Nonequilibrium Quasiparticles and2ePeriodicity in Single-Cooper-Pair Transistors

Abstract

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.