Non-Hermitian edge burst of sound

TL;DR

This paper demonstrates the phenomenon of non-Hermitian edge burst in a classical acoustic metamaterial, revealing how non-Hermitian band topology and imaginary gap closures influence boundary-localized effects.

Contribution

It introduces the concept of non-Hermitian edge burst in classical wave systems and links it to imaginary gap closures and band topology.

Findings

Edge bursts can occur at one or both boundaries depending on gap closures.

The phenomenon is demonstrated in a lossy nonreciprocal acoustic crystal.

Edge burst behavior is linked to the number and location of imaginary gap closure points.

Abstract

Non-Hermitian band topology can give rise to phenomena with no counterparts in Hermitian systems. A well-known example is the non-Hermitian skin effect (NHSE), where Bloch eigenstates localize at a boundary, induced by a nontrivial spectrum winding number. In contrast, recent studies on lossy non-Hermitian lattices have uncovered an unexpected boundary-localized loss probability-a phenomenon that requires not only non-Hermitian band topology but also the closure of the imaginary (dissipative) gap. Here, we demonstrate the non-Hermitian edge burst in a classical-wave metamaterial: a lossy nonreciprocal acoustic crystal. We show that, when the imaginary gap remains closed, edge bursts can occur at the right boundary, left boundary, or both boundaries simultaneously, all under the same non-Hermitian band topology; the latter scenario is known as a bipolar edge burst. The occurrence of each scenario depends on the number and location of the imaginary gap closure points in the eigenenergy spectra. These findings generalize the concept of edge burst from quantum to classical wave systems, establish it as an intrinsic material property, and enrich the physics of the complex interplay between non-Hermitian band topology and other physical properties in non-Hermitian systems.