118 research outputs found
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
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
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
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 is distributed with fractions 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
of the lowest subgap excitations of the finite chain defines tunneling and
entanglement between end states.We discuss the effect of a Zeeman coupling
on one of the ends of the chain only. For , the energy
difference of excitations with opposite spin orientation is ,
consistent with a spin projection . 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
Nonequilibrium Green's functions in the study of heat transport of driven nanomechanical systems
We review a recent theoretical development based on non-equilibrium Green's
function formalism to study heat transport in nanomechanical devices modeled by
phononic systems of coupled quantum oscillators driven by ac forces and
connected to phononic reservoirs. We present the relevant equations to
calculate the heat currents flowing along different regions of the setup, as
well as the power developed by the time-dependent forces. We also present
different strategies to evaluate the Green's functions exactly or approximately
within the weak driving regime. We finally discuss the different mechanisms in
which the ac driving forces deliver the energy. We show that, besides
generating heat, the forces may operate exchanging energy as a quantum engine.Comment: 14 pages, 2 figure
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
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
Does long-range antiferromagnetism help or inhibit superconductivity?
We analyze the possible existence of a superconducting state in a background
with long-range antiferromagnetism. We consider a generalized Hubbard model
with nearest-neighbor correlated hopping in a square lattice. Near half
filling, the model exhibits a d-wave-Bardeen-Cooper-Schrieffer (BCS) solution
in the paramagnetic state. The superconducting solution would be enhanced by
the antiferromagnetic background if the contribution of triplet pairs with
d-wave symmetry and total momentum (pi, pi) could be neglected. However, we
find that due to their contribution, the coexistence of superconductivity and
long-range antiferromagnetism is ruled out for large values of the Coulomb
repulsion U. Spin-density wave fluctuations (SDWF) do not change this result.Comment: 8 pages, 1 figure. Accepted for publication in Physica
Pumping charge with ac magnetic fluxes and the dynamical breakdown of Onsager symmetry
We study the transport properties of setups with one and two mesoscopic rings
threaded by ac magnetic fluxes of the form \Phi(t)=\Phi^{dc} + \Phi^{ac}
cos(\Omega_0 t + \delta) and connected to two different particle reservoirs. We
analyze the conditions to generate a pumped dc current in the adiabatic regime.
We also study the symmetry properties of the induced dc current as a function
of the static component of the flux, \Phi^{dc}, with and without a dc bias
voltage applied at the reservoirs. We analyze, in particular, the validity of
the Onsager-Casimir relations for different configurations of the setups.Comment: 12 pages, 9 figures, accepted in PRB. Added refences, corrected
typos, we now discuss in the conclusion the possibility of an experimental
realization of our findings, we now show the main quantities in terms of
universal constant
- …