  A model of the premelting fluctuations is proposed, based on the Landau mean
field theory applied to a first-order phase transition. Using the thermodynamic
potential, the nonlinear Langevin equation for the order parameter is
formulated, which describes the dynamics of the phase transition considering
microscopic thermal fluctuations. The origin of the low-frequency fluctuations,
associated with a switching in a bistable system under the influence of a
thermal noise, is shown. Analytical two-scale model of the premelting
fluctuations is developed, with characteristic frequent small-amplitude
fluctuations of chaotic motion in the vicinity of potential minima and rarer
large fluctuations due to solid-liquid transitions. Based on the numerical
simulation of the solutions to the stochastic differential equation, both the
dynamics of the order parameter fluctuations and their spectrum are obtained.
The model qualitatively reproduces experimental spectrum of isothermal
fluctuations.
