Advertising and Irreversible Opinion Spreading in Complex Social Networks

Irreversible opinion spreading phenomena are studied on small-world and scale-free networks by means of the magnetic Eden model, a nonequilibrium kinetic model for the growth of binary mixtures in contact with a thermal bath. In this model, the opinion of an individual is affected by those of their acquaintances, but opinion changes (analogous to spin flips in an Ising-like model) are not allowed. We focus on the influence of advertising, which is represented by external magnetic fields. The interplay and competition between temperature and fields lead to order–disorder transitions, which are found to also depend on the link density and the topology of the complex network substrate. The effects of advertising campaigns with variable duration, as well as the best cost-effective strategies to achieve consensus within different scenarios, are also discussed.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Diffusion-based density-equalizing maps: an interdisciplinary approach to visualizing homicide rates and other georeferenced statistical data

In every country, public and private agencies allocate extensive funding to collect large-scale statistical data, which in turn are studied and analyzed in order to determine local, regional, national, and international policies regarding all aspects relevant to the welfare of society. One important aspect of that process is the visualization of statistical data with embedded geographical information, which most often relies on archaic methods such as maps colored according to graded scales. In this work, we apply nonstandard visualization techniques based on physical principles. We illustrate the method with recent statistics on homicide rates in Brazil and their correlation to other publicly available data. This physics-based approach provides a novel tool that can be used by interdisciplinary teams investigating statistics and model projections in a variety of fields such as economics and gross domestic product research, public health and epidemiology, sociodemographics, political science, business and marketing, and many others.Instituto de Física de Líquidos y Sistemas Biológico

Stochastic resonance and dynamic first-order pseudo-phase transitions in the irreversible growth of thin films underspatially periodic magnetic fields

We study the irreversible growth of magnetic thin films under the influence of spatially periodic fields by means of extensive Monte Carlo simulations. We find first-order pseudo-phase transitions that separate a dynamically disordered phase from a dynamically ordered phase. By analogy with time-dependent oscillating fields applied to Ising-type models, we qualitatively associate this dynamic transition with the localization/delocalization transition of “spatial hysteresis” loops. Depending on the relative width of the magnetic film, L, compared to the wavelength of the external field, λ, different transition regimes are observed. For small systems (L λ), the transition is driven by Anomalous Stochastic Resonance. The origin of the latter is identified as due to the emergence of an additional relevant lengthscale, namely the roughness of the spin domain switching interface. The distinction between different stochastic resonance regimes is discussed at length, both qualitatively by means of snapshot configurations, as well as quantitatively via residence-length and order-parameter probability distributionsInstituto de Investigaciones Fisicoquímicas Teóricas y AplicadasInstituto de Física de Líquidos y Sistemas Biológico

Gregarious versus individualistic behavior in Vicsek swarms and the onset of first-order phase transitions

The standard Vicsek model (SVM) is a minimal non-equilibrium model of self-propelled particles that appears to capture the essential ingredients of critical flocking phenomena. In the SVM, particles tend to align with each other and form ordered flocks of collective motion; however, perturbations controlled by a noise term lead to a noisedriven continuous order–disorder phase transition. In this work, we extend the SVM by introducing a parameter α that allows particles to be individualistic instead of gregarious, i.e. to choose a direction of motion independently of their neighbors. By focusing on the small-noise regime, we show that a relatively small probability of individualistic motion (around 10%) is sufficient to drive the system from a Vicsek-like ordered phase to a disordered phase. Despite the fact that the α-extended model preserves the O(n) symmetry and the interaction range, as well as the dimensionality of the underlying SVM, this novel phase transition is found to be discontinuous (first order), an intriguing manifestation of the richness of the non-equilibrium flocking/swarming phenomenon.Instituto de Física de Líquidos y Sistemas BiológicosFacultad de IngenieríaFacultad de Ciencias Exacta

Gregarious versus individualistic behavior in Vicsek swarms and the onset of first-order phase transitions

The standard Vicsek model (SVM) is a minimal non-equilibrium model of self-propelled particles that appears to capture the essential ingredients of critical flocking phenomena. In the SVM, particles tend to align with each other and form ordered flocks of collective motion; however, perturbations controlled by a noise term lead to a noisedriven continuous order–disorder phase transition. In this work, we extend the SVM by introducing a parameter α that allows particles to be individualistic instead of gregarious, i.e. to choose a direction of motion independently of their neighbors. By focusing on the small-noise regime, we show that a relatively small probability of individualistic motion (around 10%) is sufficient to drive the system from a Vicsek-like ordered phase to a disordered phase. Despite the fact that the α-extended model preserves the O(n) symmetry and the interaction range, as well as the dimensionality of the underlying SVM, this novel phase transition is found to be discontinuous (first order), an intriguing manifestation of the richness of the non-equilibrium flocking/swarming phenomenon.Instituto de Física de Líquidos y Sistemas BiológicosFacultad de IngenieríaFacultad de Ciencias Exacta

