Observation of emergent Dirac physics at the surfaces of acoustic higher-order topological insulators

TL;DR

This paper demonstrates the existence and tunability of Dirac surface states in acoustic higher-order topological insulators using 3D sonic crystals, revealing topological phase transitions and hinge states with potential for controlling surface waves.

Contribution

It reports the experimental observation of Dirac surface states and topological hinge states in acoustic HOTIs, with tunable Dirac mass and zero refractive index behavior, advancing topological acoustics.

Findings

Observation of 2D Dirac surface states in acoustic HOTIs

Tunable Dirac mass via crystal geometry

Experimental confirmation of zero refractive index and hinge states

Abstract

Using three-dimensional (3D) sonic crystals as acoustic higher-order topological insulators (HOTIs), we discover two-dimensional (2D) surface states described by spin-1 Dirac equations at the interfaces between the two sonic crystals with distinct topology but the same crystalline symmetry. We find that the Dirac mass can be tuned by the geometry of the two sonic crystals. The sign reversal of the Dirac mass reveals a surface topological transition where the surface states exhibit zero refractive index behavior. When the surface states are gapped, one-dimensional (1D) hinge states emerge due to the topology of the gapped surface states. We confirm experimentally the zero refractive index behavior and the emergent topological hinge states. Our study reveals a multidimensional Wannier orbital control that leads to extraordinary properties of surface states and unveils an interesting topological mechanism for the control of surface waves.