School of Physics - Theses
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ItemErbium for optical modulation and quantum computation( 2017)Erbium (Er) is a lanthanide element, mainly used in its trivalent ionic form (Er3+), as an active dopant in optical devices, for light amplification or generation. The luminescence of Er3+ lies within the conventional wavelength band, 1530-1565 nm, for fiber-optic communication. The low noise, linear response, and stability of the optical gain provided by the Er3+ luminescence are ideal for applications in photonic systems that operate in the fiber-optic frequency. While much research has been done to understand the Er3+ luminescence in various lasing media, few studies have been conducted to tap the potential of Er for applications other than amplifiers or lasers. This thesis delves into two new areas, namely optical modulation and quantum computation, where the Er3+ luminescence may be able to be applied in a novel way. By incorporating Er3+ into a switchable optical material, an optical modulator could potentially be made that is capable of not only switching but also amplifying signal transmission or sustaining the signal intensity from propagation losses. This integrated approach could reduce device footprint and latency for on-chip as well as synchronous applications. Successful integration of Er, however, has never been demonstrated in conventional optical modulators because their reliance on electro-optic effects conflicts with the carrier-sensitive mechanism of the Er3+ luminescence. The compatibility between Er and a recently advocated optical material, namely vanadium dioxide (VO2), is examined in the first part of this thesis. VO2 exhibits a hysteretic, bistable phase transition that is accompanied by a high-contrast optical switching in infrared, including the fiber-optic, wavelength band. The phase transition can be triggered thermally as well as optically. When triggered optically, it can occur in picosecond timescale, making VO2 a promising material for ultrafast optical switching applications. Experimental characterizations of the Er3+ luminescence and the optical switching were performed on selectively prepared thin-film samples of VO2. The Er3+ luminescence could be observed after the samples were implanted with Er and then annealed between 800*C and 1000*C. The optical switching could also be measured in the implanted and annealed samples as they were thermally heated up and then cooled down past the critical temperature of the phase transition. The Er-implanted samples, however, were found to have broader hysteresis and lower switching contrasts than the pure VO2 samples. It is concluded that although Er-implanted VO2 could probably work as a combined optical switch and amplifier, the poorer switching qualities do not guarantee that a device based on the material could provide better utility than a separated system of optical switches and Er amplifiers. The Er3+ luminescence could also be utilized for quantum frequency conversion, for implementation in interconnects that interface superconducting quantum computers to a fiber-optic quantum network. For two superconducting quantum computers to be able to communicate over a fiber-optic quantum network, the frequency of the signals transmitted from either computer needs to be converted into the fiber-optic frequency, and then back into the microwave frequency upon receipt at the other computer. Early proposals suggested that the interconnect be at least comprised of Er3+ ions, a microwave resonator, and an optical resonator. The realization of this system has been attempted recently in basic experiments, but the conversion efficiency was found to be too low. The weak couplings between the Er3+ ions and the two resonators were identified as one of the main reasons for the low conversion efficiency. One way to mitigate the weak coupling in the microwave part is to have a superconducting flux qubit bridge the interaction between the Er3+ ions and the microwave resonator. The second part of this thesis presents a theoretical and simulation study of the dynamics of a coupled system consisting of Er3+ ions, a superconducting microwave resonator, an optical resonator, and a superconducting flux qubit. It is shown that quantum information can be exchanged between the Er3+ ions and the microwave resonator with a high fidelity via the qubit coupling, and the exchange process is controllable by changing the frequency of the qubit. The frequency conversion between the microwave and the optical regime is shown to be infeasible to be realized at the limit where the number of optical excitations (n) is much less than the number of ions (N). A high-efficiency frequency down-conversion is demonstrated to be achievable in the case where there is no decoherence, and both n and N are small. However, the time it takes to complete the down-conversion is very long, leaving the efficiency prone to decoherence. It is argued that for the frequency conversion to be able to be accomplished in a typical decoherent environment, both n and N need to be large. The study of the dynamics, in this case, is left for future research.
