Fluctuation Contribution to the Far-Infrared Transmission of Lead Films

The transmission in the far infrared (2-40 cm^-1) of dirty very-thin lead films has been measured in the vicinity of the transition temperature Tc. 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 cm^-1) corresponding to an increase in the ac conductivity. The results are well described by theoretical conductivities which give a characteristic frequency of the fluctuations ωF=(16kBTπ)|T-Tc|Tc both above and below Tc.