2,145 research outputs found
Self-Assembly of Monatomic Complex Crystals and Quasicrystals with a Double-Well Interaction Potential
For the study of crystal formation and dynamics we introduce a simple
two-dimensional monatomic model system with a parametrized interaction
potential. We find in molecular dynamics simulations that a surprising variety
of crystals, a decagonal and a dodecagonal quasicrystal are self-assembled. In
the case of the quasicrystals the particles reorder by phason flips at elevated
temperatures. During annealing the entropically stabilized decagonal
quasicrystal undergoes a reversible phase transition at 65% of the melting
temperature into an approximant, which is monitored by the rotation of the de
Bruijn surface in hyperspace.Comment: 4 pages, 6 figures. Physical Review Letters, in Press (April 2007
Evidence for biquadratic exchange in the quasi-two-dimensional antiferromagnet FePS
FePS is a van der Waals compound with a honeycomb lattice that is a good
example of a two-dimensional antiferromagnet with Ising-like anisotropy.
Neutron spectroscopy data from FePS3 were previously analysed using a
straight-forward Heisenberg Hamiltonian with a single-ion anisotropy. The
analysis captured most of the elements of the data, however some significant
discrepancies remained. The discrepancies were most obvious at the Brillouin
zone boundaries. The data are subsequently reanalysed allowing for unequal
exchange between nominally equivalent nearest-neighbours, which resolves the
discrepancies. The source of the unequal exchange is attributed to a
biquadratic exchange term in the Hamiltonian which most probably arises from a
strong magnetolattice coupling. The new parameters show that there are features
consistent with Dirac magnon nodal lines along certain Brillouin zone
boundaries.Comment: 8 pages, 4 figures. The following article has been accepted by the
Journal of Applied Physics. After it is published, it will be found at
(https://publishing.aip.org/resources/librarians/products/journals/). The
article was submitted as part of a special topic edition
(https://publishing.aip.org/publications/journals/special-topics/jap/2d-quantum-materials-magnetism-and-superconductivity/
Analysing star cluster populations with stochastic models: the HST/WFC3 sample of clusters in M83
The majority of clusters in the Universe have masses well below 10^5 Msun.
Hence their integrated fluxes and colors can be affected by the random presence
of a few bright stars introduced by stochastic sampling of the stellar mass
function. Specific methods are being developed to extend the analysis of
cluster SEDs into the low-mass regime. In this paper, we apply such a method to
observations of star clusters, in the nearby spiral galaxy M83. We reassess
ages and masses of a sample of 1242 objects for which UBVIHalpha fluxes were
obtained with the HST/WFC3 images. Synthetic clusters with known properties are
used to characterize the limitations of the method. The ensemble of color
predictions of the discrete cluster models are in good agreement with the
distribution of observed colors. We emphasize the important role of the Halpha
data in the assessment of the fraction of young objects, particularly in
breaking the age-extinction degeneracy that hampers an analysis based on UBVI
only. We find the mass distribution of the cluster sample to follow a power-law
of index -2.1 +/-0.2, and the distribution of ages a power-law of index -1.0
+/-0.2 for M > 10^3.5 Msun and ages between 10^7 and 10^9 yr. An extension of
our main method, that makes full use of the probability distributions of age
and mass of the individual clusters, is explored. It produces similar power-law
slopes and will deserve further investigation. Although the properties derived
for individual clusters significantly differ from those obtained with
traditional, non-stochastic models in ~30% of the objects, the first order
aspect of the age and mass distributions are similar to those obtained
previously for this M83 sample in the range of overlap of the studies. We
extend the power-law description to lower masses with better mass and age
resolution and without most of the artifacts produced by the classical method.Comment: accepted for publication in ApJ, 29 pages, 20 figure
The X-shooter Spectral Library (XSL): I. DR1. Near-ultraviolet through optical spectra from the first year of the survey
We present the first release of XSL, the X-Shooter Spectral Library. This
release contains 237 stars spanning the wavelengths 3000--10200 \AA\ observed
at a resolving power . The spectra
were obtained at ESO's 8-m Very Large Telescope (VLT). The sample contains O --
M, long-period variable (LPV), C and S stars. The spectra are flux-calibrated
and telluric-corrected. We describe a new technique for the telluric
correction. The wavelength coverage, spectral resolution and spectral type of
this library make it well suited to stellar population synthesis of galaxies
and clusters, kinematical investigation of stellar systems and studying the
physics of cool stars.Comment: 41 pages, 38 figures, 5 tables. Accepted for publication in A&A.
Webpage: http://xsl.u-strasbg.fr
The M/L ratio of massive young clusters
We point out a strong time-evolution of the mass-to-light conversion factor
\eta commonly used to estimate masses of dense star clusters from observed
cluster radii and stellar velocity dispersions. We use a gas-dynamical model
coupled with the Cambridge stellar evolution tracks to compute line-of-sight
velocity dispersions and half-light radii weighted by the luminosity. Stars at
birth are assumed to follow the Salpeter mass function in the range [0.15--17
M_\sun]. We find that , and hence the estimated cluster mass, increases
by factors as large as 3 over time-scales of 20 million years. Increasing the
upper mass limit to 50 M_\sun leads to a sharp rise of similar amplitude but
in as little as 10 million years.
Fitting truncated isothermal (Michie-King) models to the projected light
profile leads to over-estimates of the concentration par ameter c of compared to the same functional fit applied to the proj ected
mass density.Comment: Draft version of an ApJ lette
Strain and correlation of self-organized Ge_(1-x)Mn_x nanocolumns embedded in Ge (001)
We report on the structural properties of Ge_(1-x)Mn_x layers grown by
molecular beam epitaxy. In these layers, nanocolumns with a high Mn content are
embedded in an almost-pure Ge matrix. We have used grazing-incidence X-ray
scattering, atomic force and transmission electron microscopy to study the
structural properties of the columns. We demonstrate how the elastic
deformation of the matrix (as calculated using atomistic simulations) around
the columns, as well as the average inter-column distance can account for the
shape of the diffusion around Bragg peaks.Comment: 9 pages, 7 figure
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