Doping and temperature dependence of inversion symmetry breaking in La 2− xSr xCuO 4

Abstract

The doping dependence of the Raman spectra of high quality La 2− x Sr x Cu 16,18O 4 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 A 1-character of the activated modes in the symmetry reduced phase indicates a reduction from the D 2h to C 2v or D 2 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.