Asymmetric I-V characteristics and magnetoresistance in magnetic point contacts

We present a theoretical study of the transport properties of magnetic point contacts under bias. Our calculations are based on the Keldish's non-equilibrium Green's function formalism combined with a self-consistent empirical tight-binding Hamiltonian, which describes both strong ferromagnetism and charging effects. We demonstrate that large magnetoresistance solely due to electronic effects can be found when a sharp domain wall forms inside a magnetic atomic-scale point contact. Moreover we show that the symmetry of the $I$-$V$ characteristic depends on the position of the domain wall in the constriction. In particular diode-like curves can arise when the domain wall is placed off-center within the point contact, although the whole structure does not present any structural asymmetry.Comment: 7 figures, submitted to PR

Building a guide to analyse mathematics textbooks based on the didactical suitability theory

A textbook lesson can be considered as a potential or planned instructional process proposed by the author’s book, which can help the teacher to design and implement an effective instructional process. This permits the application of didactical analysis tools of the onto-semiotic approach to mathematical knowledge and instruction in order to assess the didactical suitability of the instructional process, identify possible conflicts of meaning and potential improvements. In this article, we describe the process of developing a Mathematics Textbook Lesson Analysis Guide using the didactical suitability theory and its operational breakdown into components, subcomponents and indicators. The formulation of the suitability criteria, as rules that permit to guide an informed assessment of the suitability of a teaching and learning process, has led us to carry out a content analysis of the key research on textbook analysis and the consensus adopted in the research community regarding the suitability criteria. Assuming that a mathematics teacher has decided to use a textbook lesson as a resource to help the teaching and learning process of some mathematical content, the analysis guide is presented as a tool that allows the teacher to assess the didactical suitability and facilitate the making of informed decisions about the lesson use in the classroom.Una lección de un libro de texto puede ser considerada como un proceso de instrucción potencial o planificado por el autor del libro, que sirve de apoyo al docente para diseñar e implementar un proceso de instrucción efectivo. Esto permite aplicar las herramientas de análisis didáctico del enfoque ontosemiótico del conocimiento y la instrucción matemática, para valorar la idoneidad didáctica de dicho proceso, identificar posibles conflictos de significado y decidir potenciales mejoras. En este artículo se describe el proceso de elaboración de una Guía de Análisis de Lecciones de Libros de Texto de Matemáticas utilizando la teoría de la idoneidad didáctica y su desglose operativo en componentes, subcomponentes e indicadores. La formulación de los criterios de idoneidad, en tanto reglas que permitan orientar de manera fundamentada la evaluación de la pertinencia de un proceso de enseñanza y aprendizaje, nos ha llevado a realizar un análisis de contenido de las investigaciones clave con relación al análisis de libros de texto y a los consensos adoptados en la comunidad científica. Suponiendo que un profesor de matemáticas ha decidido utilizar una lección de un libro de texto como recurso para apoyar el proceso de enseñanza y aprendizaje de algún contenido matemático, la guía de análisis le puede servir de apoyo para valorar la idoneidad didáctica de la misma y apoyar la toma de decisiones fundamentadas sobre su uso en el aula

Estrategias de cooperación inter-empresa para el desarrollo de mejores prácticas medioambientales en la industria de los astilleros colombianos

The present work aims to develop strategies that allow implementingenvironmental best practices in the Colombian shipyard industry, a sector that has been growing for the last decade and that is identified as a potential world-class industry in the country. Collaborative work was carried out with four of the biggest Colombian shipyards and an international firm to identify legal and operational issues regarding the industrial activity, to reach unified consensus to represent the whole sector. Finally, an environmentalbest practices document was elaborated to guideshipyard operationsin the country, with the potential to be constituted as an official guide of the Colombian Ministry for the Environment.El presente trabajo tiene como objetivo desarrollar estrategias que permitan incorporar buenas prácticasambientales en la industria astillera Colombiana, sector que ha mostrado un crecimiento significativo en la última década y que representa una apuesta productiva para el país. Se efectuó un trabajo colaborativo con cuatro de los astilleros más representativos en el país y una empresa internacional para identificar los aspectos legales y de operación de la actividad industrial que impactan el medio ambiente, de manera que se lograse unificar criterios que tuviesen una representatividad sectorial. Finalmente se consolidóun documento de buenas prácticas ambientales para orientar la operación de astilleros en el país, con el potencial de constituirse como una guía oficial del Ministerio de Ambiente, Vivienda y Desarrollo Territorial colombiano

