School of Physics - Theses
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ItemOn the one-dimensional bose gas( 2007-02)The main work of this thesis involves the calculation, using the Bethe ansatz, of two of the signature quantities of the one-dimensional delta-function Bose gas. These are the density matrix and concomitantly its Fourier transform the occupation numbers, and the correlation function and concomitantly its Fourier transform the structure factor. The coefficient of the delta-function is called the coupling constant; these quantities are calculated in the finite-coupling regime, both expansions around zero coupling and infinite coupling are considered. Further to this, the density matrix in the infinite coupling limit, and its first order correction, is recast into Toeplitz determinant form. From this the occupation numbers are calculated up to 36 particles for the ground state and up to 26 particles for the first and second excited states. This data is used to fit the coefficients of an ansatz for the occupation numbers. The correlation function in the infinite coupling limit, and its first order correction, is recast into a form which is easy to calculate for any N, and is determined explicitly in the thermodynamic limit.
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ItemCharacterisation of Optical Fibres Based on High Resolution Imaging( 2004-12)This thesis presents a method for accurate characterisation of optical fibres using highspatial resolution imaging. Increasing demands in the application of optical fibres hasproduced a rapid advance in fibre fabrication and optical fibres are being createdexhibiting features that are beyond the optical resolution of current characterisationtechniques.High resolution imaging of optical fibres was performed using an atomic forcemicroscope (AFM). Germanium doped fibres were chemically etched in hydrofluoricacid solution (HF) and the wet etching characteristics of germanium were studied. Amethod of calculating the refractive index profile from an AFM depth profile isdemonstrated. The refractive index profile extracted from AFM data was compared withQuantitative Phase Microscopy (QPM) and nanometer Secondary Ion MassSpectrometry (nanoSIMS) measurement techniques.In addition, AFM was employed to study the diffusion of the dopant produced by thearc fusion splicing technique using a fluorine-doped fibre and a multiple-dopant fibre.Furthermore, the phase and the refractive index profiles of a thermally expanded core(TEC) fibre were performed using quantitative phase microscopy (QPM).The application of the AFM to the resolution problem provides a fundamentalenhancement on the optical resolution from of around 400 nm to 500 nm. This providesconsiderable new structural information about the fibre.
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ItemThe thermal memory of reionization( 2012)The reionisation of cosmic hydrogen represents a significant moment in the history of the Universe. The appearance of the first stars and quasars heated and ionised the intergalactic medium (IGM), catalysing the transition in which intergalactic hydrogen changed from being predominantly neutral to its present day, highly ionised state. Understanding exactly how and when this phase transition occurred will offer significant insight into the nature of the first sources of light. Measurements of temperature in the IGM provide a potentially powerful constraint on reionisation history due to the thermal imprint left by photo-ionisation of neutral hydrogen. However, until only recently, IGM temperature measurements were limited to redshifts 2 ≤ z ≤ 4.5, restricting the ability of these data to probe the reionisation history at z > 6. In this work, we use the first direct measurements of IGM temperature in quasar proximity zones at z ~ 5.5 − 6.5 to establish new constraints on the redshift at which inhomogeneous hydrogen reionisation completed. We utilise a semi-numerical reionisation model to trace the propagation into the IGM of HI ionisation fronts produced by stellar sources. Then, calibrating the model to reproduce observational constraints on the electron scattering optical depth and the HI photoionisation rate, we compute the resulting spatially inhomogeneous temperature distribution at z = 6 for a variety of reionisation scenarios. Under suitable assumptions for the ionising spectra of population II sources, we determine that for reionization scenarios complete by zr > 9.0, the IGM settles to roughly the same asymptotic thermal state at z = 6.0 irrespective of when reionization occured. However, for zr < 9.0, the ionization of neutral hydrogen leaves behind an imprint, that is still up to 5 000K above the thermal asymptote at z = 6.0. Using our models, and the temperature measurements around quasar proximity zones, we constrain the redshift by which reionisation was completed to be zr > 7.9(6.5) at 68(95) per cent confidence. We conclude that future temperature measurements around other high redshift quasars will increase the power of this technique, enabling us to both tighten and generalise this result.
