5,528 research outputs found
Dirac point resonances due to atoms and molecules adsorbed on graphene and transport gaps and conductance quantization in graphene nanoribbons with covalently bonded adsorbates
We present a tight binding theory of the Dirac point resonances due to
adsorbed atoms and molecules on an infinite 2D graphene sheet based on the
standard tight binding model of the graphene p-band electronic structure and
the extended Huckel model of the adsorbate and nearby graphene carbon atoms.
The relaxed atomic geometries of the adsorbates and graphene are calculated
using density functional theory. Our model includes the effects of the local
rehybridization of the graphene from the sp^2 to sp^3 electronic structure that
occurs when adsorbed atoms or molecules bond covalently to the graphene. Unlike
in previous tight-binding models of Dirac point resonances, adsorbed species
with multiple extended molecular orbitals and bonding to more than one graphene
carbon atom are treated. More accurate and more general analytic expressions
for the Green's function matrix elements that enter the T-matrix theory of
Dirac point resonances than have been available previously are obtained. We
study H, F, OH and O adsorbates on graphene and for each we find a strong
scattering resonance (two resonances for O) near the Dirac point of graphene,
by far the strongest and closest to the Dirac point being the resonance for H.
We extract a minimal set of tight binding parameters that can be used to model
resonant electron scattering and electron transport in graphene and graphene
nanostructures with adsorbed H, F, OH and O accurately and efficiently. We also
compare our results for the properties of Dirac point resonances due to
adsorbates on graphene with those obtained by others using density functional
theory-based electronic structure calculations, and discuss their relative
merits. We then present calculations of electronic quantum transport in
graphene nanoribbons with these adsorbed species...Comment: 21 pages, 9 figure
Enhanced stability of hydrogen atoms at the graphene/graphane interface of nanoribbons
The thermal stability of graphene/graphane nanoribbons (GGNRs) is
investigated using density functional theory. It is found that the energy
barriers for the diffusion of hydrogen atoms on the zigzag and armchair
interfaces of GGNRs are 2.86 and 3.17 eV, respectively, while the diffusion
barrier of an isolated H atom on pristine graphene was only ~0.3 eV. These
results unambiguously demonstrate that the thermal stability of GGNRs can be
enhanced significantly by increasing the hydrogen diffusion barriers through
graphene/graphane interface engineering. This may provide new insights for
viable applications of GGNRs.Comment: 13 pages, 1 figure, 2 tables to appear in Appl. Phys. Let
CaractĂ©risation physico-chimique et microbiologique du mulet jaune (Mugil cephalus) sĂ©chĂ©-pilĂ© « Lekhlia » dâorigine Imraguen, Mauritanie
Lekhlia est un produit dâorigine Imraguen en Mauritanie, fabriquĂ© Ă base de poisson (mulet jaune sĂ©chĂ©pilĂ©). Un totale de 163 Ă©chantillons de Lekhlia ont Ă©tĂ© prĂ©levĂ©s Ă partir de 7 villages imraguen et Ă©valuĂ©s pour leurs qualitĂ©s physico-chimiques et microbiologiques. Les rĂ©sultats obtenus ont montrĂ© des teneurs moyennes de 65,33% en protĂ©ines, de 12,33% en matiĂšre grasse, de 9,21% en cendres, de 9,63% en taux dâhumiditĂ© et de 1,02 mg/100g en histamine. La valeur moyenne en FMAT est de 17,30 104 ufc/g. Les charges moyennes des germes tĂ©moins de la contamination fĂ©cale sont de 0,74 102 et 3,66 ufc/g respectivement pour les coliformes totaux et fĂ©caux. La flore fongique est de 1,70 103 ufc/g. Cependant, aucun germe pathogĂšne nâa Ă©tĂ© dĂ©tectĂ© dans les produits analysĂ©s.Mots-clĂ©s : Imraguen, mulet jaune, Lekhlia, qualitĂ©, Mauritanie
Oligosaccharides Affect Performance and Gut Development of Broiler Chickens
The effects of oligosaccharide supplementation on the growth performance, flock uniformity and GIT development of broiler chickens were investigated. Four diets, one negative control, one positive control supplemented with zinc-bacitracin, and two test diets supplemented with mannoligosaccharide (MOS) and fructooligosaccharide (FOS), were used for the experiment. Birds given MOS or FOS had improved body weight (BW) and feed efficiency (FCR), compared to those fed the negative control diet during the 35-d trial period. The effect on FCR became less apparent when the birds got older. FOS and MOS supplementation reduced the pancreas weight as a percentage of BW, with an effect similar to that of the antibiotic, at 35 d of age. Birds given MOS tended to have a heavier bursa (p = 0.164) and lower spleen/bursa weight ratio (p = 0.102) at 35 d of age. MOS and Zn-bacitracin showed a clear improvement on flock uniformity, compared to FOS. The mortality rate was not affected by FOS or MOS
Relative blocking in posets
Poset-theoretic generalizations of set-theoretic committee constructions are
presented. The structure of the corresponding subposets is described. Sequences
of irreducible fractions associated to the principal order ideals of finite
bounded posets are considered and those related to the Boolean lattices are
explored; it is shown that such sequences inherit all the familiar properties
of the Farey sequences.Comment: 29 pages. Corrected version of original publication which is
available at http://www.springerlink.com, see Corrigendu
Transitions To the Long-Resident State in coupled chaotic oscillators
The behaviors of coupled chaotic oscillators before complete synchronization
were investigated. We report three phenomena: (1) The emergence of long-time
residence of trajectories besides one of the saddle foci; (2) The tendency that
orbits of the two oscillators get close becomes faster with increasing the
coupling strength; (3) The diffusion of two oscillator's phase difference is
