25,155 research outputs found
Multi-excitonic complexes in single InGaN quantum dots
Cathodoluminescence spectra employing a shadow mask technique of InGaN layers
grown by metal organic chemical vapor deposition on Si(111) substrates are
reported. Sharp lines originating from InGaN quantum dots are observed.
Temperature dependent measurements reveal thermally induced carrier
redistribution between the quantum dots. Spectral diffusion is observed and was
used as a tool to correlate up to three lines that originate from the same
quantum dot. Variation of excitation density leads to identification of exciton
and biexciton. Binding and anti-binding complexes are discovered.Comment: 3 pages, 4 figure
Allometric growth in the Diademodontinae (Reptilia; Therapsida); a preliminary report
Main articleThe hypothesis that many, if not all, of the South African and Zambian specimens, which have
been regarded as different diademodontine genera and species, actually consitute a taxonomically
homogeneous, ontogenetic growth series is tested. The principles of allometric growth were
applied to this sample of fossils, which varied considerably in size and shape. The approach
which was followed was exclusively morphometric. The results indicate that these specimens do
represent various ontogenetic stages of a growth series of only a single species of Diademodon
Seeley.Non
Preliminary Solar Sail Design and Fabrication Assessment: Spinning Sail Blade, Square Sail Sheet
Blade design aspects most affecting producibility and means of measurement and control of length, scallop, fullness and straightness requirements and tolerances were extensively considered. Alternate designs of the panel seams and edge reinforcing members are believed to offer advantages of seam integrity, producibility, reliability, cost and weight. Approaches to and requirements for highly specialized metalizing methods, processes and equipment were studied and identified. Alternate methods of sail blade fabrication and related special machinery, tooling, fixtures and trade offs were examined. A preferred and recommended approach is also described. Quality control plans, inspection procedures, flow charts and special test equipment associated with the preferred manufacturing method were analyzed and are discussed
Exploring the geochemistry and shell ultrastructure of bivalves as archives of past ocean acidification events: recent Mytilidae in a natural acidic setting in Ischia (Italy, Mediterranean Sea)
Female release of luteinizing hormone (LH) in response to short- and long-range song in a songbird, the dark-eyed junco (Junco hyemalis)
Marine bivalve geochemistry and shell ultrastructure from modern low pH environments
Abstract. Bivalve shells can provide excellent archives of past environmental change but have not been used to interpret ocean acidification events. We investigated carbon, oxygen and trace element records from different shell layers in the mussels Mytilus galloprovincialis (from the Mediterranean) and M. edulis (from the Wadden Sea) combined with detailed investigations of the shell ultrastructure. Mussels from the harbour of Ischia (Mediterranean, Italy) were transplanted and grown in water with mean pHT 7.3 and mean pHT 8.1 near CO2 vents on the east coast of the island of Ischia. The shells of transplanted mussels were compared with M. edulis collected at pH ~8.2 from Sylt (German Wadden Sea). Most prominently, the shells recorded the shock of transplantation, both in their shell ultrastructure, textural and geochemical record. Shell calcite, precipitated subsequently under acidified seawater responded to the pH gradient by an in part disturbed ultrastructure. Geochemical data from all test sites show a strong metabolic effect that exceeds the influence of the low-pH environment. These field experiments showed that care is needed when interpreting potential ocean acidification signals because various parameters affect shell chemistry and ultrastructure. Besides metabolic processes, seawater pH, factors such as salinity, water temperature, food availability and population density all affect the biogenic carbonate shell archive.</jats:p
Fast Evaluation of Feynman Diagrams
We develop a new representation for the integrals associated with Feynman
diagrams. This leads directly to a novel method for the numerical evaluation of
these integrals, which avoids the use of Monte Carlo techniques. Our approach
is based on based on the theory of generalized sinc () functions,
from which we derive an approximation to the propagator that is expressed as an
infinite sum. When the propagators in the Feynman integrals are replaced with
the approximate form all integrals over internal momenta and vertices are
converted into Gaussians, which can be evaluated analytically. Performing the
Gaussians yields a multi-dimensional infinite sum which approximates the
corresponding Feynman integral. The difference between the exact result and
this approximation is set by an adjustable parameter, and can be made
arbitrarily small. We discuss the extraction of regularization independent
quantities and demonstrate, both in theory and practice, that these sums can be
evaluated quickly, even for third or fourth order diagrams. Lastly, we survey
strategies for numerically evaluating the multi-dimensional sums. We illustrate
the method with specific examples, including the the second order sunset
diagram from quartic scalar field theory, and several higher-order diagrams. In
this initial paper we focus upon scalar field theories in Euclidean spacetime,
but expect that this approach can be generalized to fields with spin.Comment: uses feynmp macros; v2 contains improved description of
renormalization, plus other minor change
Light-Trap: A SiPM Upgrade for Very High Energy Astronomy and Beyond
With the development of the Imaging Atmospheric Cherenkov Technique (IACT),
Gamma-ray astronomy has become one of the most interesting and productive
fields of astrophysics. Current IACT telescope arrays (MAGIC, H.E.S.S, VERITAS)
use photomultiplier tubes (PMTs) to detect the optical/near-UV Cherenkov
radiation emitted due to the interaction of gamma rays with the atmosphere. For
the next generation of IACT experiments, the possibility of replacing the PMTs
with Silicon photomultipliers (SiPMs) is being studied. Among the main
drawbacks of SiPMs are their limited active area (leading to an increase in the
cost and complexity of the camera readout) and their sensitivity to unwanted
wavelengths. Here we propose a novel method to build a relatively low-cost
pixel consisting of a SiPM attached to a PMMA disc doped with a wavelength
shifter. This pixel collects light over a much larger area than a single
standard SiPM and improves sensitivity to near-UV light while simultaneously
rejecting background. We describe the design of a detector that could also have
applications in other fields where detection area and cost are crucial. We
present results of simulations and laboratory measurements of a pixel prototype
and from field tests performed with a 7-pixel cluster installed in a MAGIC
telescope camera.Comment: Proceedings of the 35th International Cosmic Ray Conference (ICRC
2017), Bexco, Busan, Korea. Id:81
Structure and Dynamics of Metalloproteins in Live Cells
X-ray absorption spectroscopy (XAS) has emerged as one of the premier tools for investigating the structure and dynamic properties of metals in cells and in metal containing biomolecules. Utilizing the high flux and broad energy range of X-rays supplied by synchrotron light sources, one can selectively excite core electronic transitions in each metal. Spectroscopic signals from these electronic transitions can be used to dissect the chemical architecture of metals in cells, in cellular components and in biomolecules at varying degrees of structural resolution. With the development of ever-brighter X-ray sources, X-ray methods have grown into applications that can be utilized to provide both a cellular image of relative distribution of metals throughout the cell as well as a high-resolution picture of the structure of the metal. As these techniques continue to grow in their capabilities and ease of use, so to does the demand for their application by chemists and biochemists interested in studying the structure and dynamics of metals in cells, in cellular organelles and in metalloproteins
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