Magnetism in graphene nano-islands

We study the magnetic properties of nanometer-sized graphene structures with triangular and hexagonal shapes terminated by zig-zag edges. We discuss how the shape of the island, the imbalance in the number of atoms belonging to the two graphene sublattices, the existence of zero-energy states, and the total and local magnetic moment are intimately related. We consider electronic interactions both in a mean-field approximation of the one-orbital Hubbard model and with density functional calculations. Both descriptions yield values for the ground state total spin, $S$, consistent with Lieb's theorem for bipartite lattices. Triangles have a finite $S$ for all sizes whereas hexagons have S=0 and develop local moments above a critical size of $\approx 1.5$ nm.Comment: Published versio

Spin-transfer torque on a single magnetic adatom

We theoretically show how the spin orientation of a single magnetic adatom can be controlled by spin polarized electrons in a scanning tunneling microscope configuration. The underlying physical mechanism is spin assisted inelastic tunneling. By changing the direction of the applied current, the orientation of the magnetic adatom can be completely reversed on a time scale that ranges from a few nanoseconds to microseconds, depending on bias and temperature. The changes in the adatom magnetization direction are, in turn, reflected in the tunneling conductance.Comment: 5 pages, 3 figure

Giant magnetoresistance in ultra-small Graphene based devices

By computing spin-polarized electronic transport across a finite zigzag graphene ribbon bridging two metallic graphene electrodes, we demonstrate, as a proof of principle, that devices featuring 100% magnetoresistance can be built entirely out of carbon. In the ground state a short zig-zag ribbon is an antiferromagnetic insulator which, when connecting two metallic electrodes, acts as a tunnel barrier that suppresses the conductance. Application of a magnetic field turns the ribbon ferromagnetic and conducting, increasing dramatically the current between electrodes. We predict large magnetoresistance in this system at liquid nitrogen temperature and 10 Tesla or at liquid helium temperature and 300 Gauss.Comment: 4 pages, 4 figure

Performance limits of graphene-ribbon-based field effect transistors

The performance of field effect transistors based on an single graphene ribbon with a constriction and a single back gate are studied with the help of atomistic models. It is shown how this scheme, unlike that of traditional carbon-nanotube-based transistors, reduces the importance of the specifics of the chemical bonding to the metallic electrodes in favor of the carbon-based part of device. The ultimate performance limits are here studied for various constriction and metal-ribbon contact models. In particular we show that, even for poorly contacting metals, properly taylored constrictions can give promising values for both the on-conductance and the subthreshold swing.Comment: 5 pages, 4 figure

Modeling contact formation between atomic-sized gold tips via molecular dynamics

The formation and rupture of atomic-sized contacts is modelled by means of molecular dynamics simulations. Such nano-contacts are realized in scanning tunnelling microscope and mechanically controlled break junction experiments. These instruments routinely measure the conductance across the nano-sized electrodes as they are brought into contact and separated, permitting conductance traces to be recorded that are plots of conductance versus the distance between the electrodes. One interesting feature of the conductance traces is that for some metals and geometric configurations a jump in the value of the conductance is observed right before contact between the electrodes, a phenomenon known as jump-to-contact. This paper considers, from a computational point of view, the dynamics of contact between two gold nano-electrodes. Repeated indentation of the two surfaces on each other is performed in two crystallographic orientations of face-centred cubic gold, namely (001) and (111). Ultimately, the intention is to identify the structures at the atomic level at the moment of first contact between the surfaces, since the value of the conductance is related to the minimum cross-section in the contact region. Conductance values obtained in this way are determined using first principles electronic transport calculations, with atomic configurations taken from the molecular dynamics simulations serving as input structures.Comment: 6 pages, 4 figures, conference submissio

Benchmarking and viability assessment of optical packet switching for metro networks

