This dissertation contains scientific research within the realm of quantum optics, which is a branch of physics. An experimental and theoretical study is made of two-photon interference phenomena... Show moreThis dissertation contains scientific research within the realm of quantum optics, which is a branch of physics. An experimental and theoretical study is made of two-photon interference phenomena in various optical systems. Spatially entangled photon pairs are produced via the nonlinear optical process of spontaneous parametric down-conversion. These entangled photons are then passed through different optical systems to study various aspects of two-photon interference. Firstly, an experimental analysis is made of the high-dimensional entanglement that is present in the orbital angular momentum of the photons. Secondly, we present a comprehensive description of two-photon quantum interference behind a double slit. We demonstrate how to control the quantum correlations and present the first observations of complete two-photon diffraction patterns behind a double slit. Finally, we present pioneering experiments on spatially entangled two-photon states that are scattered of random media. We have observed and theoretically analyzed the structure in the random two-photon interference patterns called two-photon speckle patterns. Spatial entanglement gives two-photon speckle a much richer structure than ordinary one-photon speckle. Our experiments also demonstrate a two-photon interference phenomenon that survives averaging over different realizations of disorder. The latter results are closely related to bosonic, fermionic, and anyonic particle exchange symmetries. Show less
This thesis addresses a variety of systems in which the diffusion is anomalous, mainly motivated by recent experimental developments. The main topics discussed are: * We look at the consequences... Show moreThis thesis addresses a variety of systems in which the diffusion is anomalous, mainly motivated by recent experimental developments. The main topics discussed are: * We look at the consequences that subdiffusion on fractals has for shot noise. * The effects correlations have on superdiffusion in one dimension are examined. * We develop and demonstrate the usefulness of a method to simulate the anomalous diffusion of Dirac fermions in a computer. * A spin precession experiment in topological insulators is proposed and analyzed. * We present the mechanism for the conversion of an ordinary insulator into a topological insulator by disorder which was reported in the literature, on the basis of computer simulations, but had remained unexplained. Show less
Resonant Inelastic X-ray Scattering (RIXS) is an X-ray in, X-ray out technique that enables one to study the dispersion of excitations in solids. In this thesis, we investigated how various... Show moreResonant Inelastic X-ray Scattering (RIXS) is an X-ray in, X-ray out technique that enables one to study the dispersion of excitations in solids. In this thesis, we investigated how various elementary excitations of transition metal oxides show up in RIXS spectra. Show less
As one of the most important developments in string theory, the AdS/CFT correspondence(or in general gauge/gravity correspondence) encodes a way of using string theory to perform non-perturbative... Show moreAs one of the most important developments in string theory, the AdS/CFT correspondence(or in general gauge/gravity correspondence) encodes a way of using string theory to perform non-perturbative calculation in gauge theory which is still a complicated problem. It was firstly formulated as the correspondence between weakly coupled gravity theory with AdS (Anti de Sitter) space-time background and strongly coupled gauge theory with conformal symmetry in one lower dimension. One application of AdS/CFT is to compute some of observables of sQGP such as photon and dilepton production rates, mean-free path time of the plasma constituents, and anisotropic drag force effect to the elliptic flow. Show less
The central topic in this thesis is the effect of topological defects in two distinct types of condensed matter systems. The first type consists of graphene and topological insulators. By... Show moreThe central topic in this thesis is the effect of topological defects in two distinct types of condensed matter systems. The first type consists of graphene and topological insulators. By studying the long-range effect of lattice defects (dislocations and disclinations) we find that the graphene electrons mimic fundamental Dirac electrons in spaces with curvature and torsion. We show that these long-range effects influence interferometric transport measurements: (i) Emphasizing the importance of electron dephasing in graphene; (ii) Enabling a characterization of neutral Majorana states, which are important for quantum computation applications, and conjectured to exist in topological insulators. Considering also the microscopic structure of graphene dislocations, we interpret local tunneling experiments on graphite grain boundaries. The second type of systems we study are the high temperature cuprate superconductors, where the strongly interacting electrons lead to coexisting symmetry breaking orders in the pseudogap phase. We observe and describe the interplay of nematic (orientational) and stripe (translational) orderings in local tunneling experiments, with stripe dislocations playing the key role. We also describe the observed phonon anomaly in cuprates through the effect of metallic stripes. Show less
