Knight Shift and Leading Superconducting Instability from Spin Fluctuations in Sr2RuO4

Recent nuclear magnetic resonance studies [A. Pustogow et al., Nature 574, 72 (2019)] have challenged the prevalent chiral triplet pairing scenario proposed for Sr2RuO4. To provide guidance from microscopic theory as to which other pair states might be compatible with the new data, we perform a detailed theoretical study of spin fluctuation mediated pairing for this compound. We map out the phase diagram as a function of spin-orbit coupling, interaction parameters, and band structure properties over physically reasonable ranges, comparing when possible with photoemission and inelastic neutron scattering data information. We find that even-parity pseudospin singlet solutions dominate large regions of the phase diagram, but in certain regimes spin-orbit coupling favors a near-nodal odd-parity triplet superconducting state, which is either helical or chiral depending on the proximity of the gamma band to the van Hove points. A surprising near degeneracy of the nodal s' and d(x2-y2) wave solutions leads to the possibility of a near-nodal time-reversal symmetry broken s' + id(x2-y2) pair state. Predictions for the temperature dependence of the Knight shift for fields in and out of plane are presented for all states.