Role of oxygen on the flux pinning and vortex phase transitions in NdBa2Cu3O7-delta crystals

The origin of flux pinning is studied in NdBa2Cu3O7-delta single crystals g rown by the flux method with varying O-2 partial pressure during the growth and using different oxygen reduction schedules. The critical current densi ty, J(c) is remarkably enhanced when O-2 defects are introduced in both cry stals grown at low (0.03%) and intermediate (0.1%) O-2-partial pressure. Ho wever, for excessive O-2 vacancies, both T-c and J(c) are decreased. Our re sults demonstrate that oxygen vacancy clusters along with the substitutiona l defects and twin boundaries play the major role in flux pinning. Our resu lts also demonstrate that the first-order melting transition observed in a clean system can transform into a glassy state with increasing defect densi ty. (C) 2001 Elsevier Science B.V. All rights reserved.