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Science in Progress

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Displaying theses 11-20 of 1078 total
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V. Dadhich
Master programme: Atomic Scale Modelling of Physical, Chemical and Bio-Molecular Systems (ATOSIM), Physics / Chemistry September 5th, 2018
Institute: Other Research group: Institut Lumiere Matiere, Claude Bernard Universit Graduation thesis Supervisor: Dr. Francois Detcheverry
Simulation of active clusters
Active matter is class of systems that consists of interacting self-propelled particles (SPP) or entities. Systems of such entities can range from microscopic length scales of bacterial cell colonies to a few kilometers of flocks of birds. Active matter is currently under intense scrutiny, as it exhibits properties that are uncommon in systems at thermodynamic equilibrium. One striking instance is the cluster phase, wherein active particles spontaneously self-organize into transient groups. Such active-clusters are broadly distributed in size, constantly move and evolve through merging and splitting with other clusters. In this thesis, we will use a numerical approach to understand the cluster phase of active system and develop a minimal model of active matter.
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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: 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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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.
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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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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
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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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Scientific abstract (pdf 1K)   Full text (pdf 2198K)

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 follow-up research.
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Scientific abstract (pdf 2K)   Full text (pdf 536K)

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 surface-ligand interactions in inorganic lead halide perovskite nanocrystals with vibrational spectroscopy
Nowadays, we cannot imagine our daily life without (opto)electronic devices. Whether it’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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Scientific abstract (pdf 1K)   Full text (pdf 1749K)

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
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Building an Optical Parametric Chirped-Pulse 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.
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