215 research outputs found
Island dynamics and anisotropy during vapor phase epitaxy of m-plane GaN
Using in situ grazing-incidence x-ray scattering, we have measured the diffuse scattering from islands that form during layer-by-layer growth of GaN by metal-organic vapor phase epitaxy on the (101⎯⎯0)(101¯0)(101¯0) m-plane surface. The diffuse scattering is extended in the (0001)(0001)(0001) in-plane direction in reciprocal space, indicating a strong anisotropy with islands elongated along [12⎯⎯10][12¯10] [12¯10] and closely spaced along [0001][0001][0001]. This is confirmed by atomic force microscopy of a quenched sample. Islands were characterized as a function of growth rate F and temperature. The island spacing along [0001][0001][0001] observed during the growth of the first monolayer obeys a power-law dependence on growth rate F−nF−nF−n, with an exponent n=0.25±0.02n=0.25±0.02n=0.25±0.02. The results are in agreement with recent kinetic Monte Carlo simulations, indicating that elongated islands result from the dominant anisotropy in step edge energy and not from surface diffusion anisotropy. The observed power-law exponent can be explained using a simple steady-state model, which gives n = 1/4
Giant two-phonon Raman scattering from nanoscale NbC precipitates in Nb
High purity niobium (Nb), subjected to the processing methods used in the
fabrication of superconducting RF cavities, displays micron-sized surface
patches containing excess carbon. High-resolution transmission electron
microscopy and electron energy-loss spectroscopy measurements are presented
which reveal the presence of nanoscale NbC coherent precipitates in such
regions. Raman backscatter spectroscopy on similar surface regions exhibit
spectra consistent with the literature results on bulk NbC but with
significantly enhanced two-phonon scattering. The unprecedented strength and
sharpness of the two-phonon signal has prompted a theoretical analysis, using
density functional theory (DFT), of phonon modes in NbC for two different
interface models of the coherent precipitate. One model leads to overall
compressive strain and a comparison to ab-initio calculations of phonon
dispersion curves under uniform compression of the NbC shows that the measured
two-phonon peaks are linked directly to phonon anomalies arising from strong
electron-phonon interaction. Another model of the extended interface between Nb
and NbC, studied by DFT, gives insight into the frequency shifts of the
acoustic and optical mode density of states measured by first order Raman. The
exact origin of the stronger two-phonon response is not known at present but it
suggests the possibility of enhanced electron-phonon coupling in transition
metal carbides under strain found either in the bulk NbC inclusions or at their
interfaces with Nb metal. Preliminary tunneling studies using a point contact
method show some energy gaps larger than expected for bulk NbC.Comment: Phys. Rev. B, accepte
Reverse Hall-Petch effect in ultra nanocrystalline diamond
We present atomistic simulations for the mechanical response of ultra
nanocrystalline diamond, a polycrystalline form of diamond with grain diameters
of the order of a few nm. We consider fully three-dimensional model structures,
having several grains of random sizes and orientations, and employ
state-of-the-art Monte Carlo simulations. We calculate structural properties,
elastic constants and the hardness of the material; our results compare well
with experimental observations for this material. Moreover, we verify that this
material becomes softer at small grain sizes, in analogy to the observed
reversal of the Hall-Petch effect in various nanocrystalline metals. The effect
is attributed to the large concentration of grain boundary atoms at smaller
grain sizes. Our analysis yields scaling relations for the elastic constants as
a function of the average grain size.Comment: Proceedings of the IUTAM Symposium on Modelling Nanomaterials and
Nanosystems, Aalborg, Denmark, May 19-22 2008; to be published in the IUTAM
Bookseries by Springe
Neurology
Contains research objectives and reports on six research projects.U.S. Public Health Service (B-3055)U.S. Public Health Service (B-3090)Office of Naval Research (Nonr-1841 (70))Air Force (AF33(616)-7588)Air Force (AFAFOSR-155-63)Air Force (AFAFOSR-155-63)Army Chemical Corps (DA-18-108-405-Cml-942)National Science Foundation (Grant G-16526
Adaptive plasticity of killifish (Fundulus heteroclitus) embryos: dehydration-stimulated development and differential aquaporin-3 expression
