Physics - Theses, Dissertations, and other Required Graduate Degree Essays

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Epitaxial electrodeposition of Fe onto GaAs nanowires.

Author: 
Date created: 
2015-01-07
Abstract: 

Fe contacts with thickness ranging from 50 nanometers to 100 nanometers were selectively fabricated onto Au catalysed, Te-doped n-GaAs (111) nanowires via galvanostatic electrodeposition. An insulating polymer (SU-8) was used to fill in between the nanowires preventing deposition directly onto the GaAs substrate. Scanning transmission electron microscopy investigations combined with energy-dispersive X-ray analysis verified the existence of single-crystalline, epitaxial Fe/GaAs (110) on the sidewalls of typical nanowires. Electrical barrier heights (0.53 ± 0.02 eV) and ideality factors (1.7 to 1.9) were obtained from current-voltage measurements of individual nanowires in a scanning electron microscope.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Karen Kavanagh
Department: 
Science:
Thesis type: 
(Thesis) M.Sc.

Optimizing the Refractive Index Sensitivity of Extraordinary Optical Transmission Based Sensors

Author: 
Date created: 
2014-12-08
Abstract: 

Extraordinary optical transmission involves the resonant coupling of incident electromagnetic radiation with surface charge density oscillations called surface plasmons. The resulting propagating surface modes are known as surface plasmon polaritons. In extraordinary optical transmission the coupling mechanism is a periodic array of sub-wavelength holes perforating a thin metallic film. As light shines on the metal surface energy accumulates within the surface modes, tunnels through the holes, and is re-scattered into the far field on the opposite side of the film. The resonance condition depends intimately on the profile of the metal film, geometry of the hole array, and the optical properties of the metal film and adjacent dielectric. These surface modes are evanescently constrained to the metal-dielectric interface, and therefore make excellent probes of the local refractive index. This thesis describes a series of studies aimed at optimizing the refractive index response of nanohole arrays in thin gold films. I designed and optimized these sensors to detect the refractive index changes caused by antibodies secreted by live, microfluidically-trapped immune cells, binding to functionalized arrays. In calibration studies, the minimum detectable concentration of antibody in cell growth medium was 3 ± 1 μg/ml. In live cell studies, I was able to detect antibodies secreted from 200 trapped cells, detecting a peak shift of 10 nm above that of a control sample. Detection from lower numbers of cells was unreliable, likely due to competing reactions from non-specific binding. In the quest to improve the device sensitivity, the influence of the array geometry and the role of the nanohole array lattice on the transmission spectrum of square and hexagonal arrays were clarified. With these insights and improvements, the minimum bulk refractive index resolution (glucose solutions) increased from 2.5 ± 0.3 x10-3 to 1.5 ± 0.1 x10-3 units, ultimately limited by the optical system and the 1 nm resolution of the spectrometer used in the measurements. A superior data analysis technique based on an integrated response analysis of the entire transmission spectrum was introduced. Finally, I demonstrated a process to recycle the delicate nanohole arrays without destroying their physical and optical properties.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Karen Kavanagh
Department: 
Science:
Thesis type: 
(Thesis) Ph.D.

2H-NMR Study of Model Membranes: Investigating the Effect of Sterol Structure on the Chain Ordering and Phase Behavior of Lipid Mixtures

Author: 
Date created: 
2014-11-18
Abstract: 

