臺灣博碩士論文加值系統

近年來由於石墨烯的發現，人們對二維系統材料產生極大興趣。因單層之過渡金屬二硫化物MX2(M=Ta，Nb，V和X=S，Se)具有強自旋軌道耦合及鏡面對稱性破壞，在此篇論文中，我使用膺勢能及平面波方法配合貝里相位方程式對1T及2H結構之過渡金屬二硫化物之自旋，反常與谷霍爾電導率進行第一原理計算。晶體結構則參考實驗數據。
藉由自旋霍爾效應，我們可以在不施加外加磁場及磁性材料的情況下，便能自行操控電子的自旋流，而被視為自旋電子學中一相當重要之發現。2H結構之單層過渡金屬二硫化物因同時具有強自旋軌道耦合與鏡面對稱性破壞，使得電子在相反的谷上具有相反的貝里曲率和自旋矩，而被預期具有良好的自旋霍爾電導率和谷霍爾電導率，但在我們的結果中2H結構之單層過渡金屬二硫化物自旋霍爾電導率均較同一2H結構之塊材過渡金屬二硫化物來的小，而在1T結構下,雖失去了鏡面對稱破壞，單層之自旋霍爾電導率卻比同一結構之塊材要大，其中以單層NbSe2之1T結構具有最大的自旋霍爾電導率，單層TaSe2之2H結構具有最大的谷霍爾電導率而單層VSe2之1T結構具有最大的異常霍爾電導率。因此我們的結果可以發現過渡金屬二硫化物單原子層是一理想的材料在自旋電子學的應用上。

Because of the inversion symmetry breaking and strong spin-orbit coupling, interest in transition-metal dichalcogenides MX2 ( M = Ta, Nb, V and X = S, Se ) have emerged since the discovery of graphene. In this thesis, a systematic first principle study of spin, anomalous and valley Hall conductivities of transition metal dichalcogenides in both 1T and 2H structure is performed with full-potential projector-augmented wave method with Berry-phase formalism. The experimetal crystal structures are used.
Spin Hall effect (SHE) enables us to control spins without magnetic field or magnetic materials, which is a crucial step for spintronics. Because of the inversion symmery breaking and strong spin-orbit coupling in 2H-transition-metal dichalcogenides monolayers, charge carriers in opposite valleys carry opposite Berry curvature and spin moment, which is expected to have a good spin Hall effect and vally Hall effect. Our results show that the intrinsic spin Hall conductivity in 2H-transition-metal dichalcogenides monolayer is smaller compared to bulk. Though in 1T-structure, the intrinsic spin Hall conductivity in bulk transition-metal dichalcogenides is smaller compared to monolayer. The 1T-TaSe2 monolayer presents the largest intrinsic spin Hall conductivity and the 2H-TaSe2 monolayer exhibits the largest valley Hall conductivity and the 1T-VSe2 monolayer posseses the largest anomalous Hall conductivity. Our results demonstrate transition-metal dichalcogenides monolayers to be an ideal platform for spintronics applications.

口試委員會審定書 #
誌謝 i
中文摘要 ii
ABSTRACT iii
CONTENTS iv
LIST OF FIGURES vii
LIST OF TABLES ix
Chapter 1 Introduction 1
1.1 Overview of Anomalous, Spin and Valley Hall effects 1
1.2 Transition Metal Dichalogenides 3
Chapter 2 First Principle Calculations 5
2.1 Introduction 5
2.2 Thomas-Fermi theory 7
2.3 Density functional theory 9
2.3.1 Hohenberg-Kohn theorem 10
2.3.2 Kohn-Sham equation 11
2.4 Exchange-correlation potentials 13
2.4.1 Local density approximation (LDA) 13
2.4.2 Generalized gradient approximation (GGA) 15
2.5 Plane waves and projector augmented wave (PAW) method 15
2.5.1 Plane waves method 16
2.5.2 Projector augmented wave (PAW) method 17
2.6 Spin-orbit coupling (SOC) 19
2.7 Berry-phase 20
2.7.1 Cyclic adiabatic evolution 21
2.7.2 Anomalous, Spin and Valley Hall effects 22
Chapter 3 Anomalous, Spin and Valley Hall effect in 2H-MX2 Bulk Crystals and Monolayers 26
3.1 Spin and Valley Hall Effect in 2H-MX2 (M=Ta and Nb; X=S and Se) Bulk Crystals and Monolayers 26
3.1.1 Crystal structure and Computational details 26
3.1.2 Spin Hall Conductivity of 2H- MX2 Bulk Crystals 28
3.1.3 Spin and Valley Hall Conductivity of 2H- MX2 Monolayers 30
3.2 Spin and Anomalous Hall Effect in 2H-VX2 (X=S and Se) Bulk Crystals and Monolayers 33
3.2.1 Crystal structure and Computational details 33
3.2.2 Spin and Anomalous Hall Conductivity of 2H- VX2 Bulk Crystals 35
3.2.3 Spin and Anomalous Hall Conductivity of 2H- VX2 Monolayers 37
Chapter 4 Anomalous and Spin Hall effect in 1T-MX2 Bulk Crystals and Equation Chapter (Next) Section 1Monolayers 39
4.1 Crystal structure and Computational details 39
4.2 Spin Hall Effect in 1T-MX2 (M=Ta and Nb; X=S and Se) Bulk Crystals and Monolayers 41
4.2.1 Spin Hall Conductivity of 1T- MX2 Bulk Crystals 41
4.2.2 Spin Hall Conductivity of 1T- MX2 Monolayers 43
4.3 Spin and Anomalous Hall Effect in 1T-VX2 (X=S and Se) Bulk Crystals and Monolayers 45
4.3.1 Spin and Anomalous Hall Conductivity of 1T- VX2 Bulk Crystals 45
4.3.2 Spin and Anomalous Hall Conductivity of 1T- VX2 Monolayers 47
4.4 Spin and Anomalous Hall Effect in 1T-TaS2 Monolayer in Charge Density Wave 49
4.4.1 Crystal structure and Computational details 49
4.4.2 Spin and Anomalous Hall Conductivity of 1T-TaS2 Monolayer in Charge Density Wave 50
Chapter 5 Summary 54
REFERENCE 55

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