292 research outputs found
Geometric approach to chaos in the classical dynamics of abelian lattice gauge theory
A Riemannian geometrization of dynamics is used to study chaoticity in the
classical Hamiltonian dynamics of a U(1) lattice gauge theory. This approach
allows one to obtain analytical estimates of the largest Lyapunov exponent in
terms of time averages of geometric quantities. These estimates are compared
with the results of numerical simulations, and turn out to be very close to the
values extrapolated for very large lattice sizes even when the geometric
quantities are computed using small lattices. The scaling of the Lyapunov
exponent with the energy density is found to be well described by a quadratic
power law.Comment: REVTeX, 9 pages, 4 PostScript figures include
Geometric Approach to Lyapunov Analysis in Hamiltonian Dynamics
As is widely recognized in Lyapunov analysis, linearized Hamilton's equations
of motion have two marginal directions for which the Lyapunov exponents vanish.
Those directions are the tangent one to a Hamiltonian flow and the gradient one
of the Hamiltonian function. To separate out these two directions and to apply
Lyapunov analysis effectively in directions for which Lyapunov exponents are
not trivial, a geometric method is proposed for natural Hamiltonian systems, in
particular. In this geometric method, Hamiltonian flows of a natural
Hamiltonian system are regarded as geodesic flows on the cotangent bundle of a
Riemannian manifold with a suitable metric. Stability/instability of the
geodesic flows is then analyzed by linearized equations of motion which are
related to the Jacobi equations on the Riemannian manifold. On some geometric
setting on the cotangent bundle, it is shown that along a geodesic flow in
question, there exist Lyapunov vectors such that two of them are in the two
marginal directions and the others orthogonal to the marginal directions. It is
also pointed out that Lyapunov vectors with such properties can not be obtained
in general by the usual method which uses linearized Hamilton's equations of
motion. Furthermore, it is observed from numerical calculation for a model
system that Lyapunov exponents calculated in both methods, geometric and usual,
coincide with each other, independently of the choice of the methods.Comment: 22 pages, 14 figures, REVTeX
Relaxation of classical many-body hamiltonians in one dimension
The relaxation of Fourier modes of hamiltonian chains close to equilibrium is
studied in the framework of a simple mode-coupling theory. Explicit estimates
of the dependence of relevant time scales on the energy density (or
temperature) and on the wavenumber of the initial excitation are given. They
are in agreement with previous numerical findings on the approach to
equilibrium and turn out to be also useful in the qualitative interpretation of
them. The theory is compared with molecular dynamics results in the case of the
quartic Fermi-Pasta-Ulam potential.Comment: 9 pag. 6 figs. To appear in Phys.Rev.
On the mean-field spherical model
Exact solutions are obtained for the mean-field spherical model, with or
without an external magnetic field, for any finite or infinite number N of
degrees of freedom, both in the microcanonical and in the canonical ensemble.
The canonical result allows for an exact discussion of the loci of the Fisher
zeros of the canonical partition function. The microcanonical entropy is found
to be nonanalytic for arbitrary finite N. The mean-field spherical model of
finite size N is shown to be equivalent to a mixed isovector/isotensor
sigma-model on a lattice of two sites. Partial equivalence of statistical
ensembles is observed for the mean-field spherical model in the thermodynamic
limit. A discussion of the topology of certain state space submanifolds yields
insights into the relation of these topological quantities to the thermodynamic
behavior of the system in the presence of ensemble nonequivalence.Comment: 21 pages, 5 figure
Young stars in the periphery of the Large Magellanic Cloud
Despite their close proximity, the complex interplay between the two
Magellanic Clouds, the Milky Way, and the resulting tidal features, is still
poorly understood. Recent studies have shown that the Large Magellanic Cloud
(LMC) has a very extended disk strikingly perturbed in its outskirts. We search
for recent star formation in the far outskirts of the LMC, out to ~30 degrees
from its center. We have collected intermediate-resolution spectra of
thirty-one young star candidates in the periphery of the LMC and measured their
radial velocity, stellar parameters, distance and age. Our measurements confirm
membership to the LMC of six targets, for which the radial velocity and
distance values match well those of the Cloud. These objects are all young
(10-50 Myr), main-sequence stars projected between 7 and 13 degrees from the
center of the parent galaxy. We compare the velocities of our stars with those
of a disk model, and find that our stars have low to moderate velocity
differences with the disk model predictions, indicating that they were formed
in situ. Our study demonstrates that recent star formation occurred in the far
periphery of the LMC, where thus far only old objects were known. The spatial
configuration of these newly-formed stars appears ring-like with a radius of 12
kpc, and a displacement of 2.6 kpc from the LMC's center. This structure, if
real, would be suggestive of a star-formation episode triggered by an
off-center collision between the Small Magellanic Cloud and the LMC's disk.Comment: Accepted for publication in MNRA
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