Ideal type-II Weyl phase and topological transition in phononic crystals

TL;DR

This paper reports the experimental realization of ideal type-II Weyl points in three-dimensional phononic crystals and demonstrates a topological phase transition between Weyl semimetal and valley insulator phases, revealing unique surface states.

Contribution

It introduces the first realization of ideal type-II Weyl points in phononic crystals and explores the associated topological phase transition and surface states.

Findings

Realized ideal type-II Weyl points in phononic crystals.

Observed topological phase transition from Weyl semimetal to valley insulators.

Identified Fermi-arc and Fermi-circle surface states.

Abstract

Ideal Weyl points, which are related by symmetry and thus reside at the same frequency, could promote the deep development and utilization of the Weyl physics. Although the ideal type-I Weyl points have been achieved in photonic crystals, the ideal type-II Weyl points with tilted cone-like band dispersions, are still beyond discovery. Here we realize ideal type-II Weyl points of minimal number in three-dimensional layer-stacked phononic crystals, and demonstrate topological phase transition from Weyl semimetal to valley insulators of two distinct types. The Fermi-arc surface states exist on the interface of the Weyl and valley phases, while the Fermi-circle ones occur on that of the two distinct valley phases. We show the interesting wave partition of Fermi-circle surface states on the interfaces formed by distinct valley phases.