Thermal van der Waals Interaction between Graphene Layers

The van de Waals interaction between two graphene sheets is studied at finite temperatures. Graphene's thermal length $(\xi_T = \hbar v / k_B T)$ controls the force versus distance $(z)$ as a crossover from the zero temperature results for $z\ll \xi_T$, to a linear-in-temperature, universal regime for $z\gg \xi_T$. The large separation regime is shown to be a consequence of the classical behavior of graphene's plasmons at finite temperature. Retardation effects are largely irrelevant, both in the zero and finite temperature regimes. Thermal effects should be noticeable in the van de Waals interaction already for distances of tens of nanometers at room temperature.Comment: enlarged version, 9 pages, 4 figures, updated reference

Plasmons in layered structures including graphene

We investigate the optical properties of layered structures with graphene at the interface for arbitrary linear polarization at finite temperature including full retardation by working in the Weyl gauge. As a special case, we obtain the full response and the related dielectric function of a layered structure with two interfaces. We apply our results to discuss the longitudinal plasmon spectrum of several single and double layer devices such as systems with finite and zero electronic densities. We further show that a nonhomogeneous dielectric background can shift the relative weight of the in-phase and out-of-phase mode and discuss how the plasmonic mode of the upper layer can be tuned into an acoustic mode with specific sound velocity.Comment: 18 pages, 6 figure

Limits on the amplification of evanescent waves of left-handed materials

We investigate the transfer function of the discretized perfect lens in finite-difference time-domain (FDTD) and transfer matrix (TMM) simulations; the latter allow to eliminate the problems associated with the explicit time dependence in FDTD simulations. We argue that the peak observed in the FDTD transfer function near the maximum parallel momentum $k_{\|,\mathrm{max}}$ is due to finite time artifacts. We also find the finite discretization mesh acts like imaginary deviations from $\mu=\epsilon=-1$ and leads to a cross-over in the transfer function from constance to exponential decay around $k_{\|,\mathrm{max}}$ limiting the attainable super-resolution. We propose a simple qualitative model to describe the impact of the discretization. $k_{\|,\mathrm{max}}$ is found to depend logarithmically on the mesh constant in qualitative agreement with the TMM simulations.Comment: 4 pages, 3 figure

Projections of wind energy resources in the Caribbean for the 21st century

The Caribbean has suitable conditions for a significant wind energy development, which makes a good planning for the future renewable energy mix essential. The impact of climate change on Caribbean wind power has been analyzed by means of an ensemble of CORDEX regional climate models (RCMs) under the RCP8.5 warming scenario. The offshore wind energy resource was classified for the historical period and for the future considering wind energy factors, environmental risk factors and cost factors whose weights were estimated by a Delphi method. Future projections show a maximum annual wind increase, ∼0.4 ms−1 (8%), in most of the Caribbean, except in the Yucatán Basin. This increment occurs mainly during the wet season, ∼0.5 ms−1 (∼10%), associated with changes in the extension of the North Atlantic Subtropical High, which will strengthen the Caribbean low-level jet. Additionally, the moderate wind increase, ∼0.2 ms−1 (∼4%), projected during the dry season is restricted to the southeastern coast and it is associated with an increment in the land-ocean temperature difference (∼1 °C), which will intensify local easterly winds. The low-level jet region was classified as the richest wind energy resource in the Caribbean for the future with a larger extension compared to the historical period.publishe

Sustainability Strategies for Strengthening Agricultural Systems in Quimiag Parish, Riobamba Canton

