Universität Stuttgart
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Item Open Access Thermoelektronische Energiekonvertierung und ihre fundamentalen Grenzen(2021) Wanke, Robin; Mannhart, Jochen (Prof. Dr.)Durch einen thermoelektronischen Generator kann potenziell sehr effizient Wärmeenergie in elektrische Energie umgewandelt werden. Durch Elektronenemission von einem Emitter zu einem Kollektor kann bei richtiger Materialwahl Leistung generiert werden. Die Elektronen müssen durch ein Gitter geführt werden, um Raumladungen zu verhindern, welche den Strom verringern. Dieser Mechanismus wird in dieser Dissertation auf seine grundlegenden Grenzen untersucht. Dabei wird auf alle relevanten Komponenten geschaut und deren fundamentale und physikalischen Eigenschaften untersucht, um daraus technische Lösungen zu erarbeiten.Item Open Access Long-range order, bosonic fluctuations, and pseudogap in strongly correlated electron systems(2022) Bonetti, Pietro Maria; Metzner, Walter (Prof. Dr.)Item Open Access Automated parametric Rietveld refinement and its application to two dimensional X-ray powder diffraction experiments(2011) Rajiv, Paneerselvam; Joswig, Manfred (Prof. Dr.)Parametric Rietveld refinement has opened new possibilities to simultaneously refine multiple powder diffraction patterns collected in in situ 2D experiments; in that way the models of crystallographic variables that changes with external variables can be introduced into the refinement. The substitution of a variable with its model during the refinement has several advantages, including the improved precision of variables, direct extraction/refinement of some parameters from powder data which is otherwise impractical (e.g., activation energy), etc. The basic requirement for the realization of sequential/parametric refinements (or Whole Powder Pattern Fit-WPPF) in 2D X-ray powder diffraction (XRPD) is a robust software that handles the data and performs fast WPPF. This concern has been primarily addressed in this thesis with the help of a software, in combination with the existing total pattern analysis software (Topas). The developed software could considerably speedup and automate the sequential/parametric quantitative analysis of large number of 2D powder data, which is in general a monotonous and time consuming task. The software also provides routines that automatically determines the reconstructive phase transitions of samples from the 2D powder data and facilitates the independent refinements (or WPPFs) of the determined phases. Two practical scientific applications of parametric Rietveld refinement method have been demonstrated with the assistance of the developed program. The first application concerns the kinetic analysis of several polymorphs and polymorphs-additives mixtures of copper phthalocyanine (CuPC). The reaction rate constant and the order of reactions involving the phase transitions of various forms of CuPC were directly extracted from the isothermal experimental data by introducing the Johnson-Mehl-Avrami-Kolmogorov relation as a model of the phase fraction during the multi phase parametric Rietveld refinement. Parametric refinements could be successfully performed for most of the CuPC data collected in the experiment, however the convergence of some of the refinements showed a strong dependence on the reaction rate. In many cases, the precision of the refined parameters could be improved considerably when the data collected between the optimal time steps alone were used in the refinement. The second application demonstrates the feasibility of the parameterization of crystallite size with respect to the annealing time/temperature. Some of the data samples used in the kinetic analysis (CuPC) and the temperature dependent nanocrystalline TiO2 data were used in this demonstration. The success of the parameterization of crystallite size depended strongly on the quality of the data used, on the uniformity of the variation of the crystallite size with time/temperature and also on the correctness of the model that describes the crystallite size variation with time/temperature. This application in its present form is general; as such it can be used for stabilizing other variables during parametric refinement.Item Open Access Excitonic Fano resonances in Ta2NiSe5 and Ta2NiS5(2016) Larkin, Timofei I.; Keimer, Bernhard (Prof. Dr.)Item Open Access Raman scattering, magnetization and magnetotransport study of SrFeO3-delta, Sr3Fe2O7-delta and CaFeO3(2008) Damljanovic, Vladimir; Keimer, Bernhard (Prof. Dr.)In this thesis we have determined the Raman spectra as well as the magnetization, resistance and magnetoresistance of the compounds SrFeO3-delta, Sr3Fe2O7-delta and CaFeO3 as a function of temperature. These materials are interesting because they contain iron in the unusually high oxidation state +4, which has the same electroncic configuration as the Mn3+ ion in LaMnO3, a