ABSTRACT
This chapter presents the theoretical ideas and related phenomena which have proved important in practice. It is concerned with what happens when electrons enter a superconductor from some other system. The chapter is also concerned with topics in bcs theory which go beyond the isotropic weak-coupling model. The proximity effect is the spreading of superconducting order into normal metals which are in electrical contact with superconductors, discovered by H. Meissner. Superconductors may be out of equilibrium in two ways. The superfluid may be out of equilibrium, as would happen, for instance, if we had a Bcs wavefunction in which the uk and vk parameters did not take their equilibrium values. The excitations themselves, however, may be out of equilibrium and in the process of returning to it through elastic and inelastic scattering processes, which in some cases may be relatively slow.
