8,170 research outputs found
Comparative study between two quantum spin systems KCuCl and TlCuCl
We have performed an {\it ab initio} study of the electronic structure of two
isostructural quantum spin systems, KCuCl and TlCuCl, which have
recently attracted much attention due to their unconventional magnetic
properties. Our first-principles analysis shows unambiguously the role of Tl,
as opposed to structural differences between the two compounds, in making
TlCuCl a {\it strongly} coupled s=1/2 dimer system compared to
KCuCl which shows a {\it weakly} coupled s=1/2 dimer behavior. Good
agreement with the existing analysis of inelastic neutron scattering results
has been observed.Comment: minor changes in text with additional references; in EPL forma
A Generalized Ginzburg-Landau Approach to Second Harmonic Generation
We develop a generalized Ginzburg-Landau theory for second harmonic
generation (SHG) in magnets by expanding the free energy in terms of the order
parameter in the magnetic phase and the susceptibility tensor in the
corresponding high-temperature phase. The non-zero components of the SHG
susceptibility in the ordered phase are derived from the symmetries of the
susceptibility tensor in the high-temperature phase and the symmetry of the
order parameter. In this derivation, the dependence of the SHG susceptibility
on the order parameter follows naturally, and therefore its nonreciprocal
optical properties.
We examine this phenomenology for the magnetoelectric compound CrO as
well as for the ferroelectromagnet YMnO.Comment: European Journal of Physics B (accepted
Unicast Barrage Relay Networks: Outage Analysis and Optimization
Barrage relays networks (BRNs) are ad hoc networks built on a rapid
cooperative flooding primitive as opposed to the traditional point-to-point
link abstraction. Controlled barrage regions (CBRs) can be used to contain this
flooding primitive for unicast and multicast, thereby enabling spatial reuse.
In this paper, the behavior of individual CBRs is described as a Markov process
that models the potential cooperative relay transmissions. The outage
probability for a CBR is found in closed form for a given topology, and the
probability takes into account fading and co-channel interference (CCI) between
adjacent CBRs. Having adopted this accurate analytical framework, this paper
proceeds to optimize a BRN by finding the optimal size of each CBR, the number
of relays contained within each CBR, the optimal relay locations when they are
constrained to lie on a straight line, and the code rate that maximizes the
transport capacity.Comment: 7 pages, 4 figures, 1 table, in IEEE Military Commun. Conf. (MILCOM),
201
High-resolution CRIRES spectra of Terzan1: a metal-poor globular cluster toward the inner bulge
Containing the oldest stars in the Galaxy, globular clusters toward the bulge
can be used to trace its dynamical and chemical evolution. In the bulge
direction, there are ~50 clusters, but only about 20% have been subject of
high-resolution spectroscopic investigations. So far, the sample observed at
high resolution spans a moderate-to-high metallicity regime. In this sample,
however, very few are located in the innermost region (1.5\,Kpc and
). To constrain the chemical evolution enrichment of the
innermost region of Galaxy, accurate abundances and abundance patterns of key
elements based on high-resolution spectroscopy are necessary. Here we present
the results we obtained for Terzan 1, a metal-poor cluster located in the
innermost bulge region. Using the near-infrared spectrograph CRIRES at ESO/VLT,
we obtained high-resolution (R50,000) H-band spectra of 16 bright
giant stars in the innermost region () of Terzan1. Full spectral
synthesis techniques and equivalent width measurements of selected lines,
isolated and free of significant blending and/or contamination by telluric
lines, allowed accurate chemical abundances and radial velocities to be
derived. Fifteen out of 16 observed stars are likely cluster members, with an
average heliocentric radial velocity of +571.8\,km/s and mean iron
abundance of [Fe/H]=--1.260.03\,dex. For these stars we measured some
[/Fe] abundance ratios, finding average values of
[O/Fe]=+0.390.02\,dex, [Mg/Fe]=+0.420.02\,dex,
[Si/Fe]=+0.310.04\,dex, and [Ti/Fe]=+0.150.04\,dex The
enhancement (\,dex) found in the observed giant stars of Terzan1
is consistent with previous measurements on other, more metal-rich bulge
clusters, which suggests a rapid chemical enrichment.Comment: 7, pages, 6 figures, accepted for publication on A&
Near-Infrared photometry and spectroscopy of NGC 6539 and UKS 1: two intermediate metallicity Bulge Globular Clusters
Using the SofI imager at ESO/NTT and NIRSPEC spectrograph at KeckII, we have
obtained J,K images and echelle spectra covering the range 1.5 - 1.8 micron for
the intermediate metallicity Bulge globular clusters NGC6539 and UKS1. We find
[Fe/H]=-0.76 and -0.78, respectively, and an average alpha-enhancement of +0.44
dex and +0.31 dex, consistent with previous measurements of metal rich Bulge
clusters, and favoring the scenario of rapid chemical enrichment. We also
measure very low 12C/13C=4.5 +/-1 isotopic ratios in both clusters, suggesting
that extra-mixing mechanisms due to cool bottom processing are at work during
the evolution along the Red Giant Branch. Finally, we measure accurate radial
velocities of =+31 +/-4Km/s and =+57 +/-6Km/s and velocity dispersion
of about 8 Km/s and 11 Km/s for NGC6539 and UKS1, respectively.Comment: 10 pages, 7 figures, accepted for publication at MNRA
The Red Giant Branch in Near-Infrared Colour-Magnitude Diagrams. II: The luminosity of the Bump and the Tip
We present new empirical calibrations of the Red Giant Branch (RGB) Bump and
Tip based on a homogeneous near-Infrared database of 24 Galactic Globular
Clusters. The luminosities of the RGB Bump and Tip in the J, H and K bands and
their dependence on the cluster metallicity have been studied, yielding
empirical relationships. By using recent transformations between the
observational and theoretical planes, we also derived similar calibrations in
terms of bolometric luminosity. Direct comparison between updated theoretical
models and observations show an excellent agreement. The empirical calibration
of the RGB Tip luminosity in the near-Infrared passbands presented here is a
fundamental tool to derive distances to far galaxies beyond the Local Group, in
view of using the new ground-based adaptive optics facilities and, in the next
future, the James Webb Space Telescope.Comment: 7 pages, 7 figures, accepted for publication in MNRA
High resolution near-IR spectra of NGC 6624 and NGC 6569
We present the first abundances analysis based on high-resolution infrared
(IR) echelle spectra of NGC 6569 and NGC 6624, two moderately reddened globular
clusters located in the outer bulge of the Galaxy. We find
[Fe/H]=-0.790.02 dex and [Fe/H]=-0.690.02 dex for NGC 6569 and NGC
6624, respectively and an average -elements enhancement of
+0.430.02 dex and +0.390.02 dex, consistent with previous
measurements on other metal-rich Bulge clusters. We measure accurate radial
velocities of and
and velocity dispersions of and for NGC 6569 and NGC 6624, respectively. Finally, we find very low
isotopics ratio (7 in NGC 6624 and 5 in NGC
6569), confirming the presence extra-mixing mechanisms during the red giant
branch evolution phase.Comment: 7 pages, 3 figures, accepted for publication on MNRA
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