The theoretical description of fermionic system with strong interaction is a very challenging open problem in physics. The most notable (but far from the only) experimental realization of this... Show moreThe theoretical description of fermionic system with strong interaction is a very challenging open problem in physics. The most notable (but far from the only) experimental realization of this type of systems are the cuprate superconductors which have zero electric resistivity. Even if onehas a good microscopic model for the description of these materials it is very hard to translate it to macroscopic observables which in principle can be experimentally checked. The problem is that in case of a relevant interaction one can not Taylor expand in the coupling constant in the low-energy regime in which we are most interested. On the other hand, because of the fermion sign problem Monte Carlo numerical techniques (which are succesful with bosonic models) do not work for fermions at finite density. This thesis is devoted to the applications of several methods to the research area described above. The common theme of these techniques is that they are (partly) motivated from high-energy physics: the research area which deals with particle physics, string theory etc. Show less
The nature of the Dark Matter is one of the biggest open questions in modern cosmology and particle physics. The work in this thesis concerns a search for the observational effects of one... Show moreThe nature of the Dark Matter is one of the biggest open questions in modern cosmology and particle physics. The work in this thesis concerns a search for the observational effects of one particular class of hypothetical Dark Matter particles, namely those that are allowed to decay. In decaying, X-ray photons are emitted and should be observable. One part of the thesis details the discovery of a potential Dark Matter decay signal in X-ray spectra of galaxies and galaxy clusters, and the subsequent efforts to identify its origin. To this end archival data and new observations are compared to the respective Dark Matter masses of the observed objects. Interpretations of the signal as an instrumental effect, or due to regular astrophysical processes are unsatisfactory. Although the Dark Matter interpretation remains plausible, definitive conclusions about the origin of the signal can not be drawn yet and will require measurements by next generation observatories. The last chapter of the thesis contains the proof-of-concept of a novel technique to search for such weak signals that combines increased statistical power with the ability to determine the physical origin of a signal, while avoiding some of the disadvantages of traditional methods. Show less
Parkinson’s disease is a neurodegenerative disease characterized by the presence of abnormal deposits of aggregated proteins in the brain tissue, known as Lewy bodies. The major components of Lewy... Show moreParkinson’s disease is a neurodegenerative disease characterized by the presence of abnormal deposits of aggregated proteins in the brain tissue, known as Lewy bodies. The major components of Lewy bodies are aggregated forms of a small presynaptic protein known as α-synuclein (α-syn). In this thesis we report on the intricacies of α-syn aggregation. Using an array of biophysical techniques we were able to observe the formation of the earliest α-syn oligomeric species – relatively stable dimers and tetramers – which are more easily formed than commonly assumed. Fluorescent labelling was shown to significantly affect the morphology of α-syn aggregates, which limits the applicability of this technique. From the growth kinetics of α-syn fibrillar seeds we conclude that the elongation of fibrils proceeds by a different mechanism than primary nucleation. Further, we studied the effect of solution conditions and surface effects on the growth of the α-syn aggregates. Using total internal reflection microscopy and confocal fluorescence imaging we observed the elongation of individual fibrils in real time, showing that this process proceeds by leaps and bounds. Show less
By combining low-energy electron microscopy (LEEM) with pulsed laser deposition (PLD), we have created a unique set-up to study the first stages of growth of complex metal oxides. We... Show more By combining low-energy electron microscopy (LEEM) with pulsed laser deposition (PLD), we have created a unique set-up to study the first stages of growth of complex metal oxides. We demonstrate this by investigating the growth of SrTiO3 (STO) and LaAlO3 (LAO) on STO in real-time. We follow growth by monitoring the intensity and the full-width-half-maximum (FWHM) of the specular diffracted beam at various energies. For layer-by-layer growth, we find the anticipated intensity peaks at the completion of each layer, and an oscillatory FWHM with the maximum at half-layer coverage. In the LAO on STO case, for optimal growth conditions and a LAO thickness above the critical thickness of 4 unit cells the interface between the band insulators shows conductivity. We obtain an electronic fingerprint of the growing material, by measuring the intensity of the specular beam as a function of energy at regular intervals during growth. Extending this fingerprint with the intensity dependence on the momentum parallel to the surface allows us to extract the band dispersion of unoccupied electron states of the sample surface. Significant differences in the unoccupied band structure develop between samples which are conducting and non-conducting. Show less
