Band structure and interface engineering of two-dimensional quantum materials
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Following the breakthrough discovery of graphene, two-dimensional (2D) materials have garnered significant research interest due to their thin, flexible nature and ease of fabrication. These materials exhibit novel properties, such as 2D magnetism and superconductivity, opening doors for next-generation electronics and spintronics. Dirac and Weyl semimetals, topological materials with gapless electronic excitations protected by topology and symmetry, possess nontrivial electron band topology leading to protected surface or edge states and novel responses to applied electric and magnetic fields. This research employs molecular beam epitaxy to experimentally synthesize 2D thin films with topologically nontrivial properties. A combination of angle-resolved photoemission spectroscopy, scanning tunneling microscopy, and first-principles calculations are then used to examine the electronic and topological properties under varying interfacial stacking or strain conditions. This work primarily focuses on two different systems: type-II Dirac superconducting semimetal PdTe2 thin films and black phosphorous pnictides α-Sb and α-Bi thin films, which host Dirac and Weyl fermion states. In the former we show the topological surface state and surface resonance properties from the bulk to two-dimensional limit and discover moiré modulated strain at the substrate interface. In the latter we explore the application of strain to induce topological phase transitions in the electronic structure in α-Sb and investigate the formation of a 2D Weyl semimetal in freestanding α-Bi from puckering of the Bi sublattices.
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Ph. D