R. Osborn

Neutron-scattering investigation of the electronic ground state of neptunium dioxide

Neutron spectroscopy has been applied to study the electronic ground state of NpO2 both in the paramagnetic and in the ordered phase, below Tc=25 K. The magnetic inelastic scattering cross section shows a broad peak which is split into two components and is centred at about 55 meV. This inelastic peak is shown to originate from excitations between the Gamma 8(2) and Gamma 8(1) crystal field quartets. Its position is in agreement with the value estimated by scaling the crystal field potential of UO2 to the Np4+ case. Several mechanisms which could be responsible for the observed splitting are discussed. In the energy range below 15 meV the technique of polarization analysis has been used together with an incident polarized neutron beam to unambiguously separate magnetic from vibronic effects. At low temperature, below 25 K the authors see the appearance of an inelastic line (at an energy transfer of 6.4 meV) that indicates a lifting of the degeneracy of the Gamma 8 ground state. This supports the hypothesis that the phase transition involves the quadrupolar ordering of the Np4+ ions by a collective Jahn-Teller distortion of the oxygen sublattice. The observed amplitude of the splitting is consistent with an oxygen displacement of the order of 0.02 AA, which is below the present limits of resolution of neutron diffraction experiments.

Neutron spectroscopy of the crystalline electric field in high-Tc YbBa2Cu3O7

Abstract Inelastic neutron scattering has been employed to study the crystalline electric field (CEF) interaction in the high-T c superconductor YbBa 2 Cu 3 O 7 . The observed energy spectra exhibit three well defined CEF transitions at 88.5, 92.8 and 98.5 meV which completely determine the CEF splitting of the ground-state J-multiplet. A set of CEF parameters is derived which excellently reproduce the neutron spectroscopic data as well as the thermodynamic magnetic properties.

Magnetic properties of Nd3+ in Nd−Ba−Cu−O-compounds

Neutron diffraction, neutron spectroscopy and magnetization measurements have been employed to study the structural, electronic, and magnetic behavior of eleven compounds with the general formula Nd1+yCavBa2−y−vCu3Ox (0≦y≦0.5; 0≦v≦0.25; 6≦x≦7). The structure turned out to react to oxygen reduction similar as other 123-compounds, yielding discontinuities close to the metal-insulator-transition and the well-known relations of bond lengths as a function ofTc. The crystalline electric field (CEF) interaction, splitting the 10-fold degenerate ground-state J-multiplet of the Nd3+-ions into five doublet states, was investigated by neutron spectroscopy. The derived CEF parameters have been used to determine changes in the electronic surroundings of the Nd3+ ions. In addition, with the help of the CEF parameters the thermodynamic magnetic properties of these compounds were calculated which turn out to be in good agreement with the experimental data.

Crystal field excitations in ErMn4Al8

Abstract Distinct crystal field excitations have been observed in tetragonal ErMn4Al8 by inelastic neutron scattering. The energy level sequence and crystal field parameters have been obtained from the observed crystal field transition peak positions and intensities. The crystal field parameters account for the basal plane orientation of the Er magnetic moment in ErFe4Al8 and if extrapolated to the Fe rich compounds ErFe11Ti and ErFe10V2 also can account for the observed planar anisotropy.

Coherent diffuse neutron scattering from UO2 and ThO2 at temperatures above 2000 K

The structure of the View the MathML source reconstruction induced by adsorption of View the MathML source of a monolayer of Sn on the Si(111) surface has been determined using surface X-ray diffraction. The in-plane projection of the surface structure, obtained from structure factors near zero perpendicular momentum transfer, indicates substantial lateral displacements of the Si atoms. Intensity profiles of the fractional order rods give information concerning displacements normal to the surface. The adatoms are shown to occupy sites above second layer Si atoms. Incorporation of elastic strain minimisation into the structure factor analysis enables the identification of relaxations extending six layers into the bulk.

A neutron spectroscopy study of magnetic excitations in uranium oxysulphide

A neutron spectroscopy experiment has been performed to observe the position of the crystal-field levels in both the paramagnetic and antiferromagnetic phases of UOS. The J-4 ground manifold undergoes an overall splitting of about 100 meV in agreement with the predictions of a crystal-field model used to analyse specific heat data. Above TN, two main peaks are observed at 74 and 82 meV with a shoulder at about 87 meV. Below TN there is a redistribution of intensity together with a shift of the peak positions to 76, 84 and 92 meV at T=30 K. The experimental observations are interpreted in the framework of a refined crystal-field model.

High-resolution neutron spectroscopy of crystal-field excitations in uranium dioxide

The authors report the results of a high-resolution inelastic neutron scattering measurement of the crystal-field excitations in uranium dioxide. All the dipole-allowed transitions within the lowest multiplet have been measured, allowing the first complete determination using neutron scattering of the crystal-field potential and J-mixing wavefunctions in an actinide compound. The splitting of the cubic crystal-field levels in the antiferromagnetic phase below TN=30.8 K is discussed, and compared with calculations made assuming either a 2-k or a 3-k magnetic structure.

The defect structure of yttria-stabilized zirconia, studied by quasielastic diffuse neutron scattering

The static defect structure of the oxygen ion conductor Y203 stabilized zirconia has been studied at room temperature by coherent diffuse neutron scattering from single crystal samples containing nominally 9.4, 12, 15 and 18 mol% Y203. There are two principal contributions to the observed diffuse intensity. The first arises from tetrahedral distortions in small vacancy free regions of the crystal which decrease in volume as the dopant level increases. The second arises from correlated vacancies and their associated relaxed ions in the remainder of the crystal. The 9.4 mol% sample has been studied at elevated temperatures. The scattering becomes partly quasielastic, but the correlations persist to the highest temperatures studied (1900°C). The temperature and Q-dependence of the energy width has been studied at selected positions in reciprocal space.

Neutron spectroscopy of Nd1−v+yCavBa2−yCu3+zOx

Abstract The crystal field splitting in the high T c superconductor Nd 1- v+y Ca v Ba 2- y Cu 3+ z O x ( v =0, 0.5; y = 0, 0.25, 0.5; z =0, 0.5; 6.02⩽ x ⩽7.05) was observed by neutron spectroscopy. The charge transfer connected with doping could directly be measured by this local probe of the charge distribution around the rare earth site, and the determined values turned out to be comparable with that obtained by other methods.

Neutron inelastic experiments on actinide dioxides: Search for crystal field excitations

Abstract Neutron scattering has proved to be an effective probe of high-energy magnetic excitations. Recent measurements on AnO2 (An=U, Np, Pu) performed at spallation source-based spectrometers have made important contributions in the understanding of the crystal field potential in such systems. The experimental results are consistent with a fairly constant potential across the series and suggest that the ground state wave functions may be expressed almost entirely within a single J-manifold.