Infrared and magneto-optical studies of heavy fermion skutterudites YbFe$_4$Sb$_{12}$ and CeRu$_4$Sb$_{12}$

We will report infrared and magneto-optical results on two heavy fermion skutterudites YbFe$_4$Sb$_{12}$ and CeRu$_4$Sb$_{12}$. Detailed temperature dependence of infrared spectra will be presented for both compounds. In addition, magneto-transmission measurements on YbFe$_4$Sb$_{12}$ in magnetic field as high as 33 Tesla, and magneto-reflection measurements on CeRu$_4$Sb$_{12} $ in 17 Tesla field will be reported. The results reveal suppression of heavy fermion state with magnetic field, and recovery of a more conventional metallic state. In particular, the effective mass of charge carriers is gradual diminished. On the other hand, hybridization gap is much more insensitive to the application of magnetic field.