Coexistence of superconducting and spiral spin orders: Models of ruthenate

Abstract

The effect of a spiral spin structure on superconducting (SC) pairing in a three-band Hubbard model related to Sr2RuO4 is analyzed in the mean-field approximation. Such a structure with incommensurate vector Q=2π (1/3, 1/3) is the simplest one that removes the nesting instability of α and β bands. It is assumed that there is an intralayer pairing interaction between two types of neighbor sites, those with attraction in a singlet channel and with attraction in both two-singlet and triplet channels. In both cases, a mixed singlet-triplet SC order is observed in the γ band: a d-wave singlet order is accompanied by the formation of p-wave triplet pairs (k,-k-Q)⇈ and (k,−k+Q)⇊ with large total momenta ∓Q and the spin projections ±1 onto an axis perpendicular to the spin rotation plane of the spiral spin structure. Both the SC and normal states are states with broken time-reversal symmetry. In contradiction to the experiment, the models give different scales of T c for the γ band and for α and β bands. This fact shows that the models with intralayer interactions or with the spin structure assumed are insufficient.