61 research outputs found
The phase diagram of ice: a quasi-harmonic study based on a flexible water model
The phase diagram of ice is studied by a quasi-harmonic approximation. The
free energy of all experimentally known ice phases has been calculated with the
flexible q-TIP4P/F model of water. The only exception is the high pressure ice
X, in which the presence of symmetric O-H-O bonds prevents its modeling with
this empirical interatomic potential. The simplicity of our approach allows us
to study ice phases at state points of the T-P plane that have been omitted in
previous simulations using free energy methods based on thermodynamic
integration. The effect in the phase diagram of averaging the proton disorder
that appears in several ice phases has been studied. It is found particularly
relevant for ice III, at least for cell sizes typically used in phase
coexistence simulations. New insight into the capability of the employed water
model to describe the coexistence of ice phases is presented. We find that the
H-ordered ices IX and XIV, as well as the H-disordered ice XII, are
particularly stable for this water model. This fact disagrees with experimental
data. The unexpected large stability of ice IX is a property related to the
TIP4P-character of the water model. Only after omission of these three stable
ice phases, the calculated phase diagram becomes in reasonable qualitative
agreement to the experimental one in the T-P region corresponding to ice Ih,
II, III, V, and VI. The calculation of the phase diagram in the quantum and
classical limits shows that the most important quantum effect is the
stabilization of ice II due to its lower zero-point energy when compared to
that one of ices Ih, III, and V.Comment: 13 pages, 8 figures, 5 table
Measurement of gamma p --> K+ Lambda and gamma p --> K+ Sigma0 at photon energies up to 2.6 GeV
The reactions gamma p --> K+ Lambda and gamma p --> K+ Sigma0 were measured
in the energy range from threshold up to a photon energy of 2.6 GeV. The data
were taken with the SAPHIR detector at the electron stretcher facility, ELSA.
Results on cross sections and hyperon polarizations are presented as a function
of kaon production angle and photon energy. The total cross section for Lambda
production rises steeply with energy close to threshold, whereas the Sigma0
cross section rises slowly to a maximum at about E_gamma = 1.45 GeV. Cross
sections together with their angular decompositions into Legendre polynomials
suggest contributions from resonance production for both reactions. In general,
the induced polarization of Lambda has negative values in the kaon forward
direction and positive values in the backward direction. The magnitude varies
with energy. The polarization of Sigma0 follows a similar angular and energy
dependence as that of Lambda, but with opposite sign.Comment: 21 pages, 25 figures, submitted to Eur. Phys. J.
K0-Sigma+ Photoproduction with SAPHIR
Preliminary results of the analysis of the reaction p(gamma,K0)Sigma+ are
presented. We show the first measurement of the differential cross section and
much improved data for the total cross section than previous data. The data are
compared with model predictions from different isobar and quark models that
give a good description of p(gamma,K+)Lambda and p(gamma,K+)Sigma0 data in the
same energy range. Results of ChPT describe the data adequately at threshold
while isobar models that include hadronic form factors reproduce the data at
intermediate energies.Comment: 4 pages, Latex2e, 4 postscript figures. Talk given at the
International Conference on Hypernuclear and Strange Particle Physics
(HYP97), Brookhaven National Laboratory, USA, October 13-18, 1997. To be
published in Nucl. Phys. A. Revised version due to changes in experimental
dat
Evidence for the positive-strangeness pentaquark in photoproduction with the SAPHIR detector at ELSA
The positive--strangeness baryon resonance is observed in
photoproduction of the final state with the SAPHIR detector at
the Bonn ELectron Stretcher Accelerator ELSA. It is seen as a peak in the invariant mass distribution with a confidence level. We find
a mass MeV and an upper limit of the width
MeV at 90% c.l. From the absence of a signal in
the invariant mass distribution in at the
expected strength we conclude that the must be isoscalar.Comment: 9 pages, 4 figure
Divergent Chemical Cues Elicit Seed Collecting by Ants in an Obligate Multi-Species Mutualism in Lowland Amazonia
