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.

Figure 1

$R(T)$ as a function of $V_G$ for the 10.22 Å film. From top to bottom, gate voltages are 0, 2.5, 5, 7, 8, 9.6, 11, 12.5, 14.5, 17, 19.5, 24, 28, 33, 38, and 42.5 V. Approximately 40 data curves have been omitted for clarity. Inset: Resistance versus gate voltage for the 10.22 Å film at 65 mK.

Figure 2

Change in the screening parameter $\Delta F^{*}$ versus $V_G$. Inset: Conductance versus $\ln (T)$ for the $V_G=12.5\mathrm{V}$ curve.

Figure 3

Scaling analysis of $R(T)$ with $V_G$ as tuning parameter. The critical exponent product $\nu z=2/3$. Inset: $R(V_G)$ for isotherms from 65 to 100 mK, yielding a distinct crossing point at the critical gate voltage of 11 V.