Mechanical, Electrical, and Magnetic Properties of Ni Nanocontacts

The dynamic deformation upon stretching of Ni nanowires as those formed with mechanically controllable break junctions or with a scanning tunneling microscope is studied both experimentally and theoretically. Molecular dynamics simulations of the breaking process are performed. In addition, and in order to compare with experiments, we also compute the transport properties in the last stages before failure using the first-principles implementation of Landauer's formalism included in our transport package ALACANT.Comment: 5 pages, 6 figure

Formation of a Metallic Contact: Jump to Contact Revisited

The transition from tunneling to metallic contact between two surfaces does not always involve a jump, but can be smooth. We have observed that the configuration and material composition of the electrodes before contact largely determines the presence or absence of a jump. Moreover, when jumps are found preferential values of conductance have been identified. Through combination of experiments, molecular dynamics, and first-principles transport calculations these conductance values are identified with atomic contacts of either monomers, dimers or double-bond contacts.Comment: 4 pages, 5 figure

Coherence Resonance in Chaotic Systems

We show that it is possible for chaotic systems to display the main features of coherence resonance. In particular, we show that a Chua model, operating in a chaotic regime and in the presence of noise, can exhibit oscillations whose regularity is optimal for some intermediate value of the noise intensity. We find that the power spectrum of the signal develops a peak at finite frequency at intermediate values of the noise. These are all signatures of coherence resonance. We also experimentally study a Chua circuit and corroborate the above simulation results. Finally, we analyze a simple model composed of two separate limit cycles which still exhibits coherence resonance, and show that its behavior is qualitatively similar to that of the chaotic Chua systemComment: 4 pages (including 4 figures) LaTeX fil

Numerical Implementation of Gradient Algorithms

A numerical method for computational implementation of gradient dynamical systems is presented. The method is based upon the development of geometric integration numerical methods, which aim at preserving the dynamical properties of the original ordinary differential equation under discretization. In particular, the proposed method belongs to the class of discrete gradients methods, which substitute the gradient of the continuous equation with a discrete gradient, leading to a map that possesses the same Lyapunov function of the dynamical system, thus preserving the qualitative properties regardless of the step size. In this work, we apply a discrete gradient method to the implementation of Hopfield neural networks. Contrary to most geometric integration methods, the proposed algorithm can be rewritten in explicit form, which considerably improves its performance and stability. Simulation results show that the preservation of the Lyapunov function leads to an improved performance, compared to the conventional discretization.Spanish Government project no. TIN2010-16556 Junta de Andalucía project no. P08-TIC-04026 Agencia Española de Cooperación Internacional para el Desarrollo project no. A2/038418/1

Theoretical study of finite temperature spectroscopy in van der Waals clusters. I. Probing phase changes in CaAr_n

The photoabsorption spectra of calcium-doped argon clusters CaAr_n are investigated at thermal equilibrium using a variety of theoretical and numerical tools. The influence of temperature on the absorption spectra is estimated using the quantum superposition method for a variety of cluster sizes in the range 6<=n<=146. At the harmonic level of approximation, the absorption intensity is calculated through an extension of the Gaussian theory by Wadi and Pollak [J. Chem. Phys. vol 110, 11890 (1999)]. This theory is tested on simple, few-atom systems in both the classical and quantum regimes for which highly accurate Monte Carlo data can be obtained. By incorporating quantum anharmonic corrections to the partition functions and respective weights of the isomers, we show that the superposition method can correctly describe the finite-temperature spectroscopic properties of CaAr_n systems. The use of the absorption spectrum as a possible probe of isomerization or phase changes in the argon cluster is discussed at the light of finite-size effects.Comment: 17 pages, 9 figure

Dual partitioning and attachment effects of rhamnolipid on pyrene biodegradation under bioavailability restrictions.

7 páginas.-- 4 figuras.-- 1 tablas.-- 32 referencias.-- Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.envpol.2015.07.013.We investigated the effects of different bioavailability scenarios on the rhamnolipid-enhanced biodegradation of pyrene by the representative polycyclic aromatic hydrocarbon degrader Mycobacterium gilvum VM552. This biosurfactant enhanced biodegradation when pyrene was provided in the form of solid crystals; no effect was observed when the same amount of the chemical was preloaded on polydimethylsiloxane (PDMS). An enhanced effect was observed when pyrene was sorbed into soil but not with the dissolved compound. Synchronous fluorescence spectrophotometry and liquid scintillation were used to determine the phase exchange of pyrene. We also investigated the phase distribution of bacteria. Our results suggest that the rhamnolipid can enhance the biodegradation of pyrene by micellar solubilization and increase diffusive uptake. These mechanisms increase substrate acquisition by bacterial cells at exposure concentrations well above the half-saturation constant for active uptake. The moderate solubilization of pyrene from PDMS by the rhamnolipid and the prevention of cell attachment may explain the lack of enhancement for pyrene-preloaded PDMS.Support for this research was provided by the Spanish Ministry of Science and Innovation (CGL2010-22068-C02-01 and CGL2013-44554-R), the Andalusian Government (RNM 2337) and the FPI Programme (E.C.)Peer reviewe