Surface-induced magnetism in C-doped SnO2_{2}

The magnetism of C-doped SnO$_{2}$ (001) surfaces is studied using first-principles calculations. It is found that carbon does not induce magnetism in bulk SnO$_{2}$ when located at the oxygen site, but shows a large magnetic moment at the SnO$_{2}$ (001) surface. The magnetic moment is mainly contributed by the carbon atoms due to empty minority spins of $p$ orbitals and is localized at the surface and subsurface atoms. No magnetism is observed when the carbon atom is located at the subsurface oxygen sites. The origin of magnetism is discussed in the context of surface bonding.Comment: 3 pages, 3 figure

De la simple trasmisión de información, al “periodista” ciudadano. Retos de la comunicación social contemporánea: Desde a simples transmissão de informações ao cidadão "jornalista". Desafios da comunicação social contemporânea

Se analiza cómo tradicionalmente y hasta nuestros días, por lo general, se ha considerado comunicación a la simple trasmisión de información, a pesar de que desde hace más de cuatro décadas existe un modelo más excelso que parte de la puesta en común y la cooperación mental, entre otros elementos, como condicionantes de la verdadera comunicación interpersonal. Se aborda la importancia de considerar los intereses, necesidades, características, etc. del “otro”, como ente principal del proceso comunicacional. Finalmente se discute el concepto de “periodista ciudadano”, muy arraigado en el mundo de la comunicación social y el periodismo, y se presentan argumentos sólidos que cuestionan y difieren de esa denominación, sin rechazar la participación de los ciudadanos en el proceso de informar, pero sin que ello sea considerado periodismo y mucho menos esos ciudadanos, periodistas

Spin-filtering effect in the transport through a single-molecule magnet Mn12_{12} bridged between metallic electrodes

Electronic transport through a single-molecule magnet Mn$_{12}$ in a two-terminal set up is calculated using the non-equilibrium Green's function method in conjunction with density-functional theory. A single-molecule magnet Mn$_{12}$ is bridged between Au(111) electrodes via thiol group and alkane chains such that its magnetic easy axis is normal to the transport direction. A computed spin-polarized transmission coefficient in zero-bias reveals that resonant tunneling near the Fermi level occurs through some molecular orbitals of majority spin only. Thus, for low bias voltages, a spin-filtering effect such as only one spin component contributing to the conductance, is expected. This effect would persist even with inclusion of additional electron correlations.Comment: Accepted for publication at J. Appl. Phy

Current rectification in molecular junctions produced by local potential fields

The transport properties of a octane-dithiol (ODT) molecule coupled to Au(001) leads are analyzed using density functional theory and non-equilibrium Green functions. It is shown that a symmetric molecule can turn into a diode under influence of a local electric field created by an external charged probe. The origin of the asymmetry of the current--voltage ($I-V$) dependence is traced back to the appearance of a probe induced quasi--local state in the pseudogap of the ODT molecule. The induced state affects electron transport, provided it is close to the Fermi level of the leads. An asymmetric placement of the charged probe along the alkane chain makes the induced quasi--local state in the energy gap very sensitive to the bias voltage and results in rectification of the current. The results based on DFT are supported by independent calculations using a simple one--particle model Hamiltonian.Comment: 7 pages, 6 figure

Dynamically Stable Topological Phase of Arsenene

First-principles calculations based on density functional theory (DFT) are used to investigate the electronic structures and topological phase transition of arsenene under tensile and compressive strains. Buckling in arsenene strongly depends on compressive/tensile strain. The phonons band structures reveal that arsenene is dynamically stable up to 18% tensile strain and the frequency gap between the optical and acoustic branches decreases with strain. The electronic band structures show the direct bandgap decreases with tensile strain and then closes at 13% strain followed by band inversion. With spin-orbit coupling (SOC), the 14% strain-assisted topological insulator phase of arsenene is mainly governed by the p-orbitals. The SOC calculated bandgap is about 43 meV. No imaginary frequency in the phonons is observed in the topological phase of arsenene. The dynamically stable topological phase is accessed through Z2 topological invariant ν using the analysis of the parity of the wave functions at the time-reversal invariant momentum points. The calculated ν is shown to be 1, implying that arsenene is a topological insulator which can be a candidate material for nanoelectronic devices.G.R. acknowledges the higher education commission (HEC) of Pakistan under the project ‘electronic structure calculations using density functional theory’ and the GIK Institute for providing supercomputing facilities. V.M.G.S. thanks the Spanish Ministerio de Economía y Competitividad for funding through the project FIS2015-63918-R and the Spanish Ministerio de Ciencia, Innovación y Universidades for funding through the project PGC2018-094783-B-I00.Peer reviewe

