46 research outputs found
Universal scaling in transport out of equilibrium through a single quantum dot using the noncrossing approximation
The universal scaling behavior is studied for nonequilibrium transport
through a quantum dot. To describe the dot we use the standard Anderson
impurity model and use the non-equilibrium non-crossing approximation in the
limit of infinite Coulomb repulsion. After solving de hamiltonian, we calculate
the conductance through the system as a function of temperature and bias
voltage in the Kondo and in the mixed valent regime. We obtain a good
scaling function in both regimes. In particular, in the mixed valent regime, we
find excellent agreement with recent experiments and previous theoretical
works.Comment: 6 pages, 5 figures, Accepted Physical Review
Quantum Transport Through a Stretched Spin--1 Molecule
We analyze the electronic transport through a model spin-1 molecule as a
function of temperature, magnetic field and bias voltage. We consider the
effect of magnetic anisotropy, which can be generated experimentally by
stretching the molecule. In the experimentally relevant regime the conductance
of the unstretched molecule reaches the unitary limit of the underscreened
spin- 1 Kondo effect at low temperatures. The magnetic anisotropy generates an
antiferromagnetic coupling between the remaining spin 1/2 and a singular
density of quasiparticles, producing a second Kondo effect and a reduced
conductance. The results explain recent measurements in spin-1 molecules
[Science 328 1370 (2010)].Comment: 5 pages, 3 figures, minor changes, accepted for publication in EP
Nonequilibrium transport through magnetic vibrating molecules
We calculate the nonequilibrium conductance through a molecule or a quantum
dot in which the occupation of the relevant electronic level is coupled with
intensity to a phonon mode, and also to two conducting leads. The
system is described by the Anderson-Holstein Hamiltonian. We solve the problem
using the Keldysh formalism and the non-crossing approximation (NCA) for both,
the electron-electron and the electron-phonon interactions. We obtain a
moderate decrease of the Kondo temperature with for fixed
renormalized energy of the localized level . The meaning and value
of are discussed. The spectral density of localized electrons
shows in addition to the Kondo peak of width , satellites of this peak
shifted by multiples of the phonon frequency . The nonequilibrium
conductance as a function of bias voltage at small temperatures, also
displays peaks at multiples of in addition to the central dominant
Kondo peak near .Comment: 11 pages, 13 figures, accepted in Phys. Rev.
Impact of capacitance and tunneling asymmetries on Coulomb blockade edges and Kondo peaks in non-equilibrium transport through molecular quantum dots
We investigate theorerically the non-equilibrium transport through a
molecular quantum dot as a function of gate and bias voltage, taking into
account the typical situation in molecular electronics. In this respect, our
study includes asymmetries both in the capacitances and tunneling rates to the
source and drain electrodes, as well as an infinitely large charging energy on
the molecule. Our calculations are based on the out-of-equilibrium
Non-Crossing-Approximation (NCA), which is a reliable technique in the regime
under consideration. We find that Coulomb blockade edges and Kondo peaks
display strong renormalization in their width and intensity as a function of
these asymmetries, and that basic expectations from Coulomb blockade theory
must be taken with care in general, expecially when Kondo physics is at play.
In order to help comparison of theory to experiments, we also propose a simple
phenomenological model which reproduces semi-quantitatively the Coulomb
blockade edges that were numerically computed from the NCA in all regimes of
parameters.Comment: 9 pages, 8 figure
Replicas of the Kondo peak due to electron-vibration interaction in molecular transport properties
The low temperature properties of single level molecular quantum dots
including both, electron-electron and electron-vibration interactions, are
theoretically investigated. The calculated differential conductance in the
Kondo regime exhibits not only the zero bias anomaly but also side peaks
located at bias voltages which coincide with multiples of the energy of
vibronic mode . We obtain that the evolution with
temperature of the two main satellite conductance peaks follows the
corresponding one of the Kondo peak when , being the Kondo temperature, in agreement with recent transport measurements in
molecular junctions. However, we find that this is no longer valid when is of the order of a few times .Comment: 6 pages, 4 figures. Accepted for publication in Physical Review
Orbital Kondo spectroscopy in a double quantum dot system
We calculate the nonequilibrium conductance of a system of two capacitively
coupled quantum dots, each one connected to its own pair of conducting leads.
The system has been used recently to perform pseudospin spectroscopy by
controlling independently the voltages of the four leads. The pseudospin is
defined by the orbital occupation of one or the other dot. Starting from the
SU(4) symmetric point of spin and pseudospin degeneracy in the Kondo regime,
for an odd number of electrons in the system, we show how the conductance
through each dot varies as the symmetry is reduced to SU(2) by a pseudo-Zeeman
splitting, and as bias voltages are applied to any of the dots. We analize the
expected behavior of the system in general, and predict characteristic
fingerprint features of the SU(4) to SU(2) crossover that have not been
observed so far.Comment: 5 pages, 6 figures, submitted to Phys. Rev.
Restoring the SU(4) Kondo regime in a double quantum dot system
We calculate the spectral density and occupations of a system of two
capacitively coupled quantum dots, each one connected to its own pair of
conducting leads, in a regime of parameters in which the total coupling to the
leads for each dot are different. The system has been used recently
to perform pseudospin spectroscopy by controlling independently the voltages of
the four leads. For an odd number of electrons in the system,
, equal dot levels and sufficiently large interdot
repulsion the system lies in the SU(4) symmetric point of spin and
pseudospin degeneracy in the Kondo regime. In the more realistic case , pseudospin degeneracy is broken and the symmetry is reduced to
SU(2). Nevertheless we find that the essential features of the SU(4) symmetric
case are recovered by appropriately tuning the level difference
. The system behaves as an SU(4) Kondo one at low energies. Our
results are relevant for experiments which look for signatures of SU(4)
symmetry in the Kondo regime of similar systems.Comment: 9 pages, 10 figure