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Displaying theses 91-100 of 1078 total
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S. van der Heijden
Master programme: Physics - Advanced Matter and Energy Physics September 18th, 2017
Institute: Other Research group: ARCNL Graduation thesis Supervisor: Wim Ubachs
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Comparison of ion energy distributions from ns- and ps-laser produced tin plasmas from solid and droplet targets
For this thesis I studied the rate of expansion of tin plasmas, specifically by measuring the velocities of the ions within the plasma. To explain some of the terms: plasma is what you get when you make something so hot that even the elementary building blocks of the material are "damaged;" electrons are heated enough to escape their atoms. These particles are then called ions. I generated my plasmas by heating a tin plate with short, powerful laser pulses. Tin plasmas are interesting both from a industrial and a theoretical point of view. The industrial applications of these plasmas mostly stem from the light that they emit; tin plasmas emit extreme ultraviolet light. This light finds use for example in microscopy and in the microchip industry. The significance of doing experiments on tin plasmas for theory is that theories that try to describe the behaviour of such plasmas exist, but the falsification or verification must in the end come from observations. The results of my experiments I compared to theoretical models, which should be able to prediction the results. There was good agreement between theory and experiment for ultrashort laser pulses, but theoretical understanding for less short pulses is insufficient.
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Scientific abstract (pdf 1K)   Full text (pdf 4522K)

D. Mulder
Master programme: Physics - Theoretical Physics August 31st, 2017
Institute: ITFA Research group: MathematicalPhysics Graduation thesis Supervisor: Bernard Nienhuis
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Percolation and the Ising model in the hyperbolic plane
Percolation on the Euclidean plane has a single critical point p_c. At the critical point, the mean cluster size scales with the system size N as a N^43/48. Other quantities have similar scaling behaviour, with different critical exponents. These exponents are related through scaling laws. In the hyperbolic plane, there are two dual critical points, p_c and p_u. In between, the exponent of the mean cluster size continuously varies from 0 to 1. We have tried to find other quantities that scale as a power of N, to establish scaling relations in the hyperbolic plane. The results are not as simile to Euclidean case as expected. Since percolation is the limit case of a class of models called Potts models, we extended our scope to Potts models in general. We have studied the Ising model (which is the 2-state Potts model) by using the CTMRG-algorithm for hyperbolic lattices, as presented by Nishino et al. Using a framework given by Nogawa and Hasegawa for percolation, we were able to determine, for the first time, a second critical temperature T_u for the Ising model on hyperbolic lattices.
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Scientific abstract (pdf 3K)   Full text (pdf 3505K)

S.A.M. Lourens
Bachelor programme: Natuur- en Sterrenkunde August 29th, 2017
Institute: AMC Research group: Biomedical Engineering and Physics Graduation thesis Supervisor: M.C.G. Aalders
Fingerprint Detection with Optical Coherence Tomography
Damaged or altered fingerprints can not be traced back with conventional fingerprint detection technique, which can be a problem in especially the forensic field. A solution to this problem may be provided by Optical Coherence Tomography, which enables detecting the fingerprint from not only the surface layer, but also the dermis layer of the skin, which is not vulnerable to spoofing and superficial damage to the skin. This research focused on optimizing the process of obtaining these OCT fingerprints step by step.
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Scientific abstract (pdf 3K)   For more info or full text, mail to: m.c.aalders@amc.uva.nl

K. Gourdji
Master programme: Astronomy and Astrophysics August 28th, 2017
Institute: API Research group: Radio Pulsars Graduation thesis Supervisor: Dr. Jason Hessels
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Repeating Fast Radio Bursts
Fast Radio Bursts (FRBs) are bright, millisecond-long radio pulses of extragalactic origin. Since their discovery 10 years ago, the search for FRBs and what creates them has garnered much interest within the astronomical community and the general public. The discovery of repeat bursts from FRB 121102, the only source known to repeat, raises the possibility that all or some FRBs repeat. Furthermore, it has bolstered the search for viable emission models, as they can no longer be cataclysmic in nature, at least for this source. Studying the pulse morphology and source activity can help with this endeavour. We have observed FRB 121102 using the Arecibo telescope with unprecedented time and frequency resolution. We present a pipeline developed to find repeat FRBs, which has increased the sample of bursts from FRB 121102 from ~30 to 134 and counting. We present our findings from this sample. we address the dearth of observed repeating FRBs and test the hypothesis that the less-sensitive Parkes telescope, which has discovered the majority of the FRB population, has only been detecting the strongest bursts. We present Arecibo observations of Parkes FRB 130628, which do not reveal repeat bursts, though we continue to process observations of this sky position.
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Scientific abstract (pdf 3K)   For more info or full text, mail to: J.W.T.Hessels@uva.nl

T. Kaandorp
Bachelor programme: Natuur- en Sterrenkunde August 28th, 2017
Institute: AMC Research group: Department of Radiology Graduation thesis Supervisor: A. J. Nederveen
Accelerated 4D flow MRI, Comparing SENSE, k-t PCA and Compressed Sensing
To increase patient comfort, it is possible to accelerate a 4d flow mri using techniques such as SENSE, k-t PCA or Comrpessed Sensing(CS). The acceleration is based on collecting less data points. However by doing so the image quality decreases. One can therefore wonder what happens with the amplitude of the flow curves created by 4D flow MRI. It turns out that for accelerated SENSE and k-t PCA the amplitude of the flow curves decreases by respectfully 33 and 20 percent. The amplitude of the flow curves created with CS reconstruction only decrease with 7.5 percent. Thus making CS a more reliable acceleration method.
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Scientific abstract (pdf 1K)   Full text (pdf 2467K)

