10,021 research outputs found
Three water sites in upper mantle olivine and the role of titanium in the water weakening mechanism
Infrared spectroscopy on synthetic olivines has established that there are at least
four different mechanisms by which hydrogen is incorporated into the crystal structure.
Two mechanisms occur in the system MgO-SiO2-H2O associated with silicon and
magnesium vacancies, respectively. A third mechanism is associated with trivalent cation
substitution, commonly Fe3+ in natural olivine, while the fourth mechanism, which is
the one most prevalent in natural olivines from the spinel-peridotite facies of the Earth’s
upper mantle, is associated with Ti4+ [Berry et al., 2005]. Here first principles calculations
based on density functional theory are used to derive the structure and relative energies
of the two defects in the pure MgO-SiO2-H2O system, and possible hydrogen-bearing
and hydrogen-free point defects in Ti4+-doped forsterite. Calculated structures are used to
compare the predicted orientation of the O-H bonds with the experimentally determined
polarization. The energies are used to discuss how different regimes of chemical
environment, temperature (T), pressure (P), and both water content and water fugacity
( fH2O), impact on which of the different hydroxyl substitution mechanisms are
thermodynamically stable. We find that given the presence of Ti impurities, the most
stable mechanism involves the formation of silicon vacancies containing two protons
charge balanced by a Ti4+ cation occupying an adjacent octahedral site. This mechanism
leads to the water-mediated formation of silicon vacancies. As silicon is known to be the
most slowly diffusing species in olivine, this provides a credible explanation of the
observed water weakening effect in olivine
High-order noise filtering in nontrivial quantum logic gates
Treating the effects of a time-dependent classical dephasing environment
during quantum logic operations poses a theoretical challenge, as the
application of non-commuting control operations gives rise to both dephasing
and depolarization errors that must be accounted for in order to understand
total average error rates. We develop a treatment based on effective
Hamiltonian theory that allows us to efficiently model the effect of classical
noise on nontrivial single-bit quantum logic operations composed of arbitrary
control sequences. We present a general method to calculate the
ensemble-averaged entanglement fidelity to arbitrary order in terms of noise
filter functions, and provide explicit expressions to fourth order in the noise
strength. In the weak noise limit we derive explicit filter functions for a
broad class of piecewise-constant control sequences, and use them to study the
performance of dynamically corrected gates, yielding good agreement with
brute-force numerics.Comment: Revised and expanded to include filter function terms beyond first
order in the Magnus expansion. Related manuscripts available from
http://www.physics.usyd.edu.au/~mbiercu
Vatican Condemnation of Nazi War Crimes: Pope Pius XII’s Denunciation of Wartime Atrocities
The debate on the silence of Pope Pius XII has been rekindled by recent close examination of the Pope’s 1942 Christmas Message denouncing totalitarianism and the killing of persons “only because of their nationality and race,” along with a particular condemnation of Marxist Socialism and a call for national and international relations to be based on natural law principles guaranteeing justice, order, and peace. In particular, Michael Phayer, a historian writing on the Vatican’s relation to the Holocaust, suggested a need for renewed attention to the 1942 Christmas Message. According to Phayer:
Historians . . . have been rather too dismissive of his 1942 Christmas address. Since Pius never spoke out again in a comparable way, the Christmas address has been judged by the Pope’s critics as falling short of the mark, given the enormity of the Holocaust. This judgment rests on hindsight. Most of those who heard or read the Christmas message viewed the statement in a different light, precisely because it was the Pope’s initial comment about wartime atrocities.
Considering views such as Phayer’s, this Article will assess the 1942 Christmas Message to determine whether it in fact provided a response to the Holocaust, thus defeating the charge of “silence” which has been directed against Pius XII by revisionist historians and critics
Noise-induced behaviors in neural mean field dynamics
The collective behavior of cortical neurons is strongly affected by the
presence of noise at the level of individual cells. In order to study these
phenomena in large-scale assemblies of neurons, we consider networks of
firing-rate neurons with linear intrinsic dynamics and nonlinear coupling,
belonging to a few types of cell populations and receiving noisy currents.
Asymptotic equations as the number of neurons tends to infinity (mean field
equations) are rigorously derived based on a probabilistic approach. These
equations are implicit on the probability distribution of the solutions which
generally makes their direct analysis difficult. However, in our case, the
solutions are Gaussian, and their moments satisfy a closed system of nonlinear
ordinary differential equations (ODEs), which are much easier to study than the
original stochastic network equations, and the statistics of the empirical
process uniformly converge towards the solutions of these ODEs. Based on this
description, we analytically and numerically study the influence of noise on
the collective behaviors, and compare these asymptotic regimes to simulations
of the network. We observe that the mean field equations provide an accurate
description of the solutions of the network equations for network sizes as
small as a few hundreds of neurons. In particular, we observe that the level of
noise in the system qualitatively modifies its collective behavior, producing
for instance synchronized oscillations of the whole network, desynchronization
of oscillating regimes, and stabilization or destabilization of stationary
solutions. These results shed a new light on the role of noise in shaping
collective dynamics of neurons, and gives us clues for understanding similar
phenomena observed in biological networks
Inflammatory bowel disease-specific autoantibodies in HLA-B27-associated spondyloarthropathies: Increased prevalence of ASCA and pANCA
Aims: An association between inflammatory bowel disease (IBD) and spondyloarthropathies (SpA) has repeatedly been reported. The aim of the present study was to investigate whether serologic markers of IBD, e. g. antibodies against Saccharomyces cerevisiae (ASCA), antibodies against exocrine pancreas (PAB) and perinuclear antineutrophil cytoplasmic antibodies (pANCA) are present in HLA-B27-associated SpA. Methods: 87 patients with HLA-B27-positive SpA and 145 controls were tested for ASCA, PAB and pANCA employing ELISA or indirect immunofluorescence, respectively. Antibody-positive patients were interviewed regarding IBD-related symptoms using a standardized questionnaire. Results/Conclusion: When compared to the controls, ASCA IgA but not ASCA IgG levels were significantly increased in patients with SpA, in particular in ankylosing spondylitis (AS) and undifferentiated SpA (uSpA). pANCA were found in increased frequency in patients with SpA whereas PAB were not detected. The existence of autoantibodies was not associated with gastrointestinal symptoms but sustains the presence of a pathophysiological link between bowel inflammation and SpA. Copyright (C) 2004 S. Karger AG, Basel
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