掺杂莫特绝缘体呈现出丰富的电子态相图和物理性质，受到了凝聚态物理学研究领域的重点关注。特别是，研究和澄清掺杂莫特绝缘体的电子结构被认为是理解铜氧化物等非常规高温超导体奇异性质和微观机理的基本前提。在本论文中，我们联合分子束外延（MBE）和扫描隧道显微镜（STM）技术成功制备出两类结构简单的莫特绝缘体薄膜：无限层铜氧化物和Sn/Si(111)金属吸附表面，并深入研究了它们在欠掺杂条件下的电子结构。本论文的主要研究内容和成果如下： （1）探索了共沉积氧化物MBE生长无限层铜氧化物薄膜过程中金属原子束流比、掺杂浓度和衬底温度等参数对样品表面形貌和电子结构的影响，分别成功制备了Tb3+、Sm3+和Eu3+掺杂的高质量单晶样品，并实现了大面积、欠掺杂的CuO2面终止表面。利用原位STM/S技术重点研究了欠掺杂Sr1-xTbxCuO2样品上CuO2面的Mott-Hubbard能带随体相掺杂和表面掺杂的演化规律。我们发现，镧系元素的体相掺杂导致能带系统性平移，并在莫特能隙内产生间隙态。通过研究间隙态随能量的变化规律，发现了CuO2面内电声耦合相互作用的实验证据。而碱土金属Sr原子的表面掺杂不引起费米能级的移动，但在上Hubbard能带处产生明显的电子态密度峰，并伴随谱重转移现象。这些实验结果展示了体掺杂和表面掺杂对CuO2面电子结构的影响，为理解欠掺杂莫特绝缘体中的掺杂机制提供了新线索。 （2）通过控制Sn原子在Si(111)衬底表面上的生长获得了两种Sn/Si(111)吸附表面，即莫特绝缘相Sn/Si(111)-(√3×√3)R30°（√3相）和半导体相Sn/Si(111)-(2√3×2√3)R30°（2√3相）。进一步通过改变Si(111)衬底的掺杂类型和浓度，研究了√3相和2√3相在不同调制掺杂下的电子结构及演化规律。在欠掺杂的√3相上，我们在费米能级处发现了不依赖于磁场和缺陷的赝能隙现象。通过研究不同尺寸的√3相纳米畴的空穴掺杂浓度，获得了赝能隙随掺杂浓度的演化规律，建立了类铜氧化物的电子结构相图。此外，我们在2√3相表面观测到了界面二维电子气的形成。通过研究缺陷所导致的准粒子散射和量子限域区域中的电子驻波，获得了不同界面体系中二维电子气的能带色散关系和有效质量。我们的实验结果为理解半导体表面金属吸附体系的界面电子结构和新奇物性奠定了实验基础。
Doped Mott insulators exhibit rich electronic phase diagrams and physical properties, making them a key research focus in modern condensed matter physics. Understanding the electronic structures of doped Mott insulators is considered the most direct and effective approach to elucidating unconventional high-temperature superconductivity, especially as represented by cuprates. In this dissertation, we combined the state-of-the-arts molecular beam epitaxy (MBE) and scanning tunneling microscopy (STM) techniques to successfully grow two structurally simpler Mott insulator thin-film systems: infinite-layer cuprates and metal-adsorbed Sn/Si(111) surface. We systematically studied their electronic structures in the underdoped region. The main achievements of this dissertation are as follows. (1) Using oxide MBE, we investigated the influence of growth parameters such as flux ratio, doping concentration, and substrate temperature on the surface morphology and electronic structure of the infinite-layer cuprate thin films via a co-deposition method. We successfully prepared high-quality single-crystal samples doped with Tb3+, Sm3+, and Eu3+, enabling the real-space imaging of large-area and underdoped CuO2 surfaces. Building on this basis, we employed in-situ STM/STS to mainly investigate the evolution of the Mott-Hubbard bands of the CuO2 planes with both of the bulk and surface doping in underdoped Sr1-xTbxCuO2. Our studies revealed that bulk doping induces a systematic shift in the Fermi level and a considerable number of in-gap states within the Mott gap. Moreover, the experimental evidence of electron-phonon interactions were revealed by studying the energy-dependent in-gap states. Conversely, surface doping with alkaline earth metal Sr atoms does not modify the position of the Fermi level but induces a prominent density of states peak at the border of the upper Hubbard band, accompanied by some spectral weight transfer from higher energies. This study marks the first comparative analysis of the effects of bulk and surface doping on the electronic structure of the CuO2 plane, providing new insights into the doping mechanisms and electronic behaviors of the underdoped Mott insulators. (2) By controlling the number of Sn adatoms during MBE growth, we controllably obtained two distinct Sn/Si(111) reconstructed surfaces, namely the Mott insulator phase of Sn/Si(111)-(√3×√3)R30° and the semiconductor phase of Sn/Si(111)-(2√3×2√3)R30°. By adjusting the doping polarity and concentration of the underlying Si(111) substrate, we effectively controlled the doping levels of the two surface phases of Sn/Si(111) by using the modulation doping scheme. We then studied the evolution of the electronic structure of both the √3 phase and the 2√3 phase with the doping levels. Notably, in the underdoped region, we observed the emergence of a pseudogap in the √3 phase, independent of magnetic fields and intrinsic defects. By exploring the dependence of the pseudogap in various nano-sized domains of the √3 phase with varying doping concentrations, we established the electronic phase diagram in the √3 phase, which prominently resembles that of underdoped cuprates. Additionally, we discovered interface-induced two-dimensional electron gases in the 2√3 phase. By exploring quasiparticle interferences around the intrinsic defects and standing waves in quantum confined 2√3 domains, we extracted the band dispersions and effective masses in different heterointerface systems. Our experimental findings lay the groundwork for understanding the electronic structure and novel physical properties exhibited by the metal-adsorbed semiconductor surface.
