85,371 research outputs found
Fitting functions for dark matter density profiles
We present a unified parameterization of the fitting functions suitable for
density profiles of dark matter haloes or elliptical galaxies. A notable
feature is that the classical Einasto profile appears naturally as the
continuous limiting case of the cored subfamily amongst the double power-law
profiles of Zhao (1996). Based on this, we also argue that there is basically
no qualitative difference between halo models well-fitted by the Einasto
profile and the standard NFW model. This may even be the case quantitatively
unless the resolutions of simulations and the precisions of fittings are
sufficiently high to make meaningful distinction possible.Comment: 13 pages (6 pages main text + 5 pages appendices + 2 pages full
tables) including 5 figures and 7 tables. submitted to MNRA
PCA and K-Means decipher genome
In this paper, we aim to give a tutorial for undergraduate students studying
statistical methods and/or bioinformatics. The students will learn how data
visualization can help in genomic sequence analysis. Students start with a
fragment of genetic text of a bacterial genome and analyze its structure. By
means of principal component analysis they ``discover'' that the information in
the genome is encoded by non-overlapping triplets. Next, they learn how to find
gene positions. This exercise on PCA and K-Means clustering enables active
study of the basic bioinformatics notions. Appendix 1 contains program listings
that go along with this exercise. Appendix 2 includes 2D PCA plots of triplet
usage in moving frame for a series of bacterial genomes from GC-poor to GC-rich
ones. Animated 3D PCA plots are attached as separate gif files. Topology
(cluster structure) and geometry (mutual positions of clusters) of these plots
depends clearly on GC-content.Comment: 18 pages, with program listings for MatLab, PCA analysis of genomes
and additional animated 3D PCA plot
Electronic structure and magnetic properties of Gd-doped and Eu-rich EuO
The effects of Gd doping and O vacancies on the magnetic interaction and
Curie temperature of EuO are studied using first-principles calculations.
Linear response calculations in the virtual crystal approximation show a broad
maximum in the Curie temperature as a function of doping, which results from
the combination of the saturating contribution from indirect exchange and a
decreasing contribution from the f-d hopping mechanism. Non-Heisenberg
interaction at low doping levels and its effect on the Curie temperature are
examined. The electronic structure of a substitutional Gd and of an O vacancy
in EuO are evaluated. When the 4f spins are disordered, the impurity state goes
from single to double occupation, but correlated bound magnetic polarons are
not ruled out. At higher vacancy concentrations typical for Eu-rich EuO films,
the impurity states broaden into bands and remain partially filled. To go
beyond the homogeneous doping picture, magnetostructural cluster expansions are
constructed, which describe the modified exchange parameters near Gd dopants or
O vacancies. Thermodynamic properties are studied using Monte Carlo
simulations. The Curie temperature for Gd-doped EuO agrees with the results of
the virtual crystal approximation and shows a maximum of about 150 K. At 3.125%
vacancy concentration the Curie temperature increases to 120 K, consistent with
experimental data for Eu-rich film samples.Comment: 15 pages, 13 figures, under review in Physical Review
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