Electron tunneling through a hybrid superconducting-normal mesoscopic junction under microwave radiation

Abstract

We present a theoretical analysis for electron tunneling through a hybrid superconducting-normal mesoscopic junction consisting of a superconducting electrode, a two-level quantum dot, and a normal electrode under single mode microwave radiation. Using nonequilibrium Green's-function formalism, we incorporate Floquet basis in the Nambu space and solve the Green's function with finite matrix truncation to obtain the transport properties numerically. We studied the effects of photon-induced single-level oscillations and quantum transition between levels on the time-averaged current-voltage $(I\text{\ensuremath{-}}V)$ characteristics of the system. For quantum dot with a single localized level, the main dc resonance remains unchanged regardless of the frequency and amplitude of the radiation, and a series of secondary resonances due to multiphoton processes are present. For quantum dot with two localized levels, the sole effects from the transitions between levels produce splitting on the main dc resonance at Rabi frequency proportional to the coupling. This provides the possibility for experimental inference of the interlevel coupling strength of the driven resonant tunneling system from the bias voltage energy difference between the split resonances in the $I\text{\ensuremath{-}}V$ curve.