arXiv subjects

Orlando V. Billoni

Publications and source records attributed to Orlando V. Billoni.

At least 19 recordsLinked to original sources

arXiv2019

Universal and non-universal neural dynamics on small world connectomes: a finite size scaling analysis

Evidence of critical dynamics has been recently found in both experiments and models of large scale brain dynamics. The understanding of the nature and features of such critical regime is hampered by the relatively small size of the available connectome, which prevent among other things to determine its associated universality class. To circumvent that, here we study a neural model defined on a class of small-world network that share some topological features with the human connectome. We found that varying the topological parameters can give rise to a scale-invariant behavior belonging either to mean field percolation universality class or having non universal critical exponents. In addition, we found certain regions of the topological parameters space where the system presents a discontinuous (i.e., non critical) dynamical phase transition into a percolated state. Overall these results shed light on the interplay of dynamical and topological roots of the complex brain dynamics.

cond-mat.dis-nn↗

arXiv2018

Mitochondrial network complexity emerges from fission/fusion dynamics

Mitochondrial networks exhibit a variety of complex behaviors, including coordinated cell-wide oscillations of energy states as well as a phase transition (depolarization) in response to oxidative stress. Since functional and structural properties are often interwinded, here we characterize the structure of mitochondrial networks in mouse embryonic fibroblasts using network tools and percolation theory. Subsequently we perturbed the system either by promoting the fusion of mitochondrial segments or by inducing mitochondrial fission. Quantitative analysis of mitochondrial clusters revealed that the structural parameters of healthy mitochondria lay in between the extremes of highly fragmented and completely fusioned networks. We confirmed our results by contrasting our emprirical findings with the predictions of a recently described computational model of mitochondrial network emergence based on fission-fusion kinetics. Altogether these results not only offer an objective methodology to parametrize the complexity of this organelle but add weight to the idea that mitochondrial networks behave as critical systems and undergo structural phase transitions.

physics.bio-ph↗

arXiv2017

A short-ranged memory model with preferential growth

In this work we introduce a variant of the Yule-Simon model for preferential growth by incorporating a finite kernel to model the effects of bounded memory. We characterize the properties of the model combining analytical arguments with extensive numerical simulations. In particular, we analyze the lifetime and popularity distributions by mapping the model dynamics to corresponding Markov chains and branching processes, respectively. These distributions follow power-laws with well defined exponents that are within the range of the empirical data reported in ecologies. Interestingly, by varying the innovation rate, this simple out-of-equilibrium model exhibits many of the characteristics of a continuous phase transition and, around the critical point, it generates time series with power-law popularity, lifetime and inter-event time distributions, and non-trivial temporal correlations, such as a bursty dynamics in analogy with the activity of solar flares. Our results suggest that an appropriate balance between innovation and oblivion rates could provide an explanatory framework for many of the properties commonly observed in many complex systems.

physics.soc-ph↗

arXiv2016

A study of memory effects in a chess database

A series of recent works studying a database of chronologically sorted chess games --containing 1.4 million games played by humans between 1998 and 2007-- have shown that the popularity distribution of chess game-lines follows a Zipf's law, and that time series inferred from the sequences of those game-lines exhibit long-range memory effects. The presence of Zipf's law together with long-range memory effects was observed in several systems, however, the simultaneous emergence of these two phenomena were always studied separately up to now. In this work, by making use of a variant of the Yule-Simon preferential growth model, introduced by Cattuto et al., we provide an explanation for the simultaneous emergence of Zipf's law and long-range correlations memory effects in a chess database. We find that Cattuto's Model (CM) is able to reproduce both, Zipf's law and the long-range correlations, including size-dependent scaling of the Hurst exponent for the corresponding time series. CM allows an explanation for the simultaneous emergence of these two phenomena via a preferential growth dynamics, including a memory kernel, in the popularity distribution of chess game-lines. This mechanism results in an aging process in the chess game-line choice as the database grows. Moreover, we find burstiness in the activity of subsets of the most active players, although the aggregated activity of the pool of players displays inter-event times without burstiness. We show that CM is not able to produce time series with bursty behavior providing evidence that burstiness is not required for the explanation of the long-range correlation effects in the chess database.

physics.soc-ph↗

arXiv2016

Modulated phases in external fields: when is reentrant behavior to be expected?

