Electronic Theses and Dissertations (PhDs)
Permanent URI for this collectionhttps://hdl.handle.net/10539/38021
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Item Surface Brillouin scattering studies of high-temperature elasticity(University of the Witwatersrand, Johannesburg, 1999-03) Stoddart, Paul Randall; Comins, J. DarrelA novel technique has been developed for studying the elastic proper ties of opaque solids at high temperatures. The method is based on surface Brillouin scattering (SBS) and has the advantages of being contact-free and non-destructive. The elastic constants can be extracted from SBS measurements of the directional dependence of the surface wave velocities. An optical furnace was designed to provide the special scattering geometry required for these measurements. The technique has been evaluated on silicon and a single-crystal nickel-based superalloy, with measurements up to 800°C and 200°C respectively. Above these temperatures, measurements were precluded by a marked deterioration in the surface quality. The elastic constants for silicon compare favourably with the established ultrasonic values, particularly in terms of the changes as a function of temperature. Additional measurement were performed on silicon at temperatures up to 900°C in order to examine the well-known central mode feature. These results shed light on a major outstanding problem in SBS, because they reveal the presence of a second quasielastic mode that may be associated with scattering from diffusive excitations. Further measurements at high and low temperatures are proposed to confirm the mechanism. Silicon was also used as a test system to clarify certain aspects of the theory and practice of SBS that have not been properly dealt with before, such as the effects of surface anisotropy and of the extended collection aperture. This indicates that SBS provides effective elastic constants for the outer 300 nm of the sample surface and thus may be influenced by surface damage and surface contamination. In the case of the superalloy, the difficulties encountered in gathering data at higher temperatures suggests that modifications to the furnace arrangement are required. The larger relative error in the velocities also created problems in the extraction of the elastic constants. This difficulty was satisfactorily overcome by using the longitudinal threshold in the Lamb shoulder to fix the value of c₁₁. Although the work described here has been limited to temperatures below 900°C, it is clear that SBS provides a powerful method for probing the elastic properties of opaque solids at elevated temperatures.Item Low-temperature electronic transport of metal doped carbon nanotube molecular hybrids and Nitrogen-doped nanocrystalline diamond(University of the Witwatersrand, Johannesburg, 2024-08) Sodisetti, Venkateswara Rao; Bhattacharyya, SomnathThis thesis explores the magnetism and spin-related properties in carbon-based molecular hybrid materials, with a focus on expanding our understanding of low-dimensional carbon structures and their potential electronic applications. The investigation spans from one-dimensional systems, such as carbon nanotubes (CNTs) functionalized with single-molecule magnets (SMMs), to three-dimensional systems like nitrogen-doped ultra nanocrystalline diamond (UNCD). In these carbon structures, electronic transport is intricately tied to microstructural features, such as grain boundaries and impurity clusters, which hold significant potential for the development of all-carbon electronic devices. The research begins with a detailed examination of the chemical functionalization of multi-walled carbon nanotubes (MWCNTs) through controlled acid treatment to achieve precise metal doping. Using Raman spectroscopy and complementary techniques like ICP-MS and ToF-SIMS, we successfully demonstrate how functionalization levels influence the magnetic properties of CNT hybrids loaded with magnetic metals from the lanthanide series (Gd, Tb, Dy). The study reveals that low percentages of metal doping (0.5% to 1.0%) preserve the magnetic bistability of SMMs post-grafting, while higher doping levels lead to complex magnetic behaviors including super paramagnetism, quasi-ferromagnetism, and potential Kondo lattice behavior inCNT-heavy metal systems. We also explore the spin-phonon coupling in Gd-filled double-walled CNTs, where the onset of superparamagnetic properties at low temperatures is coupled with phonon mode stiffening observed via Raman spectroscopy. This enhanced coupling offers promising pathways for developing efficient molecular qubits through the modulation of spin-phonon interactions in one-dimensional systems. The second part of the thesis investigates into the microwave plasma-assisted chemical vapor deposition (MWCVD) growth of nitrogen-doped