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[en] Recent developments in hard x-ray resonant inelastic x-ray scattering as a probe of strongly correlated systems are reviewed. Particular attention is paid to studies of Nd2CuO4. A charge transfer excitation is observed when the incident photon energy is tuned in the vicinity of the copper K-edge. It is shown that the presence of resonant enhancements is controlled by the polarization dependence of the excitation process and by the overlap between a given intermediate state and the particular excitation being studied. This latter observation has shed light on the non-local effects present in certain intermediate states
[en] An assessment was made of the exposure fire hazards of flammable liquid spills to cable trays in utility environments. These hazards were evaluated for both open and potentially more severe enclosure (room) environments. The parameter which can be used to assess hazards to cables in enclosures is the maximum heat flux which can be predicted within an enclosure from a combination of the following: enclosure (room) ceiling height and floor area; location of the cables within the enclosure as measured from the enclosure floor; ventilation rate; and a characteristic heat release rate and time constant which are functions of the liquid type and spill size. In addition, a preliminary assessment of protection requirements for cable trays has been performed for the specific conditions studied. This assessment showed that: baffles provided protection against surface charring of insulation and electrical cable damage; a sprinkler only provided protection against electrical damage; and absence of cable protection resulted in both electrical failure and extensive surface charring. A methodology by which the test results can be applied to the assessment of exposure fire hazards to cable tray installations in utility environments is proposed
[en] Resonant x-ray scattering is a powerful experimental technique for probing orbital and charge ordering. It involves tuning the incident photon energy to an absorption edge of the relevant ion and observing scattering at previously 'forbidden' Bragg peaks, and it allows high-resolution, quantitative studies of orbital and charge order--even from small samples. Further, resonant x-ray scattering from orbitally ordered systems exhibits polarization- and azimuthal-dependent properties that provide additional information about the details of the orbital order that is difficult, or impossible, to obtain with any other technique. In the manganites, the sensitivity to charge and orbital ordering is enhanced when the incident photon energy is tuned near the Mn K absorption edge (6.539 keV), which is the lowest energy at which a 1s electron can be excited into an unoccupied state. In this process, the core electron is promoted to an intermediate excited state, which decays with the emission of a photon. The sensitivity to charge ordering is believed to be due to the small difference in K absorption edges of the Mn3+ and Mn4+ sites. For orbital ordering, the sensitivity arises from a splitting--or difference in the weight of the density of states --of the orbitals occupied by the excited electron in the intermediate state. In the absence of such a splitting, there is no resonant enhancement of the scattering intensity. In principle, other absorption edges in which the intermediate state is anisotropic could be utilized, but the strong dipole transition to the Mn 4p levels--and their convenient energies for x-ray diffraction--make the K edge well-suited to studies of manganites. The Mn 4p levels are affected by the symmetry of the orbital ordering, which makes the technique sensitive to the orbital degree of freedom. Therefore resonant x-ray scattering can be used to obtain important quantitative information concerning the details of this electronic order. Two mechanisms for splitting of the 4p states have been proposed: a Coulomb coupling of the Mn 3d and 4p states--either directly, or indirectly through the hybridization with 0 2p states --and the motion of oxygen ions due to the Jahn-Teller interaction. These two mechanisms have opposite signs in terms of the direction of the splitting, but both are consistent with experimental results to date. Which mechanism is dominant therefore remains an open question; however, in either case, the resonant scattering reflects the symmetry of the orbital ordering through the perturbation of the local electronic states at the Mn3+ sites, and the peak positions and widths determined in the x-ray experiments measure the orbital periodicity and correlation lengths, respectively. In addition, as is described below, detailed analysis of the resonant scattering provides information about the occupation of the ordered orbitals--a determination that cannot be made through the use of nonresonant scattering
[en] Carbon nanotubes (CNT) have a wide variety of interesting properties and a large number of potential aplications in electronic and optical devices. In this study we concentrate on one important aspect of their electronic stucture: the plasmon dispersions in both single- and multi-wall CNTs and their relation to those in graphite. For the first time inelastic X-ray scattering is used to study these collective electronic excitations in oriented CNT samples. The experiments were performed on the IXS instrument at beamline 9ID CMC-XOR, APS, ANL. The incident energy was defined by a Si(333) monochromator, a spherically bent Ge(733) diced analyzer at the end of a 1-m arm focused the incident radiation onto a solid-state detector. The overall resolution was ∼300 meV FWHM. The incident photons were linearly polarized perpendicular to the scattering plane. Energy loss scans were taken by varying the incident energy while keeping the exit energy fixed at 8.9805 keV. The momentum transfer was kept along the nanotubes axis. Spectra were taken at room temperature. The samples were oriented CNTs (both single- and multi-wall) grown on a Si substrate. The samples referred to as 'single-wall' were in fact a few walls at most (1-5) while the multi-walled ones had ∼12 walls. Fig. 1. shows the inelastic spectra for the single-, multi-wall, and highly oriented pyrolithic graphite (HOPG) from top to bottom. Momentum transfer was Q = 0.79 (angstrom)-1 in all cases, its direction was along the tubes for the first two samples or parallel to the sheets for graphite. The peaks at ∼10 and ∼30 eV are known as the π and σ + π plasmons respectively. Fig. 2. shows the complete dispersion curves for both plasmon modes as a function of momentum transfer for all three samples.
