Advanced spectroscopic probes of electron-phonon coupling in materials [electronic resource]

Beginning in 1986 with the discovery of LaBaCuO by Bednorz and Muller, the field of high temperature (high-Tc) superconductivity remains the biggest intellectual challenge in condensed matter physics, with theoretical (and sometimes experimental!) consensus being elusive for almost thirty years. The underlying problem of understanding how large numbers of interacting particles can form various ordered states is tremendously daunting. This thesis begins with a historical overview of superconductivity, beginning with experimental discoveries, followed by an introduction of some theoretical concepts. In particular we discuss some details about electron-phonon coupling. As the systems being studied become more complex, the tools needed to fabricate and study them must also increase in complexity. Pushing forward new experimental discoveries in high-Tc therefore requires advancing both materials growth and characterization. The characterization techniques we introduce in this thesis are angle resolved photoemission spectroscopy (ARPES) and resonant inelastic x-ray scattering (RIXS). We will discuss ARPES in detail, but present only a small work on RIXS in a later chapter, as a detailed treatment of the latter is beyond the scope of this thesis. Next we introduce the fabrication technique of molecular beam epitaxy (MBE), and discuss the design and implementation of an experimental chamber capable of performing in-situ ARPES studies of films grown via MBE. The capabilities of this chamber are demonstrated through growth and measurement of films from two classes of materials: topological insulators and iron-based superconductors. In topological insulators, we show the capability of using a thermal cracking chalcogenide source to grow intrinsically doped films, which may be useful for future studies and devices. Recently, it had been discovered that single-unit-cell-thick (1UC) iron selenide (FeSe) films grown on strontium titanate (STO) demonstrate a large increase in superconducting transition temperature compared to bulk iron selenide. We use our MBE-ARPES chamber to grow and study FeSe films of varying thicknesses down to 1UC. We then discuss spectroscopic signatures of cross-interfacial coupling between electrons in the 1UC iron selenide and the STO substrate. This electron-phonon coupling is unprecedented and until recently had not be resolved with such clarity in any other solid state system. It is furthermore unusual in that it can enhance superconductivity in many different channels. We calculate the enhancement of Tc in 1UC FeSe/STO due to this coupling and find good agreement with experimental results, and suggest that such coupling can be broadly used to enhance Tc in other films. The thesis concludes with some future prospects and directions for study. We make the case that MBE-ARPES, and more generally the improved fabrication and characterization allowed by it, will be important to definitively elucidate the interactions between particles that make up novel phases in the solid state. Lastly, in the appendix we discuss some properties of a new six-axis in-vacuum manipulator, as well as cover some theoretical details of the FeSe and RIXS experiments.

Keywords: Applied Physics.
Advisor: Shen Zhi-Xun primary advisor. Thesis advisor
Devereaux Thomas Peter
Hwang Harold Yoonsung