Organisms show a remarkable variation in sizes, yet cell sizes are surprisingly similar across species, typically ranging from 10 _m to 100 _m. A striking exception are the giant cells of the algal... Show moreOrganisms show a remarkable variation in sizes, yet cell sizes are surprisingly similar across species, typically ranging from 10 _m to 100 _m. A striking exception are the giant cells of the algal weed Chara, which can exceed 10 cm in length and 1 mm in diameter. A circulation known as cytoplasmic streaming takes place inside these cells, which has been hypothesised to mitigate the slowness of diffusion on these scales. Our work has sought to quantify how this circulation allows the organism to circumvent the diffusional bottleneck that appears to limits cell size in most organisms. We examine the circulation pattern in the cylindrical internodal cells that make up the shoot of this organism. Along the surface of these cells, two twisting bands carry the cellular fluid up and down, forming a pattern much like the red and white ribbon on a barber pole. Hydrodynamic analysis reveals that this twisting of the bands, which is most prominent during growth, may serve to enhance mixing in the cell, thereby aiding diffusional transport. Flow measurements by means of injected tracers, as well as an MRI technique known as Magnetic Resonance Velocimetry, are consistent with our hydrodymic description of the flow field. Show less
This thesis concerns the vibrational properties of different classical disordered condensed matter systems. In the first part we focus on materials that exhibit a rigidity transition as their... Show moreThis thesis concerns the vibrational properties of different classical disordered condensed matter systems. In the first part we focus on materials that exhibit a rigidity transition as their density is increased. By introducing a new method into the field, we were able to look into the localization behavior of vibrational modes of jammed packings of soft spherical particles, both in the localized regime where the localization length is much less and in the regime where it is grater than the linear system size. We also analyze the nature of vibrational modes of jammed packings of soft elliptical particles, where we uncover the change of the structure of the spectrum, compared to the simplest model of sphere packings, due to the rotational degrees of freedom of the particles. In the second part of this thesis we explore the localization properties of collective modes and response to uniform driving of bubble clouds. We find that the response is often very different from that of a typical mode because the frequency response of each mode is sufficiently wide that many modes are excited when the cloud is driven by an ultrasound. Show less
This thesis presents two lines of research. On the one hand, we investigate heterogeneity in supercooled glycerol by means of rheometry, small-angle neutron scattering, and fluorescence imaging. We... Show moreThis thesis presents two lines of research. On the one hand, we investigate heterogeneity in supercooled glycerol by means of rheometry, small-angle neutron scattering, and fluorescence imaging. We find from the rheological experiments that supercooled glycerol can behave like weak solids at temperatures well above the glass transition. This is very surprising because glycerol is supposed to be purely liquid-like in this temperature range. However, the structural origin of this solid-like state of glycerol still remains unclear. The preliminary results from small-angle neutron scattering show that the solidified glycerol is structurally different from both the supercooled liquid and the crystal. In addition, fluorescence imaging of a thin film of glycerol doped with fluorescent probes reveals heterogeneous patterns of the fluorescence intensity, which is related to long-lived and micrometer scale density fluctuations in supercooled glycerol. All these results will contribute to understanding heterogeneity or even glass transition of supercooled liquids. On the other hand, we study the conformational dynamics of polyprolines by single-molecule FRET (F_rster resonance energy transfer) combined with temperature-cycle microscopy, a novel technique developed in our group, and demonstrate the potential of this new method to address complex molecular dynamics, for example the dynamics of protein-folding, at the single-molecule level. Show less
Doped manganese oxides such as La0.67Ca0.33MnO3 (LCMO) are strongly correlated electron systems which display an insulator to metal transition upon cooling at a temperature T_MI. At low temperature... Show moreDoped manganese oxides such as La0.67Ca0.33MnO3 (LCMO) are strongly correlated electron systems which display an insulator to metal transition upon cooling at a temperature T_MI. At low temperature the material is ferromagnetic. Above the transition the material is a paramagnetic insulator in which conduction is governed by activated polaron hopping. As yet, the fundamental question of conductance on micrometer length scales (the scale on which electronic phase separation can occur) has been little studied. We investigate this question of electrical transport in LCMO, which requires the fabrication of micron sized structures in LCMO thin films. We measure current-voltage (I-V) characteristics as function of temperature, in high magnetic fields, in electric fields and with varying film thickness. In warming from the metallic to the insulating state we find strong non-linear effects in the steep part of the transition. The differential resistance is largest at zero bias, then drops with increasing current, and saturates at a current density which is the same for different samples. Resistance drops of up to 80 % are observed upon increasing the applied current. We propose that the nonlinear behavior is a direct signature of an intervening phase involving the formation of short range polaron correlations (electron-glass). The nonlinear behavior occurs when the homogeneous glass phase which now encompasses the entire microstructure is melted by the applied electrical current. We also associate the formation of inhomogeneities in the microstructure with the appearance of a strong electric field effect. Show less
