Exploring Topological Semimetallic Phases of Matter through First-Principles Calculations

Abstract

In recent years, the study of topological phases of matter have received immense interest. Following the discovery of topological insulators, topological band theory has been extended to topological semimetals, characterized by gapless electronic phases with topologically stable band crossings. Low-energy excitations around these gapless points typically obey the Dirac or Weyl equations, and are referred to as Dirac or Weyl fermions. In solid-state physics, crystal symmetries can give rise to novel quasi-particles beyond the conventional ones. In the first part of this thesis, we demonstrate that strain engineering can generate multiple Weyl fermions from unconventional multifold fermions, using first-principles calculations and low energy model Hamiltonian. Considering transition metal silicide CoSi, we show that bi-axial strain influences the distribution of topological charges in momentum space. We next investigate the manipulation of a magnetic Weyl semimetallic phase in the trivial magnetic insulator MnIn2T4 through hydrostatic pressure. Our first-principles calculations reveal that pressure effectively tunes the number of Weyl points and the anomalous Hall conductivity (AHC) near the Fermi level. Further, we propose intrinsic magneticWeyl points near the Fermi energy, in chromiumtelluride- based system Cr3Te4, highlighting the emergence of extended non-trivial Fermi arcs. Our study also demonstrates a significant value of unconventional AHC, owing to the low symmetry of the system, alongside a large conventional AHC, in these materials. In the last part of the thesis, we examine the structural dependence on the flat bands, i.e., bands with vanishingly small dispersion, in three-dimensional coupled kagome systems. We propose an analytical scheme to derive conditions for the coexistence of flat bands and Dirac fermions in these systems. To complement our analytical scheme, we analyze materials from the M3X (M = Ni, Mn, Co, Fe; X = Al, Ga, In, Sn, Cr,. . . ) family and identify a key factor related to atomic separations that significantly affects the flat band width. This facilitates the prediction and tuning of flat bands using external parameters, such as strain or pressure. Overall, this thesis provides a comprehensive study for understanding and predicting topological phases in material systems