Interacting-impurity Josephson junction: Variational wave functions and slave-boson mean-field theory

We investigate the Josephson coupling between two superconductors mediated through an infinite U Anderson impurity, adapting a variational wave-functi on approach which has proved successful for the Kondo model. Unlike the Kon do problem, however, a crossing of singlet and doublet state energies may b e produced by varying the ratio of Kondo energy to superconducting gap, in agreement with recent work of Clerk and Ambegaokar. We construct the phase diagram for the junction and discuss properties of different phases. In add ition, we find the singlet and doublet state energies within a slave-boson mean-field approach. We find the slave-boson mean-held treatment is unable to account for the level crossing.