Proximity of antiferromagnetism and superconductivity in LaFeAsO_1−xF_x: Effective Hamiltonian from ab initio studies

We report density functional theory calculations for the parent compound LaFeAsO of the recently discovered 26 K Fe-based superconductor LaFeAsO_1−xF_x. We find that the ground state is an ordered antiferromagnet, with staggered moment of about 2.3 μ_B, on the border with the Mott insulating state. We fit the bands crossing the Fermi surface, derived from Fe and As, to a tight-binding Hamiltonian using maximally localized Wannier functions on Fe 3d and As 4p orbitals. The model Hamiltonian accurately describes the Fermi surface obtained via first-principles calculations. Due to the evident proximity of superconductivity to antiferromagnetism and the Mott transition, we suggest that the system may be an analog of the electron-doped cuprates, where antiferromagnetism and superconductivity coexist.