Strong Nonequilibrium Coherent States in Mesoscopic Superconductor-Semiconductor-Superconductor Junctions

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 = 2/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 = D/L², where L is the junction length. The oscillations reappear at higher bias in a region around the superconducting energy gap V = /e corresponding to the subgap conductance peak n = 2.