Liu Pengfei, Zhang Ao, Han Jingzhi, Liu Qihang
Shenzhen Institute for Quantum Science and Engineering and Department of Physics, Southern University of Science and Technology, Shenzhen 518055, China.
School of Physics, Peking University, Beijing 100871, China.
Innovation (Camb). 2022 Oct 19;3(6):100343. doi: 10.1016/j.xinn.2022.100343. eCollection 2022 Nov 8.
Dirac semimetal is a phase of matter whose elementary excitation is described by the relativistic Dirac equation. In the limit of zero mass, its parity-time symmetry enforces the Dirac fermion in the momentum space, which is composed of two Weyl fermions with opposite chirality, to be non-chiral. Inspired by the flavor symmetry in particle physics, we theoretically propose a massless Dirac-like equation yet linking two Weyl fields with the identical chirality by assuming isospin symmetry, independent of the space-time rotation exchanging the two fields. Dramatically, such symmetry is hidden in certain solid-state spin-1/2 systems with negligible spin-orbit coupling, where the spin degree of freedom is decoupled with the lattice. Therefore, the existence of the corresponding quasiparticle, dubbed as flavor Weyl fermion, cannot be explained by the conventional (magnetic) space group framework. The 4-fold degenerate flavor Weyl fermion manifests linear dispersion and a Chern number of 2, leading to a robust network of topologically protected Fermi arcs throughout the Brillouin zone. For material realization, we show that the transition-metal chalcogenide CoNbS with experimentally confirmed collinear antiferromagnetic order is ideal for flavor Weyl semimetal under the approximation of vanishing spin-orbit coupling. Our work reveals a counterpart of the flavor symmetry in magnetic electronic systems, leading to further possibilities of emergent phenomena in quantum materials.
狄拉克半金属是一种物质相，其基本激发由相对论性狄拉克方程描述。在零质量极限下，其宇称时间对称性使得动量空间中的狄拉克费米子（由两个具有相反手性的外尔费米子组成）变为非手性的。受粒子物理学中味对称性的启发，我们在理论上提出了一个无质量的类狄拉克方程，通过假设同位旋对称性，将两个具有相同手性的外尔场联系起来，且该对称性与交换这两个场的时空旋转无关。引人注目的是，这种对称性隐藏在某些自旋 - 轨道耦合可忽略不计的固态自旋 - 1/2系统中，其中自旋自由度与晶格解耦。因此，相应的准粒子（称为味外尔费米子）的存在无法用传统的（磁性）空间群框架来解释。四重简并的味外尔费米子表现出线性色散和陈数为2，导致在整个布里渊区形成一个稳健的拓扑保护费米弧网络。对于材料实现，我们表明在自旋 - 轨道耦合可忽略的近似下，具有实验证实的共线反铁磁序的过渡金属硫族化合物CoNbS是味外尔半金属的理想材料。我们的工作揭示了磁性电子系统中味对称性的对应物，为量子材料中涌现现象带来了更多可能性。
