School of Physics - Theses
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ItemStructural instabilities in crystalline solids( 2014)Structural instabilities can be envisaged on various microstructural scales ranging from the “nano” (or atomic) to the “macro” (as one imagines for multi-phase materials). Thus the physical properties measured and techniques employed to research materials of interest for specific projects have been wide ranging. In the research summary section for each chapter, these properties and techniques have been briefly summarized. Separate thesis chapters deal with: various superconducting materials; materials exhibiting martensitic transformations; residual stresses in polycrystalline materials as studied by x-ray and neutron diffraction techniques; various dielectric materials, particularly ferro and piezoelectric materials; magnetic microstructures; and alloys developed for hollow cathodes for atomic absorption spectrometry.
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ItemAchieving micron resolution in charge integrating detectors( 2014)The advent of intense light sources, such as third generation Synchrotrons and now Free-electron Lasers (FELs), brings about new imaging techniques and drives the development of electronic detectors. This thesis reports on the developments of a novel charge interpolation technique which delivers improved spatial-resolution in acharge integrating X-ray detector. Experimental results demonstrate the technique is capable of resolving sub 2 μm structures and a sensor simulation predicts sub micron resolutions may be achieved.
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ItemContinuous and stochastic gravitational wave emission from neutron star interior flows and oscillations( 2014)This thesis investigates continuous and stochastic gravitational wave signals from neutron star interior flows and oscillations. Neutron stars provide a unique laboratory to test physics at extremes of density, gravity, and magnetism. Gravitational waves directly probe the neutron star interior, carrying information about the properties of bulk nuclear matter. Glitches are rotational irregularities occasionally observed in pulsars. We calculate analytically the nonaxisymmetric Ekman spin-up flow following a glitch and its associated gravitational wave signal in the context of an idealised model. A large glitch with $\delta\Omega/\Omega = 10^{-4}$ in a pulsar rotating at $\sim 100$ Hz may be detectable by second- and third-generation interferometers. The signal depends on the inclination angle of the pulsar and the interior viscosity, compressibility, and stratification, which can be inferred gravitational wave data. Superfluid turbulence in neutron stars, driven by crust-core differential rotation, emits stochastic gravitational radiation. We calculate the stochastic background for a Universal neutron star population and two subpopulations: radio-loud pulsars and accreting millisecond pulsars. Non-detection of the stochastic background by LIGO implies an upper limit on the relaxation parameter $\tau_d = \Delta\Omega / \dot{\Omega}$, where $\dot{\Omega}$ is the spin-down rate, of $\tau_d \lesssim 10^5$ yr for radio-loud pulsars and $\tau_d \lesssim 10^7$ yr for accreting millisecond pulsars. Turbulent convection in main-sequence stars also emits gravitational radiation. We calculate the gravitational wave strain power spectral density for an individual star and a Universal stellar population. Due to its proximity, the signal from the Sun dominates the integrated background, but both fall well below the detection threshold of proposed space-based interferometers. Inside the gravitational wave near zone, the signal scales more steeply with distance ($\propto d^{-5}$) and is amplified relative to the far-zone signal ($\propto d^{-1}$). We calculate Rømer and Doppler timing residuals for a pulsar orbiting in the near zone of a high-mass main-sequence star and compare with observed of timing noise in three high-mass systems. The largest predicted root-mean-squared residuals, $\Delta T_{rms} = 2.8$ μs for PSR J0045-7319 at periastron, are a factor $\sim 10^3$ smaller than those observed. We propose a new gravitational wave detection statistic based on a modified form of higher criticism, a statistical method designed to indirectly detect a collection of sources too weak to be detected individually. Using higher criticism to reanalyse \mathcal{C}-statistic values for a simulated search of a low mass X-ray binary, we find higher criticism is sensitive to wave strain $\sim 6\%$ lower than the \mathcal{C}-statistic threshold. Higher criticism makes fewer assumptions about the source frequency and is more robust to error caused by accretion-driven phase wandering or an incorrect orbital period. Finally, we present preliminary results from two projects studying magnetar bursts. We propose a simple model of non-linear crust cracking and shear wave propagation to investigate the transient behaviour observed in the quasiperiodic oscillations detected in magnetar giant flares. The resulting frequency-time spectrograms contain features like frequency drifting, mode splitting and rotational phase dependence of oscillation frequencies. We also extend an existing smooth-particle-magnetohydrodynamics code to build a neutron star model. We validate the code against $f$-mode oscillation frequencies, observe rotational splitting, and present early progress towards implementing a rigid crust. Future applications include simulating crust-core magnetar oscillations as well as long-term spin down and post-glitch circulation.