Prevalence of mechanisms decreasing quinolone-susceptibility among Salmonella spp. clinical isolates

Fluoroquinolone treatment failure has been reported in patients with nalidixic acid-resistant Salmonella infections. Both chromosomal- and plasmid-mediated quinolone-resistance mechanisms have been described. The objective of this study was to identify the prevalence of these mechanisms in a collection of 41 Salmonella spp. clinical isolates causing acute gastroenteritis, obtained in the Hospital Clinic, Barcelona. The minimum inhibitory concentrations (MICs) of nalidixic acid and ciprofloxacin were determined by Etest. Mutations in the quinolone-resistance determining regions (QRDRs) of the gyrA, gyrB, and parC genes and the presence of the qnr, aac(6′)-Ib-cr, and qepA genes were detected by PCR and DNA sequencing. All isolates showed constitutive expression of an efflux pump. None of the isolates were ciprofloxacin-resistant, whereas 41.5% showed nalidixic acid resistance associated with a mutation in gyrA and overexpression of an efflux pump. Although qnrS1, qnrB6, and qepA were found in four isolates, the expression of these genes was not associated with decreased quinolone susceptibility. Mutations in the gyrA gene and overexpression of an efflux pump were critical for nalidixic acid resistance and decreased susceptibility to ciprofloxacin in these isolates. However, plasmid-mediated quinolone resistance did not seem to play a major role. To our knowledge, this is the first description of qepA in Salmonella. [Int Microbiol 2010; 13(1):15-20

The cellular magnetic response and biocompatibility of biogenic zinc- and cobalt-doped magnetite nanoparticles.

The magnetic moment and anisotropy of magnetite nanoparticles can be optimised by doping with transition metal cations, enabling their properties to be tuned for different biomedical applications. In this study, we assessed the suitability of bacterially synthesized zinc- and cobalt-doped magnetite nanoparticles for biomedical applications. To do this we measured cellular viability and activity in primary human bone marrow-derived mesenchymal stem cells and human osteosarcoma-derived cells. Using AC susceptibility we studied doping induced changes in the magnetic response of the nanoparticles both as stable aqueous suspensions and when associated with cells. Our findings show that the magnetic response of the particles was altered after cellular interaction with a reduction in their mobility. In particular, the strongest AC susceptibility signal measured in vitro was from cells containing high-moment zinc-doped particles, whilst no signal was observed in cells containing the high-anisotropy cobalt-doped particles. For both particle types we found that the moderate dopant levels required for optimum magnetic properties did not alter their cytotoxicity or affect osteogenic differentiation of the stem cells. Thus, despite the known cytotoxicity of cobalt and zinc ions, these results suggest that iron oxide nanoparticles can be doped to sufficiently tailor their magnetic properties without compromising cellular biocompatibility

Physical and chemical characterization of cerium(IV) oxide nanoparticles

Chemical composition, size and structure of the nanoparticle are required to describe nanoceria. Nanoparticles of similar size and Ce(III) content might exhibit different chemical behaviour due to their differences in structure. A simple and direct procedure based on affordable techniques for all the laboratories is presented in this paper. The combination of Raman and UV-vis spectroscopy and particle impact coulometry (PIC) allows the characterization of nanoceria of small size from 4 to 65 nm at a concentration from micromolar to nanomolar, a concentration range suitable for the analysis of lab-prepared or commercial nanoparticle suspensions, but too high for most analytical purposes aimed at nanoparticle monitoring. While the PIC limits of size detection are too high to observe small nanoparticles unless catalytic amplification is used, the method provides a simple means to study aggregation of nanoparticles in the media they are needed to be dispersed for each application. Raman spectroscopy provided information about structure of the nanoparticle, and UV-vis about their chemical behaviour against some common reducing and oxidizing agents

Femtosecond laser-controlled self-assembly of amorphous-crystalline nanogratings in silicon

