1,179 research outputs found
Breaking arches with vibrations: the role of defects
We present experimental results about the stability of arches against
external vibrations. Two dimensional strings of mutually stabilizing grains are
geometrically analyzed and subsequently submitted to a periodic forcing at
fixed frequency and increasing amplitude. The main factor that determines the
granular arch resistance against vibrations is the maximum angle among those
formed between any particle of the arch and its two neighbors: the higher the
maximum angle is, the easier to break the arch. Based in an analysis of the
forces, a simple explanation is given for this dependence. From this,
interesting information can be extracted about the expected magnitudes of
normal forces and friction coefficients of the particles conforming the arches
Resonant Metalenses for Breaking the Diffraction Barrier
We introduce the resonant metalens, a cluster of coupled subwavelength
resonators. Dispersion allows the conversion of subwavelength wavefields into
temporal signatures while the Purcell effect permits an efficient radiation of
this information in the far-field. The study of an array of resonant wires
using microwaves provides a physical understanding of the underlying mechanism.
We experimentally demonstrate imaging and focusing from the far-field with
resolutions far below the diffraction limit. This concept is realizable at any
frequency where subwavelength resonators can be designed.Comment: 4 pages, 3 figure
High efficiency GaAs-Ge tandem solar cells grown by MOCVD
High conversion efficiency and low weight are obviously desirable for solar cells intended for space applications. One promising structure is GaAs on Ge. The advantages of using Ge wafers as substrates include the following: they offer high efficiency by forming a two-junction tandem cell; low weight combined with superior strength allows usage of thin (3 mil) wafers; and they are a good substrate for GaAs, being lattice matched, thermal expansion matched, and available as large-area wafers
The Distribution of High Redshift Galaxy Colors: Line of Sight Variations in Neutral Hydrogen Absorption
We model, via Monte Carlo simulations, the distribution of observed U-B, B-V,
V-I galaxy colors in the range 1.75<z<5 caused by variations in the
line-of-sight opacity due to neutral hydrogen (HI). We also include HI internal
to the source galaxies. Even without internal HI absorption, comparison of the
distribution of simulated colors to the analytic approximations of Madau (1995)
and Madau et al (1996) reveals systematically different mean colors and
scatter. Differences arise in part because we use more realistic distributions
of column densities and Doppler parameters. However, there are also
mathematical problems of applying mean and standard deviation opacities, and
such application yields unphysical results. These problems are corrected using
our Monte Carlo approach. Including HI absorption internal to the galaxies
generaly diminishes the scatter in the observed colors at a given redshift, but
for redshifts of interest this diminution only occurs in the colors using the
bluest band-pass. Internal column densities < 10^17 cm^2 do not effect the
observed colors, while column densities > 10^18 cm^2 yield a limiting
distribution of high redshift galaxy colors. As one application of our
analysis, we consider the sample completeness as a function of redshift for a
single spectral energy distribution (SED) given the multi-color selection
boundaries for the Hubble Deep Field proposed by Madau et al (1996). We argue
that the only correct procedure for estimating the z>3 galaxy luminosity
function from color-selected samples is to measure the (observed) distribution
of redshifts and intrinsic SED types, and then consider the variation in color
for each SED and redshift. A similar argument applies to the estimation of the
luminosity function of color-selected, high redshift QSOs.Comment: accepted for publication in ApJ; 25 pages text, 14 embedded figure
Sr3CrN3: a new electride with partially filled d-shells
Electrides are ionic crystals in which the electrons prefer to occupy free
space, serving as anions. Because the electrons prefer to be in the pockets,
channels, or layers to the atomic orbitals around the nuclei, it has been
challenging to find electrides with partially filled d-shells, since an
unoccupied d-shell provides an energetically favourable location for the
electrons to occupy. We recently predicted the existence of electrides with
partially filled d-shells using high-throughput computational screening. Here,
we provide an experimental support using X-ray absorption spectroscopy and
X-ray and neutron diffraction to show that Sr3CrN3 is indeed an electride
despite its partial d-shell configuration. Our findings indicate that Sr3CrN3
is the first known electride with a partially filled d-shell, in agreement with
theory, which significantly broadens the criteria for the search for new
electride materials
Height variables in the Abelian sandpile model: scaling fields and correlations
We compute the lattice 1-site probabilities, on the upper half-plane, of the
four height variables in the two-dimensional Abelian sandpile model. We find
their exact scaling form when the insertion point is far from the boundary, and
when the boundary is either open or closed. Comparing with the predictions of a
logarithmic conformal theory with central charge c=-2, we find a full
compatibility with the following field assignments: the heights 2, 3 and 4
behave like (an unusual realization of) the logarithmic partner of a primary
field with scaling dimension 2, the primary field itself being associated with
the height 1 variable. Finite size corrections are also computed and
successfully compared with numerical simulations. Relying on these field
assignments, we formulate a conjecture for the scaling form of the lattice
2-point correlations of the height variables on the plane, which remain as yet
unknown. The way conformal invariance is realized in this system points to a
local field theory with c=-2 which is different from the triplet theory.Comment: 68 pages, 17 figures; v2: published version (minor corrections, one
comment added
Prévalence des lombalgies et de leurs facteurs de risque professionnels chez les travailleurs de l'agriculture en France en 2010
Introduction
La phase pilote du programme Coset-MSA a permis d’évaluer la prévalence des lombalgies et de leurs facteurs de risque professionnels chez les travailleurs de l’agriculture, et notamment les agriculteurs exploitants et les ouvriers agricoles.
