Unraveling the origin of Kondo-like behavior in the 3 <i>d</i> -electron heavy-fermion compound YFe ₂ Ge ₂

The heavy fermion (HF) state of [Formula: see text]-electron systems is of great current interest since it exhibits various exotic phases and phenomena that are reminiscent of the Kondo effect in [Formula: see text]-electron HF systems. Here, we present a combined infrared spectroscopy and first-principles band structure calculation study of the [Formula: see text]-electron HF compound YFe[Formula: see text]Ge[Formula: see text]. The infrared response exhibits several charge-dynamical hallmarks of HF and a corresponding scaling behavior that resemble those of the [Formula: see text]-electron HF systems. In particular, the low-temperature spectra reveal a dramatic narrowing of the Drude response along with the appearance of a hybridization gap ([Formula: see text] 50 meV) and a strongly enhanced quasiparticle effective mass. Moreover, the temperature dependence of the infrared response indicates a crossover around [Formula: see text] 100 K from a coherent state at low temperature to a quasi-incoherent one at high temperature. Despite of these striking similarities, our band structure calculations suggest that the mechanism underlying the HF behavior in YFe[Formula: see text]Ge[Formula: see text] is distinct from the Kondo scenario of the [Formula: see text]-electron HF compounds and even from that of the [Formula: see text]-electron iron-arsenide superconductor KFe[Formula: see text]As[Formula: see text]. For the latter, the HF state is driven by orbital-selective correlations due to a strong Hund's coupling. Instead, for YFe[Formula: see text]Ge[Formula: see text] the HF behavior originates from the band flatness near the Fermi level induced by the combined effects of kinetic frustration from a destructive interference between the direct Fe-Fe and indirect Fe-Ge-Fe hoppings, band hybridization involving Fe [Formula: see text] and Y [Formula: see text] electrons, and electron correlations. This highlights that rather different mechanisms can be at the heart of the HF state in [Formula: see text]-electron systems.