Orbital Insulators and Orbital Order-disorder Induced Metal-Insulator   Transition in Transition-Metal Oxides
  The role of orbital ordering on metal-insulator transition of transition-metal oxides is investigated by the cluster self-consistent field approach in the strong correlation regime. A clear dependence of the insulating gap on the orbital order parameter is found in the single-particle excitation spectra. The thermal fluctuation drives the orbital order-disorder transition, diminishes the gap and leads to the metal-insulator transition. The interplay between spins and orbits results in unusual temperature dependence of the orbital polarization in the orbital insulator, which can be seen in the resonant x-ray scattering intensity. 
Dong-Meng Chen, and Liang-Jian Zou