We carried out systematic neutron scattering experiments to investigate the magnetic properties and their relationship to the high-$T_c$ superconductivity, when the materials are tuned from their antiferromagnetic (AF) parent compounds to the superconducting regime. We observed resonance mode in the electron doped cuprate Nd$_{1.85}$Ce$_{0.15}$CuO$_4$, demonstrating that the resonance is a general phenomenon in cuprate superconductors regardless of hole- or electron-doping. In Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$, the local susceptibility displays two distinct energy scales that are broadly consistent with the bosonic modes revealed by scanning tunneling microscopy experiments. These results indicate the presence of very strong electron spin excitations couplings in electron doped cuprates. Shortly after the discovery of high-$T_c$ superconductivity in the Fe pnictides, we discovered that the magnetic phase diagram of CeFeAsO$_{1-x}$F$_x$ is remarkably similar to that of the cuprates. Besides CeFeAsO, similar magnetic and lattice structures are also observed in PrFeAsO and SrFe$_2$As$_2$ systems. Neutron scattering measurements show that in SrFe$_2$As$_2$, the spectrum of magnetic excitations consists of a Bragg peak at the elastic position, a spin gap, and sharp spin-wave excitations at higher energies. Based on the observed dispersion relation, we estimated the effective magnetic exchange coupling using a Heisenberg model. In order to study the nature of the exchange interactions in the parent compound of Fe pnictides, we studied the high energy spin-wave excitations in CaFe$_2$As$_2$. Although the spin waves in the entire Brillouin zone can be described by an effective three-dimensional anisotropic Heisenberg Hamiltonian, the magnetism in this system is neither purely local nor purely itinerant; rather it is a complicated mix of the two. When the Fe pnictide is tuned into superconducting regime with doping, the low energy spin fluctuation is dominated by a resonance mode. In the optimally electron doped BaFe$_{1.9}$Ni$_{0.1}$As$_2$, application of a magnetic field that suppresses the superconductivity and superconducting gap energy also reduces the intensity and energy of the resonance. These results suggest that the energy of the resonance is proportional to the electron pairing energy, and thus indicate that spin fluctuations are intimately related to the mechanism of high $T_c$ superconductivity.

This dissertation describes a structural investigation of hyaluronan (HA) with neutron scattering techniques. HA is a natural biopolymer and one of the major components of the extracellular matrix, synovial fluid, and vitreous humor.  It is used in several biomedical applications like tissue engineering, drug delivery, and treatment of osteoarthritis. Although HA is extensively studied, very little is known about its three-dimensional conformation and how it interacts with ions and other molecules. The study aims to understand the bulk structure of a cross-linked HA hydrogel, as well as the conformational arrangement of HA at solid-liquid interfaces. In addition, the structural changes of HA are investigated by simulation of physiological environments, such as changes in ions, interactions with nanoparticles, and proteins etc. Small-angle neutron scattering and neutron reflectivity are the two main techniques applied to investigate the nanostructure of hyaluronan in its original, hydrated state. The present study on hydrogels shows that they possess inhomogeneous structures best described with two correlation lengths, one of the order of a few nanometers and the other in the order of few hundred nanometers. These gels are made up of dense polymer-rich clusters linked to each other. The polymer concentration and mixing governs the connectivity between these clusters, which in turn determines the viscoelastic properties of the gels. Surface-tethered HA at a solid-liquid interface is best described with a smooth varying density profile. The shape of this profile depends on the immobilization chemistry, the deposition protocol, and the ionic interactions. HA could be suitably modified to enhance adherence to metal surfaces, as well as incorporation of proteins like growth factors with tunable release properties. This could be exploited for surface coating of implants with bioactive molecules. The knowledge gained from this work would significantly help to develop future biomaterials and surface coatings of implants and biomedical devices.