nanocrystalline diamond (NCD) thin films on different substrates. By pioneering upgrades to the MWCVD system, I was able to achieve reliable growth of high-quality nanocrystalline diamond thin films. Notably, I observed a novel nanostructure, termed Diaphite-a previously unreported feature, in these NCD films, consisting of nanodiamond grains coherently linked with graphene-like rings. This structure, along with the non-equilibrium growth conditions induced by nitrogen doping and secondary nucleation, presents unique polymorphic features in artificially grown diamonds. Detailed low-temperature transport measurements on four different samples—ranging from 7.5% to 20% nitrogen doping—uncovered complex transport phenomena such as 3D weak localization (WL), variable-range hopping (VRH), and unusual magnetoresistance (MR) behavior. In particular, the 7.5% N2-doped UNCD film on quartz exhibited 3D weak localization (WL) at low fields and anti-weak localization (AWL) at higher fields, with distinct magnetoresistance characteristics depending on the direction of the applied magnetic field. The 20% N2-doped films on both quartz and silicon showed more metallic-like behavior, with magneto-resistance characterized by a B1/2 dependence at low temperatures, suggesting an intricate relationship between doping level, microstructure, and electron transport. These findings significantly expand our understanding of the role that microstructural and chemical modifications play in determining the electronic and magnetic properties of carbon-based materials. This work provides a foundational platform for future research into carbon electronics, offering potential breakthroughs in spintronics, molecular transistors, quantum computing, and other advanced electronic applications.Item Digital toolbox for the generation and detection of vectorial structured light(University of the Witwatersrand, Johannesburg, 2023-06) Singh, Keshaan; Dudley, Angela; Forbes, AndrewLight has been an invaluable tool in the development of the modern world, with the myriad of applications increasing along with our degree of control over it. From the development of coherent light sources, to the shaping of amplitude and phase, this development has not ceased for the past half century. The field of structured light, borne out of the necessity and desire for control over light, has been growing steadily in recent years. In the spatial domain, the control over light’s polarization (i.e., the local planes in which the electric and magnetic fields oscillate) has been the most recent avenue of improvement, providing enhancements to a variety of applications ranging form microscopy and communication to materials processing and metrology. This class of light, commonly referred to as vectorial light, often requires specialised equipment in order for its its creation before its numerous benefits can be exploited. These tools often incur high costs and suffer from limitations relating to the diversity of vectorial light they can create, wavelength dependence and slow refresh rates. This thesis follows the development of a series of digital tools for the versatile generation and analysis of vectorial light using low-cost core technologies which can operate at high rates over a broad wavelength range. We follow the development of the generation tool in the context of its application in generating novel accelerating polarization structures, emulating vectorially apertured optics, generating probes to measure birefringence and chirality and creating synthetic spin dynamics. The development of the analysis tool is explored by investigating its application in performing automated digital Stokes polarimetry measurements, completely characterizing the internal degrees of freedom of arbitrary vectorial light and acting as a polarization and wavelength independent wavefront sensor. We then demonstrate how these tools can be used, in conjunction, to investigate the fundamental invariance of vectorial light to perturbing channels and how this invariance can be exploited in a highly robust novel communication scheme. In addition to demonstrating the applicability and versatility of these vectorial light tools, the applications offered a means to highlight areas for the optimization for the design. This culminated in the ongoing prototyping of versatile, fast, broadband devices which operate stably and have a small physical footprint.Item Hunting dark matter with faint radio halos(University of the Witwatersrand, Johannesburg, 2023-10) Sarkis, Michael David; Beck, GeoffThe nature of Dark Matter (DM), the elusive substance that constitutes a significant amount of the total matter in the universe, remains an unsolved problem in modern physics despite a decades-long search effort. One approach to this problem has been to