Nematic liquid crystals in contact with a flat substrate are studied by means of the Landau-de Gennes formalism. It is assumed that the substrate provides homogenous boundary conditions and that the nematic phase undergoes a first-order transition to the smectic-A phase in the bulk. Above the bulk transition temperature, a smectic-A film with layers perpendicular to the substrate can form if the surface field is sufficiently strong. This surface phase transition is found to be continuous in the mean-field approximation. Apart from the symmetry-breaking transition, an ordinary prewetting transition may occur. The effect of smectic layer fluctuations on the stability of the smectic film is discussed. It is suggested that the continuous surface transition can be a Kosterlitz-Thouless transition, where the defects are edge dislocations.
A.Poniewierski and A.Samborski         PHYSICAL REVIEW E, Vol. 51 , 4574, (1995)