Optical spectra of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$: Effect of carrier doping on the electronic structure of the ${\mathrm{CuO}}_2$ plane

Optical reflectivity spectra are studied for single crystals of the prototypical high-${\mathit{T}}_{\mathit{c}}$ system ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ 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 ${\mathrm{CuO}}_2$ planes. They are also extended to the perpendicular polarization to study anisotropy and to discriminate the contribution from the ${\mathrm{CuO}}_2$ 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 ${\mathrm{CuO}}_2$ plane and thus to characterize the electronic structure of the ${\mathrm{CuO}}_2$ plane in the respective phase. Upon doping into the parent insulator ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ 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-${\mathit{T}}_{\mathit{c}}$ superconductivity is realized as doping proceeds and when the transfer of the spectrum weight is saturated. The resulting spectrum in the high-${\mathit{T}}_{\mathit{c}}$ regime is suggestive of a strongly itinerant character of the state in the moderately doped ${\mathrm{CuO}}_2$ 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.