Optical studies of single-crystal ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$

Optical reflectivity spectra of ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ and ${\mathrm{Nd}}_{1.8}$ ${\mathrm{Ce}}_{0.2}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ single crystals for the polarization E⊥c have been investigated over an energy range from 0.005 to 6 eV. Four ${\mathit{E}}_{\mathit{u}}$ intralayer phonons are observed for ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ at 121, 301, 347, and 507 ${\mathrm{cm}}^{\mathrm{-}1}$, in good agreement with preliminary shell-model-calculation results. The oscillator strength of a broad mid-ir band is found to depend on both oxygen deficiency and cerium doping. The far-ir optical conductivity is fit to both the recently proposed marginal-Fermi-liquid model (MFL) and the modified Drude-Lorentz model (MDL). A notch in the far-ir conductivity is observed at 24 meV, in good agreement with the MFL model. Values for the plasma frequency obtained are quite low and suggest that very large mass enhancement exists for the free carriers in these materials. A clear downshift of oscillator strength from (1–2.3)-eV charge-transfer excitation to the mid-ir band is also observed.