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Science in Progress

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Displaying theses 21-30 of 1078 total
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A.A.A. Jansma
Master programme: Physics - Theoretical Physics August 6th, 2018
Institute: ITFA Research group: Instituut voor Theoretische Fysica Graduation thesis Supervisor: Bernard Nienhuis
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E8 Symmetry Structures in the Ising Model
The Ising model is a model of very simple objects interacting among each other. It is as simple as it is powerful and ubiquitous. It is seen throughout all mathematical sciences, but more recently also in biology and the social sciences. It is easy to formulate, but contains extremely rich behaviour and is not yet fully understood in all situations. In 1989, a Russian physicist called Alexander Zamolodchikov discovered that in some situations, there are numbers appearing in the behaviour of the Ising model that are known from a seemingly unrelated, but very famous and beautiful branch of mathematics: Lie algebras, mathematical objects describing continuous symmetries. More specifically: some numbers that we see appear in the description of the Lie algebra called E8, the largest symmetry structure of all, also appear in the masses of particles that live in the Ising model. This thesis explores this connection historically and mathematically, and points into a direction that could lead to a better understanding of the role these numbers play in the Ising model.
picture that illustrates the research done
Scientific abstract (pdf 1K)   Full text (pdf 3447K)

N.D.J. Visser
Bachelor programme: Natuur- en Sterrenkunde August 4th, 2018
Institute: WZI Research group: Soft matter Graduation thesis Supervisor: Corentin Coulais
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Self-Oscillatory snap-through buckling
Actuators are devices controlling or inducing movement in mechanical systems. An actuator is responsive, so it usually turns a control signal into an action. Snap-through instabilities, which are instabilities where the system goes from one state to another in a very short time frame, can be used to create hysteretic loops, these are loops where there are multiple values of one variable depending on the direction of change of another variable. These mechanical instabilities have good properties for actuators, as a small input generates a large output. An example of such a loop is the snap-through buckling of a beam. In this thesis, we have built an autonomous actuator and tuned the behaviour by changes in the environment and the beam itself. In this project, we have shown that poroelastic constrained swelling causes oscillatory snap-through buckling in the beam, while the buckling behaviour is strongly dependent on the choice of material and solvent. We have tuned the behaviour of the beam by creating a uniholar sheet in the beam. This reduced the total oscillation time by a factor of two to approximately 1.5hrs and introduced another way of tuning the poroelastic swelling behaviour due to capillary effects in the holes.
picture that illustrates the research done
Scientific abstract (pdf 2K)   For more info or full text, mail to: coulais@uva.nl

M.A. van der Wateren
Master programme: Astronomy and Astrophysics August 1st, 2018
Institute: API Research group: General Neutron Stars Graduation thesis Supervisor: Anna Watts
Estimating mass and radius from the burst oscillations of an accreting millisecond pulsar
One of the most important goals in astrophysics today is discovering what the inside of a neutron star is made off. A promising method is by modeling the light coming from the surface of the neutron star during thermonuclear explosions. These explosions show an oscillation in their light profile. The way we observe this oscillation can be simulated using a ray tracing code. Since the light is affected by general relativistic effects, simulations can tell us something about the mass and the radius of the neutron star. We investigate the neutron star XTE J1814-338, but are not able to find a model which represents the data well enough. The model we use may be too limited. In the future we hope to improve this method, by applying a more sophisticated model.
picture that illustrates the research done
Scientific abstract (pdf 1K)   For more info or full text, mail to: A.L.Watts@uva.nl

M. Bout
Bachelor programme: Natuur- en Sterrenkunde August 1st, 2018
Institute: NIKHEF Research group: Theoretical Physics at NIKHEF Graduation thesis Supervisor: Juan Rojo
A three-dimensional profile of nuclear structure functions with neural networks
In the early sixties, Quantum ElectroDynamics (QED) produces a more and more precise description of electromagnetic interactions. For the strong nuclear force however this still needed to be done. A model that became more and more accepted was the "quark" model proposed by Gell-Mann. This model said that a nuclear particle consisted from three "quarks". IN the beginning this was seen as a purely mathematical model, however, later experiments in Deep Inelastic Scattering (DIS) showed that this model only partially described reality. Data showed that nuclear particles consist from the three quarks, gluons that keep them together and, at higher energies, from virtual quarks. By using DIS information can be gained about hte interactions in the nuclear particles and this can be used to produce an image of the nuclear structure of the particles. However, since the structure is dependent on non-perturbative variables, the structure needs to be calculated numerically. A way of doing that is by using Artificial Neural Networks (ANN), a machine learning algorithm that does not require the amount of parameters and degrees of freedom to be determined at the start.
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Scientific abstract (pdf 1K)   Full text (pdf 830K)

A.P. Basdew-Sharma
Master programme: Physics - Theoretical Physics July 31st, 2018
Institute: ITFA Research group: String theory Graduation thesis Supervisor: Dr. Diego Hofman
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On the Quantum Renormalization Group and its applications
If one applies Wilsonian RG to a matrix or vector field theory, one usually generates an infinite tower of operators. The beta functions of these operators are classical, as an initial condition fixes the trajectory of the flow. We will explain and apply a method in which the RG flow is quantized, by projecting down to the subspace of single-trace operators at each RG step. In doing this, the beta functions for the single-trace operators obtain quantum fluctuations, hence the name Quantum Renormalization Group. We will apply the method to vector models, in particular to the Complex SYK model and generate bulk actions that are invariant under (d+1)-dimensional diffeomorphisms. The dynamical fields in the bulk will arise in a natural way. A brief review of the AdS/CFT correspondence and the Holographic Renormalization Group is given to motivate the subject.
picture that illustrates the research done
Scientific abstract (pdf 1K)   For more info or full text, mail to: D.M.Hofman@uva.nl

