Ultrafast Electron Diffraction at Surfaces

In der vorliegenden Arbeit wird der Aufbau eines Experimentes zur Untersuchung der atomaren Dynamik an Kristalloberflächen mittels zeitaufgelöster Elektronenbeugung beschrieben. Dabei wird die zu untersuchende Probe mit Hilfe von 50 fs kurzen Infrarot-Laserpulsen optisch angeregt. Nach einer variablen Zeitspanne von einigen ps bis wenigen ns vor oder nach der optischen Anregung erfolgt die Abfrage des Momentanzustandes der Probe durch Streuung eines wenige ps kurzen Elektronenpulses an der Oberfläche. Aus der Intensitätsverteilung des dabei entstehenden Beugungsbildes lassen sich Rückschlüsse auf die Größe und Symmetrie der Einheitszelle, sowie auf die Temperatur der Oberfläche ziehen. Durch eine Aneinanderreihung derartiger Momentaufnahmen ist es möglich, die Relaxation des Kristallgitters nach der optischen Anregung der Oberfläche zu rekonstruieren. Das Kernstück des experimentellen Aufbaus bildet eine Elektronenkanone, in der ein 50 fs-kurzer Ultraviolett-Laserpuls durch Photoemission aus einem wenige nm dünnen Au-Film in einen ps-Elektronenpuls konvertiert wird. Die eingehende Charakterisierung der Photokathode, mit der die Konversion realisiert wird, lieferte Erkenntnisse, die auch für den Aufbau nachfolgender, verbesserter Elektronenkanonen wertvoll sein können. Die erreichbare Zeitauflösung der beschriebenen Beugungsexperimente beträgt etwa 20-30 ps. Sie ist bedingt durch den flachen Einfallswinkel der Elektronen auf die Probe, der erforderlich ist, um die Oberflächenempfindlichkeit der Messung zu gewährleisten. Als erstes Untersuchungsobjekt für die zeitaufgelösten Beugungsexperimente diente ein 5.5 nm dünner, epitaktischer Bi-Film auf einem Si(001)-Substrat, das während der Messung auf 85 K abgekühlt wurde. Die zeitliche Entwicklung der Oberflächentemperatur nach der Absorption des optischen Anregungspulses folgt keinem simplen Wärmeleitungsmodell. Stattdessen läßt sich die beobachtete exponentielle Relaxation der Oberflächentemperatur mit einer Zeitkonstante von etwa 640 ps qualitativ durch die Existenz einer endlichen Grenzflächenwärmeleitfähigkeit zwischen dem Bi-Film und dem Si-Substrat erklären. Diese ist auf die Unstetigkeit der Schallgeschwindigkeiten und Massendichten von Bi und Si zurückzuführen, die zur Reflexion eines Großteils der Phononen führen, die aus dem angeregten Bi-Film kommend auf die Grenzfläche treffen. Die Wärmeleitfähigkeit der Bi/Si-Grenzfläche wurde im Rahmen zweier einfacher Modelle berechnet und mit dem experimentell bestimmten Wert verglichen. Dabei betrug die Abweichung zwischen Experiment und Modell nur 30%, was – verglichen mit der Gesamtheit der Untersuchungen zu dieser Thematik – eine recht gute Übereinstimmung darstellt. Dieser Umstand wird auf die abrupte und glatte Bi/Si-Grenzfläche und die geringe Dichte von Gitterfehlern im Bi-Film zurückgeführt. Dünne Bi-Filme auf Si(001) stellen daher ein Modellsystem dar, an dem die Phononendynamik in Nanostrukturen ohne unnötige Komplikationen studiert werden kann. Da im vorliegenden Fall die Bi-Schichtdicke geringer als die mittlere freie Weglänge der Phononen ist, stellt sich für weitergehende Untersuchungen die Frage nach dem Einfluss der verschiedenen Phononen-Streuprozesse auf die Transmission der Phononen über die Bi/Si-Grenzfläche und somit auf das Abkühlverhalten des Bi-Filmes.In this thesis, the construction of a time-resolved electron diffraction experiment for studies of crystal surfaces is described. The setup is used to examine the atomic dynamics at surfaces after an initial optical excitation by a 50 fs-infrared laser pulse. To detect the transient state of the excited surface, a picosecond-electron pulse is directed onto the surface with a variable delay in the range of a few picoseconds up to some nanoseconds before or after the pump pulse. From the resulting diffraction pattern one can deduce information about the symmetry and size of the unit cell as well as the thermal motion of the surface atoms. Based on a series of diffraction patterns it is possible to reconstruct the transient evolution of the surface after the optical excitation. The core item of the experimental setup is the electron gun in which a 50 fs-ultraviolet laser pulse is converted into an electron bunch by photoemission from a thin Au film. The thorough characterisation of the Au photocathode delivered results which are be valuable for the construction of future improved electron guns. The achievable time resolution in the diffraction experiments reported in this thesis is in the range of 20-30 ps. It is limited by the grazing incidence of the electrons onto the crystal surface. However, a shallow incidence angle is necessary to achieve the desired surface-sensitivity of the diffraction experiment. In the first time-resolved studies the transient heating of a 5.5 nm thin, epitaxial Bi film on a Si(001) substrate was examined. The sample was held at a base temperature of 85 K during these experiments. The resulting transient surface temperature cannot be described by a simple heat conduction model. The surface temperature rather decreases exponentially with a time-constant of 640 ps, which can be explained qualitatively by the existence of a finite thermal boundary conductivity between the Bi film and the Si substrate. This barrier is caused by the abrupt change of the phonon velocities and mass densities at the interface, which leads to total internal reflection of the majority of phonons impinging onto the interface, starting from the Bi film. For a comparison with the experimental results, the thermal boundary conductivity was calculated based on two simple models. The deviation between the experiment and the two models was only about 30 %, which is a very good agreement, compared to many other studies in this field. This agreement can be explained by the smooth and abrupt Bi/Si interface and the low defect density in the Bi film. Due to the absence of many complicating effects, thin Bi films can be regarded as a model system to study the phonon dynamics in nanostructures. In the experiments described in this thesis, the phonons' mean free path in the Bi film was larger than the film thickness. This leads to the question in which way phonon scattering processes affect the phonon flux across the Bi/Si interface and thus the cooling of the Bi film

