A MICROSCOPIC THEORY FOR THE CENTRAL PEAK IN HYDROGEN-BONDED FERROELECTRICS

A microscopic theory for the central peak in hydrogen-bonded ferroelec trics is given. The transverse Ising model is treated by means of a Gr een's function technique to calculate the dynamical structure factor. It can be separated into three terms, which determine different mechan isms of the formation of the central peak: the relaxational mechanism, due to the coupling between the transverse soft mode and the longitud inal mode, the oscillatory mechanism, caused by the coupling between t ile transverse soft mode and the phonon mode, the third - ''mixed'' me chanism - can be treated as an effective coupling between the phonon a nd the longitudinal modes. The relaxational mechanism predominates in all hydrogen-bonded ferroelectrics. while the ''mixed'' mechanism play s an important role in ferroelectrics with a large value of the tunnel ing frequency Omega, such as KDP and RDP.