7,786 research outputs found
The solubility of rhenium in silicate melts: Implications for the geochemical properties of rhenium at high temperatures
The solubility of rhenium (Re) in a haplobasaltic melt (anorthite-diopside eutectic composition) has been experimentally determined using the mechanically assisted equilibration technique at 1400°C as a function of oxygen fugacity (10−12 < fO2 ≤ 10−7 bar), imposed by CO-CO2 gas mixtures. Samples were analysed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). This is a true microanalytical technique, which allows small-scale sample heterogeneity to be detected, while providing a limit of detection of 2 ppb Re. Time-resolved LA-ICP-MS spectra revealed the presence of suboptically sized micronuggets of Re in all samples, which, because they are present at the 0.5 to 10 ppm level, dominate the true solubilities of Re (<1 ppm at the conditions of the experiment) in bulk analyses of the samples. Nevertheless, the micronuggets could be filtered out from the time-resolved spectra to reveal accurate values of the true Re solubility. A number of time series of samples were taken at constant fO2 to demonstrate that the solubilities converge to a constant value. In addition, solubilities were measured after increasing and decreasing the imposed fO2. The results show that Re dissolves in the silicate melt as ReO2 (Re4+) and ReO3 (Re6+) species, with the latter predominating at typical terrestrial upper-mantle oxygen fugacities. The total solubility of Re is described by the following expression (fO2 in bars): [Re/ppb] = 9.7(±1.9) × 109 (fO2) + 4.2 (±0.3) × 1014 (fO2)1.5Assuming an activity coefficient for Re in Fe-rich metal of 1, this gives a value of DRemet/sil of 5 × 1010 at log fO2 = IW-2, appropriate for metal-silicate partitioning in an homogenously accreting Earth. Thus, Re is indeed very highly siderophile, and the mantle’s abundance cannot be explained by homogenous accretion
Enhancement of bichromatic high-harmonic generation with a high-frequency field
Using a high-frequency field superposed to a linearly polarized bichromatic
laser field composed by a wave with frequency and a wave with
frequency , we show it is possible to enhance the intensity of a
group of high harmonics in orders of magnitude. These harmonics have
frequencies about 30% higher than the monochromatic-cutoff frequency, and,
within the three-step-model framework, correspond to a set of electron
trajectories for which tunneling ionization is strongly suppressed. Particular
features in the observed enhancement suggest that the high-frequency field
provides an additional mechanism for the electron to reach the continuum. This
interpretation is supported by a time-frequency analysis of the harmonic yield.
The additional high frequency field permits the control of this group of
harmonics leaving all other sets of harmonics practically unchanged, which is
an advantage over schemes involving only bichromatic fields.Comment: 6 pages RevTex, 5 figures (ps files), Changes in text, figures,
references and equations include
Chiral Behaviour of the Rho Meson in Lattice QCD
In order to guide the extrapolation of the mass of the rho meson calculated
in lattice QCD with dynamical fermions, we study the contributions to its
self-energy which vary most rapidly as the quark mass approaches zero; from the
processes and . It turns out that in
analysing the most recent data from CP-PACS it is crucial to estimate the
self-energy from using the same grid of discrete momenta as
included implicitly in the lattice simulation. The correction associated with
the continuum, infinite volume limit can then be found by calculating the
corresponding integrals exactly. Our error analysis suggests that a factor of
10 improvement in statistics at the lowest quark mass for which data currently
exists would allow one to determine the physical rho mass to within 5%.
Finally, our analysis throws new light on a long-standing problem with the
J-parameter.Comment: 13 pages, 7 figures. Full analytic forms of the self-energies are
included and a correction in the omega-pi self-energ
Chiral Analysis of Quenched Baryon Masses
We extend to quenched QCD an earlier investigation of the chiral structure of
the masses of the nucleon and the delta in lattice simulations of full QCD.
