Transmission of Far-Infrared Radiation through Thin Films of Superconducting Amorphous Bismuth and Gallium and Beta-Phase Gallium

The ratio $\frac{T_s}{T_n}$ of the far-infrared radiation transmitted through a thin film in the superconducting state to that transmitted through it in the normal state has been measured at 1.1°K for thin-film samples of amorphous bismuth and gallium and samples of partially annealed ($\beta$-phase) gallium. The transition temperature $T_c$ of each film was also measured. The sample thicknesses were approximately in the range 50-200 Å. Numerical calculations based on the strong-coupling theory of superconductivity and on tunneling data produced theoretical values of $\frac{T_s}{T_n}$ which were in reasonably good agreement with the experimental results. The only adjustable parameter was the energy-gap width $2{\Delta }_0$. The quantities $T_c$ and $2{\Delta }_0$ decrease with increasing film resistance. The shift in $T_c$ has been previously observed by Naugle and Glover in amorphous bismuth and gallium. The abrupt resistance change in these materials at about 19°K, which is characteristic of thicker amorphous films, appeared instead as a more gradual change in resistance beginning at higher temperatures.