A 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 Cu O sub 4 show two modes in the range of O stretch vibrations two modes in the range of O stretch vibrations, near 426 cm sup (-1) 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-x) (Ba,Sr) sub x Cu O sub 4. The prominent 526 cm sup (-1) line is observed over the temperature range from 77K to 600K; 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 fluctuation.