1,654 research outputs found
Current driven quantum criticality in itinerant electron ferromagnets
We determine the effect of an in-plane current flow on the critical
properties of a 2d itinerant electron system near a ferromagnetic-paramagnetic
quantum critical point. We study a model in which a nonequilibrium steady state
is established as a result of exchange of particles and energy with an
underlying substrate. the current gives rise not only to an effective
temperature equal to the voltage drop over a distance of order the mean free
path, but also to symmetry breaking terms of the form in the effective action. The effect of the symmetry breaking on
the fluctuational and critical properties is found to be small although (in
agreement with previous results) if rotational degrees of freedom are
important, the current can make the classically ordered state dynamically
unstable.Comment: 4 pages, published versio
Electronic Transport in Fullerene C20 Bridge Assisted by Molecular Vibrations
The effect of molecular vibrations on electronic transport is investigated
with the smallest fullerene C20 bridge, utilizing the Keldysh nonequilibrium
Green's function techniques combined with the tight-binding molecular-dynamics
method. Large discontinuous steps appear in the differential conductance when
the applied bias-voltage matches particular vibrational energies. The magnitude
of the step is found to vary considerably with the vibrational mode and to
depend on the local electronic states besides the strength of
electron-vibration coupling. On the basis of this finding, a novel way to
control the molecular motion by adjusting the gate voltage is proposed.Comment: 9 pages, 4 figures, accepted for publication in Phys. Rev. Let
Enhancing laser sideband cooling in one-dimensional optical lattices via the dipole interaction
We study resolved sideband laser cooling of a one-dimensional optical lattice
with one atom per site, and in particular the effect of the dipole interaction
between radiating atoms. For simplicity, we consider the case where only a
single cooling laser is applied. We derive a master equation, and solve it in
the limit of a deep lattice and weak laser driving. We find that the dipole
interaction significantly changes the final temperature of the particles,
increasing it for some phonon wavevectors and decreasing it for others. The
total phonon energy over all modes is typically higher than in the
non-interacting case, but can be made lower by the right choice of parameters
Efficient grid-based method in nonequilibrium Green's function calculations. Application to model atoms and molecules
We propose and apply the finite-element discrete variable representation to
express the nonequilibrium Green's function for strongly inhomogeneous quantum
systems. This method is highly favorable against a general basis approach with
regard to numerical complexity, memory resources, and computation time. Its
flexibility also allows for an accurate representation of spatially extended
hamiltonians, and thus opens the way towards a direct solution of the two-time
Schwinger/Keldysh/Kadanoff-Baym equations on spatial grids, including e.g. the
description of highly excited states in atoms. As first benchmarks, we compute
and characterize, in Hartree-Fock and second Born approximation, the ground
states of the He atom, the H molecule and the LiH molecule in one spatial
dimension. Thereby, the ground-state/binding energies, densities and
bond-lengths are compared with the direct solution of the time-dependent
Schr\"odinger equation.Comment: 11 pages, 5 figures, submitted to Physical Review
Nonequilibrium transport and optical properties of model metal--Mott-insulator--metal heterostructures
Electronic properties of heterostructures in which a finite number of
Mott-insulator layers are sandwiched by semi-infinite metallic leads are
investigated by using the dynamical-mean-field method combined with the Keldysh
Green's function technique to account for the finite bias voltage between the
leads. Current across the junction is computed as a function of bias voltage.
