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

TL;DR

This paper reports the experimental realization of a three-dimensional non-Hermitian phononic crystal exhibiting an exceptional line semimetal phase, featuring topologically protected surface states and a hybrid-order skin effect, advancing non-Hermitian topological physics.

Contribution

It introduces the first experimental observation of an exceptional line semimetal in 3D non-Hermitian phononic crystals, demonstrating bulk-boundary correspondence and complex topological phenomena.

Findings

Observation of exceptional rings connected by drumhead states

Evidence of wave-function and spectral topology in the system

Demonstration of a hybrid-order skin effect in three dimensions

Abstract

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.