Transport measurements in SISNSIS single-electron transistors

We have measured IV-characteristics of a superconducting single-electron tr ansistor (SET) whose island consists of SNS (Al-Cu-Al or Al-Cr-Al) structur e with lengths 0.4, 0.15 and 0.4 mu m, respectively. In samples with copper , a supercurrent as well as resonances are observed at small bias, and the samples behave like regular SSS SETs. The amplitude of Josephson-quasiparti cle (JQP) peaks, however, displays strong variation with gate modulation, e ven though the Coulomb energy is small (island capacitance similar to 1 fF) . Chrome sections on the island result in a smooth, exponentially varying s ub-gap current that can be modeled with the voltage fluctuation theory. (C) 2000 Elsevier Science B.V. All rights reserved.