Universität Stuttgart
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Item Open Access Magnetoelektrischer Effekt in metallischen Nanostrukturen : Ab-initio Elektronentheorie und atomistische Modellierung(2011) Subkow, Sergej; Fähnle, Manfred (Prof. Dr. rer. nat.)Der magnetoelektrische Effekt in metallischen Nanostrukturen wird untersucht. Dabei wird die Veränderung der magnetokristallinen Oberflächenanisotropieenergie dünner Metallfilme als Funktion externer elektrischer Felder im Rahmen der elektronischen Spindichtefunktionaltheorie berechnet. Dadurch wird eine Parametrisierung und Modellierung des Effekt in den Systemen möglich, die wegen ihrer größe einen direkten ab-initio Zugang ausschließen.Item Open Access Passive advection of fractional Brownian motion by random layered flows(2020) Squarcini, Alessio; Marinari, Enzo; Oshanin, GlebWe study statistical properties of the process Y(t) of a passive advection by quenched random layered flows in situations when the inter-layer transfer is governed by a fractional Brownian motion X(t) with the Hurst index H ∈ (0,1). We show that the disorder-averaged mean-squared displacement of the passive advection grows in the large time t limit in proportion to t 2-H, which defines a family of anomalous super-diffusions. We evaluate the disorder-averaged Wigner-Ville spectrum of the advection process Y(t) and demonstrate that it has a rather unusual power-lawform1 f 3-H with a characteristic exponent which exceed the value 2. Our results also suggest that sample-to-sample fluctuations of the spectrum can be very important.Item Open Access Novel X-ray lenses for direct and coherent imaging(2019) Sanli, Umut Tunca; Schütz, Gisela (Prof. Dr.)Item Open Access Oxidation von CoGa Oberflächen untersucht mit oberflächensensitiver Röntgenbeugung(2003) Streitel, Reinhard; Dosch, Helmut (Prof. Dr.)Ultra dünne Oxidschichten spielen z.B. bei der Herstellung von elektronischen Bauteilen und der Realisierung von nichtflüchtigen Speichermedien (MRAM = magnetic random access memory) eine wichtige Rolle. In Speicherbausteinen findet man häufig Oxidschichten als Isolatoren. Diese Arbeit beschäftigt sich mit der Oxidation von Legierungseinkristallen und der Erzeugung ultra dünner Oxid- bzw. Isolatorschichten. Durch kontrollierte Oxidation einer CoGa(100) Oberfläche kann eine ultra dünne, geordnete Galliumoxidschicht erzeugt werden. Die Struktur der ultra dünnen Galliumoxidschicht wurde durch Oberflächenröntgenbeugung mit atomarer Auflösung bestimmt. Mit einer Dicke von etwa 0,6 nm ist die Oxidschicht wesentlich dünner als die monokline Einheitszelle der beta-Galliumoxidvolumenstruktur (1,2 nm). Struktur und Morphologie des Oxides hängen stark von den Parametern Temperatur und Sauerstoffpartialdruck ab. In situ Oxidationsuntersuchungen zeigen, dass die Oxidbildung stark von den gewählten Bedingungen abhängt und verschiedene Wachstumsmodi existieren. Bei Raumtemperatur entsteht eine ungeordnete Oxidschicht. Bei Temperaturen von 600 K bis 740 K beobachtet man die Bildung von Oxidinseln, die ein stark anisotropes Wachstumsverhalten zeigen und bevorzugt entlang den Kristallrichtungen des CoGa Substrates orientiert sind. Die mittlere Inselgröße hängt dabei stark vom gewählten Sauerstoffpartialdruck ab. Die Stabilität der ultra dünnen Galliumoxidschichten wurde durch in situ Röntgendiffraktionsexperimente analysiert. Es zeigt sich, dass die Oxidschicht bei Temperaturen über 800 K vollständig von der CoGa Oberfläche desorbiert.Item Open Access Frequency-frequency correlations of single-trajectory spectral densities of Gaussian processes(2022) Squarcini, Alessio; Marinari, Enzo; Oshanin, Gleb; Peliti, Luca; Rondoni, LambertoWe investigate the stochastic behavior of the single-trajectory spectral density S(ω,T) of several Gaussian stochastic processes, i.e., Brownian motion, the Ornstein-Uhlenbeck process, the Brownian gyrator model and fractional Brownian motion, as a function of the frequency ω and the observation time T. We evaluate in particular the variance and the frequency-frequency correlation of S(ω,T) for different values of ω. We show that these properties exhibit different behaviors for different physical cases and can therefore be used as a