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

## Cosmological tests of fundamental physics

Author:
Date created:
2019-06-11
Abstract:

The standard model of Cosmology, or the $\Lambda$CDM model, is able to describe remarkably well a plethora of observations with only six parameters. Nonetheless, several questions about its very nature have yet to be answered. Chief amongst them is the nature of Dark Energy, responsible for the observed acceleration of the Universe. While in the $\Lambda$CDM model Dark Energy is modelled via the cosmological constant $\Lambda$, its observed value cannot be reconciled with the predictions of quantum field theory, the framework at the basis of the standard model of particle physics. Modifications of General Relativity, known as modified gravity, offer an alternative approach to Dark Energy. The growth of large scale structure is modified in alternative gravity theories in ways that can be tested using cosmological data. We present a comprehensive analysis of a set of scalar-tensor theories of gravity that exhibit the chameleon screening mechanism. This mechanism hides the force mediated by the extra scalar from detection in local and solar system tests of gravity, while still leaving imprints in the cosmological observables. With the increasing precision of cosmological surveys, finer effects must be included in the theoretical predictions. This is the case for the total mass of neutrinos that affect the structure formation at the percent level in a way that could be degenerate with modified gravity. Being able to break this degeneracy requires ability to account for the mass of neutrinos while constraining modified gravity theories. We thus introduce a consistent treatment of massive neutrinos in the phenomenological $\mu-\gamma$ framework and update the popular code MGCAMB used to constrain modified gravity models. We have also introduced the option for MGCAMB to work with general background histories where Dark Energy evolves with time. It has been recently shown that a dynamical Dark Energy with density that increases with time is able to alleviate the tensions between different datasets within the $\Lambda$CDM model. Modified gravity theories provide a framework to explain such behaviour of Dark Energy and we perform a reconstruction of the Lagrangian of Generalized Brans-Dicke theories from the observed expansion history of the Universe. We then study the viability of the such theories and ways of testing them with future data. Another challenge for upcoming CMB experiments is the detection of the primordial gravitational wave background in the B-mode polarization signal. Such a background is predicted by Inflation, a period of exponential expansion in the early Universe, which is the paradigm behind the choice of the initial conditions in the $\Lambda$CDM model. CMB B-modes also offer a powerful way to test the existence of primordial magnetic fields in the early Universe. These can also arise from the inflationary mechanism or be generated during phase transitions in the early universe. We have developed a publicly available code, dubbed MagCAMB, that computes the CMB anisotropies generated by primordial magnetic fields. We also derived the tightest constrain to date on the primordial magnetic field amplitude using the CMB spectra from the Planck satellite and the B-mode measurements by the South Pole Telescope.

Document type:
Thesis
File(s):
Supervisor(s):
Levon Pogosian
Department:
Science: Department of Physics
Thesis type:
(Thesis) Ph.D.

## DC SQUID Magnetometry

Author:
Date created:
2019-10-10
Abstract:

This thesis describes the measurement of the local magnetic field of a D-Wave Systems Washington generation processor using on-chip multiplexed unshunted DC SQUID magnetometers. These measurements are used in conjunction with passive and active field compensation to minimize the magnetic field present during the superconducting transition of the chip in order to limit the number of magnetic flux lines trapped on chip. This maximizes the operability of the superconducting quantum processor.

Document type:
Thesis
File(s):
Supervisor(s):
David Broun
Department:
Science: Department of Physics
Thesis type:
(Thesis) M.Sc.

## On the hunt for a near-infrared single photon emitter in silicon — Optical characterization of radiation damage defects in 28Si

Author:
Date created:
2019-08-22
Abstract:

