제일원리 계산을 통한 고체 상태 에너지 소재의 전달 현상에 대한 연구

Abstract

This thesis consists of two parts: the first part discusses the electron transport properties of iron-based superconductors and thermoelectric (TE) materials, and the second part discusses the phonon transport properties of TE materials with various theoretical approaches.

Part I. Electron Transport Properties First, we investigated the electron correlation effect on the electronic structures and transport properties of the iron-based superconductors using density functional theory (DFT) and dynamical mean field theory (DMFT) with the Boltzmann transport equation (BTE). By considering the Fe 3d electron correlation using DMFT, the quasiparticle bandwidth near the Fermi level is found to be substantially suppressed compared to the conventional DFT calculation. Because of the different renormalization factors of each 3d orbital, DMFT gives considerably reduced electrical anisotropy compared to DFT results, which explains the unusually small anisotropic resistivity and superconducting property observed in the iron-based superconductors. Different renormalization also suppresses the Fermi surface nesting, which is an essential condition to the spin fluctuation mediated superconductivity. We have shown the origin of different electronic ground states between isostructural LiFeAs and NaFeAs compounds. Second, Seebeck coefficient and electrical conductivity, which are the most important electron transport properties of TE materials, are investigated by using DFT and BTE. Within the rigid band approximation, our calculations reproduce well experimental results. Moreover, rigid band calculation can also provide optimal doping level for high efficiency TE materials. With our prediction, the TE efficiencies of In4Se3-x and BiCuOQ (Q=S, Se, Te) compounds are well improved.

Part II. Phonon Transport Properties Lowering thermal conductivity is important issue in TE materials as well as electron transport properties. We investigated the origin of low thermal conductivity in high efficiency TE materials using two different methods: Molecular dynamics (MD) simulation and phonon BTE calculation. First, the lattice conductivity of In4Se3−x is investigated using equilibrium MD simulation, the point-defect model, and DFT calculations. The charge density distribution shows highly anisotropic structure with strong bonding along In-Se-In chain direction. Se vacancy strongly suppresses the phonon propagation along the chain direction, with little change in other directions. We show that suppressed long-range acoustic phonon transport caused by the vacancy results in anisotropic change of lattice conductivity. Second, we have investigated the origin of very low lattice conductivity in TE BiCuOQ (Q: S, Se, Te) compounds. Based on the first principles anharmonic lattice dynamics calculations, we use the single-mode relaxation time approximation of the linearized phonon BTE, which shows good agreement with experiments. Here, we found that the most important parameter for the origin of low lattice conductivity is the interlayer interaction between BiO and CuQ layers. By analyzing the phonon linewidth distribution, which indicates phonon scattering, we proposed that the interlayer interactions play crucial role on suppressing lattice conductivity.