194,065 research outputs found
Phase ordering in disordered and inhomogeneous systems
We study numerically the coarsening dynamics of the Ising model on a regular
lattice with random bonds and on deterministic fractal substrates. We propose a
unifying interpretation of the phase-ordering processes based on two classes of
dynamical behaviors characterized by different growth-laws of the ordered
domains size - logarithmic or power-law respectively. It is conjectured that
the interplay between these dynamical classes is regulated by the same
topological feature which governs the presence or the absence of a
finite-temperature phase-transition.Comment: 15 pages, 7 figures. To appear on Physical Review E (2015
Mesoscopic real space structures in aging spin-glasses: the Edwards-Anderson model
Isothermal simulational data for the 3D Edwards-Anderson spin glass are
collected at several temperatures below and, in analogy with a
recent model of dense colloidal suspensions,interpreted in terms of clusters of
contiguous spins overturned by quakes, non-equilibrium events linked to record
sized energy fluctuations. We show numerically that, to a good approximation,
these quakes are statistically independent and constitute a Poisson process
whose average grows logarithmically in time. The overturned clusters are local
projections on one of the two ground states of the model, and grow likewise
logarithmically in time. Data collected at different temperatures can be
collapsed by scaling them with , a hitherto unnoticed feature of the
E-A model, which we relate on the one hand to the geometry of configuration
space and on the other to experimental memory and rejuvenation effects. The
rate at which a cluster flips is shown to decrease exponentially with the size
of the cluster, as recently assumed in a coarse grained model of dense
colloidal dynamics. The evolving structure of clusters in real space is finally
sssociated to the decay of the thermo-remanent magnetization.
Our analysis provides an unconventional coarse-grained description of spin
glass aging as statistically subordinated to a Poisson quaking process and
highlights record dynamics as a viable common theoretical framework for aging
in different systems.Comment: 13 pages, 6 figs. Revised text and notation, several typos correcte
Kolmogorov-Sinai entropy in field line diffusion by anisotropic magnetic turbulence
The Kolmogorov-Sinai (KS) entropy in turbulent diffusion of magnetic field
lines is analyzed on the basis of a numerical simulation model and theoretical
investigations. In the parameter range of strongly anisotropic magnetic
turbulence the KS entropy is shown to deviate considerably from the earlier
predicted scaling relations [Rev. Mod. Phys. {\bf 64}, 961 (1992)]. In
particular, a slowing down logarithmic behavior versus the so-called Kubo
number (, where is the ratio of the rms magnetic fluctuation field to the magnetic field
strength, and and are the correlation lengths in respective
dimensions) is found instead of a power-law dependence. These discrepancies are
explained from general principles of Hamiltonian dynamics. We discuss the
implication of Hamiltonian properties in governing the paradigmatic
"percolation" transport, characterized by , associating it with the
concept of pseudochaos (random non-chaotic dynamics with zero Lyapunov
exponents). Applications of this study pertain to both fusion and astrophysical
plasma and by mathematical analogy to problems outside the plasma physics.
This research article is dedicated to the memory of Professor George M.
ZaslavskyComment: 15 pages, 2 figures. Accepted for publication on Plasma Physics and
Controlled Fusio
Off equilibrium properties of vortex creep in superconductors
We study a model for the dynamics of vortices in type II superconductors. In
particular, we discuss glassy ``off equilibrium'' properties and ``aging'' in
magnetic creep. At low temperatures a crossover point is found, Tg, where
relaxation times seem to diverge a' la Vogel-Tamman-Fulcher. Magnetic creep
changes by crossing Tg: above Tg power law creep is found asymptotically
followed by stretched exponential saturation; below Tg the creep is logarithmic
and vortex motion strongly subdiffusive. In this region violation of time
translation invariance is found along with important dynamical scaling
properties. A thermodynamic glassy transition point can be found at a lower
temperature Tc.Comment: published versio
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