MICROWAVE-POWER DEPENDENT SURFACE-RESISTANCE OF YBACUO SUPERCONDUCTORS - MATERIAL QUALITY LINKAGE

Three types of dependences of microwave (36 GHz) surface resistance R- s(mw) on microwave magnetic field amplitude H-omega have been found fo r YBaCuO samples of various qualities. For the poor-quality samples R- s(mw) is proportional to H-w(2) at low fields; then it changes to line ar versus H-omega and, finally, saturates at large H-omega. The interm ediate-quality samples demonstrate a threshold character of the nonlin earity with a jump-like growth of R-s(mw) at field amplitudes H-omega higher than some characteristic value, Finally, the best samples exhib it an R-s(mw)(H-omega) dependence which is initially quadratic with H- omega 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.