Tuning the electrical conductivity of nanotube-encapsulated metallocene wires

We analyze a new family of carbon nanotube-based molecular wires, formed by encapsulating metallocene molecules inside the nanotubes. Our simulations, that are based on a combination of non-equilibrium Green function techniques and density functional theory, indicate that these wires can be engineered to exhibit desirable magnetotransport effects for use in spintronics devices. The proposed structures should also be resilient to room-temperature fluctuations, and are expected to have a high yield.Comment: 4 pages, 6 figures. Accepted in Physical Review Letter

On determining defects identity in carbon nanotubes using charge probes

A metallic carbon nanotube with point-like defects under influence of a local potential due to a point charge probe is theoretically studied. A combination of density functional theory and the Landauer-Büttiker formalism is used to compute the electronic conductance in the zero-voltage limit. From a collection of the results obtained by varying the probe position around different defects the conductance maps are created. The analysis of the conductance maps allows us to formulate conditions under which several point-like defects (the Stone-Wales defect, a simple carbon vacancy, hydrogen-passivated vacancies) can be distinguished and identified in experiments with the help of scanning probe microscopy.This work has been supported by the National Science Centre under the contract DEC-2012/07/B/ST3/03412. We acknowledge that the results of this research have been achieved using the PRACE-3IP project (FP7 RI-312763) resource JUROPA based in Germany at Julich. We also thank the Spanish MICINN for the Grant No. FIS2012-34858 (V.M.G.S. and J.F.) and the Ramón y Cajal Fellowship No. RYC-2010-06053 (V.M.G.S.).Peer Reviewe

La noción de fracción en los libros de texto del grado tercero: un estudio de análisis de contenido

Este trabajo expone el primer acercamiento formal, según los libros de texto, que deben tener los estudiantes de tercer grado de educación básica primaria en el estudio de las fracciones. Nuestra investigación es fruto de la experiencia al interior del aula escolar, donde se observan dificultades en el proceso de enseñanza-aprendizaje de las fracciones. Centramos la atención en las formas como los libros de texto introducen la noción de fracción desde distintos tipos de representación: gráfica, numérica y del lenguaje natural; razón por la cual se analizaron tres textos a través de la metodología de análisis de contenido, el cual pretende ayudar a cualificar las prácticas escolares, mejorando la competencia de planificación de las clases del docente. Se identificó un favorecimiento en la enseñanza de la fracción como parte todo, con el uso de algunas figuras asociadas a este significado, cuestión que puede constituirse en una fuente de dificultad para la construcción de la noción de fracción y por ello se hace necesario un mayor compromiso por parte de editoriales y docentes para la elaboración y selección de textos escolares apropiados para la enseñanza.This paper presents the first formal approach –according to the textbooks – that students of third grade of elementary school should have when studying fractions. The research is the result of the authors’ experience in the classroom, where difficulties in the teaching-learning process of fractions are observed. The study focuses on the way textbooks introduce the notion of fraction from diverse kinds of representation: graphic, numerical, and natural language. For this reason, three textbooks were analyzed by using the Content Analysis Method, which helps qualify the school practices by improving the teacher competence in lesson planning. An improvement in the teaching of the fraction as a part of a whole, using some of the figures associated to this meaning, was identified. This usually can pose a constraint in the learner’s construction of the notion of fraction; that is why it is relevant that publishing houses and teachers commit in the production and selection of appropriate teaching textbooks