Fyzikální ústav Akademie věd ČR

Seminář Úterý, 02.08.2011 10:00

Přednášející: Natalia Kovaleva (Max-Planck-Institute for Chemical Physics of Solids, Dresden, Germany)
Místo: seminární místnost 117 ve Fyzikálním ústavu na Slovance
Jazyk: anglicky
Pořadatelé: Oddělení aplikované optiky

We present a comprehensive ellipsometric study of an untwinned, nearly stoichiometric LaMnO3 crystal in the spectral range 0.7-6.0 eV at temperatures 20 < T < 300 K. The complex dielectric response along b and c axes of the Pbnm orthorhombic unit cell, εb(ω) and єc(ω), is highly anisotropic over the spectral range covered in the experiment. The difference between εb(ω) and εc(ω) 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 TN=139.6 K. 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 eg electrons allowed us to estimate the effective intra-atomic Coulomb interaction, the Hund exchange coupling, and the Jahn-Teller splitting energy between eg 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.