[en] We report x-ray diffuse scattering studies of the iron chalcogenide Fe1+xTe as a function of doping and temperature for x = 0.03, 0.08, and 0.12. In all cases, remarkably strong, characteristic diffuse scattering is observed. This scattering extends throughout the Brillouin zone and exhibits a nonmonotonic decay away from the fundamental Bragg peaks, with a peaklike structure at a reduced q ∼ (0.3,0,0.6). We interpret this scattering as Huang diffuse scattering resulting from distortions induced by the interaction between the excess Fe and the FeTe layers. The form of the scattering indicates that this interaction is strong and extends a number of unit cells away from the interstitial Fe site. Further, the diffuse scattering shows a sudden decrease on cooling through the structural and magnetic phase transition, reflecting the first-order change of the electronic structure of FeTe.
[en] We performed soft x-ray resonant scattering at the MnL2,3- and OK edges of YMn2O5. While the resonant intensity at the MnL2,3 edges represent the magnetic order parameter, the resonant scattering at the OK edge is found to be directly related to the macroscopic ferroelectric polarization. The latter observation reveals the important role of the spin-dependent Mn-O hybridization for the multiferroicity of YMn2O5. We present details about how to obtain correct energy dependent lineshapes and discuss the origin of the resonant intensity at the OK edge.
[en] Transition metal oxides are presently the focus of much attention in condensed matter physics because of the diverse phenomena exhibited by these materials. Examples include antiferromagnetism, superconductivity and colossal magnetoresistance. The origin of these phenomena lies in the strong electron correlations present in these materials which place them between the well understood limits of band insulators and simple metals. The presence of these correlations makes these materials hard to handle theoretically, and there is a need for more detailed experimental work, in particular in regard to the electronic structure and excitations
[en] The authors report resonant x-ray scattering studies of charge and orbital order in Pr1-xCaxMnO3 with x = 0.4 and 0.5. Below the ordering temperature, TO = 245 K, the charge and orbital order intensities follow the same temperature dependence, including an increase at the antiferromagnetic ordering temperature, TN. High resolution measurements reveal, however, that long range orbital order is never achieved. Rather, an orbital domain state is formed. Above TO, the charge order fluctuations are more highly correlated than the orbital fluctuations. Similar phenomenology is observed in a magnetic field. They conclude that the charge order drives the orbital order at the transition
[en] The cross section for X-ray resonant exchange scattering is reformulated in terms of linear polarization states perpendicular and parallel to the scattering plane, a basis particularly well suited to synchrotron X-ray diffraction experiments. The explicit polarization dependence of the terms is calculated for the electric dipole and quadrupole contributions. This expression, in turn, is rewritten in an orthonormal basis to highlight the dependence of the cross section on each component of the magnetic moment. This has the benefit of providing an empirically useful expression for the cross section. Diffraction patterns from a few simple magnetic structures are calculated. Finally, the correlation function measured at each resonant harmonic is derived. (orig.)