Using the charge-conserving Floquet-Green function approach to open quantum systems driven by an external time-periodic potential, we analyze how spin current pumped by the precessing magnetization of a ferromagnetic (F) layer is injected laterally into the interface with strong spin-orbit coupling (SOC) and converted into charge current flowing in the same direction. In the case of a metallic interface with the Rashba SOC used in recent experiments [J. C. R. Sánchez, L. Vila, G. Desfonds, S. Gambarelli, J. P. Attané, J. M. De Teresa, C. Magén, and A. Fert, Nat. Commun. 4, 2944 (2013)], both spin $I^{S_{\alpha }}$ and charge $I$ current flow within the interface where $I/I^{S_{\alpha }}\simeq$ 2–8% (depending on the precession cone angle), while for a F/topological-insulator (F/TI) interface employed in related experiments [Y. Shiomi, K. Nomura, Y. Kajiwara, K. Eto, M. Novak, K. Segawa, Y. Ando, and E. Saitoh, arXiv:1312.7091] the conversion efficiency is greatly enhanced $(I/I^{S_{\alpha }}\simeq$ 40–60%) due to perfect spin-momentum locking on the surface of a TI. The spin-to-charge conversion occurs also when spin current is pumped vertically through the F/TI interface with smaller efficiency $\left(I/I^{S_{\alpha }}\sim 0.001\%\right)$, but with the charge current signal being sensitive to whether the Dirac fermions at the interface are massive or massless.

The natural transition from the radiative regime to the conductive regime of heat transfer between two identical isotropic nonmagnetic dielectric solid materials is questioned by investigating the possibility of induced phonon transfer in vacuum. We describe the process in a general way assuming a certain phonon coupling mechanism between the two identical solids, then we particularly illustrate the case of coupling through the Casimir force. We analyze how this mechanism of heat transfer compares and competes with the near field thermal radiation using a local model of the dielectric function. We show that the former mechanism can be very effective and even surpass the latter mechanism depending on the nature of the solid dielectric materials, the distance gap between them, as well as the operating temperature regime.

There is much interest in the realization of systems with $p$-wave pairing in one dimension or chiral $p$-wave pairing in two dimensions, because these are believed to support Majorana modes at the ends or inside vortices. We consider a two-component system of composite fermions and provide theoretical evidence that, under appropriate conditions, the screened interaction between the minority composite fermions is such as to produce an almost exact realization of a $p$-wave paired state described by the so-called anti-Pfaffian wave function. This state is predicted to occur at filling $\nu =3/8$ or 13/8 in GaAs when the Zeeman energy is sufficiently small, and at $\nu =\pm 3/8$ or $\pm 13/8$ in single layer graphene when either the Zeeman or the valley splitting is sufficiently small.

Based on the advanced first-principles theoretical approach, we investigate the superconducting gap structure and the pairing glue in the heavy-fermion superconductor ${\mathrm{CeCoIn}}_5$. Unexpectedly, the nesting function in the original GGA-based band structure, which is considered to be consistent with the dHvA measurement, shows a $Q$ structure incompatible with experimental observations. Instead we find the importance of the temperature-dependent Fermi surface evolution driven by electron correlations, which has been calculated by the DMFT method. Considering this effect, we obtain reasonable antiferromagnetic correlation, which can also induce the expected $d$-wave superconductivity. The system encounters the superconducting transition, before a part of the Fermi surface is formed. Similar effects can be expected in generic heavy-fermion superconductors.

We report neutron scattering and transport measurements on semiconducting ${\mathrm{Rb}}_{0.8}{\mathrm{Fe}}_{1.5}{\mathrm{S}}_2$, a compound isostructural and isoelectronic to the well-studied $A_{0.8}{\mathrm{Fe}}_y{\mathrm{Se}}_2(A=\mathrm{K},\mathrm{Rb},\mathrm{Cs},\mathrm{Tl}/\mathrm{K})$ superconducting systems. Both resistivity and dc susceptibility measurements reveal a magnetic phase transition at $T=275\mathrm{K}$. Neutron diffraction studies show that the 275 K transition originates from a phase with rhombic iron vacancy order which exhibits an in-plane stripe antiferromagnetic ordering below 275 K. In addition, the stripe antiferromagnetic phase interdigitates mesoscopically with an ubiquitous phase with $\sqrt{5}\times \sqrt{5}$ iron vacancy order. This phase has a magnetic transition at $T_N=425\mathrm{K}$ and an iron vacancy order-disorder transition at $T_S=600\mathrm{K}$. These two different structural phases are closely similar to those observed in the isomorphous Se materials. Based on the close similarities of the in-plane antiferromagnetic structures, moments sizes, and ordering temperatures in semiconducting ${\mathrm{Rb}}_{0.8}{\mathrm{Fe}}_{1.5}{\mathrm{S}}_2$ and ${\mathrm{K}}_{0.81}{\mathrm{Fe}}_{1.58}{\mathrm{Se}}_2$, we argue that the in-plane antiferromagnetic order arises from strong coupling between local moments. Superconductivity, previously observed in the $A_{0.8}{\mathrm{Fe}}_y{\mathrm{Se}}_{2-z}{\mathrm{S}}_z$ system, is absent in ${\mathrm{Rb}}_{0.8}{\mathrm{Fe}}_{1.5}{\mathrm{S}}_2$, which has a semiconducting ground state. The implied relationship between stripe and block antiferromagnetism and superconductivity in these materials as well as a strategy for further investigation is discussed in this paper.

