CURRENT-CONSERVING DESCRIPTION OF NONLINEAR TRANSPORT IN NS AND NSN STRUCTURES

Nonlinear transport in NS and NSN structures is studied by means of a self-consistent microscopic description. Current conservation requires the superfluid velocity to be nonzero, causing the various quasiparti cle channels within the superconductor to open at different voltages. This effect is responsible for major changes in the current-voltage ch aracteristics. At low temperatures, superconductivity, some times in a peculiar gapless form, may survive up to high voltages. The minimum v oltage for quasiparticle transmission is shown to decrease strongly wi th temperature and with the transmittivity of the interfaces. A transp ort regime is identified where impurities are unable to degrade the qu asiparticle electric current.