Strong nonequilibrium coherent states in mesoscopic superconductor-semiconductor-superconductor junctions

Abstract

A biased superconductor-normal metal-superconductor junction is known to be a strong nonequilibrium system, where Andreev scattering at the interfaces creates a quasiparticle distribution function far from equilibrium. A manifestation of this is the well-known subgap structure in the I/V characteristic, with peaks in the conductance at V = 2Δ/ne. We study an SNS structure consisting of highly doped diffusive GaAs with superconducting electrodes of aluminum with one electrode configured as a flux-sensitive interferometer. This enables us to probe the coherent nature of the quasiparticle states at subgap bias voltage. Oscillations in the conductance as a function of flux are seen at zero bias voltage and disappear below experimental resolution as the bias voltage exceeds the Thouless energy E t = ħD/L 2, where L is the junction length. The oscillations reappear at higher bias in a region around the superconducting energy gap V = Δ/e corresponding to the subgap conductance peak n = 2.