search for faint emission signatures that are produced indirectly from the DM present in large astrophysical structures, and thus infer properties about theoretical DM models from observational data. In recent years, the results from studies that use this type of indirect search have produced stringent constraints on the most popular DM particle candidate parameter spaces, ruling out swathes of viable DM models. These compelling results have been enabled by the arrival of sophisticated interferometric radio telescopes, which are excellent DM hunters due to their high sensitivity and resolution. In this thesis, we focus on the use of the latest data from the MeerKAT radio interferometry telescope, through the first public release of the MeerKAT Galaxy Cluster Legacy Survey, to search for DM emissions in a set of nearby galaxy clusters. Each step of this process, from the creation of theoretical DM emission models to the statistical analysis of the observational data, has been described in detail in this thesis. With this data, we find an almost universal improvement to results found with corresponding modelling scenarios in the literature. Since this work is among the first to use MeerKAT data in astrophysical DM searches, these results present a strong argument for continued work in this field. Another central focus of this thesis is the accurate modelling of the physical processes involved in the production of the DM-induced radio emissions, as the quality of current radio data requires theoretical models that are sufficiently accurate to describe the emission at such high resolutions. One aspect of the modelling that has lacked this accuracy has been the solution to the diffusion-loss equation, which is a crucial factor in determining indirect DM emissions. A new algorithm for solving this equation, which provides higher accuracy and computational efficiency than previous public methods, has thus been developed and presented in this thesis. These aspects of DM indirect detection study will become ever more important as we approach the inauguration of the Square Kilometre Array (SKA), which will provide data with unprecedented potential with which to continue the hunt for DM.Item Implementation of the DAQ software for the ALTI module in the ATLAS TileCal and the search for new physics in the four lepton final state(University of the Witwatersrand, Johannesburg, 2023-06) Tlou, Humphry Sijiye; Wilkens, Henric; Ruan, Xifeng; Mellado, BruceThe discovery of the Standard Model (SM) Higgs boson in 2012 presents new challenges and opportunities for the Large Hadron Collider (LHC) experiments. After a long period of operation, the LHC experiments needed to maintain and upgrade their detectors in order to continue and conduct research beyond the SM. As part of the upgrades, the Tile Calorimeter (TileCal) participated in Phase-I of the upgrades (December 2018 - March 2022). TileCal, the central hadronic calorimeter (|η| < 1.7) of the ATLAS experiment uses a set of Trigger and Data Acquisition (TDAQ) software to readout, transport and store physics data resulting from collisions at the LHC. In preparation for the Phase-I upgrades, the ATLAS Local Trigger Interface (ALTI) module was designed for the ATLAS experiment at CERN for TDAQ purposes. It is a 6U VME electronics board, which is a part of the Timing, Trigger and Control (TTC) system. It integrates the functionalities of four legacy modules, currently used in the experiment: Local Trigger Processor, Local Trigger Processor interface, TTC VME bus interface and the TTC emitter. The ALTI module provides the interface between the Level-1 Central Trigger Processor and the TTC optical broadcasting network to the front-end electronics of each of the ATLAS sub-detectors. This thesis discusses the development, validation and integration of the TileCal specific ALTI software in the TileCal online software by the author. A set of ALTI boards were installed in the back-end electronics of the sub-detector and fully validated for the ATLAS detector at CERN. Performance testing and maintenance of the ALTI modules and software were performed during the second half of the upgrade period, in preparation for Run 3 (2022–2025) data-taking period. The thesis also discusses the search for the presence of a new heavy resonance produced via gluon-gluon fusion and decaying to the four-lepton (4ℓ) final state, in association with missing transverse energy (EmissT), with ℓ = e, µ (where ℓ is the lepton, e is the electron and µ is the muon). The search uses 2015–2018 proton-proton collision data at √s = 13 TeV, corresponding to an integrated luminosity of 139 fb−1, collected by the ATLAS detector. The data are interpreted in terms of two models, firstly the R → SH → 4ℓ + EmissT , where R is a scalar boson, which decays to two lighter