AHARONOV-BOHM OSCILLATIONS WITH PERIOD HC/4E AND NEGATIVE MAGNETORESISTANCE IN DIRTY SUPERCONDUCTORS

We show that in a disordered superconductor near the superconductor-to-insulator transition the local superfluid density N(s) fluctuates from point to point in sign as well as magnitude. We demonstrate this explicitly with a simple model in which correlation effects produce a negative Josephson coupling between two superconducting grains. We argue more generally that correlation effects and resistance fluctuations produce random signs of N(s). This implies that the superconductor-to-insulator transition may not be in the same universality class as the Bose-superfluid-to-insulator transition. It also results in a hc/4e period of Aharonov-Bohm oscillations and a negative magnetoresistance; both occur for T > T(c) in superconducting samples and at low temperatures in insulating samples.