Transport measurements in the mixed state of oxygen-deficient YBa2Cu30 7_ 8 single crystals using the flux transformer configuration show that the flux liquid changes with increasing anisotropy from strongly correlated to uncorrelated in the field direction. For intermediate coupling, the current inducing loss of vortex correlation has a maximum near the irreversibility temperature. Thus, an effective softening of vortex lines with decreasing temperature is detected. We propose a simple model that accounts for this behavior by including the effects of the pinning potential on the dynamics of vortices.
After extensive experimental and theoretical work the importance is now well established of disorder in the static and dynamic properties of the mixed state of high-temperature superconductors. The disorder induced by thermal fluctuations is responsible for the existence of a large region in the H-T (magnetic field-temperature) phase diagram where the vortex system is a fluid [1,2]. Reducing the temperature, this vortex liquid transforms into a vortex solid [1,3] at the irreversibility temperature T~(H). The effects of static disorder are more subtle. The critical point of the melting transition in YBa2Cu307 (YBCO)