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 ${Z}_{s}{=R}_{s}+j\ensuremath{\cdot}{X}_{s}$ of high-quality YBaCuO thin films with comparable low power characteristics $[{R}_{\mathrm{res}}\ensuremath{\sim}35\char21{}60\ensuremath{\mu}\ensuremath{\Omega}$ and ${\ensuremath{\lambda}}_{L}(15\mathrm{K})\ensuremath{\sim}130\char21{}260\mathrm{nm}].$ The surface resistance ${R}_{s}$ is found to increase, decrease, or remain independent of the microwave field ${H}_{\mathrm{rf}}$ (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.