Pairing correlation effect on the zero-bias tunneling spectra of ultrasmall superconducting grains

We study the superconducting pairing correlation (SPC) effect on the resonant tunneling through an ultrasmall metallic grain with a gate electrode. The resonant conductance peaks appearing with an increase or decrease of the gate voltage are accompanied by the transition of the total electron number N on the grain, such as N <----> N +/- 1. We focus our attention oil the tunneling resonance associated with an odd-N state. In the absence of the SPC effect, the structure of the zoro-bias resonant conductance peak for N <----> N - 1 is equivalent to that for N <----> N + 1. It is shown that due to the interplay between the SPC effect and the nonuniform spacings of single-particle energy levels, we expect a notable difference between the two peak structures.