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ItemEvincing the histories of the cosmic supermassive black hole and galaxy populations with gravitational waves( 2014)Supermassive black holes (SMBHs) are inferred to exist at the centres of massive galaxies throughout the Universe. When two such galaxies merge, a binary SMBH system is likely to form, which coalesces following losses of energy and angular momentum to gravitational waves (GWs). GWs from binary SMBHs will cause metric perturbations at the Earth that affect the arrival times of pulses from radio pulsars within our Galaxy. This concept has led to the establishment of pulsar timing array (PTA) collaborations, which are primarily aimed at detecting GWs. This thesis is motivated by the prospect of gleaning insights into the assembly histories of the cosmological SMBH and galaxy populations by searching for GWs from binary SMBHs. I show that a GW background (GWB) generated by the binary SMBH population is the most promising class of signal to consider for this purpose, with the strong possibility of a detection within the forthcoming decade. In contrast, I find that GWs from individual binary SMBHs are not viable sources for current PTA searches. I also demonstrate that the statistics of pulsar timing variations induced by the GWB will be mildly non-Gaussian. By developing techniques to simulate the effects of GWs from predicted populations of binary SMBHs on PTA data, I find that these non-Gaussian statistics result in a ∼ 10% degradation in the recent Parkes PTA upper limit on the GWB. In a separate investigation, I show that interactions between binary SMBHs and their environments may cause attenuation in the GWB at frequencies up to 10^−8 Hz. Finally, upon comparing various predictions for the GWB with the most recent upper limits from the Parkes PTA, I find that a model which posits purely merger-driven growth of massive galaxies during the last 8 billion years is excluded at the 91% confidence level. I also derive a constraint on the merger timescale of massive galaxies: I find that the mean time spent between projected separations of 20 and 5 h^−1 kpc is greater than 0.1 Gyr with 95% confidence.
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ItemApplication of non-equilibrium statistical mechanics to the analysis of problems in financial markets and economy( 2014)This thesis contributes to a growing body of work in the emerging inter- disciplinary field of econophysics, where tools and techniques of statistical mechanics and other branches of physics are applied to problems in economics and finance. The thesis examines the following four topics: 1) money flows in the interbank networks, 2) wealth distributions in multi-agent exchange sys- tems, 3) variability and dynamics of the foreign exchange markets via lattice gauge theories, 4) memory loss and patterns of change in Abelian sandpiles via hidden Markov models. In economic and financial systems, interactions between the agents oc- cur on a network, whose properties depend on the system in question. Such networks are not static but rather dynamic in character, since the links com- prising the network frequently depend on the actions of the agents as they interact with one another. Moreover, the links of the network typically repre- sent flows, e.g. money flows in financial networks, which further complicates their study. These issues are addressed in the thesis in the case of the financial networks of money flows between Australian banks. In most advanced economies, including Australia, high-value transactions in the banking system are settled in real time via the so called real-time gross settlement systems, which are controlled by the central banks. Such systems have been introduced in many countries in the last ten to twenty years in order to diminish the liquidity risk in the banking system. As such systems are computerized, the information pertaining to all transactions, including their source, destination, and value, is recorded by the central banks and can be used to investigate the properties and dynamics of the interbank flows. The flows of payments in the interbank networks are not homogeneous but possess an intricate structure that reflects the nature of various payments. In- deed, some of the payments recorded by the central bank represent overnight loans extended from one bank to another and the return payments made on the following day. The work undertaken in the thesis examines the flows of overnight loans and the flows of other payments separately. It is shown that these two kinds of flows are not entirely independent. The flows of overnight loans appear to counteract the imbalances in the bank’s reserve accounts cre- ated by the flows of other payments. This is in accord with the dynamics existing in the interbank money market, where the bank’s with the surplus of reserves lend the surplus to the banks with depleted