Doctor of Philosophy (PhD)
Physics and Astronomy
This thesis develops a first-principles based approach to explore the tuning of topological properties of the tetradymite topological insulators. We begin by setting up a framework to systematically obtain the bulk and surface properties of topological insulators, treating the structural and electronic properties on an equal footing. We determine a consistent method for including the van der Waals interactions, which are responsible for the weak coupling between sets of atomic layers in this family of layered materials, which is important in obtaining accurate structural properties. We obtain close agreement with experimental values for both the bulk and surface states.
To further establish the applicability of our methodology over a wide range of parameters, we study the effects of strain on the topological nature of this family of materials. We use the c/a anisotropy ratio of lattice constants as a parameter to investigate strain, and find that the chalcogenides Bi2Se3, Sb2Se3, Bi2Te3 and Sb2Te3 display topological phase transitions from topological to trivial insulating phases as the c/a ratio is varied. We show that the transitions in each material display common features and that they are primarily driven by the competition between the Coulomb repulsion and the van der Waals attraction between the outer atomic layers of the basic layered units which comprise these materials.
The success of our methodology drives us to investigate the effects of alloying disorder in these materials. We consider supercells, including those where the atomic correlations between dopants vanish up to the third nearest neighbor shell, and establish the existence of multiple topological phase transitions as a function of doping. We investigate the electronic structure of these alloys with different compositions across the phase diagram.
These findings establish our framework as a method of choice for predictive ab-initio calculations of topological electronic materials in bulk and in heterostructures. The first steps towards using it for topological interfaces are also described in this thesis.
Shirali, Karunya Shailesh, "Tuning Properties of Topological Insulators: An Ab-initio Approach" (2022). LSU Doctoral Dissertations. 5827.