We calculate the differential conductance as a function of temperature and bias voltage, $G(T,V)$, through Au monatomic chains with a substitutional Co atom as a magnetic impurity, connected to a fourfold symmetric lead. The system was recently proposed as a possible scenario for observation of the overscreened Kondo physics. Stretching the chain, the system could be tuned through a quantum critical point (QCP) with three different regimes: overscreened, underscreened, and non-Kondo phases. We present calculations of the impurity spectral function by using the numerical renormalization group for the three different regimes characterizing the QCP. Nontrivial behavior of the spectral function is reported near the QCP. Comparison with results using the noncrossing approximation (NCA) shows that the latter is reliable in the overscreened regime, when the anisotropy is larger than the Kondo temperature. For these parameters, which correspond to realistic previous estimates, $G(T,V)$ calculated within NCA exhibits clear signatures of the non-Fermi-liquid behavior within the overscreened regime.

We have calculated the correlation energy of the homogeneous electron gas (HEG) and the dissociation energy curves of molecules with covalent bonds from an efficient implementation of the adiabatic connection fluctuation dissipation expression including the exact-exchange (EXX) kernel. The EXX kernel is defined from first-order perturbation theory and used in the Dyson equation of time-dependent density-functional theory. Within this approximation (RPAx), the correlation energies of the HEG are significantly improved with respect to the random phase approximation (RPA) up to densities of the order of $r_s\approx 10$. However, beyond this value, the RPAx response function exhibits an unphysical divergence and the approximation breaks down. Total energies of molecules at equilibrium are also highly accurate, but we find a similar instability at stretched geometries. Staying within an exact first-order approximation to the response function, we use an alternative resummation of the higher-order terms. This slight redefinition of RPAx fixes the instability in total energy calculations without compromising the overall accuracy of the approach.

We report on a pronounced prolongation of the exciton decay in strongly $p$-doped and partially compensated direct band-gap semiconductor GaAs:Mn with increasing optical excitation power. Using time-resolved photoluminescence we show that the intricate interplay of excitons, shallow and deep impurity centers in GaAs:Mn results in a complex recombination behavior that cannot be characterized in terms of simple rates. The decay can be precisely described by a model based on Shockley-Read-Hall recombination, which shows that the observed dynamics arise from a varying neutralization of shallow and deep recombination centers. This enables the investigation of the carrier dynamics in the impurity system by measuring only the exciton decay time.

The valence band of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is investigated by measuring the Shubnikov–de Haas effect as well as galvanomagnetic and thermoelectric transports. At low hole concentration, the hole Fermi surface is closed and boxlike, but at higher carrier concentrations it develops tubelike extensions that are open, in general agreement with our theoretical calculations. However, the experimentally determined density-of-states effective mass is smaller than density-functional-theory calculations predict; although we cannot give a definitive explanation for this, we suspect that the theory may lack sufficient precision to compute room-temperature transport properties, such as the thermopower, in solids with interlayer van der Waals bonds.

We study optical properties of optomagnetic metamaterials consisting of regular arrays of single and double Au nanodots (nanopillars). Using a combination of data from variable angle spectroscopic ellipsometry, transmission, and reflection measurements, we identify localized plasmon resonances of gold nanodots and measure their dependence on dot size and substrate type. We demonstrate that arrays of Au nanopillars can support narrow collective plasmon resonances coupled to in-plane and out-of-plane localized plasmon resonances. The spectral positions of these plasmon modes are extracted from the angular dependence of the transmission and reflection spectra for two beam polarizations. We show that nanoarrays exhibit dramatically different optical response on conductive and nonconductive substrates and study its angular dependence. The optical response of nanoarrays is described well by coupled dipole approximation. The procedure for extracting optical constants of metamaterials based on ellipsometry is discussed and applied to our samples resulting in a calculated negative index of refraction for double-dot arrays at green light.

