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  • Publication
    Triadic influence as a proxy for compatibility in social relationships
    (PNAS, 2023-03-28) Ruiz García, Miguel; Ozaita Corral, Juan; Pereda García, María; Alfonso, Antonio; Brañas Garza, Pablo; Cuesta Ruiz, Jose Antonio; Sánchez Valdés, Ariel; European Commission; Ministerio de Ciencia e Innovación (España); Universidad Carlos III de Madrid
    Networks of social interactions are the substrate upon which civilizations are built. Often, we create new bonds with people that we like or feel that our relationships are damaged through the intervention of third parties. Despite their importance and the huge impact that these processes have in our lives, quantitative scientific understanding of them is still in its infancy, mainly due to the difficulty of collecting large datasets of social networks including individual attributes. In this work, we present a thorough study of real social networks of 13 schools, with more than 3,000 students and 60,000 declared positive and negative relationships, including tests for personal traits of all the students. We introduce a metric -the 'triadic influence'- that measures the influence of nearest neighbors in the relationships of their contacts. We use neural networks to predict the sign of the relationships in these social networks, extracting the probability that two students are friends or enemies depending on their personal attributes or the triadic influence. We alternatively use a high-dimensional embedding of the network structure to also predict the relationships. Remarkably, using the triadic influence (a simple one-dimensional metric) achieves the best accuracy, and adding the personal traits of the students does not improve the results, suggesting that the triadic influence acts as a proxy for the social compatibility of students. We postulate that the probabilities extracted from the neural networks - functions of the triadic influence and the personalities of the students - control the evolution of real social networks, opening an avenue for the quantitative study of these systems.
  • Publication
    Chaos-based true random number generators
    (Springer, 2016-06-29) López Bonilla, Luis Francisco; Álvaro Ballesteros, Mariano; Carretero Cerrajero, Manuel; Ministerio de Economía y Competitividad (España)
    Random number (bit) generators are crucial to secure communications, data transfer and storage, and electronic transactions, to carry out stochastic simulations and to many other applications. As software generated random sequences are not truly random, fast entropy sources such as quantum systems or classically chaotic systems can be viable alternatives provided they generate high-quality random sequences sufficiently fast. The discovery of spontaneous chaos in semiconductor superlattices at room temperature has produced a valuable nanotechnology option. Here we explain a mathematical model to describe spontaneous chaos in semiconductor superlattices at room temperature, solve it numerically to reveal the origin and characteristics of chaotic oscillations, and discuss the limitations of the model in view of known experiments. We also explain how to extract verified random bits from the analog chaotic signal produced by the superlattice.
  • Publication
    Spatial dispersion in two-dimensional plasmonic crystals: Large blueshifts promoted by diffraction anomalies
    (American Physical Society, 2016-10-12) Christin, David; Christensen, Johan; Mortensen, N. Asger
    We develop a methodology to incorporate nonlocal optical response of the free electron gas due to quantum-interaction effects in metal components of periodic two-dimensional plasmonic crystals and study the impact of spatial dispersion on promising building blocks for photonic circuits. Within the framework of the hydrodynamic model, we observe significant changes with respect to the commonly employed local-response approximation, but also in comparison with homogeneous metal films where nonlocal effects have previously been considered. Notable are the emergence of a contribution from nonlocality at normal incidence and the surprisingly large structural parameters at which finite blueshifts are observable, which we attribute to diffraction that offers nonvanishing in-plane wave vector components and increases the penetration depth of longitudinal (nonlocal) modes.
  • Publication
    STM-driven transition from rippled to buckled graphene in a spin-membrane model
    (APS, 2016-11-15) Ruiz-García, M.; López Bonilla, Luis Francisco; Prados, A.; Ministerio de Economía y Competitividad (España)
    We consider a simple spin-membrane model for rippling in graphene. The model exhibits transitions from a flat but rippled membrane to a buckled one. At high temperature the transition is second order, but it is first order at low temperature for appropriate strength of the spin-spin coupling. Driving the system across the first-order phase transition in nonequilibrium conditions that mimic interaction of the graphene membrane with a scanning tunneling microscopy (STM) tip explains recent experiments. In particular, we observe a reversible behavior for small values of the STM current and an irreversible transition from a flat rippled membrane to a rigid buckled membrane when the current surpasses a critical value. This work makes it possible to test the mechanical properties of graphene under different temperature and electrostatic conditions.
