# Física

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Artículo de Publicación Periódica Time series characterization via horizontal visibility graph and Information theory(2016) Gonçalves, Bruna Amin; Capri, Laura; Rosso, Osvaldo A.; Ravetti, Martín G.Más... "Complex networks theory have gained wider applicability since methods for transformation of time series to networks were proposed and successfully tested. In the last few years, horizontal visibility graph has become a popular method due to its simplicity and good results when applied to natural and artificially generated data. In this work, we explore different ways of extracting information from the network constructed from the horizontal visibility graph and evaluated by Information Theory quantifiers. Most works use the degree distribution of the network, however, we found alternative probability distributions, more efficient than the degree distribution in characterizing dynamical systems. In particular, we find that, when using distributions based on distances and amplitude values, significant shorter time series are required. We analyze fractional Brownian motion time series, and a paleoclimatic proxy record of ENSO from the Pallcacocha Lake to study dynamical changes during the Holocene."Más... Artículo de Publicación Periódica Flow of pedestrians through narrow doors with dierent competitiveness(2016) Garcimartín, Ángel; Parisi, Daniel; Pastor, José Martín; Martín-Gómez, César; Zuriguel, IkerMás... "We report a thorough analysis of the intermittent flow of pedestrians through a narrow door. The observations include five diferent sets of evacuation drills with which we have investigated the efect of door size and competitiveness on the flow dynamics. Although the outcomes are in general compatible with the existence of the faster-is-slower efect, the temporal evolution of the instantaneous flow rate provides evidence of new features. These stress the crucial role of the number of people performing the tests, which has an influence on the obtained results. Once the transients at the beginning and end of the evacuation are removed, we have found that the time lapses between the passage of two consecutive pedestrians display heavy-tailed distributions in all the scenarios studied. Meanwhile, the distribution of burst sizes decays exponentially; this can be linked to a constant probability of finding a long-lasting clog during the evacuation process. Based on these results, a discussion is presented on the caution that should be exercised when measuring or describing the intermittent flow of pedestrians through narrow doors."Más... Artículo de Publicación Periódica Continuous-space automaton model for pedestrian dynamics(2011-05-11) Baglietto, Gabriel; Parisi, DanielMás... An off-lattice automaton for modeling pedestrian dynamics is presented. Pedestrians are represented by disks with variable radius that evolve following predefined rules. The key feature of our approach is that although positions and velocities are continuous, forces do not need to be calculated. This has the advantage that it allows using a larger time step than in force-based models. The room evacuation problem and circular racetrack simulations quantitatively reproduce the available experimental data, both for the specific flow rate and for the fundamental diagram of pedestrian traffic with an outstanding performance. In this last case, the variation of two free parameters (r min and rmax) of the model accounts for the great variety of experimental fundamental diagrams reported in the literature. Moreover, this variety can be interpreted in terms of these model parameters.Más... Artículo de Publicación Periódica Noise-assisted multibit storage device(2011-08-29) Bellomo, Guido; Patterson, Germán; Fierens, Pablo Ignacio; Grosz, DiegoMás... In this Letter we extend our investigations on noise-assisted storage devices through the experimental study of a loop composed of a single Schmitt trigger and an element that introduces a finite delay. We show that such a system allows the storage of several bits and does so more efficiently for an intermediate range of noise intensities. Finally, we study the probability of erroneous information retrieval as a function of elapsed time and show a way for predicting device performance independently of the number of stored bits.Más... Artículo de Publicación Periódica Modified nonlinear Schrödinger equation for frequency-dependent nonlinear profiles of arbitrary sign(2019) Bonetti, Juan I.; Linale, N.; Sánchez, Alfredo D.; Hernández, Santiago M.; Fierens, Pablo Ignacio; Grosz, DiegoMás... In recent times, materials exhibiting frequency-dependent optical nonlinearities, such as nanoparticle-doped glasses and other metamaterials, have gathered significant interest. The simulation of the propagation of intense light pulses in such media, by means of the nonlinear Schrödinger equation (NLSE), poses the problem in that straightforward inclusion of a frequency-dependent nonlinearity may lead to unphysical results, namely, neither the energy nor the photon number is conserved in general. Inspired by a simple quantum-mechanical argument, we derive an energy- and photon-conserving NLSE (pcNLSE). Unlike others, our approach relies only on the knowledge of the frequency-dependent nonlinearity profile and a generalization of Miller’s rule for nonlinear susceptibility, enabling the simulation of nonlinear profiles of arbitrary frequency dependence and sign. Moreover, the proposed pcNLSE can be efficiently solved by the same