Colloquia

Cooper pairs (bosons) are thought to exist in two quite distinct ground states: 1) localized in a Mott insulator or 2) condensed in a superconductor. However,recent experiments on 2D insulator-superconductor transitions indicate that there may be a third possibility: a metal with a finite resistivity at zero temperature. I will review the standard theoretical framework used to understand the insulator-superconductor transition, the recent experimental results and I will show quite generally how bosons lacking phase coherence can form a metal in the presence of disorder rather than an insulating phase. The metallic state is rather weird, however. The phase degrees of freedom are glassy. At the heart of the metallic state is the dissipation inherent in the glassy state. Bosons moving in such a glassy environment fail to localise because no true ground state exists.