In spite of unscreened Coulomb interactions close to charge neutrality, relativistic massless electrons in graphene allegedly behave as noninteracting particles. A clue to this paradox is that both interaction and kinetic energies scale with particle density in the same way. In contrast, in a dilute gas of nonrelativistic electrons, the different scaling drives the transition to Wigner crystal. Here we show that Dirac electrons in a graphene quantum dot with a mass gap localize in the manner of Wigner for realistic values of device parameters. Our theoretical evidence relies on many-body observables obtained through the exact diagonalization of the interacting Hamiltonian, which allows us to take all electron correlations into account. We predict that the experimental signatures of Wigner localization are the suppression of the fourfold periodicity of the filling sequence and the quenching of excitation energies, both of which may be accessed through Coulomb blockade spectroscopy. Our findings are relevant to other carbon-based nanostructures exhibiting a mass gap.

Coherent phonons (CPs) generated by laser pulses on the femtosecond scale have been proposed as a means to achieve ultrafast, nonthermal switching in phase-change materials such as ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ (GST). Here we use ultrafast optical pump pulses to induce coherent acoustic phonons and stroboscopically measure the corresponding lattice distortions in GST using 100-ps x-ray pulses from the European Synchrotron Radiation Facility (ESRF) storage ring. A linear-chain model provides a good description of the observed changes in the diffraction signal; however, the magnitudes of the measured shifts are too large to be explained by thermal effects alone, implying the presence of excited-state effects in addition to temperature-driven expansion. The information on the movement of atoms during the excitation process can lead to greater insight into the possibilities of using CP-induced phase transitions in GST.

We study the dynamics of spin currents in the spin-$\frac{1}{2}$ XX ladder at finite temperature. Within linear response theory, we numerically calculate autocorrelation functions for quantum systems larger than what is accessible with exact diagonalization using the concept of dynamical quantum typicality. While the spin Drude weight vanishes exponentially quickly with increasing system size, we show that this model realizes standard diffusive dynamics. Moreover, we unveil the existence of three qualitatively different dependencies of the spin-diffusion coefficient on the rung-coupling strength, resulting from a crossover from exponential to Gaussian dissipation as the rung coupling increases, in agreement with analytical predictions. We further discuss the implications of our results for experiments with cold atomic gases.

A single static magnetic impurity in a fully gapped superconductor leads to the formation of an intragap quasiparticle bound state. At temperatures much below the superconducting transition, the energy relaxation and spin dephasing of the state are expected to be exponentially suppressed. The presence of such a state can be detected in electron tunneling experiments as a pair of conductance peaks at positive and negative biases. Here we show that, for an arbitrarily weak tunneling strength, the peaks have to be symmetric with respect to the applied bias. This is in contrast to the standard result in which the tunneling conductance is proportional to the local (in general, particle-hole asymmetric) density of states. The asymmetry can be recovered if one allows for either a finite density of impurity states, or if impurities are coupled to another, nonsuperconducting, equilibrium bath.

We study an exactly solvable toric code type of Hamiltonian in three dimensions, defined on the diamond lattice with spin-$1/2$ degrees of freedom at each site. The Hamiltonian is a sum of mutually commuting plaquette operators $B_p$, all of which have eigenvalue +1 in the ground state. The excitations are “fluxes,” which are plaquettes with $B_p=-1$. Due to certain local kinematic constraints, fluxes form loops. The elementary flux-loop excitations are fermions, in contrast to other solvable spin-$1/2$ models in three dimensions, where the excitations are bosons. Furthermore, the flux loops braid nontrivially, giving rise to Abelian anyonlike statistics.

We study the effects of a finite density of negative-$U$ centers (NUCs) on the surface of a three-dimensional topological insulator. The surface Dirac fermions mediate a power-law interaction among the local Cooper pairs at the NUCs, and the interaction remains long-ranged for weak disorder. Superconductivity can be generated in the presence of a random distribution of NUCs. The NUCs play dual roles as both pair creators and pair breakers, and the competition of the two effects results in a nonmonotonic dependence of the mean-field superconducting transition temperature on the density of NUCs. Global phase coherence is established through coupling the locally superconducting puddles via Josephson coupling. Rare fluctuations play important roles, and a globally superconducting phase can only be achieved at large enough concentration of NUCs. The $p\text{−}\mathrm{wave}$ component of the superconducting order parameter gives rise to frustration among the superconducting grains, which is captured by a Potts-XY type model. New phases with chiral order, glass order, and, possibly, topological order can then emerge in the system of superconducting grains.

