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The far-infrared (10-100 cm^-1) complex conductivity of superconducting granular NbN films has been determined from reflection and transmission data. Moderately granular films, with R_=150 and 200 Ω, followed Leplae-modified Bardeen-Cooper-Schrieffer---theory predictions with no absorption below the gap frequency ωg. The derived values of 2ΔkTc were 3.8 and 4.0, slightly below the results for homogeneous films. Highly granular films, with R_≥500 Ω, displayed dc resistive tails and evidence of Josephson coupling. The real part of the far-infrared conductivity was BCS-type for frequencies above ωg but showed an anomalous absorption below ωg. This excess absorption may arise from two-dimensional fluctuations in the form of vortices or from normal conducting regions mixed in with the superconducting grains.
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Karecki et al. (1983) studied this question.
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