  • Publication
    Critical radius and temperature for buckling in graphene
    (APS, 2016-03-15) López Bonilla, Luis Francisco; Ruiz-García, Miguel; Ministerio de Economía y Competitividad (España); Ministerio de Educación, Cultura y Deporte (España)
    In this work, we find an analytical flat-membrane solution to the saddle point equations, derived by F. Guinea et al. [Phys. Rev. B 89, 125428 (2014)], for the case of a suspended graphene membrane of circular shape. We also find how different buckled membrane solutions bifurcate from the flat membrane at critical temperatures and membrane radii. The saddle point equations take into account electron-phonon coupling and this coupling provides a residual stress even for a flat graphene layer. Below a critical temperature (which is exceedingly high for an infinite layer) or above a critical size that depend on boundary conditions, different buckling modes that may be the germ of rippling appear. Our results provide the opportunity to develop new feasible experiments dealing with buckling in small suspended graphene membranes that could verify them. These experiments may also be used to fit the phonon-electron coupling constant or the bending energy.
  • Publication
    Coupled Landau-Zener-Stuckelberg quantum dot interferometers
    (APS, 2016-02-15) Gallego Marcos, Fernando; Sánchez, Rafael; Platero, Gloria; Ministerio de Economía y Competitividad (España)
    We investigate the interplay between long-range and direct photon-assisted transport in a triple quantum dot chain where local ac voltages are applied to the outer dots. We propose the phase difference between the two ac voltages as an external parameter, which can be easily tuned to manipulate the current characteristics. For gate voltages in phase opposition we find quantum destructive interferences analogous to the interferences in closed-loop undriven triple dots. As the voltages oscillate in phase, interferences between multiple paths give rise to dark states. Those totally cancel the current, and could be experimentally resolved.
  • Publication
    Wavelength selection of rippling patterns in myxobacteria
    (APS, 2016-01) López Bonilla, Luis Francisco; Glavan, Ana Maria; Marquina Vila, Antonio; Ministerio de Economía y Competitividad (España)
    Rippling patterns of myxobacteria appear in starving colonies before they aggregate to form fruiting bodies. These periodic traveling cell density waves arise from the coordination of individual cell reversals, resulting from an internal clock regulating them and from contact signaling during bacterial collisions. Here we revisit a mathematical model of rippling in myxobacteria due to Igoshin et al. [Proc. Natl. Acad. Sci. USA 98, 14913 (2001) and Phys. Rev. E 70, 041911 (2004)]. Bacteria in this model are phase oscillators with an extra internal phase through which they are coupled to a mean field of oppositely moving bacteria. Previously, patterns for this model were obtained only by numerical methods, and it was not possible to find their wave number analytically. We derive an evolution equation for the reversal point density that selects the pattern wave number in the weak signaling limit, shows the validity of the selection rule by solving numerically the model equations, and describes other stable patterns in the strong signaling limit. The nonlocal mean-field coupling tends to decohere and confine patterns. Under appropriate circumstances, it can annihilate the patterns leaving a constant density state via a nonequilibrium phase transition reminiscent of destruction of synchronization in the Kuramoto model.
  • Publication
    On the mathematical modelling of tumor-induced angiogenesis
    (American Institute of Mathematical Sciences (AIMS), 2017-02-01) López Bonilla, Luis Francisco; Capasso, Vincenzo; Álvaro Ballesteros, Mariano; Carretero Cerrajero, Manuel; Terragni, Filippo; Ministerio de Economía y Competitividad (España)
    An angiogenic system is taken as an example of extremely complex ones in the field of Life Sciences, from both analytical and computational points of view, due to the strong coupling between the kinetic parameters of the relevant branching-growth-anastomosis stochastic processes of the capillary network, at the microscale, and the family of interacting underlying biochemical fields, at the macroscale. To reduce this complexity, for a conceptual stochastic model we have explored how to take advantage of the system intrinsic multiscale structure: one might describe the stochastic dynamics of the cells at the vessel tip at their natural microscale, whereas the dynamics of the underlying fields is given by a deterministic mean field approximation obtained by an averaging at a suitable mesoscale. But the outcomes of relevant numerical simulations show that the proposed model, in presence of anastomosis, is not self-averaging, so that the "propagation of chaos" assumption cannot be applied to obtain a deterministic mean field approximation. On the other hand we have shown that ensemble averages over many realizations of the stochastic system may better correspond to a deterministic reaction-diffusion system.
