The heavy alkaline earth hydrides consist of the Group II (alkaline earth metal) hydrides, namely CaH2, SrH2, and BaH2, which adopt the cotunnite-type structure at ambient pressure and temperature conditions. There has been much recent research interest in these compounds; in the past decade numerous experimental and theoretical studies exploring the hydrides in both applied and fundamental respects have been added to the literature. The present work constitutes a study of the heavy alkaline earth hydrides subjected to extreme pressure conditions (at ambient temperature). Specifically, pressure-dependent angle-dispersive powder x-ray diffraction and Raman spectroscopy experiments, as well as first-principles calculations, have been carried out to explore the structural stability, compressibility, and optical phonon spectra of the heavy alkaline earth hydrides. To complement the work on the hydrides, a companion study on BaF2---a structural analogue---is presented, as well as a description of some selected experimental techniques used to carry out the research. The powder x-ray diffraction results reveal that all of the heavy alkaline earth hydrides undergo a pressure-induced, first-order structural phase transition. The proposed Ni2ln structure for the high-pressure phase is based on both the results of first-principles calculations and an isostructural comparison with BaF2. The transition pressures and the proposed high-pressure structure are corroborated by the results of the Raman spectroscopy measurements. Where applicable, equation of state parameters from both the experimental and theoretical studios are reported, and a comparison of these respective parameters at ultra-high pressures (greater than 50 GPa) suggests that the experimentally observed bulk moduli are overestimated due to the presence of non-hydrostatic pressure conditions.

<p> A single electron in a quantum cyclotron provides the most precise measurement of the electron magnetic moment, given in units of the Bohr magneton by <i> g</i>/2 = 1.001 159 652 180 73 (28) [0.28 ppt]. The most precise determination of the fine structure constant comes from combining this measurement with Standard Model theory, yielding &alpha;^-1 = 137.035 999 173 (34) [0.25 ppb], limited by the experimental uncertainty of the electron <i> g</i>-value. The most stringent test of CPT symmetry in leptons comes from comparing the electron and positron magnetic moments, limited by the positron uncertainty at 4.2 ppt. A new high-stability apparatus has been built and commissioned for improved measurements of the electron and positron magnetic moments, a greatly improved test of lepton CPT symmetry, and an improved determination of the fine structure constant. These new measurements require robust positron loading from a retractable radioactive source that is small enough to avoid compromising the high-precision environment of our experiment. The design and implementation of such a scheme is a central focus of this work. Robust positron loading at a rate of 1-2 e⁺/min from a 6.5 &mu;Ci ²²Na source has been demonstrated.</p>

Transport measurements and fabrication of superconductor-exchange spring magnet-superconductor systems

