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.