Dipole Interaction in Triplet Superconductivity with Horizontal Line Nodes; Orientation of the Superconducting Order Parameter in Sr2RuO4

Dipole interaction in the triplet superconductivity is studied. If the Cooper pairs are formed by the electrons at $\text{mib}r$ and $\text{mib}r+(\pm \frac{a}{2},\pm \frac{a}{2},\pm \frac{c}{2})$ in the body centered tetragonal lattice, (in this case the line nodes run horizontally on the cylindrical Fermi surface), the dipole energy is low when the $\text{mib}d$-vector is perpendicular to the direction of the angular momentum of the Cooper pairs ($\text{mib}l$-vector). This result is in contrast with the dipole energies in the ABM state of superfluid ${}^3$He, where $\text{mib}d$-vector is forced to be parallel or antiparallel to the $\text{mib}l$-vector by the dipole interaction. The recent NQR experiment in Sr${}_2$RuO${}_4$ by Ishida et al. can be explained by this result.