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Item Open Access Migration mechanisms of a faceted grain boundary(2018) Hadian, Raheleh; Grabowski, Blazej; Finnis, Michael W.; Neugebauer, JörgWe report molecular dynamics simulations and their analysis for a mixed tilt and twist grain boundary vicinal to the Σ7 symmetric tilt boundary of the type {123} in aluminum. When minimized in energy at 0K, a grain boundary of this type exhibits nanofacets that contain kinks. We observe that at higher temperatures of migration simulations, given extended annealing times, it is energetically favorable for these nanofacets to coalesce into a large terrace-facet structure. Therefore, we initiate the simulations from such a structure and study as a function of applied driving force and temperature how the boundary migrates. We find the migration of a faceted boundary can be described in terms of the flow of steps. The migration is dominated at lower driving force by the collective motion of the steps incorporated in the facet, and at higher driving forces by the step detachment from the terrace-facet junction and propagation of steps across the terraces. The velocity of steps on terraces is faster than their velocity when incorporated in the facet, and very much faster than the velocity of the facet profile itself, which is almost stationary. A simple kinetic Monte Carlo model matches the broad kinematic features revealed by the molecular dynamics. Since the mechanisms seem likely to be very general on kinked grain-boundary planes, the step-flow description is a promising approach to more quantitative modeling of general grain boundaries.Item Open Access Interlaboratory comparison measurements of aspheres(2018) Schachtschneider, R.; Fortmeier, I.; Stavridis, M.; Asfour, J.; Berger, G.; Bergmann, R. B.; Beutler, A.; Blümel, T.; Klawitter, H.; Kubo, K.; Liebl, J.; Löffler, F.; Meeß, R.; Pruß, Christof; Ramm, D.; Sandner, M.; Schneider, G.; Wendel, M.; Widdershoven, I.; Schulz, M.; Elster, C.The need for high-quality aspheres is rapidly growing, necessitating increased accuracy in their measurement. A reliable uncertainty assessment of asphere form measurement techniques is difficult due to their complexity. In order to explore the accuracy of current asphere form measurement techniques, an interlaboratory comparison was carried out in which four aspheres were measured by eight laboratories using tactile measurements, optical point measurements, and optical areal measurements. Altogether, 12 different devices were employed. The measurement results were analysed after subtracting the design topography and subsequently a best-fit sphere from the measurements. The surface reduced in this way was compared to a reference topography that was obtained by taking the pointwise median across the ensemble of reduced topographies on a 1000×1000 Cartesian grid. The deviations of the reduced topographies from the reference topography were analysed in terms of several characteristics including peak-to-valley and root-mean-square deviations. Root-mean-square deviations of the reduced topographies from the reference topographies were found to be on the order of some tens of nanometres up to 89 nm, with most of the deviations being smaller than 20 nm. Our results give an indication of the accuracy that can currently be expected in form measurements of aspheres.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.Item Open Access Hybrid materials for nonlinear optics(2018) Albrecht, Gelon; Giessen, Harald (Prof. Dr.)The goals of this thesis are to find new and more efficient material systems as well as concepts for nonlinear optics on the nanoscale. Nonlinear optical effects are mainly limited in such systems by the low nonlinear susceptibility and low photo stability of the used materials. To improve the low nonlinear susceptibility, plasmonic materials have been used for several years. These systems use the near-field enhancement of the plasmonic resonance to increase the nonlinear conversion efficiency. The efficiency can additionally be increased by using the evanescent plasmonic near-field in the vicinity of the plasmonic nanostructure. Therefore, a highly nonlinear organic polymer is deposited on the plasmonic nanostructures, creating a hybrid organic plasmonic material. Several organic materials are particularly suited due to their high nonlinear susceptibility and their simple and reproducible handling. Combined with high photo stability, these are the key requirements for a suitable polymer. However, several tested polymers did not meet these requirements. Notably, the photo stability is too low. Furthermore, for the first time it could be unambiguously proven that these hybrid materials can be improved due to an increased overall nonlinear susceptibility. Many other concepts for hybrid materials only utilize the modified near-field distribution and cannot benefit from the surrounding nonlinear medium or cannot exclude this influence. The presented layout can easily be improved by replacing the used polymer with other existing polymers that exhibit larger nonlinear susceptibilities. The hybrid plasmonic structures use gold as plasmonic material. Even if it is more photo stable than polymers, gold does