TRANSPORT CRITICAL CURRENTS AND FLUX PINNING IN MELT-GROWN YBA2CU3O7-DELTA

The critical current and flux pinning properties of melt grown YBa2Cu3O7-delta bulk material were investigated by transport measurements and V-I characteristics. A J(c) of 16 000 A cm-2 in zero field and 3000 A cm-2 in 1 T at 77 K was obtained using a criterion of 3.3-mu-V cm-1. No features of granularity could be detected as J(c) showed no hysteresis with increasing or decreasing field. By considering a distribution of activation energy U0 together with the correlation volume of flux bundles V(c) and the pinning range r(p), the measured V-I characteristics in the flux creep regime were found to be consistent with that of thermally activated flux flow (TAFF). The temperature and field dependence of the derived V(c)r(p) points to a collective pinning mechanism by point defects. The depinning current J(c), one to tow orders of magnitude higher than the transport J(c), was also obtained by using the U0 and V(c)r(p) deduced from the V-I characteristics.