Orbital ordering in cubic LaMnO3 from first principles calculations

TL;DR

This paper uses first principles calculations to show that cubic LaMnO3 exhibits orbital ordering and electron localization, leading to an insulating antiferromagnetic state without structural distortion.

Contribution

It demonstrates the presence of orbital ordering in cubic LaMnO3 from first principles, highlighting electron localization and magnetic properties without relying on lattice distortions.

Findings

Orbital ordering occurs in cubic LaMnO3 with staggered x^2-z^2 and y^2-z^2 orbitals.

The system remains insulating and antiferromagnetic in both orbital orderings.

Orbital ordering can break cubic symmetry independently of lattice distortions.

Abstract

We report on first principles Self-Interaction Corrected LSD (SIC-LSD) calculations of electronic structure of LaMnO$_{3}$ in the cubic phase. We found a strong tendency to localisation of the Mn $e_{g}$ electron and to orbital ordering. We found the ground state to be orbitally ordered with a staggered order of $x^{2}-z^{2}$ and $y^{2}-z^{2}$ orbits in one plane and this order is repeated along the third direction. The difference in energy with a solution consisting of the ordering of $3x^{2}-r^{2}$ and $3y^{2}-r^{2}$ is, however, very small. The latter ordering is similar to the one observed both experimentally and theoretically in the real distorted system. The system is in the insulating A-type antiferromagnetic ordered state in both cases. The presence of orbital ordering means breaking of the cubic symmetry and without recourse to distortion. The latter may rather be the result of the orbital ordering but the symmetry of this ordering is determined by coupling to the lattice. The strong tendency to localisation of the $e_{g}$ electron in LaMnO$_{3}$ accounts for the survival of local distortions above the structural phase transition temperature.