Orbital polarization in layered t2g 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 x RuO 4 and Bi 2 Sr 2 Co 2 O 9 . For the hole-doped Co–O triangular lattice in Bi 2 Sr 2 Co 2 O 9 , 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 CoO 2 lattice model. In Ca 2− x Sr x RuO 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 x RuO 4 . This idea is also supported by model Hartee–Fock calculations on the RuO 2 lattice model.