Spin-Controlled Mott-Hubbard Bands in LaMnO3 Probed by Optical Ellipsometry

Spectral ellipsometry is used to determine the dielectric function of an untwinned crystal of ${\mathrm{L}\mathrm{a}\mathrm{M}\mathrm{n}\mathrm{O}}_{3}$ in the range 0.5--5.6 eV at temperatures $50\ensuremath{\le}T\ensuremath{\le}300\text{ }\text{ }\mathrm{K}$. A pronounced redistribution of spectral weight is found at the N\'eel temperature ${T}_{N}=140\text{ }\text{ }\mathrm{K}$. The anisotropy of the spectral weight transfer matches the magnetic ordering pattern. A superexchange model quantitatively describes spectral weight transfer induced by spin correlations. This analysis implies that the lowest-energy transitions around 2 eV are intersite $d$-$d$ transitions, and that ${\mathrm{L}\mathrm{a}\mathrm{M}\mathrm{n}\mathrm{O}}_{3}$ is a Mott-Hubbard insulator.