numerical techniques commonly used to deal with the NLSE. Relevant simulation results supporting our theoretical approach are presented.Más... Artículo de Publicación Periódica Joint position and clock tracking of wireless nodes under mixed LOS-NLOS conditions(2022-10) Grisales Campeón, Juan Pablo; Fierenz, Pablo IgnacioMás... We propose an algorithm for the simultaneous position and clock tracking of a wireless mobile node by a set of reference nodes. Based on a protocol similar to that of two-way ranging, our algorithm efficiently estimates the position and velocity of the mobile, and the skew and offset of its clock. We take into account that the propagation conditions between each reference node and the mobile change as the latter moves. In particular, changes between line-of-sight (LOS) and several non-line-of-sight (NLOS) scenarios are considered. We study the performance of our algorithm and compare it to other relevant proposals in the literature by means of simulations, showing that our proposed method improves localization accuracy.Más... Artículo de Publicación Periódica Physical distance characterization using pedestrian dynamics simulation(2022-01-22) Parisi, Daniel; Patterson, Germán; Pagni, Lucio; Osimani, Lucía; Bacigalupo, Tomás; Godfrid, Juan; Bergagna, Federico M.; Brizi, Manuel Rodríguez; Momesso, Pedro; Gómez, Fermín; Lozano, Jimena; Baader, Juan M.; Ribas, Ignacio; Astiz Meyer, Facundo; Di Luca, Miguel; Barrera, Nicolás Enrique; Keimel Álvarez, Ezequiel Martín; Herrán Oyhanarte, Maite Mercedes; Pingarilho, Pedro Remigio; Zuberbuhler, Ximena; Gorostiaga, FelipeMás... In the present work we study how the number of simulated customers (occupancy) af-fects social distance in an ideal supermarket, considering realistic typical dimensions and processing times (product selection and checkout). From the simulated trajectories we measure social distance events of less than 2 m, and their duration. Among other observ-ables, we define a physical distance coefficient that informs how many events (of a given duration) each agent experiences.Más... Artículo de Publicación Periódica Effect of physical distancing on the speed-density relation in pedestrian dynamics(2021-04) Echeverría Huarte, Iñaki; Garcimartín, Ángel; Parisi, Daniel; Martín-Gómez, CésarMás... "We report experimental results of the speed-density relation emerging in pedestrian dynamics when individuals keep a prescribed safety distance among them. To this end, we characterize the movement of a group of people roaming inside an enclosure varying different experimental parameters: (i) global density, (ii) prescribed walking speed, and (iii) suggested safety distance. Then, by means of the Voronoi diagram we are able to compute the local density associated to each pedestrian, which is afterward correlated with its corresponding velocity at each time. In this way, we discover a strong dependence of the speed-density relation on the experimental conditions, especially with the (prescribed) free speed. We also observe that when pedestrians walk slowly, the speed-density relation depends on the global macroscopic density of the system, and not only on the local one. Finally, we demonstrate that for the same experiment, each pedestrian follows a distinct behavior, thus giving rise to multiple speed-density curves."Más... Artículo de Publicación Periódica Pedestrian dynamics at the running of the bulls evidences an inaccessible region in the fundamental diagram(2021-09) Parisi, Daniel; Sartorio, Alan G.; Colonnello, Joaquín R.; Garcimartín, Ángel; Pugnaloni, Luis Ariel; Zuriguel, IkerMás... "We characterize the dynamics of runners in the famous “Running of the Bulls” Festival by computing the individual and global velocities and densities, as well as the crowd pressure. In contrast with all previously studied pedestrian systems, we unveil a unique regime in which speed increases with density that can be understood in terms of a time-dependent desired velocity of the runners. Also, we discover the existence of an inaccessible region in the speed–density state diagram that is explained by falls of runners. With all these ingredients, we propose a generalization of the pedestrian fundamental diagram for a scenario in which people with different desired speeds coexist."Más... Artículo de Publicación Periódica A generic model for the study of supercontinuum generation in graphene-covered nanowires(2022-01) Linale, N.; Fierens, Pablo Ignacio; Vermeulen, N.; Grosz, DiegoMás... "We study supercontinuum (SC) generation in graphene-covered nanowires based on a generic model that correctly accounts for the evolution of the photon number under Kerr and two-photon absorption processes, and the influence of graphene is treated within the framework of saturable photoexcited-carrier refraction. We discuss the role of the various effects on the generation of SC by a thorough analysis of short-pulse propagation in two different kinds of graphene-covered nanowires, one made of silicon nitride and the other made of silicon. Finally, we discuss the effect of stacking graphene layers as a means to enhance SC generation with pulse powers compatible with those in integrated optical devices."Más... Artículo de Publicación Periódica Effect of physical distancing on the speed–density relation in pedestrian dynamics(2021-04) Echeverría Huarte, Iñaki; Garcimartín, Ángel; Parisi, Daniel; Cruz Hidalgo, R.; Martín-Gómez, César; Zuriguel, IkerMás... "We report experimental results of the speed-density relation emerging in pedestrian dynamics when individuals