First-principles investigation of topological phase in rare-earth compounds

Abstract

Topological insulators (TIs) have attracted scienti c community in recent years owing to their remarkable physical properties like bulk band inversion and exotic surface states. In recent years, the search for novel topological phases have been extended to topological semi-metals (TSMs) in which extremely large magnetoresistance (XMR) materials like WTe2, PtSn4, NbP, and rare-earth monopnictides (LnPn, Ln = La, Ce, Nd, Pr, etc., Pn = Bi, Sb), etc. have attracted tremendous attention. Several reports on XMR explained its origin by perfect (or nearly perfect) electron-hole compensation and topological protection. The origin of XMR by perfect electron-hole compensation is well established by two-band model. Meanwhile, some reports on XMR infer that non-trivial topological protection by time-reversal symmetry (TRS) in TSMs suppresses the electron's backscattering in the absence of magnetic eld; while the presence of magnetic eld breaks TRS, that results into XMR e ect. However, there are several materials like LaAs, LaSb, which show XMR e ect but have a lack of topological protection, indicating that the relation between topology and XMR is not well established. It is well known that spin-orbit coupling (SOC) is the main ingredient to originate topological quantum phase transition (TQPT) in a material, and the strength of SOC can be increased via making heterostructures of suitable materials or chemical doping or by applying hydrostatic pressure or strain. In this thesis, we discussed three problems based on each of these factors to investigate the topological phase in the compounds based on rare-earth monopnictides exhibiting XMR e ect. In the rst problem, we investigated the topological properties and charge compensation ratio for a heterostructure of trivial and non-trivial XMR material, LaAs and LaBi, respectively (Chapter 3). In the second problem, we investigated the in uence of doping on the topological properties of Lanthanum monopnictides (LaX; X = As and Sb). For this, we consider three doping arrangements LaAs0:5Bi0:5, LaSb0:5Bi0:5, and LaAs0:25Sb0:25Bi0:5 (Chapter 4). In the third problem, we investigated the e ect of pressure on the topological properties of topologically trivial XMR materials, YSb and TmSb. Then, we studied electron and hole charge density ratio as a function of observed topological phase under pressure (Chapter 5). The studies provide new materials as possible candidates for non-trivial topological family and possible XMR e ect in the proposed heterostructure. It also pave a path to determine the exact correlation between topology and XMR effect.