reserves. Agent-based models are gaining popularity in the efforts to understand economic and financial systems and their dynamics. The strength of these models lies in the fact that they require the formulation of local rules of interac- tion only without specifying the global constraints on the system’s behaviour, which are often poorly understood. The simulations that use agent-based models reveal the emergent behaviour of the economic and financial systems that arises as a result of the collective actions of the agents following individ- ual rules. In particular, agent-based models have been used to address the problem of the wealth and income distribution in the economies. The work conducted in the thesis examines one such model, referred to as the giver scheme, where the rule of exchange stipulates that the transfer amount from the giver to the receiver is equal to a fixed fraction of the giver’s wealth. The giver model of asset exchanges is examined in the thesis by means of multi-agent simulations. The system rapidly evolves to a steady state, in which the distribution of wealth does not vary with time, even though agents continue to exchange wealth. This model is amenable to the analysis based on the master equation, which balances the influx and outflow of agents at every wealth value. The thesis presents an investigation of the master equation by means of the Laplace transform. A novel technique for calculating the wealth distribution in the steady state is introduced and used to investigate wealth inequality. The giver model is shown to exhibit two distinct regimes in the steady state. One of them is reminiscent of exchanges that occur in the economy where the agents typically exchange a small fraction of their wealth. The other regime is more akin to gambling and is characterised by exchanges where most of the giver’s wealth is lost, so that fortunes are made and lost frequently. The first regime is characterised by relatively low inequality, whereas the second one is prone to exhibit very large inequality. In addition to the study of inequality, the thesis investigates applicability of the Boltzmann entropy as a measure of disorder in the giver model. The giver model represent a closed system with no sources or sinks of agents or wealth, i.e. it is conservative and in many respects is similar to an ideal gas. However, numerical simulations reveal that the Boltzmann entropy does not evolve monotonically in the giver model and, therefore, is not a faithful mea- sure of disorder. This paradox is resolved by observing that the exchange rules of the giver model are not time reversible, i.e. in order to reverse the dynamics of exchanges a quantitatively different rule of exchange is required. The foreign exchange market is a complex dynamical system that provides prodigious amount of financial data, which makes it an attractive subject of research. One of the approaches to modeling it relies on the lattice gauge theory, where the lattice is constructed by considering two or more currencies and discretising time and the gauge refers to the arbitrage on the lattice. A brief investigation of one such model is undertaken in the thesis. It is shown that the model is unstable in most realistic situations and thus cannot be used to model the market behaviour. Despite wealth of data, the foreign exchange market is difficult to study, since the internal structure of the market is not well known. The Abelian sandpile model, a cellular automaton that exhibits self-organised criticality, is used in the thesis as a toy model that captures some of the features of the foreign exchange market. The model is used to study temporal correlations in the observed behaviour and their relation to the underlying internal structure. A technique based on the site occupancy numbers is proposed in the thesis. It reveals that the loss of memory in the sandpile model occurs in two distinct stages, the fast stage characterised by rapid loss of memory and the subsequent slow stage, during which memory of the initial state is lost at a much reduced pace. Both stages are shown to be roughly exponential and the scaling of the time decay is investigated. The temporal correlations in Abelian sandpiles are also investigated in- dependently by means of hidden Markov models, which show exceptional ca- pabilities in detecting patterns in sequences of data. Hidden Markov models have not been applied to Abelian sandpiles despite their popularity in a broad range of applications ranging from bioinformatics to speech recognition. It is demonstrated in the thesis that hidden Markov models do detect patterns in the temporal variability of avalanche size, consistent with the results based on the occupancy numbers. However, the connection between these patterns and the internal structure of the sandpile has not been established. A number of promising directions to address this problem are proposed.
