Ultralow-frequency dispersion in macroscopic susceptibilities

A theory is presented which predicts a sample-geometry-dependent dispersion at very low frequencies in a general macroscopic susceptibility {χ} of a system, in a thermal bath, due to an adiabatic isothermal crossover. The dispersion occurs whenever the isothermal value χT is larger than the adiabatic value χS. The effect is analogous to the thermal-diffusion central peak. The dispersion is critical in the dielectric permittivity (magnetic permeability) of a system undergoing a second-order ferroelectric (ferromagnetic) phase transition. Measurements on the dielectric permittivity of ferroelectric KH₂{PO}₄$ and (${NH}₂CH₂COOH)₃{·}H₂{SO}₄$ demonstrate clearly the predicted dispersion, with features in agreement with the theory.