Observation of an emergent coherent state in the iron-based superconductor KFe2As2

Abstract

The ab-plane optical properties of KFe2As2 single crystals have been measured over a wide temperature and frequency range. Below T ∗ 155 K, where this material undergoes an incoherent-coherent crossover, a new coherent response emerges in the optical conductivity. A spectral weight analysis suggests that this feature originates from high-energy bound states. Below 75 K the scattering rate for this feature is quadratic in temperature, indicating a Fermi-liquid response. Theoretical calculations suggest this crossover is dominated by the dxy orbital. Our results indicate Kondo-type screening is the likely mechanism for the incoherent-coherent crossover in hole-overdoped KFe2As2.