Range of Validity of Landau Theory in Ferroelectric Critical Phenomena

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Abstract
The origin of a very small or no critical region observed in ferroelectric critical phenomena is discussed by the use of the Landau theory of phase transition. Whether or not the 4-th order term of the polarization fluctuation can be neglected at the free energy is adopted as a criterion for the validity of the classical theory of phase transition. In the electric dipole system where the dipole-dipole interaction acts between dipoles, only a transverse component of polarization fluctuations anomalously increases near the critical point, but the longitudinal ones do not. Taking into account the above anisotropy of the polarization fluctuations, the criterion for the validity of the classical theory is proved to be easily satisfied up to the critical point.

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