08 Fakultät Mathematik und Physik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/9
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Item Open Access Investigating superconductivity by tunneling spectroscopy using oxide heterostructures(2017) Fillis-Tsirakis, Evangelos; Mannhart, Jochen (Prof. Dr.)Item Open Access Nonlinear optical microspectroscopy with few-cycle laser pulses(2017) Wan, Hui; Wrachtrup, Jörg (Prof. Dr.)Nonlinear optical (NLO) microscopy is a powerful tool in physics, chemistry, and material science it probes intrinsic optical properties of the sample without the need of labeling. In order to investigate the ultrafast processes in nonlinear materials with high spatial resolution, we need to combine both ultrashort pulses and techniques focusing them to the diffraction limit. Previously, few-cycle laser pulses have often been tightly focused using conventional microscope objectives. However, the propagation of an ultrashort pulse in optical materials, particularly in the glass of a high numerical aperture (N.A.) microscope objective, results in spatial and temporal distortions of the pulse electric field, which can severely affect its quality in the focus. By purely passive group delay dispersion (GDD) and third-order dispersion (TOD) management, in this thesis, we experimentally demonstrate in-focus diffraction-limited and bandwidth-limited few-cycle pulses by using high N.A. objectives. Based on these achievements, the performance of a novel few-cycle NLO microscope for both second-harmonic generation (SHG) imaging and microspectroscopy in the frequency- and time-domains was characterized. The inverse linear dependence of SHG intensity on the in-focus pulse duration was demonstrated down to 7.1 fs for the first time. The application of shorter in-focus pulses for the enhancement of SHG image contrast was successfully demonstrated on a single collagen (type-I) fibril as a biological model system for studying protein assemblies under physiological conditions. Beyond imaging, a collagen fibril has been found to act as a purely non-resonant χ(2) soft matter under the present excitation conditions, and its ratio of forward- to epi-detected SHG intensities allowed for the estimation of the fibril thickness, which corresponds well with atomic force microscopy (AFM) measurements. The ultrafast dephasing of the localized surface plasmon resonance (LSPR) in the metallic nanoparticles, that only occurs on a time scale of a few femtoseconds, has gained a lot of attraction in the field of nanoplasmonics. This thesis is the first systematic experimental demonstration of time-resolving ultrashort plasmon dephasing in single gold nanoparticles by using interferometric SHG spectroscopy with in-focus 7.3 fs excitation pulses in combination with linear scattering spectroscopy performed on the same nanoparticle. For nanorods, nanodisks, and nanorectangles, strong plasmon resonance enhanced SHG is observed, where the SHG intensity strongly depends on the spectral overlap between the LSPR band and the excitation laser spectrum. For single nanorods and nanorectangles, the polarization dependence of the SHG intensity was found to follow second-order dipole scattering, and the effect of size and shape on the LSPR properties was directly observed in the time-domain. Good agreement between experimental and simulated values of dephasing times and resonance wavelengths is obtained, which confirms that a common driven damped harmonic oscillator model for the LSPR in the nanoparticle can qualitatively explain both the linear scattering spectra in the frequency-domain and the SHG response in the time-domain. Resonance bands in linear transmission and scattering spectra have also been observed for nanoholes with sizes smaller than the wavelength of the incident light in a metal film, which are assigned to LSPR modes of the electric field distribution around the nanohole with qualitatively similar resonance properties as a nanoparticle. The polarization-resolved nonlinear optical properties of the single nanoholes with different shapes and symmetries were also reported. The objective of this thesis has been systematic SHG studies of the size effect in the LSPR of single nanoholes in metal films and of their ultrafast dephasing dynamics. Although, enhancement of both the forward- and epi-detected SHG emissions from single rectangular nanoholes are observed,however,no ultrafast dephasing dynamics of LSPRs in rectangular nanoholes could be time-resolved with our in-focus 7.3 fs excitation laser pulses, which indicates that contributions from LSPR enhanced SHG to the detected SHG signal are negligible. More work needs to be done in order to overcome the current experimental limitations. However, in this thesis, the polarization dependence of the forward- and epi-detected SHG intensity from the single rectangular nanohole