Dynamic softening of vortex lines in YBa₂Cu₃O_7-δ single crystals

Transport measurements in the mixed state of oxygen-deficient YBa₂Cu₃O_7-δ 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.