Physically founded phonon dispersions of few-layer materials, and the case of borophene

An increasing number of theoretical calculations on few-layer materials have been reporting a non-zero sound velocity for all three acoustic phonon modes. In contrast with these reports, here we show that the lowest phonon dispersion branch of atomistically described few-layer materials should be quadratic, and this can have dramatic consequencies on calculated properties, such as the thermal conductivity. By reformulating the interatomic force constants (IFC) in terms of internal coordinates, we find that a delicate balance between the IFCs is responsible for this quadraticity. This balance is hard to obtain in ab-initio calculations even if all the symmetries are numerically enforced a posteriori, but it arises naturally in our approach. We demonstrate the phenomenon in the case of borophene, where a very subtle correction to the ab-initio IFCs yields the physically correct quadratic dispersion, while leaving the rest of the spectrum virtually unmodified. Such quadraticity nevertheless has a major effect on the computed lattice thermal conductivity, which in the case of borophene changes by more than a factor 2, and reverses its anisotropy, when the subtle IFC correction is put in place

Reentrant phase behaviour for systems with competition between phase separation and self-assembly

In patchy particle systems where there is competition between the self-assembly of finite clusters and liquid-vapour phase separation, reentrant phase behaviour is observed, with the system passing from a monomeric vapour phase to a region of liquid-vapour phase coexistence and then to a vapour phase of clusters as the temperature is decreased at constant density. Here, we present a classical statistical mechanical approach to the determination of the complete phase diagram of such a system. We model the system as a van der Waals fluid, but one where the monomers can assemble into monodisperse clusters that have no attractive interactions with any of the other species. The resulting phase diagrams show a clear region of reentrance. However, for the most physically reasonable parameter values of the model, this behaviour is restricted to a certain range of density, with phase separation still persisting at high densities.Comment: 13 pages, 10 figure

Resource unpredictability promotes species diversity and coexistence in an avian scavenger guild: A field experiment

Chance per se plays a key role in ecology and evolution, e.g., genetic mutation, resource spatiotemporal unpredictability. In community ecology, chance is recognized as a key factor in community assemblage, but less is known about its role in intraguild processes leading to species coexistence. Here we study the relevance of resource unpredictability per se as a promoter of intraguild positive interspecific interactions and as a biodiversity enhancer in an Old World avian scavenger guild, which has evolved to feed upon spatially and temporally unpredictable resources, i.e., carcasses. We performed a large-scale field experiment in which 58 carcasses were disposed of and observed until complete consumption, either in continuously active supplementary feeding stations (predictable carcasses) or disposed of at random in the field (unpredictable carcasses). Richness of scavenger species was similar at unpredictable and predictable carcasses, but their relative abundances were highly uneven at predictable carcasses leading to higher scavenger diversity (Shannon index) at unpredictable carcasses. Facilitatory interspecific processes only occurred at unpredictable resources but were disrupted in predictable conditions because the dominant specialist species (in our case, the Griffon Vulture Gyps fulvus) arrived earlier and in larger numbers, monopolizing the resource. Small, endangered scavengers congregated at supplementary feeding stations but profited less compared to unpredictable carcasses, suggesting that they could constitute an ecological trap. Our findings offer new insights into the relevance of unpredictability of trophic resources in promoting both positive facilitatory interspecific interactions and species diversity and thus maintaining the function of guilds. Finally, the preservation of randomness in resource availability and the processes associated with its exploitation should be a major goal of conservation strategies aimed to preserve scavenger guilds evolved under naturally unpredictable trophic resources. © 2012 by the Ecological Society of America.Peer Reviewe

The PHA Test Reflects Acquired T-Cell Mediated Immunocompetence in Birds

cological immunology requires techniques to reliably measure immunocompetence in wild vertebrates. The PHA-skin test, involving subcutaneous injection of a mitogen (phytohemagglutinin, PHA) and measurement of subsequent swelling as a surrogate of T-cell mediated immunocompetence, has been the test of choice due to its practicality and ease of use in the field. However, mechanisms involved in local immunological and inflammatory processes provoked by PHA are poorly known, and its use and interpretation as an acquired immune response is currently debated.) and plasma proteins circulating in the bloodstream, potentially involved in the immunological and inflammatory processes, through flow cytometry and electrophoresis.Our results showed stronger responses after a second PHA injection, independent of species, time elapsed and changes in body mass of birds between first and second injections, thus supporting the adaptive nature of this immune response. Furthermore, the concomitant changes in the plasma concentrations of T-lymphocyte subsets and globulins indicate a causal link between the activation of the T-cell mediated immune system and local tissue swelling.These findings justify the widespread use of the PHA-skin test as a reliable evaluator of acquired T-cell mediated immunocompetence in diverse biological disciplines. Further experimental research should be aimed at evaluating the relative role of innate immunocompetence in wild conditions, where the access to dietary proteins varies more than in captivity, and to ascertain how PHA responses relate to particular host-parasite interactions

Large scale risk-assessment of wind-farms on population viability of a globally endangered long-lived raptor

