Recently, a paper titled Weyl points and Fermi arcs in a chiral phononic crystal written by the team of Liu Zhengyou’s was published online in Nature physics, and it was the second time that Liu’s team had publishes a paper in this journal this year.
It has been another achievement in topological physics since the team published its research findings about energy valleys in Physical Review Letters last year and Nature Physics in the first half of this year. Besides, this project is supported by the 973 Program (National Basic Research Program of China) and the National Natural Science Foundation of China, etc.
Generally speaking, the elementary particles that construct the material world comprise two types: the bosons and the fermions. The fermions can be further divided into three categories: the Dirac fermions, the Weyl fermions and the Majorana fermions. The Dirac fermions are widely distributed in nature, but the other two fermions have long been questioned of their existence.
However, in recent years, the Weyl fermions and Majorana fermions have been observed in the condensed matter system, both of which are quasi-particles and elementary excitations shown by electrons under condensed structural constraints. The most distinctive property of the Weyl fermions is the existence of the Weyl points in its dispersion relation, the search on which used to be a hot spot in several branches of physics study, such as optics and cold atoms.
Does Weyl points exist in sound? The possibility of Weyl points’ existence in phononic crystal has been theoretically predicted respectively by the team of Prof. Ziting Chen, Hong Kong University of Science and Technology and the team of Prof. Baile Zhang, Nanyang Technological University, Singapore. Nevertheless, the prediction had not been confirmed by experiment.
However, the combination of the ingenious design and 3D printing technology enables Liu’s team to successfully produce chiral Weyl phononic crystals. The measurement of the dispersion relation of sound on this crystal clearly shows the Weyl points and the fermi arcs of the surface waves. Furthermore, the team demonstrates the novel transmission behavior of surface states in chiral Weyl phononic crystals: the topologically protected surface states can conduct one-way propagations with immunity to defects.
▲A photo of the 3D sample
▲Fermi arcs of the surface waves and projections of Weyl points
The macroscale of phononic crystals extends the study of Weyl physics from microcosmic to macroscopic perspectives and also brings about the convenience of both experimental preparation and measurement. The use of such crystals will definitely benefit the progress and practical application of topological physics.
Rewritten by Lingling Zheng, Edited by Shen Yuxi, Hu Sijia
