Optical properties of La1.85Sr0.15CuO4: Evidence for strong electron-phonon and electron-electron interactions

The optical properties of the high-Tc superconductor La1.85Sr0.15CuO4 have been measured over the far-infrared through ultraviolet frequency range at temperatures between 4 and 300 K. The frequency-dependent conductivity shows strong vibrational features in the far infrared (240, 355, and 495 cm-1) and two electronic transitions in the infrared (3500 cm-1/0.44 eV and 10 500 cm-1/1.3 eV). The measurements are interpreted within a model of strongly correlated electrons interacting strongly with vibrational modes.