FLUCTUATION-INDUCED RESISTIVITY IN YBCO ACROSS T-C FROM ANGULAR MEASUREMENTS IN MAGNETIC-FIELD

We present various sets of resistive transitions in an YBCO thin film, as a function of the temperature, magnetic field and field orientatio n theta with respect to the (a,b) planes. We show that the resistivity undergoes an angular scaling only above a characteristic field-depend ent temperature T-f(H), where T-f is below the mean-field transition t emperature T-C(H). The scaling function found is that expected for a s trongly coupled layered superconductor. It is shown that the interacti ng fluctuations theory describes with details the data for T>T-f(H), w ith a very reduced set of parameters. Comparison with data taken on un twinned samples reveals that the upper part of the transition is ident ical for films and single crystals, and that the same theory works wel l in both cases. In addition, Bardeen-Stephen flux flow is observed in the crystal only, and in a small region of H and T. We conclude that in a wide portion of the H-T plane the dissipation is due to intrinsic properties only (order parameter fluctuations), while only at lower t emperatures the pinning becomes relevant, as supported by the breakdow n of the angular scaling and by the fact that pure crystals and films appear to be different at low temperatures only.