Mesoscopic transport through a normal-metal–quantum-dot–superconductor system with ac responses

Abstract

We investigate the mesoscopic dc transport through the normal-metal–quantum-dot–superconductor (NDS) system with ac biases on the normal metal and on the quantum dot. The Keldysh’s nonequilibrium Green-function technique and the BCS theory are used to find the general formulas of the time-averaged current and differential conductance. The transport is associated with the energy gap Δ(T) and with the ac fields to exhibit compound effects. We compare the results of the NDS system with the normal mesoscopic transport where Δ(T)=0. There is no normal electron tunneling current in the NDS system if there is only one bias field applied to the quantum dot as 0<V<Δ(0)/e at zero temperature. However, there may exist normal electron tunneling current in the system with an ac bias applied to the normal lead even if 0<V<Δ(0)/e at zero temperature. The dc current shows steps that correspond to the resonant peaks of the differential conductance. The resonant peaks are also influenced by the energy gap. The differential conductance exhibits some discontinuities caused by the density of state in the superconducting lead, and by the ac perturbations on the normal lead.

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