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 transi tion (SIT) in homogeneously disordered films. We show that, as the supercon ducting transition temperature decreases, the transition width grows, appea ring to diverge at the SIT. In addition, structure develops in the dc curre nt-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 flu ctuations occur as the amplitude is suppressed to near zero by disorder. [S 0163-1829(99)03718-2].