Publications (128)307.31 Total impact
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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.03/2014;  [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.10/2013;  [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). · 3.08 Impact Factor  [show abstract] [hide abstract]
ABSTRACT: We characterize the phase diagram of anisotropic Heisenberg spin glasses, finding both the spin and the chiral glass transition. We remark the presence of strong finitesize effects on the chiral sector. We find a unique phase transition for the chiral and spin glass sector, in the Universality class of Ising spin glasses. We focus on keeping finitesize effects under control, and we stress that they are important to understand experiments. Thanks to large GPU clusters we have been able to thermalize cubic lattices with up to 64x64x64 spins, over a vast range of temperatures.09/2013;  [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 FPGAs, we have been able to reach times equivalent to 0.01 seconds 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 or a modecoupling crossover.07/2013;  [show abstract] [hide abstract]
ABSTRACT: Temperature chaos has often been reported in literature as a rareevent driven phenomenon. However, this fact has always been ignored in the data analysis, thus erasing the signal of the chaotic behavior (still rare in the sizes achieved) and leading to an overall picture of a weak and gradual phenomenon. On the contrary, our analysis relies on a largedeviations functional that allows to discuss the size dependencies. In addition, we had at our disposal unprecedentedly large configurations equilibrated at low temperatures, thanks to the Janus computer. According to our results, when temperature chaos occurs its effects are strong and can be felt even at short distances.EPL (Europhysics Letters) 07/2013; 103(6). · 2.26 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. · 7.94 Impact Factor  [show abstract] [hide abstract]
ABSTRACT: 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 equations of motion taking into account the complex 3D structure of fusion devices, the confining electromagnetic field and collisions with other plasma species. The Monte Carlo method used is based on the equivalence between the Fokker–Planck and Langevin equations. This allows ISDEP to run in distributed computing platforms without communication between nodes with almost linear scaling. This paper intends to be a general description and a reference paper in ISDEP.Computer Physics Communications 09/2012; 183(9):1877–1883. · 3.08 Impact Factor  [show abstract] [hide abstract]
ABSTRACT: We compare the critical behavior of the shortrange Ising spin glass with a spin glass with longrange interactions which fall off as a power sigma of the distance. We show that there is a value of sigma of the longrange model for which the critical behavior is very similar to that of the shortrange model in four dimensions. We also study a value of sigma for which we find the critical behavior to be compatible with that of the three dimensional model, though we have much less precision than in the fourdimensional case.Physical review. B, Condensed matter 07/2012; 86(13).  [show abstract] [hide abstract]
ABSTRACT: The low temperature excitations in the anisotropic antiferromagnetic Fe_{1x} Zn_x F_2 for x=0.25 and 0.31, at and just above the magnetic percolation threshold concentration x_p=0.25, were measured using inelastic neutron scattering. The excitations were simulated for x=0.31 using a localized, classical excitation model, which accounts well for the energies and relative intensities of the excitations observed in the scattering experiments.Physical review. B, Condensed matter 07/2012; 86(2). 
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).  [show abstract] [hide abstract]
ABSTRACT: We present a tethered Monte Carlo simulation of the crystallization of hard spheres. Our method boosts the traditional umbrella sampling to the point of making practical the study of constrained Gibbs' free energies depending on several crystalline order parameters. We obtain highaccuracy estimates of the fluidcrystal coexistence pressure for up to 2916 particles (enough to accommodate fluidsolid interfaces). We are able to extrapolate to infinite volume the coexistence pressure [p(co)=11.5727(10)k(B)T/σ(3)] and the interfacial free energy [γ({100})=0.636(11)k(B)T/σ(2)].Physical Review Letters 04/2012; 108(16):165701. · 7.94 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). · 1.80 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). · 9.74 Impact Factor 
Article: Finitesize scaling analysis of the distributions of pseudocritical temperatures in spin glasses
