Observation of multiple Andreev reflections in superconducting tunnel junctions

We describe measurements on high-current-density Nb-AlOₗ-Nb tunnel junctions and demonstrate that the development of excess subgap current with increasing barrier transparency is the result of multiple Andreev reflections. We present a model for the barrier consistent with measured junction resistance, the high transparency required for multiple traversals, and the observed single-particle and Josephson characteristics. We argue that excess subgap current and subharmonic gap structure in superconducting tunnel junctions are generally likely to be due to pinhole defects in the barriers.