A dynamic vortex line traces out a world sheet in spacetime. This thesis shows that the information of all its dynamic behaviour is completely contained in the world sheet. Furthermore a... Show moreA dynamic vortex line traces out a world sheet in spacetime. This thesis shows that the information of all its dynamic behaviour is completely contained in the world sheet. Furthermore a mathematical framework for order–disorder phase transitions in terms of the proliferation of such vortex world sheets is presented, leading to the prediction of quantized vortex lines of electric current in phase-disordered superconductors. Show less
Superconducting correlations can penetrate in a ferromagnet via proximity effect but over a very short length of the order of a few nanometers because ferromagnetic exchange field tries to align... Show moreSuperconducting correlations can penetrate in a ferromagnet via proximity effect but over a very short length of the order of a few nanometers because ferromagnetic exchange field tries to align the antiparallel spins of electrons in a Cooper pair. Theoretically, it was suggested that rather a long-range proximity effect can occur if spin-triplet (with parallel spins) Cooper pairs are generated at the interface that requires a magnetic inhomogeneity (spin-active interface). Experimentally, it was first observed at the Delft University using CrO2 (a half metallic ferromagnet). In these devices the induced supercurrent was not both well controlled and well understood. We have provided new evidence that supercurrents can flow through CrO2 films, deposited on sapphire substrates, over a length of 1_m. The analysis shows that these films exhibit six-fold magnetic-anisotropy that might provide the required spin-active interface. Still, low reproducibility suggests a weak control over the spin-active interface. Such a control appears better with an artificially created spin-active interface, using an additional ferromagnetic layer of Ni (2nm) with CrO2. From investigations, it suggests that there are other scattering phenomena (different spin dependent magnetic scattering from different magnetic layers) or magnetic moment disorder at the interface involved to generate the odd-frequency spin-triplet supercurrent. Show less
This thesis explores how rheology and statistical mechanics can be used to describe driven granular materials. Chapter 1 is an overview of the current knowledge of slow granular flows. In Chapter 2... Show moreThis thesis explores how rheology and statistical mechanics can be used to describe driven granular materials. Chapter 1 is an overview of the current knowledge of slow granular flows. In Chapter 2 we characterize the liquid-like behaviour of a granular system excited by flow in a split-bottom geometry. In Chapter 3 we describe the fluctuations experienced by an object floating in our granular liquid and compare these fluctuations to the Brownian motion of particles in a thermal system. In Chapter 4 we characterize the motion of objects moving in a granular system excited by oscillatory flow. Finally, in Chapter 5, we describe both the microscopic and macroscopic motion of pucks on an air hockey table. We find the the system exhibits equipartition of rotational and translational energy and that the system can be described by van der Waals' equation of state. Show less
Light is a ubiquitous carrier of information. This information can be encoded in the intensity, direction, frequency and polarisation of the light and, which was described more recently, in its... Show moreLight is a ubiquitous carrier of information. This information can be encoded in the intensity, direction, frequency and polarisation of the light and, which was described more recently, in its orbital angular momentum. Although creating light beams with orbital angular momentum is relatively easy, measuring this property has proven to be difficult. In this thesis we present two fundamental methods to solve this problem. First, we show that by analysing the interference pattern behind a multi-pinhole interferometer, we can determine the phase and amplitude of the light impinging the pinholes, making it possible to determine the orbital angular momentum of the incoming light beam. A multi-pinhole interferometer can be scaled to arbitrary sizes, making it suitable for studying optical fields that stretch out over large distances, that can be expected in, for instance, astrophysics. The second method is based on transforming the helical wave fronts that are associated with light beams with orbital angular momentum to distinguishable, titled wave fronts that can be easily sorted by a lens. This method works for single photons, making it a key piece in a high-dimensional optical communication scheme. This thesis provides theory, simulations and measurements on both detection methods. Show less