Integrated silicon photonics allows for the routine control and detection of light down to the single photon regime with low loss and in a scalable way, but to date still has the reputation of lacking fast, efficient and indistinguishable optical emitters that can be easily coupled to photonic structures. The identification of such an emitter will further enable the realization of on-chip low-energy, speed-of-light processing, and could open up exciting prospects for quantum computing and quantum communications. A multitude of radiation damage centres in silicon show bright emission in or near the attractive telecommunication band wavelength range at temperatures up to tens of Kelvin, which is compatible with the state-of-the-art superconducting nanowire single-photon detectors. These centres have been thoroughly studied in natural silicon using a wide variety of techniques, but their detailed spectral properties remained hidden by inhomogeneous line broadening due to the mixed isotopes present in natural silicon. We characterize three defect centres showing potential as single photon emitters, the C centre (790 meV, or 1571 nm), the G centre (969 meV, or 1280 nm), and the W centre (1019 meV, or 1217 nm). Performing ultra-high-resolution spectroscopy on ensembles of these defects in highly isotopically enriched 28Si, where isotopic inhomogeneous line broadening is removed, reveals a dramatic reduction in spectral linewidth of up to two orders of magnitude. We also report for the first time a quartet structure of the G centre that is revealed in 28Si, and a G linewidth that is only twice the lower bound given by the excited state lifetime. These results have direct implications for the spectral widths and fine structure to be expected from individual emitters, even in natural silicon.

Document type:
Thesis
File(s):
Supervisor(s):
Michael Thewalt
Department:
Science: Department of Physics
Thesis type:
(Thesis) M.Sc.

## Out-of-equilibrium dynamics of the Bose-Hubbard model in the strong coupling regime

Author:
Date created:
2019-07-05
Abstract:

Experimental advances have made ultracold atoms in optical lattices a favourable setting to study out-of-equilibrium phenomena and attracted considerable attention in recent years. These systems are highly versatile in that experimental parameters can be tuned over a wide range of values in real time. This facilitates the study of quantum quenches, in which parameters in the corresponding Hamiltonian are varied in time faster than the system can respond adiabatically. Such protocols open the door to a rich range of many-body physics and have been studied intensely both theoretically and experimentally. The Bose-Hubbard model (BHM) has been shown to describe interacting ultracold bosons in an optical lattice, allowing the opportunity for experiments to probe the out-of-equilibrium dynamics of the model. The BHM is a particularly convenient context for studying quantum quenches as it displays a quantum phase transition between superfluid and Mott-insulator phases. In this thesis, we develop a strong-coupling approach that allows the study of correlations in space and time in both the superfluid and Mott-insulating phases of the BHM. Specifically, we obtain a two-particle irreducible (2PI) effective action within the contour-time formalism that allows for a description of both equilibrium and out-of-equilibrium phenomena. We derive equations of motion for both the superfluid order parameter and the single-particle many-body Green's functions. First, we assess the accuracy of our formalism by studying the equilibrium solution for the homogeneous BHM and comparing our results to existing strong-coupling methods as well as to exact methods where possible. We then consider homogeneous systems that are initially thermalized in the Mott phase, and which are then subjected to quenches. We solve numerically the equations of motion for this scenario and calculate the single-particle density matrix. We demonstrate a Lieb-Robinson-like maximal propagation velocity for the spreading of single-particle correlations in one, two, and three dimensions. We study the dependence of the maximal propagation velocity on the quench protocol, chemical potential, temperature, and dimensionality. We compare our results to exact methods, existing strong-coupling approaches, and experiments where possible. Lastly, we extend our strong-coupling approach to the disordered BHM and derive equations of motion for the disorder-averaged single-particle Green's function. We discuss how these equations of motion can be used to study the phase stability of many-body localization in the disordered BHM for dimensions higher than one.

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

## Cosmological and astrophysical observables from field theory in curved backgrounds

Author:
Date created:
2019-05-03
Abstract:

The framework of effective field theory has provided valuable insights needed to understand the evolution of physical systems at different energy scales. In particular, when comparing the near-equilibrium phenomena at astrophysical scales with effects at cosmological distances. The objective of this thesis is to introduce useful tools for the evaluation of (a) the observational consistency of an effective field theory of gravity, and (b) the potential modifications of theories, equipped with diffeomorphism invariance. We calculate the evolution of gravitational observables relevant in early universe field configurations, and also in effective theories modified by contributions from higher curvature terms or semiclassical effects testable at astrophysical scales. To do so, we develop efficient numerical routines to resolve the dynamic two-point correlation functions of primordial fluctuations in inflationary and bouncing cosmologies, the accretion of scalar fields and spacetime curvature in modified gravity, and the evolution of scattering processes involving scalar and gravitational radiation. Additionally, we investigate the viability of defining gauge-invariant quantities in theories of gravity, where the canonical coordinates are deformed to incorporate extra degrees of freedom.

