23,747 research outputs found
Exact Solution of a Jamming Transition: Closed Equations for a Bootstrap Percolation Problem
Jamming, or dynamical arrest, is a transition at which many particles stop
moving in a collective manner. In nature it is brought about by, for example,
increasing the packing density, changing the interactions between particles, or
otherwise restricting the local motion of the elements of the system. The onset
of collectivity occurs because, when one particle is blocked, it may lead to
the blocking of a neighbor. That particle may then block one of its neighbors,
these effects propagating across some typical domain of size named the
dynamical correlation length. When this length diverges, the system becomes
immobile. Even where it is finite but large the dynamics is dramatically
slowed. Such phenomena lead to glasses, gels, and other very long-lived
nonequilibrium solids. The bootstrap percolation models are the simplest
examples describing these spatio-temporal correlations. We have been able to
solve one such model in two dimensions exactly, exhibiting the precise
evolution of the jamming correlations on approach to arrest. We believe that
the nature of these correlations and the method we devise to solve the problem
are quite general. Both should be of considerable help in further developing
this field.Comment: 17 pages, 4 figure
An early warning indicator for atmospheric blocking events using transfer operators
The existence of persistent midlatitude atmospheric flow regimes with
time-scales larger than 5-10 days and indications of preferred transitions
between them motivates to develop early warning indicators for such regime
transitions. In this paper, we use a hemispheric barotropic model together with
estimates of transfer operators on a reduced phase space to develop an early
warning indicator of the zonal to blocked flow transition in this model. It is
shown that, the spectrum of the transfer operators can be used to study the
slow dynamics of the flow as well as the non-Markovian character of the
reduction. The slowest motions are thereby found to have time scales of three
to six weeks and to be associated with meta-stable regimes (and their
transitions) which can be detected as almost-invariant sets of the transfer
operator. From the energy budget of the model, we are able to explain the
meta-stability of the regimes and the existence of preferred transition paths.
Even though the model is highly simplified, the skill of the early warning
indicator is promising, suggesting that the transfer operator approach can be
used in parallel to an operational deterministic model for stochastic
prediction or to assess forecast uncertainty
Mode-Coupling Theory of Colloids with Short-range Attractions
Within the framework of the mode-coupling theory of super-cooled liquids, we
investigate new phenomena in colloidal systems on approach to their glass
transitions. When the inter-particle potential contains an attractive part,
besides the usual repulsive hard core, two intersecting liquid-glass transition
lines appear, one of which extends to low densities, while the other one, at
high densities, shows a re-entrant behaviour. In the glassy region a new type
of transition appears between two different types of glasses. The complex
phenomenology can be described in terms of higher order glass transition
singularities. The various glass phases are characterised by means of their
viscoelastic properties. The glass driven by attractions has been associated to
particle gels, and the other glass is the well known repulsive colloidal glass.
These correspondences, in associations with the new predictions of glassy
behaviour mean that such phenomena may be expected in colloidal systems with,
for example, strong depletion or other short-ranged attractive potentials.Comment: 17 pages, 8 figure
Near Infrared Spectroscopy of Young Brown Dwarfs in Upper Scorpius
Spectroscopic follow-up is a pre-requisite for studies of the formation and
early evolution of brown dwarfs. Here we present IRTF/SpeX near-infrared
spectroscopy of 30 candidate members of the young Upper Scorpius association,
selected from our previous survey work. All 24 high confidence members are
confirmed as young very low mass objects with spectral types from M5 to L1,
15-20 of them are likely brown dwarfs. This high yield confirms that brown
dwarfs in Upper Scorpius can be identified from photometry and proper motions
alone, with negligible contamination from field objects (<4%). Out of the 6
candidates with lower confidence, 5 might still be young very low mass members
of Upper Scorpius, according to our spectroscopy. We demonstrate that some very
low mass class II objects exhibit radically different near infrared (0.6 -
2.5micron) spectra from class III objects, with strong excess emission
increasing towards longer wavelengths and partially filled in features at
wavelengths shorter than 1.25micron. These characteristics can obscure the
contribution of the photosphere within such spectra. Therefore, we caution that
near infrared derived spectral types for objects with discs may be unreliable.
Furthermore, we show that the same characteristics can be seen to some extent
in all class II and even a significant fraction of class III objects (~40%),
indicating that some of them are still surrounded by traces of dust and gas.
