Renormalization of fermion mixing

Precision measurements of phenomena related to fermion mixing require the inclusion of higher order corrections in the calculation of corresponding theoretical predictions. For this, a complete renormalization scheme for models that allow for fermion mixing is highly required. The correct treatment of unstable particles makes this task difficult and yet, no satisfactory and general solution can be found in the literature. In the present work, we study the renormalization of the fermion Lagrange density with Dirac and Majorana particles in models that involve mixing. The first part of the thesis provides a general renormalization prescription for the Lagrangian, while the second one is an application to specific models. In a general framework, using the on-shell renormalization scheme, we identify the physical mass and the decay width of a fermion from its full propagator. The so-called wave function renormalization constants are determined such that the subtracted propagator is diagonal on-shell. As a consequence of absorptive parts in the self-energy, the constants that are supposed to renormalize the incoming fermion and the outgoing antifermion are different from the ones that should renormalize the outgoing fermion and the incoming antifermion and not related by hermiticity, as desired. Instead of defining field renormalization constants identical to the wave function renormalization ones, we differentiate the two by a set of finite constants. Using the additional freedom offered by this finite difference, we investigate the possibility of defining field renormalization constants related by hermiticity. We show that for Dirac fermions, unless the model has very special features, the hermiticity condition leads to ill-defined matrix elements due to self-energy corrections of external legs. In the case of Majorana fermions, the constraints for the model are less restrictive. Here one might have a better chance to define field renormalization constants related by hermiticity. After analysing the complete renormalized Lagrangian in a general theory including vector and scalar bosons with arbitrary renormalizable interactions, we consider two specific models: quark mixing in the electroweak Standard Model and mixing of Majorana neutrinos in the seesaw mechanism. A counter term for fermion mixing matrices can not be fixed by only taking into account self-energy corrections or fermion field renormalization constants. The presence of unstable particles in the theory can lead to a non-unitary renormalized mixing matrix or to a gauge parameter dependence in its counter term. Therefore, we propose to determine the mixing matrix counter term by fixing the complete correction terms for a physical process to experimental measurements. As an example, we calculate the decay rate of a top quark and of a heavy neutrino. We provide in each of the chosen models sample calculations that can be easily extended to other theories.

Monte-Carlo-Simulationen zum Phasen- und Keimbildungsverhalten von Polymerlösungen

Das Phasenverhalten und die Grenzflächeneigenschaften vonPolymeren in superkritischer Lösung werden anhand einesvergröberten Kugel-Feder-Modells für das ReferenzsystemHexadekan-CO2 untersucht. Zur Bestimmung der Parameter imPotential setzt man die kritischen Punkte von Simulation undExperiment gleich. Wechselwirkungen zwischen beidenKomponenten werden durch eine modifizierteLorentz-Berthelot-Regel modelliert. Die Übereinstimmung mitden Experimenten ist sehr gut - insbesondere kann dasPhasendiagramm des Mischsystems inklusive kritischer Linienreproduziert werden. Ein Vergleich mit numerischenStörungsrechnungen (TPT1) liefert eine qualitativeÜbereinstimmung und Hinweise zur Verbesserung derverwendeten Zustandsgleichung. Aufbauend auf diesen Betrachtungen werden die Frühstadiender Keimbildung untersucht. Für das Lennard-Jones-Systemwird zum ersten Mal der Übergang vom homogenen Gas zu einemeinzelnen Tropfen im endlichen Volumen direkt nachgewiesenund quantifiziert. Die freie Energie von kleinen Clusternwird mit einem einfachen, klassischen Nukleationsmodellbestimmt und nach oben abgeschätzt. Die vorgestellten Untersuchungen wurden durch eineWeiterentwicklung des Umbrella-Sampling-Algorithmusermöglicht. Hierbei wird die Simulation in mehrereSimulationsfenster unterteilt, die nacheinander abgearbeitetwerden. Die Methode erlaubt eine Bestimmung derFreien-Energie-Landschaft an einer beliebigen Stelle desPhasendiagramms. Der Fehler ist kontrollierbar undunabhängig von der Art der Unterteilung.

