Nonlinear microwave impedance of superconductors and ac response of the critical state

The hysteretic ac response of the current-induced critical state of superconducting wires (strips of circular or rectangular cross-section) is expressed in a surface impedance formulation. The results for the rectangular strip are shown to be in good agreement with experimental results on the high power nonlinear response of superconducting microwave devices. The approach provides a proper framework to understand such nonlinear response.