Ultrafast all-optical Nth-order differentiator based on chirped fiber Bragg gratings

In this letter we present a technique for the implementation of Nth-order ultrafast temporal differentiators. This technique is based on two oppositely chirped fiber Bragg gratings in which the grating profile maps the spectral response of the Nth-order differentiator. Examples of 1st, 2nd, and 4th order differentiators are designed and numerically simulated

Adding Acanthodactylus beershebensis to the mtDNA phylogeny of the Acanthodactylus pardalis group

The phylogenetic affinities of Acanthodactylus beershebensis, a highly endangered lacertid lizard endemic to the Neguev (Israel), were assessed using mtDNA markers. Fragments of 12S and 16S rRNA were analysed and compared with already published sequences of Acanthodactylus. Results corroborate the taxonomic placement of A. beershebensis as a member of the A. pardalis group but place it within a polytomy at the same phylogenetic level as other (unnamed) African populations. This pattern of high but poorly structured genetic diversity, previously observed for other Acanthodactylus complexes, has been suggested to derive from the climatic instability of North Africa and the Middle East during the humid and dry periods of the Pleistocene as well as dune migrations. In conservation terms, if A. beershebensis is to be prioritised, then other populations of the A. pardalis group inhabiting North Africa would deserve a similar status, making their species definition urgent. These results highlight the need for considering phylogeny when establishing conservation priorities.publishe

Modifications in the distribution of met-enkephalin in the cat spinal cord after administration of clonidine. An immunocytochemical study

We have studied the modifications in the distribution of methionine-enkephalin in the cat spinal cord after intravenous or intrathecal administration of clonidine by using an immunocytochemical technique. In animals not treated with the substance, a very high density of immunoreactive fibers was found in layers I and 11; a high density in the dorso-lateral funiculus and in the reticular formation; a moderate density in layers 111, IV and V; and a low density in layer VI. However, after intravenous or intrathecal administration of clonidine a decrease in fibers containing met-enkephalin was observed in layers I and I1 (high or moderate density), the dorso-lateral funiculus, and the reticular formation (moderate or low density), and in layers IV and V (low or very low density). In all cases, the decrease in the immunoreactivity was more marked when clonidine was administered intrathecally. Our results suggest that clonidine induces the release of metenkephalin in the spinal cord. They further suggest that the opioid peptide released could be involved in the control of nociceptive transmission by inhibiting the release of neurotransmitters (e.g., substance P). In summary, our study shows that clonidine could be involved in antinociceptive mechanisms in the cat spinal cord

Proposed flat-topped pulses bursts generation using all-pass multi-cavity structures

We propose a simple lossless method for the generation of flat-topped intensity pulses bursts from a single utrashort pulse. We have found optimum solutions corresponding to different numbers of cavities and burst pulses, showing that the proposed all-pass structures of optical cavities, properly designed, can generate close to flat-topped pulse busts

Mean-field limit of systems with multiplicative noise

A detailed study of the mean-field solution of Langevin equations with multiplicative noise is presented. Three different regimes depending on noise-intensity (weak, intermediate, and strong-noise) are identified by performing a self-consistent calculation on a fully connected lattice. The most interesting, strong-noise, regime is shown to be intrinsically unstable with respect to the inclusion of fluctuations, as a Ginzburg criterion shows. On the other hand, the self-consistent approach is shown to be valid only in the thermodynamic limit, while for finite systems the critical behavior is found to be different. In this last case, the self-consistent field itself is broadly distributed rather than taking a well defined mean value; its fluctuations, described by an effective zero-dimensional multiplicative noise equation, govern the critical properties. These findings are obtained analytically for a fully connected graph, and verified numerically both on fully connected graphs and on random regular networks. The results presented here shed some doubt on what is the validity and meaning of a standard mean-field approach in systems with multiplicative noise in finite dimensions, where each site does not see an infinite number of neighbors, but a finite one. The implications of all this on the existence of a finite upper critical dimension for multiplicative noise and Kardar-Parisi-Zhang problems are briefly discussed.Comment: 9 Pages, 8 Figure

