Large cities are less green

We study how urban quality evolves as a result of carbon dioxide emissions as urban agglomerations grow. We employ a bottom-up approach combining two unprecedented microscopic data on population and carbon dioxide emissions in the continental US. We first aggregate settlements that are close to each other into cities using the City Clustering Algorithm (CCA) defining cities beyond the administrative boundaries. Then, we use data on $\rm{CO}_2$ emissions at a fine geographic scale to determine the total emissions of each city. We find a superlinear scaling behavior, expressed by a power-law, between $\rm{CO}_2$ emissions and city population with average allometric exponent $\beta = 1.46$ across all cities in the US. This result suggests that the high productivity of large cities is done at the expense of a proportionally larger amount of emissions compared to small cities. Furthermore, our results are substantially different from those obtained by the standard administrative definition of cities, i.e. Metropolitan Statistical Area (MSA). Specifically, MSAs display isometric scaling emissions and we argue that this discrepancy is due to the overestimation of MSA areas. The results suggest that allometric studies based on administrative boundaries to define cities may suffer from endogeneity bias

Fracturing the optimal paths

Optimal paths play a fundamental role in numerous physical applications ranging from random polymers to brittle fracture, from the flow through porous media to information propagation. Here for the first time we explore the path that is activated once this optimal path fails and what happens when this new path also fails and so on, until the system is completely disconnected. In fact numerous applications can be found for this novel fracture problem. In the limit of strong disorder, our results show that all the cracks are located on a single self-similar connected line of fractal dimension $D_{b} \approx 1.22$. For weak disorder, the number of cracks spreads all over the entire network before global connectivity is lost. Strikingly, the disconnecting path (backbone) is, however, completely independent on the disorder.Comment: 4 pages,4 figure

Defect-induced spin-glass magnetism in incommensurate spin-gap magnets

We study magnetic order induced by non-magnetic impurities in quantum paramagnets with incommensurate host spin correlations. In contrast to the well-studied commensurate case where the defect-induced magnetism is spatially disordered but non-frustrated, the present problem combines strong disorder with frustration and, consequently, leads to spin-glass order. We discuss the crossover from strong randomness in the dilute limit to more conventional glass behavior at larger doping, and numerically characterize the robust short-range order inherent to the spin-glass phase. We relate our findings to magnetic order in both BiCu2PO6 and YBa2Cu3O6.6 induced by Zn substitution.Comment: 6 pages, 5 figs, (v2) real-space RG results added; discussion extended, (v3) final version as publishe

Comportamiento agronómico de cuatro variedades tradicionales de arroz (Oryza sativa) a bajas dosis de nitrógeno

En Ecuador el arroz es un alimento principal en la dieta de la población. En 2005 se sembraron 324.875 hectáreas con un rendimiento de 3.4 t/ha,un nivel de productividad bajo, que no ha evolucionado favorablemente para reducir los costos unitarios. Los precios del arroz en 2005, basados en la relación de comercio con nuestros vecinos, han disminuido a niveles que desestimulan al productor, por lo que los agricultores enfrentan una situación de baja rentabilidad del cultivo. Vale señalar que se define como variedades tradicionales, aquellas sembradas en el Ecuador por décadas, cuyo origen de introducción se desconoce y que se distinguen por no pertenecer al linaje del nuevo tipo de planta de arroz (tallos cortos, hojas erectas) que se generó en la década del sesenta y cuya primera variedad entregada por el Internacional Rice Research Institute (IRRI) de Filipinas fue“IR8”. La participación de las variedades tradicionales como un recurso fitogenético en los procesos de mejoramiento ha sido muy limitada y se ha observado buena calidad de grano y productividad en las evaluaciones realizadas.5 Esto determinará una nueva línea de inves- tigación orientada a la obtención de cultivos superiores y a la sostenibilidad del ambiente, dentro de este concepto se halla este estudio