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

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Physics of Birefringent Fermions

Date created: 
2012-12-07
Abstract: 

There has been much recent interest in Dirac fermions due to their physical realizationas low energy excitations in graphene. In this thesis we introduce birefringent relativisticfermions, for which the chiral symmetry usually present for Dirac fermions is broken, andthere can be more than one Fermi velocity. We first introduce a lattice model of spinlessfermions that can arise from a scheme to introduce an artificial magnetic field for coldatoms. This model has an unusual Hofstadter-like spectrum as a function of the flux perplaquette. When there is an average of half a flux quantum per plaquette, the model hasDirac points in its spectrum and exhibits low energy excitations with two different “speedsof light”, i.e. birefringent fermions. We investigate the effects of several perturbations onthe spectrum such as staggered potentials and topological defects and we study the orderedphases that can arise from interactions. We find that sufficiently strong nearest neighbourinteractions lead to a charge density wave phase but that next-nearest neighbour interactionsallow the possibility of other phases. We also study the response of birefringent fermionsto a magnetic field and discuss how both Landau levels and the Integer Quantum Hall effectfor regular Dirac fermions are modified for birefringent fermions.

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

Terahertz Conductivity Measurements of MnSi

Author: 
Date created: 
2013-12-18
Abstract: 

This thesis presents measurements of the dynamical conductivity of a MnSi lm via terahertz time domain spectroscopy. We determine the Drude scattering rate and plasma frequency at low temperatures, and compare these to theoretical predictions. From a comparison of the plasma frequency measurement with band theory, we determine a mass renormalization of m*/m~5.5. Above the critical temperature, ts to the Drude model yield negative values for the scattering lifetime, indicating the existence of a pseudogap. At low temperatures and low frequencies, the resistivity has the well-known Fermi liquid form with the ratio of the temperature dependent term to the frequency dependent term, b>1. At the lowest temperature we estimate b ~ 4, with a large systematic uncertainty that we characterize for later improvement. This result is consistent with Fermi liquid theory predictions for electron-electron scattering.

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

Lifetime Measurement of the 6.79 MeV Excited State of 15O to Help Constrain the 14N(p,gamma)15O Reaction Rate

Author: 
Date created: 
2013-12-11
Abstract: 

In main sequence stars such as our Sun, the source of energy comes from converting hydrogen into helium. There are two competing mechanisms via which this can happen: the pp chain and CNO cycle. The latter is a cycle of reactions involving carbon, nitrogen and oxygen which are catalysts for the conversion of hydrogen into helium. The slowest reaction 14N(p,γ)15O in the cycle will affect the energy generation timescale and the amount of helium ash produced via the CNO cycle. This has several astrophysical impacts. It affects the evolutionary timescale of main sequence stars from which the ages of globular clusters can be calculated, the nucleosynthesis of heavier elements in H burning shells of red giant stars, and the fraction of energy produced by the CNO cycle compared to the pp chain in our Sun which helps determine the interior composition of the Sun. For main sequence stars the CNO cycle dominates over the pp chain for core temperatures T < 0.02 GK. For the 14N(p,γ)15O reaction this corresponds to a low center of mass energy Ecm = 30 keV. This is lower than the low energy limit of the reaction rate measurable in the laboratory. This means that we need to extrapolate down to low energy using theory. The largest remaining uncertainty in the theoretical calculations is due to the lifetime τ of the 6.79 MeV state of 15O. In this work the lifetimes of three excited states of 15O were measured using the Doppler shift attenuation method (DSAM) populating the states via the 3He(16O,α)15O reaction at a beam energy of 50 MeV. The low lifetime limit measurable using the DSAM is ∼1 fs. The lifetime of the 6.79 MeV state is near that limit, making this measurement challenging. A 1.8 fs upper limit (68.3% C.L.) on this lifetime is reported here. In addition we measured the lifetimes of the 6.17 and 6.86 MeV state in 15O which were < 2.5 fs and 13.3+0.8−1.2 fs (68.3% C.L.) respectively.

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

Theory of the electronic and magnetic structure and transport properties of single molecule magnets

Date created: 
2013-10-23
Abstract: 

Developing electronic components at the molecular scale is the ultimate goal in molecular electronics. Because of their large magnetic anisotropy barriers and associated stable magnetic moments, single molecule magnets (SMMs) bring a new dimension to this field and also raise the possibility of molecular magnetic information storage and quantum computation. Therefore the transport properties of transistors based on individual SMMs are attracting considerable experimental and theoretical interest at present. This thesis presents a theoretical investigation of the electron and spin transport and associated phenomena in SMM transistors (SMMTs). A tight binding model is developed as an alternative approach to the giant spin Hamiltonian and density functional theory (DFT) methodologies for studying SMMs. Unique aspects of this approach are that it captures more physics than the giant spin Hamiltonians do but is much simpler than DFT and it has more flexibility for modeling experimental behaviors. Because of its simplicity this model is helpful in developing a physical understanding of SMMs and their transport properties. The tight binding model yields the total Mn12 SMM spin, the spins of the individual Mn ions, the magnetic easy axis orientation, the size of the magnetic anisotropy barrier and the size of the HOMO-LUMO gap consistent with experiments. Based on this tight binding methodology, this thesis addresses the following transport problems of current interest: ligand-based transport resonances in SMMTs, gate controlled switching between Coulomb blockade and coherent resonant tunneling in SMMTs, identification of the orientation of the magnetic easy axis of a SMM, the spin filtering effect of the SMMTs, quantum dot spin valves based on SMMs which support ligand--based transport, and tunneling and cotunneling transport through Mn12 SMMs in the weak coupling regime.

