Even and Odd Order Intermodulation Nonlinearity From Superconductive Microstrip Lines

Three-tone intermodulation distortion (IMD) was used to measure the even and odd order microwave nonlinearity of superconducting Tl₂Ba₂CaCu₂O₈ and YBa₂Cu₃O₇ resonators. By using an off-resonance probe signal, the IMD generation is stimulated locally in the vicinity of the probe. Both 2nd and 3rd order IMD appear at the microwave resonant frequency, providing a synchronous comparison of the local even and odd order nonlinearities and a quantitative determination of the current densities that produce the intermodulation distortion. Comparison of synchronous 2nd and 3rd order IMD provides a characterization of the extent of time reversal symmetry breaking (TRSB) in the superconductor. As the probe is scanned across the resonator, the degree of TRSB is seen to vary in different regions, always exhibiting a significant rise in the 3rd order IMD at the critical temperature, T_c, consistent with expectation from the nonlinear Meissner effect. The ratio of 2nd order to 3rd order IMD current drops significantly at T_c, indicating a reduction in TRSB at the phase transition.