In this research, we study the oxidation process of the Transition Metal Dichalcogenide (TMD) niobium diselenide, NbSe2. TMDs are a category of van der Waals materials, of which samples are...Show moreIn this research, we study the oxidation process of the Transition Metal Dichalcogenide (TMD) niobium diselenide, NbSe2. TMDs are a category of van der Waals materials, of which samples are obtained by exfoliation. The measurements are performed with the Low Energy Electron Microscope (LEEM), which measures reflectivity at different energy, resulting in the so-called IV curves. To analyze oxidation, we have developed a new method to obtain in situ exfoliated flakes in the LEEM set-up. First, we measure in situ cleaved NbSe2 flakes (bulk). The reflectivity upon adding (pure) oxygen is followed. Additionally, the reflectivity of ex situ cleaved NbSe2 flakes is assessed. Here, an intensity boundary between flake’s edge and center is recognized. The reflectivity measurements show that the electronic structure is different across the boundary: a V-shaped reflectivity minimum appears, which sharpens over time upon oxidation. The boundary is further researched with Atomic Force Microscopy and Energy Dispersive X-ray analysis. Additionally, we perform roughness analysis and Principal Component Analysis. The latter provides an alternative method to follow the change in electronic properties over time. We propose that already degraded NbSe2 flakes are more susceptible to oxidatin damage, compared to in situ exfoliated flakes. Upon further measurements, the in situ cleaved samples do not show any degradation signs, therefore we attribute the presence of an intensity contrast with the associated IV features, to oxidized NbSe2.Show less
In this research, a recently proposed renormalization group approach for networks to the case of random directed graphs is being generalized: we present a scale-invariant description of directed...Show moreIn this research, a recently proposed renormalization group approach for networks to the case of random directed graphs is being generalized: we present a scale-invariant description of directed networks containing reciprocated edges. This allows us to neglect several strong assumptions that are currently necessary to renormalize directed networks such as financial transaction networks. As an application, a model of ING’s transaction data has been derived across multiple coarse-grained partitions. In this article we provide detailed information on how this particular model has been structured and how its parameters are obtained. We show how we can use this model to determine the expected cumulative degree and weight distributions of ING’s transaction network across multiple coarse-grained partitions of the network which we will compare to the empirical degree and weight distributions, respectively.Show less
At the moment, quantum computer development is in the NISQ (Noisy Intermediate Scale Quantum) stage. This means that quantum computers are relatively small and exhibit large amounts of noise. To...Show moreAt the moment, quantum computer development is in the NISQ (Noisy Intermediate Scale Quantum) stage. This means that quantum computers are relatively small and exhibit large amounts of noise. To run any mean- ingful computation, small noise-resistant circuits are necessary. This work proposes a new algorithm, QASNEAT, for finding small noise-resistant circuits. The performance is evaluated by ground-state energy estimation of three small molecules with shot noise and physical depolarizing noise. QASNEAT is able to find small accurate circuits both in noisy and noise- less casesShow less
One of the greatest remaining puzzles in physics is what particle dark matter consists of. For this project, the theory of dark pions is considered, a Hidden Valley model that extends the Standard...Show moreOne of the greatest remaining puzzles in physics is what particle dark matter consists of. For this project, the theory of dark pions is considered, a Hidden Valley model that extends the Standard Model with new, dark particles and a new force, dark QCD. A sensitivity study is performed to determine how many dark pions are expected to be in acceptance of the LHCb detector for Run 2 conditions; the LHCb is well-suited to search for particles in the considered O(1) GeV mass and O(1) - O(100) ps lifetime range. Additionally, a framework has been developed to study the dependence of the sensitivity on a number of theoretical parameters of the dark QCD model, namely the probability to form a dark vector meson instead of a dark pion, the number of colours in dark QCD, the dark QCD scale, and the Higgs mass. It is found that O(100) dark pions are in LHCb acceptance for different track categories, and that the considered the- oretical parameters do not drastically change the number of expected particles (with some small caveats), staying within a difference of about 20%. This is acceptable given the expected experimental uncertainty, showing theory inde- pendent searches for dark pions are possible.Show less
AdS/CFT gives a framework for using calculations from a weakly curved gravitational theory to describe phenomena in strongly correlated matter. In this work we study some holographic models this...Show moreAdS/CFT gives a framework for using calculations from a weakly curved gravitational theory to describe phenomena in strongly correlated matter. In this work we study some holographic models this has put forward, such as the Reissner-Nordstro ̈m metal and the holographic superconductor. Of these models we observe transport properties like the optical conductivity under both translational invariance and broken symmetry. Lastly, we merged the two systems into the two-charge holographic superconductor and managed to again see the phase transition, measure the optical conductivity and analyse the two- fluid model.Show less
This work evaluates the performance of both Neural Networks (NNs) and Parameterized Quantum Circuits (PQCs) in Reinforcement Learning environments. We present novel quantum gate based games that...Show moreThis work evaluates the performance of both Neural Networks (NNs) and Parameterized Quantum Circuits (PQCs) in Reinforcement Learning environments. We present novel quantum gate based games that can run on current NISQ hardware. Our results show that NNs and PQCs achieve very similar performance distributions across the different environments, showing the promise of PQCs for (Quantum) Machine Learning applications. It is also shown that the NNs tested are more sensitive to change in learning rate than our PQC models. NN performance is also more eratic with relation to the amount of parameters than PQC performance, showing hyperparameter tuning might be more predictable for PQCs. Lastly, the smallest PQC designs show strong performance, often outperforming NNs with more parameters.Show less
