6,566 research outputs found
Nanoscopic processes of Current Induced Switching in thin tunnel junctions
In magnetic nanostructures one usually uses a magnetic field to commute
between two resistance (R) states. A less common but technologically more
interesting alternative to achieve R-switching is to use an electrical current,
preferably of low intensity. Such Current Induced Switching (CIS) was recently
observed in thin magnetic tunnel junctions, and attributed to electromigration
of atoms into/out of the insulator. Here we study the Current Induced
Switching, electrical resistance, and magnetoresistance of thin
MnIr/CoFe/AlO/CoFe tunnel junctions. The CIS effect at room temperature
amounts to 6.9% R-change between the high and low states and is attributed to
nanostructural rearrangements of metallic ions in the electrode/barrier
interfaces. After switching to the low R-state some electro-migrated ions
return to their initial sites through two different energy channels. A low
(high) energy barrier of 0.13 eV (0.85 eV) was estimated. Ionic
electromigration then occurs through two microscopic processes associated with
different types of ions sites/defects. Measurements under an external magnetic
field showed an additional intermediate R-state due to the simultaneous
conjugation of the MR (magnetic) and CIS (structural) effects.Comment: 6 pages, 4 figure
Asymptotics of Reaction-Diffusion Fronts with One Static and One Diffusing Reactant
The long-time behavior of a reaction-diffusion front between one static (e.g.
porous solid) reactant A and one initially separated diffusing reactant B is
analyzed for the mean-field reaction-rate density R(\rho_A,\rho_B) =
k\rho_A^m\rho_B^n. A uniformly valid asymptotic approximation is constructed
from matched self-similar solutions in a reaction front (of width w \sim
t^\alpha where R \sim t^\beta enters the dominant balance) and a diffusion
layer (of width W \sim t^{1/2} where R is negligible). The limiting solution
exists if and only if m, n \geq 1, in which case the scaling exponents are
uniquely given by \alpha = (m-1)/2(m+1) and \beta = m/(m+1). In the diffusion
layer, the common ad hoc approximation of neglecting reactions is given
mathematical justification, and the exact transient decay of the reaction rate
is derived. The physical effects of higher-order kinetics (m, n > 1), such as
the broadening of the reaction front and the slowing of transients, are also
discussed.Comment: final version, new title & combustion reference
Diffusion-controlled annihilation with initially separated reactants: The death of an particle island in the particle sea
We consider the diffusion-controlled annihilation dynamics with
equal species diffusivities in the system where an island of particles is
surrounded by the uniform sea of particles . We show that once the initial
number of particles in the island is large enough, then at any system's
dimensionality the death of the majority of particles occurs in the {\it
universal scaling regime} within which of the particles die at
the island expansion stage and the remaining at the stage of its
subsequent contraction. In the quasistatic approximation the scaling of the
reaction zone has been obtained for the cases of mean-field ()
and fluctuation () dynamics of the front.Comment: 4 RevTex pages, 1 PNG figure and 1 EPS figur
Refined Simulations of the Reaction Front for Diffusion-Limited Two-Species Annihilation in One Dimension
Extensive simulations are performed of the diffusion-limited reaction
AB in one dimension, with initially separated reagents. The reaction
rate profile, and the probability distributions of the separation and midpoint
of the nearest-neighbour pair of A and B particles, are all shown to exhibit
dynamic scaling, independently of the presence of fluctuations in the initial
state and of an exclusion principle in the model. The data is consistent with
all lengthscales behaving as as . Evidence of
multiscaling, found by other authors, is discussed in the light of these
findings.Comment: Resubmitted as TeX rather than Postscript file. RevTeX version 3.0,
10 pages with 16 Encapsulated Postscript figures (need epsf). University of
Geneva preprint UGVA/DPT 1994/10-85
Estudos fenológicos de algumas espécies lenhosas e herbáceas da caatinga.
