Iron-Based Heavy Quasiparticles in SrFe$_{4}$Sb$_{12}$: An Infrared   Spectroscopic Study
  Temperature-dependent infrared reflectivity spectra of SrFe$_{4}$Sb$_{12}$ has been measured. A renormalized Drude peak with a heavy effective mass and a pronounced pseudogap of 10 meV develops in the optical conductivity spectra at low temperatures. As the temperature decreases below 100 K, the effective mass ($m^{*}$) rapidly increases, and the scattering rate ($1/\tau$) is quenched. The temperature dependence of $m^{*}$ and $1/\tau$ indicates that the hybridization between the Fe 3d spins and the charge carriers plays an important role in determining the physical properties of SrFe$_{4}$Sb$_{12}$ at low temperatures. This result is the clear evidence of the iron-based heavy quasiparticles. 
Shin-ichi Kimura, Takafumi Mizuno, Hojun Im, Katsuyuki Hayashi, Eiichi   Matsuoka, Toshiro Takabatake