Effect of pressure on coupled phonon and crystal-field excitations in NdBa2Cu3O7 and Pb2Sr2NdCu3O8

Abstract

We have investigated the effect of pressure on the coupling between phonon modes and Nd3+ crystal‐field excitations in the high‐Tc compounds NdBa2Cu3O7 and Pb2Sr2NdCu3O8. Polarized Raman spectra of single crystals were measured at different temperatures (10−–300 K) for pressures up to 14 GPa. For both compounds the relative intensity of the phonon–crystal‐field doublet near 300 cm−1 is essentially independent of pressure. From the pressure shift of the coupled mode frequencies at low temperature we deduce the pressure coefficients for the bare crystal field excitation energy and the phonon–crystal‐field doublet near 300 cm−1 is essentially independent of pressure. From the pressure shift of the coupled mode frequencies at low temperature we deduce the pressure coefficients for the bare crystal field excitation energy and the phonon–crystal‐field coupling constant, both of which increase with increasing pressure. The results are well reproduced by a point‐charge model calculation which is based on availabl...