Non-Hermitian Topological Phononic Metamaterials.

Non-Hermitian (NH) physics describes novel phenomena in open systems that allow generally complex spectra. Introducing NH physics into topological metamaterials, which permits explorations of topological wave phenomena in artificially designed structures, not only enables the experimental verification of exotic NH phenomena in these flexible platforms, but also enriches the manipulation of wave propagation beyond the Hermitian cases. Here, a perspective on the advances in the research of NH topological phononic metamaterials is presented, which covers the exceptional points and their topological geometries, the skin effect related to the topology of complex spectra, the interplay of NH effects and topological states in phononic metamaterials, etc.