COULOMB-BLOCKADE OF ANDREEV REFLECTION IN THE NSN SINGLE-ELECTRON TRANSISTOR

We have measured at low temperatures the current through a submicromet er superconducting island connected to normal metal leads by ultrasmal l tunnel junctions. At low bias voltages, the current changes from bei ng e-periodic in the applied gate charge to 2e-periodic. interpret thi s 2e-periodic current as a manifestation of a sequence of Andreev refl ection events which transports two electrons at a time across the isla nd. This behavior is clear evidence that there is a difference in tota l energy between ground states of differing electron number parity.