Nonlinear surface impedance in ''low'' and ''high'' T-c superconductors

The degradation of the surface impedance Z=R + iX with rf power is a principal limit for measurements and applications. Such nonlinearities occur for normal conducting cavities by heating or electron loading. But superconductors show a much richer spectrum of causes for nonlinearities. The nonlinearities in the surface resistance delta R proportional to gamma H-n and surface reactance delta X proportional to alpha H-n can be classified by the ratio r=delta X/delta R. This ratio differs in value, temperature, or frequency dependence for the different mechanisms, allowing a unique identification of those mechanisms. This identification is a prerequisite for an improved rf cavity design and improvement of superconducting materials.