MICROWAVE PROPERTIES OF YBCO THIN-FILMS ON YSZ SUBSTRATES IN THE COUPLED GRAIN MODEL APPROACH

Microstrip resonators with fundamental resonance frequency of 3 GHz we re fabricated from YBCO thin films deposited by RF magnetron sputterin g on 5 x 5 mm(2) YSZ substrates. Microwave experimental data were trea ted using the Phenomenological Loss Equivalence Method and film resist ivity, film characteristic surface resistance and superconducting pene tration depth were obtained. In spite of the high quality of the films (highly c-axis oriented, epitaxial growth in a-b-plane, high critical temperature T-c and critical current density J(c)) it was not possibl e to describe the obtained results in the framework of the two-fluid m odel. The interpretation was performed using the coupled grain model. The grain sizes were assumed to be equal to the mean distance between the microdefects observed by Atomic Force Microscopy. In this approach the intergrain penetration depth and the intergrain critical current density J(ig), were calculated, The relatively low film resistivity wa s explained as due to the residual microwave loss contributed from the grain boundaries and the film microdefects. The decrease of the Q-fac tor with the RF current density J observed at J < J(c), J(c)(ig) was i nterpreted taking into account the spatial inhomogeneous distribution of the current density in the strip.