NONLINEAR SURFACE IMPEDANCE OF YBCO THIN-FILMS - MEASUREMENTS, MODELING, AND EFFECTS IN DEVICES

High-T-c thin films continue to be of interest for passive device appl ications at microwave frequencies, but nonlinear effects may limit the performance. To understand these effects we have measured the nonline ar surface impedance Z(S) in a number of YBa2Cu3O7-x thin films as a f unction of frequency from 1 to 18 GHz, rf surface magnetic field H-rf to 1500 Oe, and temperature from 4 K to T-c. The results at low H-rf a re shown to agree quantitatively with a modified coupled-grain model a nd at high H-rf with hysteresis-loss calculations using the Bean criti cal-state model applied to a thin strip. The loss mechanisms are extri nsic properties resulting from defects in the films. We also report pr eliminary measurements of the nonlinear impedance of Josephson junctio ns, and the results are related to the models of nonlinear Z(S). The i mplications of nonlinear Z(S) for devices are discussed using the exam ple of a five-pole bandpass filter.