On-wafer measurements of nonlinear effects in high temperature superconductors

We present the results of comprehensive on-wafer microwave probe station measurements of the nonlinear properties of coplanar waveguide devices patterned from high-temperature superconductor (HTS) thin films. We introduce a sequence of measurements and analysis that is designed to describe the nonlinear response of microwave devices in increasingly general terms, and that verifies the resulting description with a number of different nonlinear measurements of different patterned devices. We demonstrate the use of this methodology to analyze the nonlinear response of YBa₂Cu₃O_7-δ (YBCO) thin films at 76 K, and find that all of our measurements can be satisfactorily explained by a current-dependent penetration depth of the form λ=λ₀[1+(J/J₀)²]. The parameter J₀ is a current-density scale that describes the strength of the material nonlinearity, and can be used as a suitable parameter for optimizing film growth techniques