  Measurements of the phonon density of states by inelastic neutron
\emph{time-of-flight} scattering and specific heat measurements along with
first principles calculations, provide compelling evidence for the existence of
an Einstein oscillator ("\emph{rattler}") at ${\omega}_{E1} \approx$ 5.0 meV in
the filled skutterudite Yb$_{0.2}$Co$_{4}$Sb$_{12}$. Multiple dispersionless
modes in the measured density of states of Yb$_{0.2}$Co$_{4}$Sb$_{12}$ at
intermediate transfer energies (14 meV $\leq$ \emph{$\omega$} $\leq$ 20 meV)
are exhibited in both the experimental and theoretical \emph{density-of-states}
of the Yb-filled specimen. A peak at 12.4 meV is shown to coincide with a
second Einstein mode at \emph{$\omega_{E2} \approx$} 12.8 meV obtained from
heat capacity data. The local modes at intermediate transfer energies are
attributed to altered properties of the host CoSb$_{3}$ cage as a result of
Yb-filling. It is suggested that these modes are owed to a complementary
mechanism for the scattering of heat-carrying phonons in addition to the mode
observed at ${\omega}_{E1} \, \approx$ 5.0 meV. Our observations offer a
plausible explanation for the significantly-higher \textit{dimensionless
figures of merit} of filled skutterudites, compared to their parent compounds.

Materials Science;Other Condensed Matter