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

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Confinement and the superfluid density in theories of the underdoped cuprates, and strongly commensurate dirty bosons in the large-N limit

Author: 
Date created: 
2005
Abstract: 

In this thesis, we study various issues arising from the QED theory of underdoped, high temperature superconductors in 2+1 dimensions. The theory breaks up roughly into two sectors: fermionic and bosonic. With regard to the fermionic sector, we consider confinement of the emergent gauge field which we take to be compact. In the absence of fermions, the interaction between monopoles is Coulombic and the well known result is that the pure gauge theory is permanently confining. With the addition of fermions, the interaction becomes logarithmic, and an analogy with the usual Kosterlitz-Thouless transition suggests a deconfinement transition for the fermions. We show, however, that, when screening is taken into account, the deconfined phase is destabilized and fermions remain permanently confined. The bosonic sector models Cooper pair phase fluctuations, whose effect on the depletion of the superfluid density we examine in two separate studies. In the first of these, we study the quantum XY model, and show that the quasi-two dimensionality, low critical temperatures and large d-wave gap characteristic of underdoped cuprates severely constrain the form of the superfluid density. Under these assumptions, we find that phase fluctuations alone are insufficient to account for recent observations of deviations from Uemura scaling, and that the quasiparticle contribution is a necessity. We use our results to satisfactorily fit the recent data. In the second study, we model the cuprates by a layered system of interacting bosons and examine the collective excitations in this system. Depending on the anisotropy and the interaction strength, we find find four different regimes of temperature dependence of the superfluid density. We argue that interactions in the underdoped cuprates are effectively short-ranged and weak. Finally, we study the related issue of disordered, interacting bosons in the large-N limit and 2 strong commensuration. Perturbatively at weak disorder and numerically at strong, we show that th screening of the random potential due to interactions is insufficient to delocalize the single-particl states so that no superfluid transition occurs from the Mott insulator.

Document type: 
Thesis
File(s): 
Department: 
Department of Physics - Simon Fraser University
Thesis type: 
Thesis (Ph.D.)

Reconstructing DNA replication kinetics from small DNA fragments

Author: 
Date created: 
2005
Abstract: 

In higher organisms, DNA replicates simultaneously from many origins. Recent in vitro experiments have yielded large amounts of data on the state of replication of DNA fragments. From measurements of the time dependence of the average size of replicated and non-replicated domains, one can estimate the rate of initiation of DNA replication origins. One problem with such estimates is that, in the experiments, the DNA is broken up into small fragments, whose finite size can bias the measured averages. Here, I present a systematic way of accounting for this bias. In particular, I derive theoretical relationships between the original domain-length distributions and fragment-domain length distributions. I also derive unbiased average-domain-length estimators, which can yield accurate results even in cases where the replicated (or nonreplicated) domains are larger than the average DNA fragment. Then I apply these estimators to previously obtained experimental data to extract replication kinetics parameters.

Document type: 
Thesis
File(s): 
Department: 
Department of Physics - Simon Fraser University
Thesis type: 
Thesis (M.Sc.)

Probing the mechanical properties of short molecules with optical tweezers

Date created: 
2010
Abstract: 

Structural proteins play vital roles in many human tissues, roles to which their mechanical properties are of direct relevance. Optical tweezers give us the remarkable ability to quantitatively probe these properties at the single-molecule level, potentially revealing a wealth of information on how such proteins fulfil their physiological functions. I have worked toward applying this technique, in which micron-sized beads chemically linked to the protein are manipulated by focussed laser beams, to structural proteins, particularly elastin. I developed methods to eliminate or account for several experimental complications presented by the fact that these proteins are short compared to other molecules studied with optical tweezers. I proceeded to design and test multiple strategies for linking elastin to beads, discovering that its unusual biochemical properties raise significant additional challenges. Some of these I overcame, and an assay I developed for linking effectiveness may be of use in overcoming others

Document type: 
Thesis
File(s): 
Senior supervisor: 
N
Department: 
Department of Physics - Simon Fraser University
Thesis type: 
Thesis (M.Sc.)

