NEGATIVE LOCAL SUPERFLUID DENSITIES - THE DIFFERENCE BETWEEN DIRTY SUPERCONDUCTORS AND DIRTY BOSE LIQUIDS

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 both correlation effects and resistance fluctuations will produce random signs of N(s). This implies that a disordered superconductor is more like a quantum XY spin glass than a disordered Bose liquid.