284 research outputs found
XANES evidence for sulphur speciation in Mn-, Ni- and W-bearing silicate melts
Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 73 (2009): 6847-6867, doi:10.1016/j.gca.2009.08.013.S K edge XANES and Mn, W and Ni XANES and EXAFS spectra of silicate glasses synthesised at 1400° C and 1 bar with compositions in the CaO-MgO-Al2O3-SiO2-S plus MnO,
NiO, or WO3 systems were used to investigate sulphur speciation in silicate glasses.
S K-edge spectra comprised a composite peak with an edge between 2470 and 2471.4 eV,
which was attributed to S2-, and a peak of variable height with an edge at 2480.2 to 2480.8
eV, which is consistent with the presence of S6+. The latter peak was attributed to sample
oxidation during sample storage. W-rich samples produced an additional lower energy peak
at 2469.8 eV that is tentatively attributed to the existence of S 3p orbitals hybridised with
the W 5d states.
Deconvolution of the composite peak reveals that the composite peak for Mn-bearing
samples fits well to a model that combines three Lorentzians at 2473.1, 2474.9 and 2476.2
eV with an arctan edge step. The composite peak for W-bearing samples fits well to the
same combination plus an additional Lorentzian at 2469.8 eV. The ratio of the proportions
of the signal accounted for by peaks at 2473.1eV and 2476.2eV correlates with Mn:Ca molar
ratios, but not with W:Ca ratios. Spectra from Ni-bearing samples were qualitatively similar
but S levels were too low to allow robust quantification of peak components. Some part of
the signal accounted for by the 2473.1 eV peak was therefore taken to record the formation
of Mn-S melt species, while the 2469.8 peak is interpreted to record the formation of W-S
melt species. The 2474.9 eV and 2476.2 eV peaks were taken to be dominated by Ca-S
and Mg-S interactions. However, a 1:1 relationship between peak components and specific
energy transitions is not proposed. This interpretation is consistent with known features of
the lower parts of the conduction band in monosulphide minerals and indicates a similarity
between sulphur species in the melts and the monosulphides. S XANES spectra cannot be
reproduced by a combination of the spectra of the component element monosulphides.
Mn-, W- and Ni- XANES and EXAFS for synthetic glasses without sulphide exsolution
did not show any sensitivity to the presence of sulphur, which is unsurprising as S:O ratios
were sufficiently low that metals would be mostly co-ordinated by O. Mn EXAFS spectra
were consistent with divalent Mn in 5 co-ordinated Mn-O melt species. W spectra were
consistent with tetrahedrally co-ordinated hexavalent W, most likely in scheelite-like melt
species, and Ni spectra were consistent with [4] co-ordinated divalent Ni. These results
indicate lower coordinations for bothWand Ni than those inferred by some previous workers.
Cation coordination may reflect the proportion of non-bridging oxygens, which is lower in
the Ca-rich and Al-poor samples investigated here than for previous studies.This work was performed with
814 support from the Australian Synchrotron Research Program (ASRP), which is funded by the
815 Commonwealth of Australia under the Major National Research Facilities Program
Quantum charges and spacetime topology: The emergence of new superselection sectors
In which is developed a new form of superselection sectors of topological
origin. By that it is meant a new investigation that includes several
extensions of the traditional framework of Doplicher, Haag and Roberts in local
quantum theories. At first we generalize the notion of representations of nets
of C*-algebras, then we provide a brand new view on selection criteria by
adopting one with a strong topological flavour. We prove that it is coherent
with the older point of view, hence a clue to a genuine extension. In this
light, we extend Roberts' cohomological analysis to the case where 1--cocycles
bear non trivial unitary representations of the fundamental group of the
spacetime, equivalently of its Cauchy surface in case of global hyperbolicity.
A crucial tool is a notion of group von Neumann algebras generated by the
1-cocycles evaluated on loops over fixed regions. One proves that these group
von Neumann algebras are localized at the bounded region where loops start and
end and to be factorial of finite type I. All that amounts to a new invariant,
in a topological sense, which can be defined as the dimension of the factor. We
prove that any 1-cocycle can be factorized into a part that contains only the
charge content and another where only the topological information is stored.
This second part resembles much what in literature are known as geometric
phases. Indeed, by the very geometrical origin of the 1-cocycles that we
discuss in the paper, they are essential tools in the theory of net bundles,
and the topological part is related to their holonomy content. At the end we
prove the existence of net representations
Flux-lattice melting in two-dimensional disordered superconductors
The flux line lattice melting transition in two-dimensional pure and
disordered superconductors is studied by a Monte Carlo simulation using the
lowest Landau level approximation and quasi-periodic boundary condition on a
plane. The position of the melting line was determined from the diffraction
pattern of the superconducting order parameter. In the clean case we confirmed
the results from earlier studies which show the existence of a quasi-long range
ordered vortex lattice at low temperatures. Adding frozen disorder to the
system the melting transition line is shifted to slightly lower fields. The
correlations of the order parameter for translational long range order of the
vortex positions seem to decay slightly faster than a power law (in agreement
with the theory of Carpentier and Le Doussal) although a simple power law decay
cannot be excluded. The corresponding positional glass correlation function
decays as a power law establishing the existence of a quasi-long range ordered
positional glass formed by the vortices. The correlation function
characterizing a phase coherent vortex glass decays however exponentially
ruling out the possible existence of a phase coherent vortex glass phase.Comment: 12 pages, 21 figures, final version to appear in Phys. Rev.
Are current accountability frameworks appropriate for degree apprenticeships?
Purpose In 2015 the Conservative-led government announced their plan to increase the number of people participating in apprenticeship to 3 million by 2020. As part of this plan there is to be an expansion of the number of degree level apprenticeships, with the government suggesting that these should be seen as a real alternative to university. Despite the governmentâs propaganda of an alternative to university, higher education institutions (HEI) have a pivotal role to play in both the development and delivery of degree level apprenticeships. However, the accountability for the success of degree level apprenticeships remains unclear. The purpose of this paper is, therefore, to consider whether existing outcome-based notions of accountability are appropriate, given the tri-partite relationship involved in apprenticeship delivery. Design/methodology/approach The paper provides an analysis of current notions of outcome-based accountability contextualised through the degree apprenticeship programme. Findings The paper illustrates that outcome-based accountability frameworks do little to support the delivery of degree level apprenticeships suggesting that there needs to be a shift to a holistic approach where student success forms just one element of an accountability framework. A conclusion is subsequently made that current accountability frameworks may end in an unnecessary confusion regarding the roles and responsibilities of individual contributors associated with degree apprenticeships, resulting in a missed opportunity to maximise on the value arising from the tri-partite delivery relationship. Originality/value This paper provides an original perspective involving accountability associated with degree apprenticeship programmes in the UK
Relativistic calculation of nuclear transparency in (e,e'p) reactions
Nuclear transparency in (e,e'p) reactions is evaluated in a fully
relativistic distorted wave impulse approximation model. The relativistic mean
field theory is used for the bound state and the Pauli reduction for the
scattering state, which is calculated from a relativistic optical potential.
Results for selected nuclei are displayed in a Q^2 range between 0.3 and 1.8
(GeV/c)^2 and compared with recent electron scattering data. For Q^2 = 0.3
(GeV/c)^2 the results are lower than data; for higher Q^2 they are in
reasonable agreement with data. The sensitivity of the model to different
prescriptions for the one-body current operator is investigated. The off-shell
ambiguities are rather large for the distorted cross sections and small for the
plane wave cross sections.Comment: 8 pages, 3 figure
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