Nonequilibrium superconductivity in mesoscopic Nb/InAs/Nb junctions

We review experiments on the response of Nb/InAs/Nb SNS junctions to high f requency irradiation. Both single junctions and multi-junction arrays show an anomalous half-integer Shapiro step, which persists to high temperatures in the absence of a critical current. The temperature and bias dependence of this effect demonstrate that it is caused by nonequilibrium ac supercurr ents flowing at twice the usual frequency. Conductance enhancement near zer o bias is also observed and attributed to nonequilibrium dynamics. A model based on time-dependent Andreev bound states, with parameters extracted fro m this enhanced conductance, successfully accounts for this half-integer Sh apiro step. (C) 1999 Academic Press.