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ItemCosmic ray measurement( 1950)The following report embraces the work carried out by the author in the Physics Department of the University of Melbourne, from January 1948 to February 1950. It is divided into five sections: Section A. Theory and Design of Ionization Chambers. B. Continuous Recorder. C. Extensive Shower Apparatus. D. Polar Continuous Recording Rack. E. Design & Construction of the Electrodynamic Electrometer. The records and apparatus described in Section B , together with the first electrodynamic electrometer, were a total loss in the hut fire. The original version of the extensive shower apparatus, the project of J.R. Prescott, was also destroyed, ant the author assisted in its modification and rebuilding. (From Introduction)
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ItemNumerical and analytical approaches to modelling 2D flocking phenomena( 2011)In the first section of this thesis, the motion of self-propelled particles (known as boids) in a 2D system with open boundaries is considered using a Lagrangian Individual-Based model. In fact, two different variations of this model are developed, one with a cohesion potential based on the soft core Morse potential and the other based on the hard core Lennard-Jones potential, both well understood in the fields of atomic and molecular physics respectively. The results obtained from these two different variations are then compared with one another, as well as with earlier work in the field, in order to determine the effectiveness and applicability of hard-core and soft-core cohesion potentials, the differences in flocking behaviour they produce, and context in which each one is applicable to real flocking systems. Some of the flocking phases and shapes obtained from these two different models are then compared to a number of specific flocking situations observed in the real world. It is shown that a flocking model with a soft-core cohesion potential is particularly good at modelling the cluster type flocks most commonly seen in bird and fish schools, whilst a hard-core cohesion potential has a tendency to produce a distinctive wavefront type flock which has been well documented in the context of large herds of mammals, particularly wildebeest. It is also found that stable vortex states, observed in systems of bacteria, are only seen with a soft-core cohesive potential. In the second section of this thesis, a novel approach is taken to model a flocking as a gas. An equation of state is derived analytically for a flocking system in 2D using the virial expansion. The relationships obtained from this equation of state are compared to the results of a simple numerical simulation in order to establish the accuracy of this new approach to modelling flocking systems. Finally, this new statistical mechanical approach to deriving a flocking model is applied to the Vicsek model, the most well understood flocking model derived from a physics perspective. It is found that the analytical relationships between a number of key variables of the gas system such as pressure, temperature, interaction length, entropy and heat capacity derived from the virial equation of state bear a close comparison to those same variables derived numerically from the standard Vicsek model, thus demonstrating the efficacy and veracity of this new approach to modelling flocking phenomena.
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ItemThe energy levels in atoms( 1928)
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ItemFrom geometric phases to intracellular sensing: new applications of the diamond nitrogen-vacancy centre( 2010)This thesis consists of two parts, each of which proposes a new application of the diamond nitrogen-vacancy (NV) centre. We first consider the NV centre as a device to detect geometric phases. We show that the Aharonov-Casher phase and Berry’s phase may be produced in the NV centre’s spin sublevels and observed using existing experimental techniques. We give the background theory to geometric phases, then show how these phases apply to the NV system. Finally, we outline a number of realistic experiments to detect these phases. The second part considers the behaviour of an NV centre within a diamond nanocrystal which rotates, in a Brownian sense, in a fluid. Our aim is to understand the effect of rotational motion on the initialisation, evolution and readout of an NV centre, motivated by the idea of using colloidal nanodiamonds for biological imaging. We first develop a model to describe the quantum evolution of a rotationally diffusing nanocrystal. The model uses theory developed in NV magnetometry and also the geometric phase theory developed in the first part of this thesis. We then explore the consequences of this model for nanoscale sensing. We show that the tumbling NV system may be used as a sensitive magnetometer with nanoscale resolution and also as a probe of its own rotational motion.