We introduce a new coarse grain model capable of describing the phase behavior of two dimensional ferromagnetic systems with competing exchange and dipolar interactions, as well as an external magnetic field. An improved expression for the mean field entropic contribution allows to compute the phase diagram in the whole temperature versus external field plane. We find that the topology of the phase diagram may be qualitatively different depending on the ratio between the strength of the competing interactions. In the regime relevant for ultrathin ferromagnetic films with perpendicular anisotropy we confirm the presence of inverse symmetry breaking from a modulated phase to a homogenous one as the temperature is lowered at constant magnetic field, as reported in experiments. For other values of the competing interactions we show that reentrance may be absent. Comparing thermodynamic quantities in both cases, as well as the evolution of magnetization profiles in the modulated phases, we conclude that the reentrant behavior is a consequence of the suppression of domain wall degrees of freedom at low temperatures at constant fields.

cond-mat.stat-mech↗

arXiv2016

Magnetization reversal in mixed ferrite-chromite perovskites with non magnetic cation on the A-site

In this work, we have performed Monte Carlo simulations in a classical model for RFe$_{1-x}$Cr$_x$O$_3$ with R=Y and Lu, comparing the numerical simulations with experiments and mean field calculations. In the analyzed compounds, the antisymmetric exchange or Dzyaloshinskii-Moriya (DM) interaction induced a weak ferromagnetism due to a canting of the antiferromagnetically ordered spins. This model is able to reproduce the magnetization reversal (MR) observed experimentally in a field cooling process for intermediate $x$ values and the dependence with $x$ of the critical temperatures. We also analyzed the conditions for the existence of MR in terms of the strength of DM interactions between Fe$^{3+}$ and Cr$^{3+}$ ions with the x values variations.

cond-mat.mtrl-sci↗

arXiv2014

Inverse transition in the dipolar frustrated Ising ferromagnet: the role of domain walls

We present a theoretical study aimed to elucidate the origin of the inverse symmetry breaking transition observed in ultrathin magnetic films with perpendicular anisotropy. We study the behavior of the dipolar frustrated Ising model in a mean field approximation as well as two other models with simple domain walls. By a numerical analysis we show that the internal degrees of freedom of the domain walls are decisive for the presence of the inverse symmetry breaking transition. In particular, we show that in a sharp domain wall model the inverse transition is absent. At high temperatures the additional degrees of freedom of the extended domain walls increase the entropy of the system leading to a reduction of the free energy of the stripe phase. Upon lowering the temperature the domain walls become narrow and with the corresponding degrees of freedom effectively frozen, which eventually induces an inverse transition to the competing homogeneous phase. We also show that, for growing external field at constant temperature, the stripe width grows strongly when approaching the critical field line and diverges at the transition. These results indicate that the inverse transition is a continuous phase transition and that the domain wall profiles as well as the temperature has little effect on the critical behavior of the period of the domain as function of the applied field.

cond-mat.stat-mech↗

arXiv2013

Innovation and Nested Preferential Growth in Chess Playing Behavior

Complexity develops via the incorporation of innovative properties. Chess is one of the most complex strategy games, where expert contenders exercise decision making by imitating old games or introducing innovations. In this work, we study innovation in chess by analyzing how different move sequences are played at the population level. It is found that the probability of exploring a new or innovative move decreases as a power law with the frequency of the preceding move sequence. Chess players also exploit already known move sequences according to their frequencies, following a preferential growth mechanism. Furthermore, innovation in chess exhibits Heaps' law suggesting similarities with the process of vocabulary growth. We propose a robust generative mechanism based on nested Yule-Simon preferential growth processes that reproduces the empirical observations. These results, supporting the self-similar nature of innovations in chess are important in the context of decision making in a competitive scenario, and extend the scope of relevant findings recently discovered regarding the emergence of Zipf's law in chess.

