6,165 research outputs found
On the use of Neumann's principle for the calculation of the polarizability tensor of nanostructures
The polarizability measures how the system responds to an applied electrical
field. Computationally, there are many different ways to evaluate this
tensorial quantity, some of which rely on the explicit use of the external
perturbation and require several individual calculations to obtain the full
tensor. In this work, we present some considerations about symmetry that allow
us to take full advantage of Neumann's principle and decrease the number of
calculations required by these methods. We illustrate the approach with two
examples, the use of the symmetries in real space and in spin space in the
calculation of the electrical or the spin response.Comment: 7 pages, 5 figures, accepted for publication in the Journal of
Nanoscience and Nanotechnolog
Jamming during the discharge of granular matter from a silo
In this work we present an experimental study of the jamming that stops the
free flow of grains from a silo discharging by gravity. When the outlet size is
not much bigger than the beads, granular material jams the outlet of the
container due to the formation of an arch. Statistical data from the number of
grains fallen between consecutive jams are presented. The information that they
provide can help to understand the jamming phenomenon. As the ratio between the
size of the orifice and the size of the beads is increased, the probability
that an arch blocks the outlet decreases. We show here that there is a power
law divergence of the mean avalanche size for a finite critical radius. Beyond
this critical radius no jamming can occur and the flow is never stopped. The
dependence of the arch formation on the shape and the material of the grains
has been explored. It has been found that the material properties of the grains
do not affect the arch formation probability. On the contrary, the shape of the
grains deeply influences it. A simple model to interpret the results is also
discussed.Comment: Submitted to Phys. Rev.
Elasticity of plagioclase feldspars
AbstractElastic properties are reported for eight plagioclase feldspars that span compositions from albite (NaSi3AlO8) to anorthite (CaSi2Al2O8). Surface acoustic wave velocities measured using Impulsive Stimulated Light Scattering and compliance sums from highâpressure Xâray compression studies accurately determine all 21 components of the elasticity tensor for these triclinic minerals. The overall pattern of elasticity and the changes in individual elastic components with composition can be rationalized on the basis of the evolution of crystal structures and chemistry across this solidâsolution join. All plagioclase feldspars have high elastic anisotropy; a* (the direction perpendicular to the b and c axes) is the softest direction by a factor of 3 in albite. From albite to anorthite the stiffness of this direction undergoes the greatest change, increasing twofold. Small discontinuities in the elastic components, inferred to occur between the three plagioclase phases with distinct symmetry (
,
, and
), appear consistent with the nature of the underlying conformation of the frameworkâlinked tetrahedra and the associated structural changes. Measured body wave velocities of plagioclaseârich rocks, reported over the last five decades, are consistent with calculated Hillâaveraged velocities using the current moduli. This confirms longâstanding speculation that previously reported elastic moduli for plagioclase feldspars are systematically in error. The current results provide greater assurance that the seismic structure of the middle and lower crusts can be accurately estimated on the basis of specified mineral modes, chemistry, and fabric
Peso Acceptance Patterns in El Paso
This paper examines the acceptance of peso payments, or currency substitution reverse dollarization, by U.S retail firms near the international border with Mexico. Survey data are drawn from a stratified random sample of 586 retailers located in El Paso, Texas, situated across the border from Ciudad Juarez, Chihuahua, Mexico. Approximately 13 percent of the participant firms accept Mexican pesos in exchange for goods and services. Empirical results indicate that factors such as a firmâs percentage of Spanish speaking employees and distance to the nearest international bridge significantly influence the decision to accept or reject Mexican pesos.Currency Substitution; Mexican Peso; Border Economics; Probit Models
A test for the search for life on extrasolar planets: Looking for the terrestrial vegetation signature in the Earthshine spectrum
We report spectroscopic observations (400 to 800nm, R = approx 100) of
Earthshine in June, July and October 2001 from which normalised Earth albedo
spectra have been derived. The resulting spectra clearly show the blue colour
of the Earth due to Rayleigh diffusion in its atmosphere. They also show the
signatures of oxygen, ozone and water vapour. We tried to extract from these
spectra the signature of Earth vegetation. A variable signal (4 to 10 +/-3%)
around 700nm has been measured in the Earth albedo. It is interpreted as being
due to the vegetation red edge, expected to be between 2 to 10% of the Earth
albedo at 700nm, depending on models. We discuss the primary goal of the
present observations: their application to the detection of vegetation-like
biosignatures on extrasolar planets.Comment: 7 pages, 7 figures. A&A, accepted 6 May 200
Development of the preterm infant gut microbiome: a research priority.
The very low birth weight (VLBW) infant is at great risk for marked dysbiosis of the gut microbiome due to multiple factors, including physiological immaturity and prenatal/postnatal influences that disrupt the development of a normal gut flora. However, little is known about the developmental succession of the microbiota in preterm infants as they grow and mature. This review provides a synthesis of our understanding of the normal development of the infant gut microbiome and contrasts this with dysbiotic development in the VLBW infant. The role of human milk in normal gut microbial development is emphasized, along with the role of the gut microbiome in immune development and gastroenteric health. Current research provides evidence that the gut microbiome interacts extensively with many physiological systems and metabolic processes in the developing infant. However, to the best of our knowledge, there are currently no studies prospectively mapping the gut microbiome of VLBW infants through early childhood. This knowledge gap must be filled to inform a healthcare system that can provide for the growth, health, and development of VLBW infants. The paper concludes with speculation about how the VLBW infants' gut microbiome might function through host-microbe interactions to contribute to the sequelae of preterm birth, including its influence on growth, development, and general health of the infant host
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