2,069 research outputs found
The mechanism of nestmate discrimination in the tropical social wasp Ropalidia marginata and its implications for the evolution of sociality
We have demonstrated that females of the primitively eusocial tropical wasp Ropalidia marginata can discriminate nestmates from nonnestmates outside the context of their nests. This was accomplished by recroding all behavioural interactions in a neutral arena and comparing tolerance levels. In order for these wasps to make such a discrimination, however, it was essential that after eclosion both the discriminated and the discriminating animals were exposed to their respective natal nests and nestmates. The results suggest that both recognition labels and templates are acquired by the animals from sources outside their body, perhaps from their nest or nestmates. It is thus unlikely that different genetic lines within a colony can be distinguished. We conclude, therefore, that genetic asymmetries created by haplodiploidy, but often broken down by multiple mating and polygyny, are not restored by preferential altruism towards full rather than half sisters by means of kin recognition. Hence we recommend caution in ascribing the multiple origins of eusociality in the Hymenoptera to haplodiploidy
Amine-Gold Linked Single-Molecule Junctions: Experiment and Theory
The measured conductance distribution for single molecule benzenediamine-gold
junctions, based on 59,000 individual conductance traces recorded while
breaking a gold point contact in solution, has a clear peak at 0.0064 G
with a width of 40%. Conductance calculations based on density functional
theory (DFT) for 15 distinct junction geometries show a similar spread.
Differences in local structure have a limited influence on conductance because
the amine-Au bonding motif is well-defined and flexible. The average calculated
conductance (0.046 G) is seven times larger than experiment, suggesting
the importance of many-electron corrections beyond DFT
Stabilizing single atom contacts by molecular bridge formation
Gold-molecule-gold junctions can be formed by carefully breaking a gold wire
in a solution containing dithiolated molecules. Surprisingly, there is little
understanding on the mechanical details of the bridge formation process and
specifically on the role that the dithiol molecules play themselves. We propose
that alkanedithiol molecules have already formed bridges between the gold
electrodes before the atomic gold-gold junction is broken. This leads to
stabilization of the single atomic gold junction, as observed experimentally.
Our data can be understood within a simple spring model.Comment: 14 pages, 3 figures, 1 tabl
Shot noise suppression at room temperature in atomic-scale Au junctions
Shot noise encodes additional information not directly inferable from simple
electronic transport measurements. Previous measurements in atomic-scale metal
junctions at cryogenic temperatures have shown suppression of the shot noise at
particular conductance values. This suppression demonstrates that transport in
these structures proceeds via discrete quantum channels. Using a high frequency
technique, we simultaneously acquire noise data and conductance histograms in
Au junctions at room temperature and ambient conditions. We observe noise
suppression at up to three conductance quanta, with possible indications of
current-induced local heating and noise in the contact region at high
biases. These measurements demonstrate the quantum character of transport at
room temperature at the atomic scale. This technique provides an additional
tool for studying dissipation and correlations in nanodevices.Comment: 15 pages, 4 figures + supporting information (6 pages, 6 figures
Orbital Interaction Mechanisms of Conductance Enhancement and Rectification by Dithiocarboxylate Anchoring Group
We study computationally the electron transport properties of
dithiocarboxylate terminated molecular junctions. Transport properties are
computed self-consistently within density functional theory and nonequilibrium
Green's functions formalism. A microscopic origin of the experimentally
observed current amplification by dithiocarboxylate anchoring groups is
established. For the 4,4'-biphenyl bis(dithiocarboxylate) junction, we find
that the interaction of the lowest unoccupied molecular orbital (LUMO) of the
dithiocarboxylate anchoring group with LUMO and highest occupied molecular
orbital (HOMO) of the biphenyl part results in bonding and antibonding
resonances in the transmission spectrum in the vicinity of the electrode Fermi
energy. A new microscopic mechanism of rectification is predicted based on the
electronic structure of asymmetrical anchoring groups. We show that the peaks
in the transmission spectra of 4'-thiolato-biphenyl-4-dithiocarboxylate
junction respond differently to the applied voltage. Depending upon the origin
of a transmission resonance in the orbital interaction picture, its energy can
be shifted along with the chemical potential of the electrode to which the
molecule is more strongly or more weakly coupled
Use of Semantic Wiki Tool to Build a Repository of Reusable Information Objects in Agricultural Education and Extension: results from a preliminary study
There has been significant interest in applying the practices of semantic web to build an
online repository of agricultural information (example: FAO-India efforts in AGROPEDIA).
We will describe the results from a preliminary investigation into the use of a semantic
tool for Mediawiki with content from the agricultural domain as the substrate.
The English AGROVOC have been used to generate 17 broad categories of terms to
harvest entries from the Wikipedia and to form the relationships. These can be browsed
at http://vasatwiki.icrisat.org/index.php/. A total of 1000 articles were downloaded and
posted onto a local server running MediaWiki software. A semantic tool
(http://ontoworld.org/wiki/Semantic_MediaWiki) was used to create additional tags in
the hosted entries and links were formed.
The searches inside the semantically re-constructed collection are more specific
compared to searches on the downloaded collection. We are conscious that this may not
be a rigorous comparison. An even more interesting possibility is the ease with which the
entries can be constituted into information objects that can be reused because their
relationships with other objects in this re-constituted domain are rich and complex. With
suitable filters the information objects can be exported to various interfaces (re-
purposing). An area of interest is the use of this approach to aggregate content for use
in education and in extension
Non-linear optical susceptibilities, Raman efficiencies and electrooptic tensors from first-principles density functional perturbation theory
The non-linear response of infinite periodic solids to homogenous electric
fields and collective atomic displacements is discussed in the framework of
density functional perturbation theory. The approach is based on the 2n + 1
theorem applied to an electric-field-dependent energy functional. We report the
expressions for the calculation of the non-linear optical susceptibilities,
Raman scattering efficiencies and electrooptic coefficients. Different
formulations of third-order energy derivatives are examined and their
convergence with respect to the k-point sampling is discussed. We apply our
method to a few simple cases and compare our results to those obtained with
distinct techniques. Finally, we discuss the effect of a scissors correction on
the EO coefficients and non-linear optical susceptibilities
LiDAR for Atmosphere Research over Africa (LARA)
International audienceThis paper describes the LIDAR for atmosphere research over Africa and current initiatives being undertaken in South Africa. A mobile LIDAR system is being developed at the Council for Scientific and Industrial Research (CSIR) National Laser Centre (NLC), Pretoria (25°5 ′ S;28°2 ′ E), South Africa, for remote sensing the atmosphere. The initial results conclude that the system is capable of providing aerosol/cloud backscatter measurements for the height region from ground to 40 km with a 10 m vertical height resolution
- …