was found to follow that of a second-order dipole pattern. While the SHG dipole pattern observed for rectangular nanoparticles is oriented parallel to its long-axis, the SHG dipole pattern of its complementary rectangular nanohole is oriented perpendicular to its long-axis. This observation represents the first experimental demonstration of Babinet’s principle in second-order nonlinear scattering of a single rectangular nanohole in a gold film.Item Open Access Exploring the growth of refractory metal and sapphire films by thermal laser epitaxy(2024) Majer, Lena N.; Mannhart, Jochen (Prof. Dr.)Item Open Access Microscopic calculation of line tensions(2008) Merath, Rolf-Jürgen Christian; Dietrich, Siegfried (Prof. Dr.)In this work the line tension has been determinded with molecular resolution, which in this context marks the forefront of research. A semi-microscopic line tension theory based on the sharp-kink approximation has been further developed. The sharp-kink results concerning wetting and line tension behavior deviate considerably from the fully microscopic results. A hybrid line tension theory has been introduced, which employs an improved effective interface potential for the SK line tension calculation. For most of the studied cases the results from this hybrid method describe the fully microscopic line tension values semi-quantitatively. However, for a tailored system with relatively strong spatial variations of the substrate potential and of the solid-liquid interfacial density the hybrid method fails and does not predict the correct order of magnitude of the line tension values. Hence in general the fully microscopic approach is required, if one is interested in quantitatively reliable line tension values or/and if the validity of the hybrid method for the considered system has not been checked. The calculation of the line tension of a liquid wedge is an important contribution for understanding the shape of very small droplets (below the micrometer range). Furthermore a proposal is given, how axisymmetric sessile droplets can be addressed efficiently within DFT.Item Open Access Dominant dimensions of finite dimensional algebras(2012) Abrar, Muhammad; König, Steffen (Prof. Dr. rer. nat.)We study the dominant dimensions of three classes of finite dimensional algebras, namely hereditary algebras, quotient algebras of trees and serial algebras. We see that a branching vertex plays a key role to establish that the dominant dimension (dom.dim) of hereditary algebras (quivers) is at most one. We define arms of a tree and split trees into two classes: trees without arms and trees with arms. Like hereditary algebras, it turns out that the dominant dimension of the quotient algebras of trees can not exceed one. For serial algebras A associated to linearly oriented quiver with n vertices, we give lower and upper bounds of dom.dimA, and show that the bounds are optimal. It is also shown that some of the algebras A satisfy the conditions in the higher dimensional version of the Auslander's correspondence. Further we consider serial algebras corresponding to one-oriented-cycle quiver Q with n vertices, and give optimal bounds for a special subclass of these algebras. We conjecture that for any non self-injective quotient algebra A of Q dom.dimA is at most 2n-3, where the number of vertices n is bigger than 2.. Finally, we construct few examples of algebras having large (finite) dominant dimensions.Item Open Access Self-organized structures and excitations in dipolar quantum fluids(2024) Hertkorn, Jens; Pfau, Tilman (Prof. Dr.)Quantum many-body phenomena at a macroscopic scale, such as superfluidity and superconductivity, are rooted in the interplay between microscopic particles, governed by the laws of quantum mechanics. Exploring how this interplay leads to quantum behavior at a large scale allows us to gain a deeper understanding of nature and to discover new quantum phases. An elusive quantum phase in which the frictionless flow of superfluids and the crystal structure of solids coexists - the supersolid - was recently realized with quantum droplets in dipolar Bose-Einstein condensates. In this thesis we investigate self-organized structures, their formation mechanism, and excitations in dipolar quantum fluids created from such Bose-Einstein condensates. We show that the supersolid formation mechanism is driven by density fluctuations due to low-energy roton excitations, leading to a crystal structure of quantum droplets that are immersed in a superfluid background. These roton excitations split into a Goldstone mode and a Higgs amplitude mode, associated to the broken translational symmetry in the supersolid. We investigate the symmetry breaking of dipolar quantum fluids in a range of confinement geometries and establish a comprehensive description of elementary excitations across the superfluid to supersolid