In epitaxial graphene irradiated by an off-resonance circularly polarized light, we demonstrate a phase transition taking place between the band insulator and Floquet topological insulator. Considering the competition between staggered sublattice potential and photon dressing, we derive the dynamical energy gap and phase diagram in the tight-binding approximation. It is found that a threshold value of light intensity is necessary to realize a Floquet topological insulator. At the phase boundary, for each set of parameters, there is a special state with only one valley that is Dirac cone gapless, but the other remains gapped; in the band insulating phase, only one valley provides low-energy electrons, and it could be switched to the other by reversing the polarization direction of light. From these results, two electronic devices are designed: one is an optical-sensing np junction, where the photodriven unusual intervalley tunneling exhibits a stronger detectable signal than the intravalley tunneling, and the other is a topological field-effect transistor, where polarized light is used to turn on or turn off a nonequilibrium current.

Noncontact atomic force microscopy (NC-AFM) features the measurement of forces with highest spatial resolution and sensitivity, resolving forces of the order of pico-Newtons with submolecular resolution. However, the measured total force is a mixture composed of various interactions. While some interactions such as electrostatic or magnetic forces can be excluded by a careful design of the experiment, the subtraction of van der Waals forces, which mainly originate from London dispersion interactions between the macroscopic tip shank and the bulk sample, remains a challenge. We present the determination of the inherently present van der Waals forces in total interaction force data from fitting a suitable model, allowing for extraction of the short-range force component. We compare the applicability of several van der Waals models based on experimental interaction data from the calcite$\left(10\overline{1}4\right)$ surface. The feasibility to fit these models to experimental data is critically discussed. We furthermore introduce criteria to assess the transition point from pure long-range interaction to mixed short- and long-range forces based on the variance of lateral and vertical force data. This determination allows us to extract the short-range interaction forces, which remained a challenge so far in NC-AFM experiments.

We present electronic transport measurements of a single wall carbon nanotube quantum dot coupled to Nb superconducting contacts. For temperatures comparable to the superconducting gap, peculiar transport features are observed inside the Coulomb blockade and superconducting energy gap regions. The observed temperature dependence can be explained in terms of sequential tunneling processes involving thermally excited quasiparticles. In particular, these channels give rise to two unusual conductance peaks at zero bias in the vicinity of the charge degeneracy point and allow one to determine the degeneracy of the ground states involved in transport. The measurements are in good agreement with model calculations.

We examine the entanglement properties of the spin-half Heisenberg model on the two-dimensional square-lattice bilayer based on quantum Monte Carlo calculations of the second Rényi entanglement entropy. In particular, we extract the dominant area-law contribution to the bipartite entanglement entropy that shows a nonmonotonous behavior upon increasing the interlayer exchange interaction: a local maximum in the area-law coefficient is located at the quantum critical point separating the antiferromagnetically ordered region from the disordered dimer-singlet regime. Furthermore, we consider subleading logarithmic corrections to the Rényi entanglement entropy scaling. Employing different subregion shapes, we isolate the logarithmic corner term from the logarithmic contribution due to Goldstone modes that is found to be enhanced in the limit of decoupled layers. At the quantum critical point, we estimate a contribution of $0.016\left(1\right)$ due to each ${90}^{\circ }$ corner. This corner term at the SU(2) quantum critical point deviates from the Gaussian theory value, while it compares well with recent numerical linked cluster calculations on the bilayer model.

Spin-polarized transport through a double quantum dot system attached to a common superconducting lead and two ferromagnetic electrodes (fork geometry) is investigated theoretically. The key objective of the analysis is to describe the influence of electrodes' ferromagnetism on the Andreev tunneling. Both direct and crossed Andreev tunneling processes are considered, in general. The other objective is a detailed analysis of the role of Coulomb interaction and its impact on the Andreev tunneling processes.

We present a model for quantum transport in a ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ slab, which is based on using a nonequilibrium Green's function approach in which bulk and surface states are modeled realistically, and the effects of phonon scatterings are included. Resistivity is computed for different temperatures and strengths of the electron-phonon coupling at various doping levels. Temperature dependence of resistivity is found to display an insulating trend when the slab is biased at the Dirac point even in the presence of strong electron-phonon coupling. In sharp contrast, for carrier doping, the material displays a metallic behavior induced by acoustic scattering effects, even though purely ballistic transport yields an insulating trend, explaining contradictory trends reported in transport experiments on ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Our analysis, furthermore, suggests an experimental strategy for obtaining a handle on the strength of electron-phonon coupling in topological insulators via temperature-dependent transport measurements.

We consider the distribution of waiting times between noninteracting fermions on a tight-binding chain. We calculate the waiting time distribution for a quantum point contact and find a crossover from Wigner-Dyson statistics at full transmission to Poisson statistics close to pinch-off as predicted by scattering theory. In addition, we consider several quantum dot structures for which we can associate oscillations in the waiting time distributions with internal energy scales of the scatterers. A detailed comparison with scattering theory and generalized master equations is provided. We focus on mesoscopic conductors, but our tight-binding models may also be realized in cold-atomic gases.

We show that a relatively sparse photonic crystal of high-permittivity magnetic garnet particles can induce a giant Faraday rotation of light transmitted through a finite slab of it. The underlying mechanism resides in wave propagation through collective Bloch modes, which are strongly localized in the particles.

