Observation of pair currents in superconductor-semiconductor contacts

An excess low-voltage conductance is observed in Nb-InGaAs contacts at low temperatures and interpreted as being due to a pair current across the superconductor-semiconductor interface. This is the first observation of a pair current, a nonequilibrium manifestation of the proximity effect, in junctions having one electrode which is not a superconductor. A model is presented which accounts for the observed behavior. In addition to the pair current, it is demonstrated that these junctions exhibit excess conductance due to Andreev scattering. © 1991 The American Physical Society.