The $c$-axis and $\mathrm{ab}$-plane optical reflectance of an electrochemically oxidized single crystal of ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_{4+\delta }$ ($T_c\sim 40$ K; $\delta \sim 0.12$) have been measured over a wide frequency range (30-40 000 ${\mathrm{cm}}^{-1\mathrm{}}$) and at temperatures between 10 and 300 K. The $\mathrm{ab}$-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 $\mathrm{ab}$-plane midinfrared conductivity obtained when the wave vector q of the light is perpendicular to the Cu${\mathrm{O}}_2$ planes in the sample exhibits structure that is absent when q is parallel to the basal planes. The $\mathrm{ab}$-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 $\mathrm{ab}$-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.

• Received 30 May 1995

DOI:https://doi.org/10.1103/PhysRevB.52.15485