RESONANT-TUNNELING THROUGH A SINGLE LOCALIZED STATE IN A NORMAL-METALSUPERCONDUCTOR JUNCTION

We have studied the conductance of normal-metal-superconductor tunnel junctions containing localized electron states in the interface potent ial barrier. The probabilities of the Andreev reflection and the quasi particle transmission are calculated in a one-dimensional model for a single localized state. It is established that the Andreev reflection processes assisted by resonant tunneling are responsible for the energ y dependence of the conductance in the subgap region at low temperatur es. Different types of conductance behavior have been found which depe nd on the relation between the gap energy in the superconductor and th e parameters of the localized states.