Méthodes
Les travailleurs ont été recrutés, dans cinq départements, par tirage au sort dans les bases de données de la MSA et invités à remplir un auto-questionnaire portant sur les expositions et les lombalgies au cours des 12 derniers mois. L’échantillon d’analyse était constitué de 1130 travailleurs, âgés de 18 à 65 ans, exerçant dans le secteur agriculture, sylviculture et pêche, et en activité au moment de l’enquête.
Résultats
La prévalence des lombalgies au cours des 12 derniers mois était élevée, chez les hommes (65 %) comme chez les femmes (64 %). Chez ces dernières, la prévalence diminuait significativement avec l’âge (de 95 % chez les moins de 30 ans à 56 % chez les 50 ans et plus, p < 0,01). Des douleurs quotidiennes étaient rapportées par 10 % des hommes et 11 % des femmes, avec une prévalence qui augmentait avec l’âge, de façon significative chez les hommes (p < 0,001). La prévalence ne différait pas significativement selon la catégorie socioprofessionnelle (CSP). Cependant, soulignons la forte prévalence des lombalgies parmi les hommes artisans, commerçants et chefs d’entreprise (surtout des jardiniers, paysagistes…) malgré des effectifs faibles. Chez les femmes, les agricultrices exploitantes et les ouvrières agricoles étaient plus nombreuses à souffrir de lombalgies, quelle que soit la durée des symptômes.
Le port de charges de plus de 25 kg plus de 2 heures par jour concernait 16 % des hommes et 5 % des femmes, se pencher en avant ou sur le côté plus de 4 heures par jour 15 % et 16 % respectivement et la conduite plus de 4 heures par jour 27 % et 10 %. Chez les hommes, les ouvriers non agricoles étaient les plus exposés aux trois facteurs (17 % au port de charges [p < 0,05], 22 % au fait de se pencher [p < 0,01], 32 % à la conduite [NS]). Chez les femmes, les ouvrières agricoles rapportaient les plus fortes expositions : 29 % se penchaient plus de 4 h/j (p < 0,001) et 16 % conduisaient plus de 4 h/j (NS).
Conclusions
La prévalence des lombalgies est particulièrement élevée dans l’agriculture, comparativement à des études portant sur tous secteurs d’activité (par exemple, l’étude Cosali menée auprès de salariés des Pays de la Loire). De plus, cette étude permet d’identifier les CSP les plus exposées aux facteurs de risque professionnels de lombalgie, qui devraient être ciblées prioritairement par les actions de prévention
Pre-logarithmic and logarithmic fields in a sandpile model
We consider the unoriented two-dimensional Abelian sandpile model on the
half-plane with open and closed boundary conditions, and relate it to the
boundary logarithmic conformal field theory with central charge c=-2. Building
on previous results, we first perform a complementary lattice analysis of the
operator effecting the change of boundary condition between open and closed,
which confirms that this operator is a weight -1/8 boundary primary field,
whose fusion agrees with lattice calculations. We then consider the operators
corresponding to the unit height variable and to a mass insertion at an
isolated site of the upper half plane and compute their one-point functions in
presence of a boundary containing the two kinds of boundary conditions. We show
that the scaling limit of the mass insertion operator is a weight zero
logarithmic field.Comment: 18 pages, 9 figures. v2: minor corrections + added appendi
Prevalence of Bartonella henselae in blood donors and risk of blood transmission in Chile
Indexación: Scopus; Scielo.Bartonella henselae es el agente causal de la enfermedad del arañazo del gato en personas inmunocompetentes y de la angiomatosis bacilar y peliosis hepatis en inmunocomprometidos. En Chile la prevalencia de anticuerpos contra B. henselae en niños y adolescentes sanos es de 13,3%, en personas con riesgo ocupacional 60,5% y en gatos 85,6%. No existen datos publicados respecto de la seroprevalencia en donantes de sangre en nuestro paÃs, por lo que determinar si B. henselae se encuentra presente en la sangre de los donantes al momento de la donación es muy importante, ya que este microorganismo puede sobrevivir hasta 35 dÃas en los eritrocitos almacenados en banco de sangre a 4 °C.
Objetivo:
Determinar la prevalencia de B. henselae en donantes de sangre.
MetodologÃa:
Se analizaron 140 muestras de sangre de donantes, para detectar la presencia de B. henselae, utilizando la técnica de la reacción de polimerasa en cadena (RPC).
Resultados:
Se obtuvo 13,6% de los donantes de sangre con RPC positiva para la B. henselae. La secuencia de los fragmentos amplificados presentó una identidad por sobre 98% con respecto a secuencias de B. henselae de referencia.
Conclusión:
El riesgo de transmisión sanguÃnea debiera ser considerado en un paÃs con alta seroprevalencia de infección por B. henselae.Bartonella henselae is the causal agent of cat scratch disease in immunocompetent persons and bacterial angiomatosis in immunocompromised patients. In Chile, the prevalence of antibodies against B. henselae in healthy children and adolescents is 13.3%, in persons with occupational risk 60.5%, and in cats 85.6%. There are no published data regarding the seroprevalence in blood donors in our country, so determining if B. henselae is present in the blood of donors at the time of donation is very important, since this microorganism can survive up to 35 days in the red blood cells stored in a blood bank at 4 °C.
Objective:
To determine the prevalence of B. henselae in blood donors.
Methodology:
140 donor blood samples were analyzed to detect the presence of B. henselae, using the polymerase chain reaction technique.
Results:
13.6% of the blood donors with positive polymerase chain reaction for B. henselae were obtained. The sequence of the amplified fragments showed an identity of over 98% with respect to B. henselae reference sequences.
Conclusion:
The risk of blood transmission is due to a country with high B. henselae infection.https://scielo.conicyt.cl/scielo.php?script=sci_arttext&pid=S0716-10182017000600539&lng=en&nrm=iso&tlng=e
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