328 research outputs found
Nucleation and Growth of GaN/AlN Quantum Dots
We study the nucleation of GaN islands grown by plasma-assisted
molecular-beam epitaxy on AlN(0001) in a Stranski-Krastanov mode. In
particular, we assess the variation of their height and density as a function
of GaN coverage. We show that the GaN growth passes four stages: initially, the
growth is layer-by-layer; subsequently, two-dimensional precursor islands form,
which transform into genuine three-dimensional islands. During the latter
stage, island height and density increase with GaN coverage until the density
saturates. During further GaN growth, the density remains constant and a
bimodal height distribution appears. The variation of island height and density
as a function of substrate temperature is discussed in the framework of an
equilibrium model for Stranski-Krastanov growth.Comment: Submitted to PRB, 10 pages, 15 figure
Step by step capping and strain state of GaN/AlN quantum dots studied by grazing incidence diffraction anomalous fine structure
The investigation of small size embedded nanostructures, by a combination of
complementary anomalous diffraction techniques, is reported. GaN Quantum Dots
(QDs), grown by molecular beam epitaxy in a modified Stranski-Krastanow mode,
are studied in terms of strain and local environment, as a function of the AlN
cap layer thickness, by means of grazing incidence anomalous diffraction. That
is, the X-ray photons energy is tuned across the Ga absorption K-edge which
makes diffraction chemically selective. Measurement of \textit{hkl}-scans,
close to the AlN (30-30) Bragg reflection, at several energies across the Ga
K-edge, allows the extraction of the Ga partial structure factor, from which
the in-plane strain of GaN QDs is deduced. From the fixed-Q energy-dependent
diffracted intensity spectra, measured for diffraction-selected iso-strain
regions corresponding to the average in-plane strain state of the QDs,
quantitative information regarding composition and the out-of-plane strain has
been obtained. We recover the in-plane and out-of-plane strains in the dots.
The comparison to the biaxial elastic strain in a pseudomorphic layer indicates
a tendency to an over-strained regime.Comment: submitted to PR
Self-assembled zinc blende GaN quantum dots grown
Zinc blende ~ZB! GaN quantum dots have been grown by plasma-assisted molecular-beam epitaxy
on AlN buffer layers using 3C-SiC~001! substrates. The two- to three-dimensional growth mode
transition is studied by following the evolution of the reflection high-energy electron diffraction
pattern. ZB GaN island layers are further examined by atomic force microscopy and transmission
electron microscopy, extracting a mean island height of 1.6 nm and a mean diameter of 13 nm at a
density of 1.331011 cm22. Embedded ZB GaN quantum dots show strong ultraviolet
photoluminescence without any thermal quenching up to room temperature.SFERERegion Rhône-AlpesConsejo Nacional de Ciencia y TecnologÃ
Subjectivities in transition: gender and sexual identities in cases of sex change and hermaphroditism in Spain, c. 1500-1800
The interplay of microscopic and mesoscopic structure in complex networks
Not all nodes in a network are created equal. Differences and similarities
exist at both individual node and group levels. Disentangling single node from
group properties is crucial for network modeling and structural inference.
Based on unbiased generative probabilistic exponential random graph models and
employing distributive message passing techniques, we present an efficient
algorithm that allows one to separate the contributions of individual nodes and
groups of nodes to the network structure. This leads to improved detection
accuracy of latent class structure in real world data sets compared to models
that focus on group structure alone. Furthermore, the inclusion of hitherto
neglected group specific effects in models used to assess the statistical
significance of small subgraph (motif) distributions in networks may be
sufficient to explain most of the observed statistics. We show the predictive
power of such generative models in forecasting putative gene-disease
associations in the Online Mendelian Inheritance in Man (OMIM) database. The
approach is suitable for both directed and undirected uni-partite as well as
for bipartite networks
Eu3+ optical activation engineering in AlxGa(1-x)N nanowires for red solid-state nano-emitters
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Deciphering the connectivity structure of biological networks using MixNet
<p>Abstract</p> <p>Background</p> <p>As biological networks often show complex topological features, mathematical methods are required to extract meaningful information. Clustering methods are useful in this setting, as they allow the summary of the network's topology into a small number of relevant classes. Different strategies are possible for clustering, and in this article we focus on a model-based strategy that aims at clustering nodes based on their connectivity profiles.</p> <p>Results</p> <p>We present MixNet, the first publicly available computer software that analyzes biological networks using mixture models. We apply this method to various networks such as the <it>E. coli </it>transcriptional regulatory network, the macaque cortex network, a foodweb network and the <it>Buchnera aphidicola </it>metabolic network. This method is also compared with other approaches such as module identification or hierarchical clustering.</p> <p>Conclusion</p> <p>We show how MixNet can be used to extract meaningful biological information, and to give a summary of the networks topology that highlights important biological features. This approach is powerful as MixNet is adaptive to the network under study, and finds structural information without any a priori on the structure that is investigated. This makes MixNet a very powerful tool to summarize and decipher the connectivity structure of biological networks.</p
Statistical methodology for the analysis of dye-switch microarray experiments
<p>Abstract</p> <p>Background</p> <p>In individually dye-balanced microarray designs, each biological sample is hybridized on two different slides, once with <it>Cy3 </it>and once with <it>Cy5</it>. While this strategy ensures an automatic correction of the gene-specific labelling bias, it also induces dependencies between log-ratio measurements that must be taken into account in the statistical analysis.</p> <p>Results</p> <p>We present two original statistical procedures for the statistical analysis of individually balanced designs. These procedures are compared with the usual ML and REML mixed model procedures proposed in most statistical toolboxes, on both simulated and real data.</p> <p>Conclusion</p> <p>The UP procedure we propose as an alternative to usual mixed model procedures is more efficient and significantly faster to compute. This result provides some useful guidelines for the analysis of complex designs.</p
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