We consider the Nernst and Hall effects in the fluctuation regime of chiral superconductors above transition temperatures, that are raised not by the conventional Lorentz force but by a mechanism that is an analog of the anomalous Nernst or Hall effects, i.e., asymmetric scattering due to chiral superconducting fluctuations. It is found that these effects can be gigantic for cleaner samples compared to conventional ones, exhibiting qualitatively distinct behavior. The results provide systematic and comprehensive understanding for recent experimental observations of the Nernst effect in a clean ${\mathrm{URu}}_2{\mathrm{Si}}_2$ sample, which is suggested to be a chiral superconductor.

Samarium hexaboride $\left({\mathrm{SmB}}_6\right)$ is the first strongly correlated material with a recognized nontrivial band-structure topology. Its electron correlations are seen by inelastic neutron scattering as a coherent collective excitation at the energy of 14 meV. Here, we calculate the spectrum of this mode using a perturbative slave-boson method. Our starting point is the recently constructed Anderson model that properly captures the band-structure topology of ${\mathrm{SmB}}_6$. Most self-consistent renormalization effects are captured by a few phenomenological parameters whose values are fitted to match the calculated and experimentally measured mode spectrum in the first Brillouin zone. A simple band-structure of low-energy quasiparticles in ${\mathrm{SmB}}_6$ is also modeled through this fitting procedure because the important renormalization effects due to Coulomb interactions are hard to calculate by ab initio methods. Despite involving uncontrolled approximations, the slave-boson calculation is capable of producing a fairly good quantitative match of the energy spectrum, and a qualitative match of the spectral weight throughout the first Brillouin zone. We find that the “fitted” band structure required for this match indeed puts ${\mathrm{SmB}}_6$ in the class of strong topological insulators. Our analysis thus provides a detailed physical picture of how the ${\mathrm{SmB}}_6$ band topology arises from strong electron interactions, and paints the collective mode as magnetically active exciton.

We study topological phases in orthorhombic perovskite iridium (Ir) oxide superlattices grown along the $\left[001\right]$ crystallographic axis. Bilayer Ir oxide superlattices display topological magnetic insulators exhibiting quantized anomalous Hall effects due to strong spin-orbit coupling of Ir $5d$ orbitals and electronic correlation effects. We also find a valley Hall insulator with counterpropagating edge currents from two different valleys and a topological crystalline insulator with edge states protected by the crystal lattice symmetry based on stacking of two layers. In a single-layer superlattice, a topological insulator can be realized, when a strain field is applied to break the symmetry of a glide plane protecting the Dirac points. It turns into a topological magnetic insulator in the presence of magnetic ordering and/or in-plane magnetic field. We discuss essential ingredients for these topological phases and experimental signatures to test our theoretical proposals.

Quantum oscillation measurements provide relevant information about the Fermi surface (FS) properties of strongly correlated metals. Here, we report on the Shubnikov–de Haas effect via high-field resistivity measurements of ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ (Eu122) and ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ (Ba122) single crystals. Although both pnictide compounds are isovalent with similar effective masses and density of states, at the Fermi level, our results reveal subtle changes in their fermiology. Remarkably, although the spin-density-wave (SDW) ordering temperature is higher in the Eu-rich end, Eu122 displays a much more isotropic and three-dimensional-like FS when compared with Ba122, in agreement with band structure calculations. Our experimental results suggest an anisotropic contribution of the Fe $3d$ orbitals to the FS in Ba122. We speculate that this orbital differentiation may be responsible for the suppression of the SDW phase in the FeAs-based compounds.

We investigate a two-dimensional electron system (2DES) under microwave illumination at cyclotron resonance subharmonics. The 2DES carries sufficient direct current, $I$, that the differential resistivity oscillates as $I$ is swept. At magnetic fields sufficient to resolve individual Landau levels, we find the number of oscillations within an $I$ range systematically changes with increasing microwave power. Microwave absorption and emission of $N$ photons, where $N$ is controlled by the microwave power, describes our observations in the framework of the displacement mechanism of impurity scattering between Hall-field tilted Landau levels.

We present a comprehensive ab initio study of structural, electronic, lattice dynamical, and electron-phonon coupling properties of the Bi(111) surface within density functional perturbation theory. Relativistic corrections due to spin-orbit coupling are consistently taken into account. Changes of interatomic couplings are confined mostly to the first two bilayers, resulting in superbulk modes with frequencies higher than the optic bulk spectrum, and in an enhanced density of states at lower frequencies for atoms in the first bilayer. We give results for the momentum-dependent electron-phonon coupling of electronic states belonging to the two surface electronic bands along $\overline{\Gamma M}$ which cross the Fermi energy. For larger momenta, the lower surface band exhibits a moderate electron-phonon coupling of about 0.45, which is larger than the coupling constant of bulk Bi. For momenta close to $\overline{\Gamma }$, states of both surface bands show even stronger couplings because of interband transitions to bulk states near $\overline{\Gamma }$ around the Fermi level. For these cases, the state-dependent Eliashberg functions exhibit pronounced peaks at low energy and strongly deviate in shape from a Debye-type spectrum, indicating that an extraction of the coupling strength from measured electronic self-energies based on this simple model is likely to fail.

