691 research outputs found
The one-component plasma: a conceptual approach
The one-component plasma (OCP) represents the simplest statistical mechanical
model of a Coulomb system. For this reason, it has been extensively studied
over the last forty years. The advent of the integral equations has resulted in
a dramatic improvement in our ability to carry out numerical calculations, but
came at the expense of a physical insight gained in a simpler analytic theory.
In this paper we present an extension of the Debye-Hueckel (DH) theory to the
OCP. The theory allows for analytic calculations of all the thermodynamic
functions, as well as the structure factor. The theory explicitly satisfies the
Stillinger-Lovett and, for small couplings, the compressibility sum rules,
implying its internal self consistency.Comment: 28 pages, 5 Postscript figures, uses elsart.sty, accepted for
publication in Physica
Test of renormalization predictions for universal finite-size scaling functions
We calculate universal finite-size scaling functions for systems with an
n-component order parameter and algebraically decaying interactions. Just as
previously has been found for short-range interactions, this leads to a
singular epsilon-expansion, where epsilon is the distance to the upper critical
dimension. Subsequently, we check the results by numerical simulations of spin
models in the same universality class. Our systems offer the essential
advantage that epsilon can be varied continuously, allowing an accurate
examination of the region where epsilon is small. The numerical calculations
turn out to be in striking disagreement with the predicted singularity.Comment: 6 pages, including 3 EPS figures. To appear in Phys. Rev. E. Also
available as PDF file at
http://www.cond-mat.physik.uni-mainz.de/~luijten/erikpubs.htm
A field guide for onfarm research with special reference to improvement of cropping systems and techniques in West and Central Africa
Spherical Model in a Random Field
We investigate the properties of the Gibbs states and thermodynamic
observables of the spherical model in a random field. We show that on the
low-temperature critical line the magnetization of the model is not a
self-averaging observable, but it self-averages conditionally. We also show
that an arbitrarily weak homogeneous boundary field dominates over fluctuations
of the random field once the model transits into a ferromagnetic phase. As a
result, a homogeneous boundary field restores the conventional self-averaging
of thermodynamic observables, like the magnetization and the susceptibility. We
also investigate the effective field created at the sites of the lattice by the
random field, and show that at the critical temperature of the spherical model
the effective field undergoes a transition into a phase with long-range
correlations .Comment: 29 page
Programming Groups of Rational Agents
Abstract. In this paper, we consider the problem of effectively pro-gramming groups of agents. These groups should capture structuring mechanisms common in multi-agent systems, such as teams, cooperative groups, and organisations. Not only should individual agents be dynamic and evolving, but the groups in which the agents occur must be open, flexible and capable of similar evolution and restructuring. We enable the description and implementation of such groups by providing an extension to our previous work on programming languages for agent-based systems based on executable temporal and modal logics. With such formalism as a basis, we consider the grouping aspects within multi-agent systems. In particular, we describe how this logic-based approach to grouping has been implemented in Java and consider how this language can be used for developing multi-agent systems.
Scaling Behavior of Anomalous Hall Effect and Longitudinal Nonlinear Response in High-Tc Superconductors
Based on existing theoretical model and by considering our longitudinal
nonlinear response function, we derive a nonliear equation in which the mixed
state Hall resistivity can be expressed as an analytical function of magnetic
field, temperature and applied current. This equation enables one to compare
quantitatively the experimental data with theoretical model. We also find some
new scaling relations of the temperature and field dependency of Hall
resistivity. The comparison between our theoretical curves and experimental
data shows a fair agreement.Comment: 4 pages, 3 figure
Universal Magnetic Properties of at Intermediate Temperatures
We present the theory of two-dimensional, clean quantum antiferromagnets with
a small, positive, zero temperature () stiffness , but with the
ratio arbitrary. Universal scaling forms for the uniform
susceptibility (), correlation length(), and NMR relaxation rate
() are proposed and computed in a expansion and by Mont\'{e}-Carlo
simulations. For large , and asymptote
to universal values, while is nearly -independent. We find good
quantitative agreement with experiments and some numerical studies on
.Comment: 14 pages, REVTEX, 1 postscript figure appende
Temperature and ac Effects on Charge Transport in Metallic Arrays of Dots
We investigate the effects of finite temperature, dc pulse, and ac drives on
the charge transport in metallic arrays using numerical simulations. For finite
temperatures there is a finite conduction threshold which decreases linearly
with temperature. Additionally we find a quadratic scaling of the
current-voltage curves which is independent of temperature for finite
thresholds. These results are in excellent agreement with recent experiments on
2D metallic dot arrays. We have also investigated the effects of an ac drive as
well as a suddenly applied dc drive. With an ac drive the conduction threshold
decreases for fixed frequency and increasing amplitude and saturates for fixed
amplitude and increasing frequency. For sudden applied dc drives below
threshold we observe a long time power law conduction decay.Comment: 6 pages, 7 postscript figure
A Simple Model for the DNA Denaturation Transition
We study pairs of interacting self-avoiding walks on the 3d simple cubic
lattice. They have a common origin and are allowed to overlap only at the same
monomer position along the chain. The latter overlaps are indeed favored by an
energetic gain.
This is inspired by a model introduced long ago by Poland and Sheraga [J.
Chem. Phys. {\bf 45}, 1464 (1966)] for the denaturation transition in DNA
where, however, self avoidance was not fully taken into account. For both
models, there exists a temperature T_m above which the entropic advantage to
open up overcomes the energy gained by forming tightly bound two-stranded
structures.
Numerical simulations of our model indicate that the transition is of first
order (the energy density is discontinuous), but the analog of the surface
tension vanishes and the scaling laws near the transition point are exactly
those of a second order transition with crossover exponent \phi=1. Numerical
and exact analytic results show that the transition is second order in modified
models where the self-avoidance is partially or completely neglected.Comment: 29 pages, LaTeX, 20 postscript figure
Reversible Pressure-Induced Amorphization in Solid C70 : Raman and Photoluminescence Study
We have studied single crystals of by Raman scattering and
photoluminescence in the pressure range from 0 to 31.1 GPa. The Raman spectrum
at 31.1 GPa shows only a broad band similar to that of the amorphous carbon
without any trace of the Raman lines of . After releasing the pressure
from 31.1 GPa, the Raman and the photoluminescence spectra of the recovered
sample are that of the starting crystal. These results indicate that
the molecules are stable upto 31.1 GPa and the amorphous carbon high
pressure phase is reversible, in sharp contrast to the results on solid
. A qualitative explaination is suggested in terms of inter- versus
intra-molecular interactions.Comment: To appear in Phys. Rev. Lett., 12 pages, RevTeX (preprint format), 3
figures available upon reques
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