Alexander Boris

Spectroscopic ellipsometry is used to determine the dielectric function of the superconducting LaFeAsO_0.9F_0.1 (Tc = 27 K) and undoped LaFeAsO polycrystalline samples in the wide range 0.01-6.5 eV at temperatures 10 ≤ T ≤ 350 K. The free charge carrier response in both samples is heavily damped with the effective carrier density as low as 0.040±0.005 electrons per unit cell. The spectral weight transfer in the undoped LaFeAsO associated with opening of the pseudogap at about 0.65 eV is restricted at energies below 2 eV. The spectra of superconducting LaFeAsO_0.9F_0.1 reveal a significant transfer of the 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.

http://arXiv.org/abs/0806.1732