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ItemUltrafast spectroscopy of nanostructures( 2017)This thesis presents studies of ultrafast laser spectroscopy of semiconductor and gold nanostructures, aiming to advance our understanding of, and consequently control, photoinduced charge carrier dynamics in nanostructures to further improve their performance in practical applications. Artificial nanostructures have drawn significant attention in applications such as optoelectronic devices, photo-catalysts, and solar cells. Compared to bulk materials, nanostructures provide unique optical properties, which more importantly can be directly and easily tailored through changing size or shapes of the structures, during their synthesis procedures. Photoinduced charge carrier dynamics in the nanostructures play an important role in the photon conversion processes. However, in contrast to the fast development of nanostructure-based devices, the mechanisms of these processes are still being experimentally unravelled. In this study, a range of ultrafast optical spectroscopy methods have been applied to investigate the carrier dynamics, with a focus on the electron transfer (ET) process. Semiconductor nanoparticles, or quantum dots (QDs), of core/shell heterostructures are promising for their good photostability and high photoluminescence quantum yields. The ET dynamics from the 1S$_\mathrm{e}$ electron state to adsorbed methyl viologen electron acceptors, in CdSe/CdS and CdSe/CdS/ZnS QDs, were studied using femtosecond transient absorption and time-resolved photoluminescence spectroscopy. By changing shell thickness or alloying the shell interface, significant modulation of the ET dynamics was observed. In CdSe/CdS QDs, the 1S$_\mathrm{e}$ ET dynamics exhibited a hole-coupled effect, which is ascribed to the Auger-assisted ET process. In CdSe/CdS/ZnS QDs, the formation of alloyed shell interfaces at elevated shelling temperatures reduced the shell potential barrier, leading to an observed greater ET rate. Photoinduced ET processes from gold nanorod and nanowire structures to TiO$_{2}$ were also investigated, using a visible pump-NIR probe transient absorption spectroscopy method. Partially embedded Au nanorods on a TiO$_{2}$ layer exhibited an enhanced but directional ET process. An Au nanowire grating supported on a TiO$_{2}$ layer structure underwent the plasmon-waveguide hybridisation mechanism. The ET dynamics from the split states showed a dependence on the light-matter coupling effect that can be varied with the Au grating period. In summary, this thesis shows the great ability of ultrafast optical spectroscopy to reveal photoinduced processes in nanostructures. Results indicate ways for rational design of nanostructure-based devices. A greater understanding in underlying physics leads to better control of the performance of these nano-systems in potential practical applications.
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ItemNon-invasive imaging techniques for the investigation of cultural objects( 2017)Laser speckle is used in cultural materials conservation to study changes in the physical condition and movement on the surface and within the structure of objects. This thesis investigates three speckle methods, Laser Speckle Contrast Imaging (LSCI), Dynamic Speckle Imaging (DSI) and Electronic Speckle Pattern Interferometry (ESPI) to: image obscured text, examine paint application methods, record physical transformations during paint drying processes, and monitor canvas strain due to climate variations in- situ. It extends the boundaries of use of speckle-based methods in conservation through a set of four case studies that cover a broad range of cultural conservation problems faced in Australian collections. These studies were contextualised within an examination the properties of viscoelastic materials, a mathematical description of speckle, an explanation of the speckle-based methods and the experimental equipment and design considerations that provide information about the challenges and considerations required when deploying speckle-based methods. In the first case study, LSCI was used to image obscured text on books that had been repaired with layers of paper. The quality of the results, which was dependent on the weight of the paper, colour of the subsurface medium, and temperature of the object, was similar to that of Infrared Photography. In the second case study LSCI was used to monitor the curing process of modified alkyd resins over 120 hours to investigate whether the application method used by the artist was robust to typical ageing conditions. The results correlated well with traditional gravimetric results, which only provide information about the mass changes to the samples with no spatial information. LSCI was able to demonstrate where and when the alkyd resin surface was moving, most likely due to off gassing processes. The third case study used DSI to monitor the drying process of artists’ acrylic paints, allowing for the novel non-invasive detection of when the paint was touch dry, a task that normally involves destructive methods or has only been applied to rapid drying industrial paints. The final case study demonstrated the in-situ application of ESPI to monitor two archetypical colonial easel paintings in assessing physical movement due to variations in the display environment. ESPI was used in-situ to record changes in the strain of the canvas. Furthermore, it was shown that DSI analysis could be applied to ESPI data to monitor for vibrational disturbances that can decorrelate results The speckle-based techniques used in this thesis were shown to satisfy six criteria for preferred conservation methods: all information is gathered non-invasively and can be analysed in a variety of ways; off the shelf optical components are used, thus it is highly suitable for fields in which specialised optical components are not available; no prior analysis about the sample is required; experimental challenges can be diagnosed and overcome through the analysis of data sets and/or computer simulations; the techniques can be applied to the investigation of a range of materials with different optical properties; and multiple speckle-based methods can be combined to provide complementary information. The case-studies explored in this thesis further demonstrated that speckle is capable of non-invasive characterisation of many physical properties and can be applied both in the laboratory and in-situ. Through the assessment of theory and case studies this work has extended the application of speckle analysis within conservation studies, delivering practical outcomes to aid conservation efforts.
