Resonant and inelastic Andreev tunneling observed on a carbon nanotube quantum dot

TL;DR

This paper reports the experimental observation of resonant and inelastic Andreev tunneling processes in a carbon nanotube quantum dot with a superconducting contact, revealing boson-assisted inelastic tunneling mechanisms.

Contribution

First experimental demonstration of both resonant and inelastic Andreev tunneling in a carbon nanotube quantum dot with a superconductor.

Findings

Resonant Andreev tunneling observed at a single QD level.

Discrete replicas indicating boson-assisted inelastic tunneling.

Magnetic field dependence suggests coupling to two different bosons.

Abstract

We report the observation of two fundamental sub-gap transport processes through a quantum dot (QD) with a superconducting contact. The device consists of a carbon nanotube contacted by a Nb superconducting and a normal metal contact. First, we find a single resonance with position, shape and amplitude consistent with the theoretically predicted resonant Andreev tunneling (AT) through a single QD level. Second, we observe a series of discrete replicas of resonant AT at a separation of $\sim145\,\mu$eV, with a gate, bias and temperature dependence characteristic for boson-assisted, inelastic AT, in which energy is exchanged between a bosonic bath and the electrons. The magnetic field dependence of the replica's amplitudes and energies suggest that two different bosons couple to the tunnel process.