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

## Cooling dynamics of a Brownian particle and the Markovian Mpemba effect

Author:
Date created:
2018-09-26
Abstract:

I experimentally and numerically investigate a Mpemba-like behaviour in a colloidal particle diffusing in a bath under the influence of an externally applied potential. Multiple particle trajectories were recorded and used to obtain the spatial probability distribution of the particle at different times. As a temperature quench is applied, the probability distribution shifts from one equilibrium distribution to another that correspond to the initial and final temperatures in the process, respectively. I experimentally and numerically study the change in value of a measure for the degree of cooling calculated from the measured probability distributions that is compatible with the characteristics of temperature when the system is at equilibrium, and can equally be applied to a system that is out-of-equilibrium. I demonstrate that probability distributions can be estimated using a limited amount of data at sufficiently high accuracy to permit experimental observation of the Markovian Mpemba effect.

Document type:
Thesis
File(s):
Supervisor(s):
John Bechhoefer
Department:
Science: Department of Physics
Thesis type:
(Thesis) M.Sc.

## Ceramide and cholesterol interactions in phospholipid membranes: A 2H NMR study

Author:
Date created:
2019-04-05
Abstract:

Sphingolipids constitute a significant fraction of cellular plasma membrane lipid content. Among sphingolipids, ceramide levels are usually very low. However, in some cell processes like apoptosis, cell membrane ceramide levels increase markedly due to activation of enzymes like sphingomyelinase. This increase can change the physical state of the membrane by promoting molecular order and inducing solid ordered (So) phase domains. This effect has been observed in a previous 2H NMR study on membranes consisting of palmitoyl sphingomyelin (PSM) and palmitoyl ceramide (PCer). Cholesterol (Chol), is also present at high concentrations in mammalian plasma membranes and has a favorable interaction with sphingomyelin (SM), together forming domains in the liquid ordered (Lo) phase in model membranes. There are reports that Chol is able to displace ceramide (Cer) in SM bilayers and abolish the So phase domains formed by SM:Cer. This ability of Chol appears to be concentration dependent; in membranes with low Chol and high Cer contents, So phase domains hypothesized to be rich in Cer coexist with the continuous fluid phase of the membrane. Here, first we study the effect of increasing PCer concentration in PSM:Chol bilayers, using 2H NMR. Chol:PCer mol ratios were 3:1, 3:2 and 3:3, at a fixed 7:3 PSM:cholesterol mol ratio. Both PSM and PCer were monitored, in separate samples, for changes in their physical state by introducing a perdeuterated palmitoyl chain in either molecule. Second, we investigate the effect of replacing PSM with DPPC to test the influence on membrane phase behavior of replacing sphingosine with a palmitoylated glycerol backbone. Third, we explore the effect of adding an unsaturated lipid present at a high level in plasma membranes, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC), to the PSM:Chol:PCer 7:3:3 lipid mixtures. This was done to study an approximate cell membrane outer leaflet mimetic lipid mixture. We found that PCer induces highly stable So phase domains in PSM:Chol, DPPC:Chol and POPC:PSM:Chol bilayers. This effect is most pronounced in bilayers with Chol:PCer 1:1 molar ratios, and below 40 oC. PCer is more effective in ordering PSM:Chol bilayers than analogous bilayers composed of DPPC:Chol.

Document type:
Thesis
File(s):
Supervisor(s):
Jenifer Thewalt
Department:
Science: Department of Physics
Thesis type:
(Thesis) Ph.D.

