Fluctuation Contribution to the Far-Infrared Transmission of Lead Films

The transmission in the far infrared (2-40 ${\mathrm{cm}}^{-1\mathrm{}}$) of dirty very-thin lead films has been measured in the vicinity of the transition temperature $T_c$. The films were evaporated onto quartz substrates at or below 40 °K and were not warmed above this temperature between the evaporation and the transmission measurements. The resistances of the films were monitored simultaneously with the far-infrared measurements. The transmission of the films is reduced at the lowest frequencies (below 5 ${\mathrm{cm}}^{-1\mathrm{}}$) corresponding to an increase in the ac conductivity. The results are well described by theoretical conductivities which give a characteristic frequency of the fluctuations ${\omega }_F=\left(\frac{16k_{B}T}{\pi \hslash }\right)\frac{|T-T_c|}{T_c}$ both above and below $T_c$.