Spektroskopische Ellipsometrie an Iridiumoxiden und Eisenpniktiden

In physics many body effects are among to the most fascinating phenomena. Rather than the huge particle number accounting for the relevant interactions and correlations is most crucial to an adequate description. Strongly correlated electron systems offer the opportunity to study many body effects with a variety of interactions with variable strength - in particular competition of the latter in driving the system might lead to unforeseen properties. In this context the presented work focuses on two material classes, which have attracted much attention recently: compounds from the iridium oxide and iron pnictide families. Since electronic correlations directly affect the charge carrier dynamics in a specific material, which is characterized by its complex dielectric function, optical spectroscopy and in particular broadband spectroscopic ellipsometry is a suitable tool to address these questions from an experimental point of view. The thesis is organized in three parts. After laying out the essential theoretical and experimental principles, we discuss the results on the iridium oxides and iron pnictides. In Sr2IrO4, considered as the prototypical member of this iridium oxide family, we detect a strong optical out-of-plane anisotropy in good agreement with calculations within density-functional theory, which including the essential spin-orbit coupling. The related compound Na3Ir3O8 shows extraordinarily strong Fano resonances in the phonon spectra, which we assign to a lack of inversion symmetry and the strong spin-orbit coupling. This points to a non-negligible electron-phonon coupling in the iridium oxides. In the case of the iron pnictides we focus on the influence of structural changes on the electronic correlations. Here, isovalent substitution of Fe(3d) by Ru(4d) reveals a systematic reduction in the correlations in accordance with the doping phase diagram. A second approach is the detailed study of the temperature and broad range (FIR to UV) spectral dependence of the dielectric response of the representative parent compounds AFe2As2 (with A = Ca, Sr, Ba) in which we detect consistently two coupled subsystems of the spin-density-wave. While the coupling strength of the respective subsystems themselves decreases with increasing atomic number, the coupling between them gets stronger. Since the three investigated materials are driven superconducting upon doping, our results may provide insights into the physics of the related superconductors.Vielteilchen-Effekte gehören zu den faszinierendsten Phänomenen der Physik und spielen auf allen Längenskalen eine wichtige Rolle in der Beschreibung physikalischer Zusammenhänge. Die Schwierigkeit in ihrer Beschreibung besteht hierbei weniger in der großen Zahl der Teilchen sondern in der Berücksichtigung der relevanten Wechselwirkungen zwischen ihnen. Stark korrelierte Elektronensysteme bieten die Möglichkeit ein solches Zusammenspiel einer ganzen Reihe verschiedener Wechselwirkungen zu studieren. Gerade die Konkurrenz verschiedener Wechselwirkungen eröffnet die Möglichkeit für neue Eigenschaften. Vor diesem Hintergrund ist die vorliegende experimentelle Arbeit Verbindungen aus zwei Materialklassen gewidmet, die zuletzt in den Fokus der Forschung gerückt sind: den Iridiumoxiden und den Eisenpniktiden. Da sich die elektronischen Korrelationen unmittelbar in der Dynamik der Ladungsträger, also der dielektrischen Funktion eines Materials widerspiegeln, ist optische Spektroskopie, insbesondere breitbandige spektroskopische Ellipsometrie besonders geeignet diesen Effekten von experimenteller Seite her nachzugehen. Die Arbeit gliedert sich im Wesentlichen in drei Teile. Nach der eingehenden Darlegung der theoretischen und experimentellen Grundlagen erfolgt die Darstellung der experimentellen Ergebnisse und deren Diskussion. Zuerst werden zwei Iridiumoxid-Verbindungen behandelt, die sich durch einen starken Einfluss der Spin-Bahn-Wechselwirkung auszeichnen. Für die prototypische Verbindung Sr2IrO4 zeigt sich eine starke Anisotropie in der dielektrischen Funktion in Einklang mit Dichtefunktionalrechnungen. Die verwandte Verbindung Na3Ir3O8 zeigt aufgrund fehlender Kristall-Inversionssymmetrie und starker Spin-Bahn-Kopplung außergewöhnlich starke Fano-Resonanzen im Phononen-Spektrum, was auf eine nicht zu vernachlässigende Elektron-Phonon-Kopplung in den Iridiumoxiden hindeutet. In den Eisenpniktiden liegt das Augenmerk auf den Auswirkungen struktureller Veränderungen auf die elektronischen Korrelationseffekte. Dabei zeigt sich zum einen bei der isovalenten Substitution von Fe(3d) durch Ru(4d) eine systematische Reduktion der Korrelationen in Übereinstimmung mit dem Dotierungsphasendiagramm. Zum anderen deckt die systematische, über einen weiten Spektralbereich (FIR - UV) erfolgte Untersuchung der Temperaturabhängigkeit der dielektrischen Funktion der undotierten Mutterverbindungen AFe2As2, A = Ca, Sr, Ba, zwei gekoppelte Untersysteme der Spin-Dichte-Welle auf. Während die Korrelationsstärke innerhalb der jeweiligen Untersysteme mit steigender Ordnungszahl abnimmt, nimmt die Kopplung zwischen diesen zu. Da die Mutterverbindungen das Ausgangsmaterial für die unter Dotierung supraleitenden Verbindungen sind, können diese Ergebnisse weitere Einblicke in die Physik der Supraleiter gewähren

