Angle-Resolved Photoemission Spectroscopic Studies Of Two-Dimensional Materials And Heterostructures

The numerous states of matter in two-dimensional（2D）materials make them an attractive platform for condensed matter physics researches.More exotic electronic prop-erties 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 Pt Se₂thin film and quasicrystalline 30°twisted bilayer graphene（30°-t BLG）.Chapter 1 of this thesis firstly gives an brief introduction to the family of 2D mate-rials 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 spec-troscopy（ARPES）measurements,together with its derived techniques including spin-ARPES and nano-ARPES,from both theoretical and experimental aspects.Other ex-perimental 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–mono-layer Pt Se₂thin film and 30°-t BLG 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 Pt Se₂monolayer,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 Pt Se₂thin 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 het-erostructure–incommensurate 30°-t BLG 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 a tight-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°-t BLG.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.