COULOMB-BLOCKADE OF ANDREEV REFLECTION

We study the subgap conductance of an NSN transistor, consisting of a superconductor grain, connected to two normal electrodes by tunnel junctions. Transport is due to electrons passing in pairs through the grain (Andreev reflection). We calculate the corresponding tunneling rate under Coulomb blockade conditions. For charging energies E(c) smaller than DELTA, the linear conductance has sharp peaks periodic in the gate voltage with a period twice larger than for a normal grain. If the applied bias voltage exceeds a certain threshold value, trapping of a single quasiparticle suppresses these peaks.