Optical properties of single-crystal La2CuO4+delta

The c-axis and ab-plane optical reflectance of an electrochemically oxidized single crystal of La2CuO4+delta (T-c similar to 40 K; delta similar to 0.12) have been measured over a wide frequency range (30-40 000 cm(-1)) and at temperatures between 10 and 300 K. The ab-plane optical conductivity shows metallic behavior at low frequency with weak phonon features whereas the c-axis spectrum is typical of an insulator, showing only phonons. Estimates of the carrier density and the oxygen concentration suggest that each excess oxygen contributes just over one mobile hole. The ab-plane midinfrared conductivity obtained when the wave vector q of the light is perpendicular to the CuO2 planes in the sample exhibits structure that is absent when q is parallel to the basal planes. The ab-plane conductivity shows a strong departure from Drude metallic response; fits to midinfrared models with frequency-dependent damping require also an additional bound-carrier contribution. A superconducting condensate is evident in the low-frequency ab-plane data; the oscillator strength of this condensate contains about 18% of the doping induced or 80% of the free carrier spectral weight in the normal state. Finally, an anomalous temperature dependence is found in the visible/ultraviolet region of the spectrum.