Hybridization gap in heavy fermion compounds.

We report the results of optical studies of new heavy fermion compounds YbFe(4)Sb(12) and CeRu(4)Sb(12). We show that these compounds, as well as several other heavy fermion materials with a nonmagnetic ground state, obey a universal scaling relationship between the quasiparticle effective mass m(*) and the magnitude of the energy gap Delta in the excitation spectrum. This result is in accord with the picture of hybridization of localized f-electron and free carrier states.