APPLYING SITUATION-SERVICE FIT TO PHYSICAL ENVIRONMENTS ENHANCED BY UBIQUITOUS INFORMATION SYSTEMS

Ubiquitous Information Systems (UIS) embedded in everyday environments are required to provide means for supporting complex behavioral task requirements by users. With the concept of a situation, we propose a knowledge level that can be used by users for understanding UIS-enhanced environments. It is discussed how the concept of situation differs from the concept of context. Situations are used to identify supporting services. At the core of this paper, we present how the Situation-Service Fit construct is used in early design stages (study 1) and later during the prototype phase (study 2). In study 2 we introduce an instrument for three more detailed fit constructs, i.e., Behavior-Service Fit, Modality-Service Fit, and Spatial-Service Fit. By additional evaluation of constructs known from various technology acceptance theories, we provide an innovative instrument for investigating UIS during the whole design cycle

Signal-to-noise ratio of temperature measurement with Cernox sensors at various supply currents

The Karlsruhe Institute of Technology (KIT) has developed a new thermal method for flow measurement, which is particularly suitable for the application in cryogenic systems. In this method, the stability and the resolution of temperature measurement is important, rather than precision. In other words, constant offsets in temperature measurements can be ignored, and the temperature sensors can be operated at supply currents beyond their nominal design value in order to gain resolution. For this application, the performance of two Cernox TM type CX-1050-SD-HT-1.4L sensors was measured in a temperature range between 300 K and 4 K. The experiments were carried out in the calibration cryostat at the Institute for Technical Physics. Sensors were connected to a Lake Shore Model 121 current source and a Keithley 2701/E digital multimeter for voltage measurements. At constant calibration temperatures, the supply currents were varied such that the resulting voltage drops lay in-between 10 mV and 100 mV. The influence on both the noise and the temperature offset are presented

Wirkungen und Grenzen von Moscheeführungen: Empirische Beobachtungen und Herausforderungen

Janzen O, Salentin K, Zick A. Wirkungen und Grenzen von Moscheeführungen: Empirische Beobachtungen und Herausforderungen. Demokratie gegen Menschenfeindlichkeit. 2016;(2)

Structure-guided mutational analysis reveals the functional requirements for product specificity of DOT1 enzymes

