The intertwining of charge order, superconductivity, and band topology has promoted the $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{K}$, Rb, Cs) family of materials to the center of attention in condensed matter physics. Underlying those mysterious macroscopic properties such as giant anomalous Hall conductivity (AHC) and chiral charge density wave is their nontrivial band topology. While there have been numerous experimental and theoretical works investigating the nontrivial band structure and especially the van Hove singularities, the exact topological phase of this family remains to be clarified. In this work, we identify ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ as a Dirac nodal line semimetal based on the observation of multiple Dirac nodal lines and loops close to the Fermi level. Combining photoemission spectroscopy and density functional theory, we identify two groups of Dirac nodal lines along the $k_z$ direction and one group of Dirac nodal loops in the $A-H-L$ plane. These nodal loops are located at the Fermi level within the instrumental resolution limit. Importantly, our first-principles analyses indicate that these nodal loops may be a crucial source of the mysterious giant AHC observed. Our results not only provide a clear picture to categorize the band structure topology of this family of materials, but also suggest the dominant role of topological nodal loops in shaping their transport behavior.

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• Topological materials
• Topological phases of matter
• Topological superconductors
• Nodal-line semimetals
• Angle-resolved photoemission spectroscopy
• Density functional calculations