Spin Triplet Superconductivity in Sr2RuO4 due to Orbital and Spin Fluctuations: Analysis by Two-Dimensional Renormalization Group Theory

摘要

We study the mechanism of the triplet superconductivity in Sr2RuO4 based onthe multiorbital Hubbard model. The electronic states are studied using therenormalization group method. Thanks to the vertex correction (VC) for thesusceptibility, which is dropped in the mean-field-level approximations, strongorbital and spin fluctuations at $Q=(2\pi/3,2\pi/3)$ emerge in the quasione-dimensional Fermi surfaces composed of $d_{xz}$ and $d_{yz}$ orbitals. Dueto the cooperation of both fluctuations, we obtain the tripletsuperconductivity in the $E_u$ representation, in which the superconducting gapis given by the linear combination of $(\Delta_x(k),\Delta_y(k))=(\sin3k_x,\sin 3k_y)$. These results are confirmed by a diagrammatic calculationcalled the self-consistent VC method.