3,216 research outputs found
Quantum-well states in ultrathin Ag(111) films deposited onto H-passivated Si(111)-(1x1) surfaces
Ag(111) films were deposited at room temperature onto H-passivated
Si(111)-(1x1) substrates, and subsequently annealed at 300 C. An abrupt
non-reactive Ag/Si interface is formed, and very uniform non-strained Ag(111)
films of 6-12 monolayers have been grown. Angle resolved photoemission
spectroscopy has been used to study the valence band electronic properties of
these films. Well-defined Ag sp quantum-well states (QWS) have been observed at
discrete energies between 0.5-2eV below the Fermi level, and their dispersions
have been measured along the GammaK, GammaMM'and GammaL symmetry directions.
QWS show a parabolic bidimensional dispersion, with in-plane effective mass of
0.38-0.50mo, along the GammaK and GammaMM' directions, whereas no dispersion
has been found along the GammaL direction, indicating the low-dimensional
electronic character of these states. The binding energy dependence of the QWS
as a function of Ag film thickness has been analyzed in the framework of the
phase accumulation model. According to this model, a reflectivity of 70% has
been estimated for the Ag-sp states at the Ag/H/Si(111)-(1x1) interface.Comment: 6 pages, 6 figures, submitted to Phys. Rev.
Estudio estructural y morfométrico de las glándulas salivales de gorrión (Passer Domesticus) durante invierno y verano
Key Words: House sparrow. Salivary glands. Histophysiology. Morphometry.An structural, cytochemical and morphometric study of the salivary glands of the House sparrow was made, with the purpose of detecting their probable histophysiological adaptatio!\u27s in relation to diet and nutritional needs according to seasons of the year. 40 sparrows from "Villa Los Aromas" (Alta Gracia) and gathered in winter and summer were used. Samples of the oral cavity were studied with H/E, PAS, Alcian Blue and Toluidine Blue. The quantitative analysis of the development and size of.the glands was perforrned according to the method of !Gelian and Cohn. No structural or cytochemical modifications were observed in the different glands. The proportion eltisting among the different glands (angularis oris, maltillary and internal and external mandibular) shows statistically significative differences, and these differences rernain constan! through the seasons of the year studied. It is concluded that these glands do not exhibit variations in winter and surnmer, perlad in which vegetation and fauna are modified and the disponibility of alimentary sources varies. The constan glandular pattern during these seasons would be a consequence of the species gene tic determination, associated to a diet that is probably similar in winterand sumrner wbere vegetable and animal components would combine in relation to seasonal rnetabolic requirements.Se realizó el estudio estructural, citoquírnico y morfométrico de glándulas salivales de gorrión con el fin de detectar adaptaciones histofisiológicas determinadas por la dieta y necesidades nutricionales durante las estaciones del año. Se emplearon 40 gorriones provenientes de Villa Los Aromas (Alta Gracia) capturados en invierno y verano. Las glándulas bucales se estudiaron con HE, PAS, Alcian Blue, y Azul de Toluidina El análisis cuantitativo del desarrollo y tamaño glandular se realizó según el método de Kielian y Cohn. No se observaron modificaciones estructurales ni citoquírnicas en las distintas glándulas.La proporción entre las diferentes glándulas (angularis mis, maxilares y mandibulares externas e internas) muestra diferencias estadísticamente significativas entre sí, que se mantienen constantes durante las estaciones del año analizadas. Se concluye que estas glándulas no muestran patrones distintos para el invierno y el verano, épocas en que la vegetación y la fauna se modifican con variaciones en la disponibilidad de recursos alimenticios. El patrón glandular constante correspondería a la determinación genética de la especie y a la dieta que en invierno y verano, se adaptaría en sus componentes animales y vegetales a los requerimientos metabólicos estacionale
Electronic properties and Fermi surface of Ag(111) films deposited onto H-passivated Si(111)-(1x1) surfaces
Silver films were deposited at room temperature onto H-passivated Si(111)
surfaces. Their electronic properties have been analyzed by angle-resolved
photoelectron spectroscopy. Submonolayer films were semiconducting and the
onset of metallization was found at a Ag coverage of 0.6 monolayers. Two
surface states were observed at -point in the metallic films,
with binding energies of 0.1 and 0.35 eV. By measurements of photoelectron
angular distribution at the Fermi level in these films, a cross-sectional cut
of the Fermi surface was obtained. The Fermi vector determined along different
symmetry directions and the photoelectron lifetime of states at the Fermi level
are quite close to those expected for Ag single crystal. In spite of this
concordance, the Fermi surface reflects a sixfold symmetry rather than the
threefold symmetry of Ag single crystal. This behavior was attributed to the
fact that these Ag films are composed by two domains rotated 60.Comment: 9 pages, 8 figures, submitted to Physical Review
Two-domains bulklike Fermi surface of Ag films deposited onto Si(111)-(7x7)
