Neutron quasielastic scattering from dynamic disorder in PbF2 and BaF2

Coherent neutron quasielastic scattering has been observed from powder samples of PbF₂, and BaF₂, at temperatures above their transition to fast-ion conduction. The scattering is found to be similar to that previously observed in SrCl₂ and CaF₂, suggesting that it stems from a mechanism common to these fluorites in their fast-ion phase.     

Spin-dependent scattering of slow neutrons on polarized 17O

The method of pseudomagnetism has been applied to measure the spin-dependent scattering length b_N of slow neutrons on ¹⁷O nuclei. A powder sample of CaO, enriched with ¹⁷O, has been used. In this sample, dynamic nuclear polarizations of ± 10 % have been achieved at 2.5 T and 0.2 K using paramagnetic impurities created by fast neutron irradiation (F-centers). b_N = 0.12 ± 0.04 fm has been found. This value is close to an earlier shell model calculation.     

Investigation of the hyperfine splitting of protons in TbH1.9 by means of incoherent spin flip scattering of neutrons

For the first time, the hyperfine field at a proton was measured by means of inelastic spin flip scattering of neutrons. The temperature dependence of the hyperfine field at the proton in TbH_1.9 was determined. The measured values could be explained by the polarization of the conduction electrons.     

CDW evidence in one-dimensional K0.3MoO3 by means of Raman scattering

Raman scattering measurements on the one-dimensional (1-D) blue bronze K_0.3MoO₃ are reported as function of temperature. At about 50 cm^-1 a Raman line of A_g(A) symmetry was found with an anomalous temperature dependence. The line symmetry, the softening of the eigenfrequency and the apparent disappearing of the coupling constant at the critical temperature, allow us to assign this line to the amplitude mode of the CDW, predicted by the theory of the Peierls transition. A Peierls precursor at room temperature was also observed and is discussed.     

Sodium diffusion in cryolite at elevated temperatures studied by quasielastic neutron scattering

Quasielastic neutron scattering (QENS) has been applied to study the sodium mobility on nanosecond time scales in the perovskite fluoride cryolite, Na₃AlF₆, at high temperatures. Up to T = 1153 K the diffusion of Na ions is well described by a diffusion process of jumps between six and eight-fold coordinated sites. Above this temperature, where a step-like increase in the electrical conductivity occurs, the jump length increases, which indicates additional jumps over larger distances. The electrical conductivity derived from the self-diffusion coefficient via the Nernst–Einstein relation and the corresponding activation energy are in excellent agreement with the previous conductivity measurements. We conclude that the jump diffusion of sodium ions is the dominant mechanism for the electrical conductivity in cryolite at high temperatures up to T = 1153 K.     

Study of the diffusion of hydrogen in LaNi5+xH6 compounds by 1H NMR relaxation

The diffusion of hydrogen in LaNi_5+xH₆ (x=?0.2, 0.0, 0.2) has been investigated by NMR from 150 to 300 K. High-temperature data of the spin-spin relaxation time T₂ and the rotating frame spin lattice relaxation time T₁? are independent of stoichiometry but the data of the spin lattice relaxation time T₁ and low-temperature T₁? data are not, and they do not fit Torrey's relaxation model.     

Investigation of diffusion processes in liquid sodium and sodium-hydrogen melt by quasielastic neutron scattering

The quasielastic neutron scattering experiments on liquid sodium (at T = 378, 573, and 693 K) and sodiumhydrogen melt (T = 693 K, hydrogen concentration ～ 0.4 at %) were performed with the DIN-2PI time-of-flight spectrometer. The characteristics of the diffusion mobility for particles comprising the liquids studied are extracted from the experimental results and analyzed with the help of the phenomenological and “relaxing cage” models. The self-diffusion coefficient in liquid sodium obtained for all temperatures is in the agreement with the values known from literature. The diffusion mobility in pure sodium and in sodium-hydrogen melt were found to be coinciding; it leads to the conclusion that in our experimental conditions hydrogen exists and diffuses in melt in the form of hydride NaH.     

