MICROWAVE HYSTERETIC LOSSES IN YBA2CU3O7-X AND NBN THIN-FILMS

Measurements of the nonlinear surface impedance of high-quality epitaxial YBCO and granular NbN films as a function of temperature (4.2-91 K) and frequency (1-13 GHz) are presented. The microwave-field-dependent surface impedance Zs(Hrf) for both YBCO and NbN films increases quadratically with increasing Hrf in the low- and intermediate-rf-field region (Hrf<50 Oe for YBCO at 77 K and for NbN at 13.4 K). In the high-rf-field region, Zs(Hrf) changes to a different functional dependence of Hrf. Through the use of a modified Bean critical-state model, the results for the high-rf-field region are explained quantitatively by hysteresis losses due to penetration of microwave vortices. The value of the microwave-vortex penetration field Hp(T) for YBCO thin films, which is nearly frequency independent, is low compared with the dc lower critical fields. The critical- current density values obtained from fits to the hysteresis model are consistent with those measured via dc transport. © 1995 The American Physical Society.