Orbital polarization in layered t_2g electron systems

By means of x-ray absorption spectroscopy and model Hartree-Fock calculations, we have investigated the electronic structure of layered t _2g electron systems such as Ca_{2 - x}Sr_xRuO ₄ and Bi₂Sr₂Co₂O₉. For the hole-doped Co-O triangular lattice in Bi₂Sr₂Co ₂O₉, the x-ray absorption spectral line shape indicates that the carriers in the t_2g band mainly have the a_1g symmetry. This result is supported by model Hartee-Fock calculations on the CoO₂ lattice model. In Ca_{2 - x}Sr_xRuO ₄, the interplay between the orbital ordering and the Mott transition is demonstrated by the x-ray absorption measurement. It is shown that the orbital ordering accompanied by the lattice distortion is important in describing the insulating state of Ca_{2 - x}Sr_xRuO ₄. This idea is also supported by model Hartee-Fock calculations on the RuO₂ lattice model.