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 mani festation of this is the well-known subgap structure in the I/V characteris tic, with peaks in the conductance at V = 2 Delta/ne. We study an SNS struc ture consisting of highly doped diffusive GaAs with superconducting electro des of aluminum with one Electrode configured as a flux-sensitive interfero meter. This enables us to probe the coherent nature of the quasiparticle st ates at subgap bias voltage. Oscillations in the conductance as a function of flux are seen at zero bias voltage and disappear below experimental reso lution as the bias voltage exceeds the Thouless energy E-t = (h) over bar D /L-2, where L is the junction length. The oscillations reappear at higher b ias in a region around the superconducting energy gap V = Delta/e correspon ding to the subgap conductance peak n = 2.