Electronic theory for superconductivity in Sr ₂ RuO ₄ : Triplet pairing due to spin-fluctuation exchange

Abstract

Using a two-dimensional Hubbard Hamiltonian for the three electronic bands\ncrossing the Fermi level in Sr$_2$RuO$_4$ we calculate the band structure and\nspin susceptibility $\\chi({\\bf q}, \\omega)$ in quantitative agreement with\nnuclear magnetic resonance (NMR) and inelastic neutron scattering (INS)\nexperiments. The susceptibility has two peaks at {\\bf Q}$_i = (2\\pi/3, 2\\pi/3)$\ndue to the nesting Fermi surface properties and at {\\bf q}$_i = (0.6\\pi, 0)$\ndue to the tendency towards ferromagnetism. Applying spin-fluctuation exchange\ntheory as in layered cuprates we determine from $\\chi({\\bf q}, \\omega)$,\nelectronic dispersions, and Fermi surface topology that superconductivity in\nSr$_2$RuO$_4$ consists of triplet pairing. Combining the Fermi surface topology\nand the results for $\\chi({\\bf q}, \\omega)$ we can exclude $s-$ and $d-$wave\nsymmetry for the superconducting order parameter. Furthermore, within our\nanalysis and approximations we find that $f$-wave symmetry is slightly favored\nover p-wave symmetry due to the nesting properties of the Fermi surface.\n