AGING OF CRITICAL CURRENTS AND IRREVERSIBILITY LINE IN MELT-TEXTURED YBA2CU3O7

The critical currents and the irreversibility line of bulk melt textured YBa2Cu3O7 have been found to evolve with the increase of the oxygenation time. A strong field-dependent reduction of the critical currents is observed which finally leads to a downward shift of the irreversibility line. Transmission electron microscopy micrographs of aged samples display a huge increase in the concentration of stacking faults near the microcracks and at the YBa2Cu3O7-Y2BaCuO5 interfaces. Low field ac susceptibility measured with H∥ab shows a strong reduction of the screening capability thus meaning that nonsuperconducting planar defects parallel to the ab planes have been created. We suggest that an enhanced two-dimensional behavior of the vortex flux lattice occurs when H∥c as a consequence of this layered defective structure.© 1995 American Institute of Physics.