Universität Stuttgart
Permanent URI for this communityhttps://elib.uni-stuttgart.de/handle/11682/1
Browse
168 results
Search Results
Item Open Access Ultrafast spectroscopy of single quantum dots(2012) Wolpert, Christian; Lippitz, Markus (Juniorprofessor Dr.)In this thesis, the coherent interaction of single semiconductor quantum dots and ultrafast optical pulses is studied. Under certain conditions, localized exciton transitions in quantum dots can be seen as semi-isolated two-level systems. While this description is sufficient for the explanation of some observations in coherent experiments, it is sometimes necessary to explicitly consider coupling of the discreet quantum states confined to the dot with the environment. We start out from simple, classical examples of coherent spectroscopy and then turn towards experiments where the interaction with the vicinity of the dot becomes an important factor. First, a novel method for transient differential reflectivity spectroscopy of single quantum systems is introduced. It is a pure far-field optical technique which does not require any sophisticated sample preparation steps which makes it applicable to a broad range of structures. Pump pulses excite the sample structure and probe pulses read out the pump-induced changes in the system after a variable delay time. In the case of a single dipole, the signal is given in the form of the spectral inteferogram between the backscattered wave from the particle and the probe light which is reflected at the sample surface. This form of homodyne detection amplifies the weak scattered wave from the particle and thus makes this kind of spectroscopy for single quantum dots feasible. In the remainder of this thesis our spectroscopic method is applied to either characterize the coherent properties of single quantum dots, to prepare and read-out a desired quantum state or to deliberately manipulate them. Coherence times and oscillator strengths are determined for localized exciton transitions. Arbitrary population states can be written by driving coherent population oscillations using resonant pulses, while entangled superpositions of two exciton states in a single dot are investigated by quantum beats on transient differential spectra. We finally exploit the interaction between the dot and a nearby absorbing layer to switch the dot's absorption spectrum on ultrafast timescales via light-induced transient electric fields.Item Open Access Ab initio thermodynamic study of defective strontium titanate(2013) Blokhin, Evgeny; Maier, Joachim (Prof. Dr.)In the presented thesis the perfect and defective SrTiO3 bulk crystals and their (001) surfaces are considered on ab initio level. Since the experimental study of the complex defective systems is comparatively expensive and difficult, and the computer performance has been greatly increased in the last years, the ab initio modeling became very efficient tool to be applied in this field. Additionally, there is a significant industrial demand for the investigation and improvements of the performance of the perovskite-based electrochemical devices, e.g. solid oxide fuel cells and permeation membranes. Finally, there is a lack of methodological studies on defect thermodynamics. The oxygen vacancies and iron impurities, as well as their complexes, are the characteristic defects in SrTiO3 perovskite material. The understanding of basic defect properties and defect-induced phenomena under realistic external conditions requires calculation of thermodynamic properties (the free energy and entropy effects). Thus, thesis is focused on atomic vibrations, i.e. phonons, which are necessary to go beyond standard 0°K approximation and to provide a link to the thermodynamic properties at finite temperatures. This is necessary for a realistic treatment of electrochemical devices. The chosen ab initio modeling scheme is found to ensure the most accurate description simultaneously for the structural, electron and phonon properties of the perfect SrTiO3. Namely, the splitting of the phonon frequencies due to the antiferrodistortive phase transition at 105°K is confirmed to be very small (2–12 cm-1). The experimental temperature dependence of the SrTiO3 heat capacity is also successfully reproduced. Further, the modeling scheme is applied for thermodynamic treatment of oxygen vacancies and iron impurities in SrTiO3 at finite temperatures. The calculated phonon densities of states and group-theoretical study of the defect-induced phonon frequencies are used for the experiment analysis. Several defect-induced local phonon modes are identified, and the experimental Raman- and IR-spectroscopy data are interpreted. The Jahn-Teller-type local lattice distortion around both Fe4+ impurity and oxygen vacancy VO is shown to result in Raman- and IR-active phonons. In particular, the experimentally observed Raman frequency near 700 cm-1 is shown to arise for both defects due to a local O ion stretching vibration nearby the Jahn-Teller defect. However, an