Topological mechanical metamaterial with nonrectilinear constraints

TL;DR

This paper introduces a new class of topological mechanical metamaterials based on non-rectilinear constraints, expanding the topological classification and enabling higher topological indices with simple unit cells.

Contribution

It generalizes the topological framework to Maxwell lattices with polygon-shaped, non-rectilinear constraints, revealing new topological states with multiple edge modes.

Findings

Generalized topological classification for non-rectilinear constraints

Achieved higher topological indices with simple unit cells

Demonstrated multiple edge states without enlarging the unit cell

Abstract

In this paper we study Maxwell lattices with non-rectilinear constraints, where the elastic energy is determined by the collective motion of three or more particles, in contrast to a rectilinear spring whose elastic energy only relies on the displacement of two particles attached to the two ends of the spring. Utilizing polygon-shaped constraints, we found that the Maxwell counting argument and the topological construction, based on the compatibility matrix and the equilibrium matrix, can be generalized in our models, and our elastic systems follow the same topological classification. In addition, we also found that non-rectilinear constraints offers a natural pass towards topological states with higher topological indices and multiple edge states, which can be achieved even for a simple unit cell with one degree of freedom per unit cell without enlarging the unit cell in the bulk or at the edge.