<p> This dissertation is a compilation of research results of extensive Fast Scanning Calorimetry studies of two non-crystalline materials: Toluene and Water. </p><p> Motivation for fundamental studies of non-crystalline phases, a brief overview of glassy materials and concepts and definitions related to them is provided in Chapter 1. Chapter 2 provides fundamentals and details of experimental apparata, experimental protocol and calibration procedure. </p><p> Chapter 3 &amp; 4 provides extensive studies of stable non-crystalline toluene films of micrometer and nanometer thicknesses grown by vapor deposition at distinct deposition rates and temperatures and probed by Fast Scanning Calorimetry. Fast scanning calorimetry is shown to be extremely sensitive to the structure of the vapor-deposited phase and was used to characterize simultaneously its kinetic stability and its thermodynamic properties. According to our analysis, transformation of vapor -deposited samples of toluene during heating with rates in excess 100,000 K/s follows the zero-order kinetics. The transformation rate correlates strongly with the initial enthalpy of the sample, which increases with the deposition rate according to sub-linear law. Analysis of the transformation kinetics of vapor deposited toluene films of various thicknesses reveal a sudden increase in the transformation rate for films thinner than 250 nm. The change in kinetics correlates with the surface roughness scale of the substrate, which is interpreted as evidence for kinetic anisotropy of the samples. We also show that out-of-equilibrium relaxation kinetics and possibly the enthalpy of vapor-deposited (VD) films of toluene are distinct from those of ordinary supercooled (OS) phase even when the deposition takes place at temperatures above the glass softening (Tg). The implications of these findings for the formation mechanism and structure of vapor deposited stable glasses are discussed. </p><p> Chapter 5 and 6 provide detailed Fast Scanning Calorimetry studies of amorphous solid water in bulk and confining geometry (ultrathin films and nano-aggregates). Bulk-like water samples were prepared by vapor-deposition on the surface of a tungsten filament near 140 K where vapor-deposition results in low enthalpy glassy water films. The vapor deposition approach was also used to grow nano-aggregates (2- 20 nm thick) and multiple ultrathin (approximately 50 nm thick) water films alternated with benzene and methanoic films of similar dimensions. When heated from cryogenic temperatures, the ultrathin water films underwent a well manifested glass softening transition at temperatures 20 degrees below the onset of crystallization. The thermograms of nano-aggregates of ASW films show two endotherms at 40 and 10 K below the onset temperatures of crystallization. However, no such transition was observed in bulk-like water samples prior to their crystallization. These results indicate that water in confined geometry demonstrates glass softening dynamics which are dramatically distinct from those of the bulk phase. We attribute these differences to water's interfacial glass transition which occurs at temperatures tens of degrees lower than that in the bulk. Implications of these finding for past studies of glass softening dynamics in various glassy water samples are discussed in chapter 5 and 6.</p>

<p> The NPDGamma Experiment is measuring the parity-violating correlation A_&gamma; between neutron spin and gamma momentum in the radiative capture of a polarized cold neutron beam on a cryogenic liquid parahydrogen target. This measurement is expected to give insight into theories that incorporate the weak interaction into what is primarily a strongly interacting system. This dissertation discusses the operation and characterization of the liquid hydrogen target, including the calibration of the instrumentation that monitors the state of the hydrogen. An important consideration is the fact that for safety reasons the instrumentation in direct contact with the hydrogen is limited, and so a detailed understanding of the target design and of the properties of hydrogen is required in order to interpret the state of the system. For this experiment, it is essential that the hydrogen be kept mostly in the para state in order to prevent the beam from being significantly depolarized before capture. Since the uncatalyzed conversion rate is slow, an ortho-para conversion loop is used to promote conversion from the room temperature orthohydrogen fraction to the fraction associated with the temperatures of the cryogenic vessel. In addition to the calibration and characterization studies, a method is introduced for placing an empirical limit on the deviation of the orthohydrogen fraction inside the vessel from the desired level associated with the temperature of the ortho-para conversion catalyst and vessel. This method, which does not require precise knowledge of the parahydrogen cross section, involves observing the transmission of the beam through the target while the rate of flow of hydrogen through the ortho-para conversion loop is changed. In addition to the studies of the hydrogen target, this dissertation discusses a calibration of some ³He ion chambers that monitor the flux of the neutron beam and that are used to perform beam transmission measurements. This calibration, which involves a study of the noise inherent in the signal due to neutron capture, does not involve comparison to a separate calibrated detector.</p>

