Microwave-induced π-junction transition in a superconductor/quantum dot/superconductor structure -: art. no. 134507

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)(h) over bar omega (n=+/-1,+/-2...), where E-0 is the resonant energy level of the quantum dot and omega 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)(h) over bar omega. The microwave-induced pi-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 can also be reversed with proper microwave strength and frequency.