SELF-CONSISTENT SCATTERING DESCRIPTION OF TRANSPORT IN NORMAL-SUPERCONDUCTOR STRUCTURES

We present a scattering description of transport in several normal-sup erconductor structures. We show that the related requirements of self- consistency and current conservation introduce qualitative changes in the transport behavior when the current in the superconductor is not n egligible. The energy thresholds for quasiparticle propagation in the superconductor are sensitive to the existence of condensate flow (v(s) not equal 0). This dependence is responsible for a rich variety of tr ansport regimes, including a voltage range in which only Andreev trans mission is possible at the interfaces, and a state of gapless supercon ductivity which may survive up to high voltages if temperature is low. The two main effects of current conservation are a shift towards lowe r voltages of the first peak in the differential conductance and an en hancement of current caused by the greater availability of charge tran smitting scattering channels.