PHONON DENSITY-OF-STATES IN R2CUO4 AND SUPERCONDUCTING R1.85CE0.15CUO4(R = ND,PR)

Inelastic-neutron-scattering measurements of the generalized phonon density of states (GDOS) for the electron-doped superconductors Nd1.85Ce0.15CuO4 and Pr1.85Ce0.15CuO4 have been carried out with the use of both a filter-detector method and time-of-flight spectroscopy. A measurement of the GDOS for the insulating Pr2CuO4 was also done for comparison with that of Pr1.85Ce0.15CuO4, while a more limited set of data was obtained for Nd2CuO4. Comparison between the GDOS spectrum of the Nd1.85Ce0.15CuO4 with that of Pr1.85Ce0.15CuO4 shows differences in the shape of some structures in the GDOS spectrum. Furthermore, there are substantial changes in the GDOS spectrum of Pr1.85Ce0.15CuO4 as compared to that of Pr2CuO4, particularly in the energy regime between 33 and 63 meV. In the Nd1.85Ce0.15CuO4 system, where electron-tunneling measurements have been performed, there are similarities between the GDOS spectrum and the Eliashberg-coupling function alpha2F(omega) obtained from the tunneling data in the energy regions where comparisons can be made. To the extent that this comparison is valid, it suggests that phonons are to some extent involved in the electron-pairing mechanism in these electron-doped superconductor systems. We have also determined the ground-state crystal-field excitations associated with the rare-earth ions (Nd3+, Pr3+). The Nd3+ crystal-field levels are observed at energies of 13, 20.7, 26.4, and 93 meV in superconducting Nd1.85Ce0.15CuO4. For the Pr2CuO4 system, the Pr3+ crystal-field excitations are found at 18.6 and 87.9 meV. These peaks broaden and split when doped to form superconducting Pr1.85Ce0.15CuO4. The observed crystal-field levels in the above systems are in good agreement with those reported by other groups.