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 $dI/dV$ subgap 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 mean-field approach, yields good agreement with the experiment.

• Received 10 March 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.126602