250 research outputs found
Shot Noise in Linear Macroscopic Resistors
We report on a direct experimental evidence of shot noise in a linear
macroscopic resistor. The origin of the shot noise comes from the fluctuation
of the total number of charge carriers inside the resistor associated with
their diffusive motion under the condition that the dielectric relaxation time
becomes longer than the dynamic transit time. Present results show that neither
potential barriers nor the absence of inelastic scattering are necessary to
observe shot noise in electronic devices.Comment: 10 pages, 5 figure
Fluctuations of Complex Networks: Electrical Properties of Single Protein Nanodevices
We present for the first time a complex network approach to the study of the
electrical properties of single protein devices. In particular, we consider an
electronic nanobiosensor based on a G-protein coupled receptor. By adopting a
coarse grain description, the protein is modeled as a complex network of
elementary impedances. The positions of the alpha-carbon atoms of each amino
acid are taken as the nodes of the network. The amino acids are assumed to
interact electrically among them. Consequently, a link is drawn between any
pair of nodes neighboring in space within a given distance and an elementary
impedance is associated with each link. The value of this impedance can be
related to the physical and chemical properties of the amino acid pair and to
their relative distance. Accordingly, the conformational changes of the
receptor induced by the capture of the ligand, are translated into a variation
of its electrical properties. Stochastic fluctuations in the value of the
elementary impedances of the network, which mimic different physical effects,
have also been considered. Preliminary results concerning the impedance
spectrum of the network and its fluctuations are presented and discussed for
different values of the model parameters.Comment: 16 Pages and 10 Figures published in SPIE Proceedings of the II
International Symposium on Fluctuation and Noise, Maspalomas,Gran
Canaria,Spain, 25-28 May 200
Challenges for artificial cognitive systems
The declared goal of this paper is to fill this gap: “... cognitive systems research needs questions or challenges that define progress. The challenges are not (yet more) predictions of the future, but a guideline to what are the aims and what would constitute progress.” – the quotation being from the project description of EUCogII, the project for the European Network for Cognitive Systems within which this formulation of the ‘challenges’ was originally developed (http://www.eucognition.org). So, we stick out our neck and formulate the challenges for artificial cognitive systems. These challenges are articulated in terms of a definition of what a cognitive system is: a system that learns from experience and uses its acquired knowledge (both declarative and practical) in a flexible manner to achieve its own goals
Modelization of Thermal Fluctuations in G Protein-Coupled Receptors
We simulate the electrical properties of a device realized by a G protein
coupled receptor (GPCR), embedded in its membrane and in contact with two
metallic electrodes through which an external voltage is applied. To this
purpose, recently, we have proposed a model based on a coarse graining
description, which describes the protein as a network of elementary impedances.
The network is built from the knowledge of the positions of the C-alpha atoms
of the amino acids, which represent the nodes of the network. Since the
elementary impedances are taken depending of the inter-nodes distance, the
conformational change of the receptor induced by the capture of the ligand
results in a variation of the network impedance. On the other hand, the
fluctuations of the atomic positions due to thermal motion imply an impedance
noise, whose level is crucial to the purpose of an electrical detection of the
ligand capture by the GPCR. Here, in particular, we address this issue by
presenting a computational study of the impedance noise due to thermal
fluctuations of the atomic positions within a rhodopsin molecule. In our model,
the C-alpha atoms are treated as independent, isotropic, harmonic oscillators,
with amplitude depending on the temperature and on the position within the
protein (alpha-helix or loop). The relative fluctuation of the impedance is
then calculated for different temperatures.Comment: 5 pages, 2 figures, Proceeding of the 18-th International Conference
on Fluctuations and Noise, 19-23 September 2005, Salamanca, Spain -minor
proofreadings
Shot-noise anomalies in nondegenerate elastic diffusive conductors
We present a theoretical investigation of shot-noise properties in
nondegenerate elastic diffusive conductors. Both Monte Carlo simulations and
analytical approaches are used. Two new phenomena are found: (i) the display of
enhanced shot noise for given energy dependences of the scattering time, and
(ii) the recovery of full shot noise for asymptotic high applied bias. The
first phenomenon is associated with the onset of negative differential
conductivity in energy space that drives the system towards a dynamical
electrical instability in excellent agreement with analytical predictions. The
enhancement is found to be strongly amplified when the dimensionality in
momentum space is lowered from 3 to 2 dimensions. The second phenomenon is due
to the suppression of the effects of long range Coulomb correlations that takes
place when the transit time becomes the shortest time scale in the system, and
is common to both elastic and inelastic nondegenerate diffusive conductors.
These phenomena shed new light in the understanding of the anomalous behavior
of shot noise in mesoscopic conductors, which is a signature of correlations
among different current pulses.Comment: 9 pages, 6 figures. Final version to appear in Phys. Rev.
