Josephson coupling through a quantum dot

We derive, via fourth-order perturbation theory, an expression for the Joseph,on current through a gated interacting quantum dot. We analyze our expression for two different models of the superconductor-dot-superconductor (SDS) system. When the matrix elements connecting dot and leads are featureless constants, we compute the Josephson coupling J(c) as a function of the gate voltage and Coulomb interaction. In the limit of a diffusive dot. we compute the probability distribution P(J(c)) of Josephson couplings. In both cases, pi junction behavior (J(c) <0) is possible. and is not simply dependent on the parity of the dot occupancy.