Resonant tunneling through a small quantum dot coupled to superconducting leads

We address the problem of nonlinear transport through discrete electronic levels in a small quantum dot coupled to superconducting electrodes. In our approach the low-temperature I-V characteristics can be calculated 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 curves with recent experimental results. When the coupling to the leads increases and Coulomb blockade effects can be neglected, the combination 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 tunneling picture qualitatively explaining the subgap structure. We suggest an experimental setup where the predicted effects could be observed.