Studies of the order parameter for spin-triplet superconductors using the Josephson effect

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The order parameter of spin-triplet superconductors, which is described in terms of the d-vector, has been investigated for UPt_3 and Sr_2RuO_4 using the Josephson effect of SNS' and SS' junctions, where S, N and S' are a spin-triplet superconductor, a normal metal and an s-wave superconductor, respectively.As for the junctions made on the UPt_3 surface perpendicular to the c-or b-axis, the quality of them, which is tested in the magnetic field dependence of the Josephson critical current Ic, has become much better than the previous ones by improving the fabrication process and decreasing the residual magnetic field. This means that the Josephson current flows almost uniformly through the junction, suggesting that the Josephson effect is allowed along b and c directions. This result excludes some of the candidates for the order parameter, such as E_<2u>. The magnetic field modulation of Ic for the junctions straddling the a-b corner has indicated no sign of the phase change in the a-b plane so far.In the case of Sr_2RuO_4, the magnetic field dependence of Ic along the c-axis suggests that the Josephson current flows through only the restricted region of the junction, although the junctions, of which area is as small as 10^<-5> cm^2 was fabricated on the cleaved flat surface perpendicular to the c-axis, which is free of steps and Ru lamellas. It is stili not clear whether the c-axis Josephson coupling, which relates to the direction of the d-vector, is allowed or not.