FIELD-INDUCED DECOUPLING OF SUPERCONDUCTING BANDS IN OXYGEN-DEFICIENT MELT-TEXTURED YBA2CU3O7-X

A layered melt-textured YBa2Cu3O7-x superconductor has been prepared consisting of alternate fully oxygenated and oxygen deficient bands stacked parallel to a-b planes. An anomalous decrease of J(c)c(H parallel-to ab) with field is observed at high temperatures. We show that this behavior is a signature of a field induced decoupling of the strong superconducting bands, and provides evidence of a decrease of vortex pinning energy in oxygen deficient regions of melt-textured YBa2Cu3O7-x. A large downwards shift of the irreversibility line results as a consequence of the substantially smaller area where a critical state sets in. When vortices are parallel to the c-axis J(c)ab(H parallel-to c) decreases due to the smaller condensation energy of the oxygen deficient bands.