Vortex rectification effects in films with periodic asymmetric pinning

We study the transport of vortices excited by an ac current in an Al film with an array of nanoengineered asymmetric antidots. The vortex response to the ac current is investigated by detailed measurements of the voltage output as a function of ac current amplitude, magnetic field and temperature. The measurements revealed pronounced voltage rectification effects which are mainly characterized by the two critical depinning forces of the asymmetric potential. The shape of the net dc voltage as a function of the excitation amplitude indicates that our vortex ratchet behaves in a way very different from standard overdamped models. Rather, as demonstrated by the observed output signal, the repinning force, necessary to stop vortex motion, is considerably smaller than the depinning force, resembling the behavior of the so-called inertia ratchets. Calculations based on an underdamped ratchet model provide a very good fit to the experimental data.Comment: 5 pages, 4 figure

Dipole-induced vortex ratchets in superconducting films with arrays of micromagnets

We investigate the transport properties of superconducting films with periodic arrays of in-plane magnetized micromagnets. Two different magnetic textures are studied: a square array of magnetic bars and a close-packed array of triangular microrings. As confirmed by MFM imaging, the magnetic state of both systems can be adjusted to produce arrays of almost point-like magnetic dipoles. By carrying out transport measurements with ac drive, we observed experimentally a recently predicted ratchet effect induced by the interaction between superconducting vortices and the magnetic dipoles. Moreover, we find that these magnetic textures produce vortex-antivortex patterns, which have a crucial role on the transport properties of this hybrid system.Comment: 4 pages, 4 figure

Tipo de estaca e concentração de ácido indol butírico no enraizamento de alecrim da chapada.

Suplemento. Edição dos Anais do 52 Congresso Brasileiro de Olericultura, Salvador, jul. 2012

Local mapping of dissipative vortex motion

We explore, with unprecedented single vortex resolution, the dissipation and motion of vortices in a superconducting ribbon under the influence of an external alternating magnetic field. This is achieved by combing the phase sensitive character of ac-susceptibility, allowing to distinguish between the inductive-and dissipative response, with the local power of scanning Hall probe microscopy. Whereas the induced reversible screening currents contribute only inductively, the vortices do leave a fingerprint in the out-of-phase component. The observed large phase-lag demonstrates the dissipation of vortices at timescales comparable to the period of the driving force (i.e. 13 ms). These results indicate the presence of slow microscopic loss mechanisms mediated by thermally activated hopping transport of vortices between metastable states.Comment: 5 pages, 2 figure

Comportamento produtivo de genótipos de mandioca em três microrregiões do Piauí.

Com o objetivo de estudar-se o comportamento produtivo de 14 genótipos de mandioca, foram conduzidos seis experimentos em três microrregiões do Estado do Piauí. Os experimentos foram instalados nos municípios de Teresina, anos agrícolas de 1995/96, 1996/97 e 1997/98; Parnaíba, em 1996/97 e 1997/98; e Jerumenha, em 1997/98. O delineamento experimental foi blocos ao acaso com quatro repetições. Os genótipos usados foram: Vermelhinha (local), Maracanã, Fio de Ouro, Branquinha, Aipim Bahia, Macaxeira Branca, Pindaré, Engana Ladrão, Manipeba Branca, Aipim Bravo, Jaburu e os clones 8707/05, 8615/09 e 8611/18. Os genótipos foram plantados em parcelas de 4,00 m x 8,40 m, no espaçamento de 1 ,O0 m entre linhas e 0,60 m dentro das linhas, com uma área útil de 14,40 m2. Realizou-se a colheita 12 meses após o plantio. Os genótipos Manipeba Branca e Vermelhinha apresentaram as maiores percentagens de amido nos três anos e locais de avaliação. Em relação as demais características avaliadas, constatou-se que, em Teresina, no ano agrícola de 1995/96, os genótipos Aipim Bahia, Pindaré e Branquinha foram os mais promissores em relação a estande final e rendimento de raízes. Em 1996/97 e 1997/98, em Teresina e Parnaíba, o clone 8707/05 apresentou maiores rendimentos de raízes frescas e de matéria seca de raízes. Ainda em Teresina, em 1995/96 e 1996/97, o genótipo Maracanã apresentou os maiores rendimentos de parte aérea; em 1997/98, o genótipo Aipim Bahia superou os demais nessa característica. Em Jerumenha, o genótipo Vermelhinha apresentou os maiores rendimentos de raízes frescas, matéria seca nas raízes e de parte aérea. Em Parnaíba, o maior rendimento de parte aérea foi obtido com o clone 8611/18 em 1996/97 e Maracanã e Aipim Bravo em 1997/98. Os genótipos testados apresentaram comportamentos diferentes, variando com o ano e local, exceto em relação a percentagem de amido.bitstream/item/35783/1/BP27.pd

Spin-polarized current and shot noise in the presence of spin flip in a quantum dot via nonequilibrium Green's functions

Using non-equilibrium Green functions we calculate the spin-polarized current and shot noise in a ferromagnet--quantum-dot--ferromagnet (FM-QD-FM) system. Both parallel (P) and antiparallel (AP) magnetic configurations are considered. Coulomb interaction and coherent spin-flip (similar to a transverse magnetic field) are taken into account within the dot. We find that the interplay between Coulomb interaction and spin accumulation in the dot can result in a bias-dependent current polarization $\wp$. In particular, $\wp$ can be suppressed in the P alignment and enhanced in the AP case depending on the bias voltage. The coherent spin-flip can also result in a switch of the current polarization from the emitter to the collector lead. Interestingly, for a particular set of parameters it is possible to have a polarized current in the collector and an unpolarized current in the emitter lead. We also found a suppression of the Fano factor to values well below 0.5.Comment: Published version. 13 pages, 7 figure

The Overlooked Potential of Generalized Linear Models in Astronomy - I: Binomial Regression

Revealing hidden patterns in astronomical data is often the path to fundamental scientific breakthroughs; meanwhile the complexity of scientific inquiry increases as more subtle relationships are sought. Contemporary data analysis problems often elude the capabilities of classical statistical techniques, suggesting the use of cutting edge statistical methods. In this light, astronomers have overlooked a whole family of statistical techniques for exploratory data analysis and robust regression, the so-called Generalized Linear Models (GLMs). In this paper -- the first in a series aimed at illustrating the power of these methods in astronomical applications -- we elucidate the potential of a particular class of GLMs for handling binary/binomial data, the so-called logit and probit regression techniques, from both a maximum likelihood and a Bayesian perspective. As a case in point, we present the use of these GLMs to explore the conditions of star formation activity and metal enrichment in primordial minihaloes from cosmological hydro-simulations including detailed chemistry, gas physics, and stellar feedback. We predict that for a dark mini-halo with metallicity $\approx 1.3 \times 10^{-4} Z_{\bigodot}$, an increase of $1.2 \times 10^{-2}$ in the gas molecular fraction, increases the probability of star formation occurrence by a factor of 75%. Finally, we highlight the use of receiver operating characteristic curves as a diagnostic for binary classifiers, and ultimately we use these to demonstrate the competitive predictive performance of GLMs against the popular technique of artificial neural networks.Comment: 20 pages, 10 figures, 3 tables, accepted for publication in Astronomy and Computin