UC Berkeley

Novel electronic phases induced by unusual electronic structures have attracted central attention in condensed matter physics communities. One of such exotic electronic structures is a dispersionless flat band. The fundamental physics and possibility of the induced phases, including high-temperature superconductivity, magnetism, and fractional quantum Hall effects, have been discussed for past decades, and such a flat band has been considered as the driver of new exotic phenomena. Hence, the system with a flat band could be fertile to many novel states of the matter. However, due to the limited number of the systems hosting the flat bands, the detail mechanisms of the flat band-induced unconventional states remain still elusive. In this dissertation, we provide the new platforms to explore what phases can be driven in the proximity of a flat band when it is coupled to a strong correlation or topological physics. We show the experimental electronic structures on two different systems, measured using angle-resolved photoemission spectroscopy (ARPES).

Chapter 1 briefly introduces the basic concept of flat band systems with examples of the unconventional states with flat bands, establishing the flat band as the tool to achieve various quantum states of matter. We also provide the motivation of each study in this dissertation. Chapter 2 introduces an overview of the primary experimental technique used in this dissertation, ARPES, a direct probe of the electronic structure. Additionally, we briefly introduce the principle of density functional theory calculation.Chapters 3, 4, and 5 describe three different studies. In Chapter 3, we build the correlation by coupling different oxides and creating the states at the interface. Unlike conventional semiconductors, oxide interface systems host strong correlations. If some unconventional phases are driven by the combination of flat band and correlation effects, this system can be the analog of cuprate high-$T_\text{c}$ superconductors. Chapters 4 and 5 are the case of topological materials. We establish a new playground where the topological physics can couple to the flat band. We investigate the bulk electronic properties of a new topological semimetal in Chapter 4. On the other hand, we focus on the surface localized states of the same material in Chapter 5. Finally, in Chapter 6, we briefly summarize the main findings and offer some concluding remarks.