Optical absorption lines characteristic of ${\mathrm{Nd}}^{3+}$ have been observed in semiconducting and metallic phases of (Nd,Ce${)}_2$${\mathrm{CuO}}_{4\mathrm{-}\mathit{x}}$ using a sensitive optical spectroscopic technique. Samples with controlled levels of impurities and controlled cerium content have been studied to verify that the lines observed are intrinsic to (Nd,Ce${)}_2$${\mathrm{CuO}}_{4\mathrm{-}\mathit{x}}$. Sharp absorption lines in the visible spectrum of these metallic materials are observed. The materials exhibit dichroism characteristic of a two-dimensional conductor. Both this dichroism and the oscillator strengths of certain transitions show variations that are correlated with the doping level, conductivity, and degree of orientation of the samples. It is believed that these variations are the result of changes, induced by the doping, in the charge distribution near the Nd ions and the indices of refraction of the material.

• Received 5 August 1991

DOI:https://doi.org/10.1103/PhysRevB.46.611