1,431 research outputs found
Rare-earth impurities in CoMnSi: an opportunity to improve Half-Metallicity at finite temperatures
We analyse the effects of doping Holmium impurities into the full-Heusler
ferromagnetic alloy CoMnSi. Experimental results, as well as theoretical
calculations within Density Functional Theory in the "Local Density
Approximation plus Hubbard U" framework show that the holmium moment is aligned
antiparallely to that of the transition metal atoms. According to the
electronic structure calculations, substituting Ho on Co sites introduces a
finite density of states in the minority spin gap, while substitution on the Mn
sites preserves the half-metallic character.Comment: 22 pages, 8 figures. published in PR
Clustering of Lyman-alpha Emitters Around Quasars at
The strong observed clustering of quasars indicates they are hosted
by massive () dark matter
halos. Assuming quasars and galaxies trace the same large-scale structures,
this should also manifest as strong clustering of galaxies around quasars.
Previous work on high-redshift quasar environments, mostly focused at ,
have failed to find convincing evidence for these overdensities. Here we
conduct a survey for Lyman alpha emitters (LAEs) in the environs of 17 quasars
at probing scales of . We measure an
average LAE overdensity around quasars of 1.4 for our full sample, which we
quantify by fitting the quasar-LAE cross-correlation function. We find
consistency with a power-law shape with correlation length of
for a fixed slope of
. We also measure the LAE auto-correlation length and find
\,cMpc (), which is
times higher than the value measured in blank fields. Taken together our
results clearly indicate that LAEs are significantly clustered around
quasars. We compare the observed clustering with the expectation from a
deterministic bias model, whereby LAEs and quasars probe the same underlying
dark matter overdensities, and find that our measurements fall short of the
predicted overdensities by a factor of 2.1. We discuss possible explanations
for this discrepancy including large-scale quenching or the presence of excess
dust in galaxies near quasars. Finally, the large cosmic variance from
field-to-field observed in our sample (10/17 fields are actually underdense)
cautions one from over-interpreting studies of quasar environments
based on a single or handful of quasar fields.Comment: 19 pages, 12 figures, submitted to the Ap
Improving SPIHT-based Compression of Volumetric Medical Data
Volumetric medical data (CT,MR) are useful tools for diagnostic investigation however their usage may be made diffcult because of the amount of data to store or because of the duration of communication over a limited capacity channel. In order to code such information sources we present a progressive three dimensional image compression algorithm based on zerotree wavelet coder with arithmetic coding. We make use of a 3D separable biorthogonal wavelet transform and we extend the zerotree SPIHT algorithm to three dimensions. Moreover we propose some improvements to the SPIHT encoder in order to obtain a better rate distortion performance without increasing the computational complexity. Finally we propose an efficient context-based adaptive arithmetic coding which eliminates high order redundancy. The results obtained on progressive coding of a test CT volume are better than those presented in recent similar works both for the mean PSNR on the whole volume and for the PSNR homogeneity between various slices
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