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 picture with a rigorous path integral formalism, we evaluate the current I through the dot and find that the interaction shifts the subharmonic pattern of the I-V curve toward higher V. For a sufficiently strong interaction the subgap current (at eV < 2Δ) is virtually suppressed.