ARPES and STM Investigation of Novel Topological Materials and Superconductors

Abstract

The field of topological physics has experienced widespread research interest in recent years. Beginning with the discovery that the quantum Hall effect is topological, a large range of topological materials including topological insulators, Dirac and Weyl semimetals, nodal line semimetals, and topological superconductors have been the subject of immense research, both for the fundamental physics novel to these systems and for the potential for application in technologies. Angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) are both powerful tools that have been used to probe the electronic states and excitations in quantum materials. In this dissertation, after providing an overview of the field of topological materials, we introduce a group of symmetry-protected topological semimetals, topological chiral crystals. ARPES measurements of the topological surface states in CoSi and RhSi provided the initial discovery of topological chiral semimetals. Additional ARPES studies of nickel-doped RhSi demonstrate its multi-gap topology. Next, we introduce topological superconductors and present STM measurements on candidate topological superconductor material PbTaSe2. We demonstrate the existence of states at zero energy in magnetic vortices and on iron impurities, which could potentially be Majorana zero modes. We further demonstrate that the topological surface states are robust against dilute levels of magnetic impurities, which provides additional support to the interpretation of the observed zero-energy states being Majorana zero modes. Finally, we look at the layered Kagome material FeSn with STM and show that the flat band is localized in real-space to the Kagome lattice. We then show that a magnetic field can be used to tune the flat band. All of these results describe novel phenomena that result from the interplay of topology and interactions with other effects in materials and together help pave the way towards new frontiers of topological quantum matter research.