Nonlinear surface impedance in “low” and “high”Tc superconductors

The degradation of the surface impedanceZ = 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 δR ∞ γ Hn and surface reactance δR ∞ α Hn can be classified by the ratior = δX/δ 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.