No description available.

No description available.

Thesis advisor: Stephen Wilson / The work performed within this thesis is divided into two parts, each focusing primarily on the study of magnetic phase behavior using neutron scattering techniques. In first part, I present transport, magnetization, and neutron scattering studies of materials within the iridium oxide-based Ruddelsden-Popper series [Srn+1IrnO3n+1] compounds Sr3Ir2O7 (n=2) and Sr2IrO4 (n=1). This includes a comprehensive study of the doped bilayer system Sr3(Ir1-xRux )2O7. In second part, I present my studies of the effect of uniaxial pressure on magnetic and structural phase behavior of the iron-based high temperature superconductor Ba(Fe1-xCox)2As2. Iridium-based 5d transition metal oxides host rather unusual electronic/magnetic ground states due to strong interplay between electronic correlation, lattice structure and spin-orbit effects. Out of the many oxides containing iridium, the Ruddelsden-Popper series [Srn+1IrnO3n+1] oxides are some of the most interesting systems to study both from the point of view of physics as well as from potential applications. My work is focused on two members of this series Sr3Ir2O7 (n=2) and Sr2IrO4 (n=1). In particular, our combined transport, magnetization and neutron scattering studies of Sr3Ir2O7 (n=2) showed that this system exhibits a complex coupling between charge transport and magnetism. The spin magnetic moments form a G-type antiferromagnetic structure with moments oriented along the c-axis, with an ordered moment of 0.35±0.06 µB/Ir. I also performed experiments doping holes in this bilayer Sr3(Ir1-xRux)2O7 system in order to study the role of electronic correlation in these materials. Our results show that the ruthenium-doped holes remain localized within the Jeff=1/2 Mott insulating background of Sr3Ir2O7, suggestive of `Mott blocking' and the presence of strong electronic correlation in these materials. Antiferromagnetic order however survives deep into the metallic regime with the same ordering q-vector, suggesting an intricate interplay between residual AF correlations in the Jeff=1/2 state and metallic nanoscale hole regions. Our results lead us to propose an electronic/magnetic phase diagram for Sr3(Ir1-xRux)2O7 system showing how the system moves from Jeff=1/2 antiferromagnetic Mott insulator (Sr3Ir2O7) to paramagnetic Fermi liquid metal (Sr3Ru2O7). On the other hand, our neutron scattering measurements on Sr2IrO4 (n=1), a prototypical Jeff=1/2 Mott insulator, showed that the spins arranged antiferromagnetically in ab-plane with an ordered moment comparable to that of Sr3Ir2O7. The second part of my work is comprised of a neutron scattering-based study of the Ba(Fe1-xCox)2As2 system, a bilayer family of iron-based high temperature superconductors. Undoped, this system exhibits either simultaneous or nearly simultaneous magnetic and structural phase transitions from a high temperature paramagnetic tetragonal phase to low temperature orthorhombic antiferromagnetic phase. With the gradual suppression of these two temperatures, the superconducting phase appears with the highest TC obtained just beyond their complete suppression. It has been proposed that these coupled magnetostructural transitions are secondary manifestations which arise as a consequence of electronic nematic ordering that occurs at a temperature higher than either of them. My work is mainly focused on probing the spin behaviors coupling to this electronic nematic phase. I devised a small device to apply uniaxial pressure along an in-plane high symmetry axis and studied the magnetic and structural behavior in series of Ba(Fe1-xCox)2As2 compounds via neutron scattering in presence of uniaxial pressure. There is an upward thermal shift in the onset of structural and magnetic transition temperature caused by this uniaxial pressure which is surprisingly insensitive to cobalt concentration in the absolute scale. Furthermore, on the first order side of the phase diagram (below the tricritical point), the structural and magnetic transitions are decoupled with magnetic transition following structural distortion. This study suggests the importance of both spin-lattice and orbital-lattice interactions in these families of compounds. / Thesis (PhD) — Boston College, 2014. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Neutron scattering at extremes of pressure is a powerful tool for studying the response of structural and magnetic properties of materials on microscopic level to applied stresses. However, experimental neutron studies require a relatively large sample volume, which makes it necessary to use rather large presses. One of the most commonly used high-pressure devices in neutron diffraction experiments is the Paris-Edinburgh (P-E) press. This compact hydraulic press has been successfully used at various neutron facilities across the world for over 20 years. However, there are some limitations imposed by the design of the press and this project addressed two of them. The first objective of this research was to solve the problem of the spatial restriction on the scattered beam imposed by the tie-rods of the P-E press which results in the limited access to the reciprocal space of a single-crystalline sample. In order to overcome this challenge a rotating device has been designed and built to work with the existing P-E press mainframe. The purpose of the device is to rotate the anvils at a high load in a controlled fashion so that neutron diffraction data can be collected from the sample at particular angles. A number of technical challenges had to be overcome in the design process. One of the problems was caused by the significant friction in the commercially available thrust roller bearings at loads above 25 tonnes. To resolve this issue a set of custom-designed hydraulic bearings has been developed. In these bearings a thin layer of oil separates the two parts that can rotate with respect to each other.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第13843号 / 工博第2947号 / 新制||工||1435(附属図書館) / 26059 / UT51-2008-C759 / 京都大学大学院工学研究科高分子化学専攻 / (主査)教授 金谷 利治, 教授 吉﨑 武尚, 教授 渡辺 宏 / 学位規則第4条第1項該当

