Evidence for strong electron-phonon coupling and polarons in the optical response of La_{2-x}Sr_xCuO_4

TL;DR

This study demonstrates that the optical response of La_{2-x}Sr_xCuO_4 in both normal and superconducting states can be explained by a multi-fluid model emphasizing strong electron-phonon coupling and polaronic effects, aligning with observed spectral features.

Contribution

It introduces a simple multi-component model incorporating strong electron-phonon interactions and polaronic states to explain La_{2-x}Sr_xCuO_4's optical properties across different states.

Findings

Strong electron-phonon interaction explains optical scattering rates.

Polaronic states near 0.15 eV are significant in the optical response.

The model accounts for penetration depth and optical response below T_c.

Abstract

The normal state optical response of La_{2-x}Sr_xCuO_4 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 La_{1.83}Sr_{0.17}CuO_4 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^{-1} is found to be consistent with strong electron-phonon interaction, which also accounts for the value of T_c. Although not strictly applicable in the superconducting state, our simple model accounts for the observed penetration depth and the optical response below T_c can be recovered by introducing a small amount of additional carriers. Our findings suggest that the optical response of La_{2-x}Sr_xCuO_4 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 mid-infrared band are adequately taken into account.