Electron microscopic investigations of neutron irradiated YBa₂Cu₃O_7-x high temperature superconducting single crystals

Abstract

There has been several recent explorations of HTSC materials particularly YBCO elucidating enhancements of critical current density 'Jc' as a result of fast neutron irradiation (~1 MeV, fluence 10¹⁶/cm²) (see e.g. Van Dover et al 1989). Although the enhancements in Jc are known to result from the flux pinning effects arising from the neutron irradiation created defects, the nature of these defects have remained unknown. The present communication elucidates the details of these defects. Employing the technique of transmission electron microscopy in normal and high resolution modes, the nature of the defects have been uncovered. It has been shown that the fast neutron irradiation creates defects corresponding to thermal and displacement spikes. In the thermal spike region the defects correspond to local removal of chain containing CuO layers and consequent creation of linear defect stripe regions with a characteristic of width of ~13.6 Å. After the defect stripe, the displacement spike corresponding to removal and rearrangement of oxygen atoms from CuO chain containing CuO layers are produced. These defects consist of linearly arranged clustures ( ~100 to 300 Å ). The effectiveness of these defects particularly the defect stripes as flux pinning centres has been outlined.