臺灣博碩士論文加值系統

在線接觸式狄拉克半金屬中，鋯矽硫 (ZrSiS) 的電子結構具有能帶交叉點並在晶格動量空間形成連續閉合的曲線。在我們的工作中，透過密度泛函原理計算以及掃描穿隧式電子顯微/能譜 (STM/STS) 量測的方式，研究鋯矽硫的電子組 態特性和表面型態。藉由觀測材料表面區域狀態密度分佈的準粒子干涉現象 (quasiparticle interference)，我們發現針對在不同晶格位置的點缺陷，準粒子散射於動量空間有很大的選擇性。因此透過計算倒空間等能量電子組態分佈 (constant energy contours) 和結合狀態密度 (joint density of states) 來模擬準粒子干涉現象，能夠清楚辨識位於不同晶格位置缺陷電子散射的貢獻，並且顯示不同電子軌域特徵的狀態在動量空間的散射會被抑制。這個觀測能幫助我們更加理解不同缺陷來源造成的散射現象與計算表面電子結構軌域特徵之間的關係。

ZrSiS is a Dirac semimetal hosting high mobility, linearly dispersive quasiparticle bands which intersect at a closed loop in momentum space, known as a Dirac-nodal line. In this work, we investigate the electronic properties and the surface morphology of ZrSiS by combining density functional theory calculations as well as scanning tunneling microscopy and spectroscopy experiments. By visualizing quasiparticle interference (QPI) in the surface local density of states in spectro-microscopy measurements, we observe that there is a strong selectivity of quasiparticle scattering channels in momentum space, depending on the lattice site of the impurity scattering center. Therefore, the calculated orbital-resolved constant-energy contours and QPI patterns at the surface of ZrSiS are presented and show that simulations of QPI mapping allow for the clear distinction of contributions in scattering of defects at different lattice sites. This implies that the scattering between the states with different orbital character is suppressed in momentum space. This observation provides a detailed insight into the relation between calculated orbital characteristics of the surface band and scattering patterns near different impurity center.

List of Figures v
List of Tables xiv
1 Overview 1
2 Introduction 3
2.1 Dirac Nodal-Line Semimetal ZrSiS ............ 3
2.2 Theory of Quasiparticle Interference........... 7
2.3 Prior Studies of Quasiparticle Interference ............... 9
3 Methods 13
3.1 Density Functional Theory...................... 13
3.1.1 The Schro ̈dinger Equation .................... 14
3.1.2 The Thomas-Fermi Model .................... 16
3.1.3 The Hohenberg-Kohn Theorems................. 17
3.1.4 The Kohn-Sham Equations.................... 18
3.1.5 The Exchange Correlation Energy................ 19
3.2 Scanning Tunneling Microscopy .................. 20
3.2.1 The Tunneling Current...................... 21
3.2.2 Bardeen’sMethod ........................ 23
3.2.3 The Tersoff-Hamann Model ................... 25
3.2.4 Scanning Tunneling Spectroscopy ................ 26
3.2.5 Ultra-High Vacuum........................ 28
3.3 Crystal Growth and Preparation Methods ............... 28
4 Surface Morphology and Electronic Properties of ZrSiS 31
4.1 Bulk and Surface Band Structure Calculations ............. 31
4.2 STS Measurements and Density of States Calculations .......... 34
4.3 Identification of Atomic Surface Lattice ............... 36
4.4 Determination of Types of Surface Defects ............... 38
5 Defect-Site Dependence of Quasiparticle Interference 43
5.1 Observations of Defects and QPI by STM ................ 43
5.2 Simulations of QPI Patterns ................. 45
5.2.1 Orbital-Resolved QPI Simulations .............. 45
5.2.2 Comparison with the Measurements............... 46
5.2.3 Observation of m-Conserving Scattering........... 51
5.3 Calculated Charge Density Distributions................. 52
5.4 Discussion............................ 56
6 Conclusion 59
Appendices 62
A Electronic Structures of Bi2Te3−xSex 63
A.1 Bulk and Surface Band Structure Calculations ............. 64
A.2 STS Measurements and Density of States Calculations ........ 68
A.3 Structural Properties of Bi2Te3−xSex ............. 70
B Simulated STM program 74
Bibliography 81

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