1,497 research outputs found
MISR stereoscopic image matchers: techniques and results
The Multi-angle Imaging SpectroRadiometer (MISR) instrument, launched in December 1999 on the NASA EOS Terra satellite, produces images in the red band at 275-m resolution, over a swath width of 360 km, for the nine camera angles 70.5/spl deg/, 60/spl deg/, 45.6/spl deg/, and 26.1/spl deg/ forward, nadir, and 26.1/spl deg/, 45.6/spl deg/, 60/spl deg/, and 70.5/spl deg/ aft. A set of accurate and fast algorithms was developed for automated stereo matching of cloud features to obtain cloud-top height and motion over the nominal six-year lifetime of the mission. Accuracy and speed requirements necessitated the use of a combination of area-based and feature-based stereo-matchers with only pixel-level acuity. Feature-based techniques are used for cloud motion retrieval with the off-nadir MISR camera views, and the motion is then used to provide a correction to the disparities used to measure cloud-top heights which are derived from the innermost three cameras. Intercomparison with a previously developed "superstereo" matcher shows that the results are very comparable in accuracy with much greater coverage and at ten times the speed. Intercomparison of feature-based and area-based techniques shows that the feature-based techniques are comparable in accuracy at a factor of eight times the speed. An assessment of the accuracy of the area-based matcher for cloud-free scenes demonstrates the accuracy and completeness of the stereo-matcher. This trade-off has resulted in the loss of a reliable quality metric to predict accuracy and a slightly high blunder rate. Examples are shown of the application of the MISR stereo-matchers on several difficult scenes which demonstrate the efficacy of the matching approach
Conservation of structure and mechanism in primary and secondary transporters exemplified by SiaP, a sialic acid binding virulence factor from Haemophilus influenzae
Extracytoplasmic solute receptors (ESRs) are important components of solute uptake systems in bacteria, having been studied extensively as parts of ATP binding cassette transporters. Herein we report the first crystal structure of an ESR protein from a functionally characterized electrochemical ion gradient-dependent secondary transporter. This protein, SiaP, forms part of a tripartite ATP-independent periplasmic transporter specific for sialic acid in Haemophilus influenzae. Surprisingly, the structure reveals an overall topology similar to ATP binding cassette ESR proteins, which is not apparent from the sequence, demonstrating that primary and secondary transporters can share a common structural component. The structure of SiaP in the presence of the sialic acid analogue 2,3-didehydro-2-deoxyN-acetylneuraminic acid reveals the ligand bound in a deep cavity with its carboxylate group forming a salt bridge with a highly conserved Arg residue. Sialic acid binding, which obeys simple bimolecular association kinetics as determined by stopped-flow fluorescence spectroscopy, is accompanied by domain closure about a hinge region and the kinking of an alpha-helix hinge component. The structure provides insight into the evolution, mechanism, and substrate specificity of ESR-dependent secondary transporters that are widespread in prokaryotes
Si/SiGe bound-to-continuum quantum cascade emitters
Si/SiGe bound-to-continuum quantum cascade emitters designed
by self-consistent 6-band k.p modeling and grown by low energy
plasma enhanced chemical vapour deposition are presented
demonstrating electroluminescence between 1.5 and 3 THz. The
electroluminescence is Stark shifted by an electric field and
demonstrates polarized emission consistent with the design.
Transmission electron microscopy and x-ray diffraction are also
presented to characterize the thick heterolayer structure
Si/SiGe quantum cascade superlattice designs for terahertz emission
Quantum cascade lasers are compact sources that have demonstrated high output powers at THz frequencies.
To date all THz quantum cascade lasers have been realized in III-V materials. Results are presented from
Si1−xGex quantum cascade superlattice designs emitting at around 3 THz which have been grown in two
different chemical vapor deposition systems. The key to achieving successful electroluminescence at THz
frequencies in a p-type system has been to strain the light-hole states to energies well above the radiative
subband states. To accurately model the emission wavelengths, a 6-band k.p tool which includes the effects
of non-abrupt heterointerfaces has been used to predict the characteristics of the emitters. X-ray diffraction
and transmission electron microscopy have been used along with Fourier transform infrared spectroscopy to
fully characterise the samples. A number of methods to improve the gain from the designs are suggested
A Current Induced Transition in atomic-sized contacts of metallic Alloys
We have measured conductance histograms of atomic point contacts made from
the noble-transition metal alloys CuNi, AgPd, and AuPt for a concentration
ratio of 1:1. For all alloys these histograms at low bias voltage (below 300
mV) resemble those of the noble metals whereas at high bias (above 300 mV) they
resemble those of the transition metals. We interpret this effect as a change
in the composition of the point contact with bias voltage. We discuss possible
explanations in terms of electromigration and differential diffusion induced by
current heating.Comment: 5 pages, 6 figure
Magnetic excitations in coupled Haldane spin chains near the quantum critical point
Two quasi-1-dimensional S=1 quantum antiferromagnetic materials, PbNi2V2O8
and SrNi2V2O8, are studied by inelastic neutron scattering on powder samples.
While magnetic interactions in the two systems are found to be very similar,
subtle differences in inter-chain interaction strengths and magnetic anisotropy
are detected. The latter are shown to be responsible for qualitatively
different ground state properties: magnetic long-range order in SrNi2V2O8 and
disordered ``spin liquid'' Haldane-gap state in PbNi2V2O8.Comment: 15 figures, Figs. 5,9, and 10 in color. Some figures in JPEG format.
