Date of Award

Winter 12-15-2022

Author's School

Graduate School of Arts and Sciences

Author's Department

Physics

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

Since their discovery, low dimensional van der Waals materials have attracted increasing research interests. They serve as ideal platforms to study novel physics in reduced-dimensional systems, and are critical in nowadays’ nanotechnology applications. Due to the reduced dielectric screening in low dimensions, strong excited state properties dictate their electronic, transport and optical properties, the study of which calls for a description of the many-particle interactions beyond the traditional density functional theory. This is where the many-body perturbation theory comes into play. In this thesis, I will present a comprehensive study of the quasiparticle and excitonic properties of a variety of two-dimensional materials using first-principles simulations with many-body effects taken into consideration.In Chapter 3, we study the phonon assisted optical excitations in monolayer MoS2 and MoS2/WS2 heterostructures. The monolayer MoS2 was shown experimentally to possess a direct band gap, whereas its bilayer form characterizes an indirect band gap. The energy difference between the valence band maximum at Γ and K is small, motivating us to study the possibility of brightening the indirect exciton states with the assistance of acoustic phonons. Since photon cannot provide momentum in optical transitions, an elementary excitation that has finite momentum is needed to assist the brightening of a momentum dark exciton. Our results not only confirm the increased oscillator strength of originally forbidden indirect exciton states, but also help identify certain phonon branches that are capable of fulfilling this process. In Chapter 4, we turn to the study of a correlated magnetic two-dimensional material, CrCl3. Due to the localized 3d orbitals of the magnetic atom Cr, its electronic band structure features flat bands around the Fermi level. The flat bands warrant large joint density of states in the system, which is promising for strong excitonic effects. We show that the exciton binding energy in monolayer CrCl3 is much larger than typical two-dimensional materials, and we further find that the exciton binding energy in bulk CrCl3 remains large due to the unique flat bands in the magnetic material. The crystallographic structure and magnetic order dependence of the excitonic effects are also studied in bilayer CrCl3. In Chapter 5, the quasiparticle-plasmon coupling in doped moiré heterostructure MoS2/WS2 is investigated. Under electron doping, the different local stackings in a moiré system renders varied strength of quasiparticle-plasmon couplings. This in turn modifies the original moiré potential landscape and results in increased moiré potential. Since the physical properties of a moiré structure are largely determined by the moiré potential, the doped moiré system experiences much stronger correlated effects and features a set of ultra-flat minibands. A quantum phase transition between the Mott and charge-transfer insulating states at half-filling is also examined, which helps us understand the rich insulating states observed in gated transition metal dichalcogenides moiré systems in recent experiments.

Language

English (en)

Chair and Committee

Li Yang

Committee Members

Alexander Seidel, Erik Henriksen, Sheng Ran, Rohan Mishra,

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