Publications (109)237.87 Total impact
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ABSTRACT: We present our analysis of a system of interacting islands of XY spins on a triangular lattice that has been introduced a few years ago by Eley et al. to account for the phenomenology in experiments on tunable arrays of proximity coupled long superconductornormal metalsuperconductor junctions. The main features of the model are the separation of a local and a global interaction energy scale and the mesoscopic character of the spin islands. Upon lowering the temperature the model undergoes two crossovers corresponding to an increasing phase coherence on a single island and to the onset of global coherence across the array; the latter is a thermodynamical phase transition in the Ising universality class. The dependence of the second transition on the island edgetoedge spacing is related to the proximityeffect of the coupling constant.  [Show abstract] [Hide abstract]
ABSTRACT: We revisited, by means of numerical simulations, the one dimensional bond diluted Levy Ising spin glasses outside the limit of validity of mean field theories. In these models the probability that two spins at distance $r$ interact (via a disordered interactions, $J_{ij}=\pm 1$) decays as $r^{\rho}$. We have estimated, using finite size scaling techniques, the infinite volume correlation length and spin glass susceptibility for $\rho=5/3$ and $\rho=9/5$. We have obtained strong evidence for divergences of the previous observables at a non zero critical temperature. We discuss the behavior of the critical exponents, especially when approaching the value $\rho=2$, corresponding to a critical threshold beyond which the model has no phase transition. Finally, we numerically study the model right at the threshold value $\rho=2$.Physical Review B 12/2014; 91(6). DOI:10.1103/PhysRevB.91.064202 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We perform equilibrium paralleltempering simulations of the 3D Ising EdwardsAnderson 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 approach to study the equilibrium behaviour of the system, by classifying the measurements as a function of a conditioning variate. We propose a finitesize scaling analysis based on the probability distribution function of the conditioning variate, which may accelerate the convergence to the thermodynamic limit. In this way, we find a nontrivial spectrum of behaviours, where a part of the measurements behaves as the average, while the majority of them shows signs of scale invariance. As a result, we can estimate the temperature interval where the phase transition in a field ought to lie, if it exists. Although this wouldbe critical regime is unreachable with present resources, the numerical challenge is finally well posed.Journal of Statistical Mechanics Theory and Experiment 03/2014; 2014(5). DOI:10.1088/17425468/2014/05/P05014 · 2.40 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study the offequilibrium dynamics of the threedimensional 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 specialpurpose computer, based on fieldprogrammable gate array (FPGAs), we have been able to reach times equivalent to 0.01 s in experiments. We have studied the system relaxation both for high and for low temperatures, clearly identifying a dynamical transition point. This dynamical temperature is strictly positive and depends on the external applied magnetic field. We discuss different possibilities for the underlying physics, which include a thermodynamical spinglass transition, a modecoupling crossover, or an interpretation reminiscent of the random firstorder picture of structural glasses.Physical Review E 03/2014; 89(31):032140. DOI:10.1103/PhysRevE.89.032140 · 2.29 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report on extensive numerical simulations of the threedimensional Heisenberg model and its analysis through finitesize scaling of LeeYang zeros. Besides the critical regime, we also investigate scaling in the ferromagnetic phase. We show that, in this case of broken symmetry, the corrections to scaling contain information on the Goldstone modes. We present a comprehensive LeeYang analysis, including the density of zeros, and confirm recent numerical estimates for critical exponents.Physical Review E 12/2013; 88(61):062117. DOI:10.1103/PhysRevE.88.062117 · 2.29 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report a highprecision finitesize scaling study of the critical behavior of the threedimensional Ising EdwardsAnderson model (the Ising spin glass). We have thermalized lattices up to L=40 using the Janus dedicated computer. Our analysis takes into account leadingorder corrections to scaling. We obtain Tc = 1.1019(29) for the critical