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Item does not contain fulltext OBJECTIVES: Evidence suggests the involvement of the dopamine D4 receptor gene (DRD4) in the pathogenesis of ADHD, but the exact mechanism is not well understood. Earlier reports on the effects of DRD4 polymorphisms on neurocognitive and neuroimaging measures are inconsistent. This study investigated the functional consequences of the 7-repeat allele of DRD4 on neurocognitive endophenotypes of ADHD in the Dutch subsample of the International Multicenter ADHD Genetics study. METHODS: Participants were 350 children (5-11.5 years) and adolescents (11.6-19 years) with ADHD and their 195 non-affected siblings. An overall measure of neuropsychological functioning was derived by principal component analysis from five neurocognitive and five motor tasks. The effects of DRD4 and age were examined using Linear Mixed Model analyses. RESULTS: The analyses were stratified for affected and non-affected participants after finding a significant three-way interaction between ADHD status, age and the 7-repeat allele. Apart from a main effect of age, a significant interaction effect of age and DRD4 was found in non-affected but not in affected participants, with non-affected adolescent carriers of the 7-repeat allele showing worse neuropsychological performance. In addition, carrying the 7-repeat allele of DRD4 was related to a significantly worse performance on verbal working memory in non-affected siblings, independent of age. CONCLUSIONS: These results might indicate that the effect of the DRD4 7-repeat allele on neuropsychological functioning is dependent on age and ADHD status.

The high speed performance of a scanning probe microscope (SPM) is improved if a microelectromechanical systems (MEMS) device is employed for the out-of-plane scanning motion. We have carried out experiments with MEMS high-speed z-scanners (189 kHz fundamental resonance frequency) in both atomic force microscope and scanning tunneling microscope modes. The experiments show that with the current MEMS z-scanner, lateral tip speeds of 5 mm/s can be achieved with full feedback on surfaces with significant roughness. The improvement in scan speed, obtained with MEMS scanners, increases the possibilities for SPM observations of dynamic processes. Even higher speed MEMS scanners with fundamental resonance frequencies in excess of a megahertz are currently under development.

We have developed a sensor to study the mechanical stiffness of atomic-size contacts. It consists of a modification of the mechanically controllable break-junction technique, using a quartz tuning fork resonator as force sensor. We present first results of measurements of the force constants in gold atomic contacts. In the formation of chains of single-metal atoms, the folding in of individual atoms from the banks into the chain can be observed. This sensor allows one to measure forces in atomic contacts for a wide variety of metals, as illustrated with the first measurements on platinum.

We find ourselves in an era of transition, not just towards a more computing- and data-driven society but also away from unsustainable fossil fuels as an energy source. This leads to a rapidly increasing demand for computing power on an ever more tight energy budget. Therefore, it is imperative to investigate novel energy-efficient computing techniques, like superconducting spintronics or neuromorphic computing using correlated electron matter. Naturally, understanding the physics governing these processes at the sub-micrometer (i.e., device) scale is crucial for this development to succeed. This thesis examines the effects of size reduction and geometry on ferromagnetic Josephson junctions and highly correlated electron matter through transport experiments. Specifically, it describes how spin-polarized supercurrents can be generated using spin texture, stabilized by carefully tuning the geometry of planar Josephson junctions, and how the bistability of these spin textures can be employed to create non-volatile superconducting memory elements. Furthermore, it reports a strong size dependence of the current density that drives the Mott-insulating-to-metal transition in Ca2RuO4 and shows how various constricted geometries can be used to localize and examine the properties of superconducting chiral domain walls in Sr2RuO4.

In the thesis we demonstrated an 85% state transfer efficiency between two mechanical modes coupled to a common optical mode via stimulated Raman adiabatic passage (STIRAP) in the classical regime. We also showed possibilities to manipulate quantum states of the mechanical modes via STIRAP.

