The structural distortion in antiferromagnetic LaFeAsO investigated by a group-theoretical approach

TL;DR

This study uses a group-theoretical approach to analyze the structural distortion in antiferromagnetic LaFeAsO, revealing how slight lattice distortions stabilize its magnetic structure consistent with experimental observations.

Contribution

It identifies the orthorhombic space group Imma as the magnetic structure's symmetry and explains how minor distortions stabilize this structure within a nonadiabatic Heisenberg model.

Findings

LaFeAsO has a magnetic band with Wannier functions compatible with Imma symmetry.

The magnetic structure is stabilized by a slight distortion from Imma to Pnn2.

Experimental atomic displacements support the theoretical group-theoretical analysis.

Abstract

As experimentally well established, undoped LaFeAsO is antiferromagnetic below 137K with the magnetic moments lying on the Fe sites. We determine the orthorhombic body-centered group Imma (74) as the space group of the experimentally observed magnetic structure in the undistorted lattice, i.e., in a lattice possessing no structural distortions in addition to the magnetostriction. We show that LaFeAsO possesses a partly filled "magnetic band" with Bloch functions that can be unitarily transformed into optimally localized Wannier functions adapted to the space group Imma. This finding is interpreted in the framework of a nonadiabatic extension of the Heisenberg model of magnetism, the nonadiabatic Heisenberg model. Within this model, however, the magnetic structure with the space group Imma is not stable but can be stabilized by a (slight) distortion of the crystal turning the space group Imma into the space group Pnn2 (34). This group-theoretical result is in accordance with the experimentally observed displacements of the Fe and O atoms in LaFeAsO as reported by Clarina de la Cruz et al. [nature 453, 899 (2008)].