scalar bosons (S and H). The S decays to a pair of neutrinos and the H decays into 4ℓ, through ZZ bosons. The second model is the A → Z(νν)H(ZZ) → 4ℓ + X, where A is considered to be a CP-odd scalar which decays to a CP-even scalar H and the Z boson. The Z boson decays to X, which can be a pair of neutrinos or jets, and the H decays to the 4ℓ final state.Item The Large N Limit of Heavy Operator Excitations(University of the Witwatersrand, Johannesburg, 2023-07) Carlson, Warren Anthony; De Mello Koch, RobertOperators with bare dimension of order N are studied. These are restricted Schur polynomials labeled by Young diagrams with two long rows or two long columns and are heavy operators in the large N limit. A dramatic simplification of the action of the dilatation operator on these states is found, where the diagonalization of the dilatation operator reduces to solving three-term recursion relations. The solutions to these recursion relations reduce the spectrum of the dilatation operator to that of decoupled harmonic oscillators, showing that these systems are integrable at large N. Then, generating functions for bound states of two giant gravitons are constructed and an extension to more than two giant gravitons is sketched. These generating functions are integrals over auxiliary variables that encode the symmetrization and anti-symmetrization of the fields in the restricted Schur polynomials and give a simple construction of eigenfunctions of the dilatation operator. These generating functions are shown to be eigenfunctions of the dilatation operator in the large N limit. As a byproduct, this construction gives a natural starting point for systematic 1/N expansions of these operators. This includes the prospect to generate asymptotic representations of the symmetric group and its characters via the restricted Schur polynomials. Finally, the asymptotic expansion of the three-point function is computed in three BMN limits by varying one parameter in the large N limit. It is argued that these asymptotic expansions encode non-perturbative effects and are related by a parametric Stokes phenomenon.Item Study on the influence of Nuclear Deformation on the Pygmy Dipole Resonance in Samarium isotopes(University of the Witwatersrand, Johannesburg, 2023) Jivan, Harshna; Sideras-Haddad, Elias; Pellegri, LunaThe past decade has seen an increase in studies dedicated to understanding the low-lying electric dipole (E1) response, commonly referred to as the Pygmy Dipole Resonance (PDR). These studies revealed that the PDR has a mixed isospin nature, and that the use of complimentary probes is needed to fully understand this response. Since majority of studies on the PDR focused on spherical nuclei, the influence that deformation has on the PDR response is yet to be understood. Preliminary relativistic proton scattering studies on 154Sm performed at RCNP (Japan), showed potential evidence for a splitting in the PDR responses similar to that of the Giant Dipole Resonance with deformation. A tentative interpretation suggested that this splitting could be connected to the splitting of the resonance structure with respect to the K quantum number. Theoretical studies considering the deformed HFB+QRPA model however, suggest that this splitting is connected to the isospin mixed character of these states as observed in spherical nuclei. The isoscalar responses of the spherical 144Sm and axially deformed 154Sm isotopes were investigated for the first time using the inelastic scattering of alpha particles at 120 MeV. The comparative experiments were performed at iThemba LABS in South Africa, coupling together for the first time, the K600 magnetic spectrometer in zero-degree mode with the BaGeL (Ball of Germanium and LaBr3:Ce detectors) array. The particle-gamma coincidence measurement was used to obtain the cross section for the population of the pygmy states. In both nuclei, the region of the PDR was excited and the E1 multipolarity of the transitions was supported by the angular correlation between the α-particles and the co-incident γ-rays measured. The total exclusive cross section measured for 144Sm amounted to 24.3 ± 3.8 mb/sr while for 154Sm to 18.8 ± 2mb/sr. The experimental results were compared with the prediction of the RQTBA and the deformed HFB+QRPA theories, respectively. The theoretical cross sections were extracted within a semiclassical coupled-channel approach. The fragmentation observed in the experiment for the 144Sm was underestimated by the calculations, although good agreement with the total cross section measured was found. In the case of the deformed 154Sm however, the experimental cross section accounted for only 52% of the predicted cross section in the same excitation region. The isoscalar response extracted in this thesis was compared with the isovector