FREQUENCY-DEPENDENCE OF THE SURFACE IMPEDANCE OF YBA2CU3O7-DELTA THIN-FILMS IN A DC MAGNETIC-FIELD - INVESTIGATION OF VORTEX DYNAMICS

We report measurements of the surface impedance Z(s) = R(s) + iomegala mbda' of YBa2Cu3O7-delta thin films in an externally applied dc magnet ic field B (parallel to the c axis) using a stripline resonator. At T = 4.3 K we. obtain the surface resistance R(s) and the microwave penet ration depth lambda' as a function of applied dc field up to 5 T and a s a function of microwave frequency f from 1.2 to 20 GHz. While lambda ' increases linearly with B for B > 1 T at all frequencies, R(s) is fo und to be roughly is-proportional-to B(alpha)(f), where alpha(f) < 1 f or f less-than-or-equal-to 10 GHz and alpha(f) almost-equal-to 1 for f greater-than-or-equal-to 10 GHz. In zero dc field, R(s) is-proportion al-to f2. For B > 1 T, R(s) shows a much weaker dependence on f. The r esults for Z(s) (f, T, B) have been quantitatively explained using a m odel developed by Coffey and Clem, based on a self-consistent treatmen t of vortex dynamics that includes the influence of vortex pinning, vi scous drag, and flux creep. The pinning force constant alpha(p), the p inning frequency omega(p), and the pinning activation energy U0(T,B) a re obtained through the fitting procedure. We find that the effects of thermally activated flux creep at 4.3 K upon the surface resistance a re significant. The low U0 almost-equal-to 35 K that we determine is i nterpreted as arising from the interaction of the vortex lattice with a dense random pinning potential as described in the collective-pinnin g theory.