Using nonequilibrium molecular dynamic simulations, the thermal conductivity of a set of Si phononic metamaterial nanowires with a twinning superlattice structure has been investigated. We first show that this latter structural modulation can yield 65% thermal-conductivity reduction compared to the straight wire case at room temperature. Second, a purely geometry-induced minimal thermal conductivity of the phononic metamaterial is observed at a specific period depending on the nanowire diameter. Mode analysis reveals that the the minimal thermal conductivity arises due to the disappearance of favored atom polarization directions. The current thermal-conductivity reduction mechanism can collaborate with the other known reduction mechanisms, such as the one related to coating, to further reduce thermal conductivity of the metamaterial. Current studies reveal that twinning superlattice nanowires could serve as a promising candidate for efficient thermoelectric conversion benefitting from the large suppression in thermal transport and without deterioration of electron-transport properties when the surface atoms are passivated.

Resonant manipulation of carbon nanotube valley-spin qubits by an electric field is investigated theoretically. We develop a new analysis of electrically driven spin resonance exploiting fixed physical characteristics of the nanotube: a bend and inhomogeneous disorder. The spectrum is simulated for an electron valley-spin qubit coupled to a hole valley-spin qubit and an impurity electron spin, and features that coincide with a recent measurement are identified. We show that the same mechanism allows resonant control of the full four-dimensional spin-valley space.

We investigate the properties of a single substitutional Mn impurity and its associated acceptor state on the (111) surface of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ topological insulator. Combining ab initio calculations with microscopic tight-binding modeling, we identify the effects of inversion symmetry and time-reversal-symmetry breaking on the electronic states in the vicinity of the Dirac point. In agreement with experiments, we find evidence that the Mn ion is in the $+2$ valence state and introduces an acceptor in the bulk band gap. The Mn acceptor has predominantly $p$ character and is localized mainly around the Mn impurity and its nearest-neighbor Se atoms. Its electronic structure and spin-polarization are determined by the hybridization between the Mn $d$ levels and the $p$ levels of surrounding Se atoms, which is strongly affected by electronic correlations at the Mn site. The opening of the gap at the Dirac point depends crucially on the quasiresonant coupling and the strong real-space overlap between the spin-chiral surface states and the midgap spin-polarized Mn-acceptor states.

We present a theoretical study of the optical conductivity of graphene with magnetic and nonmagnetic adatoms. First, by introducing an alternating potential in a pure graphene, we demonstrate a gap formation in the density of states and the corresponding optical conductivity. We highlight the distinction between such a gap formation and the so-called Pauli blocking effect. Next, we apply this idea to graphene with adatoms by introducing magnetic interactions between the carrier spins and the spins of the adatoms. Exploring various possible ground-state spin configurations of the adatoms, we find that the antiferromagnetic configuration yields the lowest total electronic energy and is the only configuration that forms a gap. Furthermore, we analyze four different circumstances leading to similar gaplike structures and propose a means to interpret the magneticity and the possible orderings of the adatoms on graphene solely from the optical conductivity data. We apply this analysis to the recently reported experimental data of oxygenated graphene.

We investigate electron transport through a finite two dimensional mesoscopic periodic potential, consisting of an array of lateral quantum dots with electron density controlled by a global top gate. We observe a transition from an insulating state at low-bias voltages to a conducting state at high-bias voltages. The insulating state shows simply activated temperature dependence, with strongly gate voltage dependent activation energy. At low temperatures the transition between the insulating and conducting states becomes very abrupt and shows strong hysteresis. The high-bias behavior suggests underdamped transport through a periodic washboard potential resulting from collective motion.

The existence of a universal interfacial thermal resistance in a broad range of systems is shown using linear response theory and computations of realistic materials. When the thermal excitation is modulated up to frequencies larger than the intrinsic resistance scattering rate defined in a previous paper [A. Rajabpour and S. Volz, J. Appl. Phys. 108, 094324 (2010)], the interfacial resistance becomes reversely proportional to frequency and only depends on the number of degrees of freedom involved in the heat transfer between both systems. We present molecular dynamics simulations of connected crystals corroborating these statements from both quantitative and qualitative viewpoints. This finding significantly impacts the thermal management of nanoelectronic systems at nanoscales where heat removal mainly relies on interfacial scattering.

