  We have investigated the nonlinear microwave properties of electron-beam
coevaporated YBaCuO films on MgO, using stripline resonators at 2.3 GHz and
temperatures 1.7-80 K. The oxygen content of the films ranged from strongly
underdoped to overdoped. Above 20 K, the nonlinear response of the resonators
was dominated by the superconductor. We could establish clear correlations
between the nonlinear surface resistance, two-tone intermodulation (IMD), and
oxygen content of the films, which indicate that the superconducting order
parameter is important for the nonlinearities. An exponential rather than a
power-law representation of the nonlinear current-voltage relation would be
required to explain our results phenomenologically. Below 20 K, the dielectric
loss tangent of MgO dominated the nonlinear response of the resonators. With
increasing power, the dissipation losses decreased markedly, accompanied by
enhanced IMD. The surface reactance passed through a shallow minimum at about 5
K, independent of power. We attribute these effects to resonant absorption by
impurity states in MgO.
