Microwave Absorption in Normal-Conducting Films on Superconducting Substrates

The microwave absorption at 9.2 kMc/sec in films of gold and copper plated onto bulk tin has been measured as a function of the temperature in the range $1.4\le T\le 4.2$°K. The results are extrapolated to zero temperature, where they indicate the presence of an energy gap of 0.61×${10}^{-23\mathrm{}}$ J at the surface of a 190-Å film of gold and at the surface of a 500-Å film of copper on tin. The normal bulk resistivity of the gold was estimated from dc measurements as well as from the microwave data. The theory of the proximity effect, that is, the influence of adjoining normal and superconducting metals, as given by de Gennes and Werthamer, was applied to the measurements, and yielded an estimate for $N\mathrm{}\left(0\right)\mathrm{}V$ for gold of 9×${10}^{-3\mathrm{}}$. Assuming that the sign of $N\mathrm{}\left(0\right)\mathrm{}V$ is positive, this leads to a very low transition temperature.