Higher-order Dirac sonic crystals

TL;DR

This paper introduces a new higher-order Dirac semimetal model protected by symmetry, demonstrates its phase transitions, and experimentally realizes the phase in a sonic crystal with observed hinge states, advancing topological wave control.

Contribution

It proposes a minimal theoretical model for a higher-order Dirac semimetal and experimentally demonstrates this phase in a sonic crystal, revealing hinge states.

Findings

Theoretical model of a higher-order Dirac semimetal protected by C6v symmetry.

Experimental realization of the higher-order topological phase in a sonic crystal.

Observation of hinge states via momentum-space spectroscopy and real-space visualization.

Abstract

Discovering new topological phases of matter is a major theme in fundamental physics and materials science. Dirac semimetal provides an exceptional platform for exploring topological phase transitions under symmetry breaking. Recent theoretical studies have revealed that a three-dimensional Dirac semimetal can harbor fascinating hinge states, a higher-order topological manifestation not known before. However, its realization in experiment is yet to be achieved. In this Letter, we propose a minimum model to construct a spinless higher-order Dirac semimetal protected by C_6v symmetry. By breaking different symmetries, this parent phase transitions into a variety of novel topological phases including higher-order topological insulator, higher-order Weyl semimetal, and higher-order nodal-ring semimetal. Furthermore, for the first time, we experimentally realize this unprecedented higher-order topological phase in a sonic crystal and present an unambiguous observation of the desired hinge states via momentun-space spectroscopy and real-space visualization. Our findings may offer new opportunities to manipulate classical waves such as sound and light.