SURFACE-RESISTANCE OF THIN PEROVSKITE FILMS - HIGH-TEMPERATURE SUPERCONDUCTORS AND GIANT MAGNETORESISTANCE MANGANITES

We present a systematic study of the surface resistance of perovskite thin films and bulk samples at microwave frequencies. The d.c. resisti vity rho of the various samples spans three orders of magnitude. For b ulk samples, we obtain the usual rho(1/2) dependence of R(s). For rho < 1 mu Omega cm, R(s) displays conventional behavior, i.e. R(s) propor tional to rho. However, for rho > 1 mu Omega cm, R(s) is not proportio nal to rho but to the conductivity sigma. Using this fact, we have mad e the first direct observation of a peak in sigma(1), just below T-c i n a-axis films of YBa2Cu3O7.