| It is a long history that transition-metal oxides have been used as a functional ma-terial by human beings. In Greek era, magnetites (Fe3O4) was already known as useful mineral[1].Recently, as the discovery of high-temperature superconductivity, colossal mag-netoresistance, multiferroics and metal-insulator transition, there is renewed interest in transition metal oxides[1]. The unusual properties of transition metal oxides, which is characterized by various magnetic and electronic properties, originate from the outer d-electrons[2]. The electronic and magnetic properties form the basis for many important applications. The interaction between spin and charge degrees of freedom lead to the magnetoelectric coupling, which result to induction of magnetization by an electric field or electric polarization by a magnetic field[3]. Transition metal oxides may be good in-sulators(TiO2), semiconductors(Feo.90), metals(ReO3) or superconductors(YBa2Cu3O7)。 And some oxides display the metal-insulator transitions as the change of temperature(VO2), pressure(V2O3), or composition(NaxW03)[4]. In the first chapter of introcuction we will introduce transition-metal oxides, and properties of multiferroics and metal-insulator transitions.Above peculiar properties of the transition metal oxides raise many physical prob-lems that need to be understood. The electrical conductivity and other properties of transition metal oxides are related to their electronic structures [4]. Therefore, we need to understand the electronic structure of transition metal oxides, and one of the most ef-fective method used today is density functional theory. In chapter 2, we will introduce this method.Because of the special localization of the d orbitals of transition metal elements, the projection bands of d orbitals of transition metal are quite narrow. The bandwidths are typically of the order 1-2 eV, compared with the 5-15 ev in normal metals. The narrow band nature leads to strong electron correlation effect in transition metal ox-ides[2]. The local electronic structure can be described in terms of atomic like states, e.g. Cu1+(d10), Cu2+(d9), Cu3+(d8) for Cu in CuO. Many transtion metal oxides are not truly three dimensional, but have low-dimensional features, for example, La2NiO4 is quasi-two-dimensional compared to LaNiO3. There are some methods, which can be used to appropriately describe and understand the electronic structure of these corre-lated system, such as LDA+Gutzwiller variational method. In chapter 3, we introduce the LDA+Gutzwiller (LDA+G) method.In chapter 4, we clarify the mechanism of high-Tc multiferroicity in CuO by com- bining first-principles calculations and an effective Hamiltonian model. We found that CuO contains two magnetic sublattices, with strong intrasublattice interactions and weakly frustrated intersublattice interactions. The weak spin frustration leads to incommensu-rate spin excitations that dramatically enhance the entropy of the multiferroic phase and eventually stabilize that phase in CuO.In chapter 5, we study the electronic structure and magnetic properties of the strongly-correlated material La2O3Fe2Se2, in which there is an ambiguity in determining the magnetic ground state, using both the LDA+U and LDA+G methods. We found that the magnetic structure of ground state obtained by first-principles calculation matches with the one from most recent experiments, but the band gap is dramatically overesti-mated with this method. Then, using LDA+G method, we found that the band-structure of La2O3Fe2Se2 is close to the metal-semiconductor transition point, which is consis-tent with experimental results. We further show that the narrow band gap nature in LDA+G calculation, is closely related to the fluctuation of atomic multiplets caused by correlation effect. On the other hand, the magnetic moments of Fe atom obtained by LDA+G calculation is obviously smaller than the ones in LDA+U, which can be well explained by the fluctuation between different atomic configurations in this system. |
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