This thesis describes a series of measurements and a theoretical analysis to get more insight into the fundamental working of superconducting systems when these are brought into close proximity... Show moreThis thesis describes a series of measurements and a theoretical analysis to get more insight into the fundamental working of superconducting systems when these are brought into close proximity with ferromagnetic systems, or when they are driven into a non-equilibrium state. The main focus is on the electronic transport properties of such coupled systems, from which the working of the individual part are well known. Show less
The growing field of bio-electronics holds many promises with regard to the integration of various organic molecules onto printed circuit-boards. These include a decrease in the cost of production,... Show moreThe growing field of bio-electronics holds many promises with regard to the integration of various organic molecules onto printed circuit-boards. These include a decrease in the cost of production, an increased sensitivity and specificity to molecular detection from various solutions (i.e. blood) and ultra-miniaturization. However, numerous challenges still face such prospects, chief among which is the retention of biological activity of the adsorbed molecules. To circumvent the possible harmful effects of the bare surfaces, we have made use of self-assembled molecular films that not only shield the proteins (i.e. azurin) off surfaces, but also help establish a spatially-defined conductive path to electrodes. At the same time, the protein itself was engineered such that the active cavity is directly connected via such molecular __wires__. Our results may help in the adsorption of more complex enzymes into future molecular devices, that will retain their activity on the surfaces and are able to integrate into biosensors. Show less
Chemotaxis, the process in which cells detect a concentration gradient of a specific substance, interpret that information, and subsequently initiate movement towards the source is an essential... Show moreChemotaxis, the process in which cells detect a concentration gradient of a specific substance, interpret that information, and subsequently initiate movement towards the source is an essential part of many biological phenomena. It___s central to the processes in wound healing, in immune defense and in the formation of a viable embryo. In this thesis I used the well characterized social amoeba Dictyostelium discoideum to investigate, in depth, the dynamics that govern the first steps in the detection of a chemical gradient. D. discoideum detects cyclic adenosine mono-phosphate (cAMP) by a special receptor protein, cAMP receptor 1 (cAR1). Inside the cell this receptor activates a G protein which subsequently initiates a complex signaling cascade. Using fluorescence single-molecule microscopy I investigated the movements of both cAR1 and its associated G protein. During chemotaxis both proteins show striking differences in mobility between the leading and trailing edge of the cell. Those differences are presumably key to our understanding of gradient sensing by cells that have been ignored in models so far. Show less
Membrane heterogeneity on the micro- and nanometer scale plays an important role for a large number of biological processes. In parallel to the conception of refined membrane models, new... Show moreMembrane heterogeneity on the micro- and nanometer scale plays an important role for a large number of biological processes. In parallel to the conception of refined membrane models, new experimental techniques to determine membrane microstructure were developed in recent years. Single molecule fluorescence has emerged as one of the leading technologies since it delivers the required spatial resolution and can be employed in living cells. In a complementary approach artificial model systems are used to study specific biophysical aspects of membranes in isolation and in a controllable way. In this thesis we show how phase separated artificial membranes can be used to gain fundamental insight into lipid composition based heterogeneity (Chap. 2) and membrane mediated interactions (Chap. 3). We demonstrate that those interactions can lead to lipid domain sorting (Chap. 4). Experiments with artificial membranes are complemented with live cell studies. We develop a robust analysis method for single molecule position data (Chap. 5) and use it to study the role of heterogeneity in cell signaling (Chap. 6). Finally, we show how protein cluster formation can be measured by counting single molecules in live cells (Chap. 7). Show less
Proteins and enzymes play a key role in all biological systems. Understanding the mechanism of biological functions and reactions in which proteins and enzymes are involved requires a detailed... Show moreProteins and enzymes play a key role in all biological systems. Understanding the mechanism of biological functions and reactions in which proteins and enzymes are involved requires a detailed characterization of protein structure and dynamics. Structure refers to geometrical structure, as a result of the local arrangement of amino-acid side chains, and electronic structure, in particular at the active site of proteins and enzymes. Dynamics refers to structural changes that proteins undergo to perform their function. The work reported in this thesis concerns both methodological developments and the application of electron paramagnetic resonance (EPR) to study protein structure and dynamics. To this end, both continuous wave (cw) and pulsed microwave excitation have been applied. In the research described in this thesis transition-metal ions, such as Cu(II) and Fe(III), and nitroxide spin labels have been used as paramagnetic probes. Show less
