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

1)YTiO_3等过渡金属氧化物的轨道极化和X射线吸收二向色性在考虑电子与晶场多极相互作用的单离子模型基础上，从理论上研究了具有铁磁性的YTiO_3中Ti离子的轨道极化与Ti 2 p X射线吸收线性二向色性（XLD）的关系。利用该模型分析了Ti的2 p XLD谱，确定了Ti的3d极化是由三重简并t<2g>态决定的。我们进一步分析了在许多具有类似模型的过渡金属氧化物中观测到的X射线吸收磁性圆二色性和XLD，得到了轨道极化的信息，并讨论了有序态出现的机制。2）用FLAPW开发了GW方法中的介电函数计算程序我们研究了介电函数的对称化矩阵元的精度对截止值的依赖性，这些矩阵元在介电函数的计算中起着重要的作用 ...更多信息 采用平面波作为基函数，是GW方法的一个重要组成部分。作为MnO的计算结果。Ni和Si，我们表明，实现了良好的精度，截止值约为20 Ry。3）BaVS_3的金属-绝缘体转变理论BaVS_3具有六方钙钛矿结构，由VS_8八面体沿着c轴构成一维链状结构。我们在精确对角化的基础上，对周期边界条件下的8V离子链模型进行了理论研究，其中考虑了电子间的多极相互作用。计算结果表明，金属-绝缘体相变是由强的电子-声子相互作用引起的。讨论了该系统中长周期轨道有序的出现和预期的V2 pXLD。少

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