Resistivity of clean YBa2Cu3O7-δ in the pinned-liquid state

Pinning interactions within the vortex-liquid state of clean YBa2C3O7-delta Single crystals have been probed using transport techniques. By means of scaling analysis, we have demonstrated that there are two distinct regimes within the vortex-liquid state. While at higher temperatures the vortex liquid is unpinned and the resistivity assumes its free-flux-flow value, at lower temperatures the vortex dynamics can be described in terms of a very viscous liquid model. Within the viscous liquid, vortex motion is controlled by thermally activated processes with high-energy barriers (U(pl)much greater than T). A comparison of samples with different oxygen contents has provided support for the idea that oxygen vacancies are the main source of pinning in the vortex-liquid phase. Furthermore, we have shown that the ratio of the activation energy to the melting temperature has a constant value [U-pl(T-m)/T(m)approximate to 12], independent of the degree of underlying disorder. This is in agreement with recent theoretical predictions for vortex-line cutting energies and implies that the vortex liquid is highly entangled. [S0163-1829(99)06237-2].