It is impossible to enumerate all the areas where diffusion plays a crucial role. Numerous studies of diffusion have been made since the observation of brownian motion by Robert Brown in 1827. In the last decades, the development of computers made it possible to carry numerical studies of diffusion. Monte Carlo algorithms are often used to model the random walk of a particle in a given system in order to measure its diffusion coefficient. In the last years, Dr Slater's research group developed an exact calculation method that allows one to compute diffusion coefficients with high precision. This calculation method, even if major modifications were necessary, is the base of the two projects presented in this thesis. The first project is a sequel of an article published in 2006 by Hickey et al. These authors derived an expression that predicts the diffusion coefficient of a point-like particle in a two-phase system, such as a hydrogel made of gelatin with maltodextrin viscous inclusions. This expression works well for a simple two-phase system but neglects numerous characteristics such hydrogels can present. In this thesis' first project, we modify this expression in order to include the possible interactions between the particle and the gel structure, the interfacial steric effects between phases and the possible incomplete phase separation. We validate these modifications by comparing them with exact numerical calculations. In preceding studies made by the research group of Dr Slater, it was assumed that the system was quenched, i.e. the obstacles didn't move. However, it is logical to believe that gel fibers inside a real hydrogel are subject to thermal motion and that they vibrate around a mean position. The second project presents a new numerical method allowing one to investigate the effect of obstacles motion on the diffusion of a particle. We vary different parameters such as the vibration frequency compared to the diffusion time scale of the particle, the amplitude of vibration, the obstacle concentration as well as their configuration (periodic or random). This new method is innovative because it makes it possible to study in details the transition of a system from a quenched state (fixed obstacles) to an annealed state (obstacles vibrating much faster than the diffusion time scale of the particle).

Abstract not available.

This thesis is a study of magnetic properties of new icosahedral quasicrystals Ag50In36Gd14, Zn77Fe7Sc 16 and a 1/1 crysta lline approximant Ag50In36Gd 14. The Ag50In36Gd14 and Zn77 Fe7Sc16 quasicrystals are shown to have a primitive six-dimensional Bravais lattice with a six-dimensional hypercubic lattice constant of, respectively, 7.805(2) and 7.087(1) A. The crystal structure of the approximant Ag50In36Gd14 is found to be of the YbCd6 type (space group lm3&macr;) with a lattice constant of 15.202(1) A. No evidence is found for a transition to a ground state with long-range magnetic order in the temperature range 2--300 K for all three alloys. These three alloys are shown to be spin glasses. The spin freezing temperature of the Ag50In36Gd14 and Zn77Fe 7Sc16 quasicrystals is, respectively, 4.25(5) and 7.75(2) K. The unusual spin freezing in the approximant is found to occur in two stages: at &sim;3.6 K spins develop short-range correlations but they co ntinue to fluctuate at low frequencies, and then long-range freezing is achieved upon further cooling to below &sim;2.4 K. The Ag50In36Gd 14 quasicrystal and its 1/1 approximant are shown to belong to a category of strongly geometrically frustrated magnets. The frequency dependence of the freezing temperature in all three alloys is shown to be equally well accounted for by the Vogel-Fulcher law and the power law. Analysis of the ageing effects observed in the Zn77Fe 7Sc16 quasicrystal leads to a major discovery that the nature of the spin-glass state in this quasicrystal is fundamentally different from that of a canonical spin glass. The effective magnetic moment at Gd atoms in the Ag50In 36Gd14 quasicrystal and its 1/1 approximant is found to be close to that of a Gd+3 ion. The effective magnetic moment at Fe atoms in the Zn77Fe7Sc16 quasicrystal is the largest ever found in an Fe-containing quasicrystal. The hyperfine magnetic fields at 155Gd and 57Fe nuclei in the Ag50In36Gd14 and Zn77 Fe7Sc16 quasicrystals are shown to set in at temperatures larger than the corresponding freezing temperatures. The presence of the distribution of the electric quadrupole splitting in the 155Gd Mossbauer spectra of the Ag50In36Gd14 quasicrystal indicate s the existence of the multiplicity of Gd sites. The binomial distribution of the electric quadrupole splitting and the hyperfine magnetic field derived from the 57Fe Mossbauer spectra Zn77Fe 7Sc16 quasicrystal indicates the existence of two classes of Fe sites. The Debye temperatures of the quasicrystals Ag50In 36Gd14, Zn77Fe7Sc16, and the approximant Ag50In36Gd14 determined from the Mossbauer spectra are, respectively, 199(2),443(8), and 199(1) K.

Thesis (M.S.)--Auburn University, 2007. / Abstract. Vita. Includes bibliographic references (ℓ. 70-73)

Thesis(M.S.)--Auburn University, 2007. / Abstract. Vita. Includes bibliographic references.