Temperature dependence of microwave losses in Y-Ba-Cu-O films

We have made measurements of the T dependence of microwave absorption (at 22.7 GHz) in films of Y1Ba2Cu3Ox on SrTiO3 and LaGaO3 substrates. The superconducting transition temperatures were near 90 K. The film on SrTiO3 had predominantly c axis normal to the film, while the film on LaGaO3 was partially a axis oriented. The microwave losses L for both films followed the temperature dependence given by L∝t/square root of 1-t, with t=T/T c. In the macroscopic theory of Ginzburg-Landau, this expression is proportional to the number of normal electrons within a penetration depth of the surface of the film, provided (1-t)≪1. However, we find that the above expression fits the data well for a wide range of T.