Exchange Constants and Neutron Spectra of Iron Pnictide Materials

TL;DR

This paper explains neutron spectra and anisotropic exchange constants in iron pnictides by proposing a model involving orbital order and an orbital selective Mott transition, highlighting the role of orbital physics in these materials.

Contribution

It introduces a simple Hamiltonian model with tetragonal symmetry that accounts for observed spin-wave spectra and suggests orbital order and an orbital selective Mott transition as key factors.

Findings

Neutron scattering spectra can be explained by orbital order.

Orbital selective Mott transition driven by superexchange is proposed.

Reduced moments are due to quasi-one-dimensionality, not frustration.

Abstract

We show that the neutron scattering spectra observed in CaFe$_2$As$_2$ by Zhao et al (arXiv:0903.2686v1) and the highly unusual spatially anisotropic exchange constants in the iron pnictides derived by Han et al (Phys. Rev. Lett. 102, 107003 (2009)), using electronic structure calculations, can be explained by assuming a role for orbital order in these materials. We write down a simple model Hamiltonian with tertagonal symmetry, whose spin-wave spectra describes the observed dispersion relations. We further argue that these materials have orbital selective Mott transition, which is driven by superexchange between neighboring iron atoms. We suggest that reduced spin and quasi-one dimensionality and not frustration are responsible for the reduced moments in these materials.