Optical conductivity of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ and soft electronic modes

We calculate the optical conductivity of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ in the inhomogeneous Hartree-Fock plus random phase approximation using the p-d model with parameters taken from first-principles calculations. The ground state evolves from a polaron state for small and moderate doping to a conventional metal for x≳1/4 in consistency with transport and optical experiments. The calculated optical conductivity shows good agreement with experiments regarding the peak positions and relative intensities for zero and small dopings. The precursor of the midinfrared band is identified with transitions from the localized states on Cu to the extended states above the Fermi level.