School of Physics - Theses
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ItemDetecting and characterising extrasolar planets in reflected light(University of Melbourne, 2009)
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ItemFault-tolerant quantum computation with local interactions(University of Melbourne, 2009)
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ItemMonte Carlo characterisation of narrow photon fields for intensity modulated radiotherapy(University of Melbourne, 2006)
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ItemFocusing of an atomic beam using a TEM01 mode lens(University of Melbourne, 2006)
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ItemThe Panoramic Deep Fields( 2001)The Panoramic Deep Fields are a deep multicolour survey of two ~ 25 ° fields at high galactic latitude. The survey images have been constructed by digitally stacking scans UK Schmidt plates. Deep images (Bj ~23.5) with low contamination have been obtained by subtracting the background from the individual plates scans and using bad pixel rejection during the stacking. The size and depth of the fields allow the accurate statistical measurement of the environments and evolution of galaxies and AGN. The clustering of galaxies and galaxy clusters has been measured from z ~0.4 until the current epoch. The clustering properties of galaxies are strongly correlated with colour and blue U – Bj selected galaxies exhibit weaker clustering than any morphologically selected sample. The weak clustering (ro ≤ 3h -1 Mpc) of blue galaxies implies galaxy colour and stellar population are more strongly correlated with environment than galaxy morphology. Despite the large fields-of-view, the clustering of red galaxies and clusters varies significantly between the two fields and the distribution of clusters is consistent with this being due to large-scale-structure at z ~0.4. The evolution and environments of AGN have been measured at intermediate redshifts with the Panoramic Deep Fields. Photometric redshifts, colour selection and the NVSS have been used to compile a catalogue of ~ 180 0.10 < z< 0.55 radio galaxies. The evolution of the radio galaxy luminosity function is consistent with luminosity evolution parameterised by L (z) ~ L(0) (1+z)3.4. The environments of UBR selected AGN and radio galaxies have been measured at z~0.5 using the Panoramic Deep Field galaxy catalogue. By applying photometric red-shifts and colour selection criteria to the galaxy catalogue, it has been possible to increase the signal-to-noise of the angular correlation function and measure the cross-correlation with specific galaxy types. Most AGN host environments are comparable to the environments of galaxies with the same morphology. However, ~6% of UBR selected AGN are in significantly richer environments. No significant correlation between AGN luminosity and environment was detected in the Panoramic Deep Fields. The richness of AGN environments is not strongly correlated with redshift and the rapid evolution of the AGN luminosity function is not caused by evolution of AGN host environments.
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ItemA spectroscopic and chromatographic study of the photochemical properties of daylight fluorescent paint( 2009)Daylight fluorescent pigments fade rapidly, accompanied by a chronology of colour change. Fluorescence is a photo-physical phenomenon which involves emission of light from an excited state. Fluorescent dyes thus have a high susceptibility of being promoted to an excited state; a characteristic in the case of organic fluorophores which infers vulnerability toward photo-bleaching. Multiple organic fluorescent dyes are routinely incorporated into a given daylight fluorescent pigment, to either additively fluoresce or interact through energy transfer. The organic fluorescent dyes employed invariably differ in photo-stability, and upon loss of each species of fluorophore an abrupt colour change is observed. The collective result of this fading behaviour is that in a short period of time a daylight fluorescent paint layer will be of a different hue, devoid of luminosity. As consequence it is almost impossible to colour match a faded daylight fluorescent paint layer without the hues diverging asynchronously, or ascertain the original palette of a daylight fluorescent artwork after a protracted period of time. The predicament is exacerbated by the fact that there is no standard method in cultural material conservation, of documenting daylight fluorescent colour in a painting photographically or colorimetrically. The objective of this thesis is to investigate the photochemical behaviour of daylight fluorescent pigments, to ensure best practice in the preservation of artworks that contain daylight fluorescent paint. Fluorimetrie and chromatographic analysis of the DayGlo daylight fluorescent pigment range at the constituent dye level, prior to and during an accelerated light ageing program formed the basis of the experimental. Given the limited selection of fluorescent dyes suitable for daylight fluorescent pigment manufacture, it is anticipated that the results attained for the DayGlo range will be applicable to all daylight fluorescent media encountered in cultural material. Experimental data revealed the manner in which the fluorescent dyes behind each DayGlo daylight fluorescent pigment were formulated, and provided explanation for the 1colour changes observed upon fading. A prognosis of when and why a daylight fluorescent palette experiences hue shift and the implications this has for display is presented. Methodology for imaging daylight fluorescence, identification of the constituent fluorescent dyes in a daylight fluorescent pigment and colour matching a daylight fluorescent paint layer are presented and applied in-situ, to case studies possessing a daylight fluorescent palette.