Various experiments on Yb₂Ti₂O₇ have shown evidence of strange magnetic behaviour at low temperatures. Specific heat measurements on powder samples of Yb₂Ti₂O₇ show evidence of a sharp peak, indicating the occurence of a first order phase transition. Meanwhile, neutron scattering, Mössbauer absorption, and μSR measurements find no evidence of long range order below the temperature of this phase transition, leaving the nature of the low temperature phase a mystery. Quantifying the magnetic interactions in this material should allow us to better understand the low temperature behaviour of this material. In this study, we fit a symmetry allowed nearest-neighbour bilinear exchange model to quasi-elastic neutron scattering data collected well above the temperature of the experimentally observed phase transition. This neutron scattering data shows evidence of rods of scattering intensity along the ⟨111⟩ crystallographic directions. Neutron scattering probes the correlations between magnetic moments in a material, so fitting an interaction model to the neutron scattering is equivalent to fitting the interactions to the magnetic correlations. These correlations are driven by the interactions between the magnetic moments, so the neutron scattering should give us direct access to the form of these interactions. Using this method we successfully identify an anisotropic nearest-neighbour bilinear exchange model that reproduces the experimentally observed quasi-elastic neutron scattering. With this model we then proceed to compute real space correlation functions, finding that the rods of neutron scattering arise from the presence of strong correlations along nearest-neighbour chains. We also compute the bulk susceptibility and local susceptibility, obtaining very good fits to experiment with no variation of the model determined from the neutron scattering. The success of these calculations provides a further independent confirmation of the success of our interaction model in describing the magnetic interactions in Yb₂Ti₂O₇. Finally, we present a brief summary of ongoing work based on our anisotropic exchange model, including mean field calculations to determine the low temperature ground state of this model and classical Monte Carlo simulations to study the phase transition present in this model. We also discuss potential further studies of this and other models.

The goal of this thesis is to study the role of single particle elasticity in the overall behavior of particulate systems. For this purpose, we use microgel particles, which are crosslinked polymer networks immersed in a solvent. In these systems, the amount of cross-linker determines their elasticity and ultimately the stiffness of the particle. For a system of hard spheres, the phase behavior is solely determined by the volume fraction occupied by the particles. Based on the volume fraction, liquid, crystal and glassy phases are observed. Interestingly, microgel particles display a richer and fascinating set of different behaviors depending on the particle stiffness. Previous results obtained in our group show that for highly cross-linked microgels, the glass phase disappears and there are only liquid and crystalline phases. By contrast, preliminary measurements indicate that for ultrasoft microgel particles the system does not show any signature of crystalline or glassy phases. The system seems to remain liquid irrespective of volume fractions. In this Thesis, we will address this striking result using light scattering as well as rheology, in order to access both static and dynamic properties in a wide range of length and time scales. In addition, we will also perform additional studies using very stiff microgels and use their swelling capabilities to change the volume fraction. We will use hydrostatic pressure to change the miscibility of the polymer network and thus change the microgel size; the use of this external variable allows fast equilibration times and homogeneous changes throughout the sample. By using neutron scattering techniques, we study the structural and dynamical properties of the system in its different phases involved.

