Quantum dot between two superconductors

Novel effects emerge from an interplay between multiple Andreev reflections and Coulomb interaction in a quantum dot coupled to superconducting leads and subject to a finite potential bias V. Combining an intuitive physical p icture with a rigorous path integral formalism, we evaluate the current I t hrough the dot and nd that the interaction shifts the subharmonic pattern o f the I-V curve toward higher V. For a sufficiently strong interaction the subgap current (at eV < 2<Delta>) is virtually suppressed.