Phonon density of states inR2CuO4and superconductingR1.85Ce0.15CuO4(R=Nd,Pr)

Abstract

Inelastic-neutron-scattering measurements of the generalized phonon density of states (GDOS) for the electron-doped superconductors ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4}$ and ${\mathrm{Pr}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4}$ 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 ${\mathrm{Pr}}_{2}$${\mathrm{CuO}}_{4}$ was also done for comparison with that of ${\mathrm{Pr}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4}$, while a more limited set of data was obtained for ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$. Comparison between the GDOS spectrum of the ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4}$ with that of ${\mathrm{Pr}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4}$ shows differences in the shape of some structures in the GDOS spectrum. Furthermore, there are substantial changes in the GDOS spectrum of ${\mathrm{Pr}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4}$ as compared to that of ${\mathrm{Pr}}_{2}$${\mathrm{CuO}}_{4}$, particularly in the energy regime between 33 and 63 meV.In the ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4}$ system, where electron-tunneling measurements have been performed, there are similarities between the GDOS spectrum and the Eliashberg-coupling function ${\mathrm{\ensuremath{\alpha}}}^{2}$F(\ensuremath{\omega}) obtained from the tunneling data in the energy regions where comparisons can be made. To the extent that this comparison is valid, it suggests that phonons are to some extent involved in the electron-pairing mechanism in these electron-doped superconductor systems. We have also determined the ground-state crystal-field excitations associated with the rare-earth ions (${\mathrm{Nd}}^{3+}$, ${\mathrm{Pr}}^{3+}$). The ${\mathrm{Nd}}^{3+}$ crystal-field levels are observed at energies of 13, 20.7, 26.4, and 93 meV in superconducting ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4}$. For the ${\mathrm{Pr}}_{2}$${\mathrm{CuO}}_{4}$ system, the ${\mathrm{Pr}}^{3+}$ crystal-field excitations are found at 18.6 and 87.9 meV. These peaks broaden and split when doped to form superconducting ${\mathrm{Pr}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4}$. The observed crystal-field levels in the above systems are in good agreement with those reported by other groups.