material that shows the giant magnetoresistance effect when doped with calcium or strontium. A novel aspect of the work described in this thesis is that it was performed on single crystals with controlled oxygen stoichiometry. In the compound SrFeO3-delta, delta can vary continuously in the range 0 to 0.5. The materialexhibits the following crystal structures due to oxygen vacancy ordering: cubic (delta=0), tetragonal (delta=0.125), orthorhombic (delta=0.25) or brownmillerite (delta=0.5). For other values of delta the material is a mixture of those phases. The cubic phase has the ideal cubic perovskite structure. In this thesis we describe the preparation of nearly stoichiometric SrFeO3-delta with delta<0.05. The Raman spectrum of a sample annealed under 5kbar of pure oxygen showed no phonon modes, as expected from a group-theoretical analysis of the ideal perovskite structure. The Mößbauer spectra on this sample shows that it contains 5.4% of the tetragonal phase. In another crystal annealed at oxygen pressure 40kbar Mößbauer spectra did not show any sign of additional phases, confirming that the sample is fully stoichiometric. In addition to the experiments we have performed lattice dynamics calculations for the ideal composition SrFeO3.00 in order to assign the phonon modes observed in infra-red experiments. The calculation accurately reproduces all frequencies observed. We have also measured the Raman spectra of the tetragonal phase in the temperature range 13K to 300K. While only three peaks can be resolved at room temperature, additional modes appear in the spectrum below the charge-ordering transition at 70K. This confirms that the crystal structure changes below this temperature. We have also measured the Raman spectra of the orthorhombic phase in the temperature range 6K to 475K. The paremeter delta in Sr3Fe2O7-delta can vary continuously between 0 and 1. We have measured the temperature dependence of the magnetization for the magnetic field along high symmetry axes of the crystal. We have also performed neutron diffraction measurements demonstrating that the magnetic moments are ordered in a helical structure. The resistivity and the magnetoresistance were measured in the range 10K to 300K. Finally we have measured the Raman spectra of the same sample in the temperature range 15K to 440K. In order to assign the observed modes, we have performed lattice dynamics calculations based on the published crystal structure of Sr3Fe2O7. The CaFeO3 compound has an orthorhombic crystal structure above 290K, which changes to monoclinic below this temperature. Here we describe the preparation of stoichiometric CaFeO3 single crystals by high pressure oxygenation of as-grown CaFeO2.5 samples, using KClO4 as an oxygen source. The powder X-ray diffraction pattern after annealing shows that the oxygen enrichment was successful. No magnetoresistance was observed within the experimental error up to magnetic fields of 9T. We have also measured Raman spectra of this material in the temperature range 15K to 300K. In contrast to tetragonal SrFeO2.875 these spectra are unaffected by the charge-ordering transition at 290K within the experimental sensitivity.Item Open Access Renormalization group analysis of order parameter fluctuations in fermionic superfluids(2014) Obert, Benjamin; Metzner, Walter (Prof. Dr.)In this work fluctuation effects in two interacting fermion systems exhibiting fermionic s-wave superfluidity are analyzed with a modern renormalization group method. A description in terms of a fermion-boson theory allows an investigation of order parameter fluctuations already on the one-loop level. In the first project a quantum phase transition between a semimetal and a s-wave superfluid in a Dirac cone model is studied. The interplay between fermions and quantum critical fluctuations close to and at the quantum critical point at zero and finite temperatures are studied within a coupled fermion-boson theory. At the quantum critical point non-Fermi liquid and non-Gaussian behaviour emerge. Close to criticality several quantities as the susceptibility show a power law behaviour with critical exponents. We find an infinite correlation length in the entire semimetallic ground state also away from the quantum critical point. In the second project, the ground state of an s-wave fermionic superfluid is investigated. Here, the mutual interplay between fermions and order parameter fluctuations is studied, especially the impact of massless Goldstone fluctuations, which occur due to spontaneous breaking of the continuous U(1)-symmetry. Fermionic gap and bosonic order parameter are distinguished. Furthermore, the bosonic order parameter is decomposed in transverse and longitudinal fluctuations. The mixing between transverse and longitudinal fluctuations is included in our description. Within a simple truncation of the fermion-boson RG flow, we describe the fermion-boson