Parity violating effects in the coherent scattering of neutrons by 209Bi

We study the rotation of the spin in the coherent scattering of polarized thermal neutrons by ²⁰⁹Bi. Contributions due to direct weak scattering by the nucleus, parity admixtures into the nuclear wave functions and scattering by the electrons are calculated. We use a parity violating potential derived in the framework of the Salam-Weinberg model and several other semi-empirical potentials. The results show that the direct scattering on nuclei is the dominant contribution and some potentials predict results up to an order of magnitude larger than expected on dimensional grounds. The sign of the effect depends on the isospin properties of the potential and can distinguish between potentials with large isotensor or large isovector contributions.     

The anomalous Debye-Waller factor of hydrogen in niobium as determined by quasielastic neutron scattering

The integrated intensity of quasielastic neutron scattering by protons in polycrystalline NbH_0.16 and in a single crystal of NbH_0.045 was investigated as a function of the scattering vector Q. Strong deviations from a harmonic Debye-Waller factor behavior were observed at elevated temperatures. The results show a temperature dependent delocalization of the proton extending as far as the neighboring sites of the interstitial lattice. Experiments on the single crystal indicate a directional dependent mean-square amplitude of the proton even at room temperature.     

On the room-temperature ferromagnetism in (ZnO)0.98(MnO2)0.02

Room-temperature ferromagnetism (RTFM) is investigated in the polycrystalline bulk (ZnO)_0.98(MnO₂)_0.02 samples prepared by a modified solid-state sintering route. Successive sintering of a sample was carried out in air at different temperatures in the range of 400-1000 °C. The study of magnetization and phase-investigation in the sample was carried out after each sintering step. The progressive suppression of impurities and the consequent reduction in RTFM is clearly observed in the samples with increase in the sintering temperature up to 800 °C. The subsequent successive sintering of the (ZnO)_0.98(MnO₂)_0.02 sample up to 1000 °C yields fully paramagnetic sample exhibiting wurtzite structure. The studies support the conjecture (Kundaliya et al., Nat. Mater. 3 (2004) 709 [18]) that RTFM in this system has an origin related to a randomly distributed impurity phase produced by local dissolution of ZnO and MnO₂.     

Study of structural phase transitions in (NH4)2SnBr6 by Raman scattering

The two successive phase transitions in (NH₄)₂SnBr₆ were examined by low- frequency Raman scattering experiments. Two low frequency modes which were assigned to the rotary modes of SnBr^2?₆ ions showed soft-mode like behavior in the lowest temperature phase. It was proposed that the higher phase transition is structural in nature associated with the soft rotary mode of the anions whereas the lower one is of an order-disorder type with respect to the relative orientations of the ammonium ions.     

Determination of zone-boundary magnon energy and damping in RbMnF3 by means of light scattering experiments

New experimental data of two-magnon Raman scattering on the cubic antiferromagnet RbMnF₃ are used to obtain information on the single spin dynamics in all the ordered phase. The comparisons with the existing second order theory and with the neutron scattering data shows that the light scattering gives correct values for energy and lifetime of the zone boundary magnetic excitations.     

The analyzing power Aγ(θ) for the elastic scattering of 12 MeV neutrons from deuterons

The analyzing power A_γ(θ) was obtained at 10° intervals between 30° (lab) to 120° (lab) for ²H(n, n)²H at 12.0 MeV. The polarized neutron beam employed in the measurement was obtained by using neutrons emitted at 0° from the polarization transfer reaction ${}^2\text{H}\text{(}\text{d}\text{,}\text{n}\text{)}{}^3\text{He}$. The accuracy in the A_γ(θ) values that was achieved ranged from ± 0.006 to ± 0.013. Comparison of the data to A_γ(θ) results obtained at 12 MeV for the charge symmetric reaction ²H(p, p)²H shows that the two A_γ(θ) distributions are equal to within the above accuracy.     

NMR evidence of two frequency scales of hydrogen jump motion in Ti2Ni-type compounds Ti2CoHx

Nuclear magnetic resonance measurements of the proton spin-lattice relaxation rate R₁ for Ti₂Ni-type compounds Ti₂CoH_x (x=0.56, 0.77 and 1.34) have been performed over the temperature range 20-510 K and the resonance frequency range 13-90 MHz. For Ti₂CoH_0.77 and Ti₂CoH_1.34 the temperature dependence of R₁ is found to exhibit an additional low-temperature peak near 280 K; the amplitude of this peak increases with increasing H content. These results give evidence for the coexistence of at least two types of hydrogen jump motion with different characteristic frequencies. For Ti₂CoH_0.56 no additional R₁ peak has been found. The concentration dependence of the additional peak is discussed in terms of the occupancy of inequivalent interstitial sites by hydrogen atoms.     