absence of such a frequency in an experimental phonon spectrum is found to be a manifestation of formation of Fe3+–VO complexes with oxygen vacancies in the first coordination sphere of iron impurities. The Gibbs formation energy calculated for the neutral oxygen vacancies in bulk SrTiO3 taking into account the phonon contribution is found to be in excellent agreement with the experiment. The phonon contribution to the formation energy is shown to increase with temperature, to about 5% above 1000°K. The predicted relative stability of several structural complexes of oxygen vacancy and iron impurity in SrTiO3 is confirmed by known experimental measurements. Several structural models of such Fe3+–VO complexes in SrTiO3 are discriminated according to the XANES and EXAFS experiments. On an example of SrO-terminated SrTiO3 ultrathin films, the one-dimensional confinement effect on the vacancy formation energy is found to be inconsiderable at 0°K. The phonon contribution to the Gibbs free energy of VO formation in such ultrathin films at finite temperatures is shown to be minor. This suggests the further account of anharmonic effects is required. The workflow developed in thesis is proposed for the modeling of wide class of defects in non-metallic solids. Several auxiliary computer tools were designed in order to simplify such possible studies.Item Open Access Polymer electrolyte membrane degradation and mobility in fuel cells : a solid-state NMR investigation(2010) Ghassemzadeh Khoshkroodi, Lida; Müller, Klaus (Prof. Dr.)It is generally believed that fuel cells will play an important role in energy technology already in the near future. Operating polymer electrolyte membrane fuel cells (PEMFCs) at temperatures higher than 100 °C and reduced humidity is anticipated to avoid most of the shortcomings associated with the low-temperature fuel cell operation, such as CO poisoning of the electrode catalysts, slow electrode kinetics of the oxygen reduction reaction and expensive water/thermal management. To date, the operation temperature of PEMFCs is limited to about 90 °C, and this limit is given by the properties of the perfluorosulfonic acid (PFSA) ionomer, Nafion, which is commonly used as a separator material. Apart from the proton conductivity decay at higher temperature and lower humidification, it is also the limited stability of Nafion preventing it from long term operation. Despite the high stability of the PTFE backbone in Nafion, severe deterioration is observed during fuel cell operation. Formation of pinholes and cracks, thinning of the membranes and decrease of ion exchange capacity were reported. The fluorine release indicated that the bond cleavage process takes place under fuel cell operating conditions. Bond cleavage was initially believed to proceed from radical attacks to the carboxyl groups terminating the PTFE backbone of Nafion, and it was claimed to be controlled by the endcapping of the polymer backbone with a CF3 group. However, the release of fluoride was reported even after endcapping of the materials. The observations proved that bond cleavage limits the stability of PFSA membranes, but the elementary reactions and consequences on the membrane microstructure are not fully understood yet. In this work, it has been tried to get new insights into the problems of long term stability of polymer electrolytes for low temperature fuel cells. The aim was to identify the changes in the chemical structure of the membrane after operating in a fuel cell. This understanding is essential for extending the operation limit of PFSA-type membranes by either improving the membrane properties or adjusting the conditions within the running fuel cell. In the present work, therefore the changes taking place in PFSA membranes after applying in-situ and ex-situ aging protocols have been investigated. While the in-situ experiments provide a global picture, the analysis of membranes after ex-situ tests, with various conditions, allows the separation of different types of reactions. In previous studies the degradation changes were mainly monitored by analyzing the released water of the fuel cell or by using the liquid ionomers. In this work with the help of solid-state NMR spectroscopy, the direct study of the chemical structure and dynamics of the polymer membranes before and after the degradation tests became possible. The structural changes in different parts of the PFSA membranes were first inspected after an in-situ aging test. These examined membranes (Nafion and Hyflon Ion) differed by the length of the side chains. The comparison of the solid-state 13C and 19F NMR data of polymers before and after the in-situ degradation test showed that changes can take place not only in the main chain of the polymer, but also within the polymer side chains, as reflected by changes of NMR signals associated with CFSO3, CF3, OCF2 and CF groups. The degree of degradation is found to decrease with increasing membrane thickness while for a given thickness the