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 thin films made by MO-CVD on (100)MgO substrate1-3. TEM observation and X-ray analysis were made for YBCO on MgO and these results were compared with those for YBCO on the SrTiO3 substrate. It was found that the YBCO films on the MgO substrates consisted of (001) oriented polycrystalline grains with the size of about 0.5-mu-m. There were two types of angular misorientation between two adjacent grains: one is a misorientation within several degrees and the other with about 45-degrees, which may be due to the 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. Jc of the YBCO on MgO is from 10(4) to 10(5)A/cm2 at 77K which is lower than that for YBCO on SrTiO3 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.