Finite-Size Corrections for Ground States of Edwards-Anderson Spin Glasses

Extensive computations of ground state energies of the Edwards-Anderson spin glass on bond-diluted, hypercubic lattices are conducted in dimensions d=3,..,7. Results are presented for bond-densities exactly at the percolation threshold, p=p_c, and deep within the glassy regime, p>p_c, where finding ground-states becomes a hard combinatorial problem. Finite-size corrections of the form 1/N^w are shown to be consistent throughout with the prediction w=1-y/d, where y refers to the "stiffness" exponent that controls the formation of domain wall excitations at low temperatures. At p=p_c, an extrapolation for $d\to\infty$ appears to match our mean-field results for these corrections. In the glassy phase, w does not approach the value of 2/3 for large d predicted from simulations of the Sherrington-Kirkpatrick spin glass. However, the value of w reached at the upper critical dimension does match certain mean-field spin glass models on sparse random networks of regular degree called Bethe lattices.Comment: 6 pages, RevTex4, all ps figures included, corrected and final version with extended analysis and more data, such as for case d=3. Find additional information at http://www.physics.emory.edu/faculty/boettcher

Violation of the Fluctuation Dissipation Theorem in Finite Dimensional Spin Glasses

We study the violation of the fluctuation-dissipation theorem in the three and four dimensional Gaussian Ising spin glasses using on and off equilibrium simulations. We have characterized numerically the function X(C) that determine the violation and we have studied its scaling properties. Moreover we have computed the function x(C) which characterize the breaking of the replica symmetry directly from equilibrium simulations. The two functions are numerically equal and in this way we have established that the conjectured connection between the violation of fluctuation dissipation theorem in the off-equilibrium dynamics and the replica symmetry breaking at equilibrium holds for finite dimensional spin glasses. These results point to a spin glass phase with spontaneously broken replica symmetry in finite dimensional spin glasses.Comment: 13 pages, 4 figures, also available at http://chimera.roma1.infn.it/index_papers_complex.htm

QCD simulations with staggered fermions on GPUs

We report on our implementation of the RHMC algorithm for the simulation of lattice QCD with two staggered flavors on Graphics Processing Units, using the NVIDIA CUDA programming language. The main feature of our code is that the GPU is not used just as an accelerator, but instead the whole Molecular Dynamics trajectory is performed on it. After pointing out the main bottlenecks and how to circumvent them, we discuss the obtained performances. We present some preliminary results regarding OpenCL and multiGPU extensions of our code and discuss future perspectives.Comment: 22 pages, 14 eps figures, final version to be published in Computer Physics Communication

Performance predictability of divide and conquer skeletons

Parallel divide and conquer computations, encompassing a wide variety of applications, can be modeled and encapsulated as a high level primitive called skeleton. The paper deals with a skeleton designed for parallel divide and conquer algorithms that provide hypercubical communications among processes The paper also introduces an accurate timing model designed for prediction of proposed primitive. The timing analysis model presented here still characterizing the communication time through architecture parameters but introduces a few novelties. The proposal is to introduce different kinds of components to the analytical model by associating a performance constant for each specific conceptual block of the skeleton. The trace files obtained from the execution of the resulting code using the skeleton are used by lineal regression techniques giving us, among other information, the values of the parameters of those blocks. An extended example showing the relative accuracy of the proposed approach concludes the paper.Workshop de Procesamiento Distribuido y Paralelo (WPDP)Red de Universidades con Carreras en Informática (RedUNCI

Four-dimensional pure compact U(1) gauge theory on a spherical lattice

We investigate the confinement-Coulomb phase transition in the four-dimensional (4D) pure compact U(1) gauge theory on spherical lattices. The action contains the Wilson coupling beta and the double charge coupling gamma. The lattice is obtained from the 4D surface of the 5D cubic lattice by its radial projection onto a 4D sphere, and made homogeneous by means of appropriate weight factors for individual plaquette contributions to the action. On such lattices the two-state signal, impeding the studies of this theory on toroidal lattices, is absent for gamma le 0. Furthermore, here a consistent finite-size scaling behavior of several bulk observables is found, with the correlation length exponent nu in the range nu = 0.35 - 40. These observables include Fisher zeros, specific-heat and cumulant extrema as well as pseudocritical values of beta at fixed gamma. The most reliable determination of nu by means of the Fisher zeros gives nu = 0.365(8). The phase transition at gamma le 0 is thus very probably of 2nd order and belongs to the universality class of a non-Gaussian fixed point.Comment: 40 pages, LaTeX, 12 figure