short side chain polymer, Hyflon Ion, appears to degrade less than Nafion. In order to understand the reason for these observations, a new ex-situ method has been developed to mimic the degradation of polymer electrolyte membranes in PEM fuel cells (caused by the cross-leakage of H2 and O2). In this ex-situ setup, it was possible to expose membranes to flows of different gases with controlled temperature and humidity. H+-form Nafion films with and without electrode layer (Pt) have been treated in the presence of different gases in order to simulate the anode and cathode side of a PEMFC. The changes of the chemical structure occurring during the degradation tests were primarily examined by solid-state 19F NMR spectroscopy. For completion, liquid-state NMR studies and ion exchange capacity measurements were performed. It was found that degradation occurs only when both H2 and O2 are present (condition of gas cross-leakage), and when the membrane is coated with Pt catalyst. The chemical degradation rate is found to be highest for H2-rich mixtures of H2 and O2, which corresponds to the conditions at the anode under OCV. It is further shown that side chain disintegration is very important for chemical degradation, although backbone decomposition also might take place. The fact that in-situ degradation effects were reproduced by the present ex-situ experiments, suggest that membrane degradation in a running fuel cell is mainly the consequence of chemical aging. Detecting the degradation for the membranes coated with Pt in the presence of both gases, H2 and O2, points toward the importance of radicals in the degradation process, which in a running fuel cell (in-situ conditions) may only form in the presence of some gas cross-over, allowing H2 and O2 to react at the Pt catalyst of the anode or cathode structure. Since the gas cross-over increases for the thinner Nafion membrane, these results indirectly explain the higher degradation rate of thin Nafion in the in-situ degradation test. The chemical degradation and stability of PFSA membranes against radical attacks was also investigated in a Fenton ex-situ degradation test. Liquid and solid-state NMR as well as ATR-FTIR spectroscopy were applied to the samples before and after the Fenton reaction. A Comparison of the degradation rate of Nafion and Hyflon Ion in the ex-situ Fenton test again proved that the Hyflon Ion membrane is more stable than Nafion. Comparing the degradation rate of the side chain in these two polymers showed that the stability of Hyflon Ion is mainly due to the shortening of the side chain in this polymer. Hence, the absence of one ether group and the tertiary carbon reduces the degradation rate of the side chain and makes this polymer less sensitive to the radical attacks than Nafion. For the performance of a membrane not only the chemical structure but also the polymer dynamics is important. Therefore the molecular mobility of the ionomer was investigated by variable temperature 19F NMR lineshape, T1 and T1ρ relaxation experiments. The decrease of the temperature dependent linewidth was explained by the reduction of static disorder in the Nafion membrane. From the relaxation data there was evidence for structural annealing, which is independent of the chemical degradation. Chemical degradation is considered to reduce the chain flexibility (i.e. the motional amplitudes), which may be explained by chain cross-linking and condensation reaction for the side chains. To overcome the problem of Nafion's low conductivity at temperatures above 100 °C and low relative humidity, also composite membranes were introduced. These membranes consist of Nafion modified by inorganic oxide additives. It has been reported that under dry conditions, these membranes show enhanced water uptake and water diffusion when compared with filler-free Nafion. In order to understand the reason for the better performance of these polymers, the impact of the oxide particles on the polymer dynamics has been investigated. [Nafion/(SiO2)x] composite membranes in the dry and wet state with x ranging from 0 to 15 w/w% were investigated by variable temperature solid-state 19F NMR spectroscopy. 19F T1 and T1ρ relaxation times and NMR lineshapes were analyzed in order to get details about the polymer mobility. It is concluded that solid oxide SiO2 particles play an important role in stabilizing the chemical structure and morphology of the polymer especially in the dry state. The filler particles lead to higher mobility of polymer chains, if the filler content has an optimized value of about 9 w/w%. The results were further supported by comparing the sideband intensity as well as the linewidth in 19F NMR and recording the 19F{1H} CP/MAS NMR spectra. Furthermore, it has been shown that the structure of composite membranes is more stable after dehydration and possible condensation reactions are less likely in these membranes. The presence of filler particles decrease the chance for morphology changes and close packing