Onset of superconductivity in ultrathin amorphous metal films

The onset of superconductivity in homogeneous ultrathin films grown incrementally is found to occur when their normal state sheet resistance falls below a value close to h/4e2, the quantum resistance for pairs. The data, which extends down to 0.4K, further suggest that in the T &rarr 0 limit such films are either superconducting or insulating, and that there may be a superconductor-insulator transition controlled by disorder.