Nonequilibrium superconductivity in mesoscopic Nb/InAs/Nb junctions

We review experiments on the response of Nb/InAs/Nb SNS junctions to high frequency 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 supercurrents flowing at twice the usual frequency. Conductance enhancement near zero bias is also observed and attributed to nonequilibrium dynamics. A model based on time-dependent Andreev bound states, with parameters extracted from this enhanced conductance, successfully accounts for this half-integer Shapiro step.