Excitation-Induced Ge Quantum Dot Growth on Si(100)-2X1 by Pulsed Laser Deposition

Self-assembled Ge quantum dots (QD) are grown on Si(100)-(2×1) with laser excitation during growth processes by pulsed laser deposition (PLD). In situ reflection-high energy electron diffraction (RHEED) and post-deposition atomic force microscopy (AFM) are used to study the growth dynamics and morphology of the QDs. A Q-switched Nd:YAG laser (λ = 1064 nm, 40 ns pulse width, 5 J/cm2 fluence, and 10 Hz repetition rate) were used to ablate germanium and irradiate the silicon substrate. Ge QD formation on Si(100)-(2×1) with different substrate temperatures and excitation laser energy densities was studied. The excitation laser reduces the epitaxial growth temperature to 250 °C for a 22 ML film. In addition, applying the excitation laser to the substrate during the growth changes the QD morphology and density and improves the uniformity of quantum dots fabricated at 390 °C. At room temperature, applying the excitation laser during growth decreases the surface roughness although epitaxial growth could not be achieved. We have also studied the surface diffusion coefficient of Ge during pulsed laser deposition of Ge on Si(100)-(2×1) with different excitation laser energy densities. Applying the excitation laser to the substrate during the growth increases the surface diffusion coefficient, changes the QD morphology and density, and improves the size uniformity of the grown quantum dots. To study the effect of high intensity ultralast laser pulses, Ge quantum dots on Si(I00) were grown in an ultrahigh vacuum (UHV) chamber (base pressure ∼7.0x10 -10 Torr) by femtosecond pulsed laser deposition. The results show that excitation laser reduces the epitaxial growth temperature to ∼70 °C. This result could lead to nonthermal method to achieve low temperature epitaxy which limits the redistribution of impurities, reduces intermixing in heteroepitaxy, and restricts the generation of defects by thermal stress. We have ruled out thermal effects and some of the desorption models. Although further studies are needed to elucidate the mechanism involved, a purely electronic mechanism of enhanced surface diffusion of Ge atoms is proposed

