The interpretation of the power dependence of the unloaded quality factor of superconductive microstrip resonators

For microstrip resonators fabricated from YBa 2 Cu 3 O 7- delta (YBCO) thin film on LaAlO 3 (001) substrates, this paper reports the experimental observation and theoretical explanation of the power dependence of the unloaded quality factor (Q 0 ). Two kinds of Q 0 behaviour were observed. In one case, with the increase of input power, Q 0 dropped at first rather quickly, then slowly and then again drastically. In the other case, Q 0 remained almost unchanged but, beyond a certain input power level, dropped very rapidly. To interpret these behaviours, two phenomenological expressions for local surface resistance (R s ) were proposed. According to these expressions R s is a function of the surface rf current density (J s ) in the strip which is strongly non-uniform spatially. By choosing proper values for the fitting parameters in these R s expressions, the dependence of Q 0 on the input power was computed, resulting in good agreement between the theoretical calculations and the experimental measurements.