Nonlinear surface resistance in YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films

A complete characterization of the nonlinearity of the surface impedance Z/sub s/ of YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films is presented. Z/sub s/(H/sub rf/, the RF magnetic field), has been measured for H/sub rf/ from 0 to approximately 1 kOe vs. temperature from 4.2 K to critical temperature T/sub c/ and vs. frequency from 1.5 to 10 GHz, using a stripline resonator. The results are compared to a coupled-grain model. Good agreement is found for R/sub s/(H/sub rf/). The imaginary part of Z/sub s/(H/sub rf/), i.e. X/sub s/(H/sub rf/), however, does not fit the model. >