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

We present a comprehensive ellipsometric study of an untwinned, nearly stoichiometric LaMnO3 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, (epsilon) over tilde (b)(nu) and (epsilon) over tilde (c)(nu), is highly anisotropic over the spectral range covered in the experiment. The difference between (epsilon) over tilde (b)(nu) and (epsilon) over tilde (c)(nu) increases with decreasing temperature, and the gradual evolution observed in the paramagnetic state is strongly enhanced by the onset of A-type antiferro-magnetic long-range order at T-N=139.6 K. In this study we focus on the analysis of excitations observed at high energy (similar to 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 similar to 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)reversible arrow 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 LaMnO3, as well as to extract experimental information concerning the type of orbital order in LaMnO3. This study identifies the lowest-energy optical transition at similar to 2 eV as an intersite d-d transition whose energy is substantially reduced compared to that obtained from the bare intra-atomic Coulomb interaction.