arXiv subjects

Julián Candia

Publications and source records attributed to Julián Candia.

35 records · Page 2Linked to original sources

arXiv2007

Irreversible Opinion Spreading on Scale-Free Networks

We study the dynamical and critical behavior of a model for irreversible opinion spreading on Barabási-Albert (BA) scale-free networks by performing extensive Monte Carlo simulations. The opinion spreading within an inhomogeneous society is investigated by means of the magnetic Eden model, a nonequilibrium kinetic model for the growth of binary mixtures in contact with a thermal bath. The deposition dynamics, which is studied as a function of the degree of the occupied sites, shows evidence for the leading role played by hubs in the growth process. Systems of finite size grow either ordered or disordered, depending on the temperature. By means of standard finite-size scaling procedures, the effective order-disorder phase transitions are found to persist in the thermodynamic limit. This critical behavior, however, is absent in related equilibrium spin systems such as the Ising model on BA scale-free networks, which in the thermodynamic limit only displays a ferromagnetic phase. The dependence of these results on the degree exponent is also discussed for the case of uncorrelated scale-free networks.

cond-mat.dis-nn↗

arXiv2006

Monte Carlo simulation of the irreversible growth of magnetic thin films

The growth of magnetic films with ferromagnetic interactions between nearest-neighbor spins is studied in $(d+1)-$dimensional rectangular geometries for $d = 1,2$. Magnetic films are grown irreversibly by adding spins at the boundaries of the growing interface. The orientation of the added spins depends on both the energetic interaction with already deposited spins and the temperature, through a Boltzmann factor. At low temperatures thin films, of thickness $L$, are constituted by a sequence of well ordered magnetic domains. Spins belonging to each domain, of average length $l_{D} \gg L$, have mostly the same orientation, but consecutive domains have opposite magnetization. Such kind of ``spontaneous magnetization reversal'' during the growth process has a short characteristic length $l_{R}$, such that $l_{D} \gg l_{R} \sim L$. At higher temperatures, a transition between ordered and disordered states is also observed. The emerging behavior is compared to that of the equilibrium Ising model.

cond-mat.stat-mech↗

arXiv2006

Irreversible growth of binary mixtures on small-world networks

Binary mixtures growing on small-world networks under far-from-equilibrium conditions are studied by means of extensive Monte Carlo simulations. For any positive value of the shortcut fraction of the network ($p>0$), the system undergoes a continuous order-disorder phase transition, while it is noncritical in the regular lattice limit ($p=0$). Using finite-size scaling relations, the phase diagram is obtained in the thermodynamic limit and the critical exponents are evaluated. The small-world networks are thus shown to trigger criticality, a remarkable phenomenon which is analogous to similar observations reported recently in the investigation of equilibrium systems.

cond-mat.stat-mech↗

arXiv2005

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

The Magnetic Eden Model (MEM) 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 non-critical (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 2 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 which 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.

cond-mat.stat-mech↗

arXiv2005

Irreversible growth of a binary mixture confined in a thin film geometry with competing walls

The irreversible growth of a binary mixture under far-from-equilibrium conditions is studied in three-dimensional confined geometries of size $L_x \times L_y \times L_z$, where $L_z \gg L_x = L_y$ is the growing direction. A competing situation where two opposite surfaces prefer different species of the mixture is analyzed. Due to this antisymmetric condition an interface between the different species develops along the growing direction. Such interface undergoes a localization-delocalization transition that is the precursor of a wetting transition in the thermodynamic limit. Furthermore, the growing interface also undergoes a concave-convex transition in the growth mode. So, the system exhibits a multicritical wetting point where the growing interface is almost flat and the interface between species is essentially localized at the center of the film.

cond-mat.stat-mech↗

arXiv2005

Detectable neutrino fluxes due to enhanced cosmic ray densities in the Galactic Center region

We examine in detail the detectability of a signal of diffuse high energy neutrinos produced in the Milky Way by the interaction of cosmic rays (CRs) with the interstellar medium (ISM). We show that highly inhomogeneous galactic CR densities arise naturally from large scale drift effects and the inhomogeneous distribution of CR sources. In particular, the CR density in the Galactic Center region (where the ISM density is maximal) could be an order of magnitude larger than the local CR density. Hence, the expected diffuse flux of neutrinos from the Galactic Center region becomes enhanced and could be detected at energies above $\sim 10^4$ GeV in a km$^3$-size neutrino telescope within 10 years of operation. The galactic anisotropy, which is the main signature of this flux, allows to discriminate this signal from other possible extraterrestrial diffuse fluxes of high energy neutrinos.