Wind-farms receive public and governmental support as an alternative energy source mitigating air pollution. However, they can have adverse effects on wildlife, particularly through collision with turbines. Research on wind-farm effects has focused on estimating mortality rates, behavioural changes or interspecific differences in vulnerability. Studies dealing with their effects on endangered or rare species populations are notably scarce. We tested the hypothesis that wind-farms increase extinction probability of long-lived species through increments in mortality rates. For this purpose, we evaluate potential consequences of wind-farms on the population dynamics of a globally endangered long-lived raptor in an area where the species maintains its greatest stronghold and wind-farms are rapidly increasing. Nearly one-third of all breeding territories of our model species are in wind-farm risk zones. Our intensive survey shows that wind-farms decrease survival rates of this species differently depending on individual breeding status. Consistent with population monitoring, population projections showed that all subpopulations and the meta-population are decreasing. However, population sizes and, therefore, time to extinction significantly decreased when wind-farm mortality was included in models. Our results represent a qualitative warning exercise showing how very low reductions in survival of territorial and non-territorial birds associated with wind-farms can strongly impact population viability of long-lived species. This highlights the need for examining long-term impacts of wind-farms rather than focusing on short-term mortality, as is often promoted by power companies and some wildlife agencies. Unlike other non-natural causes of mortality difficult to eradicate or control, wind-farm fatalities can be lowered by powering down or removing risky turbines and/or farms, and by placing them outside areas critical for endangered birds. © 2009 Elsevier Ltd. All rights reserved.Peer Reviewe

Surface roughness and thermal conductivity of semiconductor nanowires: going below the Casimir limit

By explicitly considering surface roughness at the atomic level, we quantitatively show that the thermal conductivity of Si nanowires can be lower than Casimir's classical limit. However, this violation only occurs for deep surface degradation. For shallow surface roughness, the Casimir formula is shown to yield a good approximation to the phonon mean free paths and conductivity, even for nanowire diameters as thin as 2.22 nm. Our exact treatment of roughness scattering is in stark contrast with a previously proposed perturbative approach, which is found to overpredict scattering rates by an order of magnitude. The obtained results suggest that a complete theoretical understanding of some previously published experimental results is still lacking.Comment: 11 pages, 4 figure

Propuesta preliminar de una escala de satisfacción para clientes mexicanos de supermercado

Diferentes propuestas de escalas para evaluar la satisfacción de los consumidores de distintos sectores de actividad en distintas partes del mundo han sido desarrolladas. Una buena parte de de estas escalas ha sido validada en diferentes contextos profesionales y socioculturales y otra parte ha requerido ser adaptada en virtud de que las variables inherentes a la escala original no resultaron ser suficientemente relevantes para el grupo cultural de estudio. Este estudio exploratorio propone una multiescala preliminar adaptada de satisfacción de clientes de supermercados, para su aplicación en el contexto mexicano. Su estructura a priori se definió con base en la revisión de la bibliografía especializada. Una encuesta aplicada entre 771 clientes de una cadena de supermercados de cobertura nacional aporta los datos necesarios para apoyar la multidimensionalidad del concepto de satisfacción y para comenzar a identificar aquellas dimensiones latentes que más influyen en la satisfacción del consumidor mexican

Crecimiento de becerras F1 cebú /europeo en pastoreo de Zacate Estrella-Leucaena y Estrella solo en clima Aw

En el Campo Experimental Pecuario "El Macho", en Tecuala, Nay., con un clima tropical seco Aw, se condujo un ensayo para medir el potencial de producción de carne en el pastoreo de praderas de Zacate Estrella (Cynodon plectostachyus) asociado con Leucaen

Habitat, human pressure, and social behavior: Partialling out factors affecting large-scale territory extinction in an endangered vulture

Extinctions are often the result of multiple factors that are difficult to disentangle and so methods for identifying simple and combined types of extinctions are valuable for both basic and applied ecology. We applied a modification of variance partitioning and a hierarchical partitioning analysis to test several hypotheses that attempt to explain the recent large-scale disappearance of Egyptian Vulture breeding territories in Spain. Our aim was to identify and then separate the simple (or pure) from the combined effects of habitat features, human pressure, and the social behavior of the species on the risk of extinction from a territory while controlling for spatial autocorrelation. Deviance partitioning showed that a complex mix of factors is significantly related to the disappearance of more than 400 territories throughout Spain. Abandoned territories were located in areas that are isolated from other conspecific territories and far from communal roost sites. In addition, these territories were found in places where there is a lack of natural habitats, high habitat fragmentation and reduced habitat diversity, and where food availability seems to be low and the illegal use of poison to control predators is a common practice. Deviance partitioning also showed an important spatial component in the probability of extinction. Abandoned territories were not randomly distributed; rather, they were aggregated in extinction 'hotspots', mainly related to food availability and human pressure. Deviance partitioning turned out to be an useful tool for identifying the relative contribution of a variety of factors - and their combined effects - associated with an extinction process. The deviance explained by each factor must be interpreted, however, in the context of a good knowledge of the life history of the species. Hierarchical partitioning can help rank conservation priorities and, by using as an objective criterion the relative weight of each independent variable that could be effectively managed for conservation, may provide wildlife managers with a means of saving funds and optimizing action plans. © 2006 Elsevier Ltd. All rights reserved.Peer Reviewe