In lowland Amazonian rainforests, specific ants collect seeds of several plant species and cultivate them in arboreal carton nests, forming species-specific symbioses called ant-gardens (AGs). In this obligate mutualism, ants depend on the plants for nest stability and the plants depend on ant nests for substrate and nutrients. AG ants and plants are abundant, dominant members of lowland Amazonian ecosystems, but the cues ants use to recognize the seeds are poorly understood. To address the chemical basis of the ant-seed interaction, we surveyed seed chemistry in nine AG species and eight non-AG congeners. We detected seven phenolic and terpenoid volatiles common to seeds of all or most of the AG species, but a blend of the shared compounds was not attractive to the AG ant Camponotus femoratus. We also analyzed seeds of three AG species (Anthurium gracile, Codonanthe uleana, and Peperomia macrostachya) using behavior-guided fractionation. At least one chromatographic fraction of each seed extract elicited retrieval behavior in C. femoratus, but the active fractions of the three plant species differed in polarity and chemical composition, indicating that shared compounds alone did not explain seed-carrying behavior. We suggest that the various AG seed species must elicit seed-carrying with different chemical cues
Experimental evidence for the preservation of U-Pb isotope ratios in mantle-recycled crustal zircon grains
Zircon of crustal origin found in mantle-derived rocks is of great interest because of the information it
may provide about crust recycling and mantle dynamics. Consideration of this requires understanding
of how mantle temperatures, notably higher than zircon crystallization temperatures, affected the
recycled zircon grains, particularly their isotopic clocks. Since Pb2+ diffuses faster than U4+ and Th+4,
it is generally believed that recycled zircon grains lose all radiogenic Pb after a few million years, thus
limiting the time range over which they can be detected. Nonetheless, this might not be the case
for zircon included in mantle minerals with low Pb2+ diffusivity and partitioning such as olivine and
orthopyroxene because these may act as zircon sealants. Annealing experiments with natural zircon
embedded in cristobalite (an effective zircon sealant) show that zircon grains do not lose Pb to their
surroundings, although they may lose some Pb to molten inclusions. Diffusion tends to homogenize the
Pb concentration in each grain changing the U-Pb and Th-Pb isotope ratios proportionally to the initial
206Pb, 207Pb and 208Pb concentration gradients (no gradient-no change) but in most cases the original
age is still recognizable. It seems, therefore, that recycled crustal zircon grains can be detected, and
even accurately dated, no matter how long they have dwelled in the mantle.This paper has been financed by the Spanish Grants CGL2013-40785-P and
CGL2017-84469-P
天下第一楼(剧本)
The photoproduction of the vector meson has been studied between threshold and W = 2.4 GeV with the SAPHIR detector at the Bonn electron stretcher ring ELSA. Besides, the total cross-sections angular distributions in the CMS and decay angular distributions in the helicity and Gottfried-Jackson systems have been measured
Complications related to deep venous thrombosis prophylaxis in trauma: a systematic review of the literature
Deep venous thrombosis prophylaxis is essential to the appropriate management of multisystem trauma patients. Without thromboprophylaxis, the rate of venous thrombosis and subsequent pulmonary embolism is substantial. Three prophylactic modalities are common: pharmacologic anticoagulation, mechanical compression devices, and inferior vena cava filtration. A systematic review was completed using PRISMA guidelines to evaluate the potential complications of DVT prophylactic options. Level one evidence currently supports the use of low molecular weight heparins for thromboprophylaxis in the trauma patient. Unfortunately, multiple techniques are not infrequently required for complex multisystem trauma patients. Each modality has potential complications. The risks of heparin include bleeding and heparin induced thrombocytopenia. Mechanical compression devices can result in local soft tissue injury, bleeding and patient non-compliance. Inferior vena cava filters migrate, cause inferior vena cava occlusion, and penetrate the vessel wall. While the use of these techniques can be life saving, they must be appropriately utilized
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