2,166 research outputs found
The magnetic exchange parameters and anisotropy of the quasi-two dimensional antiferromagnet NiPS
Neutron inelastic scattering has been used to measure the magnetic
excitations in powdered NiPS, a quasi-two dimensional antiferromagnet with
spin on a honeycomb lattice. The spectra show clear, dispersive magnons
with a meV gap at the Brillouin zone center. The data were fitted
using a Heisenberg Hamiltonian with a single-ion anisotropy assuming no
magnetic exchange between the honeycomb planes. Magnetic exchange interactions
up to the third intraplanar nearest-neighbour were required. The fits show
robustly that NiPS has an easy axis anisotropy with meV and
that the third nearest-neighbour has a strong antiferromagnetic exchange of
meV. The data can be fitted reasonably well with either
or , however the best quantitative agreement with high-resolution data
indicate that the nearest-neighbour interaction is ferromagnetic with meV and that the second nearest-neighbour exchange is small and
antiferromagnetic with meV. The dispersion has a minimum in the
Brillouin zone corner that is slightly larger than that at the Brillouin zone
center, indicating that the magnetic structure of NiPS is close to being
unstable.Comment: 21 pages, 7 figures, 33 reference
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/
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
Near-IR spectroscopic ages of massive star clusters in M82
Like other starburst galaxies, M82 hosts compact, massive young star clusters
that are interesting both in their own right and as benchmarks for population
synthesis models. Can spectral synthesis models at resolutions around 1000
adequately reproduce the near-IR spectral features and the energy distribution
of these clusters between 0.8 and 2.4 microns? How do the derived cluster
properties compare with previous results from optical studies?
We analyse the spectra of 5 massive clusters in M82, using data acquired with
the spectrograph SpeX on the InfraRed Telescope Facility (NASA/IRTF) and a new
population synthesis tool with a highly improved near-IR extension, based on a
recent collection of empirical and theoretical spectra of red supergiant stars.
We obtain excellent fits across the near-IR with models at quasi-solar
metallicity and a solar neighbourhood extinction law. Spectroscopy breaks a
strong degeneracy between age and extinction in the near-IR colours in the red
supergiant-dominated phase of evolution. The estimated near-IR ages cluster
between 9 and 30 Myr, i.e. the ages at which the molecular bands due to
luminous red supergiants are strongest in the current models. They do not
always agree with optical spectroscopic ages. Adding optical data sometimes
leads to the rejection of the solar neighbourhood extinction law. This is not
surprising considering small-scale structure around the clusters, but it has no
significant effect on the near-IR based spectroscopic ages. [abridged]Comment: 14 pages, 20 figures, uses aa.cl
Accounting for Stochastic Fluctuations when Analysing Integrated Light of Star Clusters. I: First Systematics
Star clusters are studied widely both as benchmarks for stellar evolution
models and in their own right. Cluster age and mass distributions within
galaxies are probes of star formation histories, and of cluster formation and
disruption processes. The vast majority of clusters in the Universe is small,
and it is well known that the integrated fluxes and colors have broad
probability distributions, due to small numbers of bright stars. This paper
goes beyond the description of predicted probability distributions, and
presents results of the analysis of cluster energy distributions in an
explicitly stochastic context. The method developed is Bayesian. It provides
posterior probability distributions in the age-mass-extinction space, using
multi-wavelength photometric observations and a large collection of Monte-Carlo
simulations of clusters of finite stellar masses. Both UBVI and UBVIK datasets
are considered, and the study conducted in this paper is restricted to the
solar metallicity. We first reassess and explain errors arising from the use of
standard analysis methods, which are based on continuous population synthesis
models: systematic errors on ages and random errors on masses are large, while
systematic errors on masses tend to be smaller. The age-mass distributions
obtained after analysis of a synthetic sample are very similar to those found
for real galaxies in the literature. The Bayesian approach on the other hand,
is very successful in recovering the input ages and masses. Taking stochastic
effects into account is important, more important for instance than the choice
of adding or removing near-IR data in many cases. We found no immediately
obvious reason to reject priors inspired by previous (standard) analyses of
cluster populations in galaxies, i.e. cluster distributions that scale with
mass as M^-2 and are uniform on a logarithmic age scale.Comment: 17 pages, 13 figures, Accepted for publication in A&A
Near-IR spectra of ISOGAL sources in the Inner Galactic Bulge
In this work we present near-IR spectra (HK-band) of a sample of 107 sources
with mid-IR excesses at 7 and 15 m detected during the ISOGAL survey.
Making use of the DENIS interstellar extinction map from Schultheis et al.
(1999) we derive luminosities and find that the vs.~
and diagrams are powerful tools for identifying
supergiants, AGB stars, giants and young stellar objects. The majority of our
sample are AGB stars (~ 80%) while we find four good supergiant candidates,
nine young stellar objects and 12 RGB candidates. We have used the most recent
relation by Jeong et al. (2002) based on recent theoretical
modeling of dust formation of AGB stars to determine mass-loss rates. However,
the uncertainties in the mass-loss rates are rather large. The mass-loss rates
of the supergiants are comparable with those in the solar neighbourhood while
the long-period Variables cover a mass-loss range from . The red giant candidateslie at the lower end of the
mass-loss rate range between . We used the
equivalent width of the CO bandhead at 2.3 , the NaI doublet and the
CaI triplet to estimate metallicities using the relation by Ram\'{\i}rez et al.
(\cite{Ramirez2000}). The metallicity distribution of the ISOGAL objects shows
a mean [Fe/H] -0.25 dex with a dispersion of which is
in agreement with the values of Ram\'{i}rez et al. (\cite{Ramirez2000}) for
Galactic Bulge fields between and . A
comparison with the solar neighbourhood sample of Lan\c{c}on & Wood (LW) shows
that our sample is ~ 0.5 dex more metal-rich on average.Comment: 23 pages, 9 figures, 2 appendix with IR spectra. accepted for A&
Kinetics of internal structures growth in magnetic suspensions
The kinetics of aggregation of non Brownian magnetizable particles in the presence of a magnetic field is studied both theoretically and by means of computer simulations. A theoretical approach is based on a system of Smoluchowski equations for the distribution function of the number of particles in linear chain-like aggregates. Results obtained in the two dimensional (2D) and three dimensional (3D) models are analyzed in relation with the size of the cell, containing the particles, and the particle volume fraction φ. The theoretical model reproduces the change of the aggregation kinetics with the size of the cell and with the particle volume fraction as long as the lateral aggregation of chains is negligible. The simulations show that lateral aggregation takes place when, roughly, φ2D>5% and φ3D>1.5%. Dependence of the average size of the chains with time can be described by a power law; the corresponding exponent decreases with the particle volume fraction in relation with the lateral aggregation. In the 3D simulations, dense labyrinthine-like structures, aligned along the applied field, are observed when the particle concentration is high enough (φ3D>5%). © 2012 Elsevier B.V. All rights reserved
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