Manifestation of Coulomb blockade effects at an arbitrary degeneracy of the levels of a molecular contact

The current-voltage (I-V) characteristics of a metal-molecule-metal structure are investigated under the condition that electron tunneling occurs only via a single molecular level of arbitrary degeneracy. A system of kinetic equations taking into account the accumulation of charge on the molecule is solved, and an exact formula for the steady-state current is obtained for the first time. In the low-temperature limit the steps on the I-V characteristic are analyzed and found to be nonequidistant with respect to current. It is shown that with increasing degeneracy of the level the initial current steps tend toward a completely equidistant spacing. In the case when the coupling parameters between the molecule and external electrodes are substantially different, the behavior of the I-V curves on parts with opposite directions of the current is found to be different: either a single current step is formed, with an amplitude proportional to the degeneracy of the level, or equidistant current steps appear in a number equal to the degeneracy of the level. It is shown that for a given polarity of the applied voltage, the matter of which of the two behaviors of the current is realized is completely conditional on whether the level via which the electron transport occurs is occupied or unoccupied by electrons. The results of the theoretical analysis of the I-V characteristics are confirmed by a numerical simulation. (C) 2002 American Institute of Physics.