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

TL;DR

This study uses spectroscopic ellipsometry to explore the optical properties of LaFeAsO and its fluorine-doped superconductor, revealing signatures of strong electronic correlations and spectral weight transfer related to pseudogap formation and superconductivity.

Contribution

It provides detailed optical spectra of LaFeAsO and LaFeAsO$_{0.9}$F$_{0.1}$, highlighting the impact of electronic correlations on their optical responses and spectral weight redistribution.

Findings

Heavy damping of free charge carriers in both samples

Spectral weight transfer associated with pseudogap in undoped LaFeAsO

Spectral weight transfer to high-energy optical band in superconducting sample

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 $\leq T \leq$ 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.