8 págs.; 5 figs.; 1 tab.Self-assembly (SA) of molecular units to form regular, periodic extended structures is a powerful bottom-up technique for nanopatterning, inspired by nature. SA can be triggered in all classes of solid materials, for instance, by femtosecond laser pulses leading to the formation of laser-induced periodic surface structures (LIPSS) with a period slightly shorter than the laser wavelength. This approach, though, typically involves considerable material ablation, which leads to an unwanted increase of the surface roughness. We present a new strategy to fabricate high-precision nanograting structures in silicon, consisting of alternating amorphous and crystalline lines, with almost no material removal. The strategy can be applied to static irradiation experiments and can be extended into one and two dimensions by scanning the laser beam over the sample surface. We demonstrate that lines and areas with parallel nanofringe patterns can be written by an adequate choice of spot size, repetition rate and scan velocity, keeping a constant effective pulse number (N ) per area for a given laser wavelength. A deviation from this pulse number leads either to inhomogeneous or ablative structures. Furthermore, we demonstrate that this approach can be used with different laser systems having widely different wavelengths (1030 nm, 800 nm, 400 nm), pulse durations (370 fs, 100 fs) and repetition rates (500 kHz, 100 Hz, single pulse) and that the grating period can also be tuned by changing the angle of laser beam incidence. The grating structures can be erased by irradiation with a single nanosecond laser pulse, triggering recrystallization of the amorphous stripes. Given the large differences in electrical conductivity between the two phases, our structures could find new applications in nanoelectronics.This work has been supported by the LiNaBioFluid project of the H2020 program of the European Commission (FETOPEN- 665337) as well as by the Spanish TEC2014-52642-C2- 1-R. MG-L and JH-R acknowledge the grants respectively awarded by the Spanish Ministry of Education and the Spanish Ministry of Economy and Competiveness.Peer Reviewe

“Muaco” un yacimiento olvidado del pleistoceno tardío de Venezuela

Venezuela presenta un alto potencial paleontológico, evidenciando en los innumerables registros fósiles reportados a lo largo de la geografía nacional, desde principios del siglo XIX. En este artículo se quiere dar continuidad a la divulgación de algunos de los yacimientos paleontológicos más importantes del territorio venezolano. Se trata del sitio Muaco (Pleistoceno Tardío), el cual fue uno de los primeros yacimientos estudiados de manera sistemática en la región durante la mitad del siglo XIX, y considerado durante mucho tiempo, el sitio con presencia de actividad humana más antiguo en el continente. La excavación del sitio estuvo liderizada por el reconocido geólogo y paleontólogo de origen español Dr. José Royo y Gómez que dejó un valioso legado científico y cultural a la ciencia paleontológica de Venezuela y de las Américas.Venezuela has a high paleontological potential. It has been evidenced with the fossil record reported throughout the national geography since the beginning of the 19th century. In this article, we want to give continuity to the disclosure of some of the most important paleontological sites in Venezuelan territory. It is the Muaco site (Late Pleistocene), which was one of the first sites systematically studied in the region during the mid-nineteenth century, and considered for a long time, the site with the oldest human activity on the continent. The excavation of the site was led by the renowned geologist and paleontologist of Spanish origin Dr. Jose Royo y Gomez, who left a valuable scientific and cultural legacy to the paleontological science of Venezuela and the Americas

Independent control of beam astigmatism and ellipticity using a SLM for fs-laser waveguide writing

We have used a low repetition rate (1 kHz), femtosecond laser amplifier in combination with a spatial light modulator (SLM) to write optical waveguides with controllable cross-section inside a phosphate glass sample. The SLM is used to induce a controllable amount of astigmatism in the beam wavefront while the beam ellipticity is controlled through the propagation distance from the SLM to the focusing optics of the writing setup. The beam astigmatism leads to the formation of two separate diskshaped foci lying in orthogonal planes. Additionally, the ellipticity has the effect of enabling control over the relative peak irradiances of the two foci, making it possible to bring the peak irradiance of one of them below the material transformation threshold. This allows producing a single waveguide with controllable cross-section. Numerical simulations of the irradiance distribution at the focal region under different beam shaping conditions are compared to in situ obtained experimental plasma emission images and structures produced inside the glass, leading to a very satisfactory agreement. Finally, guiding structures with controllable crosssection are successfully produced in the phosphate glass using this approach. © 2009 Optical Society of America.This work was partially supported by the Spanish Ministry of Science and Innovation under TEC2008-01183 project. A. R. and W. G. acknowledge their I3P-CSIC postdoctoral con- tracts (co-funded by the European Social Fund). D. P. and A. F. acknowledge their grants under Projects TEC 2005-00074 and TEC 2006-04538.Peer Reviewe