Observation of Andreev reflection in all-superconducting single-electron transistors

We present the first experimental evidence for Andreev reflection in a single-electron transistor with both the leads and the island superconducting. In this process, two electrons tunnel to form a Cooper pair, or the reverse. It provides a second mechanism for two-electron transfer in addition to Cooper pair tunneling. Andreev reflection is found to contribute to several current cycles within the Coulomb blockade of single-electron tunneling. The observation of these cycles, however, requires that the charging energy EC<Δ.