R.G. Visser
Master programme: Astronomy and Astrophysics July 29th, 2018
Institute: API Research group: Planet formation group Graduation thesis Supervisor: Carsten Dominik
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Spinning up planets with pebble accretion
Context. Classical theories of planet formation are not capable of explaining the trend in observed spin periods of Solar System Bodies due to randomized outcomes. Thus, the narrow range of spin magnitudes and the tendency to a rotation direction in line with the orbital rotation (prograde) of these bodies are yet to be reproduced from theoretical frameworks. Pebble accretion is a planetary growth mechanism in which gas drag and gravity collaborate to enhance collision cross-sections of protoplanets accreting solids (pebbles). We determine the specific angular momentum that pebbles deliver to a protoplanet to establish the net rotation a body gains in the pebble accretion framework.
picture that illustrates the research done
Scientific abstract (pdf 2K)   For more info or full text, mail to: dominik@uva.nl

D.J. van Eijnatten
Master programme: Astronomy and Astrophysics July 29th, 2018
Institute: API Research group: Black hole astrophysics group Graduation thesis Supervisor: prof. dr. Sera markoff
The impact of radiation on the dynamics and appearance of the supermassive black hole in the Galactic center
Black holes are some of the most fascinating objects in the cosmos. Our Milky Way also contains a supermassive black holes, called Sagittarius A*. Its mass is about five million times the mass of the sun and we can observe it sucking up (``accreting'') small amounts of gas that is swirling around in its neighbourhood. Almost every day we see ``flares'', burst of radiation that last a short time. To investigate this and to observe the black hole ``shadow'', the shape the black hole imprints on the surrounding gas, astronomers are taking a picture of Sagittarius A* with a very high resolution. To understand what we will see in this picture and to explain the flaring behaviour of Sagittarius A* we use computer simulations to replicate the interaction between the black hole and its surrounding gas. This gas behaves like a fluid containing a magnetic field, so the simulated physics is the laws of fluid dynamics and general relativity. I added optically thin radiation. We find that radiative energy loss has a significant impact on how the gas behaves and on what the picture from the Event Horizon Telescope will look.
picture that illustrates the research done
Scientific abstract (pdf 2K)   Full text (pdf 8597K)

J. Willemse
Master programme: Physics - Physics of Life and Health July 27th, 2018
Institute: VU / Physics & Astr. Research group: VU Biomedical Physics Group Graduation thesis Supervisor: J.F. de Boer
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Quantifying polarization properties of tissue obtained with polarization sensitive optical coherence tomography on the human brain and retina
Optical coherence tomography (OCT) is a technique which is often used to create images of the retina. An extension of OCT which is not used in the clinic yet is polarization-sensitive OCT (PS-OCT). With PS-OCT certain structures can be recognized in tissue, such as scar tissue or muscle tissue. In age-related macular degeneration (AMD)., the functionality of the different layers in the retina is distorted. In the final stage of AMD scar tissue is formed on the retina, resulting in irreversible loss of vision. With the current imaging techniques available in the clinic it is very difficult to see which part of the damaged tissue is scar tissue, and which part is still active. In this research it has been shown that PS-OCT can show which part of the retina contains scar tissue. To do this, a new mathematical method has been used which previously was only used on breast tissue. In the second part of the research, brain slices of Alzheimer patients have been examined. PS-OCT has been used to see if Alzheimer plaques could be found and correlated to microscope images. However, the plaques showed too little signal to be found with the PS-OCT setup.
picture that illustrates the research done
Scientific abstract (pdf 1K)   For more info or full text, mail to: jfdeboer@few.vu.nl

R. Reyes Garza
Master programme: Physics - Theoretical Physics July 27th, 2018
Institute: ITFA Research group: Edan Lerner Graduation thesis Supervisor: Edan Lerner
Collective mechanics of a dense suspension of active particles
Dense suspensions are fluid-like systems that behave notoriously different compared to their diluted counterparts. If the density of the suspended particles is high enough, the system undergoes into a solid-like phase through the so-called jamming transition. Toothpaste, peanut butter and mayonnaise are typical examples of jammed systems. Highways during rush hour, beehives or the crowd at a music festival are also examples of highly packed systems that resemble a dense suspension but with a notorious difference: the particles can move by themselves through their available space. In this project, we theoretically an numerically investigated the contact forces between self-propelling particles and their effective velocities when they form a dense suspension near jamming.
picture that illustrates the research done
Scientific abstract (pdf 2K)   Full text (pdf 2071K)

R. van der Werff
Master programme: Physics - Theoretical Physics July 25th, 2018
Institute: ITFA Research group: Instituut voor Theoretische Fysica Graduation thesis Supervisor: Philippe Corboz
Simulating classical spin systems using the Fixed Point Corner Method
In my research, I studied the clock model, which is an infinite square lattice with one spin on each site. Every spin can be in q possible spin states, that are represented by arrows, uniformly distributed over a circle. The model is very interesting, as the system undergoes a phase transition from an ordered to a disordered phase. When q is sufficiently high, there can also be an intervening massless phase. The two transitions to this phase are BKT-transitions. In order to simulate the clock model, I use a tensor network to represent the system. I use the Corner Transfer Matrix Renormalization Group (CTMRG) method and the Fixed Point Corner Method (FPCM) to find this tensor network. Firstly, I tested the accuracy and speed of FPCM. This was done by applying this method to the Ising model and comparing the results with results obtained with CTMRG. I conclude that the results of FPCM is as accurate as CTMRG, but need less simulation time. Secondly, I applied FPCM to the clock model in the case where q = 5 and 6. From the results I conclude that both cases have a massless phase, and that the transitions are of BKT type.
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Scientific abstract (pdf 1K)   Full text (pdf 7482K)

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