# A. Tarancón's research while affiliated with University of Zaragoza and other places

## Publications (165)

Article
Full-text available
The emergence of collective intelligence has been studied in much greater detail in small groups than in larger ones. Nevertheless, in groups of several hundreds or thousands of members, it is well-known that the social environment exerts a considerable influence on individual behavior. A few recent papers have dealt with some aspects of large grou...
Preprint
Time-reversal symmetry is spontaneously broken in spin glasses below their glass temperature. Under such conditions, the standard assumption about the equivalence of the most standard protocols (i.e.,\it{no big difference between switching the field on or off}, as it is sometimes said) is not really justified. In fact, we show here that the spin-gl...
Preprint
Full-text available
Memory and rejuvenation effects in the magnetic response of off-equilibrium spin glasses have been widely regarded as the doorway into the experimental exploration of ultrametricity and temperature chaos (maybe the most exotic features in glassy free-energy landscapes). Unfortunately, despite more than twenty years of theoretical efforts following...
Article
Full-text available
Experiments featuring non-equilibrium glassy dynamics under temperature changes still await interpretation. There is a widespread feeling that temperature chaos (an extreme sensitivity of the glass to temperature changes) should play a major role but, up to now, this phenomenon has been investigated solely under equilibrium conditions. In fact, the...
Article
The synergy between experiment, theory, and simulations enables a microscopic analysis of spin-glass dynamics in a magnetic field in the vicinity of and below the spin-glass transition temperature T g . The spin-glass correlation length, ξ ( t , t w ; T ), is analysed both in experiments and in simulations in terms of the waiting time t w after the...
Preprint
Full-text available
The synergy between experiment, theory, and simulations enables a microscopic analysis of spin-glass dynamics in a magnetic field in the vicinity of and below the spin-glass transition temperature $T_\mathrm{g}$. The spin-glass correlation length, $\xi(t,t_\mathrm{w};T)$, is analysed both in experiments and in simulations in terms of the waiting ti...
Article
The correlation length ξ, a key quantity in glassy dynamics, can now be precisely measured for spin glasses both in experiments and in simulations. However, known analysis methods lead to discrepancies either for large external fields or close to the glass temperature. We solve this problem by introducing a scaling law that takes into account both...
Preprint
Full-text available
We find a dynamic effect in the non-equilibrium dynamics of a spin glass that closely parallels equilibrium temperature chaos. This effect, that we name dynamic temperature chaos, is spatially heterogeneous to a large degree. The key controlling quantity is the time-growing spin-glass coherence length. Our detailed characterization of dynamic tempe...
Article
Full-text available
The term citizen science refers to a broad set of practices developed in a growing num- ber of areas of knowledge and characterized by the active citizen participation in some or several stages of the research process. Definitions, classifications and terminology remain open, reflecting that citizen science is an evolving phenomenon, a spectrum of...
Preprint
Full-text available
The correlation length $\xi$, a key quantity in glassy dynamics, can now be precisely measured for spin glasses both in experiments and in simulations. However, known analysis methods lead to discrepancies either for large external fields or close to the glass temperature. We solve this problem by introducing a scaling law that takes into account b...
Article
In this work, we study the impact that the withdrawal of institutions from the United Kingdom caused by BREXIT has on the European research collaboration networks. To this aim, we consider BREXIT as a targeted attack to those graphs composed by the European institutions that have collaborated in research projects belonging to the three main H2020 p...
Article
We analyse the production and networks of Nobel laureates in Economics, employing the Normalized Impact Factor (NIF) of their publications in the Journal of Citation Report (Economics), to identify the academic leaders among those laureates awarded between 1969 and 2016. Our results indicate that direct collaborations among laureates are, in genera...
Article
Full-text available
Network analysis allows us to introduce different metrics that complement the traditional indicators to measure academic performance, generally based on individual production. In this paper, we show how the use of these techniques provides a more global point of view, introducing indicators that, beyond individual merits, measure the capacity of re...
Article
The aim of this paper is to establish the similarities and differences between the way of collaboration and the production of researchers when dealing with publications or with the development of projects and whether the collaboration patterns change across disciplines. We have studied the networks of researchers formed through the collaborations i...
