转角石墨烯体系展现出了非常丰富的新奇物性，比如关联绝缘态，超导和量子反常霍尔效应等。其背后的关键物理则源自于费米能处的平带结构，但由于样品制备以及测量方面的挑战，实验上对平带的直接观测还处于比较初步的阶段。本文通过优化转角样品的质量，并结合具有空间分辨能力的角分辨光电子能谱（NanoARPES）和原子力显微镜（AFM）这两种可提供互补信息的强大实验技术，利用转角、层数以及外加电场三种手段对转角石墨烯的电子结构进行了系统性的研究和调控，并取得了以下创新性研究成果：（1）首次直接观测到双层转角石墨烯的电子结构随转角的系统性演化。结合NanoARPES 和AFM，将动量空间中的电子结构与实空间的莫尔周期一一对应起来，首先提取了层间隧穿能量和带宽等重要参数，揭示了魔角附近的带宽最窄及关联效应最强。此外，从远带间随转角演化的谱重转移揭示了存在于转角石墨烯结构中的晶格弛豫。该工作不仅为后续的研究提供了重要的参考信息，还强调了晶格弛豫在魔角石墨烯强关联物理中的重要作用。（2）在第二魔角(0.59°) 双层转角石墨烯中观测到了晶格重构对电子结构的影响。首先是发现了其与AB 堆叠双层石墨烯能带结构的相似性，并从莫尔周期结构以及对外加电场的响应两个方面论证了这一发现。此外从能带谱重的不对称分布，揭示了第二魔角附近的独特性，这一发现为第二魔角附近进一步的研究提供了新的思路和基础。（3）首次直接观测到单双层转角石墨烯中平带随外场的双向调控作用。通过施加面外电场，对单双层转角石墨烯的平带电子结构进行了有效的调控，实现了平带色散的非对称性演化以及能带谱重的选择性增强，使其呈现出更像双层转角或双层-双层转角石墨烯的能带性质。该工作为探究单双层转角石墨烯非对称的外场调控物理提供了电子结构方面的重要信息。本文通过转角、层数、外加电场等手段对转角石墨烯平带电子结构进行了有效地调控，并从能带结构的角度揭示了晶格弛豫，晶格重构以及外场调控下的非对称演化在转角石墨烯体系中的重要性，也为更好地理解转角石墨烯中新奇物性提供了重要的信息。
Twisted graphene systems display a range of novel physical properties, including correlated insulator states, superconductivity, and quantum anomalous Hall effects. The central physics behind these novel phenomena is the flat band at the Fermi energy. However, direct experimental observations of the flat bands are still in the preliminary stage due to challenges in sample preparation and measurement. In this thesis, by optimizing the quality of the twisted samples and combining the Nano-spot angle-resolved photoemission spectroscopy (NanoARPES) and atomic force microscopes (AFM) providing complementary information, we effectively manipulated the electronic structure of twisted graphene systems through three strategies: twist angle, layer number, and external electric field. The main scientific achievements are shown as follows:1. We report the first direct observation of the systematic evolution of the electronic structure in twisted bilayer graphene with the twist angle. By integrating NanoARPES and AFM, the electronic structures in momentum space were correlated with the real-space moiré periods. This led to the extraction of key parameters such as interlayer coupling and bandwidth, revealing the narrowest bandwidth and strongest correlation effects at the magic angle. Furthermore, for the first time, spectral weight transfer between bands as the angle evolves revealed lattice relaxation within the twisted graphene structure. This work not only provides important reference information for subsequent research but also underscores the significant role of lattice relaxation in the strongly correlated physics of magic-angle twisted bilayer graphene.2. We observed the impact of lattice reconstruction near the second magic angle (0.59°) on the electronic structure of twisted bilayer graphene. First, the similarity with the band structure of AB stacked bilayer graphene is discovered, and this discovery is demonstrated from two aspects: moiré periodic structure and the response to external electric field. In addition, from the asymmetric distribution of the spectral weight, the valley symmetry breaking of the electronic structure near the second magic angle is revealed. In order to understand this symmetry breaking, we also provide two possible physical mechanisms. This work provides new ideas and foundation for further research near the second magic angle.3. The external fields’ dichotomic control effect on the flat bands in twisted monolayer-bilayer graphene was observed for the first time. We directly detected the evolution of flat bands under an external field, resulting in the selective enhancement of spectral weight and the asymmetric evolution of flat band. This makes the characteristics of the graphene more similar to either twisted bilayer or twisted double bilayer graphene. The findings provide crucial information for exploring the field tunability of twisted monolayer-bilayer graphene.This thesis investigates the manipulation of the flat band electronic structure of twisted bilayer graphene through twist angle, layer number, and external electric field. From the perspective of band structures, it reveals the importance of lattice relaxation, lattice reconstruction, and asymmetric evolution under external field in twisted graphene systems, providing valuable insights for a better understanding of the novel properties in twisted bilayer graphene.
