Observation of a d-orbital flat band near the Fermi level realizing a Kondo lattice in a van der Waals material

Heavy fermions are commonly observed in f-electron Kondo systems. Several d-electron materials have been proposed as candidates for heavy fermion systems described by similar Kondo physics. In such systems, the flat d-orbital band near the Fermi energy (EF) is considered to play a role analogous to that of the f-orbital band. However, the near-EF flat band that induces Kondo coupling in a d-electron system has not been observed so far. Here, we investigate the electronic structure of a van der Waals ferromagnet Fe5−xGeTe2 with a transition temperature of about 300 K using angle-resolved photoemission spectroscopy. Our data demonstrates flat bands adjacent to the EF in both quenched and slow-cooled samples. In the slow-cooled sample, we find two flat bands with quasiparticle peaks that develop below a specific temperature of $${T}^{{\prime} }$$ (~100 K). Most importantly, one of these two flat bands develops an additional coherent spectral peak with heavy mass below 40 K, indicating that another transition occurs well below $${T}^{{\prime} }$$. Around this temperature (~40 K), we find a transition from the non-Fermi-liquid to the Fermi-liquid state and a suppression of magnetization. Furthermore, STM measurements revealed a Fano line shape, signifying a resonant interaction between localized and itinerant channels. Our data consistently indicate that Fe5−xGeTe2 forms a Kondo lattice in both slow-cooled and quenched samples. Heavy fermions are typically associated with f-electron Kondo systems but have been proposed to play a role also in d-electron systems, despite the observation of a flat d-orbital band near the Fermi level is elusive. Here, the authors use angle-resolved photoemission spectroscopy to reveal flat bands near the Fermi level in Fe5−xGeTe2, demonstrating Kondo physics behavior and a transition from non-Fermi-liquid to a heavy mass Fermi-liquid state.