A current-density scale for characterizing nonlinearity in high-T/sub c/ superconductors

In this paper, we characterize microwave nonlinearity in a high temperature superconducting (HTS) thin-film by measuring a geometry-independent current-density scale j/sub o/. The quantity j/sub o/ specifies the strength of a material-dependent nonlinearity, and can be used to calculate the nonlinear microwave response of planar superconducting transmission-line devices. Our procedure for determining j/sub o/ involves microwave measurements on superconducting coplanar waveguide devices patterned onto the HTS sample. In a YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) sample, we obtained a maximum experimental value for j/sub o/ of approximately 3.26/spl times/10/sup 8/ A/cm/sup 2/ at 25 K. From the measured temperature-dependence of j/sub o/ in our YBCO sample, and assuming a theoretical pair-breaking current density j/sub c/ equal to 3/spl times/10/sup 8/ A/cm/sup 2/, we calculated the temperature dependence of the quasiparticle factor b(T). The curve of the experimentally obtained b(T) matched well with a theoretically-predicted behavior in the scenario of d-wave symmetry in the superconducting order parameter.