4,851 research outputs found
Temperature and pressure-induced spin-state transitions in LaCoO3
We report the continuous variation of the spin moment of cobalt in LaCoO3
across its temperature and pressure-induced spin transitions evidenced with
K\beta emission spectra. The first thermal transition is best described by a
transition to an orbitally nondegenerate intermediate spin (S=1) state. In
parallel, continuous redistribution of the 3d electrons is also indicated by
partial fluorescence yield X-ray absorption spectra. At high pressure, our
study confirms that the material becomes low spin between 40 and 70 kbar at
room temperature
The Effect of External Power Output and Its Reliability on Propulsion Technique Variables in Wheelchair Users With Spinal Cord Injury
The purpose of this study was to assess 1) how treadmill slope variance affected external power output (PO) and propulsion technique reliability; and 2) how PO is associated with propulsion technique. Eighteen individuals with spinal cord injury performed two wheelchair treadmill exercise blocks (0% and 1% treadmill slope, standardized velocity) twice on two separate days. PO, velocity, and 14 propulsion technique variables were measured. In a follow-up study, N = 29 performed wheelchair treadmill drag tests. Target and actual slope were documented and PO, intraclass correlation coefficients (ICC) and smallest detectable differences (SDD) were calculated. Within and between visits, the reliability study ICCs were perfect for velocity (1.0), weak for PO (0.33-0.46), and acceptable (>0.70) for five (0% slope) and 10 (1% slope) propulsion technique variables, resulting in SDDs of 35-196%. Measured PO explained 56-90% of the variance in key propulsion technique variables. In the follow-up, PO ICCs were weak (0.43) and SDDs high. Bias between target and actual slope appeared random. In conclusion, PO variability accounts for 50-90% of the variability in propulsion technique variables when speed and wheelchair set-up are held constant. Therefore, small differences in PO between interventions could mask the effect of the interventions on propulsion technique
Multiplet ligand-field theory using Wannier orbitals
We demonstrate how ab initio cluster calculations including the full Coulomb
vertex can be done in the basis of the localized, generalized Wannier orbitals
which describe the low-energy density functional (LDA) band structure of the
infinite crystal, e.g. the transition metal 3d and oxygen 2p orbitals. The
spatial extend of our 3d Wannier orbitals (orthonormalized Nth order muffin-tin
orbitals) is close to that found for atomic Hartree-Fock orbitals. We define
Ligand orbitals as those linear combinations of the O 2p Wannier orbitals which
couple to the 3d orbitals for the chosen cluster. The use of ligand orbitals
allows for a minimal Hilbert space in multiplet ligand-field theory
calculations, thus reducing the computational costs substantially. The result
is a fast and simple ab initio theory, which can provide useful information
about local properties of correlated insulators. We compare results for NiO,
MnO and SrTiO3 with x-ray absorption, inelastic x-ray scattering, and
photoemission experiments. The multiplet ligand field theory parameters found
by our ab initio method agree within ~10% to known experimental values
Inelastic X-ray Scattering by Electronic Excitations in Solids at High Pressure
Investigating electronic structure and excitations under extreme conditions
gives access to a rich variety of phenomena. High pressure typically induces
behavior such as magnetic collapse and the insulator-metal transition in 3d
transition metals compounds, valence fluctuations or Kondo-like characteristics
in -electron systems, and coordination and bonding changes in molecular
solids and glasses. This article reviews research concerning electronic
excitations in materials under extreme conditions using inelastic x-ray
scattering (IXS). IXS is a spectroscopic probe of choice for this study because
of its chemical and orbital selectivity and the richness of information it
provides. Being an all-photon technique, IXS has a penetration depth compatible
with high pressure requirements. Electronic transitions under pressure in 3d
transition metals compounds and -electron systems, most of them strongly
correlated, are reviewed. Implications for geophysics are mentioned. Since the
incident X-ray energy can easily be tuned to absorption edges, resonant IXS,
often employed, is discussed at length. Finally studies involving local
structure changes and electronic transitions under pressure in materials
containing light elements are briefly reviewed.Comment: submitted to Rev. Mod. Phy
