DYNAMIC SOFTENING OF VORTEX LINES IN YBA2CU3O7-DELTA SINGLE-CRYSTALS

Transport measurements in the mixed state of oxygen-deficient YBa2Cu3O 7-delta 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 co upling, 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 sim ple model that accounts for this behavior by including the effects of the pinning potential on the dynamics of vortices.