrogenated $\mathrm{Sb}\left(\mathrm{S}{\mathrm{b}}_2\mathrm{H}\right)$ monolayer on a $\mathrm{LaFe}{\mathrm{O}}_3$ substrate by using ab initio methods. The substrate induces a drastic staggered exchange field in the $\mathrm{S}{\mathrm{b}}_2\mathrm{H}$ film, which plays an important role to generate the QSQAH effect. A topologically nontrivial band gap (up to 35 meV) is opened by Rashba spin-orbit coupling, which can be enlarged by strain and an electric field. To understand the underlying physical mechanism of the QSQAH effect, a tight-binding model based on $p_x$ and $p_y$ orbitals is constructed. With the model, the exotic behaviors of the edge states in the heterostructure are investigated. Dissipationless chiral charge edge states related to one valley are found to emerge along both sides of the sample, whereas low-dissipation spin edge states related to the other valley flow only along one side of the sample. These edge states can be tuned flexibly by polarization-sensitive photoluminescence controls and/or chemical edge modifications. Such flexible manipulations of the charge, spin, and valley degrees of freedom provide a promising route towards applications in electronics, spintronics, and valleytronics.

The possibility of the mechanical detection of the Casimir friction using noncontact force microscope is discussed. On a ${\mathrm{SiO}}_2$ tip situated above a graphene-covered ${\mathrm{SiO}}_2$ substrate will act the frictional drag force mediated by a fluctuating electromagnetic field produced by a current in the graphene sheet. This friction force will produce the bending of the cantilever, which can be measured by state-of-art noncontact force microscope. Both the thermal and quantum contributions to the Casimir frictional drag force can be studied using this experimental setup. This result paves the way for the mechanical detection of the Casimir friction and for the application of the frictional drag effect in micro- and nanoelectromechanical devices (MEMS and NEMS).

Despite extensive search for about a decade, specular Andreev reflection (SAR) has only recently been realized in the bilayer graphene-superconductor interface. The experimental observation of retro to specular Andreev reflection is not only fundamentally important, but also has potential application in quantum computing, etc. Here we have carried out the transport measurements at the van der Waals interface of single-layer graphene and ${\mathrm{NbSe}}_2$ superconductor. We investigate the Andreev reflection near the Dirac point by measuring the differential conductance as a function of Fermi energy and bias energy. We find that the normalized conductance ($G_{T<T_c}/G_{T>Tc}$) becomes suppressed as we pass through the Dirac cone, which manifests the transition from retro to nonretro-type Andreev reflection. The suppression indicates the blockage of Andreev reflection beyond a critical angle (${\theta }_c$) of the incident electron with respect to the normal between the single-layer graphene and the superconductor junction. However, the observation of SAR was restricted due to the finite Fermi-energy broadening. The results are compared with a theoretical model of the corresponding setup.

Multicomponent quantum Hall systems with internal degrees of freedom provide a fertile ground for the emergence of exotic quantum liquids. Here, we investigate the possibility of non-Abelian topological order in the half-filled fractional quantum Hall (FQH) bilayer system driven by the tunneling effect between two layers. By means of the state-of-the-art density-matrix renormalization group, we unveil “fingerprint” evidence of the non-Abelian Moore-Read Pfaffian state emerging in the intermediate-tunneling regime, including the ground-state degeneracy on the torus geometry and the topological entanglement spectroscopy (entanglement spectrum and topological entanglement entropy) on the spherical geometry, respectively. Remarkably, the phase transition from the previously identified Abelian (331) Halperin state to the non-Abelian Moore-Read Pfaffian state is determined to be continuous, which is signaled by the continuous evolution of the universal part of the entanglement spectrum, and discontinuities in the excitation gap and the derivative of the ground-state energy. Our results not only provide a “proof-of-principle” demonstration of realizing a non-Abelian state through coupling different degrees of freedom, but also open up a possibility in FQH bilayer systems for detecting different chiral $p\text{−}\text{wave}$ pairing states.

