Impact of noise on a dynamical system: prediction and uncertainties from a swarm-optimized neural network

In this study, an artificial neural network (ANN) based on particle swarm optimization (PSO) was developed for the time series prediction. The hybrid ANN+PSO algorithm was applied on Mackey--Glass chaotic time series in the short-term $x(t+6)$. The performance prediction was evaluated and compared with another studies available in the literature. Also, we presented properties of the dynamical system via the study of chaotic behaviour obtained from the predicted time series. Next, the hybrid ANN+PSO algorithm was complemented with a Gaussian stochastic procedure (called {\it stochastic} hybrid ANN+PSO) in order to obtain a new estimator of the predictions, which also allowed us to compute uncertainties of predictions for noisy Mackey--Glass chaotic time series. Thus, we studied the impact of noise for several cases with a white noise level ($\sigma_{N}$) from 0.01 to 0.1.Comment: 11 pages, 8 figure

Spin Pumping and Inverse Spin Hall Effect in Platinum: The Essential Role of Spin-Memory Loss at Metallic Interfaces

Through combined ferromagnetic resonance, spin-pumping and inverse spin Hall effect experiments in Co|Pt bilayers and Co|Cu|Pt trilayers, we demonstrate consistent values of spin diffusion length $\ell_{\rm sf}^{\rm Pt}=3.4\pm0.4$ nm and of spin Hall angle $\theta_{\rm SHE}^{\rm Pt}=0.051\pm0.004$ for Pt. Our data and model emphasize on the partial depolarization of the spin current at each interface due to spin-memory loss. Our model reconciles the previously published spin Hall angle values and explains the different scaling lengths for the ferromagnetic damping and the spin Hall effect induced voltage.Comment: 6 pages, 3 figures (main text) and 8 pages supplementary. Published with small modifications in Phys. Rev. Let

Experimental evidences of a large extrinsic spin Hall effect in AuW alloy

We report an experimental study of a gold-tungsten alloy (7% at. W concentration in Au host) displaying remarkable properties for spintronics applications using both magneto-transport in lateral spin valve devices and spin-pumping with inverse spin Hall effect experiments. A very large spin Hall angle of about 10% is consistently found using both techniques with the reliable spin diffusion length of 2 nm estimated by the spin sink experiments in the lateral spin valves. With its chemical stability, high resistivity and small induced damping, this AuW alloy may find applications in the nearest future

Optical photometry and spectroscopy of the 1987A-like supernova 2009mw

We present optical photometric and spectroscopic observations of the 1987A-like supernova (SN) 2009mw. Our $BVRI$ and $g'r'i'z'$ photometry covers 167 days of evolution, including the rise to the light curve maximum, and ends just after the beginning of the linear tail phase. We compare the observational properties of SN 2009mw with those of other SNe belonging to the same subgroup, and find that it shows similarities to several objects. The physical parameters of the progenitor and the SN are estimated via hydrodynamical modelling, yielding an explosion energy of $1$ foe, a pre-SN mass of $19\,{\rm M_{\odot}}$, a progenitor radius as $30\,{\rm R_{\odot}}$ and a $^{56}$Ni mass as $0.062\,{\rm M_{\odot}}$. These values indicate that the progenitor of SN 2009mw was a blue supergiant star, similar to the progenitor of SN 1987A. We examine the host environment of SN 2009mw and find that it emerged from a population with slightly sub-solar metallicty.Comment: 11 pages, 12 figures, accepted for publication in MNRA

A cluster mode-coupling approach to weak gelation in attractive colloids

Mode-coupling theory (MCT) predicts arrest of colloids in terms of their volume fraction, and the range and depth of the interparticle attraction. We discuss how effective values of these parameters evolve under cluster aggregation. We argue that weak gelation in colloids can be idealized as a two-stage ergodicity breaking: first at short scales (approximated by the bare MCT) and then at larger scales (governed by MCT applied to clusters). The competition between arrest and phase separation is considered in relation to recent experiments. We predict a long-lived `semi-ergodic' phase of mobile clusters, showing logarithmic relaxation close to the gel line.Comment: 4 pages, 3 figure

Carbon Nitrogen, and Oxygen Galactic Gradients: A Solution to the Carbon Enrichment Problem

Eleven models of Galactic chemical evolution, differing in the carbon, nitrogen,and oxygen yields adopted, have been computed to reproduce the Galactic O/H values obtained from H II regions. All the models fit the oxygen gradient, but only two models fit also the carbon gradient, those based on carbon yields that increase with metallicity due to stellar winds in massive stars (MS) and decrease with metallicity due to stellar winds in low and intermediate mass stars (LIMS). The successful models also fit the C/O versus O/H evolution history of the solar vicinity obtained from stellar observations. We also compare the present day N/H gradient and the N/O versus O/H and the C/Fe, N/Fe, O/Fe versus Fe/H evolution histories of the solar vicinity predicted by our two best models with those derived from H II regions and from stellar observations. While our two best models fit the C/H and O/H gradients as well as the C/O versus O/H history, only Model 1 fits well the N/H gradient and the N/O values for metal poor stars but fails to fit the N/H values for metal rich stars. Therefore we conclude that our two best models solve the C enrichment problem, but that further work needs to be done on the N enrichment problem. By adding the C and O production since the Sun was formed predicted by Models 1 and 2 to the observed solar values we find an excellent agreement with the O/H and C/H values of the solar vicinity derived from H II regions O and C recombination lines. One of the most important results of this paper is that the fraction of carbon due to MS and LIMS in the interstellar medium is strongly dependent on time and on the galactocentric distance; at present about half of the carbon in the interstellar medium of the solar vicinity has been produced by MS and half by LIMS.Comment: 34 pages, 6 tables, 7 figures. Accepted for publication in Ap

Quantum Phase Transitions in Josephson Junction Chains

We investigate the quantum phase transition in a one-dimensional chain of ultra-small superconducting grains, considering both the self- and junction capacitances. At zero temperature, the system is transformed into a two-dimensional system of classical vortices, where the junction capacitance introduces anisotropy in the interaction between vortices. This leads to the superconductor-insulator transition of the Berezinskii-Kosterlitz-Thouless type, as the ratios of the Josephson coupling energy to the charging energies are varied. It is found that the junction capacitance plays a role similar to that of dissipation and tends to suppress quantum fluctuations; nevertheless the insulator region survives even for arbitrarily large values of the junction capacitance.Comment: REVTeX+5 EPS figures, To appear in PRB Rapid