Raman study of coupled-phonon-crystal-field excitations in Nd1+xBa2-xCu3Oysingle crystals

We study effects due to Nd3+/Ba2+ ion substitution in Nd1+xBa2-xCu3Oy singl e crystals (0.00 less than or equal to x less than or equal to 0.33 and 6.0 less than or equal to y less than or equal to 7.0) using Raman scattering from coupled-phonon-crystal-field (CF) excitations. At low temperature the Big phonon [O(2)-O(3) out-of-phase vibration in the CuO2 planes] interacts with a nearby CF excitation of;the Nd3+ 4f electrons resulting in a double- peak structure. Systematic changes of the peak frequencies and their relati ve intensities occur as a function of x and y. Using a two-level model we c alculate from the Raman spectra the unrenormalized B-1g phonon and CF excit ation energies as well as the coupling constant. In three series of experim ents (y similar to 7.0, varying x; y similar to 6.0, varying x; x= 0, varyi ng y) we observe different but consistent changes of the superconductivity- induced B-1g phonon softenings with temperature. These changes suggest that the superconducting gap in the underdoped region continues to increase wit h respect to the phonon energy, even if T-c decreases; Ar room temperature the frequency of the B-1g phonon hardly changes with doping. We also observ e variations in the CF excitation energies that are discussed in terms of s tructural changes and charge transfer between reservoirs (e.g., in the chai ns) and the oxygen ions in the CuO2 planes. [S0163-1829(98)01345-9].