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ItemDevelopments in X-ray diffraction microscopy with applications for biological imaging( 2014)Macroscopic dynamics in biological systems are driven by signalling events between a host of microscopic sub-structures. At the molecular level, signals propagate along molecular circuitry that is mediated by signalling and receptor agents such as hormones, enzymes, neurotransmitters, cell membrane proteins, ion channels and others. At a cellular level, organelle functionality is crucial for cellular respiration and cellular division as well as protein synthesis and trafficking. Clearly, much can be learned from the size, shape and composition of the microscopic structures that regulate homeostasis. X-ray microscopy has established itself as a highly capable technique that surpasses the resolution limits of optical microscopy while circumventing the weak penetration and extensive sample preparation characteristic of electron microscopy. Lens based, optical microscopes have the advantage of producing images in real time but, apart from some forms of fluorescence microscopy, their resolution is fundamentally limited to the quality of the lens. We here discuss a powerful alternative known as X-ray Diffraction Microscopy (XDM). Removing the focussing optic means that XDM is unaffected by issues such as lens quality. Images are reconstructed from the diffraction pattern meaning that image resolution is given by the largest scattered angle detected. For biological materials the ability to record high angle is limited by radiation damage however, new techniques demonstrate that if the scattering is recorded sufficiently rapidly, radiation damage can be circumvented and wavelength limited resolution could be possible. Chapters 1 to 4 comprise introductory and review chapters that outline the theory and experimental work necessary to describe the original work presented in this thesis. In Chapter 5 we discuss the application of curved beam XDM to malaria infected red blood cells. The life cycle of this deadly disease is still not fully understood and we approach the problem from a structural biology perspective. Using a variety of infected blood cells, we explore the possibility of imaging intact cells to high resolution and without extensive sample preparation. The method is shown to be sensitive to internal cellular structure and to return high resolution images that are consistent with the accompanying SEM and optical micrographs at a resolution of ∼ 40 nm. In Chapter 6 we discuss the importance of 3D imaging and present a simple method for mounting samples in heat-thinned glass capillaries. To support the premise that this method is ideal for tomographic analysis we present preliminary results of tomographic curved beam XDM of malaria infected red blood cells mounted in glass capillaries. To address the data collection overhead imposed by imaging weakly scattering samples in a tomographic format, we challenge the notion that XDM requires fully coherent flux. Standard XDM implicitly assumes full temporal and spatial coherence. These conditions are achieved using spatial and spectral filters that remove the overwhelming majority of the radiation available. We explore the possibility of relaxing the spatial and temporal coherence requirements in order to exploit a larger fraction of the undulator radiation. In Chapter 7 we present the first demonstration of XDM data collection and image reconstruction using a broadband x-ray beam. A multi-modal reconstruction algorithm is then used to propagate the beam between real and reciprocal space. The analysis shows that by imaging structures with a partially temporally coherent beam, the data acquisition times are reduced by a factor of 60. Chapter 8 extends this work in a simulation study by analysing the effect of partial spatial coherence. We demonstrate that a reconstruction algorithm that includes the spatial coherence properties can recover a simple object with no a priori information. The results from both chapters show that if the coherence properties are accounted for in the reconstruction, significant departures from full coherence are tolerable with commensurate decreases in the required exposure time. The final chapter changes tack and focuses on the source of radiation itself. A surge of investment in new x-ray sources over the last decade is representative of the growing interest in the field. New x-ray facilities known as X-ray Free Electron Lasers (XFELs) are currently being developed and will allow methods of biological imaging that are impossible at 3rd generation sources. Unlike synchrotron sources, XFELs provide ultra short pulses of intrinsically coherent radiation. Chapter 9 describes spatial coherence measurements of individual femto-second pulses of a hard x-ray free electron laser. Using the well known “diffract and destroy” technique, we apply the classic Young’s double pinhole experiment to probe the coherence of individual pulses. Our results show that in the absence of spatial filters, the majority of the x-ray flux is contained within the fundamental spatial coherence mode and thus confirms the coherent nature of XFEL pulses.
