Dipole interaction in triplet superconductivity with horizontal line nodes:: Orientation of the superconducting order parameter in Sr2RuO4

Dipole interaction in triplet superconductivity is studied. If Cooper pairs are formed by electrons at r and r + (+/-a/2, +/-a/2, +/-c/2) in a body-centered tetragonal lattice (in this case line nodes run horizontally on a cylindrical Fermi surface), the dipole energy is low when the d-vector is perpendicular to the direction of the angular momentum of the Cooper pairs (l-vector). This result is in contrast with the dipole energies in the Anderson-Brinkman-Morel (ABM) state of superfluid He-3, where the d-vector is forced to be parallel or antiparallel to the l-vector by dipole interaction. The recent NQR experiment in Sr2RuO4 by Ishida et al. can be explained by this result.