J. J. Ramasco

Emory University, Atlanta, Georgia, United States

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Publications (15)35.83 Total impact

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    ABSTRACT: We use a discrete model to study the non-equilibrium dynamics of a slowly driven elastic string in a two-dimensional disordered medium at finite temperatures. We focus on the local activity statistics to show that it can be related to global observables like the average interface velocity and the temporal correlations of the velocity fluctuations. For low temperatures the string exhibits typical creep motion and the activity statistics follows a power law, consistent with an exponential distribution of energy barriers. However, we find that the activity statistics is essentially different when the temperature is low enough, suggesting a different relaxation mechanism as T -> 0. We argue that this is due to the generic non-equilibrium nature of our model in the absence of thermal fluctuations.
    Journal of Statistical Mechanics Theory and Experiment 01/2009; · 1.87 Impact Factor
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    ABSTRACT: The power-law form of the upper part of the distribution of individual wealth/income (Pareto's law) is very well established for many countries and years. The Pareto index is however non-universal, varying typically from $1.5$ to around $3$. A recently introduced model for wealth exchange on an evolving family-network [Physica A 353, 515 (2005)] is reviewed and compared with empirical data. While the model mimics very well recent individual wealth data in a modern society (U.K.), it fails to explain results for a feudal society, based on the number of serf families owned by nobles (Hungary, mid XVI century). The unusually low (around $1$) Pareto index found in this case is not compatible with the previous model. It is suggested that this fact may be interpreted as a result of the absence of active trading among agents.
    The European Physical Journal Special Topics 01/2007; · 1.80 Impact Factor
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    J. J. Ramasco, J. M. López, M. A. Rodríguez
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    ABSTRACT: We study numerically the out-of-equilibrium dynamics of interfaces at finite temperatures when driven well below the zero-temperature depinning threshold. We go further than previous analysis by including the most relevant non-equilibrium correction to the elastic Hamiltonian. We find that the relaxation dynamics towards the steady state shows glassy behaviour, aging and violation of the fluctuation-dissipation theorem. The interface roughness exponent α ≈ 0.7 is found to be robust to temperature changes. We also study the instantaneous velocity signal in the low-temperature regime and find long-range temporal correlations. We argue that 1/f-noise arises from the merging of local temperature-induced avalanches of depinning events.
    EPL (Europhysics Letters) 10/2006; 76(4):554. · 2.26 Impact Factor
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    ABSTRACT: In this work we study the KPZ interface growth equation subjected to both quenched and anealed disorder. We find that the interface dynamics shows a glassy behavior in the regime of low temperatures and low external force, a regime commonly known as creep in the superconductor literature. We also describe how the temperature affects to the avalanches in the front development.
    01/2006;
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    ABSTRACT: We study the behavior of the two time correlation functions and of the response function to an external field of the Contact Process, a paradigm model in the Directed Percolation (DP) universality class. In particular, we find that the autocorrelation and autoresponse exponents λ Г and λ R are equal but, in contrast to systems relaxing to equilibrium, the ageing exponents a and b are distinct. A recent proposal to define a non-equilibrium temperature through the short-time limit of the fluctuation-dissipation ratio is therefore not applicable.
    12/2005: pages 107-111;
  • R. Coelho, Z. Néda, J. J. Ramasco, M. A. Santos
    07/2005;
  • 01/2005;
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    ABSTRACT: We report an atomic-scale study on the ferromagnetic insulator manganite LaMnO3.12 using γ-γ perturbed angular correlation spectroscopy. Data analysis reveals a nanoscopic transition from an undistorted to a Jahn-Teller (JT) distorted local environment upon cooling. The percolation thresholds of the two local environments enclose a macroscopic structural transition (rhombohedral-orthorhombic). Two distinct regimes of JT distortions were found: a high-temperature regime where uncorrelated polaron clusters with severe distortions of the Mn3+O6 octahedra survive up to T≈800 K and a low-temperature regime where correlated regions have a weaker JT-distorted symmetry.
    Physical Review B 08/2004; 73(10). · 3.77 Impact Factor
