丰富的物态种类使得二维材料成为凝聚态物理学研究中一个极具吸引力的平台。新型的二维体系中，如单层薄膜或堆叠单层而成的异质结构，有望涌现更多新奇的电子特性。本论文主要利用角分辨光电子能谱及其衍生技术深入探究了两类最新实现的二维体系——单层PtSe2 和准晶30? 扭角双层石墨烯。本论文第一章中，以石墨烯和过渡族金属硫化物作为两类最具代表性的成员，简要介绍了二维材料家族。我们讨论了这两类材料在其厚度降至单层或者堆叠成范德瓦尔斯异质结的过程中所表现出来的奇特性质。第二章中，我们主要从理论及实验层面介绍了角分辨光电子能谱的测量原理以及其他两种衍生技术，包括自旋分辨以及微区分辨的角分辨光电子能谱。另外，我们还简单介绍了分子束外延以及两种电子衍射技术。在第三章里，我们仔细讨论了两种新发展的样品生长方法。其中一种利用直接硒化衬底来生长单层PtSe2 样品，另一种利用直接退火衬底来生长石墨烯样品。这两种新方法不仅具有简单高效的特点，同时生长出的样品也具有较高的质量，为这两类体系的光电子能谱研究奠定重要基础。在第四章里，我们利用自旋角分辨光电子能谱，在具有中心反演对称结构的PtSe2 单层薄膜中观测到了隐式自旋极化分布。该结果并不能用传统的Rashba 效应来解释。因此也揭示出了一种新型的自旋极化机制——局域Rashba（R-2）效应。第一性原理计算结果支持我们的结论：自旋在单层PtSe2 中虽然在能量上简并，但是会在空间上分布在两个Se 原子亚层中，形成相反的极化，因而形成了一类新的自旋结构。在第五章里，我们研究了30? 扭角双层石墨烯这种具有准晶对称性的非公度范德瓦尔斯异质结的电子结构。角分辨光电子能谱的测量结果表明，除了石墨烯原有的能带结构之外，还存在一类奇特的镜像狄拉克锥。通过进一步分析，我们发现该镜像狄拉克锥来源于一种特别的散射机制，并且该机制也得到了拉曼测量的验证。以上这些结果均表明在30? 扭角双层石墨烯中也存在着较强的层间耦合。在结尾的第六章中，我们对以上几项主要工作作了总结。我们的工作一方面表明在二维材料及其异质结中还有更多有趣的物理等待我们去探索。同时，我们的发现显示具有一定普适性，因此可以推广至其他材料结构中，从而对于未来电子学、自旋电子学等具有应用价值。
The numerous states of matter in two-dimensional (2D) materials make them an attractive platform for condensed matter physics researches. More exotic electronic properties are expected to emerge in new kinds of 2D systems, such as monolayer of novel 2D materials or the heterostructure stacked by these various single sheets. In this thesis, we report the progress in investigating the novel electronic structures of two newly realized2D systems – monolayer PtSe2 thin film and quasicrystalline 30° twisted bilayer graphene (30°-tBLG).Chapter 1 of this thesis firstly gives an brief introduction to the family of 2D materials with two representative examples – graphene and transition metal dichalcogenides (TMDCs). Their novel properties emerging when reducing to low thickness or stacking to van der Waals heterostructure are then discussed.Chapter 2 mainly discusses the principles of angle-resolved photoemission spectroscopy (ARPES) measurements, together with its derived techniques including spin-ARPES and nano-ARPES, from both theoretical and experimental aspects. Other experimental techniques such as molecular beam epitaxy and electron diffraction are also introduced.In chapter 3, newly developed growth methods for the two new 2D systems – monolayer PtSe2 thin film and 30°-tBLG are discussed in details. These two methods, by direct selenization and direct annealing of the substrate, not only exhibit high efficiency in growth procedure, but also provide high quality samples which pave way for the following experimental investigation.In chapter 4, by utilizing spin-ARPES, we reveal the hidden spin polarization with helical texture in centrosymmetric PtSe2 monolayer, which can not be explained by conventional Rashba effect. To reconcile this contradiction, we propose one kind of local Rashba (R-2) effect in such monolayer PtSe2 thin film. First-principles calculations support our interpretation that the spins are polarized locally at two Se sub-layers with opposite directions, leading to a new kind of spin-layer locking effect.Chapter 5 investigates the electronic structure of one model van der Waals heterostructure – incommensurate 30°-tBLG with quasicrystalline symmetry. By ARPES measurements, a novel mirrored Dirac cone is discovered, which is further confirmed as an intrinsic feature by nano-ARPES with spatially resolved information. Based on atight-binding model, a new kind of scattering mechanism is thus proposed which is also supported by Raman measurements. All the results indicate a strong interlayer coupling in 30°-tBLG.The thesis ends with chapter 6 for concluding our investigations on above 2D systems. On one hand, the novel electronic properties revealed in our works demonstrate the rich physics of 2D materials in various new phases and structures. On the other hand, all these discoveries are shown to be general and could have potential applications in future electronics and spintronics.