Second-harmonic scattering by domains in RbH2PO4 ferroelectrics

Second-harmonic scattering (SHS) of light by laminar ferroelectric domains of the orthorhombic phase of RbH₂PO₄ below T_c=147 K is studied in detail. CCD images of the far-field SHS pattern reveal up to six off-axis distinct intensity maxima in a plane normal to the ferroelectric axis c. These scattering maxima are interpreted on the basis of the quasi-phase matching (QPM) conditions for the three sets of polarizations allowed by the nonvanishing elements of the second-order nonlinear susceptibility tensor. On this assumption scattering angles of the three SHS modes calculated from crystal optics are shown to fit well the experimental data for any incidence of the pump beam normal to c. Scattering intensities of the two most intense modes are also measured versus the component of the wave vector perpendicular to the modulated structure. A stochastic model giving the intensity as a function of the density probability of domain widths is developed and is shown to give a good fit of the scattered intensity. Moreover, it permits the evaluation of the mean and dispersion of the ferroelectric domain widths, which are found to be, respectively, 2.6 and 0.7 μm in the crystal under study. This is consistent with the domain size of the ferroelastic counterpart revealed by polarization microscopy.     

Light scattering by single magnons in FeF3

Scattering of laser light from single magnons has been observed in the canted antiferromagnet FeF₃ as a function of temperature (260 – 360°K) and wave-vector (2 × 10⁵ ? 3 × 10⁴cm^?1). Measured magnon energies ranging up to 3.3 cm^?1 (260°K) and line widths are in good agreement with data from microwave resonance work.     

The interplay between the lattice and magnetism in La（Fe11.4Al1.6）C0.02 studied by powder neutron diffraction

The crystallographic structure and magnetic properties of La（Fell.4Alz.6）C0.02 are studied by magnetic measurernent and powder neutron diffraction with temperature and applied magnetic field. Rietveld refinement shows that La（Fe11.4Al1.6）C0.02 crystallizes into the cubic NaZn13-type with two different Fe sites： FeI （8b） and FeII （96i）, and that A1 atoms preferentially occupy the FeII site. A ferromagnetic state can he induced at a medial temperature of 39 K-139 K by an external magnetic field of 0.7 T, and a large lattice is correspondingly found at 100 K and 0.7 T. In all other conditions, La（Fe11.4Al1.6）C0.02 has no net magnetization in the paramagnetic （T 〉 TN = 182 K） or antifer- romagnetic states, and thus keeps its small lattice. Analysis of the Fe Fe bond length indicates that the ferromagnetic state prefers longer Fe-Fe distances.     

Possible resonance structure in the elastic scattering of neutrons by Y near En = 1 MeV

The total cross section as well as the differential cross section and polarization in the elastic scattering of 0.8–1.4 MeV neutrons by Y have been measured with neutron beams of energy spread less than 20 keV. Rather weak structure with widths ≈ 50 keV was observed at a few energies within this range. The data were analyzed by use of a model in which the scattering process is described in terms of resonance amplitudes superimposed on an optical-potential background. Although not completely definitive, this analysis indicates the existence of three intermediate-width resonances (two 1^? and one 1⁺) at neutron energies between ≈ 1.0 and 1.2 MeV. The properties of the 1^? resonances suggest that these are the parent states of the proposed T_> components of the El giant resonance observed near 21 MeV excitation energy in ⁹⁰Zr produced in the ⁸⁹Y(p,γ₀) reaction. The resolved resonance structure in this energy region is in reasonable agreement with a recent calculation of the energies and widths of negative-parity states in ⁹⁰Y.     

Raman scattering in CeB6

Raman spectra of phonons in CeB₆ were measured. The observed three peaks were assigned to the A_1g, E_g and F_2g phonon modes. The frequencies of the modes suggest that the Ce ion is in the trivalent state.