Optical Absorption Spectra of Neodymium-Cerium Copper-Oxygen Compounds

Sharp optical absorption lines characteristic of Nd^{3+} have been observed in semi-conducting and metallic phases of (Nd,Ce) _2CuO_{rm 4-x} using a new technique of sensitive optical spectroscopy. Careful studies of samples with controlled levels of impurities have been performed to verify that the sharp lines observed are intrinsic to (Nd,Ce)_2CuO _{rm 4-x}. This is the first observation of sharp absorption lines in a metallic material. Four (Nd,Ce)_2CuO _{rm 4-x} samples were studied: bulk polycrystalline samples with cerium dopings of 0%, 4%, and 7.5%, and a single phase film with a doping of 7.5%. The latter samples were both metallic. The electronic transitions responsible for the sharp absorption lines and the atomic levels of neodymium involved have been identified. The spectra of the four (Nd,Ce)_2CuO_{ rm 4-x} samples show intensity variations which are correlated with the doping level and conductivity of the sample. Changes in the charge environment surrounding the neodymium atoms are responsible for the intensity variations observed.