176 research outputs found
The role of the alloy structure in the magnetic behavior of granular systems
The effect of grain size, easy magnetization axis and anisotropy constant
distributions in the irreversible magnetic behavior of granular alloys is
considered. A simulated granular alloy is used to provide a realistic grain
structure for the Monte Carlo simulation of the ZFC-FC curves. The effect of
annealing and external field is also studied. The simulation curves are in good
agreement with the FC and ZFC magnetization curves measured on melt spun Cu-Co
ribbons.Comment: 13 pages, 10 figures, submitted to PR
On the signature of tensile blobs in the scattering function of a stretched polymer
We present Monte Carlo data for a linear chain with excluded volume subjected
to a uniform stretching. Simulation of long chains (up to 6000 beads) at high
stretching allows us to observe the signature of tensile blobs as a crossover
in the scaling behavior of the chain scattering function for wave vectors
perpendicular to stretching. These results and corresponding ones in the
stretching direction allow us to verify for the first time Pincus prediction on
scaling inside blobs. Outside blobs, the scattering function is well described
by the Debye function for a stretched ideal chain.Comment: 4 pages, 4 figures, to appear in Physical Review Letter
Coupled Maps on Trees
We study coupled maps on a Cayley tree, with local (nearest-neighbor)
interactions, and with a variety of boundary conditions. The homogeneous state
(where every lattice site has the same value) and the node-synchronized state
(where sites of a given generation have the same value) are both shown to occur
for particular values of the parameters and coupling constants. We study the
stability of these states and their domains of attraction. As the number of
sites that become synchronized is much higher compared to that on a regular
lattice, control is easier to effect. A general procedure is given to deduce
the eigenvalue spectrum for these states. Perturbations of the synchronized
state lead to different spatio-temporal structures. We find that a mean-field
like treatment is valid on this (effectively infinite dimensional) lattice.Comment: latex file (25 pages), 4 figures included. To be published in Phys.
Rev.
Exact formula for currents in strongly pumped diffusive systems
We analyze a generic model of mesoscopic machines driven by the nonadiabatic
variation of external parameters. We derive a formula for the probability
current; as a consequence we obtain a no-pumping theorem for cyclic processes
satisfying detailed balance and demonstrate that the rectification of current
requires broken spatial symmetry.Comment: 10 pages, accepted for publication in the Journal of Statistical
Physic
Dynamics of heteropolymers in dilute solution: effective equation of motion and relaxation spectrum
The dynamics of a heteropolymer chain in solution is studied in the limit of
long chain length. Using functional integral representation we derive an
effective equation of motion, in which the heterogeneity of the chain manifests
itself as a time-dependent excluded volume effect. At the mean field level, the
heteropolymer chain is therefore dynamically equivalent to a homopolymer chain
with both time-independent and time-dependent excluded volume effects. The
perturbed relaxation spectrum is also calculated. We find that heterogeneity
also renormalizes the relaxation spectrum. However, we find, to the lowest
order in heterogeneity, that the relaxation spectrum does not exhibit any
dynamic freezing, at the point when static (equilibrium) ``freezing''
transition occurs in heteropolymer. Namely, the breaking of
fluctuation-dissipation theorem (FDT) proposed for spin glass dynamics does not
have dynamic effect in heteropolymer, as far as relaxation spectrum is
concerned. The implication of this result is discussed
Numerical Confirmation of Late-time t^{1/2} Growth in Three-dimensional Phase Ordering
Results for the late-time regime of phase ordering in three dimensions are
reported, based on numerical integration of the time-dependent Ginzburg-Landau
equation with nonconserved order parameter at zero temperature. For very large
systems () at late times, the characteristic length grows
as a power law, , with the measured in agreement with the
theoretically expected result to within statistical errors. In this
time regime is found to be in excellent agreement with the analytical
result of Ohta, Jasnow, and Kawasaki [Phys. Rev. Lett. {\bf 49}, 1223 (1982)].
At early times, good agreement is found between the simulations and the
linearized theory with corrections due to the lattice anisotropy.Comment: Substantially revised and enlarged, submitted to PR
Chaotic Cascades with Kolmogorov 1941 Scaling
We define a (chaotic) deterministic variant of random multiplicative cascade
models of turbulence. It preserves the hierarchical tree structure, thanks to
the addition of infinitesimal noise. The zero-noise limit can be handled by
Perron-Frobenius theory, just as the zero-diffusivity limit for the fast dynamo
problem. Random multiplicative models do not possess Kolmogorov 1941 (K41)
scaling because of a large-deviations effect. Our numerical studies indicate
that deterministic multiplicative models can be chaotic and still have exact
K41 scaling. A mechanism is suggested for avoiding large deviations, which is
present in maps with a neutrally unstable fixed point.Comment: 14 pages, plain LaTex, 6 figures available upon request as hard copy
(no local report #
Random Walks with Long-Range Self-Repulsion on Proper Time
We introduce a model of self-repelling random walks where the short-range
interaction between two elements of the chain decreases as a power of the
difference in proper time. Analytic results on the exponent are obtained.
They are in good agreement with Monte Carlo simulations in two dimensions. A
numerical study of the scaling functions and of the efficiency of the algorithm
is also presented.Comment: 25 pages latex, 4 postscript figures, uses epsf.sty (all included)
IFUP-Th 13/92 and SNS 14/9
Chaotic Scattering Theory, Thermodynamic Formalism, and Transport Coefficients
The foundations of the chaotic scattering theory for transport and
reaction-rate coefficients for classical many-body systems are considered here
in some detail. The thermodynamic formalism of Sinai, Bowen, and Ruelle is
employed to obtain an expression for the escape-rate for a phase space
trajectory to leave a finite open region of phase space for the first time.
This expression relates the escape rate to the difference between the sum of
the positive Lyapunov exponents and the K-S entropy for the fractal set of
trajectories which are trapped forever in the open region. This result is well
known for systems of a few degrees of freedom and is here extended to systems
of many degrees of freedom. The formalism is applied to smooth hyperbolic
systems, to cellular-automata lattice gases, and to hard sphere sytems. In the
latter case, the goemetric constructions of Sinai {\it et al} for billiard
systems are used to describe the relevant chaotic scattering phenomena. Some
applications of this formalism to non-hyperbolic systems are also discussed.Comment: 35 pages, compressed file, follow directions in header for ps file.
Figures are available on request from [email protected]
Universality and Scaling for the Structure Factor in Dynamic Order-Disorder Transitions
The universal form for the average scattering intensity from systems
undergoing order-disorder transitions is found by numerical integration of the
Langevin dynamics. The result is nearly identical for simulations involving two
different forms of the local contribution to the free energy, supporting the
idea that the Model A dynamical universality class includes a wide range of
local free-energy forms. An absolute comparison with no adjustable parameters
is made to the forms predicted by the theories of Ohta-Jasnow-Kawasaki and
Mazenko. The numerical results are well described by the former theory, except
in the cross-over region between scattering dominated by domain geometry and
scattering determined by Porod's law.Comment: 10 pages incl. 3 figures, Revtex. Submitted to PR
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