Orbital insulators and orbital order-disorder induced metal-insulator transition in transition-metal oxides

The role of orbital ordering on metal-insulator transition in transition-metal oxides is investigated by the cluster self-consistent field approach in the strong correlation regime. A clear dependence of the insulating gap of single-particle excitation spectra on the orbital order parameter is found. The thermal fluctuation drives the orbital order-disorder transition, diminishes the gap and leads to the metal-insulator transition. The unusual temperature dependence of the orbital polarization in the orbital insulator is also manifested in the resonant X-ray scattering intensity.