Nonequilibrium quasiparticles and 2e periodicity in single-Cooper-pair transistors -: art. no. 066802

We have fabricated single-Cooper-pair transistors in which the spatial profile of the superconducting gap energy was controlled by oxygen doping. The profile dramatically affects the switching current vs gate voltage curve of the transistor, changing its period from 1e to 2e. A model based on nonequilibrium quasiparticles in the leads explains our results, including the observation that even devices with a clean 2e period are poisoned by small numbers of these quasiparticles.