Comparison of model potentials for molecular dynamics simulation of crystalline silica

Simulationen von SiO2 mit dem von van Beest, Kramer und vanSanten (BKS) entwickelten Paarpotenzial erzeugen vielezufriedenstellende Ergebnisse, aber auch charakteristischeSchwachstellen. In dieser Arbeit wird das BKS-Potenzial mitzwei kürzlich vorgeschlagenen Potenzialen verglichen, dieeffektiv Mehrteilchen-Wechselwirkungen beinhalten. Der ersteAnsatz erlaubt dazu fluktuierende Ladungen, der zweiteinduzierbare Polarisierungen auf den Sauerstoffatomen. Die untersuchten Schwachstellen des BKS Potenzialsbeinhalten das Verhältnis der zwei Gitterkonstanten a und cim Quarzübergang, das von BKS falsch beschrieben wird.Cristobalit und Tridymit erscheinen instabil mit BKS.Weiterhin zeigt die BKS-Zustandsdichte charakteristischeAbweichungen von der wahren Zustandsdichte. DerÜbergangsdruck für den Stishovit I-II Übergang wird deutlichüberschätzt. Das Fluktuierende-Ladungs-Modell verbesserteinige der genannten Punkte, reproduziert aber viele andereEigenschaften schlechter als BKS. DasFluktierende-Dipol-Modell dagegen behebt alle genanntenArtefakte. Zusätzlich wird der druckinduzierte Phasenübergang imalpha-Quarz untersucht. Alle Potentiale finden die selbeStruktur für Quarz II. Bei anschliessender Dekompressionerzeugt BKS eine weitere Phase, während die beiden anderenPotentiale wieder zum alpha-Quarz zurückkehren. Weiterhinwerden zwei Methoden entwickelt, um die piezoelektrischenKonstanten bei konstantem Druck zu bestimmen. Die Ergebnissegeben Hinweise auf eine möglicherweisenicht-elektrostatische Natur der Polarisierungen imFluktuierende-Dipole-Modell. Mit dieser Interpretation scheint das Fluktuierende-DipolPotential alle verfügbaren experimentellen Daten am bestenvon allen drei untersuchten Ansätzen zu reproduzieren.

Lagrangian transport evaluation of atmospheric chemistry in the Mediterranean region

Global observations of the chemical composition of the atmosphere are essential for understanding and studying the present and future state of the earth's atmosphere. However, by analyzing field experiments the consideration of the atmospheric motion is indispensable, because transport enables different chemical species, with different local natural and anthropogenic sources, to interact chemically and so consequently influences the chemical composition of the atmosphere. The distance over which that transport occurs is highly dependent upon meteorological conditions (e.g., wind speed, precipitation) and the properties of chemical species itself (e.g., solubility, reactivity). This interaction between chemistry and dynamics makes the study of atmospheric chemistry both difficult and challenging, and also demonstrates the relevance of including the atmospheric motions in that context. In this doctoral thesis the large-scale transport of air over the eastern Mediterranean region during summer 2001, with a focus on August during the Mediterranean Intensive Oxidant Study (MINOS) measurement campaign, was investigated from a lagrangian perspective. Analysis of back trajectories demonstrated transport of polluted air masses from western and eastern Europe in the boundary layer, from the North Atlantic/North American area in the middle end upper troposphere and additionally from South Asia in the upper troposphere towards the eastern Mediterranean. Investigation of air mass transport near the tropopause indicated enhanced cross-tropopause transport relative to the surrounding area over the eastern Mediterranean region in summer. A large band of air mass transport across the dynamical tropopause develops in June, and is shifted toward higher latitudes in July and August. This shifting is associated with the development and the intensification of the Arabian and South Asian upper-level anticyclones and consequential with areas of maximum clear-air turbulence, hypothesizing quasi-permanent areas with turbulent mixing of tropospheric and stratospheric air during summer over the eastern Mediterranean as a result of large-scale synoptic circulation. In context with the latex knowledge about the transport of polluted air masses towards the Mediterranean and with increasing emissions, especially in developing countries like India, this likely gains in importance.

