Microstructural study on 90° basal-plane-faced tilt boundaries showing d-wave paring characteristics

Two kinds of needle-like a-axis grains in a c-axis YBa2Cu3Ox thin film on a (001) MgO substrate are chosen to study the symmetry of the superconducting paring state. One kind of grain boundary (90 ° GB) is formed by (001) of the a-axis and (100) of the r-axis film while another one (45 ° GB) is formed by (001) of the a-axis grain and (110) of the c-axis film. d-Wave paring characteristics are observed for the 45 ° GB. Cross-sectional transmission electron microscopy (TEM) observation for 90 ° and 45 ° GBs shows that the a-axis grains start from the MgO substrate. Both grain boundaries are found to have atomically sharp interfaces and the distortion due to mismatch is localized only in about one atomic layer range. Further high-resolution electron microscopy (HREM) observation indicates that the (001) surface atomic layer of the a-axis grain is always a BaO layer irrespective of the orientation of the opposite grain. Moreover, these needle-like a-axis grains are found to result from small steps on MgO substrate. This offers us a chance to fabricate this kind of natural basal-plane-faced (BPF) grain boundary artificially.