Complete PostScript and PDF available from
http://papillon.phy.bnl.gov/publicat.ht
Phase transitions in the early and the present Universe
The evolution of the Universe is the ultimate laboratory to study fundamental
physics across energy scales that span about 25 orders of magnitude: from the
grand unification scale through particle and nuclear physics scales down to the
scale of atomic physics. The standard models of cosmology and particle physics
provide the basic understanding of the early and present Universe and predict a
series of phase transitions that occurred in succession during the expansion
and cooling history of the Universe. We survey these phase transitions,
highlighting the equilibrium and non-equilibrium effects as well as their
observational and cosmological consequences. We discuss the current theoretical
and experimental programs to study phase transitions in QCD and nuclear matter
in accelerators along with the new results on novel states of matter as well as
on multi- fragmentation in nuclear matter. A critical assessment of
similarities and differences between the conditions in the early universe and
those in ultra- relativistic heavy ion collisions is presented. Cosmological
observations and accelerator experiments are converging towards an
unprecedented understanding of the early and present Universe.Comment: 41 pages, 16 figures, to appear in Ann. Rev. Nucl. Part. Sci 2006.
Presentation improved, references adde
Magnetic flux jumps in textured Bi2Sr2CaCu2O(8+d)
Magnetic flux jumps in textured Bi2Sr2CaCu2O(8+d) have been studied by means
of magnetization measurements in the temperature range between 1.95 K and Tc,
in an external magnetic field up to 9 T. Flux jumps were found in the
temperature range 1.95 K - 6 K, with the external magnetic field parallel to
the c axis of the investigated sample. The effect of sample history on magnetic
flux jumping was studied and it was found to be well accounted for by the
available theoretical models. The magnetic field sweep rate strongly influences
the flux jumping and this effect was interpreted in terms of the influence of
both flux creep and the thermal environment of the sample. Strong flux creep
was found in the temperature and magnetic field range where flux jumps occur
suggesting a relationship between the two. The heat exchange conditions between
the sample and the experimental environment also influence the flux jumping
behavior. Both these effects stabilize the sample against flux instabilities,
and this stabilizing effect increases with decreasing magnetic field sweep
rate. Demagnetizing effects are also shown to have a significant influence on
flux jumping.Comment: 10 pages, 6 figures, RevTeX4, submitted to Phys. Rev.
Palaeoproterozoic magnesite: lithological and isotopic evidence for playa/sabkha environments
Magnesite forms a series of 1- to 15-m-thick beds within the approximate to2.0 Ga (Palaeoproterozoic) Tulomozerskaya Formation, NW Fennoscandian Shield, Russia. Drillcore material together with natural exposures reveal that the 680-m-thick formation is composed of a stromatolite-dolomite-'red bed' sequence formed in a complex combination of shallow-marine and non-marine, evaporitic environments. Dolomite-collapse breccia, stromatolitic and micritic dolostones and sparry allochemical dolostones are the principal rocks hosting the magnesite beds. All dolomite lithologies are marked by delta C-13 values from +7.1 parts per thousand to +11.6 parts per thousand (V-PDB) and delta O-18 ranging from 17.4 parts per thousand to 26.3 parts per thousand (V-SMOW). Magnesite occurs in different forms: finely laminated micritic; stromatolitic magnesite; and structureless micritic, crystalline and coarsely crystalline magnesite. All varieties exhibit anomalously high delta C-13 values ranging from +9.0 parts per thousand to +11.6 parts per thousand and delta O-18 values of 20.0-25.7 parts per thousand. Laminated and structureless micritic magnesite forms as a secondary phase replacing dolomite during early diagenesis, and replaced dolomite before the major phase of burial. Crystalline and coarsely crystalline magnesite replacing micritic magnesite formed late in the diagenetic/metamorphic history. Magnesite apparently precipitated from sea water-derived brine, diluted by meteoric fluids. Magnesitization was accomplished under evaporitic conditions (sabkha to playa lake environment) proposed to be similar to the Coorong or Lake Walyungup coastal playa magnesite. Magnesite and host dolostones formed in evaporative and partly restricted environments; consequently, extremely high delta C-13 values reflect a combined contribution from both global and local carbon reservoirs. A C- 13-rich global carbon reservoir (delta C-13 at around +5 parts per thousand) is related to the perturbation of the carbon cycle at 2.0 Ga, whereas the local enhancement in C-13 (up to +12 parts per thousand) is associated with evaporative and restricted environments with high bioproductivity
Edge magnetoplasmons in periodically modulated structures
We present a microscopic treatment of edge magnetoplasmons (EMP's) within the
random-phase approximation for strong magnetic fields, low temperatures, and
filling factor , when a weak short-period superlattice potential is
imposed along the Hall bar. The modulation potential modifies both the spatial
structure and the dispersion relation of the fundamental EMP and leads to the
appearance of a novel gapless mode of the fundamental EMP. For sufficiently
weak modulation strengths the phase velocity of this novel mode is almost the
same as the group velocity of the edge states but it should be quite smaller
for stronger modulation. We discuss in detail the spatial structure of the
charge density of the renormalized and the novel fundamental EMP's.Comment: 8 pages, 4 figure
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