Signatures of Electronic Correlations in Optical Properties of LaFeAsO${}_{1-x}$F${}_x$

Abstract

Spectroscopic ellipsometry is used to determine the dielectric function of the superconducting LaFeAsO${}_{0.9}$F${}_{0.1}$ ($T_c$ = 27 K) and undoped LaFeAsO polycrystalline samples in the wide range 0.01-6.5 eV at temperatures 10 $\le T\le$ 350 K. The free charge carrier response in both samples is heavily damped with the effective carrier density as low as 0.040$\pm$0.005 electrons per unit cell. The spectral weight transfer in the undoped LaFeAsO associated with opening of the pseudogap at about 0.65 eV is restricted at energies below 2 eV. The spectra of superconducting LaFeAsO${}_{0.9}$F${}_{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.

4 pages, 4 figures, units in Fig.2 added

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