This thesis examines silicon pore optics (SPO), a technology that exploits silicon wafers from the semiconductor industry to create extremely high quality X-ray optics, by studying its manufacturing process, applications, and prospects. SPO technology has become very mature thanks to the continuous development efforts to prepare for the industrial production of Athena, the largest space-borne X-ray telescope yet to be launched. In effect, SPO is also a versatile technology that can be further developed for a wide range of applications, including radiation therapy.

The elementary excitations of magnets are called spin waves, and their corresponding quasi-particles are known as magnons. The rapidly growing field of Magnonics aims at using them as information carriers in a new generation of electronic devices, (almost) free of electric currents. Encoding information in the amplitude and/or phase of these coherent waves could lead to a drastic decrease in dissipated power, typically related to the motion of electrons ("Joule" or "Ohmic" heating).This dissertation describes the development and use of a new technique to study spin waves. This technique uses the electronic spins associated with nitrogen-vacancy (NV) centers as magnetic field sensors. An NV center is a light-emitting defect in the crystal lattice of diamond. Remarkably, the brightness of its emission depends on its spin state, sensitive to magnetic fields. This way, magnetic information can be investigated optically.

In surface science there is great effort to move from studying simple, flat model surfaces in vacuum to investigating more complex model catalysts in gas environments (in situ). This thesis gives three examples of such studies using microscopy and spectroscopy.Exposure of ZnO(10-10) to moderate pressures of water in an in situ scanning tunneling microscope reveals that the surface roughens. The flat ZnO(10-10) is thus only conditionally suited as a model catalyst for reactions involving water.In the same microscope, surface gold oxide formation is observed on TiO2/Au(111) during CO oxidation at 1 bar pressure. Comparisons to the Au(111) surface suggest that the titania does not supply atomic oxygen to the Au(111) substrate as part of the reaction mechanism of the CO oxidation.Co(0001) is investigated as a model catalyst for Fischer-Tropsch synthesis, the reaction of CO and H2 to form hydrocarbons. In this thesis the oxidation behavior of the cobalt and the adsorption of carbon species during the reaction are investigated using near-ambient pressure X-ray photoelectron spectroscopy.Generally, this thesis exemplifies the significant influence that small concentrations of contaminants in gases and materials can have on the structure and behavior of surfaces in in situ studies.

This dissertation describes novel magnetic resonance imaging (MRI) surface antennas and arrays, and their applications at both 3 and 7 Tesla. While the first half of this work describes flexible lightweight antenna arrays, the other half focuses on the use of solid ceramic high-permittivity materials as a substantial part of the antenna.

This thesis explores interfacial conductance and electric field-effects in LaAlO3/SrTiO3 heterostructures. High quality epitaxial LaAlO3 films were grown on SrTiO3 substrates by 90° off-axis sputter deposition. The conductance properties of the interfaces were modulated by applying external electric fields in different geometries, namely back-gating (applying an electric field to the back side of the substrate) and ionic liquid gating (which applies an electric field on the side of the LaAlO3).

The Copenhagen interpretation of quantum mechanics states that a measurement collapses a wavefunction onto an eigenstate of the corresponding measurement operator. This causes a quantum mechanical wavefunction to break its unitary evolution described by the Schrödinger equation and is the source of the quantum measurement problem. In this thesis we take the first steps to an experiment that might shed light on this century-old problem. We envision a single-microwave-photon interferometer that has a travelling-wave parametric amplifier (TWPA) added to each of its arms. We wonder if the process of amplification of the quantum state causes the wavefunction to collapse, as it might turn into a detector while smoothly increasing the amplifier's gain. To this end we introduce necessary concepts from the field of microwave engineering. Then, we develop a quantum theory to describe TWPAs. This is followed by preliminary calculations on the expected interference visibility of the envisioned interferometer as a function of amplifier gain and losses. Furthermore, we describe how wavefunction collapse might reveal itself in the experimental results. Finally, the thesis describes our efforts to develop a low-loss TWPA based on Josephson junctions.