Based on our spectra, we select a sample of objects with spectral types of M5
to L1, whose near-infrared emission represents the photosphere only. We
recommend the use of these objects as spectroscopic templates for young brown
dwarfs in the future.Comment: 12 pages, 9 figures, Accepted in MNRA
The SONYC survey: Towards a complete census of brown dwarfs in star forming regions
SONYC, short for "Substellar Objects in Nearby Young Clusters", is a survey
program to provide a census of the substellar population in nearby star forming
regions. We have conducted deep optical and near-infrared photometry in five
young regions (NGC1333, rho Ophiuchi, Chamaeleon-I, Upper Sco, and Lupus-3),
combined with proper motions, and followed by extensive spectroscopic campaigns
with Subaru and VLT, in which we have obtained more than 700 spectra of
candidate low-mass objects. We have identified and characterized more than 60
new substellar objects, among them a handful of objects with masses close to,
or below the Deuterium burning limit. Through SONYC and surveys by other
groups, the substellar IMF is now well characterized down to ~ 5 - 10 MJup, and
we find that the ratio of the number of stars with respect to brown dwarfs lies
between 2 and 6. A comprehensive survey of NGC 1333 reveals that, down to
~5MJup, free-floating objects with planetary masses are 20-50 times less
numerous than stars, i.e. their total contribution to the mass budget of the
clusters can be neglected.Comment: to appear in the proceedings of the conference 'Brown dwarfs come of
age', May 20-24 2013, Memorie della Societa Astronomica Italian
SMEFT and the Drell-Yan Process at High Energy
The Drell-Yan process is a copious source of lepton pairs at high energy and
is measured with great precision at the Large Hadron Collider (LHC). Barring
any new light particles, beyond the Standard Model effects can be studied in
Drell-Yan production using an effective field theory. At tree level, new
4-fermion interactions dominate, while at one loop operators modifying 3-gauge
boson couplings contribute effects that are enhanced at high energy. We study
the sensitivity of the neutral Drell-Yan process to these dimension-6 operators
and compare the sensitivity to that of pair production at the LHC.Comment: 13 pages, 4 figures. v2: version accepted for publication in PR
Star polymers: A study of the structural arrest in presence of attractive interactions
Simulations and Mode-Coupling Theory calculations, for a large range of the
arm number and packing fraction have shown that the structural
arrest and the dynamics of star polymers in a good solvent are extremely rich:
the systems show a reentrant melting of the disordered glass nested between two
stable fluid phases that strongly resemble the equilibrium phase diagram.
Starting from a simple model potential we investigate the effect of the
interplay between attractive interactions of different range and ultrasoft core
repulsion, on the dynamics and on the occurrence of the ideal glass transition
line. In the two cases considered so far, we observed some significant
differences with respect to the purely repulsive pair interaction. We also
discuss the interplay between equilibrium and non equilibrium phase behavior.
The accuracy of the theoretical tools we utilized in our investigation has been
checked by comparing the results with molecular dynamics simulations.Comment: 24 pages, 14 figures, accepted for publication in Physical Review
Investigation of phase-separated electronic states in 1.5µm GaInNAs/GaAs heterostructures by optical spectroscopy
We report on the comparative electronic state characteristics of particular GaInNAs/GaAs quantum well structures that emit near 1.3 and 1.5 µm wavelength at room temperature. While the electronic structure of the 1.3 µm sample is consistent with a standard quantum well, the 1.5 µm sample demonstrate quite different characteristics. By using photoluminescence sPLd excitation spectroscopy at various detection wavelengths, we demonstrate that the macroscopic electronic states in the 1.5 µm structures originate from phase-separated quantum dots instead of quantum wells. PL measurements with spectrally selective excitation provide further evidence for the existence of composition-separated phases. The evidence is consistent with phase segregation during the growth leading to two phases, one with high In and N content which accounts for the efficient low energy 1.5 µm emission, and the other one having lower In and N content which contributes metastable states and only emits under excitation in a particular wavelength range
Quality of maternity care practices of skilled birth attendants in Cambodia
Background: The World Health Organization's recommended package of interventions for the integrated management of pregnancy and childbirth provides guidance for the use of evidence-based interventions to ensure the best outcomes for mother and newborn. However, the extent to which skilled birth attendants (SBAs) follow evidence-based guidelines is not known. There are few studies into childbirth practices of SBAs in Cambodia. The aim of this study was to observe practices of SBAs during labour, birth and the immediate post-partum period and their consistency with evidence-based guidelines. Methods: A structured non-participant observation study was undertaken. Data were collected using an observational checklist of evidence-based practices adapted from the Cambodian clinical assessment tools for associate degree in midwifery. Maternity care settings in one provincial hospital, two district referral hospitals and two health centres in one province of Cambodia were purposively selected. Results: Twenty-five SBAs who attended 40 women during labour, birth and the postnatal period were observed. The results showed that the use of the partograph was low; birth companions were not permitted; cleanliness during birth was lacking; management of the third stage of labour was inappropriate; monitoring of mother and baby in the early postnatal period was lacking; the SBAs lacked skills in neonatal resuscitation; skin-to-skin contact with the newborn and early breastfeeding were rare; and intramuscular injection of vitamin K varied. Conclusion: The findings suggest that the current SBA practices during labour, birth and the immediate postpartum period in one province of Cambodia are not consistent with evidence-based guidelines. Service improvements that address evidence-based practices are likely to have an impact on clean and safe childbirth, thereby enhancing outcomes for Cambodia women. © 2012 The Authors International Journal of Evidence-Based Healthcare © 2012 The Joanna Briggs Institute
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