Localizing softness and stress along loops in three-dimensional topological metamaterials

Abstract: Topological states can be used to control the mechanical properties of a material along an edge or around a localized defect. The surface rigidity of elastic networks is characterized by a bulk topological invariant called the polarization; materials with a well-defined uniform polarization display a dramatic range of edge softnesses depending on the orientation of the polarization relative to the terminating surface. However, in all three-dimensional mechanical metamaterials proposed to date, the topological edge modes are mixed with bulk soft modes and so-called Weyl loops. Here, we report the design of a gapped 3D topological metamaterial with a uniform polarization that displays a corresponding asymmetry between the number of soft modes on opposing surfaces and, in addition, no bulk soft modes. We then use this construction to localize topological soft modes in interior regions of the material by including defect structures---dislocation loops---that are unique to three dimensions. We derive a general formula that relates the difference in the number of soft modes and states of self-stress localized along the dislocation loop to the handedness of the vector triad formed by the lattice polarization, Burgers vector, and dislocation-line direction. Our findings suggest a novel strategy for pre-programming failure and softness localized along lines in 3D, while avoiding extended periodic failure modes associated with Weyl loops.