npj: 孙建教授和邢定钰院士等—发现系列特殊复合拓扑节点线
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近年来，拓扑材料在理论和实验上引起了科学界极大的兴趣。其无间隙相拓扑半金属节点一般被限制在布里渊区之内，例外极少，而同一体系内存在一系列例外的拓扑节点线的情况更是闻所未闻。南京大学的孙建教授和邢定钰院士团队采用对称性分析和第一性原理计算，却发现了这种稀奇现象。
来自南京大学的孙建教授和邢定钰院士领导的团队，采用对称性分析和第一性原理计算研究了正交相AgF2体系，发现其存在着环绕或穿越整个布里渊区的系列复合拓扑半金属态，包括节点面、四度简并网、节点链、“浑天仪”型节点、沙漏型狄拉克环等等。有意思的是，上述穿越布里渊区的节点线特性，均对应于不同的拓扑指标和对称性指标。在不考虑自旋轨道耦合时，正交相空间群会诱导出环绕整个布里渊区的节点面和四度简并网。在考虑自旋轨道耦合时，布里渊区的表面会出现沙漏型狄拉克环态。作者在AgF2中发现了更多特殊的简并态：1）不考虑自旋轨道耦合时，该体系表现出了穿越布里渊区的节点链与浑天仪样节点态。 2）考虑自旋轨道耦合时，体系中出现了一种新的沙漏型狄拉克环/链的构型，由一般沙漏型狄拉克圆圈态劈裂成两条穿越布里渊区的狄拉克环态而形成。他们对此定义了一个新的对称性指标，用来对所有沙漏型狄拉克环态作了分类。他们的发现为利用对称性指标理论研究高对称点、线、面上存在节点的拓扑半金属态提供了一个完美的实例。
该文近期发表于npj Computational Materials 5: 53 (2019)，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。
Composite topological nodal lines penetrating the Brillouin zone in orthorhombic AgF2
Dexi Shao, Huaiqiang Wang, Tong Chen, Pengchao Lu, Qinyan Gu, Li Sheng, Dingyu Xing & Jian Sun
It has recently been found that nonsymmorphic symmetries can bring many exotic band crossings. Here, based on symmetry analysis, we predict that materials with time-reversal symmetry in the space group of Pbca (No. 61) possess rich symmetry-enforced band crossings, including nodal surfaces, fourfold degenerate nodal lines and hourglass Dirac loops, which appear in triplets as ensured by the cyclic permutation symmetry. We take Pbca AgF2 as an example in real systems and studied its band structures with ab initio calculations. Specifically, in the absence of spin-orbit coupling (SOC), besides the above-mentioned band degeneracies, this system features a nodal chain and a nodal armillary sphere penetrating the Brillouin zone (BZ). While with SOC, we find a new configuration of the hourglass Dirac loop/chain with the feature traversing the Brillouin zone, which originates from the splitting of a Dirac loop confined in the Brillouin zone. Furthermore, guided by the bulk-surface correspondence, we calculated the surface states to explore these bulk nodal phenomena. The evolution of these interesting nodal phenomena traversing the Brillouin zone under two specific uniaxial strains is also discussed.
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