MICROSTRUCTURAL STUDY ON 90-DEGREES BASAL-PLANE-FACED TILT BOUNDARIESSHOWING 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 supe rconducting paring state. One kind of grain boundary (90 degrees GB) i s formed by (001) of the a-axis and (100) of the c-axis film while ano ther one (45 degrees GB) is formed by (001) of the a-axis grain and (1 10) of the c-axis film. d-Wave paring characteristics are observed for the 45 degrees GB. Cross-sectional transmission electron microscopy ( TEM) observation for 90 degrees and 45 degrees GBs shows that the a-ax is grains start from the MgO substrate. Both grain boundaries are foun d to have atomically sharp interfaces and the distortion due to mismat ch is localized only in about one atomic layer range. Further high-res olution electron microscopy (HREM) observation indicates that the (001 ) surface atomic layer of the a-axis grain is always a BaO layer irres pective of the orientation of the opposite grain. Moreover, these need le-like a-axis grains are found to result from small steps on MgO subs trate. This offers us a chance to fabricate this kind of natural basal -plane-faced (BPF) grain boundary artificially.