Accessing quasi-flat $\mathit{\text{f}}$-bands to harvest large Berry curvature in NdGaSi

In typical rare-earth lanthanide compounds, the localized 4$\mathit{\text{f}}$-electrons have a weak effect on the electrical conduction, limiting their influence on the Berry curvature and, hence, the intrinsic anomalous Hall effect. A comprehensive study of the magnetic, thermodynamic, and transport properties of single-crystalline NdGaSi, guided by first-principles calculations, reveals a ferromagnetic ground state that induces a splitting of quasi-flat 4$\mathit{\text{f}}$ electronic bands and positions them near the Fermi energy. The observation of an extraordinarily large intrinsic anomalous Hall conductivity of 1165 ${\mathrm{\Omega }}^{-1}$ cm${}^{-1}$ implies the direct involvement of localized states in the generation of non-trivial band crossings around the Fermi energy. The angle-resolved photoemission spectroscopy measurements provide direct evidence of non-trivial crossing of the 4$\mathit{\text{f}}$-bands with dispersive bands. These results are remarkable when compared to ferrimagnetic NdAlSi, which differs only in a non-magnetic atom (a change in the principal quantum number $\mathit{\text{n}}$ of the outer $\mathit{\text{p}}$orbital) with the same number of valence electrons and does not exhibit any measurable anomalous Hall conductivity.