sensitive probe discriminating between different kinds of random motion. These results may prove quite useful in the analysis of experimental and numerical data.Item Open Access Spin-echo resolved neutron scattering from self-organised polymer interfaces(2010) Nülle, Max; Dosch, Helmut (Prof. Dr.)This thesis focused on two main objectives: First, the clarification of the prospects of the spin-echo resolved grazing incidence neutron scattering method (SERGIS) for the investigation of buried interfaces. And second, the investigation of the self-organisation (i.e. microphase separation and dewetting) of ultrathin poly(styrene-block-isoprene) diblock copolymer films on silicon substrates by means of SERGIS and complementary techniques. SERGIS is a novel neutron scattering technique which was implemented and further developed at the new neutron / x-ray reflectometer N-REX+ at the FRM II (Garching, Germany). In contrast to conventional small-angle scattering methods, SERGIS characterises the lateral structure and morphology of interfaces and thin-film systems in real space. The technique uses a polarised primary beam, and the measured quantity is the integral polarisation of the scattered beam. By decoupling the measurement resolution and the beam divergence (in a first approximation), SERGIS aims at a good resolution and a good measurement statistics simultaneously. As a first systematic application of SERGIS to a real physical problem, the dewetting and internal structure of ultrathin poly(styrene-block-isoprene) diblock copolymer films were studied by means of SERGIS and complementary surface sensitive techniques, namely neutron and x ray reflectivity and atomic force microscopy (AFM).Item Open Access Cohesive properties of bcc and fcc rubidium from ab initio pseudopotentials(1985) Maysenhölder, Waldemar; Louie, Steven G.; Cohen, Marvin L.Total-energy calculations have been performed for Rb at zero temperature using a self-consistent ab initio pseudopotential approach within a local-density-functional scheme. The energy difference between fcc and bcc Rb, and the energy barrier between these structures, are found to be extremely small near the equilibrium volume. Agreement of the calculated cohesive properties of bcc Rb with experimental values is good in view of the softness of the material. A transition from bcc to fcc has been calculated to occur at a pressure of about 52 kbar for T=0 K, which compares favorably with the observed value of 70 kbar for this transition at room temperature.Item Open Access Structural and magnetic studies of strained thin films of La 2/3 Ca 1/3 MnO 3(2004) Carbone, Gerardina; Helmuth, Dosch (Prof. Dr. Rer. Nat.)Mn-compounds with formula A1-xBxMnO3 have been widely studied in the past 50 years because of their interesting physics. The recent discovery of phenomena of Colossal Magneto Resistance (CMR) and dense granular magneto-resistance in ferromagnetic thin films (for x~1/3) has renewed the interest towards them, as materials suitable for applications in electronic devices. This work focuses on La1-xCaxMnO3 in the x=1/3 phase (LCMO). LCMO is ferromagnetic metallic below its Curie temperature (TC ~ 280K) and presents the phenomenon of CMR. The main scope of this thesis has been the accurate structural and magnetic study of strained thin films of LCMO epitaxially grown on SrTiO3 Due to the mismatch between the crystal lattice of the two compounds mechanical strain is imposed to the film structure, which can be responsible for changes of the physical properties of the material. The interest of this research is the analysis of the strain-induced modifications to the structure of LCMO epitaxial thin films and their effect on magnetic and transport properties. The films studied, with thickness in the range 50A-600A, have been grown by Pulsed Laser Ablation by Dr.J.Klein, at the University of Cologne-Germany. their TC, of the order of 130K, is lower than the bulk, does not show thickness dependence. Structural analyses on these films have been performed with x-ray diffraction (XRD) using synchrotron radiation. The measurements have been carried out at the beamline BM28 at the ESRF (European Synchrotron Radiation Facility) in Grenoble -France, and at the