56,516 research outputs found
Segmentation and texture analysis with multimodel inference for the automatic detection of exudates in early diabetic retinopathy
published_or_final_versio
Ka-band MMIC microstrip array for high rate communications
In a recent technology assessment of alternative communication systems for the space exploration initiative (SEI), Ka-band (18 to 40 GHz) communication technology was identified to meet the mission requirements of telecommunication, navigation, and information management. Compared to the lower frequency bands, Ka-band antennas offer higher gain and broader bandwidths; thus, they are more suitable for high data rate communications. Over the years, NASA has played an important role in monolithic microwave integrated circuit (MMIC) phased array technology development, and currently, has an ongoing contract with Texas Instrument (TI) to develop a modular Ka-band MMIC microstrip subarray (NAS3-25718). The TI contract emphasizes MMIC integration technology development and stipulates using existing MMIC devices to minimize the array development cost. The objective of this paper is to present array component technologies and integration techniques used to construct the subarray modules
Artificial molecular quantum rings: Spin density functional theory calculations
The ground states of artificial molecules made of two vertically coupled
quantum rings are studied within the spin density functional theory for systems
containing up to 13 electrons. Quantum tunneling effects on the electronic
structure of the coupled rings are analyzed. For small ring radius, our results
recover those of coupled quantum dots. For intermediate and large ring radius,
new phases are found showing the formation of new diatomic artificial ring
molecules. Our results also show that the tunneling induced phase transitions
in the coupled rings occur at much smaller tunneling energy as compared to
those for coupled quantum dot systems.Comment: 10 pages, 6 figure
Neural locus of color afterimages.
After fixating on a colored pattern, observers see a similar pattern in complementary colors when the stimulus is removed. Afterimages were important in disproving the theory that visual rays emanate from the eye[1], in demonstrating inter-ocular interactions[2], and in revealing the independence of binocular-vision from eye-movements[3]. Afterimages also prove invaluable in exploring selective attention[4], filling-in[5], and consciousness[6]. Proposed physiological mechanisms for color afterimages range from bleaching of cone photo-pigments[7] to cortical adaptation[4–6, 8, 9], but direct neural measurements have not been reported. We introduce a time-varying method for evoking after-images, which provides precise measurements of adaptation and a direct link between visual percepts and neural responses[10]. We then use in vivo electrophysiological recordings to show that all three classes of primate retinal ganglion cells exhibit subtractive adaptation to prolonged stimuli, with much slower time-constants than those expected of photoreceptors. At the cessation of the stimulus, ganglion cells generate rebound responses that can provide afterimage signals for later neurons. Our results indicate that afterimage signals are generated in the retina, but may be modified like other retinal signals by cortical processes[4–6], so that evidence presented for cortical generation of color afterimages[8, 9] is explainable by spatio-temporal factors that apply to all signals
Probing the subshell closure: factor of the Mg(2) state
The first-excited state ~factor of Mg has been measured relative to
the factor of the Mg() state using the high-velocity
transient-field technique, giving . This new measurement is in
strong disagreement with the currently adopted value, but in agreement with the
-shell model using the USDB interaction. The newly measured factor,
along with and systematics, signal the closure of the subshell at . The possibility that precise -factor
measurements may indicate the onset of neutron admixtures in first-excited
state even-even magnesium isotopes below Mg is discussed and the
importance of precise excited-state -factor measurements on ~shell
nuclei with to test shell-model wavefunctions is noted.Comment: 8 pages, 5 figure
High pressure floating zone growth and structural properties of ferrimagnetic quantum paraelectric BaFeO
High quality single crystals of BaFeO were grown using the
floating zone technique in flowing oxygen pressurized to 100 atm. Single
crystal neutron diffraction was used to determine the nuclear and magnetic
structure of BaFeO at 4 K and 295 K. At both temperatures, there
exist local electric dipoles formed by the off-mirror-plane displacements of
magnetic Fe ions at the bipyramidal sites. The displacement at 4 K is
about half of that at room temperature. The temperature dependence of the
specific heat shows no anomaly associated with long range polar ordering in the
temperature range from 1.90-300 K. The inverse dielectric permittivity,
, along the c-axis shows a temperature dependence between
10 K and 20 K, with a significantly reduced temperature dependence displayed
below 10 K. Moreover, as the sample is cooled below 1.4 K there is an anomalous
sharp upturn in . These features resemble those of classic
quantum paraelectrics such as SrTiO. The presence of the upturn in
indicates that BaFeO is a critical quantum
paraelectric system with Fe ions involved in both magnetic and electric
dipole formation.Comment: 9 pages, 9 figures, submitted to APL Material
Spatially embedded random networks
Many real-world networks analyzed in modern network theory have a natural spatial element; e.g., the Internet, social networks, neural networks, etc. Yet, aside from a comparatively small number of somewhat specialized and domain-specific studies, the spatial element is mostly ignored and, in particular, its relation to network structure disregarded. In this paper we introduce a model framework to analyze the mediation of network structure by spatial embedding; specifically, we model connectivity as dependent on the distance between network nodes. Our spatially embedded random networks construction is not primarily intended as an accurate model of any specific class of real-world networks, but rather to gain intuition for the effects of spatial embedding on network structure; nevertheless we are able to demonstrate, in a quite general setting, some constraints of spatial embedding on connectivity such as the effects of spatial symmetry, conditions for scale free degree distributions and the existence of small-world spatial networks. We also derive some standard structural statistics for spatially embedded networks and illustrate the application of our model framework with concrete examples
Superstring-Inspired E_6 Unification, Shadow Theta-Particles and Cosmology
We construct a new cosmological model considering the superstring-inspired
E_6 unification in the 4-dimensional space at the early stage of the Universe.
We develop a concept of parallel existence in Nature of the ordinary and shadow
worlds with different cosmological evolutions.Comment: 7 page
Modified embedded-atom method interatomic potentials for the Mg-Al alloy system
We developed new modified embedded-atom method (MEAM) interatomic potentials
for the Mg-Al alloy system using a first-principles method based on density
functional theory (DFT). The materials parameters, such as the cohesive energy,
equilibrium atomic volume, and bulk modulus, were used to determine the MEAM
parameters. Face-centered cubic, hexagonal close packed, and cubic rock salt
structures were used as the reference structures for Al, Mg, and MgAl,
respectively. The applicability of the new MEAM potentials to atomistic
simulations for investigating Mg-Al alloys was demonstrated by performing
simulations on Mg and Al atoms in a variety of geometries. The new MEAM
potentials were used to calculate the adsorption energies of Al and Mg atoms on
Al (111) and Mg (0001) surfaces. The formation energies and geometries of
various point defects, such as vacancies, interstitial defects and
substitutional defects, were also calculated. We found that the new MEAM
potentials give a better overall agreement with DFT calculations and
experiments when compared against the previously published MEAM potentials.Comment: Fixed a referenc
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