Relation Between Local Temperature Gradients and the Direction of Heat Flow in Quantum Driven Systems

We introduce thermometers to define the local temperature of an electronic system driven out-of-equilibrium by local ac fields. We discuss the behavior of the local temperature along the sample, showing that it exhibits spatial fluctuations following an oscillatory pattern. We show explicitly that the local temperature is the correct indicator for heat flow.Comment: 3 pages, 2 figure

Nanomagnet coupled to quantum spin Hall edge: An adiabatic quantum motor

The precessing magnetization of a magnetic islands coupled to a quantum spin Hall edge pumps charge along the edge. Conversely, a bias voltage applied to the edge makes the magnetization precess. We point out that this device realizes an adiabatic quantum motor and discuss the efficiency of its operation based on a scattering matrix approach akin to Landauer-B"uttiker theory. Scattering theory provides a microscopic derivation of the Landau-Lifshitz-Gilbert equation for the magnetization dynamics of the device, including spin-transfer torque, Gilbert damping, and Langevin torque. We find that the device can be viewed as a Thouless motor, attaining unit efficiency when the chemical potential of the edge states falls into the magnetization-induced gap. For more general parameters, we characterize the device by means of a figure of merit analogous to the ZT value in thermoelectrics.Comment: 9 pages, 2 figures. Contribution to a special issue in Physica E on "Frontiers in quantum electronic transport" - in memory of Markus B"uttike

Unveiling a crystalline topological insulator in a Weyl semimetal with time-reversal symmetry

We consider a natural generalization of the lattice model for a periodic array of two layers, A and B, of spinless electrons proposed by Fu [Phys. Rev. Lett. 106, 106802 (2011)] as a prototype for a crystalline insulator. This model has time-reversal symmetry and broken inversion symmetry. We show that when the intralayer next-nearest-neighbor hoppings ta2, a = A, B vanish, this model supports a Weyl semimetal phase for a wide range of the remaining model parameters. When the effect of ta2 is considered, topological crystalline insulating phases take place within the Weyl semimetal one. By mapping to an effective Weyl Hamiltonian we derive some analytical results for the phase diagram as well as for the structure of the nodes in the spectrum of the Weyl semimetal.Comment: 8 pages, 8 figure

ac-dc voltage profile and four point impedance of a quantum driven system

We investigate the behavior of the time-dependent voltage drop in a periodically driven quantum conductor sensed by weakly coupled dynamical voltages probes. We introduce the concepts of ac-dc local voltage and four point impedance in an electronic system driven by ac fields. We discuss the properties of the different components of these quantities in a simple model of a quantum pump, where two ac voltages oscillating with a phase lag are applied at the walls of a quantum dot.Comment: 9 pages, 7 figures, accepted for publication in Physical Review

Entangled end states with fractionalized spin projection in a time-reversal-invariant topological superconducting wire

We study the ground state and low-energy subgap excitations of a finite wire of a time-reversal-invariant topological superconductor (TRITOPS) with spin-orbit coupling. We solve the problem analytically for a long chain of a specific one-dimensional lattice model in the electron-hole symmetric configuration and numerically for other cases of the same model. We present results for the spin density of excitations in long chains with an odd number of particles. The total spin projection along the axis of the spin-orbit coupling $S_z= \pm 1/2$ is distributed with fractions $\pm 1/4$ localized at both ends, and shows even-odd alternation along the sites of the chain. We calculate the localization length of these excitations and find that it can be well approximated by a simple analytical expression. We show that the energy $E$ of the lowest subgap excitations of the finite chain defines tunneling and entanglement between end states.We discuss the effect of a Zeeman coupling $\Delta_Z$ on one of the ends of the chain only. For $\Delta_Z<E$, the energy difference of excitations with opposite spin orientation is $\Delta_Z/2$, consistent with a spin projection $\pm 1/4$. We argue that these physical features are not model dependent and can be experimentally observed in TRITOPS wires under appropriate conditions.Comment: 14 pages, 8 Figure

Mesoscopic features in the transport properties of a Kondo-correlated quantum dot in a magnetic field

We study the transport behavior induced by a small bias voltage through a quantum dot connected to one-channel finite-size wires. We describe the quantum dot by the Hubbard-Kondo which is solved by means of a quantum Monte Carlo method. We investigate the effect of a magnetic field applied at the quantum dot in the Kondo regime. We identify changes in the behavior of mesoscopic oscillations introduced by the magnetic field that have an analogous behavior to those observed as a function of the temperature.Comment: 8 pages, 8 figure

"Superconductor"-insulator transitions in a Hubbard chain with nearest- neighbor and bond-charge interactions

We consider a half-filled generalized Hubbard chain with electron-hole symmetric correlated hopping and on-site and nearest-neighbor repulsions U and V respectively. In addition to the insulating charge- and spin-density wave phases for large V and U respectively, we identify a phase with dominant superconducting correlations at large distances for small U and V. Using two Berry phases (one associated to the charge and the other to the spin degrees of freedom) as discrete order parameters, we construct a phase diagram for the three thermodynamic phases.Comment: 5 pages, 2 figure