  • Publication
    Single-electron thermal devices coupled to a mesoscopic gate
    (IOP Publishing Limited, 2017-11-24) Sánchez Rodrigo, Rafael; Thierschmann, Holger; Molenkamp, Laurens W.; European Commission; Ministerio de Economía y Competitividad (España)
    We theoretically investigate the propagation of heat currents in a three-terminal quantum dot engine. Electron-electron interactions introduce state-dependent processes which can be resolved by energy-dependent tunneling rates. We identify the relevant transitions which define the operation of the system as a thermal transistor or a thermal diode. In the former case, thermal-induced charge fluctuations in the gate dot modify the thermal currents in the conductor with suppressed heat injection, resulting in huge amplification factors and the possible gating with arbitrarily low energy cost. In the latter case, enhanced correlations of the state-selective tunneling transitions redistribute heat flows giving high rectification coefficients and the unexpected cooling of one conductor terminal by heating the other one. We propose quantum dot arrays as a possible way to achieve the extreme tunneling asymmetries required for the different operations.
  • Publication
    Numerical approach for the computation of preliminary post-newtonian corrections for laser links in space
    (Hindawi, 2019-01-01) Gambi Fernández, José María; García del Pino Megía, María Luisa; Mosser, Jonathan; Weinmuller, Ewa B.
    Two systems of Earth-centered inertial Newtonian orbital equations for a spherical Earth and three systems of post-Newtonian nonlinear equations, derived from the second post-Newtonian approximation to the Earth Schwarzschild field, are used to carry out a performance analysis of a numerical procedure based on the Dormand-Prince method for initial value problems in ordinary differential equations. This procedure provides preliminary post-Newtonian corrections to the Newtonian trajectories of middle-size space objects with respect to space-based acquisition, pointing, and tracking laser systems, and it turns out to be highly efficient. In fact, we can show that running the standard adaptive ode45 MATLAB routine with the absolute and relative tolerance, TOLa=10(-16) and TOLr=10(-13), respectively, provides corrections that are final within the eclipses caused by the Earth and close to final during the noneclipse phases. These corrections should be taken into account to increase the pointing accuracy in implementing the space-to-space laser links required for ablation of designated objects or communications between space terminals.
  • Publication
    On the emergence of large and complex memory effects in nonequilibrium fluids
    (IOP, 2019-03-01) Lasanta Becerra, Antonio; Vega Reyes, Francisco; Prados, A.; Santos, A.; Ministerio de Economía y Competitividad (España); Agencia Estatal de Investigación (España)
    Control of cooling and heating processes is essential in many industrial and biological processes. In fact, the time evolution of an observable quantitymay differ according to the previous history of the system. For example, a systemthat is being subject to cooling and then, at a given time t(w) for which the instantaneous temperature is T (t(w)) = T-st, is suddenly put in contactwith a temperature source at T-st may continue cooling down temporarily or, on the contrary, undergo a temperature rebound. According to current knowledge, there can be only one 'spurious' and small peak/low. However, our results prove that, under certain conditions, more than one extremum may appear. Specifically, we have observed regionswith two extrema and a critical point with three extrema. We have also detected cases where extraordinarily large extrema are observed, as large as the order of magnitude of the stationary value of the variable of interest. We showthis by studying the thermal evolution of a low density set ofmacroscopic particles that do not preserve kinetic energy upon collision, i.e. a granular gas. We describe themechanismthat signals in this system the emergence of these complex and large memory effects, and explain why similar observations can be expected in a variety of systems.
  • Publication
    Mean field theory of chaotic insect swarms
    (APS, 2023-06) González Albaladejo, Rafael; López Bonilla, Luis Francisco; Comunidad de Madrid; Ministerio de Economía y Competitividad (España); Ministerio de Ciencia e Innovación (España)
    The harmonically confined Vicsek model displays qualitative and quantitative features observed in natural insect swarms. It exhibits a scale-free transition between single and multicluster chaotic phases. Finite-size scaling indicates that this unusual phase transition occurs at zero confinement [Phys. Rev. E 107, 014209 (2023)]. While the evidence of the scale-free-chaos phase transition comes from numerical simulations, here we present its mean-field theory. Analytically determined critical exponents are those of the Landau theory of equilibrium phase transitions plus dynamical critical exponent z = 1 and a new critical exponent φ = 0.5 for the largest Lyapunov exponent. The phase transition occurs at zero confinement and noise in the mean-field theory. The noise line of zero largest Lyapunov exponents informs observed behavior: (i) the qualitative shape of the swarm (on average, the center of mass rotates slowly at the rate marked by the winding number and its trajectory fills compactly the space, similarly to the observed condensed nucleus surrounded by vapor) and (ii) the critical exponents resemble those observed in natural swarms. Our predictions include power laws for the frequency of the maximal spectral amplitude and the winding number.