theory for the first time in a consistent manner. Several singularities appear due the Goldstone fluctuations, which partially cancel due to symmetry. Our RG flow captures the correct infrared asymptotics of the system, where the collective excitations act as an interacting Bose gas. Lowest order Ward identities and the massless Goldstone mode are fulfilled in our truncation.Item Open Access Spectroscopic study of CaMnO3/CaRuO3 superlattices and YTiO3 single crystals(2009) Yordanov, Petar; Keimer, Bernhard (Prof. Dr.)The first two sections of Chapter 1 give a general overview of the research topics and experimental methods discussed in the thesis. Further on, in Chapter 2, some of the most important characteristics and mechanisms underlying the physics of transition metal oxides are presented. As the experimental part of the thesis includes studies on manganites and titanates, these two classes of compounds are exemplified in the exposition of Chapter 2. Several recent works in the emerging research field of transition metal oxide interfaces and superlattices are also discussed along with a brief introduction in x-ray spectroscopic methods with synchrotron radiation. Chapter 3 introduces the principles of optical spectroscopy and the simplest models for dielectric function, i.e., Lorentz oscillator and Drude dielectric function. The following Chapter 4 introduces two of the experimental techniques in optical spectroscopy, reflectance and spectroscopic ellipsometry. Further on, we describe the design of a new home-built apparatus for near-normal reflectance with high magnetic fields. Several critical technical details and findings during the assembling process are also discussed. Chapter 5 represents a comprehensive experimental spectroscopic study of a prototypical superlattice system made from an antiferromagnetic insulator CaMnO3 and a paramagnetic metal CaRuO3. The resulting interface ferromagnetic state was closely investigated by means of optical spectroscopy as well as by soft x-ray scattering and absorption methods. This study led us to the conclusion that magnetic bound states, i.e. magnetic polarons, have to be considered in the description of this SL system. Chapter 6 describes a polarized far infrared reflectance study with high magnetic field on the ferromagnetic Mott insulator YTiO3, single crystals. All 25 infrared-active phonon modes were observed. The temperature and magnetic-field dependence of the phonon modes revealed a weak spin-phonon coupling in YTiO3 and largely extended temperature range (up to TM ~ 80 - 100K), for the field-induced effects on the oscillator parameters. This later observation, uncovered short-range magnetic order state which remains even at temperatures as high as three times the temperature of the actual ferromagnetic transition of Tc ~ 30K. While a quantitative theoretical description of these data is thus far not available, they point to a complex interplay between spin, orbital, and lattice degrees of freedom due to the near-degeneracy of the Ti t2g orbitals in YTiO3.Item Open Access Ultraschnelle Ladungsträgerdynamik in LTG-GaAs und ErAs:GaAs Übergittern : Grundlagen und Anwendungen(2002) Griebel, Martin; von Klitzing, Klaus (Prof. Dr.)Die Motivation für die Experimente dieser Arbeit entspringt dem Ziel, ultrahoch-zeitaufgelöste Transportmessungen an mesoskopischen Bauelementen durchzuführen. Derartige Messungen erfordern Zeitauflösungen weit unterhalb von 10 ps, gleichzeitig muss die verwendete Messmethode kompatibel zu tiefsten Temperaturen (T << 1 K) und hohen Magnetfeldern sein. Allein die geforderte Zeitauflösung schließt die Verwendung rein elektronischer Methoden zur Durchführung derartiger Experimente aus. In dieser Arbeit stellen wir ausgehend von der Methode des Photoleitungs-Samplings eine zu den Anforderungen mesoskopischer Bauelemente kompatible, integrierte Anordnung zur Durchführung hoch-zeitaufgelöster Transportexperimente vor, die sowohl die Erzeugung der hochfrequenten Signale als auch ihre Detektion und Konvertierung in quasi-DC Signale in unmittelbarer Nähe zum eigentlichen Untersuchungsobjekt innerhalb einer einzigen Probenstruktur ermöglicht. Da derartige Experimente zur Vermeidung übermäßiger Aufheizung durch Wärmestrahlung in fensterlosen Kryostaten betrieben werden sollten, wurde die Beleuchtung der Photoleitungs-Sampling Struktur durch kurze Laserpulse mithilfe optischer Fasern realisiert. Der Erfolg dieses Ansatzes hängt von der Verfügbarkeit photoleitender Materialien ab, die es einerseits erlauben, mithilfe kurzer optischer Pulse unter dem Einfluss tiefer Temperaturen und hoher Magnetfelder ultrakurze elektrische Pulse zu erzeugen und die andererseits zur monolithischen Integration mit den zur Herstellung mesoskopischer Bauelemente notwendigen Heterostrukturen geeignet sind. Der Schwerpunkt dieser Arbeit lag zunächst in der Untersuchung neuartiger photoleitender Materialien