Displaying theses 101110 of 1078 total
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H.J. van Leijen 
Bachelor programme: Natuur en Sterrenkunde  August 21st, 2017  
Institute: Other  Research group: GRAPPA institute: Gravitation and AstoParticle Phy  Graduation thesis  Supervisor: Christoph Weniger 
Mass constraints on MACHO dark matter Most scientist nowadays agree that there is something out there called dark matter, there is however still a lot of debate concerning the nature of this dark matter. The explanations for the fundamental components that form dark matter even range from ultra light axions to supermassive black holes. In this thesis I assumed that dark matter was made of MACHOs (Massive compact halo objects) and the goal of this thesis was to find mass constraints on these dark matter MACHOs. I did this by looking at their microlensing effects and by looking at the dynamical impact of dark matter MACHOs on there surrounding. 

Scientific abstract (pdf 1K) Full text (pdf 4009K) 
G. Raaijmakers 
Master programme: Astronomy and Astrophysics  August 21st, 2017  
Institute: API  Research group: Xray Timing Group  Graduation thesis  Supervisor: dr. Anna Watts 

The Dense Matter EOS, NonIdealised MassRadius Posteriors: Implications for EOS inference and observing strategy Neutron stars are the smallest and densest stars known in the Universe, with a radius of around 1015 kilometres but a mass as high as one or two solar masses. Due to the extreme compactness of a neutron star, the particles in the interior are squeezed together very tightly. At these extreme densities the behaviour of these particles is not properly understood. To answer the question of how these particles interact we need to look deep inside a neutron star. However, even with our most advanced telescopes, that is not possible. Fortunately, Einstein's theory of general relativity provides a mapping between the measurable quantities of a neutron star, such as mass and radius, to the socalled Equation of State (EOS). This EOS is a simple relation between pressure and density that fully describes the particles behaviour. In this thesis we have developed a code called MORSE to explore how the EOS depends on these mass and radius quantities. It turned out that the best constraints on the EOS can be obtained when the input neutron star masses showed a spread over the parameter space or were centred around high masses. 

Scientific abstract (pdf 2K) For more info or full text, mail to: A.L.Watts@uva.nl 
J. Haartman 
Bachelor programme: Natuur en Sterrenkunde  August 20th, 2017  
Institute: VU / Other  Research group: Physics and Medical Technology  Graduation thesis  Supervisor: dr. Jan de Munck 
Statistical analysis on the deformation of hippocampi in patients with Alzheimer’s Disease Alzheimer’s Disease is the most common form of dementia in the elderly. This disease is irreversible, but treatment can enhance quality of life, therefore an early diagnosis is desirable. Memory loss will probably be the first thing most people think about when hearing the words "Alzheimer's disease", however, beneath the surface there is already a lot going on before this is noticeable. A small, but important, part of the brain called "the hippocampus" is responsible for this. This hippocampus, shaped like a seahorse, is involved in saving memories. Research has shown that the hippocampus of people with Alzheimer's disease is smaller than the hippocampus of healthy people. By creating threedimensional reconstructions of the hippocampi of patients at two time points 12 months apart, researchers could look at the change in shape of these hippocampi. They "sliced" these 3D reconstructions into multiple sections and looked at the way they changed in one year time. It seems there may be a difference in the way these hippocampi change shape in healthy people and patients diagnosed with Alzheimer's disease. Further research has to be done, but it's a step in the right direction for diagnosing Alzheimer's disease as early as possible 

Scientific abstract (pdf 2K) For more info or full text, mail to: jc.demunck@vumc.nl 
E.J. Vermaas 
Bachelor programme: Natuur en Sterrenkunde  August 20th, 2017  
Institute: Universiteit Utrecht  Research group: Freudenthal Instituut  Graduation thesis  Supervisor: T.M. Nieuwenhuizen 
Unexpected interference in the weak decoherence condition? Quantum mechanics has a different logic than classical mechanics. For example, a thing can be at two locations at the same time in quantum mechanics which in classical logic can only have one well defined position. The behaviour of atoms is known to be described by quantum mechanics. But we are build of those atoms and we do not experience quantum logic in our daily life. Our world is well described by classical mechanics. Multiple theories and interpretations of quantum mechanics are developed to clarify this transition from quantummechanics to classical mechanics. The theory of decoherence describes classical aspects in quantum mechanics. There are two conditions formulated. If one of these conditions is satisfied, classical logic can be applied to the quantum system. One of those conditions has mathematically curious properties. In this thesis a physical example is found that satisfies this condition in order to understand what these mathematical properties mean in a physical way. 