Perception of Color Break-Up

Hintergrund. Ein farbverfälschender Bildfehler namens Color Break-Up (CBU) wurde untersucht. Störende CBU-Effekte treten auf, wenn Augenbewegungen (z.B. Folgebewegungen oder Sakkaden) während der Content-Wiedergabe über sogenannte Field-Sequential Color (FSC) Displays oder Projektoren ausgeführt werden. Die Ursache für das Auftreten des CBU-Effektes ist die sequenzielle Anzeige der Primärfarben über das FSC-System. Methoden. Ein kombiniertes Design aus empirischer Forschung und theoretischer Modellierung wurde angewendet. Mittels empirischer Studien wurde der Einfluss von hardware-, content- und betrachterbasierten Faktoren auf die CBU-Wahrnehmung der Stichprobe untersucht. Hierzu wurden zunächst Sehleistung (u. a. Farbsehen), Kurzzeitzustand (u. a. Aufmerksamkeit) und Persönlichkeitsmerkmale (u. a. Technikaffinität) der Stichprobe erfasst. Anschließend wurden die Teilnehmenden gebeten, die wahrgenommene CBU-Intensität verschiedener Videosequenzen zu bewerten. Die Sequenzen wurden mit einem FSC-Projektor wiedergegeben. Das verwendete Setup ermöglichte die Untersuchung folgender Variablen: die Größe (1.0 bis 6.0°) und Leuchtdichte (10.0 bis 157.0 cd/m2) des CBU-provozierenden Contents, das Augenbewegungsmuster des Teilnehmenden (Geschwindigkeit der Folgebewegung: 18.0 bis 54.0 °/s; Amplitude der Sakkade: 3.6 bis 28.2°), die Position der Netzhautstimulation (0.0 bis 50.0°) und die Bildrate des Projektors (30.0 bis 420.0 Hz). Korrelationen zwischen den unabhängigen Variablen und der subjektiven CBU-Wahrnehmung wurden getestet. Das ergänzend entwickelte Modell prognostiziert die CBU-Wahrnehmung eines Betrachters auf theoretische Weise. Das Modell rekonstruiert die Intensitäts- und Farbeigenschaften von CBU-Effekten zunächst grafisch. Anschließend wird die visuelle CBU-Rekonstruktion zu repräsentativen Modellindizes komprimiert, um das modellierte Szenario mit einem handhabbaren Satz von Metriken zu quantifizieren. Die Modellergebnisse wurden abschließend mit den empirischen Daten verglichen. Ergebnisse. Die hohe interindividuelle CBU-Variabilität innerhalb der Stichprobe lässt sich nicht durch die Sehleistung, den Kurzzeitzustand oder die Persönlichkeitsmerkmale eines Teilnehmenden erklären. Eindeutig verstärkende Bedingungen der CBU-Wahrnehmung sind: (1) eine foveale Position des CBU-Stimulus, (2) eine reduzierte Stimulusgröße während Sakkaden, (3) eine hohe Bewegungsgeschwindigkeit des Auges und (4) eine niedrige Bildrate des Projektors (Korrelation durch Exponentialfunktion beschreibbar, r2 > .93). Die Leuchtdichte des Stimulus wirkt sich nur geringfügig auf die CBU-Wahrnehmung aus. Generell hilft das Modell, die grundlegenden Prozesse der CBU-Genese zu verstehen, den Einfluss von CBU-Determinanten zu untersuchen und ein Klassifizierungsschema für verschiedene CBU-Varianten zu erstellen. Das Modell prognostiziert die empirischen Daten innerhalb der angegebenen Toleranzbereiche. Schlussfolgerungen. Die Studienergebnisse ermöglichen die Festlegung von Bildraten und Eigenschaften des CBU-provozierenden Content (Größe und Position), die das Überschreiten vordefinierter, störender CBU-Grenzwerte vermeiden. Die abgeleiteten Hardwareanforderungen und Content-Empfehlungen ermöglichen ein praxisnahes und evidenzbasiertes CBU-Management. Für die Vorhersage von CBU kann die Modellgenauigkeit weiter verbessert werden, indem Merkmale der menschlichen Wahrnehmung berücksichtigt werden, z.B. die exzentrizitätsabhängige Netzhautempfindlichkeit oder Änderungen der visuellen Wahrnehmung bei unterschiedlichen Arten von Augenbewegungen. Zur Modellierung dieser Merkmale können teilnehmerbezogene Daten der empirischen Forschung herangezogen werden.Background. A color-distorting artifact called Color Break-Up (CBU) has been investigated. Disturbing CBU effects occur when eye movements (e.g., pursuits or saccades) are performed during the presentation of content on Field-Sequential Color (FSC) display or projection systems where the primary colors are displayed sequentially rather than simultaneously. Methods. A mixed design of empirical research and theoretical modeling was used to address the main research questions. Conducted studies evaluated the impact of hardware-based, content-based, and viewer-based factors on the sample’s CBU perception. In a first step, visual performance parameters (e.g., color vision), short-term state (e.g., attention level), and long-term personality traits (e.g., affinity to technology) of the sample were recorded. Participants were then asked to rate the perceived CBU intensity for different video sequences presented by a FSC-based projector. The applied setup allowed the size of the CBU-provoking content (1.0 to 6.0°), its luminance level (10.0 to 157.0 cd/m2), the participant’s eye movement pattern (pursuits: 18.0 to 54.0 deg/s; saccadic amplitudes: 3.6 to 28.2°), the position of retinal stimulation (0.0 to 50.0°), and the projector’s frame rate (30.0 to 420.0 Hz) to be varied. Correlations between independent variables and subjective CBU perception were tested. In contrast, the developed model predicts a viewer’s CBU perception on an objective basis. The model graphically reconstructs the intensity and color characteristics of CBU effects. The visual CBU reconstruction is then compressed into representative model indices to quantify the modeled scenario with a manageable set of metrics. Finally, the model output was compared to the empirical data. Results. High interindividual CBU variability within the sample cannot be explained by a participant’s visual performance, short-term state or long-term personality traits. Conditions that distinctly elevate the participant’s CBU perception are (1) a foveal stimulus position on the retina, (2) a small-sized stimulus during saccades, (3) a high eye movement velocity, and (4) a low frame rate of the projector (correlation expressed by exponential function, r2 > .93). The stimulus luminance, however, only slightly affects CBU perception. In general, the model helps to understand the fundamental processes of CBU genesis, to investigate the impact of CBU determinants, and to establish a classification scheme for different CBU variants. The model adequately predicts the empirical data within the specified tolerance ranges. Conclusions. The study results allow the determination of frame rates and content characteristics (size and position) that avoid exceeding predefined annoyance thresholds for CBU perception. The derived hardware requirements and content recommendations enable practical and evidence-based CBU management. For CBU prediction, model accuracy can be further improved by considering features of human perception, e.g., eccentricity-dependent retinal sensitivity or changes in visual perception with different types of eye movements. Participant-based data from the empirical research can be used to model these features

The laser quenching technique for studying the magneto-thermal instability in high critical current density superconducting strands for accelerator magnets

Siirretty Doriast

Scholarly Information Seeking Habits and Behaviors of Missouri State University (MSU) Faculty

A group of faculty and staff of the MSU Libraries (J. Johnson, coordinator, L. Cline, W. Edgar, S. Fischer, G. Jackson-Brown, A. Miller), assisted by W. Meadows of the MSU Department of Anthropology, conducted an ethnographic study using direct observation and semi-structured interviews of a sample of MSU faculty members to gain knowledge about faculty scholarly information seeking habits and behaviors to provide insight into the following research questions: ● What information sources and technologies are used by MSU faculty members in their scholarship?● Where do faculty members conduct their research?● Whom do faculty members consult for research, writing, and information seeking assistance?https://bearworks.missouristate.edu/reports-lib/1000/thumbnail.jp

