Crystal growth and characterization of doped perovskite titanates ATiO3 and single-layered manganites R1-xA1+xMnO4

In this thesis, different single-crystalline perovskite-type transition metal oxides are studied. Two of these materials are doped perovskite titanates with an insulating parent compound. Commercial Sr1-xCaxTiO3 single crystals with a calcium content of x = 0.009 and charge-carrier densities tuned from the insulating, ferroelectric parent material to n ≃ 60⋅10¹⁹ 1/cm³ are investigated by thermal-expansion measurements. Pronounced α/T anomalies, signaling the ferroelectric transition of pristine Sr1-xCaxTiO3, persist upon increasing charge-carrier density n in reduced Sr1-xCaxTiO3-δ, confirming earlier findings from resistivity measurements. Though, in contrast to the latter, the anomalies in α/T do not completely vanish as a function of n but change in character at a certain threshold carrier density n* ≈ 1.3⋅10¹⁹ 1/cm³. This might indicate a structural phase transition when crossing n*, analogously to the parent compound Sr1-xCaxTiO3 that changes from non-centrosymmetric ferroelectric to antiferroelectric as a function of x involving the recovery of an inversion center. However, an almost vertical phase boundary in a TC(n) diagram is not easily detectable by temperature-dependent measurements like thermal expansion. This puzzle can only be resolved by structural analyses of Sr1-xCaxTiO3-δ crystals with carrier concentrations on both sides of n*. A sign change of α as a function of n, as is expected for a quantum phase transition where n is the control parameter, could not be observed. Single crystals of EuTiO3 are grown by the floating-zone method and characterized. Similar to Sr1-xCaxTiO3, the material's properties strongly depend on the actual oxygen content. The as-grown crystal is found to be oxygen-excessive by comparing its saturation magnetization to that of a pure Eu2+ system. Other key parameters as Néel temperature, Weiss temperature, and the Barrett behavior of the permittivity agree with literature values. To induce metallic conductivity, pieces of the as-grown EuTiO3 crystal were reduced by annealing. The metallic EuTiO3-δ samples show the full saturation magnetization of 7 µB. A metal-insulator transition is induced being similar to that in SrTiO3-δ but appearing at a charge-carrier concentration nc larger by a factor of 10⁴ due to the smaller permittivity of EuTiO3 implying a smaller effective Bohr radius aB*. Doped crystals of EuTiO3, SrTiO3, and KTaO3 obey a scaling behavior ∛(nc)⋅aB* = K resembling the original Mott criterion for a metal-insulator transition but at a much larger K ≈ 10. At low temperature, the electronic mobility of metallic EuTiO3-δ and SrTiO3-δ systematically increases upon decreasing n across both materials. For the first time, an AT² resistivity is observed in metallic EuTiO3-δ, which is similar to that reported for doped SrTiO3. The T² prefactor A scales with the charge-carrier density n and its absolute values match that of doped SrTiO3 with the same n. A simple three-band model is used to describe the A(n) scaling of both materials over a large range of n. Several single-layered manganites R1-xA1+xMnO4 with various element combinations R/A and doping levels x were grown in single-crystalline form to outperform the doping range of the parent compound La1-xSr1+xMnO4 and shift the limit for x to new heights, where a maximum of x = 0.73 is achieved in Pr1-xSr1+xMnO4. The transition to the charge and orbital order at TCOO signals itself by characteristic features in resistivity, magnetization, and heat capacity. The tetragonal R1-xSr1+xMnO4 systems with a large ion-size mismatch exhibit comparably low transition temperatures, but their respective TCOO systematically increases as a function of x. In contrast, systems with small ion-size disorder like Pr1-xCa1+xMnO4 and Nd1-xCa1+xMnO4 show the highest transition temperatures TCOO of all investigated manganites. In these two compounds, the evolution of TCOO with x is not monotonic as it is observed for Sr manganites. Instead, the highest transition temperatures are reached at commensurate doping levels x = 1/2, 2/3

Photoluminescence quantum efficiency investigations on thin film light-emitting diodes during electrical operation

