ANDREEV SCATTERING IN WEAK LINKS

Quasiparticles in inhomogeneous superconductors are subject to two off-diagnoal forces. One, due to the gradient in the modulus of the pair potential, is responsible for electron - hole (Andreev) scattering; the other, recently discovered one results from the gauge invariant Cooper pair velocity of interface supercurrents and should be important in high field type II superconductors. In clean metallic weak links Andreev scattering is one basic mechanism which transfers charge 2e and establishes phase coherence between the superconducting banks. The competitive interplay between Andreev scattering and conventional scattering from ordinary surface and interface potentials and mismatches of the Fermi velocities determines the electronic structure and the transport properties of microbridges, point contacts, heterojunctions, including semi-superconducting hybrides, and layered structures. Modeling these systems by SNS junctions, one can show, how (multiple) Andreev scattering may lead to phenomena experimentally observed in Josephson junctions with non-tunnel-type conductivity, such as excess currents, subharmonic gap structures, negative differential conductivity and high low-voltage conductivity.