Displaying theses 110 of 1066 total
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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. 

Scientific abstract (pdf 1K) Full text (pdf 3429K) 
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. 

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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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O.E. Huijgen 
Master programme: Physics  Theoretical Physics  August 24th, 2018  
Institute: ITFA  Research group: Statistical Physics and Condensed Matter Theory  Graduation thesis  Supervisor: Prof. B. Nienhuis 

Conserved Quantities in the O(n) loop model In statistical physics, we often describe the (magnetic) properties of a material by looking at the interactions of spin particles on a lattice. One can compare the spin of a particle to a tiny magnetic field. In quantum mechanics, a spin particle usually has a fixed number of states that it can be in. As it turns out, we can make an equal description to some of these spin models by drawing loops around clusters of spin particles that are in the same state. We take a look at one of such models: the dense O(n) loop model. In physics, a conserved quantity is something of which the total amount stays constant in a system. In the dense O(n) loop model, there is a set of conserved quantities growing with the size of the system. While the first few can be calculated quite easily, calculating larger ones becomes hard. Furthermore, a precise formula is not known. We calculate as many of these quantities as possible and find an exact description for these quantities from the results. Additionally, we try to find the exact probability on a cluster of particles of the same spin value appearing. 

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C.A.J. Keijzer 
Bachelor programme: Natuur en Sterrenkunde  August 23rd, 2018  
Institute: VU / Physics & Astr.  Research group: VU Atoms, Molecules & Lasers  Graduation thesis  Supervisor: Dr. Wim Vassen 
Deflecting Metastable Helium Atoms with Resonant Light Photons do not have a mass, but they do carry momentum. In this thesis, a research on the deflection of metastable helium atoms using resonant 1083 nm laser light is described. By scattering photons, the metastable helium atoms gain the momentum of the absorbed photons in the direction of illumination. This results in a deflection of the atom beam. The main goal of this research is to determine the relation between the deflection of the atom beams and the scattering force of the resonant laser light. This is done by varying parameters like the atom beam width, the direction of illumination, the light beam width, the light intensity and the interaction time. The results are quantitatively analyzed and compared to each other. The results will be discussed by means of the theory of the scattering force, showing the limits of the theory and the effects of the assumptions made to test it. This results in new ideas and suggestions for followup research. 

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B.M. Vermaas 
Bachelor programme: Natuur en Sterrenkunde  August 10th, 2018  
Institute: VU / Physics & Astr.  Research group: Hybrid Solar Energy Conversion  Graduation thesis  Supervisor: Elizabeth von Hauff 
Elucidation of surfaceligand interactions in inorganic lead halide perovskite nanocrystals with vibrational spectroscopy Nowadays, we cannot imagine our daily life without (opto)electronic devices. Whether itâ€™s about our phones, computers or the television: you probably cannot live a day without them. And everyday we want a bit more. Bigger, faster, more flexible, but more often also sustainable. Unfortunately the development of these devices is slowed down by high fabrication costs and inefficient and/or nonflexible parts. The use of nanocrystals could be the solution. The production of nanocrystals is often simple and cheap and results in devices with qualitatively high electronic properties. But there is one problem to solve before this solves our problems: these nanocrystals are not very stable and of course you would expect your new phone to keep running for a while. To be able to solve this problem, we first need to know a bit more about what kind of interactions actually keep this nanocrystal stable. This why this research will focus on the interactions that take place on the surface of lead halide perovskite nanocrystals. 

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J.N.R.M. Mathijssen 
Master programme: Physics  Advanced Matter and Energy Physics  August 9th, 2018  
Institute: VU / Other  Research group: Advanced recearch center for nanolithography  EUV  Graduation thesis  Supervisor: Stefan Witte 

Building an Optical Parametric ChirpedPulse Amplifier In this thesis, we show the experimental realization of an optical parametric chirped pulse amplification system. This laser system will be used to perform research on laser ablation of tin in the femtosecond regime. Additionally, we show such an amplification system provides a way to amplify short femtosecond pulses across a large wavelength range to high energies. This is realized by exploiting a nonlinear effect called difference frequency generation, also called optical parametric amplification. In this process, two laser beams interact in a nonlinear manner to produce a laser beam at the difference frequency. Thereby, photons from the laser with the highest frequency are split in two photons, one with the difference frequency of the two interacting beams, and one at the frequency of the lowest frequency input beam. This then results in energy transfer from a high intensity laser to two other beams, of which one is amplified and one is generated. 

Scientific abstract (pdf 2K) For more info or full text, mail to: witte@arcnl.nl 
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 

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. 

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