## Investigations of the deep double donor magnesium in silicon

Author:
Date created:
2018-01-28
Abstract:

The deep double donor levels of substitutional chalcogen impurities in silicon have unique optical properties which may enable a spin/photonic quantum technology. The interstitial magnesium impurity in silicon is also a deep double donor but has not yet been studied in the same detail as have the chalcogens. In this study we look at the neutral and singly ionized Mgi absorption spectra in natural silicon and 28-silicon in more detail, looking in particular for the 1s(A1) to 1s(T2) transitions which are very strong for the chalcogens and are central to the proposed spin/photonic quantum technology. We further observe the presence of another donor (Mgi∗) that may result from Mgi in a reduced symmetry configuration, due to either complexing or the occupation of an interstitial site with symmetry lower than the usual tetrahedral site. The neutral species of Mgi∗ reveal additional low lying ground state levels detected through temperature dependence studies. We also observe a new shallow donor which we identify as an Mg-B pair center. Additionally we present photoluminesence spectra that verify the existence of an Mg-Mg isoelectronic pair center through its isotopic fingerprint. We further include the results of a temperature dependence and photoluminesence lifetime study of this same center, revealing unusual behaviour of the main no-phonon line and associated phonon replicas.

Document type:
Thesis
File(s):
Supervisor(s):
Michael Thewalt
Department:
Science: Department of Physics
Thesis type:
(Thesis) Ph.D.

## Morphological studies of bulk heterojunction films made of polymers showing stable photovoltaic properties

Author:
Date created:
2018-06-01
Abstract:

In this project, the morphology of bulk heterojunction photovoltaic cells employing a low-bandgap polymer as donor is studied. The polymer is based on a poly(thieno[3,4-b] thiophene-benzo[1,2-b:4,5-b0 ] dithiophene) (PTB) backbone with tetrahydropyranyl (THP) terminated side chains that are cleaved upon thermal treatment. Photovoltaic properties of the devices have been previously shown to demonstrate a stable performance during an accelerated aging process. I have used optical microscopy, transmission electron microscopy, and grazing incidence small angle X-ray scattering (GISAXS) to investigate the morphology of films made from blends of the polymer with PCBM before (PTB(THP):PCBM) and after (dPTB:PCBM) cleaving at micro and nanoscale. I have further studied the self-organization of these polymers and their blend solution with PCBM using grazing incidence wide angle X-ray scattering (GIWAXS). Results are compared to films made from blends of poly- (3-hexylthiophene) (P3HT:PCBM) and the version of the PTB series whose structure is closest to that of PTB(THP), PTB4 (PTB4:PCBM). Morphological investigations demonstrate that phase separation is suppressed at micro and nanoscale in the dPTB:PCBM films, while large micron-sized PCBM aggregates develop during thermal annealing in P3HT:PCBM, PTB4:PCBM and PTB(THP):PCBM films. Consequently, I have concluded that the removal of THP on the side chains can lead to stable morphology which results in stable performance of photovoltaic cells. GIWAXS studies show that PTB(THP) polymer show considerably stronger ability to self-organize compared with PTB4. Although deprotection process could deteriorate this self-organization but dPTB polymer still show better self-organization along ????????- stack. While this study has been focused on comparison to PTB4 blends, the results should be transferable to other polymers in the PTB series.

Document type:
Thesis
File(s):
Supervisor(s):
Barbara Frisken
Department:
Science: Department of Physics
Thesis type:
(Thesis) Ph.D.

## Installation, commissioning, and acceptance measurements of EMMA

Author:
Date created:
2018-12-13
Abstract:

The ElectroMagnetic Mass Analyzer EMMA is a vacuum mode recoil mass spectrometer that is capable of horizontally dispersing reaction recoils according to their mass/charge ratio at its focal plane station. The recoils enter into two consecutive gas-filled proportional counters, one that detects their positions and the other to measure their energy loss per unit length as well as the residual energy so that the recoils may be uniquely identified. EMMA was designed to exhibit excellent beam suppression so that reaction channels that are weakly populated may be extracted from the unreacted beam and high-yield background channels. EMMA has undergone several commissioning tests to determine how it performs compared to its design specifications. This thesis covers a subset of the tests which involved using a radioactive alpha source as well as accelerated ion beam backscattering to determine its energy/charge and angular acceptances as well as its mass/charge dispersion and acceptance as part of the commissioning of the spectrometer.

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