Coupling of phonons to crystal-field excitations in ${\mathrm{NdBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

We demonstrate the existence of a coupled elementary excitation involving a phonon and a ${\mathrm{Nd}}^{3+}$ 4f-electron crystal-field excitation in the high-${\mathit{T}}_{\mathit{c}}$ superconductor ${\mathrm{NdBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. It explains in a natural way the recent observation of a double-peak structure in the Raman spectra as well as in the inelastic neutron scattering spectra near 300 ${\mathrm{cm}}^{\mathrm{-}1}$ at low temperature. Our assignment is supported by measurements of the dependence of this feature on temperature, isotopic substitution, and polarization. A model based on strong coupling of extended phonon states to localized crystal-field excitations describes the observed behavior well.