Josephson effect and small-angle grain boundary in YBCO thin film bridge

Clear AC and DC Josephson effects have been observed only by using the YBCO (YBa₂Cu₃O_7-x) thin films made by MO-CVD on (100) MgO substrates. TEM observation and X-ray analysis of YBCO on MgO were made and these results were compared with those for YBCO on the SrTiO₃ substrate. It was found that the YBCO films on the MgO substrate consisted of (001) oriented polycrystalline grains with the size of about 0.5 μm. There were two types of angular misorientation between two adjacent grains: one is a misorientation within several degrees and the other with about 45°, which may be due to lattice mismatching between YBCO and (100) MgO substrate. The small-angle grain boundaries were clearly observed by TEM observation. On the other hand, there was a relatively small misorientation at the grain boundary of YBCO on SrTiO₃ substrate. J_c of the YBCO on MgO is from 10⁴ to 10⁵ A/cm ² at 77 K, which is lower than that for YBCO on SrTiO₃ by one order of magnitude, possibly related to the difference of the grain boundary structures. For YBCO on (100) MgO, only a few grains with the small-angle grain boundary exist at the bridge region of the device, and the grain boundaries work as effective barriers for Josephson junctions