Stabilization of s -wave superconductivity through arsenic p -orbital hybridization in electron-doped BaFe2As2

Using random-phase approximation spin-fluctuation theory, we study the influence of the hybridization between iron d orbitals and pnictide p orbitals on the superconducting pairing state in iron-based superconductors. The calculations are performed for a 16-orbital Hubbard-Hund tight-binding model of BaFe2As2 that includes the As-p orbital degrees of freedom in addition to the Fe-d orbitals and compared to calculations for a 10-orbital Fe-d only model. In both models we find a leading s± pairing state and a subleading dx2-y2-wave state in the parent compound. Upon doping, we find that the s± state remains the leading state in the 16-orbital model up to a doping level of 0.475 electrons per unit cell, at which the hole Fermi-surface pockets at the zone center start to disappear. This is in contrast to the 10-orbital model, where the d-wave state becomes the leading state at a doping of less than 0.2 electrons. This improved stability of s± pairing is found to arise from a decrease of dxy orbital weight on the electron pockets due to hybridization with the As-p orbitals and the resulting reduction of near (π,π) spin-fluctuation scattering which favors the competing d-wave state. These results show that the orbital dependent hybridization of Fermi-surface Bloch states with the usually neglected p-orbital states is an important ingredient in an improved itinerant pairing theory.