DOT1 enzymes are conserved methyltransferases that catalyse the methylation of lysine 79 on histone H3 (H3K79). Most eukaryotes contain one DOT1 enzyme, whereas African trypanosomes have two homologues, DOT1A and DOT1B, with different enzymatic activities. DOT1A mediates mono-and dimethylation of H3K76, the homologue of H3K79 in other organisms, whereas DOT1B additionally catalyses H3K76 trimethylation. However, it is unclear how these different enzymatic activities are achieved. Here we employ a trypanosomal nucleosome reconstitution system and structure-guided homology modelling to identify critical residues within and outside the catalytic centre that modulate product specificity. Exchange of these residues transfers the product specificity from one enzyme to the other, and reveals the existence of distinct regulatory domains adjacent to the catalytic centre. Our study provides the first evidence that a few crucial residues in DOT1 enzymes are sufficient to catalyse methyl-state-specific reactions. These results might also have far-reaching consequences for the functional understanding of homologous enzymes in higher eukaryotes

Structure-guided mutational analysis reveals the functional requirements for product specificity of DOT1 enzymes

DOT1 enzymes are conserved methyltransferases that catalyse the methylation of lysine 79 on histone H3 (H3K79). Most eukaryotes contain one DOT1 enzyme, whereas African trypanosomes have two homologues, DOT1A and DOT1B, with different enzymatic activities. DOT1A mediates mono-and dimethylation of H3K76, the homologue of H3K79 in other organisms, whereas DOT1B additionally catalyses H3K76 trimethylation. However, it is unclear how these different enzymatic activities are achieved. Here we employ a trypanosomal nucleosome reconstitution system and structure-guided homology modelling to identify critical residues within and outside the catalytic centre that modulate product specificity. Exchange of these residues transfers the product specificity from one enzyme to the other, and reveals the existence of distinct regulatory domains adjacent to the catalytic centre. Our study provides the first evidence that a few crucial residues in DOT1 enzymes are sufficient to catalyse methyl-state-specific reactions. These results might also have far-reaching consequences for the functional understanding of homologous enzymes in higher eukaryotes

Stability of the replica-symmetric saddle-point in general mean-field spin-glass models

Within the replica approach to mean-field spin-glasses the transition from ergodic high-temperature behaviour to the glassy low-temperature phase is marked by the instability of the replica-symmetric saddle-point. For general spin-glass models with non-Gaussian field distributions the corresponding Hessian is a $2^n\times 2^n$ matrix with the number $n$ of replicas tending to zero eventually. We block-diagonalize this Hessian matrix using representation theory of the permutation group and identify the blocks related to the spin-glass susceptibility. Performing the limit $n\to 0$ within these blocks we derive expressions for the de~Almeida-Thouless line of general spin-glass models. Specifying these expressions to the cases of the Sherrington-Kirkpatrick, Viana-Bray, and the L\'evy spin glass respectively we obtain results in agreement with previous findings using the cavity approach

Isotopic study of Raman active phonon modes in β-Ga2O3

Holding promising applications in power electronics, the ultra-wide band gap material gallium oxide has emerged as a vital alternative to materials like GaN and SiC. The detailed study of phonon modes in β-Ga2O3 provides insights into fundamental material properties such as crystal structure and orientation and can contribute to the identification of dopants and point defects. We investigate the Raman active phonon modes of β-Ga2O3 in two different oxygen isotope compositions (16O,18O) by experiment and theory: By carrying out polarized micro-Raman spectroscopy measurements on the (010) and (-201) planes, we determine the frequencies of all 15 Raman active phonons for both isotopologues. The measured frequencies are compared with the results of density functional perturbation theory (DFPT) calculations. In both cases, we observe a shift of Raman frequencies towards lower energies upon substitution of 16O with 18O. By quantifying the relative frequency shifts of the individual Raman modes, we identify the atomistic origin of all modes (Ga-Ga, Ga-O or O-O) and present the first experimental confirmation of the theoretically calculated energy contributions of O lattice sites to Raman modes. The DFPT results enable the identification of Raman modes that are dominated by the different, inequivalent O- or Ga-atoms of the unit cell. We find that oxygen substitution on the OII site leads to an elevated relative mode frequency shift compared to OI and OIII sites. This study presents a blueprint for the future identification of different point defects in Ga2O3 by Raman spectroscopy