Nonlinear surface impedance of YBCO thin films: Measurements, modeling, and effects in devices

High-T thin films continue to be of interest for passive device applications at microwave frequencies, but nonlinear effects may limit the performance. To understand these effects we have measured the nonlinear surface impedance Z in a number of YBaCuO thin films as a function of frequency from 1 to 18 GHz, rf surface magnetic field H to 1500 Oe, and temperature from 4 K to T. The results at low H are shown to agree quantitatively with a modified coupled-grain model and at high H with hysteresis-loss calculations using the Bean critical-state model applied to a thin strip. The loss mechanisms are extrinsic properties resulting from defects in the films. We also report preliminary measurements of the nonlinear impedance of Josephson junctions, and the results are related to the models of nonlinear Z. The implications of nonlinear Z for devices are discussed using the example of a five-pole bandpass filter. © 1995 Plenum Publishing Corporation.