13 pages, 7 figures, 3 tablesEmbryos of the marine killifish Fundulus heteroclitus are adapted to survive aerially. However, it is unknown if they are able to control development under dehydration conditions. Here, we show that air-exposed blastula embryos under saturated relative humidity were able to stimulate development, and hence the time of hatching was advanced with respect to embryos continuously immersed in seawater. Embryos exposed to air at later developmental stages did not hatch until water was added, while development was not arrested. Air-exposed embryos avoided dehydration probably because of their thickened egg envelope, although it suffered significant evaporative water loss. The potential role of aquaporins as part of the embryo response to dehydration was investigated by cloning the aquaporin-0 (FhAqp0), -1a (FhAqp1a), and -3 (FhAqp3) cDNAs. Functional expression in Xenopus laevis oocytes showed that FhaAqp1a was a water-selective channel, whereas FhAqp3 was permeable to water, glycerol, and urea. Expression of fhaqp0 and fhaqp1a was prominent during organogenesis, and their mRNA levels were similar between water- and air-incubated embryos. However, fhaqp3 transcripts were highly and transiently accumulated during gastrulation, and the protein product was localized in the basolateral membrane of the enveloping epithelial cell layer and in the membrane of ingressing and migrating blastomers. Interestingly, both fhaqp3 transcripts and FhAqp3 polypeptides were downregulated in air-exposed embryos. These data demonstrate that killifish embryos respond adaptively to environmental desiccation by accelerating development and that embryos are able to transduce dehydration conditions into molecular responses. The reduced synthesis of FhAqp3 may be one of these mechanisms to regulate water and/or solute transport in the embryo.This study was supported by the European Commission New and Emerging
Science and Technologies (NEST) program (contract no. 012674-2 Sleeping
Beauty) and by a grant from the Spanish Ministry of Education and Science
(MEC; AGL2004-00316/ACU) to J. Cerda`. Participation of C. Zapater and F.
Chauvigne´ was financed by a predoctoral fellowship from MEC (Spain) and by
the European Commission [Marie Curie Research Training Network Aqua
(glycero)porins, MRTN-CT-2006-035995], respectively.Peer reviewe
Spectroscopic study of impurities and associated defects in nanodiamonds from Efremovka (CV3) and Orgueil (CI) meteorites
The results of spectroscopic and structural studies of phase composition and
of defects in nanodiamonds from Efremovka (CV3) and Orgueil (CI) chondrites
indicate that nitrogen atomic environment in meteoritic nanodiamonds (MND) is
similar to that observed in synthetic counterparts produced by detonation and
by the Chemical Vapour Deposition (CVD)-process. Most of the nitrogen in MND
appears to be confined to lattice imperfections, such as crystallite/twin
boundaries and other extended defects, while the concentration of nitrogen in
the MND lattice is low. It is suggested that the N-rich sub-population of MND
grains may have been formed with high growth rates in environments rich in
accessible N (i.e., N in atomic form or as weakly bonded compounds). For the
first time the silicon-vacancy complex (the "silicon" defect) is observed in
MND by photoluminescence spectroscopy.Comment: 33 pages, 5 figures, submitted to Geochimica et Cosmochimica Act
Sildenafil attenuates pulmonary arterial pressure but does not improve oxygenation during ARDS
OBJECTIVE: Pulmonary hypertension is a characteristic feature of acute respiratory distress syndrome (ARDS) and contributes to mortality. Administration of sildenafil in ambulatory patients with pulmonary hypertension improves oxygenation and ameliorates pulmonary hypertension. Our aim was to determine whether sildenafil is beneficial for patients with ARDS. DESIGN: Prospective, open-label, multicenter, interventional cohort study. SETTING: Medical-surgical ICU of two university hospitals. PATIENTS: Ten consecutive patients meeting the NAECC criteria for ARDS. INTERVENTIONS: A single dose of 50 mg sildenafil citrate administered via a nasogastric tube. MAIN RESULTS: Administration of sildenafil in patients with ARDS decreased mean pulmonary arterial pressure from 25 to 22 mmHg (P = 0.022) and pulmonary artery occlusion pressure from 16 to 13 mmHg (P = 0.049). Systemic mean arterial pressures were markedly decreased from 81 to 75 mmHg (P = 0.005). Sildenafil did not improve pulmonary arterial oxygen tension, but resulted in a further increase in the shunt fraction. CONCLUSION: Although sildenafil reduced pulmonary arterial pressures during ARDS, the increased shunt fraction and decreased arterial oxygenation render it unsuitable for the treatment of patients with ARD
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