Article
Full-text available
Experiments on spin glasses can now make precise measurements of the exponent z(T) governing the growth of glassy domains, while our computational capabilities allow us to make quantitative predictions for experimental scales. However, experimental and numerical values for z(T) have differed. We use new simulations on the Janus II computer to resol...
Article
Full-text available
Significance The Mpemba effect, wherein an initially hotter system relaxes faster when quenched to lower temperatures than an initially cooler system, has attracted much attention. Paradoxically, its very existence is a hot topic. Using massive numerical simulations, we show unambiguously that the Mpemba effect is present in the archetypical model...
Article
Full-text available
Experiments on spin glasses can now make precise measurements of the exponent $z(T)$ governing the growth of glassy domains, while our computational capabilities allow us to make quantitative predictions for experimental scales. However, experimental and numerical values for $z(T)$ have differed. We use new simulations on the Janus II computer to r...
Article
Full-text available
We first reproduce on the Janus and Janus II computers a milestone experiment that measures the spin-glass coherence length through the lowering of free-energy barriers induced by the Zeeman effect. Secondly we determine the scaling behavior that allows a quantitative analysis of a new experiment reported in the companion Letter [S. Guchhait and R....
Article
Full-text available
Significance The unifying feature of glass formers (such as polymers, supercooled liquids, colloids, granulars, spin glasses, superconductors, etc.) is a sluggish dynamics at low temperatures. Indeed, their dynamics are so slow that thermal equilibrium is never reached in macroscopic samples: in analogy with living beings, glasses are said to age....
Article
Full-text available
We present a new tool, Kampal (http:// kampal. unizar. es), developed to help to analyze the academic productivity of a research institution from the point of view of Complex Networks. We will focus on two main aspects: paper production and funding by research grants. Thus, we define a network of researchers and define suitable ways of describing t...
Article
Full-text available
We perform equilibrium parallel-tempering simulations of the 3D Ising Edwards-Anderson spin glass in a field. A traditional analysis shows no signs of a phase transition. Yet, we encounter dramatic fluctuations in the behaviour of the model: Averages over all the data only describe the behaviour of a small fraction of it. Therefore we develop a new...
Article
Full-text available
We study the off-equilibrium dynamics of the three-dimensional Ising spin glass in the presence of an external magnetic field. We have performed simulations both at fixed temperature and with an annealing protocol. Thanks to the Janus special-purpose computer, based on field-programmable gate array (FPGAs), we have been able to reach times equivale...
Article
Full-text available
We report a high-precision finite-size scaling study of the critical behavior of the three-dimensional Ising Edwards-Anderson model (the Ising spin glass). We have thermalized lattices up to L=40 using the Janus dedicated computer. Our analysis takes into account leading-order corrections to scaling. We obtain Tc = 1.1019(29) for the critical tempe...
Article
Full-text available
This paper describes the architecture, the development and the implementation of Janus II, a new generation application-driven number cruncher optimized for Monte Carlo simulations of spin systems (mainly spin glasses). This domain of computational physics is a recognized grand challenge of high-performance computing: the resources necessary to stu...
Chapter
In this chapter we describe the Janus supercomputer, a massively parallel FPGA-based system optimized for the simulation of spin-glasses, theoretical models that describe the behavior of glassy materials. The custom architecture of Janus has been developed to meet the computational requirements of these models. Spin-glass simulations are performed...
Article
In this paper we present a general description of the ISDEP code (Integrator of Stochastic Differential Equations for Plasmas) and a brief overview of its physical results and applications so far. ISDEP is a Monte Carlo code that calculates the distribution function of a minority population of ions in a magnetized plasma. It solves the ion equation...
Conference Paper
We describe Janus, an application-driven architecture for Monte Carlo simulations of spin glasses. Janus is a massively parallel architecture, based on reconfigurable FPGA nodes; it offers two orders of magnitude better performance than commodity systems for spin glass applications. The first generation Janus machine has been operational since earl...
Article
Full-text available
It is not fully understood why we cooperate with strangers on a daily basis. In an increasingly global world, where interaction networks and relationships between individuals are becoming more complex, different hypotheses have been put forward to explain the foundations of human cooperation on a large scale and to account for the true motivations...
