Search results for " Neutron scattering"

Symmetry-species conversion in CD3systems

1993

The rates for symmetry-species conversion of CD3 groups are calculated using a model in which the interaction between the quadrupolar moment of the deuterons with electric-field gradient at the site of the nucleus causes symmetry-changing transitions. Just the same phonons are considered for energy conservation as are used to describe the temperature dependence of inelastic neutron scattering experiments. For the conversion rate, a similar temperature dependence is found as has already been obtained for CH3. For temperatures around the tunnelling energy, a behaviour is predicted for CD3 that is different from the behaviour in protonated systems according to all theories known to the authors…

NH3 dynamics in Zn(NH3)4I2 and Zn(NH3)4Br2 investigated by incoherent quasielastic neutron scattering

1997

We report on studies of NH3 dynamics in Zn(NH3)4I2 and Zn(NH3)4Br2 by means of incoherent quasielastic neutron scattering in the temperature range from 150 to 300 K. Quasielastic experiments were carried out on the Fermi chopper time-of-flight spectrometer at NIST. The Lorentzian full-widths at half-maximum, ƒw, of the quasielastic line follow an Arrhenius law and increase from 0.39 [0.25] meV at 150 K to 1.33 [0.80] meV at 300 K. We find an activation energy Ea for the NH3 reorientations of 32(1) and 30(3) meV for Zn(NH3)4I2 and Zn(NH3)4Br2, respectively.

MAGPACK1A package to calculate the energy levels, bulk magnetic properties, and inelastic neutron scattering spectra of high nuclearity spin clusters

2001

The Lineshape of Inelastic Neutron Scattering in Relaxor Ferroelectrics

2005

We show that a microscopic reason for the steep drop of the optical phonon branch into an acoustic one (the so-called waterfall effect) in relaxor ferroelectrics may be the coupling of phonons with defects and impurities of different kinds, which is always present in relaxors. Namely, we do not specify the type of impurities but rather represent them as an ensemble of so-called two-level systems (TLS). This approach makes it possible to trace the evolution of the “waterfall” with temperature and the TLS concentration. To facilitate the planning of experiments on inelastic neutron scattering, we present a modification of the so-called Latin hypercube sampling method, which, based on some sig…

Relaxation of the field-induced structural anisotropy in a rotating magnetic fluid

2009

The relaxation of field-induced anisotropy in a magnetic fluid with dominant repulsion is theoretically modeled and experimentally measured by small angle neutron scattering on a sample rotating at angular velocity ω. The scattered pattern distortion scales as the Mason number Mn=ω·τq, τq being the q-dependent diffusion time of nanoparticles. The model accounts for the magnetophoretical drift in the non-homogeneous self-magnetic field of the assembly, continuously created by the thermal noise. The Mn-dependence of the pattern distortion is well described without any adjustable parameter.

Inelastic neutron scattering study of the rotational excitations in(KBr)1−x(KCN)xin the paraelastic and structural glass state

1984

The coupled rotational-translational excitations in ${(\mathrm{KBr})}_{1\ensuremath{-}x}{(\mathrm{KCN})}_{x}$ were studied by inelastic neutron scattering for concentrations $0.008\ensuremath{\le}x\ensuremath{\le}0.20$. We followed the ${A}_{1g}\ensuremath{-}{T}_{2g}$ tunneling transition and the ${A}_{1g}\ensuremath{-}{E}_{g}$ librational excitation through the transition from the paraelastic to the structural glass state. We found that these two excitations and their coupling to the lattice strains exhibit a very different temperature dependence in the glass state. While the tunneling transition, which triggers reorientations of the ${\mathrm{CN}}^{\ensuremath{-}}$ ions, shows a drastic r…

Some comments on fluctuating-elasticity and local oscillator models for anomalous vibrational excitations in glasses

2011

Abstract An overview is given on the present status of the theoretical description of vibrational spectra of glasses, as seen by inelastic neutron, X-ray and light (Raman) scattering. Using the language of Green's/response functions the merits and shortcomings of a local oscillator and a generalized elasticity-theory point of view are discussed. It is pointed out that in both cases the interaction of phonons with disorder-induced irregularities leads to Rayleigh scattering (mean free path l ∝ ω − 4 ) at low enough frequencies and temperatures. In disordered solids at ambient temperature the Rayleigh scattering is usually masqued by Akhiezer-like anharmonic scattering l ∝ ω − 2 , but it can …

Spin Dynamics of the Half-Integer-Spin Quasi-One-Dimensional Heisenberg Antiferromagnet CsMnI3

1994

Magnetic excitations of CsMnI 3 , a quasi-one-dimensional Heisenberg antiferromagnet with S =5/2, have been measured by means of inelastic neutron scattering. Magnetic excitations in the low temperature phase are in good agreement with the predictions of the conventional linear spin-wave theory. In particular, in accordance with the linear spin-wave theory, we found three separate modes at Q =(0, 0, 1) instead of a threefold degenerate mode as seen in CsNiCl 3 ( S =1). It confirms that the spin dynamics of the integer spin value system are very different from those of the half-integer spin value system, even in their three-dimensionally ordered phase. Magnetic excitations in the intermediat…

Elastic electron scattering from the multipole moment distributions ofMg25

1977

4He+Ni elastic scattering near the Coulomb barrier and optical model parameters

2002

Cross sections for the elastic scattering system of 4He+Ni have been investigated. Natural nickel was bombarded by 4He ions in the laboratory energy range of 3.0–14.3 MeV and the intensity of helium backscattering through laboratory angles of 96°, 117°, and 137° was observed. From measured yield data, the cross sections were extracted for the direct Ni(4He,4He)Ni scattering process as well as for the inverse recoil scattering process 4He(58Ni,4He)58Ni by calculating the kinematical reversal of the reaction. The experimental helium scattering angles convert to helium recoil angles of 20°, 30°, and 40°, and the incident helium ion energies to 43.5–207 MeV for incident 58Ni ions in the reverse…