Development of empirical potentials for amorphous silica

Development of empirical potentials for amorphous silica Amorphous silica (SiO2) is of great importance in geoscience and mineralogy as well as a raw material in glass industry. Its structure is characterized as a disordered continuous network of SiO4 tetrahedra. Many efforts have been undertaken to understand the microscopic properties of silica by classical molecular dynamics (MD) simulations. In this method the interatomic interactions are modeled by an effective potential that does not take explicitely into account the electronic degrees of freedom. In this work, we propose a new methodology to parameterize such a potential for silica using ab initio simulations, namely Car-Parrinello (CP) method [Phys. Rev. Lett. 55, 2471 (1985)]. The new potential proposed is compared to the BKS potential [Phys. Rev. Lett. 64, 1955 (1990)] that is considered as the benchmark potential for silica. First, CP simulations have been performed on a liquid silica sample at 3600 K. The structural features so obtained have been compared to the ones predicted by the classical BKS potential. Regarding the bond lengths the BKS tends to underestimate the Si-O bond whereas the Si-Si bond is overestimated. The inter-tetrahedral angular distribution functions are also not well described by the BKS potential. The corresponding mean value of theSiOSi angle is found to be ≃ 147◦, while the CP yields to aSiOSi angle centered around 135◦. Our aim is to fit a classical Born-Mayer/Coulomb pair potential using ab initio calculations. To this end, we use the force-matching method proposed by Ercolessi and Adams [Europhys. Lett. 26, 583 (1994)]. The CP configurations and their corresponding interatomic forces have been considered for a least square fitting procedure. The classical MD simulations with the resulting potential have lead to a structure that is very different from the CP one. Therefore, a different fitting criterion based on the CP partial pair correlation functions was applied. Using this approach the resulting potential shows a better agreement with the CP data than the BKS ones: pair correlation functions, angular distribution functions, structure factors, density of states and pressure/density were improved. At low temperature, the diffusion coefficients appear to be three times higher than those predicted by the BKS model, however showing a similar temperature dependence. Calculations have also been carried out on crystalline samples in order to check the transferability of the potential. The equilibrium geometry as well as the elastic constants of α-quartz at 0 K are well described by our new potential although the crystalline phases have not been considered for the parameterization. We have developed a new potential for silica which represents an improvement over the pair potentials class proposed so far. Furthermore, the fitting methodology that has been developed in this work can be applied to other network forming systems such as germania as well as mixtures of SiO2 with other oxides (e.g. Al2O3, K2O, Na2O).

Travelling emblems of power: the Ghanaian ‘Seat of State’ /// Arbeitspapiere / Institut für Ethnologie und Afrikastudien ; 94

This paper presents a case study of the self-confident and creative fusion of European and African political symbols and rituals that is characteristic of Ghanaian statehood and nation-making. It explores the aesthetic and historical genealogy of the Ghanaian ‘Seat of State’, a throne-like stool on which the President sits when attending Parliament on important state occasions. The Seat was crafted in the early 1960s by Kofi Antubam, one of the chief ‘state artists’ during the Nkrumah regime, and incorporates symbols of Asante royal authority, European aristocratic imagery as well as Ghanaian neo-traditional emblems such as the Black Star. The discussion of the Seat of State’s political meaning is followed by some more general observations on the history of party politics and parliamentary procedure in Ghana as examples of travelling political paradigms.

Petrogenetic evolution of Uralian-Alaskan-type mafic-ultramafic complexes in the southern and middle Urals, Russia

The PhD thesis at hand consists of three parts and describes the petrogenetic evolution of Uralian-Alaskan-type mafic ultramafic complexes in the Ural Mountains, Russia. Uralian-Alaskan-type mafic-ultramafic complexes are recognized as a distinct class of intrusions. Characteristic petrologic features are the concentric zonation of a central dunite body grading outward into wehrlite, clinopyroxenite and gabbro, the absence of orthopyroxene and frequently occurring platinum group element (PGE) mineralization. In addition, the presence of ferric iron-rich spinel discriminates Uralian-Alaskan-type complexes from most other mafic ultramafic rock assemblages. The studied Uralian-Alaskan-type complexes (Nizhnii Tagil, Kytlym and Svetley Bor) belong to the southern part of a 900 km long, N–S-trending chain of similar intrusions between the Main Uralian Fault to the west and the Serov-Mauk Fault to the east. The first chapter of this thesis studies the evolution of the ultramafic rocks tracing the compositional variations of rock forming and accessory minerals. The comparison of the chemical composition of olivine, clinopyroxene and chromian spinel from the Urals with data from other localities indicates that they are unique intrusions having a characteristic spinel and clinopyroxene chemistry. Laser ablation-ICPMS (LA-ICPMS ) analyses of trace element concentrations in clinopyroxene are used to calculate the composition of their parental melt which is characterized by enriched LREE (0.5-5.2 prim. mantle) and other highly incompatible elements (U, Th, Ba, Rb) relative to the HREE (0.25-2.0 prim. mantle). A subduction-related geotectonic setting is indicated by a positive anomaly for Sr and negative anomalies for Ti, Zr and Hf. The mineral compositions monitor the evolution of the parental magmas and decipher differences between the studied complexes. In addition, the observed variation in LREE/HREE (for example La/Lu = 2-24) can be best explained with the model of an episodically replenished and erupted open magma chamber system with the extensive fractionation of olivine, spinel and clinopyroxene. The data also show that ankaramites in a subduction-related geotectonic setting could represent parental magmas of Uralian-Alaskan-type complexes. The second chapter of the thesis discusses the chemical variation of major and trace elements in rock-forming minerals of the mafic rocks. Electron microprobe and LA-ICPMS analyses are used to quantitatively describe the petrogenetic relationship between the different gabbroic lithologies and their genetic link to the ultramafic rocks. The composition of clinopyroxene identifies the presence of melts with different trace element abundances on the scale of a thin section and suggests the presence of open system crustal magma chambers. Even on a regional scale the large variation of trace element concentrations and ratios in clinopyroxene (e.g. La/Lu = 3-55) is best explained by the interaction of at least two fundamentally different magma types at various stages of fractionation. This requires the existence of a complex magma chamber system fed with multiple pulses of magmas from at least two different coeval sources in a subduction-related environment. One source produces silica saturated Island arc tholeiitic melts. The second source produces silica undersaturated, ultra-calcic, alkaline melts. Taken these data collectively, the mixing of the two different parental magmas is the dominant petrogenetic process explaining the observed chemical variations. The results further imply that this is an intrinsic feature of Uralian-Alaskan-type complexes and probably of many similar mafic-ultramafic complexes world-wide. In the third chapter of this thesis the major element composition of homogeneous and exsolved spinel is used as a petrogenetic indicator. Homogeneous chromian spinel in dunites and wehrlites monitors the fractionation during the early stages of the magma chamber and the onset of clinopyroxene fractionation as well as the reaction of spinel with interstitial liquid. Exsolved spinel is present in mafic and ultramafic rocks from all three studied complexes. Its composition lies along a solvus curve which defines an equilibrium temperature of 600°C, given that spinel coexists with olivine. This temperature is considered to be close to the temperature of the host rocks into which the studied Uralian-Alaskan-type complexes intruded. The similarity of the exsolution temperatures in the different complexes over a distance of several hundred kilometres implies a regional tectonic event that terminated the exsolution process. This event is potentially associated with the final exhumation of the Uralian-Alaskan-type complexes along the Main Uralian Fault and the Serov-Mauk Fault in the Uralian fold belt.

