LINEAR AND NONLINEAR MICROWAVE DYNAMICS OF VORTICES IN YBA2CU3O7-DELTA THIN-FILMS

We report the results of a study of the nonlinear microwave surface im pedance Z(s), i.e., its dependence on microwave current Z(s)(I-rf), re sulting from vortex motion in YBa2Cu3O7-delta thin films in a dc magne tic field applied parallel to the film's c axis. Using the technique o f stripline resonators we have measured the nonlinear Z(s) at frequenc ies of 1.23-8.45 GHz, at temperatures from 5 to 30 K, and in magnetic fields from 0 to 4 T. In the mixed state, there is a significant incre ase in R-s from the zero-field value, particularly at lower frequencie s, causing an R-s proportional to f(1.2) dependence at all measured te mperatures and microwave powers. We also review our previously reporte d measurements and modeling of the linear Z(s) because the nonlinear r esults can be explained by an extension of the model that we have prev iously used to describe the dependence of Z(s) on frequency, temperatu re, and magnetic field in the linear I-rf regime. This model explains the linear Z(s) data through the thermal activation of vortex segments between metastable vortex states separated by distances of order of t he coherence length similar to xi and by a distribution of energy barr iers U-b whose magnitudes extend from U-b similar to 0 K to at least s everal, hundred K. We further show that the behavior of Z(s) at high m icrowave powers is fully consistent with the same thermal activation o f vortex segments, but the effective magnitudes of the energy barriers (U) over tilde(b) have been reduced in proportion to the microwave cu rrent density J(rf)(r,t) such that (U) over tilde(b)=U-b-J(rf()r,t)phi (0) xi l. [S0163-1829(97)03142-1].