Ab initio calculations of the electric polarization of correlation-driven insulating materials, namely Mott insulators, have not been possible so far. Using a combination of density functional theory and dynamical-mean-field theory we study the electric polarization of the Mott insulator Sr0.5Ba0.5MnO3. We predict a ferroelectric polarization of ≃16.5μC/cm2 in the high temperature paramagnetic phase and recover the measured value of ≃13.3μC/cm2 in the low temperature antiferromagnetic phase. Our calculations reveal that the driving force for the ferroelectricity, the hybridization between Mn eg and O p orbitals, is suppressed by correlations, in particular by the Hund coupling and by the onset of magnetic order. They also confirm that the half-filled Mn t2g orbitals give rise to the antiferromagnetic Mott phase. This magnetic ordering leads to changes in the ionic polar displacement and in turn to the electronic polarization. In addition, for a fixed ionic displacement, we find that there is a reduction in the electronic contribution due to partial magnetic polarization of the Mn eg orbitals. The reduction of the polarization due to ionic displacement dominates over the additional electronic part, hence the net magneto-electric coupling is negative.
@article{arxiv.1412.3379,
title = {Electric polarization of Sr$_{0.5}$Ba$_{0.5}$MnO$_{3}$: a multiferroic Mott insulator},
author = {R. Nourafkan and G. Kotliar and A. -M. S. Tremblay},
journal= {arXiv preprint arXiv:1412.3379},
year = {2014}
}