Browsing by Author "Habicht, K"

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    Gains from the upgrade of the cold neutron triple-axis spectrometer FLEXX at the BER-II reactor
    (Elsevier, 2013-11-21) Le, MD; Quintero-Castro, DL; Toft-Petersen, R; Groitl, F; Skoulatos, M; Rule, KC; Habicht, K
    The upgrade of the cold neutron triple-axis spectrometer FLEXX is described. We discuss the characterisation of the gains from the new primary spectrometer, including a larger guide and double focussing monochromator, and present measurements of the energy and momentum resolution and of the neutron flux of the instrument. We found an order of magnitude gain in intensity (at the cost of coarser momentum resolution), and that the incoherent elastic energy widths are measurably narrower than before the upgrade. The much improved count rate should allow the use of smaller single crystals samples and thus enable the upgraded FLEXX spectrometer to continue making leading edge measurements. © 2013, Elsevier Ltd.
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    Jahn-Teller versus quantum effects in the spin-orbital material LuVO 3
    (American Physical Society, 2015-04-13) Skoulatos, M; Toth, S; Roessli, B; Enderle, M; Habicht, K; Sheptyakov, D; Cervellino, A; Freeman, PG; Reehuis, M; Stunault, A; McIntyre, GJ; Tung, LD; Marjerrison, C; Pomjakushina, E; Brown, PJ; Khomskii, DI; Rüegg, A; Kreyssig, A; Goldman, AI; Goff, JP
    We report on combined neutron and resonant x-ray scattering results, identifying the nature of the spin-orbital ground state and magnetic excitations in LuVO3 as driven by the orbital parameter. In particular, we distinguish between models based on orbital-Peierls dimerization, taken as a signature of quantum effects in orbitals, and Jahn-Teller distortions, in favor of the latter. In order to solve this long-standing puzzle, polarized neutron beams were employed as a prerequisite in order to solve details of the magnetic structure, which allowed quantitative intensity analysis of extended magnetic-excitation data sets. The results of this detailed study enabled us to draw definite conclusions about the classical versus quantum behavior of orbitals in this system and to discard the previous claims about quantum effects dominating the orbital physics of LuVO3 and similar systems. © 2015 American Physical Society
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    Neutron study of the magnetism in NiCl2•4SC(NH2)2
    (IOP Publishing Ltd., 2013-05-29) Tsyrulin, N; Batista, CD; Zapf, VS; Jaime, M; Hansen, BR; Niedermayer, C; Rule, KC; Habicht, K; Prokes, K; Kiefer, K; Ressouche, E; Paduan, A; Kenzelmann, M
    We study the strongly anisotropic quasi-one-dimensional S = 1 quantum magnet NiCl2 center dot 4SC(NH2)(2) using elastic and inelastic neutron scattering. We demonstrate that a magnetic field splits the excited doublet state and drives the lower doublet state to zero energy at a critical field H-c1. For H-c1 < H < H-c2, where H-c2 indicates the transition to a fully magnetized state, three-dimensional magnetic order is established with the AF moment perpendicular to the magnetic field. We mapped the temperature/magnetic field phase diagram, and we find that the total ordered magnetic moment reaches m(tot) = 2.1 mu(B) at the field mu H-0 = 6 T and is thus close to the saturation value of the fully ordered moment. We study the magnetic spin dynamics in the fully magnetized state for H > H-c2, and we demonstrate the presence of an AF interaction between Ni2+ on the two interpenetrating sublattices. In the antiferromagnetically ordered phase, the spin-waves that develop from the lower-energy doublet are split into two modes. This is most likely the result of the presence of the AF interaction between the interpenetrating lattices.© 2013 IOP Publishing LTD.
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    Observation of phasons in the magnetic shape memory alloy Ni2MnGa
    (IOP Sciences, 2007-03) Shapiro, SM; Vorderwisch, P; Habicht, K; Hradil, K; Schneider, H
    An inelastic neutron scattering study of the lattice dynamics of the martensite phase of the ferromagnetic shape memory alloy, Ni2MnGa, reveals the presence of well-defined phasons associated with the charge density wave (CDW) resulting from Fermi surface (FS) nesting. The velocity and the temperature dependence of the phason are measured as well as the anomalous [110]-TA(2) phonon. © 2007, EPLA
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    Phason mode in the incommensurate martensitic phase of Ni2MnGa - observed by neutron spectroscopy
    (Australian Institute of Physics, 2006-12-04) Vorderwisch, P; Shapiro, SM; Habicht, K; Hradil, K; Schneider, H
    The ferromagnetic shape-memory alloy Ni2MnGa, of technological interest as a magnetically driven actuator, undergoes a martensitic transformation from a cubic austenite to a tetragonally distorted martensite structure. Previous inelastic neutron scattering experiments, performed in the martensite phase, show an anomaly in the phonon dispersion for the [110] transverse acoustic branch TA2 [1]. This anomaly is identified (in ab-initio calculations through examination of Fermi-surface nesting and electron-phonon coupling) as a Kohn anomaly [2] and occurs at a wave vector incommensurate with the underlying lattice. Elastic neutron scattering reveals a modulation of the martensite phase with this incommensurate wave vector [1]. In very recent inelastic experiments, performed with cold- and thermal-neutron triple-axis-spectrometers, in addition to the known acoustic branch a new dispersive branch was found, showing all characteristics of a phason mode. In our interpretation, the new mode is caused by a charge-density-wave in the incommensurately distorted lattice. Inelastic neutron scattering is the only technique able to unambiguously detect a phason branch - and successful examples are rare. [1] P. Vorderwisch and S.M. Shapiro, submitted [2] C. Bungaro et al., Phys. Rev. B 68 (2003) 134104