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 YB a2Cu3O7-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 nega tive to a positive curvature with increasing current. Collective-creep or vortex-glass theory are in agreement with the experimental observa tion below the corresponding inflection point, whereas above, the expo nential E(j) dependence of the Anderson model is the better approximat ion. The exponent mu from the collective creep model was used for the analysis of the E(j) dependence in both regimes. Because mu varies wit h 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 a t which mu changes from positive to negative. Finally, it is argued th at the maximum of the current at intermediate magnetic fields (the fis htail effect) is not based on relaxation.