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Title:Transport properties of quantum dot molecules and electronic structure of graphene quantum dot qubits
Author(s):Chen, Chih-Chieh
Director of Research:Chang, Yia-Chung
Doctoral Committee Chair(s):Stone, Michael
Doctoral Committee Member(s):Wagner, Lucas K; Bezryadin, Alexey
Department / Program:Physics
Discipline:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):Electron transport
Quantum Dot
Graphene
Qubit
Abstract:Coupled Quantum dots systems, or quantum dot molecules (QDMs), have been suggested as good candidates for nanoelectronics, spintronics, thermoelectrics, and quantum computing applications. The knowledge in the transport and electronic properties of QDMs is important in making progress toward practical devices. We use many-body equation of motion method for Hubbard-Anderson model to study non-equilibrium charge and thermal transport properties of QDMs connected to metallic electrodes in the Coulomb blockade regime. An exterior algebra method is developed to construct the equation of motion computationally, taking into account all correlation functions. The quantum interference (QI) effect of triangular quantum dot molecule (TQDM) resulting from electron coherent tunneling between quantum dots is revealed. The spectra of electrical conductance of TQDM with charge filling from one to six electrons clearly depict the many-body and topological effects. The calculated charge stability diagram for conductance and total occupation numbers match well with the recent experimental measurements. We also demonstrate that the destructive QI effect on the tunneling current of TQDM is robust with respect to temperature variation, making the single electron QI transistor feasible at higher temperatures. The thermoelectric properties of QDMs with high figure of merit are also illustrated. Graphene nanoribbon quantum dot qubits have been proposed as promising candidates for quantum computing applications to overcome the spin-decoherence problems associated with GaAs quantum dot qubits. We perform theoretical studies of the electronic structures of graphene nanoribbon quantum dots by solving the Dirac equation with appropriate boundary conditions. We then evaluate the exchange splitting based on an unrestricted Hartree-Fock method for the Dirac particles. The electronic wave function and long-range exchange coupling due to the Klein tunneling and the Coulomb interaction are calculated for various gate configurations. It is found that the exchange coupling between qubits can be significantly enhanced by the Klein tunneling effect. The implications of our results for practical qubit construction and operation are discussed.
Issue Date:2015-08-25
Type:Thesis
URI:http://hdl.handle.net/2142/88937
Rights Information:Copyright 2015 Chih-Chieh Chen
Date Available in IDEALS:2016-03-02
Date Deposited:2015-12


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