npj：非常規外爾系統—三端外爾複合體

近年來，凝聚態系統中的本徵拓撲序受到了廣泛的關注。人們發現，在某些拓撲體系中存在著對應於量子場論中相對論費米子的準粒子激發，這為研究高能物理中的基本粒子及其相關現象提供了絕妙的平臺。和基本粒子不同，晶體中的準粒子不受龐加萊對稱性的約束，只需要考慮晶體對稱性，因此，除了標準模型中常規的狄拉克，外爾和馬約拉納粒子之外，在凝聚態物理中還存在著高能物理中沒有對應物的非常規拓撲準粒子。

來自南方科技大學物理系的徐虎課題組通過對稱性分析和第一性原理計算，提出在具有立方晶格結構的SrSi2中存在著手性相反且拓撲電荷不相等的聲子外爾點。這些聲子外爾點受到晶體旋轉對稱性的保護，共同形成了非常規的三端外爾複合體。在SrSi2中，每一組三端外爾複合體都包含了單個拓撲電荷為+2的雙外爾點和一對拓撲電荷為-1的單外爾點，使得總的拓撲電荷守恆。他們研究發現，這種三端外爾複合體的聲子表面態呈現出清晰的雙表面弧特徵，每一支表面弧都連接著一個雙外爾點和一個單外爾點在表面上的投影，從而形成了非常規的拓撲聲子表面態。這一發現與通常所認為的現象不同，當晶體中同時存在二次型雙外爾點和線性單外爾點時，通常認為表面弧會連接具有相反手性但拓撲電荷數相同的外爾點，而三端外爾複合體卻展現了另一種可能，在一定程度上拓寬了人們對非常規拓撲準粒子的認知。為了進一步研究對稱性對三端外爾複合體的影響，徐虎課題組採用單軸拉伸應變的方法，沿z軸方向施加了0.5%的晶格應變，發現拓撲電荷為+2的雙外爾點在四重螺旋對稱性破缺時會演變成為一對拓撲電荷為+1的單外爾點，最終演變成一支表面弧連接一對手性相反的單外爾點的常規外爾系統。

該文近期發表於npj Computational Materials 6: 87 (2020)，英文標題與摘要如下，點擊https://www.nature.com/articles/s41524-020-00354-y可以自由獲取論文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.