303 research outputs found
Non-equilibrium dynamics of Andreev states in the Kondo regime
The transport properties of a quantum dot coupled to superconducting leads
are analyzed. It is shown that the quasiparticle current in the Kondo regime is
determined by the non-equilibrium dynamics of subgap states (Andreev states)
under an applied voltage. The current at low bias is suppressed exponentially
for decreasing Kondo temperature in agreement with recent experiments. We also
predict novel interference effects due to multiple Landau-Zener transitions
between Andreev states.Comment: Revtex4, 4 pages, 4 figure
The phase-dependent linear conductance of a superconducting quantum point contact
The exact expression for the phase-dependent linear conductance of a weakly
damped superconducting quantum point contact is obtained. The calculation is
performed by summing up the complete perturbative series in the coupling
between the electrodes. The failure of any finite order perturbative expansion
in the limit of small voltage and small quasi-particle damping is analyzed in
detail. In the low transmission regime this nonperturbative calculation yields
a result which is at variance with standard tunnel theory. Our result predicts
the correct sign of the quasi-particle pair interference term and exhibits an
unusual phase-dependence at low temperatures in qualitative agreement with the
available experimental data.Comment: 12 pages (revtex) + 1 postscript figure. Submitted to Phys. Rev. Let
Real space finite difference method for conductance calculations
We present a general method for calculating coherent electronic transport in
quantum wires and tunnel junctions. It is based upon a real space high order
finite difference representation of the single particle Hamiltonian and wave
functions. Landauer's formula is used to express the conductance as a
scattering problem. Dividing space into a scattering region and left and right
ideal electrode regions, this problem is solved by wave function matching (WFM)
in the boundary zones connecting these regions. The method is tested on a model
tunnel junction and applied to sodium atomic wires. In particular, we show that
using a high order finite difference approximation of the kinetic energy
operator leads to a high accuracy at moderate computational costs.Comment: 13 pages, 10 figure
Low frequency shot noise in double-barrier resonant-tunneling structures in a strong magnetic field
Low frequency shot noise and dc current profiles for a double-barrier
resonant-tunneling structure (DBRTS) under a strong magnetic field applied
perpendicular to the interfaces have been studied. Both the structures with 3D
and 2D emitter have been considered. The calculations, carried out with the
Keldysh Green's function technique, show strong dependencies of both the
current and noise profiles on the bias voltage and magnetic field. The noise
spectrum appears sensitive to charge accumulation due to barriere capacitances
and both noise and dc-current are extremely sensitive to the Landau levels'
broadening in the emitter electrode and can be used as a powerful tool to
investigate the latter. As an example, two specific shapes of the levels'
broadening have been considered - a semi-elliptic profile resulting from
self-consistent Born approximation, and a Gaussian one resulting from the
lowest order cumulant expansion.Comment: 15 pages Revtex, 8 Postscript figures included. To be published in
Journal of Physics: Condensed matte
Universal features of electron-phonon interactions in atomic wires
The effect of electron-phonon interactions in the conductance through
metallic atomic wires is theoretically analyzed. The proposed model allows to
consider an atomic size region electrically and mechanically coupled to bulk
electrodes. We show that under rather general conditions the features due to
electron-phonon coupling are described by universal functions of the system
transmission coefficients. It is predicted that the reduction of the
conductance due to electron-phonon coupling which is observed close to perfect
transmission should evolve into an enhancement at low transmission. This
crossover can be understood in a transparent way as arising from the
competition between elastic and inelastic processes.Comment: 5 pages, 5 figure
Microscopic origin of the conducting channels in metallic atomic-size contacts
We present a theoretical approach which allows to determine the number and
orbital character of the conducting channels in metallic atomic contacts. We
show how the conducting channels arise from the atomic orbitals having a
significant contribution to the bands around the Fermi level. Our theory
predicts that the number of conducting channels with non negligible
transmission is 3 for Al and 5 for Nb one-atom contacts, in agreement with
recent experiments. These results are shown to be robust with respect to
disorder. The experimental values of the channels transmissions lie within the
calculated distributions.Comment: 11 pages, 4 ps-figures. Submitted to Phys. Rev. Let
Josephson and Andreev transport through quantum dots
In this article we review the state of the art on the transport properties of
quantum dot systems connected to superconducting and normal electrodes. The
review is mainly focused on the theoretical achievements although a summary of
the most relevant experimental results is also given. A large part of the
discussion is devoted to the single level Anderson type models generalized to
include superconductivity in the leads, which already contains most of the
interesting physical phenomena. Particular attention is paid to the competition
between pairing and Kondo correlations, the emergence of \pi-junction behavior,
the interplay of Andreev and resonant tunneling, and the important role of
Andreev bound states which characterized the spectral properties of most of
these systems. We give technical details on the several different analytical
and numerical methods which have been developed for describing these
properties. We further discuss the recent theoretical efforts devoted to extend
this analysis to more complex situations like multidot, multilevel or
multiterminal configurations in which novel phenomena is expected to emerge.
These include control of the localized spin states by a Josephson current and
also the possibility of creating entangled electron pairs by means of non-local
Andreev processes.Comment: 39 pages, 54 figures, corresponds to a review article as submitted to
Advances in Physic
Resonant Andreev reflections in superconductor-carbon-nanotube devices
Resonant Andreev reflection through superconductor-carbon-nanotube devices
was investigated theoretically with a focus on the superconducting proximity
effect. Consistent with a recent experiment, we find that for high transparency
devices on-resonance, the Andreev current is characterized by a large value and
a resistance dip; low-transparency off-resonance devices give the opposite
result. We also give evidence that the observed low-temperature transport
anomaly may be a natural result of Andreev reflection process
Extraordinary Temperature Dependence of the Resonant Andreev Reflection
An extraordinary temperature dependence of the resonant Andreev reflection
via discrete energy level in a normal-metal / quantum-dot / superconductor
(N-QD-S) system is predicted theoretically by using Green function technique.
The width of zero bias conductance peak in N-QD-S is about and does not exhibit thermal broadening, where and
are the coupling strength between QD and leads. Considering the
intra-dot Coulomb interaction, the Coulomb blockade oscillations conducted by
Andreev reflection differs dramatically from that in N-QD-N. Instead of thermal
broadening, finite temperature induces more resonant peaks around the
oscillation peaks of zero temperature. This effect can be applied to determine
the coupling strength and QD level spacing in N-QD-S.Comment: 11 pages, 3 figures, LaTe
Direct link between Coulomb blockade and shot noise in a quantum coherent structure
We analyze the current-voltage characteristic of a quantum conduction channel
coupled to an electromagnetic environment of arbitrary frequency-dependent
impedance. In the weak blockade regime the correction to the ohmic behavior is
directly related to the channel current fluctuations vanishing at perfect
transmission in the same way as shot noise. This relation can be generalized to
describe the environmental Coulomb blockade in a generic mesoscopic conductor
coupled to an external impedance, as the response of the latter to the current
fluctuations in the former.Comment: 12 pages, 2 figures, submitted to Phys. Rev. Let
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