The formation of tunable bound states in the continuum (BICs) within photonic crystal (PC) slabs has been investigated by using a semianalytical coupled-wave theory framework. An analytic expression of the radiative wave has been derived in order to depict the condition of BICs. As a result, in addition to well-known symmetry-protected BICs, a novel type of vertical-cancellation BIC can be realized through continuously varying a given parameter to eliminate radiative waves at the boundaries. We investigated one-dimensional and two-dimensional (2D) periodic structures, and found that such tunable BICs can occur for a wide range of wave vectors by the selection of appropriate slab thicknesses. For a 2D PC slab, a ring of high-$Q$ modes is predicted and confirmed by numerical simulation.

In a partially filled flat Bloch band electrons do not have a well defined Fermi surface and hence the low-energy theory is not a Fermi liquid. Nevertheless, under the influence of an attractive interaction, a superconductor well described by the Bardeen-Cooper-Schrieffer (BCS) wave function can arise. Here we study the low-energy effective Hamiltonian of a generic Hubbard model with a flat band. We obtain an effective Hamiltonian for the flat band physics by eliminating higher-lying bands via the perturbative Schrieffer-Wolff transformation. At first order in the interaction energy we recover the usual procedure of projecting the interaction term onto the flat band Wannier functions. We show that the BCS wave function is the exact ground state of the projected interaction Hamiltonian, if a simple uniform pairing condition on the single-particle states is satisfied, and that the compressibility is diverging as a consequence of an emergent $\text{SU}\left(2\right)$ symmetry. This symmetry is broken by second-order interband transitions resulting in a finite compressibility, which we illustrate for a one-dimensional ladder with two perfectly flat bands. These results motivate a further approximation leading to an effective ferromagnetic Heisenberg model. The gauge-invariant result for the superfluid weight of a flat band can be obtained from the ferromagnetic Heisenberg model only if the maximally localized Wannier functions in the Marzari-Vanderbilt sense are used. Finally, we prove an important inequality $D\ge {\mathcal{W}}^2$ between the Drude weight $D$ and the winding number $\mathcal{W}$, which guarantees ballistic transport for topologically nontrivial flat bands in one dimension.

A variety of two-dimensional materials possess a band structure with an energy extremal ridge along a ring in momentum space. Examples are biased bilayer graphene and surfaces and interfaces with a Rashba spin-orbit interaction where at low doping the carriers fill an annulus. This topological feature causes an anomalous screening behavior, which we study using the Thomas-Fermi theory. Specifically, reducing the doping is predicted to enhance the linear screening response, whereas at zero doping the size of the screening cloud surrounding a Coulomb impurity is found to increase as the cube root of the impurity charge.

Using the two-loop analysis and the background field method we demonstrate that the local pure scaling operators without derivatives in the Finkel'stein nonlinear sigma model can be constructed by straightforward generalization of the corresponding operators for the noninteracting case. These pure scaling operators demonstrate multifractal behavior and describe mesoscopic fluctuations of the single-particle Green's function. We determine anomalous dimensions of all such pure scaling operators in the interacting theory within the two-loop approximation.

We revisit the thermodynamic behavior of the random-anisotropy O($N$) model by investigating its large-$N$ limit. We focus on the system at zero temperature where the mean-field-like artifacts of the large-$N$ limit are less severe. We analyze the connection between the description in terms of self-consistent Schwinger-Dyson equations and the functional renormalization group. We provide a unified description of the phase diagram and critical behavior of the model and clarify the nature of the possible “glassy” phases. Finally we discuss the implications of our findings for the finite-$N$ and finite-temperature systems.

