Three-dimensional valley-contrasting sound

TL;DR

This paper introduces a novel 3D acoustic crystal that demonstrates valley-contrasting physics, enabling topological wave transport with unique properties such as robustness and valley-specific localization, expanding valley physics into three dimensions.

Contribution

The study develops the first 3D acoustic crystal exhibiting valley-contrasting physics, extending valley phenomena from 2D materials to three-dimensional structures.

Findings

Demonstration of 3D valley-contrasting physics in acoustic crystals

Observation of robust topological surface states

Valley-specific wave localization and refraction phenomena

Abstract

Spin and valley are two fundamental properties of electrons in crystals. The similarity between them is well understood in valley-contrasting physics established decades ago in two-dimensional (2D) materials like graphene--with broken inversion symmetry, the two valleys in graphene exhibit opposite orbital magnetic moments, similar to the spin-1/2 behaviors of electrons, and opposite Berry curvature that leads to a half topological charge. However, valley-contrasting physics has never been explored in 3D crystals. Here, we develop a 3D acoustic crystal exhibiting 3D valley-contrasting physics. Unlike spin that is fundamentally binary, valley in 3D can take six different values, each carrying a vortex in a distinct direction. The topological valley transport is generalized from the edge states of 2D materials to the surface states of 3D materials, with interesting features including robust propagation, topological refraction, and valley-cavity localization. Our results open a new route for wave manipulation in 3D space.