Critical dynamics of displacive structural phase transitions and light and neutron scattering below Tc

The critical dynamics of the displacive structural phase transitions with the short-range interaction and one-component order parameter in the scaling region below Tc has been studied. The two-point retarded correlators of the order parameter and its square describing the spectral shape of light and neutron scattering have been calculated. It is shown that the observed anomalies in the low-frequency scattering near Tc are caused by the critical slowing down of two-particle collective excitations-the fluctuations of the photon density, and their coupling with soft phonons.