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