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

Spectroscopic ellipsometry is used to determine the dielectric function of superconducting ${\mathrm{LaFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$ ($T_c=27\mathrm{K}$) and undoped LaFeAsO polycrystalline samples in the wide range 0.01–6.5 eV at temperatures $10\le T\le 350\mathrm{K}$. 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 ${\mathrm{LaFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$ 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.

Figure 1

(a) Imaginary and (b) real parts of the dielectric function of ${\mathrm{LaFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$ measured at $T=300\mathrm{K}$ (blue heavy lines) and represented by the total contribution [(a) shaded area, (b) red line] of separate Lorenzian bands determined by the dispersion analysis, as described in the text. The dark gray-shaded area shows the free charge carrier contribution to ${\epsilon }_2(\omega )$.

Figure 2

(a) Optical conductivity of the ${\mathrm{LaFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$ (black line) and LaFeAsO (gray line) measured at $T=300$. Low-energy part of the spectra for (b) undoped and (c) F-doped samples at 300 (red line) and 10 K (blue line). The dark gray lines and shaded areas in (b) and (c) show the low-energy tail from the interband transitions and the remaining intraband charge carrier contribution, respectively. Arrows mark the pseudogap opening at ${\omega }_{\mathrm{PG}}\approx 0.65\mathrm{eV}$ (${\omega }_{\mathrm{PG}}^{*}\approx 0.16\mathrm{eV}$).

Figure 3

Difference spectra (a) and (b) $\Delta {\sigma }_1(T,\omega )={\sigma }_1(T,\omega )-{\sigma }_1(10\mathrm{K},\omega )$ and (c) and (d) $\Delta {\epsilon }_1(T,\omega )={\epsilon }_1(T,\omega )-{\epsilon }_1(10\mathrm{K},\omega )$ for LaFeAsO (upper panels) and ${\mathrm{LaFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$ (lower panels). (e) and (f) Temperature dependence of ${\sigma }_1(\omega )$ and ${\epsilon }_1(\omega )$ measured at representative energies 1.2, 2.5, and 4.8 eV as marked by green, blue, and red arrows in (a)–(d), respectively. Cooling-down and warming-up curves are consistent and averaged.

Figure 4

Spectral weight changes $\Delta N_{\mathrm{eff}}(\omega )=N_{\mathrm{eff}}(\omega ,300\mathrm{K})-N_{\mathrm{eff}}(\omega ,10\mathrm{K})$ in ${\mathrm{LaFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$ (black line) and LaFeAsO (gray line). The black arrow marks the same energy ${\omega }_{\mathrm{PG}}\approx 0.65\mathrm{eV}$ as in Figs. 2b, 2c, 3a, 3b.