The Crystal and Electronic Structures of Cd3As2, the Three-Dimensional Electronic Analogue of Graphene

The structure of Cd3As2, a high-mobility semimetal reported to host electrons that act as Dirac particles, is reinvestigated by single-crystal X-ray diffraction. It is found to be centrosymmetric rather than noncentrosymmetric as previously reported. It has a distorted superstructure of the antifluorite (M2X) structure type with a tetragonal unit cell of a = 12.633(3) and c = 25.427(7) Å in the centrosymmetric I41/acd space group. The antifluorite superstructure can be envisioned as consisting of distorted Cd6□2 cubes (where □ = an empty cube vertex) in parallel columns, stacked with opposing chirality. Electronic structure calculations performed using the experimentally determined centrosymmetric structure are similar to those performed with the inversion symmetry absent but with the important implication that Cd3As2 is a three-dimensional (3D)-Dirac semimetal with no spin splitting; all bands are spin degenerate and there is a 4-fold degenerate bulk Dirac point at the Fermi energy along Γ–Z in the Brillouin zone. This makes Cd3As2 a 3D electronic analogue of graphene. Scanning tunneling microscopy experiments identify a 2 × 2 surface reconstruction in the (112) cleavage plane of single crystals; needle crystals grow with a [110] long axis direction.

本研究对高迁移率半金属二砷化三镉（Cd3As2）的晶体结构进行了重新表征，该材料此前被报道存在可充当狄拉克粒子的电子。研究采用单晶X射线衍射（single-crystal X-ray diffraction）技术开展，结果发现其实际为中心对称结构，而非此前报道的非中心对称结构。
该材料具有以反萤石（M2X）结构型为基础的畸变超结构，在中心对称空间群I4₁/acd中，其四方晶胞参数为a = 12.633(3)、c = 25.427(7) Å。该反萤石超结构可被视为由平行排列的畸变Cd6□2立方单元（其中□代表空穴晶位）构成，这些立方单元以相反手性方式堆叠形成柱体。
基于实验测定的中心对称结构开展的电子结构计算结果，与忽略反演对称性的计算结果整体相似，但存在一项关键差异：二砷化三镉是一种无自旋劈裂的三维（3D）狄拉克半金属，所有能带均存在自旋简并，且在布里渊区沿Γ–Z方向的费米能级处存在四重简并体相狄拉克点。这一特性使其成为三维电子体系中的石墨烯类似物。
扫描隧道显微镜（scanning tunneling microscopy）实验观测到，单晶的(112)解理面存在2×2表面重构；针状晶体的长轴方向沿[110]晶向生长。