  • Publication
    Tracking collective cell motion by topological data analysis
    (PLOS, 2020-12-23) López Bonilla, Luis Francisco; Carpio, Ana; Trenado, Carolina; Agencia Estatal de Investigación (España); Ministerio de Ciencia, Innovación y Universidades (España)
    By modifying and calibrating an active vertex model to experiments, we have simulated numerically a confluent cellular monolayer spreading on an empty space and the collision of two monolayers of different cells in an antagonistic migration assay. Cells are subject to inertial forces and to active forces that try to align their velocities with those of neighboring ones. In agreement with experiments in the literature, the spreading test exhibits formation of fingers in the moving interfaces, there appear swirls in the velocity field, and the polar order parameter and the correlation and swirl lengths increase with time. Numerical simulations show that cells inside the tissue have smaller area than those at the interface, which has been observed in recent experiments. In the antagonistic migration assay, a population of fluidlike Ras cells invades a population of wild type solidlike cells having shape parameters above and below the geometric critical value, respectively. Cell mixing or segregation depends on the junction tensions between different cells. We reproduce the experimentally observed antagonistic migration assays by assuming that a fraction of cells favor mixing, the others segregation, and that these cells are randomly distributed in space. To characterize and compare the structure of interfaces between cell types or of interfaces of spreading cellular monolayers in an automatic manner, we apply topological data analysis to experimental data and to results of our numerical simulations. We use time series of data generated by numerical simulations to automatically group, track and classify the advancing interfaces of cellular aggregates by means of bottleneck or Wasserstein distances of persistent homologies. These techniques of topological data analysis are scalable and could be used in studies involving large amounts of data. Besides applications to wound healing and metastatic cancer, these studies are relevant for tissue engineering, biological effects of materials, tissue and organ regeneration.
  • Publication
    Track-to-track association methodology for operational surveillance scenarios with radar observations
    (Springer Nature, 2023-07) Pastor Rodríguez, Alejandro; Sanjurjo Rivo, Manuel; Escobar, Diego; Comunidad de Madrid
    This paper proposes a novel track-to-track association methodology able to detect and catalogue resident space objects (RSOs) from associations of uncorrelated tracks (UCTs) obtained by radar survey sensors. It is a multi-target multi-sensor algorithm approach able to associate data from surveillance sensors to detect and catalogue objects. The association methodology contains a series of steps, each of which reduces the complexity of the combinational problem. The main focus are real operational environments, in which brute-force approaches are computationally unaffordable. The hypotheses are scored in the measurement space by evaluating a figure of merit based on the residuals of the observations. This allows us to filter out most of the false hypotheses that would be present in brute-force approaches, as well as to distinguish between true and false hypotheses. The suitability of the proposed track-to-track association has been assessed with a simulated scenario representative of a real operational environment, corresponding to 2 weeks of radar survey data obtained by a single survey radar. The distribution and evolution of the hypotheses along the association process is analysed and typical association performance metrics are included. Most of the RSOs are detected and catalogued and only one false positive is obtained. Besides, the rate of false positives is kept low, most of them corresponding to particular cases or objects with high eccentricity or limited observability.
  • Publication
    Edge observables of the Maxwell-Chern-Simons theory
    (American Physical Society (APS), 2022-07-15) Barbero G., J. Fernando; Díaz Jiménez, Bogar; Margalef Bentabol, Juan; Sánchez Villaseñor, Eduardo Jesús; Comunidad de Madrid; European Commission; Ministerio de Ciencia, Innovación y Universidades (España); Universidad Carlos III de Madrid
    We analyze the Lagrangian and Hamiltonian formulations of the Maxwell-Chern-Simons theory defined on a manifold with boundary for two different sets of boundary equations derived from a variational principle. We pay special attention to the identification of the infinite chains of boundary constraints and their resolution. We identify edge observables and their algebra [which corresponds to the well-known U (1) Kac-Moody algebra]. Without performing any gauge fixing, and using the Hodge-Morrey theorem, we solve the Hamilton equations whenever possible. In order to give explicit solutions, we consider the particular case in which the fields are defined on a 2-disk. Finally, we study the Fock quantization of the system and discuss the quantum edge observables and states.
