Spin-triplet superconductivity in Sr2RuO4: anisotropic antiferromagnetic spin-fluctuation

In the conventional theories of spin-fluctuation mechanism for superconductivity, the ferromagnetic spin-fluctuation leads to triplet pairing, while the antiferromagnetic one causes singlet pairing. However, recently a mechanism leading to spin-triplet superconductivity due to antiferromagnetic spin-fluctuation has been proposed for Sr2RuO4. It was found that the anisotropy in spin fluctuation and the nesting property of the Fermi surface play crucial roles for the Cooper pairing. In the previous studies, however, the spin anisotropy was assumed phenomenologically. We study the origin of the anisotropy using the more realistic three-band Hubbard model with spin–orbit coupling and interband interactions. In this model an anisotropy is naturally obtained, which becomes large enough, depending on the parameters, to support the superconducting mechanism for triplet pairing. It is found that the Hund coupling between the orbitals plays an important role.