Subgap features due to quasiparticle tunneling in quantum dots coupled to superconducting leads

TL;DR

This paper develops a microscopic theory for quasiparticle tunneling in quantum dots with superconducting leads, explaining subgap transport features and providing a framework for spectroscopy of excited states.

Contribution

It introduces a master equation approach to describe quasiparticle tunneling in quantum dot systems coupled to superconductors, explaining experimental subgap features.

Findings

Subgap transport occurs due to thermally excited quasiparticles at higher temperatures.

The theory explains experimental subgap features observed in quantum dot systems.

Subgap spectroscopy can reveal excited states of quantum dots.

Abstract

We present a microscopic theory of transport through quantum dot set-ups coupled to superconducting leads. We derive a master equation for the reduced density matrix to lowest order in the tunneling Hamiltonian and focus on quasiparticle tunneling. For high enough temperatures transport occurs in the subgap region due to thermally excited quasiparticles, which can be used to observe excited states of the system for low bias voltages. On the example of a double quantum dot we show how subgap transport spectroscopy can be done. Moreover, we use the single level quantum dot coupled to a normal and a superconducting lead to give a possible explanation for the subgap features observed in the experiments published in Appl. Phys. Lett. 95, 192103 (2009).