Rare-earth energy levels in Nd2CuO4, Pr2CuO4, and the electron superconductor Pr1.85Ce0.15CuO4

Abstract

The magnetic excitation spectra of the electron superconductor ${\mathrm{Pr}}_{1.85}$ ${\mathrm{Ce}}_{0.15}$ ${\mathrm{CuO}}_4$ and related parent compounds ${\mathrm{Pr}}_2$ ${\mathrm{CuO}}_4$ and ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_4$ have been determined by inelastic neutron scattering. We observe crystal-field transitions up to about 100 meV within the ${\mathrm{Pr}}^{3+}$ ${}_{}{}^3{}_{}{}^{}$ ${\mathit{H}}_4$ and ${\mathrm{Nd}}^{3+}$ ${}_{}{}^4{}_{}{}^{}$ ${\mathit{I}}_{9/2}$ Russell-Saunders ground multiplets in these materials. We find that a crystal-field treatment of the ${\mathrm{Pr}}^{3+}$ and ${\mathrm{Nd}}^{3+}$ ions can adequately explain the observed excitation spectra for both ${\mathrm{Pr}}_2$ ${\mathrm{CuO}}_4$ and ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_4$. The obtained crystal-field parameters are close to values estimated from a superposition-model calculation and Mössbauer data. The ${\mathrm{Pr}}^{3+}$ and ${\mathrm{Nd}}^{3+}$ wave functions are found to have significant (≊20%) admixture of some states belonging to higher J multiplets. The calculated contributions to the susceptibility of ${\mathrm{Pr}}_2$ ${\mathrm{CuO}}_4$ and ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_4$ from the crystal-field states agree well with experiments. The observed crystal-field transitions in the superconductor ${\mathrm{Pr}}_{1.85}$ ${\mathrm{Ce}}_{0.15}$ ${\mathrm{CuO}}_4$ shift to slightly lower energies and are broadened significantly relative to the present compound due to chemical disorder from Ce doping.