Electron tunneling determination of the order-parameter amplitude at the superconductor-insulator transition in 2D

We have investigated the behavior of the superconducting energy gap Δ in ultrathin films of quench condensed Bi near the superconductor-insulator (SI) transition. From electron tunneling measurements on these films, we conclude that Δ becomes very small and approaches zero at the SI transition. We studied high-sheet-resistance films with ${\mathit{T}}_{\mathit{c}0}$’s as low as 0.19 K. This is a factor of 40 lower than the low-sheet-resistance film ${\mathit{T}}_{\mathit{c}0}$ of 6.4 K.