temperature, \nu = 2.562(42) for the thermal exponent, \eta = 0.3900(36) for the anomalous dimension and \omega = 1.12(10) for the exponent of the leading corrections to scaling. Standard (hyper)scaling relations yield \alpha = 5.69(13), \beta = 0.782(10) and \gamma = 6.13(11). We also compute several universal quantities at Tc.Physical Review B 10/2013; 88(22). DOI:10.1103/PhysRevB.88.224416 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: This paper describes the architecture, the development and the implementation of Janus II, a new generation applicationdriven number cruncher optimized for Monte Carlo simulations of spin systems (mainly spin glasses). This domain of computational physics is a recognized grand challenge of highperformance computing: the resources necessary to study in detail theoretical models that can make contact with experimental data are by far beyond those available using commodity computer systems. On the other hand, several specific features of the associated algorithms suggest that unconventional computer architectures, which can be implemented with available electronics technologies, may lead to order of magnitude increases in performance, reducing to acceptable values on human scales the time needed to carry out simulation campaigns that would take centuries on commercially available machines. Janus II is one such machine, recently developed and commissioned, that builds upon and improves on the successful JANUS machine, which has been used for physics since 2008 and is still in operation today. This paper describes in detail the motivations behind the project, the computational requirements, the architecture and the implementation of this new machine and compares its expected performances with those of currently available commercial systems.Computer Physics Communications 10/2013; 185(2). DOI:10.1016/j.cpc.2013.10.019 · 3.11 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A Comment on the Letter by B. Yucesoy, H. G. Katzgraber, and J. Machta, Phys. Rev. Lett. 109, 177204 (2012). The authors of the Letter offer a Reply.Physical Review Letters 05/2013; 110(21):219701. DOI:10.1103/PhysRevLett.110.219701 · 7.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In this chapter we describe the Janus supercomputer, a massively parallel FPGAbased system optimized for the simulation of spinglasses, 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. Spinglass simulations are performed using Monte Carlo methods that lead to algorithms characterized by (1) intrinsic parallelism allowing us to implement many Monte Carlo update engines within a single FPGA; (2) rather small data base (2 MByte) that can be stored onchip, significantly boosting bandwidth and reducing latency. (3) need to generate a large number of goodquality long (≥ 32 bit) random numbers; (4) mostly integer arithmetic and bitwise logic operations. Careful tailoring of the architecture to the specific features of these algorithms has allowed us to embed up to 1024 special purpose cores within just one FPGA, so that simulations of systems that would take centuries on conventional architectures can be performed in just a few months.HighPerformance Computing Using FPGAs, Edited by Wim Vanderbauwhede, Khaled Benkrid, 02/2013: chapter 16: pages 481506; Springer New York., ISBN: 9781461417903  [Show abstract] [Hide abstract]
ABSTRACT: We have characterized numerically, using the Janus computer, the LeeYang complex singularities related to the overlap in the 3D Ising spin glass with binary couplings in a wide range of temperatures (both in the critical and in the spinglass phase). Studying the behavior of the zeros at the critical point, we have obtained an accurate measurement of the anomalous dimension in very good agreement with the values quoted in the literature. In addition, by studying the density of the zeros we have been able to characterize the phase transition and to investigate the EdwardsAnderson order parameter in the spinglass phase, finding agreement with the values obtained using more conventional techniques.Journal of Statistical Mechanics Theory and Experiment 12/2012; 2013(02). DOI:10.1088/17425468/2013/02/P02031 · 2.40 Impact Factor 
Conference Paper: Spin glass simulations on the janus architecture: a desperate quest for strong scaling
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ABSTRACT: We describe Janus, an applicationdriven 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 early 2008; we are currently developing a new generation, that will be on line in early 2013. In this paper we present the Janus architecture, describe both implementations and compare their performances with those of commodity systems.Proceedings of the 18th international conference on Parallel processing workshops; 08/2012 
