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 GHz) surface impedance $Z_s=R_s+j{X}_s$ of high-quality YBaCuO thin films with comparable low power characteristics [$R_{res}\sim 35--60\mu \mathrm{\Omega }$ and $\lambda_L(15 K)\sim 130–260 nm$]\mathrm{.}Thesurfaceresistance$R_s$ 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.