13,744 research outputs found
Excitonic and Quasiparticle Life Time Effects on Silicon Electron Energy Loss Spectrum from First Principles
The quasiparticle decays due to electron-electron interaction in silicon are
studied by means of first-principles all-electron GW approximation. The
spectral function as well as the dominant relaxation mechanisms giving rise to
the finite life time of quasiparticles are analyzed. It is then shown that
these life times and quasiparticle energies can be used to compute the complex
dielectric function including many-body effects without resorting to empirical
broadening to mimic the decay of excited states. This method is applied for the
computation of the electron energy loss spectrum of silicon. The location and
line shape of the plasmon peak are discussed in detail.Comment: 4 pages, 3 figures, submitted to PR
Multiple scattering of ultrasound in weakly inhomogeneous media: application to human soft tissues
Waves scattered by a weakly inhomogeneous random medium contain a predominant
single scattering contribution as well as a multiple scattering contribution
which is usually neglected, especially for imaging purposes. A method based on
random matrix theory is proposed to separate the single and multiple scattering
contributions. The experimental set up uses an array of sources/receivers
placed in front of the medium. The impulse responses between every couple of
transducers are measured and form a matrix. Single-scattering contributions are
shown to exhibit a deterministic coherence along the antidiagonals of the array
response matrix, whatever the distribution of inhomogeneities. This property is
taken advantage of to discriminate single from multiple-scattered waves. This
allows one to evaluate the absorption losses and the scattering losses
separately, by comparing the multiple scattering intensity with a radiative
transfer model. Moreover, the relative contribution of multiple scattering in
the backscattered wave can be estimated, which serves as a validity test for
the Born approximation. Experimental results are presented with ultrasonic
waves in the MHz range, on a synthetic sample (agar-gelatine gel) as well as on
breast tissues. Interestingly, the multiple scattering contribution is found to
be far from negligible in the breast around 4.3 MHz.Comment: 35 pages, 11 figures, final version, contains the appendix of the
original articl
Anisotropic thermal expansion of bismuth from first principles
Some anisotropy in both mechanical and thermodynamical properties of bismuth
is expected. A combination of density functional theory total energy
calculations and density functional perturbation theory in the local density
approximation is used to compute the elastic constants at 0 K using a finite
strain approach and the thermal expansion tensor in the quasiharmonic
approximation. The overall agreement with experiment is good. Furthermore, the
anisotropy in the thermal expansion is found to arise from the anisotropy in
both the directional compressibilities and the directional Gr\"uneisen
functions.Comment: accepted for publication in PR
Distributed Maximum Likelihood for Simultaneous Self-localization and Tracking in Sensor Networks
We show that the sensor self-localization problem can be cast as a static
parameter estimation problem for Hidden Markov Models and we implement fully
decentralized versions of the Recursive Maximum Likelihood and on-line
Expectation-Maximization algorithms to localize the sensor network
simultaneously with target tracking. For linear Gaussian models, our algorithms
can be implemented exactly using a distributed version of the Kalman filter and
a novel message passing algorithm. The latter allows each node to compute the
local derivatives of the likelihood or the sufficient statistics needed for
Expectation-Maximization. In the non-linear case, a solution based on local
linearization in the spirit of the Extended Kalman Filter is proposed. In
numerical examples we demonstrate that the developed algorithms are able to
learn the localization parameters.Comment: shorter version is about to appear in IEEE Transactions of Signal
Processing; 22 pages, 15 figure
Is MS1054-03 an exceptional cluster? A new investigation of ROSAT/HRI X-ray data
We reanalyzed the ROSAT/HRI observation of MS1054-03, optimizing the channel
HRI selection and including a new exposure of 68 ksec. From a wavelet analysis
of the HRI image we identify the main cluster component and find evidence for
substructure in the west, which might either be a group of galaxies falling
onto the cluster or a foreground source. Our 1-D and 2-D analysis of the data
show that the cluster can be fitted well by a classical betamodel centered only
20arcsec away from the central cD galaxy. The core radius and beta values
derived from the spherical model(beta = 0.96_-0.22^+0.48) and the elliptical
model (beta = 0.73+/-0.18) are consistent. We derived the gas mass and total
mass of the cluster from the betamodel fit and the previously published ASCA
temperature (12.3^{+3.1}_{-2.2} keV). The gas mass fraction at the virial
radius is fgas = (14[-3,+2.5]+/-3)% for Omega_0=1, where the errors in brackets
come from the uncertainty on the temperature and the remaining errors from the
HRI imaging data. The gas mass fraction computed for the best fit ASCA
temperature is significantly lower than found for nearby hot clusters,
fgas=20.1pm 1.6%. This local value can be matched if the actual virial
temperature of MS1054-032 were close to the lower ASCA limit (~10keV) with an
even lower value of 8 keV giving the best agreement. Such a bias between the
virial and measured temperature could be due to the presence of shock waves in
the intracluster medium stemming from recent mergers. Another possibility, that
reconciles a high temperature with the local gas mass fraction, is the