This work covers two closely related topics: a theoretical study on the origins of friction and an experimental study on the growth of graphene. Both fundamental studies are focusing on the atomic... Show moreThis work covers two closely related topics: a theoretical study on the origins of friction and an experimental study on the growth of graphene. Both fundamental studies are focusing on the atomic processes involved. The study on friction treats the dissipation that takes places at one single friction contact. We show that the current explanations result in a discrepancy that we solve by evalutation of the mass involved: this mass is orders of magnitude smallar than assumed. The very small and dynamic mass at a friction contact forms an efficient channel of dissipation. This explanation allows us to understand and predict the friction behavior of surfaces at both the small and large scale. The study of graphene growth investigates the growth process of graphene at the atomic scale with a Scannning Tunneling Microscope in situ. We use our high- and, variable-temperature STM to determine the lowest nucleation temperature of graphene on Ir(111). Additionaly, individual steps that follow up each other during growth are clarified and presented. The graphene film closure is studied as well, which showed that graphene introduces internal strain in order to prevent local lattice defects. Our results are important for the improvement of the quality of graphene. Show less
We present a novel strategy to overcome this limitation and create programmable me chanical metamaterials, where the response of a single structure is determined and can be changed by the... Show more We present a novel strategy to overcome this limitation and create programmable me chanical metamaterials, where the response of a single structure is determined and can be changed by the amount of lateral confinement. Show less
A foam is not a homogeneous material, but consists of a large number of small air bubbles. The whole is more than the sum of its parts: even though the behavior of a single bubble is easy... Show more A foam is not a homogeneous material, but consists of a large number of small air bubbles. The whole is more than the sum of its parts: even though the behavior of a single bubble is easy to understand, their collective behavior is much more complicated and completely different. The reason for this is that a deformation of the whole is only reflected partially in the deformation of individual bubbles. Instead, a large part of the deformation is reflected in a change in the structure of the foam. In this thesis, we investigate this change in structure. In the first part of this thesis, we use a simple microscopic computer model to simulate the response of a foam when it is deformed by a tiny amount, and measure how far we need to deform the foam until it is irreversibly deformed. In the second part of this thesis, we focus on an experiment, where we measure the response of a two-dimensional foam when it is deformed at the edges. We observe that the response of bubbles in the center of the foam qualitatively depends on how densely the bubbles are packed together. Show less
The Lead Zeppelin is a small piece of Lead that is made to float in a magnetic field. At low temperatures - 4.2 K in this thesis, the boiling point of Helium - Lead is a superconductor, and... Show more The Lead Zeppelin is a small piece of Lead that is made to float in a magnetic field. At low temperatures - 4.2 K in this thesis, the boiling point of Helium - Lead is a superconductor, and acts as a ‘mass on a spring’, where the spring is formed by the magnetic field. As such, it is incredibly sensitive to externally applied forces. Other techniques are already capable of measuring forces in the range of zeptoNewtons; the Lead Zeppelin has the potential to be even more sensitive. The development of such increasingly sensitive force sensors paves the way to all sorts of interesting experiments, such as probing the quantum mechanics of heavy objects. At this moment, the Lead Zeppelin is already so sensitive that the motion of the laboratory, which is only a few nanometers at 10 Hz, is picked up so easily that it dominates the Lead Zeppelin. Show less
Combining ferromagnetism and superconductivity can lead to the development of a completely new generation of technology, with unique and powerful characteristics, called superconducting spintronics... Show moreCombining ferromagnetism and superconductivity can lead to the development of a completely new generation of technology, with unique and powerful characteristics, called superconducting spintronics. The task of developing this, however, is challenging because at the microscopic level the superconducting and ferromagnetic states are intrinsically incompatible. Under certain conditions, however, the conventional (singlet) superconducting state can be converted into the triplet one, with the spins of the electrons forming the Cooper pairs aligned parallel. The triplet state can coexist with ferromagnetism and is very interesting both for applications and from a fundamental point of view. In this thesis we study the electrical properties of small hybrid devices that mainly consist of superconducting and ferromagnetic layers. By measuring the electrical resistance of these devices as a function of parameters such as the temperature or the applied magnetic field, it is possible to indirectly infer important information about the state of the (super)conductivity in the different layers. We investigate different types of devices (multilayers, triplet spin valves and Josephson junctions), in order to address different aspects related to the generation of the triplet state, for a better and better control of the process. Show less
Dissertation. We study the topological properties of strongly externally driven quantum non-interacting quantum systems, focussing on the example of the quantum walk and closely related systems.
