Antiferromagnetic Exchange, Hund's Coupling and the Origin of the Charge Gap in LaMnPO

Abstract: We present inelastic neutron scattering and magnetization measurements of the antiferromagnetic insulator LaMnPO that are well described by a Heisenberg spin model. These measurements are consistent with the presence of two-dimensional magnetic correlations up to a temperature T${max}$ $\approx$ 700 K >> T${N}$ = 375 K, the N\'eel temperature. Optical transmission measurements show the T = 300 K direct charge gap $\Delta$ = 1 eV has decreased only marginally by 500 K and suggest it decreases by only 10% at T${max}$. 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 onsite Hubbard U = 8 eV. These results show the direct charge gap in LaMnPO is rather insensitive to antiferromagnetic exchange coupling and instead is a result of the local physics governed by U and J$_{H}$.