Normalized IMD Spectra of High-T _C and Low-T _C Superconducting Resonators

Analyzing the results of the two-tone intermodulation distortion (IMD) measurements for the 2.3 GHz YBa₂Cu₃O_{7− x} and GdBa₂Cu₃O_{7− x} microstrip resonators we described the IMD spectra by the normalized values of P _IMDn/P ₀, where P _IMDn is the output power of the n-th order IMD product and P ₀ is that of the fundamental tone. The circulating power in the resonator was normalized by the power of the third-order intercept: P _circ/P _TOI. It was found, that the examined resonators exhibited the same dependence of P _IMDn/P ₀ versus P _circ/P _TOI and the same dependence of P _IMDn/P ₀ versus the change of the microwave surface resistance (ΔR _S) at any measurement temperature. We reproduced these measurements for the indium microstrip resonators of the same geometry and obtained the same results. We conclude, that the nonlinear response of the superconducting microstrip resonators in the high power range is strongly affected by a geometry-dominated hf loss mechanism (likely the hf hysteretic losses due to magnetic flux penetration), which overshadows other nonlinear mechanisms related to the intrinsic material properties. This conclusion is in agreement with the results of the indium disk resonator measurements.