A topological insulator (TI) is a novel electronic state of quantum matter characterize by a bulk insulating bandgap and spin-polarized metallic surface states. Recently, the idea of topology protected surface states extended to semimetallic/metallic systems such as Dirac and Weyl semimetals. Unlike topological insulators where only surface states are interesting and topologically protected, Dirac and Weyl semimetals feature unusual bands in both on the surface and bulk. Dirac semimetals show photon-like linear band dispersion and exhibit a variety of exotic properties that include surface Fermi arc, large magnetoresistance, and high carrier mobility, etc. Recently, a new type of topological phase known as the topological nodal-line phase has been discovered where band touching over 1D line/loop in momentum space and requires extra symmetry protection. By utilizing angle-and time-resolved photoemission spectroscopy (ARPES) study in parallel with first-principles calculations, we reveal the presence of nodal-line in ZrSiS and study the detailed electronic structure. Our study discovers a naturally tuned nodal-line semimetal with the 2D Dirac fermion in ZrGeTe. Although nodal semimetal can originate in both time-reversal (T) or inversion symmetry broken materials but most of the experimentally discovered TNLS are nonmagnetic. Our study reveals the first magnetic nodal-line state in GdSbTe where a combination of broken T-symmetry and roto-inversion symmetry provides topological protection. Similar to the Fermi arc in Weyl semimetal, the drumhead surface state is known as the characteristic signature of TNLS. Using ARPES and quantum oscillation study, we discuss the first in-plane and clean drumhead surface state in SrAs3. Furthermore, all the above discussed topological states are observed in different materials. We discuss our ARPES and first-principles study to identify weak and strong topological insulator state along with a Dirac node arc in Hf2Te2P.
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Doctor of Philosophy (Ph.D.)
College of Sciences
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Hosen, MD Mofazzel, "Discovery of New Topological Quantum Materials by Photoemission" (2020). Electronic Theses and Dissertations, 2020-. 442.