astro-ph↗

arXiv2005

Quasi-wetting and morphological phase transitions in confined far-from-equilibrium magnetic thin films

The growth of confined magnetic films with ferromagnetic interactions between nearest-neighbor spins is studied in a stripped $(1+1)-$dimensional rectangular geometry. Magnetic films are grown irreversibly by adding spins at the boundaries of the growing interface. A competing situation with two opposite short range surface magnetic fields of the same magnitude is analyzed. Due to the antisymmetric condition considered, an interface between domains with spins having opposite orientations develops along the growing direction. Such interface undergoes a localization-delocalization transition that is identified as a quasi-wetting transition, in qualitative agreement with observations performed under equilibrium conditions. In addition, the film also exhibits a growing interface that undergoes morphological transitions in the growth mode. It is shown that, as a consequence of the nonequilibrium nature of the investigated model, the subtle interplay between finite-size effects, wetting, and interface growth mechanisms leads to more rich and complex physical features than in the equilibrium counterpart. Indeed, a phase diagram that exhibits eight distinct regions is evaluated and discussed. In the thermodynamic limit, the whole ordered phase (which contains the quasi-wetting transition) collapses, while within the disordered phase, standard extrapolation procedures show that only two regions are present in the phase diagram of the infinite system.

cond-mat.stat-mech↗

arXiv2005

Non-equilibrium wetting transition in a magnetic Eden model

Magnetic Eden clusters with ferromagnetic interaction between nearest-neighbor spins are grown in a confined 2d-geometry with short range magnetic fields acting on the surfaces. The change of the growing interface curvature driven by the field and the temperature is identified as a non-equilibrium wetting transition and the corresponding phase diagram is evaluated.

cond-mat.stat-mech↗

arXiv2005

Corner wetting in a far-from-equilibrium magnetic growth model

The irreversible growth of magnetic films is studied in three-dimensional confined geometries of size $L\times L\times M$, where $M\gg L$ is the growing direction. Competing surface magnetic fields, applied to opposite corners of the growing system, lead to the observation of a localization-delocalization (weakly rounded) transition of the interface between domains of up and down spins on the planes transverse to the growing direction. This effective transition is the precursor of a true far-from-equilibrium corner wetting transition that takes place in the thermodynamic limit. The phenomenon is characterized quantitatively by drawing a magnetic field-temperature phase diagram, firstly for a confined sample of finite size, and then by extrapolating results, obtained with samples of different size, to the thermodynamic limit. The results of this work are a nonequilibrium realization of analogous phenomena recently investigated in equilibrium systems, such as corner wetting transitions in the Ising model.

cond-mat.stat-mech↗

arXiv2004

Propagation of High Energy Galactic Cosmic Rays

We explore several aspects related to the propagation of high energy cosmic rays (CRs) of galactic origin (i.e. in the range $E\simeq 10^{15}- 10^{18}$ eV). In particular, we study in detail the diffusion/drift scenario, a rigidity-dependent model that explains the origin of the knee and the second knee of the CR spectrum, as well as the most recent observations of composition and anisotropies. Moreover, the behavior of the diffusion tensor for charged particles propagating in highly turbulent magnetic fields is determined numerically. We also investigate the high energy neutrino fluxes induced by CRs, and we show that a rigidity-dependent scenario for the CR knee reduces the background of atmospheric neutrinos, hence encouraging the search for astrophysical neutrino sources. After exploring this effect in the detection channel of lepton tracks, we investigate the less studied ``shower'' channel, which turns out to be particularly effective for isolating the prompt atmospheric neutrino component. Finally, we consider in detail the nuclear photodisintegration model as an alternative explanation for the knee in the CR spectrum, in which CR nuclei emitted by a source are disintegrated during their propagation through a surrounding optical/UV photon field. In this case, the composition is predicted to turn lighter across the knee, in constrast to the results obtained in rigidity-dependent scenarios.