MPI Surface Scattering Beamline at ANKA in Karlsruhe - Germany. Grazing Incidence Diffraction (GID) measurements has allowed the observation of a particular microstructure of the films, consisting of twin domains of similar size periodically arranged in the plane of the growth. The signature of this periodic microstructure, is the presence of broad superlattice (SL) peaks around in-plane Bragg peaks. No SL peaks have been found around purely out-of-plane peaks for the thin film (100A). For thicker films, instead, a broad peak has been observed together with the sharp Bragg component of [0 0L]-type peaks. Such broad component can also be ascribed to a poorly correlated out-of-plane periodicity. Such twin domain modulation (TDM) is the structural response of the film to the strain induced by the substrate. In this special case of epitaxial growth, the TDM creates an average structure which helps reducing the mismatch between film and substrate. A more detailed description of the model is found in the PhD thesis of U. Gebhardt. In our proposed model, the size of the twin domains is statistically distributed around an average value L0 with a standard deviation DeltaL0.For the 100A film the distribution of twin domains, has an average domain size of LO = 32u.c. and a distribution width of DeltaL0 = 16 u.c. For higher thickness, as for the 400A film, 3 different in-plane TD distributions are needed to reproduce the data (: L01 = 32u.c. and DeltaL01 = 14u.c, L02 = 64u.c and DeltaL02 = 24u.c, L03 = 79u.c and DeltaL03 = 49u.c.) This can be justified with a model of increased disorder in the domain distribution in the direction of the growth. The presence of the broad peak found at [0 0L]-type positions has been explained, within the same model, as due to an out-of-plane TD modulation due to the presence of a monoclinic distortion of the LCMO unit cell CTR measurements confirm the presence, in thick films, of a form of strain-relaxation along the direction of the growth, consisting in a variation of the c-spacing both at the surface of the films and at the substrate- film interface. Magnetic measurements have been performed both with both SQUID and polarized neutron reflectivity (PNR) techniques. The magnetisation of the samples does not saturate completely up to the maximum value of external field applied H = 6Tesla. This effect can be ascribed either to a canting of the magnetic moments, or to the presence of magnetic domains. Furthermore, the value of the saturation magnetisation of the films, calculated at the maximum field measured decreases with increasing thickness of the samples, following an exponential law. These results have been confirmed by PNR measurements, performed at EVA@ILL (Grenoble - France). For one of the thick samples, 400A, PNR give evidence of a strong gradient in the magnetic profile of the film. The average magnetic moment found is maximum at the interface, which decreases exponentially along the direction of the growth. These peculiar magnetic properties, together with the model of change in distribution of twin domains and variation of c-axis, can be attributed to an increase of disorder in the domains, which reflects in the disorder in magnetic domains. All the measurements converge in a model of epitaxial growth, achieved by twin domain periodicity, with a distribution of domain sharper at the interface and broader at the surface.Item Open Access High-density hydrogen monolayer formation and isotope diffusion in porous media(2019) Balderas-Xicohténcatl, Rafael; Schütz, Gisela (Prof. Dr.)This thesis is focused on two fundamental aspects of hydrogen physisorption. First, the density of a single layer of adsorbed hydrogen molecules is studied at temperatures close the boiling point (20 K). These systematic experiments give an explanation to high H2 monolayer capacity as a high-density phase of adsorbed hydrogen. Secondly, the diffusion of the hydrogen molecules through a porous material (ZIF-8) is studied using a commercially available adsorption apparatus. Establishing that gas adsorption experiments can be used to study hydrogen diffusion and isotope separation at temperatures close the boiling point.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.