5,309 research outputs found
Actions for axisymmetric potentials
We give an algorithm for the economical calculation of angles and actions for
stars in axisymmetric potentials. We test the algorithm by integrating orbits
in a realistic model of the Galactic potential, and find that, even for orbits
characteristic of thick-disc stars, the errors in the actions are typically
smaller than 2 percent. We describe a scheme for obtaining actions by
interpolation on tabulated values that significantly accelerates the process of
calculating observables quantities, such as density and velocity moments, from
a distribution function.Comment: 5 pages accepted for publication in MNRA
Numerical Study of Competing Spin-Glass and Ferromagnetic Order
Two and three dimensional random Ising models with a Gaussian distribution of
couplings with variance and non-vanishing mean value are studied
using the zero-temperature domain-wall renormalization group (DWRG). The DWRG
trajectories in the () plane after rescaling can be collapsed on two
curves: one for and other for . In the first case
the DWRG flows are toward the ferromagnetic fixed point both in two and three
dimensions while in the second case flows are towards a paramagnetic fixed
point and spin-glass fixed point in two and three dimensions respectively. No
evidence for an extra phase is found.Comment: a bit more data is taken, 5 pages, 4 eps figures included, to appear
in PR
The Astrophysical Multipurpose Software Environment
We present the open source Astrophysical Multi-purpose Software Environment
(AMUSE, www.amusecode.org), a component library for performing astrophysical
simulations involving different physical domains and scales. It couples
existing codes within a Python framework based on a communication layer using
MPI. The interfaces are standardized for each domain and their implementation
based on MPI guarantees that the whole framework is well-suited for distributed
computation. It includes facilities for unit handling and data storage.
Currently it includes codes for gravitational dynamics, stellar evolution,
hydrodynamics and radiative transfer. Within each domain the interfaces to the
codes are as similar as possible. We describe the design and implementation of
AMUSE, as well as the main components and community codes currently supported
and we discuss the code interactions facilitated by the framework.
Additionally, we demonstrate how AMUSE can be used to resolve complex
astrophysical problems by presenting example applications.Comment: 23 pages, 25 figures, accepted for A&
Einselection and Decoherence from an Information Theory Perspective
We introduce and investigate a simple model of conditional quantum dynamics.
It allows for a discussion of the information-theoretic aspects of quantum
measurements, decoherence, and environment-induced superselection
(einselection).Comment: Proceedings of the Planck constant centenary meeting. Uses
annalen.cls and fleqn.st
A new approach to the study of the ground-state properties of 2D Ising spin glass
A new approach known as flat histogram method is used to study the +/-J Ising
spin glass in two dimensions. Temperature dependence of the energy, the
entropy, and other physical quantities can be easily calculated and we give the
results for the zero-temperature limit. For the ground-state energy and entropy
of an infinite system size, we estimate e0 = -1.4007 +/- 0.0085 and s0 = 0.0709
+/- 0.006, respectively. Both of them agree well with previous calculations.
The time to find the ground-states as well as the tunneling times of the
algorithm are also reported and compared with other methods.Comment: 11 pages, 4 figure
Simulating the universe on an intercontinental grid of supercomputers
Understanding the universe is hampered by the elusiveness of its most common
constituent, cold dark matter. Almost impossible to observe, dark matter can be
studied effectively by means of simulation and there is probably no other
research field where simulation has led to so much progress in the last decade.
Cosmological N-body simulations are an essential tool for evolving density
perturbations in the nonlinear regime. Simulating the formation of large-scale
structures in the universe, however, is still a challenge due to the enormous
dynamic range in spatial and temporal coordinates, and due to the enormous
computer resources required. The dynamic range is generally dealt with by the
hybridization of numerical techniques. We deal with the computational
requirements by connecting two supercomputers via an optical network and make
them operate as a single machine. This is challenging, if only for the fact
that the supercomputers of our choice are separated by half the planet, as one
is located in Amsterdam and the other is in Tokyo. The co-scheduling of the two
computers and the 'gridification' of the code enables us to achieve a 90%
efficiency for this distributed intercontinental supercomputer.Comment: Accepted for publication in IEEE Compute
Tempering simulations in the four dimensional +-J Ising spin glass in a magnetic field
We study the four dimensional (4D) Ising spin glass in a magnetic
field by using the simulated tempering method recently introduced by Marinari
and Parisi. We compute numerically the first four moments of the order
parameter probability distribution . We find a finite cusp in the
spin-glass susceptibility and strong tendency to paramagnetic ordering at low
temperatures. Assuming a well defined transition we are able to bound its
critical temperature.Comment: 6 Pages including 5 figures, Revte
Local excitations in mean field spin glasses
We address the question of geometrical as well as energetic properties of
local excitations in mean field Ising spin glasses. We study analytically the
Random Energy Model and numerically a dilute mean field model, first on
tree-like graphs, equivalent to a replica symmetric computation, and then
directly on finite connectivity random lattices. In the first model,
characterized by a discontinuous replica symmetry breaking, we found that the
energy of finite volume excitation is infinite whereas in the dilute mean field
model, described by a continuous replica symmetry breaking, it slowly decreases
with sizes and saturates at a finite value, in contrast with what would be
naively expected. The geometrical properties of these excitations are similar
to those of lattice animals or branched polymers. We discuss the meaning of
these results in terms of replica symmetry breaking and also possible relevance
in finite dimensional systems.Comment: 7 pages, 4 figures, accepted for publicatio
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