Role of Defects in Metal Oxides for Applications in Quantum Computing and Pseudocapacitive Charge Storage

Holder, Aaron Matthew

Graduate Thesis Or Dissertation

Role of Defects in Metal Oxides for Applications in Quantum Computing and Pseudocapacitive Charge Storage Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/w0892b125

Abstract

The accurate prediction of materials properties and atomistic mechanisms is a significant challenge in condensed matter theory and computation that is made increasingly possible by ab initio methods. In this thesis we computationally investigate defects in metal oxides that are relevant to applications in quantum computing and pseudocapacitive charge storage.

We perform ab initio calculations of hydrogen-based tunneling defects in Al₂O₃ to identify deleterious two-level systems (TLS) in superconducting qubits. The formation energies of the defects are computed to give the likelihood of defect occurrence during growth. The potential energy surfaces and the corresponding dipole moments are evaluated to determine the coupling of the defects to an electric field. The tunneling energy is then computed for the hydrogen defect and the analogous deuterium defect, providing an estimate of the TLS energy and the corresponding frequency for photon absorption. We predict that hydrogenated cation vacancy defects will form a significant density of GHz frequency TLS in Al₂O₃.

Electrochemical supercapacitors utilizing pseudocapacitive materials offer the possibility of both high power density and high energy density. From first principles, we derive a detailed pseudocapacitive charge storage mechanism of MnO₂ and predict the effect of operating conditions on charge storage using a combined theoretical electrochemical and band structure analysis. We identify the importance of the band gap, work function, the point of zero charge, and the tunnel sizes of the electrode material, as well as the pH and stability window of the electrolyte in determining the charge storage viability of a given electrode material. The high capacity of α-MnO₂ results from cation induced charge-switching states in the band gap that overlap with the scanned potential allowed by the electrolyte. The charge-switching states originate from interstitial and substitutional cation defects. We calculate the equilibrium electrochemical potentials at which these states are reduced and predict the effect of the electrochemical operating conditions on their contribution to charge storage. The mechanism and theoretical approach we report is general and can identify new materials with high densities of thermodynamically accessible charge-switching states and optimal alignment of the relevant electrochemical potentials for improved pseudocapacitive charge storage.

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