Rf-induced transport of Cooper pairs in superconducting single electron transistors in a dissipative environment

TL;DR

This paper studies how radio-frequency signals induce transport of Cooper pairs in superconducting single-electron transistors within dissipative environments, revealing mechanisms of sequential tunneling and cotunneling influenced by gate voltage and quasiparticle poisoning.

Contribution

It demonstrates the role of RF-induced Cooper pair transport in superconducting transistors with high charging energy and develops a master equation model to explain the observed tunneling behaviors.

Findings

Sequential tunneling dominates near specific gate voltages.

Cotunneling prevails away from these points.

High-frequency gate cycling reveals tunneling mechanisms.

Abstract

We investigate low-temperature and low-voltage-bias charge transport in a superconducting Al single electron transistor in a dissipating environment, realized as on-chip high-ohmic Cr microstrips. In our samples with relatively large charging energy values Ec > EJ, where EJ is the energy of the Josephson coupling, two transport mechanisms were found to be dominating, both based on discrete tunneling of individual Cooper pairs: Depending on the gate voltage Vg, either sequential tunneling of pairs via the transistor island (in the open state of the transistor around the points Qg = CgVg = e mod(2e), where Cg is the gate capacitance) or their cotunneling through the transistor (for Qg away of these points) was found to prevail in the net current. As the open states of our transistors had been found to be unstable with respect to quasiparticle poisoning, high-frequency gate cycling (at f ~ 1 MHz) was applied to study the sequential tunneling mechanism. A simple model based on the master equation was found to be in a good agreement with the experimental data.