Coupled-mode theory for photonic band-gap inhibition of spatial instabilities
We study the inhibition of pattern formation in nonlinear optical systems using intracavity photonic crystals. We consider mean-field models for singly and doubly degenerate optical parametric oscillators. Analytical expressions for the new (higher) modulational thresholds and the size of the "band gap" as a function of the system and photonic crystal parameters are obtained via a coupled-mode theory. Then, by means of a nonlinear analysis, we derive amplitude equations for the unstable modes and find the stationary solutions above threshold. The form of the unstable mode is different in the lower and upper parts of the band gap. In each part there is bistability between two spatially shifted patterns. In large systems stable wall defects between the two solutions are formed and we provide analytical expressions for their shape. The analytical results are favorably compared with results obtained from the full system equations. Inhibition of pattern formation can be used to spatially control signal generation in the transverse plane
Enhanced thermal stability and fracture toughness of TiAlN coatings by Cr, Nb and V-alloying
The effect of metal alloying on mechanical properties including hardness and fracture toughness were investigated in three alloys, Ti~0.33Al0.50(Me)~0.17N (Me¿=¿Cr, Nb and V), and compared to Ti0.50Al0.50N, in the as-deposited state and after annealing. All studied alloys display similar as-deposited hardness while the hardness evolution during annealing is found to be connected to phase transformations, related to the alloy's thermal stability. The most pronounced hardening was observed in Ti0.50Al0.50N, while all the coatings with additional metal elements sustain their hardness better and they are harder than Ti0.50Al0.50N after annealing at 1100¿°C. Fracture toughness properties were extracted from scratch tests. In all tested conditions, as-deposited and annealed at 900 and 1100¿°C, Ti0.33Al0.50Nb0.17N show the least surface and sub-surface damage when scratched despite the differences in decomposition behavior and h-AlN formation. Theoretically estimated ductility of phases existing in the coatings correlates well with their crack resistance. In summary, Ti0.33Al0.50Nb0.17N is the toughest alloy in both as-deposited and post-annealed states.Peer ReviewedPostprint (author's final draft
Managing family conflict and resilience. Results of a universal socio-educative family drugs prevention program developed in school settings
This paper assesses changes in family conflict and resilience among families participating in a socio-educative short universal drug prevention program (PCF-U 11-14). A pre-post test quasi experimental design with control and experimental groups was implemented with 275 families. The work addresses the convenience of family training in social and parenting skills to strengthen families’ capacity to cope with difficulties and boosting family cohesion, but it also highlights the need to research deeper into the factors that affect parent and adolescent conflict to create new training strategies for families.Este trabajo evalúa los cambios en los conflictos familiares y la resiliencia entre las familias que participan en un programa socioeducativo universal, de corta duración, para la prevención de drogas (PCFU 11-14). Se implementó un diseño cuasi experimental pre-post test con grupos control y experimental con 275 familias. Se aborda la conveniencia de la capacitación familiar en habilidades sociales y de crianza para fortalecer la capacidad de las familias para hacer frente a las dificultades y fomentar la cohesión familiar, pero también se destaca la necesidad de investigar más a fondo los factores que afectan al conflicto entre padres y adolescentes para crear nuevas estrategias de capacitación para familias
Stationary states and phase diagram for a model of the Gunn effect under realistic boundary conditions
A general formulation of boundary conditions for semiconductor-metal contacts
follows from a phenomenological procedure sketched here. The resulting boundary
conditions, which incorporate only physically well-defined parameters, are used
to study the classical unipolar drift-diffusion model for the Gunn effect. The
analysis of its stationary solutions reveals the presence of bistability and
hysteresis for a certain range of contact parameters. Several types of Gunn
effect are predicted to occur in the model, when no stable stationary solution
exists, depending on the value of the parameters of the injecting contact
appearing in the boundary condition. In this way, the critical role played by
contacts in the Gunn effect is clearly stablished.Comment: 10 pages, 6 Post-Script figure
Phase-space structure of two-dimensional excitable localized structures
In this work we characterize in detail the bifurcation leading to an
excitable regime mediated by localized structures in a dissipative nonlinear
Kerr cavity with a homogeneous pump. Here we show how the route can be
understood through a planar dynamical system in which a limit cycle becomes the
homoclinic orbit of a saddle point (saddle-loop bifurcation). The whole picture
is unveiled, and the mechanism by which this reduction occurs from the full
infinite-dimensional dynamical system is studied. Finally, it is shown that the
bifurcation leads to an excitability regime, under the application of suitable
perturbations. Excitability is an emergent property for this system, as it
emerges from the spatial dependence since the system does not exhibit any
excitable behavior locally.Comment: 10 pages, 9 figure
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