The past decades have shown a rise in skin cancer. This creates the need for prevention and efficient treatment. The most common skin cancer (melanoma) can only be treated when detected early. In...Show moreThe past decades have shown a rise in skin cancer. This creates the need for prevention and efficient treatment. The most common skin cancer (melanoma) can only be treated when detected early. In this thesis we propose a method of increasing awareness for people with a high risk of skin cancer as well as allowing for early detection. Skin cancer is hard to detect, even for experiences healthcare professionals. One of the signals of potential harm full lesions is change over time. We propose to develop an application with which changes in skin lesions can be identified early. By allowing patients to film their body with a mobile phone camera we aim to track the development of lesions. If a patient films their body regularly changes can be detected and the application can urge the patient to consult a dermatologist. In this thesis we explore the possibility of combining the frames of these films into an overview displaying the patients complete back or arm. Combining frames is called stitching. Different stitching techniques found in literature are explored and tested for effectiveness. The optimizations performed are reported and the final result is presented. The location of the different lesions on an overview of the body is needed to show the patient and the healthcare professional where potential harmful lesions are located on the body. This allows for further inspection at the dermatology department.Show less
In this thesis the relative spectral energy density of stochastic primordial gravitational waves is investigated. Decoupling of Standard Model particles and neutrino free-streaming affect the...Show moreIn this thesis the relative spectral energy density of stochastic primordial gravitational waves is investigated. Decoupling of Standard Model particles and neutrino free-streaming affect the expansion history of the universe and thus leave characteristic signatures on the amplitude of the gravitational wave spectrum. Adding extra light or heavy particles damps the spectrum at frequencies before the particle decouples. Including an extra neutrino species amplifies the spectrum at larger wave numbers, but damps it at shorter wave numbers. Measuring these primordial gravitational waves reveals the thermal history of the universe. One possible non-standard thermal history is early matter domination due to the inflaton. It is shown that, in this cosmology, the end of early matter domination and beginning of the radiation era depend linearly on the reheating temperature.Show less
The intergalactic medium (IGM) contains most of the baryonic matter of the Universe and serves as a suitable environment for probing the thermal history of the Universe. The crucial moment in IGM...Show moreThe intergalactic medium (IGM) contains most of the baryonic matter of the Universe and serves as a suitable environment for probing the thermal history of the Universe. The crucial moment in IGM evolution is the Epoch of Reionization, corresponding to the transition from neutral to ionized IGM. However, due to the observational limitations, this period is still not well understood. In this thesis, we focus on constraining IGM thermal history by using Lyman-alpha forests data. This method is applicable in a wide range of temperatures, densities, and ionization fractions of cosmic gas at z about 2 − 5. Observations show that the longitudinal flux power spectrum of the Lyman-α forest exhibits a cut-off at small scales. This phenomenon is caused by thermal Doppler broadening, peculiar velocities along the line of sight (LOS), Hydrogen pressure smoothing, and warm dark matter. The first two effects act only along LOS, while the last two affect all spatial directions. To separate the one-dimensional and three-dimensional effects, we used the method of close quasar pairs, which is based on studying the correlations between Lyman-alpha forests of close quasar pairs. We used the Kolmogorov-Smirnov test to analyze the differences between distributions of phase difference, which characterizes correlations between Lyman-alpha forests. The calculations were performed for various thermal histories, parameters characterizing IGM, LOS separations, and wavenumbers, and accounting for different effects (Doppler broadening and peculiar velocities). Our results indicate that this method can distinguish various thermal histories regardless of the IGM thermal state and one-dimensional effects. Moreover, at separations of the order of pressure broadening, there is a prominent feature caused by different influences of pressure smoothing at large and small scales. In addition, this simple and powerful approach has the potential to distinguish scenarios with warm dark matter.Show less
In this thesis we present an experimental realisation of a double loop type Magnetic Paul Trap. We show that a microgram heavy NdFeB permanent magnet can stably be levitated for hours at room...Show moreIn this thesis we present an experimental realisation of a double loop type Magnetic Paul Trap. We show that a microgram heavy NdFeB permanent magnet can stably be levitated for hours at room temperature in this trap. Magnetic levitation of a magnetized particle is theoretically possible with this trap by generating opposed alternating magnetic fields. We show the fabrication of a printed circuit structure capable of producing these fields, as well as the engineering behind the realisation of the trap. Both by optical and magnetic readout we characterize the motion of the trapped magnet and show that its center of mass motion frequencies $\omega_z = 2\omega_{x,y} \approx 20Hz$. We characterize the damping on these modes and find that at low pressure the quality factor is strongly limited (to $Q \approx 90$) by coupling to the environment through generation of Eddy currents.Show less
