Spatial Patterns Induced Purely by Dichotomous Disorder

We study conditions under which spatially extended systems with coupling a la Swift-Hohenberg exhibit spatial patterns induced purely by the presence of quenched dichotomous disorder. Complementing the theoretical results based on a generalized mean-field approximation, we also present numerical simulations of particular dynamical systems that exhibit the proposed phenomenology

Universal Behavior of the Coefficients of the Continuous Equation in Competitive Growth Models

The competitive growth models involving only one kind of particles (CGM), are a mixture of two processes one with probability $p$ and the other with probability $1-p$. The $p-$dependance produce crossovers between two different regimes. We demonstrate that the coefficients of the continuous equation, describing their universality classes, are quadratic in $p$ (or $1-p$). We show that the origin of such dependance is the existence of two different average time rates. Thus, the quadratic $p-$dependance is an universal behavior of all the CGM. We derive analytically the continuous equations for two CGM, in 1+1 dimensions, from the microscopic rules using a regularization procedure. We propose generalized scalings that reproduce the scaling behavior in each regime. In order to verify the analytic results and the scalings, we perform numerical integrations of the derived analytical equations. The results are in excellent agreement with those of the microscopic CGM presented here and with the proposed scalings.Comment: 9 pages, 3 figure

Análisis comparativo entre un tutor circular y uno monolateral en elongaciones óseas

El presente trabajo compara la funcionalidad de dos tutores externos utilizados para elongación: el del Dr. Ilizarov y el tutor HG, desarrollado en nuestra institución. De 131 pacientes tratados con elongación ósea en 147 huesos largos se seleccionaron al azar 25 huesos por cada aparato anteriormente mencionado. Para objetivar los resultados se registraron estadísticamente variables independientes y dependientes en sus características subjetivas y objetivas, tales como: edad, sexo, tipo de hueso elongado, tolerancia psíquica, sensación de confort, facilidad de higiene y control, las infecciones, las rigideces articulares por retracción músculo tendinosa, y la deformación ósea residual. El objetivo fue comparar y establecer si el cambio en la elección del sistema fue ventajoso para nuestros pacientes. En el intento comparativo se enfrentaron dos variables, que a nuestro criterio eran las más importantes para establecer diferencias: la calidad del callo del hueso sometido a elongación y la presencia de complicaciones tanto transitorias como definitivas.In this work we compare the results obtained with two different external fixation devices in patients undergoing bone lengthening. The devices studied were the Ilizarov type and the HG, an apparatons developed in our institution. Out of 131 patientes treated by bone lengthening in 147 long bones, 25 bones lengthened with each device were selected at random. Different subjective and objective variables were assessed: age, sex, type of bone, psichological tolerance, patients, confort, nursing, infections, joint stiffness due to musculotendinous retractions, and residual bone deformity. The aim of the study was to analyze if the monolateral frame entailed advantages for our patients. Two main factors were more deeply analysed, namely the quality of the bone callus subjected to lengthening and the presence of both transitory and definitive complications

Transition from anomalous to normal hysteresis in a system of coupled Brownian motors: a mean field approach

We address a recently introduced model describing a system of periodically coupled nonlinear phase oscillators submitted to multiplicative white noises, wherein a ratchet-like transport mechanism arises through a symmetry-breaking noise-induced nonequilibrium phase transition. Numerical simulations of this system reveal amazing novel features such as negative zero-bias conductance and anomalous hysteresis, explained resorting to a strong-coupling analysis in the thermodynamic limit. Using an explicit mean-field approximation we explore the whole ordered phase finding a transition from anomalous to normal hysteresis inside this phase, estimating its locus and identifying (within this scheme) a mechanism whereby it takes place.Comment: RevTex, 21 pgs, 15 figures. Submited to Physical Review E (2000

Nonequilibrium coupled Brownian phase oscillators

A model of globally coupled phase oscillators under equilibrium (driven by Gaussian white noise) and nonequilibrium (driven by symmetric dichotomic fluctuations) is studied. For the equilibrium system, the mean-field state equation takes a simple form and the stability of its solution is examined in the full space of order parameters. For the nonequilbrium system, various asymptotic regimes are obtained in a closed analytical form. In a general case, the corresponding master equations are solved numerically. Moreover, the Monte-Carlo simulations of the coupled set of Langevin equations of motion is performed. The phase diagram of the nonequilibrium system is presented. For the long time limit, we have found four regimes. Three of them can be obtained from the mean-field theory. One of them, the oscillating regime, cannot be predicted by the mean-field method and has been detected in the Monte-Carlo numerical experiments.Comment: 9 pages 8 figure

