We have been able to reversibly alter the ground state of an ultrathin film from insulating to superconducting using the electric field effect. At the same time the mode of conduction changes from strong to weak localization. The quantum critical point scenario that evolves is very different from what would be implied by phase-only models of the insulator-superconductor transition. These same films have been driven back into the insulating state by the application of parallel magnetic fields. Finite size scaling analyses (temperature scaling) can be carried out for both the parallel-field, and electrostatically tuned transitions with exponent products consistent with a (2+1) dimensional XY model.
The previously reported intermediate metallic regime separating superconducting and insulating regimes, at least for ultrathin films, appears to be a hot electron effect resulting from the failure of the electrons to cool in the presence of even a small measuring current. Also nonlinear effects that have been analyzed using electric-field (or in-plane voltage) scaling can be shown to be hot electron effects and thus mapped directly onto temperature scaling by relating measured voltages to elevated electron temperatures. This implies that the determination of the critical exponents describing superconductor-insulator transitions in ultrathin films by combining the results of electric field and temperature scaling does not work. An independent determination of either the dynamical critical exponent or the correlation length exponent is needed to fully characterize these transitions.
Work performed in collaboration with K. Parendo and K. H. Sarwa B. Tan, and supported by the National Science Foundation.