Lossy phononic metamaterials for valley manipulation

TL;DR

This paper demonstrates how coupling loss in phononic metamaterials can be used to control valley degrees of freedom, leading to novel non-Hermitian effects such as valley filtering, skin effects, and boundary-dependent edge states.

Contribution

It introduces a new approach to valley manipulation using pure loss in phononic metamaterials, expanding non-Hermitian physics beyond gain and nonreciprocity.

Findings

Demonstrated valley-resolved nonreciprocity acting as a valley filter

Observed valley-dependent skin effects localizing states at opposite boundaries

Identified valley-projected edge states with boundary-dependent lifetimes

Abstract

Non-Hermitian physics characterized by complex band spectra has established a new paradigm in condensed matter systems and metamaterials. Recently, non-Hermitian gain and nonreciprocity are deliberately introduced to valley manipulation, leading to various phenomena beyond the Hermitian scenarios, such as the amplified topological whispering gallery modes as an acoustic laser. In contrast, pure loss is inevitable in practice and generally regarded as a detrimental factor. Here, we reveal that the coupling loss can manipulate valley degrees of freedom in a phononic metamaterial. Three distinct valley-related effects, including valley-resolved nonreciprocity that functions as a valley filter, valley-dependent skin effects where bulk states from different valleys localize at opposite boundaries, and valley-projected edge states with boundary-dependent lifetimes that leads to an anomalous beam splitting, are demonstrated through theoretical analysis and airborne sound experiments. Owing to the easy preparation of loss, our findings shed light on both non-Hermitian and valley physics and may facilitate innovative applications of valley-related devices.