Nonequilibrium quasiparticles and $2e$ periodicity in single-Cooper-pair transistors

TL;DR

This paper demonstrates how controlling the superconducting gap in single-Cooper-pair transistors influences their periodicity, revealing the impact of nonequilibrium quasiparticles on device behavior.

Contribution

It introduces a method to tune the superconducting gap profile via oxygen doping and explains the resulting periodicity changes with a quasiparticle-based model.

Findings

Switching current period changes from 1e to 2e with gap profile control.

Even clean 2e devices are affected by quasiparticle poisoning.

Quasiparticles significantly influence superconducting transistor behavior.

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 \textit{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 surprising observation that even devices with a clean $2e$ period are ``poisoned'' by small numbers of these quasiparticles.