Nature of anisotropic fluctuation modes in ordered systems

The nature of anisotropic fluctuation modes in an ordered system is analysed using general symmetry arguments. It is shown that the anisotropic fluctuation modes in a periodic phase can be classified using a wave vector within the irreducible Brillouin zone and a band index. The spatial profiles of the fluctuation modes are described by Bloch functions. These general features enable a study of the stability and kinetic pathway of complex ordered polymeric structures to be carried out. The utility of the theory is illustrated using the Landau-Brazovskii theory of weak crystallization.