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 YBa2CU3O7-8 (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 lambda=lambda (0)[l+(J/J(0))(2)]. The parameter J(0) 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.