Thick metallic silver films have been deposited onto Si(111)-(7x7) substrates
at room temperature. Their electronic properties have been studied by using
angle resolved photoelectron spectroscopy (ARPES). In addition to the
electronic band dispersion along the high-symmetry directions, the Fermi
surface topology of the grown films has been investigated. Using ARPES, the
spectral weight distribution at the Fermi level throughout large portions of
the reciprocal space has been determined at particular perpendicular
electron-momentum values. Systematically, the contours of the Fermi surface of
these films reflected a sixfold symmetry instead of the threefold symmetry of
Ag single crystal. This loss of symmetry has been attributed to the fact that
these films appear to be composed by two sets of domains rotated 60 from
each other. Extra, photoemission features at the Fermi level were also
detected, which have been attributed to the presence of surface states and
\textit{sp}-quantum states. The dimensionality of the Fermi surface of these
films has been analyzed studying the dependence of the Fermi surface contours
with the incident photon energy. The behavior of these contours measured at
particular points along the Ag L high-symmetry direction puts forward
the three-dimensional character of the electronic structure of the films
investigated.Comment: 10 pages, 12 figures, submitted to Physical Review
Effective Lagrangian Approach to Weak Radiative Decays of Heavy Hadrons
Motivated by the observation of the decay by
CLEO, we have systematically analyzed the two-body weak radiative decays of
bottom and charmed hadrons. There exist two types of weak radiative decays: One
proceeds through the short-distance transition and the other
occurs through -exchange accompanied by a photon emission. Effective
Lagrangians are derived for the -exchange bremsstrahlung processes at the
quark level and then applied to various weak electromagnetic decays of heavy
hadrons. Predictions for the branching ratios of and
\Xi_b^0\to\xip_c^0\gamma are given. In particular, we found . Order of magnitude
estimates for the weak radiative decays of charmed hadrons: and
are also presented. Within this approach, the decay asymmetry for antitriplet
to antitriplet heavy baryon weak radiative transitions is uniquely predicted by
heavy quark symmetry. The electromagnetic penguin contribution to
is estimated by two different methods and its
branching ratio is found to be of order . We conclude that
weak radiative decays of bottom hadrons are dominated by the short-distance
mechanism.Comment: 28 pages + 3 figures (not included), CLNS 94/1278, IP-ASTP-04-94.
[Main changes in this revised version: (i) Sect 2 and subsection 4.1 are
revised, (ii) A MIT bag method for calculating the decay rate of is presented, (iii) All predictions are updated using the
newly available 1994 Particle Data Group, and (iv) Appendix and subsections
3.3 and 4.4 are deleted.
P-P Total Cross Sections at VHE from Accelerator Data
Comparison of P-P total cross-sections estimations at very high energies -
from accelerators and cosmic rays - shows a disagreement amounting to more than
10 %, a discrepancy which is beyond statistical errors. Here we use a
phenomenological model based on the Multiple-Diffraction approach to
successfully describe data at accelerator energies. The predictions of the
model are compared with data On the basis of regression analysis we determine
confident error bands, analyzing the sensitivity of our predictions to the
employed data for extrapolation. : using data at 546 and 1.8 TeV, our
extrapolations for p-p total cross-sections are only compatible with the Akeno
cosmic ray data, predicting a slower rise with energy than other cosmic ray
results and other extrapolation methods. We discuss our results within the
context of constraints in the light of future accelerator and cosmic ray
experimental results.Comment: 26 pages aqnd 11 figure
All-particle cosmic ray energy spectrum measured by the HAWC experiment from 10 to 500 TeV
We report on the measurement of the all-particle cosmic ray energy spectrum
with the High Altitude Water Cherenkov (HAWC) Observatory in the energy range
10 to 500 TeV. HAWC is a ground based air-shower array deployed on the slopes
of Volcan Sierra Negra in the state of Puebla, Mexico, and is sensitive to
gamma rays and cosmic rays at TeV energies. The data used in this work were
taken from 234 days between June 2016 to February 2017. The primary cosmic-ray
energy is determined with a maximum likelihood approach using the particle
density as a function of distance to the shower core. Introducing quality cuts
to isolate events with shower cores landing on the array, the reconstructed
energy distribution is unfolded iteratively. The measured all-particle spectrum
is consistent with a broken power law with an index of prior to
a break at ) TeV, followed by an index of . The
spectrum also respresents a single measurement that spans the energy range
between direct detection and ground based experiments. As a verification of the
detector response, the energy scale and angular resolution are validated by
observation of the cosmic ray Moon shadow's dependence on energy.Comment: 16 pages, 11 figures, 4 tables, submission to Physical Review
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