Condensed Matter Seminars

According to the scaling theory of localization (1979), simple metallic phases should not exist in two dimensional electronic systems. Experiments showing the divergence of the low temperature resistance in ultrathin films of metallic elements and 2d electron gases in semiconductor heterostructures tended to support this prediction for about 15 years. More recently, however, evidence of metallic transport has begun to emerge, popping up in thin normally superconducting films and in very low density, high mobility 2D electron gas systems. The physics behind these metallic behaviors is not known although it is generally agreed that explanations must go beyond the scaling theory paradigm and include electron-electron interactions. In an effort to uncover an understandable metallic phase in two dimensions, we are studying ultrathin films composed of superconducting (S) and normal metal (N) elements. Interactions are essential to their superconducting state and such "SN" systems have been predicted to undergo a quantum superconductor to metal transition (SMT) as N is increased. I will describe how our transport and tunneling experiments on SN (Pb/Ag) bilayer films exhibit deviations from standard superconductor proximity effect theories that are consistent with an impending SMT. For example, the quasiparticle density of states of superconducting bilayers acquires a hybrid superconductor-metal appearance. This characteristic suggests that coexisting but separate superconducting and metal quasiparticle populations develop in the approach to the metallic phase.