We report dc and high-frequency transport properties of antiferromagnetic $\mathrm{S}{\mathrm{r}}_3\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$. Temperature-dependent resistivity measurements show that the activation energy of this material can be tuned by an applied dc electrical bias. The latter allows for continuous variations in the sample resistivity of as much as 50% followed by a reversible resistive switching at higher biases. Such a switching is of high interest for antiferromagnetic applications in high-speed memory devices. Interestingly, we found the switching behavior to be strongly affected by a high-frequency (microwave) current applied to the sample. The microwaves at 3–7 GHz suppress the dc switching and produce resonancelike features that we tentatively associated with the dissipationless magnonics recently predicted to occur in antiferromagnetic insulators subject to ac electric fields. We have characterized the effects of microwave irradiation on electronic transport in $\mathrm{S}{\mathrm{r}}_3\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ as a function of microwave frequency and power, strength and direction of external magnetic field, strength and polarity of applied dc bias, and temperature. Our observations support the potential of antiferromagnetic materials for high-speed/high-frequency spintronic applications.

We generalize the multiband typical medium dynamical cluster approximation and the formalism introduced by Blackman, Esterling, and Berk so that it can deal with localization in multiband disordered systems with both diagonal and off-diagonal disorder with complicated potentials. We also introduce an ansatz for the momentum-resolved typical density of states that greatly improves the numerical stability of the method while preserving the independence of scattering events at different frequencies. Starting from the first-principles effective Hamiltonian, we apply this method to the diluted magnetic semiconductor ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{N}$, and find the impurity band is completely localized for Mn concentrations $x<0.03$, while for $0.03<x<0.10$ the impurity band has delocalized states but the chemical potential resides at or above the mobility edge. So, the system is always insulating within the experimental compositional limit ($x\approx 0.10$) due to Anderson localization. However, for $0.03<x<0.10$ hole doping could make the system metallic, allowing double-exchange mediated, or enhanced, ferromagnetism. The developed method is expected to have a large impact on first-principles studies of Anderson localization.

We study the contribution of the Higgs amplitude mode on the nonlinear optical response of superconductors beyond the BCS approximation by taking into account the retardation effect in the phonon-mediated attractive interaction. To evaluate the vertex correction in nonlinear optical susceptibilities that contains the effect of collective modes, we propose an efficient scheme which we call the “dotted DMFT” based on the nonequilibrium dynamical mean-field theory (nonequilibrium DMFT), to get around the difficulty of solving the Bethe-Salpeter equation and analytical continuation. The vertex correction is represented by the derivative of the self-energy with respect to the external driving field, which is self-consistently determined by the differentiated (“dotted”) DMFT equations. We apply the method to the Holstein model, a prototypical electron-phonon-coupled system, to calculate the susceptibility for the third-harmonic generation including the vertex correction. The results show that, in sharp contrast to the BCS theory, the Higgs mode can contribute to the third-harmonic generation for general polarization of the laser field with an order of magnitude comparable to the contribution from the pair breaking or charge density fluctuations. The physical origin is traced back to the nonlinear resonant light–Higgs coupling, which has been absent in the BCS approximation.

This paper describes a semiclassical field-theory approach to the topological properties of spatially featureless Affleck-Kennedy-Lieb-Tasaki type valence bond solid ground states of antiferromagnets in spatial dimensions one to three. Using nonlinear sigma models set in the appropriate target manifold and augmented with topological terms, we argue that the path integral representation of the ground-state wave functional can correctly distinguish symmetry-protected topological ground states from topologically trivial ones. The symmetry-protection feature is demonstrated explicitly in terms of a dual field theory, where we take into account the nontrivial spatial structure of topological excitations, which are caused by competition among the relevant ordering tendencies. A temporal surface contribution to the action originating from the bulk topological term plays a central role in our study. We discuss how the same term governs the behavior of the so-called strange correlator. In particular, we find that the path integral expression for the strange correlator in two dimensions reduces to the well-known Haldane expression for the two point spin correlator of antiferromagnetic spin chains.

Considering the interband correlation, we present a generalized multiple-scattering approach of Green's function to investigate the effects of electron-impurity scattering on the density of states in silicene at zero temperature. The reduction of energy gaps in the case of relatively high chemical potential and the transformation of split-off impurity bands into band tails for low chemical potential are found. The dependency of optical conductivity on the impurity concentration is also discussed for frequency within the terahertz regime.