Crystal-field-excitation--phonon coupling in Pb2Sr2NdCu3O8+δ

We have observed a double-peak structure around 300 cm-1 in the Raman spectrum of Pb2Sr2NdCu3O8+δ single crystals. Polarization-dependent measurements show clearly the B1g symmetry character of both peaks. Their dependence on temperature, isotopic substitution, and polarization can be interpreted by postulating a coupling between a crystal-field (CF) excitation of the Nd3+ 4f electrons and the B1g phonon mode. We have fitted the spectra obtained at different temperatures in Pb2Sr2NdCu3O8+δ single crystals, and also earlier observations in NdBa2Cu3O7, with a theoretical model taking the occupation of CF excitations and phonons into account. From these fits we derive the CF-excitation--phonon coupling constant V for the two materials, the unperturbed energies of the phonon ωPh and the CF excitation ωCF, and a parameter κ, which accounts for the temperature dependence of V. For NdBa2Cu3O7 the data reveal a self-energy anomaly of the B1g mode near Tc. We have also estimated the coupling constant V for Pb2Sr2NdCu3O8 using experimental CF parameters from NdBa2Cu3O6 and structural information of Pb2Sr2NdCu3O8. We find |V|=21.7 cm-1, in good agreement with the experimental value of (23±3) cm-1 at 10 K