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Coming dissertations at TekNat

  • Theoretical Investigations of Two-Dimensional Materials : Studies on Electronic, Magnetic, Mechanical, and Thermal Properties

    Author: Xin Chen
    Publication date: 2020-11-09 07:50

    Two-dimensional (2D) materials have been paid enormous attention since the first realization of graphene in 2004, in connection to high-speed flexible electronics, 2D magnetism, optoelectronics, and so on. Apart from graphene, many new 2D materials with special properties have been predicted and synthesized. For the understanding of several interesting phenomena and prediction of new 2D materials, materials-specific density functional theory (DFT) plays a very important role.

    In this thesis, based on first-principles calculations, structural, magnetic, electronic, mechanical, and thermal transport properties of two kinds of 2D systems are investigated.

    The first kind of 2D materials is based on the synthesized material or the predicted structure with ultralow energy. These materials were functionalized by adsorbing transition metal atoms or oxygen atoms, which makes a significant difference in the properties. A part of the thesis covers the study of the self-assembly process of 3d transition metal hexamers on graphene with different defects. Interestingly, it is found that the easy axis of magnetization can be tuned between in-plane and out-of-plane directions in...

  • Learning Physics through Transduction : A Social Semiotic Approach

    Author: Trevor Stanton Volkwyn
    Publication date: 2020-11-06 10:30

    This doctoral thesis details the introduction of the theoretical distinction between transformation and transduction to Physics Education Research. Transformation refers to the movement of meaning between semiotic resources within the same semiotic system (e.g. between one graph and another), whilst the term transduction refers to the movement of meaning between different semiotic systems (e.g. diagram to graph). A starting point for the thesis was that transductions are potentially more powerful in learning situations than transformations, and because of this transduction became the focus of this thesis. The thesis adopts a social semiotic approach. In its most basic form, social semiotics is the study of how different social groups create and maintain their own specialized forms of meaning making. In physics education then, social semiotics is interested in the range of different representations used in physics, their disciplinary meaning, and how these meanings may be learned. Students need to gain representational competence in interpreting and using the different representations they meet in their physics education and this thesis...

  • Supersymmetric Localization : A Journey from Seven to Three Dimensions

    Author: Konstantina Polydorou
    Publication date: 2020-11-04 12:40

    Quantum Field Theory has been a dominating framework in elementary particle physics during the last century. Within this framework, supersymmetric theories have attracted a lot of attention due to their mathematical structure, simplicity and insight into the problems of unification, dark matter and hierarchy. Even though the boundaries of our perturbative undestanding of supersymmetric theories have been pushed far, there is generically no systematic way to obtain exact results. Especially for strongly coupled theories, where perturbative techniques cannot be applied, methods for exact computations are crucial. A powerful technique to obtain exact results of partition functions and correlators of curtain protected operators is supersymmetric localization. This thesis studies physical and geometrical properties of localization results in different supersymmetric theories.

    The first model considered is maximally supersymmetric Yang-Mills placed on a 7-dimensional Sasaki-Einstein manifold. After redefining the fields to differential forms, the cohomological complex is formed and a localization computation of the partition function is performed using data from the...

  • Towards quinone/pEDOT conducting redox polymers as energy storage materials

    Author: Huan Wang
    Publication date: 2020-10-30 09:24

    With the increased demand for electrical energy storage devices, e.g. electric vehicles and smart grids, organic batteries have caught increasing attention due to their advantages, such as sustainability, environmental friendliness and cheap raw materials over traditional inorganic energy storage materials. Quinones, as an organic energy storage materials have high specific capacity, show fast 2e− redox reactions and provide a wide structural diversity, making them promising candidates as active battery materials. However, quinones often suffer from dissolution problem and limited electronic conductance. In this thesis, quinones are functionalized onto conducting polymer backbones to make a conducting redox polymer (CRP), where the quinone provides capacity while the conducting polymer backbone provides electronic conductance and prevents dissolution of the quinone. One fundamental requirement to make a functional CRP is that redox matching between the quinone redox potential and the conducting region of the conducting polymer is achieved. To that end, the quinone redox chemistry as well as the conductance behavior of conducting polymers must be fully understood. In...

  • Numerical and experimental modelling for wave energy arrays optimization

    Author: Marianna Giassi
    Publication date: 2020-10-29 13:28

    Many wave power conversion devices, especially point-absorbers, do not provide alone the necessary amount of converted electricity to be cost effective, instead they are designed to be deployed in arrays of many units. Such arrays, or parks, can satisfy a large-scale energy demand, reduce the costs of the produced electricity and improve the reliability of the system.

    The performance of a wave energy park is affected by multiple and mutually interacting parameters, and the complex problem that arises during its design is called array optimization.

    The scope of the present thesis is to study such systems and their design, by the development of an optimization routine able to predict the best layout of a wave energy park under fixed constraints. The wave energy converter considered is the point-absorber developed at Uppsala University, which consists of a linear electric generator located on the seabed and a floating buoy at the surface.

    An optimization routine based on a genetic algorithm was created, which allows simultaneous optimization of the geometry of the buoys, the damping coefficient of the linear generators and the geometrical layout of the park.

