The Discussion for Applicable Scope of Landau Theory of Phase Transition in Flunction of the Critical Point Nearby

The Landau theory of phase transition is a powerful tool which studies all kinds of structural phase transition of crystal. In the critical point nearby, some order parameters fluctuate heavily and it causes that the Landau theory of phase transition has not work. It needs to consider the Ginzburg gradient item in free energy density expansion. By dint of the relative probability and the Fourier component of connection function , we analyse the fluctuation of the order para- meters in the critical point nearby, and thus we obtain the applicable scope of the Landau theory of phase transition.