MICROWAVE-ABSORPTION OF YBA2CU3O THIN-FILM

The microwave surface resistance R-s of YBa2Cu3O thin film on LaAlO2 h as been measured as a function of temperature and magnetic field throu gh a resonant cavity technique. At zero field, the temperature depende nce of R-s shows a BCS-like variation at higher temperatures and then tends to level off at lower temperatures. A general linear and square- root field dependence of R-s at low and high fields, respectively, has been obtained for both flux flow and pinning dominant regions based o n the flux motion theory. The cross-over field H from linear to squar e-root R-s(H) dependence was found to be very different for these two regions, In the flux flow region, H linearly depends on the microwave frequency, white it is independent of the frequency in the flux pinni ng region. The experimental results for R-s(H) at about 10 GHz show a linear relationship at high temperatures and a transition from lineari ty to nonlinearity al low temperatures. The temperature dependence of the slope of the linear R-s(H) has been derived and can be well accoun ted for in terms of the flux flow model. The anisotropy of R-s(H) was measured in different magnetic and electrical field orientations. The results can be qualitatively expressed in terms of the flux motion the ory.