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ItemChannelled ion beam analysis of diamond( 2010)The remarkable attributes of diamond make it a promising material for the implementation of emerging quantum technologies for information processing and communication. In particular, the colour centres of diamond have been shown to have desirable properties for use as single photon sources and for the storage and manipulation of quantum states that can act as optically readable quantum bits. Successful exploitation of these attributes requires the development of new methods to controllably insert dopant atoms into the diamond lattice and to fabricate photonic structures around the emitters. In particular, monolithic diamond has important advantages so that emitters can be embedded in the centre of the optical mode. In this thesis, both the creation of centres, and the creation of photonic structures is addressed. Colour centres based on Ni containing diamond have desirable luminescence in the near infra-red but the location of Ni in the diamond lattice is not known. Here the lattice location of dopants in Ni containing high pressure, high temperature diamond is investigated by Rutherford backscattering with ion channelling and use of the ion induced X-ray signal from the trace Ni. This work reveals that, despite the dense packing of the diamond lattice, Ni is substituted into the diamond lattice. Thin diamond membranes, with promising attributes for photonic devices, can be fabricated by deep ion beam lithography. A crucial attribute for devices in these membranes is the quality of the membrane surface which is examined here with ion channelling. This is highly sensitive to the membrane surface damage produced by the lithography process. In this case the surface is shown to contain significant residual ion beam damage. Transmission electron microscopy is used to directly observe this interface. Residual surface damage is further quantified by complementary measurements with X-ray photoelectron spectroscopy and X-ray absorption fine structure measurements which probe the atomic bonding.
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ItemA model of fermion masses without a Higgs mechanism( 2010)The purpose of this thesis is to understand the origin of fermion masses in a model where the electroweak symmetry is not a fundamental gauge symmetry. Electroweak symmetry is instead a global symmetry of a new strongly interacting sector, with the electromagnetic symmetry remaining as a gauged subgroup. The W and Z-bosons are thus composite vector mesons, which obtain their mass from strong dynamics rather than the traditional Higgs mechanism in spontaneously broken gauge theories. As a result, the hierarchy problem is evaded since the compositeness scale is exponentially small compared to the Planck scale MP. Fermions obtain mass by Yukawa coupling to a scalar vacuum expectation value, which breaks electroweak symmetry at the Planck scale. Interestingly, the natural scale of fermion masses is << M_P and the hierarchical pattern of masses and mixings can be solved by Planck-scale physics. After reviewing important background material on flavor physics, extra dimensions and branes, we describe the relationship between warping and compositeness using the holographic principle inspired by the anti de-Sitter/conformal field theory correspondence (AdS/CFT). Despite the non-perturbative nature of the physics underlying composite models, the technology of AdS/CFT is employed to construct a higher-dimensional, weakly-coupled description of fermions interacting with composite electroweak vector bosons in four spacetime dimensions. The techniques of electroweak precision analysis are explained and then applied to the model, using the higher-dimensional model as a calculational tool. It is demonstrated that despite the composite nature of the W/Z-bosons, the model is consistent with electroweak precision data for the first two generations, while the third generation leads to a tension with indirect bounds. Moreover, the model is shown to predict definite deviations from the Standard Model parameters at energies that will be explored by the LHC.
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ItemRelative intensity squeezing: by four-wave mixing in rubidium( 2010)This thesis is a theoretical and experimental study of the production of relative intensity squeezed light through four-wave mixing in a rubidium vapour. Relative intensity squeezing enhances measurement precision by using quantum-correlated “twin beams” to eliminate photon shot-noise. The “double-^” four-wave mixing process produces twin beams by stimulating a four-stage cyclical transition resulting in the emission of time-correlated “probe” and “conjugate” photons. Measuring and subtracting the corresponding beam intensities cancels the photon shot-noise, enabling measurements beyond the shot-noise limit. An ab initio analysis of the double-^ scheme determined the experimental phase-matching conditions required to generate efficient mixing. Expressions for the expected level of squeezing were then derived. Deviations from perfect matching were considered and a spatial bandwidth for the mixing process was derived. This bandwidth was used to explain recent experiments obtaining multi-mode squeezed light from this system. Optical losses are an experimental inevitability that destroy quantum correlations by randomly ejecting photons. Expressions were derived to quantify the degradation of squeezing caused by losses. Sensitivity to unbalanced losses was exhibited and an optimum level of relative beam loss was observed. Near-resonant absorption within the vapour causes losses to compete with squeezing, so an interleaved gain/loss model was formulated to analyse the interplay of the two processes. A novel theoretical framework was developed and used to derive expressions for the level of squeezing produced in the presence of absorption. Four-wave mixing resonances were observed experimentally and the intensity noise spectra of the resulting beams were characterised. Gain dependence on beam power, cell temperature and laser detuning was determined. Relative intensity squeezing of 3 dB was demonstrated and physical insight into the experimental results was gained through analysis with the theoretical model. Factors limiting the measured level of squeezing are discussed and design improvements proposed.