  Inelastic neutron scattering experiments on poly crystalline sample of
heavy-fermion compound YbCo$_2$Zn$_{20}$ were carried out in order to obtain
microscopic insights on the ground state and its magnetic field response. At
zero field at 300 mK, inelastic response consists of two features: quasielastic
scattering and a sharp peak at 0.6 meV. With increasing temperature, a broad
peak comes up around 2.1 meV, whereas quasielastic response gets broader and
the peak at 0.6 meV becomes unclear. By applying magnetic field, the
quasielastic response exhibits significant broadening above 1 T, and the peak
at 0.6 meV is obscure under fields. The peaks in inelastic spectra and its
temperature variation can be ascribed to the suggested crystal-field model of
${{\Gamma}_6}$ - ${{\Gamma}_8}$ - ${{\Gamma}_7}$ with the overall splitting of
less than 3 meV. The observed quasielastic response and its rapid broadening
with magnetic field indicates that the heavy-electron state arises from the
ground state doublets, and are strongly suppressed by external field in
YbCo$_2$Zn$_{20}$.

Strongly Correlated Electrons