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ItemClassical processing for topological quantum error correction( 2017)This thesis deals with a series of issues implementing the classical software stack to control a large-scale quantum computer. Due to the relatively high error rates of quantum hardware expected for the foreseeable future, high threshold error correction schemes such as the topological cluster state and the surface code, will be essential to performing robust quantum computation. The work begins by bringing new functionality to the Autotune software library to support the presence of runtime qubit loss when using the topological cluster state for error correction. Through simulation, it is shown that <1% runtime loss is required for minimal space-time overhead. The thesis continues by constructing new software, LogiQSim, which enables fully-fault-tolerant quantum simulation and error analysis of logical quantum gates. Through the implementation of byproduct operator tracking and the placement of defects and correlation fragments, it was possible to demonstrate a full quantum simulation of a working logical controlled NOT gate (CNOT), as well as estimate its logical error rate directly through simulation. This approach was further improved to allow the simulation of the Clifford portion of A-state distillation, significant as this circuit is the predominant source of overhead in quantum computation when using the topological cluster state. The simulated overhead of the distillation process was compared to estimates calculated from the results of the logical CNOT simulations in order to determine the accuracy of estimating a larger circuit using the simulations of a smaller one. Furthermore, we found that the relationship between the structure of a correlation fragment and its contribution to the logical error rate is complex. This is due to an asymmetry in the frequency of logical errors that are the result of error chains connecting two defects as compared to rings of errors around defects. The work presented in this thesis makes substantial progress towards the understanding and development of a practical classical software stack that can be used to run a topologically error corrected quantum computer, and utilizes this (in the absence of physical hardware) to simulate and provide more accurate overhead estimates for key logical operations.
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ItemNano-electronics at graphene-electrolyte interfaces for biological application( 2017)Research of graphene for life science has attracted significant attention from not only the scientific community but also the general public. Graphene possesses many exciting properties which are beneficial for biological applications, such as biocompatibility, tunable chemical properties, atomic thickness, among others. Detection of biomolecules in liquids, such as the electrolytes encountered in biological environments, is a key application. However, the physical processes at the graphene-electrolyte interface are not fully understood. This thesis presents a theoretical and experimental study of this system, and demonstrates its application in bio-sensing. Electrical double layer (EDL) is a critical component necessary for the investigation of the processes at solid-electrolyte interfaces. A new ionic vibration model for the EDL as an improvement to the conventional Gouy-Chapman-Stern (GCS) theory is proposed in chapter 3, as the GCS fails to describe the EDL capacitance over a wider range of electrode potentials. This new theory elucidates the mechanism behind the ionic dynamics at the interface of solid-electrolyte systems, as ions are found to vibrate near the electrode surface in response to the applied electric field. The calculated results are consistent with measurements, and this provide useful insight not only important for biosensing (as we may need to change the potential on graphene over a wide range), but also for energy storage, neuron stimulation, among others. Since some of these applications (e.g.: neuron stimulation) require large capacitance, and this theory indicates that the EDL capacitance