【作者】 陈牧；

【导师】 薛其坤； 马旭村；

【作者基本信息】 清华大学 ， 物理学， 2012， 博士

【摘要】 三维拓扑绝缘体是一类新的量子物质态,为近年来物理学的热点前沿课题之一。这类材料体内是绝缘体,表面存在无能隙的、受时间反演对称性保护的金属表面态。利用分子束外延技术精确可控地生长高质量的拓扑绝缘体薄膜,是研究拓扑绝缘体的新奇量子现象和探索其在自旋电子学和量子计算等应用的重要基础。Bi2Te3和Sb2Te3是表面具有单个狄拉克锥结构的第二代拓扑绝缘体,可在石墨烯衬底上用分子束外延生长得到。在本博士论文中,我们以生长在石墨烯上的Bi2Te3和Sb2Te3薄膜为衬底,进行了Bi薄膜的异质结生长和元素掺杂研究,利用原位的低温扫描隧道显微镜(STM)系统地研究了这两类体系在纳米尺度下的物理性质,得到的主要结论如下：(1)表面态如果被表面边界囚禁,则可以实现类似光学谐振腔的封闭体系。此封闭体系内的无质量Dirac费米子由于受到边界处多次散射,会形成量子干涉现象。我们在Bi2Te3上沉积亚单层的Bi(111)薄膜,实现了限制Dirac费米子的三角形“量子围栏”。通过围栏内的STM空间局域态密度成像、扫描隧道谱,我们观察到了这种量子干涉行为。结合该体系受限表面态共振散射的具体机制,我们分析了限制表面态寿命的主要来源。我们还系统地研究了Bi膜生长动力学机理、Bi膜对Bi2Te3薄膜的掺杂效应以及两种薄膜的功函数差别,并最终提出了Bi/Bi2Te3界面处能带弯曲的模型。(2)在室温掺杂Sb的Bi2Te3薄膜中,我们观测到了杂质中心的强烈共振峰。通过分析共振峰随空间和能量的变化情况,我们证明该中心处的共振峰与Dirac点是共存的。我们还研究了表面态在杂质周围的散射,结合Bi2Te3等能面构形,定量分析了散射波矢的来源：连接次近邻Γ-K方向电子波矢的q2。对照三类不同沉积条件的样品,我们研究了杂质在薄膜中的占位情况。(3)(BixSb1-x)2Te3合金薄膜是一类电子结构介于Bi2Te3和Sb2Te3之间的拓扑绝缘体材料。我们通过控制掺杂比例,实现了Dirac点相对费米能级的可控移动。在表面6掺杂Bi的Sb2Te3薄膜中,我们分析了不同掺杂浓度下表面态的色散关系和在强磁场下Landau量子化行为的演化,认为Bi掺杂抑制了原表面态内束缚态扰动,表面态更趋于线性色散。我们也观察到了Sb2Te3与Bi2Te3类似的表面态在杂质周围的准粒子相干条纹。更多还原

【Abstract】 As a new state of quantum matter, three-dimensional topological insulatorhas recently attracted tremendous attention in the physics community. The bulkof a topological insulator is insulating, but there exist gapless metallic surfacestates, which are topologically protected by time-reversal symmetry. Bymolecular beam epitaxy technique, one can grow high-quality topologicalinsulator thin films with precisely controlled properties and can be used forexploring novel quantum phenomena in topological insulators and potentialapplications in spintronics and quantum computation.The surface states of the second-generation topological insulators Bi₂Te₃and Sb₂Te₃consist of a single Dirac cone structure on each surface. High qualitythin films of Bi₂Te₃and Sb₂Te₃have been successfully grown on graphenesubstrate by molecular beam epitaxy. In this thesis, we perform heteroepitaxialgrowth of Bi films and chemical doping on Bi₂Te₃and Sb₂Te₃substrates. Usingin-situ low temperature scanning tunneling microscopy/spectroscopy（STM/STS）, we systematically study the physical properties of the two systemsin nanoscale. The main conclusions are as follows:1. Confining topological insulator surface states with a closed boundary isfascinating and experimentally challenging. In this closed system,multiple-scattering of massless Dirac fermions at boundaries is predicted toinduce quantum interference. We have successfully confined the Dirac fermionsin Bi₂Te₃by bilayer Bi（111） films, which forms a new quantum resonatorsystem in analogy to optical cavity. The quantum interference nature isidentified by both spatial local density of state maps of STM and byenergy-dependent STS. The underlying physics of surface state resonantscattering is clarified, and the origin of surface state lifetime is discussed. Wealso investigate the growth kinetics of Bi film, its doping effect to Bi₂Te₃, andthe work functions of Bi and Bi₂Te₃films. We propose a schematic energy-banddiagram, which reveals band bending effect at the Bi/Bi₂Te₃interface.2. In Sb-doped Bi₂Te₃films, we observe a strong resonance peak at the impurity center. By probing its spatial and energy dependent STS, we demonstrate the coexistence of Dirac point and resonance peak at the impurity center. Coherent scattering of surface states around impurities is analyzed in real-and q-spaces. Based on the constant energy contour of Bi2Te3, we quantitatively discuss the origin of the wave vector involved in the scattering: nesting vectors q2connected to the electron vectors from next nearest Γ-K directions. We compare three kinds of samples prepared under different growth conditions, and determine the atomic structure of the impurity in films.3.（BixSb1-x）2Te3alloy films are a new promising class of topological insulator materials, by combining the electronic and structural properties of Bi2Te3and Sb2Te3. By fine-tuning composition ratio of Bi, we are able to control the position of the Dirac point with respect to the Fermi level. In the Bi-δ-doped Sb2Te3films, we analyze the dispersion change of and the evolution of Landau levels of surface states under magnetic field, as a function of doping level. Bi dopants suppress bound states induced by native Sb vacancies and make surface states approaching linear dispersions. Quasiparticle interference patterns related to surface impurities are also observed in the same samples. It is found that they are similar to the surface state scattering behavior in Bi2Te3.更多还原