  • Publication
    Theory of non-Hermitian topological whispering gallery
    (Springer Nature, 2022-11-18) Pernas-Salomón, René; Zheng, Li-Yang; Zhang, Zhiwang; Gao, Penglin; Liu, Xiaojun; Cheng, Ying; Christensen, Johan; European Commission
    Topological insulators have taken the condensed matter physics scenery by storm and captivated the interest among scientists and materials engineers alike. Surprisingly, this arena which was initially established and profoundly studied in electronic systems and crystals, has sparked a drive among classical physicists to pursue a wave-based analogy for sound, light and vibrations. In the latest efforts combining valley-contrasting topological sound with non-Hermitian ingredients, B. Hu et al. [Nature 597, 655 (2021)] employed thermoacoustic coupling in sonic lattices whose elementary building blocks are coated with electrically biased carbon nanotube films. In this contribution, we take a theoretical and numerical route towards understanding the complex acoustic interplay between geometry and added acoustic gain as inspired by the aforesaid publication. Besides complex bulk and edge states predictions and computations of mode-split resonances using whispering gallery configurations, we also predict an acoustic amplitude saturation in dependence on the activated coated elements. We foresee that our computational advances may assist future efforts in exploring thermoacoustic topological properties.
  • Publication
    Consistent and non-consistent deformations of gravitational theories
    (Springer, 2022-05) Barbero G., J. Fernando; Basquens, Marc; Díaz Jiménez, Bogar; Sánchez Villaseñor, Eduardo Jesús; Agencia Estatal de Investigación (España); Universidad Carlos III de Madrid; European Commission
    We study the internally abelianized version of a range of gravitational theories, written in connection tetrad form, and study the possible interaction terms that can be added to them in a consistent way. We do this for 2+1 and 3+1 dimensional models. In the latter case we show that the Cartan-Palatini and Holst actions are not consistent deformations of their abelianized versions. We also show that the Husain-Kuchař and Euclidean self-dual actions are consistent deformations of their abelianized counterparts. This suggests that if the latter can be quantized, it could be possible to devise a perturbative scheme leading to the quantization of Euclidean general relativity along the lines put forward by Smolin in the early nineties.
  • Publication
    Proof of the equivalence of the symplectic forms derived from the canonical and the covariant phase space formalisms
    (American Physical Society, 2022-05-15) Margalef Bentabol, Juan; Sánchez Villaseñor, Eduardo Jesús; Comunidad de Madrid; Ministerio de Ciencia e Innovación (España); Universidad Carlos III de Madrid
    We prove that, for any theory defined over a space-time with boundary, the symplectic form derived in the covariant phase space is equivalent to the one derived from the canonical formalism.
  • Publication
    Exponentially accelerated approach to stationarity in Markovian open quantum systems through the Mpemba effect
    (American Physical Society, 2021-08-06) Carollo, Federico; Lasanta Becerra, Antonio; Lesanovsky, Igor; Ministerio de Ciencia, Innovación y Universidades (España)
    Ergodicity breaking and slow relaxation are intriguing aspects of nonequilibrium dynamics both in classical and quantum settings. These phenomena are typically associated with phase transitions, e.g., the emergence of metastable regimes near a first-order transition or scaling dynamics in the vicinity of critical points. Despite being of fundamental interest the associated divergent timescales are a hindrance when trying to explore steady-state properties. Here we show that the relaxation dynamics of Markovian open quantum systems can be accelerated exponentially by devising an optimal unitary transformation that is applied to the quantum system immediately before the actual dynamics. This initial "rotation" is engineered in such a way that the state of the quantum system no longer excites the slowest decaying dynamical mode. We illustrate our idea - which is inspired by the so-called Mpemba effect, i.e., water freezing faster when initially heated up - by showing how to achieve an exponential speeding-up in the convergence to stationarity in Dicke models, and how to avoid metastable regimes in an all-to-all interacting spin system.
  • Publication
    Gaussian RBF-FD weights and its corresponding local truncation errors
    (Elsevier, 2012-09) Bayona Revilla, Víctor; Moscoso, Miguel; Kindelan Segura, Manuel; Comunidad de Madrid; Ministerio de Ciencia e Innovación (España)
    In this work we derive analytical expressions for the weights of Gaussian RBF-FD and Gaussian RBF-HFD formulas for some differential operators. These weights are used to derive analytical expressions for the leading order approximations to the local truncation error in powers of the inter-node distance h and the shape parameter є. We show that for each differential operator, there is a range of values of the shape parameter for which RBF-FD formulas and RBF-HFD formulas are significantly more accurate than the corresponding standard FD formulas. In fact, very often there is an optimal value of the shape parameter є+ for which the local error is zero to leading order. This value can be easily computed from the analytical expressions for the leading order approximations to the local error. Contrary to what is generally believed, this value is, to leading order, independent of the internodal distance and only dependent on the value of the function and its derivatives at the node.