Electron spectral functions in the interacting region are shown to evolve by an
applied bias voltage. This effect is measurable by photoemission spectroscopy
and scanning tunneling microscopy. Further predictions are made for the optical
conductivity under a bias voltage as a possible tool to detect a deformed
density of states. A general discussion of correlated-electron based
heterostructures and future prospect is given.Comment: 11 pages, 11 figures, published versio
Schwinger-Keldysh approach to out of equilibrium dynamics of the Bose Hubbard model with time varying hopping
We study the real time dynamics of the Bose Hubbard model in the presence of
time-dependent hopping allowing for a finite temperature initial state. We use
the Schwinger-Keldysh technique to find the real-time strong coupling action
for the problem at both zero and finite temperature. This action allows for the
description of both the superfluid and Mott insulating phases. We use this
action to obtain dynamical equations for the superfluid order parameter as
hopping is tuned in real time so that the system crosses the superfluid phase
boundary. We find that under a quench in the hopping, the system generically
enters a metastable state in which the superfluid order parameter has an
oscillatory time dependence with a finite magnitude, but disappears when
averaged over a period. We relate our results to recent cold atom experiments.Comment: 22 pages, 7 figure
Competing order and nature of the pairing state in the iron pnictides
We show that the competition between magnetism and superconductivity can be
used to determine the pairing state in the iron arsenides. To this end we
demonstrate that the itinerant antiferromagnetic phase (AFM) and the
unconventional sign-changing superconducting state (SC) are near the
borderline of microscopic coexistence and macroscopic phase separation,
explaining the experimentally observed competition of both ordered states. In
contrast, conventional pairing is not able to coexist with magnetism.
Expanding the microscopic free energy of the system with competing orders
around the multicritical point, we find that static magnetism plays the role of
an intrinsic interband Josephson coupling, making the phase diagram sensitive
to the symmetry of the Cooper pair wavefunction. We relate this result to the
quasiparticle excitation spectrum and to the emergent SO symmetry of
systems with particle-hole symmetry. Our results rely on the assumption that
the same electrons that form the ordered moment contribute to the
superconducting condensate and that the system is close to particle-hole
symmetry. We also compare the suppression of SC in different regions of the
FeAs phase diagram, showing that while in the underdoped side it is due to the
competition with AFM, in the overdoped side it is related to the disappearance
of pockets from the Fermi surface.Comment: 24 pages, 13 figures; revised versio
Low-Energy Structures in Strong Field Ionization Revealed by Quantum Orbits
Experiments on atoms in intense laser pulses and the corresponding exact ab
initio solutions of the time-dependent Schr\"odinger equation (TDSE) yield
photoelectron spectra with low-energy features that are not reproduced by the
otherwise successful work horse of strong field laser physics: the "strong
field approximation" (SFA). In the semi-classical limit, the SFA possesses an
appealing interpretation in terms of interfering quantum trajectories. It is
shown that a conceptually simple extension towards the inclusion of Coulomb
effects yields very good agreement with exact TDSE results. Moreover, the
Coulomb quantum orbits allow for a physically intuitive interpretation and
detailed analysis of all low-energy features in the semi-classical regime, in
particular the recently discovered "low-energy structure" [C.I. Blaga et al.,
Nature Physics 5, 335 (2009) and W. Quan et al., Phys. Rev. Lett. 103, 093001
(2009)].Comment: 4 pages, 3 figures, REVTe
Bias-induced insulator-metal transition in organic electronics
We investigate the bias-induced insulator-metal transition in organic
electronics devices, on the basis of the Su-Schrieffer-Heeger model combined
with the non-equilibrium Green's function formalism. The insulator-metal
transition is explained with the energy levels crossover that eliminates the
Peierls phase and delocalizes the electron states near the threshold voltage.
This may account for the experimental observations on the devices that exhibit
intrinsic bistable conductance switching with large on-off ratio.Comment: 6 pages, 3 figures. To appear in Applied Physics Letter
Dynamic equation for quantum Hall bilayers with spontaneous interlayer coherence: The low-density limit
The bilayer systems exhibit the Bose-Einstein condensation of excitons that
emerge due to Coulomb pairing of electrons belonging to one layer with the
holes belonging to the other layer. Here we present the microscopic derivation
of the dynamic equation for the condensate wave function at a low density of
electron-hole () pairs in a strong magnetic field perpendicular to the
layers and an electric field directed along the layers. From this equation we
obtain the dispersion law for collective excitations of the condensate and
calculate the electric charge of the vortex in the exciton condensate. The
critical interlayer spacing, the excess of which leads to a collapse of the
superfluid state, is estimated. In bilayer systems with curved conducting
layers, the effective mass of the pair becomes the function of the
pair coordinates, the regions arise, where the energy of the pair is
lowered (exciton traps), and lastly pairs can gain the polarization in
the basal plane. This polarization leads to the appearance of quantized
vortices even at zero temperature.Comment: 8 page
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