740 research outputs found
Heat transfer and Fourier's law in off-equilibrium systems
We study the most suitable procedure to measure the effective temperature in
off-equilibrium systems. We analyze the stationary current established between
an off-equilibrium system and a thermometer and the necessary conditions for
that current to vanish. We find that the thermometer must have a short
characteristic time-scale compared to the typical decorrelation time of the
glassy system to correctly measure the effective temperature. This general
conclusion is confirmed analyzing an ensemble of harmonic oscillators with
Monte Carlo dynamics as an illustrative example of a solvable model of a glass.
We also find that the current defined allows to extend Fourier's law to the
off-equilibrium regime by consistently defining effective transport
coefficients. Our results for the oscillator model explain why thermal
conductivities between thermalized and frozen degrees of freedom in structural
glasses are extremely small.Comment: 7 pages, REVTeX, 4 eps figure
Why charges go to the surface: a generalized Thomson problem
We study a generalization of a Thomson problem of n particles confined to a
sphere and interacting by a 1/r^g potential. It is found that for g \le 1 the
electrostatic repulsion expels all the charges to the surface of the sphere.
However for g>1 and n>n_c(g) occupation of the bulk becomes energetically
favorable. It is curious to note that the Coulomb law lies exactly on the
interface between these two regimes
Ground state of a large number of particles on a frozen topography
Problems consisting in finding the ground state of particles interacting with
a given potential constrained to move on a particular geometry are surprisingly
difficult. Explicit solutions have been found for small numbers of particles by
the use of numerical methods in some particular cases such as particles on a
sphere and to a much lesser extent on a torus. In this paper we propose a
general solution to the problem in the opposite limit of a very large number of
particles M by expressing the energy as an expansion in M whose coefficients
can be minimized by a geometrical ansatz. The solution is remarkably universal
with respect to the geometry and the interaction potential. Explicit solutions
for the sphere and the torus are provided. The paper concludes with several
predictions that could be verified by further theoretical or numerical work.Comment: 9 pages, 9 figures, LaTeX fil
Crystalline Order on a Sphere and the Generalized Thomson Problem
We attack generalized Thomson problems with a continuum formalism which
exploits a universal long range interaction between defects depending on the
Young modulus of the underlying lattice. Our predictions for the ground state
energy agree with simulations of long range power law interactions of the form
1/r^{gamma} (0 < gamma < 2) to four significant digits. The regime of grain
boundaries is studied in the context of tilted crystalline order and the
generality of our approach is illustrated with new results for square tilings
on the sphere.Comment: 4 pages, 5 eps figures Fig. 2 revised, improved Fig. 3, reference
typo fixe
Relativistic integro-differential form of the Lorentz-Dirac equation in 3D without runaways
It is well known that the third-order Lorentz-Dirac equation admits runaway
solutions wherein the energy of the particle grows without limit, even when
there is no external force. These solutions can be denied simply on physical
grounds, and on the basis of careful analysis of the correspondence between
classical and quantum theory. Nonetheless, one would prefer an equation that
did not admit unphysical behavior at the outset. Such an equation - an
integro-differential version of the Lorentz-Dirac equation - is currently
available either in 1 dimension only, or in 3 dimensions only in the
non-relativistic limit.
It is shown herein how the Lorentz-Dirac equation may be integrated without
approximation, and is thereby converted to a second-order integro-differential
equation in 3D satisfying the above requirement. I.E., as a result, no
additional constraints on the solutions are required because runaway solutions
are intrinsically absent. The derivation is placed within the historical
context established by standard works on classical electrodynamics by Rohrlich,
and by Jackson.Comment: (updated journal reference & email address
Limits on models of the ultrahigh energy cosmic rays based on topological defects
An erratum exists for this article. Please see the description link below for details.Using the propagation of ultrahigh energy nucleons, photons, and electrons in the universal radiation backgrounds, we obtain limits on the luminosity of topological defect scenarios for the origin of the highest energy cosmic rays. The limits are set as a function of the mass of the X particles emitted by the cosmic strings or other defects, the cosmological evolution of the topological defects, and the strength of the extragalactic magnetic fields. The existing data on the cosmic ray spectrum and on the isotropic 100 MeV gamma-ray background limit significantly the parameter space in which topological defects can generate the flux of the highest energy cosmic rays, and rule out models with the standard X-particle mass of 10¹⁶GeV and higher.R. J. Protheroe and Todor Stane
The nature of the highest energy cosmic rays
Ultra high energy gamma rays produce electron--positron pairs in interactions
on the geomagnetic field. The pair electrons suffer magnetic bremsstrahlung and
the energy of the primary gamma ray is shared by a bunch of lower energy
secondaries. These processes reflect the structure of the geomagnetic field and
cause experimentally observable effects. The study of these effects with future
giant air shower arrays can identify the nature of the highest energy cosmic
rays as either gamma-rays or nuclei.Comment: 15 pages of RevTeX plus 6 postscript figures, tarred, gzipped and
uuencoded. Subm. to Physical Review
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