超薄Bi薄膜的电子态研究

发布时间：2018-07-10 04:02

本文选题：铋 + 超薄薄膜　；参考：《上海交通大学》2015年博士论文

【摘要】：近年来,石墨烯、拓扑绝缘体和过渡族金属硫族化合物等一系列二维晶体材料由于其新奇的电学、磁学、光学等性质而被广泛研究。与上述材料类似,铋(Bi)的111取向的超薄薄膜(Bi(111))也是一种具有特殊性质的二维电子材料。它以双原子层(bilayer,BL)为一个单元结构,由于层间作用力较弱,因而是一种稳定的层状材料。研究表明Bi(111)薄膜具有一系列特殊的物理性质:(1)由于Bi的体载流子浓度小且具有较长的费米波长,Bi(111)薄膜是研究量子尺寸效应的极佳材料。(2)体材料Bi具有非常稳定而特殊的金属性表面态。由于非常强的Rashba型自旋轨道耦合,Bi的表面态具有自旋劈裂(动量空间)特性,并且相对体有很高的载流子浓度,因此Bi有望成为一种高性能的二维自旋电子学材料。(3)第一性原理计算表明,超薄Bi(111)薄膜在某个临界厚度会发生半金属-半导体相变。同时最新的理论计算也显示,超薄(5BL)的Bi(111)薄膜是一种二维的拓扑绝缘体。因此研究超薄Bi(111)的电子能带结构、自旋结构以及拓扑性质对于了解这种二维材料的物理性质和应用具有重要的意义。本文主要围绕超薄Bi(111)的电子结构和自旋性质以及背后所蕴含的物理机理,利用分子束外延(MBE)、角分辨光电子能谱(ARPES)、自旋分辨角分辨光电子能谱(SR-ARPES)和第一性原理计算对其开展了系统研究,并取得了以下结果:1)在Bi2Te3基底上,我们利用MBE制备出极高质量的单个双原子层的Bi(111)薄膜。通过ARPES结合第一性原理计算,在Bi(111)/Bi2Te3的界面上发现了一种由Bi(111)的Rashba劈裂态和Bi2Te3本征狄拉克锥杂化而成的新的螺旋型狄拉克锥(Helical Dirac Cone)。在这个杂化狄拉克锥的狄拉克点附近发现了能带重整为垂直拉长的无色散形态。我们发现,这种奇特的能带重整效应起源于强烈电子-电子多体相互作用。这是第一次利用能谱技术在类拓扑绝缘体的螺旋型狄拉克锥发现强多体相互作用的存在。2)在Bi2Te3和Bi2Se3两种不同基底上外延的单个双原子层Bi(111)体系中,存在三种不同物理起源的螺旋型狄拉克锥:本征拓扑型,Rashba型以及杂化型。我们结合SR-ARPES和第一性原理计算发现,虽然三者的类型不同,但是其自旋构型具有完全相同的轨道依赖性,决定它们自旋构型的因素是是Rashba型自旋轨道耦合,和拓扑性质无关。3)我们研究了Bi(111)薄膜的电子结构随厚度的演化。在Bi2Te3基底上制备不同厚度的Bi(111)薄膜,研究发现2BL及以上的Bi薄膜中的电子不再和Bi2Te3的拓扑表面态发生杂化,但是和基底体能带的杂化一直延伸至离界面3BL距离(1.2nm)。厚度低于15BL的薄膜的面内晶格都受到压缩。晶格压缩改变了Bi的两条自旋劈裂的表面态的色散关系以及费米面的形状。
[Abstract]:In recent years, a series of two-dimensional crystal materials, such as graphene, topological insulators and transition metal sulfur compounds, have been widely studied for their novel electrical, magnetic and optical properties. Similar to the above materials, Bi (111) thin film with 111 orientation is also a two-dimensional electronic material with special properties. The bilayer layer (BL) is a kind of stable layered material because of its weak interlayer force. The results show that Bi (111) thin films have a series of special physical properties: (1) Bi (111) thin films are excellent materials for studying quantum size effect due to their small bulk carrier concentration and longer Fermi wavelength. (2) Bi (111) bulk material is very stable and stable. Special gold attribute surface state. Because the surface state of Rashba type spin-orbit coupling Bi has spin-splitting (momentum space) characteristics, and the relative body has a high carrier concentration, As a result, Bi is expected to be a high performance two-dimensional spin electronics material. (3) First-principle calculations show that ultrathin Bi (111) thin films have semimetal-semiconductor phase transitions at a certain critical thickness. The latest theoretical calculations also show that the ultrathin (5BL) Bi (111) thin film is a two-dimensional topological insulator. Therefore, the study of the electron band structure, spin structure and topological properties of ultra-thin Bi (111) is of great significance in understanding the physical properties and applications of this two-dimensional material. This paper focuses on the electronic structure and spin properties of ultrathin Bi (111) and the physical mechanism behind them. The molecular beam epitaxy (MBE), angular resolved photoelectron spectroscopy (ARPES), spin resolved angle resolved photoelectron spectroscopy (SR-ARPES) and first-principles calculations were used to study them. The following results were obtained: 1) on Bi2Te3 substrate. High quality Bi (111) thin films with a single diatomic layer were prepared by MBE. By means of ARPES and first-principle calculations, a new helical Dirac cone (Helical Dirac Cone) has been found on the interface of Bi (111) / Bi _ 2TE _ 3, which is composed of Bi (111) Rashba split state and Bi _ 2TE _ 3 intrinsic Dirac cone hybrid. Near the Dirac point of the hybrid Dirac cone, the band is reorganized into a vertical elongated non-dispersive form. We find that this peculiar band reforming effect originates from the strong electron-electron multi-body interaction. This is the first time that the existence of strong multibody interaction has been found in the helical Dirac cone of a topological insulator by using the energy spectrum technique. 2) in Bi _ 2TE _ 3 and Bi _ 2SE _ 3 single diatomic layer Bi (111) system epitaxial on two different substrates. There are three kinds of helical Dirac cones with different physical origins: intrinsic topological type Rashba type and hybrid type. In combination with SR-ARPES and first-principles calculations, we find that although the three have different types, their spin configurations have exactly the same orbital dependence, and their spin configurations are determined by the Rashba spin-orbit coupling. We have studied the evolution of electronic structure with thickness in Bi (111) thin films. Bi (111) thin films with different thickness were prepared on Bi2Te3 substrates. It was found that the electrons in 2BL and above Bi thin films were no longer hybridized with the topological surface states of Bi2Te3, but the hybridization with the substrate physical band extended to the 1.2nm distance from the interface. The in-plane lattice of thin films with thickness less than 15BL is compressed. Lattice squeezing changes the dispersion relation of two spin-splitting surface states of Bi and the shape of Fermi surface.
【学位授予单位】：上海交通大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：O484

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