Date: May 17, 2007

Prof. Allen Goldman, University of Minnesota

Title: Inducing Superconductivity Using the Electric Field Effect

Abstract: 
Superconductivity has been induced in insulating ultra-thin films of
amorphous bismuth using the electric field effect. At the same time
that superconductivity is induced, the temperature dependence of the
resistance of the films changes from insulating, governed by Mott
variable range hopping, to two-dimensional "metallic" with the usual
quantum corrections. For films which become superconductive in the low
temperature limit, the transition temperature and the screening of the
electronelectron interaction are both found to increase with
increasing carrier concentration.  The data for the electrical
resistance vs. temperature at different carrier concentrations of this
electrostatically tuned transition can be analyzed using finite size
scaling. The quantum phase transition implied by this analysis belongs
to the universality class of the three dimensional XY model, assuming
that the dynamical critical exponent is unity. These same films can be
driven back into the insulating state with applied parallel magnetic
field. The resultant quantum phase transition appears to belong to the
same universality class as the electrostatically tuned transition.