Superconducting-gap symmetry study using a/c boundary Josephson junctions in YBa2Cu3O7-delta films

We have investigated gap symmetry of high-T-c superconductors using Josephson junctions formed along a boundary of an a-axis-oriented YBa2Cu3O7-delta (YBCO) grain surrounded by c-axis-oriented YBCO grains (a/c boundary). There are two types of a/c boundary junctions; one is the boundary between (001) of a-axis-oriented grains and (100) of c-axis-oriented grains, and the other is the boundary between (001) of a-axis-oriented grains and (110) of c-axis-oriented grains. TEM observation shows clean, sharp, and nearly single-facet interface along the grain boundaries in both types. In the case of the (100)-(001) type a/c boundary junction, typical resistively shunted junction (RSJ)-type I-V curves, Shapiro steps under microwave irradiation, and Fraunhofer-like diffraction pattern of I-c under magnetic field are observed, indicating that the boundary works as a Josephson junction. In the case of the (110)-(001)-type a/c boundary junction, typical RSJ type I-V curves and Shapiro steps under microwave irradiation are also observed. However, the diffraction pattern of I-c under magnetic field has the minimum value at zero magnetic field. This property is analogous to one observed for a corner junction which is formed between Pb and YBCO. These results show that the (110) of YBCO has a phase difference of pi and d(x)(2)-(2)(y) superconducting-gap symmetry is in CuO2 planes of YBCO. But, the (001) of YBCO has no phase difference of pi in spite of the existence of d-wave symmetry in YBCO.