A wide family of singularity-free cosmological models

In this paper a family of non-singular cylindrical perfect fluid cosmologies is derived. The equation of state corresponds to a stiff fluid. The family depends on two independent functions under very simple conditions. A sufficient condition for geodesic completeness is provided.Comment: 7 pages, RevTeX

Search for medicinal plants with pharmaceutical use by integration of the physical and biotic informations in a geographic information system: The interface steppe region - Saharan region (Algeria)

Algeria is a large, rich country in flora and fauna. The objective of this study was the integration of biological and physical data in a GIS for research of medicinal plants.Postprint (published version

On the uniqueness of (p,h)(p,h)-gonal automorphisms of Riemann surfaces

Let $X$ be a compact Riemann surface of genus $g\geq 2$. A cyclic subgroup of prime order $p$ of $Aut(X)$ is called properly $(p,h)$-gonal if it has a fixed point and the quotient surface has genus $h$. We show that if $p>6h+6$, then a properly $(p,h)$-gonal subgroup of $Aut(X)$ is unique. We also discuss some related results.Comment: final version, 9 pages, minor improvements, added 2 reference

Información y monitoreo: base esencial para el manejo integrado de playas en Latinoamérica

El artículo demuestra la importancia de información bien organizada como una base esencial para el manejo integrado de playas. En este contexto se trata de información sobre los varios aspectos de la zona litoral, como física, química, biológica, así como social, económica y cultural. Para obtener tal informacion, hoy en día existen diferentes técnicas in situ y por medios remotos. La aplicación de metodologías y herramientas numéricas para el modelado de procesos hidrodinámicos permite conocer, entender y pronosticar el comportamiento de la costa, con el fin de proponer soluciones que ayuden a un desarrollo sostenible, en equilibrio con el entorno social, cultural y medioambiental. Se presentan ejemplos de tecnologías y metodologías para el monitoreo y la modelación de playas, como una base para su adecuado manejo y desarrollo sostenible.The paper demonstrates the importance of well-organised information as an essential basis for integrated management of beaches. This refers to information about various aspects of the coastal zone: physical, chemical, biological as well as social, economical and cultural. Today different techniques are available to obtain such information, in situ as well as remote. The application of methodologies and numeric tools for the modelling of hydrodynamic processes allows knowing, understanding and predicting the behaviour of the coast, with the aim of proposing solutions that will support sustainable development, in balance with the social, cultural and environmental surroundings. Examples of technologies and methodologies will be presented for the monitoring and modelling of beaches, as a basis for their adequate management and sustainable development

Symmetry breaking from Scherk-Schwarz compactification

We analyze the classical stable configurations of an extra-dimensional gauge theory, in which the extra dimensions are compactified on a torus. Depending on the particular choice of gauge group and the number of extra dimensions, the classical vacua compatible with four-dimensional Poincar\'e invariance and zero instanton number may have zero energy. For SU(N) on a two-dimensional torus, we find and catalogue all possible degenerate zero-energy stable configurations in terms of continuous or discrete parameters, for the case of trivial or non-trivial 't Hooft non-abelian flux, respectively. We then describe the residual symmetries of each vacua.Comment: 24 pages, 1 figure, Section 4 modifie

Aportaciones a la flora del sureste ibérico

New records to the flora of SE Spain.Palabras clave: Corología, plantas vasculares, Murcia, Albacete, España.Key words: Chorology, vascular plants, Murcia, Albacete, Spain

Influence of the sulphurization time on the morphological, chemical, structural and electrical properties of Cu2ZnSnS4 polycrystalline thin films

The effects of the sulphurization annealing time on the morphological, chemical, structural and electrical properties of CZTS thin films were investigated by scanning electron microscopy, X-ray energy dispersive spectroscopy, Hall effect and electrical conductivity measurements in samples annealed during different time intervals. The increase of the annealing time was found to improve the chemical composition of the samples and to, slightly, increase the crystallite size. Small amounts of Na were measured in the samples. However, the concentration of Na does not increase significantly with the annealing time and should not modify the characteristics of the CZTS thin films. It was also found that at high temperature the electrical conductivity is dominated by thermal emission of carriers over the inter-grain potential barriers. As the temperature decreases different hopping conduction mechanisms start to dominate. At first with nearest-neighbour hopping and successively changing to variable range hopping conduction with a crossover from Mott and Efros–Shklovskii behavior. The electrical conductivity, the concentration of free holes, acceptors and donors, traps0 density at the grain boundaries and the grain potential barriers height were found to increase with the annealing time. However, a significant drop in the compensation ratio from 0.8 to 0.5 was also detected.info:eu-repo/semantics/publishedVersio

Superconductivity in carbon nanotube ropes

We investigate the conditions in which superconductivity may develop in ropes of carbon nanotubes. It is shown that the interaction among a large number of metallic nanotubes favors the appearance of a metallic phase in the ropes, intermediate between respective phases with spin-density-wave and superconducting correlations. These arise in samples with about 100 metallic nanotubes or more, where the long-range Coulomb interaction is very effectively reduced and it may be overcome by the attractive interaction from the exchange of optical phonons within each nanotube. We estimate that the probability for the tunneling of Cooper pairs between neighboring nanotubes is much higher than that for single electrons in a disordered rope. The effect of pair hopping is therefore what establishes the intertube coherence, and the tunneling amplitude of the Cooper pairs dictates the scale of the transition to the superconducting state.Comment: 12 page

Anomalous persistent photoconductivity in Cu2ZnSnS4 thin films and solar cells

A persistent photoconductivity effect (PPC) has been investigated in Cu2ZnSnS4 thin films and solar cells as a function of temperature. An anomalous increase of the PPC decay time with temperature was observed in all samples. The PPC decay time activation energy was found to increase when temperature rises above a crossover value, and also to grow with the increase of the sulfurization temperature and pressure. Both the anomalous behavior of the PPC decay time and the existence of two different activation energies are explained in terms of local potential fluctuations in the band edges of CZTS