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ItemQuantum technology for 3D imaging of single molecules( 2018)Biochemical processes are conducted by interactions of individual molecules that comprise cells. It is the transient physical shape of proteins that dictates their specific functionality. However, imaging individual instances of single molecular structures is one of the notable challenges in structural biology. Presently available protein structure reconstruction techniques, Nuclear Magnetic Resonance (NMR) spectroscopy, X-ray crystallography and cryogenic Electron microscopy (cryo-EM), cannot provide images of individual molecules. Despite their power and their complementary capabilities, said techniques produce only average molecular information. They achieve this by sampling large ensembles of molecules in nearly identical conformational states. As a result, individual instances of a generic, inhomogeneous or unstable atomic structures presently remain beyond reach. We seek to address this problem in a novel way by leveraging quantum technologies. In quantum computing, qubits are usually arranged in grids and coupled to one another in a highly organised manner. However, what if a qubit was coupled to an organic cluster of nuclear spins instead, e.g. that of a single molecule? What can be done with such a system in the context of quantum control and 3D imaging of individual molecular systems? What are its ultimate limits and possibilities? We explore those questions in stages throughout the chapters of this thesis. We begin in Chapter 2 by investigating dipole-dipole interactions present between the nuclear spins in a target molecule, on one side, and between an electron-spin based qubit and each of the nuclear target spins on the other. We consider the Nitrogen Vacancy (NV) centre in diamond as an example of a suitable qubit with an active community interest as a biocompatible nano-magnetometer. Our intention is to lay down foundations that will help us advance from magnetometry to 3D molecular imaging. Our inspiration comes from drawing parallels between the single molecule sensing in the qubit-target system and the clinical Magnetic Resonance Imaging (MRI). An MRI machine directly images a single, specific sample in its native state regardless of its characteristics. That is precisely what we would like to achieve on the molecular level. In Chapter 3, we develop a framework that allows a spin qubit to serve as a platform for 3D atomic imaging of molecules with Angstrom resolution. It uses an electron spin qubit simultaneously as a detector and as a gradient field provider for MRI-style imaging. We develop a theoretical quantum control methodology that allows dipole-dipole decoupling sequences used in solid-state NMR to be interleaved with the gradient field provided by the qubit. In Chapter 4, we propose group-V donors in silicon as a novel qubit platform for bioimaging. Actively researched for quantum computing purposes, such qubits have not been considered in the biological context. A prime example of this class of qubits is the phosphorus donor in silicon (Si:P). We show how its specific set of properties, including long coherence times, large wave function and low operational temperatures can be leveraged for the purposes of atomic level imaging. Finalising the work in Chapter 5, we simulate the imaging process for one transmembrane protein of the influenza virus embedded in a lipid membrane. This demonstration highlights the potential of silicon spin qubits in the future development of in situ single molecule imaging at sub-Angstrom resolution.
