Recently, in collaboration with the group of Prof. Baile Zhang in Nanyang Technological University, the group of physical acoustics in our school has experimentally observed a twofold symmetry-enforced nodal surface in a three-dimensional (3D) chiral acoustic crystal. The research results have been published in《Nature Communications》entitled " Observation of a topological nodal surface and its surface-state arcs in an artificial acoustic crystal" (DOI: 10.1038/s41467-019-13258-3). Yang Yihao, postdoctoral fellow in Nanyang Technology University, and Xia Jianping, master in our school are the co-first authors. Prof. Sun Hongxiang in our school, Prof. Zhang Baile and Prof. Chong Yidong in Nanyang Technology University are co-corresponding authors.

Topological semimetallic materials are the research hotspot in condensed matter physics, optics and acoustics. 3D gapless topological phases can be classified by the dimensionality of the band degeneracies, including zero-dimensional nodal points, one-dimensional nodal lines, and two-dimensional nodal surfaces. Both nodal points and nodal lines have been realized recently in photonics and acoustics. However, a nodal surface has never been observed in any classical-wave system.

In this work, the authors report on the experimental observation of a twofold symmetry-enforced nodal surface in a 3D chiral acoustic crystal. In particular, the demonstrated nodal surface carries a topological charge of 2, constituting the first realization of a higher-dimensional topologically-charged band degeneracy. Using direct acoustic field measurements, the authors observe the projected nodal surface and its Fermi-arc-like surface states and demonstrate topologically-induced robustness of the surface states against disorders. The discovery of a higher-dimensional topologically-charged band degeneracy paves the way toward further explorations of the physics and applications of new topological semimetal phases.

Article link：https://www.nature.com/articles/s41467-019-13258-3.