Einstein Modes in the Phonon Density of States of the Single-Filled Skutterudite Yb$_{0.2}$Co$_{4}$Sb$_{12}$

Measurements of the phonon density of states by inelastic neutron 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 ${\omega}_{E2} \approx$ 12.8 meV obtained from heat-capacity data. The emergence of local modes at intermediate transfer energies is 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.