keep a prescribed safety distance among them. To this end, we characterize the movement of a group of people roaming inside an enclosure varying different experimental parameters: (i) global density, (ii) prescribed walking speed, and (iii) suggested safety distance. Then, by means of the Voronoi diagram we are able to compute the local density associated to each pedestrian, which is afterward correlated with its corresponding velocity at each time. In this way, we discover a strong dependence of the speed-density relation on the experimental conditions, especially with the (prescribed) free speed. We also observe that when pedestrians walk slowly, the speed-density relation depends on the global macroscopic density of the system, and not only on the local one. Finally, we demonstrate that for the same experiment, each pedestrian follows a distinct behavior, thus giving rise to multiple speed-density curves."Más... Artículo de Publicación Periódica Analytical study of coherence in seeded modulation instability(2016-09) Bonetti, Juan I.; Hernández, Santiago M.; Fierens, Pablo Ignacio; Grosz, DiegoMás... "We derive analytical expressions for the coherence in the onset of modulation instability, in excellent agreement with thorough numerical simulations. As usual, we start by a linear perturbation analysis, where broadband noise is added to a cw pump; then, we investigate the effect of adding a deterministic seed to the cw pump, a case of singular interest as it is commonly encountered in parametric amplification schemes. Results for the dependence of coherence on parameters such as fiber type, pump power, propagated distance, and seed signal-to-noise ratio are presented. Finally, we show the importance of including higher-order linear and nonlinear dispersion when looking at longer-wavelength regions"Más... Artículo de Publicación Periódica Libor at crossroads: stochastic switching detection using information theory quantifiers(2016-07) Fernández Bariviera, Aurelio; Guercio, M. Belén; Martinez, Lisana B.; Rosso, Osvaldo A.Más... "This paper studies the 28 time series of Libor rates, classified in seven maturities and four currencies, during the last 14 years. The analysis was performed using a novel technique in financial economics: the Complexity-Entropy Causality Plane. This planar representation allows the discrimination of different stochastic and chaotic regimes. Using a temporal analysis based on moving windows, this paper unveils an abnormal movement of Libor time series around the period of the 2007 financial crisis. This alteration in the stochastic dynamics of Libor is contemporary of what press called "Libor scandal", i.e. the manipulation of interest rates carried out by several prime banks. We argue that our methodology is suitable as a market watch mechanism, as it makes visible the temporal redution in informational efficiency of the market."Más... Artículo de Publicación Periódica Experimental investigation of noise-assisted information transmission and storage via stochastic resonance(2010-05) Patterson, Germán; Goya, Andrés F.; Fierens, Pablo Ignacio; Ibáñez, Santiago Agustín; Grosz, DiegoMás... "We present experimental results on the information transmission and storage via stochastic resonance in circuits designed and built around Schmitt triggers (STs). First, we investigate the performance of a transmission line comprised of five STs and show it to exhibit stochastic resonance. Each ST in the line is fed with white Gaussian noise, and the first ST is driven by a non-return-to-zero pseudo-random bit sequence with sub-threshold amplitude. Parameters such as bit error rate (Q-factor) are measured (calculated) and shown to exhibit a minimum (maximum) for an optimum amount of noise. Interestingly, we find that system performance degrades with the number of STs as if the system were linear and impaired only by additive Gaussian noise. We then propose and build a 1-bit storage device based on two STs in a loop configuration. We demonstrate that such a system is capable of storing one bit of information only in the presence of noise, and that there is a regime where the efficiency of such a device increases with increasing noise. Our results point to the feasibility of building 'blocks' that can transmit, store and eventually process information, whose performance is not only robust against noise, but can actually benefit from it. © 2010 Elsevier B.V. All rights reserved."Más... Artículo de Publicación Periódica Permutation min-entropy: an improved quantifier for unveiling subtle temporal correlations(2015-01) Zunino, Luciano; Olivares, Felipe; Rosso, Osvaldo A.Más... "The aim of this letter is to introduce the permutation min-entropy as an improved symbolic tool for identifying the existence of hidden temporal correlations in time series. On the one hand, analytical results obtained for the fractional Brownian motion stochastic model theoretically support this hypothesis. On the other hand, the analysis of several computer-generated and experimentally observed time series illustrate that the proposed symbolic quantifier is a versatile and practical tool for identifying the presence of subtle temporal structures in complex dynamical systems. Comparisons against the results obtained with other tools confirm its usefulness as an alternative and/or complementary measure of temporal correlations."Más... Artículo de Publicación Periódica Thermal effects on the switching kinetics of silver/manganite memristive systems(2014-10) Stoliar, Pablo; Sánchez, M. J.; Patterson, Germán; Fierens, Pablo