Lossy acoustic metamaterials for higher-order Weyl exceptional ring

A research team led by Prof. Zhengyou Liu from the School of Physics and Technology, Wuhan University, has successfully realized a non-Hermitian higher-order Weyl exceptional ring semimetal in a lossy acoustic metamaterial. The lattice model of the metamaterials is constructed by stacking the breathing Kagome lattice along the z direction via cross-linked couplings (IMAGE 1). The metamaterial sample is fabricated by 3D printing technology, and controllable loss is achieved by filling sponges into the walls of specific coupling tubes. The metamaterial hosts Weyl exceptional rings with dual topological charges in terms of quantized Chern number and spectral winding number, as the first-order topological invariants, which give rise to Fermi arc surface states and surface-dependent skin effect of the bulk states (IMAGE 2), respectively. More importantly, second-order topological features are revealed by topological hinge states protected by bulk polarization, along with a hinge-dependent skin effect that stems from the interplay between Fermi arc surface states and the non-Hermitian skin effect (IMAGE 3). Unlike trivial hinge states, which are almost unobservable, the topological hinge states exhibit counterintuitive purely real frequencies-an attribute that ensures robust hinge propagation of acoustic waves against the introduced loss.

These results broaden the understanding of the interplay between topological semimetals and non-Hermiticity. Given the unavoidable intrinsic loss in both natural and engineered systems, the real hinge states induced by the Weyl exceptional ring hold significant potential for applications in topological waveguides and sensors.

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