•Maarten Wegewijs^{1}, Matthias Hettler^{2}, Herbert
Schoeller^{1} und Wolfgang Wenzel^{2}
^{1}Institut für Theoretische Physik - Lehrstuhl A RWTH
Aachen 52056 Aachen
^{2}Forschungszentrum Karlsruhe, Institut für Nanotechnologie,
76021 Karlsruhe, Germany

The nonlinear transport through a benzene molecule coupled weakly to electrodes is analyzed theoretically. A negative differential conductance effect (NDC) is predicted. It is caused by the population of a certain excited state of the molecule-anion at a finite bias voltage. This state can spontaneously decay under photon emission to a lower lying state which does not couple to the electron tunneling. Therefore, the flow of current is blocked above the threshold voltage.

This effect is linked to the symmetry properties of the molecule with respect to the transport direction and is found to be generic. This may provide a clue for finding molecules with desired current-switching properties.

From electronic structure calculations an effective interacting Hamiltonian for the pi-electrons is constructed which serves as input for a transport calculation. Most importantly, Coulomb blockade effects are accounted for properly. Furthermore, the relevance of diffuse orbitals (so-called Rydberg states) for an extension of the effective model is discussed.