Free-standing, axially-symmetric diffraction gratings for neutral matter-waves: experiments and fabrication

Since de Broglie’s famous postulation of the wave-nature of material particles and its subsequent experimental verification, it is known that atoms and molecules behave like waves on a small scale. The investigation of matter waves is a fundamental topic in quantum mechanics. Furthermore, matter waves have many applications in a number of scientific fields. For example they are used to investigate materials by studying the diffraction of atoms (Helium Atom Scattering) or electrons (Low-energy electron-diffraction) from the material’s surface. The topic of this thesis has been to make nano-structured optical elements for matter-waves and use them in a number of experiments. A major result of this thesis is the performance of the classical Poisson-spot experiment with neutral matter-waves. The Poisson-spot refers to the bright interference spot observed in the shadow of a circular obstacle. The experiment gave convincing evidence of the wave nature of light at the beginning of the 19th century. The first two articles (A&B) in this thesis are concerned with the Poisson-spot experiment for matter waves and its applications. The experiment was performed using low-energy deuterium molecules (normal D2, 23.5 meV), which is described in the first article. In the second article the feasibility of using the Poisson-spot experiment to demonstrate the wave-nature of large molecules is studied. The wave properties of large molecules are an important topic in current research. The remaining three articles are concerned with Fresnel zone-plates. Fresnel zoneplates are axially symmetric diffraction gratings that can be used to focus waves. Loosely formulated the relationship between Fresnel zone-plates and the circular object in the Poissonspot experiment is the same as the relationship between multiple-slit gratings and a single straight edge. Paper C studies the application of microscopy with neutral atoms and molecules and the limitations of focusing matter waves with zone plates. The fabrication of the nanostructured free-standing zone-plates using electron-beam lithography is the topic of paper D. A second, new application of Fresnel zone-plates is introduced in paper E: There a zone plate is used for a direct size measurement of the so-called virtual source in a supersonic expansion. The main-result from the Poisson-spot experiment was the observation of the Poisson spot with a molecular-deuterium beam and the successful comparison of the collected experimental data with Fresnel-diffraction theory. The wave-length independence of the on-axis interferencecondition in the Poisson-spot experiment and the weak constraints on angular alignment and position of the circular object led us to conclude that the Poisson spot is a good candidate for demonstrating the wave nature of larger molecules. This idea is studied further in paper B, where among other things the feasibility of a Poissonspot experiment with the fullerene molecule C70 is examined. The main conclusion from this article was that the wave-nature of a molecular-beam can be demonstrated in diffraction experiments with circular discs that have varying amount of intended edge-corrugation. This is because the dependence of the bright spot on the edge-corrugation in the particle-model is different than in the wave-model. The result for the C70 Poisson-spot was that the experiment would be very challenging with count rates as low as 10−4 s−1. However, one possibility would be, due to the simplicity of the setup, to parallelize the experiment and measure the diffraction pattern behind many circular discs at once. This could be realized using the fabrication techniques discussed in chapter 4. Other applications of the Poisson spot could include the study of the Casmir-Polder potential, molecule-lithography and diffraction experiments with atom-lasers from Bose-Einstein condensates. While Fresnel zone-plates can be used to focus any type of wave they suffer from strong chromatic aberration since their focal length is inversely proportional to wavelength. In the case of neutral atom and molecule focusing this is what limits resolution. Paper C presents the highest resolution helium transmission-images hitherto and results from the first focusing of molecular deuterium with a zone plate. In addition the paper discusses the resolution-limit and finds that with presently available techniques a minimal spot size of 300 nm full-width-athalf- maximum is feasible using a zone plate of 200 μm diameter and a beam with speed ratio of about 500. The Fresnel zone-plates used to focus the supersonic-expansion beams need to be freestanding, since the low-energy atoms and molecules do not penetrate any material. In paper D an electron-beam lithography fabrication-process for free-standing zone-plates is presented. The process uses a 200-nm-thin layer of low-stress silicon-nitride for the material of the zone plates. The fabricated zone plates were tested in the supersonic-expansion beam apparatus. The transmission and first-order diffraction-efficiency are close to the theoretical prediction for the smaller 190 μm-diameter zone plates patterned with a single electron-beam write-field. A reduction to 70 % for the 388-μm-diameter zone-plate was observed which was attributed to stitching errors since it was stitched together from four write-fields. In chapter 4 additional unpublished results are presented on spatial-phase-locked electron-beam lithography, which aims at the reduction of stitching errors and other pattern-placement errors in electron-beam lithography. Finally in paper E one of the fabricated zone plates in addition to a previously existing one is used to measure the virtual source-width of a molecular deuterium supersonic-expansion beam. The virtual source-width is a measure of the beam’s temperature perpendicular to the beam axis. The beam’s energy distribution along the beam axis is measured using the timeof- flight method. Data sets for source temperatures T0=310 K and T0=106 K were collected in the stagnation pressure ranges p0=3-171 bar and p0=3-131 bar, respectively. The measured parameters were compared to a simple model of the expansion which explicitly includes the coupling between translational and rotational degrees of freedom. The data generally corresponded very well to the model, except for the virtual source size which was systematically about 2/3 of the model’s prediction. For the cold source-conditions the beam is increasingly heated due to condensation effects at increasing pressures, which results in a clear deviation from the model. This could be used to estimate the fraction of condensed molecules in the beam

Invasive pulmonary aspergillosis complicating COVID-19 in the ICU - A case report.

