Coulomb repulsion and correlation strength in LaFeAsO from Density Functional and Dynamical Mean-Field Theories

TL;DR

This study uses first-principles calculations combined with DMFT to determine Coulomb and Hund's parameters in LaFeAsO, revealing weak correlations consistent with experimental spectra, regardless of the basis set used.

Contribution

It provides a detailed first-principles estimation of Coulomb and Hund's parameters in LaFeAsO using LDA+DMFT, highlighting the dependence on basis set choice and the resulting weak electronic correlations.

Findings

Parameters depend on basis set choice.

Both parameter sets yield weakly correlated electronic structures.

Results agree with experimental spectra.

Abstract

LDA+DMFT (Local Density Approximation combined with Dynamical Mean-Field Theory) computation scheme has been used to calculate spectral properties of LaFeAsO -- the parent compound for new high-T$_c$ iron oxypnictides. Coulomb repulsion $U$ and Hund's exchange $J$ parameters for iron 3d electrons were calculated using \textit {first principles} constrained density functional theory scheme in Wannier functions formalism. Resulting values strongly depend on the number of states taken into account in calculations: when full set of O-$2p$, As-$4p$, and Fe-3d orbitals with corresponding bands are included, computation results in $U=3\div$4 eV and J=0.8 eV. In contrast to that when the basis set is restricted to Fe-3d orbitals and bands only, computation gives much smaller parameter values $F^0$=0.8 eV, $J$=0.5 eV. However, DMFT calculations with both parameter sets and corresponding to them choice of basis functions result in weakly correlated electronic structure that is in agreement with experimental X-ray and photoemission spectra.