Substrate effect for correlated chalcogenide thin films
Keywords: Physics.

The intersection between correlated electron systems and thin films provides exciting research opportunities. Quantum confinement effects, interfacial interactions and other substrate effects can be engineered to acquire desired physical properties in heterostructures. A brief introduction of the correlated electron problem and thin film research is presented in Chapter 1. To create and characterize complex thin films, precise tools for film growth and electron property measurement are needed. We provide an experimentalist's introduction of two major experimental tools in this dissertation: Angle-resolved Photoemission Spectroscopy (ARPES) for electronic property measurement (Chapter 2) and Molecular Beam Epitaxy (MBE) for thin film growth (Chapter 3). Monolayer (1ML) FeSe films on SrTiO$_3$ (STO) substrates demonstrate a large increase in electron doping and superconducting transition temperature. This is an example of how thin films and interfaces provide new opportunities in correlated electron systems. We use our MBE-ARPES chamber to grow and study FeSe films on various substrates in order to elucidate the reason behind this superconductivity enhancement. In Chapter 4, we describe the fabrication and study of 1ML FeSe/TiO$_2$ (rutile). By comparing 1ML FeSe/TiO$_2$ to 1ML FeSe/STO and other superconductors in iron chalcogenide family, we argue that doping and interfacial electron-phonon interaction are the two key factors to the enhanced superconductivity. In Chapter 5, we further study the origin of the doping effects in 1ML FeSe/titanate thin films using a versatile platform: 1ML FeSe/LaTiO$_3$ (LTO) /STO. To our surprise, although the charge density of two-dimensional electron gases (2DEGs) on LTO/STO can be increased to a few times higher than FeSe carrier density, the doping of 1ML FeSe on top of LTO/STO heterostructure does not increase. This provides strong confinements to theories that aim to explain the doping of 1ML FeSe/titanates. Our research on 1ML FeSe with TiO$_2$ and LTO/STO as substrates helps researchers understand the origin of increased superconductivity in this material system and provides insights on how to apply similar mechanisms to other materials. We switch gears and discuss the substrate effects for TiSe$_2$/TiO$_2$ heterostructure in Chapter 6. TiSe$_2$, both as bulk single crystals and as thin films on graphene, is in an ordered charge density wave (CDW) state at low temperatures. We find that hexagonal TiSe$_2$ can be grown epitaxially on tetragonal TiO$_2$ substrates, and the resulting films show large electron doping and no CDW order. Our further experiments demonstrate that the doping and suppression of CDW order is due to Se vacancies, which are created by substrate induced change in the film growth condition. This indirect substrate effect of doping may be applied to other transition metal dichalcogenide (TMDC) films on transition metal oxide (TMO) substrates as a powerful way to tune the electrical and optical properties of such heterostructures. This dissertation is summarized in Chapter 7, where we also propose possible future investigations based on this work. With quantitative understanding and precise control of thin film growth in MBE and advanced \textit{in situ} ARPES for measurements, our strength in creating and understanding correlated electron systems will be further improved