Topological kagome magnets and superconductors: a view with scanning tunneling spectro-microscopy

Abstract

The field of topological physics has seen an explosion in research efforts and interest in a wide variety of the field within recent years. It began with an experimental discovery of the quantum Hall effect in 1980s and within only a few decades or so, it has expanded into a rich and wide field of study including recent discoveries of topological insulators, Weyl and Dirac semimetals, topological superconductors and so forth. In the meantime, scanning tunneling microscopy has been used as one of the most powerful instruments to study exotic phenomena in topological systems due to its unique capability to probe atomically resolved electronic quantum states. In this dissertation, we present the experimental realization on novel topological systems including several materials consisting of a kagome layer(s) which is a promising new platform to explore novel quantum many-body physics. In the first chapter, I will discuss two systems whose electronic structures are related to topological insulators; in TbMn6Sn6, we discovered a quantum-limit Chern phase, and in Bi4Br4, we demonstrated the possibility of the room temperature quantum spin Hall edge state which is also a higher-order topological insulator. In the second chapter, I will explore three compounds with intrinsic magnetic properties; in HoSbTe, we found the magnetism-driven transition of the electronic states; in CoSn, our results provided the first example of flat-band phonon coupled in electronic excitations and its strong interaction with fermionic degrees of freedom in kagome system; and in Co3Sn2S2 with In doping, we demonstrated the strong spin-orbit effect of the single-atomic impurity at the quantum level, which is of importance in quantum information science. Finally, two topological superconductor candidate systems will be discussed, KV3Sb5 and RbV3Sb5 where we discovered unconventional chiral charge order. These results featured novel quantum phenomena and phases which arise from a delicate interplay among spin-orbit coupling, band topology, many-body interactions, and uniqueness of lattice geometry. These novel topological magnets and superconductors constitute new platforms for further discovery of exotic quantum phenomena relevant to quantum information science in future experiments.