Resumo: Este trabalho, desenvolvido na Fazenda Experimental do Vale do Curu, Pentecoste, Ceará, teve por objetivo estudar a fenologia de espécies lenhosas e herbáceas da caatinga com ênfase nas de interesse apícola cola. A área experimental foi demarcada por quatro transetos que se originaram a partir de um apiário e seguiram a direcão dos pontos cardeais. Ao longo destes transetos 10 plantas arbóreas das espécies mais importantes foram identificadas e etiquetadas. Também, 20 parcelas de 0,5m2 cada, foram marcadas permanentemente para estudos da vegetação herbácea. Durante as estações seca e úmida dos anos de 1986 a 1988 foram feitas avaliações feno lógicas da vegetação a intervalos semanais, no período úmido e quinzenais, no seco. Das espécies lenhosas, o marmeleiro (Croton sonderianus Mül. Arg.), o pau-branco (Auxemma oncocalyx Taub.), o pereiro (Aspidosperma pirifolium Mart.) o sabiá (Mimosa caesalpiniaefolia Bentn.J floresceram no período das cnuvas, enquanto o anglco (Anadenanthera macrocarpa (Benth.) Brenan), a jurema preta (Mimosa acutistipula Benth.) e o Juazeiro (Zizyphus joazeiro Mart.) o fizeram ao longo da estaçãos .e_cTa.o das as espécies herbáceas, que eram anuais, floresceram no período úmido, concentrando sua florada nos meses de abril e maio. Os resultados sugerem que a ocorrência de floradas ao longo de todo o ano torna a vegetação da caatinga adequadamente adaptada à exploração apícola sustentável. [Phenologic studies of wood on herbaceous species of the caatinga]
The incidence of bacterial endosymbionts in terrestrial arthropods.
Intracellular endosymbiotic bacteria are found in many terrestrial arthropods and have a profound influence on host biology. A basic question about these symbionts is why they infect the hosts that they do, but estimating symbiont incidence (the proportion of potential host species that are actually infected) is complicated by dynamic or low prevalence infections. We develop a maximum-likelihood approach to estimating incidence, and testing hypotheses about its variation. We apply our method to a database of screens for bacterial symbionts, containing more than 3600 distinct arthropod species and more than 150 000 individual arthropods. After accounting for sampling bias, we estimate that 52% (CIs: 48-57) of arthropod species are infected with Wolbachia, 24% (CIs: 20-42) with Rickettsia and 13% (CIs: 13-55) with Cardinium. We then show that these differences stem from the significantly reduced incidence of Rickettsia and Cardinium in most hexapod orders, which might be explained by evolutionary differences in the arthropod immune response. Finally, we test the prediction that symbiont incidence should be higher in speciose host clades. But while some groups do show a trend for more infection in species-rich families, the correlations are generally weak and inconsistent. These results argue against a major role for parasitic symbionts in driving arthropod diversification.This is the author accepted manuscript. The final version is available via Royal Society Publishing at http://rspb.royalsocietypublishing.org/content/282/1807/20150249#ack-1
Approach to competences, human development and higher education
Tendo como fundamento a psicologia do desenvolvimento humano, apresenta-se, neste artigo, uma ampliação conceitual à compreensão de competência. Esse é um termo polissêmico, com multiplicidade de concepções epistemológicas, éticas e ideológicas que imprimem complexidade aos processos formativos, especialmente em relação à formação pessoal e profissional na educação superior. Defende-se a noção de competência ancorada na mobilização intencional de diversos recursos próprios ao desenvolvimento humano: processos psicológicos, comportamentos, conhecimentos, afetos, crenças, habilidades, escolhas éticas e estéticas, que devem ser mobilizados pelo sujeito. Apresenta-se, a partir dessa ampliação, uma categorização de competências transversais e possibilidades para sua avaliação. A reflexão pode fundamentar uma atuação crítica de psicólogos e educadores para a mediação do desenvolvimento de competências dos atores do ensino superior.Grounded on human development psychology, this paper presents a conceptual expansion of the understanding about competence. This is a polysemic construct, with a wide range of epistemological, ethical and ideological concepts that reinforce the complexity of training processes, especially in relation to personal and professional training in the context of higher education. This paper defends the notion of competence rooted in intentional mobilization of various resources inherent to human development: psychological processes, behaviors, knowledge, emotions, beliefs, aptitudes, ethical and aesthetic choices that the subject must mobilize. From this conceptual expansion, a categorization of transversal competences and possibilities of assessment are presented. Analyses can support critical actions of psychologists and educators to mediate the development of competences of higher education actors.info:eu-repo/semantics/publishedVersio
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