Browsing by Author "Fouquet, P"

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    Global optimization of an entire neutron guide hall
    (Wiley-Blackwell, 2011-06-01) Bentley, PM; Fouquet, P; Bohm, M; Sutton, I; Dewhurst, CD; Andersen, KH
    This paper describes the optimization of an entire neutron guide system, from the moderator to the sample position for several instruments simultaneously, using no more than a desktop computer and a few days of CPU time. This is made possible by merging two relatively advanced computational techniques. Neutron acceptance diagram shading is a fast new method for modelling neutron beams, using an approach based on polygons similar to those featuring in computer games. Optimization algorithms based on swarm intelligence are efficient and reliable ways to maximize numerically calculable figures of merit with many strongly coupled geometry parameters. Recent developments in these methods are described, as well as their combination to optimize the geometry of the H5 beamlines at the Institut Laue-Langevin. The optimization was such that all instruments simultaneously gain as much on-sample flux as possible by taking unused phase space from their neighbours, whilst no instrument suffers any losses in its useful flux. © 2011, Wiley-Blackwell.
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    Hindered water motions in hardened cement pastes investigated over broad time and length scales.
    (American Chemical Society, 2009-10-28) Bordallo, HN; Aldridge, LP; Fouquet, P; Pardo, LC; Unruh, T; Wuttke, J; Yokaichiya, F
    We investigated the dynamics of confined water in different hydrated cement pastes with minimized contributions of capillary water. It was found that the water motions are extremely reduced compared to those of bulk water. The onset of water mobility, which was modified by the local environment, was investigated with elastic temperature scans using the high-resolution neutron backscattering instrument SPHERES. Using a Cauchy−Lorenz distribution, the quasi-elastic signal observed in the spectra obtained by the backscattering spectrometer was analyzed, leading to the identification of rotational motions with relaxation times of 0.3 ns. Additionally, neutron spin echo (NSE) spectroscopy was used to measure the water diffusion over the local network of pores. The motions observed in the NSE time scale were characterized by diffusion constants ranging from 0.6 to 1.1 × 10−9 m2 s−1 most likely related to water molecules removed from the interface. In summary, our results indicate that the local diffusion observed in the gel pores of hardened cement pastes is on the order of that found in deeply supercooled water. Finally, the importance of the magnetic properties of cement pastes were discussed in relation to the observation of a quasi-elastic signal on the dried sample spectra measured using the time-of-flight spectrometer. © 2009, American Chemical Society
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    Low energy spin dynamics in the spin ice Ho2Sn2O7
    (Intitite of Physics Publishing Ltd, 2012-02-22) Ehlers, G; Huq, A; Diallo, SO; Adriano, C; Rule, KC; Cornelius, AL; Fouquet, P; Pagliuso, PG; Gardner, JS
    The magnetic properties of Ho 2 Sn 2 O 7 have been investigated and compared to other spin ice compounds. Although the lattice has expanded by 3% relative to the better studied Ho 2 Ti 2 O 7 spin ice, no significant changes were observed in the high temperature properties, T greater than or similar to 20 K. As the temperature is lowered and correlations develop, Ho 2 Sn 2 O 7 enters its quantum phase at a slightly higher temperature than Ho 2 Ti 2 O 7 and is more antiferromagnetic in character. Below 80 K a weak inelastic mode associated with the holmium nuclear spin system has been measured. The hyperfine field at the holmium nucleus was found to be ≈700 T. (c) 2012 IOP Publishing LTD
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    Magnetic fluctuations and correlations in MnSi: Evidence for a chiral skyrmion spin liquid phase
    (American Physical Society, 2011-06-20) Pappas, C; Lelièvre-Berna, E; Bentley, PM; Falus, P; Fouquet, P; Farago, B
    We present a comprehensive analysis of high-resolution neutron scattering data involving neutron spin echo spectroscopy and spherical polarimetry, which confirm the first-order nature of the helical transition in MnSi. The experiments reveal the existence of a totally chiral dynamic phase in a very narrow temperature range above T-C. This unconventional magnetic short-range order has a topology similar to that of a skyrmion liquid or the blue phases of liquid crystals. © 2011, American Chemical Society.