Superconductivity-enhanced bias spectroscopy in carbon nanotube quantum dots

TL;DR

This paper investigates how superconducting niobium leads influence low-temperature transport in carbon nanotube quantum dots, revealing enhanced cotunneling processes and unique sub-gap features.

Contribution

It demonstrates the impact of superconducting leads on cotunneling in nanotube quantum dots and explains the gate-dependent behavior of inelastic cotunneling lines.

Findings

Enhanced conductance peaks due to superconducting coherence peaks

Gate-dependent inelastic cotunneling lines

Pronounced sub-gap structures in Coulomb diamonds

Abstract

We study low-temperature transport through carbon nanotube quantum dots in the Coulomb blockade regime coupled to niobium-based superconducting leads. We observe pronounced conductance peaks at finite source-drain bias, which we ascribe to elastic and inelastic cotunneling processes enhanced by the coherence peaks in the density of states of the superconducting leads. The inelastic cotunneling lines display a marked dependence on the applied gate voltage which we relate to different tunneling-renormalizations of the two subbands in the nanotube. Finally, we discuss the origin of an especially pronounced sub-gap structure observed in every fourth Coulomb diamond.