Electrical current-driven pinhole formation and insulator-metal transition in tunnel junctions

Current Induced Resistance Switching (CIS) was recently observed in thin tunnel junctions (TJs) with ferromagnetic (FM) electrodes and attributed to electromigration of metallic atoms in nanoconstrictions in the insulating barrier. The CIS effect is here studied in TJs with two thin (20 \AA) non-magnetic (NM) Ta electrodes inserted above and below the insulating barrier. We observe resistance (R) switching for positive applied electrical current (flowing from the bottom to the top lead), characterized by a continuous resistance decrease and associated with current-driven displacement of metallic ions from the bottom electrode into the barrier (thin barrier state). For negative currents, displaced ions return into their initial positions in the electrode and the electrical resistance gradually increases (thick barrier state). We measured the temperature (T) dependence of the electrical resistance of both thin- and thick-barrier states ($R_b$ and R$_B$ respectively). Experiments showed a weaker R(T) variation when the tunnel junction is in the $R_b$ state, associated with a smaller tunnel contribution. By applying large enough electrical currents we induced large irreversible R-decreases in the studied TJs, associated with barrier degradation. We then monitored the evolution of the R(T) dependence for different stages of barrier degradation. In particular, we observed a smooth transition from tunnel- to metallic-dominated transport. The initial degradation-stages are related to irreversible barrier thickness decreases (without the formation of pinholes). Only for later barrier degradation stages do we have the appearance of metallic paths between the two electrodes that, however, do not lead to metallic dominated transport for small enough pinhole radius.Comment: 10 pages, 3 figure

Protogynous Interval in Elephantgrass (Pennisetum purpureum Schum.)

A study was carried out to examine the protogyny duration in elephantgrass inflorescences. Nine accessions were selected, divided into three groups, according to early, intermediate or late flowering. From each cultivar, 10 panicles were sampled for evaluation. The criterion for estimating the duration of protogyny was the time interval (number of days) from the first visual detection of exserted stigmas up to the first visual appearance of open anthers (±5%). Duration of protogyny averaged 7.43±1.37 d, confirming that the species is protoginous. Late flowering accessions exhibited an average of 8.97 d for the duration of protogyny, which was significantly higher than the values observed for the intermediate and early flowering accessions. Data also indicate that the duration of protogyny in elephantgrass is efficient to prevent panicle selfcrossing

The compound Poisson limit ruling periodic extreme behaviour of non-uniformly hyperbolic dynamics

We prove that the distributional limit of the normalised number of returns to small neighbourhoods of periodic points of non-uniformly hyperbolic dynamical systems is compound Poisson. The returns to small balls around a fixed point in the phase space correspond to the occurrence of rare events, or exceedances of high thresholds, so that there is a connection between the laws of Return Times Statistics and Extreme Value Laws. The fact that the fixed point in the phase space is a repelling periodic point implies that there is a tendency for the exceedances to appear in clusters whose average sizes is given by the Extremal Index, which depends on the expansion of the system at the periodic point. We recall that for generic points, the exceedances, in the limit, are singular and occur at Poisson times. However, around periodic points, the picture is different: the respective point processes of exceedances converge to a compound Poisson process, so instead of single exceedances, we have entire clusters of exceedances occurring at Poisson times with a geometric distribution ruling its multiplicity. The systems to which our results apply include: general piecewise expanding maps of the interval (Rychlik maps), maps with indifferent fixed points (Manneville-Pomeau maps) and Benedicks-Carleson quadratic maps.Comment: To appear in Communications in Mathematical Physic

Germinação de Eragrostis plana Nees sob efeito do extrato de Achyrocline satureioides (Lam.) DC.

Resumo 31

Variabilidade genética entre acessos de pinhão-manso na fase juvenil.

O pinhão-manso está entre as oleaginosas que podem ser cultivadas para aumentar a produção de biodiesel. Este trabalho teve como objetivo estudar a variabilidade genética entre acessos de pinhão-manso do Banco Ativo de Germoplasma da Universidade Federal de Mato Grosso do Sul, com base em características tomadas na fase juvenil das plantas. Foram avaliados dez acessos de pinhão-manso, oriundos de diferentes locais, tomando-se para avaliação as características: altura de plantas, altura de inserção da primeira folha, diâmetro do caule e número de folhas. A análise estatística foi baseada em modelos mistos do tipo REML/BLUP. Os valores genotípicos preditos dos acessos foram utilizados na realização da análise multivariada para cômputo de distâncias genéticas entre procedências e agrupamento pelo método de Tocher. Os resultados indicaram a presença de considerável variabilidade entre os dez acessos. A herdabilidade de plantas individuais no sentido amplo foi baixa, porém para a herdabilidade da média de acesso foram obtidos altos coeficientes: 56% para altura de plantas, 82% para altura da primeira folha, 76% para diâmetro de caule e 89% para número de folhas. Tais valores indicam a possibilidade de grande precisão na seleção entre populações. No estudo da divergência, os acessos avaliados foram separados em dois grupos distintos: o primeiro grupo com sete acessos similares geneticamente, e o segundo com três acessos

