•Axel Thielmann1, Matthias H. Hettler1, Jürgen
König2 und Gerd Schön1,3
1Forschungszentrum Karlsruhe, Institut für Nanotechnologie,
76021 Karlsruhe, Germany
2Institut für Theoretische Physik III, Ruhr-Universität
Bochum, 44780 Bochum, Germany
3Institut für Theoretische Festkörperphysik ,
Universtität Karlsruhe, 76128 Karlsruhe, Germany
We consider charge transport through single molecules coupled weakly
to metal electrodes via tunneling barriers. The molecule is represented
by the Anderson impurity model which is the simplest model that includes
Coulomb interactions U. We calculate the current and current noise within
a first-order perturbation expansion in the coupling strength using a diagrammatic
technique. Analytical results for current, noise and the Fano factor, F,(depending
on the ratio of the couplings to the left and right electrodes) are presented
and discussed for all transport regimes involving a single interacting
(molecular) level (Phys. Rev. B 68, 115105 (2003)). Furthermore
an extended model with two levels will be considered, where the effects
of asymmetric coupling and photon-relaxation can be studied. Negative differential
conductance (NDC) behavior leading to super-poissonian noise (F
> 1) can be observed in the case of asymmetric coupling to the leads
and is distroyed by strong relaxation rates. In addition to the numerical
evaluation analytical expressions for several transport regimes will be
presented.