Physics – Condensed Matter – Statistical Mechanics
Scientific paper
2001-08-29
Physics
Condensed Matter
Statistical Mechanics
latex, 11 pages, 5 figures
Scientific paper
The standard map is a paradigmatic one-parameter (noted $a$) two-dimensional conservative map which displays both chaotic and regular regions. This map becomes integrable for $a=0$. For $a \ne 0$ it can be numerically shown that the usual, Boltzmann-Gibbs entropy $S_1(t)=-\sum_{i} p_i(t)\ln{p_i(t)}$ exhibits a {\it linear} time evolution whose slope hopefully converges, for very fine graining, to the Kolmogorov-Sinai entropy. However, for increasingly small values of $a$, an increasingly large time interval emerges, {\it before} that stage, for which {\it linearity} with $t$ is obtained only for the generalized nonextensive entropic form $S_q(t)=\frac{1-\sum_{i}[p_i(t)]^{q}}{q-1}$ with $q = q^*\simeq 0.3$. This anomalous regime corresponds in some sense to a power-law (instead of exponential) mixing. This scenario might explain why in isolated classical long-range $N$-body Hamiltonians, and depending on the initial conditions, a metastable state (whose duration diverges with $1/N\to 0$) is observed before it crosses over to the BG regime.
Baldovin Fulvio
Schulze Bruno
Tsallis Constantino
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