Recently, in collaboration with the groups of Prof. Baile Zhang and Prof. Chong Yidong in Nanyang Technological University, the group of physical acoustics in our university has first experimentally observed a topological threefold band degeneracy points (BDPs) in a three-dimensional phononic crystal, as well as doubled Fermi arcs of surface states consistent with a topological charge of 2. Additionally, the phenomenon of topologically protected negative refraction of surface states has been demonstrated experimentally. Our group of physical acoustics mainly undertakes the experimental design and measurement, and participates in the analysis and discussion of the results in this work. The research results have been published in Nature Physics (IF: 22.73, SCI 1) entitled "Topological triply degenerate point with double Fermi arcs" (DOI: 10.1038/s41567-019-0502-z). Postdoctoral Yang Yihao in Nanyang Technology University and Associate Professor Hongxiang Sun in our university are the co-authors.

In this work, using direct acoustic field mapping, we demonstrate the existence of the threefold BDP in the bulk bandstructure, as well as doubled Fermi arcs of surface states consistent with a topological charge of 2. Another novel BDP, similar to a Dirac point but carrying nonzero topological charge, is connected to the threefold BDP via the doubled Fermi arcs. The Fermi arcs form double helicoids spanning a broad frequency range (relative bandwidth >25%). We show that the non-contractibility of these arcs, which is due to the nontrivial topological charge, gives rise to the phenomenon of topologically protected negative refraction of surface states on all surfaces of the sample. Our findings pave the way to using these unconventional particles for exploring new emergent physical phenomena, and may find applications in symmetry-stabilized 3D zero-index metamaterial.

Article link：https://www.nature.com/articles/s41567-019-0502-z