decreases with the electrode potential, one of the approaches is to increase the surface area of the electrode material (graphene). Conformally coating graphene onto a nanoporous three-dimensional (3D) structure gives 3D graphene with very large surface area. A novel method of directly fabricating 3D graphene by plasma assisted graphitization is presented in chapter 4. The obtained material shows very large electrochemical capacitance values, as high as 2.1 mF for a sample of 10 mm$^3$. It also exhibits excellent chemical stability, providing a good platform for electrochemical applications (e.g.: energy storage, neuron stimulation, etc.). This sample is then used to capacitively detect a protein (bovine serum albumin) as a non-destructive label-free method. The results can inspire the work on developing bio-sensors which are able to work as bio-filters simultaneously in future, due to the nano-porous structure. However, the unclear sensing mechanism (whether EDL capacitance or quantum capacitance is dominant), low sensitivity, and long response time of the sensor raise the task for searching for different sensing mechanisms. Based on the discussion of electron transport of graphene in solution in chapter 5, a new method for the detection of biomolecules at very low concentrations using Hall effect measurements on liquid gated graphene based devices is proposed in chapter 6. The results of L-histidine detection suggests high sensitivity in the pM range, exceeding the performance of the conventional amperometric and potentiometric techniques under the same conditions. Through the determination of changes of the mobility and the charge carrier density in graphene we provide new clues for understanding different mechanisms of molecular detection on graphene-based devices in liquids. Theoretical modeling of the experiments indicates that the asymmetric electron-hole mobilities can explain the enhanced detection sensitivity. Quantum capacitance is only dominant near the "Dirac" point, while EDL capacitance dominates at higher potential. This explains the drastic changes of the measured electronic properties close to the "Dirac" point. The detailed theoretical and experimental studies presented in this thesis provide a deep understanding for graphene-electrolyte systems, which are important not only for biological applications (such as biosensing, neuron stimulation and recording, bionic implants, etc), but also for energy storage systems.
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ItemInvestigation of B+ mesons decay to K+K−π+ at the Belle experiment( 2017)Charmless decays of B mesons to three charged hadrons are suppressed in the Standard Model, and thus provide an opportunity to search for physics beyond the Standard Model. An unexpected excess and a large CP asymmetry in the low invariant mass spectrum of the K+K− system for the decay B+ → K+K−π+ were observed by BaBar and LHCb in recent years. We present the measurements of branching fraction and direct CP asymmetry of the charmless decay B+ → K+K−π+. This analysis is performed on a data sample of 772 × 10^6BB pairs produced at the Υ(4S) resonance by the KEKB asymmetric-energy e+e− collider and collected by the Belle detector. We perform a blind analysis, examining signal reconstruc- tion and background suppression with Monte Carlo simulated samples, and extract signal yield and direct CP asymmetry with a 2D extended maximum likelihood fit to the data. The measured branching fraction and direct CP asymmetry are B(B+ → K+K−π+) = (5.38 ± 0.40 ± 0.35) × 10^−6 and ACP = −0.170 ± 0.073 ± 0.017, respectively, where the first uncertainties are statistical and the second are systematic. These results are in agreement with the current world average. We extract the branching fraction and direct CP asymmetry as a function of the K+K− invariant mass. The K+K− invariant mass distribution of the signal candidates shows an excess in the region below 1.5 GeV/c^2, which is consistent with the previous studies from BaBar and LHCb. Strong evidence of a large direct CP asymmetry of −0.90 ± 0.17 ± 0.03 with 4.8σ significance is found in the K+K− low-invariant-mass region.
