CURRENT-VOLTAGE RELATION OF A NORMAL-METAL-SUPERCONDUCTOR JUNCTION

We calculate the electrical and heat currents flowing through a narrow, mesoscopic, normal-metal-superconductor (NS) junction containing a single point impurity. The NS junction exhibits large subgap conduction steps in its current-voltage (I-V) relation when the impurity is located in the normal metal. One new subgap step appears in the I-V characteristic whenever an additional quasiparticle is trapped between the impurity and NS interface. Locating the impurity inside the superconductor produces both an ''excess current'' and oscillations in the I-V characteristic analogous to the Tomasch effect. A maximum excess current of 2e Delta/h, slightly less than the 8e Delta/3h found for a ballistic NS junction, is obtained when the impurity is located a few coherence lengths inside the superconductor.