Quantum capacitance of a superconducting subgap state in an electrostatically floating dot-island

TL;DR

This paper investigates the quantum capacitance of subgap states in a floating superconducting quantum dot device using radio-frequency reflectometry, revealing thermal screening effects and modeling with an Anderson impurity framework.

Contribution

It introduces a measurement technique for quantum capacitance in floating hybrid devices and demonstrates its effectiveness in analyzing subgap states with a theoretical model.

Findings

Capacitance loading is suppressed by thermal excitations in small gaps.

Resonance shifts are explained by a single-level Anderson impurity model.

Method is applicable to complex hybrid superconducting devices.

Abstract

We study a hybrid device defined in an InAs nanowire with an epitaxial Al shell that consists of a quantum dot in contact with a superconducting island. The device is electrically floating, prohibiting transport measurements, but providing access to states that would otherwise be highly excited and unstable. Radio-frequency reflectometry with lumped-element resonators couples capacitatively to the quantum dot, and detects the presence of discrete subgap states. We perform a detailed study of the case with no island states, but with quantum-dot-induced subgap states controlled by the tunnel coupling. When the gap to the quasi-continuum of the excited states is small, the capacitance loading the resonator is strongly suppressed by thermal excitations, an effect we dub "thermal screening". The resonance frequency shift and changes in the quality factor at charge transitions can be accounted for using a single-level Anderson impurity model. The established measurement method, as well as the analysis and simulation framework, are applicable to more complex hybrid devices such as Andreev molecules or Kitaev chains.