2,777 research outputs found
Xe films on a decagonal Al-Ni-Co quasicrystal surface
The grand canonical Monte Carlo method is employed to study the adsorption of
Xe on a quasicrystalline Al-Ni-Co surface. The calculation uses a semiempirical
gas-surface interaction, based on conventional combining rules and the usual
Lennard-Jones Xe-Xe interaction. The resulting adsorption isotherms and
calculated structures are consistent with the results of LEED experimental
data. In this paper we focus on five features not discussed earlier (Phys. Rev.
Lett. 95, 136104 (2005)): the range of the average density of the adsorbate,
the order of the transition, the orientational degeneracy of the ground state,
the isosteric heat of adsorption of the system, and the effect of the vertical
cell dimension.Comment: 6 pages, 5 pic
Noble gas films on a decagonal AlNiCo quasicrystal
Thermodynamic properties of Ne, Ar, Kr, and Xe adsorbed on an Al-Ni-Co
quasicrystalline surface (QC) are studied with Grand Canonical Monte Carlo by
employing Lennard-Jones interactions with parameter values derived from
experiments and traditional combining rules. In all the gas/QC systems, a
layer-by-layer film growth is observed at low temperature. The monolayers have
regular epitaxial fivefold arrangements which evolve toward sixfold
close-packed structures as the pressure is increased. The final states can
contain either considerable or negligible amounts of defects. In the latter
case, there occurs a structural transition from five to sixfold symmetry which
can be described by introducing an order parameter, whose evolution
characterizes the transition to be continuous or discontinuous as in the case
of Xe/QC (first-order transition with associated latent heat). By simulating
fictitious noble gases, we find that the existence of the transition is
correlated with the size mismatch between adsorbate and substrate's
characteristic lengths. A simple rule is proposed to predict the phenomenon.Comment: 19 pages. 8 figures. (color figures can be seen at
http://alpha.mems.duke.edu/wahyu/ or
http://www.iop.org/EJ/abstract/0953-8984/19/1/016007/
Brightest Cluster Galaxies and Core Gas Density in REXCESS Clusters
We investigate the relationship between brightest cluster galaxies (BCGs) and
their host clusters using a sample of nearby galaxy clusters from the
Representative XMM Cluster Structure Survey (REXCESS). The sample was imaged
with the Southern Observatory for Astrophysical Research (SOAR) in R band to
investigate the mass of the old stellar population. Using a metric radius of
12h^-1 kpc, we found that the BCG luminosity depends weakly on overall cluster
mass as L_BCG \propto M_cl^0.18+-0.07, consistent with previous work. We found
that 90% of the BCGs are located within 0.035 r_500 of the peak of the X-ray
emission, including all of the cool core (CC) clusters. We also found an
unexpected correlation between the BCG metric luminosity and the core gas
density for non-cool core (non-CC) clusters, following a power law of n_e
\propto L_BCG^2.7+-0.4 (where n_e is measured at 0.008 r_500). The correlation
is not easily explained by star formation (which is weak in non-CC clusters) or
overall cluster mass (which is not correlated with core gas density). The trend
persists even when the BCG is not located near the peak of the X-ray emission,
so proximity is not necessary. We suggest that, for non-CC clusters, this
correlation implies that the same process that sets the central entropy of the
cluster gas also determines the central stellar density of the BCG, and that
this underlying physical process is likely to be mergers.Comment: 16 pages, 8 figures, accepted Astrophysical Journa
Reconstructing palaeotemperatures using leaf floras – case studies for a comparison of leaf margin analysis and the coexistence approach
AbstractIn the past the problems and advantages of the nearest-living-relative (NLR) and leaf physiognomy approaches have been repeatedly discussed and it has been demonstrated that both approaches frequently show broad agreement with each other. However, detailed comparisons of the various methods for accuracy in estimation of palaeoclimate at individual localities are still lacking. Such studies are needed before data obtained from different approaches can be integrated in palaeoclimate maps and models. Moreover, there are some indications that leaf physiognomy and NLR approaches may lead to different results. In this study we applied a physiognomic method based on leaf margin analysis and the coexistence approach, a recent variation of the NLR approach, to two Tertiary palaeofloras (Schrotzburg, Middle Miocene, south Germany; Kleinsaubernitz, Upper Oligocene, east Germany). We demonstrated that both approaches can produce reasonable and consistent results if the standard error of the leaf physiognomy palaeoclimate data is taken into account. However, our results and interpretations indicate that reconstructions based on leaf physiognomy are influenced by factors not related to climate, such as sample size and differential preservation or transport. In contrast, reconstructions for the same fossil assemblages based on the coexistence approach seem to be less affected by taphonomic variables, but may be less sensitive to minor climate changes
Background Measurements in the Gran Sasso Underground Laboratory
The gamma background flux below 3000 keV in the Laboratori Nazionali del Gran
Sasso (LNGS), Italy, has been measured using a 3" diameter NaI(Tl) detector at
different underground positions: In hall A, hall B, the interferometer tunnel,
and inside the Large Volume Detector (LVD). The integrated flux is 0.3--0.4
scm at the first three locations, and is lower by two orders of
magnitude inside LVD. With the help of Monte Carlo simulations for every
location, the contribution of the individual primordial isotopes to the
background has been determined. Using an 11" diameter NaI(Tl) detector, the
background neutron flux in the LNGS interferometer tunnel has been estimated.
Within the uncertainties, the result agrees with those from other neutron
measurements in the main halls.Comment: 6 pages, 6 figures, accepted versio
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