Conference Paper: Janus2: an FPGAbased supercomputer for spin glass simulations
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ABSTRACT: 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 speedup 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 on stateoftheart FPGA technology, and provided almost two order of magnitude improvement in terms of cost/performance and power/performance ratios. More than four years later, commercial facilities are closingup in terms of performance, but FPGA technology has largely improved. A new generation supercomputer, Janus2, will be able to improve by more than one orders of magnitude with respect to the previous one, and will accordingly be again the best choice in Monte Carlo simulations of Spin Glasses for several years to come with respect to commercial solutions.Future HPC Systems: the Challenges of PowerConstrained Performance, 2012, Newport Beach, CA, USA; 06/2012 
Article: Numerical test of the CardyJacobsen conjecture in the sitediluted Potts model in three dimensions
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ABSTRACT: We present a microcanonical Monte Carlo simulation of the sitediluted Potts model in three dimensions with eight internal states, partly carried out in the citizen supercomputer Ibercivis. Upon dilution, the pure model's firstorder transition becomes of the secondorder at a tricritical point. We compute accurately the critical exponents at the tricritical point. As expected from the CardyJacobsen conjecture, they are compatible with their Random Field Ising Model counterpart. The conclusion is further reinforced by comparison with older data for the Potts model with four states.Physical review. B, Condensed matter 05/2012; 86(18). DOI:10.1103/PhysRevB.86.184428 · 3.66 Impact Factor 
Article: Reconfigurable computing for Monte Carlo simulations: Results and prospects of the Janus project
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ABSTRACT: We describe Janus, a massively parallel FPGAbased computer optimized for the simulation of spin glasses, theoretical models for the behavior of glassy materials. FPGAs (as compared to GPUs or manycore processors) provide a complementary approach to massively parallel computing. In particular, our model problem is formulated in terms of binary variables, and floatingpoint operations can be (almost) completely avoided. The FPGA architecture allows us to run many independent threads with almost no latencies in memory access, thus updating up to 1024 spins per cycle. We describe Janus in detail and we summarize the physics results obtained in four years of operation of this machine; we discuss two types of physics applications: long simulations on very large systems (which try to mimic and provide understanding about the experimental nonequilibrium dynamics), and lowtemperature equilibrium simulations using an artificial parallel tempering dynamics. The time scale of our nonequilibrium simulations spans eleven orders of magnitude (from picoseconds to a tenth of a second). On the other hand, our equilibrium simulations are unprecedented both because of the low temperatures reached and for the large systems that we have brought to equilibrium. A finitetime scaling ansatz emerges from the detailed comparison of the two sets of simulations. Janus has made it possible to perform spinglass simulations that would take several decades on more conventional architectures. The paper ends with an assessment of the potential of possible future versions of the Janus architecture, based on stateoftheart technology.The European Physical Journal Special Topics 04/2012; 210(1). DOI:10.1140/epjst/e2012016369 · 1.40 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: 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 a spin glass in a field. In this context, the space dimension is crucial. Simulations are easier to interpret in a large number of dimensions, but one must work below the upper critical dimension (i.e., in d<6) in order for results to have relevance for experiments. Here we show conclusive evidence for the presence of a phase transition in a fourdimensional spin glass in a field. Two ingredients were crucial for this achievement: massive numerical simulations were carried out on the Janus specialpurpose computer, and a new and powerful finitesize scaling method.Proceedings of the National Academy of Sciences 02/2012; 109(17). DOI:10.1073/pnas.1203295109 · 9.67 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study the sampletosample fluctuations of the overlap probability densities from largescale equilibrium simulations of the threedimensional EdwardsAnderson 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 tested against numerical