Microwave properties of thin films of high-T_c superconductor Y-Ba-Cu-O

Measurements are reported of the magnitude and phase of microwave transmission through thin films of Y-Ba-Cu-O on MgO substrate in the presence of a small magnetic field. Such films are used for the construction of high-Q superconducting cavity. The real and imaginary parts of the complex conductivity and their dependence on temperature and magnetic field are evaluated. It is shown that the presence of a constant magnetic field appreciably changes the magnitude and phase of the microwave transmission through superconducting films, while the presence of an oscillating field modulates it with the generation of many high-order harmonics. This strong nonlinear behavior may be explained by a model that assumes an array of Josephson junctions between superconducting grains.