physics.soc-ph↗

arXiv2013

Memory and long-range correlations in chess games

In this paper we report the existence of long-range memory in the opening moves of a chronologically ordered set of chess games using an extensive chess database. We used two mapping rules to build discrete time series and analyzed them using two methods for detecting long-range correlations; rescaled range analysis and detrented fluctuation analysis. We found that long-range memory is related to the level of the players. When the database is filtered according to player levels we found differences in the persistence of the different subsets. For high level players, correlations are stronger at long time scales; whereas in intermediate and low level players they reach the maximum value at shorter time scales. This can be interpreted as a signature of the different strategies used by players with different levels of expertise. These results are robust against the assignation rules and the method employed in the analysis of the time series.

physics.data-an↗

arXiv2013

Disorder-induced mechanism for positive exchange bias fields

We propose a mechanism to explain the phenomenon of positive exchange bias on magnetic bilayered systems. The mechanism is based on the formation of a domain wall at a disordered interface during field cooling (FC) which induces a symmetry breaking of the antiferromagnet, without relying on any ad hoc assumption about the coupling between the ferromagnetic (FM) and antiferromagnetic (AFM) layers. The domain wall is a result of the disorder at the interface between FM and AFM, which reduces the effective anisotropy in the region. We show that the proposed mechanism explains several known experimental facts within a single theoretical framework. This result is supported by Monte Carlo simulations on a microscopic Heisenberg model, by micromagnetic calculations at zero temperature and by mean field analysis of an effective Ising like phenomenological model.

cond-mat.mes-hall↗

arXiv2012

Finite temperature phase diagram of ultrathin magnetic films without external fields

We analyze the finite temperature phase diagram of ultrathin magnetic films by introducing a mean field theory, valid in the low anisotropy regime, i.e., close to de Spin Reorientation Transition. The theoretical results are compared with Monte Carlo simulations carried out on a microscopic Heisenberg model. Connections between the finite temperature behavior and the ground state properties of the system are established. Several properties of the stripes pattern, such as the presence of canted states, the stripes width variation phenomenon and the associated magnetization profiles are also analyzed.

cond-mat.stat-mech↗

arXiv2012

Smart random walkers: the cost of knowing the path

In this work we study the problem of targeting signals in networks using entropy information measurements to quantify the cost of targeting. We introduce a penalization rule that imposes a restriction to the long paths and therefore focus the signal to the target. By this scheme we go continuously from fully random walkers to walkers biased to the target. We found that the optimal degree of penalization is mainly determined by the topology of the network. By analyzing several examples, we have found that a small amount of penalization reduces considerably the typical walk length, and from this we conclude that a network can be efficiently navigated with restricted information.

cond-mat.stat-mech↗

arXiv2011

The exchange bias phenomenon in uncompensated interfaces: Theory and Monte Carlo simulations

We performed Monte Carlo simulations in a bilayer system composed by two thin films, one ferromagnetic (FM) and the other antiferromagnetic (AFM). Two lattice structures for the films were considered: simple cubic (sc) and a body center cubic (bcc). In both lattices structures we imposed an uncompensated interfacial spin structure, in particular we emulated a FeF2-FM system in the case of the (bcc) lattice. Our analysis focused on the incidence of the interfacial strength interactions between the films J_eb and the effect of thermal fluctuations on the bias field H_EB. We first performed Monte Carlo simulations on a microscopic model based on classical Heisenberg spin variables. To analyze the simulation results we also introduced a simplified model that assumes coherent rotation of spins located on the same layer parallel to the interface. We found that, depending on the AFM film anisotropy to exchange ratio, the bias field is either controlled by the intrinsic pinning of a domain wall parallel to the interface or by the stability of the first AFM layer (quasi domain wall) near the interface.

cond-mat.mes-hall↗

arXiv2011

Static and dynamic properties of Single-Chain Magnets with sharp and broad domain walls

We discuss time-quantified Monte-Carlo simulations on classical spin chains with uniaxial anisotropy in relation to static calculations. Depending on the thickness of domain walls, controlled by the relative strength of the exchange and magnetic anisotropy energy, we found two distinct regimes in which both the static and dynamic behavior are different. For broad domain walls, the interplay between localized excitations and spin waves turns out to be crucial at finite temperature. As a consequence, a different protocol should be followed in the experimental characterization of slow-relaxing spin chains with broad domain walls with respect to the usual Ising limit.

