Acoustic phonons in the aperiodic layered crystal of Bi2Sr2CaCu2O8+δ

We present neutron scattering investigations of the structural dynamical properties of the aperiodic layered compound, Bi2Sr2CaCu2O8+delta. Acoustic phonons behave identically around the assumed average structure Bragg reflection and around the first satellite. This observation prove that the collective dynamics related to the incommensurate structure are quite different from what is expected in a modulated incommensurate model. Our data indicate two acoustic-like longitudinal branches along the incommensurate direction. Such behavior is compatible with a composite incommensurate structure where these two Bragg peaks correspond to the main reflections of each subsystem.