Field induced vanishing of the vortex glass temperature in Tl2Ba2CaCu2O8 thin films

The current density j(s) and relaxation rate Q have been measured on a ring shaped Tl2Ba2CaCu2O8 him between 4.2 K and the irreversibility temperature in magnetic fields up to 7 T. The temperature and field dependent exponent mu(T,B-e) used to parametrize the activation energy U in the form U(j(s),T,B-e)=(U-c/mu)[(j(c)/j(s))(mu)-1] is determined for many different fields. In the B-e-T phase diagram we associate the mu=0 line with the,glass temperature T-g and find that T-g essentially drops to 0 K at B-e approximate to 0.7 T, which indicates a dimensional crossover at this field. The mu > 0 region (B-e < 0.7 T, T less than or equal to 55 K) corresponds to 3D elastic vortex motin while the mu < 0 region corresponds to 2D dislocation mediated flux creep.