High temperature superconductors at high frequencies

Many of the high frequency applications of superconductors depend on their low high frequency losses. However, the much weaker frequency dependence for normal metals means that at high enough frequencies the surface impedance of say cold copper is comparable to that of the superconductor. This crossover frequency is a simple measure of material performance. It should rise as the sample is cooled further. At present the best in-situ films can have crossover frequencies well over 300 GHz when measured at 77 K. Even post-annealed films can get above 50 GHz, depending on the superconductor orientation and the presence of second phases. At lower frequencies the margin of advantage over normal metals increases. There is every reason to believe that the present day performance is not fundamental, and that further improvements in the crystallinity and purity of the films will be reflected in their high frequency performance