cond-mat.stat-mech↗

arXiv2011

Inverse transition in the two dimensional dipolar frustrated ferromagnet

We show that the mean field phase diagram of the dipolar frustrated ferromagnet in an external field presents an inverse transition in the field-temperature plane. The presence of this type of transition has recently been observed experimentally in ultrathin films of Fe/Cu(001). We study a coarse-grained model Hamiltonian in two dimensions. The model supports stripe and bubble equilibrium phases, as well as the paramagnetic phase. At variance with common expectations, already in a single mode approximation, the model shows a sequence of paramagnetic-bubbles-stripes-paramagnetic phase transitions upon lowering the temperature at fixed external field. Going beyond the single mode approximation leads to the shrinking of the bubbles phase, which is restricted to a small region near the zero field critical temperature. Monte Carlo simulations results with a Heisenberg model are consistent with the mean field results.

cond-mat.stat-mech↗

arXiv2010

Stability as a natural selection mechanism on interacting networks

Biological networks of interacting agents exhibit similar topological properties for a wide range of scales, from cellular to ecological levels, suggesting the existence of a common evolutionary origin. A general evolutionary mechanism based on global stability has been proposed recently [J I Perotti, O V Billoni, F A Tamarit, D R Chialvo, S A Cannas, Phys. Rev. Lett. 103, 108701 (2009)]. This mechanism is incorporated into a model of a growing network of interacting agents in which each new agent's membership in the network is determined by the agent's effect on the network's global stability. We show that, out of this stability constraint, several topological properties observed in biological networks emerge in a self organized manner. The influence of the stability selection mechanism on the dynamics associated to the resulting network is analyzed as well.

cond-mat.dis-nn↗

arXiv2010

T=0 phase diagram and nature of domains in ultrathin ferromagnetic films with perpendicular anisotropy

We present the complete zero temperature phase diagram of a model for ultrathin films with perpendicular anisotropy. The whole parameter space of relevant coupling constants is studied in first order anisotropy approximation. Because the ground state is known to be formed by perpendicular stripes separated by Bloch walls, a standard variational approach is used, complemented with specially designed Monte Carlo simulations. We can distinguish four regimes according to the different nature of striped domains: a high anisotropy Ising regime with sharp domain walls, a saturated stripe regime with thicker walls inside which an in-plane component of the magnetization develops, a narrow canted-like regime, characterized by a sinusoidal variation of both the in-plane and the out of plane magnetization components, which upon further decrease of the anisotropy leads to an in-plane ferromagnetic state via a spin reorientation transition (SRT). The nature of domains and walls are described in some detail together with the variation of domain width with anisotropy, for any value of exchange and dipolar interactions. Our results, although strictly valid at $T=0$, can be valuable for interpreting data on the evolution of domain width at finite temperature, a still largely open problem.

cond-mat.dis-nn↗

arXiv2009

Domain walls, canted states and stripe width variation in ultrathin magnetic films with perpendicular anisotropy

Stripe width variation in ultrathin magnetic films is a well known phenomenon still not well understood. We analyze this problem considering a 2D Heisenberg model with ferromagnetic exchange interactions, dipolar interactions and perpendicular anisotropy, relevant e.g. in Fe/Cu(001) films. By extending a classic result of Yafet & Gyorgy (YG) and using Monte Carlo simulations we calculate the complete zero temperature phase diagram of the model. Through this calculation we analyze the correlation between domain walls structure and stripe width variation, as the perpendicular anisotropy changes. In particular, we found evidences that the recently detected canted state becomes the ground state of the system close to the Spin Reorientation Transition (SRT) for any value of the exchange to dipolar couplings ratio. Far away of the SRT the canted ground state is replaced by a saturated stripes state, in which in--plane magnetization components are only present inside the walls. We find that the domain wall structure strongly depends on the perpendicular anisotropy: close to SRT it is well described by YG approximation, but a strong departure is observed in the large anisotropy limit. Moreover, we show that stripe width variation is directly related to domain wall width variation with the anisotropy.

cond-mat.dis-nn↗