The interplay between localization and superconductivity has been studied in uniform ultrathin films of both high- and low-temperature superconductors. In both types, a decrease of the transition temperature with decreasing thickness and increasing sheet resistance is found, and there appears to be a critical sheet resistance for superconductivity. The important issue is whether the threshold is material-dependent, or universal, with a value close to the quantum resistance for pairs, h/4e2 (6450Ω), the value found for films of Bi and DyBaCuO. The universality of this result would have important implications for the fundamental understanding of this problem.
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