Displaying theses 110 of 1071 total
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E.B. Ippel 
Master programme: Physics  Theoretical Physics  November 15th, 2018  
Institute: ITFA  Research group: Instituut voor Theoretische Fysica  Graduation thesis  Supervisor: Bernard Nienhuis 
Universality in onedimensional models displaying selforganized criticality The concept of selforganized criticality is considered to be one of the mechanisms by which selfsimilar fractal structures and complexity in nature arise. In this thesis, we first introduce the concept of selforganized criticality and give a brief overview of selforganized criticality in one dimension. We then proceed by analysing various models, all of which are governed by different stochastic toppling rules. These models quickly evolve towards a steady state which is characterized by the occurrence of avalanches of varying size, which do not necessarily have a characteristic scale, and possibly display powerlaw behaviour in the frequency of their occurrence. Indeed, we find that three of our models exhibit critical behaviour in the form of distributions of avalanches following a powerlaw, all of which are characterized by different scaling exponents. Furthermore, we find certain critical properties intrinsic to the steady state of two of our models. Lastly, we investigate whether the critical behaviour emerging through the dynamics of our models is universal. We indeed find that the critical behaviour emerging in the nonlocallimited model is universal between a class of different models, where the universality class to which these models belong can be characterized by four critical exponents. 

Scientific abstract (pdf 2K) Full text (pdf 952K) 
J.H. Bootsma 
Master programme: Astronomy and Astrophysics  October 29th, 2018  
Institute: API  Research group: Planet formation group  Graduation thesis  Supervisor: Carsten Dominik 
Constraining starspot fraction and temperature in weaklined T Tauri stars using spectroscopy Whether the gas situated in the innermost regions of a protoplanetary disk (PPD) is optically thin or optically thick determines the radial distance upward of which rocky planets (or planetary cores) can form. Determining the opacity of this gas, however, is impeded by a number of phenomena. The presence of starspots on the surface of the stars that host these protoplanetary disks, is one of these. This, because the starspots and the inner gas are believed to have similar temperatures. In this work we present a method that constrains the starspot fraction and temperature on the surface of T Tauri stars using spectroscopy. We select starspotsensitive absorption lines of which we compare the line profile with synthetic line profiles (derived from PHOENIX models). 

Scientific abstract (pdf 2K) Full text (pdf 8307K) 
S.A.S. de Wit 
Master programme: Astronomy and Astrophysics  September 28th, 2018  
Institute: API  Research group: Massive Star Group  Graduation thesis  Supervisor: Prof. Dr. A. de Koter 
Characterizing the properties of the VFTS 176 binary in the 30 Dor. starburst region In this work we present an analysis of the massive binary system VFTS 176. VFTS 176 is a close binary, with two massive main sequence components and a short orbital period (~0.1 AU). It is thus very likely that the stars have either interacted in the past or will interact while still on the main sequence. We investigate the possibility of a past phase of masstransfer. However, since this system is a known spectroscopic binary, the observed spectra consists of features from both components. To tackle this problem, we use a spectral disentangling technique to separate both contributions. We then found a set of properties for both stars by means of quantitative spectroscopy, including their masses, stellar radii and effective temperature. By then fitting evolutionary models, we find the posterior age distributions. Based on the posterior age distributions, the absence of tidal synchronization and the absence of a stripped primary, we reject the possibility of a past phase of masstransfer. We do however, predict that the primary will donate mass to its companion around an age of 4.20 Myr, drastically changing the final fate of this system. 

Scientific abstract (pdf 2K) For more info or full text, mail to: a.dekoter@uva.nl 
J.A.L. Seneca 
Master programme: Physics  Particle and Astroparticle Physics  September 26th, 2018  
Institute: NIKHEF  Research group: KM3NeT  Graduation thesis  Supervisor: Ronald Bruijn 

Models of Particle Signatures in KM3NeT ORCA The neutrinos are elusive particles which are constantly produced in the Earth's atmosphere. Neutrinos are able to travel through the entire Earth barely affected, but the way they are affected can tell us about the particle's properties; their masses. Our detector, KM3NeT ORCA, is specialized in detecting these particles. When a neutrino eventually interacts in the detector, it produces a myriad of other particles which produce light. Based on the light we see from these particles, we can extract information about the neutrino which produced them. Understanding the light signature produced by these product particles is a challenge in itself which has been a matter of research for almost a century. This knowledge is also useful for KM3NeT, as lets us work backwards from the light signatures to the properties of the original particle, and thus, that of the neutrino. For the first time in KM3NeT, we create models of these light signatures for individual particles and present them in all their gory details. We then use these models to estimate neutrino information from simulations and to perform simulations of our own. The results are promising, but more work is needed for practical use of the models. 

