MECHANICAL DAMPING EFFECTS BY FLUX LINES IN YBA2CU3O6.9 THIN-FILMS

We present new results of internal friction (IF) measurements on epitaxial YBa2Cu3O6.9 thin films in magnetic fields up to 3 T. For fields perpendicular to the c axis of the films we find a sharp maximum in the temperature dependence of the IF slightly below T-c. For a small tilt of the samples with respect to the magnetic field this maximum broadens and shifts to lower temperatures. It is explained within the framework of thermally assisted flux flow where the position is related to an activation energy which describes thermal diffusion of flux lines. In order to account for the width of the maximum, a distribution of activation energies has to be taken into account.