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Tomas R.,CERN | Bach T.,CERN | Calaga R.,CERN | Langner A.,CERN | And 7 more authors.
Physical Review Special Topics - Accelerators and Beams

The LHC is currently operating with a proton energy of 4 TeV and β * functions at the ATLAS and CMS interaction points of 0.6 m. This is close to the design value at 7 TeV (β *=0. 55m) and represented a challenge for various aspects of the machine operation. In particular, a huge effort was put into the optics commissioning and an unprecedented peak β beating of around 7% was achieved in a high energy hadron collider. Source

Chatterji T.,Laue Langevin Institute | Ouladdiaf B.,Laue Langevin Institute | Henry P.F.,ESS AB | Bhattacharya D.,Indian Central Glass and Ceramic Research Institute
Journal of Physics Condensed Matter

We have investigated magnetoelastic effects in multiferroic YMnO 3 below the antiferromagnetic phase transition, T N70K, using neutron powder diffraction. The a lattice parameter of the hexagonal unit cell of YMnO 3 decreases normally above T N, but decreases anomalously below T N, whereas the c lattice parameter increases with decreasing temperature and then increases anomalously below T N. The unit cell volume also undergoes an anomalous contraction below T N. By fitting the background thermal expansion for a non-magnetic lattice with the Einstein-Grüneisen equation, we determined the lattice strains Δa, Δc and ΔV due to the magnetoelastic effects as a function of temperature. We have also determined the temperature variation of the ordered magnetic moment of the Mn ion by fitting the measured Bragg intensities of the nuclear and magnetic reflections with the known crystal and magnetic structure models and have established that the lattice strain due to the magnetoelastic effect in YMnO 3 couples with the square of the ordered magnetic moment or the square of the order parameter of the antiferromagnetic phase transition. © 2012 IOP Publishing Ltd. Source

Nilsen G.J.,Laboratory for Quantum Magnetism | Nilsen G.J.,University of Tokyo | Nilsen G.J.,University of Edinburgh | Coomer F.C.,University of Edinburgh | And 7 more authors.
Physical Review B - Condensed Matter and Materials Physics

We present spatial and dynamic information on the s=1/2 distorted kagome antiferromagnet volborthite, Cu3V2O7(OD) 2•2D2O, obtained by polarized and inelastic neutron scattering. The instantaneous structure factor, S(Q), is dominated by nearest-neighbor pair correlations, with short-range order at wave vectors Q1=0.65(3) -1 and Q2=1.15(5) -1 emerging below 5 K. The excitation spectrum, S(Q,ω), reveals two steep branches dispersing from Q1 and Q2, and a flat mode at ωf=5.0(2)meV. The results allow us to identify the crossover at T*∼1 K in 51V NMR and specific-heat measurements as the buildup of correlations at Q1. We compare our data to theoretical models proposed for volborthite, and also demonstrate that the excitation spectrum can be explained by spin-wave-like excitations with anisotropic exchange parameters, as suggested by recent local-density calculations. © 2011 American Physical Society. Source

Du Plessis H.E.,Sasol Limited | De Villiers J.P.R.,University of Pretoria | Kruger G.J.,University of Johannesburg | Steuwer A.,ESS AB | And 2 more authors.
Journal of Synchrotron Radiation

Fischer-Tropsch (FT) synthesis is an important process in the manufacturing of hydrocarbons and oxygenated hydrocarbons from mixtures of carbon monoxide and hydrogen (syngas). The reduced iron catalyst reacts with carbon monoxide and hydrogen to form bulk Fe5C2 Hägg carbide (χ-HC) during FT synthesis. Arguably, χ-HC is the predominant catalyst phase present in the working iron catalyst. Deactivation of the working catalyst can be due to oxidation of χ-HC to iron oxide, a step-wise decarburization to cementite (θ-Fe3C), carbon formation or sintering with accompanying loss of catalytic performance. It is therefore critical to determine the precise crystal structure of χ-HC for the understanding of the synthesis process and for comparison with the first-principles ab initio modelling. Here the results of high-resolution synchrotron X-ray powder diffraction data are reported. The atomic arrangement of χ-HC was confirmed by Rietveld refinement and subsequent real-space modelling of the pair distribution function (PDF) obtained from direct Fourier transformation. The Rietveld and PDF results of χ-HC correspond well with that of a pseudo-monoclinic phase of space group P1-[a = 11.5661 (6) Å, b = 4.5709 (1) Å, c = 5.0611 (2) Å, α = 89.990 (5)°, β = 97.753 (4)°, γ = 90.195 (4)°], where the Fe atoms are located in three distorted prismatic trigonal and one octahedral arrangement around the central C atoms. The Fe atoms are distorted from the prismatic trigonal arrangement in the monoclinic structure by the change in C atom location in the structure. © 2011 International Union of Crystallography. Source

Tsirlin A.A.,Max Planck Institute for Chemical Physics of Solids | Abakumov A.M.,University of Antwerp | Ritter C.,Laue Langevin Institute | Henry P.F.,Helmholtz Center Berlin | And 3 more authors.
Physical Review B - Condensed Matter and Materials Physics

We present a comprehensive study of the crystal structure, magnetic structure, and microscopic magnetic model of (CuBr)LaNb 2O 7, the Br analog of the spin-gap quantum magnet (CuCl)LaNb 2O 7. Despite similar crystal structures and spin lattices, the magnetic behavior and even peculiarities of the atomic arrangement in the Cl and Br compounds are very different. The high-resolution x-ray and neutron data reveal a split position of Br atoms in (CuBr)LaNb 2O 7. This splitting originates from two possible configurations developed by [CuBr] zigzag ribbons. While the Br atoms are locally ordered in the ab plane, their arrangement along the c direction remains partially disordered. The predominant and energetically more favorable configuration features an additional doubling of the c lattice parameter that was not observed in (CuCl)LaNb 2O 7. (CuBr)LaNb 2O 7 undergoes long-range antiferromagnetic ordering at T N=32 K, which is nearly 70% of the leading exchange coupling J 4≃48 K. The Br compound does not show any experimental signatures of low-dimensional magnetism because the underlying spin lattice is three-dimensional. The coupling along the c direction is comparable to the couplings in the ab plane, even though the shortest Cu-Cu distance along c (11.69 Å) is three times larger than nearest-neighbor distances in the ab plane (3.55 Å). The stripe antiferromagnetic long-range order featuring columns of parallel spins in the ab plane and antiparallel spins along c is verified experimentally and confirmed by the microscopic analysis. © 2012 American Physical Society. Source

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