الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 126 |  |  |
| | | from | 126 | | |
Abstract
Over the past decade, quantum materials have been one of the most exciting topics in condensed matter physics. These quantum materials have a strong electronic correlation system. The field of quantum materials encompasses a wide range of topics, but one common theme is identifying and investigating materials whose properties cannot be explained by traditional condensed matter theories. As the name suggests, the term quantum material simply means a material that does not follow the laws of classical physics, for example, superconductors, multiferroics, lower dimension systems, and topological materials. Many novel technologies can be developed thanks to these materials, including quantum computers, improved optical sensors, and levitating trains. In many classes of quantum materials, there are newly formed particles called quasiparticles whose properties can differ from the properties of the underlying electrons. At the microscopic level, the four fundamental degrees of freedom (charge, spin, orbit, and lattice) combine to produce complex electronic states. The electronic correlations in functional quantum materials were investigated by using angle-resolved photoemission spectroscopy (ARPES). First, the low-energy electronic structure of p-type transparent conducting oxide (TCO) CuAlO2 was investigated by ARPES. Transparent conducting oxides are an intriguing class of materials with many potential applications, including transparent electrodes for photovoltaic cells, touch panels, electrochromic mirrors, electrochromic windows, and optoelectronic devices. A fascinating phenomenon occurs when the electronic structure of these materials is examined and it is found that they behave as conductors, yet also act as optical transparent materials. This is because most electrical conductors are primarily opaque, while most optically transparent solids serve as insulators. Copper-based delafossite oxides are excellent candidates for the p-type TCO, which is essential in realizing transparent semiconductor applications. It was found that the band structure near the top of the valence band was characterized by hole bands with their maxima along the Brillouin zone boundary. Furthermore, the effective masses along the Γ–M and Γ–K directions were found to be (0.6 ± 0.1) m0 and (0.9 ± 0.1) m0, respectively, which impose an important benchmark against the existing band calculations. XII Second, the ARPES was combined with the scanning tunneling microscopy (STM), and the scanning tunneling spectroscopy (STS) to demonstrate the emergence of a unique insulating 2 × 1 dimer ground state in monolayer (ML) transition metal dichalcogenide (TMD) 1T-IrTe2. A fascinating class of materials, two-dimensional (2D) TMDs have numerous potential applications such as nanoelectronics, photonics, sensing, energy storage, and optoelectronics. It is interesting when you achieve quantum materials by lowering the size of a system. Therefore, the 2D system can exhibit novel phenomena, unlike its three-dimensional (3D) counterpart. Nonetheless, the mechanisms by which such phenomena might be realized, and their experimental evidence remain unknown. As a result, the most critical question is how to learn strong electronic correlations in the 2D system and manipulate the fundamental physical properties. Monolayers of 2D van der Waals materials exhibit novel electronic phases distinct from their bulk due to the symmetry breaking and reduced screening in the absence of the interlayer coupling. ML 1T-IrTe2 has a large bandgap in contrast to the metallic bilayer-to-bulk forms of this material. First-principles calculations reveal that phonon and charge instabilities as well as local bond formation collectively enhance and stabilize a charge-ordered ground state. These findings provide novel insights into the subtle balance of interactions having similar energy scales that occurs in the absence of strong interlayer coupling, which offers new opportunities for engineering the properties of 2D monolayers. In this dissertation, we use ARPES to reveal the momentum-resolved electronic structure of TCO (CuAlO2). The results provide an essential benchmark for the available theoretical calculations and future studies on the electronic properties of CuAlO2. In addition, we describe the successful growth of ML TMD (IrTe2) by molecular beam epitaxy as well as its electronic and atomic structures. The combined ARPES, STM/STS, and first- principles study of ML 1T-IrTe2 has revealed a 2 × 1 dimer structure in the monolayer with a band gap larger than 1 eV, which establishes it as a unique platform to investigate the charge order in layered 2D materials.