VORTEX-LIQUID ENTANGLEMENT IN BI2SR2CACU2O8+DELTA FILMS IN THE PRESENCE OF QUENCHED DISORDER

We have investigated the thermally activated behavior of the in-plane electrical resistivity of Bi2Sr2CaCu2O8+delta films for magnetic field s B less than or equal to 10(4) G applied parallel to the c axis. The activation energy in the vortex-liquid state changes suddenly at a cro ssover field B-cr. The anisotropy reduction generated by oxygen anneal ing leads to the increase of the crossover field. For B<B-cr, the acti vation energy U is weakly magnetic-field dependent. For B>B-cr, U(B,T) similar to(1-T/T-c0)/B-1/2, which corresponds to an entangled vortex f luid. The observation of vortex-liquid entanglement in the presence of relevant quenched disorder is discussed in connection with the relati on between the theoretically predicted entanglement length for a clean system and the collective pinning length along the field direction. O ur results suggests that, in the case of a pronounced anisotropy and s ignificant collective pinning, the entanglement field B-E=B(cr)approxi mate to Phi(0)/gamma(2)s(2), where s is the interlayer spacing.