Direct Evidence of the Importance of Electron-Phonon Coupling in La2CuO4: Photoinduced IR Active Vibrational Modes

Abstract

Abstract We present direct evidence of the importance of the electron-phonon interaction in pure La2CuO4. Photoinduced IR active vibrational modes (IRAV) and associated bleaching of the La2CuO4 phonon modes are observed in the spectral range below 700 cm−1. The observation of photoinduced IRAV implies the existence of structural deformation around the photoexcited carriers, indicative of coupling of the photoexcitations to the lattice. We find, in addition, a broad photoinduced absorption which peaks at ≈ 0.5 eV, indicating an electronic transition deep within the energy gap. Some specific insight into the nature of the distortion around the photoexcitations can be obtained from comparison of the photoinduced bleaching at 218 cm−1 and 706 cm−1 and the doping induced bleaching (at 218 cm−1 and 704 cm−1). This remarkable similarity implies that locally in the vicinity of a photogenerated carrier, the CuO6 octahedra become tetragonal (as in the Sr-doped case). The possible importance of such a specific large local structural deformation around a carrier to the occurrence of high temperature superconductivity is quite obvious.