Juan M. Lopez

Juan M. Lopez

Instituto de Física de Cantabria
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Juan M. Lopez has a total of 102 co-authors. Here are the top co-authors by number of shared publications:

Latest Publications

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    Ciencia de película. La hormiga que soñaba con agujeros negros

    by José Luis Bernal, Francisco J. Carrera Troyano, Maite Ceballos Merino, Yael Gutiérrez, Diego Herranz Muñoz, Lara Lloret Iglesias, Juan M. López, Álvaro López García, César Moreno Sierra, María Dolores Ortiz, et al.

    ¿Qué buscas cuando vas al cine? Que no te quepa duda, buscas emocionarte. Y no importa si vas a ver una película de terror, una comedia romántica, o una película de acción con explosiones. TODAS (con mayúsculas) las películas transmiten emociones, que pueden ir desde el miedo, la tristeza y el aburrimiento, hasta la sorpresa o la alegría. Si ahora te digo que todas las películas tienen ciencia, es muy probable que tu primer pensamiento sea «todas, todas… igual no». TODAS. Créeme. En El Padrino podemos hablar —por ejemplo— de la ciencia en las armas de fuego, o de la biología de los gatos. En Cantando bajo la lluvia, una película musical, tenemos para elegir entre las ondas sonoras y el ciclo del agua. ¿Titanic? Anda que no hay ciencia en un barco... o en el iceberg que lo hundió. Hasta en Ratatouille encontramos ciencia entre los fogones. El cine es un medio fantástico para descubrir cosas que desconocemos, para ver conexiones que nunca nos habríamos planteado. Así, en cada capítulo de este libro partimos de una película para contar un relato sobre la ciencia a la que nos dedicamos. Las películas elegidas son La ventana indiscreta (1954), Contact (1997), Her (2013), Interstellar (2014), Big Hero 6 (2014), Ant-Man (2015), Figuras ocultas (2016), y Tenet (2020). Disfruta.

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    Information theory approach to plasma turbulence

    by Juan M López, José A Mier, Omar F Castellanos

    The nature of intermittency in turbulent plasma transport-whether bursts occur as independent events or exhibit memory-is a fundamental question with direct implications for confinement prediction and control. Conventional empirical time-series diagnostics, such as autocorrelation functions and power spectra, are second-order measures and, when used alone, may not fully characterize statistical dependence in intermittent, non-Gaussian plasma fluctuations dominated by nonlinear bursty avalanche-like events. Here we employ information-theoretic tools-block entropy, excess entropy, complexity index, and mutual information-to systematically probe temporal correlations in fluctuation-induced particle flux time series from the Santander Linear Plasma Machine. Experimental data are first converted into binary event/calm sequences via thresholding. Analysis of block entropy reveals that the complexity index grows with block length and remains unsaturated for the outermost radial positions, indicating significant temporal memory and structure. At the plasma edge, mutual information decays slowly and remains detectable over many consecutive bursty events, indicating that information propagates across the avalanche sequence far into the future. This provides a direct evidence that edge plasma turbulence exhibits non-renewal, long-range memory in burst occurrence: the timing of each large event contains predictive information about avalanches many events ahead. The complexity index and quiet-time mutual information together offer sensitive, model-free, quantitative markers of proximity to marginal stability and confinement regime transitions.

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    Fragility of Kardar-Parisi-Zhang universality class in the presence of temporally correlated noise

    by Enrique Rodríguez-Fernández, Alejandro Alés, Jorge Martín-Álvarez, Juan M. López

    We study numerically a family of surface growth models that are known to be in the universality class of the Kardar-Parisi-Zhang equation when driven by uncorrelated noise. We find that, in the presence of noise with power-law temporal correlations with exponent θ, these models exhibit critical exponents that differ both quantitatively and qualitatively from model to model. The existence of a threshold value for θ below which the uncorrelated fixed point is dominant occurs for some models but not for others. In some models the dynamic exponent z(θ) is a smooth decreasing function, while it has a maximum in other cases. Despite all models sharing the same symmetries, critical exponents turn out to be strongly model dependent. Our results clearly show the fragility of the universality class concept in the presence of long-range temporally correlated noise.

    Cited by 4
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    Avalanche statistics of fluctuation-induced fluxes from the SLPM and the W7-AS stellarator

    by J A Mier, J Blanco, R Sánchez, O F Castellanos, D E Newman, E Anabitarte, J M López

    Measurements of fluctuating floating potentials and ion saturation currents at different radial locations in the Santander Linear Plasma Machine (Castellanos et al 2005 Plasma Phys. Control. Fusion 47 2067) and at the edge of the W7-AS stellarator by means of radially movable Langmuir probes allow to infer the corresponding fluctuation-induced radial flux temporal series. Avalanche-like transport events are identified in the time series and statistically characterized in terms of avalanche size/duration/quiet-time distributions and size-duration scaling relations. Transport is diffusive in the inner and intermediate radial region of the SLPM r < r tr ≈ 2.6 cm, undergoing a transition at r tr , becoming non-diffusive in the outermost region of the device, r > r tr . The results obtained at the edge of the W7-AS stellarator are similar to those found in SLPM for r > r tr , i.e. consistent with what would be expected for scale-free, self-similar plasma transport dynamics near a critical state.

