PHASE-DIAGRAM OF FLUX-CREEP IN MELT-TEXTURED AND SINGLE-CRYSTALLINE YBA2CU3O7-DELTA

Voltage-current characteristics E(j) were obtained from relaxation and magnetization measurements in melt-textured and single-crystalline YBa2Cu3O7-delta. Both samples show a change of the E(j) behavior, which is in accordance with a vortex-glass-vortex-liquid transition. In the (B, T) range below this transition, logE(logj) curves pass from a negative to a positive curvature with increasing current. Collective-creep or vortex-glass theory are in agreement with the experimental observation below the corresponding inflection point, whereas above, the exponential E(j) dependence of the Anderson model is the better approximation. The exponent mu from the collective creep model was used for the analysis of the E(j) dependence in both regimes. Because mu varies with the current, mean values were taken within a constant current window for consistency. With increasing magnetic fields, mu passes a maximum, approaches zero, and becomes - 1. The transformation from collective creep into the Anderson creep occurs at the end of the plateau of S at which mu changes from positive to negative. Finally, it is argued that the maximum of the current at intermediate magnetic fields (the fishtail effect) is not based on relaxation.