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.
@article{arxiv.cond-mat/0703082,
title = {Even-odd effect in Andreev Transport through a Carbon Nanotube Quantum Dot},
author = {A. Eichler and M. Weiss and S. Oberholzer and C. Schonenberger and A. Levy Yeyati and J. C. Cuevas},
journal= {arXiv preprint arXiv:cond-mat/0703082},
year = {2009}
}