We employed deuterium nuclear magnetic resonance spectroscopy (2H-NMR) to investigate the effect of sterol structure on lipid membrane organization. Cholesterol is the major sterol component of mammalian cell plasma membranes. It strongly affects the properties of phospholipid membranes. For example, incorporating cholesterol in liquid crystalline membranes increases lipid acyl chain order, and induces the liquid ordered phase which is considered to have biological importance. We first measured the chain ordering in pure bilayers of 1-cholesterylhemisuccinoyl-2-palmitoyl(d31)-sn-glycero-3-phosphocholine (CholPC), a sterol-modified phospholipid with a cholesterol moiety covalently attached to the phospholipid glycerol backbone in place of one of the lipid acyl chains. We then compared CholPC’s chain ordering with that of 1-palmitoyl-2-palmitoyl-d31-sn-glycero-3-phosphocholine (DPPC-d31)/cholesterol and found that constrainded cholesterol’s ability to order adjacent acyl chains is greatly reduced. Several sterols, broadly similar in structure to cholesterol but with specific chemical modifications, are prevalent in plant or fungal cell plasma membranes. We used 2H-NMR to study the influence of sterol structure on its effectiveness in modifying the acyl chain order of a 1-palmitoyl(d31)-2-oleoyl-sn-glycero-3-phosphocholine (POPC-d31) membrane. Spectra of POPC-d31 multilamellar vesicles containing campesterol, β-sitosterol, brassicasterol or stigmasterol were taken at 25oC for sterol concentrations up to 45 mol% and compared to previous observations obtained using cholesterol, 7-dehydrocholesterol (7-DHC) or ergosterol. Among the sterol structural modifications we compared, the C22 double bond reduced the sterol’s ordering ability the most, followed by a C24 ethyl or methyl substituent. Finally we used 2H-NMR to study the effect of sterol structure on the propensity of sterols to induce phase separation in equimolar DPPC/POPC/sterol membranes containing 7-DHC, brassicasterol or stigmasterol. The results were compared to previous observations obtained for membranes containing cholesterol or ergosterol, which highlighted the significance of sterol structure on phase separation promoting properties. Such comparative studies are prerequisites to establishing the underlying principles of sterol/phospholipid interactions.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Jenifer Thewalt
Martin Zuckermann
Department: 
Science:
Thesis type: 
(Thesis) Ph.D.

Evidence for Standard Model Higgs boson decays to tau lepton pairs in the ATLAS detector supported by a search in the fully hadronic final state

Author: 
Date created: 
2014-12-04
Abstract: 

This thesis presents the results of a search for Standard Model Higgs bosons decaying to tau lepton pairs, where both tau leptons decay hadronically. The search is performed using the proton-proton collision data collected in 2012 by the ATLAS detector at the Large Hadron Collider at CERN. The search is complicated by a significant source of background events from non-resonant multijet processes and by di-tau events from Z boson decays that share similar features with the Higgs signal. The multijet background is estimated from a control region in data, and the Z→ττ background is estimated from Z→μμ events in data where the muons are replaced by simulated tau leptons. Other minor backgrounds are estimated from simulation. A multivariate analysis using boosted decision trees is optimized to search for Higgs events produced via gluon-gluon fusion and vector boson fusion at a Higgs mass of 125 GeV. A cut-based analysis is also developed and presented in support of the multivariate analysis. An excess of events over the expected background is observed with a statistical significance of 2.9 standard deviations. When combined with excesses observed by searches in the fully leptonic and semi-leptonic final states, the statistical significance reaches 4.5 standard deviations. The combined excess is compatible with and provides direct evidence for the coupling of the recently discovered Higgs boson to fermions.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Dugan O'Neil
Department: 
Science:
Thesis type: 
(Thesis) Ph.D.

Towards silicon-based quantum Dits: optical study of group-V donors 75As:28Si and 121Sb:28Si

Author: 
Date created: 
2014-12-12
Abstract: 

Silicon-based quantum information processing devices show great promise, being one of the most advanced candidates for the basis of the quantum computer. The spin state of phosphorus atoms in isotopic silicon has been demonstrated to have the longest solid-state coherence times. In this thesis, I describe experiments characterizing the other group-V shallow donors arsenic and antimony in silicon, in the context of developing quantum information processing technology. A sample of isotopic silicon containing these dopants was commissioned. I report here the results of photoluminescence excitation spectroscopy, nuclear magnetic resonance (NMR) and electron paramagnetic resonance spectroscopy, and pulsed NMR experiments. I found that antimony is not a suitable candidate in this context by virtue of being unpolarizable by our technique; however, arsenic is sufficiently polarizable. I report a measured coherence time of 2.0 s for the neutral arsenic nuclear spin.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Michael Thewalt
Department: 
Science:
Thesis type: 
(Thesis) M.Sc.

Tissue-specific and trait-associated variation of human DNA methylation

Author: 
Date created: 
2014-08-14
Abstract: 

Epigenetics refers to a host of different mechanisms that regulate the expression levels of genes in an organism that do not rely on direct changes to the DNA sequence. These marks are heritable and sensitive to external signals. One epigenetic mechanism is DNA methylation, which consists on the attachment of a methyl group to a CpG dinucleotide. With the use of microarrays, DNA methylation studies typically interrogate the methylation status of hundreds of thousands of CpG sites in cohorts that range from tens to hundreds of individuals. In this thesis we identify tissue-specific and trait-specific patterns of methylation and highlight biological features associated with them. We also quantify the extent to which DNA methylation can optimally encode information about certain human traits.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Eldon Emberly
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) M.Sc.

