From Hund's insulator to Fermi liquid: Optical spectroscopy study of K doping in BaMn2As2

We present optical transmission measurements that reveal a charge gap of 0.86 eV in the local-moment antiferromagnetic insulator BaMn2As2, an order of magnitude larger than previously reported. Density functional theory plus dynamical mean-field theory (DFT+DMFT) calculations correctly reproduce this charge gap only when a strong Hund's coupling is considered. Thus, BaMn2As2 is a member of a wider class of Mn pnictide compounds that are Mott-Hund insulators. We also present optical reflectance for metallic 2%-K-doped BaMn2As2 that we use to extract the optical conductivity at different temperatures. The optical conductivity σ1(ω) exhibits a metallic response that is well described by a simple Drude term. Both σ(ω→0,T) and ρ(T) exhibit Fermi-liquid temperature dependencies. From these measurements, we argue that a more strongly correlated Hund-metal version of the parent compounds of the iron pnictide superconductors has not yet been realized by doping this class of Hund insulators.