strength obtain from an experiment performed at RCNP using the relativistic proton scattering at forward angles. The double hump observed in the isovector channel was not found in the case of the isoscalar strength. This implies that the difference obtained between these two experiments is related to the “isospin splitting” of the PDR rather than a splitting of thestrength connected with the K quantum number.Item Development of TileCoM firmware and software for the off-detector electronics of the ATLAS Tile Calorimeter at the HL-LHC(University of the Witwatersrand, Johannesburg, 2023-08) Gololo, Mpho Gift Doctor; Argos, Fernando Carrio; Mellado, BruceIn 2010 the LHC started to operate as the energy frontier particle accelerator in the world, situated close to Geneva and 100 m below the French and Swiss border in a circular tunnel of 27 km. The HL-LHC which is an upgrade of the LHC is envisioned to maximize the instantaneous luminosity of L = 1 × 1034 cm−2s −1 by a factor of 5 to fully exploit the physics potential at the energy frontier. During 10 years of operation, an improved TDAQ system architecture will have the capability to accommodate the trigger rates and the amount of data generated from the HL-LHC. TileCal is the ATLAS central hadronic calorimeter, a sampling calorimeter with iron as passive medium and plastic scintillator tiles as active medium. The ATLAS TileCal Phase-II upgrades will prepare the ATLAS experiment for the HL-LHC and includes new requirements in terms of radiation levels, an increase in data bandwidth, and clock distribution. To meet the requirements of the HL-LHC, a completely new readout electronics is designed to support the data acquisition system of TileCal. As part of the new readout electronics, this thesis is focused on the design of the TileCoM and Tile GbE Switch. The Tile GbE Switch PCB is manufactured by two South African companies, Trax Interconnect and Jemstech. The PCBs are fully func tional and have been integrated with new readout electronics. Three main function alities are implemented on the TileCoM in software and firmware implementation as key elements of the TDAQ and DCS of the ATLAS TileCal at the HL-LHC. The TileCoM and Tile GbE Switch are successfully integrated with the ATLAS Phase II TileCal upgrade electronics. This is achieved by successful remote control and monitoring of the ATLAS TileCal Phase-II upgrade electronics. This thesis shows monitoring results based on voltage, current and other parameters.Item Transfer reactions to populate the pygmy dipole resonance in 96Mo(University of the Witwatersrand, Johannesburg, 2023) Khumalo, Thuthukile Charmane; Pellegri, L.; Wiedeking, M.The presence of a low-lying dipole strength in neutron-rich nuclei has been established and its location in the vicinity of the neutron threshold (Sn) has implications in nucleosynthesis and specifically in neutron-capture reaction rate calculations. Additionally, a correlation of this low-lying dipole strength with neutron-skin thickness has been discussed. Since its observation, there has been a great deal of work in an attempt to understand its nature, both theoretically and experimentally. Some of the characteristics of this low-lying dipole strength include isospin mixing, which allows the use of different experimental probes to study it. In addition, compared to the IVGDR, the degree to which the low-lying dipole states are collective is under scrutiny and remains an open question of interest. This study was aimed at addressing the question of collectivity of these dipole states and one-nucleon transfer reactions were the chosen probes as they have been shown to be powerful in probing the single-particle property of nuclei. In particular the (p,d) and (d,p) reactions have been instrumental in such measurements. To allow the investigation from both neutron addition and removal, the 96Mo nucleus is particularly attractive as it can be populated via both mechanisms, with the availability of stable targets as a bonus. In addition, the (d,p) has been successfully used recently used for PDR related measurements on 120Sn and 208Pb with results alluding to a strong single-particle contribution, hence conducting the investigation on 96Mo provides access to a different mass region. 