National Autonomous University of Mexico

About: National Autonomous University of Mexico is a education organization based out in Mexico City, Distrito Federal, Mexico. It is known for research contribution in the topics: Population & Galaxy. The organization has 72868 authors who have published 127797 publications receiving 2285543 citations. The organization is also known as: UNAM & Universidad Nacional Autónoma de México.

Light transmission in quasiperiodic multilayers of porous silicon

Abstract: Porous silicon is an efficient photo- and electro-luminescent material and represents a promising candidate for opto-electronic applications On the other hand, quasiperiodic structures have been shown to be effective media for light localization and third harmonic generation In this work, we present a photonic model for quasiperiodic multilayer structures, which are built experimentally by alternating porous silicon layers with high and low refractive indices The analysis of the light propagation through these structures is based on the transfer matrix theory The theoretical reflectance spectrum is compared with experimental data, observing a good agreement

Antiproton Flux, Antiproton-to-Proton Flux Ratio, and Properties of Elementary Particle Fluxes in Primary Cosmic Rays Measured with the Alpha Magnetic Spectrometer on the International Space Station

Abstract: A precision measurement by AMS of the antiproton flux and the antiproton-to-proton flux ratio in primary cosmic rays in the absolute rigidity range from 1 to 450 GV is presented based on 3.49 × 105 antiproton events and 2.42 × 109 proton events. The fluxes and flux ratios of charged elementary particles in cosmic rays are also presented. In the absolute rigidity range ∼60 to ∼500 GV, the antiproton ¯p, proton p, and positron eþ fluxes are found to have nearly identical rigidity dependence and the electron e− flux exhibits a different rigidity dependence. Below 60 GV, the ( ¯ p=p), ( ¯ p=eþ), and (p=eþ) flux ratios each reaches a maximum. From ∼60 to ∼500 GV, the ( ¯ p=p), ( ¯ p=eþ), and (p=eþ) flux ratios show no rigidity dependence. These are new observations of the properties of elementary particles in the cosmos.

Somatosensory discrimination based on cortical microstimulation

Abstract: The sensation of flutter is produced when mechanical vibrations in the range of 5-50Hz are applied to the skin. A flutter stimulus activates neurons in the primary somatosensory cortex (S1) that somatotopically map to the site of stimulation. A subset of these neurons-those with quickly adapting properties, associated with Meissner's corpuscles-are strongly entrained by periodic flutter vibrations, firing with a probability that oscillates at the input frequency. Hence, quickly adapting neurons provide a dynamic representation of such flutter stimuli. However, are these neurons directly involved in the perception of flutter? Here we investigate this in monkeys trained to discriminate the difference in frequency between two flutter stimuli delivered sequentially on the fingertips. Microelectrodes were inserted into area 3b of S1 and the second stimulus was substituted with a train of injected current pulses. Animals reliably indicated whether the frequency of the second (electrical) signal was higher or lower than that of the first (mechanical) signal, even though both frequencies changed from trial to trial. Almost identical results were obtained with periodic and aperiodic stimuli of equal average frequencies. Thus, the quickly adapting neurons in area 3b activate the circuit leading to the perception of flutter. Furthermore, as far as can be psychophysically quantified during discrimination, the neural code underlying the sensation of flutter can be finely manipulated, to the extent that the behavioural responses produced by natural and artificial stimuli are indistinguishable.

Climate-driven declines in arthropod abundance restructure a rainforest food web.

Abstract: A number of studies indicate that tropical arthropods should be particularly vulnerable to climate warming. If these predictions are realized, climate warming may have a more profound impact on the functioning and diversity of tropical forests than currently anticipated. Although arthropods comprise over two-thirds of terrestrial species, information on their abundance and extinction rates in tropical habitats is severely limited. Here we analyze data on arthropod and insectivore abundances taken between 1976 and 2012 at two midelevation habitats in Puerto Rico’s Luquillo rainforest. During this time, mean maximum temperatures have risen by 2.0 °C. Using the same study area and methods employed by Lister in the 1970s, we discovered that the dry weight biomass of arthropods captured in sweep samples had declined 4 to 8 times, and 30 to 60 times in sticky traps. Analysis of long-term data on canopy arthropods and walking sticks taken as part of the Luquillo Long-Term Ecological Research program revealed sustained declines in abundance over two decades, as well as negative regressions of abundance on mean maximum temperatures. We also document parallel decreases in Luquillo’s insectivorous lizards, frogs, and birds. While El Nino/Southern Oscillation influences the abundance of forest arthropods, climate warming is the major driver of reductions in arthropod abundance, indirectly precipitating a bottom-up trophic cascade and consequent collapse of the forest food web.