183 research outputs found
Electron-hole symmetry in a semiconducting carbon nanotube quantum dot
Optical and electronic phenomena in solids arise from the behaviour of
electrons and holes (unoccupied states in a filled electron sea). Electron-hole
symmetry can often be invoked as a simplifying description, which states that
electrons with energy above the Fermi sea behave the same as holes below the
Fermi energy. In semiconductors, however, electron-hole symmetry is generally
absent since the energy band structure of the conduction band differs from the
valence band. Here we report on measurements of the discrete, quantized-energy
spectrum of electrons and holes in a semiconducting carbon nanotube. Through a
gate, an individual nanotube is filled controllably with a precise number of
either electrons or holes, starting from one. The discrete excitation spectrum
for a nanotube with N holes is strikingly similar to the corresponding spectrum
for N electrons. This observation of near perfect electron-hole symmetry
demonstrates for the first time that a semiconducting nanotube can be free of
charged impurities, even in the limit of few-electrons or holes. We furthermore
find an anomalously small Zeeman spin splitting and an excitation spectrum
indicating strong electron-electron interactions.Comment: 12 pages, 4 figure
Mechanical properties of freely suspended atomically thin dielectric layers of mica
We have studied the elastic deformation of freely suspended atomically thin
sheets of muscovite mica, a widely used electrical insulator in its bulk form.
Using an atomic force microscope, we carried out bending test experiments to
determine the Young's modulus and the initial pre-tension of mica nanosheets
with thicknesses ranging from 14 layers down to just one bilayer. We found that
their Young's modulus is high (190 GPa), in agreement with the bulk value,
which indicates that the exfoliation procedure employed to fabricate these
nanolayers does not introduce a noticeable amount of defects. Additionally,
ultrathin mica shows low pre-strain and can withstand reversible deformations
up to tens of nanometers without breaking. The low pre-tension and high Young's
modulus and breaking force found in these ultrathin mica layers demonstrates
their prospective use as a complement for graphene in applications requiring
flexible insulating materials or as reinforcement in nanocomposites.Comment: 9 pages, 5 figures, selected as cover of Nano Research, Volume 5,
Number 8 (2012
A review of the catfish genus Pterocryptis (Siluridae) in Vietnam, with the description of two new species
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73958/1/j.1095-8649.2001.tb02367.x.pd
Local strain engineering in atomically thin MoS2
Tuning the electronic properties of a material by subjecting it to strain
constitutes an important strategy to enhance the performance of semiconducting
electronic devices. Using local strain, confinement potentials for excitons can
be engineered, with exciting possibilities for trapping excitons for quantum
optics and for efficient collection of solar energy. Two-dimensional materials
are able to withstand large strains before rupture, offering a unique
opportunity to introduce large local strains. Here, we study atomically thin
MoS2 layers with large local strains of up to 2.5% induced by controlled
delamination from a substrate. Using simultaneous scanning Raman and
photoluminescence imaging, we spatially resolve a direct bandgap reduction of
up to 90 meV induced by local strain. We observe a funnel effect in which
excitons drift hundreds of nanometers to lower bandgap regions before
recombining, demonstrating exciton confinement by local strain. The
observations are supported by an atomistic tight-binding model developed to
predict the effect of inhomogeneous strain on the local electronic states in
MoS2. The possibility of generating large strain-induced variations in exciton
trapping potentials opens the door for a variety of applications in atomically
thin materials including photovoltaics, quantum optics and two-dimensional
optoelectronic devices.Comment: Supp.Info. not included here, available following a link included in
the tex
Seasonality of Leaf and Fig Production in Ficus squamosa, a Fig Tree with Seeds Dispersed by Water
The phenology of plants reflects selection generated by seasonal climatic factors and interactions with other plants and animals, within constraints imposed by their phylogenetic history. Fig trees (Ficus) need to produce figs year-round to support their short-lived fig wasp pollinators, but this requirement is partially de-coupled in dioecious species, where female trees only develop seeds, not pollinator offspring. This allows female trees to concentrate seed production at more favorable times of the year. Ficus squamosa is a riparian species whose dispersal is mainly by water, rather than animals. Seeds can float and travel in long distances. We recorded the leaf and reproductive phenology of 174 individuals for three years in Chiang Mai, Northern Thailand. New leaves were produced throughout the year. Fig production occurred year-round, but with large seasonal variations that correlated with temperature and rainfall. Female and male trees initiated maximal fig crops at different times, with production in female trees confined mainly to the rainy season and male figs concentrating fig production in the preceding months, but also often bearing figs continually. Ficus squamosa concentrates seed production by female plants at times when water levels are high, favouring dispersal by water, and asynchronous flowering within male trees allow fig wasps to cycle there, providing them with potential benefits by maintaining pollinators for times when female figs become available to pollinate