A general form of a many-body Hamiltonian is considered, which includes an interacting fermionic subsystem coupled to noninteracting extended fermionic and bosonic systems. We show that the exact dynamics of the extended bosonic system can be derived from the reduced density matrix of the subsystem alone, despite the fact that the latter contains information about the subsystem only. The advantage of the formalism is immediately clear: While the reduced density matrix of the subsystem is readily available, the formalism offers access to observables contained in the full density matrix, which is often difficult to obtain. As an example, we consider an extended Holstein model and study the nonequilibrium dynamics of the so-called “reaction mode” for different model parameters. The effects of the phonon frequency, the strength of the electron-phonon couplings, and the source-drain bias voltage on the phonon dynamics across the bistability are discussed.

Traditional routes to charge density wave (CDW) in transition-metal dichalcogenides, relying on Fermi surface nesting or Jahn-Teller instabilities, have recently been brought into question. While this calls for exploration of alternative views, a paucity of theoretical guidance sustains lively controversy on the origin of, and interplay between, CDW and superconductive orders in transition-metal dichalcogenides. Here, we explore a preformed excitonic liquid route, heavily supplemented by modern correlated electronic-structure calculations, to an exci-tonic CDW order in $1T\text{−}{\mathrm{TiSe}}_2$. We show that orbital-selective dynamical localization arising from preformed excitonic liquid correlations is somewhat reminiscent of states proposed for $d$ and $f$ band quantum criticality at the border of magnetism. Excellent quantitative explication of a wide range of spectral and transport responses in both normal and CDW phases provides strong support for our scenario, and suggests that soft excitonic liquid fluctuations mediate superconductivity in a broad class of transition-metal dichalcogenides on the border of CDW. This brings the transition-metal dichalcogenides closer to the bad actors (where the metallic state is clearly not a Fermi liquid) in $d$ and $f$ band systems, where anomalously soft fluctuations of electronic origin are believed to mediate unconventional superconductivity on the border of magnetism.

Nonintentionally doped 200-nm-thick ${\mathrm{In}}_{0.16}{\mathrm{Al}}_{0.84}\mathrm{N}/n^{+}\text{−}\mathrm{GaN}$ samples were grown by metal-organic vapor phase epitaxy and used for the electrical characterization of InAlN. In the temperature range 180–400 K, the forward current of Schottky diodes is dominated by a tunneling mechanism below 1.2 V. Capacitance and conductance-temperature characteristics were measured at 1 MHz in the 90–400 K range and at various voltages. The conductance vs temperature reveals two peaks ${\mathrm{D}}_1$ and ${\mathrm{D}}_2$, which are attributed to bulk states in InAlN. Their characterization by admittance spectroscopy gives thermal activation energies of $\approx 68$ meV and 290 meV, and thermal capture cross section of $9.7\times {10}^{-17}$ ${\mathrm{cm}}^2$ and $\approx 6.2\times {10}^{-15}$ ${\mathrm{cm}}^2$, respectively. The same levels are also revealed by extracting the temperature dependence of the carrier density in the neutral region of InAlN from $I\text{−}V\text{−}T$ characteristics on the Schottky diode. A partial carrier freeze out is demonstrated and discussed in the framework of an existing theory for DX centers. The use of this approach is supported by the evidence of persistent photoconductivity effects, which strongly indicate the presence of DX centers in our material. It results that each donor in InAlN would exist in two distinct lattice configurations, a substitutional one (${\mathrm{D}}_1$, hydrogenic state) and a lattice-distorted one (${\mathrm{D}}_2$, DX state). From secondary ion mass spectrometry data, theoretical grounds, and previous experimental evidence in the $\mathrm{Al}{}_x\mathrm{Ga}{}_{1-x}\mathrm{N}$ system, oxygen is the most probable candidate for such an unintentional dopant.

Coherent multisubband plasmons in doped semiconductor quantum wells have recently attracted large interest as they allow us to strongly enhance light-matter interaction via collective Coulomb coupling among different intersubband transitions. In this work, we develop a semiclassical theory of intersubband plasmons in quantum wells, on the basis of nonlocal electrodynamics. The nonlocal treatment provides a proper description of collective effects in the electromagnetic response of the system and, in the long-wavelength approximation, it predicts the same resonance frequencies as the quantum mechanical description. The nonlocal formalism is applied to the study of the radiative decay rate of multisubband plasmons and plasmon polaritons, both in the case of an isolated quantum well and of a planar microcavity. We show that subpicosecond radiative lifetimes are to be expected for intersubband plasmons in semiconductor quantum wells, similarly to quantum well excitons. The theory is formulated in the context of the transfer-matrix method and it can be applied in a straightforward way to stratified geometries of any degree of complexity.