CRYSTAL-FIELD-EXCITATION PHONON COUPLING IN PB2SR2NDCU3O8+DELTA

We have observed a double-peak structure around 300 cm-1 in the Raman spectrum of Pb2Sr2NdCu3O8+delta single crystals. Polarization-dependen t measurements show clearly the B1g symmetry character of both peaks. Their dependence on temperature, isotopic substitution, and polarizati on can be interpreted by postulating a coupling between a crystal-fiel d (CF) excitation of the Nd3+ 4f electrons and the B1g phonon mode. We have fitted the spectra obtained at different temperatures in Pb2Sr2N dCu3O8+delta single crystals, and also earlier observations in NdBa2Cu 3O7, 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 ener gies of the phonon omega(Ph) and the CF excitation omega(CF), and a pa rameter kappa, which accounts for the temperature dependence of V. For NdBa2Cu3O7 the data reveal a self-energy anomaly of the B1g mode near T(c). We have also estimated the coupling constant V for Pb2Sr2NdCu3O 8 using experimental CF parameters from NdBa2Cu3O6 and structural info rmation of Pb2Sr2NdCu3O8. We find Absolute value of V = 21.7 cm-1, in good agreement with the experimental value of (23 +/- 3) cm-1 at 10 K.