    Cited by 2
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    Lyapunov vectors and excited energy states of the directed polymer in random media

    by Enrique Rodríguez-Fernández, Juan M. López

    The scaling behavior of the excited energy states of the directed polymer in random media is analyzed numerically. We find that the spatial correlations of polymer energies scale as ∼k−δ for small enough wave numbers k with a nontrivial exponent δ≈1.3. The equivalence between the stochastic-field equation that describes the partition function of the directed polymer and that governing the time evolution of infinitesimal perturbations in space-time chaos is exploited to connect this exponent δ with the spatial correlations of the Lyapunov vectors reported in the literature. The relevance of our results for other problems involving optimization in random systems is discussed.

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    Lyapunov vectors and excited energy levels of the directed polymer in random media

    by Enrique Rodriguez, Juan M. Lopez

    The scaling behavior of the excited energy levels of the directed polymer in random media is analyzed numerically. We find that the spatial correlations of polymer energies scale as $\sim k^{-δ}$ for small enough wavenumbers $k$ with a nontrivial exponent $δ\approx 1.3$. The equivalence between the stochastic-field equation that describes the partition function of the directed polymer and that governing the time evolution of infinitesimal perturbations in space-time chaos is exploited to connect this exponent $δ$ with the spatial correlations of Lyapunov vectors reported in the literature. The relevance of our results for other problems involving optimization in random systems is discussed.

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    Melting of crystals of polarization vortices and chiral phase transitions in oxide superlattices

    by Fernando Gómez-Ortiz, Pablo García-Fernández, Juan M. López, Javier Junquera

    We study the equilibrium arrangements of polarization vortices in (PbTiO3)n/(SrTiO3)n superlattices by means of second-principles simulations. We find that, at low temperatures, polarization vortices organize in a regular arrangement in which clockwise and counterclockwise vortices alternate positions, leading to a crystal-like structure with well-defined handedness. This chiral crystal melts at a critical temperature TM into a chiral liquid where long-range order is lost but handedness is preserved. At even higher temperatures TC, a second phase transition occurs at which the chiral liquid of polarization vortices loses its handedness. Both phase transitions can be readily identified by the adequate choices of order parameters.

    Cited by 26
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    Roughening of the anharmonic elastic interface in correlated random media

    by Alejandro Alés, Juan M. López

    We study the roughening properties of the anharmonic elastic interface in the presence of temporally correlated noise. The model can be seen as a generalization of the anharmonic Larkin model, recently introduced by Purrello, Iguain, and Kolton [ Phys. Rev. E 99, 032105 (2019)], to investigate the effect of higher-order corrections to linear elasticity in the fate of interfaces. We find analytical expressions for the critical exponents as a function of the anharmonicity index n, the noise correlator range θ∈[0,1/2], and dimension d. In d=1 we find that the interface becomes faceted and exhibits anomalous scaling for θ>1/4 for any degree of anharmonicity n>1. Analytical expressions for the anomalous exponents αloc and κ are obtained and compared with a numerical integration of the model. Our theoretical results show that anomalous roughening cannot exist for this model in dimensions d>1.

    Cited by 3
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    Nonuniversal large-size asymptotics of the Lyapunov exponent in turbulent globally coupled maps

    by David Velasco, Juan M. López, Diego Pazó

    Globally coupled maps (GCMs) are prototypical examples of high-dimensional dynamical systems. Interestingly, GCMs formed by an ensemble of weakly coupled identical chaotic units generically exhibit a hyperchaotic “turbulent” state. A decade ago, Takeuchi et al. [ Phys. Rev. Lett. 107, 124101 (2011)] theorized that in turbulent GCMs the largest Lyapunov exponent (LE), λ(N), depends logarithmically on the system size N: λ∞−λ(N)≃c/lnN. We revisit the problem and analyze, by means of analytical and numerical techniques, turbulent GCMs with positive multipliers to show that there is a remarkable lack of universality, in conflict with the previous prediction. In fact, we find a power-law scaling λ∞−λ(N)≃c/Nγ, where γ is a parameter-dependent exponent in the range 0<γ≤1. However, for strongly dissimilar multipliers, the LE varies with N in a slower fashion, which is here numerically explored. Although our analysis is only valid for GCMs with positive multipliers, it suggests that a universal convergence law for the LE cannot be taken for granted in general GCMs.

    Cited by 5
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    Efficient harmonic oscillator chain energy harvester driven by colored noise

    by M. Romero-Bastida, Juan M. López

    We study the performance of an electromechanical harmonic oscillator chain as an energy harvester to extract power from finite-bandwidth ambient random vibrations, which are modelled by colored noise. The proposed device is numerically simulated and its performance assessed by means of the net electrical power generated and its efficiency in converting the external noise-supplied power into electrical power. Our main result is a much enhanced performance, both in the net electrical power delivered and in efficiency, of the harmonic chain with respect to the popular single oscillator resonator. Our numerical findings are explained by means of an analytical approximation, in excellent agreement with numerics.

    Cited by 4
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