Thermally-induced nonlinearities in the surface impedance of superconducting YBCO thin films

We have measured the microwave power dependence of the surface impedance Z/sub g/ of YBa/sub 2/Cu/sub 3/O/sub x/ (YBCO) thin films up to very high microwave power levels. Experiments have been performed, using both frequency and pulsed time domain techniques, in a 14 GHz TE/sub 011/ dielectric cavity. Our results demonstrate that heating of the superconducting film by the applied RF field contributes significantly to the nonlinearity of the measured surface resistance at high input power levels. Analysis of the electromagnetic response of the cavity to a pulsed input signal has been performed using a dielectric resonator model combined with appropriate thermal balance equations for the film-cavity system. Both the frequency- and time-dependent nonlinear electromagnetic responses of the cavity have been calculated as functions of input power, and are in excellent agreement with our data.