Antiferromagnetic order in multi-band Hubbard models for iron-pnictides

TL;DR

This paper uses Gutzwiller variational theory to study multi-band Hubbard models for iron-pnictides, revealing the importance of Hund's-rule exchange in antiferromagnetic order and aligning theoretical moments with experimental data.

Contribution

It demonstrates that Gutzwiller theory better captures antiferromagnetic order in iron-pnictides than Hartree-Fock approaches, emphasizing the role of Hund's-rule exchange and electron polarization.

Findings

Antiferromagnetic order requires significant Hund's-rule exchange.

Three-band model's moments match experimental data.

Five-band model shows e_g electrons polarize t_2g electrons.

Abstract

We investigate multi-band Hubbard models for the three iron 3$d$-$t_{2g}$ bands and the two iron 3$d$-$e_g$ bands in ${\rm La O Fe As}$ by means of the Gutzwiller variational theory. Our analysis of the paramagnetic ground state shows that neither Hartree--Fock mean-field theories nor effective spin models describe these systems adequately. In contrast to Hartree--Fock-type approaches, the Gutzwiller theory predicts that antiferromagnetic order requires substantial values of the local Hund's-rule exchange interaction. For the three-band model, the antiferromagnetic moment fits experimental data for a broad range of interaction parameters. However, for the more appropriate five-band model, the iron $e_g$ electrons polarize the $t_{2g}$ electrons and they substantially contribute to the ordered moment.