Conference Paper
Full-text available
We describe the past and future of the Janus project. The collaboration started in 2006 and deployed in early 2008 the Janus supercomputer, a facility that allowed to speed-up Monte Carlo Simulations of a class of model glassy systems and provided unprecedented results for some paradigms in Statistical Mechanics. The Janus Supercomputer was based o...
Article
Full-text available
We describe Janus, a massively parallel FPGA-based computer optimized for the simulation of spin glasses, theoretical models for the behavior of glassy materials. FPGAs (as compared to GPUs or many-core processors) provide a complementary approach to massively parallel computing. In particular, our model problem is formulated in terms of binary var...
Article
Full-text available
Spin glasses are a longstanding model for the sluggish dynamics that appears at the glass transition. However, spin glasses differ from structural glasses for a crucial feature: they enjoy a time reversal symmetry. This symmetry can be broken by applying an external magnetic field, but embarrassingly little is known about the critical behaviour of...
Article
PireGrid [1] (Project Number EFA35/08) is an INTERREG IV A project which has two main objectives, namely the deployment of a production Grid Computing infrastructure in the regions of Aragon, Aquitaine and Midi-Pyrenees and the achievement of successful cases in execution of applications from small and medium size companies of the named regions, in...
Article
Full-text available
The number of people using online social networks in their everyday life is continuously growing at a pace never saw before. This new kind of communication has an enormous impact on opinions, cultural trends, information spreading and even in the commercial success of new products. More importantly, social online networks have revealed as a fundame...
Article
Full-text available
We study the sample-to-sample fluctuations of the overlap probability densities from large-scale equilibrium simulations of the three-dimensional Edwards-Anderson spin glass below the critical temperature. Ultrametricity, Stochastic Stability and Overlap Equivalence impose constraints on the moments of the overlap probability densities that can be...
Article
Full-text available
We numerically study the aging properties of the dynamical heterogeneities in the Ising spin glass. We find that a phase transition takes place during the aging process. Statics-dynamics correspondence implies that systems of finite size in equilibrium have static heterogeneities that obey Finite-Size Scaling, thus signaling an analogous phase tran...
Article
Full-text available
We present a massive equilibrium simulation of the three-dimensional Ising spin glass at low temperatures. The Janus special-purpose computer has allowed us to equilibrate, using parallel tempering, L=32 lattices down to T=0.64 Tc. We demonstrate the relevance of equilibrium finite-size simulations to understand experimental non-equilibrium spin gl...
Article
Full-text available
We study the 3D disordered Potts model with p = 5 and 6. Our numerical simulations (that severely slow down for increasing p) detect a very clear spin glass phase transition. We evaluate the critical exponents and the critical value of the temperature, and we use known results at lower p values to discuss how they evolve for increasing p. We do n...
Chapter
Physics simulations can be very resource demanding, and the fusion ones are a good example. Grid computing, which is continuously improving its features, is a promising architecture for the execution of these simulations. On the other hand, the visualization of generated data could be significantly extended by the use of an interactive grid environ...
Article
Full-text available
This paper presents the latest results on confinement studies in the TJ-II stellarator. The inherently strong plasma--wall interaction of TJ-II has been successfully reduced after lithium coating by vacuum evaporation. Besides H retention and low Z , Li was chosen because there exists a reactor-oriented interest in this element, thus giving special...
Article
The flux-expansion divertor concept is theoretically explored in TJ-II. This concept seems to be most suitable for this flexible stellarator in which the positions and the order of the resonances can be varied. As a first step, a family of configurations that exhibit flux expansion is identified. Then, a three-dimensional map of the collisional par...
Article
The validity of the traditional local diffusive approach and of the use of monoenergetic calculations has been studied for the stellarator TJ-II [ Alejaldre et al., Fusion Technol. 17, 131 (1990) ]: it is shown to be doubtful, under some circumstances, even in a purely collisional description of transport. The diffusion in physical space starting f...
Article
We have investigated the phase transition in the Heisenberg spin glass using massive numerical simulations to study larger sizes, 48x48x48, than have been attempted before at a spin glass phase transition. A finite-size scaling analysis indicates that the data is compatible with the most economical scenario: a common transition temperature for spin...
Article
Full-text available
This paper presents the latest results on confinement studies in the TJ-II stellarator. The inherently strong plasma--wall interaction of TJ-II has been successfully reduced after lithium coating by vacuum evaporation. Besides H retention and low Z , Li was chosen because there exists a reactor-oriented interest in this element, thus giving special...
