A STUDY OF TOPOLOGICAL QUANTUM PHASE TRANSITION IN SOME SELECTED BINARY AND TERNARY SYSTEMS

Abstract

The modern era of condensed matter physics and materials research has been significantly shaped by the concepts of topology and symmetry. The materials with unusual metallic edge/surface whilst having insulating bulk are known as topological materials (TM) and have been studied intensively in recent years. The TMs can be divided into various categories such as topological insulators (TIs), topological crystalline insulators (TCIs), Dirac semimetals (DSMs), Weyl semimetals (WSMs), nodal line semimetals (NLSMs), ℤ2 topological semimetals, and triply degenerate node semimetals, etc. The metallic Dirac cone like electronic states on the surface of the crystal with insulating bulk are the signature of the existence of a topological phase in materials. These metallic edge/surface states appear in the presence of spin-orbit coupling (SOC), which are protected by time-reversal symmetry (TRS) and provide robust spin-polarized conduction channels. In recent times, the search for new TMs via alteration of the SOC strength has become a hot area of research. This alteration in the SOC leads to the topological quantum phase transition (TQPT) or topological phase transition (TPT) in materials. The SOC strength can be tuned via hydrostatic pressure or strain, which is essentially a non- destructive method and does not disturb the charge neutrality and stoichiometry of the material. In this thesis, we investigate the TPT under the effect of applied hydrostatic pressure and strain, and identify the novel topological phases in some binary rare-earth semimetals, ternary Zintl, and chalcogenide families. The brief description of the work performed in this thesis is as follows; Chapter 1 starts with a brief introduction, the origin of the topological phase of matter, as well as its different categories and their signatures for the identification of xvi topological phase transitions in materials. Further, it also includes an overview of the literature, motivation for this research work, and the objectives of the thesis work. Chapter 2 describes the methodology used to analyze the topological phase of the matter in different binary and ternary systems. The chapter describes the mathematical background of the density functional theory (DFT), the Wannierization method, and the Green’s function approach used in this work. Chapter 3 includes the study of topological phase transitions in rare-earth semimetallic materials such as YX (X = As, Bi), YbAs, and their heterostructure stackings with applied hydrostatic pressures and epitaxial strains. We analysed the electronic, structural stability, and topological phase transitions under the effect of hydrostatic pressure and epitaxial strain in these materials. Chapter 4 presents the study of the electronic, structural, and topological phase in experimentally synthesized intermetallic Zintl compounds RbZn4X3 (X = P, As) under the effect of hydrostatic pressure and epitaxial strain. The existence of topological surface states and the Fermi arc in the (001) plane verified the topologically non-trivial nature of these materials. Chapter 5 has been drafted with the investigation of the TPT from the trivial to TI phase to TCI phase in the Sn-based ternary chalcogenides family PbSnX2 (X = S, Se, Te) under the hydrostatic pressure. We have analysed the structural, electronic, as well as different TPTs have been analysed with an accurately identified exchange correlation functional. Chapter 6 concludes the topological phase study of the phase change chalcogenide materials Ge2Sb2Te5 and Si2Sb2Te5 in Kooi and De Hosson, as well as the Petrov sequence, under the effect of applied hydrostatic pressure and epitaxial strain. The observed surface xvii Dirac cones along the (001) plane have also verified the topologically non-trivial nature of these materials. Chapter 7 summarizes the relevant conclusions based on the results obtained in the previous chapters and outlines the future scope of the work.