Superconductivity has been induced in insulating ultrathin films of amorphous bismuth using the electric field effect. The screening of the electron-electron interaction was found to increase with electron concentration in a manner correlated with the tendency towards superconductivity. This does not preclude an increase in the density of states being important in the development of superconductivity. The superconductor-insulator transition appears to belong to the universality class of the three dimensional XY model.

The parallel magnetic field tuned two-dimensional superconductor-insulator transition has been investigated in ultrathin films of amorphous Bi. The resistance is found to be independent of temperature on both sides of the transition below approximately 120 mK. Several observations suggest that this regime is not intrinsically "metallic" but results from the failure of the films' electrons to cool. The onset of this temperature-independent regime can be moved to higher temperatures by either increasing the measuring current or the level of electromagnetic noise. Temperature scaling is successful above 120 mK. Electric field scaling can be mapped onto temperature scaling by relating the electric fields to elevated electron temperatures. These results cast doubt on the existence of an intrinsic metallic regime and on the independent determination of the correlation length and dynamical critical exponents obtained by combining the results of electric field and temperature scaling.

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The two-dimensional superconductor-insulator transition in disordered ultrathin amorphous bismuth films has been tuned both by electrostatic electron doping using the electric field effect and by the application of parallel magnetic fields. Electrostatic doping was carried out in both zero and nonzero magnetic fields, and magnetic tuning was conducted at multiple strengths of electrostatically induced superconductivity. The various transitions were analyzed using finite size scaling to determine their critical exponent products. For the electrostatically tuned transition the exponent product νz = 0.7 ± 0.1, using data from intermediate temperatures down to 60 mK. Here ν is the correlation length exponent and z is the dynamical critical exponent. In the case of electrostatically tuned transitions in field, and the field-tuned transtions at various values of electrostatically induced superconductivity, from intermediate temperatures down to about 100 to 150 mK scaling was successful with νz = 0.65 ± 0.1. The parallel critical magnetic field, Bc, increased with electron transfer as (∆nc − ∆n) 0.33 , and the critical resistance decreased linearly with ∆n. However at lower temperatures, in the insulating regime, the resistance became larger than expected from extrapolation of its temperature dependence at higher temperatures, and scaling failed. These observations imply that although the electrostatic-and parallel magnetic field-tuned superconductor-insulator transitions would appear to belong to the same universality class and to be delineated by a robust phase boundary that can be crossed either by tuning ∆n or B, in the case of the field-tuned transition at the lowest temperatures, some different type of physical behavior turns on in the insulating regime.

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