Article
Full-text available
The influence of magnetic ripple on ion collisional transport in ITER (Shimada et al 2007 Progress in the ITER Physics Basis: chapter 1. Overview and summary Nucl. Fusion 47 S1) is calculated using the Monte Carlo orbit code ISDEP (Castejón et al 2007 Plasma Phys. Control. Fusion 49 753). The ripple is introduced as a perturbation to the 2D equilib...
Article
Full-text available
This paper presents the latest results on confinement studies in the TJ-II stellarator. The inherently strong plasma--wall interaction of TJ-II has been successfully reduced after lithium coating by vacuum evaporation. Besides H retention and low Z , Li was chosen because there exists a reactor-oriented interest in this element, thus giving special...
Book
Full-text available
This work deals with the problems solved in fusion research by means of grid computing. The computing necessities for fusion are discussed and the applications that have been ported to grid as well as their main physical results are described. The range of plasma physics research covered by this set of tools is analysed and the future of grid compu...
Article
Full-text available
We perform numerical simulations, including parallel tempering, on the Potts glass model with binary random quenched couplings using the JANUS application-oriented computer. We find and characterize a glassy transition, estimating the location of the transition and the value of the critical exponents. We show that there is no ferromagnetic transiti...
Article
Full-text available
Using the dedicated computer Janus, we follow the nonequilibrium dynamics of the Ising spin glass in three dimensions for eleven orders of magnitude. The use of integral estimators for the coherence and correlation lengths allows us to study dynamic heterogeneities and the presence of a replicon mode and to obtain safe bounds on the Edwards-Anderso...
Article
The role of the divertor in a stellarator‐based fusion reactor is discussed, making emphasis on the flux‐expansion concept. In this context, the possibility of having a flux‐expansion divertor in TJ‐II is explored. As a first step, the three‐dimensional map of the particle flux has been estimated using the code ISDEP in two different plasma regimes...
Article
Full-text available
We study numerically the nonequilibrium dynamics of the Ising spin glass, for a time spanning 11 orders of magnitude, thus approaching the experimentally relevant scale (i.e., seconds). We introduce novel analysis techniques to compute the coherence length in a model-independent way. We present strong evidence for a replicon correlator and for over...
Article
The ion Drift Kinetic Equation (DKE) which describes the ion collisional transport is solved for the TJ-II device plasmas. This non-linear equation is computed by performing a mean field iterative calculation. In each step of the calculation, a Fokker-Planck equation is solved by means of the Langevin approach: one million particles are followed in...
Article
We introduce a new code of plasma transport based on evolving the Boltzmann equation in guiding center approximation where collisions has been taken into account. The spatial geometry is discretized using high order elements in space and a moment expansion in velocity space. First calculations with non-evolving electric field agree with the particl...
Conference Paper
Janus is an FPGA-based computer optimized for the simulation of spin glasses or similar condensed matter systems. Computing requirements in this area, are still riot met by available commercial systems, so all application-driven machine, boosting performance by approximately a factor 100x is in this case the only viable option to simulate large sys...
Article
Full-text available
In TJ-II stellarator plasmas, in the electron cyclotron heating regime, an increase in the ion temperature is observed, synchronized with that of the electron temperature, during the transition to the core electron-root confinement (CERC) regime. This rise in ion temperature should be attributed to the joint action of the electron–ion energy transf...
Article
Full-text available
We study numerically the nonequilibrium dynamics of the Ising Spin Glass, for a time that spans eleven orders of magnitude, thus approaching the experimentally relevant scale (i.e. {\em seconds}). We introduce novel analysis techniques that allow to compute the coherence length in a model-independent way. Besides, we present strong evidence for a r...
Article
We explore the possibility of having a flux-expansion divertor in TJ-II. As a first step, the three-dimensional map of the particle flux has been obtained for two different plasma regimes using the code ISDEP, which computes the ion guiding-centre trajectories. We consider the particle trajectories rather than the field lines due to the fact that,...
Article
Full-text available
In order to calculate the heating effects in a realistic way we have to leave the linear approximation, implicit in the Langevin approach, and modify the background temperature profile. Otherwise we would never find a real increase in the temperature because collisions would tend to thermalize every test particle. This obstacle is avoided by perfor...
