Observation of critical amplitude fluctuations near the two-dimensional superconductor-insulator transition

We report results of transport measurements in the quantum critical regime of the disorder tuned, two-dimensional (2D) superconductor-insulator transition (SIT) in homogeneously disordered films. We show that, as the superconducting transition temperature decreases, the transition width grows, appearing to diverge at the SIT. In addition, structure develops in the dc current-voltage characteristics of films closest to the SIT indicating that the 2D superconductivity is driven into a regime of extreme inhomogeneity. The data suggest a picture of the phase transition in which large amplitude fluctuations occur as the amplitude is suppressed to near zero by disorder. [S0163-1829(99)03718-2].