droplet phase transition. The droplets are stabilized by an interplay between interactions and the presence of quantum fluctuations. We show how this interplay can be used to find regimes where droplets are immersed in a high superfluid background, allowing for frictionless flow throughout the crystal. Moreover we show that towards higher densities beyond the quantum droplet phase, this interplay leads to several new self-organized structures in the phase diagram of dipolar quantum fluids. We theoretically predict new supersolid honeycomb, amorphous labyrinth, and other phases in oblate dipolar quantum fluids. Finally, we present a new experimental setup for the exploration of self-organized phases in dipolar quantum fluids and which also lays the foundation for the implementation of a quantum gas microscope. The results of this thesis present a complete framework for understanding and creating exotic phases in dipolar quantum fluids. The versatile structure formation, governed by a competition of controllable interactions and the presence of quantum fluctuations, positions dipolar quantum fluids as a model system for exploring self-organized equilibrium in weakly-interacting quantum many-body systems.Item Open Access Adaptive higher order discontinuous Galerkin methods for porous-media multi-phase flow with strong heterogeneities(2018) Kane, Birane; Siebert, Kunibert (Prof. Dr.)In this thesis, we develop, analyze, and implement adaptive discontinuous Galerkin (DG) finite element solvers for the efficient simulation of porous-media flow problems. We consider 2d and 3d incompressible, immiscible, two-phase flow in a possibly strongly heterogeneous and anisotropic porous medium. Discontinuous capillarypressure functions and gravity effects are taken into account. The system is written in terms of a phase-pressure/phase-saturation formulation. First and second order Adams-Moulton time discretization methods are combined with various interior penalty DG discretizations in space, such as the symmetric interior penalty Galerkin (SIPG), the nonsymmetric interior penalty Galerkin (NIPG) and the incomplete interior penalty Galerkin (IIPG). These fully implicit space time discretizations lead to fully coupled nonlinear systems requiring to build a Jacobian matrix at each time step and in each iteration of a Newton-Raphson method. We provide a stability estimate of the saturation and the pressure with respect to initial and boundary data. We also derive a-priori error estimates with respect to the L2(H1) norm for the pressure and the L∞(L2)∩L2(H1) norm for the saturation. Moving on to adaptivity, we implement different strategies allowing for a simultaneous variation of the element sizes, the local polynomial degrees and the time step size. These approaches allow to increase the local polynomial degree when the solution is estimated to be smooth and refine locally the mesh otherwise. They also grant more flexibility with respect to the time step size without impeding the convergence of the method. The aforementioned adaptive algorithms are applied in series of homogeneous, heterogeneous and anisotropic test cases. To our knowledge, this is the first time the concept of local hp-adaptivity is incorporated in the study of 2d and 3d incompressible, immiscible, two-phase flow problems. Delving into the issue of efficient linear solvers for the fully-coupled fully-implicit formulations, we implement a constrained pressure residual (CPR) two-stage preconditioner that exploits the algebraic properties of the Jacobian matrices of the systems. Furthermore, we provide an open-source DG two-phase flow simulator, based on the software framework DUNE, accompanied by a set of programs including instructions on how to compile and run them.Item Open Access Nanoscale magnetic resonance spectroscopy with nitrogen-vacancy centers in diamond(2021) Paone, Domenico; Wrachtrup, Jörg (Prof. Dr.)Stickstoff-Fehlstellen (NV-Zentren) in Diamant bilden interessante Quantensysteme, welche für Quanten-Sensing Protokolle genutzt werden können. In der vorliegenden Arbeit, werden NV-Zentren genutzt, um einzelne Molekülsysteme auszulesen und supraleitende Proben lokal zu charakterisieren. Zusätzlich werden Methoden entwickelt, um die Spineigenschaften der NV-Zentren zu optimieren, welche dann Einfluss auf das Sensorikverhalten des Systems haben.Item Open Access Ambient pressure oxidation of Ag(111) surfaces : an in-situ X-ray study(2008) Reicho, Alexander; Dosch, Helmut (Prof. Dr.)The oxidation of metals plays an outstanding role in everyday life. Typical phenomena are the formation of rust on steel or oxide scales on copper, showing up as a green patina. The formation of metal oxides is not always an unwanted process. The functionality of many materials is directly related to their controlled oxidation. The most prominent examples are passivating oxide layers