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Introduction
In quantum materials, many distinct mechanisms can create flat electronic bands, which are realized when the kinetic energy of electrons (W) is smaller than the on-site Coulomb repulsion (U), i.e., U/W > 11,2. The kinetic energy can be quenched due to band folding (e.g., in moiré systems3) or quantum interference (e.g., in frustrated lattices, such as kagome and pyrochlore4), whereas the Coulomb repulsion can be boosted due to reduced dimensionality or strong electronic correlations in local-moment systems (e.g., in 3d-, 4f-, and 5f-based materials5). When flat bands occur near or at the Fermi level (EF), sought-after many-body phenomena are known to emerge6. Recent examples include the fractional Chern insulating states in moiré systems7,8 and odd-parity superconductivity in f-electron UTe29,10.
Recent developments also highlight the intriguing possibility of combining two distinct flat-band mechanisms in a single material. For example, in CuV2S4, topological flat bands are pinned to the Fermi level due to the cooperation between the large on-site Coulomb repulsion from vanadium 3d electrons and the quenched kinetic energy due to the pyrochlore lattice geometry11,12. In layered materials, distinct layer-dependent mechanisms may offer a route towards tunable interlayer coupling. In particular, the family of layered materials RM6X6 (R = rare earth, M = transition metal, X = post transition metal) has recently provided a unique platform to investigate the interplay between highly-localized moments from rare-earth layers and either magnetic or nonmagnetic transition-metal kagome layers, whose electronic structure hosts Dirac crossings, van Hove singularities, and flat bands13,14. For example, the magnetic exchange coupling between 3d and 4f electrons plays an important role in the ground state properties of RMn6Sn615,16, and the uniaxial anisotropy of Tb in TbMn6Sn6 has been argued to provide ideal conditions for the opening of a Chern insulating gap in the Dirac dispersion17.
Given that 5f wavefunctions are typically more extended than their 4f counterparts, here we investigate the hypothesis that the d-f hybridization and...