Magnetodynamics of Heavy Fermion YbCo$_{2}$Zn$_{20}$ and YbFe$_{2}$Zn$_{20}$

We have performed inelastic neutron scatting experiments on the recently discovered heavy Fermion systems YbCo$_{2}$Zn$_{20}$ and YbFe$_{2}$Zn$_{20}$. The magnetic excitation spectrum demonstrates that YbCo$_{2}$Zn$_{20}$ exhibits crystal field dynamics with an overall splitting 2.72 meV. Despite this, the splitting between the ground state and first excited state is large enough such that the large low temperature electronic specific heat is not due to crystal field entropy and hence YbCo$_{2}$Zn$_{20}$ is a truly heavy Fermion material. YbFe$_{2}$Zn$_{20}$ exhibits significantly different spin dynamics. At 200 K the scattering is quasielastic and as the temperature is lowered the spectral weight is progressively shifted to finite energy transfers. This temperature dependence is characteristic of an intermediate valence system with an unusually low Kondo temperature of $\sim $50 K.