Doping evolution of the phonon density of states and electron-lattice interaction in Nd 2- x Ce x CuO 4+δ

We use inelastic neutron scattering to study the evolution of the generalized phonon density of states (GDOS) of the n-type high-T-c superconductor Nd2-xCexCuO4+delta, from the half-filled Mott-insulator (x=0) to the T-c=24 K superconductor (x=0.15). Upon doping the CuO2 planes in Nd2CuO4+delta with electrons by Ce substitution, the most significant change in the GDOS is the softening of the highest phonon branches associated with the Cu-O bond stretching and out-of-plane oxygen vibration modes. However, most of the softening occurs within the first few percent of Ce doping, and is not related to the electron-doping-induced nonsuperconducting-superconducting transition at xapproximate to0.12. These results suggest that the electron-lattice coupling in the n-type high-T-c superconductors is weaker than that in the p-type materials.