Microwave absorption of YBa2Cu3O thin film

The microwave surface resistance Rs of YBa2Cu3O thin film on LaAlO2 has been measured as a function of temperature and magnetic field through a resonant cavity technique. At zero field, the temperature dependence of Rs 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 Rs at low and high fields, respectively, has been obtained for both flux flow and pinning dominant regions based on the flux motion theory. The cross-over field H* from linear to square-root Rs(H) dependence was found to be very different for these two regions. In the flux flow region, H* linearly depends on the microwave frequency, while it is independent of the frequency in the flux pinning region. The experimental results for Rs(H) at about 10 GHz show a linear relationship at high temperatures and a transition from linearity to nonlinearity at low temperatures. The temperature dependence of the slope of the linear Rs(H) has been derived and can be well accounted for in terms of the flux flow model. The anisotropy of Rs(H) was measured in different magnetic and electrical field orientations. The results can be qualitatively expressed in terms of the flux motion theory. (author)