Effect of a barrier at the superconducting-normal metal interface

A superconducting-normal metal sandwich, in which there is a potential barrier at the interface, is considered. The Tomasch effect amplitude is shown to be reduced by a factor related to the barrier transmission coefficient. We then derive from first principles the boundary conditions obeyed by the kernel in the Gor'kov gap equation at the interface, within the diffusion approximation. This represents the first ab initio calculation of the ''second'' de Gennes boundary condition