Temporal Griffiths Phases

Disorder is an unavoidable ingredient of real systems. Spatial disorder generates Griffiths phases (GPs) which, in analogy to critical points, are characterized by a slow relaxation of the order parameter and divergences of quantities such as the susceptibility. However, these singularities appear in an extended region of the parameter space and not just at a (critical) point, i.e. there is generic scale invariance. Here, we study the effects of temporal disorder, focusing on systems with absorbing states. We show that for dimensions $d \geq 2$ there are Temporal Griffiths phases (TGPs) characterized by generic power-law spatial scaling and generic divergences of the susceptibility. TGPs turn out to be a counterpart of GPs, but with space and time playing reversed roles. TGPs constitute a unifying concept, shedding light on the non-trivial effects of temporal disorder.Comment: 4 pages, 3 figures; Accepted in PR

Orden y estructuras algebraicas mediante nuevas tecnologías

En un conjunto, los elementos son comparables o no según cierto criterio (tamaño, edad, peso, longitud, categoría,...). Así en el conjunto N de los números naturales (los que utilizamos para contar) todos sus elementos son comparables por un criterio conocido, “ser mayor o menor que”:0 ≤ 1 ≤ 2 ≤ 3 ≤ 4 ≤ 5 ≤ 6 ≤ ... ≤ n ≤ n + 1 ≤ ....A esto se le llama “orden”. La ordenación no siempre será tan sencilla como en el caso anterior, ya que puede haber elementos que no estén relacionados entre si. Otro problema que encuentra el alumno al trabajar con este tipo de conjuntos es la similitud entre los diversos elementos notables de un conjunto ordenado, a saber, máximo, mínimo, supremo, ínfimo, elementos maximales y minimales.Algunos órdenes dan lugar a la estructura algebraica de retículo, y en particular, a las álgebras de Boole. Las mismas herramientas nos valen para estudiar si un conjunto ordenado es un retículo o un álgebra de Boole. Todos estos conceptos son relevantes, por ejemplo por su aplicación a lainformática: órdenes usados en bases de datos relacionales, estudio de problemas relativos a clasificación, como ejemplo de estructura de álgebra de Boole tenemos el conjunto de las funciones booleanas cuya aplicación a circuitos electrónicos es directa... De hecho, toda la informática está basada en la estructura de álgebra de Boole.Teniendo en cuenta lo abstracto que resultan tales conceptos y el escaso contacto que el alumno universitario ha tenido con ellos, nos ha resultado de gran ayuda el uso del ordenador para motivar y facilitar la comprensión de estos contenidos abstractos. De una forma muy sencilla, se traducen conceptos eminentemente matemáticos al lenguaje de programación, lenguaje al que debe estar acostumbrado el alumno de informática

Divisibilidad de los números enteros: El “secreto” de los números primos. (Una experiencia práctica de clase)

Las matemáticas y en particular la matemática discreta es una materia abstracta, tradicionalmente considerada como compleja y bastante novedosa para el alumnado; la monotonía de los contenidos, el abandono durante el curso de la asignatura, la falta de motivación e interés, el poco o ningún conocimiento de contenidos previos, los prejuicios de los alumnos, son los principales problemas a los que se enfrenta el docente. Ajustándonos a un tema particular, mostramos como podemos acercar al alumno estos contenidos; a modo de ejemplo, y muy en concreto, basándonos en un concepto sobradamente conocido por todos como el de número primo; en general, no suele quedar patente la importancia de éste en la vida cotidiana y tampoco su uso generalizado, especialmente en temas relacionados con la informática. Nos valemos de ello para motivar de manera efectiva al alumnado (especialmente a un alumnado de la Ingeniería de Informática de Gestión)

Crystallization and melting of bacteria colonies and Brownian Bugs

Motivated by the existence of remarkably ordered cluster arrays of bacteria colonies growing in Petri dishes and related problems, we study the spontaneous emergence of clustering and patterns in a simple nonequilibrium system: the individual-based interacting Brownian bug model. We map this discrete model into a continuous Langevin equation which is the starting point for our extensive numerical analyses. For the two-dimensional case we report on the spontaneous generation of localized clusters of activity as well as a melting/freezing transition from a disordered or isotropic phase to an ordered one characterized by hexagonal patterns. We study in detail the analogies and differences with the well-established Kosterlitz-Thouless-Halperin-Nelson-Young theory of equilibrium melting, as well as with another competing theory. For that, we study translational and orientational correlations and perform a careful defect analysis. We find a non standard one-stage, defect-mediated, transition whose nature is only partially elucidated.Comment: 13 Figures. 14 pages. Submitted to Phys. Rev.