Cai Xiangxi, Ye Liping, Qiu Chunyin, Xiao Meng, Yu Rui, Ke Manzhu, Liu Zhengyou
1Key Laboratory of Artificial Micro-Structures and Nano-Structures of Ministry of Education and School of Physics and Technology, Wuhan University, 430072 Wuhan, China.
2Institute for Advanced Studies, Wuhan University, 430072 Wuhan, China.
Light Sci Appl. 2020 Mar 10;9:38. doi: 10.1038/s41377-020-0273-4. eCollection 2020.
Dirac semimetals, the materials featuring fourfold degenerate Dirac points, are critical states of topologically distinct phases. Such gapless topological states have been accomplished by a band-inversion mechanism, in which the Dirac points can be annihilated pairwise by perturbations without changing the symmetry of the system. Here, we report an experimental observation of Dirac points that are enforced completely by the crystal symmetry using a nonsymmorphic three-dimensional phononic crystal. Intriguingly, our Dirac phononic crystal hosts four spiral topological surface states, in which the surface states of opposite helicities intersect gaplessly along certain momentum lines, as confirmed by additional surface measurements. The novel Dirac system may release new opportunities for studying elusive (pseudo) and offer a unique prototype platform for acoustic applications.
狄拉克半金属是具有四重简并狄拉克点的材料，是拓扑不同相的关键状态。这种无带隙拓扑态是通过能带反转机制实现的，其中狄拉克点可以通过微扰成对湮灭而不改变系统的对称性。在此，我们报告了利用非对称三维声子晶体通过晶体对称性完全实现狄拉克点的实验观测。有趣的是，我们的狄拉克声子晶体拥有四个螺旋拓扑表面态，正如额外的表面测量所证实的那样，相反螺旋度的表面态沿着某些动量线无带隙相交。这个新型狄拉克系统可能为研究难以捉摸的（赝）态带来新机遇，并为声学应用提供一个独特的原型平台。