Copper and zinc stable isotope ratios as tracers of biogeochemical processes, sources and transport of Cu and Zn in soils

Copper and Zn are essential micronutrients for plants, animals, and humans; however, they may also be pollutants if they occur at high concentrations in soil. Therefore, knowledge of Cu and Zn cycling in soils is required both for guaranteeing proper nutrition and to control possible risks arising from pollution.rnThe overall objective of my study was to test if Cu and Zn stable isotope ratios can be used to investigate into the biogeochemistry, source and transport of these metals in soils. The use of stable isotope ratios might be especially suitable to trace long-term processes occurring during soil genesis and transport of pollutants through the soil. In detail, I aimed to answer the questions, whether (1) Cu stable isotopes are fractionated during complexation with humic acid, (2) 65Cu values can be a tracer for soil genetic processes in redoximorphic soils (3) 65Cu values can help to understand soil genetic processes under oxic weathering conditions, and (4) 65Cu and 66Zn values can act as tracers of sources and transport of Cu and Zn in polluted soils.rnTo answer these questions, I ran adsorption experiments at different pH values in the laboratory and modelled Cu adsorption to humic acid. Furthermore, eight soils were sampled representing different redox and weathering regimes of which two were influenced by stagnic water, two by groundwater, two by oxic weathering (Cambisols), and two by podzolation. In all horizons of these soils, I determined selected basic soil properties, partitioned Cu into seven operationally defined fractions and determined Cu concentrations and Cu isotope ratios (65Cu values). Finally, three additional soils were sampled along a deposition gradient at different distances to a Cu smelter in Slovakia and analyzed together with bedrock and waste material from the smelter for selected basic soil properties, Cu and Zn concentrations and 65Cu and 66Zn values.rnMy results demonstrated that (1) Copper was fractionated during adsorption on humic acid resulting in an isotope fractionation between the immobilized humic acid and the solution (65CuIHA-solution) of 0.26 ± 0.11‰ (2SD) and that the extent of fractionation was independent of pH and involved functional groups of the humic acid. (2) Soil genesis and plant cycling causes measurable Cu isotope fractionation in hydromorphic soils. The results suggested that an increasing number of redox cycles depleted 63Cu with increasing depth resulting in heavier 65Cu values. (3) Organic horizons usually had isotopically lighter Cu than mineral soils presumably because of the preferred uptake and recycling of 63Cu by plants. (4) In a strongly developed Podzol, eluviation zones had lighter and illuviation zones heavier 65Cu values because of the higher stability of organo-65Cu complexes compared to organo-63Cu complexes. In the Cambisols and a little developed Podzol, oxic weathering caused increasingly lighter 65Cu values with increasing depth, resulting in the opposite depth trend as in redoximorphic soils, because of the preferential vertical transport of 63Cu. (5) The 66Zn values were fractionated during the smelting process and isotopically light Zn was emitted allowing source identification of Zn pollution while 65Cu values were unaffected by the smelting and Cu emissions isotopically indistinguishable from soil. The 65Cu values in polluted soils became lighter down to a depth of 0.4 m indicating isotope fractionation during transport and a transport depth of 0.4 m in 60 years. 66Zn values had an opposite depth trend becoming heavier with depth because of fractionation by plant cycling, speciation changes, and mixing of native and smelter-derived Zn. rnCopper showed measurable isotope fractionation of approximately 1‰ in unpolluted soils, allowing to draw conclusions on plant cycling, transport, and redox processes occurring during soil genesis and 65Cu and 66Zn values in contaminated soils allow for conclusions on sources (in my study only possible for Zn), biogeochemical behavior, and depth of dislocation of Cu and Zn pollution in soil. I conclude that stable Cu and Zn isotope ratios are a suitable novel tool to trace long-term processes in soils which are difficult to assess otherwise.rn