In this work, a near-zero stiffness mechanical filter is designed for use in STMs in a cryogenic environment. The filter is a Geometric anti-spring (GAS) filter which consists of a set of blades...Show moreIn this work, a near-zero stiffness mechanical filter is designed for use in STMs in a cryogenic environment. The filter is a Geometric anti-spring (GAS) filter which consists of a set of blades with a payload attached. This design allows for a low resonance frequency of 0.27 Hz and thus a low cutoff frequency in terms of filtering. First, a theoretical model is described in order to determine the relevant properties of the filter and its approximate workings. Second, the model was experimentally verified. From this, the resonance and damping of the filter were found. A limited amount of vibration measurements were also done to check if the filter is functioning as expected, however, due to instrumental limitations this was not conclusive.Show less
Quantum tomography is a method of reconstructing a quantum state based on information about the probability distributions associated with the quadrature operators, linear combinations of position...Show moreQuantum tomography is a method of reconstructing a quantum state based on information about the probability distributions associated with the quadrature operators, linear combinations of position and momentum. The theory says that if one knows these distributions precisely, the quantum state can be faithfully reconstructed. This thesis delves into the mathematical formalism behind quantum tomography, how it can be applied to quantum systems one might encounter in practice. We rigorously define the quantum tomogram of a state and give an example of an experimental set-up which allows one to measure the quadratures of single mode light.Show less
Galileon models form a class of models where an additional scalar field is added to the Lagrangian describing the general theory of relativity. The addition of the scalar field causes a wide array...Show moreGalileon models form a class of models where an additional scalar field is added to the Lagrangian describing the general theory of relativity. The addition of the scalar field causes a wide array of phenomena within our universe to change. Among those phenomena are both the expansion of the universe and the formation of large scale structures. We will study how they are both changed within a subclass of the Galileon models, called the Galileon ghost condensate models. First, we explore the parameter space of the model to find the values that give rise to non-singular evolu- tions of the expansion of the universe. Then, we examine how the large scale structures would form within those universes. To do that we use the spherical collapse model, in which the evolution of a spherical overden- sity is tracked. The spherical overdensity models how a relatively small perturbation leads to the formation of dark matter halos. We will show that the Galileon ghost condensate models still allow for a large degree of freedom within the spherical collapse, which would allow further research to constrain its parameter space.Show less
This research aims to improve the existing capabilities of Scanning Tunneling Microscopy (STM) in a dry-dilution refrigerator, at the Oosterkamp group. The group currently has an STM setup that can...Show moreThis research aims to improve the existing capabilities of Scanning Tunneling Microscopy (STM) in a dry-dilution refrigerator, at the Oosterkamp group. The group currently has an STM setup that can resolve step edges in highly oriented pyrolytic graphite (HOPG) and is poised to achieve atomic resolution [1]. Currently the STM is mounted in the dry-dilution refrigerator, which allows for the study of samples at milli-Kelvin temperatures. The use of a dry-dilution refrigerator gives rise to some complications. Specifically we analyse the contributions of the compressor from the pulse tube cooling system. It was found that this compressor is the dominant source of mechanical noise in setup. We determined that the vibrations enter through both the compressor lines and via the ground through the outer frame. The compressors vibrations gave rise to 0.88 ̊ARMS noise. By placing the compressor on compressed gas springs and running the compressor lines through a trough filled with sandbags, we were able to reduce this noise to 0.13 ̊ARMS. This should no longer stand in the way of reaching atomic resolution STM.Show less
On the hunt for spin triplet superconductivity in ferromagnet-superconductor multilayers, additions were made to an RF insert with an open co-planar waveguide. The low loss setup, with a -1dB per...Show moreOn the hunt for spin triplet superconductivity in ferromagnet-superconductor multilayers, additions were made to an RF insert with an open co-planar waveguide. The low loss setup, with a -1dB per GHz characteristic, was used to obtain FMR spectra with a vector network analyzer. A superconducting coil was designed to generate magnetic fields in situ at cryogenic temperatures. Many cobalt-niobium thin film multilayers were created by means of e-beam evaporation. The FMR spectra were recorded at room temperature and cryogenic temperatures as low as 4.5K. The spectra were analyzed to determine the linewidth of the FMR resonance above and below the critical temperature of different thicknesses of the superconducting layer. The initial results are promising, but not sufficient to conclude decreased gilbert damping in the ferromagnet below the Tc of the superconductor.Show less
This thesis investigates the mathematical and physical foundations of topological defects. We first introduce the mathematical background, which consists of the theory of Lie groups and their...Show moreThis thesis investigates the mathematical and physical foundations of topological defects. We first introduce the mathematical background, which consists of the theory of Lie groups and their corresponding Lie algebras, and fibre bundles, principal bundles and connections on principal bundles. We also give an introduction to classical field theory, and present the Lagrangian formalism for fields and Yang-Mills theory. We cover spontaneous symmetry breaking, and we explain how this can lead to topological defects using the Kibble mechanism. Finally, we classify topological defects using homotopy groups, for which we develop the underlying framework.Show less
