18,781 research outputs found
Nonmonotonic Trust Management for P2P Applications
Community decisions about access control in virtual communities are
non-monotonic in nature. This means that they cannot be expressed in current,
monotonic trust management languages such as the family of Role Based Trust
Management languages (RT). To solve this problem we propose RT-, which adds a
restricted form of negation to the standard RT language, thus admitting a
controlled form of non-monotonicity. The semantics of RT- is discussed and
presented in terms of the well-founded semantics for Logic Programs. Finally we
discuss how chain discovery can be accomplished for RT-.Comment: This paper appears in the proceedings of the 1st International
Workshop on Security and Trust Management (STM 2005). To appear in ENTC
A VLT/FORS2 Multi-Slit Search for Lyman-alpha Emitting Galaxies at z~6.5
We present results from a deep spectroscopic search in the 9150A atmospheric
window for z~6.5 Lyman-alpha emitting galaxies using the VLT/FORS2. Our
multi-slit+narrow-band filter survey covers a total spatial area of 17.6 sq.
arcmin in four different fields and reaches fluxes down to 5x10^(-18)
erg/s/cm^2 (7 sigma detection). Our detection limit is significantly fainter
than narrow-band searches at this redshift and fainter also than the unlensed
brightness of Hu et al.'s HCM6A at z=6.56, and thus provides better overlap
with surveys at much lower redshifts. Eighty secure emission line galaxies are
detected. However, based on their clear continuum emission shortward of the
line or the presence of multiple lines, none of these can be Ly-alpha emission
at z~6.5. Our null result of finding no z~6.5 Ly-alpha emitters suggests that
the number density of Ly-alpha emitters with L>2x10^(42) erg/s declines by ~2
between z~3 and z~6.5.Comment: accepted by ApJ Letters (originally submitted June 11, 2004
Survival of Massive Star-forming Galaxies in Cluster Cores Drives Gas-Phase Metallicity Gradients : The Effects of Ram Pressure Stripping
Recent observations of galaxies in a cluster at z=0.35 show that their
integrated gas-phase metallicities increase with decreasing cluster-centric
distance. To test if ram pressure stripping (RPS) is the underlying cause, we
use a semi-analytic model to quantify the "observational bias" that RPS
introduces into the aperture-based metallicity measurements. We take integral
field spectroscopy of local galaxies, remove gas from their outer galactic
disks via RPS, and then conduct mock slit observations of cluster galaxies at
z=0.35. Our RPS model predicts a typical cluster-scale metallicity gradient of
-0.03 dex/Mpc. By removing gas from the outer galactic disks, RPS introduces a
mean metallicity enhancement of +0.02 dex at a fixed stellar mass. This gas
removal and subsequent quenching of star formation preferentially removes low
mass cluster galaxies from the observed star-forming population. As only the
more massive star-forming galaxies survive to reach the cluster core, RPS
produces a cluster-scale stellar mass gradient of -0.05 log(M_*/M_sun)/Mpc.
This mass segregation drives the predicted cluster-scale metallicity gradient
of -0.03 dex/Mpc. However, the effects of RPS alone can not explain the higher
metallicities measured in cluster galaxies at z=0.35. We hypothesize that
additional mechanisms including steep internal metallicity gradients and
self-enrichment due to gas strangulation are needed to reproduce our
observations at z=0.35.Comment: 17 pages, 21 figures, accepted for publication Ap
A molecular perspective on the limits of life: Enzymes under pressure
From a purely operational standpoint, the existence of microbes that can grow
under extreme conditions, or "extremophiles", leads to the question of how the
molecules making up these microbes can maintain both their structure and
function. While microbes that live under extremes of temperature have been
heavily studied, those that live under extremes of pressure have been
neglected, in part due to the difficulty of collecting samples and performing
experiments under the ambient conditions of the microbe. However, thermodynamic
arguments imply that the effects of pressure might lead to different organismal
solutions than from the effects of temperature. Observationally, some of these
solutions might be in the condensed matter properties of the intracellular
milieu in addition to genetic modifications of the macromolecules or repair
mechanisms for the macromolecules. Here, the effects of pressure on enzymes,
which are proteins essential for the growth and reproduction of an organism,
and some adaptations against these effects are reviewed and amplified by the
results from molecular dynamics simulations. The aim is to provide biological
background for soft matter studies of these systems under pressure.Comment: 16 pages, 8 figure
Enhancement of the Spin Accumulation at the Interface Between a Spin-Polarized Tunnel Junction and a Semiconductor
We report on spin injection experiments at a Co/AlO/GaAs interface
with electrical detection. The application of a transverse magnetic field
induces a large voltage drop at the interface as high as 1.2mV for a
current density of 0.34 nA.. This represents a dramatic increase of
the spin accumulation signal, well above the theoretical predictions for spin
injection through a ferromagnet/semiconductor interface. Such an enhancement is
consistent with a sequential tunneling process via localized states located in
the vicinity of the AlO/GaAs interface. For spin-polarized carriers
these states act as an accumulation layer where the spin lifetime is large. A
model taking into account the spin lifetime and the escape tunneling time for
carriers travelling back into the ferromagnetic contact reproduces accurately
the experimental results
Transport properties in Simplified Double Exchange model
Transport properties of the manganites by the double-exchange mechanism are
considered. The system is modeled by a simplified double-exchange model, i.e.
the Hund coupling of the itinerant electron spins and local spins is simplified
to the Ising-type one. The transport properties such as the electronic
resistivity, the thermal conductivity, and the thermal power are calculated by
using Dynamical mean-field theory. The transport quantities obtained
qualitatively reproduce the ones observed in the manganites. The results
suggest that the Simplified double exchange model underlies the key properties
of the manganites.Comment: 5 pages, 5 eps figure
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