Observation of fourfold Dirac nodal line semimetal and its unconventional surface responses in sonic crystals

TL;DR

This paper reports the experimental discovery of fourfold Dirac nodal line semimetals in sonic crystals, revealing unique topological surface responses and contrasting with conventional Weyl NLSMs, opening new avenues for acoustic wave manipulation.

Contribution

First experimental observation of fourfold Dirac NLSMs in sonic crystals with distinct topological properties and surface responses compared to Weyl NLSMs.

Findings

Observed Dirac NLSMs with fourfold degeneracy in sonic crystals.

Identified unique surface responses including torus surface states and absence of surface states.

Demonstrated topological surface arcs changing from open to closed contours.

Abstract

Three-dimensional nodal line semimetals (NLSMs) provide remarkable importance for both enrich topological physics and wave management. However, NLSMs realized in acoustic systems are twofold bands degenerate, which are called Weyl NLSMs. Here, we first report on the experimental observation of novel Dirac NLSMs with fourfold degenerate in sonic crystals. We reveal that the topological properties of the Dirac NLSMs are entirely different than that of the conventional Weyl NLSMs. The Berry phase related to the Dirac nodal line (DNL) is 2{\pi}, which results in the surface responses of the Dirac NLSMs with two radically different situations: a torus surface state occupying the entire surface Brillouin zone (SBZ) and without any surface state in the SBZ. We further reveal that topological surface arcs caused by DNL can change from open to closed contours. The findings of Dirac NLSMs and their unique surface response may provoke exciting frontiers for flexible manipulation of acoustic surface waves.