astro-ph↗

arXiv2004

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 {\it 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 {\it 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 JCAP {\bf 0309}, 005). We also improve the corresponding calculation of the neutrino flux induced by cosmic ray collisions with the interstellar medium.

hep-ph↗

arXiv2004

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.

astro-ph↗

arXiv2003

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 ($Δα\simeq 0.3$) observed in the total spectrum. We show that this implies that the very high energy ($E_ν>10^{14}$ 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.

astro-ph↗

arXiv2003

Cosmic ray spectrum and anisotropies from the knee to the second knee

We consider the scenario in which the knee in the cosmic ray spectrum is due to a change in the escape mechanism of cosmic rays from the Galaxy from one dominated by transverse diffusion to one dominated by drifts. We show that this scenario explains not only the changes in spectral slope at the knee and at the second knee, but can also account for the main characteristics of the observed energy dependent anisotropy amplitude and phase of first harmonic in the energy range between $10^{15}$ and $10^{18}$ eV. This provides a useful handle to distinguish this diffusion/drift model from other scenarios proposed to explain the knee in the spectrum.

astro-ph↗

arXiv2002

Turbulent diffusion and drift in galactic magnetic fields and the explanation of the knee in the cosmic ray spectrum

We reconsider the scenario in which the knee in the cosmic ray spectrum is explained as due to a change in the escape mechanism of cosmic rays from the Galaxy from one dominated by transverse diffusion to one dominated by drifts. We solve the diffusion equations adopting realistic galactic field models and using diffusion coefficients appropriate for strong turbulence (with a Kolmogorov spectrum of fluctuations) and consistent with the assumed magnetic fields. We show that properly taking into account these effects leads to a natural explanation of the knee in the spectrum, and a transition towards a heavier composition above the knee is predicted.

astro-ph↗

arXiv2002

Order-disorder criticality, wetting, and morphological phase transitions in the irreversible growth of far-from-equilibrium magnetic films

An exhaustive numerical investigation of the growth of magnetic films in confined $(d+1)$-dimensional stripped geometries ($d=1,2$) is carried out by means of extensive Monte Carlo simulations. Thin films in contact with a thermal bath are grown by adding spins with two possible orientations and considering ferromagnetic (nearest-neighbor) interactions. At low temperatures, it is observed that the films exhibit ``spontaneous magnetization reversals'' during the growth process. Furthermore, it is found that for $d=1$ the system is non-critical, while a continuous order-disorder phase transition at finite temperature takes place in the $d=2$ case. Using standard finite-size scaling procedures, the critical temperature and some relevant critical exponents are determined. Finally, the growth of magnetic films in $(2+1)$ dimensions with competing short-range magnetic fields acting along the confinement walls is studied. Due to the antisymmetric condition considered, an interface between domains with spins having opposite orientation develops along the growing direction. Such an interface undergoes a localization-delocalization transition that is the precursor of a wetting transition in the thermodynamic limit. Furthermore, the growing interface also undergoes morphological transitions in the growth mode. A comparison between the well-studied equilibrium Ising model and the studied irreversible magnetic growth model is performed throughout. Although valuable analogies are encountered, it is found that the nonequilibrium nature of the latter introduces new and rich physical features of interest.

cond-mat↗

arXiv2002

Interfacial phase transitions in a far-from-equilibrium magnetic growth model

The irreversible growth of a magnetic film with spins having two possible orientations is studied in three-dimensional confined geometries of size $L\times L\times M$, where $M\gg L$ is the growing direction. A competing situation with two opposite short range surface magnetic fields $H$ of the same magnitude is analyzed. Due to the antisymmetric condition considered, an interface between domains with spins having opposite orientations develops along the growing direction. Such an interface undergoes a localization-delocalization transition that is the precursor of a wetting transition in the thermodynamic limit, in qualitative agreement with observations performed under equilibrium conditions. However, in contrast to its equilibrium counterparts, the film also exhibits a growing interface that undergoes a concave-convex transition in the growth mode. The phase diagram on the $H$ versus $T$ plane is first obtained for a finite system, and exhibits eight different regions. Subsequently, the phase diagram corresponding to the thermodynamic limit is obtained by extrapolation. It is shown that in the latter only six regions remain. The relevant physical properties of all of these regions are discussed in detail.

cond-mat↗