Socio-economic characterization of olive groves in a region in the south of Portugal

Serpa and Moura region, in the South of Portugal, have important agricultural potential as well as important agricultural resources that can and should be used and developed, in spite of economic and social depression signs reinforced by demographic difficulties that can lead to desertification in some areas. The main farms' Technical-Economic Orientation (TEO) is olives production, to produce olive oil, since this region has excellent conditions for the activity development. Some mobilizations traditionally made in olive groves lead to soil erosion and make the farm vehicles mobility much more difficult, namely those used in cultural operations. The maintenance of a soil cover in the olive grove space between lines is good, both because it favours the vehicles mobility, it promotes the rain water infiltration and, last but not the least, the soil airing. The objective of this work has been to make a socio-economic characterization of a zone of Serpa and Moura Councils in which a project of soil covers under olive trees is being developed. At the same time cultural accounts were made for the different situations under the study - irrigated olive grove with spontaneous vegetation in the space between lines, irrigated olive grove with seeded vegetation in the space between lines, irrigated olive grove with herbicide application in the space between lines, dry olive grove with spontaneous vegetation in the space between lines, dry olive grove with seeded vegetation in the space between lines and dry olive grove with herbicide application in the space between lines. The survey shows farmers in this area are younger then usual, although they don't have a high level of formal education, general or specific in agriculture. Farm areas are usually high, which can be determinant for the proposed technology adoption. Installation and operation costs for the olive grove are higher on the irrigated olive grove, but of course expected productions are also higher. For this production technology profits coming from olives production are higher then the operation costs. However, in the dry olive grove technology costs are always higher then the profits, being profitability only due to subsidies. The importance of soil cover maintenance, in this region, beside the benefits in what concerns erosion, infiltration and soil airing, is also due to its capacity to minimize the risk farmer's face

The variability behavior of CoRoT M-giant Stars

For 6 years the Convection, Rotation, and Planetary Transits (CoRoT) space mission has acquired photometric data from more than one hundred thousand point sources towards and directly opposite from the inner and outer regions of the Galaxy. The high temporal resolution of the CoRoT data combined with the wide time span of the observations has enabled the study of short and long time variations in unprecedented detail. From the initial sample of 2534 stars classified as M-giants in the CoRoT databasis, we selected 1428 targets that exhibit well defined variability, using visual inspection. The variability period and amplitude of C1 stars (stars having Teff < 4200 K) were computed using Lomb-Scargle and harmonic fit methods. The trends found in the V-I vs J-K color-color diagram are in agreement with standard empirical calibrations for M-giants. The sources located towards the inner regions of the Galaxy are distributed throughout the diagram while the majority of the stars towards the outer regions of the Galaxy are spread between the calibrations of M-giants and the predicted position for Carbon stars. The stars classified as supergiants follow a different sequence from the one found for giant stars. We also performed a KS test of the period and amplitude of stars towards the inner and outer regions of the Galaxy. We obtained a low probability that the two samples come from the same parent distribution. The observed behavior of the period-amplitude and period-Teff diagrams are, in general, in agreement with those found for Kepler sources and ground based photometry, with pulsation being the dominant cause responsible for the observed modulation. We also conclude that short-time variations on M-Giant stars do not exist orare very rare and the few cases we found are possibly related to biases or background stars.Comment: 11 pages, 6 figure

Two-magnon Raman scattering in spin-ladder geometries and the ratio of rung and leg exchange constants

We discuss ways in which the ratio of exchange constants along the rungs and legs of a spin-ladder material influences the two-magnon Raman scattering spectra and hence can be determined from it. We show that within the Fleury-Loudon-Elliott approach, the Raman line-shape does not change with polarization geometries. This lineshape is well known to be difficult to calculate accurately from theory. However, the Raman scattering intensities do vary with polarization geometries, which are easy to calculate. With some assumptions about the Raman scattering Hamiltonian, the latter can be used to estimate the ratio of exchange constants. We apply these results to Sugai's recent measurements of Raman scattering from spin-ladder materials such as La$_6$Ca$_8$Cu$_{24}$O$_{41}$ and Sr$_{14}$Cu$_{24}$O$_{41}$.Comment: 5 pages, revtex. Latest version focuses on ladder materials, with a detailed examination of the role of Heisenberg-like coupling constants which appear in the Fleury-Loudon-Elliott scattering operator but are rarely discussed in the literatur