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ABSTRACT: Using the results of large scale numerical simulations we study the probability distribution of the pseudo critical temperature for the threedimensional EdwardsAnderson Ising spin glass and for the fully connected SherringtonKirkpatrick model. We find that the behavior of our data is nicely described by straightforward finitesize scaling relations.Journal of Statistical Mechanics Theory and Experiment 08/2011; 2011(10). · 1.87 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;  [show abstract] [hide abstract]
ABSTRACT: The steadystate distribution function of neutral beam injection (NBI) fast ions is calculated numerically for the LHD and TJII stellarators using the code ISDEP (Integrator of Stochastic Differential Equations for Plasmas). ISDEP is an orbit code that solves the guiding centre motion of fast ions using Cartesian coordinates in position space, allowing arbitrary magnetic configurations and the reentering of particles in the plasma. It takes into account collisions of fast ions with thermal ions and electrons using the Boozer and KuoPetravic collision operator. The steadystate distribution function is computed with a time integral following Green's function formalism for a timeindependent source. The rotation profiles of the fast ions are also estimated, thus computing their contribution to the total plasma current. In addition, energy slowing down time and escape distribution are studied in detail for both devices.Nuclear Fusion 07/2011; 51(8):083040. · 2.73 Impact Factor 
Article: Tethered Monte Carlo: Managing rugged freeenergy landscapes with a Helmholtzpotential formalism
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ABSTRACT: Tethering methods allow us to perform Monte Carlo simulations in ensembles with conserved quantities. Specifically, one couples a reservoir to the physical magnitude of interest, and studies the statistical ensemble where the total magnitude (system+reservoir) is conserved. The reservoir is actually integrated out, which leaves us with a fluctuationdissipation formalism that allows us to recover the appropriate Helmholtz effective potential with great accuracy. These methods are demonstrating a remarkable flexibility. In fact, we illustrate two very different applications: hard spheres crystallization and the phase transition of the diluted antiferromagnet in a field (the physical realization of the random field Ising model). The tethered approach holds the promise to transform cartoon drawings of corrugated freeenergy landscapes into real computations. Besides, it reduces the algorithmic dynamic slowingdown, probably because the conservation law holds nonlocally.Journal of Statistical Physics 07/2011; 144. · 1.40 Impact Factor  [show abstract] [hide abstract]
ABSTRACT: We study the critical behavior of the Diluted Antiferromagnet in a Field with the Tethered Monte Carlo formalism. We compute the critical exponents (including the elusive hyperscaling violations exponent $\theta$). Our results provide a comprehensive description of the phase transition and clarify the inconsistencies between previous experimental and theoretical work. To do so, our method addresses the usual problems of numerical work (large tunneling barriers and selfaveraging violations).06/2011;  [show abstract] [hide abstract]
ABSTRACT: We present a numerical study of the threedimensional Diluted Antiferromagnet in a Field (DAFF), one of the experimental realizations of the Random Field Ising Model. We work in a constrained ensemble (tethered ensemble) where the Helmholtz effective potential is featured, rather than the free energy. Our method cures the problem of a strong violation of selfaveraging, thus allowing us to compute the correlation length for systems sizes up to L=32. This quantity, when measured in units of the lattice size, is independent of the system size at the critical point, a strong indication of a secondorder phase transition. This scale invariance allows us to apply finitesize scaling in the form of Nightingale's phenomenological renormalization. We obtain accurate estimates of the critical exponents. Since our method reconstructs the effective potential, we can also compute accurately the hyperscaling violation exponent. We perform as well an investigation of the geometrical properties of the instantonlike configurations, namely, the minimal cost configurations joining the two ordered phases. This study sheds light on previous claims of a firstorder phase transition in this system.03/2011;
Publication Stats
795  Citations  
307.31  Total Impact Points  
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Institutions

1995–2013

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


2006–2012

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


2003–2012

University of Zaragoza
 • Department of Theoretical Physics
 • Faculty of Sciences (CIENCIAS)
Caesaraugusta, Aragon, Spain 
Scuola Normale Superiore di Pisa
Pisa, Tuscany, Italy


1999–2011

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


2008

Universita degli studi di Ferrara
 Department of Physics and Earth Sciences
Ferrara, EmiliaRomagna, Italy
