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

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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.

Studies of Synthetic Molecular Motors

Author: 
Date created: 
2013-04-18
Abstract: 

To better understand the underlying principles by which biological motors operate, recent work has focused both on understanding their operational principles, and on designing new molecular motors ab initio. Here, by studying and designing motors which use Brownian motion and track asymmetry to bias the direction of motion, I gained insight into the underlying principles by which such motors operate. "Molecular spiders" [JACS. 128, 12693 (2006)] are one example of synthetic biomolecular walkers able to generate biased motion by coupling the chemical asymmetry arising from substrate binding and cleavage to bias their mechanical stepping.These DNA-based motors diffuse to their substrate track where productive binding between a molecular spider’s DNAzyme leg and a ssDNA substrate facilitates cleavage of the substrate. Once cleaved, the decreased binding affinity between the DNAzyme and resulting product allows the motor to diffuse along the track and form new interactions with uncleaved substrate molecules. Toinvestigate the origin of biased motion of molecular spiders, I have performed Monte Carlo simulations. Using my simulations, I also investigated their performance as molecular motors, and determined how to optimize their motor properties by modifying tunable experimental parameters in spider design. These studies assisted us in the design and construction of a novel protein-based synthetic motor, the "Lawnmower", which uses a burnt–bridges type of mechanism, the same as spiders, to autonomously and diffusively move forward. The lawnmower has trypsin proteases as blades, linked to a quantum dot hub, that interact with a one-dimensional peptide substrate track via binding to and cleavage of the substrates. Experimentally, it is confirmed with kinetic assays that ourlawnmower is an active motor and that there are an average number of 8 blades on each motor. I also outlined the synthesis and characterization of a highly modified DNA-peptide construct, which acts as the track for the lawnmower. For this, I employed PCR to generate a densely labeled DNA and click chemistry for peptide conjugation to the functionalized DNA. As an additional motors-relatedproject, I present the synthesis of a long one-dimensional DNA track with periodically repeating elements that provide specific binding sites for the "Tumbleweed" molecular motor.

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

First determination of the 8Li valence neutron asymptotic normalization coefficient using the 7Li(8Li,7Li)8Li reaction

Date created: 
2013-03-19
Abstract: 

Solar neutrino experiments have often been plagued with large uncertainties. With the recent results from the Borexino Collaboration, for the first time the total uncertainty in the 7Be solar neutrino flux measurement is smaller than the uncertainty of standard solar model (SSM) predictions. Improvement in neutrino experiments must be followed by refinements to the SSM; to do so requires reduced uncertainties on the parameters used in SSM calculations. One such value is the astrophysical S-factor describing the 7Be(p,gamma)8B reaction, S_{17}(0). We report here a determination of the asymptotic normalization coefficient (ANC) of the valence neutron in 8Li from a measurement of the angular distribution of the single neutron transfer between 8Li and 7Li via the 7Li(8Li,7Li)8Li reaction at 11 MeV. Using isospin symmetry the 8B ANC has also been calculated and used to infer a value for S_{17}(0) of 20.2 +/- 4.4 eV b.

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

Contribution of non-linear structure to the kinetic Sunyaev-Zeldovich power spectrum

Author: 
Date created: 
2013-02-06
Abstract: 

In the last two decades, a satisfactory standard cosmological model has been established. On the other side, a wealth of new precise data from various astronomical observations is becoming available, leading to increased interest in details of the CMB foreground physics. In this thesis we investigate a very promising foreground effect, the kinetic Sunyaev-Zeldovich (kSZ) effect, and calculate its contribution to the CMB power spectrum. The kSZ effect is due to the interaction of CMB photons with a moving structure between the observer and the surface of last scattering, such as a cluster of galaxies, that contains ionized gas and moves with a certain peculiar velocity. The strength of the effect is determined by both the non-linear density distribution of matter inside clusters and the peculiar velocity which is governed by the linearly perturbed matter density around them. We analyze the relative importance of the the linear and non-linear contributions to the kSZ considering only gravitational interactions. We find that the non-linear contribution significantly change the power spectrum in very small scales, thus we believe including baryonic physics which also governing the nonlinear evolution of the structures is necessary. The effect of baryons is to redistribute matter toward the centres of halos, which can be dramatic when baryon component is allowed to cool. We calculate this deviation as well as its impact on the kSZ CMB power spectrum.

