, Brown University
[Host: Joe Poon]
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.
Condensed Matter Seminar
Thursday, October 20, 2005
Physics Building, Room 204
Note special time.
Note special room.
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