Infrared optical conductivity of the Nd-Ce-Cu-O system

The reflectivity of seven ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_4$ single crystals has been measured at room temperature between 0.005 and 3 eV for 0≤x≤0.21. At x=0, four transverse and longitudinal ${\mathit{E}}_{\mathit{u}}$ phonon modes have been determined. Two bands have been also identified at 1.6 eV and at 2.8 eV, which lose their intensity with doping. Such intensity is transferred to the well-known Drude and midinfrared contributions, and to a band at ∼0.1 eV, which can be attributed to oxygen vacancies. At the insulator-to-metal transition, the Drude term abruptly increases, the 1.6-eV band vanishes. An overall strong similarity with the closing of the optical gap in highly doped semiconductors, is found. The reflectivity of a reduced sample with x=0.07 suggests that the midinfrared band may not originate from oxygen vacancy states. Present results are compared with predictions of different theoretical models.