The Quimiag parish in Riobamba canton is purely agricultural. For this reason, this study aimed to develop sustainability strategies to strengthen its agricultural systems. 24 communities were involved in the research and 312 Agricultural Productive Units (UPAs) were surveyed. Basic information was collected and then a proposal was developed using the logical framework matrix methodology and focusing on 3 sustainability components: social, productive and economical. In terms of the social component, it was observed that producers were on average 46 years old and 65% of producers had a primary education. In terms of the production component, we found that maize, beans and potato are the main agricultural products, while dairy cattle, cuyes and pigs are the main livestock in the area. Capacity building, production technology, production credit management and the generation of partnerships with state institutions and private companies are the main sustainability strategies proposed for the Quimiag parish, and adopting these would lead to improvements in profitability rates.  Keywords: sustainability, agricultural systems, economic indicators, Quimiag. RESUMEN La parroquia Quimiag perteneciente al cantón Riobamba, es netamente agropecuaria, por este motivo el presente estudio  tiene  por objetivo  crear  estrategias  de  sostenibilidad  para  fortalecer  los  sistemas agropecuarios  de  sus productores. Para realizar la investigación se contó con la participación de 24 comunidades y de ellas se muestreo a 312 Unidades Productivas Agropecuarias (UPAs), a las cuales se les aplicó una encuesta que permitió levanter información base para posteriormente elaborar la propuesta utilizando la metodología de matriz de marco lógico, y la cual está enfocada en 3 componentes de sostenibilidad que son: social, productivo y económico. En el componente social se obtuvo que los productores tienen una edad promedio de 46 años y el 65% de los productores tienen instrucción primaria. En el componente productivo se obtuvo que el maíz, fréjol y papa son los principales productos agrícolas, mientras que el ganado lechero, cuyes y cerdos son los principales rubros pecuarios de la zona. El fortalecimiento de capacidades, tecnificación de la producción, gestión de créditos productivos y generación de alianzas con instituciones del estado y empresas privadas, son las principales estrategias para la sostenibilidad de la producción de la parroquia Quimiag con los cuales se mejoraría los índices de rentabilidad. Palabras claves: sostenibilidad, sistemas agropecuarios, indicadores económicos, quimiag. &nbsp

A data science approach for spatiotemporal modelling of low and resident air pollution in Madrid (Spain): Implications for epidemiological studies

Model developments to assess different air pollution exposures within cities are still a key challenge in environmental epidemiology. Background air pollution is a long-term resident and low-level concentration pollution difficult to quantify, and to which population is chronically exposed. In this study, hourly time series of four key air pollutants were analysed using Hidden Markov Models to estimate the exposure to background pollution in Madrid, from 2001 to 2017. Using these estimates, its spatial distribution was later analysed after combining the interpolation results of ordinary kriging and inverse distance weighting. The ratio of ambient to background pollution differs according to the pollutant studied but is estimated to be on average about six to one. This methodology is proposed not only to describe the temporal and spatial variability of this complex exposure, but also to be used as input in new modelling approaches of air pollution in urban areas. (c) 2018 The Author

Differences in coastal and oceanic SST trends north of Yucatan Peninsula

The coastal area north of Yucatan has experienced a cooling SST trend from 1982 to 2015 during the upwelling season (May–September) that contrasts with the warming observed at the adjacent ocean area. Different drivers were analyzed to identify the possible causes of that unusual coastal cooling. Changes in coastal upwelling and in sea-atmosphere heat fluxes are not consistent with the observed coastal cooling. The eastward shift of the Yucatan Current observed over the last decades is hypothesized as the most probable cause of coastal cooling. This shift enhances the vertical transport of cold deeper water to the continental shelf from where it is pumped to the surface by upwelling favorable westerly winds.publishe

Interdependent network reciprocity in evolutionary games

Besides the structure of interactions within networks, also the interactions between networks are of the outmost importance. We therefore study the outcome of the public goods game on two interdependent networks that are connected by means of a utility function, which determines how payoffs on both networks jointly influence the success of players in each individual network. We show that an unbiased coupling allows the spontaneous emergence of interdependent network reciprocity, which is capable to maintain healthy levels of public cooperation even in extremely adverse conditions. The mechanism, however, requires simultaneous formation of correlated cooperator clusters on both networks. If this does not emerge or if the coordination process is disturbed, network reciprocity fails, resulting in the total collapse of cooperation. Network interdependence can thus be exploited effectively to promote cooperation past the limits imposed by isolated networks, but only if the coordination between the interdependent networks is not disturbe