20,741 research outputs found
Endothelial cell density in relation to morphology
Corneal endothelium of 121 normal corneas was studied with the clinical specular microscope, and the relationship between cell density, cell morphology, and age was examined. Our observations indicate a decrease in cell density with age in homomegethous endothelium but no such correlation in a polymegethous endothelium
Communications and information research: Improved space link performance via concatenated forward error correction coding
With the development of new advanced instruments for remote sensing applications, sensor data will be generated at a rate that not only requires increased onboard processing and storage capability, but imposes demands on the space to ground communication link and ground data management-communication system. Data compression and error control codes provide viable means to alleviate these demands. Two types of data compression have been studied by many researchers in the area of information theory: a lossless technique that guarantees full reconstruction of the data, and a lossy technique which generally gives higher data compaction ratio but incurs some distortion in the reconstructed data. To satisfy the many science disciplines which NASA supports, lossless data compression becomes a primary focus for the technology development. While transmitting the data obtained by any lossless data compression, it is very important to use some error-control code. For a long time, convolutional codes have been widely used in satellite telecommunications. To more efficiently transform the data obtained by the Rice algorithm, it is required to meet the a posteriori probability (APP) for each decoded bit. A relevant algorithm for this purpose has been proposed which minimizes the bit error probability in the decoding linear block and convolutional codes and meets the APP for each decoded bit. However, recent results on iterative decoding of 'Turbo codes', turn conventional wisdom on its head and suggest fundamentally new techniques. During the past several months of this research, the following approaches have been developed: (1) a new lossless data compression algorithm, which is much better than the extended Rice algorithm for various types of sensor data, (2) a new approach to determine the generalized Hamming weights of the algebraic-geometric codes defined by a large class of curves in high-dimensional spaces, (3) some efficient improved geometric Goppa codes for disk memory systems and high-speed mass memory systems, and (4) a tree based approach for data compression using dynamic programming
Pressure effects on the magnetic structure in La0.5Ca0.5-xSrxMnO3 (0.1 -< x -< 0.4) manganites
The effect of high pressure (0 - 8 GPa) on the magnetic structure of
polycrystalline samples of La0.5Ca0.5-xSrxMnO3 (0.1 -< x -< 0.4) manganites at
5 K is investigated using neutron diffraction technique. Application of
pressure is found to modify the previously reported magnetic structure,
observed under ambient conditions, in these compounds [I. Dhiman et al., Phys.
Rev. B 77, 094440 (2008)]. In x = 0.1 composition, at 4.6(2) GPa and beyond,
A-type antiferromagnetic structure is found to coexist with CE-type
antiferromagnetic phase, observed at ambient pressure, with TN ~ 150 K. For x =
0.3 sample, as a function of pressure the CE-type phase is fully suppressed at
2.3(1) GPa and A-type antiferromagnetic phase is favored. Further Sr doping at
x = 0.4, the A-type antiferromagnetic phase is observed at ambient pressure and
for T < TN (~ 250K). This phase is retained in the studied pressure range.
However, the magnetic moment progressively reduces with increasing pressure,
indicating the suppression of A-type antiferromagnetic phase. The present study
brings out the fragile nature of the CE-type antiferromagnetic state in half
doped manganites as a function of pressure and disorder \sigma 2. We observe
that pressure required for destabilizing the CE-type antiferromagnetic state is
reduced with increasing disorder \sigma 2. External pressure and changing
A-site ionic radii have analogous effect on the magnetic structure.Comment: 9 pages, 6 figures, 1 table, To appear in Physical Review
The evolutionary status of the semiregular variable QYSge
Repeated spectroscopic observations made with the 6m telescope of yielded new
data on the radial-velocity variability of the anomalous yellow supergiant
QYSge. The strongest and most peculiar feature in its spectrum is the complex
profile of NaI D lines, which contains a narrow and a very wide emission
components. The wide emission component can be seen to extend from -170 to +120
km/s, and at its central part it is cut by an absorption feature, which, in
turn, is split into two subcomponents by a narrow (16km/s at r=2.5) emission
peak. An analysis of all the Vr values leads us to adopt for the star a
systemic velocity of Vr=-21.1 km/s, which corresponds to the position of the
narrow emission component of NaI. The locations of emission-line features of
NaI D lines are invariable, which point to their formation in regions that are
external to the supergiant's photosphere. Differential line shifts of about
10km/s are revealed. The absorption lines in the spectrum of QYSge have a
substantial width of FWHM~45 km/s. The method of model atmospheres is used to
determine the following parameters: Teff=6250K, lg g=2.0, and microturbulence
Vt=4.5km/s. The metallicity of the star is found to be somewhat higher than the
solar one with an average overabundance of iron-peak elements of [Met/H]=+0.20.
