Raman study ofLa2CuO4: Possibility of local symmetry breaking

Abstract

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 ${\mathrm{cm}}^{-1\mathrm{}}$. The latter line exhibits a very large intensity and has polarization features consistent with an in-plane O stretch vibration. The 426 ${\mathrm{cm}}^{-1\mathrm{}}$ line is assigned to the axial O stretch mode, also found in superconducting ${\mathrm{La}}_{2-x}{(\mathrm{B}\mathrm{a},\mathrm{S}\mathrm{r})}_x\mathrm{Cu}{\mathrm{O}}_4$. The prominent 526 ${\mathrm{cm}}^{-1\mathrm{}}$ 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.