Fluctuational dissipation in YBCO under magnetic fields

We present some experimental facts which suggest that fluctuational phenomena dictate the dissipative behaviour in a wide region of fields and temperatures across the critical temperature T-c. In particular, we discuss resistivity data on an YBCO thin film as a function of the field orientation theta with respect to the crystallographic axes of the sample. We also compare our data with those taken on an untwinned single crystal. Both analyses define a temperature T-f(H) marking the onset 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 whether the intrinsic behaviour is due to fluctuations of the order parameter or to the free motion of vortex lines.