Tailored magnetic nanostructures on surfaces

Nanostructuring has introduced us to a new world of tunable, artificially structured materials. An exciting aspect of this new world is that we control where the atoms, or layers of atoms, are arranged in materials and have learned that this can awaken new properties in them. But, we are only at the beginning stages in developing this control and an understanding of what can be done with it. This. dissertation is about an important part of finding our way in this new world; learning to tailor magnetic nanostructures on surfaces. We begin by showing ways in which the magnetic properties of ultrathin films, nanostripes, and isolated nanoclusters can be systematically varied in order to teach us about their behavior. The ultrathin films are from the historically challenging Fe/Cu(l00) system. We use small fractions of a single layer of cobalt capping atoms to control their magnetization direction and find a completely new way to cause the magnetization direction to reorient. The nanostripes are made of alloys of iron and cobalt on a tungsten surface. We explore how the magnetic ordering in these stripes is affected by variation of their composition. We then show how changing the size and spacing of isolated Fe dots on a copper surface can teach us about magnetic interactions between them. Finally, we show how our ability to synthesize the dots represented the last piece in an important puzzle. This work enables us to make the first direct observation of how the magnetic properties of a particular amount of a single material change as it is prepared in the form of an ultrathin film, wire array, or dot assembly on a common template

Contours in Visualization

This thesis studies the visualization of set collections either via or defines as the relations among contours. In the first part, dynamic Euler diagrams are used to communicate and improve semimanually the result of clustering methods which allow clusters to overlap arbitrarily. The contours of the Euler diagram are rendered as implicit surfaces called blobs in computer graphics. The interaction metaphor is the moving of items into or out of these blobs. The utility of the method is demonstrated on data arising from the analysis of gene expressions. The method works well for small datasets of up to one hundred items and few clusters. In the second part, these limitations are mitigated employing a GPU-based rendering of Euler diagrams and mixing textures and colors to resolve overlapping regions better. The GPU-based approach subdivides the screen into triangles on which it performs a contour interpolation, i.e. a fragment shader determines for each pixel which zones of an Euler diagram it belongs to. The rendering speed is thus increased to allow multiple hundred items. The method is applied to an example comparing different document clustering results. The contour tree compactly describes scalar field topology. From the viewpoint of graph drawing, it is a tree with attributes at vertices and optionally on edges. Standard tree drawing algorithms emphasize structural properties of the tree and neglect the attributes. Adapting popular graph drawing approaches to the problem of contour tree drawing it is found that they are unable to convey this information. Five aesthetic criteria for drawing contour trees are proposed and a novel algorithm for drawing contour trees in the plane that satisfies four of these criteria is presented. The implementation is fast and effective for contour tree sizes usually used in interactive systems and also produces readable pictures for larger trees. Dynamical models that explain the formation of spatial structures of RNA molecules have reached a complexity that requires novel visualization methods to analyze these model\''s validity. The fourth part of the thesis focuses on the visualization of so-called folding landscapes of a growing RNA molecule. Folding landscapes describe the energy of a molecule as a function of its spatial configuration; they are huge and high dimensional. Their most salient features are described by their so-called barrier tree -- a contour tree for discrete observation spaces. The changing folding landscapes of a growing RNA chain are visualized as an animation of the corresponding barrier tree sequence. The animation is created as an adaption of the foresight layout with tolerance algorithm for dynamic graph layout. The adaptation requires changes to the concept of supergraph and it layout. The thesis finishes with some thoughts on how these approaches can be combined and how the task the application should support can help inform the choice of visualization modality

OBSERVED NONLINEAR RESPONSES IN PATTERNED SUPERCONDUCTING, FERROMAGNETIC, AND INTERACTING THIN FILMS

Many advances in technology ranging from biology and medicine through engineering and computer science to fundamental physics and chemistry depend upon the capability to control the fabrication of materials and devices at the submicron scale. Quantum mechanical effects become increasingly important to atomic and molecular interactions as the distances between neighbors decrease. These effects will provide materials and device designers with additional flexibility to establish properties of the designers choice, but the cost of this additional flexibility must be paid in the complexity of nonlinearities entering the interactions and the design process. The work presented here has provided several early results on three such interactions among closely-spaced submicron material structures: 1) the properties of superconductivity have been studied, 2) the properties of ferromagnetism have been studied, and 3) the interactions between superconductivity and ferromagnetism have been studied. Since our work was published, there has been considerable interest in all three of these wide-open areas and hundreds or thousands of additional results are now in the literature. We have used standard methods from the semiconductor industry as well as innovative methods to fabricate micron and submicron devices for observation. Standard optical lithography and standard electron beam lithography have been implemented to shape micron and submicron structures, respectively. Additionally, a laser interferometric lithography method has been invented and used to shape submicron structures. The materials used were vanadium, niobium, nickel, and/or permalloy. We have utilized SQUID magnetometry and Hall effect magnetometry to observe the properties of superconductor structures and superconductorferromagnetic mixed systems. We have used SQUID magnetometry and ferromagnetic resonance to observe the physical properties of ferromagnetic structures and the interactions between adjacent structures. Using these materials and methods we have discovered an unusual paramagnetic Meissner effect in thin Nb films that exists at igh-applied magnetic fields. We have discovered fluxoid matching anomalies at low sample temperature. And we have discovered interactions between electron exchange and magnetic dipole forces. Additionally, we have found clear evidence to support several past hypotheses advanced by other authors