Phase diagram of the strongly disordered s -wave superconductor NbN close to the metal-insulator transition

We present a phase diagram as a function of disorder in three-dimensional NbN thin films as the system enters the critical disorder for destruction of the superconducting state. The superconducting state is investigated using a combination of magnetotransport and scanning tunneling spectroscopy measurements. Our studies reveal three disorder regimes. At low disorder (kFl {~} 10--4), the system follows the mean field Bardeen-Cooper-Schrieffer behavior, where the superconducting energy gap vanishes at the temperature at which electrical resistance appears. For stronger disorder (1<kFl 4), a ``pseudogap'' state emerges, where a gap in the electronic spectrum persists up to temperatures much higher than Tc, suggesting that Cooper pairs continue to exist in the system even after the zero resistance state is destroyed. Finally, at even stronger disorder (kFl 1), the global superconducting ground state is destroyed even though superconducting correlations continue to survive, as evidenced from a pronounced magnetoresistance peak at low temperatures.