POWER-DEPENDENT MICROWAVE PROPERTIES OF SUPERCONDUCTING YBA2CU3O7-X FILMS ON BUFFERED POLYCRYSTALLINE SUBSTRATES

We have studied the microwave properties of 0.4 mu m thick YBa2Cu3O7-x (YBCO) films on polycrystalline substrates with ion-beam-assisted-dep osited yttria-stabilized zirconia buffer layers using a parallel-plate resonator technique at 10 GHz. The YBCO films with similar in-plane t exture grown on both forsterite and Ni-based alloy substrates show sim ilar microwave properties. We measure low-power surface resistance R-s values of about 0.5 m Omega at 76 K and 0.15 m Omega at 4 K for films with an in-plane mosaic spread of about 7 degrees. Single-tone power- dependence measurements show that the surface resistance and the surfa ce reactance increase linearly and by the same amount with increasing microwave field level. At intermediate power levels, the intermodulati on measurements show odd-order intermodulation products that increase quadratically with two-tone input power. These results indicate a hyst eretic vortex penetration mechanism in the weak links as the most plau sible source of the observed nonlinearities in these films. (C) 1997 A merican Institute of Physics.