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ItemElectron diffraction using a cold atom source( 2017)Observing matter on atomic length and time scales simultaneously is now routinely achieved in ultrafast electron and X-ray imaging techniques, but continued advances in both approaches promise to deliver huge leaps in our understanding of all kinds of atomic structures and processes. Both technologies rely on the generation of ultrabright, ultrashort duration electron bunches, with these bunches being used directly to probe the sample in electron diffraction, or to generate ultrabright X-ray pulses in X-ray free electron lasers. The Cold Atom Electron and Ion Source (CAEIS) was conceived as a possible way to generate ultrafast electron bunches that are brighter than can currently be produced, with the aim of enabling next-generation structural determination techniques, particularly those based on electron diffraction. The CAEIS generates electrons by near-threshold photoionisation of an atomic gas, which has been shown to produce electron bunches with temperature as low as 10K. Such cold electron bunches have the potential to be much brighter than those generated from solid photocathode sources, which typically have temperatures in the thousands of Kelvin range. Extremely cold ions are also generated in the CAEIS, which show great potential for use in ion microscopy and milling. This thesis presents work on a number of different aspects of the continued development of the cold atom electron and ion source, with a particular emphasis on progress towards ultrafast single-shot electron diffraction based experiments. The brightness degrading effects of space-charge repulsion are investigated using nanosecond duration ion bunches as analogues of ultrafast, picosecond duration electron bunches. Ion bunch shaping was achieved through tailoring of the spatial profile of lasers used in ionisation of the atomic gas. It was found that atomic fluorescence could substantially reduce the fidelity with which the ion bunch profile could be controlled, but methods were developed to circumvent the fluorescence problem. The improved shaping procedures allowed generation of uniformly filled ellipsoidal bunches, which theoretically will not suffer emittance degradation under space-charge expansion. Emittance measurements following space-charge driven expansion showed that these uniformly filled ellipsoids did indeed have reduced emittance growth compared to other profiles. Photoexcitation and field-ionisation processes involved in generation of cold electrons on ultrafast timescales were investigated, with the aim of determining the mechanisms that affect the ultimate electron bunch duration. Bunch duration was measured for a range of excitation conditions, with the finding that previously assumed ultrafast excitation pathways in fact generated fairly slow nanosecond long bunches. Ionisation time could also be a million times slower than assumed if atoms were excited below the classical ionisation threshold. Identification of the conditions required for ultrafast excitation and ionisation ultimately allowed generation of ultrafast cold electron bunches with duration of tens of picoseconds. Electron diffraction using nanosecond long electron bunches was achieved in both transmission and reflection modes for a variety of large samples of inorganic crystals. Bunches were of sufficiently high charge to allow identification of features of a crystal structure using only a single shot. Bragg peaks could also be identified by averaging together many images formed using ultrafast, but low-charge bunches. Simulations were performed to determine the feasibility of using electrons generated in the CAEIS for electron coherent diffractive imaging of nanoscale apertures. It was found that it should be possible to successfully reconstruct the object plane wavefield, even taking into account realistic experimental parameters for partial coherence and noise.
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ItemNovel plasmonic elements for the generation, manipulation and detection of polarised light( 2017)Plasmonics provides an opportunity to develop nanoscale optical devices, where the spectral, angular and polarisation response can be tailored. The aim of this research is to determine which designs prove suitable as polarisation sensitive features, how novel nanofabriction techniques can be employed to scale up the production and in what settings can we use these scalable plasmonic polarising devices to address current nanophotonics challenges. Presented here is a study of potential plasmonics based methods of manipulating polarisation, including converting the polarisation state of a beam from linear to circular with carefully designed cross nanoapertures in a metallic film, strong filtering of left and right circular polarised light using 2D chiral geometries as well as creating a compact nanoantenna-enabled metal--semiconductor--metal photodetector to determine the polarisation state of a beam. To ensure the economic feasibility of the devices, special attention is also paid to novel scalable nanofabrication techniques. The cost of a feature is of great concern, as it is of little use to have an expensive feature for applications in consumer photonics. To that end a direct imprinting technique for the low cost production of plasmonic metasurfaces is investigated, including a study of the optical phenomena achievable and some potential applications are discussed. Finally, altering the quantum properties of emitters coupled to these scalable plasmonic features is investigated. Particular attention is paid to increasing the emission rate and polarising or focussing light from quantum sources using plasmonic nanocavities. These plasmon-exciton devices could see a reduction in the energy requirements of LED displays.
