Harmonic emission at microwave frequencies in Ba0.6K0.4BiO3 crystals

Nonlinear microwave emission by Ba0.6K0.4BiO3 crystals is investigated. Second- and third-harmonic signals are studied as a function of the temperature and the applied magnetic field at different values of the input power level. The experimental data are discussed in the framework of models previously reported in the literature. We show that there are several mechanisms responsible for harmonic emission in Ba0.6K0.4BiO3 crystals: they play a different role depending on the temperature and the intensity of the applied field. We suggest that harmonic emission at temperatures far from Tc is due to nonlinear processes in weak links or flux motion, while nonlinearity near Tc is ascribable to the time variation of the order parameter induced by the microwave field.