Correlation, doping, and interband effects on the optical conductivity of iron superconductors

Electronic interactions in multiorbital systems lead to nontrivial features in the optical spectrum. In iron superconductors, the Drude weight is strongly suppressed with hole-doping. We discuss why the common association of the renormalization of the Drude weight with that of the kinetic energy, used in single-band systems, does not hold in multiorbital systems. This applies even in a Fermi liquid description when each orbital is renormalized differently, as happens in iron superconductors. We estimate the contribution of interband transitions at low energies. We show that this contribution is strongly enhanced by interactions and dominates the coherent part of the spectral weight in hole-doped samples at frequencies currently used to determine the Drude weight.

Figure 1

Solid lines: Optical spectrum corresponding to an undoped ($n=6$) and a hole-doped system ($n=5.5$). Dotted line: Interband part of the optical spectrum for $n=6$. The $\delta$ functions that appear in the expressions for the optical conductivity have been broadened to Lorentzians with half-width $\Gamma =160{\mathrm{cm}}^{-1}$ to mimic a constant scattering rate. Only the coherent contribution is included; see the text.

Figure 2

Main figure: Doping dependence of $R_{\text{inter}}\left({\omega }_c\right)$, the ratio of the spectral weight coming from interband transitions to the total (coherent) spectral weight, for the nonrenormalized and renormalized cases. The optical conductivity is integrated up to a frequency cutoff ${\omega }_c=1500$ ${\mathrm{cm}}^{-1}$. A broadening factor $\Gamma =160{\mathrm{cm}}^{-1}$ is used. The interband contribution amounts to one-fifth of the total spectral weight in undoped compounds (six electrons per Fe, vertical dotted line) and dominates the coherent part in hole-doped samples. The inset shows the total coherent spectral weight ${\text{SW}}_{\text{coh}}\left({\omega }_c\right)$ and the interband part ${\text{SW}}_{\text{inter}}\left({\omega }_c\right)$ for the renormalized case as a function of doping.

Figure 3

(a) Drude weight corresponding to the noninteracting (top line) and interacting models (bottom). The Drude weight is strongly suppressed due to interactions, especially with increasing hole-doping. (b) Comparison among the doping-dependent orbital quasiparticle weights $Z_m$ (solid lines), the renormalized Drude weight $Z_D$, and the renormalized kinetic energy $Z_K$; see the text for discussion.