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Abstract: A systematic analysis of resonant x-ray Bragg diffraction data for UPd3, with signal enhancement at the U M-IV edge, including possible structural phase transitions leads to a new determination of the space groups of the material in the phases between T-0 = 7.8 K and T+1 = 6.9 K, as P222(1), and between T-1 = 6.7 K and T-2 = 4.4 K, as P2(1). In addition, the quadrupolar order parameters, < Q(ab)>, inferred from diffraction data for the phase between T-1 and T-2, are < Q(xz)> and < Q(yz)> at the ( 103) Bragg reflection and < Q(xy)> at the ( 104) reflection.

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Abstract: The hole confinement in Ge dots fabricated on a wetting layer in Stranski-Krastanov (S-K) growth was directed by an applied bias. At medium bias voltage, the holes overflowed from the small dots, indicating a moderate potential barrier without a notch at the boundary. The electrostatic force of the confined holes attracted excessive holes to the wetting layer. The system was energetically stabilized by the formation of a "virtual dot" in an open space enclosed by dots. At a high bias voltage, the virtual dot disappeared since the holes in the wetting layer were emitted from the surface. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3220065]

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Abstract: The longitudinal coherence function at the Advanced Photon Source beamline 34-ID-C has been measured by a novel method and the coherence length (?L) determined to be, ?L = 0.66 ± 0.02µm. Three dimensional Coherent X-ray Diffraction (CXD) patterns were measured for multiple Bragg reflections from two Zinc Oxide (ZnO) nanorods with differing aspect ratios. The visibility of fringes corresponding to the 002 crystal direction for each reflection were found to be different and used to map the coherence function of the incident radiation. Partial coherence was found to be associated with amplitude ‘hot’ spots in three dimensional reconstructions of the crystal structure.

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Abstract: Epitaxially grown magnetic tunnel junctions (MTJs) with a stacking structure of Co2MnSi/MgO/CoFe were fabricated. Their tunnel magnetoresistance (TMR) effects were investigated. The TMR ratio and tunnelling conductance characteristics of MTJs were considerably different between those with an MgO barrier prepared using sputtering (SP-MTJ) and those prepared using EB evaporation (EB-MTJ). The EB-MTJ exhibited a very large TMR ratio of 217% at room temperature and 753% at 2?K. The bias voltage dependence of the tunnelling conductance in the parallel magnetic configuration for the EB-MTJ suggests that the observed large TMR ratio at RT results from the coherent tunnelling process through the crystalline MgO barrier. The tunnelling conductance in the anti-parallel magnetic configuration suggests that the large temperature dependence of the TMR ratio results from the inelastic spin?flip tunnelling process.

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Abstract: The change in structure of glassy GeS2 with pressure increasing to {\simeq }5~\mathrm {GPa} at ambient temperature was explored by using in situ neutron and x-ray diffraction. Under ambient conditions, the glass structure is made from a mixture of corner- and edge-sharing Ge(S1/2)4 tetrahedra where 47(5)% of the Ge atoms are involved in edge-sharing configurations. The network formed by these tetrahedra orders on an intermediate range as manifested by the appearance of a pronounced first sharp diffraction peak in the measured total structure factors at a scattering vector k = 1.02(2) Å−1 which has a large contribution from Ge–Ge correlations. The intermediate range order breaks down when the pressure on the glass increases above ≈2 GPa but there does not appear to be a significant alteration of the Ge–S coordination number or corresponding bond length with increasing density. The results for the glass are consistent with a densification mechanism in which there is a replacement of edge-sharing by corner-sharing Ge centred tetrahedral motifs and/or a reduction in the Ge–\hat {\mathrm {S}} –Ge bond angle between corner-sharing tetrahedral motifs with increasing pressure. The change in structure with increasing temperature at a pressure of {\simeq }5~\mathrm {GPa} was also investigated by means of in situ x-ray diffraction as the glass crystallized and then liquefied. At 5.2(1) GPa and 828(50) K the system forms a tetragonal crystal, with space group I\bar {4}2d and cell parameters a = b = 4.97704(12) and c = 9.5355(4) Å, wherein corner-sharing Ge(S1/2)4 tetrahedra pack to form a dense three-dimensional network. A method is described for correcting x-ray diffraction data taken in situ under high pressure, high temperature conditions for a cylindrical sample, container and gasket geometry with a parallel incident beam and with a scattered beam that is defined using an oscillating radial collimator. A method is also outlined for obtaining coordination numbers from direct integration of the peaks in measured x-ray total pair distribution functions.