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ItemStrong gravitational fields and radiation from neutron stars( 2017)Part 1 of this thesis is dedicated to the study of the gravitational field in the strong field regime. In particular, we focus on modified theories of gravity, exploring the implications of high-energy corrections to Einstein's equations in the context of black holes and gravitational waves. Chapter 1 offers a literature review of general relativity and modified theories of gravity. In general relativity, multipole moment constructions allow one to decompose a general metric tensor in order to analyse the physical properties of the solution in detail. For example, the Kerr metric has two free parameters in it, which can be identified as the mass and angular momentum of the black hole through multipole analysis. In Chapter 2 we generalise these multipole moment constructions, with the goal of classifying black holes in modified theories of gravity. One can attempt to design experiments aimed at searching for `non-Einstein' black hole parameters. We explore whether the frequencies of quasi-periodic oscillations from galactic microquasars are consistent with a particular generalised-Kerr black hole solution, after a suitable interpretation of non-Kerr parameters has been made. We use X-ray measurements of the microquasars GRS $1915+105$ and GRO J$1665-40$ to constrain the properties of their black holes. The Ernst formalism of general relativity is a tool used to reduce the tensorial Einstein equations to a single, non-linear differential equation for a complex-valued scalar function. The formalism can be used to generate exact solutions to the Einstein equations. In Chapter 3 we present a generalisation to the $f(R)$ theory of gravity, which allows us to find new exact solutions. We find some exact solutions which represent the gravitational field outside of ellipsoidal, compact objects, and we find some solutions representing exact, solitonic gravitational waves which propagate with arbitrary phase velocity $v \neq c$. We show that neutron stars can be arbitrarily `hairy' in some modified theories of gravity, in the sense that the metric coefficients can depend on an arbitrary number of free parameters. Using the Ernst formalism developed in Chapter 3, we focus on the nature of causality in modified gravity. In Chapter 4, we show that some theories predict the existence of exact, faster-than-light gravitational waves, even when the linearised theory forbids them. The implication is that gravitational information may propagate at a different speed to electromagnetic information, which would have astrophysical consequences for highly relativistic systems such as black holes surrounded by plasma. We show that some theories permitting tachyonic gravitational waves are consistent with data coming from solar system and pulsar timing experiments. In Chapter 5 we investigate black hole structure in modified theories of gravity. We ask the question of what restrictions are placed on a modified theory of gravity if one assumes that event horizons are spherical; a feature of black holes expected from our intuitive understanding of gravitational collapse. We show that spherical horizons are guaranteed if a certain differential inequality is satisfied. This inequality can be used to place constraints on any given theory of gravity. In the context of $f(R)$ gravity, we show that the spherical-horizons condition is consistent with the `no-ghosts' (no negative energy modes) condition. Chapters 2 through 5 are therefore geared towards trying to reduce the pool of possible modified theories of gravity through various theoretical and empirical tests. When considering modified theories of gravity, one faces a degeneracy problem of sorts; non-Einstein gravity with ordinary matter might look like Einstein gravity but with exotic matter, or vice-versa. Part 2 of this thesis is concerned with the other half of the field equations; the matter portion. We focus in particular on the physics of neutron stars with asymmetric mass-density and fluid-velocity distributions, i.e. on continuous gravitational radiation from neutron stars. Our goal is to quantitatively explore the astrophysical properties of neutron stars from gravitational and electromagnetic observations. Chapter 6 serves as a literature review aimed at exploring the physics of neutron stars in various settings. Time-dependent multipole moments lead to gravitational radiation as the star spins. A detection of this radiation would then give us some information, otherwise invisible in the electromagnetic spectrum, about how the star is behaving. In Chapter 7 we consider quantum spin effects of the neutrons, protons, and electrons which comprise