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

Abstract

The heavy fermion (HF) state of d -electron systems is of great current interest since it exhibits various exotic phases and phenomena that are reminiscent of the Kondo effect in f -electron HF systems. Here, we present a combined infrared spectroscopy and first-principles band structure calculation study of the 3 d -electron HF compound YFe 2 Ge 2 . The infrared response exhibits several charge-dynamical hallmarks of HF and a corresponding scaling behavior that resemble those of the f -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 ( Δ ∼ 50 meV) and a strongly enhanced quasiparticle effective mass. Moreover, the temperature dependence of the infrared response indicates a crossover around T ∗ ∼ 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 2 Ge 2 is distinct from the Kondo scenario of the f -electron HF compounds and even from that of the d -electron iron-arsenide superconductor KFe 2 As 2 . For the latter, the HF state is driven by orbital-selective correlations due to a strong Hund’s coupling. Instead, for YFe 2 Ge 2 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 3 d and Y 4 d electrons, and electron correlations. This highlights that rather different mechanisms can be at the heart of the HF state in d -electron systems.