The star is found to be slightly overabundant in carbon and nitrogen,
[C/Fe]=+0.25, [N/Fe]=+0.27. The alpha-process elements Mg, Si, and Ca are
slightly overabundant [alpha/H]=+0.12. The strong sodium excess, [Na/Fe]=+0.75,
is likely to be due to the dredge-up of the matter processed in the NeNa cycle.
Heavy elements of the s-process are underabundant relative to the Sun. On the
whole, the observed properties of QYSge do not give grounds for including this
star into the group of RCrB or RVTau-type type objects.Comment: 29 pages, 8 figures, 4 tables; accepted by Astrophys. Bulleti
Electronic structure and chemical bonding of 3d-metal dimers ScX, X=Sc-Zn
The electronic and geometrical structures of the ground and excited states of the homonuclear Sc2, mixed ScTi, ScV, ScCr, ScMn, ScFe, ScCo, ScNi, ScCu, and ScZn 3d-metal dimers and their anions have been calculated using the density functional theory with generalized gradient approximation for the exchange-correlation potential. The ground states of the neutral dimers are found to be 5Σ−u (Sc2), 6Σ+ (ScTi), 7Σ+ (ScV), 4Σ+ (ScCr), 3Σ+ (ScMn), 2Δ(ScFe), 1Σ+ (ScCo), 2Σ+ (ScNi), 3Δ(ScCu), and 4Σ+ (ScZn). A natural bond analysis reveals an antiferrimagnetic spin coupling in the ground states of ScCr, ScMn, and ScFe. This is due to the electron transfer from Sc to the opposite atom and specific bond formations. While each dimer has a unique chemical bonding pattern, most curious is the localization of two 4s electrons at both atomic sites in the ground 5Σ−u state of Sc2, which leads to formation of two lone pairs and the bonding scheme: (3d+3d)3α(4s+4s)1β. No appreciable sd hybridization is found for the ground states of the ScX dimers except for ScNi. Even though the electron affinities of the ScX dimers are relatively low and do not exceed 1 eV, each ScX− (except ScCo−) possesses at least two states stable towards detachment of an extra electron
Tuning Fermi-surface properties through quantum confinement in metallic meta-lattices: New metals from old atoms
We describe a new class of nanoscale structured metals wherein the effects of
quantum confinement are combined with dispersive metallic electronic states to
induce modifications to the fundamental low-energy microscopic properties of a
three-dimensional metal: the density of states, the distribution of Fermi
velocities, and the collective electronic response.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let
Phonon dispersion and electron-phonon interaction in peanut-shaped fullerene polymers
We reveal that the periodic radius modulation peculiar to one-dimensional
(1D) peanut-shaped fullerene (C) polymers exerts a strong influence on
their low-frequency phonon states and their interactions with mobile electrons.
The continuum approximation is employed to show the zone-folding of phonon
dispersion curves, which leads to fast relaxation of a radial breathing mode in
the 1D C polymers. We also formulate the electron-phonon interaction
along the deformation potential theory, demonstrating that only a few set of
electron and phonon modes yields a significant magnitude of the interaction
relevant to the low-temperature physics of the system. The latter finding gives
an important implication for the possible Peierls instability of the C
polymers suggested in the earlier experiment.Comment: 9 pages, 8 figure
Nonlinear Interactions Between Gravitational Radiation and Modified Alfven Modes in Astrophysical Dusty Plasmas
We present an investigation of nonlinear interactions between Gravitational
Radiation and modified Alfv\'{e}n modes in astrophysical dusty plasmas.
Assuming that stationary charged dust grains form neutralizing background in an
electron-ion-dust plasma, we obtain the three wave coupling coefficients, and
calculate the growth rates for parametrically coupled gravitational radiation
and modified Alfv\'{e}n-Rao modes. The threshold value of the gravitational
wave amplitude associated with convective stabilization is particularly small
if the gravitational frequency is close to twice the modified Alfv\'en
wave-frequency. The implication of our results to astrophysical dusty plasmas
is discussed.Comment: A few typos corrected. Published in Phys. Rev.
- …