Presence of quantum diffusion in two dimensions: Universal resistance at the superconductor-insulator transition

We argue that whenever the transition between the insulating and superconducting phases of a disordered two-dimensional Fermi system at zero temperature (T=0) is continuous, the system behaves like a normal metal right at the transition; i.e., the resistance has a finite, nonzero value at T=0. This value is universal—independent of all microscopic details. These features, consistent with recent measurements on disordered films, are hypothesized to apply to other 2D transitions at T=0, such as Anderson localization with spin-orbit coupling, and the quantum Hall effect.