Long-range interacting classical systems: universality in mixing weakening

Abstract

Through molecular dynamics, we study the $d=2,3$ classical model of $N$ coupled rotators (inertial XY model) assuming a coupling constant which decays with distance as $r_{ij}^{-α}$ ($α\ge 0$). The total energy $<H>$ is asymptotically $\propto N {\tilde N}$ with ${\tilde N} \equiv [N^{1-α/d}-(α/d)]/[1-α/d]$, hence the model is thermodynamically extensive if $α/d>1$ and nonextensive otherwise. We numerically show that, for energies above some threshold, the (appropriately scaled) maximum Lyapunov exponent is $\propto N^{-κ}$ where $κ$ is an {\it universal} (one and the same for $d=1,2$ and 3, and all energies) function of $α/d$, which monotonically decreases from 1/3 to zero when $α/d$ increases from 0 to 1, and identically vanishes above 1. These features are consistent with the nonextensive statistical mechanics scenario, where thermodynamic extensivity is associated with {\it exponential} mixing in phase space, whereas {\it weaker} (possibly {\it power-law} in the present case) mixing emerges at the $N \to \infty$ limit whenever nonextensivity is observed.