Because of the unique electronic properties, intriguing novel phenomena, and potentiality in quantum device applications, the quantum materials with non-trivial band structures have enticed a bulk of research works over the last two decades. The experimental discovery of the three-dimensional topological insulators (TIs) - bulk insulators with surface conduction via spin-polarized electrons - kicked off the flurry of research interests towards such materials, which resulted in the experimental discovery of new topological phases of matter beyond TIs. The topological semimetallic phase in Dirac, Weyl, and nodal-line semimetals is an example, where the classification depends on the dimensionality, degeneracy, and symmetry protection of the bulk band touching. The field of topology has extended to the materials that possess non-trivial topological states at/along lower-dimensional regions of the crystals as well. A class of such materials is the higher-order topological insulator in which both bulk and surface are insulating, but symmetry-protected conducting channels can appear along the hinges or corners of the crystal. Recently, significant focus has been given to the study of the interplay among various physical parameters such as topology, geometry, magnetism, and electronic correlation. Kagome systems have emerged as fertile ground to study the interaction among such parameters in a material class. Charge density wave (CDW) order in quantum materials remains an important topic of study given its co-existence or competence with superconductivity and magnetic ordering. In this dissertation, we study the electronic structure of quantum material systems beyond TIs, particularly the lanthanide element-based and correlated systems, by utilizing state-of-art angle-resolved photoemission spectroscopy with collaborative support from first-principles calculations and transport and magnetic measurements. The lanthanide-based materials are interesting because of the possible magnetic ordering and electron correlations that the lanthanide 4f electrons may bring into the table. Our work on the Europium-based antiferromagnetic material EuIn2As2 highlights this material as a promising ground to study the interplay of different kinds of topological orders including higher-order topology with magnetism. Temperature-dependent measurements reveal a band splitting near the Fermi level below the antiferromagnetic transition. Another study on the samarium- and neodymium-based materials SmSbTe and NdSbTe shows the presence of multiple nodal lines that remain gapless even in the presence of spin-orbit coupling. We also studied a van der Waals kagome semiconductor Nb3I8, where we observed flat and weakly dispersing bands in its electronic structure. These bands are observed to be sensitive to light polarization and originate from the breathing kagome plane of niobium atoms. Next, our study in Gadolinium-based van der Waals material GdTe3 shows the presence of a momentum-dependent CDW gap and the presence of antiferromagnetic ordering that could prove important to study the interaction of CDW and magnetic orders in this material. Overall, the works under this dissertation reveal the electronic properties in correlated systems that range from insulator to metals/semimetals and from topological insulator to topological semimetals, kagome semiconductor, and CDW material.
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Doctor of Philosophy (Ph.D.)
College of Sciences
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Regmi, Sabin, "Observation of Novel Phases of Quantum Matter Beyond Topological Insulator" (2022). Electronic Theses and Dissertations, 2020-. 1718.