Proximity of antiferromagnetism and superconductivity in LaFeAsO 1-x F 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 ${\text{LaFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$. We find that the ground state is an ordered antiferromagnet, with staggered moment of about $2.3\text{ }{\ensuremath{\mu}}_{B}$, 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 $\text{Fe}\text{ }3d$ and $\text{As}\text{ }4p$ 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.