Directed percolation depinning models: Evolution equations

We present the microscopic equation for the growing interface with quenched noise for the model first presented by Buldyrev et al. [Phys. Rev. A 45, R8313 (1992)]. The evolution equation for the height, the mean height, and the roughness are reached in a simple way. The microscopic equation allows us to express these equations in two contributions: the contact and the local one. We compare this two contributions with the ones obtained for the Tang and Leschhorn model [Phys. Rev A 45, R8309 (1992)] by Braunstein et al. [Physica A 266, 308 (1999)]. Even when the microscopic mechanisms are quiet different in both model, the two contribution are qualitatively similar. An interesting result is that the diffusion contribution, in the Tang and Leschhorn model, and the contact one, in the Buldyrev model, leads to an increase of the roughness near the criticality.Comment: 10 pages and 4 figures. To be published in Phys. Rev.

A Simple Entropic-Driving Separation Procedure of Low-Size Silver Clusters, Through Interaction with DNA

Synthesis and purification of metal clusters without strong binding agents by wet chemical methods are very attractive for their potential applications in many research areas. However, especially challenging is the separation of uncharged clusters with only a few number of atoms, which renders the usual techniques very difficult to apply. Herein, we report the first efficient separation of Ag2 and Ag3 clusters using the different entropic driving forces when such clusters interact with DNA, into which Ag3 selectively intercalates. After sequential dialysis of the samples and denaturalizing the DNA-Ag3 complex, pure Ag2 can be found in the dialysate after extensive dialysis. Free Ag3 is recovered after DNA denaturation

Stochastic Resonance in Spatially Extended Systems: The Role of Far from Equilibrium Potentials

Previous works have shown numerically that the response of a ``stochastic resonator'' is enhanced as a consequence of spatial coupling. Also, similar results have been obtained in a reaction-diffusion model by studying the phenomenon of stochastic resonance (SR) in spatially extended systems using "nonequilibrium potential" (NEP) techniques. The knowledge of the NEP for such systems allows us to determine the probability for the decay of the metastable extended states, and approximate expressions for the correlation function and the signal-to-noise ratio (SNR). Here, exploiting known forms of the NEP, we have investigated the role of NEP's symmetry on SR, the enhancement of the SNR due to a "selectivity" of the coupling or diffusion parameter, and discussed competition between local and nonlocal (excitatory) coupling.Comment: RevTex, 22 pgs, 6 figures. Invited Talk STATPHYS21, Proceedings to be published in Physica

Nonequilibrium phase transitions induced by multiplicative noise: effects of self-correlation

A recently introduced lattice model, describing an extended system which exhibits a reentrant (symmetry-breaking, second-order) noise-induced nonequilibrium phase transition, is studied under the assumption that the multiplicative noise leading to the transition is colored. Within an effective Markovian approximation and a mean-field scheme it is found that when the self-correlation time of the noise is different from zero, the transition is also reentrant with respect to the spatial coupling D. In other words, at variance with what one expects for equilibrium phase transitions, a large enough value of D favors disorder. Moreover, except for a small region in the parameter subspace determined by the noise intensity and D, an increase in the self-correlation time usually preventsthe formation of an ordered state. These effects are supported by numerical simulations.Comment: 15 pages. 9 figures. To appear in Phys.Rev.

Controlling roughening processes in the stochastic Kuramoto–Sivashinsky equation

We present a novel control methodology to control the roughening processes of semilinear parabolic stochastic partial differential equations in one dimension, which we exemplify with the stochastic Kuramoto-Sivashinsky equation. The original equation is split into a linear stochastic and a nonlinear deterministic equation so that we can apply linear feedback control methods. Our control strategy is then based on two steps: first, stabilize the zero solution of the deterministic part and, second, control the roughness of the stochastic linear equation. We consider both periodic controls and point actuated ones, observing in all cases that the second moment of the solution evolves in time according to a power-law until it saturates at the desired controlled value