Orbital rotations induced by charges of polarons and defects in doped vanadates

We explore the competition of doped holes and defects that leads to the loss of orbital order in vanadate perovskites. In compounds such as La1−x Ca x VO3 spin and orbital order result from superexchange interactions described by an extended three-orbital degenerate Hubbard-Hund model for the vanadium t2g electrons. Long- range Coulomb potentials of charged Ca2+ defects and e-e interactions control the emergence of defect states inside the Mott gap. The quadrupolar components of the Coulomb fields of doped holes induce anisotropic orbital rotations of degenerate orbitals. These rotations modify the spin-orbital polaron clouds and compete with orbital rotations induced by defects. Both mechanisms lead to a mixing of orbitals, and cause the suppression of the asymmetry of kinetic energy in the C-type magnetic phase. We find that the gradual decline of orbital order with doping, a characteristic feature of the vanadates, however, has its origin not predominantly in the charge carriers, but in the off-diagonal couplings of orbital rotations induced by the charges of the doped ions.