Effective Hamiltonian for FeAs-based superconductors

Abstract

The recently discovered FeAs-based superconductors show intriguing behavior and unusual dynamics of electrons and holes which occupy the Fe d orbitals and As 4s and 4p orbitals. Starting from the atomic limit, we carry out a strong-coupling expansion to derive an effective Hamiltonian that describes the electron and hole behaviors. The hopping and the hybridization parameters between the Fe d and As s and p orbitals are obtained by fitting the results of our density-functional-theory calculations to a tight-binding model with nearest-neighbor interactions and a minimal orbital basis. We find that the effective Hamiltonian, in the strong on-site Coulomb-repulsion limit, operates on three distinct subspaces coupled through Hund's rule. The three subspaces describe different components (or subsystems): (a) one spanned by the d(x)(2)-y(2) Fe orbital, (b) one spanned by the degenerate atomic Fe orbitals d(xz) and d(yz), and