Metallic scattering lifetime measurements with terahertz time-domain spectroscopy

Author: 
Date created: 
2010
Abstract: 

The momentum scattering lifetime is a fundamental parameter of metallic conduction that can be measured with terahertz time-domain spectroscopy. This technique has an important strength over optical reflectance spectroscopy: it is capable of measuring both the phase and the amplitude of the probing radiation. This allows simultaneous, independent measurements of the scattering lifetime and resistivity. Broadly, it is the precision of the phase measurement that determines the precision of scattering lifetime measurements. This thesis describes milliradian-level phase measurement refinements in the experimental technique and measures the conductivity anisotropy in the correlated electron system CaRuO3. These phase measurement refinements translate to femtosecond-level refinements in scattering lifetime measurements of thin metallic films.

Document type: 
Thesis
File(s): 
Senior supervisor: 
J
Department: 
Department of Physics - Simon Fraser University
Thesis type: 
Thesis (M.Sc.)

An experimental model for MRI-detected measurements of oxygen uptake in the lungs

Date created: 
2010
Abstract: 

Oxygen, which is paramagnetic, influences the nuclear relaxation rate of Helium-3, and can be the limiting factor when hyperpolarized (HP) gases are used as inhaled contrast agents in MR lung imaging. The decay rate of a HP 3He NMR signal, acquired during a single breath-hold, will thus change as oxygen is exchanged with the bloodstream. We model this effect in vitro using a time-varying magnetic field gradient to mimic oxygen uptake in the lung. The goal of the study is to investigate the expected accuracy and precision with which oxygen concentrations and uptake rates can be extracted from lung imaging experiments performed in vivo. Particular emphasis is placed on the effects of varying signal-to-noise ratio and acquisition time. These experiments are expected to inform developments in the field of hyperpolarized gas MRI, and may lead to new tools for the treatment and detection of lung diseases and disorders.

Document type: 
Thesis
File(s): 
Senior supervisor: 
M
Department: 
Department of Physics - Simon Fraser University
Thesis type: 
Thesis (M.Sc.)

Measurement of single top quark production in the Tau+jets channel using boosted decision trees at D0

Author: 
Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2009
Abstract: 

The top quark is the heaviest known matter particle and plays an important role in the Standard Model of particle physics. At hadron colliders, it is possible to produce single top quarks via the weak interaction. This allows a direct measurement of the CKM matrix element Vtb and serves as a window to new physics. The first direct measurement of single top quark production with a tau lepton in the final state (the tau+jets channel) is presented in this thesis. The measurement uses 4.8 fb^−1 of Tevatron Run II data in p\bar{p} collisions at \sqrt{s}=1.96 TeV acquired by the D0 experiment. After selecting a data sample and building a background model, the data and background model are in good agreement. A multivariate technique, boosted decision trees, is employed in discriminating the small single top quark signal from a large background. The expected sensitivity of the tau+jets channel in the Standard Model is 1.8 standard deviations. Using a Bayesian statistical approach, an upper limit on the cross section of single top quark production in the tau+jets channel is measured as 7.3 pb at 95% confidence level, and the cross section is measured as 3.4+2.0−1.8 pb. The result of the single top quark production in the tau+jets channel is also combined with those in the electron+jets and muon+jets channels. The expected sensitivity of the electron, muon and tau combined analysis is 4.7 standard deviations, to be compared to 4.5 standard deviations in electron and muon alone. The measured cross section in the three combined final states is sigma(p\bar{p}->tb+X,tqb+X) = 3.84+0.89−0.83 pb. A lower limit on |Vtb| is also measured in the three combined final states to be larger than 0.85 at 95% confidence level. These results are consistent with Standard Model expectations.

Document type: 
Thesis
File(s): 
Senior supervisor: 
D
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (Ph.D.)

Microwave flux-ow impedance measurements of Type-II superconductors

Author: 
Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2009
Abstract: 

The electrodynamic properties of type-II superconductors have been studied in strong magnetic elds using microwave spectroscopy to access the ux- ow regime of vortex motion. The measurements give insight into two important physical quantities: ux- ow resistivity, which is intimately related to dissipation from electronic states near the vortex cores; and pinning forces, from the collective interactions of the vortex lattice with material defects. A notable aspect of this work is the ability to accurately separate contributions from viscous and elastic forces. This has been made possible by the development of microwave apparatus that allows the separate measurement of in-phase and out-of- phase components of surface impedance, and through the use of well-controlled, high quality samples. Measurements on conventional superconducting systems Nb, NbSe2 and V3Si have been used to test both the experimental technique and the subsequent data analysis, with the results in good accord with established data on these materials. In addi- tion, some interesting new features have been observed, likely associated with the multiband nature of these systems. The primary focus of this work is on the ux- ow dynamics of cuprate high temperature superconductors. Two material systems, YBa2Cu3O6+x and Tl2Ba2CuO6+x, have been used to carry out measurements that span the entire superconducting region of the cuprate phase diagram. For each sample, the pinning force constant and ux- ow resistivity have been extracted across the superconducting temperature range. As has been previously reported by our group, there is an anomalous logarithmic up- turn in ux- ow resistivity at low temperatures, previously attributed to localization physics. The main discovery of this work is that the logarithmic behaviour persists across the phase diagram, even in highly overdoped Tl2Ba2CuO6+x samples in which the normal state is known to be a metallic, conventional Fermi liquid. This suggests,for the frst time, that the resistivity upturns in cuprates are in fact intimately con- nected to the presence of vortices, which in turn has strong implications for the nature of the underdoped cuprate normal state. i