Stereophonic hands-free communication system based on microphone array fixed beamforming: real-time implementation and evaluation
Recent advances in food allergy
Food allergy is a public health issue that has significantly increased worldwide in the past decade, affecting consumers’ quality of life and making increasing demands on health service resources. Despite recent advances in many areas of diagnosis and treatment, our general knowledge of the basic mechanisms of the disease remain limited i.e., not at pace with the exponential number of new cases and the explosion of new technologies. Many important key questions remain: What defines a major allergen? Why do some individuals develop food allergies and others do not? Which are the environmental factors? Could the environmental factors be monitored through epigenetics or modified by changes in the microbiome? Can tolerance to food be induced? Why are some foods more likely to trigger allergies than others? Does the route and timing of exposure have any role on sensitization? These and many other related questions remain unanswered. In this short review some of these topics are addressed in the light of recent advances in the area
Electromagnetic Wave Theory and Applications
Contains table of contents for Section 3, reports on four research projects and a list of publications.National Aeronautics and Space Administration Grant NAGW-1617National Aeronautics and Space Administration Agreement 958461National Aeronautics and Space Administration Grant NAGW-1272U.S. Army Corp of Engineers Contract DACA39-87-K-0022U.S. Navy - Office of Naval Research Grant N00014-89-J-1107U.S. Navy - Office of Naval Research Grant N00014-92-J-1616Digital Equipment CorporationJoint Services Electronics Program Contract DAAL03-92-C-0001U.S. Navy - Office of Naval Research Grant N00014-90-J-1002U.S. Navy - Office of Naval Research Grant N00014-89-J-1019U.S. Department of Transportation Agreement DTRS-57-88-C-00078TTD13U.S. Department of Transportation Agreement DTRS-57-88-C-00078TTD30U.S. Department of Transportation Agreement DTRS-57-92-C-00054TTD1DARPA/Consortium for Superconducting Electronics Contract MDA972-90-C-0021National Science Foundation Fellowship MIP 88-5876
The Chemical Information Ontology: Provenance and Disambiguation for Chemical Data on the Biological Semantic Web
Cheminformatics is the application of informatics techniques to solve chemical problems in silico. There are many areas in biology where cheminformatics plays an important role in computational research, including metabolism, proteomics, and systems biology. One critical aspect in the application of cheminformatics in these fields is the accurate exchange of data, which is increasingly accomplished through the use of ontologies. Ontologies are formal representations of objects and their properties using a logic-based ontology language. Many such ontologies are currently being developed to represent objects across all the domains of science. Ontologies enable the definition, classification, and support for querying objects in a particular domain, enabling intelligent computer applications to be built which support the work of scientists both within the domain of interest and across interrelated neighbouring domains. Modern chemical research relies on computational techniques to filter and organise data to maximise research productivity. The objects which are manipulated in these algorithms and procedures, as well as the algorithms and procedures themselves, enjoy a kind of virtual life within computers. We will call these information entities. Here, we describe our work in developing an ontology of chemical information entities, with a primary focus on data-driven research and the integration of calculated properties (descriptors) of chemical entities within a semantic web context. Our ontology distinguishes algorithmic, or procedural information from declarative, or factual information, and renders of particular importance the annotation of provenance to calculated data. The Chemical Information Ontology is being developed as an open collaborative project. More details, together with a downloadable OWL file, are available at http://code.google.com/p/semanticchemistry/ (license: CC-BY-SA)
Varying coefficient analysis for indeterminate species interactions with non-parametric estimation, exemplifying with a fig-fig wasp system
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