Role of oxygen on the flux pinning and vortex phase transitions in NdBa 2Cu 3O 7− δ crystals

Abstract

The origin of flux pinning is studied in NdBa 2Cu 3O 7− δ single crystals grown by the flux method with varying O 2 partial pressure during the growth and using different oxygen reduction schedules. The critical current density, J c is remarkably enhanced when O 2 defects are introduced in both crystals grown at low (0.03%) and intermediate (0.1%) O 2-partial pressure. However, for excessive O 2 vacancies, both T c and J c are decreased. Our results demonstrate that oxygen vacancy clusters along with the substitutional defects and twin boundaries play the major role in flux pinning. Our results also demonstrate that the first-order melting transition observed in a clean system can transform into a glassy state with increasing defect density.