Origin of the charge gap in LaMnPO

Abstract

We present high temperature inelastic neutron scattering and magnetic susceptibility measurements of the antiferromagnetic insulator LaMnPO that are consistent with the presence of two-dimensional magnetic correlations up to a temperature ${T}_{\text{max}}\ensuremath{\approx}700\phantom{\rule{0.28em}{0ex}}\text{K}\ensuremath{\gg}{T}_{N}=375$ K, the N\'eel temperature. Optical transmission measurements show the $T=300$ K direct charge gap $\ensuremath{\Delta}=1$ eV has decreased only marginally by 500 K and suggest it decreases by only 10% at ${T}_{\text{max}}$. Density functional theory and dynamical mean-field theory calculations reproduce a direct charge gap in paramagnetic LaMnPO only when a strong Hund's coupling ${J}_{H}=0.9$ eV is included, as well as on-site Hubbard $U=8$ eV. Our results show that LaMnPO is a Mott-Hund's insulator, in which the charge gap is rather insensitive to antiferromagnetic exchange coupling.