Dynamical study of a central-mode in KDP

Abstract A central-mode, which grows up near the phase transition temperature Tc of KDP, has been observed in the x(yx)y geometry by the use of a high-resolution light-scattering system. The spectral analysis of the central-mode has been carried out by using either a damped harmonic-oscillator model or a relaxational Debye model. The spectral curves calculated by both models are in good agreement with experimental curves. However, in the damped harmonic-oscillator model, the frequency and the damping constant of oscillator do not follow the prediction of the soft-mode model. On the other hand, in the case of Debye model, it is found that relaxation time shows a critical slowing down and static susceptibility obeys the Curie-Weiss law. Therefore, it is concluded that the origin of the central-mode is the fluctuating polarization which become divergent near Tc .