Scientific abstract (pdf 34K) Full text (pdf 765K) 
M. van der Eyden 
Master programme: Physics  Theoretical Physics  August 19th, 2017  
Institute: UvA / Other  Research group: UvA / Other  Extra internship  Supervisor: Edan Lerner 
Discrete element simulations of micropenetration in cohesive granular materials The SnowMicroPenetrometer (SMP) is an instrument that was developed to characterize snow stratigraphy for various applications. It penetrates the snow and measures the penetration force signal with high spatial resolution. While some structural parameters can be estimated from the measured force signal within a simple, stochastic penetration model, their relation to the true microstructural and micromechanical properties of snow is yet poorly understood. To improve the understanding of the penetration process we have analyzed Discrete Element Simulations of a small cone penetrating into cohesive sphere assemblies. We have used initial sphere configurations covering a variety of packing fractions and (cohesive) coordination numbers and conducted simulations for different (cohesive) bond strengths and cone sizes. Our main results show that the mean penetration force is primarily controlled by the bond failure rate, with a superlinear dependence indicating a feedback of failed material. A secondary influence stems from particle jamming in front of the cone. These influences are incorporated in a proposal towards a new model that describes the penetration force in terms of simulation parameters and failure processes around the cone. 

Scientific abstract (pdf 1K) Full text (pdf 5985K) 
J.M. van Urk 
Bachelor programme: Natuur en Sterrenkunde  August 18th, 2017  
Institute: WZI  Research group: Quantum Gases & Quantum Information  Graduation thesis  Supervisor: Robert Spreeuw 

Characterization of a magnetooptical trap of Rubidium atoms and building the repump laser The development of quantum simulators and quantum chips is nowadays a widely examined part in physics. Quantum simulators manipulate simple quantum systems to get a better understanding of more difficult systems. One way of realising systems for quantum computation or quantum simulation is by creating atom chips. In these chips small clouds of ultracold atoms are loaded into magnetic traps. This process starts with trapping ultra cold atoms in a magnetooptical trap (MOT). A MOT uses a combination of a strong magnetic field and lasers, made for cooling and trapping neutral atoms. The light from the lasers need the right properties, like wavelength, frequency and polarisation, so the photons can excite Rubidium atoms to a higher energy state. In this report the complete setup for one of the lasers is elaborated. To optimise to actual experiment for quantum simulation, it is important to know the properties of the MOT, like the size of the cloud and the number of atoms inside of it. Two techniques to determine this are used in this research: measuring fluorescence with a photodiode and taking absorption images with a camera. 

Scientific abstract (pdf 1K) Full text (pdf 17749K) 
T. Bouma 
Bachelor programme: Natuur en Sterrenkunde  August 17th, 2017  
Institute: ITFA  Research group: Instituut voor Theoretische Fysica  Graduation thesis  Supervisor: Vladimir Gritsev 
Derivation of the shift charge current for 3D Bi_2X_3 (X = Te,Se) Topological Insulators, using the Floquet formalism combined with Keldysh’s Green’s function method Topological insulators lie at the foundation of a new type of electronics: spintronics. These devices are very much similar to the more general electronics, except they have one additional degree of freedom: spin. Spin is an intrinsic characteristic of particles, and manipulating it could allow us to store more information on the same surface. Topological Insulators can be defined as conducting at the edge, but insulating at the bulk, a state which is defined by the topology of the system. The topology, in turn, can roughly be defined as the geometrical shape of the energy spectrum of the system. Optical spintronics are a type of Topological Insulators, where the topological behaviour is induced by or related to optical excitation. For instance, charge currents can be induced at the surface of the Topological Insulator, called the shift charge current. This study aims to describe the direction and magnitude of the shift charge current on the surface of 3D Bi_2 X_3 (X=Te,Se) TIs, by looking at the process of exciting an electron inside the material and creating a pair of an electron and a hole (in the energy spectrum). The excited electron then travels through the material, creating a current. 