Scientific abstract (pdf 89K) Full text (pdf 11439K) 
N. Petropoulos 
Master programme: Physics  Theoretical Physics  September 11th, 2018  
Institute: ITFA  Research group: String theory  Graduation thesis  Supervisor: Erik Verlinde 
A DoubleHeaded Centaur in a Black de Sitter Universe In my thesis we study black holes in the early and late phases of our Universe. Also, we probe to find the nature of these black holes and of the socalled cosmological horizon. This is a horizon which seems to surround us when we look at the Universe through our telescopes. 

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R. Koch 
Master programme: Physics  Theoretical Physics  August 31st, 2018  
Institute: ITFA  Research group: Statistical Physics and Condensed Matter Theory  Graduation thesis  Supervisor: JeanSebastien Caux 
Excitations of the Gapped XXZ Heisenberg Spin1/2 Chain In quantum mechanics particles only occur in certain states  there are quantized. A famous quantum mechanical model is the spin1/2 chain, which is a onedimensional model that describes spinparticles fixed to points on a line. It turns out that there is no classification of one particular spinchain, the XXZ gapped Heisenberg spin1/2, so that one cannot obtain the correct number of states. In this thesis, such a classification is presented. 

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I.H.A. Knottnerus 
Master programme: Physics  Advanced Matter and Energy Physics  August 31st, 2018  
Institute: WZI  Research group: Quantum Gases & Quantum Information  Graduation thesis  Supervisor: Florian Schreck 

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Scientific abstract (pdf 0K) For more info or full text, mail to: f.schreck@uva.nl 
I.H. Stammes 
Bachelor programme: Natuur en Sterrenkunde  August 29th, 2018  
Institute: WZI  Research group: Soft matter  Graduation thesis  Supervisor: Sander Woutersen 
Two amorphous phases of mannitol. Water has many properties that are hard to explain. One possible explanation is a liquidliquid transition in water. The liquidliquid transition in water happens in an experimentally inaccessible region, so other ways have to be found to research liquidliquid transitions. Mannitol is the first pure substance with a waterlike liquidliquid transition. We have studied the hydrogen bond strength in both liquid forms of mannitol by taking IRspectra of the OHstretch mode. The OHstretch mode is very sensitive to the nature of the hydrogen bond and therefore a good probe to investigate the hydrogen bond. We found that the hydrogen bonds tighten as mannitol transitions from one liquid to the other. We also tried to measure the viscosity of both liquids. A way to measure this is by using single fluorescent molecules as probes and measure at what speed the single molecules rotates. This rotating speed is related to the viscosity of the substance. Previous single molecule measurements on a polymer were successfully reproduced. Hence, the single molecule method seems promising for measuring viscosity in mannitol's two liquid phases. 

Scientific abstract (pdf 1K) For more info or full text, mail to: s.woutersen@uva.nl 
P.C.G. Vlaar 
Master programme: Physics  Theoretical Physics  August 25th, 2018  
Institute: ITFA  Research group: Condensed Matter Theory  Graduation thesis  Supervisor: Philippe Corboz 

A simplified approach for 3D tensor network simulations Tensor network methods have been very successful to accurately simulate one and twodimensional quantum models, with White's densitymatrix renormalization group (DMRG) as a famous example [PRL 69, 2863 (1992)]. A way to generalize tensor network methods to three dimensions would be highly desirable, especially for simulating fermionic and frustrated models which in many cases are hard or impossible to solve using other methods. In this project, we have implemented a new way of contracting threedimensional tensor networks using clusters. These clusters consist of a certain number of tensors which are contracted exactly or with a higher accuracy as compared to the environment of the cluster, which is approximated effectively. The method is implemented on the square and cubic lattices and benchmarked with results obtained from other methods. Also, the method is applied to the SU(3) Heisenberg model and different ground state candidates are compared to each other. 

Scientific abstract (pdf 1K) Full text (pdf 2887K) 
D.J. Hemminga 
Master programme: Physics  Theoretical Physics  August 25th, 2018  
Institute: ITFA  Research group: Condensed Matter Theory  Graduation thesis  Supervisor: prof.dr. Kareljan Schoutens 

Geometric quantum computing with supersymmetric lattice models In this project we consider a new approach to quantum computation. We investigate quantum control of qubits defined by a supersymmetric lattice model. Periodic chains with length L=3n produce two degenerate ground states, which can be interpreted as the computational states. After the introduction of a staggering parameter we can move along a closed adiabatic path which results in a nonAbelian Berry phase. This unitary matrix can be interpreted as a geometric quantum gate. The triangle (L=3) and hexagon (L=6) configurations allow us to investigate a onequbit quantum gate. We can define the phase shift gate and rotation gate by a geometric procedure, which are sufficient for the construction of a general onequbit gate. In the bow tie lattice, constructed by two connected triangles, we investigate a nontrivial twoqubit quantum gate. We explore its nontrivial nature using the entanglement entropy. We find that the procedure based on the nonAbelian Berry phase can produce nearmaximal entanglement entropy. While the nontrivial nature of the constructed twoqubit quantum gate is shown, the proof of equivalence to known nontrivial twoqubit quantum gates is not given in this work. 

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