Optical spectra of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">La</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn><mml:mi mathvariant="normal">−</mml:mi><mml:mi mathvariant="italic">x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Sr</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="italic">x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CuO</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>: Effect of carrier doping on the electronic structure of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CuO</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>plane

Optical reflectivity spectra are studied for single crystals of the prototypical high-${\mathit{T}}_{\mathit{c}}$ system ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ over a wide compositional range 0\ensuremath{\le}x\ensuremath{\le}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 \ensuremath{\omega}=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\ensuremath{\sim}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.

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