Abstract
Recent experiments on granular films have shown an apparently universal maximum normal state sheet resistance of around 6.5 kΩ above which superconductivity cannot be established. A possible explanation for this phenomenon is presented which is based on the competition between quantum fluctuations in the phase of the superconducting order parameter and dissipative effects. The films are modelled by locally superconducting grains coupled through Josephson junctions. Quantum fluctuations in the phases of individual grains may lead to a state in which the Cooper pair number rather than its conjugate phase is a good quantum number and superconductivity is destroyed. Dissipative effects tend to suppress these fluctuations and lead to the appearance of a new type of superconducting state possessing only short range phase coherence. This occurs for normal state resistances smaller than the quantum unit h/4e 2=6.5 kΩ, in agreement with the experimentally observed threshold.
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