Phase diagram of flux creep in melt-textured and single-crystalline ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

Voltage-current characteristics E(j) were obtained from relaxation and magnetization measurements in melt-textured and single-crystalline ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. 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 μ from the collective creep model was used for the analysis of the E(j) dependence in both regimes. Because μ varies with the current, mean values were taken within a constant current window for consistency. With increasing magnetic fields, μ 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 μ 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.