Nonequilibrium effects and subgap structure in superconducting contacts

We calculate the current-voltage characteristics of a superconductor---normal-metal---superconductor junction, including the effects of multiple Andreev reflections and of a non-equilibrium quasiparticle distribution in the superconducting regions adjacent to the normal-superconductor interfaces. We find that only in ideal point contacts can nonequilibrium effects be neglected. Model calculations are performed in two limiting cases in which the nonequilibrium quasiparticle distributions are readily determined. The results are in good qualitative agreement with experimental observations of subgap structure.