Adiabatic and isothermal order-parameter susceptibilities and the thermal diffusion mode

Studies of the adiabatic order-parameter susceptibility near symmetry-breaking phase transitions show that the ratio S(t)S(-t) (in the limit t0+), and the proportion of the order-parameter spectral intensity associated with the thermal diffusion mode (in the limit t0-), are universal. The universal quantities are tabulated and discussed in the light of experiments on structural phase transitions, with particular emphasis on the dielectric properties of ferroelectrics and the scattering properties of ordered phases. New experiments are suggested. © 1980 The American Physical Society.