The main goal of this project is to study the conductivity of pentacene thin films in the herringbone standing-up (HSU) phase as a function of layer count and to investigate the role of film...Show moreThe main goal of this project is to study the conductivity of pentacene thin films in the herringbone standing-up (HSU) phase as a function of layer count and to investigate the role of film defects in the charge transport through these pentacene layers. To grow these films, atomically flat and spatially uniform substrates are required, for which hexagonal boron-nitride (hBN) is used. Several techniques to clean these substrates are tested and multiple growth experiments are performed on our samples. We also develop a very general approach, which we have named the Laplace conductivity algorithm, to extract local conductivities on a sample from experimental potential maps. This algorithm is tested using simulations, with very promising results being achieved.Show less
Active particles that transform energy into directed or persistent motion show collective behavior and self-organization. One type of self-organization is in the form of crystal structures. However...Show moreActive particles that transform energy into directed or persistent motion show collective behavior and self-organization. One type of self-organization is in the form of crystal structures. However, an active crystal with tunable properties has not been achieved. Inspired by an experiment in which vibrated polar discs form ordered domains, and an experiment in which metallic spheres, that move by contact charge electrophoresis, form a passive crystal, we create active particles in the shape of a cylinder composed of a metallic and a dielectric part, that move thanks to Quincke rotation on the dielectric side and an electrostatic force on the metallic side due to an external electric field. We study the behavior of these particles by characterizing the persistence of their motion, their ability to repel each other, and by studying their instantaneous speed as a function of the electric field and their size. Our results suggest that the particles have the potential to form an active crystal and seem to be compatible with the passive crystal existent at our lab.Show less
The repositioning of nucleosomes in DNA plays a vital role in accessing the encoded information. This repositioning can be induced through remodelers sitting on the DNA wrapped around the histone...Show moreThe repositioning of nucleosomes in DNA plays a vital role in accessing the encoded information. This repositioning can be induced through remodelers sitting on the DNA wrapped around the histone cores. These introduce twist defects which can propagate through the nucleosomal DNA and escaping on one end which leads to single base pair repositioning of the nucleosome. However the precise dynamics at play here and how it interacts with the larger DNA sequence are not fully clear. Here we will demonstrate some of the properties of the twist defects on the wrapped DNA and how this affects the nucleosome repositioning along a sequence. This is done through setting up a Monte-Carlo Markov chain for the rigid base pair model. We find that a build-up of twists occurs on hard to traverse DNA regions and that this causes multiple steps in quick succession. We also see that the positional preference of the nucleosome with remodeler is less sensitive to the global landscape as opposed to free nucleosome repositioning. This quick succession of steps may explain an earlier work by Sabantsev et al. where this effect was seen in an experiment without an explanation. The current work can be used as guidance for future studies investigating the dynamics of simultaneous defects around the nucleosome.Show less
Since the 80s, strange metals, metals where the electrons are so densely entangled that the conventional condensed matter paradigm of Short Ranged Entanglement fails, have eluded any form of study...Show moreSince the 80s, strange metals, metals where the electrons are so densely entangled that the conventional condensed matter paradigm of Short Ranged Entanglement fails, have eluded any form of study due to the sign problem, which renders numerical calculations impossible. However, holography, a duality between strongly coupled quantum field theory problems and classical general relativity problems of one spatial dimension higher, grants us a way to circumvent the sign problem. In this thesis, we will run a modified version of code that was once used to simulate binary black holes on a supercomputer to calculate the properties of two $2+1$-dimensional holographic models for strange metals, the Reissner-Nordstr\"om metal and the Gubser-Rocha metal, subject to an ionic lattice potential: the code needed to simulate the Gubser-Rocha metal was only finished last year. We then investigate whether the DC electrical conductivity $\sigma$, thermopower $\alpha$ and thermal conductivity $\bar{\kappa}$ obey four different Drude models: one basic relativistic model and three models with different extra incoherent terms, models A, B and C. We find that model A, the most conventional model, fails, while the conductivities obey model C ($\kappa$-dominated transport) for low lattice strength $A$ and model B ($\sigma_{Q=0}$-dominated transport) for high $A$. We suspect this surprising result is caused by a pole collision causing a crossover between two regimes, but more research needs to be done to verify this.Show less