on stainless steel. Relevant for this thesis are industrially applied heterogeneous catalytic reactions for the synthesis of many chemical products, where gaseous reactants are in contact with the solid surface of the catalyst. Oxidation reactions are very important in this context, leading to a big need of understanding of these processes in research and development. Thereby, the active oxygen species on the surface and selectivity and poisoning of the catalyst have to be studied on an atomic scale. The high temperature and high pressure oxidation of the 4d transition metals Ru, Rh, Pd and Ag is a matter of particular interest, because these metals are widely used as oxidation catalysts. On Ruthenium one observes the formation of RuO2(110) bulk oxide islands at elevated temperatures and oxygen pressure. In the case of the Pd(100) and Rh(111) surface oxidation can lead to the formation of so-called surface oxides. These oxides are structurally related to the bulk oxide of the respective element. Furthermore, surface oxides are ultra thin oxides containing one metallic layer surrounded by two oxygen layers, giving rise to an oxygen-metal-oxygen sequence perpendicular to the surface plane. A future vision is to get a direct microscopic control of the emerging surface structures and ultimately of the real-time oxidation/reduction dynamics allowing one to tailor such catalytic reactions to better performance. A necessary prerequisite to the microscopic control is the full atomistic understanding of the surface structures which form at high temperature and at high oxygen pressures. Silver plays a unique role in heterogeneous catalysis. Supported Ag catalysts are used for the selective oxidation ('epoxidation') of ethylene and for the partial oxidation of methanol to formaldehyde. Ethylene oxide and its derivates are basic chemicals for industry, used in a many technologies with a world-wide production of more than 10 million tons as in medicine for disinfection, sterilization, or fumigation, or in transport and energy technologies for engine antifreeze and heat transfer. Because of its ability to kill most bacteria, formaldehyde is extensively used as disinfectant and as preservative in vaccinations. Therefore, the optimisation of these two Ag-supported catalytic reactions is of paramount importance. Current strategies employed in the industrial process to enhance selectivity include the empirical use of inhibitors (Cl) and promoters (Cs), however, on the way to a knowledge-based control of these reactions one has first to understand the surface structure of oxidized silver under relevant conditions in full detail. The formation of extended Ag(111) facets is observed on polycrystalline silver during the above industrial catalytic oxidation reactions, in turn fundamental research (experiment and theory) has been devoted to the detailed understanding of oxidation of this surface. The formation of an oxygen induced p(4x4) reconstruction on the Ag(111) surface is known since the early 70s. A surface oxide trilayer model, based on a three-layer slab of Ag2O(111), was proposed. Accordingly, the Ag(111) surface seemed to show a similar behaviour like Pd and Rh, being neighbours in the periodic table. Further theoretical calculations predicted the stability of this reconstruction under industrially relevant conditions. Nevertheless, several questions remained unsolved: the stability of the p(4x4) reconstruction under industrially relevant conditions was not checked experimentally, the structural model of the p(4x4) structure was not proven by a crystallographic method and previously unknown structures might play an important role for the catalytic activity of Ag(111) facets. Our experimental approach is based on the nowadays routinely available highly brilliant x-ray radiation produced by third generation synchrotron light sources. This radiation is used by us in three surface sensitive x-ray techniques. In-situ surface x-ray diffraction (SXRD) allows the identification and determination of structural models of surface reconstructions under industrially relevant conditions. This technique is combined with high resolution core level spectroscopy (HRCLS) and normal incidence x-ray standing wave absorption (NIXSW), giving insight into the local binding geometry of the oxygen and silver atoms.Item Open Access From Hermitian to non-Hermitian topological phases of matter(2019) Rui, Wenbin; Metzner, Walter (Prof. Dr.)The focus of this thesis lies on extending the theory of topological phases of matter from the Hermitian to the non-Hermitian regime. This includes not only the extension of conventional concepts such as topological invariants and topological boundary states in the theory of Hermitian topological phases, but also the exploration and characterization of entirely new topological phases unique to non-Hermitian systems.