D-wave pairing from spin fluctuations in the K_xFe _2-ySe₂ superconductors

Angle-resolved photoemission spectroscopy measurements on the recently discovered superconductors in the KFe₂Se₂ family with critical temperatures up to ~33 K suggest that no Fermi pockets of hole character centered on the Γ point of the Brillouin zone are present, in contrast to all other known ferropnictide and ferrochalcogenide superconductors. Using a fluctuation exchange approximation and a five-orbital tight-binding description of the band structure, we calculate the effective pairing interaction. We find that the pairing state in this system is most likely to have d-wave symmetry due to pair scattering between the remaining electron Fermi pockets at wave vector q~(π,π), but without any symmetry-imposed nodes for the given Fermi surface. We propose experimental tests of this result, including the form of the resonance spectrum probed by inelastic neutron scattering.