POLARONS AND STRONG ELECTRON-PHONON COUPLING IN THE OPTICAL-RESPONSE OF LA2-XSRXCUO4

A multiliquid approach, based on free carriers with strong electron-ph onon interaction, localized polaronic states near 0.15 eV, and a mid-i nfrared band at 0.5 eV, is applied to model the optical response of La 2-xSrxCuO4. The normal state reflectance and absorbance of La1.83Sr0.1 7CuO4 are investigated and their temperature dependence is explained. Both the ac and de response are recovered, and the quasilinear behavio r of the optical scattering rate up to 3000-4000 cm(-1) is found to be consistent with strong electron-phonon interaction, which also accoun ts for the value of T-c. In the superconducting state a comparison wit h optical data indicates the presence of additional carriers in the co nduction band. Model calculations suggest the 0.5 eV mid-infrared band as the source of these carriers and indicate a bipolaronic origin of this band.