MAGNETIC-FIELD AND TEMPERATURE-DEPENDENCE OF FLUX CREEP IN ORIENTED GRAINED AND SINGLE-CRYSTALLINE YBA2CU3OX

Thermally activated flux creep of oriented grained and single-crystalline YBa2Cu3Ox was studied in fields up to 12 T and at temperatures ranging between 4 and 90 K. In fixed fields the activation energy U0 of both samples was found to increase with temperature, pass through some maximum and drop to the order of kBT around the irreversibility line. While at constant temperature U0 of the oriented grained sample showed a monotonous decrease with field; in the case of the single crystal it was found to follow a characteristic minimum-maximum structure paralleled by the previously observed field dependence of the shielding current. This clearly demonstrates the influence of the coupling properties, i.e., bulk behavior of the oriented grained sample and granularity of the single crystal, on relaxation. Therefore, models exclusively based either on a pinning or on a junction approach alone could not describe our experimental findings. A more appropriate explanation is based on the properties of the defect structure. Depending on field and temperature, defective regions are driven into the normal state whereby additional pinning centers are created which in turn give rise to increasing activation energies. The connectivity of the sample then depends on size and density of these defects.