24 research outputs found
Shot noise in coupled dots and the "fractional charges"
We consider the problem of shot noise in resonant tunneling through double
quantum dots in the case of interacting particles. Using a many-body quantum
mechanical description we evaluate the energy dependent transmission
probability, the total average current and the shot noise spectrum. Our results
show that the obtained reduction of the noise spectrum, due to Coulomb
interaction, can be interpret in terms of non--interacting particles with
fractional charge like behavior.Comment: some clarifications added, to appear in Phys. Lett.
Charging effects in biased molecular devices
The influence of the charging effects on the transport characteristics of a
molecular wire bridging two metallic electrodes in the limit of weak contacts
is studied by generalized Breit-Wigner formula. Molecule is modeled as a
quantum dot with discrete energy levels, while the coupling to the electrodes
is treated within a broad band theory. Owing to this model we find
self-consistent occupation of particular energy levels and orbital energies of
the wire in the presence of transport. The nonlinear conductance and
current-voltage characteristics are investigated as a function of bias voltage
in the case of symmetric and asymmetric coupling to the electrodes. It is shown
that the shape of that curves are determined by the combined effect of the
electronic structure of the molecule and by electron-electron repulsion.Comment: 5 pages, 3 figures; accepted in Physica
Weak coupling approximations in non-Markovian Transport
We study the transport properties of the Fano-Anderson model with a
Lorentzian-shaped density of states in one of the electronic reservoirs. We
explicitly show that the energy dependence of the density of states can cause
non-Markovian effects and that the non-Markovian master equation may fail if
these effects are strong. We evaluate the stationary current, the zero
frequency current noise and the occupation dynamics of the resonant level by
means of a quantum master equation approach within different approximation
schemes and compare the results to the exact solution obtained by scattering
theory and Green's functions.Comment: 9 pages, 6 figures; due to suggestions of a referee we have added an
appendix where our kernel is derived in detail; a few typos are correcte
Photon-Assisted Transport Through Ultrasmall Quantum Dots: Influence of Intradot Transitions
We study transport through one or two ultrasmall quantum dots with discrete
energy levels to which a time-dependent field is applied (e.g., microwaves).
The AC field causes photon-assisted tunneling and also transitions between
discrete energy levels of the dot. We treat the problem by introducing a
generalization of the rotating-wave approximation to arbitrarily many levels.
We calculate the dc-current through one dot and find satisfactory agreement
with recent experiments by Oosterkamp et al. . In addition, we propose a novel
electron pump consisting of two serially coupled single-level quantum dots with
a time-dependent interdot barrier.Comment: 16 pages, Revtex, 10 eps-figure
Conduction in molecular junctions: Inelastic effects
The effect of a thermal environment on electron (or hole) transfer through
molecular bridges and on the electron conduction properties of such bridges is
studied. Our steady state formalism based on an extension of the Redfield
theory (D. Segal et al, J. Phys. Chem. B 104 (2000) 3817; Chem. Phys. 268
(2001) 315) is extended in two ways: First, a better description of the weak
coupling limit, which accounts for the asymmetry of the energy dependence of
the quasi-elastic component of the transmission is employed. Secondly, for
strong coupling to the thermal bath the small polaron transformation is
employed prior to the Redfield expansion. It is shown that the thermal coupling
is mainly characterized by two physical parameters: The reorganization energy
that measures the coupling strength and the correlation time (or its inverse -
the spectral width) of the thermal bath. Implications for the observed
dependence of the bridge-length dependence of the transmissions are discussed.
It is argued that in the intermediate regime between tunneling behavior and
site-to-site thermally induced hopping, the transport properties may depend on
the interplay between the local relaxation rate and the transmission dynamics.Comment: 42 pages, 9 figures, Chem. Phys., in pres
Non Linear Current Response of a Many-Level Tunneling System: Higher Harmonics Generation
The fully nonlinear response of a many-level tunneling system to a strong
alternating field of high frequency is studied in terms of the
Schwinger-Keldysh nonequilibrium Green functions. The nonlinear time dependent
tunneling current is calculated exactly and its resonance structure is
elucidated. In particular, it is shown that under certain reasonable conditions
on the physical parameters, the Fourier component is sharply peaked at
, where is the spacing between
two levels. This frequency multiplication results from the highly nonlinear
process of photon absorption (or emission) by the tunneling system. It is
also conjectured that this effect (which so far is studied mainly in the
context of nonlinear optics) might be experimentally feasible.Comment: 28 pages, LaTex, 7 figures are available upon request from
[email protected], submitted to Phys.Rev.