  In the quest for novel quantum states driven by topology and correlation,
kagome lattice materials have garnered significant interest due to their
distinctive electronic band structures, featuring flat bands (FBs) arising from
the quantum destructive interference of the electronic wave function. The
tuning of the FBs to the chemical potential would lead to the possibility of
liberating electronic instabilities that lead to emergent electronic orders.
Despite extensive studies, direct evidence of FBs tuned to the chemical
potential and their participation in emergent electronic orders have been
lacking in bulk quantum materials. Here using a combination of Angle-Resolved
Photoemission Spectroscopy (ARPES) and Density Functional Theory (DFT), we
reveal that the low-energy electronic structure of the recently discovered
Cr-based kagome metal superconductor CsCr3Sb5 is dominated by a pervasive FB in
close proximity to, and below the Fermi level. A comparative analysis with
orbital-projected DFT and polarization dependence measurement uncovers that an
orbital-selective renormalization mechanism is needed to reconcile the
discrepancy with the DFT calculations, which predict the FB to appear 200 meV
above the Fermi level. Furthermore, we observe the FB to shift away from the
Fermi level by 20 meV in the low-temperature density wave-ordered phase,
highlighting the role of the FB in the emergent electronic order. Our results
reveal CsCr3Sb5 to stand out as a promising platform for further exploration
into the effects of FBs near the Fermi level on kagome lattices, and their role
in emergent orders in bulk quantum materials.
