Acoustic Crystals with a Möbius Twist

T he Möbius loop is an iconic geometry that shows the oddity of a topological transformation. One can easily construct this one-sided surface by twisting a strip of paper by 180° and sticking the ends of the strip together (Fig. 1). As topology has turned into a central topic in condensed-matter physics, an interesting question arises: Can a Möbius-like geometry interact with topological states and enrich them? Seeking an answer to that question, two groups of researchers from Nanyang Technological University, Singapore, and Wuhan University, China, respectively, have coupled a Möbius loop with the topological edge states of acoustic lattices—periodic arrays of acoustic resonators [1, 2]. The experimental observations indicating that this system hosts a new class of “Möbius-twisted” topological bands will inspire new fundamental research directions in topology and may lead to new devices for controlling acoustic or electromagnetic waves.