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B.M. van Bloppoel 
Bachelor programme: Natuur en Sterrenkunde  August 17th, 2017  
Institute: ITFA  Research group: MathematicalPhysics  Graduation thesis  Supervisor: dhr. dr. M.L. (Marcel) Vonk 
At the shoal of the resurgent depths When physicists solve problems, the first approach is to solve by expressing solutions in exact mathematical representations. But in most problems, we won't find exact expressions, so we use something which is called a power series. A power series is a series of coefficients and powers of a variable. Every power of x is multiplied by a coefficient. In physics, these are asymptotic. The first term will be near to the answer and every subsequent term will bring us closer to the answer. In this case, the series converges. This is behaviour we wish from series, because we calculate many terms and get a better approximation to our answer. But the course of physics has brought us to realise that many modern problems produce divergent series, where summing all the terms leads us to infinite answers. These divergent series leave us with no way to solve problems. So we need tools to calculate divergent series. Borel's resummation technique is the first point of attention in my thesis. We find that this technique allows us to find finite answers to divergent series in a lot of cases. In the cases where this doesn't work, we turn to resurgence theory for answers. 

Scientific abstract (pdf 2K) Full text (pdf 574K) 
K.H. Taris 
Master programme: Physics  Physics of Life and Health  August 14th, 2017  
Institute: VU / Physics & Astr.  Research group: VU Physics of living systems  Graduation thesis  Supervisor: Erwin Peterman 
Intraflagellar Transport in C. elegans with Light Sheet Microscopy IntraFlagellar Transport (IFT) in C. elegans is a process that transports cargo along axonemes with motor proteins. IFT is involved in multiple signalling pathways. In the Erwin Peterman group, epifluorescence microscopy has been used to image IFT dynamics. However, epifluorescence microscopy has limits in its signaltobackground ratio. Light Sheet Microscopy (LSM) illuminates only a thin slice of the sample, which lowers phototoxicity and low outoffocus fluorescent protein bleaching, which can lead to a high signaltobackground ratio. LSM uses two different perpendicular orientated objectives, which adds a rotational degree of freedom. Here we show that our light sheet microscope is not yet capable of determining IFTdynein’s velocity, as the kymographs have few distinguishable trains. Our LSM has a lower signaltobackground ratio than epifluorescence, despite having a better axial resolution and an added rotation stage. We consider this light sheet microscope to be a project in progress, where potential improvements include using thicker circular samples with a low agarose concentration, changing the detection objective to be a fluorite plan objective with higher N.A., or adding a second detection objective. 

Scientific abstract (pdf 2K) For more info or full text, mail to: e.j.g.peterman@vu.nl 
V. Ricciardulli 
Master programme: Physics  Particle and Astroparticle Physics  August 14th, 2017  
Institute: ITFA  Research group: String theory  Graduation thesis  Supervisor: Alejandra Castro Anich 
Localization in supersymmetric ChernSimons theories The study of interacting quantum field theories has been possible, to great extent, thanks to perturbation theory. However, there is an inherent limitation to the amount of information that one can extract in this way. The need to develop nonperturbative techniques is then crucial to understand a field theory for all values of the coupling constant. In some cases, it is possible to exploit the symmetry of the theory to obtain such results and one particularly fruitful setting is represented by supersymmetric theories. In this thesis, we introduce fundamental concepts in N=2 supersymmetric ChernSimonsmatter theories on the threesphere and describe the procedure of supersymmetric localization, which provides an exact computation of the partition function of such theories. 

Scientific abstract (pdf 2K) Full text (pdf 784K) 
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