<p>This thesis is comprised of three original works presented in Chapters 4,5, and 6. The three projects are comprised of three different problems in condensed matter physics, the commonality between the topics is the technique used to study the materials, diffraction. Two of which are in the form of journal articles that have been peer-reviewed and published in Physical Review B, while the third is that of an unpublished work. The unpublished work is presented first and uses neutron scattering techniques, while two published papers were performed using x-ray diffraction.</p> <p>The neutron scattering experiments were performed by myself with the assistance of William J .L. Buyers and B.D. Gaulin. These experiments were performed at Chalk River Laboratories in northern Ontario. The x-ray scattering experiments were carried out on the rotating anode x-ray laboratory at McMaster University and the Advanced Photon Source in Argonne II. All data analysis was performed by myself under the guidance of B.D. Gaulin.</p> / Thesis / Doctor of Philosophy (PhD)

Block copolymers have attracted interests for potential application ranging from dynamic photonic sensors to solid-state ion conductors. However, due to nucleation and growth mechanism, block copolymer inherently forms granular microstructure with defects such as grain boundaries. Understanding the microstructure of block copolymer is thus crucial in many applications because the microstructure determines the transport property of functional fillers such as ions in block copolymer template. Previous research has shown that athermal filler segregated to grain boundary of lamellae block copolymer and retards the grain coarsening. However, the kinetics of this grain boundary segregation during thermal annealing has not been revealed. Polystyrene-b-polyisoprene blended with deuterated polystyrene is used for neutron scattering study on studying the kinetics of grain boundary segregation. Deuterated polystyrene will segregate to grain boundaries, therefore, decorate grain boundary. The filler segregation behavior will be studied by comparing neutron scattering of polystyrene-b-polyisoprene/deuterated polystyrene with different annealing times (at T=130 deg C, duration of 0hr, 3hr, 1day, 3day and 7day, respectively). Invariant (Q) analysis along with grain mapping is conducted to quantitatively analyze the kinetics of grain boundary segregation. This kinetic was in good agreement with the McLean’s kinetic model for grain boundary segregation in metals. By applying Langmuir-Mclean’s segregation isotherm equation, we have predicted the equilibrium concentration of filler in grain boundary by calculating the strain energy stored in grain boundary.

