Ab-initio tight-binding models for twisted bilayer transition metal dichalcogenides
File(s)
Author(s)
Atalar, Kemal
Type
Thesis
Abstract
Atomically thin semiconductors, such as transition metal dichalcogenides (TMDs), exhibit unique electronic and optical properties, and introducing a twist angle between two TMD layers can lead to exotic quantum phases. To navigate the complex landscape of materials and twist angles and understand the nature of these emergent states, this thesis presents the development and application of \emph{ab initio} tight-binding Hamiltonians for the study of twisted homo- and heterobilayers of MoS$_2$, MoSe$_2$, WS$_2$ and WSe$_2$.
A novel \emph{ab initio} tight-binding model for twisted bilayer TMDs is introduced, incorporating a position-dependent interlayer interaction that considers metal-chalcogen hopping and explicitly depend on the distance between layers. This model enhances the qualitative and quantitative description of moir{\'e} valence bands and extends to ten different homo- and heterobilayers. Additionally, advancements are made to include long-range intralayer hopping terms to provide a very accurate representation of the conduction band manifold.
The application of this tight-binding model reveals various moiré flat bands and investigates the effects of atomic reconstructions on flat-band properties in twisted bilayer TMDs. Insights are gained regarding the material-specific band flattening and band gap evolution as a function of twist angle, alongside an analysis of the ordering variations of moiré valence states originating from the monolayer $\Gamma$ and $K$ valleys.
Further developments extend the model to include position-dependent intralayer interactions, employing separate Wannier Hamiltonian and band structure fitting approaches. In addition to the Slater-Koster description of the hopping integrals, a machine learning model is developed to map atomic environments to Wannier on-site energies. The accuracy and transferability of both Wannier and band-fitted tight-binding models are assessed for the cases of biaxial strain, random displacements, and temperature dependent band structures.
A novel \emph{ab initio} tight-binding model for twisted bilayer TMDs is introduced, incorporating a position-dependent interlayer interaction that considers metal-chalcogen hopping and explicitly depend on the distance between layers. This model enhances the qualitative and quantitative description of moir{\'e} valence bands and extends to ten different homo- and heterobilayers. Additionally, advancements are made to include long-range intralayer hopping terms to provide a very accurate representation of the conduction band manifold.
The application of this tight-binding model reveals various moiré flat bands and investigates the effects of atomic reconstructions on flat-band properties in twisted bilayer TMDs. Insights are gained regarding the material-specific band flattening and band gap evolution as a function of twist angle, alongside an analysis of the ordering variations of moiré valence states originating from the monolayer $\Gamma$ and $K$ valleys.
Further developments extend the model to include position-dependent intralayer interactions, employing separate Wannier Hamiltonian and band structure fitting approaches. In addition to the Slater-Koster description of the hopping integrals, a machine learning model is developed to map atomic environments to Wannier on-site energies. The accuracy and transferability of both Wannier and band-fitted tight-binding models are assessed for the cases of biaxial strain, random displacements, and temperature dependent band structures.
Date Issued
2024-08-16
Date Awarded
01/03/2025
License URL
(https://creativecommons.org/licenses/by-nc/4.0/
Advisor
Mostofi, Arash
Lischner, Johannes
Sponsor
Engineering and Physical Sciences Research Council
Publisher Department
Department of Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)