97Mo(p,d)96Mo and 95Mo(d,p)96Mo transfer reactions were performed in normal kinematics using the MAGNEX magnetic spectrometer at INFN-LNS. The 25 MeV/u proton beam and 5 MeV/u deuteron beam from the Tandem accelerator interacted with the 97Mo and 95Mo targets, respectively. The MAGNEX spectrometer was utilised to analyse the scattered particles based on their momentum prior to being detected at the focal-plane. Excitation energy spectra were obtained and angular distributions were computed for the bound states and the higher excitation energy region of interest (above Ex = 4 MeV). These were fitted, using the MDA with DWBA calculations considering different single-particle configurations from a simplistic shell model. Comparing spectra from the two reactions, same excitation energy regions were populated. The results from the MDA of the (p,d) data, show a strong single-particle component in the Ex region that was analysed, with one particular configuration that excites 1− states dominating. The QPM was used for the theoretical interpretation and below 6 MeV, the configuration ((2d5 2 )+1 N(1g9 2 )−1) that populates 2+ states dominates but in the experimental data, this configuration was found to be suppressed as the momentum matching conditions were optimized for l=1 momentum transfer. When considering the QPM predictions involving only the sp configurations of momentum transfer of l=1, 2 and 3, an agreement with the data was found. Extraction of reliable angular distributions from the (d,p) was not possible thus future (d,pγ) experiments are envisagedItem Optimization of gallium oxide (ga2o3) nanomaterials for gas sensing applications(University of the Witwatersrand, Johannesburg, 2024) Gatsi, Nyepudzai CharslineGas sensors are needed for monitoring different gases in indoor and outdoor environments, food quality assessment, and health diagnostics. Among materials studied for these applications, semiconducting metal oxides (SMOs) have generated a lot of interest due to their excellent sensitivity, simple circuit, and low cost. One-dimensional (1𝐷) 𝐺𝑎2𝑂3 nanomaterials are part of the promising candidates explored for the sensing of different gases due to their excellent electrical conductivity, high catalytic behavior, and chemical and thermal stability. This study reports the optimization of crystal structure, morphology, and surface chemistry of 𝐺𝑎2𝑂3 nanostructures for use in the detection of various gases. A set of unmodified and noble metal modified 1𝐷 𝐺𝑎2𝑂3 nanomaterials were synthesized by microwave-assisted hydrothermal method followed by heat-treatment at different temperatures and their gas sensing performances were systematically studied. The samples were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), Raman analysis, scanning electron microscope (SEM), transmission electron microscope (TEM), Brunauer-Emmett-Teller (BET), photoluminescence (PL), diffuse reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS) methods. The effects of heat-treatment temperatures on phase transformations and gas sensing performances of various 𝐺𝑎2𝑂3 polymorphs were investigated. The 𝛼 − 𝐺𝑎2𝑂3, 𝛽 − 𝐺𝑎2𝑂3 and 𝛼/𝛽 − 𝐺𝑎2𝑂3 crystal structures were synthesized and evaluated for gas sensing. The 𝛽 − 𝐺𝑎2𝑂3 sensing layers presented selective response coupled with fast response/recovery times towards carbon monoxide (𝐶𝑂) compared to the 𝛼 − 𝐺𝑎2𝑂3 and 𝛼/𝛽 − 𝐺𝑎2𝑂3 crystal structures. The observed variations in the gas sensing performances of these three crystal structures were attributed to controlled properties of different 𝐺𝑎2𝑂3 polymorphs. Furthermore, the 𝛽 − 𝐺𝑎2𝑂3 polymorph was prepared in the form of regular and hierarchical nanorod-based morphological features which demonstrated different gas sensing behaviors. The 𝛽 − 𝐺𝑎2𝑂3 regular nanorods showed better capabilities of detecting isopropanol than the nanobundle-like and nanodandelion-like features, and these differences were attributed to changes in textural, porosity, and compositional properties related to different morphologies. The effects of incorporating 𝐴𝑔 and 𝐴𝑢 noble metal nanocrystals on regular 𝛽 − 𝐺𝑎2𝑂3 nanorods surfaces on their gas sensing behaviour were also investigated. The results revealed that surface modification of 𝛽 − 𝐺𝑎2𝑂3 nanorods with 0.5 and 1.0 𝑚𝑜𝑙% 𝐴𝑔 and 𝐴𝑢 noble metals significantly lowered the sensor operating temperature compared to that of unmodified 𝛽 − 𝐺𝑎2𝑂3 nanorods towards the detection of ethylene. In addition, surface incorporation of 1.0 𝑚𝑜𝑙% 𝐴𝑔 dramatically increased the sensor sensitivity and selectivity and reduced the response/recovery times towards ethylene gas, and these positive changes were attributed to the electronic and chemical sensitization effects stimulated by the catalytic activity of 𝐴𝑔 nanocrystals incorporated on the surface of 𝛽 − 𝐺𝑎2𝑂3 nanorods. This study unambiguously optimized the crystal structure, morphology, and surface chemistry of 𝐺𝑎2𝑂3 nanostructures for the detection of carbon monoxide, ethylene and isopropanol gases. These sensors may potentially be used in real-time detection of carbon monoxide and isopropanol for indoor air quality monitoring to improve human health. In additional they have also demonstrated capabilities for the precise and economical detection of ethylene around plants and fruits, which could be beneficial to the horticultural and agricultural industries