Fragility of local moments against hybridization with flat bands

Abstract

The Kondo screening of a localized magnetic moment crucially depends on the spectral properties of the electronic bath to which it is coupled. Unlike textbook examples, realistic systems as well as dynamical mean-field theory of correlated lattice models force us to explicitly consider sharp features in the hybridization function near the Fermi energy. A case currently under the spotlight is twisted bilayer graphene, where the hybridization function of the heavy-fermion-like bands to the itinerant ones displays a divergence. We clarify how this impacts the screening mechanisms by means of a toy model with a tunable $\delta$-peak in the hybridization function, superimposed to a regular part. Our analysis unveils an unexpectedly big impact on the Kondo screening already for a parametrically small weight of the flat band in the bath.