Observation of Acoustic Non-Hermitian Bloch Braids and Associated Topological Phase Transitions

Topological features embedded in ancient braiding and knotting arts endow significant impacts on our daily life and even cutting-edge science. Recently, fast growing efforts are invested to the braiding topology of complex Bloch bands in non-Hermitian systems. This new classification of band topology goes far beyond those established in Hermitian counterparts. Here, we present the first acoustic realization of the topological non-Hermitian Bloch braids, based on a two-band model easily accessible for realizing any desired knot structure. The non-Hermitian bands are synthesized by a simple binary cavity-tube system, where the long-range, complex-valued, and momentum-resolved couplings are accomplished by a well -controlled unidirectional coupler. In addition to directly visualizing various two-band braiding patterns, we unambiguously observe the highly elusive topological phase transitions between them. Not only do our results provide a direct demonstration for the non-Hermitian band topology, but also the experimental techniques open new avenues for designing unconventional acoustic metamaterials.

Automated summary

The topological features in ancient braiding and knotting arts have significant impacts on daily life and cutting-edge science. Recent efforts have been focused on the braiding topology of complex Bloch bands in non-Hermitian systems, surpassing the established classifications in Hermitian systems. In this study, the first acoustic realization of topological non-Hermitian Bloch braids was presented, using a two-band model that can easily achieve any desired knot structure. The non-Hermitian bands were synthesized using a simple binary cavity-tube system, enabling the direct visualization of various two-band braiding patterns and observation of highly elusive topological phase transitions.