not withstand high illumination intensities due to its low dimensional stability. This is a major drawback since most applications require a stable plasmon resonance. To overcome this issue a simple but effective way to significantly increase the thermal stability as well as the photo stability of gold nanostructures is presented. The improved properties are due to an alumina protective coating. The alumina coating can be as thin as 4 nm maintaining access to the enhanced near-field of the plasmonic nanostructure. With this concept a platform for nonlinear optics and high temperature applications is available that is stable in air at temperatures up to 900°C and still has excellent optical properties. Moreover this system withstands laser intensities at least up to 10 GW/cm² , one order of magnitude more than usually used intensities for nonlinear spectroscopy on gold nanostructures. Finally, common and more uncommon plasmonic materials are surveyed to determine their linear and nonlinear optical properties. Furthermore, the thermal and chemical stability with and without a protective alumina coating is investigated. Based on the collected data silver, gold, copper, magnesium, and aluminum could be identified and confirmed to be suitable materials for nonlinear applications. Moreover, nickel, palladium, platinum, germanium, and YH2 are investigated for their plasmonic and thermal properties, however suitable nonlinear properties have not been observed. Based on this survey a comparison of the presented materials is possible, which surprisingly did not exist until this survey. Bi2Te2Se is investigated as an unusual plasmonic material that exhibits edge state plasmons. These edge state plasmons arise from the topological properties of the material. Up to now these edge state plasmons have only been observed via electron excitation. To reveal the predicted localized modes nanostructures are fabricated by several methods and dark field spectroscopy is applied. However, no optical plasmonic response could be identified, most likely due to the small scattering rate of the material.Item Open Access Double light-cone dynamics establish thermal states in integrable 1D Bose gases(2018) Langen, Tim; Schweigler, Thomas; Demler, Eugene; Schmiedmayer, JoergWe theoretically investigate the non-equilibrium dynamics in a quenched pair of one-dimensional Bose gases with density imbalance. We describe the system using its low-energy effective theory, the Luttinger liquid model. In this framework the system shows strictly integrable relaxation dynamics via dephasing of its approximate many-body eigenstates. In the balanced case, this leads to the well-known light-cone-like establishment of a prethermalized state, which can be described by a generalized Gibbs ensemble. In the imbalanced case the integrable dephasing leads to a state that, counter-intuitively, closely resembles a thermal equilibrium state. The approach to this state is characterized by two separate light-cone dynamics with distinct characteristic velocities. This behavior is a result of the fact that in the imbalanced case observables are not aligned with the conserved quantities of the integrable system. We discuss a concrete experimental realization to study this effect using matterwave interferometry and many-body revivals on an atom chip.Item Open Access Hydrogen transport in thin films : Mg-MgH2 and Ti-TiH2 systems(2018) Hadjixenophontos, EfiHydrogen storage has become progressively important due to increasing energy demand. Magne-sium (Mg/MgH2) is one of the most promising elements of hydrogen uptake, however, the slow kinetics and need for high temperatures during dehydrogenation make this material challenging for mobile applications. Meanwhile, Titanium (Ti/TiH2/TiO2) draws attention due to its catalytic effect in hydrogenation of other metals with higher capacities. A comprehensive way to quantitatively char-acterize the kinetics of hydride formation in both systems (Mg and Ti) is shown here. A technique allowing a large range of pressures and temperatures (room temperature to 300 °C and from 0.05 bar up to 100 bar) is developed successfully. Thin films (50-1000 nm), deposited by ion beam sput-tering (PVD), are used because of their smooth surface and defined structure. In order to study hydrogen transport precisely, X-ray diffraction (XRD), electron microscopy (SEM/FIB/TEM) and electric resistance measurements are used. In the case of Mg, while a Pd coating is used as catalyst, the hydride is formed from the surface towards the substrate and transformation in the morpholo-gy is observed. Parabolic law is followed and the diffusion coefficient of hydrogen in MgH2 is ob-tained at room temperature (2.67 · 10-17 cm2/s). Additionally, a model is created to fit the experi-mental change in resistance during hydrogen loading and shows the changes in the behavior of thicker layers. The interface between Pd/Mg is discussed, since Mg5Pd2 and Mg6Pd are formed at high temperatures and are most dominant over dehydrogenation. However, at room temperature, this interface appears to be more stable. The activation energy of hydrogenation is calculated ex-perimentally from an Arrhenius plot to be equal to Ea = 22.6 ± 2.0 kJ/mol and the pre-factor