Raman study of La/sub 2/CuO/sub 4/: Possibility of local symmetry breaking

Raman spectra on flux-grown single crystals of La/sub 2/CuO/sub 4/ show two modes in the range of O stretch vibrations, near 426 and 526 cm/sup -1/. The latter line exhibits a very large intensity and has polarization features consistent with an in-plane O stretch vibration. The 426 cm/sup -1/ line is assigned to the axial O stretch mode, also found in superconducting La/sub 2-//sub x/(Ba,Sr)/sub x/CuO/sub 4/. The prominent 526 cm/sup -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.