Optical Pseudogap from Iron States in Filled Skutterudites $A{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ ($A=\mathrm{Yb},\mathrm{Ca},\mathrm{Ba}$)

Optical investigations are presented of the filled skutterudites $A{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ with divalent cations $A=\mathrm{Yb},\mathrm{Ca},\mathrm{Ba}$. For each of these compounds a very similar pseudogap structure in the optical conductivity develops in the far-infrared spectral region at temperatures below 90 K. Highly accurate local-density approximation electronic band structure calculations can consistently explain the origin of the pseudogap structure generated largely by transition metal $3d$ states. In particular, a $4f$-conduction electron hybridization or strong correlations can be ruled out as origin for the pseudogap.

Figure 1

Reflectivity spectra of $A{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ at $T=4$ and 300 K. Inset shows the spectrum of ${\mathrm{BaFe}}_4{\mathrm{Sb}}_{12}$ at $T=300\mathrm{K}$ in the complete accessible spectral range.

Figure 2

Optical conductivity ${\sigma }_1(\omega )$ of $A{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ ($A=\mathrm{Ca},\mathrm{Ba},\mathrm{Yb}$) at $T=4,30,90,300\mathrm{K}$. Inset shows ${\sigma }_1(T)$ at $\hslash \omega =10\mathrm{meV}$ for ${\mathrm{BaFe}}_4{\mathrm{Sb}}_{12}$.

Figure 3

(a) Optical conductivity ${\sigma }_1(\omega ,T=4\mathrm{K})$ related to ${\sigma }_1(\omega ,T=90\mathrm{K})$. (b) Comparison of ${\sigma }_1(\omega )$ of ${\mathrm{CaFe}}_4{\mathrm{Sb}}_{12}$ and ${\mathrm{BaFe}}_4{\mathrm{Sb}}_{12}$ with optical phonons in ${\mathrm{CoSb}}_3$. Triangles indicate phonon structures in ${\sigma }_1(\omega )$ of ${\mathrm{CaFe}}_4{\mathrm{Sb}}_{12}$ and ${\mathrm{BaFe}}_4{\mathrm{Sb}}_{12}$. Arrow indicates a structure with no phonon counterpart.

Figure 4

Total DOS of $A{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ near the Fermi level. For $A=\mathrm{Yb}$ the scalar relativistic result is shown without spin orbit coupling (see text). For $A=\mathrm{Ca},\mathrm{Ba}$ the spin orbit coupling was included leading to an upward shift and broadening of the peak $\approx 50\mathrm{meV}$ above $E_{\mathrm{F}}$.