Tuning the superconductor-insulator transition by use of the electric field effect and parallel magnetic fields

Two-dimensional superconductor-insulator quantum phase transitions have been investigated in nominally homogeneous, ultrathin, amorphous Bi films by the use of the electric field effect and parallel magnetic fields. The properties of these films depend strongly on disorder, which is determined in part by the films' thickness. In an insulating film, superconductivity was reversibly induced without changing the thickness by electrostatic electron charging using the electric field effect. Electrostatically induced superconductivity was quenched by the application of parallel magnetic fields for various values of electron transfer. Temperature scaling analyses were conducted on electrostatically tuned transitions in fields of 0 and 2.5 T with the electron density transfer, Deltan, as the tuning parameter and on the magnetic-field tuned transitions at various electron transfers with field, B, as the tuning parameter. Each analysis yielded a critical exponent product, nuz, of 0.65 or 0.7, suggesting that these transitions are in the same universality class. The critical field increased as a power of Deltan and the critical resistance decreased linearly with Deltan. In superconducting films, magnetic-field tuned transitions were conducted as a function of bias current, with the expectation that independent values of nu and z could be determined from electric field scaling analyses. However, it was determined that the dominant effect of increased bias current was to heat the film's electrons out of equilibrium with their environment. Investigations were performed to determine the dependence of the electron temperature on heating due to elevated bias current and electromagnetic noise. The results suggest that temperature-independent resistances that occur below about 60 mK in zero applied magnetic field and below about 100 mK in finite fields are not evidence of a metallic regime, but are rather a consequence of electrons failing to cool as well as the dilution refrigerator.