Polarization Fluctuations in a Ferroelectric Crystal

The theory of thermal fluctuations of electrical polarization in a ferroelectric crystal is considered with special attention to temperatures in the neighborhood of the Curie point. Models giving a second-order transition are discussed thermo-dynamically, and the free energy with its derivatives yield the mean value and variance of dipole moment. The fluctuations and their spectral density are related respectively to the real and imaginary parts of the susceptibility of the crystal in conformity with Nyquist's theorem, which is not violated by the presence of a constant applied electric field. The g–r. theorem is shown to be applicable to equilibrium and non-equilibrium conditions, and its extension to lambda-point phenomena generally is discussed.