As crystals are classified by space groups, nematic liquid crystals should be in principle classified by point groups. Conventionally, the study of nematic liquid crystals has mainly been focused... Show moreAs crystals are classified by space groups, nematic liquid crystals should be in principle classified by point groups. Conventionally, the study of nematic liquid crystals has mainly been focused on a very small subset of the whole nematic family, partly because of limitations of traditional methods. In this thesis, we introduce a non-Abelian gauge theory that can treat nematic phases with arbitrary point group symmetries in a unified framework in an efficient way. The proposed gauge theory allows us to compare nematic phases against a common reference. We are therefore able to quantify the orientational fluctuations of nematic orders with different symmetries and identify a novel chiral liquid phase. Moreover, this gauge theory can act as an order-parameter generator, and we thus achieve a full classification of nematic-order-parameter tensors, which has never been done before. Finally, we show that the gauge theory provides a convenient way to access the anisotropy of axial orders, by which we generalize the extensively studied biaxial-uniaxial transition of D2h nematics to a much broader class. Show less
Since all astrophysical objects spin, it is important to study the dynamics of spinning objects in curved space-time. The dynamics of spinning particles are described with a covariant Hamiltonian... Show moreSince all astrophysical objects spin, it is important to study the dynamics of spinning objects in curved space-time. The dynamics of spinning particles are described with a covariant Hamiltonian formalism. In this formalism, the closed set of equations of motion are obtained from Poisson-Dirac brackets. Since the brackets are model-independent, a large class of Hamiltonians are used to describe a variety of models. We apply our formalism to study the dynamics of spinning particles in Schwarzschild background, and establish a number of new results. First, we obtain the equations with minimal Hamiltonian for the circular orbit of the particle in the equatorial plane. By generalizing the method of geodesic deviation in General Relativity to world lines of particles carrying spin, we derive the complete first-order solution for the non- circular bound orbits. This solution leads to the surprising insight that the periastron is not only subject to an angular shift, but show radial variations as well. Then analyzing the stability criterion for circular motion we find the Innermost Stable Circular Orbits as a function of spin. Finally we show that the spin-orbit coupling leads to geodetic precession of the bound orbits. Show less
We developed a new FRET-based technique, “Fluredox”, which allows fluorescence readout of the redox state of oxido-reductases at single molecule level. Commercially available red-absorbing... Show moreWe developed a new FRET-based technique, “Fluredox”, which allows fluorescence readout of the redox state of oxido-reductases at single molecule level. Commercially available red-absorbing fluorophore ATTO655 was selected for labeling Azurin, a small blue mononuclear copper protein. Single molecule fluorescence correlation spectroscopy (FCS) of the fluorescently labeled Copper azurin in solution reveals how the position of the label in the 3-D structure of the protein affects the redox kinetics of the redox center as well as the label. Under certain redox conditions, we have been able to observe a microsecond dynamics for intramolecular ET reaction between the label and the metal center in azurin. Our results show that this FRET technique can be profitably used to study the enzyme activity of dye-labeled oxidoreductases. Show less
Catalysis is one of the most important technical and scientific developments, on which present-day society is based. For example, it is crucial to the production of fertilizers or clean... Show more Catalysis is one of the most important technical and scientific developments, on which present-day society is based. For example, it is crucial to the production of fertilizers or clean fuels and needed for the abatement of exhaust gases. Frequently, the employed catalysts are being discovered in a very empirical way; by trial and error. However, designing catalysts based on detailed understanding is preferred. Obtaining understanding is very difficult, because catalysts are very complex materials. Furthermore, its properties often depend on the atmosphere surrounding the catalysts, i.e., the temperature and pressure of reactants and products, which they are exposed to, and these properties also change over time. The major part of this thesis focuses on structural changes of Pt model catalysts exposed to high oxygen pressures at elevated temperatures. The changes were measured with a ReactorSTM, a special version of a scanning tunneling microscope (STM) adapted to operate at high pressure and temperatures. These observations show that various surface oxide with a single-layer thickness form under reaction conditions. These oxides are structurally and chemically different from the Pt bulk oxides. The second part describes a set of experiments to understand the role of low-coordinated atoms and water in Au-catalyzed CO oxidation. Show less
