Even-odd effect in Andreev Transport through a Carbon Nanotube Quantum Dot

TL;DR

This study investigates how the even-odd electron occupancy in a carbon nanotube quantum dot affects Andreev reflection and conductance features when coupled to superconductors, revealing a hidden Kondo resonance.

Contribution

It demonstrates the presence of an even-odd effect in Andreev transport and introduces a theoretical model that explains the observed phenomena.

Findings

Enhanced sub-gap conductance for odd charge states

Absence of zero-bias Kondo peak in superconducting state

Good agreement between experiment and Anderson model simulation

Abstract

We have measured the current(I)-voltage(V) characteristics of a single-wall carbon nanotube quantum dot coupled to superconducting source and drain contacts in the intermediate coupling regime. Whereas the enhanced differential conductance dI/dV due to the Kondo resonance is observed in the normal state, this feature around zero bias voltage is absent in the superconducting state. Nonetheless, a pronounced even-odd effect appears at finite bias in the $dI/dV$ sub-gap structure caused by Andreev reflection. The first-order Andreev peak appearing around V=\Delta/e is markedly enhanced in gate-voltage regions, in which the charge state of the quantum dot is odd. This enhancement is explained by a `hidden' Kondo resonance, pinned to one contact only. A comparison with a single-impurity Anderson model, which is solved numerically in a slave-boson meanfield ansatz, yields good agreement with the experiment.