近年来，凝聚态系统中的本征拓扑序受到了广泛的关注。人们发现，在某些拓扑体系中存在着对应于量子场论中相对论费米子的准粒子激发，这为研究高能物理中的基本粒子及其相关现象提供了绝妙的平台。和基本粒子不同，晶体中的准粒子不受庞加莱对称性的约束，只需要考虑晶体对称性，因此，除了标准模型中常规的狄拉克，外尔和马约拉纳粒子之外，在凝聚态物理中还存在着高能物理中没有对应物的非常规拓扑准粒子。
来自南方科技大学物理系的徐虎课题组通过对称性分析和第一性原理计算，提出在具有立方晶格结构的SrSi2中存在着手性相反且拓扑电荷不相等的声子外尔点。这些声子外尔点受到晶体旋转对称性的保护，共同形成了非常规的三端外尔复合体。在SrSi2中，每一组三端外尔复合体都包含了单个拓扑电荷为 2的双外尔点和一对拓扑电荷为-1的单外尔点，使得总的拓扑电荷守恒。他们研究发现，这种三端外尔复合体的声子表面态呈现出清晰的双表面弧特征，每一支表面弧都连接着一个双外尔点和一个单外尔点在表面上的投影，从而形成了非常规的拓扑声子表面态。这一发现与通常所认为的现象不同，当晶体中同时存在二次型双外尔点和线性单外尔点时，通常认为表面弧会连接具有相反手性但拓扑电荷数相同的外尔点，而三端外尔复合体却展现了另一种可能，在一定程度上拓宽了人们对非常规拓扑准粒子的认知。为了进一步研究对称性对三端外尔复合体的影响，徐虎课题组采用单轴拉伸应变的方法，沿z轴方向施加了0.5%的晶格应变，发现拓扑电荷为 2的双外尔点在四重螺旋对称性破缺时会演变成为一对拓扑电荷为 1的单外尔点，最终演变成一支表面弧连接一对手性相反的单外尔点的常规外尔系统。
该文近期发表于npj Computational Materials 6: 87 (2020)，英文标题与摘要如下，点击https://www.nature.com/articles/s41524-020-00354-y可以自由获取论文PDF。
Three-terminal Weyl complex with double surface arcs in a cubic lattice
Zhenqiao Huang, Zhongjia Chen, Baobing Zheng and Hu Xu
Exploring unconventional topological quasiparticles and their associated exotic physical properties has become a hot topic in condensed matter physics, thus stimulating extensive interest in recent years. Here, in contrast to the double-Weyl phonons (the topological chiral charge 2) in the trigonal and hexagonal crystal systems, we propose that the unconventional double-Weyl without counterparts in high-energy physics can emerge in the phonons of cubic structures, i.e., SrSi2. Employing a two-band k⋅p Hamiltonian, we prove that the quadratic double-Weyl nodes are protected by the fourfold screw rotational symmetry
. Strikingly, we find that the surface arcs are terminated with the Weyl nodes that possess unequal topological charges with opposite sign (i.e., 2 and −1), leading to unique three-terminal Weyl complex (one quadratic double-Weyl and two linear single-Weyl) with double surface arcs in SrSi2. In addition, we apply a uniaxial tensile strain along z-axis to examine the evolution of the three-terminal Weyl complex when the corresponding symmetries are broken. Our work not only provides an ideal candidate for the realization of the quadratic double-Weyl and the corresponding unique surface arc states, but also broadens the understanding of topological Weyl physics.
