Electronic Theses and Dissertations (PhDs)

Permanent URI for this collectionhttps://hdl.handle.net/10539/38021

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Start date: 2020Subject: search.filters.subject.SDG-4: Quality education

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    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, Somnath
    This 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.
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    Digital toolbox for the generation and detection of vectorial structured light
    (University of the Witwatersrand, Johannesburg, 2023-06) Singh, Keshaan; Dudley, Angela; Forbes, Andrew
    Light 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.
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    The Large N Limit of Heavy Operator Excitations
    (University of the Witwatersrand, Johannesburg, 2023-07) Carlson, Warren Anthony; De Mello Koch, Robert
    Operators 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.