Application of the Cluster Variation Method to Anisotropic Polarization Fluctuations in KD₂PO₄-Type Crystals above and below the Transition Temperature

抄録

The cluster variation method (CVM) in the cactus approximation is applied to a pseudo-spin Ising Hamiltonian of the Slater-Takagi model for KD₂PO₄-type hydrogen-bonded ferroelectrics to calculate the wave-number dependent susceptibility χ(q), mainly focussing on the ferroelectric phase.The strong anisotropy of polarization fluctuations along the easy z-axis is shown to appear not only in the paraelectric phase but also in the ferroelectric phase when the ice-rule limit is approached. An analytical expression of the spontaneous polarization is fully utilized for calculations on χ(q) below the transition temperature.