HIGH-PRESSURE RAMAN-STUDY OF THE COUPLING OF CRYSTAL-FIELD EXCITATIONS TO PHONONS IN ND-CONTAINING CUPRATES

We have investigated the effect of pressure (up to 14 GPa) an the coup ling between a Nd3+ crystal-field excitation and a CuO2-plane oxygen p honon near 300 cm(-1) in the two high-temperature superconductors NdBa 2Cu3O7 and Pb2Sr2NdCu3O8. Raman spectra of single crystals as well as ceramic samples with O-16 and O-18 isotopes were measured at different temperatures (10-300 K). From the low-temperature pressure shift of t he coupled phonon-crystal-held doublet we deduce the pressure coeffici ents for the bare crystal-field excitation energy and the phonon-cryst al-held coupling constant, both of which incr ease with increasing pre ssure. For NdBa2Cu3O7 the experimental results are compared to a point -charge model calculation of the pressure dependence of the crystal-fi eld levels and coupling constant.