Nonlinear effects in YBa2Cu3O7−x microstrip resonators on sapphire

Power-induced nonlinear effects, i.e., the reduction of the quality factor, the distortion of the resonance peaks, and the two-tone intermodulation distortion (IMD), were experimentally examined by using 2.3 GHz microstrip resonators prepared from double-sided YBa2Cu3O7−x (YBCO) films on CeO2-buffered sapphire. The resonators exhibited advanced performance with an unloaded quality factor of 80 000 at 63 K up to a circulating power Pcirc of 0.5 W and an IMD third-order interception point estimated at Pcirc≈400 W. We found that the nonlinear effects in these resonators were produced by the power-dependent surface resistance Rs of the YBCO films, whereas the variation of the surface reactance was negligible. Both the presence of the high-order IMD products and the dependence of their amplitude on the microwave power, indicate a Rs(H)∝cosh H relationship, where H is the amplitude of the microwave magnetic field at the film surface. Such a dependence agrees with the direct measurements of the power handling capability.