Low-energy Mott-Hubbard excitations in LaMnO3 probed by optical ellipsometry

We present a comprehensive ellipsometric study of an untwinned, nearly stoichiometric LaMnO 3 crystal in the spectral range 1.2–6.0 eV at temperatures 20≤T≤300 K . The complex dielectric response along b and c axes of the Pbnm orthorhombic unit cell, ε ̃ b (ν) and ε ̃ c (ν) , is highly anisotropic over the spectral range covered in the experiment. The difference between ε ̃ b (ν) and ε ̃ c (ν) increases with decreasing temperature, and the gradual evolution observed in the paramagnetic state is strongly enhanced by the onset of A -type antiferromagnetic long-range order at T N =139.6 K . In this study we focus on the analysis of excitations observed at high energy (∼4–5 eV) and show that the observed temperature changes of their spectral weight are opposite to those found for the lowest-energy gap excitation at ∼2 eV . We used a classical dispersion analysis to quantitatively determine the temperature-dependent optical spectral-weights shifts between low- and high-energy optical bands. Based on the observation of a pronounced spectral-weight transfer between both features upon magnetic ordering, they are assigned to high-spin and low-spin intersite d 4 d 4 ⇌d 3 d 5 transitions by Mn electrons. The anisotropy of the lowest-energy optical band and the spectral-weight shifts induced by antiferromagnetic spin correlations are quantitatively described by an effective spin-orbital superexchange model. An analysis of the multiplet structure of the intersite transitions by Mn e g electrons allowed us to estimate the effective intra-atomic Coulomb interaction, the Hund exchange coupling, and the Jahn-Teller splitting energy between e g orbitals in LaMnO 3 , as well as to extract experimental information concerning the type of orbital order in LaMnO 3 . This study identifies the lowest-energy optical transition at ∼2 eV as an intersite d-d transition whose energy is substantially reduced compared to that obtained from the bare intra-atomic Coulomb interaction.