Calculating electron momentum densities and Compton profiles using the linear tetrahedron method

A method for computing electron momentum densities and Compton profiles from ab initio calculations is presented. Reciprocal space is divided into optimally-shaped tetrahedra for interpolation, and the linear tetrahedron method is used to obtain the momentum density and its projections such as Compton profiles. Results are presented and evaluated against experimental data for Be, Cu, Ni, Fe3Pt, and YBa2Cu4O8, demonstrating the accuracy of our method in a wide variety of crystal structures.

Measurement of magnetic exchange in ferromagnet-superconductor La2/3Ca1/3MnO3/YBa2Cu3O7 bilayers

The existence of coherent magnetic correlations in the normal phase of cuprate high-temperature superconductors has proven difficult to measure directly. Here we report on a study of ferromagnetic-superconductor bilayers of La2/3Ca1/3MnO3/YBa2Cu3O7 (LCMO/YBCO) with varying YBCO layer thicknesses. Using x-ray magnetic circular dichroism, we demonstrate that the ferromagnetic layer induces a Cu magnetic moment in the adjacent high-temperature superconductor. For thin samples, this moment exists at all temperatures below the Curie temperature of the LCMO layer. However, for a YBCO layer thicker than 12 unit cells, the Cu moment is suppressed for temperatures above the superconducting transition, suggesting this to be a direct measurement of magnetic coherence in the normal state of a superconducting oxide.

Real-time synchrotron X-ray diffraction study on the isothermal martensite transformation of maraging steel in high magnetic fields

The isothermal austenite-to-martensite transformation kinetics in a maraging steel have been studied by time-dependent microbeam diffraction measurements with high-energy X-rays. The transformation kinetics are shown to be accelerated significantly when a magnetic field of 8 T is applied. The average phase behaviour, obtained from a Rietveld refinement of the powder-averaged diffraction data, demonstrates that the martensite formation does not lead to a macroscopic strain in the austenite and martensite phases. An analysis of individual austenite reflections in the microbeam diffraction patterns, however, indicates that within the transforming austenite grains a transformation strain develops as a result of the formed martensite. The development of elastic strains during the transformation is explained by a partial strain confinement within the untransformed part of the austenite grain. The strain relaxation to the surrounding austenite grains is found to be dependent on the austenite volume. For a set of individual austenite grains the martensite nucleation is correlated with the initial austenite volume and the strain developed prior to the transformation as a result of martensite formation in the neighbouring grains.

Depth-dependent magnetism in epitaxial MnSb thin films: effects of surface passivation and cleaning

Depth-dependent magnetism in MnSb(0001) epitaxial films has been studied by combining experimental methods with different surface specificities: polarized neutron reflectivity, x-ray magnetic circular dichroism (XMCD), x-ray resonant magnetic scattering and spin-polarized low energy electron microscopy (SPLEEM). A native oxide ∼4.5 nm thick covers air-exposed samples which increases the film's coercivity. HCl etching efficiently removes this oxide and in situ surface treatment of etched samples enables surface magnetic contrast to be observed in SPLEEM. A thin Sb capping layer prevents oxidation and preserves ferromagnetism throughout the MnSb film. The interpretation of Mn L(3,2) edge XMCD data is discussed.

Bulk spin polarization of Co(1-x)Fe(x)S2

We report on a new method to determine the degree of bulk spin polarization in single crystal Co(1-x)Fe(x)S2 by modeling magnetic Compton scattering with ab initio calculations. Spin-dependent Compton profiles were measured for CoS2 and Co0.9Fe0.1S2. The ab initio calculations were then refined by rigidly shifting the bands to provide the best fit between the calculated and experimental directional profiles for each sample. The bulk spin polarizations, P, corresponding to the spin-polarized density of states at the Fermi level, were then extracted from the refined calculations. The values were found to be P=-72+/-6% and P=18+/-7% for CoS2 and Co0.9Fe0.1S2, respectively. Furthermore, determinations of P weighted by the Fermi velocity (v(F) or v(F)2) were obtained, permitting a rigorous comparison with other experimental data and highlighting the experimental dependence of P on v(F).

The Coordination Number of Silicon in Silicon−Oxygen Compounds: The Special Case of 6-Fold Coordination in Thaumasite

In most silicon-oxygen compounds, the silicon atom is 4-fold coordinated. Exceptionally, silicon is found in 6-fold coordination, as in the mineral thaumasite, which also may be formed in the degradation, and sometimes subsequent weakening, of certain concretes. The connection between coordination number and chemical bonding in oxidic compounds generally is explored using the optical basicity concept to estimate the extent of negative charge provided by oxide(-II) donor atoms in the coordination sphere. A correlation is established between this charge and the heat of formation of silicate (per oxide(-II) atom), which indicates that reduction in charge decreases thermodynamic stability. Calculations for thaumasite show that 4-fold coordination would provide a very small charge, but that 6-fold provides a charge comparable with that for stable 4-fold coordinated silicates such as K2Si2O5.

