Proton-Phonon Coupling in Cs${\mathrm{H}}_2$ As${\mathrm{O}}_4$ and K${\mathrm{H}}_2$As${\mathrm{O}}_4$

We have observed strong anharmonic coupling between the pseudospin "tunneling" mode and an optical mode of ${\beta }_2$ symmetry in the paraelectric phase of cesium dihydrogen arsenate. At temperatures above $T_c=143\mathrm{}$ °K this interaction is manifest as an antiresonance interference shape in the Raman spectrum, similar to that in AlP${\mathrm{O}}_4$; however, the interacting modes in Cs${\mathrm{H}}_2$AS${\mathrm{O}}_4$ are heavily damped, in contrast to AlP${\mathrm{O}}_4$. The observed spectra are in good agreement with a spectral distribution function obtained from a Green's-function calculation. Very similar results are obtained for K${\mathrm{H}}_2$As${\mathrm{O}}_4$.