Optical spectra of La_2-xSr_xCuO₄: Effect of carrier doping on the electronic structure of the CuO₂ plane

Optical reflectivity spectra are studied for single crystals of the prototypical high-T_c system La_2-xSr_xCuO₄ over a wide compositional range 0≤x≤0.34, which covers insulating, superconducting, and normal metallic phases. The measurements are made at room temperature over an energy range from 0.004 to 35 eV for the polarization parallel to the CuO₂ planes. They are also extended to the perpendicular polarization to study anisotropy and to discriminate the contribution from the CuO₂ plane. The present study focuses on the x dependence of the optical spectrum, which makes it possible to sort out the features of the excitations in the CuO₂ plane and thus to characterize the electronic structure of the CuO₂ plane in the respective phase. Upon doping into the parent insulator La₂CuO₄ with a charge-transfer energy gap of about 2 eV the spectral weight is rapidly transferred from the charge-transfer excitation to low-energy excitations below 1.5 eV. The low-energy spectrum is apparently composed of two contributions; a Drude-type one peaked at ω=0 and a broad continuum centered in the midinfrared range. The high-T_c superconductivity is realized as doping proceeds and when the transfer of the spectrum weight is saturated. The resulting spectrum in the high-T_c regime is suggestive of a strongly itinerant character of the state in the moderately doped CuO₂ plane while appreciable weight remains in the charge-transfer energy region. The spectrum exhibits a second drastic change for heavy doping (x∼0.25) corresponding to the superconductor-to-normal-metal transition and becomes close to that of a Fermi liquid. The results are universal for all the known cuprate superconductors including the electron-doped compounds, and they reconcile the dc transport properties with the high-energy spectroscopic results.