We propose a model for the consistent interpretation of the transport behavior of manganese perovskites in both the metallic and insulating regimes. The concept of polarons as charge carriers in the metallic ferromagnetic phase of manganites also solves the conflict between transport models, which usually neglects polaron effects in the metallic phase, and, on the other hand, optical conductivity, angle-resolved spectroscopy, and neutron scattering measurements, which identify polarons in the metallic phase of manganites down to 6 K. Transport characterizations of epitaxial ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ thin films in the thickness range of 5–40 nm and temperature interval of 25–410 K have been accurately collected. We show that taking into account polaron effects allows us to achieve an excellent fit of the transport curves in the whole temperature range. The current carriers density collapse picture accurately accounts for the properties variation across the metal-insulator transitions. The electron-phonon coupling parameter γ estimations are in a good agreement with theoretical predictions. The results promote a clear and straightforward quantitative description of the manganite films involved in charge transport device applications and promises to describe other oxide systems involving a metal-insulator transition.

Collective excitations reveal fundamental properties and potential applications of superconducting states. We theoretically study macroscopic quantum tunneling (MQT) in a Josephson junction composed of multiband superconductors, focusing on a phase mode induced by interband fluctuations: the Josephson-Leggett (JL) collective excitation mode. Using the imaginary-time path-integral method, we derive a formula for the MQT escape rate for low-temperature switching events. We clarify that the JL mode has two major effects on the MQT: (i) the zero-point fluctuations enhance the escape rate, and (ii) the quantum dissipation induced by the couplings to the gauge-invariant phase difference suppresses the quantum tunneling. We show that the enhancement exceeds the suppression for a wide range of junction parameters. This enhancement originates from the single-mode interaction between the tunneling variable and the interband fluctuations.

We present a comprehensive analytical theory of localized nonlinear excitations—dark solitons—supported by an incoherently pumped, spatially homogeneous exciton-polariton condensate. We show that, in contrast to dark solitons in conservative systems, these nonlinear excitations “relax” by blending with the background at a finite time, which critically depends on the parameters of the condensate. Our analytical results for trajectory and lifetime are in excellent agreement with direct numerical simulations of the open-dissipative mean-field model. In addition, we show that transverse instability of quasi-one-dimensional dark stripes in a two-dimensional open-dissipative condensate demonstrates features that are entirely absent in conservative systems, as creation of vortex-antivortex pairs competes with the soliton relaxation process.

A scheme is proposed to generate a maximally entangled state between two qubits by means of a dissipation-driven process. To this end, we entangle the quantum states of qubits that are mutually coupled by a plasmonic nanoantenna. Upon enforcing a weak spectral asymmetry in the properties of the qubits, the steady-state probability to obtain a maximally entangled, subradiant state approaches unity. This occurs despite the high losses associated with the plasmonic nanoantenna that are usually considered as being detrimental. The entanglement scheme is shown to be quite robust against variations in the transition frequencies of the quantum dots and deviations in their prescribed position with respect to the nanoantenna. Our work paves the way for applications in the field of quantum computation in highly integrated optical circuits.

Spin-dependent transport has been computationally studied for an open-shell singlet di-iron-oxo cluster. Currents and magnetoresistances have been investigated as a function of spin state within the nonequilibrium Green's function approach. The applied bias can be used to tune the sign of the observed magnetoresistance. A colossal magnetoresistance has been determined for hydrogen anchoring. Applied biases lower than 0.3 V, in conjunction with sulfur anchoring, induce a negative magnetoresistance due to lowering of the anchor-scatterer tunneling barrier. The di-iron-oxo cluster displays nearly perfect spin filtering for parallel alignment of the iron magnetic moments due to the energetic proximity, relative to the Fermi level, of its highest occupied molecular orbitals.

We study the effects of extended and localized potentials and a magnetic field on the Dirac electrons residing at the surface of a three-dimensional topological insulator like ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. We use a lattice model to numerically study the various states; we show how the potentials can be chosen in a way which effectively avoids the problem of fermion doubling on a lattice. We show that extended potentials of different shapes can give rise to states which propagate freely along the potential but decay exponentially away from it. For an infinitely long potential barrier, the dispersion and spin structure of these states are unusual and these can be varied continuously by changing the barrier strength. In the presence of a magnetic field applied perpendicular to the surface, these states become separated from the gapless surface states by a gap, thereby giving rise to a quasi-one-dimensional system. Similarly, a magnetic field along with a localized potential can give rise to exponentially localized states which are separated from the surface states by a gap and thereby form a zero-dimensional system. Finally, we show that a long barrier and an impurity potential can produce bound states which are localized at the impurity, and an “L”-shaped potential can have both bound states at the corner of the L and extended states which travel along the arms of the potential. Our work opens the way to constructing wave guides for Dirac electrons.

We present a strange metal, described by a holographic duality, which reproduces the famous linear resistivity of the normal state of the copper oxides, in addition to the linear specific heat. This holographic metal reveals a simple and general mechanism for producing such a resistivity, which requires only quenched disorder and a strongly interacting, locally quantum critical state. The key is the minimal viscosity of the latter: unlike in a Fermi liquid, the viscosity is very small and therefore is important for the electrical transport. This mechanism produces a resistivity proportional to the electronic entropy.

Normally $4d$ or $5d$ transition metals are in a low-spin state. Here using first-principles calculations, we report on a rare case of a high-spin $S=1$ magnetic state for the ${\mathrm{Ag}}^{3+}$ ion in the double perovskite ${\mathrm{Cs}}_2$${\mathrm{KAgF}}_6$. We also explored a possibility of a conventional low-spin $S=0$ ground state and find an associated tetragonal distortion to be 0.29 Å. However, the lattice elastic energy cost and the Hund exchange loss exceed the ${\mathrm{e}}_g$ crystal-field energy gain, thus making the low-spin tetragonal structure less favorable than the high-spin cubic structure. We conclude that the compact perovskite structure of ${\mathrm{Cs}}_2$${\mathrm{KAgF}}_6$ is an important factor in stabilizing the unusual high-spin ground state of ${\mathrm{Ag}}^{3+}$.