Unusual features in the nonlinear microwave surface impedance of Y-Ba-Cu-O thin films

Striking features have been found in the nonlinear microwave (8.0 GHz) surface impedance Z(s)=R-s +j.X-s of high-quality YBaCuO thin films with comparable low power characteristics [R(res)similar to 35-60 mu Omega and lambda(L)(15 K)similar to 130-260 nm]. The surface resistance R-s is found to increase, decrease, or remain independent of the microwave field H-rf (up to 60 mT) at different temperatures and for different samples. However, the surface reactance X-s always follows the same functional form. Mechanisms which may be responsible for the observed variations in R-s and X-s are briefly discussed. [S0163-1829(98)052142-1].