Doping and temperature dependence of inversion symmetry breaking in La2−xSrxCuO4

The doping dependence of the Raman spectra of high quality La2−xSrxCu16,18O4 polycrystalline compounds has been investigated at low temperatures. It is shown that symmetry forbidden bands peaked at ∼150 cm−1, ∼280 cm−1, and ∼370 cm−1 are activated in the (xx/yy) polarization Raman spectra due to the local breaking of the inversion symmetry mainly at low temperatures and for doping concentrations for which the compound is superconducting. The apparent A1-character of the activated modes in the symmetry reduced phase indicates a reduction from the D2h to C2v or D2 crystal symmetries, which associates the observed modes to specific IR-active phonons with eigenvectors mainly along the c-axis. The temperature and doping dependence of this inversion symmetry breaking and the superconducting transition temperature are very similar, though the symmetry reduction occurs at significantly higher temperatures.