of polymer chains in the dry state. Also the decrease of ionic exchange capacity after dehydration is less severe for the composite membrane as compared to filler-free Nafion. In conclusion, the present results provide a complete picture of solid membrane before and after degradation and of possible mechanisms for radical formation and radical attacks to the polymer. In addition, it is shown which changes can occur in the morphology of polymer chains in low humidification and high temperature. Some general suggestions for the better performance of polymer electrolyte membrane are therefore: For improving the performance of polymer electrode membrane, the sources for the radical formation in the fuel cell should be controlled. This can be possible to some extend by avoiding the use of iron end plates in the fuel cells. Also the chance for the gas crossover through the membrane should be decreased. Thicker membranes show less gas cross-over. By taking into account the higher resistivity of thicker membranes, an optimized membrane thickness should be selected. Hydrocarbon sulfonated polyetherketones possess narrower hydrophilic channels which significantly reduce electroosmotic drag, water permeation as well as gas cross-over. Also the short side chain perfluorinated polymer, Hyflon Ion, with lower electroosmotic drag of water should possess a reduced gas cross-over though the membrane. The more efficient way for decreasing degradation is to use membranes which are stable against radical attacks. At this point the perfluorinated polymers are still the best available membranes. Endcapping of the backbone in these polymers and decreasing the concentration of reactive end groups like COOH during the polymer manufacturing process can significantly decrease degradation. To minimize degradation of the side chains in perfluorinated polymers, short side chain polymers are suggested because of less reactive groups for the radical attacks and higher concentration of acidic groups. When higher operation temperatures are required, composite Nafion membranes might be used. The higher stability of these membranes makes them advantageous for operating at evaluated temperatures and low relative humidity. The novel results from the present work lead to a better understanding of membrane degradation, which still represents a serious problem for fuel cells under operation conditions, and provide important indications for future developments of membranes with improved performance for alternative energy conversion devices.Item Open Access Fluorine-promoted intramolecular aryl-aryl coupling : toward the isomer-specific fullerene synthesis(2013) Kabdulov, Mikhail; Jansen, Martin (Prof. Dr. Dr. h. c.)Direct synthesis of fullerenes is of considerable interest as a method to access new fullerenes which cannot be obtained in the uncontrolled process of graphite evaporation or form in low yields as a “hard-to-isolate” mixture. The general strategy of the direct approach to fullerenes is based on the synthesis of polycyclic aromatic hydrocarbons (PAH) that already contain the required carbon framework. Such “unrolled” molecules can be “rolled up” to form fullerenes through intramolecular Aryl-Aryl condensation under flash vacuum pyrolysis (FVP) conditions. The presence of chlorine or bromine in the initial precursor is essential for effective Aryl-Aryl condensation via free radical mechanism. On the other hand the use of chlorine and bromine functionalizations reaches its limits in the case of large molecules such as fullerene precursors. Availability of alternative promoters which do not have these disadvantages is a key prerequisite for successful direct fullerene synthesis. In this work various functional groups have been tested as alternative promoters of Aryl-Aryl intramolecular condensation under FVP conditions. Methyl and fluorine functionalization has been found to be promising approaches. Unexpected high selectivity in cyclization was observed for fluorine derivatives. It was found that HF elimination is a synchronous process leading directly to the target molecule without any intermediates, thus producing no side products. The small size and low molecular weight of fluorine as well as high thermostability of the C-F bond, makes fluorine a “perfect” activating group for rational fullerene synthesis. Since fluorine can promote the desired ring closure only if hydrogen is placed neighboring in space in the precursor structure, full control in the direction of the condensation can be achieved. It was shown that the use of fluorine, as an activating group, solves the problem of selectivity in FVP and provides an effective conversion of the respective PAH precursors. Several fullerene precursors containing fluorine atoms in key positions have been synthesized and investigated as a precursor for direct fullerene synthesis. Furthermore optimization has led to the discovery of a highly effective alternative solid-state strategy for intramolecular Aryl-Aryl coupling via