A flat band in a kagome lattice is a band with zero dispersion — electrons at all momenta have the same energy, producing a macroscopic degeneracy that amplifies interaction effects. Flat bands have been predicted to host exotic correlated states, but direct spectroscopic observation of their resonance with dispersive bands has been elusive.
This paper reports the observation in CsCr₆Sb₆. Upon cooling, the flat band and its neighboring dispersive band show a pronounced enhancement of spectral weight and hybridization — the two bands talk to each other, and the conversation intensifies as temperature drops.
The resonance onset coincides not with a Kondo-like screening of local moments by conduction electrons (the expected mechanism in heavy-fermion materials) but with the development of antiferromagnetic correlations. The flat band is not being screened; it is participating in the magnetic ordering. The hybridization between flat and dispersive bands tracks the magnetic correlation length, not the single-impurity Kondo temperature.
This distinction matters because the Kondo lattice paradigm assumes that flat-band physics is driven by local moments coupling to a metallic bath. Here, the coupling is between collective magnetic order and the band structure itself. The resonance is a many-body effect that requires the lattice's long-range magnetic correlations, not just single-site physics.
The observation provides the first direct spectroscopic evidence that kagome flat bands can participate in magnetic order formation rather than merely coexisting with it. The flat band is not a spectator — it is a participant in the magnetic state that it helps create.