Thesis advisor: Ilija Zeljkovic / Kagome lattice is a versatile platform that can host both strongly correlated electronic phenomena and topological Bloch electrons. Correlated electronic states in kagome metals show some resemblance to those in high-temperature superconductors, such as cuprates and iron-based superconductors, where rotational and/or translational symmetries of the electronic structure are spontaneously broken. Many of the kagome materials are now also known to break time-reversal symmetry, even if spin magnetism is entirely absent. In our studies, we use scanning tunneling microscopy/spectroscopy (STM/S) to discover novel emergent phenomena in several representative families of kagome metals.In the first part of the thesis, I focus on a family of kagome superconductors AV3Sb5 (A = Cs, Rb, K). Using STM/S, we visualize a surprising C6 to C2 rotation symmetry breaking in the charge density wave (CDW) state of AV3Sb5. Moreover, we discover distinct temperature scales associated with a two-fold symmetric 2a_0×2a_0 CDW (~70+ K), a unidirectional 4a0 stripe-charge order (~50-60 K), and unidirectional coherent states in AV3Sb5 (~30-35 K). In isostructural system CsTi3Bi5 Kagome crystal, we revealed rotational symmetry breaking, or electronic nematicity, without the underlying CDW state. Our experiments shed light on a rich phase diagram hosting a variety of symmetry-breaking electronic phases in kagome metals.
In the second part of the thesis, we focus on the topological aspects of the electronic band structure of kagome metals. When electrons hop (nearly) freely in kagome lattices, spin-orbit coupling can open a topological Dirac gap at the K point and induce either quantum anomalous Hall or quantum spin Hall phases when the Fermi level is positioned in the gap. In strongly spin-orbit coupled kagome metals YMn6Sn6 and TbV6Sn6, we discovered enormous crystal momentum-dependent magnetic-field induced electronic band renormalization, which could be attributed to unusual orbital magnetization. Modern orbital magnetization theory describes that orbital magnetization comes from (spin) Berry Curvature associated with the Chern Dirac band. Using quasiparticle interference imaging, we map the Dirac band renormalization under external magnetic field and measure the values of orbital magnetic moments as a function of crystal momenta. Our experiments provide the first effort to resolve momentum-space orbital magnetic moments in a single crystal with atomic resolution. / Thesis (PhD) — Boston College, 2024. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.
| Identifer | oai:union.ndltd.org:BOSTON/oai:dlib.bc.edu:bc-ir_110023 |
| Date | January 2024 |
| Creators | Li, Hong |
| Publisher | Boston College |
| Source Sets | Boston College |
| Language | English |
| Detected Language | English |
| Type | Text, thesis |
| Format | electronic, application/pdf |
| Rights | Copyright is held by the author. This work is licensed under a Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0). |
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