HF elimination. The efficiency of the approach has been demonstrated by quantitative transformation of the precursor molecules to the desired PAHs and buckybowl structures. The quantitative conversion to the extended C46 buckybowl, representing more than 75% of the C60 fullerene connectivity, demonstrates the high potential of the technique for construction of extended non-planar carbon based nanostructures, including higher fullerenes, giant buckybowls and nanotubes. The results obtained point the way to the fabrication fullerenes as well as other carbon based nanostructures such as single walled nanotubes and nanoribbons in a fully controllable manner.Item Open Access Role of disorder and interactions on the surface of topological superconductors(2015) Queiroz, Raquel; Metzner, Walter (Prof. Dr.)In this work we study the surface properties of topological systems, with a special focus on topological superconductors without inversion symmetry. These materials provide a rich playground for multiple topological phenomena, showing boundary modes with linear and (or) flat dispersion arising from complex nodal structures. A remarkable characteristic of topological phases is their robustness to local perturbations. In the present work, we explore the extent to which this robustness can be generalized to gapless topological phases. We numerically test the robustness of topological boundary modes against local disorder and explore the contrast between different disorder strengths and distributions. Additionally, we study the interplay between topology and electron-electron interactions at the surface of nodal superconductors, where the infinitely degenerate flat bands are susceptible to spontaneous symmetry breaking. Finally, we briefly look into possible symmetry preserving interactions that can lead to the destruction of the boundary modes.Item Open Access New ternary and quaternary metal oxides of ruthenium and osmium(2006) Mogare, Kailash M.; Jansen, Martin (Prof. Dr. Dr. h. c.)Ternary and quaternary oxides of platinum metals are of particular interest because of their intriguing physical, especially magnetic, properties. A very few alkali metal ruthenates and osmates have been reported so far with ruthenium and osmium exhibiting high oxidation states. The scope of the present thesis is to synthesize and characterize new ternary and quaternary alkali metal ruthenates and alkali metal osmates with an acquisition of high oxidation states by the transition metals. To achieve this, high oxygen pressures at elevated temperatures have been applied by using steel autoclaves as reaction vessels. The synthesized alkali metal ruthenates and osmates are investigated qualitatively and quantatively and further characterized using techniques such as powder and single crystal X-ray diffraction, powder neutron diffraction, infra-red/Raman spectroscopy, thermal analysis and measurements of conductivity and magnetic properties. In the case of alkali metal oxo-ruthenates, new hexavalent ruthenates of the compositions A2RuO4 and A3B(RuO4)2 have been explored. Within this family, the compounds display a trend of increasing magnetic interactions with decrease in the size of alkali metals. These interactions are independent from the crystal structure type and can be correlated to the shortest Ru-Ru distances among the Ru6+ ions. Neutron diffraction studies are carried out on Na2RuO4, the ternary oxo-ruthenate displaying the highest magnetic transition temperature in this family, and the low temperature magnetic structure has been deduced from the same. A number of new ternary and quaternary alkali metal oxo-osmates containing heptavalent and octavalent osmium have been explored. Some more high valent osmium containing oxides, which have been predicted and sketchily studied earlier by spectroscopic methods, have now been characterized with their crystal structures and physical properties. Some of the new compounds explored in this work belong to the family of double or triple perovskites (A2BMO6 and A3BM2O9, respectively). Only a few double and triple perovskite oxides with alkali metal cations as the A and B cations are known till today. The new compounds represent the first osmium perovskites containing alkali metal cations at the A site. Sr2CrOsO6, a cubic double perovskite, is re-investigated and found to exhibit the highest Curie temperature known so far in the family of double perovskites. The compound belongs to the increasing list of double perovskites with the task of discovering high ferromagnetic ordering temperature for potential technological applications.Item Open Access Phase transitions in single layer and bilayer quantum Hall ferromagnets(2004) Höppel, Lutz W.; Klitzing, Klaus von (Prof. Dr.)A remotely doped GaAs wide quantum well (WQW) of 77 nm width is used to explore ferromagnetic phase transitions (FPTs), which occur due to crossings of Landau levels (LLs), using magneto-transport at low temperatures. An in-situ grown back gate and an evaporated