Josephson Coupling through a Magnetic Impurity

We analyze nonperturbatively the behavior of a Josephson junction in which two BCS superconductors are coupled through an Anderson impurity. We recover earlier perturbative results which found that a δ0ex0ex=0ex0exπ phase difference is preferred when the impurity is singly occupied and the on-site Coulomb interaction is large. We find a novel intermediate phase in which one of δ0ex0ex=0ex0ex0 and δ0ex0ex=0ex0exπ is stable while the other is metastable, with the energy E(δ) having a kink somewhere in between. As a consequence of the kink, the I-V characteristics of the junction are modified at low voltages.