PERSISTENT CURRENTS IN A SUPERCONDUCTING RING WITH A NORMAL AND SPIN-FLIP JUNCTION

We study the persistent currents in a one-dimensional ring with a junc tion, described by the negative-U Hubbard model using exact diagonaliz ation methods. The hopping in the junction has two terms: a direct hop ping t' and a hopping J through a magnetic impurity. For sufficiently large values of J/t' the persistent current changes sign, giving rise to what is known as a pi-junction. For high values of J and t' non-lin ear effects are observed.