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ItemElectrical Characterisation of Ion Implantation Induced Defects in Silicon Based Devices for Quantum Applications( 2020)Quantum devices that leverage the manufacturing techniques of silicon-based classical computers make them strong candidates for future quantum computers. However, the demands on device quality are much more stringent given that quantum states can decohere via interactions with their environment. In this thesis, a detailed investigation of ion implantation induced defects generated during device fabrication in a regime relevant to quantum device fabrication is presented. We identify different types of defects in Si using various advanced electrical characterisation techniques. The first experimental technique, electrical conductance, was used for the investigation of the interface state density of both n- and p-type MOS capacitors after ion implantation of various species followed by a rapid thermal anneal. As precise atomic placement is critical for building Si based quantum computers, implantation through the oxide in fully fabricated devices is necessary for some applications. However, implanting through the oxide might affect the quality of the Si/SiO_2 interface which is in close proximity to the region in which manipulation of the qubits take place. Implanting ions in MOS capacitors through the oxide is a model for the damage that might be observed in other fabricated devices. It will be shown that the interface state density only changes significantly after a fuence of 10^13 ions/cm^2 except for Bi in p-type silicon, where significant increase in interface state density was observed after a fuence of 10^11 Bi/cm^2. The second experimental technique, deep level transient spectroscopy, was used to study the defects in the substrate of Si after ion implantation. As Er has the potential of interfacing electrical and optical properties of Si based quantum computers, it is important to know what defects will be present after the implantation because of its large atomic mass. H and Er implantation damages were compared to demonstrate the more complex defect evolution for Er implantation. Although defects were still present after a 400 C anneal, the concentration was reduced by at least one order of magnitude. The last experimental technique, charge pumping, was used on MOSFETs to study the interface state density directly in device structures that can be directly used in, for example, magnetic resonance and quantum sensing applications. Charge pumping has the potential of allowing measurement and manipulation of both electronic and magnetic properties of the interface defects and defects in the MOSFET channel. For such applications it may be necessary to operate the device close to absolute zero temperature. The work presented here represents a first step towards device and technique development with the ultimate aim of pushing measurements to mK temperatures where quantum device operations typically operate.
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ItemFlexible electrodes for neural recording, stimulation and neurochemical sensing( 2020)This thesis focuses on the development of implantable neural interfaces to perform multifunctional neural recording, neural stimulation and biochemical sensing. Neural interfacing devices using penetrating electrodes have emerged as an important tool in both neuroscience research and medical applications. These implantable electrodes enable communication between man-made devices and the nervous system by detecting and/or evoking neuronal activities. Recent years have seen a rapid development of electrodes fabricated using flexible, ultrathin microwires/microfibers. Compared to the arrays fabricated with rigid materials and larger cross sections, these microwires/microfibers have been shown to reduce foreign body response after implantation, with improved signal-to-noise ratio for neural recording and enhanced resolution for neural stimulation. Carbon fibers (CFs) are considered for implant into particular tissue types since they have small size, cause less tissue damage, and are flexible. CF recording electrodes have shown promise as recording electrodes and have the properties necessary to form sensing electrodes. Micron-scale electrodes such as CFs are expected to evoke localized neural responses due to localized electric fields. CFs are traditionally used with fast-scan cyclic voltammetry to study rapid neurotransmitter changes in vivo and in vitro, as they allow real-time detection of catecholamines with high sensitivity and selectivity. However, they possess narrow usable voltage range, which limits their application for neural stimulation. Additionally, surface fouling occurs with certain neurochemicals potentially obstructing further neurotransmitter adsorption onto the electrode surface. Therefore, they need to be coated with other materials to boost their electrochemical properties for neural stimulation. In this thesis, diamond and diamond-like materials, in particular nitrogen doped ultrananocrystalline diamond (NUNCD) hybrid and boron doped carbon nanowall (B-CNW) are considered as coatings for CFs to enhance properties towards neural interface applications. A focus is finding acceptable properties for recording, stimulation and neurochemical sensing. Novel fabrication techniques were developed to deposit the films onto the surface of CFs. Firstly, the surface of CFs was amine-functionalised and covalent bonds were formed with oxygen terminated nanodiamonds. Films were grown on the treated/seeded fibers using plasma-assisted chemical vapor deposition. To fabricate single fiber electrodes, individual fibers were insulated with capillary glass with 100 micrometer of fiber exposed. The physical and chemical properties of NUNCD hybrid and B-CNW were characterized and studied. The results from electrochemical characterization, in conjunction with both in vitro and in vivo assessments, suggest that these electrodes offer a highly functional alternative to conventional electrode materials for both recording and stimulation, yielding safe charge injection capacities up to 25.08 +-12.37 mC/cm2. To test the capability of electrodes for neural stimulation in vitro, explanted wholemount rat retina was used. The electrodes could elicit localized stimulation responses in the explanted retina. These electrodes with micron -scale cross sections have the potential to improve the spatial resolution for stimulation while minimizing axon bundle activation. In vivo and in vitro single-unit recording showed that the electrodes could detect signals with high signal-to-noise ratios up to 8.7. NUNCD hybrid coated CFs were able to electrochemically detect dopamine with high sensitivity and selectivity. Such electrodes are needed for the next generation of miniaturized, closed-loop implants that can self-tune therapies by monitoring both electrophysiological and biochemical biomarkers.
