Berlin 2008 – scientific programme
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TT: Fachverband Tiefe Temperaturen
TT 32: Correlated Electrons: Poster Session
TT 32.62: Poster
Thursday, February 28, 2008, 14:00–18:00, Poster B
Mott-Hubbard vs. charge-transfer type: optical conductivity of LaSrMnO4 — •A. Gössling, M.W. Haverkort, M. Benomar, H. Wu, D. Senff, T. Möller, M. Braden, J.A. Mydosh, and M. Grüninger — Institute of Physics II, University of Cologne, Germany
Using ellipsometry we study the optical conductivity of insulating LaSrMnO4 between 0.75 and 5.8 eV from 15 to 330 K. The layered structure gives rise to a pronounced anisotropy. Based on a local multiplet calculation we obtain (i) an excellent description of our data, (ii) a detailed peak assignment in terms of the multiplet splitting of Mott-Hubbard and charge-transfer (CT) bands, and (iii) effective electronic parameters, e.g. the on-site Coulomb repulsion Ueff=2.2 eV, the in-plane CT energy Δa=4.5 eV, and the crystal-field parameters (10 Dq=1.2 eV, Δeg=1.4 eV, and Δt2g=0.2 eV). The spectral weight of the lowest peak (at 1-2 eV) changes by a factor of 2 as a function of temperature, which can be attributed to the change of the nearest-neighbor spin-spin correlation function. Interpreting LaSrMnO4 effectively as a Mott-Hubbard insulator naturally explains this strong T dependence, the relative weight of the different peaks, and the pronounced anisotropy. From the transmittance we determine the onset of the optical gap Δopta = 0.4-0.45 eV at 15 K and 0.1-0.2 eV at 300 K. Our data show that the crystal-field splitting is too large to explain the anomalous T dependence of the c-axis lattice parameter by thermal occupation of excited crystal-field levels. Alternatively, we propose that a thermal population of the upper Hubbard band gives rise to the shrinkage of the c-axis lattice parameter.