Using the force landscape of an active solid to predict plastic deformation

Amorphous solids possess a set of soft quasilocalized excitations that control many aspects of the material's mechanical and thermodynamic properties, similar to lattice defects in crystals. In non-active systems, a nonlinear analysis of the potential energy landscape around an equilibrium reference configuration reveals quasilocalized excitations with low energy barriers, estimated to third order. However, many materials of interest (especially biological systems) do not have a well-defined energy landscape, as they are driven out of equilibrium by active forces or non-reciprocal interactions. Do these non-Hamiltonian materials still possess similar quasilocalized excitations? To address this question, we generalize these nonlinear excitations by analyzing the force landscape of systems with active forces that can not be derived from a global energy functional, and apply this formalism to packings of self-propelled rods at densities well above the jamming transition. We numerically compute force-based soft excitations at different levels of activity, and compare these with particle rearrangements that occur as the magnitude of the active forces is quasistatically increased in a creep protocol. We find that the nonlinear excitations with the lowest energy barriers are highly correlated with upcoming plastic events, even multiple rearrangements in advance. Future extensions include applying this methodology to non-reciprocal systems, such as odd elastic materials.