K. H. Sarwa B. Tan

Electrostatic tuning of the superconductor-insulator transition in two dimensions.

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.

Electrostatic and parallel-magnetic-field tuned two-dimensional superconductor-insulator transitions

The 2D 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 transitions were analyzed using finite size scaling with critical exponent products nu*z = 0.65-0.7. The parallel critical magnetic field increased with electron transfer as (dn_c-dn)^0.33, where dn is the electron transfer and dn_c is its critical value, and the critical resistance decreased linearly with dn. 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 electron density or magnetic field, in the case of the field-tuned transition at the lowest temperatures, some different type of physical behavior turns on in the insulating regime.

Evidence of spatially inhomogeneous pairing on the insulating side of a disorder-tuned superconductor-insulator transition

Measurements of transport properties of amorphous insulating indium oxide thin films have been interpreted as evidence of the presence of superconducting islands on the insulating side of a disorder-tuned superconductor-insulator transition. Although the films are not granular, the behavior is similar to that observed in granular films. The results support theoretical models in which the destruction of superconductivity by disorder produces spatially inhomogenous pairing with a spectral gap.

Anomalous insulating state induced by parallel magnetic field in ultrathin bismuth films

Recent investigations of the magnetic‐field induced insulator in two‐dimensional superconducting systems suggest that unexpected physics occurs. However, direct comparisons with the intrinsic insulator have not been possible, nor have most experiments studied the effects of parallel magnetic fields. We have applied parallel magnetic fields to an insulating, amorphous Bi film, which exhibited Mott variable range hopping in the absence of field. We have also applied fields to an insulating film in which superconductivity was induced by electrostatic electron doping. Comparison of these magnetic‐field induced insulators with the intrinsic insulator reveals the appearance of an anomalous insulating phase at low temperatures.