Abstract: A photoluminescence measurement setup was constructed to investigate on the optical emitter properties of thin film light-emitting diodes during electrical operation. Firstly, investigations - with this setup - on organic LEDs containing either phosphorescent Ir-based molecules or TADF based emitters were compared. Furthermore, the influence of polar materials used in adjacent charge transport layers was studied. The introduction of a sizeable Giant Surface Potential lead to a significant amount of Photoluminescence Quantum Efficiency reduction already prior to the electroluminescent turn-on of the LED. Secondly, the performance of Quantum-Dot-LEDs (QDLEDs) based on InP/ZnSe/ZnS Core-Shell-Shell structures was investigated under electrical bias. While a considerable amount of field induced efficiency losses was observed in the reverse bias regime, a developed theoretical model clearly identified Auger quenching as the dominant loss process under forward operation. Observed metastabilities in these QD-based systems show the need for further research in this field, as a model to explain these effects could be proposed to be linked to ion migration in the QD- and adjacent charge transport layers, however definitive proof is still missing

Evaluating State-of-the-Art Web Component Frameworks

Websites are getting more complex over the years because it is not just used to display information, but to use some applications. One challenge is to maintain these services and keep them up to date. To reuse existing code, Web Components are introduced to outsource complex structures as an HTML element including its functionality. Therefore, some frameworks available which help to create Web Components by providing some useful tools and make the development process easier for developers. Because there are numerous different frameworks available for the development, it is not easy to find the right one for the own project. Because the standards are changing fast in computer science, the development process should always be State-of-the-Art. The aim of this thesis is to give a brief overview over Web Component frameworks and find out which framework is a good choice for given use cases. First some frameworks are introduced. Three selected frameworks are more detailed introduced. For the evaluation of these frameworks, the used criteria are introduced. With the use of the analytical hierarchy process, three scenarios are evaluated to get the best framework for each scenario. This shows that when the requirements differ, also the choice of the framework can change

Parallel Algorithms Align with Neural Execution

Neural algorithmic reasoners are parallel processors. Teaching them sequential algorithms contradicts this nature, rendering a significant share of their computations redundant. Parallel algorithms however may exploit their full computational power, therefore requiring fewer layers to be executed. This drastically reduces training times, as we observe when comparing parallel implementations of searching, sorting and finding strongly connected components to their sequential counterparts on the CLRS framework. Additionally, parallel versions achieve strongly superior predictive performance in most cases.Comment: 8 pages, 5 figures, To appear at the KLR Workshop at ICML 202

Genese und Struktur der Erkenntnistheorie Rudolf Steiners mit besonderer Rücksicht auf ihre Fundierung in Denk- und Bewusstseinserlebnissen

Die vorliegende Arbeit beschäftigt sich mit Genese und Struktur der Erkenntnistheorie Rudolf Steiners. Zunächst von Goethes Naturwissenschaften ausgehend, versuchte Steiner mit seinen erkenntnistheoretischen Arbeiten die Frage zu klären, in wie weit das menschliche Erkenntnisvermögen in der Lage ist, die Welt zu erkennen, wie sie wirklich ist. Steiner sucht den Weg aus der Subjektivität heraus, in die sich die Philosophie spätestens mit Kant eingesponnen hatte. Er unterzieht dafür das Denken einer phänomenologischen Untersuchung. Dabei bedient er sich der Methode der Denk-Beobachtung, die dem Menschen ein neues Organ aufschließen soll, das ihn das Denken seinem Wesen nach erkennbar macht. Das Denken erweist sich dabei als transsubjektiv, da es überhaupt erst die Gegensätze von Subjekt und Objekt konstituiert. Die menschliche Organisation bzw. das gewöhnliche menschliche Bewusstsein löscht für den ersten Anblick der Dinge aus, was ihrem Wesen nach zu ihnen gehört. Die rein sinnliche Welt wird damit zum Schein, zur Illusion, die der ideellen Ergänzung bedarf, um Wirklichkeit zu werden. Im Gegensatz zu einem platonischen Idealismus sieht Steiner aber nicht im Ideellen die volle Wirklichkeit, sondern immer in der Einheit von Idee und Anschauung. Dadurch erweist sich Steiners Erkenntnistheorie und das aus ihr fließende Wissenschaftsverständnis als unbedingt empirisch. Steiner versucht mit seiner Erkenntnistheorie auf dem Weg philosophischer Argumentation zu dem Zusammenhang zwischen Ideellem und Sinnlichen hinzuführen. Die eigentliche Einholung dieses Zusammenhanges kann aber letztlich nur empirisch über die Erfahrungen der Denk-Beobachtung erfolgen. Steiner erweist sich mit seiner Denk-Beobachtung schließlich auch als Vorläufer der Denkpsychologischen Experimente zu Beginn des 20. Jahrhunderts

