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

We have investigated gap symmetry of high-Tc superconductors using Josephson junctions formed along a boundary of an a-axis-oriented YBa2Cu3O7-δ (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 Ic 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 Ic 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 π and dx2-y2 superconducting-gap symmetry is in CuO2 planes of YBCO. But, the (001) of YBCO has no phase difference of π in spite of the existence of d-wave symmetry in YBCO. copyright 1997 The American Physical Society