Topological insulators are electronic materials that have a conventional energy gap as an insulator or semiconductor in the bulk, but possess gapless conducting states around their boundary. They are novel topological states of quantum matters and exhibit a series of exotic physics, such as quantum spin Hall effect, single valley Dirac fermions, Majorana fermions, topological magnetoelectric effect, etc. The conducting edge and surface states have topological origin of the electron band structure, and are protected by time-reversal symmetry such that they are robust or immune against local perturbation. In this dissertation, an effective continuous model for surface states is established from the three-dimensional modified Dirac model, and a theory of ultrathin film for topological insulators is developed. The established electronic model helps us explore spin physics of massive Dirac fermions. The theory has been successfully applied to explain an energy gap opening of the surface states in Bi2Se3 thin film in the measurement of angle-resolved photoemission spectroscopy (ARPES). In-gap bound states are also considered due to vacancy and impurity in topological insulators. It is found that a vacancy can always induce in-gap bound states in both two- and threedimensional topological insulators, and a half quantum magnetic flux inside the vacancy can result in helical Dirac zero modes. Finally the effect of random impurities on the surface transport in topological insulators is investigated, particularly the weak anti-localization of surface electrons in the quantum diffusion regime. It is found that the spin-orbit scattering may suppress the weak localization behaviors of massive Dirac fermions, which suggests an experiment to detect the weak localization in the topological insulator thin film. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

In recent years, the discovery of topological insulators brought a topological classification of materials and opened a new field in condensed matter physics. Due to the nontrivial topological properties, the topological insulators have insulating bulk and metallic edge/surface relating to some exotic physics such as quantum anomalous Hall effect, quantum spin Hall effect, and magneto-electric effect. Followed realizations of the Z2 topological insulators in two and three dimensions, the quantum anomalous Hall effect was realized in the magnetic-doped topological insulators very recently, which attracts intensive interest. In this thesis, the magnetic topological insulators as a consequence of time-reversal symmetry breaking in the Z2 topological insulators in two or three dimensions are studied. As an introduction, a review of the topological insulators including some relevant theories is given. The approaches involved in this study are also presented. The results can be summarized in two parts. First, the quantum anomalous Hall effect can be found on the two-dimensional decorated lattice with spin-orbit coupling and electron-electron interaction. Without interaction, this model exhibits the quantum spin Hall effect and has at bands in the middle of the spectra. A at-band ferrimagnetism which breaks the time-reversal symmetry and a charge-density wave can be induced by the electron-electron interaction. Altogether they can modulate the Chern number of the system and give rise to the quantum anomalous Hall effect. In the second part, the realization of the quantum anomalous Hall effect in magnetic-doped topological insulator thin films is investigated. With an effective Hamiltonian of the surface states of a topological insulator thin _lm, the condition of the quantum anomalous Hall effect and the behavior of the longitudinal and Hall conductivity is given, which agrees with the experimental results. The effects of the structural inversion asymmetry potential and the particle-hole symmetry breaking term are studied. With a thin _lm model of the three-dimensional topological insulator, it is shown that the lateral surface states account for the non-quantized value of the Hall conductance and the nonzero longitudinal conductance. The quantized Hall conductance restores when the lateral surface state electrons are thoroughly localized by disorder. The quantum anomalous Hall phase in magnetic topological insulator thin film in the present of disorder is also studied. The disorder will shrink the regime of the quantum anomalous Hall effect in a thick film and becomes an obstacle to the realization of the quantum anomalous Hall effect. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

No description available.

The Dark Matter Experiment with Liquid Argon Using Pulse Shape Discrim- ination (DEAP) is a dark matter experiment based in the SNOLAB facility in Sudbury, Ontario. Its aim is to detect WIMPs, Weakly Interacting Massive Par- ticles, that may make up the missing component of the matter in our universe by the scintillation of liquid argon from nuclear recoils. A 7 kg prototype, DEAP-1, is currently in operation with work underway to scale up to a 1 tonne detector, DEAP-3600, by 2012. For DEAP-3600 to be a competitive dark matter search, a limit of 0.2 fiducial surface alpha events is required in the energy region of interest for three years of run time, or 150 total surface events. Of particular concern to the DEAP experiment is 222Rn and its daughter prod- ucts, as the alpha decays of these isotopes may create events in the detector that mimic a WIMP signature. The first half of this thesis concerns the testing and suc- cessful use of an activated carbon trap to eliminate 222Rn from the argon gas source in DEAP-1. The Carbo-Act F2/F3 grain activated carbon brand was tested as a potential ultra-low activity candidate for a DEAP-3600 filtration system and was found to have an upper limit for its 222Rn emanation rate of 284 atoms/day/kg. A temperature swing system is proposed. If operated at 110 K, an upper limit of five atoms of 222Rn can be expected to enter the detector from the trap. An indirect relationship between the number of low energy nuclear recoil events in the DEAP WIMP region of interest and the number of radon alpha decays was found. The ratio between the low energy events in the WIMP region of interest and the high energy alphas was found to be 0.18 ± 0.03 in the detector. From this, the upper limit of the contribution from the proposed radon trap to the WIMP background in DEAP-3600 will be be ten events for three years of run time, which is within acceptable limits. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2011-01-10 10:27:19.303

Kohn-Sham density functional theory (DFT) is a popular method used to investigate the properties of matter. Although exact in principle, DFT is in practice limited by a single approximation for the exchange-correlation functional - the quantity that describes the many-body interactions between electrons. This thesis is concerned with developing improved exchange-correlation functionals for use in practical DFT calculations. The standard functional currently used in solid state physics, and also popular in quantum chemistry, is the generalised gradient approximation (GGA), which requires only the local density, n(r), and the density gradient, ∆n(r), as input. A flexible semi-empirical GGA form, containing 15 free parameters that are fitted to near-exact molecular data is implemented within the plane-wave pseudopotential (PW-PP) Kohn-Sham scheme, to assess the possibility of employing semi-empirical GGAs in solid state applications. Self-consistent calculations performed for several bulk semiconductor properties using this GGA reveal that, overall, no improvement is attained over a conventional non-empirical GGA used in solid state physics. The remainder of the thesis focuses on a fully non-local functional known as the weighted density approximation (WDA), which utilises the global density of a system, n(r'), as input. An efficient computational algorithm is devised for use within the PW-PP formalism which enables fully self-consistent WDA calculations to be performed. Physical properties are shown to be intimately related to the particular form used for the pair-correlation function, Gwda/xc(r,r'), and by comparing with recent variational Monte Carlo (VMC) data, it is shown that the forms that provide a good description of exchange-correlation holes, nxc(r,r'), also lead to the most accurate bulk properties. For strongly inhomogeneous electron gas systems, the WDA provides close agreement with the VMC method for a variety of exchange-correlation quantities. The success of the fully non-local approach given by the WDA for other model electron gas systems studied suggests that the WDA is a very promising functional.

Thesis (Ph. D)--UCLA, 2007. / Vita. Includes bibliographical references (leaves 178-182).

Thesis (Ph.D.)--Syracuse University, 2008. / "Publication number: AAT 3333585."

Thesis (Ph.D.)--Syracuse University, 2008. / "Publication number: AAT 3323084."

Thesis (Ph. D)--Ohio State University, 2003. / Title from first page of PDF file. Document formatted into pages; contains xvi, 134 p. : ill. (some col.). Includes abstract and vita. Advisor: Arthur J. Epstein, Dept. of Physics. Includes bibliographical references (p. 128-134).

Thesis (Ph. D.)--University of Wisconsin--Madison, 1964. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.