Symmetry breaking on the phonon Raman spectra only at the superconductor compositions in La_2-xSr_xCuO₄

Phonons in La_2-xSr_xCuO₄ were systematically investigated by Raman and infrared spectroscopy covering from the insulator, superconductor, to normal metal compositions. In the Raman spectra, modes which appear only at the superconductor compositions were found. Many of those modes were assigned to the infrared active transverse optical modes with the polarization vectors parallel to the CuO₂ plane. The Raman and infrared activity is mutually exclusive in the crystal with inversion symmetry as in the present crystal structure. The appearance of the infrared active modes in the Raman spectra implies that the doped carriers take the states which break the inversion symmetry of phonons only at the superconductor compositions. The larger scattering intensity of those modes than of the natively active modes indicates that the deformation-potential-type electron-phonon interactions are very large. The experimental fact that the longitudinal optical modes do not appear in the Raman spectra indicates that the Fröhlich-type electron-phonon interactions induced by the macroscopic electric field of the longitudinal optical modes do not work.