Quasiparticle and Optical Properties of Quasi-Two-Dimensional System

Since the experimental isolation of graphene in 2004, there has been tremendous interest in studying quasi-two-dimensional (quasi-2D) systems. These atomically thin materials display a number of unique properties not found in their bulk counterparts, such as large self-energy and excitonic effects due to weaker screening in 2D. However, simple dimensionality arguments alone often fail to give quantitative---and sometimes qualitative---explanation of physical phenomena in these systems. Many low-energy excitation processes in these materials involve length scales comparable to the extent of these materials along the confined direction. Thus, many of these interesting properties are a result of the interplay of the physics of 2 and 3 dimensions.;In order to predict quasiparticle and optical properties in these materials, it is therefore highly important to use methods that capture the explicit quasi-2D crystal structure and rely on as little experimental input as possible. Ab initio formalisms are well-tested, mature, and predictive methods for calculating physical properties of systems with arbitrary crystal structure and dimensionality. In particular, the ab initio GW and GW plus Bethe-Salpeter equation (BSE) approaches are reliable methods to compute quasiparticle and optical properties of materials without experimental parameters and for systems with arbitrary electronic structure and dimensionality.;In this dissertation, we study the quasiparticle and optical properties of quasi-2D systems, with emphasis on graphene and monolayer transition metal dichalcogenides. This dissertation is divided into three parts. In the first part, we introduce the formalisms that allow us to compute quasiparticle and optical properties of material. We include a brief review of the quasiparticle approximation, and connect it to Green's function methods. We then introduce the GW approximation and the BSE as tools to compute quasiparticle and optical properties of materials, respectively. We include a simplified derivation of these two formalisms in terms of many-body perturbation theory and diagrammatic series. We also review how the GW approximation and the BSE are implemented into ab initio electronic-structure codes, such as BerkeleyGW.;In the second part of the dissertation, we show our theoretical works on the quasiparticle and optical properties of quasi-2D systems. We compute the quasiparticle bandstructure, optical absorption spectrum, and excitonic series on monolayer MoS2, a prototypical quasi-2D semiconductor. We also understand the origin of novel physics in these materials, such as the presence of excitonic states that cannot be understood in terms of a 2D hydrogenic model. We understand these unique phenomena in terms of the unique features of the screening in 2D, and also show how this leads to severe challenges in applying the GW and GW-BSE approaches to system with reduced dimensionality. We then develop new methods that efficiently capture these fast variations of the screening, and reduce the computational cost of GW and GW-BSE approaches on these systems by orders of magnitude.;Finally, in the third part of the dissertation, we show a variety of projects that are collabo- rations between our theoretical group at Berkeley and various experimental groups. In the first collaboration, we perform a joint work with Prof. Tony Heinz's experimental group, wherein we demonstrate how excitonic effects on graphene can be tuned by carrier doping. Our work goes beyond the independent-particle picture, and includes, without adjustable parameters, the effect of finite quasiparticle lifetimes due to electron-electron and electron-phonon interactions on the optical absorption of graphene. The second project in this part---a collaboration with the experimental groups of Profs. Mike Crommie and Feng Wang---directly measures the exciton binding energy in MoSe2. Because these measurements are performed on a substrate of bilayer graphene, we develop a new method to include the effect of screening from the substrate into our ab initio formalism. Finally, the third joint theory-experiment work was a collaboration with Prof. Mike Crommie's group, wherein we compute the quasiparticle properties of few-layer MoSe2 and simulate the corresponding scanning-tunneling spectroscopy curves. Our work shows how the electronic structure of MoSe 2 evolves with layer number, and elucidates the role of layer hybridization, self-energy effects, and intrinsic/extrinsic screening in the quasiparticle properties of few-layer transition metal dichalcogenides.