front gate allow to control the electron density confined to the WQW and its distribution. FPTs are indicated by the loss of the quantum Hall effect (QHE) of a particular filling factor (ff) over a critical density range. A theoretical description is developed which assigns a pseudospin to the two crossing LLs. The pseudospin anisotropy energy determines how the system evolves when passing an FPT. In the regime of the integer QHE (IQHE) the electron density is adjusted that high that the two lowest subbands (offset by BAB of the order of 1 meV) are occupied (bilayer regime) leading to two intersecting LL fan-charts. The two FPTs involving ff = 4 exhibit narrow critical density ranges in contrast to ff = 3 and ff =5 signaling easy-axis and easy-plane anisotropy, respectively. The critical densities or magnetic fields (BC) shift to higher values with imbalance s (defined as the normalized density difference between the two layers) in a quadratic manner due to the equivalent increase of BAB. The critical density ranges of ff = 4 widen with s most probably due to a gradual changeover to easy-plane anisotropy, as predicted. A quantitative analysis of the FPTs yields a six-fold enhancement (Z) of the Zeeman splitting (EZ). Substituting one electron and two flux quanta by one composite Fermion (CF) maps the regime of the fractional QHE (FQHE) to the one of the IQHE. The orbital energy splitting is quantified by the CF cyclotron energy (EC) which scales as the Coulomb energy. The ff = 2/3 with two fully occupied CF LL (CF-ff = 2) gives rise to two FPTs. One FPT involves a change from a spin unpolarized to a spin polarized system (spin FPT) and occurs as a competition between EC and EZ. The finite thickness l of a single electron layer is tuned by varying the tilt of the WQW potential thus influencing EC since EC depends inversely on the average inter-electron distance. The latter is a function of the magnetic length lB and l (measured as FWHM and deduced from self-consistent simulations). EZ depends linearly on the effective g factor (-0.44 for bulk GaAs) and the enhancement Z. BC is found to vary from 4.5 down to 2.5 T by changing the electron distribution from squeezed at one of the edges of the WQW to very broad when centered in the WQW. A fit of BC in dependence of l based on the equality EC = EZ yields Z = 6 as upper limit. In the case of narrow (< 22 nm) QWs the reduction of the absolute value of g due to confinement effects increases BC. Widely spreading BC values, as reported in the literature, thus arise from heterostructure design and sample specific properties affecting l and g. Spin flip-flop processes connecting electron and nuclear spin system via the hyperfine coupling become apparent at the spin FPT because the nuclear magnetic field (BN), which is a function of the nuclear spin polarization, exclusively affects EZ thus altering BC. BC is thus a probe for the electron-nuclear-spin interaction. The strength of the hyperfine coupling is controlled by changing the ff. The time scale t1 to equilibrium between the two spin systems is found to be 75 s at ff = 0.5 and less than 3 s at ff = 0.9. An as short t1 time at ff = 1 as at ff = 0.9 indicates that low energy excitations, which promote flip-flop processes are not only provided by Skyrmions. BN is 0.4 T at the lowest temperature (25 mK) and opposite to the external field and an increasing DC-current (up to 60 nA) reduces BN by up to 100 mT. The Curie-temperature of the spin FPT is determined to be 170 mK. The second FPT involves a mixing of CF LLs from different subbands and occurs as a competition between BAB and EC in the bilayer regime and leads to a loss of coherence between the electron layers, known as one-component to two-component phase transition, due to the layer separation which increases with density. BC is found to be 8.7 T when s = 0. A thermal activation study reveals that an excitation of the system close to the FPT is accompanied by up to eight pseudospin flips suggesting easy-axis anisotropy and the existence of bilayer Skyrmions as predicted. BC increases with s in a quadratic manner. The bilayer unique QHE at ff = 1/2 is investigated and found to be most pronounced at a density close to the 1C-2C phase transition supporting theory which attributes the stabilization of this QHE to comparable inter- (BAB) and intra-layer (EC) Coulomb correlation. The finite longitudinal resistance close to the spin FPT is accompanied by sudden jumps if explored in a 18 nm narrow QW at the lowest temperature and slow sweep rates (< 0.05 T/min). These jumps resemble similarity to Barkhausen jumps and may reflect discontinuous changes of electron spin domains.Item Open Access Untersuchung von Druckumwandlungen an Oxiden und Fluoriden und Synthese neuer Verbindungen(2007) Orosel, Denis; Jansen, Martin (Prof. Dr. Dr. h. c.)In der vorliegenden Arbeit wird das Verhalten von verschiedenen Oxiden und Fluoriden bei hohen Drücken und Temperaturen untersucht. Die Untersuchung des alpha-SeO2 bei