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

Bringing the Higgs Boson to Rest

Date created: 
2013-01-31
Abstract: 

Within the Standard Model of particle physics, the Higgs boson can decay to a pair of W bosons which decay leptonically. Despite its relatively large branching ratio, this is a challenging channel in which to search for the Higgs boson since we cannot detect neutrinos directly with the ATLAS detector. The matrix element method is a first principles approach that allows for better separation of signal and background by responding to subtle differences in the measured event kinematics. A straightforward implementation of the method is to assume that the Higgs boson is produced at rest in the transverse plane. However, this is often not the case due to next-to-leading order effects like initial state radiation. In order to improve the sensitivity of the matrix element analysis, we developed an estimator for the transverse momentum of the Higgs boson that allows us to boost it into its transverse rest frame. Using a regression tree algorithm to estimate the transverse kinematics of the Higgs boson on an event-by-event basis, we observe a 15% improvement in sensitivity. The application of this technique to Higgs boson property measurements, such as the determination of its spin, is also explored.

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

Cross Section Measurement and Search for New Physics with Top Quark Pair Events Involving a Tau Lepton in ATLAS

Date created: 
2013-03-15
Abstract: 

In 2011, the ATLAS detector recorded an integrated luminosity of over 5 fb−1 of proton-proton collisions delivered by the LHC at a centre-of-mass √s=7 TeV. The first of two analyses is a test of the standard model and the world’s most precise measurement of the top quark pair production cross section for final states which include a hadronically decaying tau lepton. The second analysis uses the same dataset to search for a charged Higgs boson, also resulting in the world’s best limits for the search channel. In the cross section measurement, 2.1fb−1 of ATLAS proton-proton collision data is used to measure the top quark pair production cross section in events containing an isolated electron or muon and a tau lepton decaying hadronically. After initial event requirements, the leading background comes from top quark pairs with jets faking tau leptons. A fit to a tau lepton identification variable is used to determine the signal yield. The measured cross section 186±13(stat.)+20-19(syst.)±7(lumi.)pb, is in good agreement with the standard model prediction. Several extensions to the standard model predict the existence of at least one charged Higgs boson, H±. According to these extensions, the top quark can decay into a bottom quark and a light charged Higgs boson in addition to the standard model decay to a bottom quark and aW boson. In the second analysis, event yield ratios between different final states are measured using 4.6fb−1 of ATLAS data. This is compared to simulation to search for a violation of lepton universality. This ratio-based method reduces the impact of systematic uncertainties in the analysis. No significant deviations from the standard model predictions are observed. With the assumption that the charged Higgs boson branching ratio to a tau lepton and a neutrino is 100%, upper limits in the range 3.2%–4.4% can be placed on the top quark to charged Higgs branching ratio for 90≤mH±≤140 GeV. After combination with results from a search for charged Higgs bosons in t¯t decays using the thad+jets final state, upper limits on this branching ratio can be set in the range 0.8%–3.4%, for 90≤mH±≤140 GeV.

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

Chiral symmetry breaking in high-temperature superconductors and birefringent cold atoms, helicity modulus in layered bosons and phase diagram of superconductor-insulator transition

Date created: 
2012-06-26
Abstract: 

This work is a compilation of several research projects with them main theme being high-temperature superconductivity. We construct the field theory of underdoped cuprates beginning with a well-defined d-wave superconductor and adding the vortex degree of freedom using a singular gauge transformation. The symmetries of the theory both in the presence and absence of a quasi-particle mass are studied. Nodal quasi-particles are known to obey a relativistic Lorentz symmetry while their massless nature represent another symmetry which we will identify as a chiral SU(2) symmetry. It is shown that 2+1 quantum electrodynamics is the effective theory that describes underdoped cuprates in the zero- temperature pseudogap regime. We focus on the mechanism of dynamical mass generation in three dimensional quantum elec- trodynamics and theories with four-fermion interactions. This is a field that has been subject of extensive research in last two decades. However, our momentum-shell renormalization group ap- proach is new to the field and through that we are able to estimate the conditions for the mass generation mechanism and also work out the phase diagram of the theory for charge and inter- action strength. We discuss the applications of momentum-shell renormalization group to other four-fermionic theories in the absence of a gauge field. The justification for this is the fact that in the superconducting regime the system can be described by a massive gauge field theory coupled to relativistic quasi-particles which effectively represent a four-fermionic theory. Inspired by the field theory constructed for underdoped cuprates we discuss the superfluid re- sponse of the underdoped materials using an anisotropic bosonic model and compare it to experiment. The idea is to see how c-axis superfluid density measurements can help one set the parameters in our field theory for underdoped cuprates. The behaviours of the superfluid responses in both out- of-plane and in-plane measurements has been detailed as a function of temperature and density (doping) and it is shown that there is disagreement with the measured c-axis response using the iii conjectured bosonic Hamiltonian.

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