  We have investigated the nonlinear surface impedance and two-tone
intermodulation distortion of nine epitaxial YBa2Cu3O7-d films on MgO
substrates, using stripline resonators, at frequencies f=2.3-11.2GHz and
temperatures T=1.7K-Tc. The power dissipation decreased by up to one order of
magnitude as the microwave electric field was increased to about 100V/m for
T<20 K, while the reactance Xs showed only a weak increase. The minimum of the
losses correlated with a plateau in the intermodulation signal. The same
features were observed for a Nb film on MgO. The anomalous response is due to
nonlinear dielectric losses in the substrate, which can be described by defect
dipole relaxation.
