A microscopic theory for the central peak in hydrogen-bonded ferroelectrics

A microscopic theory for the central peak in hydrogen-bonded ferroelectrics is given. The transverse Ising model is treated by means of a Green's function technique to calculate the dynamical structure factor. It can be separated into three terms, which determine different mechanisms of the formation of the central peak: the relaxational mechanism, due to the coupling between the transverse soft mode and the longitudinal mode, the oscillatory mechanism, caused by the coupling between the transverse soft mode and the phonon mode, the third - ''mixed'' mechanism - can be treated as an effective coupling between the phonon and the longitudinal modes. The relaxational mechanism predominates in all hydrogen-bonded ferroelectrics while the ''mixed'' mechanism plays an important role in ferroelectrics with a large value of the tunneling frequency Omega, such as KDP and RDP.