Date of Award

December 2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Physics

First Advisor

Prasenjit N Guptasarma

Committee Members

Paul F Lyman, Michael T Weinert, Daniel F Agterberg, Carol J Hirschmugl

Keywords

Bi2Se3, Charge Density Wave, Charge order, Intercalation, Lattice order, Topological Insulators

Abstract

Bi2X3 (X=Se/Te) is a topological insulator, as well as a layered dichalcogenide. The topological properties of Bi2Se3 have gained a lot of interest over the past decade. However, as a layered chalcogenide, much of its uniqueness has not been fully discovered, e.g. hosting Charge Density Wave as reported in most other chalcogenides. With intercalation of Nb, Cu and Sr, Bi2Se3 becomes an unconventional superconductor. Together with its topological properties, A-Bi2X3 (A=Nb, Cu and Sr) have been proposed to be potential Topological superconductors. However, the mechanism of the unconventional SC in these compounds is still under discussion.

For my PhD research, I discovered charge density wave (CDW) order in self-doped Bi2Se3 and metal intercalated Bi2X3 together with superconducting transitions. Together with collaborators, I identified these phase transitions through studying and analyzing their crystal structures, electronic structures, and local nuclear environment. I further found that certain growth conditions (annealing and quenching temperatures) can help maintain the intercalation/defect phase and play as an important factor for observations of the intertwined electronic ground states. With intercalation or self-doped defects, the layered nature of Bi2X3 can easily be driven with a periodic lattice distortion. This lattice disorder can further lead to a local charge density distortion, with concomitant changes in the electronic structure. Depending on the relationship between the periodicity of the charge density and the underlying lattice constant, it can lead to I-CDW or CDW. In Cu doped Bi2Te2Se, I studied lattice and charge order due to different concentration of Cu into Bi2Te2Se. In self-doped Bi2Se3, I found CDW order with energy gap of 10meV. With Nb doped Bi2Se3, both superconductivity (SC) and CDW were found in this material. Both SC and CDW are broken symmetries at the ground state. The underlying relationship between these two states has long been under debate. Based on recent reports and my experimental observations, the fermiology seems play an important role for the electronic properties in A-Bi2X3. It is possible in doped Bi2X3 that both SC and CDW originate from the intercalation effect where the change of lattice structure symmetry leads to electronic symmetry broken. This work examines how lattice order leads to charge order in doped Bi2X3. It also discusses possible origins for the underlying electronic intertwined states and how they are all related to each other.

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