Spin Excitations and Flat Electronic Bands in a Kagome Superconductor CsCr₃Sb₅

In the pursuit of discovering new quantum states driven by topology and electronic correlations, kagome lattice materials have attracted considerable attention due to their unique electronic band structures, particularly flat bands (FBs) that emerge from quantum destructive interference of the electronic wave functions. The tuning of the FBs to the chemical potential would lead to the possibility of liberating electronic instabilities that give rise to emergent electronic orders. Despite extensive studies, direct evidence of FBs tuned to the chemical potential and their participation in emergent electronic orders has remained elusive in bulk quantum materials. By combining angle-resolved photoemission spectroscopy, resonant inelastic X-ray scattering, and density functional theory, we reveal that the low-energy electronic structure of the recently discovered Cr-based kagome metal superconductor CsCr3Sb5 is dominated by a pervasive FB near the Fermi level. Moreover, we observe low-energy magnetic excitations, whose evolution across the low-temperature phase transition is associated with the observed shift of the electronic FB. Therefore, our results establish that the low-temperature order contains a magnetic origin and that the kagome FBs near the Fermi level likely play a dominant role in the induction of this order, as well as provide a plausible mechanism for the emergent unconventional superconductivity upon suppression of the magnetic order.