Microwave-induced π-junction transition in a superconductor/quantum dot/superconductor structure

Using the nonequilibrium Green function, we show that microwave irradiation can reverse the supercurrent flowing through a superconductor/quantum dot/superconductor structure. In contrast with the conventional sideband effect in normal-metal/quantum dot/normal-metal junctions, the photon-assisted structures appear near $E_0=(n/2\mathrm{})\mathrm{}ħ\omega (n=\pm 1,\pm 2\cdot \cdot \cdot ),$ where $E_0$ is the resonant energy level of the quantum dot and ω is the frequency of microwave field. Each photon-assisted structure is composed of a negative and a positive peak, with an abrupt jump from the negative peak to the positive peak around $E_0=(n/2\mathrm{})\mathrm{}ħ\omega .$ The microwave-induced π-junction transition is interpreted in the picture of photon-assisted Andreev bound states, which are formed due to multiple photon-assisted Andreev reflection between the two superconductors. Moreover, the main resonance located at $E_0=0\mathrm{}$ can also be reversed with proper microwave strength and frequency.