Graphene nanoribbons (GNRs) are used as a current carrying substrate in investigation of current-induced forces in a low-temperature STM (chapter 2). We demonstrate induced migration of Co adatoms on GNRs and on Au(111) using voltage pulses from the STM tip and we argue that motion is due to thermal excitations rather than the wind force. In chapter 3 we show that voltage signal is induced in a graphene strip when a droplet of ionic liquid is moved across its surface. Here we show that even deionized water can induce voltage over charged graphene surface due to the polarizability of water molecules. In chapter 4 we present a method for fabrication of graphene nanoelectrodes which we further test electrically in a modified STM. For the first time we demonstrate that the gap between two graphene nanoelectrodes can be tuned with subnanometric precision

Triplet superconductivity refers to a condensate of equal-spin Cooper pairs (pairs of electrons with equal spin). While exceptionally rare in nature, triplet pairing of electrons can occur if either the temporal or spatial component of the superconducting wavefunction can be represented by an odd function. These are often referred to as odd-frequency and odd-parity triplets, respectively. We use hybrid magnetic devices to study the former, while the latter is investigated in mesoscopic structures of strontium ruthenate (Sr\tss{2}RuO\tss{4}).

The research presented in this thesis makes use of small molecules (as H2 , D2 and O2 ) on well-defined single crystal surfaces (flat Pt(111), flat Cu(211) and curved Pt(111)) to elucidate the role of surface structure and degrees of freedom in the reactant in specific surface reactions. For D2 on Pt(111), we find at most a very weak signature of geometric corrugation at large polar angles. For D2 on Cu, we find an anomalous reduced dissociative sticking probability for the stepped Cu(211) surface compared to Cu(111). For hydrogen on curved Pt(111), the HD formation increases linearly with the step density at low incident energy. A surface reconstruction on curved Pt(111) surface is observed on both A- and B-step side when the crystal is annealed at 1200 K. For O2 on curved Pt(111), at low incident energy, steps dominate reactivity by providing an indirect dynamical trapping mechanism. At higher impact energy, a direct chemisorption mechanism dominates. The step facet favors molecules impacting with their internuclear axis parallel to its surface.

This thesis is devoted to an in-depth examination of the various effects of disorder in the cuprate high-temperature superconductors. Disorder is ubiquitous in these materials and is central to a number of phenomena observed in various phases. We revisit several phenomena in the cuprates in light of what is now known about the nature of disorder present in these materials. First, the phenomenon of quasiparticle scattering interference is revisited using a number of different realistic models of distributed disorder which go beyond the single-impurity paradigms used in much of the literature. Next, we study the manner in which a finite DOS at the Fermi energy, seen in a large number of experiments, is generated by various models of disorder, and consider the possibility that smooth disorder from off-plane dopants explains this phenomenon. In addition the localization properties of the superconducting quasiparticles in the presence of various types of disorder are studied. Finally, we show how a number of nontrivial interaction effects in the superconducting and normal states of the cuprates could be visualized by scanning tunneling spectroscopy experiments.

Electron microscopy has become an extremely important techniquein a wide variety of elds. The resolving power is vastly superiorto light microscopes and electron microscopy has proven tobe valuable in elds ranging from archaeology and geology to biology andcondensed-matter physics.A major disadvantage is that the electron energy used in conventional ElectronMicroscopy (EM) ranges from 10’s to 100’s of keV. Such energetic electronscan signicantly damage the specimen. This is especially relevant in thestudy of biological samples and organic materials in general. Major eorts arebeing made to avoid this radiation damage from interfering with the studyof such materials. There are several approaches to minimize damage in EM.These include developing better detectors such that lower electron doses aresucient to form an image, and lowering the electron energies to several keV.In this dissertation I present the development of, and measurements with, atransmission electron microscope that uses electron energies ve orders ofmagnitude lower than in conventional Transmission Electron Microscopes(TEMs). The energies we use are in the order of a few eV. Hence, we call ourtechnique ’eV-TEM’.