D0 = 3904 cm2/s. Additional attention is given to magnesium hydride as an anode electrode in Li-ion bat-teries. TEM investigations of thin film electrodes demonstrate the complete lithiation of the mate-rial however, with drastic volume changes, leading to bad reversibility. In Ti the thin oxide layer naturally formed on the surface, appears to play a dominant role in the kinetics of hydrogen transport leading to a linear kinetics. A pressure dependency is observed, while an experimental evaluation of the permeation coefficient in the oxide is also discussed. Important information on the hydrogen transport is obtained in both systems, giving an input for further improvements of such hydrides.Item Open Access Optische Phänomene in Natur und Alltag(2018) Haist, TobiasDieses eBook soll Ihnen helfen, optische Phänomene in Natur und Alltag zu sehen und diese zu verstehen. Es ist als Begleitung (Skript) zur Vorlesung "Optische Phänomene" am Institut für Technische Optik der Universität Stuttgart entstanden. Hauptzielgruppe sind daher Studenten der Physik, des Maschinenbaus und des Studium Generale.Item Open Access Ions and electrons interacting with ultracold atoms : novel approaches based on Rydberg excitations(2018) Kleinbach, Kathrin Sophie; Pfau, Tilman (Prof. Dr.)In this thesis, the interaction of ions and electrons with ultracold atoms was investigated at the example of Rubidium-87. Thereto, a Rydberg atom was excited in a dense ultracold cloud. Both the interaction of the ionic core of the Rydberg atom and the Rydberg electron itself with neighboring neutral atoms was studied. Two main achievements were reported: First, photo-association of hybrid Trilobite Rydberg molecules was demonstrated. This is a special type of homonuclear molecules with a large electric dipole moment, which is bound by the electron-atom interaction. Second, it was shown for the first time that the ion-atom interaction between the Rydberg ionic core and the neutral atom can be accessed experimentally. The ion-atom interaction was observed at submicrokelvin temperature, once the nearest neighbor spacing in BEC is small and the electron-atom interaction is suppressed.Item Open Access Photo-excited dynamics in the excitonic insulator Ta2NiSe5(2018) Werdehausen, Daniel; Takayama, Tomohiro; Albrecht, Gelon; Lu, Yangfan; Takagi, Hidenori; Kaiser, StefanThe excitonic insulator is an intriguing correlated electron phase formed of condensed excitons. A promising candidate is the small band gap semiconductor Ta2NiSe5. Here we investigate the quasiparticle and coherent phonon dynamics in Ta2NiSe5 in a time resolved pump probe experiment. Using the models originally developed by Kabanov et al for superconductors (Kabanov et al 1999 Phys. Rev. B 59 1497), we show that the material’s intrinsic gap can be described as almost temperature independent for temperatures up to about 250 K to 275 K. This behavior supports the existence of the excitonic insulator state in Ta2NiSe5. The onset of an additional temperature dependent component to the gap above these temperatures suggests that the material is located in the BEC-BCS crossover regime. Furthermore, we show that this state is very stable against strong photoexcitation, which reveals that the free charge carriers are unable to effectively screen the attractive Coulomb interaction between electrons and holes, likely due to the quasi 1D structure of Ta2NiSe5.Item Open Access PFG-NMR studies of ATP diffusion in PEG-DA hydrogels and aqueous solutions of PEG-DA polymers(2018) Majer, Günter; Southan, AlexanderAdenosine triphosphate (ATP) is the major carrier of chemical energy in cells. The diffusion of ATP in hydrogels, which have a structural resemblance to the natural extracellular matrix, is therefore of great importance to understand many biological processes. In continuation of our recent studies of ATP diffusion in poly(ethylene glycol) diacrylate (PEG-DA) hydrogels by pulsed field gradient nuclear magnetic resonance (PFG-NMR), we present precise diffusion measurements of ATP in aqueous solutions of PEG-DA polymers, which are not cross-linked to a three-dimensional network. The dependence of the ATP diffusion on the polymer volume fraction in the hydrogels, φ, was found to be consistent with the predictions of a modified obstruction model or the free volume theory in combination with the sieving behavior of the polymer chains. The present measurements of ATP diffusion in aqueous solutions of the polymers revealed that the diffusion coefficient is determined by φ only, regardless of whether the polymers are cross-linked or not. These results seem to be inconsistent with the free volume model, according to which voids are formed by a statistical redistribution of surrounding molecules, which is expected to occur more frequently in the case of not cross-linked polymers. The present results indicate that ATP diffusion takes place only in the aqueous regions of the systems, with the volume fraction of the polymers, including a solvating water layer, being blocked for the ATP molecules. The solvating water layer increases the effective volume of the polymers by 66%. This modified obstruction model is most appropriate to correctly describe the ATP diffusion in PEG-DA hydrogels.