Morphology of Columnar Defects Dependent on Irradiation Direction in High-T_cSuperconductors

The morphologies of ion tracks tilted at different angles were systematically investigated by TEM observations on GdBCO coated conductors irradiated with 80 MeV Xe ions. The 80 MeV Xe ion beam along the c-axis produced shortly segmented (discontinuous) columnar defects (CDs) along the ion path throughout the sample. When the ion beam was tilted at slight angle of 20° from the c-axis, by contrast, not discontinuous CDs but thick and elongated (continuous) ones were formed in the irradiation direction. The formation of continuous CDs was observed to a deeper depth from the surface for the irradiation tilted at larger angle from the c-axis. The variation of the microstructures against the irradiation angles was caused by the anisotropic temperature spread during the formation of CDs, which was attributed to the anisotropic thermal diffusivity in high-Tc superconductors. We also demonstrated that discontinuous CDs could be formed at the irradiation angle of ±45° relative to the c-axis into YBCO thin films by using 50 MeV Kr ions, which was the irradiation condition estimated backwards from the directional dependent morphology of CDs.