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

We have measured at low temperatures the current through a submicrometer superconducting island connected to normal-metal leads by ultrasmall tunnel junctions. At low bias voltages, the current changes from being e-periodic in the applied gate charge to 2e-periodic. We interpret this 2e-periodic current as a manifestation of a sequence of Andreev reflection events which transports two electrons at a time across the island. This behavior is clear evidence that there is a difference in total energy between ground states of an even or odd number of electrons.