Observation of Antihelical Edge States in Acoustic Metamaterials

As a hallmark of the quantum Hall effect, chiral edge modes (CEMs) counterpropagate along the two parallel edges of a ribbon structure. However, recent studies demonstrate counterintuitive anti-CEMs that copropagate along the parallel edges. Analogous to the established extension of the CEMs to helical edge modes (HEMs) in the quantum spin Hall effect, it is natural to extend the anti-CEMs to anti-HEMs, which comprise a pair of time-reversal-related anti-CEMs. In this Letter, we report the first observation of the anti-HEMs based on a bilayer model that features staggered positive and negative interlayer hoppings. Experimentally, we implement this antihelical model on an acoustic platform and provide compelling evidence for the anti-HEMs by selectively exciting different spin subspaces, along with identifying the energy-biased Dirac points in bulk spectra. Our findings may offer new insights into topological phases of matter and potentially pave the way for designing novel devices with unique edge transport properties.