Subgap resonant quasiparticle transport in normal-superconductor quantum dot devices

TL;DR

This paper investigates thermally activated quasiparticle transport resonances in a carbon nanotube quantum dot device with superconducting Pb contacts, revealing how these resonances depend on temperature and magnetic field, and demonstrating the device's ideal transport properties.

Contribution

The study demonstrates reproducible subgap resonances in CNT quantum dots with optimized Pb contacts, providing insights into quasiparticle transport and a method to determine tunnel couplings.

Findings

Resonances occur due to finite quasiparticle populations at elevated temperatures.

Superconducting gaps close monotonically with increasing magnetic field and temperature.

Resonant tunneling model accurately describes subgap resonances.

Abstract

We report thermally activated transport resonances for biases below the superconducting energy gap in a carbon nanotube (CNT) quantum dot (QD) device with a superconducting Pb and a normal metal contact. These resonances are due to the superconductor's finite quasi-particle population at elevated temperatures and can only be observed when the QD life-time broadening is considerably smaller than the gap. This condition is fulfilled in our QD devices with optimized Pd/Pb/In multi-layer contacts, which result in reproducibly large and "clean" superconducting transport gaps with a strong conductance suppression for subgap biases. We show that these gaps close monotonically with increasing magnetic field and temperature. The accurate description of the subgap resonances by a simple resonant tunneling model illustrates the ideal characteristics of the reported Pb contacts and gives an alternative access to the tunnel coupling strengths in a QD.