Inelastic Scattering Measurements Research Articles

Magnetic excitations in amorphous isotropic ferromagnets

Neutron-scattering techniques have been used to investigate the magnetic excitations in the structurally amorphous ideal isotropic ferromagnet (${T}_{c}=513$ K) ${\mathrm{Fe}}_{40}$${\mathrm{Ni}}_{40}$${\mathrm{P}}_{14}$${\mathrm{B}}_{6}$. Small-angle inelastic-scattering measurements were taken with a triple-axis spectrometer to determine the temperature dependence of the spin-wave dispersion relation at long wavelengths. Most of the measurements, however, were concentrated in the momentum region near the first peak (${Q}_{0}=3.1$ ${\mathrm{\AA{}}}^{\ensuremath{-}1}$) in the static structure factor, and were made at 295 and 17 K with a pulsed polarized-beam time-of-flight spectrometer that uses the cross-correlation technique of data collection to obtain a high signal-to-noise ratio. It is found that there is a "cone" of scattering as a function of energy with its apex at ${Q}_{0}$, in general agreement with the powder-averaged model proposed by Shirane et al. However, not all of the features of the data can be explained by this model; in particular at ${Q}_{0}$ there is a broad distribution of scattering as a function of energy with a maximum at $E\ensuremath{\sim}12$ meV. Time-of-flight results are also presented for a single crystal and a powdered crystalline material in order to gain a better understanding of the corrections needed to obtain accurate data, and to establish that the instrument performs properly. The advantages as well as the limitations of this polarized-beam technique are discussed.

Neutron-Scattering Determination of the Momentum Distribution and the Condensate Fraction in LiquidHe4

The momentum distribution function, $n(\stackrel{\ensuremath{\rightarrow}}{\mathrm{p}})$, is determined for liquid $^{4}\mathrm{He}$ at 1.00, 2.12, 2.27, and 4.27 K from new neutron inelastic-scattering measurements at large momentum transfers. An improved method for extracting the condensate fraction, ${n}_{0}$, from $n(\stackrel{\ensuremath{\rightarrow}}{\mathrm{p}})$ is presented and used to obtain new values for ${n}_{0}$ at 1.00 and 2.12 K and a revised value at 1.1 K.

Effect of neutron irradiation on single-crystalV3Si: Neutron diffraction studies

Elastic and inelastic neutron-diffraction measurements have been made on a single crystal of ${\mathrm{V}}_{3}$Si as a function of fast-neutron irradiation. After irradiation to a fluence of 22.2\ifmmode\times\else\texttimes\fi{}${10}^{18}$ n/${\mathrm{cm}}^{2}$ ($Eg1$ MeV), refinement of the Bragg intensity data reveals a slight decrease in the long-range-order parameter to a value of about 0.9, and an increase in the average value of the mean-square displacement $〈{u}^{2}〉$ at low temperature of about 0.001 ${\mathrm{\AA{}}}^{2}$. Inelastic-scattering measurements show that the softening of the transverse-acoustical mode associated with the martensitic transition is absent in the irradiated crystal.

Collective motions in classical liquids. IV. Liquid rubidium

The coherent scattering function is calculated for liquid rubidium at its melting point on the basis of the theory developed in paper II of this series and using the static structure factor obtained by Rahman from computer simulation with a potential determined by Price et al. It is noted that for q up to 1 A −1 , the theory indicates the existence of propagating density waves unlike liquid argon, but the shape of S(q, ω) is found to disagree with the experimental observations. For intermediate values of q, the agreement between the theoretical and experimental results is not so good, but the disagreement is always less than 40%. However, for q≥3 A −1 , the theoretical results for S(q,ω) are quantitatively in good agreement with the recent neutron inelastic-scattering measurements on liquid rubidium.

