Metal-Insulator Transition with Hund's Coupling in Multiband Systems

DoctorI have studied the metal-insulator transition (MIT) in multiband systems by using density functional theory and its combination with dynamical mean field theory. The Mott type MIT induced by strong electron-electron correlation has been intensively studied almost over the past century. Most of our understanding, however, is still based on the one-band Hubbard model while most of the realistic materials have multiband nature. Since the discovery of Fe-based superconductor, there has been an increasing interest in strong correlation effects in multiband systems with Hund's coupling. The strongly correlated multiband systems are affected by not only Coulomb interaction but also Hund's coupling and crystal field splitting, unlike the one-band system. In the presence of the Hund's coupling, the multiband systems show various fascinating phenomena like orbital-selective Mott transition, high-spin to low-spin transition (spin state transition), non-Fermi liquid spin freezing phase, and Hund's metal. In this dissertation, I will discuss the MIT in strongly correlated multiband systems with several energy scales, especially the Hund's coupling. In the first part, I will discuss the origin of negligible spin-orbit coupling (SOC) effects on the ground state properties of UO2. Although the SOC is expected to be essential to describe physical properties of UO2, the previous theoretical results neglecting the SOC are in reasonable agreement with experiment. Via the charge density distribution, we show that the SOC does not affect the U-O bonding states which are directly related to the bonding properties (e.g., lattice constant, bulk modulus, and oxidation/reduction energies). In the second part, I will discuss the band type MIT and the Mott-type MIT accompanied by the spin state transition in FeO2. FeO2 is recently discovered under the high-pressure condition and receives great attention because it suggests an alternative explanation for several geophysical phenomena such as the Great Oxidation Event. The electronic and magnetic properties of FeO2 under high pressure, however, are still poorly understood and there has been controversy surrounding the oxidation state of FeO2. We show that the electronic structure of FeO2 is sensitively affected by the O2 dimer bond length. Our study suggests that the localized nature of not only Fe 3d orbitals but also O2 molecular orbitals should be correctly treated to describe structural and electronic properties of FeO2. In the last part, I will discuss the Hund's coupling effect on MIT in NiS2-xSex. In the last decade, there has been remarkable progress in the understanding of Hund's coupling effect on the correlated systems far from the Mott insulating state. However, it has not been discussed how the Hund's coupling can be realized in the vicinity of the Mott transition. NiS2-xSex shows the bandwidth-control type Mott MIT and is the most suitable multiband system to study Hund's coupling effect on MIT. Our study suggests that the traditional Brinkman-Rice picture can be modified in the multiband systems in the presence of Hund's coupling