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Coming dissertations at TekNat

  • Configurational Thermodynamics of the CeO2-Gd2O3 System : A Combined DFT, Cluster Expansion and Monte Carlo Approach to Bulk and Interfaces

    Author: Pjotrs A. Žguns
    Publication date: 2018-10-05 11:47

    In this thesis, we study the configurational thermodynamics of Ce1-xGdxO2-x/2 x ≤ 1 or CGO. We apply a combined Density Functional Theory (DFT), cluster expansion and Monte Carlo (MC) approach in which the configurational energy of CGO is described by means of the Ising-type Hamiltonian. The interactions are determined by the cluster expansion of total energies calculated with DFT for a set of various cation–anion configurations. This allows one to perform on- the-fly calculations of the configurational energies in MC simulations of cation and anion ordering.

    The cluster interactions are essentially electrostatic and long-range, and describe the configurational energetics in the entire range of concentrations rather well.

    The phase diagram obtained in the MC simulations allows one to rationalise existing experimental data and is largely in agreement with that. We observe the phase separation into pure oxides, in equilibrium, below ca. 1000 K for x ≤ 1 (however it is kinetically hindered as diffusion of cations below ca. 1500 K is slow). We also observe the C-type oxygen–vacancy ordering in the x...

  • Magnetism in Transition Metal Systems : Interplay between structure, dimensionality and electron correlation

    Author: Samara Keshavarz Hedayati
    Publication date: 2018-10-05 10:07

    In this thesis, an ab initio study of electronic structures and magnetic properties of transition metal systems has been presented, covering bulk, interface and surface geometries. Among them are Fe, Co, Ni, CaMnO3, Co2MnSi, a ferromagnetic Heusler alloy, as well as double-perovskites oxides such as Sr3(Fe1.25Ni0.75)O6 and Nd2NiMnO6.

    Their electronic structures have been obtained within the framework of the density functional theory (DFT) in combination with Hubbard type interaction such as the static correction evaluated within the Hartree-Fock method (DFT+U) or within the more sophisticated method of dynamical mean-field theory (DFT+DMFT). Using many-body approaches enables us to treat the correlation effects such as non-quasiparitcle states above the Fermi level for Co2MnSi and in the half-metallic side of the Co2MnAl/CoMnVAl heterostructure.

    Based on the converged electronic structure, the magnetic excitations were mapped onto the Heisenberg Hamiltonian. Among various ways to extract exchange parameters, in this Thesis the method of infinitesimal rotation of the spins has been applied in the framework of the local magnetic force approach. It is...

  • Metabolic Engineering of Synechocystis PCC 6803 for Butanol Production

    Author: Rui Miao
    Publication date: 2018-10-05 09:37

    There is an urgent demand for renewable alternatives to fossil fuels since the extraction and utilization cause a series of environmental problems in the world. Thus, the utilization of solar energy has attracted much attention in the last decades since there is excess amount of light on Earth. Photosynthetic microorganisms, such as cyanobacteria, can be a good biological chassis to convert solar energy directly to chemical energy. It has been demonstrated that cyanobacteria can produce various compounds which can be used asfourth-generation biofuels. This thesis focuses on the photo-autotrophic production of two biofuel compounds, isobutanol and 1-butanol, in the unicellular cyanobacterial strain Synechocystis PCC 6803. In the studies of isobutanol production, the endogenous alcohol dehydrogenase of Synechocystis encoded by slr1192 showed impressive activity in isobutanol formation. In addition, a-ketoisovalerate decarboxylase (Kivd) was identified as the only heterologous enzyme needed to be introduced for isobutanol production in Synechocystis. Kivd was further recognized as a bottleneck in the isobutanol production pathway. Therefore,...

  • Electron energization in near-Earth space : Studies of kinetic scales using multi-spacecraft data

    Author: Elin Eriksson
    Publication date: 2018-10-02 14:20

    Plasma, a gas of charged particles exhibiting collective behavior, is everywhere in the Universe. The heating of plasma to millions of degrees and acceleration of charged particles to very high energies has been observed in many astrophysical environments. How and where the heating and acceleration occur is in many cases unclear. In most astrophysical environments, plasma consists of negative electrons and positive ions. In this thesis we focus on understanding the heating and acceleration of electrons. Several plasma processes have been proposed to explain the observed acceleration. However, the exact heating and acceleration mechanisms involved and their importance are still unclear. This thesis contributes toward a better understanding of this topic by using observations from two multi-spacecraft missions, Cluster and the Magnetospheric MultiScale (MMS), in near-Earth space.

    In Article I we look at magnetic nulls, regions of vanishing magnetic field B believed to be important in particle acceleration, in the Earth's nightside magnetosphere. We find that nulls are common at the nightside magnetosphere and that the characterization of the B...

  • Search for a new light boson in meson decays

    Author: Damian Pszczel
    Publication date: 2018-10-02 11:52

    The subject of the presented work lies in the field of experimental particle physics. The main topic is the study of e+ e− pairs from η meson decays. The data sample used in this work was collected by the WASA-at-COSY collaboration in proton-proton collisions at 1.4 GeV kinetic beam energy. The experiment took place in 2012 at Forschungzentrum Jülich in Germany at the COSY storage ring. An internal proton beam interacted with a pellet target of frozen hydrogen.

    We implemented a set of selection criteria in order to extract the η → e+ e− γ event candidates. This is a rare electromagnetic decay of the η meson with branching ratio equal to 6.9·10−3. The resulting set of events served as the basis for three analyses.

    First, we extracted the η transition form factor that is a function depending on the inner quark and gluon structure of the meson. We implemented a specific method to reduce the contribution of background channels from direct pion production.

    The second analysis was the search for a narrow structure on the e+ e− invariant mass in the selected sample of η → e+ e− γ candidates. Many theoretical models and some astrophysical and particle physics...

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