the stellar fluid. We find that the resulting quantum force terms may be large $(\chi \gtrsim 1)$ or small $(\chi \lesssim 10^{-3})$ in a neutron star environment depending on the degree of saturation physics, such as the formation of magnetic domains, limiting the value of the magnetic susceptibility $\chi$. We show that the terms are likely to be small except perhaps in magnetar $\left[|\boldsymbol{B}| \gtrsim 10^{13} \left(T_{e}/10^{9} \text{ K}\right) \text{ G}\right]$ environments, where paramagnetic forces can amplify (up to a factor $\sim 10$) the strength of any and all gravitational radiation. In some neutron stars, there is an observational discrepancy between magnetic field strengths inferred from cyclotron lines and spin-down estimates. It has been suggested that strong, non-dipole components may be present in the magnetic field structure, which would account for the apparent inconsistency. In Chapter 8 we investigate a non-ideal magnetohydrodynamic phenomena, known as Hall drift, in the context of neutron star models. In particular, we show that strong, non-dipole components can emerge naturally through Hall drift on timescales of $t \gtrsim 10^{4} \text{ yr}$. The Hall drift necessarily modifies the equilibrium structure of the star, which bolsters the expected gravitational wave luminosity. We find that, if Hall drift is responsible for the magnetic field strength discrepancy, old $(\gtrsim 10^{5} \text{ yr})$ radio pulsars should emit continuous gravitational wave signals which are large enough $(h_{0} \gtrsim 10^{-26})$ to be measurable in the near future with ground-based interferometers such as the Laser Interferometer Gravitational-Wave Observatory (LIGO). In accreting neutron star systems like low-mass X-ray binaries (LMXBs), the time-averaged spin-up torque from accretion is expected to add angular momentum to the neutron star at a rate of $N_{a} \approx \dot{M} \left( G M_{\star} R_{\star} \right)^{1/2}$, where $\dot{M}$ is the accretion rate. It is therefore puzzling that we observe no neutron stars spinning faster than $\nu_{\text{observed}} \lesssim 700 \text{ Hz}$, which is much less than the theoretical maximum set by the break-up limit $\nu_{\text{max}} \gtrsim 1.5 \text{ kHz}$. In Chapter 9 we consider mass-density asymmetries in the form of physical mounds (`magnetic mountains') which build up on neutron star surfaces, when matter is accreted from a main sequence companion. The built-up mountains reduce the global dipole moment of the neutron star by forcing magnetic field lines to buckle underneath infalling matter, and also excite gravitational radiation in the process. The gravitational radiation reaction associated with the magnetic mountain may be responsible for capping the spin frequency of the star. We investigate the characteristics of magnetic mountains when thermal conduction is treated for the first time in self-consistent, numerical simulations. We find that thermal conduction has the effect of softening the equation of state of accreted material on timescales $t \gtrsim 10 \text{ s}$, thereby amplifying the gravitational wave signal (factor $\sim 2$). This strengthens the argument for targeting LMXBs such as Sco X-1 in searches with facilities like LIGO. We show that the clumping of accreted matter leads to the formation of hot regions $(T \gtrsim 10^{8} \text{ K})$ throughout the mountain body, which has implications for type I X-ray burst activity. The excitation of inertial $(r-)$ modes within neutron stars leads to a fluid-velocity asymmetry, which generates a time-dependent current quadrupole moment. Depending on the equation of state, the resulting gravitational radiation may cause $r$-mode amplitudes to grow through an instability known as the Chandrasekhar-Friedman-Schutz (CFS) instability. In particular, it has been suggested that the gravitational radiation reaction resulting from the CFS instability may explain the observation that LMXBs have a narrow range of spin frequencies $0.1 < \nu / \left( \text{ kHz}\right) < 0.7$. However, for barotropic neutron stars, $r$-modes are often described by hyperbolic boundary-value problems (HBVPs). HBVPs are known to admit singularities and other pathological properties in certain circumstances, indicating that some aspects of the model may be unphysical. In Chapter 10 we present models for $r$-modes in nonbarotropic (stratified) neutron stars, and demonstrate that the HBVP nature of the problem cannot arise in certain, well-defined circumstances. We find that nonbarotropic $r$-modes are subject to the CFS instability for magnetic field strengths satisfying $|\boldsymbol{B}| \lesssim 10^{12} \text{ G}$, and spin frequencies in the range $3.0 \times 10^{-2} \leq \nu / \left( \text{ kHz} \right) \leq 1.5$.
