Kondo effect in normal-superconductor quantum dots

We study the transport properties of a quantum dot coupled to a normal and a superconducting lead. The dot is represented by a generalized Anderson model. Correlation effects are taken into account by an appropriate self-energy that interpolates between the limits of weak and strong coupling to the leads. The transport properties of the system are controlled by the interplay between the Kondo effect and Andreev reflection processes. We show that, depending on the parameters' range, the conductance can either be enhanced or suppressed as compared to the normal case. In particular, by adequately tuning the coupling to the leads one can reach the maximum value 4e²/h for the conductance.