Researchers in mechanical engineering have published new findings that explore the properties of mechanical lattices, as opposed to atomic lattices, which are twisted relative to each other. The research is part of the new field of “twistronics” in physics and has potential applications in the design of new materials that are able to transmit or absorb sound.
Published in Nature: Communications Physics in June, the article is titled “Topological gaps by twisting” and is authored by Matheus Rosa and Professor Massimo Ruzzene from CU Boulder along with physics Professor Emil Prodan from Yeshiva University.
Simplified abstract
Researchers in physics have shown that the stacking of two graphene sheets which are twisted with respect to each other produces unique properties, such as electrical superconductivity. These findings have opened a new field of research, which is now known as “twistronics." In this context, we explore the properties of mechanical lattices, as opposed to atomic lattices, which are twisted relative to each other. Similarly to the findings on graphene, we found that the twist angle between the layers is a key parameter, which can be tuned to produce “topological band gaps”. These are frequency bands wherein the energy associated with an elastic wave deforming the lattice cannot propagate in the interior, but can only travel along the boundaries of the lattice. This is an effect that is reminiscent of the Quantum Hall Effect, which manifests in materials that conduct electricity only along the edges, and not in their interior, when subjected to an external magnetic field. In mechanical lattices, we do not apply an external magnetic field, instead, we achieve a similar effect through the relative sliding of the layers at a fixed twist angle. Hence, our results show a strong analogy with the behavior of graphene layers, and illustrate how bi-layer and multi-layer systems can be designed to support the propagation of elastic waves only along the edges, with the relative sliding of the layers producing the same effect that a magnetic field has in atomistic systems. The existence of these edge modes, and our ability to control them through twisting and sliding has strong implications on the design of new materials that are able to confine waves resulting from, for example, impacts, or that are able to transmit and absorb sound in very unusual and non-intuitive ways. The findings can therefore find application in sound absorbing panels made of bilayered lattice structures, or in personal protection systems (body armors, helmets) designed to absorb and reduce the effects of an impact.