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ItemHeterogenous catalysis in a non-equilibrium atmospheric plasma( 2017)The long-established Haber-Bosch process for the manufacture of ammonia is not suitable for small scale production. An alternative to the Haber process is plasma catalysis, which, despite showing early signs of promise, has not yet been proven to be a viable alternative route for ammonia fabrication. This is at least in part due to our lack of understanding of the complex mechanisms underlying the plasma-catalyst interactions. Gaining such an understanding is a prerequisite step for exploiting the potential of plasma catalysis for ammonia production. In this study, by controlling the surface functionality of carbon material such as nanodiamonds and diamond-like carbon, the significant role of surface reactivity has been investigated along with a kinetic modelling in a non-equilibrium atmospheric-pressure N2-H2 plasma. The experiments used a packed-bed dielectric barrier discharge reactor, with functionalized-nanodiamond and diamond-like-carbon coatings on α-Al2O3 spheres used as catalysts. An extensive zero-dimensional model of the plasma kinetic processes, including surface interactions, was developed, and used to simulate the reactions occurring in the reactor. The results provide improved our understanding of the crucial role of surface chemical functionality in plasma catalysis and also the influence of catalytic materials on plasma characteristics. Oxygenated nanodiamonds were found to increase the production yield of ammonia, while hydrogenated nanodiamonds decreased the yield. Neither type of nanodiamond affected the plasma properties significantly. Using diffuse-reflectance FT-IR and XPS, the role of different functional groups on the catalyst surface was investigated. Evidence is presented that the carbonyl group is associated with an efficient surface adsorption and desorption of hydrogen in ammonia synthesis on the surface of the nanodiamonds, and an increased production of ammonia. Conformal diamond-like-carbon coatings, deposited by plasma-enhanced chemical vapour deposition, led to a plasma with a significantly higher electron density, and increased the production of ammonia. Moreover, the voltage at which a discharge could be sustained was significantly reduced. The detailed plasma kinetic model was applied to understand the mechanisms of ammonia synthesis in a low electron energy N2–H2 atmospheric-pressure discharge. The model considers the vibrational kinetics, including excited N2 (X, ν>0) and H2 (X, ν>0) species, and surface reactions such as those occurring by the Eley–Rideal and Langmuir–Hinshelwood mechanisms and dissociative adsorption of molecules. The predictions of the model were compared to the measured ammonia concentration produced in the packed-bed dielectric barrier discharge reactor as a function of process parameters such as input gas composition and applied voltage. The predictions of the model were found to agree reasonably well with the experimental observations. The model was used to provide a detailed understanding of the important species and reactions in ammonia formation. The dominant mechanisms differ considerably from those previously suggested for atmospheric-pressure plasmas. In particular, surface-adsorbed atomic hydrogen was found to be of much greater importance than surface-adsorbed atomic nitrogen, which has previously been postulated to be the main precursor. The surface-absorbed atomic species, were found to be predominantly generated through the dissociative adsorption of molecules. NH radicals were also found to be important, as was the surface reactivity of the catalyst material. Important differences were also identified between the important mechanisms at atmospheric pressure, and those that have been identified in typical low-pressure plasma processes. For example, under the plasma conditions considered in our work (reduced electric field in the range 30 to 50 Td, electron density of the order 10^8 cm^-3), the influence of ions was found not to be significant. Instead, the reactions between radicals and vibrationally-excited molecules are more important. A maximum production yield of 2.3 [% H_2 conv.] was obtained in the experiments presented, demonstrating the potential of plasma catalysis for small-scale ammonia production using affordable carbon coatings under atmospheric-pressure and close to ambient temperature. Although the yield is modest, it should be recalled that it is at present not possible to generate significant amounts of ammonia molecules under ambient conditions using conventional thermal equilibrium processes. There are several possible avenues for further improvement of the process. In particular, the diamond-like carbon coating allowed a plasma discharge to be initiated and sustained at a significantly lower input voltage than an uncoated catalyst. This indicates that the approach has promise as a practical method to produce ammonia on demand with an inexpensive and portable system.