Diffusion and drift of cosmic rays in highly turbulent magnetic fields

We determine numerically the parallel, perpendicular, and antisymmetric diffusion coefficients for charged particles propagating in highly turbulent magnetic fields, by means of extensive Monte Carlo simulations. We propose simple expressions, given in terms of a small set of fitting parameters, to account for the diffusion coefficients as functions of magnetic rigidity and turbulence level, and corresponding to different kinds of turbulence spectra. The results obtained satisfy scaling relations, which make them useful for describing the cosmic ray origin and transport in a variety of different astrophysical environments.Instituto de Física La Plat

Rigidity dependent knee and cosmic ray induced high energy neutrino fluxes

Scenarios in which the knee of the cosmic ray spectrum depends on the particle rigidities usually predict that the cosmic ray composition becomes heavier above the knee and have associated a change in the spectral slope of each individual nuclear component which is steeper than the change (Δα≃0.3) observed in the total spectrum. We show that this implies that the very high energy (Eν>1014 eV) diffuse neutrino fluxes produced by cosmic rays hitting the atmosphere or colliding with the interstellar medium in the Galaxy will be significantly suppressed, making their detection harder but also reducing the background for the search of other (more challenging) astrophysical neutrino sources.Facultad de Ciencias Exacta

Comparative study of an Eden model for the irreversible growth of spins and the equilibrium Ising model

The magnetic Eden model (MEM) [N. Vandewalle and M. Ausloos, Phys. Rev. E 50, R635 (1994)] with ferromagnetic interactions between nearest-neighbor spins is studied in (d+1)-dimensional rectangular geometries for d = 1,2. In the MEM, magnetic clusters are grown by adding spins at the boundaries of the clusters. The orientation of the added spins depends on both the energetic interaction with already deposited spins and the temperature, through a Boltzmann factor. A numerical Monte Carlo investigation of the MEM has been performed and the results of the simulations have been analyzed using finite-size scaling arguments. As in the case of the Ising model, the MEM in d = 1 is noncritical (only exhibits an ordered phase at T = 0). In d = 2 the MEM exhibits an order-disorder transition of second order at a finite temperature. Such transition has been characterized in detail and the relevant critical exponents have been determined. These exponents are in agreement (within error bars) with those of the Ising model in two dimensions. Further similarities between both models have been found by evaluating the probability distribution of the order parameter, the magnetization, and the susceptibility. Results obtained by means of extensive computer simulations allow us to put forward a conjecture that establishes a nontrivial correspondence between the MEM for the irreversible growth of spins and the equilibrium Ising model. This conjecture is certainly a theoretical challenge and its confirmation will contribute to the development of a framework for the study of irreversible growth processes.Facultad de Ciencias ExactasInstituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Shower power: isolating the prompt atmospheric neutrino flux using electron neutrinos

At high energies, the very steep decrease of the conventional atmospheric component of the neutrino spectrum should allow the emergence of even small and isotropic components of the total spectrum, indicative of new physics, provided that they are less steeply decreasing, as generically expected. One candidate is the prompt atmospheric neutrino flux, a probe of cosmic ray composition in the region of the knee as well as small-x QCD, below the reach of collider experiments. A second is the diffuse extragalactic background due to distant and unresolved AGNs and GRBs, a key test of the nature of the highest-energy sources in the universe. Separating these new physics components from the conventional atmospheric neutrino flux, as well as from each other, will be very challenging. We show that the charged-current electron neutrino 'shower' channel should be particularly effective for isolating the prompt atmospheric neutrino flux, and that it is more generally an important complement to the usually considered charged-current muon neutrino 'track' channel. These conclusions remain true even for the low prompt atmospheric neutrino flux predicted in a realistic cosmic ray scenario with heavy and varying composition across the knee (Candia and Roulet, 2003 J. Cosmol. Astropart. Phys. JCAP09(2003)005). We also improve the corresponding calculation of the neutrino flux induced by cosmic ray collisions with the interstellar medium.Instituto de Física La Plat

Nonequilibrium critical behavior of magnetic thin films grown in a temperature gradient

We investigate the irreversible growth of (2+1)-dimensional magnetic thin films under the influence of a transverse temperature gradient, which is maintained by thermal baths across a direction perpendicular to the direction of growth. Therefore, different longitudinal layers grow at different temperatures between T1 and T2, where T1 chom 2 and Tchom = 0.69(1) is the critical temperature of films grown in homogeneous thermal baths. We find a far-from-equilibrium continuous order-disorder phase transition driven by the thermal bath gradient. We characterize this gradient-induced critical behavior by means of standard finite-size scaling procedures, which lead to the critical temperature T<subc=0.84(2) and a new universality class consistent with the set of critical exponents ν=3/2, γ=5/2, and β=1/4. In order to gain further insight into the effects of the temperature gradient, we also develop a bond model that captures the magnetic films growth dynamics. Our findings show that the interplay of geometry and thermal bath asymmetries leads to growth bond flux asymmetries and the onset of transverse ordering effects that explain qualitatively the shift observed in the critical temperature. The relevance of these mechanisms is further confirmed by a finite-size scaling analysis of the interface width, which shows that the growing sites of the system define a self-affine interface.Instituto de Física de Líquidos y Sistemas Biológico