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TT: Fachverband Tiefe Temperaturen

TT 32: Correlated Electrons: Poster Session

TT 32.62: Poster

Donnerstag, 28. Februar 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 U_eff=2.2 eV, the in-plane CT energy Δ_a=4.5 eV, and the crystal-field parameters (10 Dq=1.2 eV, Δ_eg=1.4 eV, and Δ_t2g=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 Δ_opt^a = 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.