Observation of exceptional line semimetal in three-dimensional non-Hermitian phononic crystals

Non-Hermitian topological phases, which exhibit unique features such as skin effect and exceptional points originated from nontrivial band topologies in complex plane, have attracted enormous attention in condensed-matter physics and metamaterials. Here we report the realization of an exceptional line semimetal in a three-dimensional non-Hermitian phononic crystal. A pair of exceptional rings with opposite topologies are connected by the drumhead bulk states in the first Brillouin zone. The exceptional rings not only possess wave-function topology and thus result in the drumhead surface states, but also host spectral topology and thereby give rise to the hybrid-order geometry-dependent skin effect in three dimensions. Our experimental results evidence the complete non-Hermitian bulk-boundary correspondence of the three-dimensional exceptional line semimetal, and may pave the way for designing non-Hermitian acoustic devices.