Observation of Single Pair of Type-III Weyl Points in Sonic Crystals

TL;DR

This paper reports the first experimental observation of type-III Weyl points in a sonic crystal, demonstrating a minimal pair of Weyl points and confirming associated topological surface states, thus advancing topological physics in artificial systems.

Contribution

It is the first to realize and confirm type-III Weyl points in a sonic crystal, challenging previous beliefs about minimal Weyl point numbers in nonmagnetic systems.

Findings

Observation of a single pair of type-III Weyl points

Confirmation of noncontractible Fermi arc surface states

Detection of chiral edge states

Abstract

In electronics systems, the Weyl points can be classified into three types based on the geometry of the Fermi surface, and each type exhibits various unique and intriguing phenomena. While the type-I and type-II Weyl points have been achieved in both spinful and spinless systems, the realization of type-III Weyl points remains challenging, and has not been reported in artificial periodic systems. Here, we for the first time report the experimental observation of the type-III Weyl points in a sonic crystal. Remarkably, a single pair of type-III Weyl points are observed as the only band crossings in a frequency range, experimentally disproving a common belief in the field, namely, the minimal number of Weyl points in nonmagnetic systems is four. The consistency between experimental results and theoretical predictions confirms the existence of type-III Weyl points, noncontractible Fermi arc surface states, and chiral edge states. Our work not only fill the gap of the type-III Weyl point in sonic crystal but also stimulate related researches in other systems, such as photonic, mechanical and cold atom systems.