Hybridization gap in heavy fermion compounds

We report the results of optical studies of new heavy fermion compounds YbF e4Sb12 and CeRu4Sb12. We show that these compounds, as well as several othe r heavy fermion materials with a nonmagnetic ground state, obey a universal scaling relationship between the quasiparticle effective mass m* and the m agnitude of the energy gap Delta in the excitation spectrum. This result is in accord with the picture of hybridization of localized f-electron and fr ee carrier states. DOI: 10.1103/PhysRevLett.86.684.