Signatures of electronic correlations in optical properties of LaFeAsO1-xFx

Spectroscopic ellipsometry is used to determine the dielectric function of superconducting LaFeAsO0.9F0.1 (Tc=27K) and undoped LaFeAsO polycrystalline samples in the wide range 0.01-6.5 eV at temperatures 10≤T≤350K. The charge carrier response in both samples is heavily damped. The spectral weight transfer in LaFeAsO associated with an opening of the pseudogap at about 0.65 eV is restricted to energies below 2 eV. The spectra of superconducting LaFeAsO0.9F0.1 reveal a significant transfer of spectral weight to a broad optical band above 4 eV with increasing temperature. Our data may imply that the electronic states near the Fermi surface are strongly renormalized due to electron-phonon and/or electron-electron interactions.