G-protein–coupled receptors are the largest superfamily in the human genome, and involved in critical cellular signaling processes in living cells. Protein structural fluctuations are the key for GPCR function that is driven and modulated by a variety of factors that are not well understood. This dissertation focusses on understanding the activation of GPCRs using the visual receptor, rhodopsin as the prototype. Rhodopsin is an ideal candidate for this study, as it represents the largest class of GPCRs, and is known to demonstrate more noticeable structural changes upon activation compared to the other GPCRs. What structural fluctuations occur, the role of water, and how the retinal cofactor regulates the protein dynamics during rhodopsin activation are specific research problems addressed in this work. Hypothesizing an ensemble activation mechanism, experiments were conducted using a variety of techniques to probe structural and dynamical fluctuations of rhodopsin in native membranes, as well as in membrane mimetics such as detergent micelles. Time-resolved wide-angle X-ray scattering (TR-WAXS), small-angle neutron scattering (SANS), quasielastic neutron scattering (QENS), and electronic spectroscopy are among the prominent techniques used to gain insights into the photo-intermediates that are key to understanding the rhodopsin activation process. The small-angle neutron scattering (SANS) experiments revealed a volumetric expansion of the protein molecule upon photoactivation of rhodopsin. Electronic spectroscopy together with the differential hydration study revealed the crucial role of water in rhodopsin signaling process and signal amplification by water. The quasielastic neutron scattering study conducted on powdered rhodopsin probed the changes in the local dynamics that are regulated by the retinal cofactor of the rhodopsin molecule. The increased local steric crowding in the ligand-free opsin is consistent with collapsing of the apoprotein structure in the absence of the retinal chromophore leading to inactive opsin conformation. Finally, a time-resolved wide-angle X-ray scattering study was conducted using the X-ray free electron laser at the SLAC national laboratory to probe the early structural fluctuations in rhodopsin photoactivation. The preliminary pump-probe experiments conducted on rhodopsin in CHAPS detergent micelles revealed a light-triggered protein quake that occurs during the early activation stages of rhodopsin photoactivation. Thus the protein fluctuations underlying the GPCR function are revealed by neutrons, X-rays, and other photons in a combined implementation of both spectroscopic and scattering techniques as applied to the investigation of rhodopsin activation.

This thesis describes the experimental investigation of three different strongly correlated transition-metal oxide systems. The magnetic behaviour of each has been probed using inelastic neutron spectroscopy. A distinctive hour-glass excitation spectrum has been observed in the layered cobaltate La_1.75Sr_0.25CoO₄. This spectrum is similar to that measured in a related cobaltate La_1.67Sr_0.33CoO₄, although it appears broader. The spectrum has been reproduced using a spin wave model derived from a disordered cluster spin glass ground state. Signatures of spin glass behaviour have also been observed in bulk magnetisation measurements of La_1.75Sr_0.25CoO₄. These findings, once more, demonstrate the emergence of an hour-glass spectrum from a ground state that combines quasi-one dimensional magnetic correlations and disorder. Additionally, this study shows that charge and magnetic stripe order persists to lower dopings in La_2-xSr_xCoO₄ than previously thought. The complete magnetic excitation spectrum of the multiferroic compound CuO has been measured for the first time. A high energy, one-dimensional magnetic spectrum is observed and modelled using the Muller ansatz derived for the S=1/2 Heisenberg antiferromagnetic chain. At lower energies, a three-dimension spectrum is observed. The measured spectrum is inconsistent with all previous theoretical estimates of the dominant inter-chain exchange interactions in CuO. The inter-chain dispersion is successfully described by a phenomenological model based on linear spin wave theory. The third material investigated, LuFe₂O₄ demonstrates complex charge and magnetic order, the precise nature of which is still under debate. The full spectrum of in-plane excitations in LuFe₂O₄ has been measured and a complicated dispersion consistent with six magnetic modes is observed. These findings are compatible with structures described by a magnetic unit cell containing six spins. The dispersion can be described by a spin wave model derived from a bilayer structure comprised of charge-rich and charge-poor monolayers. This structure is consistent with the original site-specific model for the 3D magnetic ordering in LuFe₂O₄.

Synthesis and structural characterisation of metal hydrides in two important systems are presented. The first system presented is low valent cobalt and nickel complex hydrides with the compositions BaMg5Co2H10, RbMg5CoNiH10, SrMg2CoH7and Sr4Mg4Co3H19 featuring nickel with oxidation state of 0 and cobalt with oxidation state +I and -I. The second system presented is polyanionic gallium complex hydrides with the compositions RbGaH2, RbxK(1−x)GaH2 (0.5≤x≤1), CsxRb(8−x)Ga5H15 (0≤x≤8) and Cs10Ga9H25 featuring novel hydrogenous polyanionic gallium hydride clusters mimicking common hydrocarbons. The syntheses of the compounds were performed at elevated temperatures and at moderate hydrogen pressures (50-100 bar). The structural investigations were mainly done by X-ray powder diffraction (XRPD) and neutron powder diffraction (NPD). The metal-hydrogen bond was investigated by vibrational spectroscopy using Fourier Transform IR-spectroscopy (FTIR) and Inelastic Neutron Scattering (INS).By subtle changes in the compositions of the hydrides it was possible to induce major changes in band gaps, oxidation states and structures. / <p>At the time for the doctoral defence the following papers were unpublished and had a status as follows: Paper 1: Manuscript; Paper 2: Accepted; Paper 5: Manuscript</p>