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

Structural Study of Anion Exchange Polymer Electrolyte Membranes, mes-PBI, mes-PDMBI and the Blends Using X-ray Scattering Techniques and Molecular Dynamics Simulations

Date created: 
2014-01-08
Abstract: 

In this work, I studied the structure of novel anion exchange polymer membranes (AEMs) using X-ray scattering techniques combined with molecular simulations as a complementary method. I simulated a monomer and a chain of each sample consisting of 3 monomers using ChemSketch. Good agreement between our simulations for the monomers and results found in the literature was observed. Comparing WAXS data to the simulation results, I attribute some of the features observed in the scattering data for each sample to specific repeat units along the chain of each polymer. A side-to-side and a stacking distance were measured for the pure samples. I observed an increase of 0.22 A in side-to-side distance and 0.17 A in stacking distance for pairs of PBI-PDMBI and mesPBI-mesPDMBI, respectively. I attribute this increase in both cases to adding mesitylene group to the structure.

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

Non equilibrium dynamics of quantum Ising chains in the presence of transverse and longitudinal magnetic fields

Author: 
Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2013-08-26
Abstract: 

There has been much recent interest in the study of out of equilibrium quantum systems due to experimental advances in trapped cold atomic gases that allow systematic studies of these effects. In this work we study a quantum Ising model in the presence of transverse and longitudinal magnetic fields. The model lacks an analytical solution in the most general case and has to be treated numerically. We first investigate the static properties of the model, including the ground state energy and the zero temperature phase diagram, using the Density Matrix Renormalization Group (DMRG) method. We identify ordered and disordered phases separated by a quantum phase transition. We then consider the time evolution of the system following an abrupt quench of the fields. We study the response of the order parameter to the quenches within the ordered and disordered phases as well as quenches between the phases using the time-dependent DMRG method. The results are in qualitative agreement with experiments and the implemented numerical method in this work can be extended to allow for more general interactions.

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

Morphology and conductivity tuning of III-V semiconductor nanowires

Date created: 
2013-07-29
Abstract: 

Semiconductor nanowires (NWs) offer a wide range of opportunities to explore the fundamentals of the crystal growth process as well as the possibility to design and fabricate novel nano-scale devices. The rational design of these devices requires the understanding of the NW growth mechanism in order to control the NW size and morphology, the NW doping, and the ability to grow coherent heterostructures. In this thesis, we grew gold-catalyzed III-V NWs via the vapour-liquid-solid (VLS) mechanism using the metalorganic vapour phase epitaxy (MOVPE) technique. We explored the details of VLS growth mechanisms by analyzing the dependence of NW growth rate on NW diameter. We presented a systematic study of the effect of precursor chemistry (i.e. group III precursor, and CBr4 dopant) on the growth of GaAs NWs. We showed that precursor chemistry can be employed as a useful mean to grow axial or radial heterostructures. A model to estimate the critical dimensions of core/shell NWs based on elasticity theory was presented. The numerical calculations were carried out for various III-V core/shell NWs and showed excellent agreement with experimental results obtained for various core/shell NWs. We demonstrated both core and shell doping of GaAs NWs using Te- (n-type) and C- (p-type) dopants. Electrical measurements were performed using a nanoprobe within a scanning electron microscope on individual NWs and doping levels and conduction mechanisms were determined. The incorporation pathways of dopants into the NWs were discussed based on the analysis of the observed dependence of measured resistivity on NW diameter.

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

The jet energy scale at atlas using Z+jet events

Date created: 
2013-03-22
Abstract: 

Jets, collimated sprays of subatomic particles, are an important component of the final state in high- energy proton-proton scattering. A correct jet energy scale is therefore essential to the success of the ATLAS experiment. In this thesis the missing transverse projection fraction method is used to measure the absolute jet response in Z+jet events where the Z decays into a pair of leptons. This measurement complements similar measurements made using γ+jet events while extending the calibration to lower energies. The possibility of taking advantage of the differing fraction of events in each sample with gluon-initiated jets as a method for deriving a parton-dependent jet response is also explored. Preliminary results are shown to agree with Monte Carlo predictions within their statistical uncertainty.

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

Observation of the Castaing instability in a trapped ultracold Bose gas

Author: 
Date created: 
2013-04-15
Abstract: 

Ultracold trapped atomic systems are ideal model systems to investigate physical phenomena in the quantum regime. In this work we studied instabilities in spin dynamics of a sample of nondegenerate trapped $^{87}$Rb gas. This is the first experimental investigation of the so-called Castaing instability in such systems. The Castaing instability is an instability in the spin dynamics of spin-polarized systems as a result of introducing sharp spin gradients in their spin profile. We used an optical technique via the ac Stark effect to initialize arbitrary spin profiles in a sample of evaporatively cooled nondegenerate $^{87}$Rb gas. The experimental results manifest evidence for the presence of the Castaing instability in both transverse and longitudinal spin channels. The results agree reasonably well with theoretical studies and results of a numerical solution to the spin transport equation. Some imperfection in the longitudinal spin gradient due to the spin-state preparation technique makes the signature of the instability less clear in the transverse channel. We propose another preparation technique to overcome this shortcoming in future experiments.

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

Measuring the quark and gluon jet energy response in proton-proton collisions at 7 tev center-of-mass energy with the atlas detector

Date created: 
2013-04-25
Abstract: 

The jet energy scale is one of the largest systematic uncertainties in physics analyses at ATLAS, making it vital to understand and minimize. This thesis provides a comprehensive study of the quark and gluon jet responses, by comparing results of the missing transverse energy projection fraction method in the established gamma+jet events and the newly developed analysis of dijet events. A likelihood discriminator is used to tag jets according to their calorimeter response, and correct the response to one for the dijet study. The mean energy of quark and gluon tagged jets is shown to differ by 4-7%, depending on the energy of the jet.

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