Title: Plasticity in amorphous systems: which perturbations matter?

Abstract:  Hard spheres provide an archetypal model of amorphous systems that have both a glass and jamming transition which are well described even in two and three dimensions by a mean field theory. However, real systems deform and are generally modeled to first approximation as soft overlapping spheres at an energy minimum. First, I will show that this small soft sphere perturbation introduces contact changes which are not associated with saddle points in the energy landscape. This has the consequence that the response of soft spheres at finite pressure to shear is not a smooth perturbation away from hard spheres, and that the nonlinear response of soft spheres is tied to the linear response, both of which are singular at zero pressure. Additionally, I will show that the response of the soft sphere system to shear can be generalized to a broader class of random perturbations with the same general response. Ultimately, these two stories are connected by nontrivial structural information contained in the energy landscape of allowed configurations.