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 superco nducting, In this process, two electrons tunnel to form a Cooper pair, or the reverse. It provides a second mechanism for two-electron trans fer 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 E-C<Delta.