data. We found small deviations from the GhirlandaGuerra predictions, which get smaller as system size increases. We also focus on the shape of the overlap distribution, comparing the numerical data to a meanfieldlike prediction in which finitesize effects are taken into account by substituting delta functions with broad peaksPhysical review. B, Condensed matter 07/2011; 84(17). DOI:10.1103/PhysRevB.84.174209 · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We use a highprecision Monte Carlo simulation to determine the universal specificheat amplitude ratio A+/A in the threedimensional Ising model via the impact angle \phi of complex temperature zeros. We also measure the correlationlength critical exponent \nu from finitesize scaling, and the specificheat exponent \alpha through hyperscaling. Extrapolations to the thermodynamic limit yield \phi = 59.2(1.0) degrees, A+/A = 0.56(3), \nu = 0.63048(32) and \alpha = 0.1086(10). These results are compatible with some previous estimates from a variety of sources and rule out recently conjectured exact values.Journal of Statistical Mechanics Theory and Experiment 07/2011; 30(14). DOI:10.1088/17425468/2011/09/P09019 · 2.40 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: 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. Staticsdynamics correspondence implies that systems of finite size in equilibrium have static heterogeneities that obey FiniteSize Scaling, thus signaling an analogous phase transition in the thermodynamical limit. We compute the critical exponents and the transition point in the equilibrium setting, and use them to show that aging in dynamic heterogeneities can be described by a FiniteTime Scaling Ansatz, with potential implications for experimental work.Physical Review Letters 10/2010; 105(17):177202. DOI:10.1103/PhysRevLett.105.177202 · 7.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A spinglass transition occurs both in and out of the limit of validity of meanfield theory on a diluted one dimensional chain of Ising spins where exchange bonds occur with a probability decaying as the inverse power of the distance. Varying the power in this longrange model corresponds, in a onetoone relationship, to change the dimension in spinglass shortrange models. Using different finite size scaling methods evidence for a spinglass transition is found also for systems whose equivalent dimension is below the upper critical dimension at zero magnetic field. The application of a new method is discussed, that can be exported to systems in a magnetic field.Philosophical Magazine A 06/2010; 91(13). DOI:10.1080/14786435.2010.534741  [Show abstract] [Hide abstract]
ABSTRACT: We present a massive equilibrium simulation of the threedimensional Ising spin glass at low temperatures. The Janus specialpurpose computer has allowed us to equilibrate, using parallel tempering, L=32 lattices down to T=0.64 Tc. We demonstrate the relevance of equilibrium finitesize simulations to understand experimental nonequilibrium spin glasses in the thermodynamical limit by establishing a timelength dictionary. We conclude that nonequilibrium experiments performed on a time scale of one hour can be matched with equilibrium results on L=110 lattices. A detailed investigation of the probability distribution functions of the spin and link overlap, as well as of their correlation functions, shows that Replica Symmetry Breaking is the appropriate theoretical framework for the physically relevant length scales. Besides, we improve over existing methodologies to ensure equilibration in parallel tempering simulations. Comment: 48 pages, 19 postscript figures, 9 tables. Version accepted for publication in the Journal of Statistical MechanicsJournal of Statistical Mechanics Theory and Experiment 03/2010; 2010(6). DOI:10.1088/17425468/2010/06/P06026 · 2.40 Impact Factor
Publication Stats
2k  Citations  
237.87  Total Impact Points  
Top Journals
Institutions

20022014

Universidad de Extremadura
 • Department of Physics
 • Faculty of Science
Ara Pacis Augustalis, Extremadura, Spain


19922014

Complutense University of Madrid
 • Department of Theoretical physics I
 • Facultad de Ciencias Físicas
Madrid, Madrid, Spain


2011

Coventry University
 Applied Mathematics Research Centre
Coventry, England, United Kingdom


20062008

Institute for Biocomputation and Physics of Complex Systems
Caesaraugusta, Aragon, Spain


2000

Abdus Salam International Centre for Theoretical Physics
Trst, Friuli Venezia Giulia, Italy


19942000

Sapienza University of Rome
 Department of Physics
Roma, Latium, Italy


1998

Università degli studi di Cagliari
 Department of Physics
Cagliari, Sardinia, Italy


19961998

University of Rome Tor Vergata
 Dipartimento di Fisica
Roma, Latium, Italy