Temperature dependent Eu 3d-4f X-ray Absorption and Resonant Photoemission Study of the Valence Transition in
We study the mixed valence transition ( 80 K) in
EuNi(SiGe) using Eu 3 X-ray absorption
spectroscopy (XAS) and resonant photoemission spectroscopy (RESPES). The
Eu and Eu main peaks show a giant resonance and the spectral
features match very well with atomic multiplet calculations. The spectra show
dramatic temperature ()-dependent changes over large energies (10 eV)
in RESPES and XAS. The observed non-integral mean valencies of 2.35
0.03 ( = 120 K) and 2.70 0.03 ( = 40 K) indicate homogeneous
mixed valence above and below . The redistribution between
Eu+ and Eu+ states is attributed to
a hybridization change coupled to a Kondo-like volume collapse.Comment: 4 pages, 3 figure
Kondo Resonance in PrTi2Al20: Photoemission spectroscopy and single-impurity Anderson model
The Kondo resonance at the Fermi level is well-established for the electronic
structure of Ce (f1 electron) and Yb (f1 hole) based systems. In this work, we
report complementary experimental and theoretical studies on the Kondo
resonance in Pr-based f2 system, PrTi2Al20. Using Pr 3d-4f resonant
photoemission spectroscopy and single impurity Anderson model (SIAM)
calculations including the full multiplets of Pr ions, we show that an f2
system can also give rise to a Kondo resonance at the Fermi level. The Kondo
resonance peak is experimentally observed through a final-state-multiplet
dependent resonance and is reproduced with properly tuned hybridization
strength in SIAM calculations
A hard X- ray probe to study doping-dependent electron redistribution and strong covalency in La_{1-x}Sr_{1+x}MnO_4
The effect of doping on the electronic structure at the Mn sites in the
La_{1-x}Sr_{1+x}MnO_4 series (x=0, 0.3 and 0.5) was studied by means of
non-resonant hard X- ray emission spectroscopy (XES). We observe a linear
dichroism in the Mn K-beta main lines (3p to 1s transitions) that is strongest
for x=0 and decreases with increasing x to 0.5. The Mn K-beta main lines in the
poly-crystalline samples change considerably less upon increasing the hole
doping (substitution of La by Sr) than it would be expected based on the change
of formal valence. From this we conclude that the charge and spin density at
the Mn sites are only little affected by doping. This implies that holes
injected in the La_{1-x}Sr_{1+x}MnO_4 series mainly result in a decrease of
charge density on the oxygen atoms, i.e. oxygen takes part in the charge
balancing. These findings are supported by many-body cluster calculations
Adoption of Instant Messaging Technologies by University Students
The main objective of this paper is to better understand the nature and patterns of students’ socialization patterns in relation to the adoption of Instant Messaging (IM) systems. A model based on the Extended Planned Behavior Theory (EPBT) was applied to a sample of 80 students of software engineering at the University of New South Wales, Australia. Based on the EPBT model, a questionnaire was administered to these students. A number of key concepts were identified in relation to the students’ adoption of IM. It was also found that students use IM to support a number of task-related purposes such as collaborating with their classmates about group work and assignments, as well as for scheduling and coordinating meetings and significant results were obtained
Unquenched large orbital magnetic moment in NiO
Magnetic properties of NiO are investigated by incorporating the spin-orbit
interaction in the LSDA+U scheme. It is found that the large part of orbital
moment remains unquenched in NiO. The orbital moment contributes about mu_L =
0.29 mu_B to the total magnetic moment of M = 1.93 mu_B, as leads to the
orbital-to-spin angular momentum ratio of L/S = 0.36. The theoretical values
are in good agreement with recent magnetic X-ray scattering measurements.Comment: 4 pages, 2 figure
Signatures of technetium oxidation states: a new approach
A general strategy for the determination of Tc oxidation state by new approach involving X-ray absorption near edge spectroscopy (XANES) at the Tc L-3 edge is shown. A comprehensive series of Tc-99 compounds, ranging from oxidation states I to VII, was measured and subsequently simulated within the framework of crystal-field multiplet theory. The observable trends in the absorption edge energy shift in combination with the spectral shape allow for a deeper understanding of complicated Tc coordination chemistry. This approach can be extended to numerous studies of Tc systems as this method is one of the most sensitive methods for accurate Tc oxidation state and ligand characterization
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