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ItemResonant Leptogenesis and Quark-Lepton Unification with Low-Scale Seesaws( 2020)The seesaw mechanism, where a hierarchy exists between the moduli of different entries of a mass mixing matrix, is a simple and theoretically attractive explanation for the observed large hierarchy between the neutral- and charged-fermion masses of the Standard Model. The simplest neutrino mass seesaw predicts that, upon diagonalisation, the physical mass states will either all be Majorana or all form pseudo-Dirac pairs. Non-minimal variants of this seesaw often generate a hybrid scenario with the physical mass states being a combination of both Majorana and pseudo-Dirac pairs. Such models often predict unique phenomenology and also allow for much lower mass scales of new physics. This thesis explores the implications such non-minimal variants can have beyond the simple generation of neutrino mass, particularly the possible role they may have in explaining the observed matter-antimatter asymmetry as well as implications for particular models of quark-lepton unification. Chapter 1 reviews the current experimental evidence for neutrino mass and discusses some possible tree-level origins. The matter-antimatter asymmetry is introduced and the conditions necessary for the dynamical generation of this observed asymmetry are reviewed. The idea of thermal leptogenesis is outlined as a simple mechanism for generating an asymmetry dynamically at an epoch between the the period of reheating and the electroweak phase transition of the early universe. Finally, the idea that quarks and leptons are related by hidden symmetries are discussed with a particular emphasis on the quark-lepton unifying Pati-Salam gauge group. In Chapter 2 we consider the leptogenesis implications for the Standard Model extended by two gauge-singlet fermions for each generation of charged lepton. We focus on the possibility of resonant scenarios without the need for inter-generational mass degeneracies and therefore do not require a possible flavour symmetry origin. The possible connection between neutrino parameters measureable in low-energy experiments and the generation of a matter-antimatter asymmetry is explored. In Chapter 3 we extend the analysis of the previous chapter and highlight how a flavour symmetry can allow for leptogenesis in a much wider region of parameter space for the extended seesaw used in \Cref{Chapter2}. The benefits of this extended seesaw, compared to the minimal seesaw scenario, when the proposed flavour symmetry is included are discussed and implications for low-energy flavour-violation experiments are explored. In Chapter 4 different possible Pati-Salam models are discussed with an emphasis on the connection between the scale of Pati-Salam breaking and the scale of heavy neutrino masses. Models allowing for the breaking scale to occur close to the electroweak scale are introduced. The dominant experimental probe of Pati-Salam is discussed and the current limits on the scale of breaking are calculated. Simple extensions of this model are proposed which both break an undesired mass degeneracy in the theory and allow for a significant reduction in the experimental limits on Pati-Salam breaking. A thorough analysis of the possible allowed parameter space in which both of these effects occur is explored and any possible connection to the symmetries of the theory is made. Chapter 5 briefly concludes.