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

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

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.

Towards optical readout of Si:Se+

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

The demonstration of a qubit system in silicon, with efficient optical control and readout of robust electronic and nuclear spin states, would change the current dominant industrial trends in quantum devices. Singly ionized deep double donors in silicon (Si:Se+) have shown promise as examples of such industry-changing qubit candidates. The (Si:Se+) system possesses a long-lived spin qubit with photonic access through a spin-selective optical transition. Under the assumption that this optical transition is radiatively efficient, it has been proposed that this optical transition be exploited for direct emission-based spin-state readout, or alternatively used as a much-sought-after silicon-integrated single-photon source. In the first part of this thesis, we present the measurement of the T1 lifetime of the optically excited state which in turn allowed us to determine a natural radiative efficiency of 0.80(1)%. Fortunately, this spin-photon interface can be coupled to photonic cavity modes for indirect spin-state read-out or to improve the emission rate through the Purcell effect. In the second part of this thesis, we present the hardware and software details of an adaptable automated photonics testing system that can be used to characterize integrated photonic devices.

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

Mid-IR waveguides and grating couplers for 2.7-2.9 μm

Author: 
Date created: 
2018-12-04
Abstract: 

Integrated silicon photonics strip waveguides and grating couplers are developed for mid-infrared (mid-IR) wavelengths at 2.7 μm and 2.9 μm. Waveguide loss is measured as a function of width for 2.7 μm light in the vicinity of a prominent OH absorption band, and a loss of less than 2 dB/cm is recorded for the 1.0 μm width waveguide. A fabrication bias metric is determined for accurately developing grating couplers at 2.9 μm on a 500 nm silicon-on-insulator (SOI), 3 μm buried-oxide substrate. A high resolution measurement scheme is motivated and measurements indicate that these devices will be capable of studying the Se+ donor spin qubit cavity coupling platform proposed by researchers at Simon Fraser University.

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

Spin transport in an ultra-cold trapped non-condensed 87rb gas

Author: 
Date created: 
2018-09-11
Abstract: 

Ultra-cold trapped atoms, with their high degree of tunability, provide ideal model systems to study physical phenomena with applications in many different fields of research. This thesis describes studies on spin transport phenomenon in a trapped ultra-cold spin-polarized 87Rb gas at temperatures above quantum degeneracy. This work is focused on the less studied regime of cross-over between classical and quantum transport. Diffusion is a fundamental dissipative process that tends to relax any system towards a state of minimum inhomogeneity. In this work we study longitudinal spin diffusion as a special case of spin transport. The system studied here consists of two anti-parallel longitudinal spin domains separated by a helical domain-wall. We report that the diffusion process manifests a significant deviation from classical diffusion due to purely quantum mechanical modifications. The two-domain spin textures are prepared using optical and microwave pulse techniques, and the dynamics of the spin structure is studied as it relaxes towards the final equilibrium state. Generally, there is a wide range of parameter space that could be studied. In this work we focused our studies on the effects related to the degree of coherence in the domainwall as well as the effective magnetic field acting on the spins. By controllably tuning these experimental parameters, we studied in detail how the spatiotemporal behaviour of the diffusion dynamics is modified. Our results show that the longitudinal spin diffusion time scales depend sensitively on the domain-wall degree of coherence. External magnetic field gradients also alter the dynamics noticeably, manifesting a significant dependence on the sign of the applied field gradients.

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