165 research outputs found
Effective Monte Carlo simulation on System-V massively parallel associative string processing architecture
We show that the latest version of massively parallel processing associative
string processing architecture (System-V) is applicable for fast Monte Carlo
simulation if an effective on-processor random number generator is implemented.
Our lagged Fibonacci generator can produce random numbers on a processor
string of 12K PE-s. The time dependent Monte Carlo algorithm of the
one-dimensional non-equilibrium kinetic Ising model performs 80 faster than the
corresponding serial algorithm on a 300 MHz UltraSparc.Comment: 8 pages, 9 color ps figures embedde
Cluster approximation solution of a two species annihilation model
A two species reaction-diffusion model, in which particles diffuse on a
one-dimensional lattice and annihilate when meeting each other, has been
investigated. Mean field equations for general choice of reaction rates have
been solved exactly. Cluster mean field approximation of the model is also
studied. It is shown that, the general form of large time behavior of one- and
two-point functions of the number operators, are determined by the diffusion
rates of the two type of species, and is independent of annihilation rates.Comment: 9 pages, 7 figure
Non-equilibrium phase transitions in one-dimensional kinetic Ising models
A family of nonequilibrium kinetic Ising models, introduced earlier, evolving
under the competing effect of spin flips at {\it zero temperature} and nearest
neighbour random spin exchanges is further investigated here. By increasing the
range of spin exchanges and/or their strength the nature of the phase
transition 'Ising-to-active' becomes of (dynamic) mean-field type and a first
order tricitical point is located at the Glauber () limit.
Corrections to mean-field theory are evaluated up to sixth order in a cluster
approximation and found to give good results concerning the phase boundary and
the critical exponent of the order parameter which is obtained as
.Comment: 15 pages, revtex file, figures available at request from
[email protected] in postscript format, submitted to J.Phys.
Slow Logarithmic Decay of Magnetization in the Zero Temperature Dynamics of an Ising Spin Chain: Analogy to Granular Compaction
We study the zero temperature coarsening dynamics in an Ising chain in
presence of a dynamically induced field that favors locally the `-' phase
compared to the `+' phase. At late times, while the `+' domains still coarsen
as , the `-' domains coarsen slightly faster as . As
a result, at late times, the magnetization decays slowly as, . We establish this behavior both analytically within an
independent interval approximation (IIA) and numerically. In the zero volume
fraction limit of the `+' phase, we argue that the IIA becomes asymptotically
exact. Our model can be alternately viewed as a simple Ising model for granular
compaction. At late times in our model, the system decays into a fully compact
state (where all spins are `-') in a slow logarithmic manner , a fact that has been observed in recent experiments on granular systems.Comment: 4 pages Revtex, 3 eps figures, supersedes cond-mat/000221
Urban segregation with cheap and expensive residences
In this paper we study urban segregation of two different communities A and
B, poor and rich, distributed randomly on finite samples, to check cheap and
expensive residences. For this purpose we avoid the complications of the
Schelling model which are not necessary and instead we use the Ising model on
500 x 500 square lattice, which give similar results, with random magnetic
field at lower and higher temperatures (kT/J = 2.0, 99.0) in finite times equal
to 40, 400, 4000 and 40,000. This random-field Ising magnet is a suitable
model, where each site of the square lattice carries a magnetic field h which
is randomly up (expensive) or down (cheap). The resulting addition to the
energy prefers up spins on the expensive and down spins on the cheap sites. Our
simulations were carried out using a 50-lines FORTRAN program. We present at a
lower temperature (2.0) a time series of pictures, separating growing from
non-growing domains. A small random field (h = +- 0.1) allows for large
domains, while a large random field (h = +- 0.9) allows only small clusters. At
higher temperature (99.0) we could not obtain growing domains.Comment: 11 pages, large figures, shortened version will be prepared for IJMP
The generalized contact process with n absorbing states
We investigate the critical properties of a one dimensional stochastic
lattice model with n (permutation symmetric) absorbing states. We analyze the
cases with by means of the non-hermitian density matrix
renormalization group. For n=1 and n=2 we find that the model is respectively
in the directed percolation and parity conserving universality class,
consistent with previous studies. For n=3 and n=4, the model is in the active
phase in the whole parameter space and the critical point is shifted to the
limit of one infinite reaction rate. We show that in this limit the dynamics of
the model can be mapped onto that of a zero temperature n-state Potts model. On
the basis of our numerical and analytical results we conjecture that the model
is in the same universality class for all with exponents , and . These exponents
coincide with those of the multispecies (bosonic) branching annihilating random
walks. For n=3 we also show that, upon breaking the symmetry to a lower one
(), one gets a transition either in the directed percolation, or in the
parity conserving class, depending on the choice of parameters.Comment: 10 pages, RevTeX, and 10 PostScript figures include
Environmental drivers of forest biodiversity in temperate mixed forests – A multi-taxon approach
Harmonization of timber production and forest conservation is a major challenge of modern silviculture. For the establishment of ecologically sustainable forest management, the management-related environmental drivers of multi-taxon biodiversity should be explored. Our study reveals those environmental variables related to tree species diversity and composition, stand structure, litter and soil conditions, microclimate, landscape, and land-use history that determine species richness and composition of 11 forest-dwelling organism groups. Herbs, woody regeneration, ground-floor and epiphytic bryophytes, epiphytic lichens, terricolous saprotrophic, ectomycorrhizal, and wood-inhabiting macrofungi, spiders, carabid beetles, and birds were sampled in West Hungarian mature mixed forests. The correlations among the diversities and compositions of different organism groups were also evaluated. Drivers of organism groups were principally related to stand structure, tree species diversity and composition, and microclimate, while litter, soil, landscape, and land-use historical variables were less influential. The complex roles of the shrub layer, deadwood, and the size of the trees in determining the diversity and composition of various taxa were revealed. Stands with more tree species sustained higher stand-level species richness of several taxa. Besides, stands with different dominant tree species harbored various species communities of organism groups. Therefore, landscape-scale diversity of dominant tree species may enhance the diversity of forest-dwelling communities at landscape level. The effects of the overstory layer on forest biodiversity manifested in many cases via microclimate conditions. Diversity of organism groups showed weaker relationship with the diversity of other taxa than with environmental variables. According to our results, the most influential drivers of forest biodiversity are under the direct control of the actual silvicultural management. Heterogeneous stand structure and tree species composition promote the different organism groups in various ways. Therefore, the long-term maintenance of the structural and compositional heterogeneity both at stand and landscape scale is an important aspect of ecologically sustainable forest management
On universality in aging ferromagnets
This work is a contribution to the study of universality in
out-of-equilibrium lattice models undergoing a second-order phase transition at
equilibrium. The experimental protocol that we have chosen is the following:
the system is prepared in its high-temperature phase and then quenched at the
critical temperature . We investigated by mean of Monte Carlo simulations
two quantities that are believed to take universal values: the exponent
obtained from the decay of autocorrelation functions and the
asymptotic value of the fluctuation-dissipation ratio . This
protocol was applied to the Ising model, the 3-state clock model and the
4-state Potts model on square, triangular and honeycomb lattices and to the
Ashkin-Teller model at the point belonging at equilibrium to the 3-state Potts
model universality class and to a multispin Ising model and the Baxter-Wu model
both belonging to the 4-state Potts model universality class at equilibrium.Comment: 17 page
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