existence of a non zero cosmological constant.Comment: 12 pages, 5 figures, accepted for publication in Ap
Neutron Stars as Dark Matter Probes
We examine whether the accretion of dark matter onto neutron stars could ever
have any visible external effects. Captured dark matter which subsequently
annihilates will heat the neutron stars, although it seems the effect will be
too small to heat close neutron stars at an observable rate whilst those at the
galactic centre are obscured by dust. Non-annihilating dark matter would
accumulate at the centre of the neutron star. In a very dense region of dark
matter such as that which may be found at the centre of the galaxy, a neutron
star might accrete enough to cause it to collapse within a period of time less
than the age of the Universe. We calculate what value of the stable dark
matter-nucleon cross section would cause this to occur for a large range of
masses.Comment: 8 pages, 7 figure
The Role of Calcium in Osteoporosis
Calcium requirements may vary throughout the lifespan. During the growth years and up to age 25 to 30, it is important to maximize dietary intake of calcium to maintain positive calcium balance and achieve peak bone mass, thereby possibly decreasing the risk of fracture when bone is subsequently lost. Calcium intake need not be greater than 800 mg/day during the relatively short period of time between the end of bone building and the onset of bone loss (30 to 40 years). Starting at age 40 to 50, both men and women lose bone slowly, but women lose bone more rapidly around the menopause and for about 10 years after. Intestinal calcium absorption and the ability to adapt to low calcium diets are impaired in many postmenopausal women and elderly persons owing to a suspected functional or absolute decrease in the ability of the kidney to produce 1,25(OH)2D2. The bones then become more and more a source of calcium to maintain critical extracellular fluid calcium levels. Excessive dietary intake of protein and fiber may induce significant negative calcium balance and thus increase dietary calcium requirements. Generally, the strongest risk factors for osteoporosis are uncontrollable (e.g., sex, age, and race) or less controllable (e.g., disease and medications). However, several factors such as diet, physical activity, cigarette smoking, and alcohol use are lifestyle related and can be modified to help reduce the risk of osteoporosis
Characterisation of biodiversity in improved rubber agroforests in West-Kalimantan, Indonesia. Real and Potential uses for spontaneous plants
Since the introduction of rubber at the turn of the 20th century smallholders have developed an original complex agroforestry system called jungle rubber in which non selected young rubber trees (seedlings) are managed extensively alongside secondary forest re-growth. The issue of improving smallholder rubber productivity at affordable capital investments and levels of inputs while maintaining the environmental benefits of jungle rubber has been addressed by the Smallholder Rubber Agroforestry Project (SRAP: a joint project run by ICRAF, GAPKINDO and CIRAD). In 1995-1996, 27 trials (with a total of 100 plots) were set up in three provinces in Indonesia to assess the possibility of associating clonal rubber with agroforestry practices under smallholder conditions (Penot, 1997). Two RAS types were selected for this study: RAS n° 1 and n° 3. RAS n° 1 is basically improved jungle rubber using clonal planting material (see a description of RAS types in annexe 1). The rubber trees are in competition with spontaneous vegetation in the inter-row but results show that there are no negative consequences for rubber growth during the immature period. RAS n° 3 was designed for areas infested by Imperata cylindrica, with the establishment of shrubby leguminous cover crops and fast-growing tree species in the inter-rows with the aim of shading out weeds. The other type, RAS n° 2, is based on intercropping clonal rubber with various annual and perennial crops, including fruit and timber trees (Penot et al, 1994). In all cases, RAS have a planting density of 550 clonal rubber trees/ha and a variable number of associated fruit, timber or fast growing shade trees (from 92 to 256/ha). In addition to the RAS experimental plots, “RAS sendiri” (or “endogenous RAS”) are rubber agroforests improved by farmers without outside assistance. The district of Sanggau in the province of West Kalimantan was identified by SRAP as representative of traditional jungle-rubber-based local farming systems that have developed over the last 90 years. The district of Sanggau is located in the central area of the Kapuas river basin, between 1° N and 0°6' S and 09°8' W and 11°33' E. The district covers 18 302 km2, i.e. 13 % of the province. The trial plots described in this study are located in the villages of Embaong, Engkayu, Kopar, and Trimulia (the last being in the transmigration area). Most soils in the province of West-Kalimantan are acrisoils associated with ferralitic soils. Such soils have relatively good physical characteristics but poor chemical value and become acid. Rubber is widely grown in this area as it can grow in poor soils. The landscape is dominated by logged-over forest, secondary forest and a mosaic of jungle rubber and fallow with secondary forest re-growth. Large scale logging activities took place from 1950s to the 1980s at the expense of primary forest. At present, forested areas are located in hilly or remote areas and are very limited in extent. Oil palm and Acacia mangium plantations developed exponentially in the 1990s increasing the conversion of degraded forest areas into Estates that cultivate perennial crops. The main objective of this study is to assess existing plant biodiversity in RAS systems compared to that of jungle rubber. The second objective is to review the current uses of certain plants and their market potential.
- …