Resonant transmission through electronic quantum states that exist at the zero points of a magnetic field gradient inside a ballistic quantum wire is reported. Since the semiclassical motion along such a line of zero magnetic field takes place in the form of unidirectional snake trajectories, these states have no classical equivalence. The existence of such quantum states has been predicted more than a decade ago by theoretical considerations. We further show how their properties depend on the amplitude of the magnetic field profile as well as on the Fermi energy.

The polarization response of a material to a strain gradient, known as flexoelectricity, holds great promise for novel electromechanical applications. Despite considerable recent progress, however, the effect remains poorly understood. From both the fundamental and practical viewpoints, it is of crucial importance to know whether the coupling coefficients are primarily governed by the properties of the bulk material or by the details of the sample surface. Here we provide, by means of first-principles calculations, quantitative evidence supporting the latter scenario. In particular, we demonstrate that a ${\mathrm{SrTiO}}_3$ film can yield a positive or negative voltage upon bending, depending on whether it is terminated by a ${\mathrm{TiO}}_2$ or SrO layer. This result points to a full control of the flexoelectric effect via surface/interface engineering, opening exciting new avenues for device design.

We report torque magnetometry measurements of an oxide heterostructure consisting of an amorphous ${\mathrm{Al}}_2{\mathrm{O}}_3$ thin film grown on a crystalline ${\mathrm{SrTiO}}_3$ substrate ($a$-AO/STO) by atomic layer deposition. We find a torque response that resembles previous studies of crystalline ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ (LAO/STO) heterointerfaces, consistent with strongly anisotropic magnetic ordering in the plane of the interface. Unlike crystalline LAO, amorphous ${\mathrm{Al}}_2{\mathrm{O}}_3$ is nonpolar, indicating that planar magnetism at an oxide interface is possible without the strong internal electric fields generated within the polarization catastrophe model. We discuss our results in the context of current theoretical efforts to explain magnetism in crystalline LAO/STO.

We demonstrate an elastically tunable self-assembled quantum dot in a nanowire antenna that emits single photons with resolution-limited spectral linewidths. The single-photon device is composed of a single quantum dot embedded in a top-down fabricated nanowire waveguide integrated onto a piezoelectric actuator. Nonresonant excitation leads to static (fluctuating) charges likely at the nanowire surface, causing DC Stark shifts (inhomogeneous broadening); for low excitation powers, the effects are not observed, and resolution-limited linewidths are obtained. Despite significant strain-field relaxation in the high-aspect-ratio nanowires, we achieve up to 1.2-meV tuning of a dot's transition energy. Single-photon sources with high brightness, resolution-limited linewidths, and wavelength tunability are promising for future quantum technologies.

The electronic structure at the interface between a topological band insulator and a Mott insulator is studied within layer dynamical mean field theory. To represent the bulk phases of these systems, we use the generalized Bernevig-Hughes-Zhang model that incorporates the Hubbard-like on-site Coulomb energy $U$ in addition to the spin-orbit coupling term that causes band inversion. The topological and Mott insulating phases are realized by appropriately choosing smaller and larger values of $U$, respectively. As expected, the interface is found to be metallic because of the localized edge state. When the Coulomb energy in the Mott insulator is close to the critical value, however, this edge state exhibits its largest amplitude deep within the Mott insulator rather than at the interface. This finding corresponds to a new type of proximity effect induced by the neighboring topological band insulator and demonstrates that, as a result of spin-orbit coupling within the Mott insulator, several layers near the interface convert from the Mott insulating phase to a topological insulating phase. Moreover, we argue that the ordinary proximity effect, whereby a Kondo peak is induced in a Mott insulator by neighboring metallic states, is accompanied by an additional reverse proximity effect, by which the Kondo peak gives rise to an enhancement of the density of states in the neighboring metallic layer.

With x-ray absorption spectroscopy we investigated the orbital reconstruction and the induced ferromagnetic moment of the interfacial Cu atoms in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7/{\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_3$ (YBCO/LCMO) and ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4/{\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_3$ (LSCO/LCMO) multilayers. We demonstrate that these electronic and magnetic proximity effects are coupled and are common to these cuprate/manganite multilayers. Moreover, we show that they are closely linked to a specific interface termination with a direct Cu-O-Mn bond. We furthermore show that the intrinsic hole doping of the cuprate layers and the local strain due to the lattice mismatch between the cuprate and manganite layers are not of primary importance. These findings underline the central role of the covalent bonding at the cuprate/manganite interface in defining the spin-electronic properties.