Article
Within the framework of the int.eu.grid project, IVISDEP, which stands for Interactive Visualizer of an Integrator of Stochastic Differential Equations for Plasmas, has been successfully ported and executed. It is an example of integration of many of the innovative features that this grid infrastructure offers. The application itself simulates and...
Article
Full-text available
We investigate the critical properties of the four-state commutative random permutation glassy Potts model in three and four dimensions by means of Monte Carlo simulation and of a finite size scaling analysis. Thanks to the use of a field programmable gate array we have been able to thermalize a large number of samples of systems with large volume....
Article
This paper describes JANUS, a modular massively parallel and reconfigurable FPGA-based computing system. Each JANUS module has a computational core and a host. The computational core is a 4x4 array of FPGA-based processing elements with nearest-neighbor data links. Processors are also directly connected to an I/O node attached to the JANUS host, a...
Article
Full-text available
We study magnetic-monopole excitations in the framework of the lattice Georgi-Glashow model. With the appropriate definition of the lattice e.m. stress tensor we find that the vacuum in the confining phase is characterized by a large density of magnetic monopoles, and the Higgs phase by the presence of monopoles as particles in the spectrum. An ord...
Article
A Reply to the Comment by I. A. Campbell and H. Kawamura.
Article
We perform intensive numerical simulations of the three-dimensional site-diluted Ising antiferromagnet in a magnetic field at high values of the external applied field. Even if data for small lattice sizes are compatible with second-order criticality, the critical behavior of the system shows a crossover from second-order to first-order behavior fo...
Article
Full-text available
The ion collisional transport is estimated for the TJ-II device without the assumption of radially narrow particle trajectories, which is usually made in neoclassical transport studies. One million particles are followed in a realistic TJ-II magnetic configuration, taking into account collisions and the electric field. Global features of transport,...
Article
We describe the hardwired implementation of algorithms for Monte Carlo simulations of a large class of spin models. We have implemented these algorithms as VHDL codes and we have mapped them onto a dedicated processor based on a large FPGA device. The measured performance on one such processor is comparable to O(100) carefully programmed high-end P...
Conference Paper
Full-text available
This paper describes the architecture and FPGA-based implementation of a massively parallel processing system (IANUS), carefully tailored to the computing requirements of a class of simulation problems relevant in statistical physics. We first discuss the system architecture in general and then focus on the configuration of the system for Monte Car...
Article
Full-text available
Volunteer computing has come up as a new form of distributed computing. Unlike other computing paradigms like Grids, which use to be based on complex architectures, volunteer computing has demonstrated a great ability to integrate dispersed, heterogeneous computing resources with ease. This article presents ZIVIS, a project which aims to deploy a c...
Article
ETH Lab- Eurotech Group, I-33020 Amaro (Italy) This paper describes IANUS, a modular massively parallel and reconfigurable FPGA-based computing system. Each IANUS module has a computational core and a host. The computational core is a 4x4 array of FPGA-based processing elements with nearest-neighbor data links. Processors are also directly connecte...
Article
Full-text available
It is shown, by means of Monte Carlo simulation and finite size scaling analysis, that the Heisenberg spin glass undergoes a finite-temperature phase transition in three dimensions. There is a single critical temperature, at which both a spin glass and a chiral glass ordering develop. The Monte Carlo algorithm, adapted from lattice gauge theory sim...
Article
It was observed previously that the ion temperature profile of low-detisity electron cyclotron resonance-heated TJ-II plasmas is almost flat and that energetic ions are present well outside the last closed magnetic surface. The heat diffusivity obtained for such ion temperature profiles is very high, and therefore, transport cannot be described by...
Article
Using the decay of the out equilibrium spin-spin correlation function we compute the equilibrium Edward-Anderson order parameter in the three dimensional binary Ising spin glass in the spin glass phase. We have checked that the Edward-Anderson order parameter computed from out of equilibrium numerical simulations follows with good precision the cri...
Article
Full-text available
With Ianus, a next-generation field-programmable gate array (FPGA)-based machine, the authors hope to build a system that can fully exploit the performance potential of FPGA devices. A software platform that simplifies Ianus programming will extend its intended application range to a wide class of interesting and computationally demanding problems.