Nanoscale Investigations of Monolayer Thin Films and Heavy Element Materials

Abstract

During my Ph.D, my work were divided into two parts. First, I spent a lot of time and effort using molecular beam epitaxy (MBE) to grow monolayer thin films that have novel quantum properties. Second, I used the scanning tunneling microscope (STM) to study cleavable single crystals, including materials with a strong surface Rashba effect and Kondo effect. This dissertation focuses on a subset of the work that I have done in my Ph.D. Part I is about film growth using MBE, which includes three chapters. Chapter 1 gives a general introduction to the characterization tools I used for film growth, including STM, X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), scanning transmission electron microscope (STEM), and electron energy loss spectroscopy (EELS). Chapter 2 appears in its entirety in the manuscript: Samantha O’Sullivan, Ruizhe Kang, Jules A. Gardener, Austin J. Akey, Christian E. Matt, and Jennifer E. Hoffman ”Imaging Se diffusion across the FeSe/SrTiO3 interface.” Physical Review B 105, 165407 (2022) There has been a long debate on the exact structure of the FeSe/SrTiO3 interface. Some groups reported a clean interface between the FeSe and the SrTiO3 surface while others observed an additional Se layer. In this chapter, we provided evidence aiming to put an end to this debate. Even though we didn’t observe an ordered Se layer between the film and the substrate, we discovered a significant amount of Se diffused across the monolayer FeSe/SrTiO3 interface using EELS. This work shines light for a possible factor that affect the high-temperature superconductivity at FeSe/SrTiO3 interface. Chapter 3 demonstrates my efforts in growing a monolayer honeycomb bismuth film (bismuthene) on hydrogen-passivated SiC substrates. In this chapter, I have demonstrated the importance of the hydrogen passivation of the SiC substrate and provided evidence of the air sensitivity of the bismuthene film. The second part of this thesis is about STM studies on two cleavable materials composed of heavy elements, BiTeI and UTe3. Chapter 4 is adapted from this manuscript: Ruizhe Kang, Jian-Feng Ge, Yang He, Zhihuai Zhu, Daniel T. Larson, Mohammed Saghir, Jason D. Hoffman, Geetha Balakrishnan, Jennifer E. Hoffman. ”Nanoscale variation of the Rashba energy in BiTeI.” arXiv.2402.18779 The strong spin-orbit coupling (SOC) leads to a huge Rashba effect in BiTeI. In this chapter, we observed ring-like charging states on the iodine surface of BiTeI, which could be used as a probe of the local electric field. We extracted the local Rashba energies by fitting the van Hove singularities observed in our scanning tunneling spectroscopy. We discovered that the Rashba energies have nanoscale variations, which positively correlate with the local electric field probed by the charging ring states. Chapter 5 reports the first-ever STM measurement on UTe3, where we measured Kondo resonance. In this chapter, we demonstrate how the Kondo holes affect the local Kondo resonance. We discovered that the Kondo holes in UTe3 will reduce the local q factors and shift the Kondo resonance energies towards the valance band. However, the hybridization factor Γ shows a very weak correlation with the Kondo hole locations, indicating that the Kondo holes could induce some hybridization disorder. This manuscript is in preparation.