Observation of Topological Flat Bands in Chalcogen-Substituted Nb₃Cl₈ by ARPES

In kagome lattice materials, destructive interference of electronic wave functions can generate highly degenerate topological flat bands (TFBs), leading to distinctive electronic behavior. Nb₃Cl₈-based compounds, which realize a breathing kagome lattice, have recently drawn considerable interest due to such TFBs and their associated emergent phenomena. Here, we investigate a series of chalcogen-substituted Nb₃Cl₈ compounds using high-resolution angle-resolved photoemission spectroscopy (ARPES) and track the evolution of their electronic structures. For a representative compound, Nb₃SeCl₇, ARPES along high-symmetry directions resolves multiple dispersive bands at higher binding energies, consistent with the expected Brillouin-zone periodicity, and—crucially—pronounced flat-band features at low binding energies. Comparison with density functional theory calculations corroborates the nontrivial topology of these flat bands. We further demonstrate that these TFBs are robust across the chalcogen-substituted series, while their energy positions can be systematically tuned via chemical substitution. Our results establish a spectroscopic foundation for exploring novel quantum states in breathing kagome systems and provide design principles for topological quantum materials with controllable flat-band energetics.