Dielectric and Neutron Inelastic Scattering Measurements on Phenanthrene

The static dielectric constants and the linear coefficients of thermal expansion of high purity single crystals of phenanthrene were measured as a function of temperature in three mutually perpendicular directions and indicate a small increase in the dielectric constant in the a direction at about 72°C. Cold neutron inelastic scattering from polycrystalline phenanthrene was investigated at room temperature, 60, 75, and 85°C, using the Brookhaven Slow Chopper facility. Changes in the density of phonon states above and below 72°C were examined by difference analysis and loss of some low-lying states above 72°C was detected. Some implications of these dielectric measurements and the neutron spectra as well as earlier electrical and heat capacity measurements are discussed. The conclusion is that phenanthrene cannot at present be described as one of the known types of solids. It is suggested that the observed phenomena are not inconsistent with a model in which hindered vibration of the 4, 5 hydrogens above 72°C introduce a new degree of freedom.

Phonon Frequencies in Copper at 49 and 298°K

Phonon frequencies for wave vectors along the principal symmetry directions in copper have been determined at 49 and 298\ifmmode^\circ\else\textdegree\fi{}K from neutron inelastic-scattering measurements. In general, the temperature dependences of the frequencies were found to be smaller for the higher-frequency modes. For the lower frequencies ($\ensuremath{\nu}\ensuremath{\lesssim}3\ifmmode\times\else\texttimes\fi{}{10}^{12}$ cps), the frequency changes measured are consistent with the 3-4% changes estimated from the isothermal elastic constants. For higher frequencies the relative changes are much smaller, often being 1% or less. Axially symmetric force models, which included interactions to the sixth nearest neighbors, were fitted to the data and have been used to calculate a frequency distribution function $g(\ensuremath{\nu})$ at each temperature. A comparison of the temperature dependences of the moments of these distributions with various Gr\"uneisen parameters leads to the conclusion that Cu does not satisfy the assumption of the quasiharmonic model. The Debye temperature ${\ensuremath{\Theta}}_{C}$ versus temperature curve calculated with the 49\ifmmode^\circ\else\textdegree\fi{}K $g(\ensuremath{\nu})$ is in excellent agreement with results from specific-heat measurements in the entire 0 to 298\ifmmode^\circ\else\textdegree\fi{}K range. A fairly strong temperature dependence for the widths of some well-focused phonons was observed.

Inelastic Scattering Measurements in a Fast Reactor by the Spherical Shell Method

The spherical shell method for investigating inelastic scattering cross sections has been used in a fast-reactor core environment. The changes in 238U/ 235U, 236U/ 235U, and 234U/ 235U fission rati...

Proton Dynamics in Hydrogen-Bonded Ferroelectrics

A comparison between infrared absorption and neutron inelastic-scattering measurements indicates the existence of a broad low-energy (around 400 cm−1) protonic level in KH2PO4 above the Curie point. This level is only weakly discerned in the ir but is seen quite clearly in neutron scattering. The mode is explained as resulting from splitting of the ground level of the proton in a slightly asymmetric double minimum potential well where tunneling of the proton takes place. The asymmetry is caused by the interaction between the different protons, and it changes slowly with time as the result of the collective motions of the protons. Prominent changes in ir spectra in the 400-cm−1 range were found on cooling through the Curie point, indicating the disappearance of the tunneling mode. This picture allows a simplified treatment of the system, leading to the existence of two correlated phase transition points, and a negative thermal expansion coefficient of the H bond at temperatures between these points. The two transition points are identified as the Curie point and the melting or dissociation point of the crystal. The theory also suggests an increase in the dielectric constant as the melting point is approached, corresponding to the Curie—Weiss behavior as the Curie point is approached.

Solid-Superfluid Transition inHe4at Absolute Zero

A solid-superfluid phase transition applicable to ${\mathrm{He}}^{4}$ at the absolute zero of temperature is described, utilizing a cell model. Below the critical density which marks the onset of the superfluid, the system has these properties: (i) The energy for adding a particle to the system without creating excitations becomes equal to that for removing a particle; (ii) the excitation spectrum changes from quadratic to linear in momentum for low-momentum excitations; (iii) the zero-momentum state exhibits macroscopic occupation. We estimate the maximum occupation of the zero-momentum level to be about 6%. An estimate of the effective repulsive energy between a pair of ${\mathrm{He}}^{4}$ atoms confined within a specific volume yields about 16\ifmmode^\circ\else\textdegree\fi{}K. The excitation spectra obtained from averaging on a simple cubic, a body-centered cubic, and a face-centered cubic system are compared with the experimental curve from neutron inelastic-scattering measurements on liquid helium.