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

TL;DR

This study investigates the magnetic and electronic properties of LaMnPO, revealing that its charge gap is primarily determined by local interactions like Hund's coupling and Hubbard U, rather than magnetic exchange.

Contribution

It demonstrates that the charge gap in LaMnPO is mainly influenced by Hund's coupling and Hubbard U, not antiferromagnetic exchange, supported by neutron scattering, optical, and theoretical calculations.

Findings

Magnetic correlations persist well above the Néel temperature.

The charge gap decreases marginally with temperature, remaining close to 1 eV.

Theoretical models require strong Hund's coupling to reproduce the observed charge gap.

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$_{H}$.