Acoustic phonons in the aperiodic layered crystal of Bi2Sr2CaCu2O8+delta

We present neutron scattering investigations of the structural dynamical pr operties of the aperiodic layered compound, Bi2Sr2CaCu2O8+delta. Acoustic p honons behave identically around the assumed average structure Bragg reflec tion and around the first satellite. This observation prove that the collec tive dynamics related to the incommensurate structure are quite different f rom 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 whe re these two Bragg peaks correspond to the main reflections of each subsyst em.