753 research outputs found
Dynamical quenching and annealing in self-organization multiagent models
We study the dynamics of a generalized Minority Game (GMG) and of the Bar
Attendance Model (BAM) in which a number of agents self-organize to match an
attendance that is fixed externally as a control parameter. We compare the
usual dynamics used for the Minority Game with one for the BAM that makes a
better use of the available information. We study the asymptotic states reached
in both frameworks. We show that states that can be assimilated to either
thermodynamic equilibrium or quenched configurations can appear in both models,
but with different settings. We discuss the relevance of the parameter that
measures the value of the prize for winning in units of the fine for losing. We
also provide an annealing protocol by which the quenched configurations of the
GMG can progressively be modified to reach an asymptotic equlibrium state that
coincides with the one obtained with the BAM.Comment: around 20 pages, 10 figure
Glassy dynamics, metastability limit and crystal growth in a lattice spin model
We introduce a lattice spin model where frustration is due to multibody
interactions rather than quenched disorder in the Hamiltonian. The system has a
crystalline ground state and below the melting temperature displays a dynamic
behaviour typical of fragile glasses. However, the supercooled phase loses
stability at an effective spinodal temperature, and thanks to this the Kauzmann
paradox is resolved. Below the spinodal the system enters an off-equilibrium
regime corresponding to fast crystal nucleation followed by slow activated
crystal growth. In this phase and in a time region which is longer the lower
the temperature we observe a violation of the fluctuation-dissipation theorem
analogous to structural glasses. Moreover, we show that in this system there is
no qualitative difference between a locally stable glassy configuration and a
highly disordered polycrystal
Dynamics of the Time Horizon Minority Game
We present exact analytic results for a new version of the Minority Game (MG)
in which strategy performance is recorded over a finite time horizon. The
dynamics of this Time Horizon Minority Game (THMG) exhibit many distinct
features from the MG and depend strongly on whether the participants are fed
real, or random, history strings. The THMG equations are equivalent to a Markov
Chain, and yield exact analytic results for the volatility given a specific
realization for the quenched strategy disorder.Comment: Latex file, 11 pages, 6 figure
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Alterations in T1 of normal and reperfused infarcted myocardium after Gd-BOPTA versus GD-DTPA on inversion recovery EPI.
This study tested whether Gd-BOPTA/Dimeg or Gd-DTPA exerts greater relaxation enhancement for blood and reperfused infarcted myocardium. Relaxivity of Gd-BOPTA is increased by weak binding to serum albumin. Thirty-six rats were subjected to reperfused infarction before contrast (doses = 0.05, 0.1, and 0.2 mmol/kg). delta R1 was repeatedly measured over 30 min. Gd-BOPTA caused greater delta R1 for blood and myocardium than did Gd-DTPA; clearance of both agents from normal- and infarcted myocardium was similar to blood clearance; plots of delta R1 myocardium/delta R1 blood showed equilibrium phase contrast distribution. Fractional contrast agent distribution volumes were approximately 0.24 for both agents in normal myocardium, 0.98 and 1.6 for Gd-DTPA and Gd-BOPTA, respectively, in reperfused infarction. The high value for Gd-BOPTPA was ascribed to greater relaxivity in infarction versus blood. It was concluded that Gd-BOPTA/Dimeg causes a greater delta R1 than Gd-DTPA in regions which contain serum albumin
A novel method for evaluating the critical nucleus and the surface tension in systems with first order phase transition
We introduce a novel method for calculating the size of the critical nucleus
and the value of the surface tension in systems with first order phase
transition. The method is based on classical nucleation theory, and it consists
in studying the thermodynamics of a sphere of given radius embedded in a frozen
metastable surrounding. The frozen configuration creates a pinning field on the
surface of the free sphere. The pinning field forces the sphere to stay in the
metastable phase as long as its size is smaller than the critical nucleus. We
test our method in two first-order systems, both on a two-dimensional lattice:
a system where the parameter tuning the transition is the magnetic field, and a
second system where the tuning parameter is the temperature. In both cases the
results are satisfying. Unlike previous techniques, our method does not require
an infinite volume limit to compute the surface tension, and it therefore gives
reliable estimates even by using relatively small systems. However, our method
cannot be used at, or close to, the critical point, i.e. at coexistence, where
the critical nucleus becomes infinitely large.Comment: 12 pages, 15 figure
On the stationary points of the TAP free energy
In the context of the p-spin spherical model, we introduce a method for the
computation of the number of stationary points of any nature (minima, saddles,
etc.) of the TAP free energy. In doing this we clarify the ambiguities related
to the approximations usually adopted in the standard calculations of the
number of states in mean field spin glass models.Comment: 11 pages, 1 Postscript figure, plain Te
Geometric approach to the dynamic glass transition
We numerically study the potential energy landscape of a fragile glassy
system and find that the dynamic crossover corresponding to the glass
transition is actually the effect of an underlying geometric transition caused
by a qualitative change in the topological properties of the landscape.
Furthermore, we show that the potential energy barriers connecting local glassy
minima increase with decreasing energy of the minima, and we relate this
behaviour to the fragility of the system. Finally, we analyze the real space
structure of activated processes by studying the distribution of particle
displacements for local minima connected by simple saddles
Saddle Points and Dynamics of Lennard-Jones Clusters, Solids and Supercooled Liquids
The properties of higher-index saddle points have been invoked in recent
theories of the dynamics of supercooled liquids. Here we examine in detail a
mapping of configurations to saddle points using minimization of , which has been used in previous work to support these theories. The
examples we consider are a two-dimensional model energy surface and binary
Lennard-Jones liquids and solids. A shortcoming of the mapping is its failure
to divide the potential energy surface into basins of attraction surrounding
saddle points, because there are many minima of that do not
correspond to stationary points of the potential energy. In fact, most liquid
configurations are mapped to such points for the system we consider. We
therefore develop an alternative route to investigate higher-index saddle
points and obtain near complete distributions of saddles for small
Lennard-Jones clusters. The distribution of the number of stationary points as
a function of the index is found to be Gaussian, and the average energy
increases linearly with saddle point index in agreement with previous results
for bulk systems.Comment: 14 pages, 7 figure
Continuum time limit and stationary states of the Minority Game
We discuss in detail the derivation of stochastic differential equations for
the continuum time limit of the Minority Game. We show that all properties of
the Minority Game can be understood by a careful theoretical analysis of such
equations. In particular, i) we confirm that the stationary state properties
are given by the ground state configurations of a disordered (soft) spin
system; ii) we derive the full stationary state distribution; iii) we
characterize the dependence on initial conditions in the symmetric phase and
iv) we clarify the behavior of the system as a function of the learning rate.
This leaves us with a complete and coherent picture of the collective behavior
of the Minority Game. Strikingly we find that the temperature like parameter
which is introduced in the choice behavior of individual agents turns out to
play the role, at the collective level, of the inverse of a thermodynamic
temperature.Comment: Revised version (several new results added). 12 pages, 5 figure
Sterols and their d<sup>13</sup>C signature in suspended particles in the Southern Ocean water column
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