Evolution towards superconductivity in granular films of bismuth

Abstract

The thickness-tuned two-dimensional insulator-superconductor transition has been investigated in ultrathin, granular films of amorphous bismuth. ``Double reentrance'' was observed in the resistance vs temperature. We suggest that the reentrance into the insulating state results from the opening of the energy gap on superconducting grains or clusters coupled by quasiparticle tunneling, and that the reentrance into the superconducting state is a consequence of the onset of intercluster Josephson coupling leading to global phase coherence. Measurements of voltage fluctuations have also been carried out. For insulators, the first power spectra exhibit $1∕{f}^{2}$ frequency dependence and the second spectra are white, which is consistent with uncorrelated fluctuations. The first power spectra gradually change from having a $1∕{f}^{2}$ frequency dependence in the insulating state to being independent of frequency in the superconducting state. There is no specific feature of the noise that can be associated with the onset of superconductivity