Abstract
We have measured the current ()-voltage () 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 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 subgap structure caused by Andreev reflection. The first-order Andreev peak appearing around 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
©2007 American Physical Society