Orbital ordering in cubic LaMnO3 from first principles calculations

We report on first principles self-interaction corrected LSD (SIC-LSD) calculations of electronic structure of LaMnO3 in the cubic phase. We found a strong tendency to localization of the Mn eg electron and to orbital ordering. We found the ground state to be orbitally ordered with a staggered order of x2 - z2 and y2 - z2 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 3x2 - r2 and 3y2 - r2 is, however, very small. 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 localization of the eg electron in LaMnO3 accounts for the survival of local distortions above the structural phase transition temperature