High-pressure Raman study of the coupling of crystal-field excitations to phonons in Nd-containing cuprates.

Abstract

We have investigated the effect of pressure (up to 14 GPa) on the coupling between a ${\mathrm{Nd}}^{3+}$ crystal-field excitation and a ${\mathrm{CuO}}_{2}$-plane oxygen phonon near 300 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the two high-temperature superconductors ${\mathrm{NdBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and ${\mathrm{Pb}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{NdCu}}_{3}$${\mathrm{O}}_{8}$. Raman spectra of single crystals as well as ceramic samples with $^{16}\mathrm{O}$ and $^{18}\mathrm{O}$ isotopes were measured at different temperatures (10--300 K). From the low-temperature pressure shift of the coupled phonon-crystal-field doublet we deduce the pressure coefficients for the bare crystal-field excitation energy and the phonon-crystal-field coupling constant, both of which increase with increasing pressure. For ${\mathrm{NdBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ the experimental results are compared to a point-charge model calculation of the pressure dependence of the crystal-field levels and coupling constant.