Nonlinear microwave effects in superconducting microstrip resonators based on YBCO thin films

Linear microstrip resonators based on YBaCuO thin films made by RF magnetron sputtering on MgO substrates have been characterized with transmission microwave measurements as a function of temperature (from 10 K to T/sub c/) and the incident microwave power (from -30 to 10 dBm). The variation of the resonant frequency with temperature has been used to determine the intrinsic parameters (/spl lambda//sub 0/, T/sub c/) of the thin film using a fitting procedure and appropriate models. For low microwave powers (<-18 dBm), the variation of the quality factor, the insertion loss and the surface resistance do not exhibit any nonlinear effect. For higher microwave powers nonlinear effects appear and the variation of the quality factor has shown that they vary in a different way according to three temperature zones. Various theoretical models have been used to discuss the experimental results. These zones can be correlated to the temperature dependence of the surface resistance, in particular to grain boundaries resistance and to penetration of microwave Josephson vortices into the weak links. Moreover, the variation of the measured surface resistance as a function of the RF surface magnetic field is discussed considering different models.