FLUCTUATIONAL DISSIPATION IN YBCO UNDER MAGNETIC-FIELDS

We present some experimental facts which suggest that fluctuational ph enomena dictate the dissipative behaviour in a wide region of fields a nd temperatures across the critical temperature T-c. In particular, we discuss resistivity data on an YBCO thin film as a function of the fi eld orientation theta with respect to the crystallographic axes of the sample. We also compare our data with those taken on an untwinned sin gle crystal. Both analyses define a temperature T-f(H) marking the ons et of extrinsic effects on the dissipation. Finally, the comparison of the data with the predictions of the interacting fluctuations theory and with the Bardeen-Stephen flux-flow expression is used to clarify w hether the intrinsic behaviour is due to fluctuations of the order par ameter or to the free motion of vortex lines.