It is not yet known, if critically ill COVID-19 patients are prone to fungal infections. We report a 69-year-old patient without typical risk factors for invasive pulmonary aspergillosis (IPA), who developed IPA two weeks after onset of symptoms. Our report shows that IPA may occur in critically ill COVID-19 patients

Schutz der Ackerwildkräuter in Thüringen - Eine Erfolgsgeschichte des Naturschutzes

Der Anbau von Feldfrüchten wird durch Pflanzen beeinträchtigt, die ohne bewusstes Zutun des Menschen auftreten und zu einer Minderung der Erträge führen können. Derartige Kräuter und Gräser werden allgemein als Unkräuter bezeichnet. Die Definition der Unkräuter als unerwünschte und Schaden verursachende Pflanzen erweist sich als einseitig wirtschaftsorientiert. Unter ökologischen Aspekten handelt es sich um Pflanzen, die zusammen mit den Nutzpflanzen auftreten und in ihrer Lebensweise und ihren Standortansprüchen den Kulturpflanzen angepasst sind

Interaction Control of Parallel Robots Based on Skill Primitives

Roboter sind als universell einsetzbare Werkzeuge unverzichtbarer Bestandteil des weiterhin bestehenden Trends hin zur Automatisierung der Produktion. Die vorliegende Dissertation leistet einen Beitrag das besondere Potential von Parallelrobotern, das diese gegenüber den hauptsächlich in der Industrie eingesetzten seriellen Robotern besitzen, zu nutzen. In der Arbeit wird eine umfassendes Kontaktregelungskonzept für Parallelroboter auf der Basis von Aktionsprimitiven vorgestellt, das eine sensorbasierte und fehlertolerante Programmierung von Roboteraufgaben ermöglicht. Eine unterlagerte Antriebsregelung, deren Stabilität durch einen Beweis auf Basis der Passivitätstheorie gewährleistet wird, berücksichtigt die dynamischen Eigenschaften von Parallelrobotern. Die Kontaktregelung umfasst die Gesamtheit der Vorgänge zwischen dem Endeffektor des Parallelroboters und der Umgebung während einer Montageaufgabe: Eine entwickelte Systemdynamik im Umgebungskontakt berücksichtigt die im Task-Frame des Aktionsprimitivs auftretenden Kräfte und Momente in realistischer Weise. Abpralleffekte bei der Aufnahme des Umgebungskontakts durch den Endeffektor werden mit den Gesetzmäßigkeiten der nicht-glatten Mechanik beschrieben und durch die Regelung bedämpft. Die Stabilität des Systemverhaltens in diesem Zustand wird durch Moreaus Stoßprozess validiert. Experimentelle Untersuchungen zeigen abschließend die Leistungsfähigkeit der vorgestellten Regelungskonzepte in der praktischen Anwendung.The trend toward a fully automated production of goods is still a major topic for the industry. Robots as universal and programmable automation devices play an important role in this process. In comparison to serial robots, parallel kinematic machines posses some characteristics that make them most suitable for high-speed assembly tasks. In this thesis, an interaction control concept for parallel kinematic machines based on robot programming with skill primitives is presented that utilizes the capabilities of parallel kinematic machines. The use of skill primitives allows a sensor-based and fault tolerant programming of robot tasks. A subordinated drive control takes the special characteristics of parallel kinematic machines into account. Its stability is verified using a passivity based approach. A global approach describing all effects occurring while the end-effector establishes contact with a surface of the environment characterizes the interaction control concept. The constraint system dynamics introduces an environment dependent interaction frame of the skill primitive. Thus, forces and torques in the task frame are modeled realistically during a robot task. Impacts of the robot's end-effector on the environment surface and the system behavior during the transition phase are described using non-smooth mechanics. The impacts are damped by the interaction control and stability of the algorithms is validated by simulations using Moreau's sweeping process. Finally, experiments proof the performance of the introduced control concept for realistic tasks of the parallel robot

Dynamic magnetic resonance angiography of the arteries of the hand. A comparison between an extracellular and an intravascular contrast agent

