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We study Andreev transport through double quantum dots connected in seriesnormal and superconducting (SC) leads, using the numerical renormalizationgroup. The ground state of this system shows a crossover between a localCooper-pairing singlet state and a Kondo singlet state, which is caused by thecompetition between the Coulomb interaction and the SC proximity. We show thatthe ground-state properties reflect this crossover especially for small valuesof the inter-dot coupling $t$, while in the opposite case, for large $t$,another singlet with an inter-dot character becomes dominant. We find that theconductance for the local SC singlet state has a peak with the unitary-limitvalue $4e^2/h$. In contrast, the Andreev reflection is suppressed in the Kondoregime by the Coulomb interaction. Furthermore, the conductance has twosuccessive peaks in the transient region of the crossover. It is furtherelucidated that the gate voltage gives a different variation into thecrossover. Specifically, as the energy level of the dot that is coupled to thenormal lead varies, the Kondo screening cloud is deformed to a long-rangesinglet bond.

Correlated electron transport through double quantum dots coupled to normal and superconducting leads