Orbital Order and Spontaneous Orthorhombicity in Iron Pnictides

TL;DR

This paper investigates the origin of electronic anisotropy in iron pnictides, proposing that orbital order involving $d_{xz}$ and $d_{yz}$ orbitals causes orthorhombic distortions, with potential detection methods discussed.

Contribution

It introduces the idea that orbital order from coupled spin-orbital degrees of freedom explains anisotropies in iron pnictides, emphasizing the distinction between total orbital occupation and near-Fermi-level polarization.

Findings

Orbital order may be small in total occupation but significant near the Fermi level.

X-ray absorption linear dichroism can detect orbital order.

Electronic anisotropies can originate from orbital degrees of freedom.

Abstract

A growing list of experiments show orthorhombic electronic anisotropy in the iron pnictides, in some cases at temperatures well above the spin density wave transition. These experiments include neutron scattering, resistivity and magnetoresistance measurements, and a variety of spectroscopies. We explore the idea that these anisotropies stem from a common underlying cause: orbital order manifest in an unequal occupation of $d_{xz}$ and $d_{yz}$ orbitals, arising from the coupled spin-orbital degrees of freedom. We emphasize the distinction between the total orbital occupation (the integrated density of states), where the order parameter may be small, and the orbital polarization near the Fermi level which can be more pronounced. We also discuss light-polarization studies of angle-resolved photoemission, and demonstrate how x-ray absorption linear dichroism may be used as a method to detect an orbital order parameter.