799 research outputs found

    HORACE: software for the analysis of data from single crystal spectroscopy experiments at time-of-flight neutron instruments

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    The HORACE suite of programs has been developed to work with large multiple-measurement data sets collected from time-of-flight neutron spectrometers equipped with arrays of position-sensitive detectors. The software allows exploratory studies of the four dimensions of reciprocal space and excitation energy to be undertaken, enabling multi-dimensional subsets to be visualized, algebraically manipulated, and models for the scattering to simulated or fitted to the data. The software is designed to be an extensible framework, thus allowing user-customized operations to be performed on the data. Examples of the use of its features are given for measurements exploring the spin waves of the simple antiferromagnet RbMnF3_{3} and ferromagnetic iron, and the phonons in URu2_{2}Si2_{2}.Comment: 14 pages, 6 figure

    Spin excitations used to probe the nature of the exchange coupling in the magnetically ordered ground state of Pr0.5_{0.5}Ca0.5_{0.5}MnO3_{3}

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    We have used time-of-flight inelastic neutron scattering to measure the spin wave spectrum of the canonical half-doped manganite Pr0.5_{0.5}Ca0.5_{0.5}MnO3_{3}, in its magnetic and orbitally ordered phase. The data, which cover multiple Brillouin zones and the entire energy range of the excitations, are compared with several different models that are all consistent with the CE-type magnetic order, but arise through different exchange coupling schemes. The Goodenough model, i.e. an ordered state comprising strong nearest neighbor ferromagnetic interactions along zig-zag chains with antiferromagnetic inter-chain coupling, provides the best description of the data, provided that further neighbor interactions along the chains are included. We are able to rule out a coupling scheme involving formation of strongly bound ferromagnetic dimers, i.e. Zener polarons, on the basis of gross features of the observed spin wave spectrum. A model with weaker dimerization reproduces the observed dispersion but can be ruled out on the basis of discrepancies between the calculated and observed structure factors at certain positions in reciprocal space. Adding further neighbor interactions results in almost no dimerization, i.e. recovery of the Goodenough model. These results are consistent with theoretical analysis of the degenerate double exchange model for half-doping, and provide a recipe for how to interpret future measurements away from half-doping, where degenerate double exchange models predict more complex ground states.Comment: 14 pages, 11 figure

    Double exchange magnets: Spin-dynamics in the paramagnetic phase

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    The electronic structure of perovskite manganese oxides is investigated in terms of a Kondo lattice model with ferromagnetic Hund coupling and antiferromagnetic exchange between t2gt_{2g}-spins using a finite temperature diagonalization technique. Results for the dynamic structure factor are consistent with recent neutron scattering experiments for the bilayer manganite La1.2_{1.2}Sr1.8_{1.8}Mn2_2O7_7 . The susceptibility shows Curie-Weiss behaviour and is used to derive a phase diagram. In the paramagnetic phase carriers are characterized as ferromagnetic polarons in an antiferromagnetic spin liquid.Comment: Revtex, 4 pages with 5 postscript figures include

    Absence of strong magnetic fluctuations in the iron phosphide superconductors LaFePO and Sr2ScO3FeP

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    We report neutron inelastic scattering measurements on polycrystalline LaFePO and Sr2ScO3FeP, two members of the iron phosphide families of superconductors. No evidence is found for any magnetic fluctuations in the spectrum of either material in the energy and wavevector ranges probed. Special attention is paid to the wavevector at which spin-density-wave-like fluctuations are seen in other iron-based superconductors. We estimate that the magnetic signal, if present, is at least a factor of four (Sr2ScO3FeP) or seven (LaFePO) smaller than in the related iron arsenide and chalcogenide superconductors. These results suggest that magnetic fluctuations are not as influential on the electronic properties of the iron phosphide systems as they are in other iron-based superconductors.Comment: 7 pages, 5 figure

    A Comparison of the High-Frequency Magnetic Fluctuations in Insulating and Superconducting La2-xSrxCuO4

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    Inelastic neutron scattering performed at a spallation source is used to make absolute measurements of the dynamic susceptibility of insulating La2CuO4 and superconducting La2-xSrxCuO4 over the energy range 15<EN<350 meV. The effect of Sr doping on the magnetic excitations is to cause a large broadening in wavevector and a substantial change in the spectrum of the local spin fluctuations. Comparison of the two compositions reveals a new energy scale of 22 meV in La1.86Sr0.14CuO4.Comment: RevTex, 7 Pages, 4 postscript figure

    Leaving the hospital behind? An anthropological study of group homes in two London boroughs.

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    The thesis examines the impact of leaving a psychiatric hospital, which is due to close, on a group of long-stay patients. Using anthropological method, the study provides a small scale, in-depth analysis of the transition processes involved. It aims to examine the differing perceptions produced from different positions within an institutional setting, focusing particularly on the residents' perspective, and to compare the experience of transition from the clients' viewpoints with the carers' knowledge and assumptions. The first two chapters outline the context of the study, the historical and sociological background to community care policy and the methodological and theoretical approaches taken. Chapter three uses a 'life history' approach, which recounts the experiences of the residents of a particular group home project. These accounts are compared qualitatively with the official summaries of their case histories, suggesting that rehabilitation requires a different method of understanding the patients experiences, abilities, problems and needs. Chapters four and five outline the processes of selection and preparation for leaving the hospital. The experience of leaving hospital is analysed as a 'life crisis' and the professional handling of the move is examined. In chapters six and seven, group home life is described in detail and compared to hospital life, bringing out key features of the environment, routines, and interactions within the home and beyond it. I focus on several areas of change and continuity in the lifestyles of the residents, their use of time and their relations to staff and others. The final section explores the conceptual models of the group home, as an aspect of community care philosophy. I focus particularly on the 'family model' of care, its bases and its implications for the quality of life of the residents, in terms of rehabilitation, social integration and the personal and social identities of the residents

    Inelastic neutron scattering studies of the quantum frustrated magnet clinoatacamite, γ\gamma-Cu2(OD)3Cl, a proposed valence bond solid (VBS)

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    The frustrated magnet clinoatacamite, γ\gamma-Cu2_2(OH)3_3Cl, is attracting a lot of interest after suggestions that at low temperature it forms an exotic quantum state termed a Valence Bond Solid (VBS) made from dimerised Cu2+^{2+} (S=1/2S=1/2) spins.\cite{Lee_clinoatacamite} Key to the arguments surrounding this proposal were suggestions that the kagom\'e planes in the magnetic pyrochlore lattice of clinoatacamite are only weakly coupled, causing the system to behave as a quasi-2-dimensional magnet. This was reasoned from the near 95^\circ angles made at the bridging oxygens that mediate exchange between the Cu ions that link the kagom\'e planes. Recent work pointed out that this exchange model is inappropriate for γ\gamma-Cu2_2(OH)3_3Cl, where the oxygen is present as a μ3\mu_3-OH.\cite{Wills_JPC} Further, it used symmetry calculations and neutron powder diffraction to show that the low temperature magnetic structure (T<6T<6 K) was canted and involved significant spin ordering on all the Cu2+^{2+} spins, which is incompatible with the interpretation of simultaneous VBS and N\'eel ordering. Correspondingly, clinoatacamite is best considered a distorted pyrochlore magnet. In this report we show detailed inelastic neutron scattering spectra and revisit the responses of this frustrated quantum magnet.Comment: Proceedings of The International Conference on Highly Frustrated Magnetism 2008 (HFM2008
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