Document type: 
Thesis
File(s): 
Senior supervisor: 
D
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (Ph.D.)

Active shielding of cylindrical transmit-coils for low-field magnetic resonance imaging

Author: 
Date created: 
2005
Abstract: 

Simple yet effective cylindrical transmit and receive coils for low frequency magnetic reso nance imaging (MRI) can be fabricated from a sine-phi distribution of windings. This thesi introduces the novel concept of an actively-shielded transmit coil. This coil produces a uni form oscillating magnetic field (a "B1 field") transverse to the bore of a cylindrical volum (the patient volume) yet at the same time produces very little magnetic field outside of thi region. In practice, this allows one to minimize undesirable inductive coupling betweei the B1 coil and the myriad of other coils (Bo, gradient, shim) that are needed to implemen MRI. Exact analytical solutions are derived for active shielding of the elementary build ing blocks of typical B1 coils. Specifically, solutions for a single straight wire, a completl current loop, and a sine-phi current distribution on the surface of an infinite cylinder ar~ derived. Following this, the results of an experimental study examining the magnetic field produced by actively shielded B1 coils is presented. The concepts described in this thesi are expected to be useful for emerging applications in low field nuclear magnetic resonance and magnetic resonance imaging.

Document type: 
Thesis
File(s): 
Department: 
Department of Physics - Simon Fraser University
Thesis type: 
Thesis (M.Sc.)

Structural investigation of Carbopol ETD2050 by light scattering

Author: 
Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2009
Abstract: 

Carbopol, a family of cross-linked acrylic acid-based polymers and a well-known thickener used in personal care, household and industrial products, is often used as a model yield-stress fluid because it is transparent and its rheological properties can be precisely tuned by sample preparation conditions. Carbopol dispersions behave as an elastic solid, but they will flow when the applied stress exceeds a sample-dependent yield value. Both the yield-stress phenomena and the range of potential applications motivate the study of the microscopic structure and properties of yield-stress fluids. These investigation will lead to a fundamental understanding of the yield-stress phenomenon of Carbopol. Light scattering experiments investigating Carbopol ETD2050 dispersed in water have revealed that the Carbopol microstructure consists of a small and a large length scale, possibly described by different cross-linking densities. Experiments also show that the length scales and mass fractal dimension depend on the sample concentration and pH.

Document type: 
Thesis
File(s): 
Senior supervisor: 
B
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (M.Sc.)

The effect of cholesterol on the partitioning of 1-octanol into POPC vesicles

Peer reviewed: 
No, item is not peer reviewed.
Date created: 
2009
Abstract: 

Microcalorimetry has become a method of choice for sensitive characterization of biomolecular interactions. In this study, isothermal titration calorimetry (ITC) was used to measure the partitioning of 1-octanol into lipid bilayers composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), a semi-unsaturated lipid, and cholesterol, a steroid, as a function of cholesterol molar concentration. The ITC instrument measures the heat evolved or absorbed upon titration of a liposome dispersion, at concentrations ranging from 0 to 40% cholesterol, into a suspension of 1-octanol in water. A model function was fit to the data in order to determine the partition coefficient of octanol into POPC bilayers and the enthalpy of interaction. I found that the partition coefficient increases and the heat of interaction becomes less negative with increasing cholesterol content, in contrast to results found by other groups for partitioning of alcohols into lipid-cholesterol bilayers containing saturated lipids. The heat of dilution of vesicles was also measured.

Document type: 
Thesis
File(s): 
Senior supervisor: 
B
Department: 
Dept. of Physics - Simon Fraser University
Thesis type: 
Thesis (M.Sc.)