  Striking features have been found in the nonlinear microwave (8 GHz) surface
impedance $Z_s=R_s + jX_s$ of high-quality YBaCuO thin films with comparable
low power characteristics [$R_{res}\sim 35--60 \mu\Omega$ and $\lambda_L(15
K)\sim 130--260 nm$]. 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.
