Far-infrared transmission of superconducting homogeneous NbN films: Scattering time effects

The transmission of superconducting homogeneous NbN films has been measured between 20 and 95 ${\mathrm{cm}}^{-1\mathrm{}}$. The results could be fitted with the Leplae extension of dirty-limit Mattis-Bardeen theory, which includes the effects of electronic scattering. The fits yield $\frac{2\Delta }{k_B}{T}_c=4.4\mathrm{} \mathrm{and} 4.1$ for $T_c$'s of 17 and 14 K, and scattering times of (1-5)×${10}^{-14\mathrm{}}$ sec. These values are in substantial agreement with other determinations. The results, together with theoretical considerations and analysis of previous data in Pb, show that the Leplae approach simply describes optical behavior even in very strongly coupled materials. A refined description, however, probably requires consideration of both scattering and strong-coupling effects.