Optical packet switching (OPS) has been proposed as a strong candidate for future metro networks. This paper assesses the viability of an OPS-based ring architecture as proposed within the research project DAVID (Data And Voice Integration on DWDM), funded by the European Commission through the Information Society Technologies (IST) framework. Its feasibility is discussed from a physical-layer point of view, and its limitations in size are explored. Through dimensioning studies, we show that the proposed OPS architecture is competitive with respect to alternative metropolitan area network (MAN) approaches, including synchronous digital hierarchy, resilient packet rings (RPR), and star-based Ethernet. Finally, the proposed OPS architectures are discussed from a logical performance point of view, and a high-quality scheduling algorithm to control the packet-switching operations in the rings is explained

Determinación de curvas para la estimación del contenido de humedad de diez maderas de frondosas mediante medidas de resistencia eléctrica

Accuracy in moisture content measurement is of great importance in the assurance of wood product quality and isnecessary to meet administrative and normative requirements. Improving the accuracy of resistance-type moisture meters,and meeting the normative demands of their annual calibration, requires the use of optimised curves relating electricalresistance to moisture content for the most commercially important wood types. The Samuelsson model, adjusted bylinear regression techniques, was used to describe the relationship between the electrical resistance and moisture contentof seven boreal and three tropical hardwoods available on the Spanish market. The curves produced can be used to predictthe moisture contents of these hardwoods via the measurement of their electrical resistance with an error of just ± 1.0%.These curves should also prove of great use in the calibration of wood resistance-type moisture meters.La precisión en la medida del contenido de humedad juega un papel fundamental no solo en el aseguramiento de lacalidad de los productos de la madera sino, también, en el cumplimiento de las exigencias normativas y administrativasque gravitan sobre los productos. Para ser capaces de mejorar la precisión de la medida tomada por los xilohigrómetrosde resistencia y cumplir con las exigencias normativas de calibración anual de los equipos, es necesario obtener curvasoptimizadas que relacionen la resistencia eléctrica con la humedad en las maderas de mayor importancia comercial.El modelo de Samuelsson, Log[Log(R) + 1] = a.h + b, ajustado mediante técnicas de regresión lineal, fue usado paradescribir la relación existente entre la resistencia eléctrica de la madera (R) y el contenido de humedad (h) de sietemaderas de frondosas boreales y tres de frondosas tropicales comercialmente disponibles en el mercado español. Lascurvas obtenidas pueden ser usadas para la predicción del contenido de humedad de dichas maderas, con un errormáximo de ± 1.0%, mediante la medición de su resistencia eléctrica. Estas curvas serían también de interés para lacalibración de medidores de humedad de madera por resistencia eléctrica

La reforma de la formación inicial del profesorado de ciencias de secundaria: propuesta de un diseño del currículo basado en competencias

El principal objetivo de este trabajo es proponer un mecanismo de selección de competencias específicas para la formación inicial del profesorado de ciencias experimentales, contando para ello con distintas fuentes relevantes para dicha formación. Con tal fin hemos hecho uso de un procedimiento consistente en ir incorporando de forma crítica las competencias procedentes de las siguientes fuentes curriculares: los modelos de formación inicial del profesorado vigentes en la literatura educativa, el currículo actual de ciencias en la educación secundaria obligatoria en España, el Programa internacional de evaluación PISA, la opinión del profesorado en activo mediante una encuesta cuyos resultados más relevantes son mostrados, las concepciones epistemológicas del profesorado, la profesionalización docente y las demandas que genera y, finalmente, la dimensión social de la ciencia.The primary objective of this paper is to propose a mechanism for selecting specific competences for preservice science teacher training, referring to relevant sources for such training. To this end we have used an trans-disciplinary procedure consistent with incorporating critical competences from the following curricular sources: Models of current preservice teacher training in educational literature, the current curriculum of Science in Compulsory Secondary Education in Spain, the Program for International Student Assessment (PISA), the views of in-service secondary school teachers (through a survey whose most relevant results are given), teachers' epistemological conceptions, the professionalization of teaching and the demands it generates and, finally the social dimension of science