Far Infrared Transmission through Superconducting Films

Abstract

The far infrared transmission through films of superconducting and normal lead, tin, indium, and mercury has been measured in the wavelength region between 0.1 and 1.1 mm. The transmission data have been analyzed to find the ratio of the complex conductivity in the superconducting state to that in the normal state, as a function of frequency. The width of the energy gap at 0°K may be estimated from the frequency of the extrapolated cutoff of the real part, ${\sigma }_1\left(\omega \right)$, of the superconducting conductivity. The values so obtained are 4.0±0.5, 3.3±0.2, and $3.9\pm 0.3k{T}_c$ for lead, tin, and indium, respectively. These values are in good agreement with those obtained in other experiments on bulk samples. The frequency dependence of $\frac{{\sigma }_1\left(\omega \right)}{{\sigma }_N}$ is in qualitative agreement with the results of a calculation by Mattis and Bardeen based on the theory of Bardeen, Cooper, and Schrieffer, except for an unexpected hump in ${\sigma }_1\left(\omega \right)$ for lead and (tentatively) mercury at low frequencies. This hump may be due to the production of collective excitations or an anisotropy in the energy gap. It has also been found that a magnetic field as high as 8000 gauss applied in the plane of a lead film about 12 A thick has only a very small effect on the electromagnetic properties of the film. This is not surprising, in view of the results of the microwave experiments of Pippard and of Spiewak.