Identity formation at the beginning of the twenty-first century: Intersubjectivity, art, and medicine in Siri Hustvedt’s works

„Ich bin, weil du bist“ – so lautet eines der Schlüsselzitate in What I Loved, dem 2003 erschienenen dritten Roman der zeitgenössischen amerikanischen Autorin Siri Hustvedt. Die Bedeutung von Beziehung und Interaktion für die Identitätsbildung spielt eine zentrale Rolle nicht nur in diesem Roman, sondern auch in ihrem Gesamtwerk, das vier Romane, ein memoir, drei Essay-Sammlungen und einen Lyrikband umfasst. Hustvedt erforscht die Identität als ein vielschichtiges Produkt bewusster und unbewusster Verknüpfungen innerhalb der sozialen und biologischen Umwelt. Das Bewusstsein wird als eine dialogisch geprägte Entität gezeigt, dessen Identität erst durch die Beziehung auf ein Anderes geformt werden kann. Um dem Mysterium der menschlichen Identitätsfindung nachzuspüren, bedient sich Hustvedt sowohl philosophischer, psychoanalytischer, biologischer als auch kunsttheoretischer Diskurse. In ihren Romanen stellt sich die Frage nach der Erklärung von Identität als komplexe Problematik dar: Ist die Beziehung zu anderen Menschen vor allem durch unsere Entwicklung als Kind und die Nähe zu Bezugspersonen geprägt? In welchem Ausmaß ist das Empfinden von Subjektivität beeinflusst von körperlichen und unbewussten Mechanismen? Inwiefern ist die Wahrnehmung visueller Kunst eine Kooperation zwischen Betrachter und Künstler? rnDiesen und anderen Fragen geht diese Dissertation nach, indem sie Hustvedts Werk als Anlass für eine Analyse intersubjektiver Strukturen der Identität nimmt. Die Intersubjektivitätsphiloso¬phien von Hegel, Buber, Bakhtin, Husserl, und Merleau-Ponty dienen hierbei als Ausgangspunkt für die Interpretation von relationaler Identität in Hustvedts Werken. Die Dissertation konzentriert sich auf Hustvedts Darstellung der Beziehung zwischen Selbst und Anderem in der Photographie und in der Malerei, der Überschreitung von Körpergrenzen in Hysterie und Anorexie sowie der Auswirkung des Verlustes von Bezugspersonen auf die persönliche Identität. Entscheidend für den Hustvedtschen Kunstbegriff ist das Zusammenspiel von Kunstobjekt, Künstler und Betrachter. Die Grenzen zwischen Innerem und Äußeren werden aufgelöst: mal wird der Rezipient Teil des Kunstwerks, mal verschmilzt der Künstler förmlich mit seinem Objekt. Auch hier wird wiederum deutlich, dass Identität nur in Wechselbeziehung und als zwischenmenschliche Kooperation entsteht. Hustvedt betritt durch ihre einzigartige Auseinandersetzung mit den Wechselbeziehungen und fragilen Grenzen zwischen Ich und Umwelt Neuland auf dem Gebiet der literarischen Identitätsforschung, da sie ihr Prinzip des „mixing,“ des unausweichlichen Eindringens fremder Substanz in die eigene Identität, aus dem Blickwinkel dieser verschiedenen Erklärungsansätze beleuchtet. rn

Electron spectroscopy of novel charge transfer systems based on polycyclic aromatic hydrocarbons

