Crystal Symmetry and Magnetic Order in Iron Pnictides: a Tight Binding Wannier Function Analysis

TL;DR

This study constructs Wannier functions for Fe 3d orbitals in iron pnictides to analyze magnetic order and electronic interactions, revealing how stripe antiferromagnetism influences orbital localization and hopping, with implications for superconductivity.

Contribution

It provides a detailed Wannier function analysis of Fe 3d orbitals in LaFeAsO and LaFePO, linking orbital localization, hopping parameters, and magnetic order, highlighting the influence of pnictide atoms.

Findings

Stripe antiferromagnetism causes localization of specific Fe 3d orbitals.

Antiferromagnetic order modifies hopping parameters, increasing isotropy in certain orbitals.

Differences between LaFeAsO and LaFePO are linked to Wannier functions and hopping integrals.

Abstract

To perform a local orbital analysis of electronic and magnetic interactions, we construct the Wannier functions (WFs) of the Fe $3d$ orbitals in the parent compound of the recently discovered iron pnictide superconductors, LaFeAsO, and a comparison material LaFePO. Comparing the WFs for the stripe antiferromagnetic order with those for no magnetic order, the difference is a significant spreading (``{\it de}localization'') of specifically the $d_{xy}$ and $d_{xz}$ (but not $d_{yz}$) WFs, where parallel Fe spins lie along the $x$ direction. The WF basis gives a tight-binding representation of the first principles, density functional based Fe-derived bands. Comparing hopping parameters, it is found that changes due to stripe antiferromagnetism, even if it is weak, enables more isotropic hopping involving spin-majority electrons in the Fe $3d_{xz}$ (but not the $3d_{yz}$) orbital. This change, counterintuitively, actually reinforces electronic anisotropy. Further insight is gained by comparing the WFs of LaFeAsO and LaFePO, identifying how the difference in WFs is related to the difference in hopping integrals and showing how the pnictide atom is influential in forming the stripe antiferromagnetism. Kinetic energy considerations suggest that orbital fluctuation, in addition to spin fluctuation, may contribute to the decrease in observed ordered moment compared to the calculated values.