RESONANT-TUNNELING THROUGH A SMALL QUANTUM-DOT COUPLED TO SUPERCONDUCTING LEADS

We address the problem of nonlinear transport through discrete electro nic levels in a small quantum dot coupled to superconducting electrode s. In our approach the low-temperature T-V characteristics can be calc ulated including all multiple quasiparticle and Andreev processes. The Limit of very weak coupling to the leads and large charging energies is briefly analyzed comparing the calculated line shapes of the I-V cu rves with recent experimental results. When the coupling to the leads increases and Coulomb blockade effects can be neglected, the combinati on of multiple Andreev processes and resonant transmission gives rise to a rich subgap structure which largely differs from the one found in the more studied S-N-S systems. We concentrate on this regime showing how multiple processes can be included within a simple sequential tun neling picture qualitatively explaining the subgap structure. We sugge st an experimental setup where the predicted effects could be observed .