Microwave power dependent surface resistance of YBaCuO superconductors: material quality linkage

Abstract

Three types of dependences of microwave (36 GHz) surface resistance on microwave magnetic field amplitude have been found for YBaCuO samples of various qualities. For the poor-quality samples is proportional to at low fields; then it changes to linear versus and, finally, saturates at large . The intermediate-quality samples demonstrate a threshold character of the nonlinearity with a jump-like growth of at field amplitudes higher than some characteristic value. Finally, the best samples exhibit an dependence which is initially quadratic with and then grows faster with the microwave field. The results are analysed in the framework of various models for the nonlinear surface impedance of type II superconductors.