Atoms and molecules are the basic units of matter. If we keep dividing a bar of gold or a glass of water into smaller parts, at the end we are left with a single gold atom or a water molecule. We could not divide them further without them losing their identity. Molecular electronics is the study of how electrons, which are fundamental particles in nature, flow through these basic units of the matter once connected between two electrical leads. At such small dimensions matter starts to lose its macroscopic properties and its behaviour is governed by the rules of quantum mechanics. A molecule can have various electronic and mechanical degrees of freedom (or eigenstates) with discrete energies. The electrons flowing through the molecule can interact with these degrees of freedom making single molecule devices. Better control and understanding of these interactions and study of how the atomic structure of the macroscopic electrical leads affect the electronic-transport forms the main focus of this thesis. Controlled manipulation of single atoms and molecules using a low-temperature scanning tunnelling microscope, probing charge transport using high-frequency shot-noise measurement and the use of graphene as possible electrodes are the three directions which are investigated here.

There are many interpretations of quantum mechanics, and ultimately experiments are needed to verify or falsify these interpretations. In this thesis we work towards an experiment that should make the study of the existence of gravitational-induced collapse of the wave function feasible. We propose to use a Magnetic Resonance Force Microscope where we let the magnetic tip interact with a controlled Nitrogen-Vacancy center to study the superposition of the tip’s position (Ch.1 and Ch. 5). We have shown theoretically (Ch. 2) and experimentally (Ch.3) how spins on the surface and in the bulk can change the resonance frequency of the resonator and increase the dissipation. Using this theory we have measured the bulk impurity and surface spin density of diamond and performed local spin resonance on the bulk spins (Ch. 4). An important part of this thesis forms the partly new techniques and instrumentation that this experiment needs (Ch. 6). One of these techniques led to the study of the tip-sample capacitance in general SPM-applications which can be used to do fast pre-approaches and in-situ tip characterizations (Ch. 7).

This thesis consists of two subjects, that are both a consequence of radiation pressure. In optomechanics, light is used to influence the motion of a trampoline resonator. It is possible to slow down this motion, cooling it from room temperature to an effective temperature of several milllikelvins, The other subject is how superconductivity influences the Casimir force between a niobium-titanium-nitride plate and a gold microsphere. This has been investigated both theoretically and experimentally.

The thesis describes experimental steps towards reduction of friction on the macroscopic scale by scenarios of thermo- and superlubricity well-known on the nanoscale. The friction study involves experiments on tailored Si nanopillar arrays, micropatterned Diamond-Like Carbon coating and high-quality graphene.

In the early nineties, Sidles (1991) came with a solution to combine the force microscopy techniques sensitive to atoms with that of magnetic resonance techniques: Magnetic Resonance Force Microscopy (MRFM) was born. The technique was promising, big steps were taken, and the holy grail of atomic resolution imaging of biological tissues seemed within an arm’s reach. Unfortunately, the last steps are the most difficult. The technique is experimentally challenging and so far, the images of biological structures are no better than those obtained by other conventional techniques. In order to be an attractive technique, MRFM needs to be scientifically relevant while the technique is further improved towards the holy grail of imaging biological structures on the nanometer scale. In this thesis, we show how MRFM can usefully contribute to the field of condensed-matter.

Biological molecules, especially the proteins, have a special and important function. We study their structure to understand their functions, and further make application, like the medical research. The routine method is diffraction, but not work for molecules which cannot grow into crystal and molecules which their crystal are too small. Cryo-EM technique provides another way to solve their structures through their images, it does not need crystals. Meanwhile, electron diffraction can work small crystals (micro- and nano-crystals) after the sample preparation was improved. Hence, we try to build a method that can restore the structure from the crystal’s image. We collect images of protein nano-crystal, and these images were processed to enhance their contrast. The key step is to find the orientations of these images in the procedure of reconstruction, therefore, we create a method that calculates these orientations. Nano-crystals, which cannot be used in diffraction method, then can be used in this method.

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 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.