New low-frequency optic phonon behaviour in single crystal incommensurate Bi-2212

Room temperature Brillouin light scattering experiments on single crystal Bi-2212 were conducted between 0-6 cm^-1 at 532 nm through use of an ultra-high contrast Fabry-Perot interferometer. From spectra collected very close to the crystallographic c-axis, an extremely low-lying optic phonon mode was observed near 6 cm^-1 (~200 GHz), while higher angle spectra show this mode to dissipate. This behaviour is highly reminiscent of other low-lying optic phonons in Bi-2212 whose origins have been recently connected to the crystals incommensurate structure. This work opens many questions surrounding phonon behaviour in BSCCO compounds, such as their role in unconventional superconductivity via electron-phonon coupling.