Seeded localized nonlinear excitations in the Fermi-Pasta-Ulam-Tsingou system – Stability, delocalization and journey to equilibrium

It is well known that the Fermi-Pasta-Ulam-Tsingou system admits localized nonlinear excitations. We show that these excitations exhibit nearly periodic behavior at early times but delocalize by leaking energy in the form of solitary waves and other metastable excitations in the absence of phonons. Further, we show that the excitations share qualitative features with the strongly nonlinear Duffing oscillator. After the delocalization is complete, the system enters a quasi-equilibrium phase characterized by a Gaussian velocity distribution, high kinetic energy fluctuations and no equipartitioning of energy. Finally, at late times, we calculate the specific heat capacity of the system and compare it to analytical results to show that the system transitions past the quasi-equilibrium phase to equilibrium.