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
The goal of this thesis is to study charge transport phenomena in organic materials. This is done optically by means of single-moleculespectroscopy in a field-effect transistor based on a molecular... Show moreThe goal of this thesis is to study charge transport phenomena in organic materials. This is done optically by means of single-moleculespectroscopy in a field-effect transistor based on a molecular crystal.We present (in Chapter 2) a fundamental requirement for single-moleculespectroscopy concerning the energy levels of the guest molecule withrespect to the ones of the host molecule. Following this constraint, westudy (in Chapters 3 and 4) the photophysics of a new system forhigh-resolution spectroscopy at cryogenic temperatures, which consistsof dibenzoterrylene molecules inserted in a crystal of anthracene. Wethen characterise (in Chapter 5) the electrical properties of theanthracene field-effect transistor with 'conventional' methods. In Chapter 6, wefinally use the influence of an electric field on the spectroscopicproperties of fluorescent molecules to investigate locally the chargecarrier transport phenomena in a crystal of anthracene.The goal of this thesis is to study charge transport phenomena inorganic materials. This is done optically by means of single-moleculespectroscopy in a field-effect transistor based on a molecular crystal.We present (in Chapter 2) a fundamental requirement for single-moleculespectroscopy concerning the energy levels of the guest molecule withrespect to the ones of the host molecule. Following this constraint, westudy (in Chapters 3 and 4) the photophysics of a new system forhigh-resolution spectroscopy at cryogenic temperatures, which consistsof dibenzoterrylene molecules inserted in a crystal of anthracene. Wethen characterise (in Chapter 5) the electrical properties of theanthracene field-effect transistor with 'conventional' methods. In Chapter 6, wefinally use the influence of an electric field on the spectroscopicproperties of fluorescent molecules to investigate locally the chargecarrier transport phenomena in a crystal of anthracene.The goal of this thesis is to study charge transport phenomena inorganic materials. This is done optically by means of single-moleculespectroscopy in a field-effect transistor based on a molecular crystal.We present (in Chapter 2) a fundamental requirement for single-moleculespectroscopy concerning the energy levels of the guest molecule withrespect to the ones of the host molecule. Following this constraint, westudy (in Chapters 3 and 4) the photophysics of a new system forhigh-resolution spectroscopy at cryogenic temperatures, which consistsof dibenzoterrylene molecules inserted in a crystal of anthracene. Wethen characterise (in Chapter 5) the electrical properties of theanthracene field-effect transistor with 'conventional' methods. In Chapter 6, wefinally use the influence of an electric field on the spectroscopicproperties of fluorescent molecules to investigate locally the chargecarrier transport phenomena in a crystal of anthracene. Show less
The central topic of the Thesis concerns light scattering experiments with entangled photons. Specifically, we study the effect of scattering processes on polarization-entanglement of twin-photons.... Show moreThe central topic of the Thesis concerns light scattering experiments with entangled photons. Specifically, we study the effect of scattering processes on polarization-entanglement of twin-photons. The main idea is that scattering generally couples polarization and spatial degrees of freedom of photons. The details of this coupling depend on the characteristics of the scattering medium. Such coupling, in turn, can reduce the entanglement of twin-photons, if the photon pairs are detected in a momentum-insensitive way. We have investigated a broad range of optical scattering media ranging from milk to polymer fibers. By manipulating the parameters of these samples we were able to generate a broad range of quantum states, proving for the first time that scattering processes are a substantial tool for mixed-state engineering. This is of great importance for quantum information since any real-world application thereof has to deal with mixed (as opposed to pure) states. Show less
The initial goal of this thesis was to demonstrate chaos in an open two-mirror resonator. We have designed a bifocal mirror that forms a resonator with an unstable inner and a stable outer part. To... Show moreThe initial goal of this thesis was to demonstrate chaos in an open two-mirror resonator. We have designed a bifocal mirror that forms a resonator with an unstable inner and a stable outer part. To be able to distinguish phenomena unique for configuration from phenomena also present in conventional resonators, i.e., roughness-induced scattering and aberrations, the performance of a conventional stable resonator is investigated first. Roughness-induced scattering turns out to affect the cavity finesse as well as the average power throughput and produces mode coupling close to frequency-degenerate points. We demonstrate, furthermore, a method to accurately determine aberrations by measuring the Gouy phase of subsequent higher-order modes around frequency-degeneracy. The bifocal mirror is not fabricated by traditional grinding and polishing, but by diamond-machining. The eigenmodes of a resonator with one diamond-machined bifocal mirror turn out to be Laguerre-Gaussian. We demonstrate furthermore, the coupling of two resonators based on transmission spectra and patterns, and report on the ability of the configuration to fulfill the basic requirements to obtain chaos. Show less
The research in this thesis comprises two separate topics: single-molecule spectroscopy and resonant Raman spectroscopy. The first part concerns single-molecule (SM) spectroscopy on polyethylene ... Show moreThe research in this thesis comprises two separate topics: single-molecule spectroscopy and resonant Raman spectroscopy. The first part concerns single-molecule (SM) spectroscopy on polyethylene (PE) films. Ultra thin (200 nm) films of pure high density PE were produced by spincoating. By determining the position (accuracy 10 nm) and in-plane orientation (acc. 5 deg.) of single 2.3,8.9-dibenzanthanthrene (DBATT) guest molecules, by means of SM microscopy and spectroscopy respectively, we demonstrated that these thin PE films have a shish-kebab morphology, instead of the spherulitic morphology common in thicker PE films. We have also investigated the alignment process of individual quest chromophores by stretching thicker (50 um), low density PE films. Using SM spectroscopy, we have shown that individual guest chromophores are not better aligned along the stretch direction, as draw ratio increases. Instead alignment occurs suddenly, due to the destruction and (oriented) reformation of local crystalline regions and subsequent adsorption of chromophores. Each chromophore's orientation is determined by specific interactions with the oriented PE crystal surface. The second part of this thesis concerns a quantum-chemical analysis of the resonant Raman spectrum of the carotenoid spheroidene reconstituted in the photosynthetic reaction center (RC) of Rhodobacter sphaeroides. Our analysis demonstrates that spheroidene can adopt at least two cis forms in the RC. One of these has been conclusively shown to be the 15,15'-cis structure. Show less
