We present optical transmission measurements that reveal a charge gap of 0.86 eV in the local-moment antiferromagnetic insulator ${\mathrm{BaMn}}_2{\mathrm{As}}_2$, an order of magnitude larger than previously reported. Density functional theory plus dynamical mean-field theory ($\text{DFT}+\text{DMFT}$) calculations correctly reproduce this charge gap only when a strong Hund's coupling is considered. Thus, ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ 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 ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ that we use to extract the optical conductivity at different temperatures. The optical conductivity ${\sigma }_1\left(\omega \right)$ exhibits a metallic response that is well described by a simple Drude term. Both $\sigma (\omega \to 0,T)$ and $\rho \left(T\right)$ 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.

• Received 31 May 2015

DOI:https://doi.org/10.1103/PhysRevB.92.115142