Power dependencies of the surface impedance of Y-Ba-Cu-O

The application of HTS is obstructed by the nonlinear behavior of Z(T,f,H-rf) with rf field. The study of those nonlinearities profits by carefully calibrated rf measurements in the frequency and time domain, which yield deltaR(S)(H) proportional to H-n p deltaX(S)(H). In contrast to deltaR(S)(H) proportional to H-n, the ratio r=deltaX(S)/deltaR(S) is independent of geometry factor and shows distinct T and f dependencies for various mechanisms and rf set-ups. The high power rf response in the frequency domain yields nonlinear resonance curves, n and r values, The time domain measurements give useful insight about dynamics of the underlying nonlinear mechanisms. The results r approximate to 1 and n approximate to 1-2 are in line with Josephson fluxons in weak links, which may interact with de fluxons threading through the weak links.