Magnetic Compton scattering study of the magnetocaloric material Gd(7)Pd(3)

The spin-dependent momentum density of Gd(7)Pd(3) was probed by the magnetic Compton scattering technique with elliptically polarized synchrotron radiation. A contribution to the spin moment from Pd 4d electrons was observed, at 2 and 280 K, alongside a large Gd 4f moment and a smaller Gd 5d moment. The total spin moment, at 2 K, was determined as 50.8 ± 0.7 μ(B) (f.u.)(-1). The Gd 4f contribution to the spin moment was determined as 43.4 ± 1.8 μ(B) (f.u.)(-1), the Gd 5d moment as 4.4 ± 0.7 μ(B) (f.u.)(-1) and the Pd 4d spin moment contribution as 2.9 ± 1.1 μ(B) (f.u.)(-1), where f.u. represents a formula unit. At 280 K the total spin moment was 27.3 ± 0.9 μ(B) (f.u.)(-1) with individual contributions determined as a Gd 4f spin moment of 23.8 ± 1.1 μ(B) (f.u.)(-1), a Gd 5d contribution of 2.2 ± 0.5 μ(B) (f.u.)(-1) and a Pd 5d spin moment of 1.2 ± 0.6 μ(B) (f.u.)(-1).

Ionic−Covalent Character of Metal and Nonmetal Oxides

The acid-base properties of oxidic media are quantified in terms of the optical basicity concept, which serves to correlate many properties with chemical constitution. Optical basicity values, Lambda, have been assigned to 25 oxides such that they relate to Lambda for crystalline CaO being taken as unity. Since Lambda for an oxide is proportional to the degree of negative charge borne by the oxide-(-II) atom or ion, it follows that optical basicity should go hand-in-hand with the ionic/covalent nature of the cation-oxide-(-II) bonding. Unfortunately, this assumption produces many anomalies and trends that do not fit normal inorganic trends. The problem is resolved by adjusting the influence of ionic forms to the bonding by taking into account the heats of formation. In contrast to the (Pauling) electronegativity treatment of oxides, this procedure allows assignment of percentage ionicity to the bonding, and the trends in these in the Periodic Table are as expected for inorganic oxides.

Direct observation of the multisheet Fermi surface in the strongly correlated transition metal compound ZrZn2

The existence of flat areas of a Fermi surface (FS), predicted by electronic structure calculations and used in models of both magnetically mediated and phonon-mediated Fulde-Ferrell-Larkin-Ovchinnikov superconducting states, is reported in the paramagnetic phase of the ferromagnetic superconductor ZrZn2 using positron annihilation. The strongly mass-renormalized FS sheet, dominating the Fermi level density of states, is seen for the first time. The delocalization of the magnetization is studied using measured and calculated magnetic Compton profiles.

Performance of phase plates on the XMaS beamline at the ESRF

Two phase plates, a 0.78 mm-thick natural diamond and a 0.3 mm-thick synthetic diamond, were used to convert linearly polarized X-rays into a circularly polarized beam, to cover an energy range of 3-9 keV. The performance of these plates followed theoretical predictions as indicated by polarization analyses and X-ray magnetic circular dichroism measurements. The use of the device is illustrated by resonant magnetic reflectivity measurements on UAs/Co multilayers.

A comparative study of resident, family, and administrator expectations for service quality in nursing homes

This article compares the service quality expectations of residents, family, and administrators within a long-term-care context. Expectations for service quality are measured using the SERVQUAL scale. The results support the reliability of the SERVQUAL instrument for nursing home residents. Resident expectations were significantly different from administrators' expectations on all dimensions of quality. The findings are interpreted in terms of expectation formation and the nature of service encounters.

Polarized-neutron scattering study of the cooper-pair moment in Sr2RuO4

We report a study of the magnetization density in the mixed state of the unconventional superconductor Sr2RuO4. On entering the superconducting state we find no change in the magnitude or distribution of the induced moment for a magnetic field of 1 T applied within the Ru O2 planes. Our results are consistent with a spin-triplet Cooper pairing with spins lying in the basal plane. This is in contrast with similar experiments performed on conventional and high- T(c) superconductors.