In this thesis we investigate diverse aspects of spatial coherence of light. Non-classical fields containing two photons can be generated by a nonlinear optical process known as spontaneous... Show moreIn this thesis we investigate diverse aspects of spatial coherence of light. Non-classical fields containing two photons can be generated by a nonlinear optical process known as spontaneous parametric down conversion (SPDC). Among the questions we consider are: What is so special about spatial entanglement? How is it revealed in the fourth-order correlations? What are the differences between a highly entangled and a classically correlated state? How can the number of modes be manipulated and measured? For a two-photon system, we measure both intensities and two-photon correlations. To get deeper insights into how coherence affects interference, we also investigate completely classical sources. Show less
In eukaryotic cells, genomic DNA is organized in chromatin fibers composed of nucleosomes as structural units. A nucleosome contains 1.7 turns of DNA wrapped around a histone octamer and is... Show moreIn eukaryotic cells, genomic DNA is organized in chromatin fibers composed of nucleosomes as structural units. A nucleosome contains 1.7 turns of DNA wrapped around a histone octamer and is connected to the adjacent nucleosomes with linker DNA. The folding of chromatin fibers effectively increases the compaction of genomic DNA, but it remains accessible for enzymatic reactions. This apparent paradox motivates a detailed study of the dynamics of chromatin. A structural model at the molecular level will shed light on how cells regulate the compaction and dynamics of genomic DNA. This thesis presents the results of an experimental study on the dynamics of chromatin higher-order folding. Using magnetic tweezers, we observed force-dependent structural changes within chromatin fibers at the single nucleosome level. Show less
Optical frequency conversion is important nonlinear process for generating coherent radiation in spectral regions where there are no convenient laser sources. For example, nonlinear processes are... Show moreOptical frequency conversion is important nonlinear process for generating coherent radiation in spectral regions where there are no convenient laser sources. For example, nonlinear processes are used to generate ultraviolet radiation for biomedical applications or mid-infrared radiation in the wavelength range of 3–12 μm for remote sensing of atmosphere. Besides these practical applications, frequency conversion can be used to create entangled photon pairs in quantum optics experiments to test the fundamental laws of quantum mechanics. This thesis describes an experimental investigation of second harmonic generation in III-V semiconductor photonic structures. These nanostructures have both wavelength and subwavelength dimensions. In particular, ensembles of aligned gallium phosphide nanowires and two-dimensional aluminum gallium arsenide photonic crystal slabs with a square lattice of holes are studied. In both cases, the III-V semiconductor material provides a large second-order nonlinearity, while the special arrangement of dielectric material of the nanostructure introduces additional dispersion. This extra dispersion can be used to phase match the nonlinear process in order to make frequency conversion efficient. We demonstrate that it is possible to resonantly couple to leaky modes of a photonic crystal slab at both the fundamental and second harmonic frequency and enhance the second harmonic signal ~10,000 times. Show less
This thesis describes the construction of a second generation high-pressure, high-temperature scanning tunneling microscope, the ReactorSTM, with which the surfaces of catalysts can be studied... Show moreThis thesis describes the construction of a second generation high-pressure, high-temperature scanning tunneling microscope, the ReactorSTM, with which the surfaces of catalysts can be studied under relevant reaction conditions. Furthermore, the thesis describes three separate catalytic systems at ~ 1 bar, and elevated temperatures. Firstly, NO reduction on Pt(100), in which a mathematical model for the reaction mechanism, following Langmuir-Hinshelwood kinetics, is proposed. Secondly, CO oxidation, in which the Pt(110) surface is atomically resolved at high p, T. Thirdly, the thesis describes a successful pilot experiment about the hydrodesulphurization of thiophene, which is catalytically activated by molybdenum disulphide nano-crystallites on an Au(111) support. Show less
