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 that the direction of photon-assisted tunneling (PAT) current sensitively changes depending on the external frequency and on the electron energy level of 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 between the two connecting leads.X11sciescopu