Extensive packet excitations in FPU and Toda lattices

At low energies, the excitation of low frequency packets of normal modes in the Fermi-Pasta-Ulam (FPU) and in the Toda model leads to exponentially localized energy profiles which resemble staircases and are identified by a slope $\sigma$ that depends logarithmically on the specific energy $\varepsilon=E/N$. Such solutions are found to lie on stable lower dimensional tori, named $q$-tori. At higher energies there is a sharp transition of the system's localization profile to a straight-line one, determined by an $N$-dependent slope of the form $\sigma \sim (\varepsilon N)^{-d}$, $d>0$. We find that the energy crossover $\varepsilon_c$ between the two energy regimes decays as $1/N$, which indicates that $q$-tori disappear in the thermodynamic limit. Furthermore, we focus on the times that such localization profiles are practically frozen and we find that these "stickiness times" can rapidly and accurately distinguish between a power-law and a stretched exponential dependence in $1/\varepsilon$.