Raman study ofLa2CuO4: Possibility of local symmetry breaking

Raman spectra on flux-grown single crystals of ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ show two modes in the range of O stretch vibrations, near 426 and 526 ${\mathrm{cm}}^{\ensuremath{-}1}$. The latter line exhibits a very large intensity and has polarization features consistent with an in-plane O stretch vibration. The 426 ${\mathrm{cm}}^{\ensuremath{-}1}$ line is assigned to the axial O stretch mode, also found in superconducting ${\mathrm{La}}_{2\ensuremath{-}x}{(\mathrm{B}\mathrm{a},\phantom{\rule{0ex}{0ex}}\mathrm{S}\mathrm{r})}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$. The prominent 526 ${\mathrm{cm}}^{\ensuremath{-}1}$ line is observed over the temperature range from 77 to 600 K; i.e., even in the high-temperature tetragonal phase, where, by symmetry arguments, no in-plane O stretch mode is Raman allowed. We believe that the presence of this mode indicates local symmetry breaking, possibly due to antiferromagnetic fluctuations.