IgnacioMás... "We investigate the switching kinetics of oxygen vacancy (Ov) diffusion in La(5/8-y)Pr(y) Ca(3/8)MnO(3)-Ag (LPCMO–Ag) memristive interfaces by performing experiments on the temperature dependence of the high resistance state under thermal cycling. Experimental results are well reproduced by numerical simulations based on thermally activated Ov diffusion processes and fundamental assumptions relying on a recent model proposed to explain bipolar resistive switching in manganite-based cells. The confident values obtained for activation energies and the diffusion coefficient associated to Ov dynamics constitute a validation test for both model predictions and Ov diffusion mechanisms in memristive interfaces."Más... Artículo de Publicación Periódica Noise on resistive switching: a Fokker–Planck approach(2016-05) Patterson, Germán; Grosz, Diego; Fierens, Pablo IgnacioMás... "We study the effect of internal and external noise on the phenomenon of resistive switching. We consider a non-harmonic external driving signal and provide a theoretical framework to explain the observed behavior in terms of the related Fokker–Planck equations. It is found that internal noise causes an enhancement of the resistive contrast and that noise proves to be advantageous when considering short driving pulses. In the case of external noise, however, noise only has the effect of degrading the resistive contrast. Furthermore, we find a relationship between the noise amplitude and the driving signal pulse width that constrains the persistence of the resistive state. In particular, results suggest that strong and short driving pulses favor a longer persistence time, an observation that may find applications in the field of high-integration high-speed resistive memory devices."Más... Artículo de Publicación Periódica Measuring self-steepening with the photon-conserving nonlinear Schrödinger equation(2020-08-15) Linale, N.; Fierens, Pablo Ignacio; Bonetti, Juan I.; Sánchez, Alfredo D.; Hernández, Santiago M.; Grosz, DiegoMás... "We propose an original, simple, and direct method to measure self-steepening (SS) in nonlinear waveguides. Our proposal is based on results derived from the recently introduced photon-conserving nonlinear Schrödinger equation (NLSE) and relies on the time shift experienced by soliton-like pulses due to SS upon propagation. In particular, a direct measurement of this time shift allows for a precise estimation of the SS parameter. Furthermore, we show that such an approach cannot be tackled by resorting to the NLSE. The proposed method is validated through numerical simulations, in excellent agreement with the analytical model, and results are presented for relevant spectral regions in the near infrared, the telecommunication band, and the mid infrared, and for realistic parameters of available laser sources and waveguides. Finally, we demonstrate the robustness of the proposed scheme against deviations expected in real-life experimental conditions, such as pulse shape, pulse peak power, pulsewidth, and/or higher-order linear and nonlinear dispersion."Más... Artículo de Publicación Periódica Narrowband and ultra-wideband modulation instability in nonlinear metamaterial waveguides(2020-11-01) Linale, N.; Fierens, Pablo Ignacio; Hernández, Santiago M.; Bonetti, Juan I.; Grosz, DiegoMás... "Waveguides based on metamaterials may exhibit strongly frequency-dependent nonlinearities. In this work, we focus on the phenomenon of modulation instability in this type of waveguide, departing from a new modeling equation that ensures strict conservation of both the energy and photon number of the parametric process. In particular, we analyse the case of a waveguide with a linearly frequency-dependent nonlinear coefficient, revealing unique features such as narrowband and ultra-wideband gain spectra and the suppression of the power cutoff giving rise to an ever-growing MI gain. These markedly distinct regimes are enabled by self-steepening (SS) and manifest themselves depending upon the magnitude and sign of the SS parameter.We believe these findings to be most relevant in the context of mid-IR supercontinuum sources."Más... Artículo de Publicación Periódica Soliton solutions and self-steepening in the photon-conserving nonlinear Schrödinger equation(2020-12-09) Hernández, Santiago M.; Bonetti, Juan I.; Linale, N.; Grosz, Diego; Fierens, Pablo IgnacioMás... "We have recently introduced a new modeling equation for the propagation of pulses in optical waveguides, the photon-conserving Nonlinear Schrödinger Equation (pcNLSE) which, unlike the canonical NLSE, guarantees strict conservation of both the energy and the number of photons for any arbitrary frequency-dependent nonlinearity. In this paper, we analyze some properties of this new equation in the familiar case where the nonlinear coefficient of the waveguide does not change sign. We show that the pcNLSE effectively adds a correction term to the NLSE proportional to the deviation of the self-steepening (SS) parameter from the photon-conserving condition in the NLSE. Furthermore, we describe the role of the self-steepening parameter in the context of the conservation of the number of photons and derive an analytical expression for the relation of the SS parameter with the time delay experienced by pulses upon propagation. Finally, we put forth soliton-like solutions of the pcNLSE that, unlike NLSE solitons, conserve the number of photons for any arbitrary SS parameter. "Más...

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