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/sub rf/) with rf field. The study of those nonlinearities profits by carefully calibrated rf measurements in the frequency and time domain, which yield /spl delta/R/sub S/(H)/spl prop/H/sup n//spl prop//sub S/(H). In contrast to /spl delta/R/sub S/(H)/spl prop/H/sup n/, the ratio r=/spl delta/X/sub S///spl delta/R/sub 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/spl ap/1 and n/spl ap/1-2 are in line with Josephson fluxons in weak links, which may interact with dc fluxons threading through the weak links.