Theory of orbital polarization and x-ray absorption dichroism in d-and f-electron systems

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1) Orbital polarization and x-ray absorption dichroism in transition-metal oxides including YTiO_3The relation between the orbital polarization of Ti ion and Ti 2p x-ray absorption linear dichroism (XLD) in YTiO_3, which shows ferromagnetism, is investigated theoretically on the basis of a single-ion model including the multipole interaction between electrons and crystal field for Ti ion. By analyzing the observed Ti 2p XLD spectrum with the model, we determined the Ti 3d polarization in terms of the triply degenerate t_<2g>state. We furthermore analyzed the observed x-ray absorption magnetic circular dichroism and XLD in many transition-metal oxides with similar models and obtained information about orbital polarization and discussed mechanisms of the occurrence of ordered states.2) Development of computational code of dielectric function used in GW method with FLAPWWe investigated the cut-off dependence of the accuracy of symmetrized matrix elements of dielectric function, which play … More s an essential role in GW method, by adopting the plane wave as basis functions. As results of calculation for MnO. Ni and Si, we showed that a good accuracy is realized with the cut-off of approximately 20 Ry. The cut-off value is same as that for LAPW basis and the usefulness of the plane wave basis in the calculation of dielectric function is therefore demonstrated.3) Theory of metal-insulator transition in BaVS_3BaVS_3 has a hexagonal perovskite structure with one-dimensional chain composed of VS_8 octahedron along the c-axis. We investigated theoretically the mechanism of metal-insulator transition in this system on the basis of exact diagonalization for a 8 V-ion chain model with periodic boundary condition where the multipole interaction between electrons is included. As a result of calculation, the metal-insulator transition is found to be caused by strong electron-phonon interaction. The occurrence of the orbital ordering with a long periodicity in this system and an expected V 2p XLD are also discussed. Less