A.M.L. Edstam
Master programme: Astronomy and Astrophysics August 27th, 2017
Institute: API Research group: BinCosmos Graduation thesis Supervisor: Dr. S. E. de Mink
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Binary black hole formation through alternative formation channels
In 2015 gravitational waves were detected in a direct manner for the first time. Gravitational waves stretch and compress space. It is impossible to measure these effects using a ruler as the ruler's length changes too. The gravitational wave detections have been results of the merger of two black holes orbiting each other. Black holes are dense objects that have such strong gravitational pull that nothing that comes close can escape. These black holes originate from massive stars. For the merger to occur fast enough to produce an observable gravitational wave source, the black holes must have been closer to each other than Mercury is to the Sun. This is a challenge as the sizes of the massive stars forming black holes are larger than this during the course of their lives. Astronomers have several theories about how merging black-hole systems are formed. We look into two recently proposed formation theories. Three black-hole mergers have been observed as gravitational wave sources, too few to be able to distinguish between formation theories. More detections are expected as more observations are performed, the detectors are made more sensitive and new observatories are built, making it possible to distinguish between formation theories.
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Scientific abstract (pdf 3K)   For more info or full text, mail to: S.E.deMink@uva.nl

G. de Haan
Master programme: Physics - Advanced Matter and Energy Physics August 25th, 2017
Institute: Other Research group: ARCNL Graduation thesis Supervisor: Paul Planken
Femtosecond Pump-Probe Measurements on Thin Beta-Sn Layers in the Ablation Regime
By firing femtosecond laser pulses onto Sn samples, interesting dynamics are measured using the pump probe technique. These dynamics take place at an ultrashort time scale in the range of femto- and picoseconds. By measuring the transmission change of the Sn as a result of the laser we are able to resolve the electron behaviour of the Sn.
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Scientific abstract (pdf 1K)   For more info or full text, mail to: p.c.m.planken@uva.nl

D.M. Chernowitz
Master programme: Physics - Theoretical Physics August 25th, 2017
Institute: ITFA Research group: Condensed Matter Theory Graduation thesis Supervisor: Vladimir Gritsev
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Thermalization in Quantum Information Geometry
A boiling, bubbling glass of water will, over time, calm and cool down when left alone in a room. After a while, all such glasses of water look the same, despite starting under dramatically different conditions. It becomes hard to recover the initial conditions. On smaller scales in quantum mechanics, the same happens: we often see that a system of quantum particles or spins on lattice sites tends to some predictable state. Conventionally, no information can be lost in quantum mechanics. In order to understand how the initial conditions then become irrelevant, and systems become indistinguishable, a standard framework with a 'bath' or environment is constructed. This is used to describe which information can be extracted, which is inaccessible, and when we can distinguish a state from another state. From here, we make geometric arguments about what a typical quantum system can do, and how information is lost over time, going so far as to derive several statistical statements about the disorder of the average quantum system. We also spend some time on how to treat varying systems as points separated by distances in a space of systems, and explain the difficulties that arise in this description.
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Scientific abstract (pdf 2K)   Full text (pdf 8825K)

S. Smit
Master programme: Physics - Advanced Matter and Energy Physics August 24th, 2017
Institute: WZI Research group: QMat Amsterdam Graduation thesis Supervisor: Prof. Mark Golden
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X-ray Spectroscopy and Electron Diffraction on VO2
Vanadium dioxide (VO2) is a very interesting material, as it is possible to change it from electrically insulating to conducting simply by heating up. This property has a lot of interesting physics behind it, and many people are thinking of applying this in future electronic devices. In this master thesis this material has been investigated by means of electrons and X-rays, and in particular the moment at which it switches from one state to another. There are many factors that play a role, such as the physical structure of the atoms within the material, and the behaviour of the electrons that surround them. The atomic structure is investigated by bombarding the material with electrons, and investigating how they scatter from the crystal. The electronic behaviour was tested by looking at how these electrons absorb certain energies of X-rays. Looking at the time-scales of the changes within the material, one can hope to learn how to use this material most effectively in future devices.
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Scientific abstract (pdf 2K)   Full text (pdf 13511K)

J.G.B. Henke
Master programme: Physics - Advanced Matter and Energy Physics August 22nd, 2017
Institute: WZI Research group: QMat Amsterdam Graduation thesis Supervisor: Mark Golden
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The search for 2D Dirac fermions in a 3D topological system
Recently, a central goal of condensed matter physics has been to understand and characterise so-called topological phases of matter. One such phase is a two-dimensional (2D) Dirac semimetal, which hosts fermions that behave according to the relativistic Dirac equation. Such a material would also be a fantastic conductor, hosting (nearly) massless charge carriers with a high mobility. Graphene came to fame as being such a material in 2005, although these properties were later shown not to be robust. In 2015, Young and Kane proposed that the presence of certain symmetries will result in a 2D Dirac semimetal phase that is inherently robust. In this thesis, we study one specific material – SrMnSb2 – using a range of experimental and theoretical methods to determine whether it is indeed the 2D Dirac semimetal we've been looking for. We show it to be a quasi-2D material hosting low-mass, high-mobility charge carriers, but despite these hopeful signs, the Dirac fermions we were hoping to find were not those involved in conduction. Our study proves the value of the combination of thorough experimental and theoretical research, and opens new avenues of research into the role of magnetism in (otherwise) Young and Kane-like systems.
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Scientific abstract (pdf 2K)   Full text (pdf 14591K)

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