In a time of thriving Gravitational Wave physics, we study Black Hole Quasi-Normal Modes emitted in the post-ringdown phase of merger events. By using Boundary Effective Field Theoretical methods,...Show moreIn a time of thriving Gravitational Wave physics, we study Black Hole Quasi-Normal Modes emitted in the post-ringdown phase of merger events. By using Boundary Effective Field Theoretical methods, we search for modifications to General Relativity in the strong-field limit, for scalar as well as gravitational field perturbations. Going beyond General Relativity, Black Holes are predicted to produce echoing signals, for which we characterise observational parameters, by searching for leading order Boundary Conditions near the Black Hole horizon. For a scalar field, we discuss a parity symmetric and a shift symmetric configuration, while for a gravitational field, parity and diffeomorphism symmetry are implemented. The diffeomorphism symmetric Boundary Condition oddly seems to mix modes. Given the intimate relationship between Effective Field Theory and renormalisation techniques, we also comment on recent first principle arguments that have been brought up regarding the supposed impossibility of echo observation. We find that these arguments over-interpret a regularisation cut-off. From our perspective, the arguments still have merit, but do not form the no-go theorem that it seems to. In the end, only observation can give the decisive answer on the existence of Black Hole echoes.Show less
Performing real-time non-invasive nanoscale microscopy on surfaces is essential to studies of plasmonics, electrochemistry and protein dynamics. Optical near-field electron microscopy (ONEM) is a...Show morePerforming real-time non-invasive nanoscale microscopy on surfaces is essential to studies of plasmonics, electrochemistry and protein dynamics. Optical near-field electron microscopy (ONEM) is a proposed imaging technique that achieves this by converting an optical near-field image to an electron image through a low-workfunction thin-film photocathode and imaging these electrons with the detection optics of a low-energy electron microscope (LEEM). In this thesis, we perform theory, sample fabrication and microscopy experiments as a stepping stone towards ONEM. We focus on photocathode properties and resolution considerations.Show less
Real-world networks often possess scale-free characteristics, such as degree or node strengths distributions. These features are not necessarily related to the physical concept of renormalization....Show moreReal-world networks often possess scale-free characteristics, such as degree or node strengths distributions. These features are not necessarily related to the physical concept of renormalization. On the other hand, the fact that empirical information about networks is usually provided at different hierarchical levels (e.g. flows of money are known sometimes for individuals, sometimes for firms or entire sectors) calls for the introduction of renormalizable, scale-invariant network models. In this work, we develop such a renormalizable model for random weighted graphs. The discovered probability distribution (called \textit{Bombass}) with no parameters besides the connectivity and scale constants allows us to tweak both the network topology and the weights. We study the World Trade Web network of 2000 and find a very good agreement between empirical and simulated edges for the same fitnesses. We also derive the general form of a continuous renormalizable probability distribution and analyze whether the already known ones satisfy the criteria. We discover the representation of continuous renormalizable distributions in terms of Erlang compounding with a specific discrete distribution that has to satisfy the specific necessary and sufficient conditions that we also provide. Finally, we provide the recurrent formulae for constructing such discrete distributions.Show less
Neural networks have been an active field of research for years, but relatively little is understood of how they work. Specific types of Neural Networks have a layer structure with decreasing width...Show moreNeural networks have been an active field of research for years, but relatively little is understood of how they work. Specific types of Neural Networks have a layer structure with decreasing width which acts like coarse-graining, reminiscent of the renormalization group (RG). We examine the Restricted Boltzmann Machine (RBM) and discuss it’s possible relation to RG. The RBM is trained on the 1D and 2D Ising model, as well as the MNIST dataset. In particular for the 2D Ising model showing a flow towards the critical point Tc ≈ 2.27, opposite to the RG-flow. Examining the behaviour of the RBM on the MNIST dataset shows that sparse datasets can allow multiple fixed points which can be removed by artificially creating new samples. We conclude that this RBM-flow exists due to the multiple relevant length scales at the critical point and we briefly discuss why.Show less
Current imaging is crucial to condensed matter physics, materials research and industry. State-of-the-art current imaging setups revolve around SQUID-on-tip (SOT) probes, that scan over a sample to...Show moreCurrent imaging is crucial to condensed matter physics, materials research and industry. State-of-the-art current imaging setups revolve around SQUID-on-tip (SOT) probes, that scan over a sample to locally measure magnetic fields and temperature. The resolution of such systems is presently limited by the lack of a robust method to control the probe-sample distance. In this thesis, we develop probes for hybrid microscopy that combine SOT with STM. We theoretically investigate interesting systems, and find that our approach would considerably improve on past magnetic investigations of vortex matter. We use focused-ion-beam milling to fabricate SOT probes on top of a commercial AFM-cantilever, and show these to be very sensitive to changes in applied magnetic field and temperature. We develop a novel readout scheme to simultaneously measure a magnetic and a tunneling signal. We present a proof-of-concept STMSOT probe that displays magnetic sensitivity inside a cryogenic STM setup, and use it as an STM probe to see the topography of a NbSe$_2$ crystal. Our approach will culminate in the development of a STMSOT setup in the near future.Show less
