강한 스핀-궤도 결합 및 전자간 상관 효과를 보이는 무거운 원소 물질계의 전자 구조 특성에 대한 연구

Abstract

We have studied the electronic structure of heavy element compounds to explain their physical properties including charge density wave (CDW), thermoelectric properties, and heavy fermion behavior, using the first-principles electronic structure calculation based on density functional theory (DFT) and dynamical mean field theory (DMFT). In the transport calculation, Boltzmann transport equation (BTE) and Green-Kubo (GK) formula are used. Here, we focus on the electron correlation effect and spin-orbit coupling (SO) to describe the electronic structure of heavy element compounds correctly. The detailed analysis on the transport properties shows that various physical properties such as dimensionality and scattering properties are hardly captured by the level of DFT, but the consideration of the electron correlation and SO effect is essential.

This thesis consists of three parts: the first part discusses the effect of quasi-one-dimensionality on the charge density wave (CDW) phenomenon of the rare-earth intermetallic compounds, the second part deals with the electronic structure and scattering properties of thermoelectric (TE) materials with layered hetero structure, and the last part treats the effect of crystalline electric field (CEF) on the electronic structure and the Kondo physics of heavy fermion compounds.

Part I. The Quasi One Dimensionality of CDW Systems First, we investigated the chemical and hydrostatic pressure effects on the CDW phase of SmNiC2, using the DFT calculation. Using the calculation of Fermi surface and electrical susceptibility, we captured the change of Fermi surface nesting feature of SmNiC2 under pressure. Based on the transport calculation, we found that SmNiC2 becomes more one dimensional (1D) as pressure increases. This results in the enhancement of the CDW phase, which corresponds to the increase of the CDW transition temperature (TCDW). Second, the dimensionality crossover of CeTe2-xSbx was studied using DFT calculation. Recent experiment on CeTe2-xSbx reported that it shows the enhanced TE properties due to increased band gap size at doping of x = 0.1 ~ 0.2. Using the band structure calculation, we found that 3D band structures near -Z symmetry line disappear at similar Sb doping x = 0.15 ~ 0.3. As a result of more 2D like electronic structure, the reduced dimensionality enhances the CDW phase and enlarges the CDW band gap. We confirm that the control of the dimensionality is a way to improve the TE properties.

Part II. The Electronic structure and Scattering Properties of Layered Hetero-structure Thermoelectric Materials The layered hetero-structure TE materials are useful for the band engineering of the electronic structure as well as the low lattice thermal conductivity due to phonon scattering at interface. First, we focused [GeTe]m[Bi2Te3]n (GBT) compounds to analyze the change of electronic structures and their effect on the TE properties as changing their composition (m, n) and doping level. We found the effective tensile (strain) effect on the electronic structure of GeTe (Bi2Te3) and the quantum confinement effect of Bi2Te3 bands near Fermi level as changing GeTe ratio. Using the BTE calculation, we predicted the enhancement of TE properties (ZT ~ 1.4) of moderately doped p-type (p = 1019 cm-3) GBT compounds with m=8, n=1 ratio by the combination of two effects. Second, the electron scattering of GBT, SBT (SnTe-Bi2Te3), and PBT (PbTe-Bi2Te3) compounds was studied. Because the BTE uses the constant scattering time approximation (CSTA) for simplicity of calculation process, it is hard to reproduce the experimental temperature and doping dependent electrical conductivity using the BTE. We suggest the semi-empirical way for electron scattering rate by combining the theoretical conductivity tensor and the experimental electrical conductivity. We verified the electron-acoustic phonon scattering properties of GBT, SBT, and PBT compounds at intermediate temperature ranges (300 ~ 600 K).

Part III. The Effect of Crystalline Electric Field on the Electronic Structure of Heavy Fermion Compounds The recent spectroscopy measurements (XAS, ARPES, etc.) reveal the importance of the anisotropic 4f orbital occupation and the crystalline electric field (CEF) of heavy fermion compounds at low temperature. It was suggested that they should be correctly described to understand the complicated ground state phases of local anti-ferromagnetism (AFM), unconventional superconductivity (SC), heavy Fermi liquid (HFL), etc. First, we investigated the role of temperature and CEF splitting in the orbital anisotropy of the heavy fermion compounds and their effect on Kondo physics, using the first principles DFT+DMFT calculation. We noticed that the orbital anisotropy sensitively changes with the temperature and the resulting anisotropic hybridization strength has crucial role in the determination of local Kondo temperature (~ 150 K). Also, we found two temperature scales: one is coherence temperature (T^*) given from the ground state orbital at low temperature and the other one is the CEF splitting Δ_CEF given by the depopulation from the ground state to the excited state at high temperature. We suggest that the orbital anisotropy should be qualitatively considered in the analysis of heavy fermion materials. Second, we reproduced the detailed 3D electronic structure of CeCoIn5 in comparison with ARPES measurements, using the DFT+DMFT calculation in consideration of CEF basis. We investigated the temperature evolution of heavy quasi-particle bands and related properties (Fermi momentum k_F, Fermi velocity v_F, etc.) to reveal a small-to-large Fermi surface change by the participation of Ce 4f electron in the formation of Fermi surface. The temperature dependent resonant 4f spectrum near Fermi level provides the evidence of hybridization between conduction of Ce 4f electrons well above T*. Also, we verified the enhancement of hybridization strength of Sn-doped CeRhIn5, compared to pure one and Cd-doped one by checking the frequency dependent self-energy Σ(ω) and spectral functions A(ω) at low temperature.