PROGRAM | Materials Science & Engineering

By: Sivakumar Vishnuvardhan Mambakkam Chair: Stephanie Law

ABSTRACT

Quantum computers offer a promising development in computing technology due to the unique way in which information is stored, known as the quantum bit or qubit. Qubits can take the form of superpositions of discrete electronic energy states in a material. Encoding information in this way can enable certain operations to be performed on a much faster time scale than what could be achieved by a normal computer. What prevents quantum computers from being realized on a wider scale is an issue referred to as decoherence. This is the change in the superposition state of the qubit electron due to interactions with its surroundings. This can irreversibly change the state, resulting in the loss of information. Currently, the most common method for mitigating this issue involves using cryogenics to cool the system, and surrounding the system with large amounts of cladding and noise-isolation.

One potential route to solving this issue of decoherence is through the use of topological insulators or TIs. TIs are a class of materials which feature linearly dispersed, spin-momentum locked electronic states which are delocalized on the outer surface of the material. Spin-momentum locking in these TI surface states provides the electrons occupying them with a form of scattering resistance, as they require both a change in momentum and a spin-flip before the electron will scatter into a degenerate state. In theory, if a system of energetically separated TI surface states was created, an electron could be supported in a superposition state that would retain the scattering resistance, and thus serve as a robust quantum bit. To explore this possibility, we attempted to use quantum confinement in the topological insulator Bi₂Se₃ to energetically separate the surface states by creating Bi₂Se₃ nanoparticles.

In this thesis, we focus on answering two main questions: 1) How can we create Bi₂Se₃ nanoparticles with controllable dimensions, and 2) Can we observe evidence of energetically discrete TI surface states in these nanoparticles? Over the course of this project, we were able to demonstrate the creation of Bi₂Se₃ nanoparticles via both top-down and bottom-up approaches and examined the relative advantages and disadvantages of each approach. We then applied various spectroscopic techniques to probe the electronic band structure of these particles. In our study, we were unable to find definitive signatures of the surface states but did obtain valuable insights into the materials and characterization challenges involved in this type of measurement. Lastly, we evaluated the potential for “healing” damage along the sidewalls of etched TI nanostructures via post-fabrication annealing. We found that this method was ineffective and discussed what future work may aid in addressing etch-induced damage in TI nanomaterials.

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