3,952 research outputs found
A Simplified Cellular Automaton Model for City Traffic
We systematically investigate the effect of blockage sites in a cellular
automaton model for traffic flow. Different scheduling schemes for the blockage
sites are considered. None of them returns a linear relationship between the
fraction of ``green'' time and the throughput. We use this information for a
fast implementation of traffic in Dallas.Comment: 12 pages, 18 figures. submitted to Phys Rev
A Monte Carlo study of the three-dimensional Coulomb frustrated Ising ferromagnet
We have investigated by Monte-Carlo simulation the phase diagram of a
three-dimensional Ising model with nearest-neighbor ferromagnetic interactions
and small, but long-range (Coulombic) antiferromagnetic interactions. We have
developed an efficient cluster algorithm and used different lattice sizes and
geometries, which allows us to obtain the main characteristics of the
temperature-frustration phase diagram. Our finite-size scaling analysis
confirms that the melting of the lamellar phases into the paramgnetic phase is
driven first-order by the fluctuations. Transitions between ordered phases with
different modulation patterns is observed in some regions of the diagram, in
agreement with a recent mean-field analysis.Comment: 14 pages, 10 figures, submitted to Phys. Rev.
Numerical Studies of the Compressible Ising Spin Glass
We study a two-dimensional compressible Ising spin glass at constant volume.
The spin interactions are coupled to the distance between neighboring particles
in the Edwards-Anderson model with +/- J interactions. We find that the energy
of a given spin configuration is shifted from its incompressible value, E_0, by
an amount quadratic in E_0 and proportional to the coupling strength. We then
construct a simple model expressed only in terms of spin variables that
predicts the existence of a critical value of the coupling above which the
spin-glass transition disappears.Comment: REVTeX, 4 pages, 4 figures. Submitted to Phys. Rev. Let
Self-guided wakefield experiments driven by petawatt class ultra-short laser pulses
We investigate the extension of self-injecting laser wakefield experiments to
the regime that will be accessible with the next generation of petawatt class
ultra-short pulse laser systems. Using linear scalings, current experimental
trends and numerical simulations we determine the optimal laser and target
parameters, i.e. focusing geometry, plasma density and target length, that are
required to increase the electron beam energy (to > 1 GeV) without the use of
external guiding structures.Comment: 15 pages, 8 figure
Quickest Paths in Simulations of Pedestrians
This contribution proposes a method to make agents in a microscopic
simulation of pedestrian traffic walk approximately along a path of estimated
minimal remaining travel time to their destination. Usually models of
pedestrian dynamics are (implicitly) built on the assumption that pedestrians
walk along the shortest path. Model elements formulated to make pedestrians
locally avoid collisions and intrusion into personal space do not produce
motion on quickest paths. Therefore a special model element is needed, if one
wants to model and simulate pedestrians for whom travel time matters most (e.g.
travelers in a station hall who are late for a train). Here such a model
element is proposed, discussed and used within the Social Force Model.Comment: revised version submitte
Exact Results for the One-Dimensional Self-Organized Critical Forest-Fire Model
We present the analytic solution of the self-organized critical (SOC)
forest-fire model in one dimension proving SOC in systems without conservation
laws by analytic means. Under the condition that the system is in the steady
state and very close to the critical point, we calculate the probability that a
string of neighboring sites is occupied by a given configuration of trees.
The critical exponent describing the size distribution of forest clusters is
exactly and does not change under certain changes of the model
rules. Computer simulations confirm the analytic results.Comment: 12 pages REVTEX, 2 figures upon request, dro/93/
Excitations of Ellipsoid Packings near Jamming
We study the vibrational modes of three-dimensional jammed packings of soft
ellipsoids of revolution as a function of particle aspect ratio and
packing fraction. At the jamming transition for ellipsoids, as distinct from
the idealized case using spheres where , there are many
unconstrained and non-trivial rotational degrees of freedom. These constitute a
set of zero-frequency modes that are gradually mobilized into a new rotational
band as increases. Quite surprisingly, as this new band is
separated from zero frequency by a gap, and lies below the onset frequency for
translational vibrations, , the presence of these new degrees of
freedom leaves unaltered the basic scenario that the translational spectrum is
determined only by the average contact number. Indeed, depends
solely on coordination as it does for compressed packings of spheres. We also
discuss the regime of large , where the two bands merge.Comment: 6 pages, 4 figure
Far infrared study of the two dimensional dimer spin system SrCu_2(BO_3)_2
Using far-infrared spectroscopy in magnetic fields up to 12T we have studied
a two-dimensional dimer spin gap system SrCu_2(BO_3)_2. We found several
infrared active modes in the dimerized state (below 10K) in the frequency range
from 3 to 100cm^-1. The measured splitting from the ground state to the excited
triplet M_S=0 sublevel is Delta_1=24.2cm^-1 and the other two triplet state
sublevels in zero magnetic field are 1.4cm^-1 below and above the M_S=0
sublevel. Another multiplet is at Delta_2=37.6cm^-1 from the ground state. A
strong electric dipole active transition polarized in the (ab)-plane is
activated in the dimer spin system below 15K at 52cm^-1.Comment: 4 pages including 5 figures, submitted to PRB, instrumental arte
facts remove
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