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    ABSTRACT: The recently proposed Langevin equation, aimed to capture the relevant critical features of stochastic sandpiles, and other self-organizing systems is studied numerically. This equation is similar to the Reggeon field theory, describing generic systems with absorbing states, but it is coupled linearly to a second conserved and static (non-diffusive) field. It has been claimed to represent a new universality class, including different discrete models: the Manna as well as other sandpiles, reaction-diffusion systems, etc. In order to integrate the equation, and surpass the difficulties associated with its singular noise, we follow a numerical technique introduced by Dickman. Our results coincide remarkably well with those of discrete models claimed to belong to this universality class, in one, two, and three dimensions. This provides a strong backing for the Langevin theory of stochastic sandpiles, and to the very existence of this new, yet meagerly understood, universality class. Comment: 4 pages, 3 eps figs, submitted to PRL
    07/2003;
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    ABSTRACT: The kinetic roughening of a stable oil-air interface moving in a Hele-Shaw cell that contains a quenched columnar disorder (tracks) has been studied. A capillary effect is responsible for the dynamic evolution of the resulting rough interface, which exhibits anomalous scaling. The three independent exponents needed to characterize the anomalous scaling are determined experimentally. The anomalous scaling is explained in terms of the initial acceleration and subsequent deceleration of the interface tips in the tracks coupled by mass conservation. A phenomenological model that reproduces the measured global and local exponents is introduced.
    Physical Review Letters 08/2002; 89(2):026102. · 7.94 Impact Factor
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    J J Ramasco, J M López, M A Rodríguez
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    ABSTRACT: We study the pinning-depinning phase transition of interfaces in the quenched Kardar-Parisi-Zhang model as the external driving force F goes towards zero. For a fixed value of the driving force, we induce depinning by increasing the nonlinear term coefficient lambda, which is related to lateral growth, up to a critical threshold. We focus on the case in which there is no external force applied (F=0) and find that, contrary to a simple scaling prediction, there is a finite value of lambda that makes the interface to become depinned. The critical exponents at the transition are consistent with directed percolation depinning.
    Physical Review E 01/2002; 64(6 Pt 2):066109. · 2.31 Impact Factor
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    JJ Ramasco, JM Lopez, MA Rodriguez
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    ABSTRACT: We study the dynamic scaling hypothesis in invariant surface growth. We show that the existence of power-law scaling of the correlation functions (scale invariance) does not determine a unique dynamic scaling form of the correlation functions, which leads to the different anomalous forms of scaling recently observed in growth models. We derive all the existing forms of anomalous dynamic scaling from a new generic scaling ansatz. The different scaling forms are subclasses of this generic scaling ansatz associated with bounds on the roughness exponent values. The existence of a new class of anomalous dynamic scaling is predicted and compared with simulations.
    Physical Review Letters 04/2000; 84(10):2199-202. · 7.94 Impact Factor
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    ABSTRACT: We comment on a recent Letter by Braunstein and Buceta [PRL vol.81, 630 (1998)], in which a novel equation has been proposed to describe the dynamics of interfaces in the presence of quenched disorder. We argue that the ansatz Braunstein and Buceta presented is just an effective description of the very short times regime and has no significance for the macroscopic (large scale) behaviour of the interface. We calculate an expression for the short time behaviour of the interface that is valid for any q (and not only at q_c). Comment: 1 page, 1 figure, uses epsf
    03/1999;
  • J M Lopez, J J Ramasco, M A Rodriguez
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    ABSTRACT: A Comment on the Letter by L. A. Braunstein and R. C. Buceta, Phys. Rev. Lett. 81, 630 (1998). The authors of the Letter offer a Reply.
    Physical Review Letters 01/1999; 82(6):1337-1337. · 7.94 Impact Factor

Publication Stats

117 Citations
35.83 Total Impact Points

Institutions

  • 2004–2013
    • Emory University
      • Department of Physics
      Atlanta, Georgia, United States
  • 2002–2009
    • Instituto de Física de Cantabria
      Santander, Cantabria, Spain
  • 2000–2002
    • Universidad de Cantabria
      • Department of Modern Physics
      Santander, Cantabria, Spain