Drücken bis 19,5 GPa und Temperaturen bis 820 °C ergab zwei neue metastabile Hochdruckphasen, nämlich beta- und gamma-SeO2. Beide kristallisieren in der Raumgruppe Pmc21 (Nr. 26). Der Hauptunterschied in den Kristallstrukturen zwischen alpha-, beta- und gamma-SeO2 besteht in der Orientierung dieser Ketten zueinander. Des weiteren konnte im p-T-Diagramm die Phasengrenze zwischen der beta- und gamma-Modifikation des SeO2 bestimmt werden. Es wurden Hochdruckexperimenten mit kubischen (alpha) und orthorhombischen (beta) Sb2O3 zu einem Druck von 19,5 GPa und Temperaturen bis 400 °C durchgeführt. Es konnte die Struktur einer neuen Hochdruckmodifikation gamma-Sb2O3 gelöst werden. Diese kristallisiert in der Raumgruppe P212121 (Nr.19). Die Struktur wird aus SbO3E-Einheiten aufgebaut. Drei dieser Einheiten sind zu Sb3O3-Ringen verknüpft, die jeweils über eine weitere SbO3E-Einheit unendlichen Ketten in Richtung der a-Achse bilden. Die Ketten sind über eine Sauerstoffbrücke miteinander verbunden. Von Sb2O4 sind eine Raumtemperatur- (alpha) und eine Hochtemperaturmodifikation (beta) in der Literatur bekannt. Das Verhalten von alpha-Sb2O4 wurde durch Versuche mit einer Diamantstempelzelle bis 27,3 GPa Druck bei Raumtemperatur und in der Belt-Presse bis 6 GPa Druck und bei Temperaturen bis 400 °C untersucht. Aus den experimentellen Daten mit der Diamantstempelzelle konnten auch die Kompressionsmodule für die alpha- und beta-Phase berechnet werden. Die Versuche in den Hochdruckpresse zeigten bei p = 4 GPa und T = 400 °C und p = 6 GPa und T = 375 °C eine vollständige Umwandlung von alpha- zu beta-Sb2O4. Das Hochdruckverhalten von alpha-BiF3 wurde bis zu einem Druck von 20 GPa und Temperaturen bis 700 °C untersucht. In der Literatur wurde schon eine dem Tysonit (LaF3) ähnliche trigonale Hochdruckmodifikation „T-BiF3“ erwähnt, aber die Struktur wurde nicht vollständig gelöst. Bei der Indizierung wurde eine andere Zelle gefunden. Diese Hochdruckmodifikation (beta-BiF3) kristallisiert in der hexagonalen Raumgruppe P63/mmc. Das Verhalten von alpha-As2O5 ist zum einen mit Hilfe von Experimenten in Diamantstempelzellen bei Drücken bis 19,5 GPa und Raumtemperatur untersucht worden. Aus den Daten konnte das Kompressionsmodul für alpha-As2O5 berechnet werden. Zum anderen wurde alpha-As2O5 mit Hilfe von Hochdruckpressen bis zu 19 GPa Druck und Temperaturen bis 1400 °C untersucht. Um die Stabilität der Hochdruckmodifikationen zu bestimmen, wurden temperaturabhängigen Pulverdiffraktogramme aufgenommen. Es konnte die Umwandlung der Hochdruckmodifikationen über eine intermediäre Phase (gamma-As2O5) in die alpha-Modifikation beobachtet werden. Das intermediäre gamma-As2O5 kristallisiert monoklin in der Raumgruppe P21/n. Es wurde das Verhalten von alpha-V2O5 bei Drücken bis 29 GPa und Temperaturen bis 1450 °C untersucht. Aus den Daten der Hochdruckversuche konnte das p-T-Phasendiagramm erweitert werden. Es sind aber keine neuen Hochdruckphasen in dem Druckbereich gefunden. Die Struktur einer Hochdruckmodifikation (beta-V2O5) war schon aufgeklärt, die der anderen Hochdruckmodifikation (delta-V2O5) war noch nicht vollständig bestimmt. Die Verbindung Hexakaliumnonaoxodiselenat(VI) K6(SeO4)(SeO5) ist durch eine Festkörperreaktion aus K2O und K2SeO4 hergestellt worden und stellt das erste bekannte Kaliumorthoselenat(VI) dar. Die Verbindung kristallisiert in der tetragonalen Raumgruppe P41212 mit a = 812,6 pm, c = 1749,5 pm und V = 1155,2·106 pm3. Es besitzt zwei unterschiedliche Baueinheiten des Selenats(VI). Einmal das typische SeO4-Tetraeder und die trigonale Bipyramide SeO5. Halbiert man die Summenformel K6Se2O9 und reduziert man sie auf die K- und Se-Positionen, dann gehört es zu der großen Familie der anorganischen A3B-Strukturtypen, für die Li3Bi den Aristotyp darstellt. Eine weitere Verbindung, die bei hohen Drücken und Temperaturen dargestellt wurde ist die Arsen(III,V)oxosäure H6As14O31. Es kristallisiert in der Raumgruppe P63 (Nr. 173). Die Struktur ist ein dreidimensionales Netzwerk aus den bekannten AsO4 Tetraedern für As5+ und AsO3E Tetraedern für As3+. Aber sie enthält auch ein neues Koordinationspolyeder für das Kation As5+, nämlich AsO5 als quadratischen Pyramide. Die Struktur von H6As14O31 weist auch scheinbar leere Kanäle auf, die parallel der c-Achse durch die Struktur verlaufen. Durch die Synchrotronpulveraufnahmen konnten die H-Atome innerhalb der Hohlräume lokalisiert werden. Es wurde die Verbindung Palladium(II)metaarsenat PdAs2O6 dargestellt, welche das erste paramagnetische Oxid des Pd2+ ist. Es kristallisiert in der Raumgruppe P-31m (Nr. 162). Bei hohen Temperaturen zeigt es das erwartete paramagnetische Verhalten, bei Temperaturen unter 150 K findet man eine magnetische Ordnung. Die magnetische Struktur wurde durch Neutronenbeugung bestimmt.Item Open Access High pressure study of high-temperature superconductors(2014) Souliou, Sofia-Michaela; Keimer, Bernhard (Prof. Dr.)The current thesis studies experimentally the effect of high external pressure on