Observation of a three-dimensional vortex-line liquid in a highly c-axis-oriented (Bi,Pb)2Sr2Ca2Cu3Ox silver-sheathed tape

We have measured the temperature dependence of resistance of a highly c-axis-oriented (Bi,Pb)2Sr2Ca2Cu3Ox silver-sheathed tape in the mixed state using the dc flux transformer geometry. In the vortex fluid regime, we have observed the onset of a nonlocal conductivity which corresponds to the peak (Tpeak) in Rbot(T) curves. This can be explained qualitatively in terms of the vortex decoupling which arises from an increased thermal fluctuation. At T<Tpeak, the vortices are three dimensional (3D) in character. This behavior is quite different from the results obtained recently by Safar et al. [Phys. Rev. Lett. 69, 824 (1992)] and Busch et al. [Phys. Rev. Lett. 69, 522 (1992)] in Bi2Sr2CaCu2Oy single crystals using the same measuring method. Their results show that the vortices are two dimensional in character. The difference of the vortex dimensionality between (Bi,Pb)2Sr2Ca2Cu3Ox and Bi2Sr2CaCu2Oy superconductors in the vortex liquid regime may be attributed to different anisotropies due to their different crystal structures and pinning disorders. In a 3D vortex line liquid, the results reveal that dissipation seems to originate from the thermally activated plastic motion of a pinned vortex line liquid. The field and temperature dependence of the activation energy is as follows: U(H,T)=U0(1-T/Tc)H-0.45. copyright 1996 The American Physical Society