im Hinblick auf die Erfüllung dieser Kriterien. Besonderes Gewicht wurde auf die Aufklärung der für die Ladungsträgerdynamik relevanten physikalischen Mechanismen gelegt. Im Anschluss daran wurden die zur Erzeugung, Propagation und Detektion elektrischer Pulse mit Bandbreiten von einigen Terahertz notwendigen Methoden entwickelt und zur Verwendung unter dem Einfluss tiefer Temperaturen und hoher Magnetfelder optimiert. Ein Ausgangsmaterial für Photoleitungsschalter, das, wie wir zeigen konnten, den gestellten Anforderungen genügt, besteht aus einem Übergitter äquidistanter ErAs-Inselschichten in einer GaAs Matrix (ErAs:GaAs). Die Ladungsträgerdynamik in ErAs:GaAs wurde als Funktion der Übergitterperiode mithilfe von Autokorrelationsmessungen an Photoleitungsschaltern sowie mithilfe von Pulspropagationsexperimenten an koplanaren Streifenleitungen untersucht. Wir beobachten einen Anstieg der Elektronenlebensdauer um nahezu zwei Größenordnungen von 190 fs auf 17 ps bei einer Erhöhung der Übergitterperiode von 10 auf 400 nm. Dieses Verhalten kann im Rahmen eines Diffusionsmodells verstanden werden, das in Übereinstimmung mit den experimentellen Daten eine quadratische Abhängigkeit der Elektronenlebensdauer von der Übergitterperiode vorhersagt. ErAs:GaAs zeichnet sich nicht nur durch seine sehr kurze Elektronenlebensdauer aus, sondern besitzt darüber hinaus eine bislang in diesem Maße nicht beobachtete Abstimmbarkeit der Elektronenlebensdauer, eine hohe thermische Stabilität sowie zusätzliche, unabhängige Materialparameter zur Kontrolle des Dunkelstroms. Unter Verwendung dieses neuartigen Substratmaterials wurden Photoleitungssampling-Experimente durchgeführt, indem jeweils zwei Photoleitungsschalter in eine koplanare Wellenleiterstruktur integriert und über optische Monomodenfasern mit Subpikosekunden-Laserpulsen beleuchtet wurden. Für eine Propagationsdistanz von 1.5 mm konnten wir dabei eine Zeitauflösung von weniger als 2.0 ps erreichen. In Experimenten unter kryogenen Bedingungen erwies sich diese Zeitauflösung sowohl als temperatur- als auch als magnetfeldunabhängig. Ein numerisches Modell zur Beschreibung der Generation, Propagation und Detektion der elektrischen Pulse konnte die Dispersion der koplanaren Wellenleiter sowie die Kapazitäten der Photoleitungsschalter als limitierende Faktoren der Zeitauflösung identifizieren. Durch die Verringerung der Bandlücke des die ErAs-Inseln umgebenden Matrixmaterials können Photoleitungsschalter bei längeren Wellenlängen betrieben werden. Als besonders attraktiv erweist sich in diesem Zusammenhang InGaAs mit einer an InP Substrate angepassten Komposition. Dieses Material absorbiert bis zu Wellenlängen von 1.67 µm, so dass die für Kommunikationszwecke relevanten Wellenlängen abgedeckt werden können. In Photostrom-Autokorrelationsmessungen an Proben mit einer Übergitterperiode von 40 nm konnten wir eine Elektronenlebensdauer von 1.1 ps bei einer Wellenlänge von 800 nm erzielen. Eine Veränderung der Wellenlänge von 750 nm auf 1 µm ergab keine Veränderung der Ladungsträgerlebensdauer, so dass kurze Elektronenlebensdauern bis hin zu 1.67 µm erwartet werden können.Item Open Access A study of lattice dynamics in iron-based superconductors by inelastic light scattering(2013) Um, Youngje; Keimer, Bernhard (Prof. Dr.)After the discovery of high temperature (high Tc) superconductivity in copper oxide-based materials (cuprates) in 1986, this phenomenon was a unique property of the cuprates for more than 20 years. The origin of high Tc superconductivity is still under debate. In 2008, high Tc superconductivity was discovered in iron-based compounds. This discovery presents new opportunities for the development of a fundamental understanding of high Tc superconductivity. Density functional calculations indicate a weak electron-phonon coupling strength in iron-based superconductors and these suggest that superconductivity is not mediated by phonons. However, experimental report of a large isotope effect of the iron atoms on the superconductivity Tc suggests that phonons play an important role in iron-based superconductors. Motivated by these findings, this thesis presents a Raman scattering study of the lattice dynamics of the iron-based superconductors Fe1+yTe1-xSex, LiFeAs and NaFe1-xCoxAs as a function of chemical composition and temperature. In Fe1+yTe1-xSex, an unconventional linewidth broadening of the c-axis polarized Fe phonon of B1g symmetry is found with decreasing temperature, which indicates an unusual coupling between the phonon and iron excessinduced magnetic fluctuations in this compound. In LiFeAs, the Raman scattering data provide evidence for a weak electron-phonon coupling, which is consistent with non-phonon mediated Cooper pairing in this compound. In NaFe1-xCoxAs, upon cooling two features are observed: (i) an unconventional linewidth broadening of several phonons, which is indicative of spin