Experimental observation of non-Abelian earring nodal links in phononic crystals

TL;DR

This paper reports the experimental observation of non-Abelian earring nodal links in phononic crystals, demonstrating unique topological phenomena and providing a new platform for studying non-Abelian band topology.

Contribution

The study designs and verifies earring nodal links in phononic crystals, experimentally observing non-Abelian topological charges in a simple acoustic system.

Findings

Observed two types of earring nodal links experimentally.

Demonstrated that nodal chain and line order can switch after band inversion.

Showed that the link structure cannot be severed despite band order changes.

Abstract

Nodal lines are symmetry-protected one-dimensional band degeneracies in momentum space, which can appear in numerous topological configurations such as nodal rings, chains, links, and knots. Very recently, non-Abelian topological physics has been proposed in space-time inversion (PT) symmetric systems, and attract widespread attention. One of the most special configurations in non-Abelian system is the earring nodal link, composing of a nodal chain linking with an isolated nodal line, is signature of non-Abelian topology and cannot be elucidated using Abelian topological classifications. However, the earring nodal links have not been yet observed in real system. Here we design the phononic crystals with earring nodal links, and verify its non-Abelian topologicial charge in full-wave simulations. Moreover, we experimentally observed two different kinds of earring nodal links by measuring the band structures for two phononic crystals. Specifically, we found that the order of the nodal chain and line can switch after band inversion but their link cannot be severed. Our work provides experimental evidence for phenomena unique to non-Abelian band topology and our simple acoustic system provides a convenient platform for studying non-Abelian charges.