Current reversal of photon-assisted tunneling in a quantum dot connected to superconducting leads

We study the current reversal of the photon-assisted electron transport in a single level quantum dot connected to two superconductors. It is found th at the direction of photon-assisted tunneling (PAT) current sensitively cha nges depending on the external frequency and on the electron energy level o f the quantum dot. The frequency region for the reversal of the PAT current is determined by a small applied de-bias or the energy gap difference betw een the two connecting leads.