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自拓扑绝缘体开始,拓扑材料便成为了凝聚态物理领域的一个的热点话题。由于拓扑能带理论和计算方法的发展,基于对称性分析的非磁性拓扑材料数据库在近期被成功建立。这些数据库中不仅包含了拓扑绝缘体,还有部分拓扑半金属态,这些拓扑半金属拥有位于高对称路径或高对称点的简并点费米子。但是对于可以位于整个布里渊区任何位置的Weyl费米子,这种对称性的方法是行不通的,因为传统Weyl半金属并不受晶体对称性保护。因此对于Weyl半金属的预测,需要其他的方法。而传统基于带隙的方法不仅难以操作,还会浪费大量的人力以及计算资源,这使得拓扑Weyl半金属数据库始终没能成功建立。
来自德国马普固体化学物理研究所的孙岩和Claudia Felser与莱布尼茨固体材料研究所的Jeroen van den Brink的合作团队通过对贝利相的高通量计算,高效地得到了非磁性Weyl半金属数据库并对其非线性光学做了系统的研究,从中发现一些Weyl半金属具有很强的二阶光伏效应。该研究不仅克服了之前所提到的计算难题,而且能够简单有效地从晶体结构数据库出发找到Weyl半金属以及Weyl点在能量和k-空间中的分布。这套计算网格贝利相位进而局部缩小范围的方法除了可用于非磁性非中心反演Weyl半金属体系,为后期磁性拓扑半金属数据库的发展提供了便利。除了第一类Weyl半金属和第二类Weyl半金属,我们还得到了9个具有手性结构的材料,这类材料有望实现量子化旋光光伏效应(Q-CPGE)。随着Weyl半金属材料数据库的建立,非磁性拓扑材料数据库终于完备。本研究提出的贝利相位计算方法可大大的加速拓扑材料数据库的发展,同时也揭示了拓扑材料具有出色的光学响应特性。
该文近期发表于npj Computational Materials 6: 32 (2020),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。
Comprehensive scan for nonmagnetic Weyl semimetals with nonlinear optical response
Qiunan Xu, Yang Zhang, Klaus Koepernik, Wujun Shi, Jeroen van den Brink, Claudia Felser & Yan Sun
First-principles calculations have recently been used to develop comprehensive databases of nonmagnetic topological materials that are protected by time-reversal or crystalline symmetry. However, owing to the low symmetry requirement of Weyl points, a symmetry-based approach to identifying topological states cannot be applied to Weyl semimetals (WSMs). To date, WSMs with Weyl points in arbitrary positions are absent from the well-known databases. In this work, we develop an efficient algorithm to search for Weyl points automatically and establish a database of nonmagnetic WSMs with Weyl points near the Fermi level based on the experimental non-centrosymmetric crystal structures in the Inorganic Crystal Structure Database (ICSD). In total, 46 Weyl semimetals were discovered to have nearly clean Fermi surfaces and Weyl points within 300 meV of the Fermi level. Nine of them are chiral structures which may exhibit the quantized circular photogalvanic effect. In addition, the nonlinear optical response is studied and the giant shift current is explored. Besides nonmagnetic WSMs, our powerful tools can also be used in the discovery of magnetic topological materials.
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