Organische Ladungstransfersysteme weisen eine Vielfalt von konkurrierenden Wechselwirkungen zwischen Ladungs-, Spin- und Gitterfreiheitsgraden auf. Dies führt zu interessanten physikalischen Eigenschaften, wie metallische Leitfähigkeit, Supraleitung und Magnetismus. Diese Dissertation beschäftigt sich mit der elektronischen Struktur von organischen Ladungstransfersalzen aus drei Material-Familien. Dabei kamen unterschiedliche Photoemissions- und Röntgenspektroskopietechniken zum Einsatz. Die untersuchten Moleküle wurden z.T. im MPI für Polymerforschung synthetisiert. Sie stammen aus der Familie der Coronene (Donor Hexamethoxycoronen HMC und Akzeptor Coronen-hexaon COHON) und Pyrene (Donor Tetra- und Hexamethoxypyren TMP und HMP) im Komplex mit dem klassischen starken Akzeptor Tetracyanoquinodimethan (TCNQ). Als dritte Familie wurden Ladungstransfersalze der k-(BEDT-TTF)2X Familie (X ist ein monovalentes Anion) untersucht. Diese Materialien liegen nahe bei einem Bandbreite-kontrollierten Mottübergang im Phasendiagramm.rnFür Untersuchungen mittels Ultraviolett-Photoelektronenspektroskopie (UPS) wurden UHV-deponierte dünne Filme erzeugt. Dabei kam ein neuer Doppelverdampfer zum Einsatz, welcher speziell für Milligramm-Materialmengen entwickelt wurde. Diese Methode wies im Ladungstransferkomplex im Vergleich mit der reinen Donor- und Akzeptorspezies energetische Verschiebungen von Valenzzuständen im Bereich weniger 100meV nach. Ein wichtiger Aspekt der UPS-Messungen lag im direkten Vergleich mit ab-initio Rechnungen.rnDas Problem der unvermeidbaren Oberflächenverunreinigungen von lösungsgezüchteten 3D-Kristallen wurde durch die Methode Hard-X-ray Photoelectron Spectroscopy (HAXPES) bei Photonenenergien um 6 keV (am Elektronenspeicherring PETRA III in Hamburg) überwunden. Die große mittlere freie Weglänge der Photoelektronen im Bereich von 15 nm resultiert in echter Volumensensitivität. Die ersten HAXPES Experimente an Ladungstransferkomplexen weltweit zeigten große chemische Verschiebungen (mehrere eV). In der Verbindung HMPx-TCNQy ist die N1s-Linie ein Fingerabdruck der Cyanogruppe im TCNQ und zeigt eine Aufspaltung und einen Shift zu höheren Bindungsenergien von bis zu 6 eV mit zunehmendem HMP-Gehalt. Umgekehrt ist die O1s-Linie ein Fingerabdruck der Methoxygruppe in HMP und zeigt eine markante Aufspaltung und eine Verschiebung zu geringeren Bindungsenergien (bis zu etwa 2,5eV chemischer Verschiebung), d.h. eine Größenordnung größer als die im Valenzbereich.rnAls weitere synchrotronstrahlungsbasierte Technik wurde Near-Edge-X-ray-Absorption Fine Structure (NEXAFS) Spektroskopie am Speicherring ANKA Karlsruhe intensiv genutzt. Die mittlere freie Weglänge der niederenergetischen Sekundärelektronen (um 5 nm). Starke Intensitätsvariationen von bestimmten Vorkanten-Resonanzen (als Signatur der unbesetzte Zustandsdichte) zeigen unmittelbar die Änderung der Besetzungszahlen der beteiligten Orbitale in der unmittelbaren Umgebung des angeregten Atoms. Damit war es möglich, präzise die Beteiligung spezifischer Orbitale im Ladungstransfermechanismus nachzuweisen. Im genannten Komplex wird Ladung von den Methoxy-Orbitalen 2e(Pi*) und 6a1(σ*) zu den Cyano-Orbitalen b3g und au(Pi*) und – in geringerem Maße – zum b1g und b2u(σ*) der Cyanogruppe transferiert. Zusätzlich treten kleine energetische Shifts mit unterschiedlichem Vorzeichen für die Donor- und Akzeptor-Resonanzen auf, vergleichbar mit den in UPS beobachteten Shifts.rn

Band structure of Heusler compounds studied by photoemission and tunneling spectroscopy