We study the shear flow of two-dimensional foams, i.e., a monolayer of bubbles floating on a soapy solution. We successfully connect local and global flow behaviour
The effect of spin-orbit coupling on various quantum transport phenomena is considered. The main topics discussed are: * How spin-orbit coupling can induce shot noise through trajectory splitting. ... Show moreThe effect of spin-orbit coupling on various quantum transport phenomena is considered. The main topics discussed are: * How spin-orbit coupling can induce shot noise through trajectory splitting. * How spin-orbit coupling can degrade electron-hole entanglement (created by a tunnel barrier) by mode mixing. * Mesoscopic Spin Hall effect: longitudinal charge current leads to transverse spin currents in a chaotic electron cavity which has universal fluctuations around a zero mean. * How smooth disorder increases the conductivity of a graphene sheet. In addition a detailed introduction is given to both the origin of spin-orbit coupling and the consequences of time reversal symmetry in quantum systems. Show less
A low-temperature scanning tunneling microscope is used to perform spin-excitations on individual magnetic transition metal atoms when placed onto a crystal surface. By following these excitations... Show moreA low-temperature scanning tunneling microscope is used to perform spin-excitations on individual magnetic transition metal atoms when placed onto a crystal surface. By following these excitations while applying external magnetic fields the precise influence of the anisotropic crystal field on the spins of iron and manganese atoms is investigated. Similar experiments on an individual cobalt atom lead to the observation of a Kondo resonance. This can be explained by showing that the crystal field reduces the 3/2 spin state to an effective spin-1/2 Kramers doublet. The influence of the local environment (i.e. magnetic field, crystal field anisotropy and spin-coupling to neighboring atoms) on the characteristics of the Kondo resonance is studied. Show less
A surface plasmon is a purely two-dimensional electromagnetic excitation bound to the interface between metal and dielectric and quickly decaying away from it. A surface plasmon is able to... Show moreA surface plasmon is a purely two-dimensional electromagnetic excitation bound to the interface between metal and dielectric and quickly decaying away from it. A surface plasmon is able to concentrate light on sub-wavelength scales __ a feature that is attractive for nano-photonics and integrated optics. Light couples to a surface plasmon via surface roughness or an irregularity and vice versa. Most experimental and theoretical work on surface plasmons is concerned with metallic structures that contain one or two-dimensional arrays of close-lying sub-wavelength holes or slits. In this thesis a much simpler system of only a few parallel sub-wavelength-wide slits is used as a playground for surface-plasmon studies. Amplitude and phase characteristics of the light-plasmon-slit interaction events, like coupling, reflection and plasmon hopping over a slit, are studied via corresponding interference phenomena. Moreover, a single subwavelength-wide slit displays a wave-plate behavior, as a result of a finite conductivity of the metal film. Show less
This thesis describes the formation and physical properties of atomic chains consisting of metal atoms and incorporated small molecules. Small molecules like oxygen and hydrogen modify the... Show moreThis thesis describes the formation and physical properties of atomic chains consisting of metal atoms and incorporated small molecules. Small molecules like oxygen and hydrogen modify the electrical and mechanical properties of these wires, resulting in new one-dimensional conductors. Show less
Gold nanoparticles are spherical clusters of gold atoms, with diameters typically between 1 and 100 nanometers. The applications of these particles are rather diverse, from optical labels for... Show moreGold nanoparticles are spherical clusters of gold atoms, with diameters typically between 1 and 100 nanometers. The applications of these particles are rather diverse, from optical labels for biological experiments to data carrier for optical data storage. The goal of my project was to develop new methods to study the physical properties of single gold nanoparticles on ultra-short timescales. Exciation with a short laser pulse brings a nanoparticle out of equilibrium, which makes it vibrate with a period that depends on the particle diameter and the speed of sound in gold. The vibrational period of a gold nanoparticle with a diameter of 60 nanometer is 20 picoseconds. This acoustic vibration has been detected by us for the first time for single particles. The main advantage of single-particle studies over bulk detection of these particles lies in the fact that all particles in an ensemble vibrate at slightly different frequencies, which causes increased damping due to dephasing. The damping of the vibrations of single particles only depends on the elastic coupling between the particle and its environment, which offers the possibility of using these particles as mechanical nanosensors. Show less