The purpose of this study was to compare the image quality of the intravascular contrast agent gadofosveset with the extracellular contrast agent gadoterate meglumine in time-resolved three-dimensional magnetic resonance (MR) angiography of the human arteries of the hand. The value of cuff compression technique for suppression of venous enhancement for both contrast agents was also investigated. Three-dimensional MR angiograms of both hands of 11 healthy volunteers were acquired for each contrast agent at 1.5-T, while subsystolic cuff compression was applied at one side. Quantitative and qualitative evaluation were performed and analyzed with Student's t-test. Visualization of vessels was superior in the images acquired with gadofosveset, especially in the late phases. Quantitative and qualitative evaluation showed significantly higher values for gadofosveset. The cuff compression at the lower arm proved to be an effective method to enhance arterial vessels. In conclusion the blood pool agent gadofosveset is superior for the dynamic imaging of the vessels of the hand when compared with the extracellular contrast agent gadoterate meglumine. To fully utilize the advantages of intravascular contrast agents, venous overlay has to be delayed or reduced, which can be achieved effectively by subsystolic lower arm cuff compressio

Magnetotransport Properties of Ferromagnetic Nanoparticles in a Semiconductor Matrix Studied by Precise Size-Selective Cluster Ion Beam Deposition

The combination of magnetic and semiconducting properties in one material system has great potential for integration of emerging spintronics with conventional semiconductor technology. One standard route for the synthesis of magnetic semiconductors is doping of semiconductors with magnetic atoms. In many semiconductor–magnetic–dopant systems, the magnetic atoms form precipitates within the semiconducting matrix. An alternative and controlled way to realize such nanocomposite materials is the assembly by co-deposition of size-selected cluster ions and a semiconductor. Here we follow the latter approach to demonstrate that this fabrication route can be used to independently study the influence of cluster concentration and cluster size on magneto-transport properties. In this case we study Fe clusters composed of approximately 500 or 1000 atoms soft-landed into a thermally evaporated amorphous Ge matrix. The analysis of field and temperature dependent transport shows that tunneling processes affected by Coulomb blockade dominate at low temperatures. The nanocomposites show saturating tunneling magnetoresistance, additionally superimposed by at least one other effect not saturating upon the maximum applied field of 6 T. The nanocomposites’ resistivity and the observed tunneling magnetoresistance depend exponentially on the average distance between cluster surfaces. On the contrary, there is no notable influence of the cluster size on the tunneling magnetoresistance

Investigation wettability of carbon components with the polyurethane binder of resistive composites

The research paper deals with influence wettability of carbon components with the polyurethane binder of resistive composites. We have revealed that the polyurethane lacquer wets up to 25 wt. % of C-1 and GE-3 particles, which results in their uniform distribution over the entire volume of the binder and homogeneity of RCMs. Zero wetting of the same weight percentage is observed in the case of K-163 carbon black, which gives rise to caking and distortion of homogeneity of the composite coating after its formation and its further cracking after heat treatment. The wetting capacity of K-163 carbon black in the polyurethane lacquer is improved using a DISPERBYK-2155 dispersing agent in the amount of 10 wt. % with respect to the binder. The resulting experimental data indicate that C-1 particles exhibit the best wettability, hence their adhesion strength with the binder would be higher than that of GE-3 graphite element and K-163 carbon black

Security and Privacy in Unified Communication

The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.The use of unified communication; video conferencing, audio conferencing, and instant messaging has skyrocketed during the COVID-19 pandemic. However, security and privacy considerations have often been neglected. This paper provides a comprehensive survey of security and privacy in Unified Communication (UC). We systematically analyze security and privacy threats and mitigations in a generic UC scenario. Based on this, we analyze security and privacy features of the major UC market leaders and we draw conclusions on the overall UC landscape. While confidentiality in communication channels is generally well protected through encryption, other privacy properties are mostly lacking on UC platforms

Fast resolution change in neutral helium atom microscopy

In neutral helium atom microscopy, a beam of atoms is scanned across a surface. Though still in its infancy, neutral helium microscopy has seen a rapid development over the last few years. The inertness and low energy of the helium atoms (less than 0.1 eV) combined with a very large depth of field and the fact that the helium atoms do not penetrate any solid material at low energies open the possibility for a non-destructive instrument that can measure topology on the nanoscale even on fragile and insulating surfaces. The resolution is determined by the beam spot size on the sample. Fast resolution change is an attractive property of a microscope because it allows different aspects of a sample to be investigated and makes it easier to identify specific features. However up till now it has not been possible to change the resolution of a helium microscope without breaking the vacuum and changing parts of the atom source. Here we present a modified source design, which allows fast, step wise resolution change. The basic design idea is to insert a moveable holder with a series of collimating apertures in front of the source, thus changing the effective source size of the beam and thereby the spot size on the surface and thus the microscope resolution. We demonstrate a design with 3 resolution steps. The number of resolution steps can easily be extended.publishedVersio