Microwave properties of YBa2Cu3O7-delta thin films in linear and nonlinearregime in a dc magnetic field

We report an experimental and theoretical study of the linear and nonlinear microwave impedance of thin YBa2Cu3O7-delta films in a de magnetic field o f different orientations. The measurements were done using the parallel-pla te resonator technique at 5.7 GHz, 21 K<T<80 K, and for H<0.8 T. We demonst rate that the contributions to the microwave response from nonlinearity and from the de magnetic field are almost additive for our films (excluding ve ry low microwave power). The increase of microwave current simply leads to a parallel shift of the de magnetic field dependences of the surface impeda nce. This result shows that widely discussed nonlinear mechanisms related t o Vortices produced by the de field are not relevant for our experimental c onditions. The theoretical analysis of other nonlinear mechanisms points ou t that the most probable source of nonlinearity is vortices produced by mic rowave currents in weak links. A qualitative description of the experimenta lly observed nonlinear microwave response is provided by the critical-state Bean model, which was modified for thin films.