Crystalline guanine adducts of natural and synthetic trioxacarcins suggest a common biological mechanism and reveal a basis for the instability of trioxacarcin A

a b s t r a c t X-ray crystallographic characterization of products derived from natural and fully synthetic trioxacarcins, molecules with potent antiproliferative effects, illuminates aspects of their reactivity and mechanism of action. Incubation of the fully synthetic trioxacarcin analog 3, which lacks one of the carbohydrate residues present in the natural product trioxacarcin A (1) as well as oxygenation at C2 and C4 yet retains potent antiproliferative effects, with the self-complimentary duplex oligonucleotide d(AACCGGTT) led to production of a crystalline covalent guanine adduct (6). Adduct 6 is closely analogous to gutingimycin (2), the previously reported guanine adduct derived from incubation of natural trioxacarcin A (1) with duplex DNA, suggesting that 3 and 1 likely share a common basis of cytotoxicity. In addition, we isolated a novel, dark-red crystalline guanine adduct (7) from incubation of trioxacarcin A itself with the selfcomplimentary duplex oligonucleotide d(CGTATACG). Crystallographic analysis suggests that 7 is an anthraquinone derivative, which we propose arises by a sequence of guanosine alkylation within duplex DNA, depurination, base-catalyzed elimination of the trioxacarcinose A carbohydrate residue, and oxidative rearrangement to form an anthraquinone. We believe that this heretofore unrecognized chemical instability of natural trioxacarcins may explain why trioxacarcin analogs lacking C4 oxygenation exhibit superior chemical stabilities yet, as evidenced by structure 3, retain a capacity to form lesions with duplex DNA. Ó 2014 Elsevier Ltd. All rights reserved. The trioxacarcins are structurally complex bacterial metabolites that exhibit potent cytotoxicity in cultured human cancer cells and have demonstrated antibacterial and antimalarial activities

Large resistivity change and phase transition in the antiferromagnetic semiconductors LiMnAs and LaOMnAs

Antiferromagnetic semiconductors are new alternative materials for spintronic applications and spin valves. In this work, we report a detailed investigation of two antiferromagnetic semiconductors AMnAs (A = Li, LaO), which are isostructural to the well-known LiFeAs and LaOFeAs superconductors. Here we present a comparison between the structural, magnetic, and electronic properties of LiMnAs, LaOMnAs, and related materials. Interestingly, both LiMnAs and LaOMnAs show a variation in resistivity with more than five orders of magnitude, making them particularly suitable for use in future electronic devices. Neutron and x-ray diffraction measurements on LiMnAs show a magnetic phase transition corresponding to the N´eel temperature of 373.8 K, and a structural transition from the tetragonal to the cubic phase at 768 K. These experimental results are supported by density functional theory calculations

Reflections on Social Wellbeing and the Values of Small-Scale Fisheries: Implications for Research, Policy and Management

The contributors to this volume engaged in different ways with social wellbeing as an approach through which to investigate, identify and make visible a broad range of values associated with small-scale fisheries. In this concluding chapter, we highlight four themes that emerge from these contributions that are crucial for thinking about the diverse values of small-scale fisheries: 1) the broader context of transition; 2) integrating environmental considerations into wellbeing through co-construction and place; 3) recognizing the fertile, yet productively unsettled idea that value represents for small-scale fisheries, and; 4) putting into practice the social wellbeing approach to values that this volume develops. We point to connections between our approach and the FAO Voluntary Guidelines for Securing Sustainable Small-scale Fisheries in the Context of Food Security and Poverty Eradication