We have investigated the buckling and snapping unstabilities of beams with a slit, both experimentally and numerically, for different geometries. We find that beams with a slit display non-linear...Show moreWe have investigated the buckling and snapping unstabilities of beams with a slit, both experimentally and numerically, for different geometries. We find that beams with a slit display non-linear symmetry breaking. Specifically they display asymmetric buckled states after symmetrically buckling, a property that can be used in metamaterial design to propagate and amplify symmetry breaking perturbations. We find hysteresis between the "closed" and "open" post-buckling states of the beam and that the transitions between these are snappy. This hysteresis implies that, under compression, these beams are tristable. Exploring two of the states connected by a hysterectic transition, we can regard them as hysterons under a compression field. We find that we can tune the degree of asymmetry as well as the regime where there is hysteresis by modifying the geometrical parameters of the beam. Both the non-linear symmetry breaking and its characteristic as hysterons make beams with a slit useful tools to achieve functionality in metamaterial design.Show less
In the past decades experiments have found condensed matter systems which could not be described by the conventional methods of condensed matter theory, these are densely entangled strange metals....Show moreIn the past decades experiments have found condensed matter systems which could not be described by the conventional methods of condensed matter theory, these are densely entangled strange metals. During the same period, the string theory community has developed the AdS/CFT correspondence, a duality between field theories and gravitational systems. This duality may be used to understand condensed matter field theory from a gravitational perspective. It is especially useful for densely entangled quantum matter, which can be described according to the duality by charged black hole systems of classical gravity. In this thesis we will consider the Gubser-Rocha black hole of the Einstein-Maxwell-Dilaton action to describe a metal. To understand charge and heat transport in these metals, one needs a mechanism to dissipate momentum. This is explicitly implemented by introducing a periodic lattice in the condensed matter system. Using heavy numerical codes to calculate the gravitational differential equations that are dual to the metal, we can find the transport properties of our metal. In this metal a linear in T resistivity is found, which is a famous property of the strange metals. Furthermore we find empirically a saturation of the conductivity, which could be the instance of Planckian dissipation and the minimal viscosity of the strange metal.Show less
This works investigates the possibility of building a full-field X-ray fluorescence microscope using Silicon Pore Optics (SPO) technology in the Advanced Kirkpatrick-Baez (AKB) configuration. AKB...Show moreThis works investigates the possibility of building a full-field X-ray fluorescence microscope using Silicon Pore Optics (SPO) technology in the Advanced Kirkpatrick-Baez (AKB) configuration. AKB system focuses the X-ray beam by reflection from four curved mirrors. Such an imaging system is an aberration-free and relatively easy to manufacture optics. We start with a simple two-mirror Kirkpatrick-Baez (KB) system as a first step and a basis for understanding and modelling of Advanced KB. Using the derived equations, a KB system is parameterized and characterized. A sensitivity study is conducted to find the optimal system parameters under constraints enforced by microscopy application and SPO technology. Performance of an optical system is assessed on the basis of solid angle, tightness of focal spot and field of view. The conclusions made for the KB system are relevant for the AKB system as well. SPO technology was initially developed to enable light-weight large area telescopes with resolution of a few arc seconds. It uses the concept of stacking of slightly curved reflective mirrors of high quality. We were interested to explore whether SPO can improve X-ray microscopy. However, it was found that using SPO technology for the case of full-field X-ray microscopy presents no benefits in terms of increase of solid angle. Nonetheless, the technology can be used to provide self-standing stacks with only upper reflecting plate for AKB-based microscopy setup.Show less
Pentacene, a semiconducting organic molecule that is able to grow in crystalline structures, is a versatile material for many semiconductor applications. In this thesis, the transmissivity and...Show morePentacene, a semiconducting organic molecule that is able to grow in crystalline structures, is a versatile material for many semiconductor applications. In this thesis, the transmissivity and reflectivity of low energy electrons (0-10 eV) through standing-up phase pentacene in the transversal direction is studied. From these values, the total, inelastic and elastic mean free path (MFP) can be obtained from which the latter contains information about the unoccupied bands present in the pentacene. Using a graphene-hBN substrate on a TEM-grid, pentacene is epitaxially grown into 2 or 3 layer thick dendrites. Low energy electron microscopy (LEEM) and low energy transmissive electron microscopy (eV-TEM) are then used to find the transmissivity and reflectivity. First the substrate is studied, then, using a mathematical model to disentangle the MFP of pentacene from the substrate it is found that standing-up phase pentacene contains an unoccupied band around 3.2 eV. Furthermore, it is found that the disordered layer of non-crystalline pentacene molecules have a large influence on the measured reflectivity and transmissivity of the substrate.Show less
