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The optical properties of KFe$_2$As$_2$ have been measured for lightpolarized in the a-b planes over a wide temperature and frequency range. Below$T^\ast\simeq 155$ K, where this material undergoes an incoherent-coherentcrossover, we observe a new coherent response emerging in the opticalconductivity. A spectral weight analysis suggests that this new feature arisesout of high-energy bound states. Below about $T_{\rm FL} \simeq 75$ K thescattering rate for this new feature is quadratic in temperature, indicating aFermi-liquid response. Theory calculations suggest this crossover is dominatedby the $d_{xy}$ orbital. Our results advocate for Kondo-type screening as themechanism for the orbital-selective incoherent-coherent crossover inhole-overdoped KFe$_2$As$_2$.

Observation of an Emergent Coherent State in the Iron-Based Superconductor KFe$_\mathbf{2}$As$_\mathbf{2}$