Emergent physics of surfaces and interfaces : an exploration of thin films using MBE and ARPES

The goal of this thesis is to provide insight and motivation for the study of condensed matter physics, particularly the physics which emerges at the surfaces and interfaces of materials. Theoretical understanding of these materials can be daunting, thanks to the many body problem in solids, which is introduced in Chapter 1. High temperature superconductivity is one such phenomena which can arise to do these many body interactions and is one of the biggest open problems in physics. A brief introduction to superconductivity is also provided in Chapter 1. Two techniques, particularly when paired together, provide many opportunities of studying and developing new material systems; they are molecular beam epitaxy (MBE) and angle-resolved photoemission spectroscopy (ARPES). ARPES is used for detailed electronic property measurement particularly for surfaces of materials and is introduced in Chapter 2. MBE is a technique for thin film growth and is described in Chapter 3. Together, these two techniques are central to all of the work presented throughout this thesis. The next three chapters will focus on the application of these tools two three different material systems, TiSe$_2$, SrTiO$_3$ and FeSe. In Chapter 4, we discuss the surprising growth of TiSe$_2$ on TiO$_2$ and study the substrate effects which arise. We compare TiSe$_2$ films grown on different substrates. In TiSe\textsubscript{2} with heavy substrate interaction, we find that the charge density wave (CDW) state found in more bulk like TiSe$_2$ samples is gone. The lack of CDW is caused by the large electron doping found in this material, due to selenium vacancies. This study provides a new methodology for growing MBE films and could pave the way for the development of more exotic heterostructures. In Chapter 5, we discuss the 2D electron gas (2DEG) found in SrTiO$_3$ (STO). Contrary to expectations and previous measurements on the system, we found that the 2DEG is associated with the SrO termination and not the TiO$_2$ termination. Growth and termination control of the STO films was done using MBE, while characterization of the 2DEG was done with ARPES. With the combination of these tools, we were able to establish the mechanism behind the formation of the STO 2DEG and help reinterpret other historical results. In Chapter 6, we begin discussion of the high temperature superconductor 1 monolayer FeSe on STO, which is the main topic for this thesis. We examine the role of the STO substrate in the the superconducting mechanism, by replacing it with another titanate, rutile TiO$_2$. We find the similar electronic properties, despite substantial differences between the two substrates. As a result, we argue that doping and interfacial electron-phonon interaction are the two key factors for superconductivity. We continue to explore the necessity of the double TiO$_2$ termination found so far in every one of the six demonstrations of FeSe grown on titanates. We replace the typical commercial substrate used for the other FeSe growths with MBE grown STO. With MBE we can precisely control the surface terminations and discover that changing the surface termination of STO between SrO, TiO$_2$ and double TiO$_2$ appears to have minimal effect on the properties of the FeSe film. Finally we discus possible origins of the replica band feature found in monolayer FeSe and show how ARPES can be used to help differentiate between them. The dissertation is then summarized and concluded in Chapter 7. Providing some insights in how MBE and ARPES were used in the works presented in this thesis and the potential uses for them in the future to help progress the study of correlated electron systems

Keywords: Applied Physics.
Advisor: Shen Zhi-Xun 
Devereaux Thomas Peter
Lee Young Sang