Inhalt der vorliegenden Dissertation ist die Untersuchung der elektronischen und magnetischen Eigenschaften der Schwerfermionensubstanz CeCu2. Dafür wurden makroskopische Messungen der Magnetisierung, der Suszeptibilität sowie der spezifischen Wärme aus der Literatur miteinander verglichen und eigenen Neutronenstreuexperimenten gegenübergestellt. Ein Teil der Arbeit widmet sich der Entwicklung des neuen Dreiachsenspektrometers PANDA am FRM2 München. Für dieses wurden umfangreiche Simulationen durchgeführt, um zukünftige Leistungsdaten zu spezifizieren. Mit dem Ray - Trace - Simulationspaket McStas wurde das Neutronenstreugerät modelliert und das Zusammenspiel der neutronenoptischen Bauelemente optimiert. Der zweite Teil der Arbeit beschäftigt sich mit Messungen und Rechnungen zum magnetischen Phasendiagramm von CeCu2. Dazu wurden eigene Messungen der Magnetisierung (VSM) und elastische Neutronenstreuexperimente (D23, ILL Grenoble) einer McPhase - Rechnung gegenübergestellt. Der nächste Abschnitt der Arbeit befasst sich mit elastischen und inelastischen Neutronenstreuexperimenten (IN12 & IN20, ILL Grenoble) an CeCu2 Einkristallen. Diese Messungen untersuchen die statischen (Struktur, Phasendiagramm) sowie dynamischen Eigenschaften (Anregungen) in dieser Substanz. Insbesondere beschäftigen sich diese Messungen mit den Dispersionsrelationen der Magnonen in hohen magnetischen Feldern bis zu mu0H = 12 Tesla. Für diese Experimente wurde eine neue Probenbox für den Einsatz in Hochfeldmagneten entwickelt, welche die Probe fixiert und in einer Schutzgasatmosphäre einschließt. In den Messungen wurde eine unerwartete, dispersionslose Anregung gefunden, welche mit dem derzeitigen Spinwellenmodell unverträglich ist. Bei Erhöhung des Magnetfeldes steigt die Anregungsenergie doppelt so schnell, wie von gewöhnlichen Magnonen erwartet wird. Zur Erklärung wird ein neutroneninduzierter pi/2 - Spinflop - Prozess vorgeschlagen. In einer anderen Messung wurde mit polarisierten Neutronen nach dem bisher indirekt nachgewiesenen, ersten angeregten Kristallfeldniveau bei 9 Millielektronenvolt gesucht. Bei wachsenden Temperaturen konnte kaum magnetische Streuintensität unterhalb 10 Millielektronenvolt gefunden werden. Jedoch wurde eine stark verbreiterte Intensität von 8 bis 15 Millielektronenvolt gemessen. Diese Energie korreliert mit dem Übergang zwischen den angeregten Kristallfeldniveaus. Als Ursache für die Verbreiterung wird eine Kristallfeld - Phonon - Wechselwirkung angegeben. Aus dem Vergleich der in verschiedenen Richtungen gemessenen magnetischen Streuintensitäten des 23 Millielectronvolt Kristallfeldüberganges können dessen Übergangsmatrixelemente abgeschätzt werden. Diese wurden den errechneten Werten der veröffentlichten Kristallfeldparametersätze gegenübergestellt. Es zeigte sich, dass die alten Parametersätze die neuen Messdaten nicht beschreiben können. Deshalb wurde ein neuer Satz aufgestellt, welcher erstmals sowohl die makroskopischen Messungen als auch die Neutronenstreuexperimente erklärt. / In this thesis, the electronic and magnetic properties of the heavy-fermion system CeCu2 are investigated. For that purpose, measurements of susceptibility, magnetisation and specific heat from literature are compared with each other and are compared with neutron-scattering experiments performed for the present work. One part of the thesis is addressed to the design of the new triple-axis spectrometer PANDA, which is situated at the research reactor FRM2 in Munich. Extensive simulations were performed with the ray - tracing simulation package McStas in order to model this neutron scattering instrument and to optimise its neutron optic elements. The second part of this work deals with measurements and calculations of the magnetic phase diagram of CeCu2. The experiments include the macroscopic magnetisation (VSM) and elastic neutron-scattering measurements (D23, ILL Grenoble). The McPhase calculations are in a good agreement with the experiments. The next part presents elastic and inelastic neutron-scattering experiments (IN12 & IN20, ILL Grenoble) at CeCu2 single crystals. These measurements investigate the static (phase diagram) as well as dynamic properties (excitations) of this compound. Especially, the experiments are concentrated on the investigation of the dispersion relation of magnons in magnetic fields up to mu0H = 12 tesla. For these measurements, a new sample holder suitable for use in high magnetic fields was designed, which fixes the sample in a hermetically sealed containment. In the experiments an unexpected dispersionless excitation was observed, which does not fit the standard magnon model. With increasing magnetic field the energy of this excitation increases with a slope of twice the value expected for regular magnons. A neutron - induced pi/2 spin - flop process is proposed to explain this excitation. In another measurement with polarised neutrons the first excited crystal - field level at 9 millielectronvolt, which has not been directly detected up to now, was investigated. With increasing temperature, only small magnetic scattering intensity occurs below 10 millielectronvolt. >From 8 to 15 millielectronvolt a broad signal was detected, which corresponds to the energy of the transition between the two excited levels. Due to a strong crystal field - phonon interaction, the width is unusually broadened. The transition matrix elements can be estimated from the comparison of the components of the magnetic scattering intensities of the 23 millielectronvolt crystal - field transition. These values were compared with the calculated values from published crystal-field parameter sets. The old sets cannot explain the new measurements. A new set was derived which accounts for the macroscopic measurements and for the neutron - scattering data, too.

