On the mechanism on amplification in superconducting films

Nethercot,' in seeking an alternate explanation for the microwave amplification observed in superconducting films,* suggests that thermal effects might have been responsible. The purposes of the present note are to point out that the mechanism described by Nethercot could not have been responsible for the observed amplification and to present further evidence supporting the explanation based on parametric action. The amplifier under discussion is illustrated in Fig. 1. Most of the heat generated in the tin film is carried into the crystal quartz substrate; some of it subsequently escapes into the superfluid helium which surrounds the substrate, and the remainder crosses into the wall of the silver-plated stainless steel guide onto which the device is held with a cement of high thermal conductivity. As mentioned previously,* the pump power available to the device was only 2 X 10-7 watts and the device temperature was 2'K. The dimensions of the film wereapproximately0.45cmX0.4cmX250~.