This thesis is devoted to the effects of disorder on two-dimensional systems of Dirac fermions. Disorder localizes the usual electron system governed by the Schroedinger equation. The influence of... Show moreThis thesis is devoted to the effects of disorder on two-dimensional systems of Dirac fermions. Disorder localizes the usual electron system governed by the Schroedinger equation. The influence of disorder on Dirac fermions is qualitevely different. We concentrate on a random mass term in the Dirac equation. We have discovered that Dirac fermions in graphene are localized by a random mass, without any transition into metallic state. The situation is entirely different for Dirac fermions in a p-wave superconductor. There electrostatic disorder appears in the Dirac equation as a random mass, which localizes the excitation, but only if the disorder is relatively weak. For large mass fluctuations a transition into metallic state appears. This qualitatively different response to disorder in graphene and in p-wave superconductors is explained by the appearance of Majorana bound states, which allow for resonant tunneling and metallic state. Electrostatic disorder in a d-wave superconductor represented as random vector potential in the Dirac equation. We look at the transmission of Dirac fermions for electrostatic potential with long- and short-range fluctuations. We study the interplay of electrical and mechanical properties of suspended graphene by calculating the correction to the conductivity due to its deformation by a gate electrode. Show less
By tuning control parameters like pressure, magnetic field or doping, a fermionic system can be driven to a state with vanishing Fermi energy and power law behavior in many observables. Such... Show moreBy tuning control parameters like pressure, magnetic field or doping, a fermionic system can be driven to a state with vanishing Fermi energy and power law behavior in many observables. Such fermionic quantum critical states have been identified in systems including heavy fermions, cuprates and pnictides. We study the superconducting transition in such systems. We propose that the superconducting instability, which is marginal in Fermi liquids, becomes relevant, leading naturally to a high transition temperature. This picture has been verified numerically in the 2-dimensional Hubbard model by using the Dynamical Cluster Approximation. We also propose tunneling experiments which can distinguish our mechanism from others. Show less
In this thesis we present a new method to simulate realistic three-dimensional networks of biopolymers under shear. These biopolymer networks are important for the structural functions of cells and... Show moreIn this thesis we present a new method to simulate realistic three-dimensional networks of biopolymers under shear. These biopolymer networks are important for the structural functions of cells and tissues. We use the method to analyze these networks under shear, and consider the elastic modulus, the non-affinity during deformation, the normal modes and the density of states. We expand our analysis to composite networks consisting of stiff and floppy filaments. In the final chapter of the thesis we incorporate the thermal and viscous interactions of the surrounding medium in the calculations, and present the results of the simulations of the shear frequency dependence of the network response. We find that non-affine reorientations are important for understanding the network response. These non-affine reorientations make the networks relatively soft under shear and delay the onset of stiffnening. In composite networks non-affine reorientations allow for an intricate interplay between the stiff and floppy filaments. The frequency-dependent response shows a transition from a soft, non-affine regime at low frequencies to a stiff, close-to-affine response at high frequencies. Show less
Quantum entanglement is a fundamental trait of quantum mechanics that causes the information about the properties of two (or more) objects to be inextricably linked. When a measurement on one of... Show moreQuantum entanglement is a fundamental trait of quantum mechanics that causes the information about the properties of two (or more) objects to be inextricably linked. When a measurement on one of the objects is performed, the state of the other object is immediately altered, even when these objects are separated at arbitrary distances. In this thesis, we explore the rich properties of entanglement in a high-dimensional mode space. Experimentally, we have implemented the high dimensionality by use of the orbital-angular-momentum degree of freedom of entangled photon pairs. The emphasis is on the question how to quantify the dimensionality of the entanglement as measured in an experiment. We introduce the Shannon dimensionality as a useful quantifier of measured entanglement. Furthermore, we discuss various production methods of optical phase plates, which we use to manipulate the orbital-angular-momentum states of light. Finally, we present an experimental feasibility study on the potential of orbital-angular-momentum entanglement for free-space quantum communication through the atmosphere Show less