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ItemDomain-wall brane phenomenology in five and six dimensions( 2014)This thesis explores the construction and phenomenology of models based on domain-wall branes in five and six dimensions. In these models, the extra dimensions are infinite. In 5D, we explore how a model with a single domain-wall brane can account for the fermion mass spectra. In 6D, we construct a model with intersecting domain-wall branes and show how we can localize scalars, fermions and gauge bosons. We first analyze the fermion mass hierarchy problem as well as the problems of generating quark and lepton mixing in the context of the 4+1-dimensional SU(5) domain-wall brane model first proposed by Davies, George and Volkas. We exploit the split-fermion mechanism which naturally arises in the model to show that the fermion mass hierarchy as well as the Cabibbo-Kobayashi-Maskawa (CKM) mixing angles can be accounted for naturally. We later suggest that the same mechanism cannot be used to generate acceptable lepton mixing. We then modify the original SU(5) model by including a discrete $A_{4}$ flavor symmetry. The SM fermions and scalars are then assigned to appropriate $SU(5)\times{}A_{4}$ representations and we give an example parameter choice for which the fermion mass hierarchy and the CKM mixing angles are generated by the split fermion mechanism while realistic, large lepton mixing angles are produced from the special properties of $A_{4}$. We show that the splitting of scalars in the extra dimension can solve the vacuum alignment problem inherent to most models with discrete flavor symmetries. In the second half of the thesis, we deal with models in six dimensions. We show that a $\mathbb{Z}_{2}\times{}\mathbb{Z}_{2}$-invariant scalar field theory with four scalar fields can generate two intersecting domain walls, with two of the fields condensing in the interiors of the walls to form `lumps'. We show that for a special parameter choice, one can obtain analytic solutions. We also discuss how the same mechanisms used to localize fermions and scalars in five-dimensional models can be used to localize these fields to the domain-wall intersection. Lastly, we deal with how to trap gauge fields to the domain-wall intersection in the previous six-dimensional model. We achieve this by giving the fields that form `lumps' gauge charges, so that a confining, non-Abelian gauge group G is spontaneously broken to subgroups $H_{1}$ and $H_{2}$, localizing the associated gauge fields to the respective walls by the Dvali-Shifman mechanism. On the intersection there is a further breaking to $H_{1}\cap{}H_{2}$ and we then outline the conditions under which gauge fields are then localized to the intersection. We show that this mechanism can localize the Standard Model gauge fields starting from an $SU(7)$ theory. We thus find that the domain-wall brane model-building framework represents an interesting approach to reproducing the essential components of the Standard Model. We also find that this framework is very flexible, with the possibility to extend it with additional extra dimensions, as well as with larger gauge groups as previously shown.
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ItemThe role of thermal scattering in the imaging of condensed matter at atomic resolution( 2014)Transmission electron microscopy is a technique capable of determining specimen composition and structure at atomic resolution. Since the invention of the first microscopes in the 1930’s, a series of instrumentation and theoretical advances have led to a mature field where one can routinely obtain images of individual columns of atoms with high precision. Despite the extraordinary success of the microscope and its prevalence in laboratories around the world, interpretation of experimental images is a non-trivial matter. The strong interaction of electrons with the charged particles making up the target specimen (~10^4 times greater than that for x-rays) results in interesting and complicated scattering dynamics which can make direct interpretation of experimental images difficult. A particularly important scattering mechanism at high incident energies is thermal scattering, whereby the incident electron excites a phonon (that is, a vibrational mode) within the specimen. In this thesis we will present a new model for thermal scattering which affords new physical insights as compared with previous models. In particular the new model distinguishes between elastic and thermal scattering of the fast electron and can predict the individual contributions to the scattered intensity from both types of scattering. We will use this model to gain new insights into the role of thermal scattering in a number of different imaging modes.
