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 $\omega$ is studied in terms of the Schwinger-Keldysh nonequilibrium Green functions. The nonlinear time dependent tunneling current $I(t)$ 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 $I_{n}$ is sharply peaked at $n=\frac {\Delta E} {\hbar \omega}$, where $\Delta E$ is the spacing between two levels. This frequency multiplication results from the highly nonlinear process of $n$ 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.