Observation of Z ₂ Non‐Hermitian Skin Effect in Projective Mirror‐Symmetric Acoustic Metamaterials

Abstract

Abstract 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 Z 2 NHSE, featuring oppositely accumulated skin modes. Here, a 1D bilayer Z 2 NHSE model based on projective mirror symmetry is proposed, making Z 2 NHSE possible in spinless systems. Experimentally, both uniform and bidirectional Z 2 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.