Physics and Astronomy in Progress

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Displaying theses 1-10 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 one-dimensional models displaying self-organized criticality
The concept of self-organized criticality is considered to be one of the mechanisms by which self-similar fractal structures and complexity in nature arise. In this thesis, we first introduce the concept of self-organized criticality and give a brief overview of self-organized 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 power-law 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 power-law, 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 nonlocal-limited 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.
picture that illustrates the research done
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 weak-lined 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 starspot-sensitive absorption lines of which we compare the line profile with synthetic line profiles (derived from PHOENIX models).
picture that illustrates the research done
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 mass-transfer. 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 mass-transfer. 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.
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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
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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.
picture that illustrates the research done
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 Double-Headed 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 so-called cosmological horizon. This is a horizon which seems to surround us when we look at the Universe through our telescopes.
picture that illustrates the research done
Scientific abstract (pdf 1K)   Full text (pdf 3431K)

R. Koch
Master programme: Physics - Theoretical Physics August 31st, 2018
Institute: ITFA Research group: Statistical Physics and Condensed Matter Theory Graduation thesis Supervisor: Jean-Sebastien Caux
Excitations of the Gapped XXZ Heisenberg Spin-1/2 Chain
In quantum mechanics particles only occur in certain states - there are quantized. A famous quantum mechanical model is the spin-1/2 chain, which is a one-dimensional model that describes spin-particles fixed to points on a line. It turns out that there is no classification of one particular spin-chain, the XXZ gapped Heisenberg spin-1/2, so that one cannot obtain the correct number of states. In this thesis, such a classification is presented.
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Scientific abstract (pdf 1K)   Full text (pdf 3184K)

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 liquid-liquid transition in water. The liquid-liquid transition in water happens in an experimentally inaccessible region, so other ways have to be found to research liquid-liquid transitions. Mannitol is the first pure substance with a water-like liquid-liquid transition. We have studied the hydrogen bond strength in both liquid forms of mannitol by taking IR-spectra of the OH-stretch mode. The OH-stretch 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.
picture that illustrates the research done
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
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A simplified approach for 3D tensor network simulations
Tensor network methods have been very successful to accurately simulate one- and two-dimensional quantum models, with White's density-matrix 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 three-dimensional 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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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
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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 non-Abelian 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 one-qubit 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 one-qubit gate. In the bow tie lattice, constructed by two connected triangles, we investigate a non-trivial two-qubit quantum gate. We explore its non-trivial nature using the entanglement entropy. We find that the procedure based on the non-Abelian Berry phase can produce near-maximal entanglement entropy. While the non-trivial nature of the constructed two-qubit quantum gate is shown, the proof of equivalence to known non-trivial two-qubit quantum gates is not given in this work.
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Scientific abstract (pdf 1K)   Full text (pdf 1370K)

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