Abstract
We present a comprehensive ellipsometric study of an untwinned, nearly stoichiometric crystal in the spectral range 1.2–6.0 eV at temperatures . The complex dielectric response along and axes of the orthorhombic unit cell, and , is highly anisotropic over the spectral range covered in the experiment. The difference between and increases with decreasing temperature, and the gradual evolution observed in the paramagnetic state is strongly enhanced by the onset of -type antiferromagnetic long-range order at . In this study we focus on the analysis of excitations observed at high energy and show that the observed temperature changes of their spectral weight are opposite to those found for the lowest-energy gap excitation at . 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 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 electrons allowed us to estimate the effective intra-atomic Coulomb interaction, the Hund exchange coupling, and the Jahn-Teller splitting energy between orbitals in , as well as to extract experimental information concerning the type of orbital order in . This study identifies the lowest-energy optical transition at as an intersite transition whose energy is substantially reduced compared to that obtained from the bare intra-atomic Coulomb interaction.
7 More- Received 28 July 2009
DOI:https://doi.org/10.1103/PhysRevB.81.235130
©2010 American Physical Society