Heavy fermions in frustrated Hund's metal with portions of incipient flat bands

Flat bands induced by destructive interference of hoppings in frustrated lattices such as kagome metals, have been extensively studied in recent years. However, such flat bands usually appear in small portions of the Brillouin zone and are away from the Fermi level in the noninteracting limit (dubbed as ``incipient''). Whether such incipient flat-band portions can induce very strong electron correlations is an open question. Recently, such incipient flat bands are found in frustrated kagome CsCr₃Sb₅ and triangular CrB₂, which show moderately heavy-fermion behaviors and unconventional superconductivity. Here, by density functional theory plus dynamical mean-field theory calculations, we show that both compounds are typical Hund's metals. The incipient flat-band portions induce dips in hybridization functions, which further enhance the Kondo-like effect of Hund's metal, resulting in moderately heavy-fermion behaviors. This explains the origin of the heavy-fermion behaviors and pave the way for understanding the unconventional superconductivity in CsCr₃Sb₅ and CrB₂. Our work demonstrates a flexible route for generating d-electron heavy fermions and for inducing unconventional superconductivity based on frustrated Hund's metals with portions of incipient flat bands. A large family of materials with kagome, triangular, and pyrochlore lattices consisting of Cr (d^4), Ru (d^4), and Fe (d^6) ions are promising candidates.