Observation of the wave-vector dependence of the central mode by hyper-Raman scattering and the $A_u$-optical-phonon–central-mode coupling in ${\mathrm{CsH}}_2$${\mathrm{PO}}_4$

Wave-vector dependencies of the central mode in quasi-one-dimensional hydrogen-bonded ferroelectric ${\mathrm{CsH}}_2$ ${\mathrm{PO}}_4$ are observed by hyper-Raman scattering at room temperature. The intensity and the half-width of the central mode are found to vary with the direction of the scattering wave vector because of the anisotropy of the dipolar interactions between the polarizations. The central mode also shows a prominent coupling with the 75-${\mathrm{cm}}^{\mathrm{-}1}$ $A_u$ optical-phonon mode.