Excitation of travelling multibreathers in anharmonic chains

We study the dynamics of the “externally” forced and damped Fermi–Pasta–Ulam (FPU) 1D lattice. The forcing has the spatial symmetry of the Fourier mode with wavenumber p and oscillates sinusoidally in time with the frequency ω. When ω is in the phonon band, the p-mode becomes modulationally unstable above a critical forcing, which we determine analytically in terms of the parameters of the system. For ω above the phonon band, the instability of the p-mode leads to the formation of a travelling multibreather, that, in the low-amplitude limit could be described in terms of soliton solutions of a suitable driven-damped nonlinear Schrödinger (NLS) equation. Similar mechanisms of instability could show up in easy-axis magnetic structures that are governed by such NLS equations.