Flat Band and ℤ₂ Topology of Kagome Metal CsTi₃Bi₅

The simple kagome-lattice band structure possesses Dirac cones, flat band, and saddle point with van Hove singularities in the electronic density of states, facilitating the emergence of various electronic orders. Here we report a titanium-based kagome metal CsTi₃Bi₅ where titanium atoms form a kagome network, resembling its isostructural compound CsV₃Sb₅. Thermodynamic properties including the magnetization, resistance, and heat capacity reveal the conventional Fermi liquid behavior in the kagome metal CsTi₃Bi₅ and no signature of superconducting or charge density wave (CDW) transition anomaly down to 85 mK. Systematic angle-resolved photoemission spectroscopy measurements reveal multiple bands crossing the Fermi level, consistent with the first-principles calculations. The flat band formed by the destructive interference of hopping in the kagome lattice is observed directly. Compared to CsV₃Sb₅, the van Hove singularities are pushed far away above the Fermi level in CsTi₃Bi₅, in line with the absence of CDW. Furthermore, the first-principles calculations identify the nontrivial ℤ₂ topological properties for those bands crossing the Fermi level, accompanied by several local band inversions. Our results suppose CsTi₃Bi₅ as a complementary platform to explore the superconductivity and nontrivial band topology.