Even after including the meson-loop self-energies which give rise to the
leading and next-to-leading non-analytic behaviour (and hence the most rapid
variation in the region of light quark mass), we find surprisingly little
curvature in the quenched case. Replacing these meson-loop self-energies by the
corresponding terms in full QCD yields a remarkable level of agreement with the
results of the full QCD simulations. This comparison leads to a very good
understanding of the origins of the mass splitting between these baryons.Comment: 23 pages, 6 figure
Quantum Effects in Small-Capacitance Single Josephson Junctions
We have measured the current-voltage (I-V) characteristics of
small-capacitance single Josephson junctions at low temperatures (T=0.02-0.6
K), where the strength of the coupling between the single junction and the
electromagnetic environment was controlled with one-dimensional arrays of dc
SQUIDs. The single-junction I-V curve is sensitive to the impedance of the
environment, which can be tuned IN SITU. We have observed Coulomb blockade of
Cooper-pair tunneling and even a region of negative differential resistance,
when the zero-bias resistance R_0' of the SQUID arrays is much higher than the
quantum resistance R_K = h/e^2 = 26 kohm. The negative differential resistance
is evidence of coherent single-Cooper-pair tunneling within the theory of
current-biased single Josephson junctions. Based on the theory, we have
calculated the I-V curves numerically in order to compare with the experimental
ones at R_0' >> R_K. The numerical calculation agrees with the experiments
qualitatively. We also discuss the R_0' dependence of the
single-Josephson-junction I-V curve in terms of the superconductor-insulator
transition driven by changing the coupling to the environment.Comment: 11 pages with 14 embedded figures, RevTeX4, final versio
Neutrino-Deuteron Scattering in Effective Field Theory at Next-to-Next-to Leading Order
We study the four channels associated with neutrino-deuteron breakup
reactions at next-to-next to leading order in effective field theory. We find
that the total cross-section is indeed converging for neutrino energies up to
20 MeV, and thus our calculations can provide constraints on theoretical
uncertainties for the Sudbury Neutrino Observatory. We stress the importance of
a direct experimental measurement to high precision in at least one channel, in
order to fix an axial two-body counterterm.Comment: 32 pages, 14 figures (eps
Chiral extrapolation of lattice data for B-meson decay constant
The B-meson decay constant fB has been calculated from unquenched lattice QCD
in the unphysical region. For extrapolating the lattice data to the physical
region, we propose a phenomenological functional form based on the effective
chiral perturbation theory for heavy mesons, which respects both the heavy
quark symmetry and the chiral symmetry, and the non-relativistic constituent
quark model which is valid at large pion masses. The inclusion of pion loop
corrections leads to nonanalytic contributions to fB when the pion mass is
small. The finite-range regularization technique is employed for the
resummation of higher order terms of the chiral expansion. We also take into
account the finite volume effects in lattice simulations. The dependence on the
parameters and other uncertainties in our model are discussed.Comment: 11 pages, 3 Postscript figures, accepted for publication in EPJ
Spin-3/2 Nucleon and Delta Baryons in Lattice QCD
We present first results for masses of spin-3/2 N and Delta baryons in
lattice QCD using Fat-Link Irrelevant Clover (FLIC) fermions. Spin-3/2
interpolating fields providing overlap with both spin-3/2 and spin-1/2 states
are considered. In the isospin-1/2 sector, we observe, after appropriate spin
and parity projection, a strong signal for the J^P=3/2^- state together with a
weak but discernible signal for the 3/2^+ state with a mass splitting near that
observed experimentally. We also find good agreement between the 1/2^+/- masses
and earlier nucleon mass simulations with the standard spin-1/2 interpolating
field. For the isospin-3/2 Delta states, clear mass splittings are observed
between the various 1/2^+/- and 3/2^+/- channels, with the calculated level
orderings in good agreement with those observed empirically.Comment: 17 pages, 8 figures, 2 table
Towards hardware acceleration of neuroevolution for multimedia processing applications on mobile devices
This paper addresses the problem of accelerating large artificial neural networks (ANN), whose topology and weights can evolve via the use of a genetic algorithm. The proposed digital hardware architecture is capable of processing any evolved network topology, whilst at the same time providing a good trade off between throughput, area and power consumption. The latter is vital for a longer battery life on mobile devices. The architecture uses multiple parallel arithmetic units in each processing element (PE). Memory partitioning and data caching are used to minimise the effects of PE pipeline stalling. A first order minimax polynomial approximation scheme, tuned via a genetic algorithm, is used for the activation function generator. Efficient arithmetic circuitry, which leverages modified Booth recoding, column compressors and carry save adders, is adopted throughout the design
Singular Continuous Spectrum for the Laplacian on Certain Sparse Trees
We present examples of rooted tree graphs for which the Laplacian has
singular continuous spectral measures. For some of these examples we further
establish fractional Hausdorff dimensions. The singular continuous components,
in these models, have an interesting multiplicity structure. The results are
obtained via a decomposition of the Laplacian into a direct sum of Jacobi
matrices
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