Heusler compounds are key materials for spintronic applications. They have attracted a lot of interest due to their half-metallic properties predicted by band structure calculations.rnThe aim of this work is to evaluate experimentally the validity of the predictions of half metallicity by band structure calculations for two specific Heusler compounds, Co2FeAl0.3Si0.7 and Co2MnGa. Two different spectroscopy methods for the analysis of the electronic properties were used: Angular Resolved Ultra-violet Photoemission Spectroscopy (ARUPS) and Tunneling Spectroscopy.rnHeusler compounds are prepared as thin films by RF-sputtering in an ultra-high vacuum system. rnFor the characterization of the samples, bulk and surface crystallographic and magnetic properties of Co2FeAl0.3Si0.7 and Co2MnGa are studied. X-ray and electron diffraction reveal a bulk and surface crossover between two different types of sublattice order (from B2 to L21) with increasing annealing temperature. X-ray magnetic circular dichroism results show that the magnetic properties in the surface and bulk are identical, although the magnetic moments obtained are 5% below from the theoretically predicted.rnBy ARUPS evidence for the validity of the predicted total bulk density of states (DOS) was demonstrated for both Heusler compounds. Additional ARUPS intensity contributions close to the Fermi energy indicates the presence of a specific surface DOS. Moreover, it is demonstrated that the crystallographic order, controlled by annealing, plays an important role on brodening effects of DOS features. Improving order resulted in better defined ARUPS features.rnTunneling magnetoresistance measurements of Co2FeAl0.3Si0.7 and Co2MnGa based MTJ’s result in a Co2FeAl0.3Si0.7 spin polarization of 44%, which is the highest experimentally obtained value for this compound, although it is lower than the 100% predicted. For Co2MnGa no high TMR was achieved.rnUnpolarized tunneling spectroscopy reveals contribution of interface states close to the Fermi energy. Additionally magnon excitations due to magnetic impurities at the interface are observed. Such contributions can be the reason of a reduced TMR compared to the theoretical predictions. Nevertheless, for energies close to the Fermi energy and for Co2MnGa, the validity of the band structure calculations is demonstrated with this technique as well.

Adsorption of macromolecules on interfaces studied by isothermal titration calorimetry

The mixing of nanoparticles with polymers to form composite materials has been applied for decades. They combine the advantages of polymers (e.g., elasticity, transparency, or dielectric properties) and inorganic nanoparticles (e.g., specific absorption of light, magneto resistance effects, chemical activity, and catalysis etc.). Nanocomposites exhibit several new characters that single-phase materials do not have. Filling the polymeric matrix with an inorganic material requires its homogeneous distribution in order to achieve the highest possible synergetic effect. To fulfill this requirement, the incompatibility between the filler and the matrix, originating from their opposite polarity, has to be resolved. A very important parameter here is the strength and irreversibility of the adsorption of the surface active compound on the inorganic material. In this work the Isothermal titration calorimetry (ITC) was applied as a method to quantify and investigate the adsorption process and binding efficiencies in organic-inorganic–hybrid-systems by determining the thermodynamic parameters (ΔH, ΔS, ΔG, KB as well as the stoichiometry n). These values provide quantification and detailed understanding of the adsorption process of surface active molecules onto inorganic particles. In this way, a direct correlation between the adsorption strength and structure of the surface active compounds can be achieved. Above all, knowledge of the adsorption mechanism in combination with the structure should facilitate a more rational design into the mainly empirically based production and optimization of nanocomposites.

Electronic structure and physical properties of Heusler compounds for thermoelectric and spintronic applications

This thesis focuses on synthesis as well as investigations of the electronic structure and properties of Heusler compounds for spintronic and thermoelectric applications.rnThe first part reports on the electronic and crystal structure as well as the mechanical, magnetic, and transport properties of the polycrystalline Heusler compound Co2MnGe. The crystalline structure was examined in detail by extended X-ray absorption fine structure spectroscopy and anomalous X-ray diffraction. The low-temperature magnetic moment agrees well with the Slater-Pauling rule and indicates a half-metallic ferromagnetic state of the compound, as is predicted by ab-initio calculations. Transport measurements and hard X-ray photoelectron spectroscopy (HAXPES) were performed to explain the electronic structure of the compound.rnA major part of the thesis deals with a systematical investigation of Heusler compounds for thermoelectric applications. Few studies have been reported on thermoelectric properties of p-type Heusler compounds. Therefore, this thesis focuses on the search for new p-type Heusler compounds with high thermoelectric efficiency. The substitutional series NiTi1−xMxSn and CoTi1−xMxSb (where M = Sc, V and 0 ≤ x ≤ 0.2) were synthesized and investigated theoretically and experimentally with respect to electronic structure and transport properties. The results show the possibility to create n-type and p-type thermoelectrics within one Heusler compound. The pure compounds showed n-type behavior, while under Sc substitution the system switched to p-type behavior. A maximum Seebeck coefficient of +230 μV/K (at 350 K) was obtained for NiTi0.26Sc0.04Zr0.35Hf0.35Sn, which is one of the highest values for p-type thermoelectric compounds based on Heusler alloys up to now. HAXPES valence band measurement show massive in gap states for the parent compounds NiTiSn, CoTiSb and NiTi0.3Zr0.35Hf0.35Sn. This proves that the electronic states close to the Fermi energy play a key role for the behavior of the transport properties. Furthermore, the electronic structure of the gapless Heusler compounds PtYSb, PtLaBi and PtLuSb were investigated by bulk sensitive HAXPES. The linear behavior of the spectra close to εF proves the bulk origin of Dirac-cone type density of states. Furthermore, a systematic study on the optical and transport properties of PtYSb is presented. The compound exhibits promising thermoelectric properties with a high figure of merit (ZT = 0.2) and a Hall mobility μh of 300 cm2/Vs at 350 K.rnThe last part of this thesis describes the linear dichroism in angular-resolved photoemission from the valence band of NiTi0.9Sc0.1Sn and NiMnSb. High resolution photoelectron spectroscopy was performed with an excitation energy of hν = 7.938 keV. The linear polarization of the photons was changed using an in-vacuum diamond phase retarder. Noticeable linear dichroism is found in the valence bands and this allows for a symmetry analysis of the contributing states. The differences in the spectra are found to be caused by symmetry dependent angular asymmetry parameters, and these occur even in polycrystalline samples without preferential crystallographic orientation.rnIn summary, Heusler compounds with 1:1:1 and 2:1:1 stoichiometry were synthesized and examined by chemical and physical methods. Overall, this thesis shows that the combination of first-principle calculations, transport measurements and high resolution high energy photoelectron spectroscopy analysis is a very powerful tool for the design and development of new materials for a wide range of applications from spintronic applications to thermoelectric applications.rn

