Intermodulation distortion and third-harmonic generation in YBCO films of varying oxygen content

We have measured the nonlinear surface impedance, intermodulation distortion (IMD), and third-harmonic generation (THG) in a series of identically prepared YBaCuO films that have been carefully annealed to produce a controlled oxygen stoichiometry. These are also compared with an unannealed film. The measurements were performed using a stripline-resonator technique as a function of temperature at a fundamental frequency of 2.3 GHz; the IMD tone separation was 10 kHz. We have found that over-doping films with oxygen substantially lowers the IMD relative to optimally and underdoped films. We have also observed differences in the slopes of the THG and IMD, with IMD slope of close to 2:1 while the same film shows THG slope of 3:1. A possible explanation of the differences lies in the different time scales to which IMD and THG are sensitive.