Observation of Z₂ Non-Hermitian Skin Effect in Projective Mirror-Symmetric Acoustic Metamaterials

Non-Hermitian skin effect (NHSE), where eigenstates localize at the boundary of non-Hermitian lattices, has gained significant attention in various fields. This phenomenon, driven by the point-gap topology of complex energy bands, occurs even without special symmetries. Nevertheless, additional symmetry may significantly enrich the NHSE. Notably, time-reversal symmetry protects a Z₂ NHSE, featuring oppositely accumulated skin modes. Here, a 1D bilayer Z₂ NHSE model based on projective mirror symmetry is proposed, making Z₂ NHSE possible in spinless systems. Experimentally, both uniform and bidirectional Z₂ NHSEs are observed in acoustic metamaterials, where the necessary non-Hermitian elements-gain and loss-are achieved through active feedback circuits. These findings open new avenues for exploring symmetry-enriched non-Hermitian topological phenomena and pave the way for potential applications in wave manipulation, sensing, and beyond.