Orbital order and electron itinerancy in CoVO and MnCoVO from first principles.

In view of the recent experimental predictions of a weak structural transition in CoVO we explore the possible orbital order states in its low temperature tetragonal phases from first principles density functional theory calculations. We observe that the tetragonal phase with I41/amd symmetry is associated with an orbital order involving complex orbitals with a reasonably large orbital moment at vanadium sites while in the phase with I41/a symmetry, the real orbitals with quenched orbital moment constitute the orbital order. Further, to study the competition between orbital order and electron itinerancy we considered MnCoVO as one of the parent compounds, CoVO, lies near itinerant limit while the other, MnVO, lies deep inside the orbitally ordered insulating regime. Orbital order and electron transport have been investigated using first principles density functional theory and Boltzmann transport theory in CoVO, MnVO and MnCoVO. Our results show that as we go from MnVO to CoVO there is enhancement in the electron's itinerancy while the nature of orbital order remains unchanged.