    ...
  • Carbothermal synthesis of nano-structured metal, carbides and nitrides, using solution processed precursors; from precursor to product

    Author: Sarmad Naim Katea
    Publication date: 2020-10-29 13:27

    This thesis describes how solution chemical based carbothermal syntheses can be used for the synthesis of nanostructured zirconium carbide and nitride nanophase powders as well as a nickel-metal coating on NbC for use as a sintering agent. The focus is on the development of novel synthesis routes using atomic or small nanoscale level mixing of the metal oxide and carbon source in the starting precursors, enabling the use of lower synthesis temperatures and non-agglomerated nanosized powders. Detailed characterisation of the final materials and those found during the phase-evolution were performed using TG-DTA, dilatometry, XPS, XRD, IR and Raman spectroscopy, ToF-ERDA, PIXE, SEM-EDX/EBSD/TKD, (S)TEM-EDX/HAADF/EELS and Vickers micro-hardness.

    Starting materials with various carbon-to-zirconia ratios, composed of 2-4 nm sized ZrO2 particles embedded in amorphous carbon, were prepared by hydrolysis of Zr(OPrn)4 and sucrose. Following heating in argon and nitrogen gas yielded nano-phase ZrC and ZrN powders, respectively. Heating in argon gas to 1495 oC, yielded ca. 75 nm sized, loosely agglomerated ZrC particles after 30 minutes annealing when using a sucrose-C:Zr ratio of 5:...

  • On Ingeae Systematics of synandrous mimosoids

    Author: Julia Ferm
    Publication date: 2020-10-29 12:52

    Background: The Ingeae tribe (Caesalpiniodeae, Fabaceae) has a long history of taxonomic complexity with genera being merged and split, and many species with former placements in several different genera. Moreover, phylogenetic studies have shown that the tribe is non-monophyletic with the genus Acacia (of tribe Acacieae) nested within Ingeae. This problem of non-monophyly is also reflected at the generic and specific level of ingoid taxa. Phylogenetic relationships have been difficult to resolve, with unsettled generic delimitations as a result. In this thesis, I investigated the systematics and phylogeny of the Ingeae-Acacia complex, with reflections on taxonomy and biogeography. Methods: Molecular data of plastid (matKpsbAtrnL-trnFycf1) and nuclear (ETS, ITS) DNA sequences were analysed using Bayesian inference and Ultrafast Bootstrap in order to investigate phylogenetic relationships of the Ingeae-Acacia complex. Results: In paper I, Marmaroxylon was shown to be included in Zygia and Zygia inundata in ...

  • Learning physics with Controllable Worlds : Perspectives for examining and augmenting physics students' engagement with digital learning environments

    Author: Elias Euler
    Publication date: 2020-10-28 13:34

    In this thesis I present a collection of case studies involving small groups of participants using ‘Controllable Worlds’—i.e., a particular class of physics digital learning environment (DLE) including simulations, ‘microworlds,’ and educational games that provides users with control over manipulable virtual environments. Throughout the thesis I employ and develop several perspectives for the interpretation, analysis, and instructional guidance of physics students’ engagement with DLEs. While this thesis focuses in particular on participants’ use of the 2D Newtonian software Algodoo and the PhET simulation My Solar System, I also contribute to a more general scholarly discussion on student interaction and technology use in physics education. One such contribution, which relates to my development of an overarching taxonomy for learning environments, is the theoretical distinctions between ‘constrained’ and ‘less-constrained’ DLEs and between DLEs with high and low degrees of ‘semi-formality.’

    The work of this thesis is largely based on five peer-reviewed publications, the content of which can be organized into three broader themes. In Theme 1,...

  • Topological superconductivity in multiorbital materials

    Author: Johann Schmidt
    Publication date: 2020-10-28 13:33

    Multiorbital materials add a new intricacy to the already complex phenomenon of superconductivity. The additional orbital degree of freedom requires leaving behind the established band picture, but also opens the possibility of more complicated order parameters with new properties. This thesis summarizes theoretical studies of two examples of multiorbital superconductors.

    The first part focuses on superconductivity in Kitaev materials. The unusual interactions in these materials are shown to give rise to spin triplet superconducting pairing on both two-and three-dimensional lattice structures. A symmetry characterization enables the analysis of the stable superconducting order parameters, revealing several nodal states on the 3D harmonic honeycomb lattices and a competition between nematic and chiral superconductivity on the 2D honeycomb lattice. The following topological classification uncovers a number of topologically non-trivial superconducting states protected by various symmetries, giving rise to flat bands, Fermi arcs, or dispersing Majorana excitations on their surface.

    The thesis’ second part spotlights odd-frequency superconductivity in a doped topological...

  • Ultrafast Magnetization Dynamics : Element-selective studies of magnetic alloys using ultra short XUV pulses

    Author: Rameez Saeed Malik
    Publication date: 2020-10-16 10:13

    In this thesis, I investigate the ultrafast magnetization dynamics in 3d ferromagnets and their alloys with ultrashort laser pulses. The high harmonics generation (HHG) setup provides extreme-ultraviolet photons with energies 35-72 eV, which is the energy range where 3d metals have their M2,3 absorption edges.  By employing HHG with the transverse magneto-optical Kerr effect, the magnetization of multiple elements in a magnetic system is probed and their dynamics are resolved separately on femtoseconds time scales.

    The magneto-optical response of elemental Fe and Ni during demagnetization is investigated. This magneto-optical response is measured as an asymmetry in the intensity of reflected light for two opposite sample magnetization directions. Experiment and density functional theory calculations show that for Fe, the asymmetry is strongly dependent on the particular type of magnetic excitation. However, for Ni, it is relatively insensitive to the magnetic excitation. Next, the element-specific magnetization dynamics of FeNi alloys are investigated. A time delay in the Ni demagnetization relative to Fe is observed for all alloy compositions. This Ni-delay depends on...

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