This thesis explores the basic mechanism of the CO oxidation reaction on a palladium catalyst. Two new setups are presented to investigate catalytic reactions in general. Using a variety of... Show moreThis thesis explores the basic mechanism of the CO oxidation reaction on a palladium catalyst. Two new setups are presented to investigate catalytic reactions in general. Using a variety of techniques, reaction oscillations are analysed and modeled to examine the basic structure of the palladium catalyst in operating conditions. Show less
Gas bubbles in liquids are important in biomedical applications such as ultrasound imaging, drug release, or photothermal therapy. We create vapor nanobubbles in liquids around laser-heated gold... Show moreGas bubbles in liquids are important in biomedical applications such as ultrasound imaging, drug release, or photothermal therapy. We create vapor nanobubbles in liquids around laser-heated gold nanoparticles and investigate their time-dependent properties using optical techniques. Vapor nanobubbles can show explosive behavior on the (sub-)nanosecond (10^-9s) time scale, even under continuous heating. They are highly sensitivity to acoustic pressures. From our study, we conclude that vapor nanobubbles are very interesting systems for fast all-optical light modulation and for nanoscale acoustic-wave sensing. Conjugated polymers are important organic materials in organic light emitting diodes, thin-film transistors, solar cells, or as chemical sensors. We use photothermal microscopy to study the optical absorption of single conjugated polymer molecules (MEH-PPV). Immersing the molecules in supercritical xenon as the photothermal transducing medium, we detect single conjugated polymer molecules via their absorption, and correlate this signal with photoluminescence. We measure the number of monomers in individual polymer chains and the apparent quantum yield of single conjugated molecules. The information provided by our technique will generate a better understanding of trapping and non-radiative deactivation channels of optically-induced excitations in conjugated polymers and thus facilitate the design and optimization of devices based on this important class of materials. Show less
Nuclear magnetic resonance force microscopy (MRFM) is a technique which combines magnetic resonance imaging (MRI) with scanning probe microscopy (SPM). The final goal is to develop this technique... Show moreNuclear magnetic resonance force microscopy (MRFM) is a technique which combines magnetic resonance imaging (MRI) with scanning probe microscopy (SPM). The final goal is to develop this technique to such a level that the atomic structure of a virus or protein can be revealed by this microscope. This thesis shows nuclear magnetic resonance force measurements on copper in which the interaction of the magnetic moments of the nuclei of copper with a magnetic cantilever has delivered a detectable signal at a temperature of 50 millikelvin. Furthermore, we show measurements, which support a new theory where at low magnetic field and low temperature, non contact friction between the magnetic cantilever and paramagnetic electron spins is described. These measurements were enabled by technical improvements such as vibration reduction in a cryogen free dilution refrigerator. As a benchmark for the low vibration, we show atomic resolution scanning tunneling microscopy at 15 millikelvin temperature on graphite. We also show a method to create small magnets for MRFM from a thin magnet film. With these small magnets the field gradient and therefore the sensitivity may be significantly enhanced. Show less
In this thesis we study some aspects of the very early Universe. We focus on the period of inflation, and show how the presence of many fields during this period can affect its low energy... Show moreIn this thesis we study some aspects of the very early Universe. We focus on the period of inflation, and show how the presence of many fields during this period can affect its low energy description. We focus on the predictions for the two and three point correlation functions of the curvature perturbations, in the case in which apart from the inflaton field there is a very heavy isocurvature field participating in the dynamics. We also treat briefly the case in which the isocurvature field is light, in particular applied to the case of so-called natural inflation. Show less
