Quantum Dot between two Superconductors
  Novel effects emerge from an interplay between multiple Andreev reflections and Coulomb interaction in quantum dot coupled to superconducting leads and subject to a finite potential bias $V$. Combining an intuitive physical picture with 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 is shifted toward higher $V$. For sufficiently ^M strong interaction the subgap current (at $eV < 2 \Delta$) is virtually suppressed. 
Yshai Avishai, Anatoly Golub and Andrei D. Zaikin