In dieser Arbeit wurde die durch Bestrahlung mit schnellen Neutronen bedingte Materialalterung von Reaktordruckbehälterstählen untersucht. Das Probenmaterial umfasste unbestrahlte, bestrahlte und ausgeheilte RDB-Stähle russischer und westlicher Reaktoren sowie Eisenbasis-Modelllegierungen. Mittels Neutronen-Kleinwinkelstreuung ließen sich bestrahlungsinduzierte Leerstellen/Fremdatom-Cluster unterschiedlicher Zusammensetzung mit mittlerem Radius um 1.0 nm nachweisen. Ihr Volumenanteil steigt mit der Strahlenbelastung monoton, aber im allgemeinen nicht linear an. Der Einfluss der Elemente Cu, Ni und P auf den Prozess der Clusterbildung konnte herausgearbeitet werden. Eine Wärmebehandlung oberhalb der Bestrahlungstemperatur reduziert den Anteil der Strahlendefekte bis hin zu deren vollständiger Auflösung. Die Änderungen der mechanischen Eigenschaften der Werkstoffe lassen sich eindeutig auf die beobachteten Gefügemodifikationen zurückführen. Die abgeleiteten Korrelationen können als Hilfsmittel zur Vorhersage des Materialverhaltens bei fortgeschrittener Betriebsdauer von Leistungsreaktoren mit herangezogen werden.

No description available.

The hydrogen molecule dynamics in tetrahydrofuran-H2-H2O clathrate hydrate has been studied by high-resolution inelastic neutron scattering and Raman light scattering. Several intense bands in the neutron spectrum are observed that are due to H2 molecule excitations. These are rotational transitions, center-of-mass translational transitions (rattling) of either para- or ortho-H2, and combinations of rotations and center-of-mass transitions. The rattling of the H2 molecule is a paradigmatic example of the motion of a quantum particle in a non-harmonic three-dimensional potential well. Both the H2 rotational transition and the fundamental of the rattling transition split into triplets. Raman spectra show a similar splitting of the S0(0) rotational transition, due to a significant anisotropy of the potential with respect to the orientation of the molecule in the cage. The comparison of our experimental values for the transition frequencies to a recent quantum mechanical calculation gives qualitative agreement, but shows some significant difference.

The response of low energy nuclear recoil in NaI(Tl) is investigated in the following experiment. Such detectors have been used recently to search for evidence of dark matter in the form of weakly interacting massive particles (WIMPs). NaI(Tl) crystal response to nuclear recoil energy deposition is a key element in these searches. I discuss the cosmological and experimental motivations for these experiments, followed by an overview of the physics of direct detection and current relevant WIMP search experiments. With the experiment motivations covered, the details of NaI(Tl) detectors are reviewed. The specifics of our experiment are laid out including the neutron production, neutron beam calibration, shielding optimization, experimental design and setup. Then the crystal response calibration studies and Geant4 simulations are discussed followed by the final quenching factor values and uncertainties. This experiment measured quenching factors for sodium recoils in the energy range of (9 keV-40 keV) of 19%-27% QF. These results are similar to current published measurements. Interesting features of the QF measurements include an increase at low energies and a dip in the values at 30 keV, the iodine K-shell absorption edge. The goal of this experiment was to add valuable measurements of nuclear recoils at low energies that are relevant to low-mass WIMP experiments. Future plans will improve and expand on these measurements in order to better understand the response of NaI(Tl) at low energies.

This project proposes a reactor-based instrument, which retains most of the above advantages of spallation source engineering instruments, while maintaining the advantages inherent to steady-state instruments. The main idea is to allow the entire white beam from the reactor onto the sample---no monochromation. Diffraction is then allowed at a fixed angle, and information from two sample directions can be obtained simultaneously. Since a white beam is incident onto the sample, the diffraction condition is satisfied for multiple sample planes in the fixed angle of diffraction. Multiple analyzer/detector pairs are placed downstream from the sample and tuned to scatter only one of the diffracted wavelength bands. Monte Carlo methods were used to create models of both a standard two-axis engineering diffractometer, found on current reactor sources, and the proposed white beam instrument. There models were validated by experiments performed on a standard two-axis instrument, which was also modified to operate in a white beam configuration, in which the position of the sample and monochromator were interchanged. Both the models and the experiments of the white beam instrument showed proof of concept for this design and identified areas of concern that required special attention. Upon a comparison on the results from the standard two-axis instrument to the results from the white beam instrument (both simulation and experimental), it was found that the standard diffractometer showed a better performance in all aspects. However, this project proposes numerous areas where the white beam design can be improved upon in order to enhance its performance as an engineering instrument. The most important of these is the design of an appropriate analyzer/detector system, as the results overwhelmingly show this to be the area of greatest concern. Ideas for a few such designs are also given. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2009-04-23 17:46:59.419