fluctuation-phonon coupling, and (ii) a superconductivity-induced phonon lineshape renormalization, which can not be explained by standard model calculations.Item Open Access Neutron scattering studies on layered ruthenates(2018) Krautloher, Maximilian; Keimer, Bernhard (Prof. Dr.)Transition metal oxides (TMOs) exhibit a large variety of magnetic, electronic, and structural phases and have received much attention from the community. The tight competition between different interactions and ordering phenomena typical for such systems result in phase diagrams which are characterized by a multitude of transitions. These often depend on external variables, including temperature, magnetic or electric fields, pressure, and chemical doping. Early research focused on oxides of light transition metals exhibiting flat electronic bands and strongly correlated systems. Prominent examples include the families of copper oxides (cuprates) that exhibit high-temperature superconductivity, and manganites that show colossal magnetoresistance. For a long time, oxides of heavier transition metals were not expected to exhibit particular exciting phenomena: with increasing atomic mass and ionic radii, the Coulomb repulsion decreases while the extension of the d orbitals enlarges, consequently increasing the orbital overlap and the electronic bandwidth W. Such heavy-metal based systems were therefore expected to be metallic, without the intricate competition between different ordering phenomena seen in their lighter analogues. Recently, however, it was recognized that the spin-orbit coupling (SOC) can profoundly change the phase behavior of 4d- and 5d-electron materials. The strength of SOC scales with the atomic number Z as ∝Z^4 , which—in contrast to systems including 3d TMOs—renders SOC a driving force in oxides of heavy transition metals. As the interplay between SOC and electronic correlations brings about novel quantum ground states, these systems have received increasing interest during the last decade. One such systems is Sr2IrO4, where this interplay generates a Mott-insulating state with total angular momentum J_{eff} = 1/2 . 4d-electron compounds, which are characterized by moderate SOC, have until recently been modeled akin to oxides of 3d-electron systems, treating the SOC as a minor perturbation only. However, even moderate SOC proved to be enough to realize exotic phenomena that are not captured by such approaches, and can lead to a variety of competing structural and magnetic phases. Consequently, the role of SOC in 4d TMOs has been underestimated, calling for re-evaluation of the underlying physics. In this work we focus on the antiferromagnetic Mott insulator Ca2RuO4, in which the interaction is limited to the two-dimensional layers of RuO6 octahedra. The low-spin 4d^4 configuration of Ru^{4+} leads to a S = 1 spin, while the lattice symmetry results in an effective orbital momentum of L_{eff} = 1. Previous studies have shown that Ca2RuO4 undergoes an insulator-metal transition upon heating and exhibits a series of phase transitions upon isovalent substitution with Sr. The wide variety of phases makes Ca2RuO4 a prime material platform to investigate the role of moderate SOC in magnetism. We concentrate on the magnetic excitation spectrum, which reflects the combined influence of the exchange interactions between the Ru ions and the inter-ionic SOC. The first part of this PhD project is dedicated to the growth of high-quality crystals of Ca2RuO4 and related ruthenium oxides. To this end, we used the optical floating zone technique. The several hundred crystal shards were then co-aligned to be used in inelastic neutron scattering experiments. With a map of the magnetic scattering intensity in the full magnetic Brillouin zone, we observe and distinguish all transverse magnon (Goldstone) modes as well as a longitudinal amplitude (Higgs) mode. The results can be consistently interpreted in an excitonic magnetism model with a dominant influence of the SOC. We then used inelastic neutron scattering to investigate the magnetic excitations of the Sr-substituted Ca2RuO4 crystals and found a modified set of exchange interactions. We also investigated the closely related Ca3Ru2O7 system; here, a double-layers of RuO6 octahedra are interleaved by a CaO barrier layer. We find that this bilayer system exhibits a metallic phase where the impact of the SOC is less pronounced. Surprisingly, a chemical substitution of the 4d^4 Ru^{4+} ions with magnetically inactive 3d^0 Ti^{4+} ions renders the system insulating even for Ti concentrations less than 1 %. In this phase that the system’s magnetic excitations are similar to Ca2RuO4 suggesting the same excitonic magnetism. Our studies demonstrate the crucial role of SOC for the magnetic properties of ruthenium oxides, and call for a general reevaluation of the impact of SOC on the ground state and excitations of 4d-electron systems.