Evidence for strong electron–phonon coupling and polarons in the optical response of La2−xSrxCuO4

The normal state optical response of La2−xSrxCuO4 is found to be consistent with a simple multi–component model, based on free carriers with strong electron–phonon interaction, localized polaronic states near 0.15 eV and a mid–infrared band at 0.5 eV. Normal state reflectance and absorbance of La1.83Sr0.17CuO4 are investigated and their temperature dependence is explained. Both, the ac and dc response are recovered and the quasi–linear behavior of the optical scattering rate up to 3000 − 4000 cm is found to be consistent with strong electron–phonon interaction, which also accounts for the value of Tc. Although not strictly applicable in the superconducting state, our simple model accounts for the observed penetration depth and the optical response below Tc can be recovered by introducing a small amount of additional carriers. Our findings suggest that the optical response of La2−xSrxCuO4 could be explained both, in the normal and superconducting state, by a simple multi–fluid model with strong electron–phonon interaction if the gap symmetry and the temperature dependence of the 0.5 eV MIR band are adequately taken into account. 74.20.De, 74.25Fy, 74.25Gz, 74.72Dn Typeset using REVTEX 1