A simple model is described for the frequency-dependent conductivity of normal-superconducting composites. The superconducting component is characterized by the Mattis-Bardeen conductivity, and the composite conductivity is determined via the effective-medium approximation. Near the percolation threshold pc there is strong absorption below twice the superconducting gap. Scaling arguments indicate that our results have validity beyond mean-field theory. The model is in qualitative agreement with recent experiments on granular NbN films.
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Garner et al. (1983) studied this question.
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