Observation of flat band, Dirac nodal lines and topological surface states in Kagome superconductor CsTi₃Bi₅

Kagome lattices of various transition metals are versatile platforms for achieving anomalous Hall effects, unconventional charge-density wave orders and quantum spin liquid phenomena. Here, we use laser-based angle-resolved photoemission spectroscopy in combination with density functional theory calculations to investigate the electronic structure of the newly discovered kagome superconductor CsTi₃Bi₅. We directly observe a striking flat band derived from the local destructive interference of Bloch wave functions within the kagome lattice. We identify type-II and type-III Dirac nodal lines and their momentum distribution in CsTi₃Bi₅. Around the Brillouin zone centre, Z₂ nontrivial topological surface states are also observed due to band inversion mediated by strong spin-orbit coupling.

Jiangang Yang, Xinwei Yi, Zhen Zhao, Yuyang Xie, Taimin Miao, Hailan Luo, Hao Chen, Bo Liang, Wenpei Zhu, Yuhan Ye, Jing-Yang You, Bo Gu, Shenjin Zhang, Fengfeng Zhang, Feng Yang, Zhimin Wang, Qinjun Peng, Hanqing Mao, Guodong Liu, Zuyan Xu, Hui Chen, Haitao Yang, Gang Su, Hongjun Gao, Lin Zhao, X. J. Zhou