[论文解读] New classes of chiral topological nodes with non-contractible surface Fermi arcs in CoSi

Zhicheng Rao, Hongcheng Li+19 more|arXiv (Cornell University)|Jan 10, 2019

Topological Materials and Phenomena参考文献 34被引用 96

一句话总结

本论文报道了在 CoSi 中通过 ARPES 实验发现自旋-1 和电荷-2 Dirac 手性节点，并在表面布里渊区连接的不可收缩表面费米弧被观测到。

ABSTRACT

In condensed matter systems, chiral topological nodes are robust band crossing points in momentum space that carry nonzero Chern numbers. The chirality is manifested by the presence of surface Fermi arcs connecting the projections of nodes with opposite Chern numbers. In addition to the well-known Weyl nodes, theorists have proposed several other types of chiral topological nodes in condensed matter systems, but the direct experimental evidence of their existence is still lacking. Here, using angle-resolved photoemission spectroscopy, we reveal two types of new chiral nodes, namely the spin-1 nodes and charge-2 Dirac nodes, at the band crossing points near the Fermi level in CoSi, the projections of which on the (001) surface are connected by topologically protected surface Fermi arcs. As these chiral nodes in CoSi are enforced at the Brillouin zone (BZ) center and corner by the crystalline symmetries, the surface Fermi arcs connecting their projections form a non-contractible path traversing the entire (001) surface BZ, in sharp contrast to pairs of Weyl nodes with small separation. Our work marks the first experimental observation of chiral topological nodes beyond the Weyl nodes both in the bulk and on the surface in condensed matter systems.

研究动机与目标

• 促使在凝聚态物理中探索超越 Weyl 节点的手性拓扑节点的可能性。
• 证明 CoSi 中新型节点类型（spin-1 和 charge-2 Dirac）的存在及其表面连通性。
• 显示晶体对称性约束在布里渊区高对称点强制这些节点的存在。

提出的方法

• 使用角分辨光电子能谱（ARPES）来探测费ermi 能级附近的体带交叉。
• 在布里渊区中心和角点识别并表征自旋-1 和电荷-2 Dirac 节点。
• 在（001）表面绘制表面费米弧并确立其跨越表面 BZ 的不可收缩拓扑性连接。
• 利用晶体对称性约束来解释节点的强制存在及表面态的连通性。

实验结果

研究问题

• RQ1 自旋-1 和电荷-2 Dirac 类手性节点在 CoSi 的费ermi 能级附近是否存在？
• RQ2 表面费米弧是否能将节点投影在（001）表面 BZ 中以不可收缩路径连接？
• RQ3 在 CoSi 的特定布里渊区点（中心和角点）是否由对称性强制这些节点存在？

主要发现

• 观测到两种新的手性节点类型：自旋-1 节点和靠近费ermi 能级的 CoSi 中的电荷-2 Dirac 节点。
• 这些节点在（001）表面的投影被拓扑保护的表面费米弧连接。
• 表面费米弧形成穿越整个（001）表面布里渊区的不可收缩路径。
• 这些节点由晶体对称性在布里渊区中心和角点处强制存在。
• 这是 condensed matter 系统中超越 Weyl 节点的手性拓扑节点的首次实验观测。