Ab initio Derivation of Low-energy Model for Iron-Based Superconductors LaFeAsO and LaFePO

TL;DR

This paper derives effective low-energy Hamiltonians for LaFeAsO and LaFePO from first-principles calculations, providing a foundation for understanding superconductivity and electron correlations in iron-based superconductors.

Contribution

It introduces a first-principles downfolding scheme to construct effective Hubbard models for LaFeAsO and LaFePO, including screened Coulomb and exchange interactions.

Findings

Onsite Coulomb interaction estimated at 2.2-3.3 eV.

Transfer integrals between Fe-3d orbitals are 0.2-0.3 eV.

Similar screened interactions for As and P compounds.

Abstract

Effective Hamiltonians for LaFeAsO and LaFePO are derived from the downfolding scheme based on first-principles calculations and provide insights for newly discovered superconductivity in the family of LnFeAsO$_{1-x}$F$_x$, Ln = La, Ce, Pr, Nd, Sm, and Gd. Extended Hubbard Hamiltonians for five maximally localized Wannier orbitals per Fe are constructed dominantly from five-fold degenerate iron-3$d$ bands. They contain parameters for effective Coulomb and exchange interactions screened by the polarization of other electrons away from the Fermi level. The onsite Coulomb interaction estimated as 2.2-3.3 eV is compared with the transfer integrals between the nearest-neighbor Fe-3$d$ Wannier orbitals, 0.2-0.3 eV, indicating moderately strong electron correlation. The Hund's rule coupling is found to be 0.3-0.6 eV. The derived model offers a firm basis for further studies on physics of this family of materials. The effective models for As and P compounds turn out to have very similar screened interactions with slightly narrower bandwidth for the As compound.