THz measurements of semiconductor nanostructures

Date created: 
2014-09-15
Abstract: 

This thesis describes measurements made using terahertz (THz) spectroscopy applied to semiconductor nanostructures. Our samples take the form of either thin continuous films or nanowire fields. We fit the Drude model to our results on thin continuous films to obtain their carrier density and mobility, and compare these to DC transport measurements. We compare our measurements of nanowire fields to expectations based on the Bruggeman effective medium theory.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Steven Dodge
Department: 
Science:
Thesis type: 
(Thesis) M.Sc.

f(R) Gravity and Spherical Collapse

Author: 
Date created: 
2014-08-15
Abstract: 

This thesis studies f(R) model building, cosmological dynamics, Solar System tests of f(R) gravity, and spherical collapse in f(R) gravity. We apply the running coupling idea to gravity. We describe several well-known f(R) models in a simple way in terms of infrared renormalization group flow. We explore two logarithmic models, produced by the flows. These two models generate a large hierarchy between the Planck scale and the cosmological constant scale. We study the cosmological dynamics of a range of f(R) models, presenting generic features of phase-space dynamics in f(R) cosmology. New techniques to explore phase space dynamics are developed. These techniques are very general and can be applied to other similar dynamical systems. We investigate the Solar System tests of f(R) gravity. The metric is rederived by directly focusing on the equations of motion. The chameleon mechanism in the Jordan frame is considered. These approaches provide a more intuitive understanding of the Solar System tests of f(R) gravity. We explore spherical scalar collapse in f(R) gravity numerically. We study the dynamics throughout the collapse. Mesh refinement and asymptotic analysis are implemented in the vicinity of the singularity of the formed black hole. The Kasner solution for spherical scalar collapse in f(R) gravity is obtained. These results support the Belinskii-Khalatnikov-Lifshitz conjecture well in the context of black hole physics.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Andrei Frolov
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) Ph.D.

Optical NMR Study of 31P Donor Spins in Isotopically Enriched 28Si

Date created: 
2014-07-14
Abstract: 

A quantum computer requires a quantum system that is isolated from its environment, but can be integrated into devices, and whose states can be measured with high accuracy. Nuclear spin qubits using shallow neutral donors in semiconductors have been studied extensively as quantum memories due to their promise of long coherence lifetimes. However, the nuclear spins of neutral donors are not only difficult to initialize into known states and detect with high sensitivity, they are limited to use at cryogenic temperatures. The nuclear spins of ionized donors, on the other hand, have the potential for high-temperature operation. In this thesis, I show how the distinctive optical properties of enriched 28Si enable the use of hyperfine-resolved optical transitions of donor bound excitons, as previously applied to great effect for isolated atoms and ions in vacuum. Together with efficient Auger photoionization, these optical transitions permit rapid nuclear hyperpolarization and electrical spin readout. These techniques are combined to detect nuclear magnetic resonance from dilute 31P in an isotopically purified 28Si sample, at concentrations inaccessible to conventional NMR techniques. Dynamical decoupling is used to measure cryogenic coherence times of over 180 seconds and 3 hours for an ensemble of neutral and ionized 31P nuclear spins in 28Si, respectively. A room-temperature coherence time of over 39 minutes is demonstrated in the latter system, which is more than an order of magnitude longer than the previous solid-state coherence time record. I further show that a coherent spin superposition can be cycled from 4.2 Kelvin to room temperature and back.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Mike Thewalt
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) Ph.D.

Evidence for the Standard Model Higgs Boson Decaying to One Semi-hadronically Decaying Tau and One Leptonically Decaying Tau at ATLAS

Date created: 
2014-05-29
Abstract: 

This thesis reports the search for the Higgs boson decaying to a pair of tau leptons, where one tau decays leptonically and the other decays semi-hadronically. The search was conducted by the ATLAS experiment on the proton-proton collisions supplied by the LHC in 2012. It is complicated by background from a mix of known processes from the Standard Model of Particle Physics. It makes use of boosted decision trees to separate the Higgs signal from the background. The gluon-gluon fusion and the vector boson fusion production mechanisms are searched for separately. The statistical significance of the excess observed over the background-only hypothesis is 3.3 standard deviations, and reaches 4.1 standard deviations when combined with other ditau channels. The observed signal appears fully compatible with the Higgs boson of the Standard Model.

Document type: 
Thesis
Senior supervisor: 
Dugan O'Neil
Department: 
Science: Department of Physics
Thesis type: 
(Thesis) Ph.D.