621 research outputs found
A two stage vector quantization approach via self-organizing map
In this paper, a two-stage algorithm for vector quantization is proposed based on a self-organizing map (SOM) neural network. First, a conventional self-organizing map is modified to deal with dead codebooks in the learning process and is then used to obtain the codebook distribution structure for a given set of input data. Next, subblocks are classified based on the previous structure distribution with a prior criteria. Then, the conventional LBG algorithm is applied to these sub-blocks for data classification with initial values obtained via the SOM. Finally, extensive simulations illustrate that the proposed two-stage algorithm is very effective.<br /
Electronic structure interpolation via atomic orbitals
We present an efficient scheme for accurate electronic structure
interpolations based on the systematically improvable optimized atomic
orbitals. The atomic orbitals are generated by minimizing the spillage value
between the atomic basis calculations and the converged plane wave basis
calculations on some coarse -point grid. They are then used to calculate the
band structure of the full Brillouin zone using the linear combination of
atomic orbitals (LCAO) algorithms. We find that usually 16 -- 25 orbitals per
atom can give an accuracy of about 10 meV compared to the full {\it ab initio}
calculations. The current scheme has several advantages over the existing
interpolation schemes. The scheme is easy to implement and robust which works
equally well for metallic systems and systems with complex band structures.
Furthermore, the atomic orbitals have much better transferability than the
Shirley's basis and Wannier functions, which is very useful for the
perturbation calculations
Ab initio study of the formation of transparent carbon under pressure
A body-centered tetragonal carbon (bct-Carbon) allotrope has been predicted
to be a transparent carbon polymorph obtained under pressure. The structural
transition pathways from graphite to diamond, M-Carbon, and bct-Carbon are
simulated and the lowest activation barrier is found for the graphite-bct
transition. Furthermore, bct-Carbon has higher shear strength than diamond due
to its perpendicular graphene-like structure. Our results provide a possible
explanation for the formation of a transparent carbon allotrope via the cold
compression of graphite. We also verify that this allotrope is hard enough to
crack diamond.Comment: [email protected] or [email protected]
Stable isotope compositions (δ2H, δ18O and δ17O) of rainfall and snowfall in the central United States
Stable isotopes of hydrogen and oxygen (δ2H, δ18O and δ17O) can be used as natural tracers to improve our understanding of hydrological and meteorological processes. Studies of precipitation isotopes, especially 17O-excess observations, are extremely limited in the mid-latitudes. To fill this knowledge gap, we measured δ2H, δ18O and δ17O of event-based precipitation samples collected from Indianapolis, Indiana, USA over two years and investigated the influence of meteorological factors on precipitation isotope variations. The results showed that the daily temperature played a major role in controlling the isotope variations. Precipitation experienced kinetic fractionation associated with evaporation at the moisture source in the spring and summer and for rainfall, while snowfall, as well as precipitation in the fall and winter, were mainly affected by equilibrium fractionation. The 17O-excess of both rainfall and snowfall were not affected by local meteorological factors over the whole study period. At the seasonal scale, it was the case only for the spring. Therefore, 17O-excess of rainfall, snowfall and the spring precipitation could be considered as tracers of evaporative conditions at the moisture source. This study provides a unique precipitation isotope dataset for mid-latitudes and provides a more mechanistic understanding of precipitation formation mechanisms in this region
CMB Temperature and Matter Power Spectrum in a Decay Vacuum Dark Energy Model
In this paper, a decay vacuum model
is revisited by detailed analysis of background evolution and perturbation
equations. We show the imprints on CMB temperature and matter power spectrum
from the effective coupling terms between dark sectors by comparing to the
standard cosmological constant model and observational data points (WMAP7 and
SDSS DR7). We find that the decay vacuum model can describe the expansion rate
at late times as well as the standard cosmological constant model but it fails
to simultaneously reproduce the observed CMB and matter power spectrum. Its
generalization is also
discussed. Detailed analysis of the background evolution shows that the
dimensionless parameter would be zero to avoid the unnatural 'fine
tuning' and to keep the positivity of energy density of dark matter and dark
energy in the early epoch
Scattering of scalar perturbations with cosmological constant in low-energy and high-energy regimes
We study the absorption and scattering of massless scalar waves propagating
in spherically symmetric spacetimes with dynamical cosmological constant both
in low-energy and high-energy zones. In the former low-energy regime, we solve
analytically the Regge-Wheeler wave equation and obtain an analytic absorption
probability expression which varies with , where is the
central mass and is cosmological constant. The low-energy absorption
probability, which is in the range of , increases monotonically
with increase in . In the latter high-energy regime, the scalar
particles adopt their geometric optics limit value. The trajectory equation
with effective potential emerges and the analytic high-energy greybody factor,
which is relevant with the area of classically accessible regime, also
increases monotonically with increase in , as long is less
than or of the order of . In this high-energy case, the null cosmological
constant result reduces to the Schwarzschild value .Comment: 12 pages, 6 figure
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