MULTILEVEL EFFECT ON THE JOSEPHSON CURRENT THROUGH A QUANTUM-DOT

Josephson current through a multilevel quantum dot with strong Coulomb repulsion is studied by means of a Hartree-Fock approximation. We pay attention to the offdiagonal Cooper pair tunneling process in which a n up-spin electron and a down-spin electron are transferred to the opp osite bank through different single-particle levels in the dot. It is shown that; if the offdiagonal tunneling process is relevant, the syst em can behave as a pi junction even when the dot is nonmagnetic withou t a localized spin. It is also found that pi-junction behavior is enha nced with increasing the number of the dot levels. On the other hand, if the offdiagonal tunneling process is completely negligible, the mul tilevel structure suppresses pi-junction behavior. These results indic ate that we should take the multilevel effects into account in analyzi ng the Josephson current through a correlated quantum dot.