Semiclassical transport and magnetotransport in topological systems: effects of anisotropy, trivial bands, and excitons = 위상계에서의 준고전 수송 및 자기수송:비등방성, 비위상적 밴드, 엑시톤의 영향

다국어 초록 (Multilingual Abstract)

Topological materials have attracted considerable attention in modern condensed matter physics due to their novel quantum phenomena. To explore the properties of these systems, it is essential to understand their transport behaviors, since such understanding provides crucial information about the band structure, Fermi surface, and relevant scattering mechanisms. In this thesis, within the framework of a semiclassical approach, we investigate how anisotropies in band dispersion and the presence of trivial bands affect transport phenomena in topological systems. First, we analyze the density response and the corresponding diffusion constant in anisotropic multiband
systems using a diagrammatic approach, and demonstrate that a proper consideration of the componentwise transport relaxation time is essential for understanding transport in such systems. Next, we develop a semiclassical Boltzmann magnetotransport theory for topological systems with a nonvanishing Berry curvature, fully incorporating both field-driven and intrinsic anisotropies in the band structure. We then apply this theory to investigate the longitudinal magnetoconductivity of Weyl semimetals, revealing that trivial bands near the Weyl nodes play a crucial role, leading to a Fermi energy dependence distinct from the predictions of conventional theories, in good agreement with experimental observations. Finally, we construct an electrostatic model to capture the experimentally observed stiffening of the interlayer breathing mode with increasing fluence in a WSe2/WS2 heterobilayer system.

국문 초록 (Abstract)

위상 물질은 그들의 독특한 양자 현상으로 인해 현대 응집물질물리에서 많은 관심을 받아왔다. 이러한 계의 특성을 탐구하기 위해서는 수송 거동을 이해하는 것이 필수적인데, 이는 밴드 구조, 페르미 면, 그리고 주요 산란 메커니즘에 대한 중요한 정보를 제공하기 때문이다. 본 학위논문에서는 준고전적 접근법을 기반으로, 밴드 분산의 비등방성과 비위상적 밴드의 존재가 위상계에서의 수송 현상에 어떤 영향을 미치는지를 조사한다. 먼저, 우리는 도식적 접근법을 이용하여 비등방적 다중밴드 계에서 밀도 응답과 이에 대응하는 확산 상수를 분석하고, 성분별 수송 이완 시간을 적절히 고려하는 것이 이러한 계의 수송을 이해하는 데 필수적임을 보인다. 다음으로, 우리는 베리 곡률을 가지는 위상계에 대해, 밴드 구조의 장 유도 및 내재적 비등방성을 모두 반영한 반고전적 볼츠만 자기수송 이론을 전개한다. 그리고 이 이론을 바일 준금속의 종방향 자기전도도 분석에 적용하여, 바일 노드 근처의 비위상적 밴드가 중요한 역할을 하며, 그 결과 기존 이론의 예측과는 구별되는 페르미 에너지 의존성이 나타나고, 이것이 실험 결과와도 잘 일치함을 밝힌다. 마지막으로, 우리는 WSe2/WS2 이질이중층에서 플루언스가 증가함에 따라 층간 호흡 모드가 경직되는 실험 결과를 설명하기 위해 정전기적 모델을 구성한다.

목차 (Table of Contents)

• Abstract i
• List of Contents v
• List of Figures xi
• List of Tables xii
• List of Publications xiii
• Chapter 1. Introduction 1
• Chapter 2. Diffusive density response of electrons in anisotropic multiband systems 5
• 2.1. Introduction 5
• 2.2. Vertex corrections to the density-density response function 7
• 2.2.1. Intrinsic susceptibility in the static limit 9
• 2.2.2. Evaluation of the charge vertex 11
• 2.2.3. Extrinsic susceptibility driven by disorder 16
• 2.3. Evaluation of the diffusion constants in anisotropic systems 17
• 2.4. Discussion 21
• Chapter 3. Semiclassical Boltzmann magnetotransport theory in anisotropic systems with a nonvanishing Berry curvature 24
• 3.1. Introduction 24
• 3.2. Magnetotransport equation in electron gas systems 26
• 3.3. Magnetotransport equation in anisotropic systems with a nonvanishing Berry curvature 31
• 3.4. Magnetoconductivity 36
• 3.5. Numerical calculations for Weyl semimetals 37
• 3.6. Discussion 40
• Chapter 4. Effect of trivial bands on chiral anomaly induced longitudinal magnetoconductivity inWeyl semimetals 42
• 4.1. Introduction 42
• 4.2. Evaluation and analysis of LMC at zero temperature 45
• 4.2.1. Dependence on the magnetic field 48
• 4.2.2. Analytic analysis of LMC in the weak internode and node-trivial band scattering limit 50
• 4.2.3. Dependence on the Fermi energy 53
• 4.3. Temperature dependence of LMC 56
• 4.3.1. Analytic analysis for LMC at finite temperature 56
• 4.3.2. Evaluation of LMC at finite temperature 59
• 4.4. Discussion 62
• Chapter 5. Electrostatic interpretation of ultrafast control of coherent interlayer coupling in WSe2/WS2 heterobilayers 65
• 5.1. Introduction 65
• 5.2. Experimental results 67
• 5.3. Electrostatic model 70
• 5.4. Discussion 74
• Chapter 6. Conclusion 79
• 6.1. Summary 79
• 6.2. Future work 80
• 6.2.1. Dissipationless orbital magnetic moment induced by interlayer exciton superfluids 81
• 6.2.2. Extension of Boltzmann theory through non-Abelian wave-packet dynamics 83
• Appendix A. Diffusive density response of electrons in anisotropic multiband systems 85
• A.1. Alternative derivations for the density response 85
• A.2. Detailed numerical calculations for the diffusion constants 91
• A.2.1. Anisotropic 2D electron gas 91
• A.2.2. Anisotropic graphene 94
• A.2.3. Few-layer black phosphorous 96
• A.2.4. Anisotropy of the diffusion constant 101
• Appendix B. Semiclassical Boltzmann magnetotransport theory in anisotropic systems with a nonvanishing Berry curvature 104
• B.1. Conventional derivation of the magnetotransport equation in electron gas systems 104
• B.2. Justification of the ansatz for the distribution function 105
• B.3. Consistency between Secs. 3.2 and 3.3 107
• B.4. Details of the numerical calculation in Sec. 3.5 108
• Appendix C. Effect of trivial bands on chiral anomaly induced longitudinal magnetoconductivity inWeyl semimetals 113
• C.1. Details for the calculation of LMC 113
• C.2. Constraint for the number conservation 121
• C.3. Asymptotic form of LMC at finite temperature 123
• C.3.1. Chemical potential 123
• C.3.2. Anomalous conductivity 125
• Appendix D. Electrostatic interpretation of ultrafast control of coherent interlayer coupling in WSe2/WS2 heterobilayers 127
• D.1. Additional experimental results and theoretical calculations 127
• Bibliography 131
• Abstract in Korean 142