Observation of Dirac-like energy band and ring-torus Fermi surface associated with the nodal line in topological insulator CaAgAs
Observation of Dirac-like energy band and ring-torus Fermi surface associated with the nodal line in topological insulator CaAgAs
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DOI：
10.1038/s41535-017-0074-z
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发表时间：
2017-08
影响因子：
5.7
通讯作者：
D. Takane;K. Nakayama;S. Souma;Taichi Wada;Y. Okamoto;K. Takenaka;Y. Yamakawa;A. Yamakage;T. Mitsuhashi;K. Horiba;H. Kumigashira;Takashi Takahashi;Takafumi Sato
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作者：
D. Takane;K. Nakayama;S. Souma;Taichi Wada;Y. Okamoto;K. Takenaka;Y. Yamakawa;A. Yamakage;T. Mitsuhashi;K. Horiba;H. Kumigashira;Takashi Takahashi;Takafumi Sato
当前材料研究的关键挑战之一是寻找具有倒置体能带结构的新型拓扑材料。在拓扑绝缘体中，由强自旋轨道耦合引起的能带反转导致整个布里渊区的带隙开放，而额外的晶体对称性，如点群和非对称对称性，有时会阻止动量空间中特定点或线上的差距开放，从而产生拓扑半金属。尽管许多拓扑绝缘体/半金属与这种晶体对称性的理论预测，实验实现仍然相对稀缺。在这里，使用角分辨光电子能谱与体敏感的软X射线光子，我们实验证明，六角磷属元素化物CaAgAs属于一个新的家庭的拓扑绝缘体，其特征在于由反转的能带结构和晶体的镜面反射对称性。我们建立了三维布里渊区的体价带结构，观察到了与线节点相关的类Dirac-like能带和环形环面费米面，其中体价带和导电带在动量空间中相交于一条线上，自旋轨道耦合可以忽略.有趣的是，我们发现没有其他带穿过费米能级，因此低能激发的唯一特征是Dirac-like带。CaAgAs为研究低能电子动力学、晶体对称性和奇异拓扑性质之间的相互作用提供了一个很好的平台。
One of key challenges in current material research is to search for new topological materials with inverted bulk-band structure. In topological insulators, the band inversion caused by strong spin–orbit coupling leads to opening of a band gap in the entire Brillouin zone, whereas an additional crystal symmetry such as point-group and nonsymmorphic symmetries sometimes prohibits the gap opening at/on specific points or line in momentum space, giving rise to topological semimetals. Despite many theoretical predictions of topological insulators/semimetals associated with such crystal symmetries, the experimental realization is still relatively scarce. Here, using angle-resolved photoemission spectroscopy with bulk-sensitive soft-x-ray photons, we experimentally demonstrate that hexagonal pnictide CaAgAs belongs to a new family of topological insulators characterized by the inverted band structure and the mirror reflection symmetry of crystal. We have established the bulk valence-band structure in three-dimensional Brillouin zone, and observed the Dirac-like energy band and ring-torus Fermi surface associated with the line node, where bulk valence and conducting bands cross on a line in the momentum space under negligible spin–orbit coupling. Intriguingly, we found that no other bands cross the Fermi level and therefore the low-energy excitations are solely characterized by the Dirac-like band. CaAgAs provides an excellent platform to study the interplay among low-energy electron dynamics, crystal symmetry, and exotic topological properties.