Dynamic central peak in ferroelectric KH2PO4

A dynamic central peak of width ∼50 MHz has been observed in the depolarized-light-scattering spectrum of K${\mathrm{H}}_2$P${\mathrm{O}}_4$. It is seen only in the ferroelectric phase within ∼0.1°K of the transition, where its intensity exceeds that of the strongest Brillouin component, the $\mathrm{xy}$ transverse acoustic mode. We show that this strongly temperature-dependent feature arises from the coupling of the ferroelectric soft mode to the thermal diffusion mode in the ferroelectric (but not in the paraelectric) phase through the temperature dependence of the order parameter, the spontaneous polarization $P_3\mathrm{}\left(T\right)\mathrm{}$.