first enhanced and then suppressed. There are exact correspondences among these
phenomena. The mechanism of these correspondences is explored. These phenomena
uncover the route to synchronization of coupled chaotic oscillators.Comment: 3 pages, 5 figure
Simulation study on effect of chassis water cooling on solidification of eleven tons flat steel ingot
In this paper, the solidification process of eleven tons flat steel ingot is simulated by the finite element analysis software PROCAST, and the solidification state of the ingot with and without chassis is analyzed and compared. The results show that the forced cooling chassis makes low temperature area of the bottom ingot enlarged. And it has little influence on the temperature field and the solidification speed of the upper ingot. For the small flat steel ingot, the forced cooling chassis will deteriorate the shrinkage
Effects of Dietary Additives and Early Feeding on Performance, Gut Development and Immune Status of Broiler Chickens Challenged with
The effects of dietary additives and holding time on resistance and resilience of broiler chickens to Clostridium perfringens challenge were investigated by offering four dietary treatments. These were a negative control (basal), a positive control (Zn-bacitracin) and two dietary additives, mannanoligosaccharides (MOS), and acidifier. Two holding times included (a) immediate access to feed and water post hatch (FED) and (b) access to both feed and water 48 h post hatch (HELD). Chicks fed Zn-bacitracin had no intestinal lesions attributed to necrotic enteritis (NE), whereas chicks fed both MOS or acidifier showed signs of NE related lesions. All dietary treatments were effective in reducing the numbers of C. perfringens in the ileum post challenge. The FED chicks had heavier body weight and numerically lower mortality. The FED chicks also showed stronger immune responses to NE challenge, showing enhanced (p<0.05) proliferation of T-cells. Early feeding of the MOS supplemented diet increased (p<0.05) IL-6 production. The relative bursa weight of the FED chicks was heavier at d 21 (p<0.05). All the additives increased the relative spleen weight of the HELD chicks at d 14 (p<0.05). The FED chicks had increased villus height and reduced crypt depth, and hence an increased villus/crypt ratio, especially in the jejunum at d 14 (p<0.05). The same was true for the HELD chicks given dietary additives (p<0.05). It may be concluded that the chicks with early access to dietary additives showed enhanced immune response and gut development, under C. perfringens challenge. The findings of this study shed light on managerial and nutritional strategies that could be used to prevent NE in the broiler industry without the use of in-feed antibiotics
Fossil evidence for a pharyngeal origin of the vertebrate pectoral girdle
The origin of vertebrate paired appendages is one of the most investigated and debated examples of evolutionary novelty. Paired appendages are widely considered key innovations that allowed new opportunities for controlled swimming and gill ventilation and were prerequisites for the eventual transition from water to land. The last 150 years of debate has been shaped by two contentious theories: the ventrolateral fin-fold hypothesis and the archipterygium hypothesis. The latter proposes that fins and girdles evolved from an ancestral gill arch. Although tantalizing developmental evidence has revived interest in this idea, it is apparently unsupported by fossil evidence. Here we present fossil evidence of a pharyngeal basis for the vertebrate shoulder girdle. We use CT scanning to reveal details of the braincase of Kolymaspis sibirica, a placoderm fish from the Early Devonian of Siberia that suggests a pharyngeal component of the shoulder. We combine these findings with refreshed comparative anatomy of placoderms and jawless outgroups to place the origin of the shoulder girdle on the sixth branchial arch. These findings provide a novel framework for understanding the origin of the pectoral girdle. Our new evidence clarifies the location of the presumptive head-trunk interface in jawless fishes and explains the constraint on branchial arch number in gnathostomes. The results revive a key aspect of the archipterygium hypothesis, but also reconciles it with the ventrolateral fin fold model
What Powers Lyman alpha Blobs?
Lyman alpha blobs (LABs) are spatially extended lyman alpha nebulae seen at
high redshift. The origin of Lyman alpha emission in the LABs is still unclear
and under debate. To study their heating mechanism(s), we present Australia
Telescope Compact Array (ATCA) observations of the 20 cm radio emission and
Herschel PACS and SPIRE measurements of the far-infrared (FIR) emission towards
the four LABs in the protocluster J2143-4423 at z=2.38. Among the four LABs, B6
and B7 are detected in the radio with fluxes of 67+/-17 microJy and 77+/-16
microJy, respectively, and B5 is marginally detected at 3 sigma (51+/-16
microJy). For all detected sources, their radio positions are consistent with
the central positions of the LABs. B6 and B7 are obviously also detected in the
FIR. By fitting the data with different templates, we obtained redshifts of
2.20 for B6 and 2.20 for B7 which are
consistent with the redshift of the lyman alpha emission within uncertainties,
indicating that both FIR sources are likely associated with the LABs. The
associated FIR emission in B6 and B7 and high star formation rates strongly
favor star formation in galaxies as an important powering source for the lyman
alpha emission in both LABs. However, the other two, B1 and B5, are
predominantly driven by the active galactic nuclei or other sources of energy
still to be specified, but not mainly by star formation. In general, the LABs
are powered by quite diverse sources of energy.Comment: 7 pages and 3 figurs, accepted by A&
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