Strain and Spin-Orbit Coupling Induced Orbital-Ordering in Mott Insulator BaCrO3

TL;DR

This study uses ab initio calculations to reveal how strain, correlation, and spin-orbit coupling induce orbital ordering and a metal-insulator transition in the Mott insulator BaCrO3, a rare phenomenon in 3d systems.

Contribution

It demonstrates the combined effects of strain, correlation, and SOC in driving orbital ordering and metal-insulator transition in BaCrO3, highlighting a novel mechanism in 3d transition metal oxides.

Findings

Strain enhances the volume of BaCrO3.

Inclusion of correlation and SOC causes a metal-insulator transition.

Orbital ordering involves alternating $d_{xz}+id_{yz}$ and $d_{xz}-id_{yz}$ orbitals.

Abstract

Using ab initio calculations, we have investigated an insulating tetragonally distorted perovskite BaCrO$_3$ with a formal $3d^2$ configuration, the volume of which is apparently substantially enhanced by a strain due to SrTiO$_3$ substrate. Inclusion of both correlation and spin-orbit coupling (SOC) effects leads to a metal-insulator transition and in-plane zigzag orbital-ordering (OO) of alternating singly filled $d_{xz}+id_{yz}$ and $d_{xz}-id_{yz}$ orbitals, which results in a large orbital moment $M_L$ ~ -0.78 $\mu_B$ antialigned to the spin moment $M_S$ ~ $2|M_L|$ in Cr ions. Remarkably, this ordering also induces a considerable $M_L$ for apical oxygens. Our findings show metal-insulator and OO transitions, driven by an interplay among strain, correlation, and SOC, which is uncommon in 3d systems.