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ItemAn all-diamond hermetic encapsulation for a high-acuity retinal prosthesis( 2014)Bionic vision through electrical stimulation of the retina is fast becoming a reality. To date, clinical trials have allowed blind patients to see a lover’s smile and navigate night scenes. (K Stingl et al, 2013) This kind of data has encouraged an abundance of research activity. Bionic Vision Australia, among others, is developing a retinal prosthesis to restore high visual acuity. One of its flagship technologies is a diamond electrode array, which will form part of the encapsulation for the implanted electronics. The remainder of the encapsulation also needs to be constructed from leak-proof, or hermetic, materials. The aim of this work was to design and test feasibility of a hermetic encapsulation that incorporated the diamond electrode array. An all-diamond hermetic encapsulation design was proposed, in which a diamond box-shaped capsule was bonded to the diamond array, with the electronics contained inside. Diamond capsules were made from polycrystalline diamond. Laser micromachining was found to be the optimal fabrication method. Hermetic joints were made in diamond using vacuum brazing with precious metal braze alloys. Several brazes were investigated for their ability to wet and form strong bonds with diamond. Bond interfaces were studied for morphology, chemical composition and hermeticity. Brazed diamond capsules were sealed at room temperature using laser microwelding. Welds were optimised for smooth surface morphology and hermeticity. The results demonstrated a hermetic all-diamond encapsulation. Combining the hermetic capsule, the brazing technique, and the welding technique with the diamond electrode array formed a retinal prosthesis technology that can protect against degradation for the lifetime of the patient.
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ItemMany- and few-body methods for strongly correlated quantum gases( 2014)The experimental realisation of ultracold quantum gases offers a new paradigm for studying fundamental physics and quantum phenomena on a macroscopic scale. In this thesis we analyse quantum few- and many-body systems under external confinement with tuneable interactions. Underpinning the many-body dynamics in ultracold gases lies few-body systems and these are critical in understanding strongly-interacting Fermi gases. By exactly solving the energies we can determine vast information about the system. We analytically determine the properties of two- and three-fermions in a harmonic trap subject to an external rotation. Many-body thermodynamic quantities such as the entropy and energy are calculated using the quantum virial expansion. As the external rotation is increased, the regime over which the virial expansion is valid is extended to lower temperatures. By parametrising the solutions in the rotating frame we find that the energy and entropy are universal for all rotations in the strongly interacting regime. Additionally, we find that rotation suppresses the onset of itinerant ferromagnetism in strongly interacting repulsive three-body systems. Extending the number of fermions to be analysed, we present a new approach for studying few-body physics in ultracold two-component atomic Fermi gases. A stochastically variational gaussian expansion method is applied, focusing on the two-body correlations present in these systems in the strongly interacting, or unitary, limit. The groundstate energies of the four-, six- and eight-body problem with equal spin populations are calculated with high accuracy and minimal computational effort. We also calculate structural properties of these systems and discuss their implication for the many-body ultracold gas. In the second part of this work we consider the effects of long-range dipolar interactions to a Bose-Einstein condensates. We perform a theoretical study into how dipole-dipole interactions modify the properties of superfluid vortices within the context of a two-dimensional atomic Bose gas of co-oriented dipoles. The reduced density at a vortex acts like a giant antidipole, changing the density profile and generating an effective dipolar potential centred at the vortex core whose most slowly decaying terms go as $1/\rho^2$ and $\ln(\rho)/\rho^3$. These effects modify the vortex-vortex interaction which, in particular, becomes anisotropic for dipoles polarized in the plane. Striking modifications to vortex-vortex dynamics are demonstrated, including anisotropic corotation dynamics and the suppression of vortex annihilation.