6,016 research outputs found
Study of a microcanonical algorithm on the spin glass model in d=3
We consider a microcanonical local algorithm to be applied on the
spin glass model. We have compared the results coming from a microcanonical
Monte Carlo simulation with those from a canonical one: Thermalization times,
spin glass susceptibilities and Binder parameters. For a fixed lattice size we
found different results between the two thermodynamic ensembles, which tend to
vanish at bigger volumes. Moreover, microcanonical thermalization times are
longer than the canonical ones. Finally we have checked that one of the Guerra
relations is satisfied with good precision for the two largest lattices.Comment: Revised version. Latex 14 pages, 6 figures. To be published in
Comput. Phys. Commu
Scaling above the upper critical dimension in Ising Models
We rederive the finite size scaling formula for the apparent critical
temperature by using Mean Field Theory for the Ising Model above the upper
critical dimension. We have also performed numerical simulations in five
dimensions and our numerical data are in a good agreement with the Mean Field
theoretical predictions, in particular, with the finite size exponent of the
connected susceptibility and with the value of the Binder cumulant.Comment: 9 pages and 3 figures, available at
http://chimera.roma1.infn.it/index_papers_complex.htm
Universality in the off-equilibrium critical dynamics of the diluted Ising model
We study the off-equilibrium critical dynamics of the three dimensional
diluted Ising model. We compute the dynamical critical exponent and we show
that it is independent of the dilution only when we take into account the
scaling-corrections to the dynamics. Finally we will compare our results with
the experimental data.Comment: Final Version, 5 Latex pages (RevTeX) plus 3 eps figure
A numerical study of planar arrays of correlated spin islands
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 superconductor-normal metal-superconductor 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 edge-to-edge spacing is related to the proximity-effect of the coupling
constant.Comment: 6 pages, 5 figures, accepted for publication in EPJ
Generalized off-equilibrium fluctuation-dissipation relations in random Ising systems
We show that the numerical method based on the off-equilibrium
fluctuation-dissipation relation does work and is very useful and powerful in
the study of disordered systems which show a very slow dynamics. We have
verified that it gives the right information in the known cases (diluted
ferromagnets and random field Ising model far from the critical point) and we
used it to obtain more convincing results on the frozen phase of
finite-dimensional spin glasses. Moreover we used it to study the Griffiths
phase of the diluted and the random field Ising models.Comment: 20 pages, 10 figures, uses epsfig.sty. Partially presented at
StatPhys XX in a talk by one of the authors (FRT). Added 1 reference in the
new versio
A Proposal of a Renormalization Group Transformation for Lattice Field Theories
We propose a new Real Space Renormalization Group transformation useful for
Monte Carlo calculations in theories with global or local symmetries. From
relaxation arguments we define the block-spin transformation with two tunable
free parameters, adapted to the system's action. Varying them it is possible to
place the fixed point very close to the simulation point. We show how the
method works in a simple model with global symmetry: the three dimensional XY
model.Comment: 26 pages, uuencoded compressed postscript single file, 8 figures
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