Spectroscopic investigations of functional bulk and thin film Heusler alloys

Functional and smart materials have gained large scientific and practical interest in current research and development. The Heusler alloys form an important class of functional materials used in spintronics, thermoelectrics, and for shape memory alloy applications. An important aspect of functional materials is the adaptability of their physical properties. In this work functional polycrystalline bulk and epitaxial thin film Heusler alloys are characterized by means of spectroscopic investigation methods, X-ray magnetic circular dichroism (XMCD) and energy dispersive X-ray analysis (EDX). With EDX the homogeneity of the samples is studied extensively. For some cases of quaternary compounds, for example Co2(MnxTi1−x)Sn and Co2(Mn0.5Dy0.5)Sn, an interesting phase separation in two nearly pure ternary Heusler phases occurs. For these samples the phase separation leads to an improvement of thermoelectric properties. XMCD as the main investigation method was used to study Co2TiZ (Z = Si, Sn, and Sb), Co2(MnxTi1−x)Si, Co2(MnxTi1−x)Ge, Co2Mn(Ga1−xGex), Co2FeAl, Mn2VAl, and Ni2MnGa Heusler compounds. The element-specific magnetic moments are calculated. Also, the spin-resolved unoccupied density of states is determined, for example giving hints for half-metallic ferromagnetism for some Co-based compounds. The systematic change of the magnetic moments and the shift of the Fermi energy is a proof that Heusler alloys are suitable for a controlled tailoring of physical properties. The comparison of the experimental results with theoretical predictions improves the understanding of complex materials needed to optimize functional Heusler alloys.

Interactions of gold nanoparticles with polymeric membranes

This thesis focuses on the interactions of nanoparticles with artificial membranes. The synthesis of the block copolymer poly(dimethylsiloxane)-block-poly(2-methyloxazoline) (PDMS-b-PMOXA) is described, as well as the formation of polymersomes in water. These polymersomes act as minimal cell models, consisting of an artificial bilayer membrane only, allowing the study of the interactions between nanoparticles and polymeric membranes. Both spherical and rod-shaped gold nanoparticles (AuNPs) were used in this study and they were characterized using light scattering (PCS), transmission electron microscopy (TEM), UV/Vis spectroscopy, and polarization anisotropy measurements. The polymer grafting on the spherical cores is asymmetric (shell asphericity) but is parallel to the inherent, due to polycrystallinity, core anisotropy, resulting in a characteristic scattering of the AuNPs in PCS.rnInteractions of polymersomes and AuNPs were investigated by PCS, cryo-TEM and UV/Vis. Three possible scenarios upon mixing of polymersomes and AuNPs can be distinguished by using only PCS: (i) no interactions between particles and vesicles, (ii) attachment of the particles to the outer side of the vesicles (decoration), and (iii) uptake of particles into the vesicles. It is shown that all three scenarios are possible, solely depending on the particle’s surface functionalization. In addition, it was revealed that the AuNPs need to be attached to the inner side of the membrane instead of diffusing freely within the vesicle. The present experimental findings essentially help with the understanding of the interactions of nanoparticles with membranes and show that the process of endocytosis can be attributed to physical processes only. rn