A light-scattering survey of critical behavior in the phase diagram of KH2PO4

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Abstract
Recent Brillouin-Rayleigh scattering experiments performed on the xy shear mode of KDP near the ferroelectric transition temperature are reviewed. The static central peak has been removed by annealing, producing paraelectric spectra in perfect agreement with mean-field theory. Application of a static electric field causes the transition to become non-symmetry breaking. This suppresses the soft mode and generates a heat-diffusion central peak in the paraelectric phase. At the critical field there are no microscopic critical fluctuations, and hence no opalescence, although several thermodynamic quantities are predicted to diverge classically. Under sufficient hydrostatic pressure, the transition becomes continuous in the absence of field, and the spectra exhibit an increased relaxation rate.