Evidence for Incommensurate Composite Behaviour in Bi-2212 Crystals by Brillouin Light Scattering Spectroscopy

Room-temperature phonon dynamics in crystals of the high-Tc superconductor Bi-2212 were probed using Brillouin light scattering spectroscopy along the incommensurate c-axis. The spectra contain peaks due to the Rayleigh surface acoustic mode, two quasi-transverse bulk acoustic modes, and two quasi-longitudinal bulk acoustic modes. The presence of two distinct quasi-longitudinal modes, instead of the usual one, is interpreted as evidence of the incommensurate nature of Bi-2212, and is consistent with the so-called composite model for incommensurate materials. Furthermore, a weak, higher-energy peak was observed in Brillouin spectra at a frequency shift of ~95 GHz and may be another manifestation of the incommensurate character of Bi-2212.