high-Tc superconductors. The structure and lattice dynamics of several members of the high-Tc cuprate and Fe-based superconductors families were investigated by means of Raman spectroscopy and x-ray diffraction under well-controlled, hydrostatic high pressure and low temperature conditions. The lattice dynamics of the high-Tc superconductor YBa(2)Cu(3)O(6+x) have been investigated systematically by Raman spectroscopy as a function of doping (x = 0.95, 0.75, 0.60, 0.55, and 0.45) and external pressure. Under ambient pressure conditions, in addition to the Raman modes expected from group theory, we observe new Raman active phonons upon cooling the underdoped samples, at temperatures well above the superconducting transition temperature. The doping dependence and the onset temperatures of the new Raman features suggest that they are associated with the incommensurate charge density wave (CDW) state recently discovered in underdoped cuprates using synchrotron x-ray scattering techniques. Under high pressure conditions (from 2 to 12 GPa), our Raman measurements on highly ordered underdoped YBa(2)Cu(3)O(6.55) samples do not show any of the new Raman phonons seen at ambient pressure. High pressure and low temperature Raman measurements have been performed on the underdoped superconductor YBa(2)Cu(4)O(8). A clear renormalization of some of the Raman phonons is seen below Tc as a result of the changes in the phonon self-energy upon the opening of the superconducting gap, with the most prominent one being that of the B1g-like buckling phonon mode. The amplitude of this renormalization strongly increases with pressure, resembling the effect of hole doping in YBa(2)Cu(3)O(6+x). At 10 GPa, the system undergoes a reversible pressure-induced structural phase transition to a non-centrosymmmetric structure (space group Imm2). The structural transition is clearly reflected in the high pressure Raman data through the appearance of several new modes, allowing us to map in detail the (P,T) phase diagram and determine the transition line between the two phases. In the new phase, the renormalization of the buckling mode is completely suppressed, while no anomalies are observed in any of the other Raman active phonons. According to ab initio calculations, the coupling of the buckling mode to the electronic system is not significantly affected by the structural phase transition. The absence of phonon renormalizations in the presence of sizable electron-phonon coupling, indicate that, in contrast to earlier transport studies, YBa(2)Cu(4)O(8) is not superconducting anymore under hydrostatic pressures higher than 10 GPa. Finally we proceeded with the investigation of the high pressure structural and vibrational properties of SmFeAsO, a member of the "1111" family (space group P4/nmm) of the Fe-based superconductors, in which superconductivity is commonly induced either by substituting F/H for O or by applying high pressures on the parent magnetic compound. The magnetic transition of the undoped compound is accompanied with a tetragonal-to-orthorhombic structural distortion, both of which are commonly suppressed upon the emergence of superconductivity. In the SmFeAsO(x)F(1-x) system while the magnetic transition is totally suppressed already at low doping levels, structural studies have reported either the gradual suppression of the orthorhombic distortion or its retention over a wide regime of the superconducting phase. We addressed this controversy using high pressure as an alternative tuning parameter to suppress the magneto-structural transition and induce superconductivity in the parent compound. Our high pressure, low temperature x-ray diffraction measurements on single crystals of SmFeAsO have revealed that the tetragonal-to-orthorhombic transition survives with the application of high pressures up to 85 kbars. In addition, our Raman data reveal a linewidth renormalization of the c-axis polarized A1g and B1g Raman phonons through the magnetic transition, similar to the one previously observed in the "122" family and attributed to the opening of the spin density wave gap. The renormalization is gradually suppressed under high pressure, in line with an earlier high pressure magnetization study which reported the pressure-induced decrease of the magnetic transition temperature. The linewidth anomaly disappears at 8 GPa suggesting the complete suppression of the magnetic transition at this pressure.Item Open Access Interaction of superconductivity and ferromagnetism in YBCO/LCMO heterostructures(2005) Soltan, Soltan; Dressel, Martin (Prof. Dr.)It has been shown that bilayers grown out of spin-polarized LCMO and superconducting YBCO show a variety of new physical phenomena. The transition temperatures T_(c) and T_(Curie), the critical current density in the superconductor j_(c) and even the normal state resistance can be influenced by external parameters and/or the sample geometry. These bilayers might be good candidates for technical applications in the near future.