Mature dendritic cells infected with canarypox virus elicit strong anti-human immunodeficiency virus CD8+ and CD4+ T-cell responses from chronically infected individuals

Recombinant canarypox virus vectors containing human immunodeficiency virus type 1 (HIV-1) sequences are promising vaccine candidates, as they replicate poorly in human cells. However, when delivered intramuscularly the vaccines have induced inconsistent and in some cases transient antigen-specific cytotoxic T-cell (CTL) responses in seronegative volunteers. An attractive way to enhance these responses would be to target canarypox virus to professional antigen-presenting cells such as dendritic cells (DCs). We studied (i) the interaction between canarypox virus and DCs and (ii) the T-cell responses induced by DCs infected with canarypox virus vectors containing HIV-1 genes. Mature and not immature DCs resisted the cytopathic effects of canarypox virus and elicited strong effector CD8+ T-cell responses from chronically infected HIV+ individuals, e.g., cytolysis, and secretion of gamma interferon (IFN-γ) and β-chemokines. Furthermore, canarypox virus-infected DCs were \u3e30-fold more efficient than monocytes and induced responses that were comparable to those induced by vaccinia virus vectors or peptides. Addition of exogenous cytokines was not necessary to elicit CD8+ effector cells, although the presence of CD4+ T cells was required for their expansion and maintenance. Most strikingly, canarypox virus-infected DCs were directly able to stimulate HIV-specific, IFN-γ-secreting CD4 helper responses from bulk as well as purified CD4+ T cells. Therefore, these results suggest that targeting canarypox virus vectors to mature DCs could potentially elicit both anti-HIV CD8+ and CD4+ helper responses in vivo

Lubricating Oil Consumption Measurement on Large Gas Engines

Increasing the reliability of combustion engines while further reducing emissions and life cycle costs are the main drivers for optimizing lubricating oil consumption (LOC). However, in order to reduce the lube oil consumption of an engine, it is crucial to measure it accurately. Therefore, a LOC measurement device based on the use of the stable isotope deuterium has been developed. Previous publications have focused on the use of passenger car engines. This publication describes the first application of this newly developed method on a large gas engine. This is of particular interest as large-bore engines might show different oil consumption behavior, much higher LOC in gram per hour and the bigger oil reservoir need larger amounts of tracer. Additionally, a different type of fuel has an effect on oil consumption measurement as well, as presented in this paper. The results showed this method can be applied to large gas engines as well after conducting minor changes to the measurement setup. However, other than liquid fuels, the origin and isotopic composition of the natural gas has to be monitored. Ideally, gas from large storage is used for carrying out these measurements

Deuterium Tracer for Accurate Online Lube-Oil-Consumption Measurement: Stability, Compatibility and Tribological Characteristics

Because of the impact of lubrication on the efficiency and the lifecycle cost and emissions, the lubricating-oil consumption (LOC) is one of the key indicators in the research and development of internal combustion engines. State-of-the-art methods for LOC measurement are based on the use of a certain tracer to track the oil consumption. However, all of the currently available tracers have their downsides (e.g., the use of a radioactive tracer, corrosive emissions, etc.). Therefore, in the course of this research project, a new tracer substance that is based on a stable nonradioactive isotope of hydrogen—deuterium—was developed and tested thoroughly. The LOC is monitored by a hydrogen/deuterium isotopic ration in the exhaust gas by using an isotopic water analyzer. Tribologically important properties, such as the viscosity, stability, and compatibility of the tracer were investigated by laboratory experiments by using several tools, such as infrared spectroscopy, gas chromatography, thermogravimetry, etc. The properties relevant to the applicability of the method, such as the accuracy and the reproducibility, were investigated by engine test-bench experiments. Finally, long-term stability tests of the tracer were conducted with a field test