COUPLING OF PHONONS TO CRYSTAL-FIELD EXCITATIONS IN NDBA2CU3O7-DELTA

We demonstrate the existence of a coupled elementary excitation involving a phonon and a Nd3+ 4f-electron crystal-field excitation in the high-T(c) superconductor NdBa2Cu3O7-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 cm-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.